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01_Intro.pptx 02_SystemOverview.pptx 03_OpticalChannel.pptx 04_Lasers.pptx 05_Modulation.pptx 06_Receivers.pptx 07_Amplifiers.pptx 08_Filters.pptx 09_Systems.pptx 10_Simulation.ppt 11_Outlook.pptx Lecture Optical Communications Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke Overview • Organization • History of Optical Communications • Structure of the Lecture • Goals 2 01 - Introduction Contact Slides of the lecture and excercises can be found here: http://www.tf.uni-kiel.de/etit/NT Password: LNT E-Mail: [email protected] E-Mail (for the excercises): [email protected] 3 01 - Introduction Organization • The lecture takes place each week on Monday from 10:15 – 11:45 • Exercises are scheduled every second week on Tuesday from 12:15 – 13:45 (starting 19.4.2016) • Examination: written examination (90 min) in the examination period after the course (currently planned for 06.10.16 9.00-10.30) 4 01 - Introduction Curriculum vitae • Born in Dortmund, 1977 • Master studies at City University, London Degree: MSc in Information Engineering, 2001 • Diploma studies at TU Dortmund Degree: Dipl.-Ing., 2002 • PhD studies at TU Dortmund – Chair of High Frequency Technology, funded by Siemens AG, Munich Degree: Dr.-Ing., 2005 • Diploma studies at Fernuni, Hagen Degree: Dipl.-Wirt.-Ing., 2005 5 01 - Introduction CV (contd.) • 2007-2011 Principal Engineer (Oberingenieur) at Chair of High Frequency Technology, TU Dortmund • 2011-2015 in various positions at ADVA Optical Networking SE (last as Principal Engineer in the CTO Office) • • • Project leader and manager (also as consortium lead) of several EUfunded research projects in the area of next-generation optical access networks and structural and functional convergence of fixed and mobile networks Development of novel system concepts for the second generation of 100 Gb/s transmission systems and next generation systems with 400 Gb/s data rate Support of the standardization activities in FSAN and ITU-T (Study Group 15, Question 2 and Study Group 15, Question 6) • Since April 2016 Professor of Communications at ChristianAlbrechts-Universität zu Kiel 6 01 - Introduction History 7 01 - Introduction History of Optical Communications (1) Smoke signals, light signals 1900 1925 1938 Analog transmission, one channel per telephone line Vacuum tube frequency division multiplexing Coaxial cables 1960 Invention of the laser (Maiman) 8 01 - Introduction History of Optical Communications (2) 1966 Proposal: Use of optical fibers for guiding of light (Nobel prize for Charles Kao in 2009) 1970 Reduction of fiber losses to 17dB/km (Corning) 1977 1.3 µm InGaAsP laser (Fiber attenuation @ 1.3 µm: 1 dB/km) 1978 First installation of an optical transmission system Multimode fiber, =0.8 µm, 50 Mb/s, 10 km regenerator distance 1980 Optimized optical transmission systems Multimode fiber, =1.3 µm, 100 Mb/s, 20 km regenerator distance 9 01 - Introduction History of optical communications (3) 1981 First demonstration of single-mode fiber (SSMF) 1987 Invention of the Erbium-Doped Fiber Amplifier (Univ. of Southampton) 1988 Further generations of optical transmission systems SSMF, =1.3 µm, 1 Gb/s, 50 km regenerator distance Reach limited by fiber attenuation (0.5 dB/km) 1990 10 Improved systems by use of single mode laser SSMF, =1.3 or 1.55 µm, 2.5 Gb/s, 100 km regenerator distance 01 - Introduction History of Optical Communications (4) 1992 1995 1996 1998 11 First work on optical transmission systems using fiber amplifiers and wavelength division multiplexing (WDM) First generation of optical WDM transmission systems SSMF, =1.55 µm, 8x2.5 Gb/s, 1200 km regenerator distance point-to-point transmission (static, manually configured) First field trials with Optical Cross-Connects (OXC) Availability of first commercial Optical Add-Drop Multiplexer (OADM) 01 - Introduction History of Optical Communications (5) 2000 Second generation of optical WDM transmission systems SSMF, =1.55 µm, 160x10.7 Gb/s, 1500 km regenerator distance automatic reconfigurability by OADM/OXC 2001 2002 12 Introduction of Automatically Switched Transport Networks (ASTN) standards by ITU-T 01 - Introduction History of Optical Communications (6) Introduction of phase modulated transmission systems 2004 Next generation of optical transmission systems SSMF, =1.55 µm, 96x43 Gb/s, 1500 km regenerator distance Introduction of coherent transmission systems (electronic pre-distortion, equalizer, DSP…) 2006 2010 Future? 13 First field trials of 112 Gb/s WDM transmission systems Next generation of optical transmission systems SSMF, =1.55 µm, 96x112 Gb/s, 3000 km regenerator distance Fiber-to-the-home (passive optical networks), Software-Defined Optics, 400 Gb/s WDM transmission systems, Optical Fronthauling (connection of mobile phone antenna sites) 01 - Introduction Bandwidth Increase Source: DE-CIX Internet Exchange New services (e.g. video streaming), higher access data rates (DSL, FTTH etc.) lead to exponential increase of the bandwidth. Challenges: Affordable, dynamic, energy efficient networks 14 01 - Introduction What happens in 1 min? 15 01 - Introduction Network View Edge Metro Access Core Long Haul/Ultra Long Haul Regional Metro Core Wavelength-Division Multiplexing (WDM) Network Element Time-Division Multiplexing (TDM) Network Element OXC DWDM mesh OADM Capacity EDFA Internet Service Provider (ISP) Metro DWDM Rings or Mesh TDM mesh Rings or Mesh Business User Campus EXC ADM Rings Video (O)ADM: (Optical) Add/Drop Multiplexer OXC: Optical Crossconect EDFA: Erbium-Doped Fiber Amplifier EXC: Electrical Crossconnect Data Voice Distance 16 01 - Introduction Contents (1) 1.Introduction 1.1 Contents of the Course 1.2 Overview: Optical Communications 1.2.1 Review 1.2.2 Relevance of Optical Communications 1.3 Basic System Overview 1.3.1 Generic Block Diagram of Optical Transmission 1.3.2 Basic Properties of the Optical Fiber Channel 1.4 Basic Concepts in Optical Communications 1.4.1 Time division multiplexing (TDM) 1.4.2 Wavelength division multiplexing (WDM) 1.4.3 Optical Time Division Multiplex (OTDM) 1.5 Evolution of Optical Networks 17 01 - Introduction Contents (2) 2. The Optical Transmission Channel 2.1 Optical Signals 2.2 Basic properties: Fiber Loss 2.3 Basic properties: Fiber Dispersion 2.4 Linear Channel Model of Optical Fiber 2.4.1 System Responses of Fiber 2.4.2 Influence of Dispersion on Data Transmission 2.4.3 Dispersion Compensation in Long Distance Links 2.5 Dispersion in Fibers 2.5.1 Polarisation Mode Dispersion 2.6 Nonlinear Fiber Effects 2.6.1 Linear and Nonlinear Fiber Properties 2.6.2 Optical Kerr-Effect 2.6.5 Influence of SPM on Signal Transmission 2.6.5 Soliton Transmission 2.7 Propagation Modes in Fibers, Characteristics of MMF and POF 18 01 - Introduction Contents (3) 3. Optical Transmitters and Modulators 3.1 Semiconductor Lasers 3.1.1 Materials 3.1.2 Basic Principle 3.1.3 Bandgap Model of Semiconductor Lasers 3.1.4 Laser Beam Confinement 3.1.5 Recombination 3.1.6 Technical Implementation 3.1.7 Fabry-Perot Resonator, Lasing Condition 3.1.8 Single Mode Lasers 3.1.9 Rate Equations 3.1.10 Light-Power-Current Characteristics 3.1.11 Direct Modulation of a Laser 3.1.12 Laser-Chirp 3.1.13 Small-Signal Properties, Laser-Frequency Response 3.2 External Modulators 3.2.1 Electro-Absorption-Modulator (EAM) 3.2.2 Mach-Zehnder-Modulator (MZM) 19 01 - Introduction Contents (4) 4. Optical Receivers 4.1 Block Diagram 4.2 Optical to Electrical Conversion 4.3 Noise Performance 4.3.1 Quantum Limit 4.3.2 Noise Performance of Practical Receivers 5. Optical Amplifiers 5.1 Overview, Basic Concepts 5.2 Semiconductor Laser Amplifiers 5.3 Fiber-Optic Amplifiers 5.3.1 Building Blocks of an Erbium-Doped Fiber Amplifier 5.3.2 Optical Amplification 5.3.3 EDFA Noise 6. Optical Filters 6.1 Fabry-Perot Filter 6.2 Fiber Gratings 6.3 Optical Delay Line Filters 6.3.1 Basics 6.3.2 Mach-Zehnder Filters 20 01 - Introduction Contents (5) 7. Optical Transmission Systems 7.1 Modulation Formats 7.2 Transmitter and Receiver Design 7.3 Coherent Transmission 8. Simulation 8.1 Propagation Equation, Nonlinear Schrödinger Equation 8.2 Numerical Solution: Split-Step Fourier Method 9. Outlook 21 01 - Introduction 22 01 - Introduction Literature Fundamentals of Optical Communications • E. Voges, K. Petermann, „Optische Kommunikationstechnik“ ISBN: 3-540-67213-3 (only in German) • G. P. Agrawal, „Fiber-Optic Communication Systems“ ISBN: 0-471-21571-6 • I. P. Kaminow, T. Li, A. E. Willner, „Optical Fiber Telecommunications V B: Systems and Networks“ ISBN: 0-12-374172-1 • M. Seimetz: “High-Order Modulation for Optical Fiber Transmission”. Springer Series in Optical Sciences, 2009 23 01 - Introduction Literature Optical Networks • R. Ramaswami, K. N. Sivarajan, „Optical Networks“ ISBN: 1-55860-655-6 • B. Mukherjee, „Optical WDM Networks“ ISBN: 0-387-29055-9 • S. Pachnicke, „Fiber-Optic Transmission Networks“ ISBN: 978-3642210549 24 01 - Introduction Lecture Optical Communications Basic System Properties Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke Click To Edit Master Title Style Overview: Optical Communications Review Optical telegraph network in France 1830 – 1840 Source: V. Aschoff 2 02 – Basic System Properties Click To Edit Master Network Title Style Optical Telegraph in France 1830-1840 "Transmitter" 3 "Receiver" 02 – Basic System Properties Global Data Traffic (PB/Month) Click To of Edit Master in Title Style Increase Capacity Communication Networks Quelle: R. Essiambre, et al, Proc. of IEEE, 2012. Year Exponential growth of traffic volume 4 02 – Basic System Properties Click To Edit Master Title Style Relevance of Optical Communications Advantages of fiber optics: • Very high usable bandwidth • Electromagnetic compatibility (EMC) Disadvantages: • Cables/fibers necessary (compared to radio) • Expensive optical components, lower price reduction (compared to electronics) Applications: • Long distance digital transmission at high capacity in wide area networks (WAN) and metropolitan areas (MAN) • Fiber to the home (FTTH), Fiber to the curb (FTTC) – used for VDSL, hybrid fibercable TV distribution systems (CATV) – used for DOCSIS • Replacement of electrical lines, if EMC problems exist (optical cabling in cars, airplanes, ships, big data centres, optical “bus" at back-plane cabling) • "Radio over Fiber" RF-signal feeding of mobile radio base stations • Optical wireless communications: Free space transmission, terrestrial (short) as well as in space (very long distance), submarine, in-door visible light communications 5 02 – Basic System Properties Optical Network Deutsche Telekom Click ToFiber Edit Master TitleofStyle (Core Network) D Ä N E M A R K • Approx. 130,000 km of optical cables • Approx. 40 fibers per cable (average) 5.2 Mio. km deployed fibers • Mostly Standard Single Mode Fiber (SSMF) • Data Rates (interface rates): up to 100 Gb/s per fiber and WDM channel NIEDERLANDE • Typically WDM-links installed (e.g. 96 wavelenghts @ 40Gb/s per fiber) BELGIEN LUX G BUR EM TSCHECHOSLOWAKEI Offenbach/ Straßburg Passau/ ? FRANKREICH Traunstein/ Salzburg SCHWEIZ 6 LIECHTENSTEIN ÖSTERREICH 02 – Basic System Properties Click To EditOptical MasterSubmarine Title Style Network Worldwide Source: Telegeography 7 02 – Basic System Properties Click To Submarine-Systems Edit Master Title Style Optical • Channel data rate 10 Gb/s … 100 Gb/s • Maximum link-length: 9000 km • Optical amplification: typ. 40km distance • 25 years of continuous operation Submarine-Cable 8 Trans-Pacific-System 02 – Basic System Properties Click MasterPremises/ Title StyleCurb (FTTx) FiberTo to Edit the Home/ GPON TDMA-PON Passive Splitter Central Office NG-PON2 WDM-PON Central Office WDM Mux/Demux + 1 fiber to the central office + 1 transceiver in the central office – shared multi-point connection – low reach (ca. 10 km) – Bit rate / fiber = N x end-user bitrate + Physical point-to-point connection + Medium reach(ca. 40 km) + 1 fiber in the central office + 1 transceiver in the central office + Bit rate / fiber = end user bitrate Acronyms: TDMA=Time Division Multiple Access; WDM=Wavelength Division Multiplex GPON=Gigabit capable passive optical networks; NG-PON2=Next Generation–PON2 • • • • 9 lower data rates (up to 10Gb/s/l) short distance (typ. ~ 20 km) low cost required passive optical networks (PON) no electrically power components required in the field 02 – Basic System Properties Click To EditOverview Master Title Style Basic System Laser diode A basic optical transmission system Intensity modulation with digital on-off keying at transmitter (TX) opt. noise ok_1_optkanal.dsf Digital Source Detector Modulator Direct detection with photo diode at receiver (RX) Transmitter A state of the art (high-end) optical transmission system QPSK-modulation, optical I/Qmodulator, dual polarizations (x/y) at TX Coherent detection, local laser, balanced photo receivers, ADC and digital signal processing at RX 10 electr. noise 02 – Basic System Properties opt. ampl. opt. filter opt. fibre Optical channel Photo diode Receiver electronics Receiver Digital sink Generic Diagram of Optical Click ToBlock Edit Master Title Style Transmission Using Intensity Modulation / Direct Detection Laser diode opt. noise electr. noise ok_1_optkanal.dsf Digital Source Detector Modulator Transmitter opt. ampl. opt. filter opt. fibre Optical channel Photo diode Digital sink Receiver electronics Receiver Basic optical transmission link with transmitter, channel, and receiver. Optical signals (thick, green) and electrical signals (thin, black). Optical signals are carrier signals at optical frequency (~193 THz) modulated with the digital data signal. 11 02 – Basic System Properties Click To Edit Master Title Style Laser Diode Laser diode opt. noise electr. noise ok_1_optkanal.dsf Digital Source Detector Modulator Transmitter opt. ampl. opt. filter opt. fibre Optical channel Photo diode Digital sink Receiver electronics Receiver Laser diode: Generates (nearly) single frequency light with well defined wavelength = carrier signal at frequency f C c / lC (Electromagnetic wave). f C Lies in the range of several hundred THz! (1 THz = 1012 Hz). The light signal, i.e. the electromagnetic wave is usually simply called "electric field“ E(t) 12 02 – Basic System Properties Click To Edit Master Title Style Modulator Laser diode opt. noise electr. noise ok_1_optkanal.dsf Digital Source Detector Modulator Transmitter opt. ampl. opt. filter opt. fibre Optical channel Photo diode Digital sink Receiver electronics Receiver Modulator: The digital data signal is modulated on the carrier (here light) by the modulator. Most modulators have the capability to modulate the power of the light, i.e. the instantaneous optical power (=light intensity) is proportional to the modulating data signal of e.g. 10 Gb/s. The traditional basic modulation format is On-Off-Keying (OOK) and Intensity Modulation (IM) With binary signal: “1“ = Light on, “0“= Light (almost) off. In some (low cost) applications with relatively low bit rates the modulator can be omitted and the laser can be directly modulated by varying the input (injection) current into the laser diode (directly modulated laser, DML) 13 02 – Basic System Properties Click Fiber To Edit Master Title Style Optical Laser diode opt. noise electr. noise ok_1_optkanal.dsf Digital Source Detector Modulator Transmitter opt. ampl. opt. filter opt. fibre Optical channel Photo diode Digital sink Receiver electronics Receiver Optical fiber: Guided wave propagation in silica glass fiber. Fiber typically measures 125 mm in diameter with a "core" in center of ≈ 9 mm (single mode) or ≈ 50 mm (multi mode) in diameter, where core and the outer cladding both are silica glass, the core has slightly higher refractive index (through doping). Very low loss, huge bandwidth! The fiber channel may be modeled as a linear system (to first order approx.!) with transfer function H(f) of the electrical field E1(t) at the fiber input and E2(t) at the fiber output. 14 02 – Basic System Properties Click Amplifiers To Edit Master Title Style Optical and Filters Laser diode opt. noise electr. noise ok_1_optkanal.dsf Digital Source Detector Modulator Transmitter opt. ampl. opt. filter opt. fibre Optical channel Photo diode Digital sink Receiver electronics Receiver Optical amplifiers: Broadband amplifiers for optical signals. At amplifier input, light of a pump laser (high power) and the optical signal to be amplified are combined and coupled into several meters of Erbium doped fiber (EDFA=erbium doped fiber amplifier). This leads to amplification of the incident wave by stimulated emission. But amplification of EDFAs is only possible in the wavelength range of 1530nm ...1570nm (191THz ... 197THz), i.e. in third optical window! Opt. amplifiers are used as booster amplifiers (at transmitter), in-line amplifier (repeater in long distance links), and pre-amplifier (at receiver input). Optical filters: Applications (i) filtering of broadband optical noise produced e.g. by optical amplifiers (ii) equalizers of optical signals e.g. dispersion compensation. Various physical realizations, described by transfer functions, bandwidth, etc. (as usual) 15 02 – Basic System Properties ClickDiode To Edit Master Title Style Photo Laser diode opt. noise electr. noise ok_1_optkanal.dsf Digital Source Detector Modulator Transmitter opt. ampl. opt. filter opt. fibre Optical channel Photo diode Digital sink Receiver electronics Receiver Photo Diode: Optical-to-electrical (o/e) conversion by detection of received optical power (envelope detector). PIN (positive, intrinsic, negative)-diode (very fast, low sensitivity) or APD = Avalanche Photo Diode. (high sensitivity, slower, more noise) The received instantaneous light power results in a proportional electrical signal (photo current), which drives a trans-impedance (TI)-amplifier so that a voltage can be processed in the detector (clock and data recovery, CDR) where the bits are retrieved. Noise: 16 Is present in the optical domain (mainly from optical amplifiers) as well as in the electrical domain (from electronics) and from o/e-conversion itself (shot noise) 02 – Basic System Properties Click To Edit Master Title Style Modelling Laser diode opt. noise electr. noise ok_1_optkanal.dsf Digital Source Detector Modulator Transmitter Remark: opt. ampl. opt. filter opt. fibre Optical channel Photo diode Digital sink Receiver electronics Receiver The generic transmitter / channel receiver configuration shown above uses intensity modulation and direct detection (IM/DD configuration). In terms of communications theory this is a pretty simple approach. However, it has been used widespread until recently. Sufficient for data rates up to ~10 Gb/s. Today much more advanced transmitter and receiver setups are under investigation and are implemented in new (e.g. 100-400 Gb/s) systems. The full range of tools from communications theory is used (like coding, equalisation, signal processing, multilevel QAM, etc.) 17 02 – Basic System Properties Click ToCommunications: Edit Master Title Style Optical Some Impressions… Lab view (Coriant) Lab view (NT Kiel) Lab equipment with several 100 km of fiber spools (Dt. Telekom) Very high speed photo diode 18 02 – Basic System Properties High speed receiver SiGe-chip Click Edit Master TitleOptical Style Fiber Channel BasicTo Properties of the Electromagnetic Spectrum 19 02 – Basic System Properties Frequencies and Wavelengths of Click To Edit Master Title Style Optical Signal Transmission 1000 Multimode Optical Fiber 900 Singlemode Optical Fiber Polymer Optical Fiber Frequency [THz] 800 700 f c l 600 500 400 Visible Range 300 200 Infrared Light (IR) 100 300 400 500 600 700 800 900 1000 1100 Wavelength l [nm] 20 02 – Basic System Properties 1200 1300 1400 1500 1600 Loss Single Mode ClickofToStandard Edit Master Title StyleFibers (SSMF) over Wavelength 1.4 C-Band L-Band O-Band Typical Values 1.2 Loss in dB/ km 1 0.4 dB/km @ 1310nm 0.2 dB/km @ 1550nm OH-Absorption 0.8 SiO2 (IR)Absorption RayleighScattering 0.6 0.4 0.2 0 1250 21 Bendinglosses 1300 1350 1400 1450 1500 Wavelength in nm 1550 02 – Basic System Properties 1600 1650 Chromatic Dispersion of Different Click To Edit Master Title Style Single Mode Fiber Types over Wavelength 25 O-Band C-Band L-Band Dispersion in ps/ (nmkm) 20 15 10 5 Dispersion Shifted Fiber 0 -5 -10 1250 22 1300 1350 1400 1450 1500 Wavelength in nm 1550 02 – Basic System Properties 1600 1650 Click To Edit Master Title Style Basic Mathematical Model of Single Mode Fiber Loss and dispersion Linear system Equivalent LP Transfer Function All-pass with quadratic phase response: H LP a dB L ( f ) 10 20 23 c DL f 2 e loss L: l c: adB: D: j lc2 dispersion fiber length in km wavelength loss in dB/km dispersion parameter Response to a rectangular pulse (25ps ≙ 40Gb/s data rate) 02 – Basic System Properties Click To Edit Master Title Style Multimode and Single Mode Silica Fibres 24 02 – Basic System Properties Click To Edit Master Title Style Standard Single Mode Fiber 25 02 – Basic System Properties Cross Sections of Commercial Click To Edit Master Title Style Optical Fiber Cables 26 02 – Basic System Properties Click ToConcepts Edit Master in Title Style Communications Basic Optical Time division multiplexing (TDM) Data streams of • several bit rates (e.g. 64 kb/s, 2.048 Mb/s, 140 Mb/s) from • several users Principle of TDM Multiplexer ok_1_optkanal.dsf ch1 are aggregated to (very) high speed data rates for transmission in the network ch2 MUX ch3 ch4 clock 27 02 – Basic System Properties Click To Edit Master Title Style Increase of Transmission Capacity (E)TDM (electrical) Time Division Multiplexing 2.5 Gb/s 10 Gb/s 40 Gb/s (100 Gb/s / 400 Gb/s) steps in the SDH-Hierarchy. Each time factor of 4 starting from an STM-1 Container with 155 Mb/s data rate. A single optical carrier with bandwidth according to data rate. Problems: Availability of high speed electronics. Transmission problems due to high signal bandwidth Chromatic Dispersion (CD) and Polarisation mode dispersion (PMD). Today: Data rate (here: the interface rate) is limited to approx. 400 Gb/s. The Baud rate (symbols per sec.) is limited to approx. 100 GBaud 28 02 – Basic System Properties Click To Edit Master Title Style Wavelength Division Multiplexing (WDM) Data streams of several light paths from • several transmitters with lasers at different wavelengths • each carrying a high speed data signal are combined to a multiwavelength optical WDM signal and transmitted over a single fibre Principle of WDM Multiplexer l1 l2 l3 l4 ch1 ch2 ch3 ch4 WDMMUX l1 l2 l3 l4 ok_1_optkanal.dsf l1 l 2 l 3 l 4 29 02 – Basic System Properties Click To Edit MasterAllocation Title StyleGrid for WDM Networks The ITU-T Channel ITU-T recommendation G.694.1 in 2002 Dense Wavelength Division Multiplexing L-Band C-Band S-Band 186,00 … 190.90 191,00 … 195.90 196,00 … 200.90 1611.79 … 1570,42 1569.59 … 1530,33 1529.55 … 1492,25 Channel Spacing: 100GHz 200/50/25/12.5GHz also allowed No. of channels: 96 in each band (at 50 GHz spacing) Center frequency: 193.10THz = 1552.52nm 30 02 – Basic System Properties A WDM Multi-Span Transmission Link with Click To Edit Master Title Style Gain- and Dispersion-Management The transmission link is segmented in several "spans" where each span consists of transmission fiber (typically 40…120km), an optical amplifier (loss compensation) and a dispersion compensator (e.g. DCF=dispersion compensating fiber) Data 1 ocn_7_wdm system.dsf l1 Data 2 1 span l2 M U X Data 3 l3 Data 1 l1 Data N lN LD MOD optical amplifiers & dispersion compensation Data 2 D l E 2 M l U 3 X Data 3 lN BP LD: Laser Diode, MOD: Modulator 31 BP: Band-pass, PD: Photo Diode 02 – Basic System Properties Data N PD Click To Time Edit Master Title Style Optical Division Multiplexing (OTDM) an ch nel an 1 ch nel an 2 ch ne an l 3 ne l4 ch ... ch an ch nel an 1 ne l2 TDM in optical domain. Binary data (Bits) are represented as very short optical pulses within a short time slot for each channel. ... ok_124_systeme_wdm_und_otdm.dsf 100ps time --> Problems: Generation of short pulses. Timing jitter of pulses (phase fluctuations), very high signal bandwidth (distortions by dispersion, PMD) 32 02 – Basic System Properties Click To Edit Master Title Style Wrap Up What you should recall from this chapter: • • • • • • • • • • • • 33 Describe in detail the various multiplexing principles: WDM, OTDM, (E)TDM. What kind of network elements are typically used in optical networks? What is the meaning of “span” in optical communication links? What are the dimensions of core and cladding in SMF and MMF? Where is zero dispersion in SSMF; what is the value of the dispersion parameter at 1550 nm? What is the centre wavelength of C- and L-bands; what are the related frequencies? What are typical loss coefficients dB in the C-band and O-band? What are the approximate wavelengths of red and violet visible light? Plot the generic block diagram of optical transmission and explain each block in detail. What is the typical span length in optical submarine systems? What is the value for the vacuum speed of light? Describe fiber optic applications in various parts of optical communication networks. 02 – Basic System Properties Lecture Optical Communications Optical Channel Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke Click To Edit Master Title Style The Optical Transmission Channel Optical Signals Physically the optical signal is an electromagnetic wave travelling through a wave guide (i.e. the fiber). Wave propagation is described by Maxwell’s Equations (ME). However, for communications purposes, we simplify and use a “signals & systems” notation. This is exactly the same approach as e.g. in radio or electrical wire lines communications! In addition, for communication purposes we have to modulate the wave, either in amplitude or in phase or both. 2 03 – Optical Channel Click To Edit Master Title Style Communications in a bandpass channel: sBP (t ) a (t ) cos wc t + (t ) + Re a (t )e j (t ) e j (wct + ) (2.1-1) In Optical Communications it is common use to denote sBP (t ) E (t ) and z (t ) A(t ) E (t ) a (t ) cos wc t + (t ) + Re a (t )e j (t ) e j (wct + ) (2.1-2) wc 2 fc 2 c / c carrier frequency (from laser=carrier) typically ≈ 200THz A(t ) 3 Complex envelope typically a few GHz bandwidth 03 – Optical Channel Click To Edit Master Title Style Power, Intensity Power, Intensity It is relatively easy to exploit the intensity of an optical signal, whereas it is more complicated to deal with the full optical signal, including carrier frequency and phase (coherent optics) Examples: A photo diode “measures” the intensity because its output current is proportional to the incoming light power ( optical receiver). Laser output light power is proportional to the injection current ( optical intensity modulation) The signal (i.e. the electric field) E(t) has instantaneous optical power (=intensity) P (t ) A(t ) A* (t ) | A(t ) |2 | a (t ) |2 (2.1-3) The intensity or instantaneous power is typically a time domain signal with GHz bandwith and should not be mixed up with the mean optical power (to be measured by a power meter) 4 03 – Optical Channel Click To Edit Master Title Style Optical Polarization Two polarizations of an optical signal If we look closer into wave propagation (resulting from Maxwell’s Equations), we find that there exist two orthogonal polarization planes, namely a horizontal and a vertical polarization plane, also called x- and y-polarization. It is possible to extract or combine the signals from their individual polarization planes (polarization beam splitter/combiner PBS or PBC). Thus we can extend our signal description E x (t ) a x (t ) Eˆ x cos wct + x (t ) + x E(t ) ˆ E ( t ) a ( t ) E cos w t + ( t ) + y y y y y c (2.1-4) We have now two signals that propagate in the fiber. Each of these signals may be modulated individually. denotes the relative phase difference and Ê denotes the relative amplitude difference of the x- and y-polarisation signals. 5 03 – Optical Channel Wave To Propagation Click Edit Master Title Style In communications we are interested in the signals at the input and at the output of the channel, here the fiber with length L. I. e. we observe the wave at fixed positions z ( z 0 at fiber input and z L at fiber output). If we are interested how the wave propagates along the fiber, we have to evaluate the wave equations resulting from ME. It turns out (by solving the wave equation for the boundary conditions given by the properties of the optical fiber and assuming a harmonic steady state solution and considering one polarization and assuming lossless fiber) that the light wave signal can be described as E ( z , t ) Re Eˆ e - j z e jwC t e j Eˆ cos - z + w0t + Phase (2.1-5) This is a single frequency i.e. unmodulated, continuous wave (CW) signal at frequency w0 [1/s] that travels along the z-axis, i.e. the longitudinal axis of the fibre with propagation constant [1/m]. 6 03 – Optical Channel Click To Edit Master Title Style For a fixed time instant t t1 we see a cos-function vs. distance z with period n For a fixed location z z1 we see a cos-function vs. time t with period T t=t 1 t=t1 +Dt Dz z E ( z , t ) Re Eˆ e - j z e jwC t e j Eˆ cos - z + w0t + Phase . We observe the propagating wave at a fixed location on z-axis. During the time interval T (i.e. one period in time), the wave has moved a distance n 2 2 w0 (i.e. one full cycle in space) which is the wave’s wavelength. This distance n depends on the travelling speed of the wave. It is n v p T , where v p is called the phase velocity of the light wave, which depends on the refractive index n of the material within which the wave is travelling. We have: 7 n 1.00 for vacuum = free space ≈ air n 1.45 for silica glass (fibre) at 3rd optical window, slightly depending on wavelength 03 – Optical Channel Click To Edit Master Title Style Actually the refractive index is calculated by dividing the speed of light c in a vacuum by the speed of light v p in some other medium. The refractive index in vacuum is therefore 1, by definition. Thus vp c n (2.1-6) Note: Compared to light propagation in vacuum light travels through fiber ( n 1.45 ), with reduced speed ( c / n 200, 000 km / s ) with reduced wavelength ( c / n ) frequency remains unchanged! Note: Wavelengths are specified as vacuum (free space) wavelengths c e.g. a laser radiates at c 1550 nm it’s frequency is f c 8 03 – Optical Channel c c 193.548 THz Propagation Click To EditConstant Master Title Style The propagation constant is frequently used for characterising wave propagation wc w 2 c n n k n wc 0 0 r vp c c (2.1-7) k propagation constant in vacuum (also called wave number) c 1 0 0 speed of light in vacuum n r relative electric field constant (relative permittivity) in lossless material After travelling a distance L through the fiber we get (assume 0 in (2.1-5)): L E ( L, t ) Re Eˆ e - j L e jw0t Eˆ cos w0t - L Eˆ cos w0 (t ) w 0 (2.1-8) I.e. we observe a phase shift L or equivalently a time delay of L / wo phase shift L of the carrier wave (many millions of 2 cycles!) propagation delay L / wo of the carrier (a few ms per km fibre) sometimes called "phase delay" 9 03 – Optical Channel Click To Edit Master Title Style Exercise: Calculate the phase shift and the phase delay per km of fiber. Calculate the “length” of a 100 ps bit duration Note As refractive index and thus propagation constant are slightly wavelength and thus frequency dependent, each frequency component of a modulated optical signal experiences a slightly different delay/phase shift. fiber dispersion, phase distortions due to fiber dispersion! 10 03 – Optical Channel Other To Light Models Click Edit Master Title Style Geometrical optics = ray optics (objects/dimensions > ) Light as particle = quantum mechanic model emission and absorption Wave optics (small Objects ) electromagnetic waves Maxwell’s equations for electric and magnetic field (vectors) Light propagation: = Change of field with time and space E x x, y , z , t E r , t E y x, y , z , t E x , y , z , t z 11 (2.1-9) 03 – Optical Channel Fiber Loss Click To Edit Master Title Style 12 03 – Optical Channel Attenuation Constant Click To Edit Master Title Style In a bandwidth up to several 10 GHz, attenuation is constant and thus not wavelength or frequency dependent. (Remember: | Df | c | D | / C2 , i.e. 1nm corresponds to 125 GHz at 1550nm!). Therefore, fiber loss reduces the optical signal by a constant factor (we do not have to consider a frequency dependent attenuation by a transfer function H (w ) ) Power of light decreases exponentially with fiber length L. P adB L -10lg L P0 a - dB L P ( L) 10 10 P (0) also used: attenuation coefficient (in Neper) a (ln10) 13 adB 10 P( L) e- a L P(0) 03 – Optical Channel Attenuation Mechanisms Click To Edit Master Title Style In fibers we observe: (i) absorption loss, (ii) scattering loss, (iii) bending loss, In a fiber communication link we have additional (iv) insertion loss Absorption losses: Dominates at high wavelengths (>1600nm). Light interacts with fiber material. Impurities cause mechanical vibrations heat absorption of energy. Impurities of water vapour (OH ions) large absorption peaks at 1400nm, 1240nm, 950nm "transmission windows" (peaks can be nearly eliminated in today's extremely pure production processes "all wave fiber™") Scattering losses: Dominant at low wavelengths (<100nm). Scattering losses arise from microscopic variation in material density (structural inhomogeneities). Rayleigh scattering is proportional to -4 (sharp decrease with ). 14 03 – Optical Channel Click To Edit Master Title Style Bending losses: Energy is radiated out of the fiber due to macrobending (large curvature) and microbending (random microscopic bends of fiber axis due to stress in cables or irregularities of core/cladding boundary) Insertion losses determines the total fiber link loss, due to splices, connectors or components (couplers, filters,...) 15 03 – Optical Channel Fiber Dispersion Click To Edit Master Title Style Dispersion is a major impairment in optical fiber transmission. The background is as follows: We have seen that the fiber loss is not frequency dependent. This means that the magnitude of the fiber’s frequency response | H ( f ) | is constant over the frequencies of interest. Therefore we do not expect any filtering effects (LP or BP) on the transmit data signal caused by magnitude response. The phase of the fiber’s frequency response b( f ) - arg H ( f ) , however, turns out to be frequencydependent and is not a linear function of frequency (not of linear phase). Thus due to the nonlinear phase property we expect signal distortions caused by the phase response. From (2.1-5): L E ( L, t ) Re Eˆ e- j L e jw0t Eˆ cos w0t - L Eˆ cos w0 (t ) w0 We know that an unmodulated wave with frequency w0 experiences a phase shift of L and a phase delay of L / w0 16 03 – Optical Channel Click To Edit Master Title Style We assume now a modulation of the optical signal with carrier frequency wc . j (t ) - L E ( L, t ) Re Eˆ a (t )e e jwC t Eˆ a(t ) cos wct + (t ) - L A modulated signal always has a spectrum of non-zero width, i.e. spectral components in the vicinity of the carrier frequency. As is frequency-dependent, each spectral component experiences a different phase shift L w0 c n(w ) L (w ) L b(w ) . (2.3-1) In frequency domain the phase response is described by the exp-function: H (w ) e - j (w )L e- jb (w ) Thus we have a phase response b(w ) from which we can derive the group delay, which is per definition: g d b(w ) / dw . 17 (2.3-2) 03 – Optical Channel Click To Edit Master Title Style Note: Unlike phase delay, which gives the propagation delay time (latency), group delay is not a physical time delay. It is primarily used to describe the phase response. We know that deviations from a linear shape of the phase response b(w ) result in signal distortions (called phase distortions). Thus from the definition of group delay, deviations from a constant value of the group delay g (w ) result in these distortions. It is simply more convenient to evaluate deviations from a constant than from a linear function. 18 03 – Optical Channel TaylorTo Series of the Phase Click EditExpansion Master Title Style Response b(w ) L (w ) For small deviations Dw w - wc from (light) carrier frequency, i. e. for w wc : w D (w ) c + (w - wc ) + w w w c 1 1 2 (w - wc )2 2 w 2 w w c + 2 1 3 (w - wc )3 + ... 6 w 3 w w c 3 (2.3-3) Therefore for small Dw : 1 1 b(Dw ) L (Dw ) L c + 1Dw + 2 Dw 2 + 3Dw 3 + 2 6 const. linear squared cubic Lc constant phase shift of carrier "phase dealy" L1 linear phase contribution constant group delay" L 2 quadratic phase contribution L3 cubic phase contribution 19 03 – Optical Channel (2.3-4) Click To Edit Master Title Style As group delay can be easily measured, it is common to define a dispersion parameter D , which finally characterizes the influence of frequency-dependent phase response on optical signal transmission. D 1 d g L d (2.3-5) D is given in ps (group delay variation)/nm (wavelength variation) per km (fiber length). D is also called Chromatic Dispersion (CD) parameter or group velocity dispersion parameter. 20 03 – Optical Channel Dispersion over Wavelength Click To Edit Master Title Style @1310nm: D 0 ps/ (nm km) 2nd optical window @1550nm: D 17 ps/ (nm km) 3rd optical window Dispersion in ps/(nm km) 40 30 D 20 10 0 -10 -20 1,1 1,2 1,3 1,4 1,5 1,6 Wavelength in µm 21 03 – Optical Channel 1,7 Phase Response of the Fiber Click To Edit Master Title Style By zooming in, we find that D is constant over a reasonably wide -range (and thus w-range) for a given c D const. g linear over g linear over w b quadratic over w Now we calculate for small deviations D in the neighborhood of c DL d g d g (D ) DL d D DLD + c. c c2 with D Dw we find group delay as a function of Dw 2 c g (Dw ) - DL c2 Dw + c. 2 c The linear group delay term results in a quadratic phase term b(Dw ) g (Dw )d Dw - DL 22 c2 Dw 2 + c.Dw + d . 2 c 2 03 – Optical Channel Click To Edit Master Title Style 1 1 b(Dw ) L (Dw ) L c + 1Dw + 2 Dw 2 + 3Dw 3 + 2 6 const. linear squared (2.3-4) cubic From the Taylor series expansion, we find D- 2 c c2 2 - 2 c2 D 2 c (2.3-6) In third optical window: c 1550nm , 23 D 17 ps / ( nm km ) , 2 -21.75 ps²/km 03 – Optical Channel Dispersion Slope Click To Edit Master Title Style If we need to include an additional, cubic term in the Taylor series, we have to consider dispersion slope S dD ( ) d c This is required, if we have a very wideband signal, or if the quadratic term is small as e.g. at zero dispersion (1310nm for SSMF). Dispersion is then considered as a linear function: (2.3-7) D ( D ) D1 + S D We use D1 here to denote the constant dispersion and derive group delay over D as above 1 2 g (D ) L D(D )d D D1LD + SL(D )2 + c. We use also a more accurate relationship D f (Dw ) in order to find g as a function of Dw D w w - wc , 24 D - c , 2 c w 03 – Optical Channel Click To Edit Master Title Style 1 2 g (D ) L D ( D )d D D1LD + SL(D ) 2 + c. We use also a more accurate relationship D f (Dw ) in order to find g as a function of Dw Dw w - wc , D - c , 2 c w Using these relations D Dwc / (2 c) c 2 c - c - c -c Dwc wc + Dw 1 + D w / (2 c ) c 1+ 2 c Dwc / (2 c ) c2 c3 -c Dw + Dw 2 2 2 c (2 c ) 1 + Dwc / (2 c ) Introducing D in the above g (D ) and integrating for the phase we find finally: 2 2 2D 3 T S + 1 2 c T 25 (2.3-8) 03 – Optical Channel Click To Editand Master TitleDispersion Style Anomalous Normal D> 0 = anomalous dispersion: increasing increasing delay slower propagation g ("blue faster than red") D< 0 = normal dispersion: increasing decreasing delay faster propagation ("red faster than blue") g Material and Waveguide Dispersion Chromatic dispersion caused by Material dispersion: refractive index of silica glass material Waveguide dispersion: fiber waveguide properties (core geometry) D DM + DW 26 03 – Optical Channel Click Types To Edit Other of Master Fiber Title Style 27 03 – Optical Channel Click To Edit Master Overview of Fiber TypesTitle Style 25 O-Band C-Band L-Band Dispersion in ps/ (nmkm) 20 15 10 5 Dispersion Shifted Fiber 0 -5 -10 1250 28 1300 1350 1400 1450 1500 Wavelength in nm 1550 03 – Optical Channel 1600 1650 Click To Edit Master Title Style to Dispersion Parameter According ITU-T Standard 29 03 – Optical Channel Measurement ofMaster the Dispersion Parameter Click To Edit Title Style 30 03 – Optical Channel Linear Channel Model 31 Transmission Model for Digital Binary Intensity Modulation Simple transmission schemes use intensity modulation with “on-off-keying” ("1"= light on, "0"= light off). Intensity of light = instantaneous power of light. From (2.1-3) P(t ) A(t ) A* (t ) | A(t ) |2 | a(t ) |2 TX: A directly modulated laser (DML) translates variations of the injection current in proportional variations of the optical intensity P1 (t ) ~ v1 (t ) RX: A photo diode is able to detect the optical intensity and translates variations of the optical intensity in proportional variations of the output current v2 (t ) ~ P2 (t ) Transmitter Fibre Receiver ok _232_s ignalubertrag.ds f v1(t)>0 const P1(t) P1(t) Ain (t) 32 |...| 2 h TP(t) Aout(t) P2(t) const v2(t) Click ToChannel Edit Master Title Linear Model of Style Optical Fiber From previous sections we conclude that the optical fiber channel is modeled as a linear system, that can be described (w close to carrier frequency wc) by a transfer function H (w ) | H(w ) | e - jb (w ) with constant magnitude | H(w ) | and a phase b (w ) , where the most important contribution is a quadratic function in w . H (w ) 10 - adB L / 20 e - j (w ) L | H (w ) | e - jb (w ) Input and output signals are the optical fields at carrier frequency acc. to (2.1-2). 33 03 – Optical Channel Click To Edit Master Title Style Bandpass-to-Lowpass Transformation It is common in communications to represent bandpass systems and bandpass signals in lowpass domain. H LP (w ) 1 2 H BP (w + wc ) w -wc 0 w -wc 2 scaling factor 1/2 for preservation of energy here: H LP (w ) | H (w + wc ) | e - j (w +wc ) L for -wB w +wB where wB is the bandwidth that is of interest for signal transmission 34 03 – Optical Channel Click To Edit Master Title Style 1 1 b(Dw ) L (Dw ) L c + 1Dw + 2 Dw 2 + 3Dw 3 + 2 6 With Taylor series expansion (eq. (2.3-3)) for (w ) around w wc we find with eq. (2.3-4) for small w Dw in the lowpass domain: H LP (w ) | H | e - j c L e - j 1 Lw 1 - j 2 Lw 2 e 2 1 - j 3 Lw 3 e 6 (2.4-1) for -wB w +wB Input and output signals are the complex envelopes of optical fields at input A(t , z 0) Ain (t ) and output A(t , z L) Aout (t ) Ain (t ) hLP (t ) of fiber (see (2.1-2)) 35 03 – Optical Channel Click To Edit Master Title Style Oberservations Phase b(w ) (w ) L is not an odd function Lowpass impulse response hLP(t) – • HLP(w) is complex valued! Complex envelope Aout (t ) at fiber output is complex valued, i.e. has magnitude and phase (or real and imaginary part), even if the fiber input is real valued 2 Impact of dispersion is proportional to Lw High frequency components (high bit rates) are extremely critical: At twice the bit rate transmission distance must be reduced to one quarter in order to achieve the same impact (i.e. signal distortions) of dispersion. Attention: input and output signals of fiber H(f) is electric field, not power or intensity ! 36 03 – Optical Channel Click To Edit Master Style System Responses of Title the Fiber Impulse response regarding 1st order dispersion and non-causal representation we have H LP 1 - j 4 2 2 Lf 2 2 (f)e b2 D T2 L 2 + j 2 f 2 2c e e + j 2 b2 f (2.4-2) 2 Define: L b2 D T - 2 L , b2 0 for D 0 (anomalous Dispersion) 2c hLP (t ) F-1{H LP ( f )} | hLP (t ) | 1 e 2b2 1 const. t 2b2 j 2 ( - t2 1 + ) 4b2 8 (2.4-3) arg{hTP (t )} Parabola 2 in time and frequency domain same functional relationship ( e - j x )! Compare Gaussian function see following figures (for all figures: D = 17ps/nm/km) 37 03 – Optical Channel Click To Response Edit Master Style Impulse ofTitle Fiber with Dispersion 38 03 – Optical Channel Step and Rectangular ClickResponse To Edit Master Title Style Pulse Response Step and rectangular pulse responses can be computed from the impulse response t h-1 (t ) h( )d - hR (t ) h-1 (t ) - h-1 (t - Tb ) 39 03 – Optical Channel Click To Edit Title Style Response to aMaster Gaussian-Impulse Model for an isolated "1" in binary transmission A(t ) a(t )e j (t ) g (t ) Gaussian ok_231_gaussimpuls.dsf g(t) ok_231_gaussimpuls.dsf G(f) 1 1 e- 2 =0,606 -T0 t T0 - g (t ) e fbw= - 1 2T0 t2 2T02 f 1 2T0 G ( f ) 2 T0 e - 1 2T0 T0 2 (2 f ) 2 2 at fibre output: Aout ( f ) Ain ( f ) H LP ( f ) G ( f ) H LP ( f ) 2 T02 (2 f ) 2 2 2 T0e e j 2 b2 f - - Aout (t ) F-1{ Aout ( f )} T0 b2 e b t 2 (T02 + j 2 ) t2 2(T02 - j b2 ) ck e T02 - j 40 ck compl. const. 03 – Optical Channel b 2[T04 + ( 2 )2 ] Click To Edit Master Title Style - T0 Aout (t ) F-1{ Aout ( f )} b2 e b t 2 (T02 + j 2 ) t2 2(T02 - j b2 ) ck e T02 - j ck compl. const. - | Aout (t ) || ck | e 41 t 2T02 b 2[T04 + ( 2 )2 ] remains Gaussian! 03 – Optical Channel b 2[T04 + ( 2 )2 ] Click To Edit Master Title Style Pulse width (decay by 1/ e ) at fiber output: - ! t 2T02 1 b 2 2[T04 + ( 2 ) 2 ] t2 T1 b 1 + ( 2 2 )2 T0 T0 b T 4 [1 + ( 2 2 ) 2 ] T12 2 0 T0 T0 1 2 DL T2 1+ 2 Pulse broadening ! T0 2 c 2L b D T 2 2c Dispersion length: Def.: Fiber length LD for which T1 2 T0 DL T2 ! 1 T02 2 c L T1 1+ T0 LD L LD T02 2 c DT2 2 (2.4-4) Example: T0=50ps (corresponding to Tb=100ps) LD=115.4 km For L ~ LD Pulse broadening Inter-symbol-interference Bit errors LD dispersion limited transmission length 42 03 – Optical Channel Influence of Dispersion onStyle Data Click To Edit Master Title Transmission Binary on-off keying transmission with on-off keying intensity modulation 2 Transitions "0"-"1" and "1"-"0" with smooth edges (cos -roll-off) Eye diagram after 0 km (left) and 6.25 km (right) at 40 Gb/s and SSMF (D = 17 ps/nm/km) 43 03 – Optical Channel Examples of Eye Distortions Click To Edit Master Title Style Eye diagrams and time signals at 10 Gb/s transmission over standard single mode fiber (SSMF) with D=17ps/nm/km. Signals shown are instantaneous power signals 0 km 20 km 80 km 44 03 – Optical Channel Dispersion Limits Click To Edit Master Title Style Dispersion limit is proportional to the square of the bit rate D Lmax c 4 2 fbit2 Dispersion compensation is required for transmission over (for NRZ-OOK modulation) 900 km SSMF @ 2.5 Gb/s (15000 ps/nm) 60 km SSMF @ 10 Gb/s (1000 ps/nm) 4 km SSMF @ 40 Gb/s (65 ps/nm) Alternative: Modulation formats (e.g. duobinary) with higher dispersion tolerances 45 03 – Optical Channel Click To Edit Master Title Style Extinction Ratio from eye diagram (time signal on oscilloscope, trigger with bit clock) we find Def.: ex P ("0") P ("1") (2.4-5) ex dB 10lg(ex) typically: ex dB -10dB - 16dB, ex 0.1 0.025 P("1") P("0") 46 03 – Optical Channel Dispersion Compensation inStyle Long Click To Edit Master Title Distance Links Transmission distance L >> Dispersion limit LD (wide area networks WAN L >100km ... several 1000km) Due to fiber loss, the received signal is attenuated optical amplifiers for loss compensation Due to dispersion, the received signal is heavily distorted! However, as dispersion results in linear distortions, we can compensate by applying equalization: H equal ( f ) 1/ H fibre ( f ) As the fiber transfer function is basically a phase filter with quadratic phase response, we need an optical filter as equalizer with inverted sign quadratic phase shape. optical dispersion compensators (opt. filters or DCF) 47 03 – Optical Channel Click To Edit Master Title Style Optical Dispersion Management Typically long distance fiber links are organised in several spans. In each span optical amplifiers are used for loss compensation and DCF (Dispersion compensating fiber) is used for dispersion compensation. DCF has an inverted dispersion coefficient: DDCF -(5...15) DSSMF LDCF LSSMF / (5...15) For linear transmission (small optical power) full compensation of dispersion is possible. Usually we use higher power (e.g. 10dBm) non-linear transmission signal distortions due to both nonlinearity and chromatic dispersion 48 03 – Optical Channel Click To Edit Master Title Style Electronic Dispersion Compensation Can we apply electrical equalisers at RX (after optical/electrical conversion) for dispersion compensation (electrical filters are cheaper and more flexible!)? Yes, if phase distortions on the optical signal can be transformed in exactly the same phase distortions in the electrical domain (after optical front end). Such a transformation is possible with coherent optical receivers. No, if we use direct detection as optical front end, because photo diode detects only magnitude squared and no phase of the optical signal. Thus we have nonlinearity |...|2 in our system and a simple linear equalizer filters fail or results in only limited compensation performance. 49 03 – Optical Channel Optical Polarization 50 Click To EditinMaster Polarisation FibersTitle Style Solution of wave equations result (approximately) in a fundamental (single) mode, which is a linearly polarized transversal wave. I.e. regarding a plane z=const the Evector has (approximately) no longitudinal (in z-direction) component and moves in this plane on a straight line with a certain angle against x-axis. vector can be decomposed in x- and y-components (orthogonal components) 51 03 – Optical Channel Click To Edit Master Title Style Jones Vector E(r , z, t ) Re{Ex (r , z, t )e x + E y (r , z, t )e y } ˆ - jx z E0 x (r )e e jwT t Re - j z Eˆ 0 y (r )e y J=Jones-Vector State of Polarization (SOP) is considered in plane z=const Linear Polarisation: E0xE0y, x=xz=±y=yz components Circular Polarisation: E0x=E0y, y=x/2 are in-phase (+ right, - left circular) Eˆ 0 x e jx fig. 2.-3 J Eˆ e j (x /2) 0x Eˆ 0 x e jx fig. 2.-2 J Eˆ 0 y e jx Elliptical Polarisation: E0xE0y, xy Eˆ 0 x e jx J Eˆ 0 y e j y 52 03 – Optical Channel fig. 2.-5 Click To&Edit Master Polarized Title StyleLight Linearly Circularly 53 03 – Optical Channel Click To Edit Master Light Title Style Elliptically Polarized See a nice animated tutorial on polarization at: https://www.keysight.com/main/editorial.jspx?cc=DE&lc=ger&ckey=2475067&i d=2475067&cmpid=zzfindpolarization-form 54 03 – Optical Channel Click To Master Title Style States of Edit Polarisation (SOP) in SMF Polarized light: SOP in plane z=const remains constant Un-polarized light: SOP changes Ideal case: for perfect cylindrical symmetry the SOP (e.g. linear) of incoming light is preserved along the fiber Real world: Distortions from cylindrical symmetry through manufacturing process (core ellipticity) or fiber installation (stress, bending) slightly different propagation constants of orthogonal modes = Birefringence along the fiber the SOP changes within some cm. (SMF is “polarization maintaining” only over some cm). "Beat length" approx. 10m 55 03 – Optical Channel ClickLength To Edit Master Title Style Beat Consequences: Polarization-dependent optical components and systems (e.g. semiconductor opt. amplifiers, Mach-Zehnder modulators) need polarizers and polarization maintaining fibers (special geometry of core) for interconnection of these components Optical receivers for intensity modulation detect optical power (direct detection) independent of polarization 56 03 – Optical Channel Click To Edit Mode MasterDispersion Title Style Polarisation PMD is a serious impairment of the fiber channel for high speed transmission! PMD stems from slightly different propagation velocities (=birefringence) of the orthogonal modes (x- and y-component) of the light due to (i) non-ideal core geometry and (ii) environmental influences on installed cable (pressure, bending). PMD is changing along fiber and with time stochastic process. 57 03 – Optical Channel Click To Edit Master Title Style Birefringence In the case of birefringence we have a fast and a slow polarization axis. With D | gx - gy | Differential Group Delay (DGD) A perfectly birefringent fiber is called PM-fibre (polarization maintaining), where the slow and fast axes remain constant over the entire length and D g is relatively high (Fig.12.10a) In typical transmission fiber, birefringence is maintained only over a short distance. The orientation of the axes changes with distance "mode coupling". Hence the PMD of a fiber can be modeled as pieces of birefringent fiber with random axes rotation. This model is called "waveplate model" (Fig. 12.10b). Due to this mode coupling, D increases not linearly (as in PM-fiber) but with the square root of the fiber length L. 58 03 – Optical Channel Click To Edit Model Master Title Style Waveplate 59 03 – Optical Channel Pulse Outage Click ToBroadening Edit Masterand Title Style Probability due to PMD 60 03 – Optical Channel Click Edit Master Title Style PMD To Characterization It is common to characterize PMD by the DGD D, which is a random parameter. DGD D has a Maxwellian-probability density function (pdf) D 2 2 D 2 - 2 q2 pD (D ) e q3 pD(D ) where q 8 E{D } Maxwell pdf ok _24_maxwell.dsf D=DGD E(D ) Definitions: DGD-value: D [ps] PMD-value: E{D} [ps] PMD-coefficient: E{D } km [ps/√km] Typical values for SSMF: PMD-coeff. E{D km } 0,05 ... 2 61 ps km 03 – Optical Channel Click To Edit Master Title Style Influence of PMD on Digital Transmission New fibers have very low PMD-coefficients. Old installed fibers (‘legacy fiber’) had sometimes no PMD-specification (today < 0.1 ps/sqrt(km) specified!) high PMD in transmission link is possible worst case must be considered for network design! 62 03 – Optical Channel Click Impairments To Edit Master Title Style PMD PMD in IM/DD Systems Rule of thumb: (Estimation from statistical and experimental investigations) E{D} ≈ 0.1 T b 10% of bit duration is the limit of a tolerable PMD-value. PMD is critical for high bit rates of 10 Gb/s and above! PMD compensation is required! PMD in Coherent Transmission systems PMD can be compensated by linear equalizers in the electrical domain after opt/elect conversion by means of digital signal processing x- and y-polarizations are detected in parallel paths. 2x2 MIMO equalizer separates polarizations. 63 03 – Optical Channel Click To EditPMD Master Title Style First-order 64 03 – Optical Channel Click To EditPMD Master Title Style First-order – System Model ok_232_signalubertrag.dsf |...| 2 x(t)= Px(t) + D |...| 2 y(t)= P(t) 1- Delay Optical power detected by photo diode P(t ) | E x (t ) |2 + (1 - ) | E x (t - D ) | 2 Px (t ) + (1- )Px (t - D ) where = power ratio in fast/slow axis (varies statistically!), uniformly distrib. in [0,1] For detected power: PMD behaves as a linear multipath system! Compensation (equalisation) by an inverse system e.g. in electrical part of receiver (after o/e conversion) is possible 65 03 – Optical Channel Waveplate Model of PMD forStyle “all- order” PMD Click To Edit Master Title Cascade of birefringent wave plates with arbitrarily rotated fast and slow axis. In equivalent LP-domain: 1 propagation constant in fast axis: fast ( f ) C fast + 1 fast f + 2 fast f 2 + ... 2 1 propagation constant in slow axis: slow ( f ) Cslow + 1slow f + 2 slow f 2 + ... 2 neglecting dispersion, results in transfer function for a single wave plate H f (w ) e - j ( Cf L + 1 f Lw ) H s (w ) e- j ( Cs L + 1s Lw ) 66 03 – Optical Channel System Model Click To Edit Master Title Style H f (w ) e - j ( Cf L + 1 f Lw ) H s (w ) e - j ( Cs L + 1s Lw ) oc_24_PMD.dsf fast +j e slow 67 -j e D 2 D 2 +j e -j e D g 2 D g 2 2 f 2 f 03 – Optical Channel Waveplate Model of PMD Click To Edit Master Title Style Vectorial Description Light incident to the fiber has linear polarization in x,y-coordinates. We rotate the components by angle a to the fast and slow axis (x',y') of the wave plate. Rotation in matrix form = Jones y matrix calculus: y' E x' 0 cos a E 'y 0 - sin a oc_24_PMD.dsf x' sin a E x 0 cos a E y 0 a x Concatenated wave plates are rotated (with resp. to fast and slow axis) by arbitrary angles ai . 68 03 – Optical Channel Click To Edit Master Title Style y y' E x' 0 cos a E 'y 0 - sin a oc_24_PMD.dsf x' sin a E x 0 cos a E y 0 a x Concatenated wave plates are rotated (with resp. to fast and slow axis) by arbitrary angles ai . Therefore we can derive the model: e(+jD1/2) e(+jD1w/2) + n si + e(-jD1/2) e(-jD1w/2) Transfer matrix of one rotated birefringent element: cos a2 cos ai - sin a i ... -s in a 2 a -s in 1 oc_24_PMD-model.dsf a3 a2 a1 si na e(+jD3/2) e(+jD3w/2) cos a3 + e(+jD2/2) e(+jD2w/2) + n si in -s cos a1 69 cos a2 3 cos a1 + e(-jD2/2) e(-jD2w/2) + j Di + D gi 2 f 2 2 sin a i e cos a i 0 03 – Optical Channel cos a3 + e(-jD3/2) e(-jD3w/2) cos a 0 sin a i +1 i +1 - sin ai +1 cos a i +1 D D g i -j i2 f 2 2 e Conclusions Click To Edit Master Title Style 10 to 15 wave plates result in sufficient statistical accuracy for modelling PMD of fibers. Implementation as a simulation model a i and i are chosen randomly from uniform distribution in [0, 2 ] D gi group delay difference is chosen from (near) Gaussian distribution according to desired PMD value. 70 03 – Optical Channel Click To EditNonlinearity Master Title Style Fiber 71 03 – Optical Channel Click To Edit Master Title Style Non-Linear Fiber Characteristics For large and medium optical power (from approx. P>= 1mW i.e. 0dBm) optical fibers behave non-linearly Non-linear influence depends on: mean power at fiber input Peak-to-average power ratio (PAPR) of instantaneous optical power Spatial power distribution over the fiber-core cross section effective area fiber length and exponential decrease of signal power along the fiber 72 03 – Optical Channel Click To Edit Length Master L Title Style The Effective eff Influence of fiber nonlinearity decreases along z-axis due to exponential decrease of power with length. Simplified model: assume constant power over a certain fiber length L ≤ actual eff length L effective length Leff : Typical value: 1 P0 L L P ( z )dz z 0 e -a z dz z 0 1 - e -a L a Exercise: For a multi-span link (span length Lspan with in-line amplifiers of total link length Ltot ) we find: Leff 1 - e-a L Ltot a Lspan The effective length can be interpreted as the length of an equivalent fiber with no loss, but having the same nonlinear impact 73 03 – Optical Channel Clickeffective To Edit Area Master The AeffTitle Style Influence of fiber nonlinearity depends on the distribution of light power inside the fiber-core cross section For SMF: Field distribution has approx. Gaussian-function Define an effective cross-sectional area: circle of area A eff where the power is assumed to be uniformly distributed(see fig. 4.25) A is approximately equal to the core area A = 50…80 m for SMF , A eff 2 eff 74 eff 2 = 20 m for DCF 03 – Optical Channel Click To EditProperties Master Title Style Linear Fiber Linear Fiber Effects Attenuation 75 Dispersion Polarization Mode Dispersion 03 – Optical Channel Click To Edit Master Title Style Nonlinear Fiber Properties Nonlinear Fiber Effects Inelastic Elastic (Kerr-Effect) SPM XPM FWM SPM: Self-phase modulation XPM: Cross-phase modulation FWM: Four-wave mixing 76 SRS SBS SRS: Stimulated Raman Scattering SBS: Stimulated Brillouin Scattering 03 – Optical Channel Click To Edit Master Scattering Title Style Stimulated Brillouin SBS = interaction between light and acoustic waves signal is backscattered, if a certain threshold with resp. to spectral power density (in W/Hz) is exceeded! SBS critical, if power is concentrated in small bandwidth, B (e.g. 100 MHz) = Laser Q with small spectral linewidth. SBS-threshold: Pth, SBS BQ 77 21 Aeff g B Leff BB 03 – Optical Channel Click To Edit Master Title Style SBS-threshold: Pth, SBS BQ 21 Aeff g B Leff BB A ≈50…80(m)2, effective mode field area, depends on field distribution eff L ≈ 20km, effective length of nonlinear interaction eff B ≈ 20MHz, SBS gain bandwidth B g ≈ 4 10-11m/W, SBS gain coefficient B Typical result: Pth, SBS 6.5mW for BQ 100MHz SBS cancellation: Slight spectral broadening of power e.g. by a slowly varying (e.g. 100 kHz) frequency modulation of light carrier (=laser) linewidth enhancement 78 03 – Optical Channel Click To Edit Master Title Style Stimulated Raman Scattering Raman Scattering = interaction between light (photons) and molecular vibrations of the fiber’s silica molecules. The molecules absorb some energy (E=hf ) from the photons reduces frequency of scattered light by Df R Stimulated Raman Scattering (SRS) An optical wave with frequency f < f is amplified through stimulation by a second 1 wave of frequency f = f +Df 2 1 2 R SRS crosstalk in WDM WDM with channel spacing ≈ Df : lower frequencies are amplified gain tilt R 79 03 – Optical Channel Click To Edit Master Title Style Spontaneous vs. Stimulated Raman Scattering Spontaneous Raman Scattering E Scattered Photon Pump-Photon Phonon Stimulated Raman Scattering E Signal-Photon Signal-Photon Signal-Photon Pump-Photon 80 Phonon 03 – Optical Channel Click ToTilt Edit Master Title Style Raman dPS g -a S PS + R Pp PS dz Aeff dPp dz -a pPp - S g R PS Pp p Aeff PS: Signal power PP: Pump power a: Attenuation constant Aeff: effective fiber area gR: Raman gain coefficient 81 03 – Optical Channel ClickEffect To Edit Master Title Style SRS Copropagating channels log(P) Local Dispersion Accumulated Dispersion CH1 Power transfer from lower to higher wavelengths through SRS Accumulation of the individual segments Input CH2 CH1 log-lin tilt(~ Ptot BWtot) and noise-like interference (multichannel, multisegment) on the “1” of the IM signal Output CH2 82 03 – Optical Channel Click To Edit Master Title Style Raman Amplifier An optical wave at frequency f can be amplified through stimulation by a co 1 propagating second wave of frequency f = f +Df 2 1 R from a (high power) “Raman pump laser” Raman Gain Raman effect is most efficient for Df ≈ 10..16THz R (i.e. “bad” for SRS cross talk, “good” for Raman amplifier) 83 03 – Optical Channel Click To EditEffect Master Title Style Optical Kerr Refractive index depends on light intensity = Kerr nonlinearity | E |2 n n(w, E ) nlin (w ) + n2 Aeff Propagation constant is related to refractive index (w , E ) wc w w n n(w , E ) c nlin (w ) + c 2 | E |2 lin + | E |2 c c c Aeff wc n2 c Aeff nonlinearity parameter Phase shift of light signal is related to propagation constant (b=·L) 2 e- j (w ,E )L e- j[ lin (w )+ |E| 84 ]L 03 – Optical Channel Click To Edit Master Title Style Self-Phase Modulation Intensity modulation (IM) of the signal t Phase modulation (PM) due nonlinear refractive index Accumulated Dispersion PM-IM conversion Signal distortion (Intersymbol interference) D>0 85 D<0 03 – Optical Channel t Click To of Edit Master Title Style Influence SPM on Signal Transmission Dispersion and loss management Optical long distance transmission = wide area networks WAN (>100km … several 1000km): Loss compensation optical amplifiers Dispersion compensation DCF (Dispersion compensating fibre) For linear transmission (small optical power) full compensation of dispersion is possible with DCF. Usually we use higher power (e.g. 0…10dBm) non-linear transmission signal distortions due to NL and chromatic dispersion (CD) link design for minimum distortions (=“Dispersion management“). Criteria: required receiver input power for fixed BER, e.g. BER=10-9 (minimum) eye diagram with open eye (eye opening = maximum) 86 03 – Optical Channel Non-linear Intermodulation Effects Click To Edit Master Title Style in WDM Transmission In communications theory well known: Nonlinear channel properties may result in new spectral components These new spectral components may interfere with neighbour channels, if frequency division multiplexing transmission scheme is used Example: CATV, highly linear amplifiers required! Therefore we expect intermodulation effects in optical WDM transmission systems, where we transmit multiple channels with different carrier wavelengths separated by a given channel spacing. 87 03 – Optical Channel Click To EditExplanation Master Title Style Qualitative Kerr-effect is a nonlinear impairment single-channel transmission nonlinear distortions of complex envelope (Self Phase Modulation=SPM) multi-channel transmission (=WDM) generation of intermodulation products (Cross Phase Modulation=XPM, Four Wave Mixing=FWM) Question: All impairments (SPM, XPM, FWM) are due to Kerr effect. Why are they separated into SPM, XPM and FWM? Answer: The way SPM, XPM and FWM disturb signal transmission is fairly different although all of them stem from the Kerr effect. It makes sense to treat them separately. 88 03 – Optical Channel Click To Edit Master Title Style Quantitative Derivation First issue: How to express the complex envelope of a multi-channel (i.e. WDM) signal? Well known: bandpass-lowpass transform of a single-channel (i.e. single-carrier) signal bandpass signal: sBP (t ) a(t ) cos[2 f ct + (t )] analytical signal: s BP (t ) a(t ) e j (t )e j 2 fct complex envelope: A(t ) a (t ) e j (t ) Now apply formalism on multi-channel signal (N channels) N bandpass signal: sBP (t ) ai (t ) cos[2 f cit + i (t )] i 1 with ai(t): amplitude modulation of channel number i fci: carrier frequency of channel number i i(t): phase modulation of channel number i 89 03 – Optical Channel Click To Edit Master Title Style N N i 1 i 1 sBP (t ) ai (t )e ji (t ) e j 2 fcit Ai (t ) e j 2 fcit analytical signal: with i(t): complex envelope of channel number i complex envelope: WDM signal has more than one carrier. Which frequency f c should be used for quadrature mixing into baseband? For practical reasons choose f c such that it falls right into the center of the WDM spectrum. Result for complex envelope: N AWDM (t ) Ai (t )e i 1 j 2 fcit - j 2 fct e N Ai (t )e j 2Dfit i 1 With Dfi f ci - f c difference between carrier frequency of channel i and fc 90 03 – Optical Channel Click To Edit Master Title Style Example Properties of AWDM (t ) : baseband signal multi-carrier signal Example for N=5: 91 03 – Optical Channel Click To Edit Master Title Style Cross-Phase Modulation (XPM) Intensity Modulation (IM) of copropagating channels Local Dispersion Phase Modulation (PM) due to nolinear refractive index Accumulated Dispersion PM-IM Conversion Noiselike crosstalk (multi-channel, multi-segment) on the “1” of an NRZ signal from 010101... pattern In contrast to SPM, for XPM the origin of the induced phase modulation is not the optical power of the signal itself but the sum of the powers of all co-propagating WDM channels. 92 03 – Optical Channel Click To EditofMaster Properties XPM Title Style XPM from each disturbing channel is twice as strong as SPM XPM only results in phase modulation not a problem for direct detection systems however fiber dispersion converts phase modulation into intensity modulation results in signal degradation also for direct detection XPM may add up to strong phase modulation for high number of channels But: o disturbing channel and disturbed channel have different group velocity (for dispersive fiber, represented by 1~D) o group delay difference (walk-off) is higher the larger the spacing between the channels impact of XPM is averaged out fiber dispersion is required to mitigate impact of XPM ! only direct neighbours have significant impact more impact for tight WDM channel packing 93 03 – Optical Channel Click To Edit Mixing Master(FWM) Title Style Four-Wave Copropagating channels atfi,fj, fk -20 cw, full resolution -30 Power @ Pre (dBm) Power @ Pre (dBm) -25 -35 -40 -45 -50 -55 -60 -65 1553 1554 1555 1556 1557 Local Dispersion Accumulated Dispersion Coherent mixing: Sideband generation through nonlinear refractive index at fijk = fi+fj-fk with k i,j Accumulation of the individual mixing products 1558 Noiselike distortion (multi-channel, multi-segment) on the “1” of the signal 94 03 – Optical Channel Click To Edit Master Title Style Description Origin of name: three waves generate a fourth wave four wave mixing precondition: equidistant channel spacing (in almost all cases fulfilled) Normal FWM P w4 = w2 + w3 - w1 w1 w2 w3 w4 w Degenerate FWM P w1 = 2 w2 - w3 w4 = 2 w3 - w2 Dw Dw Dw w1 95 w2 w3 w4 03 – Optical Channel w Click ToMismatch Edit Master Title Style Phase Mechanism of phase mismatch: FWM is generated continuously along the fiber if all contributions along the fiber add up in-phase: phase mismatch D=0 strong FWM if contributions along the fiber do not add up in-phase: phase mismatch D >0 weak FWM Phase mismatch depends heavily on fiber dispersion parameter D=0 2=0 (i.e. dispersion shifted fibre, DSF): no phase mismatch strong FWM WDM on DSF is near to impossible! D>0 | 2|>0 phase mismatch higher the larger spacing of contributing channels only neighbouring channels have impact on SSMF: FWM generally weaker than XPM 96 03 – Optical Channel Click To Edit Master Title Style Example 97 03 – Optical Channel Click To EditManagement Master Title Style Dispersion dispersion Full compensation of the accumulated dispersion after each span 0 distance 0 . . . SSMFN-1 SSMF1 acc. dispersion Distributed undercompensation of the dispersion after each span 0 DC 1 DC SSMFN N-1 DCN distance 98 03 – Optical Channel Click To EditUnmanaged Master TitleTransmission Style Dispersion Links In modern (“high-end”) transmission links with coherent receivers, it is preferred to avoid DCF in the link. DC is rather compensated for by linear equalizers in the receiver by a digital signal processing unit. This is possible, since coherent receivers are “linear transducers”. They transform the optical signal into the lowpass domain, including all distortions. 99 03 – Optical Channel Click To Edit Master Title Style Propagation Modes 100 03 – Optical Channel Propagation ModesTitle in Fibres, Click To Edit Master Style Characteristics of MMF and POF Concept of modes Electro-magnetic waves in fibers propagate according to the wave equation (WE), which can be derived from Maxwell’s equations. In general there are many solutions to the WE fiber modes Each (fiber) mode has a different propagation speed (group velocity) explained by ray optics field distribution (intensity pattern) over the fiber’s cross section The number and types of modes depend on the refractive index profile (n1, n2) and on core diameter d. Modes in fibers are (approximation of "weakly guided" waves ) ≈ linear polarized (LP modes). The different modes are sorted by designating order numbers: LP 101 jm where j (azimuth wave number) and m (radial wave number). 03 – Optical Channel Click To Edit Multimode andMaster Single Title ModeStyle Condition The following plot shows the normalized propagation constant: B : ( / k ) 2 - n22 n12 - n22 / k - n2 n1 - n2 / k - n2 n1D n -n with D 1 2 n1 over normalized frequency V wC d 2 2 n1 - n2 c the larger V, the more modes (high order modes) only one mode if V < 2.405 102 Single mode condition 03 – Optical Channel Click To Edit Master Title Style bV Characteristics B 1,0 0,8 LP01 0,6 11 02 0,4 21 31 • 22 12 51 0,2 41 32 03 61 13 0 0 2 4 6 8 10 V 103 • 03 – Optical Channel the larger V, the more modes (higher order modes) only one mode, if V < 2.405 (Single mode condition) FieldToDistribution Modes Click Edit MasterofTitle Style (Intensity Patterns) 104 03 – Optical Channel Click To Edit Master Title Style Intensity Distribution MMF Many modes co-propagate in a fiber (multi-mode fiber) lower order modes travel in core center higher order modes travel off center SMF: 105 LP field distribution is approximately Gaussian 01 is the only mode that propagates (single mode fiber) 03 – Optical Channel Click To Edit Master Title Style LP01 – Approximation weakly guiding fiber Exact solution for step-index fiber 106 03 – Optical Channel Transmission Characteristics of SMF Click To Edit Master Title Style SMF: for high speed, long distance applications WAN core diameter: ≈ 9µm sophisticated handling due to small core diameter, e.g. connectors launching laser light n nk nm r Single Mode Step-Index Fiber 107 03 – Optical Channel Transmission Characteristics of Click To Edit Master Title Style MMF & FMF MMF: n for high speed, short distance applications LAN core diameter: ≈ 50µm or ≈ 62.5µm simple handling and light power launching due to large core diameter main limitation: mode dispersion Graded index MMF much better than step index MMF Challenge: 10Gb/s (Ethernet protocol) over 300m ? Channel model of MMF very different from SMF! Channel model not fixed as MMF properties differ in brought range! nk nm r e.g. dips or holes in refractive index profile! Multimode Graded Index Fiber FMF (few mode fiber) Core diameter 10…20m Only a few modes (2 …6) propagate Used for mode division multiplexing (MDM): Use each mode for transmitting a separate data stream! capacity 108 03 – Optical Channel Transmission Characteristics of Click To Edit Master Title Style POF (Plastic Optical Fiber) POF: polymer (plastic) fiber, material e.g. PMMA (polymethylmethacrylate), no glass! core diameter up to ≈1000 m (1mm) large core diameter, thousands of modes short reach, low cost applications loss: typ. ~100dB/km in 500nm…650nm window (advanced POF: ~10dB/km at 1050nm) very simple handling (connectors, bending radius,…) Applications: automobiles sensor networks In-house LAN 109 03 – Optical Channel Click ToUp Edit Master Title Style Wrap What you should recall from this chapter: • • • • • • • • • • • 110 What are the bit durations for 10 Gb/s and 40 Gb/s transmission? Give the loss (in dB) and the dispersion (in ps/nm) of 80 km SSMF in O-, C- and L-bands Explain PMD of 1st order, give the model What is linear polarization and circular polarization? What is the value of cut-off (normalized) frequency V for single mode condition? Look at the equation for Aeff and identify the impact on fiber nonlinearity What is the definition of the extinction ratio? What are the nonlinear fiber effects in single channel (i.e. no WDM) transmission? Explain the wave-plate model for PMD What is the origin of PMD in transmisson fibers and what is the impact on transmission? Give the dispersion length for 40 Gb/s data transmission 03 – Optical Channel Click To Edit Master Title Style Appendix 111 03 – Optical Channel Click To Edit Master Title Style Soliton Transmission Soliton: special pulse form, which results as a stable solution (i.e. pulse shape is maintained for all z) in attenuation-free (a=0) fiber transmission dA z , t dz 2 - j A z , t A z , t + j 2 2 d A z, t 2 dt 2 st Soliton of 1 Order: 1 A z, t T0 112 2 e j 2 2T02 z t sech T0 03 – Optical Channel Click To Edit Master Title Style 1 A z, t T0 "FWHM" 1.76 T0 solution holds for <0 only, i.e. D > 0 (anomalous dispersion) sech x 1/ cosh x 1 soliton-amplitude: T0 2 2 e x + e- x 2 T2 is fixed for fixed dispersion 2 D 2 c w n non-linearity parameter ( T 2 ), and pulse duration T0 c Aeff 113 03 – Optical Channel 2 e j 2 2T02 z t sech T0 Click To Edit Master Title Style Soliton Transmission Applications |A(z,t)| is independent from z (no pulse distortion due to dispersion) Due to interaction non-linearity dispersion, soliton maintains its pulse shape, (theoretically) over arbitrary long distances. For compensation of loss (we assumed a=0!) we use optical amplifiers (regular amplifier spacing 30 ... 50km) In general T0<<Tb (no pulse overlap) Soliton = RZ-(Return to Zero) pulse. It is sufficient to generate roughly soliton-like pulses at transmitter, along transmission fibre, the soliton pulse shape is created automatically Therefore RZ-pulse shape has better performance (e.g. longer transmission distance) than NRZ-pulse shape in nonlinear fibre regime (more robust!) Proposed applications: ultra long haul submarine systems (however not used in commercial systems) 114 03 – Optical Channel Lecture Optical Communications Lasers Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke Click To Edit Master Title Style Optical Sources LED (Light emitting diode) Broadband emission spectrum ( Dl 4060nm ) not a single frequency carrier dispersion limit at approx. 10 Mbit/s. Low-cost applications. Semiconductor Laser (Laser=Light Amplification by Stimulated Emission of Radiation) • small size • high reliability • narrow emission spectrum (quasi single frequency carrier possible) • emission wavelength in optical window 1 (~850 nm), 2 (~1310 nm), and 3 (~1550 nm) possible • direct modulation possible up to several Gbit/s • small emission radius of light beam (coupling to optical fiber) 2 04 – Lasers Click To Edit Master Title Style Semiconductor Lasers pn-diode operating with forward bias. Light emission due to special semiconductor material. Materials Materials: III-V compounds: • GaAs in 1st optical window (850nm) • InGaAsP in 2nd and 3rd optical windows 3 04 – Lasers Click To Edit Master Title Style Basic Principle Oscillator (energy source, amplifier, resonator) • Energy source (pump) = electrical current through pn-junction. • Active gain medium (active layer) = volume charge zone of pn-junction stimulated emission of photons by recombination of electrons with holes. • Resonator (cavity) = wave guide with facettes as reflecting mirrors (coating). 4 04 – Lasers Click To Edit Master Title Style Energy Band Model Physical structure: Special, heavily doped (degenerated) pn-junction with a double-hetero structure ("Hetero"=layers with different band gaps). I.e. cavity (recombination zone = active layer) with defined width where photons are generated by recombination and which serves also as dielectric wave guide (confinement zone which is guiding the laser beam). heavily p-doped slightly p-doped heavily n-doped (degener. WF>WV) Conduction band Wg2 Wg3 WF Valence band Wg2 <Wg1 ,Wg3 Active layer (recombinations zone) ok_313_bändermodell 2010.dsf -x 5 U=0, (thermodynamic equilibrium)Wg1 04 – Lasers Click To Edit Master Title Style Energy Band Model (Forward Bias) heavily p-doped slightly p-doped heavily n-doped (degener. WF>WV) Conduction band Wg2 Wg3 WF Valence band Wg2 <Wg1 ,Wg3 Active layer (recombinations zone) ok_313_bändermodell 2010.dsf -x 6 04 – Lasers Click To Edit Master Title Style Guiding of Laser Beam Recombination zone Energy p-doped Conduction band Electrons Band gap Valence band Field distrib. Refract. index n n-doped Holes n1 n2 n2 ok_314_laserführung.dsf ~2m 7 04 – Lasers -x Click To Edit Master Title Style Recombination Mechanism Non-radiating (electron-hole) recombination in indirect band-gap semiconductors (e.g. Si) results in thermal radiation (heat). Here unwanted! Radiating recombination in direct band-gap semiconductors (e.g. GaAs) results in light radiation (photon) active layer - - - Conduction band = excited state WC Wc hf>Wg hf>Wg Wg=WC-WV Wg hf>Wg Wg WV Wv Valence band = ground state a)Absorption Light attenuation in laser 8 z b)Spontaneous emission random direction and phase (LED) 04 – Lasers ok_124_laser_rekombination.dsf c)Stimulated emission direction, phase, frequency, polarisation same as in incoming light (Laser) Click To Edit Master Title Style Physical Structure Horizontal confinement of active laser cavity (x-direction) by double heterostructure Lateral confinement (y-direction) Gain-guided or index-guided (preferred today) 9 04 – Lasers Click To Edit Master Title Style Fabry-Perot Resonator, Lasing Condition Volume with rectangular cross section = resonance cavity (active layer). Reflections at planar boundaries: Laser-material (e.g. GaAs n=3,6) / air (n=1) standing longitudinal waves complex envelope of forward travelling wave E ( z ) A0 e s g 2 Air n=1 n~3,6 (GaAs) ok_317_FP_resonator_laser.dsf Air n=1 z j z e after one round trip: forward length L, reflection r1, backwards length L, reflection r2: Facette L~100...500m z Amplification=gain g by stimul. emission Attenuation=loss S ! E ( z 0 |2 L ) r1r2 A0 e ( s g ) L e j 2 L E ( z 0 |0 ) A0 constructive interference compensation of loss 10 04 – Lasers Click To Edit Master Title Style Phase Condition l 2 L 2 m kn 2 l 2nL m m integer, n~3,6 (refr. index) n Resonance wavelength. Amplitude condition: with R1=|r1|2, R2=|r2|2 power reflection coefficients gth s 1 1 ln 2 L R1R2 "Lasing modes" minimum gain necessary for lasing: Gain g Line-width (<0,1nm) g gth gth Resonance modes ok_317_FP_resonator_laser.dsf m-1 m m+1 Laser emission 11 04 – Lasers l ~0,3nm Click To Edit Master Title Style Single-Mode Lasers FP-Laser has some disadvantages: (i) not single-moded (= e.g. high dispersion sensitivity), (ii) “Mode hopping“ = center mode changes from lm to lm+1 with changes in modulation current, (iii) "Mode partition noise" = change of mode distribution of emitted power with changes in modulation current. For (high end) communication systems: • DFB-laser (distributed feedback), • DBR-laser (distributed Bragg reflector), • MQW-laser (Multi quantum well) Mirrors of FP-laser replaced by Bragg-grating (BG). BG acts as a mirror, reflects however only one wavelength (only center mode) filter Bragg-grating = cascade of partly reflecting mirrors in distance m l 2 , m integer Implemented in semiconductor laser by periodic variation of refractive index 12 04 – Lasers Click To Edit Master Title Style DFB and DBR Lasers Laser structures using built-in frequency-selective resonator gratings. DBR DFB Tunable Lasers: Tuning of emission wavelength by (i) changing cavity-length (mechanical stretching) (ii) changing refractive index (temperature control) 13 04 – Lasers Click To Edit Master Title Style VCSELs (Vertical Cavity Surface-Emitting Laser ) Light emission is perpendicular to semiconductor surface Advantages: • integration of multiple lasers in an array WDM application • On-waver testing possible 14 04 – Lasers Click To Edit Master Title Style Laser Rate Equations Coupled differential equations describing interaction of photons and electrons within active region dN (t ) I (t ) N (t ) G (t ) S (t ) dt qV te dS (t ) N (t ) S (t ) G (t ) S (t ) d dt te tp Terms used: = carrier (electrons) density [1/m3] = photon density [1/m3] =1,6 10-19As = electron charge = volume of active layer (≈ 0.510-16m3) te = lifetime of carriers (≈ 10-9s at 300K) tp = lifetime of photons (≈ 10-12s) d = fraction of spontaneous emission, that contributes to lasing (<10-4). G(t) = g0 [N(t)-N0 ] gain coefficient of stimulated emission N0 = carrier density at transparency (1.210-24m-3) N(t) S(t) q V 15 04 – Lasers Click To Edit Master Title Style Laser Modeling We are interested in: 1.) P(t) = optical power (P(t) ~ S(t) ) 2.) Df(t) = frequency modulation (Df(t) ~ N(t) )("Chirp") 3.) Noise I(t) Numerical solution of rate-equations (e.g. Runge-Kutta) S(t) P(t) N(t) Df(t) Modulation current 16 04 – Lasers z jDf(t) Fibre P(t) e Df(t) ok_319_Lasermodell.dsf Compl. envelope at fibre-output Click To Edit Master Title Style Power - Current Characteristics From stationary solution (dN(t)/dt=0, dS(t)/dt=0, I(t)=const.) of rate eq. minimum required carrier density N>NTh for lasing (lasing threshold) minimum required current I>ITh for lasing (lasing threshold) ITH qV te NTH qV 1 N0 t e g0t p te in (3.1-4) strongly temperature dependent empirical formula T T0 ITH I 0e , for T0 50...100 K Relationship: light-power and injection current: P hf T h d ( I I TH ) , 2q I I TH h h = 6.626 10-34 Ws2 (Planck's constant) hd = Quantum-efficiency (e.g. 22%) h= slope of light-current characteristic 17 04 – Lasers Click To Edit Master Title Style Direct Modulation of a Laser From stationary solution (dN(t)/dt=0, dS(t)/dt=0, I(t)=const.) of rate eq. minimum required carrier density N>NTh for lasing (lasing threshold) minimum required current I>ITh for lasing (lasing threshold) Step response: Bias required in order to avoid turn-on delay 18 04 – Lasers Click To Edit Master Title Style On-Off-Keying with Bias Bias, so that extinction ratio>0 P ok_3111_direktmod_laser.dsf Laser driver driver circuitry for delivering sufficient current amplitudes to laser modulation input. P(t) Pmax Slope= h Pmin t 0 ITH IBias I Extinktion ratio: Imod(t) t 19 04 – Lasers ex= Pmin ___ , exdB=10 lg ex Pmax Click To Edit Master Title Style Large Signal Modulation 20 04 – Lasers Click To Edit Master Title Style Laser Chirp Change in injection current change in carrier density. refractive index depends on carrier density change in refractive index variation of phase (and thus frequency) with time = chirp df (t ) 1 1 g0 ( N (t ) N 0 ) dt 2 t p = Line-width enhancement factor (= Henry-factor = chirp parameter) From rate eq. (with d<< 1) dS (t ) 1 g 0 ( N (t ) N 0 ) S (t ) dt t p I(t) t f(t) we find relationship df/dt (Chirp) with S(t) (Photon density) df (t ) 1 dS (t ) d ln S (t ) 2Df (t ) dt 2 S (t ) dt 2 dt t Df(t) ok_3113_chirp.dsf rough approximation: t df (t ) dS (t ) dP (t ) dI (t ) ~ ~ ~ ~ Df (t ) dt dt dt dt Chirp produces frequency modulation (FM) with amplitude (frequency shift) of several GHZ transmitted spectrum is widened (= line-width enhancement) after dispersive fiber we expect (in most cases) additional distortion! 21 04 – Lasers Click To Edit Master Title Style Small Signal Performance, Laser Frequency-Response Frequency response for sinusoidal modulation I(t)=I0 cos(mt) above threshold. Gives information on modulation bandwidth maximum bitrate From rate-equations with exponential ansatz and some simplifying assumptions: linear Diff.eq. 2nd order 1 H (m ) 2 2D 1 m j 02 m 0 0 with: 0 I BIAS / ITH 1 t et p D0 I BIAS 2 0t e ITH 1 D0 < 1: overshoot in step-response Relaxation oscillations 22 04 – Lasers Click To Edit Master Title Style Laser Noise Spontaneous emission in laser: 1. Intensity noise Relative Intensity Noise (RIN) Line-width enhancement () 2. Phase noise RIN: Power Spectral Density (=power in 1 Hz bandwidth) relative to the mean output power P, with B = (narrow) measurement bandwidth P 2 2 /B ok_3_Laserrauschen.dsf Pn2 (t ) / B P RIN-LDS[dB/Hz] P(t ) P RIN 2 ok_3_Laserrauschen.dsf P P(t) -120 P(mW) 1 -160 0,1 t 23 Typical shape: 04 – Lasers 2 1 3 f/GHz 10 Click To Edit Master Title Style Phase Noise (Laser Linewidth) PSD of electrical field with phase noise: P 1 1 S Laser ( f ) 2 2 Df 2( f fT ) 1 2( f fT ) 1 Df Df Df results from 3dB-bandwidth, typically 10kHz-100MHz P/Df P/2Df Df ok_3_laserrauschen.dsf fT 24 04 – Lasers Lorentzian line Click To EditTunable Master Title Broadband LaserStyle (DBR) Y-branch laser Setup: ► ► ► ► 2 Reflector sections (Branches) 1 Multi-Mode-Interferometer (MMI) 1 Phase section 1 Gain section Principal of operation: ► Wideband tunability (Dl = 40 nm) by Vernier effect in reflector sections ► Fine adjustment by phase section with 0,35 nm wavlength span 25 04 – Lasers Vernier Click ToEffekt Edit Master Title Style 2 scales with 10% difference A shift of one scale by dx, leads to a shift of the point where both scales fall together by Dx = 9 dx. 26 04 – Lasers LaserTo Resonator Click Edit Master Title Style Example: Resonator 1 Reflection Resonator 2 Wavlength [nm] Tuning of resonator 2 by 0.34 nm leads to a shift of the overall reflection by 1.7 nm. Problem: Temperature stability ► Usually requires use of a thermo-electric cooler (TEC) 27 04 – Lasers Click To Edit Master Title Style Wrap Up What you should recall from this chapter: • • • • • • • • • 28 Explain, why a directly modulated laser should be driven above threshold Plot the PSD of a laser output for a laser linewidth of 10MHz Plot the PSD of a laser output for a laser linewidth of 0MHz Explain direct modulation of a laser with the power-current characteristics What kind of resonator is used in a DFB and a DBR laser? What is the advantage of VCSELs? Explain the difference between single-mode lasers and Fabry-Perot lasers What is the twofold purpose of the active zone in a laser? Explain the tunability of a broadband DBR laser 04 – Lasers Lecture Optical Communications Modulators and Modulation Formats Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke External Modulators Directly modulated laser Chirp! Alternative: cw-laser followed by an additional external modulator. Two Types: 1. Electro-absorption modulator (EAM) 2. Mach-Zehnder modulator (MZM) 2 05 – Modulation Electro-Absorption Modulator Intensity modulation by changing the absorption properties of modulator material (III-V semiconductor pn-junction) as a function of applied external voltage physical effect: "Franz-Keldish-Effect" P1(t)=const. I=const. Laser EAM cw P2(t) f(t) ok_32_modulator.dsf u(t), Modulation Modules with DFB-laser und EAM commercially available up to 20 Gb/s P2 (t ) T {u (t )} P1 (t ) Transmission characteristic s(exponential shape) d (t ) a 1 dP2 (t ) 2pf (t ) dt 2 P2 (t ) dt a = chirp parameter (approx. 10 times smaller compared to direct modulation) 3 05 – Modulation Transmission and Chirp Characteristics of an EAM Measured transmission of an EAM versus applied drive voltage [Lucent Technologies] 4 Measured Chirp parameter of an EAM versus drive voltage [Lucent Technologies] 05 – Modulation Phase Modulator Phasor Diagramm Q Phase modulator V1 I Ein Applied voltage U1, U2 changes refractive index through "Linear electro-optical effect" ("Pockels-Effect") propagation constant changes resulting in a phase change. 5 05 – Modulation Mach-Zehnder Modulator (MZM) MZM works as an interferometer: Splits light into 2 wave-guide arms (coupler). Applied voltage U1, U2 changes refractive index through "Linear electro-optical effect" ("Pockels-Effect") propagation constant changes resulting in a phase change. Recombining signal (coupler): constructive (in-phase) or destructive (out-of phase) interference Waveguide ocn_32_modulator.dsf U1 1 Ein ,Pin Eout ,Pout U2 2 MZM available as integrated optical component (e.g. PLC=planar lightwave circuit) LiNbO3 (Lithium-Niobath) 6 05 – Modulation Coupler For splitting and combining (optical) signals we need a coupler (optical component). Here: 3dB coupler with 2 inputs and 2 outputs (2x2 coupler) Ei1 Eo1 ok_63_bilder.dsf Input Output Ei2 L Eo2 coupling length Implementation: melting fibers together (= fiber optic solution), or wave-guides in integrated optics L= coupling length (a few mm) Coupler model 2x2 coupler is a MIMO system described by a 2x2 transmission matrix (scattering matrix): S3dB 7 1 j L 1 j e 2 j 1 05 – Modulation MZM Model (Block View) Ei1 Ei2 1 + 1/ 2 e j1 j j j j 1 + 1/ 2 e j2 oc_32_modulator.dsf 8 1 05 – Modulation 1 + 1/ 2 Eo1 + 1/ 2 Eo2 MZM Model (Mathematical Model) Eo 2 jEi1 1 j1 j2 e e 2 cos 1 2 e 2 j 1 2 2 H MZM With MZM arbitrary amplitude and/or phase modulation of optical carriers possible! 9 05 – Modulation Push-Pull Operation of an MZM Push-pull operation of an MZM: 1 2 For U D Vp U1 U 2 UD 1 p / 2 2 zero output (destructive interference) 1 p 2Vp U D 2 Vp is a characteristic parameter of an MZM In Push-pull mode, an MZM is an amplitude modulator (no phase modulation, no "chirp") p 2 H cos 1 2 cos 1 cos 1 cos UD 2 2 2Vp | H |2 10 p cos 2 UD 2Vp 05 – Modulation MZM in Push-Pull Operation ocn_32_modulator.dsf Im{H} Hpush-pull Hpush-pull 12 21 1 2 Re{H} Phasor representation (upper and lower branches) 0 2 Vp UD Field (blue) and power (red) transfer function Phasor representation (output power) 11 Vp 05 – Modulation MZM in Push-Pull Operation (Modulation) Pout UBias Pmax UD1 p 2Vp UD Ein + - Eout p UD2 2Vp ubias oc_32_modulator.dsf Vp MZM-Model ocn_32_modulator.dsf Amplitude (intensity) modulation: Amplitude (intensity) modulation with MZM: Small drive voltage swing at quadrature bias point: ≈ linear P-U-characteristic ≈ √- field-ampl.-U-charact. 12 05 – Modulation 2Vp UD MZM (Lab Photo) 13 05 – Modulation IQ-Modulator IQ-Modulator V1 I Ein Q I 90° Q V3 V2 Nested configuration of two MZMs with 90° phase shift in lower branch. 14 05 – Modulation Usable for arbitrary IQ-constellations Modulation Formats 15 05 – Modulation Basic Formats (OOK / BPSK) Q Q I 0 I -A A BPSK: Binary Phase Shift Keying OOK: On-Off Keying Coding of one bit per symbol (MZM sufficient as modulator) 16 A 05 – Modulation Higher-Order Modulation Higher-order modulation: Several bits are coded in one symbol Reduced symbol rate / tighter spectrum Higher data rates (> 100 Gb/s) Increase in spectral efficiency Satisfies future capacity requirements Disadvantages: 17 • Reduced transmission reach • Higher complexity of transmitter and receiver 05 – Modulation Amplitude Shift Keying (ASK) Q 01 11 10 00 Q I 000 0 110 101 I 001 010 111 100 4-ASK 8-ASK Coding of 2 bits per symbol Coding of 3 bits per symbol Gray coding of all symbols 18 011 05 – Modulation Phase Shift Keying (PSK) 101 -jA Q 0110 0111 0010 0011 jA 0101 0001 0100 0000 I 1100 -A A 1000 1101 1001 1111 1010 1110 -jA 1011 QPSK: Quadrature Phase Shift Keying 8PSK: 8 Phase Shift Keying 16PSK: 16 Phase Shift Keying 2 bits/symbol 3 bits/symbol 4 bits/symbol Q 11 Q 011 010 jA 01 I 10 -A A 00 -jA 001 jA 000 I 110 -A A 111 100 Requires use of phase modulator 19 05 – Modulation Quadrature Amplitude Modulation (QAM) 0000 Q 011 Q 11 0100 01 001 010 0001 0101 Q 1100 1000 1101 1001 000 I I I 110 10 00 100 111 0011 0111 1111 1011 0010 0110 1110 1010 101 4QAM: 4 Quadrature Amplitude Modulation 8QAM: 8 Quadrature Amplitude Modulation 16QAM: 16 Quadrature Amplitude Modulation 2 bits/symbol 3 bits/symbol 4 bits/symbol Requires use of IQ-modulator 20 05 – Modulation System Configuration 21 05 – Modulation Transmitter Setup with IQ-Modulator Digital-AnalogConverter Driver Amplifier (linear) Data (I) Data (Q) Q I I 90° Q Laser f 193.1 THz (C-Band) 22 IQ-Modulator 05 – Modulation Transmitter Setup for Polarization Multiplexed Transmission Data (Ix) Data (Qx) Q Polarization Beam Splitter I I X-Polarization 90° Q I 90° Y-Polarization Q Q I Data (Iy) Data (Qy) 23 05 – Modulation Wrap Up What you should recall from this chapter: • Explain the structures of an EAM and of an MZM • Assume a MZM in push-pull configuration with Vp=5V. Plot the phasors of both arms for 0V, 2.5V, 5V, 10V DC voltage • Plot the MZM phasors for quadrature biasing. • Give the biasing and drive voltage conditions for an MZM with (nearly) linear intensity modulation • Give both, the power and the electrical field for a MZM at quadrature bias • Explain how to connect the 4 contacts of an MZM for (i) push-pull configuration and for (ii) pure phase modulation • Is it possible to use an EAM as a phase modulator? • How does an optical IQ-modulator work? • What higher modulation formats are used in optical communications? • How does a transmitter look like for optical IQ-modulation and polarization multiplexed transmission? 24 05 – Modulation Lecture Optical Communications Receivers Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke Optical Receivers Block diagram Optical Optical preBPamplifier filter Photo diode =O/E-conv. Transimpedance amplifier EDFA p1(t) Pulsformerfilter Limiteramplifier Sampling+ decision device =slicer TIA p(t) i(t) Data u(t) Clock Optical front-end oc_5_empf.dsf 2 06 – Receivers CDR (Clock&Data Recovery) Clock recovery Optical Receivers: Components • Optical preamplifier: Optional e. g. for long distance transmission. Increases overall receiver sensitivity, however, adds ASE (amplified spontaneous emission) broadband noise optical BP filter • Photo diode: optical/electrical converter • Trans-impedance amplifier (TIA): low noise preamplifier • Pulse-former filter (Matched filter, Nyquist filter): optional or implemented in TIA. • Limiting amplifier for eliminating signal level variations (overshoots) prior to CDR • Clock and data recovery (CDR) e. g. with PLL for clock recovery. Clock for sampler and decision device (slicer) and successive electronic stages (e.g. SDH-demultiplexer) 3 06 – Receivers Optical-Electrical Converter Photo-diodes are realized either as PIN-diode or APD-diode Requirements: • high sensitivity Responsivity and quantum efficiency for pin-diodes • low intrinsic noise • high bandwidth Semiconductor materials: Material with band gap suitable for received wavelength. Si (~0.8 nm) suitable for joint integration of photo diode and electronic circuitry on one single chip, opto-electronic ICs InGaAs, InGaAsP, Ge (~1.2-1.6 nm) large range of wavelengths 4 06 – Receivers Absorption Coefficient • • • 5 06 – Receivers Wavelength c at which becomes 0 is called cutoff wavelength Material can only be used as photodetector for < c Indirect bandgap materials (Si, Ge) show can be used as photodetector (however, with reduced absorption edge steepness) PIN Photo Diode PIN-Diode: pn-junction with intrinsic (i.e. weakly doped) layer of some m thickness where absorption of photons takes place, pn-junction backward biased Absorption of photons pairs of electrons/holes electrical current i(t ) R P(t ) ~ R | E (t ) |2 q R R=Responsivity hc =Quantum efficiency<1 (e.g.90%), q=electron charge, h=Planck's constant (6.63 10-34 J/Hz) Voltage/current characteristic of a photodiode 6 06 – Receivers Reverse Biased Operation Depletion region Diffusion region w~1/2 • Elctron-hole pairs are generated through absorption Large electric field inside of depletion region accelerates electrons and holes to opposite directions Drift component dominates over diffusion component Resulting flow current is proportional to the incident optical power • • • 7 06 – Receivers Drift region Rise Time • Rise time Tr is defined as the time during which the response increases from 10 to 90% of its final output value When the input voltage across an RC circuit changes instantaneously from 0 to V0 the output voltage changes as • = • 1 − exp − The rise time is then calculated by = 9 • In a photodetector a transit time tr needs to be added to consider the time before the carriers are collected after their generation through absorption of photons • tr can be thus reduced by decreasing the width W of the intrinsic region. However, for W<3 the quantum efficiency decreases significantly. • There is a tradeoff between bandwidth and responsitivity (speed versus sensitivity) of a photodetector 8 06 – Receivers APD (Avalanche Photo Diode) Very high backwards-bias voltage (>100V) Photons produce electron/hole pairs additional pairs of electrons/holes by impact ionisation = avalanche effect q RAPD 1 , RAPD M R, M =avalanche gain (e.g. 100) hc high gain, but usually lower bandwidth, more intrinsic noise • • • 9 An accelerated electron can generate a new electronhole pair The energetic electron gives part of ist kinetic energy to another electron in the conduction band (leaving behind a hole) Many secondary electrons and holes can be generated 06 – Receivers Front-End Amplifiers Photo diode with parasitic C (small signal equivalent circuits) Load ok_5_empf.dsf Ip Cp Photo diode RL +A - Load Rp=RL High-impedance amplif. Ip RL -A + Cp Photo diode Load Rp=RL/(A+1) Trans-impedance amplif. 1.) low noise power (resistor noise): value of RL as large as possible! 2.) high bandwidth: load resistor RP seen from photo diode as small as possible! usually transimpedance amplifier (TIA) used (additional advantage: high dynamic range for optical input signal), which is a current-to-voltage converter 10 06 – Receivers Noise Performance Quantum Limit The process of current generation from incident photons is of statistical nature modelled as a shot-noise process with Poisson probability distribution (approximately Gaussian) Photon rate (= number of photons arriving at photo diode (PD) per second), [rp]=1/s rp P(t ) P(t ) T , where hfT Energy of one photon [Ws] hf hc T We consider binary on/off keying (0,1 signalling) with P1=optical received power at photo diode for “1”-level. Then: PT N 1b hfT =mean number of photons arriving at PD during the "1"=bit intervall of duration Tb and from Poisson probability distribution, the probability that a number of n photons arrive during the “1”-bit interval is: n PT 1 b PT 1 b hfT e hfT n! 11 06 – Receivers Minimum Number of Photons An ideal on/off receiver (photo detector) expects zero photons to arrive during the “0” bit interval and at least one photon to arrive during the “1” bit interval Bit error, if in “1” bit interval (dk=1) n=0 photons arrive W {n 0 | d k 1} 1 2 e PT 1 b hfT 1 N e BER 2 W {d k 1} Quantum-limit: 1 BER e N 10(lg 2 lg e N ) 10(0.3 0.4343 N ) 2 BER = 10-9 N =20 photons per Tb minimum received power P1 BER = 10-12 N =27 photons per Tb minimum received power P1 12 06 – Receivers Noise Performance of Real Receivers Noise Contributions: 1. Shot noise (see above), Poisson probability density function 2. Thermal noise of receiver electronics, Gaussian 3. Optical noise, if optical preamplifier is used first we consider only 1. and 2. (3. will follow in next lecture: optical amplifiers) 13 06 – Receivers Approximation for Shot Noise (PIN-Diodes) Photo current = Superposition of (filtered) current pulses occurring at photon rate rp. If the photo current is well above quantum limit, "0" i (t ) isignal (t ) ishot (t ) "1" "0" ok_5_rauschen.dsf isignal (t ) R P(t ) mean value (≈const. during Tb) ishot (t) (shot noise) ≈ Gaussian random process with zero mean and variance 2shot within electrical bandwidth Be: 2 shot 2q Be isignal (t ) 2q R P(t ) Be Shot noise power is proportional to signal power (P1(t)) “1” is more severely disturbed than “0”! For APD-Diodes Useful signal isignal (t ) M R P(t ) Shot noise: (increases with increasing avalanche gain) 2 shot 2q Be M 2 FA ( M ) R P(t ) with FA ( M ) k A M (1 k A )( 2 1 / M ) = excess noise factor kA = ratio (0<kA<1) of ionisation coefficients 14 06 – Receivers Tb Photons t Excess Noise Factor Excess noise factor as a function of the average APD gain M for several values of the ionization-coefficient ration kA 15 06 – Receivers Thermal Noise Thermal noise based on resistor noise power spectrum density (PSD) Noise power measured in receiver bandwidth Be kT 2 therm 2 2 Be R It2 with: k = Boltzmann's const. 1,38·10-23 Ws/K T = absolute temperature R = load resistor PSD of current (current density), typically It≈10…20 pA/√Hz Electrical receiver bandwidth: according to Nyquist bandwidth for cancelling ISI (Inter-Symbol-Interference) e.g. 10 Gb/s Tb=100ps ok_5_rauschen.dsf 5 GHz in practice: Be 1 2Tb 1 Tb -Tb 0 +Tb 100ps (5 ... 10 GHz at 10Gb/s) 16 06 – Receivers t -fb/2 0 +fb/2 f Thermal Noise Impact of Trans-Impedance Amplifier with Noise Figure Fn Definition of the noise figure: (SNR)in Fn ( SNR )in ( SNR )out (SNR)out ok_5_rauschen.dsf Thermal noise power after amplifier: 2 therm 4kT Fn Be R Total noise power Shot noise and thermal noise are statistically independent total noise power: 2 2 2 shot therm 2qR Be P (t ) 17 4kT Be RL 06 – Receivers Common Values 18 06 – Receivers Receiver Front-End Model Simulation model with noise sources: Poisson noise source 2 _ E(t) R= P(t) q __ hf T + + + H(f) i(t) isignal(t) id(t) Gaussian dark current noise source Magnitude squared operation of photo diode Dark current = residual current with no light input (reverse biased diode) 19 06 – Receivers ok_5_empf.dsf Clock Recovery 20 06 – Receivers Decision Circuit 21 06 – Receivers Wrap Up What you should recall from this chapter: • • • • • • • • • • 22 Give the quantum limit for BER=10-3 in terms of average photons per bit and in received power Which semiconductor materials are suitable in 3rd optical window (1550 nm region)? Discuss the use of Si as semiconductor material in optical communications Compare the shot noise variance and thermal noise variance using typical parameter values. The mean optical power at receiver is 3dBm and the extinction ratio is 10 dB. Calculate the power levels for “0” and for “1” Explain why optical detection is always random in nature Calculate the signal to noise power (S/N) for the electrical current at optical front-end output, assuming shot noise and thermal noise Shot noise power is proportional to signal power. Explain how this fact can be noticed in the eye diagram. Explain how the threshold in the slicer must be adjusted for the above effect Compare pin-diode and APD noise performance 06 – Receivers Lecture Optical Communications Optical Amplifiers Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke Optical Amplifiers General Concepts Applications: a) compensation of loss (fiber, insertion loss of components) b) (rarely) for making use of non-linear optical properties of optical elements (optical network elements like switches, wavelength converters,…) Application areas: a) In digital long-haul transmission systems as power amplifier (booster-ampl. in transmitter), pre-amplifier (in front of photo diode), in-line amplifier (ampl. within transmission link) b) In access networks (PONs - Passive Optical Networks) at the central office (optical line terminal): compensates for the loss of 1:N-star couplers at the remote node 2 07 – Amplifiers 3-R Regeneration Prior to the optical amplifier era (≈1995): Digital transmission with o/e/o repeaters (regenerators) inserted in transmission link with 3-R-Regeneration: 1. Reamplification (signal amplification) 2. Reshaping (signal shaping e.g. through limiting amplifier) 3. Retiming (clock recovery and clock jitter cleaning) o/e electrical receiver transmitter amplification clock recovery data decision laser driver e/o ok4_repeater.dsf • complex and expensive device • works at fixed bit rate and modulation format, i.e. not transparent • for WDM not useful due to high cost (one repeater per WDM channel!) • very good signal cleaning 3 07 – Amplifiers Example of 3R Regeneration vs. Optical Amplification 3R Regeneration 80km Significant costs due to optical-electrical conversion 80km 80km 80km 80km 80km 80km 80km 80km 1550 1550 1550 1550 1550 1550 1550 1550 10G 10G 1550 1550 1550 1550 1550 1550 1550 1550 RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 10G 1550 1550 1550 1550 1550 1550 1550 1550 10G RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 1550 1550 1550 1550 1550 1550 1550 1550 RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 10G 1550 1550 1550 1550 1550 1550 1550 1550 RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 10G 1550 1550 1550 1550 1550 1550 1550 1550 10G RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 10G 1550 1550 1550 1550 1550 1550 1550 1550 10G RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 1550 1550 1550 1550 1550 1550 1550 1550 10G RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 1550 1550 1550 1550 1550 1550 1550 1550 10G 10G RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 1550 1550 1550 1550 1550 1550 1550 1550 RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 10G 1550 1550 1550 1550 1550 1550 1550 1550 RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 10G 1550 1550 1550 1550 1550 1550 1550 1550 RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 10G 1550 1550 1550 1550 1550 1550 1550 1550 RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 10G 1550 1550 1550 1550 1550 1550 1550 1550 RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 10G 1550 1550 1550 1550 1550 1550 1550 1550 10G RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G 1550 1550 1550 1550 1550 1550 1550 1550 10G RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR 10G RPTR RPTR RPTR RPTR RPTR RPTR RPTR RPTR Optical Amplification 10G 10G 10G 10G 10G 10G 10G WDM 10G 10G TERM 10G 10G 10G 10G 10G 10G 10G 4 ~80 km EDFA ~80 km 07 – Amplifiers EDFA ~80 km 10G 10G 10G 10G 10G 10G 10G 10G WDM 10G TERM 10G 10G 10G 10G 10G 10G 10G Optical Amplifiers • only 1R-Regeneration, no improvement (signal cleaning) of transmission impairments as e.g. dispersion, PMD, jitter • reshaping (optical limiting amplifiers) in optical domain very difficult • transparent • amplifies many WDM-channels with only 1 amplifier (wide amplifier bandwidth of several THz) • 2 Types: a) fiber amplifiers (Erbium-Doping: EDFA for 3rd opt. window, Praseodymium-Doping: PDFA for 2nd opt. window) and b) semiconductor opt. amplifiers (SOA, 2nd und 3rd opt. window) 5 07 – Amplifiers Semiconductor Optical Amplifiers Description and properties: • optically active medium = semiconductor resonator (anti-reflection coating) => similar to semiconductor laser without • due to semiconductor geometry (rectangular geometry of active zone) dependent => polarization • Problem of fiber-chip interconnect => coupling loss (insertion loss) • non-linear saturation effects => intermodulation in a WDM system with many wavelength channels => for WDM-amplifier not a good solution • Gain at both = 1.3 m and 1.55 m with very large gain bandwidth (>100nm) => useful for both optical windows • as integrated optical component well suitable for miniaturization • Applications as optical switch and wavelength converter in WDM-networks (makes use of non-linear properties) Block diagram: 6 07 – Amplifiers Fiber Optical Amplifiers Silica (glass) fiber doped with rare earth elements is used as active amplifying medium. Typically used dopents: a) Erbium (amplification in 3rd opt. window), commercially available, widely used b) Praseodymium (amplification in 2nd opt. window), low commercial interest here: EDFA = Erbium Doped Fiber Amplifier Er fiber Signal ASE 980 nm 7 l 1480 nm 07 – Amplifiers Output spectrum Noise λ Isolator Filter Filter Tap VOA Coupler Isolator EDFA Block Diagram • Coupler adds pump laser signal and (modulated) input signal • Erbium-doped fiber: silica fiber where the core is doped with Er-Ions (length: several meters e.g. l = 10m) • Optical isolator to reduce oscillations due to reflection at connectors, components, etc. • Pump laser: provision of (optical) energy. Wavelength is given through energy levels of Er.: pump = 980nm or pump = 1480nm, Ppump ~ 50mW • Optical filter for noise reduction (optional) • other arrangements are possible: reverse pumping, bi-directional pumping, remote pumping (undersea systems) 8 07 – Amplifiers Setup of a 2-Stage Optical Amplifier Stage 1 Preamp In C/L Stage 2 VOA Booster 95:5 95:5 Mon IN OSC/RX Inter Stage 12 dB Pump upgrade Mon OUT OSC/TX OSC: Optical Supervisory Channel VOA: Variable Optical Attenuator only for C-Band L-Band L-Band 22 dB 21 dB 12 dB 30 dB log(P) VOA:1-13 dB max. 8 dB l 9 07 – Amplifiers Out C/L Interaction between Er-Ions and Photons Absorption Spontaneous emission Stimulated emission An arriving photon hits an Er-ion in ground state and raises an electron up into laser level. The photon is absorbed An electron in excited state (laser level) falls spontaneously from laser level to ground level and a photon is generated. A photon hits an Er-ion in laser level and stimulates its transition to ground level. Another photon is emitted. This photon has the same frequency, phase, polarization and direction as the incoming photon which stimulated this radiation process. This is a light amplification process. Laser level Ground level 10 07 – Amplifiers Energy Levels in Erbium Energy Level Nr. Pump Three energy levels of Er-ions in silica material (3-level scheme): Energy difference Wavelength Stimulated Emission 11 E E pump E ground h f 07 – Amplifiers hc pump Optical Gain pump level laser lavel 980nm energy E3 E2 s p 1530nm absorption spontan. emission ok_432_energieniveau ground level E1 stimulated emission • Due to energy of pump laser (980nm), carriers move from E1 E3 by absorption • Carriers at level E3 have short lifetime of several s. Transition to E2 by emission of heat. • Carriers at level E2 have long lifetime of several ms! meta-stable state • Inversion state with high carrier density at level E2: N2 > N1 • Therefore signal light at ~1530 nm is amplified by stimulated emission (E2 E1). • Pump energy levels are not discrete but are split in many sub levels ("Stark-splitting"). Therefore we have a broad gain bandwidth in the range of 1525-1570nm with wavelength dependent gain • Spontaneous emission noise within signal wavelength is also amplified Amplified Spontaneous Emission (ASE): un-polarised wideband noise distortion 12 07 – Amplifiers Energy Broadening of the Energy Levels • • 13 Homogeneous broadening of the energy levels arises from the interactions with phonons of the glass. Inhomogeneous broadening is caused by differences in the glass sites where different ions are hosted. 07 – Amplifiers EDFA Characteristics Typical gain spectrum of an EDFA Sopt() ok_432_energieniveau 1530 Pump power and Saturation 14 07 – Amplifiers 1560 /nm Gain Flattening Filter Issues: 15 • Synthesis of the filter function (reproducability) • Temperature dependence of the active fiber • Polarization dependence of the gain • Inhomogeneous saturation of the EDFA • Dependence of the gain spectrum on the pump wavelength 07 – Amplifiers Gain in dB Dynamic Tilt of the Gain Spectrum Wavelength in µm 16 07 – Amplifiers • Gain must be adjusted to the attenuation on the link • A change in gain tilts the spectrum Gain in dB Amplifier Saturation Output Power in dBm 17 07 – Amplifiers Optical Power in dBm Signal Power Evolution Pump power Signal power ASE+ ASE- z in m 18 07 – Amplifiers Comparison of Major SOA and EDFA Characteristics 19 Type of amplifier Pumping source Carrier lifetime Insertion losses Center wavelength (gain bandwidth) Physical length Inputoutput gain Typical noise figure SOA Electrical (~100mA) ~1 ns ~3dB 1.3 or 1.55 m (50-75nm) ~500m 10-15dB 8-12dB EDFA Optical (~20100mW) ~1 ms ~0.2dB 1.55 m (30-45nm) ~10m 10-40dB 4-5dB 07 – Amplifiers Noise in Optical Amplifers 20 07 – Amplifiers EDFA Noise EDFA noise = amplified spontaneous emission (ASE) noise = additive white Gaussian-noise in the optical domain with mean optical noise power per each of the two polarizations (x and y): x PASE nsp h fc (G 1) Bopt x N ASE Bopt -fT x S ASE (f) 1 x 1 N ASE nsp h f c (G 1) 2 2 ok_434_EDFA_rauschen.dsf NASE/2 0 +fT f nsp = spontaneous emission factor nsp >1, typically nsp ≈1.5…3 G = optical power gain of EDFA, e.g. G ≈400 (26dB) Bopt = optical bandwidth h = Planck’s constant ASE noise is unpolarized and occurs in both polarizations (x-pol. and y-pol.) with total noise power in both polarizations: x PASE 2 nsp h f c (G 1) Bopt 2 N ASE Bopt x N ASE 21 07 – Amplifiers Noise Calculation with EDFA Pre-Amplifier Optical field at photo receiver input E (t ) 2GPrec (t ) cos(2 f ct ) nASE (t ) Decompose BP-noise into in-phase and quadrature component E (t ) 2GPrec (t ) cos(2 fct ) niASE (t ) cos(2 f ct ) nqASE (t ) sin(2 f ct ) Optical instantaneous power (squaring of BP optical field and taking low pass components only) 1 2 2 E 2 (t ) GPrec (t ) 2GPrec (t ) niASE (t ) niASE (t ) nqASE (t ) 2 22 07 – Amplifiers Signal-Spontaneous Beat Noise Power Photo current: iP (t ) R E 2 (t ) Calculate ACF Ripip(t) and by F.T. power spectrum density PSD Sipip(f) of ip(t) Si pi p ( f ) / R 2 G 2 Prec 2 2GPrec N ASE Bopt ( N ASE Bopt ) 2 0 ( f ) 2GPrec SiASE ( f ) 2 SiASE ( f ) SiASE ( f ) Signal-spontaneous beat noise power after electrical filter of single sided Bandwidth Be: 2 2 sig spont 4 R GPrec N ASE Be 23 07 – Amplifiers Spontaneous-Spontaneous Beat Noise Power Si pi p ( f ) / R 2 G 2 Prec 2 2GPrec N ASE Bopt ( N ASE Bopt ) 2 0 ( f ) 2GPrec SiASE ( f ) 2 SiASE ( f ) SiASE ( f ) Spontaneous-spontaneous beat noise power after electrical filter of single sided Bandwidth Be: 2 2 2 2 2 spont spont 2 R N ASE 2 Bopt Be Be 4 R N ASE Bopt Be may be reduced by reducing optical Bandwidth Bopt: typical values: G>10dB, Bopt>2Be, , therefore 2 sig spont 2 spont spont 2 shot Signal-spontaneous beat-noise is dominating! 24 07 – Amplifiers Noise Figure of an EDFA Typically the noise contribution of an EDFA is given by the noise figure Fn Fn SNRinp SNRout 2nsp nsp 1 where SNR is measured with a photo detector in the electrical domain Therefore we find: "Best case" noise-figure: 3dB, typically: 4...7dB 25 07 – Amplifiers Optical Signal-to-Noise Ratio (OSNR) OSNR can be measured with OSA (Optical Spectrum Analyzer). Typical result of OSA measurement of WDM signal: OSA measurement principle: optical BP BW=Bm variable fcenter A/D conv. Signal proc. Display PD Measurement of optical power within the resolution bandwidth (RBW) Bm of opt. BP for various BP center frequencies (frequency sweep). 26 07 – Amplifiers Amplifier Chain Booster Inline RX TX Power Distance Noise Figure (G >> 1) F PASE h f ref G Bref > 3dB, typ. 6.5 dB F h f ref Bref 1 P P ASE ASEopt OSNRlin , Bref PAv G Pin Pinopt 1 EDFA: OSNRdB , Bref 0,1nm 58dBm 10 log F Pinopt Amplifer chain with N similar fiber spans (F and Pin in dB) : Fi OSNRdB , Bref 0,1nm 58dBm 10 log opt 58dBm F Pinopt 10 log N i Pin 27 07 – Amplifiers OSNR Measurement Measurement of optical power within the resolution bandwidth (RBW) Bm of opt. BP for various BP center frequencies (frequency sweep). x Noise pwr. measurement: Pnoise PASE 2 N ASE Bm Signal pwr. measurement Psig ,m PBP (t ) S signal ( f fc ) Bm if Bm <≈ signal BW Psig ,m PBP (t ) Psignal if Bm >≈ signal BW with Ssignal = PSD of modulated opt. signal Full signal power is directly measured, if RBW Bm is larger than signal BW! 28 07 – Amplifiers OSNR Calculation According to standardization OSNR is given with resp. to 0.1 nm noise BW, i.e. 12.5 GHz BW in Hz @1550nm OSNRref OSNRref 29 Psignal Psignal Bm Psignal Bm Mean opt. signal power x x Noise power in 0.1nm ref. BW 2 N ASE P Bref 2 N ASE Bm Bref noise Bref GPrec x 2 N ASE Bref GPrec Prec 2 nsp h fc (G 1) Bref 07 – Amplifiers 1 Fn 1 h fc Bref Wrap Up What you should recall from this chapter: • Give the bandwidth of an EDFA in THz. How many WDM channels fit in this bandwidth (approximately)? • Give applications for SOAs • Explain 3-R-regeneration • Why does an EDFA amplify only in the 3rd optical window? • Compare basic OSA and EDFA properties • What is the physical source for EDFA noise • Consider an inline EDFA: Describe the noise model in the optical domain. • Consider an EDFA pre-amplifier: Describe the noise model after the photo detector • Identify the noise power contributions of a pre-amplified optical receiver • The noise figure of an EDFA is given (data sheet) as Fn=5dB. Calculate nsp 30 07 – Amplifiers Lecture Optical Communications Optical Filters Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke Optical Filters Applications: • Noise reduction (ASE noise of optical amplifiers) • De-multiplexer in WDM-systems Bandpass filter bank for selection of (wavelength-) channels, bandwidth ≈ channel spacing 1 2 3 4 1 2 3 4 1 2 3 4 MUX DMUX 1 2 3 4 • Mux and Demux in network elements (add-drop multiplexer, wavelength routers) 2 08 – Filters Applications • Dispersion compensation (delay filter with linear group delay) g(Df)|Filter fT f g(Df)|SSMF ok_6_filteranwend.dsf • Equalization of frequency response of EDFA (power gain spectrum is not flat!) . 3 08 – Filters Technology & Properties Technologies: • Integrated optical wave-guides based on Si, InGaAs, polymer planar light-wave circuits (PLC) = photonic integrated circuits (PIC) • Free space optical filters • Filters based on optical fibre Properties: • Optical filters can be described by transfer functions H(f) (magnitude |H(f)| and phase b(f) = -arg{H(f)} response) for the optical field power transfer function |H(f)|2 • Filter function in bandpass domain at optical fc. May be transformed to equivalent LP domain by BP-LP transformation • We may want to tune the center frequency (tuneable filters) or the filter bandwidth 4 08 – Filters Filter Parameters Attenuation Bandwidth (1dB, 3dB) Crosstalk (Isolation) CW 1dB To adjacent channels To non-adjacent channels I ADJ Phase response I NADJ Dispersion Ripples Polarization dependence Polarization mode dispersion (PMD) Polarization dependent loss (PDL) Passive / active Is temperature control required? 5 08 – Filters Ci Ci+1 Ci+2 Fabry-Perot Filter (Dielectric Filter) Also called “etalon” (e.g. used as laser resonator) Cavity (Resonanzraum) - z E0 t1e t1 n E0t1t2e- L r1 r 2 t2 E0t1t2r1r2e- L L Facet (Spiegel) oc_6_filter.dsf z t1,t2: transmission- r1,r2 reflection coefficients of facets 2 n 2 n f = /2 j Propagation constant: = / 2 j = / 2 j c E-field at right output of mirror (facet): Et = E0t1t2e 6 L L 1 r1r2e 2 L (r1r2e 2 L ) 2 = t1t2e E 1 r r e 2 L 0 12 08 – Filters Calculation of Transfer Function GFP | Et |2 | t1t2 e L |2 = = | E0 |2 |1 r1r2 e2 L |2 With power reflection coefficient: R1 = r12 , R2 = r22 and no loss at facets: GFP finally for t1 = 1 r12 , t2 = 1 r22 (1 R1 )(1 R2 )e L = (1 R1 R2 e L ) 2 4 R1 R2 e L sin 2 ( L) R1 = R2 = R, 0 GFP ( f ) = 7 t12 r12 = 1 1 2 L 2 R 2 n L O 1 M sin( f )P 1 R c N Q 08 – Filters Power Transfer Function In f periodic transfer function GFP(f) = max if: f =k c , k = 0,1,2,... 2nL i.e. choose n and L such that waves add constructively at filter output 8 08 – Filters Calculation of Free Spectral Range Free Spectral Range (FSR) = distance between peaks Df = c 2nL Full Width at Half Maximum (FWHM) bandwidth: BFWHM = FG H c 1 R arcsin nL 2 R IJ K Finesse: F= Df BFWHM R 1 R for large R 1 Typical values: insertion loss 1…2dB, Finesse > 150 9 08 – Filters Filter Tuning Change Cavity-length L or refractive index n by • Mechanical displacement of mirrors (facets) • Piezo-electrical material Cavity length is reduced by ext. appl. voltage Multistage Filters Cascade of F-P filters with various values of FSR 10 08 – Filters Bragg Gratings A Bragg Reflector is a periodic array of reflective “mirrors” (made from any boundary surface like refractive index step changes). Maximum reflectivity occurs, if distance between mirrors is such that reflected waves from all mirrors superimpose constructively. I.e. if the distance of mirrors is related to half wavelength, resulting in full wavelength optical path difference of all reflected waves. 11 08 – Filters Fiber Bragg Gratings Piece of single mode fiber, where “mirrors” are implemented by refractive index variations of fiber core along z. Interaction between forward and backward travelling waves described by a pair of coupled mode equations Solution for reflected optical field (for uniform grating) 12 08 – Filters Fiber Bragg Grating Properties Frequency response for the reflected optical field: r ( L, ) = | A2 ( z = 0) | j B sinh (L) = | A1 ( z = 0) | j cosh (L) Dk sinh (L) with: B = (n3 n2 ) h Dk = 1 1 /(2neff ) = B2 (Dk ) 2 where h = fraction of wave intensity in core (e.g. 0,6) Dn = n3 n2 typically 10-5 …10-3 Maximum power reflectivity R=|r|2 at Bandwidth: Bragg wavelenght: D = 2 Rmax = R ( Dk = 0) = tanh ( B L) 13 08 – Filters B2 neff L 2 ( B L) 2 Spectra 14 08 – Filters Applications Band pass filters: a circulator drops the reflected (backwards) wave (4) at port 3 Band stop filter: transmitted wave Add-Drop-Multiplexer in WDM-Networks BP und Band stop filter for channel selection 15 08 – Filters Fiber Gratings for Dispersion Compensation Dispersion in anomalous regime (e.g. D=+17ps/(nm km)): Increasing wavelength increasing group delay, linearly increasing group delay over therefore we need: a filter, with linearly decreasing group delay and constant magnitude in a wide frequency range flat top BP filter Chirped Fibre-Bragg-Grating 16 08 – Filters Working Principle ok_6_FBG_dispersionskomp.dsf L large small max min large (red) are reflected at beginning of FBG short delay small (blue) are reflected at end of FBG large delay delay compensation = Dispersion compensation 17 08 – Filters Optical Delay-Line Filters Basics IIR-filter (Infinite Impulse Response), recursive filter, filter degree n (=ARMA-filter (Auto-Regressive Moving Average)) x(t) bn + -cn ok_64_dig_filter.dsf bn-1 T + T -c n-1 b1 + b0 T y(t) -c1 y (t ) = bn x(t nT ) bn 1x(t (n 1)T ) ... b1x(t T ) b0 x(t ) cn y (t nT ) cn 1 y (t (n 1)T ) ... c1 y (t T ) n b z H ( z) = Y ( z) = =0 n X ( z) 1 c z =1 Filter with poles and zeroes in complex z-plane 18 08 – Filters FIR Filter FIR-filter (Finite Impulse Response), non-recursive filter, filter degree n (MA-filter) x(t) T b0 T b1 T ok_64_dig_filter.dsf bn + + n y (t ) = y(t) n b x(t T ) H ( z) = =0 b z v =0 Filter with zeroes in z-plane, all poles at z=0 n H( f ) = b e =0 19 j 2 Tf n h0 (t ) = b 0 (t T ) =0 08 – Filters coeff. = impulse response Phase and Frequency Response FIR-filter has linear phase, if all coefficients are symmetrical: m1 = multiplicity of zero at e jT = z = 1 b = ( 1) m1 bn FIR and IIR: frequency response periodic with = 2 / T H ( ) = H ( m 2 ) T m integer m = 0, 1, 2,... Normalisation: e j 2 fT = e jT = e j = z 1 = e sT 20 08 – Filters Digital vs. Optical Filters Digital filters: T=sampling interval (delay by 1 sampling interval), frequency response is periodic with f s = 1/ T . We are interested (in most cases) in the spectral range 0 f f s = 1/ T Optical filters: T=time delay of a delay line (e.g) (we assume time delays are integer multiples of an elementary delay T in order to be able to apply z-transform). Periodicity = free spectral range (FSR). We are interested in the frequency range (i.e. the period) close to optical carrier frequency (e.g. 193 THz) dig. LP-filter ok_64_dig_filter.dsf 0 0 1/T 1 2/T 2 f opt. BP-filter(=LP in equiv. baseband for compl. envelope) fT ok_64_dig_filter.dsf 0 0 21 1/T 1 (m-1)/T m-1 m/T (m+1)/T m m+1 optical carrier frequency 08 – Filters f Optical Delay-Line Filters Basic building blocks: Signal splitting, delay, coefficients, adders by choosing appropriate Requirements: Adaptive, reconfigurable parameters (coefficients, flexible frequency resp. designs degree, etc.) Delay lines: wave-guides of appropriate length DL : speed of wave: v = c m DL 2 108 = n s T DL T FSR 2cm 100ps 10GHz 0,2cm 10ps 100GHz for n 1.46 (fibre) Signal splitting and combining: directional couplers Coefficients: coupling ratio of splitting couplers attenuator / opt. amplifier (EDFA) phase shifter (negative and complex coefficients are possible!) modulators (EAM,MZM) fast reconfigurable adaptive optical filter 22 08 – Filters Mach-Zehnder-Filters Single stage optical FIR filter Schematic coupler 1 coupler 2 Input 1 Output 1 Length difference DL ok_63_bilder.dsf Input 2 Output 2 Two couplers connected via two paths of different lengths. coupler 1: c S1 = 1 js1 c1,2 = cos(1,2 L1,2 ) s1,2 = sin(1,2 L1,2 ) 23 js1 c1 coupler 2: c S2 = 2 js2 where L= coupling length, = coupling coefficient 08 – Filters js2 c2 Mathematical Model length difference DL Phase shift: DL = DL delay: DT = 2 n = DL 2 n f = D c D n = DL 2 f c delay in frequency domain: e j DT = e j 2DT f = z 1 = e j ( = DT ) Model MZ-filter: Ei1 Ei2 c1 + js1 js2 js1 js2 c1 + 1 Eo1 s1s2 c1c2 z = Eo 2 j ( s1c2 c1s2 z 1 ) 24 c2 z-1 + Eo1 ok_63_bilder.dsf c2 + Eo2 j ( s1c2 c1s2 z 1 ) Ei1 H11 ( z ) H12 ( z ) Ei1 = 1 Ei 2 H 21 ( z ) H 22 ( z ) Ei 2 c1c2 s1s2 z 08 – Filters = non-recursive filter functions of 1st order Filter coefficients: coupling ratio in coupler 1 and 2. Complex filter coefficients are possible! Example Example: coupler 1 = coupler 2 = 3dB-coupler: c1 = c2 = 1/ 2 , s1 = s2 = 1/ 2 Ei 2 = 0 Transfer functions at out 1 and out 2 1 1 1 1 1 Eo1 ( z ) = H11 ( z ) Ei1 ( z ) = (1 z 1 ) Ei1 ( z ) = z 2 z 2 z 2 Ei1 ( z ) 2 2 1 1 1 j j Eo 2 ( z ) = H 21 ( z ) Ei1 ( z ) = (1 z 1 ) Ei1 ( z ) = z 2 z 2 z 2 Ei1 ( z ) 2 2 25 08 – Filters Example: coupler 1 = coupler 2 = 3dB-coupler: c1 = c2 = 1/ 2 , s1 = s2 = 1/ 2 Ei 2 = 0 Transfer functions at out 1 and out 2 1 1 1 j j 2 2 1 Eo 2 ( z ) = H 21 ( z ) Ei1 ( z ) = (1 z ) Ei1 ( z ) = z z z 2 Ei1 ( z ) 2 2 Magnitude of frequency response: | H11 |= | H 21 |= | Eo1 | | Ei1 | | Eo 2 | | Ei1 | ~ sin 1 1 1 1 1 2 2 1 Eo1 ( z ) = H11 ( z ) Ei1 ( z ) = ( 1 z ) Ei1 ( z ) = z z z 2 Ei1 ( z ) 2 2 1 = sin DT 2 2 ~ cos 1 = cos DT 2 2 Periodic frequency response with free spectral range: =D =D f FSR = Interleaving transfer functions at out 1 and out 2. 26 08 – Filters 1 DT Mutli Stage Filters 27 08 – Filters Optical FIR-Filter for Equalization of Signal Distortion cascaded Mach-Zehnder Filter coupler 1 coupler 3 coupler 2 coupler N coupler N+1 Input 1 Output 1 DL *** DL Input 2 DL Output 2 single-input single-output device the output 1 can be reached from input 1 by following N+1 different paths characterized by delay times from 0 to N DT therefore the time domain output response y(t) for an input signal x(t) is: N y (t ) = cn x(t nT ) n =0 which is the response of a FIR-Filter! equalization of all single channel distortions: chromatic dispersion, SelfPhase Modulation, Polarization mode Dispersion, Group Delay Ripple 28 08 – Filters Arrayed Waveguide Grating 29 • The incoming WDM signal (1) is coupled into an array of planar waveguides after passing through a free-propagation region (2) in the form of a lens. • In each waveguide (3), the WDM signal experiences a different phase shift because of different lengths of waveguides. Moreover, the phase shifts are wavelength dependent because of the frequency dependence of the mode-propagation constant. • As a result, different channels focus (4) to different output waveguides (5) when the light exiting from the array diffracts in another free-propagation region. The net result is that the WDM signal is demultiplexed into individual channels. 08 – Filters Application Example in WDM-System Channel 1 C-Band C-Band 192 193 194 195 196THz 186 187 188 189 190 191 192 193 194 195 196THz even odd 160 Channels 50 GHz Basic technologies : Dielectric filters Fiber gratings 30 C-/L-Band Filter 192 193 194 195 196THz even odd 80 Channels 50 GHz Interleaver L-Band Basic technologies : MZI (fiber, integrated optical) 08 – Filters 40 Channels even,100 GHz ... even C-Band 100 GHz DEMUX 195 196THz 40 Channels odd,100 GHz Basic technologies: Dielectric filters Arrayed Waveguide Grating Gratings Fiber gratings Wrap Up What you should recall from this chapter: • Give the requirements for an optical filter to be used as EDFA gain equalizer • Give the Bragg condition for reflection by (i) a formula and (ii) by your own words • An optical filter has center wavelength 1555 nm and spectral width (FWHM) of 0.1 nm. Calculate the center frequency and the 3-dB-bandwidth in Hz. • Explain how FBG filters can be fabricated • Explain how an Add-drop Multiplexer with circulator and FBG works • Explain the working principle of an FBG, which is used for dispersion compensation • Explain the pros and cons, if an FBG instead of a DCF is used as dispersion compensator • Which types of optical filters do you know? • Which components are required for designing an optical FIR filter? • Explain how an interleaver based on a single stage Mach-Zehnder filter works. • Calculate the FSR and DL of an interleaver based WDM demultiplexer for 100 GHz channel spacing • Explain the working principle of an AWG 31 08 – Filters Lecture Optical Communications Optical Transmission Systems Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke Setup of an Optical Transmission System Optical Multiplexer Electrical Switch Optical Demultiplexer Reconfigurabel Optical RamanAdd-Drop Multiplexer Amplifier ROADM Dispersion- Transmission Fiber Compensation Module (optional) Optical Cross-Connects OXC Transponder RX / TX S ... ... ... X ... S Transponder RX / TX Optical Amplifier (EDFA) 2 Optical Line Amplifier (EDFA) 09 – Optical Transmission Systems Optical Pre-Amplifier ... ... Transponder RX / TX Transmission Impairments ASE SBS Raman Tilt/ Raman XTK Loss PDGL ROADM ISI GVD (CD) Tilt Ripple ISI – Intersymbol Interference GVD – Group Velocity Dispersion CD – Chromatic Dispersion ASE – Amplifier Spontaneous Emission (Noise) PDGL – Polarisation Dependent Gain/Loss XTK - Crosstalk 3 PDGL SPM, FWM, XPM XTK ISI PMD XTK ISI PMD – Polarisation Mode Dispersion THN – Thermal Noise SPM – Self-Phase Modulation XPM – Cross-Phase Modulation FWM – Four-Wave Mixing SRS/SBS – Stimulated Raman/Brillouin Scattering 09 – Optical Transmission Systems ISI THN Setup of an Optical Transponder Modul 10-100 Gb/s in (shortreach) Photodiode CDR/DMUX Electrical Amplifier Digital Signal Processing (DSP) FEC OTH/SDH CLOCK RZ Generator RZ 10-100 Gb/s out (longhaul) VOA MUX DATA NRZ External Modulator Laser Bias Control Laser Control CDR: clock & data recovery VOA: variable optical attenuator MSA Modul Internal IF (e.g.. 10x10 Gb/s) 4 09 – Optical Transmission Systems Optical Modules (Client Side) Left: CFP Modul (C form-factor pluggable module) 100 Gb/s (C = centum) Reach up to 40 km Bottom: hot-pluggable SFP+ Module (small form-factor pluggable) 10 Gb/s, different bit rates supported (e.g. 1 GbE, 10 GbE, CPRI mobile fronthaul, OTH, ..) Various reach classes up to 80 km 5 09 – Optical Transmission Systems Important Optical Modulation Formats 1. 3. 2. 6 09 – Optical Transmission Systems 4. Recap: Transmitter Setup with IQ-Modulator Driver Digital-Analog- Amplifier Converter (linear) Data (I) Data (Q) Q I I 90° Q Laser f 193.1 THz (C-Band) 7 IQ-Modulator 09 – Optical Transmission Systems Coherent Reception Balanced Detection Input Signal ES 90-DegreeHybrid E1 I E2 = ELO E3 Local Oscillator Laser +j Q E4 1 2 1 = 2 1 = 2 1 = 2 = Homodyne Reception + = − = ( ) ( )− ( ) = − = ( ) ( )− ( ) − + − Control of local oscillator phase and frequency required (or estimation in DSP) 8 09 – Optical Transmission Systems Recap: Transmitter Setup for Polarization Multiplexed Transmission Data (Ix) Data (Qx) Q Polarization Beam Splitter I I X-Polarization 90° Q I 90° Y-Polarization Q Q I Data (Iy) Data (Qy) 9 09 – Optical Transmission Systems DPSK Transmission Balanced DPSK Receiver using Delay-Line-Interferometer Balanced Receiver A T Input Signal B DI DI: Delay-Line Interferometer T = Bit duration A: „Destructive“ port B: „Constructive“ port Suitable for differentially encoded PSK (DI converts differentially encoded PSK to IM) 10 09 – Optical Transmission Systems DQPSK Transmission CSRZ Differential Quaternary Phase-Shift Keying 10 GHz Vp CSRZ-DQPSK -Vp 50ps +p/4 DPSK CSRZ LD aI aQ dI DQPSK Precoder dQ -Vp/2Vp/2 Square 90° PM LPF -Vp/2Vp/2 Square RI 40 Gb/s 50ps -p/4 LPF Very high spectral efficiency – supports 40G at 50GHz grid Very robust against PMD due to reduced Baud rate Two DI Demodulators / Balanced Receivers for I + Q May be susceptible to crosstalk and phase noise 11 09 – Optical Transmission Systems ^ dI ^ aI ^ dQ DQPSK ^ Decoder aQ RQ Im DQPSK Re Duobinary Modulation 1 Binary Input Data 0 1 0 0 1 1 1 Q NRZ (electrical field) 0 -1 I 0 200 400 600 800 1000 1200 1400 1 Duobinary signal (electrical field) Q 0 -1 I 0 200 400 600 800 1000 1200 1400 1 Electrical signal (Direct detection) 0.5 0 Optical Carrier (symbolic) 0 200 400 600 800 1000 1200 1400 3-level signal offers potential for bandwidth reduction Coding rule: an uneven number of zeros inverts the following ones Can be received with standard direct detection RX 12 09 – Optical Transmission Systems Transmitter Setup E Realisierung -Vp Delay&Add bk Coding z-1 + Vp -E ck LPF MZ driver MZ modulator MZ driver MZ modulator fc=3fb/4 Delay&Add Coding Filtered Duobinary Signal bk LPF fc=fb/4 Delay & Add coding is similar to fb-periodic low-pass filter Electrical fb/4 low-pass filter (similar properties) is simpler to realize Both implementations need manual optimization of the filter bandwidths for optimum performance 13 09 – Optical Transmission Systems Example of Signal Coding ck=bk-1+ bk bk=bk-1 d’k dk clock ck divide-by-2 counter AND LPF diff. pre-coding MZ driver c’k Ek MZ modulator Fiber =fb/2 ffcg=f b/4 k dk -1 0 0 1 0 0 0 -Vp E 1 1 0 1 1 1 0 0 0 2 1 0 1 2 Vp -E 1 3 0 1 0 1 0 0 0 4 0 1 1 1 0 0 0 5 1 0 1 2 Vp -E 1 6 1 0 1 2 Vp -E 1 d’k bk ck c’k Ek E ek=dk 0 -Vp Differential pre-coding: standard Rx, no error propagation 14 Direct dection 09 – Optical Transmission Systems Vp -E ck c’k 0 -Vp 1 0 2 +Vp ek Eye Diagrams Delay&Add Coding Filtered Duobinary Signal Electrical Input Signal to MZM Received Eye (back-to-back) V-shaped eye for filtered duobinary signal leads to Back-to-Back Penalty 15 09 – Optical Transmission Systems Eye Diagram after Transmission Filtered Duobinary Signal NRZ 200km SSMF 100km SSMF back-to-back Delay&Add Coding Filtered duobinary signals allows much higher system reach 16 09 – Optical Transmission Systems Optical Spectra 50 Duobinary Filter Gaussian Filter 1st order, Bandwidth 10GHz DB filtering DB encoder, 7.5GHz LPF NRZ, ideal EXT 40 30 Duobinary Coding Delay&Add Coding and Gaussian Filter 2nd order, Bandwidth 7.5GHz PDS [dB] 20 10 0 NRZ With ideal extinction -10 Receiver Bessel Filter 5th order, Bandwidth 7.5GHz -20 -30 -40 -20 -15 -10 -5 0 f-f [GHz] 5 10 15 20 c Data rate 10 Gbit/s Reduced spectral width: Improved dispersion tolerance and spectral efficiency 17 09 – Optical Transmission Systems Futher Optical Modulation Formats 50 NRZ DPSK RZ-DPSK DQPSK RZ-DQPSK DB (filtered) 40 30 PSD [dB] 20 DB Q I DPSK Q 10 I 0 DQPSK 01 -10 Q 00 -20 I -30 11 -40, -20 -15 -10 -5 0 5 f-fc [GHz] 10 15 10 20 PSK and QPSK are relevant for 40-100 Gb/s (spectral efficiency, sensitivity, robustness) 18 09 – Optical Transmission Systems 90° QPSK Coder Filter + Driver PBS LO PC 90° 90° Hybr. 0° 50 GHz DWDM with 2 (bit/s)/Hz spectral efficiency 2500 km reach on uncompensated fiber (~2000 km with inline DCF) DSP as standard component 19 09 – Optical Transmission Systems Client I/F (CFP) PBC 90° FEC, Framing, Monitoring PC PBS PC ADC CW LD 0° Digital Filter (FFE) 90° 90° Hybr. ADC Filter + Driver ADC FEC, Framing, Monitoring Client I/F (CFP) QPSK Coder ADC Coherent Dual-Polarization Transmission System Digital Signal Processing 20 09 – Optical Transmission Systems Block Diagram (Single Polarization) ADC IQCorr. CDComp. Timing Recov. ADC Freq. & Phase Est. Data Recov. First signal flow for reception of only one polarization axis is shown (i.e. 1 complex signal) 21 09 – Optical Transmission Systems Block Diagram (Polmux) ADC X-Pol IQCorr. CDComp. ADC Timing Freq. & Phase Est. Data Recov. Freq. & Phase Est. Data Recov. Pol. Equal. Recov. ADC Y-Pol IQCorr. CDComp. ADC 22 09 – Optical Transmission Systems IQ-Imbalance Correction The (maximum) voltage swings of both channels (I/Q) can be different. The angle between I- and Q-axis in the 90°-hybrid can be different from 90°. Q- channel with 1.2x amplitude and angle between I- and Q of 100°. 23 After (automatic) correction the constellation diagram is circular. 09 – Optical Transmission Systems Timing Recovery The sampling rates of Tx and Rx can be (slightly) different. The optimum sampling point (i.e. center of a bit) may not be hit exactly. Input: IQ-imbalance corrected signal 24 Output: Circular constellation diagram 09 – Optical Transmission Systems Algorithm Gardner Algorithm F. M. Gardner, IEEE Trans. Comm., vol. 34, no. 5, pp. 423-429, May 1986. Square Timing Recovery M. Oerder, H. Meyr, IEEE Trans. Commun., vol. 36, no. 5, pp. 605-612, May 1988. In the following short explanation of the Gardner algorithm working principle: Uses 2 samples per symbol (bipolar signal required) Error is determined by en yn yn 2 yn 1 with a T / 2 distance between two samples Typically PI-control loop to find optimum sampling time Correct Timing en=(-1-1)0 = 0 25 Too late sampling en=(-0,8-0,8)(-0,2) = 0,32 09 – Optical Transmission Systems Too early sampling en=(-0,8-0,8)0,2 = -0,32 Extensions for Polarization Multiplexed Transmission Gardner Algorithmus verliert Regelinformation für DGD = 0.5 Tbit Lösung: Addiere Gardner Phase Detector (GPD) Terme für Signale mit und ohne Entzerrung Verwende Schmetterlingsfilter als Entzerrer mit gleichgewichteten Koppeltermen C. Hebebrand, et al, „Clock Recovery with DGD-tolerant Phase Detector for CP-QPSK Receivers“, SPPCom 2010. 26 09 – Optical Transmission Systems Equalization (only Polmux) (Linear) Polarization effects (PMD & PDL) lead to crosstalk. Equalizer (butterfly structure) approximates Jones matrix to compensate for crosstalk. Input: Retimed Signal (T/2 DGD + PDL) 27 Output: Equalized signal (10 dB OSNR) 09 – Optical Transmission Systems Algorithm Constant Modulus Algorithmus (CMA) S. J. Savory, Optics Express, vol. 16, no. 2, 2008. C.R. Johnson, et al, Proc. IEEE, vol. 86, 1998. Working principle: For each polarization axis the amplitude should be constant (circular constellation diagram). Adapt complex filter coefficients (hxx, hxy, hyx, hyy) iteratively by minimizing the root mean square error (RMSE). 28 09 – Optical Transmission Systems Frequency & Phase Estimation The carrier frequencies of Tx and Rx lasers are not identical. Phase noise can additionally degrade the signal. Input: Retimed Signal 29 Output: Frequency & Phase corrected signal 09 – Optical Transmission Systems Algorithm Viterbi & Viterbi Algorithm A. J. Viterbi, A. N. Viterbi, IEEE Trans. Inf. Theory, vol. 29, no. 4, pp. 543-551, 1983 Recommended: S. Hoffmann, et al, IEEE PTL, vol. 20, no. 18, September 2008. Working principle carrier frequency estimation: Normalize input data (if multi-level modulation formats are used) Determine phase difference between neighobring symbols (Modulo p / 2) Average over a high number of samples (e.g. 500) & correct for frequency offset Working principle phase estimation: Remove modulation by exponentiating (e.g. raise to the power of 4 for QPSK) Determine average value with symmetrically decaying coefficients Determine (average) phase & corrected sampling values using estimated phase 30 09 – Optical Transmission Systems Data Recovery & BER Calculation Divide constellation diagram into sectors Allocate bit sequence to sectors Count bit errors by comparison with original sequence at Tx Calculate BER I = 0, Q=1 I = 0, Q=0 31 I = 1, Q=0 I = 1, Q=1 09 – Optical Transmission Systems Outlook to Higher System Capacity 32 09 – Optical Transmission Systems Development of Transmission System Capacity 33 09 – Optical Transmission Systems Developments towards Higher System Capacity Im OOK Im DPSK Im DP-QPSK Im DC-DPQPSK Im Re Re or Im DP-16QAM DC-DPRe QPSK Re Re Re 1T Intensity Modulation Phase Modulation Direct Detection Self-coherent Detection Phase Modulation Phase Modulation, QAM Single / Dual Carrier Digital coherent Intradyne Detection Single Polarization 34 Dual Polarization 09 – Optical Transmission Systems QAM O-OFDM Maximum Fiber Capacity Spectral Efficiency [bits/s/Hz] 9 Shannon Limit 8 256QAM 7 2000 km Fiber Capacity Limit [1] 6 64QAM 5 8000 km 4 16QAM 3 2 QPSK 1 BPSK [1] Essiambre, et al., “Capacity Limits of Optical Fiber Networks,” JLT, vol. 28, no. 4, Feb. 2010 Shannon Limit Gaussian Ch. 0 0 5 10 15 20 25 SNR/bit [dB] Boundary conditions: Transmission on a single polarization, SSMF, 100-km fiber spans, ideal amplification with Raman amplifiers, no inline DCF, 5-channel WDM Total fiber capacity is limited due to nonlinear fiber effects 35 09 – Optical Transmission Systems Modulation Formats for 400 Gb/s Transmission Format DP-16QAM DP-QPSK DP-QPSK PS-QPSK DP-8QAM DP-64QAM Symbol Rate 60 GBd 60 GBd 30 GBd 40 GBd 40 GBd 40 GBd Bits / Symbol 8 4 4 3 6 12 # Subcarriers 1 2 4 4 2 1 Req. BW DP-64QAM Reach Penalty vs. 100G 7 dB 1.2 dB 1.2 dB -1.8 dB 5.2 dB 14.4 dB 100 GHz 150 GHz 150 GHz 200 GHz 100 GHz 50 GHz Capacity x Reach = const. DP-8QAM DP-16QAM DP-QPSK PS-QPSK 100G DP-QPSK 36 09 – Optical Transmission Systems Options for 1 Tb/s Transmission Format DP-16QAM DP-8QAM PS-16QAM DP-QPSK Symbol Rate 38 GBd 40 GBd 40 GBd 60 GBd Bits / Symbol 8 6 5 4 # Subcarriers 4 5 6 5 Req. BW 150 GHz 200 GHz 250 GHz 300 GHz Reach Penalty vs. 100G 9.2 dB 5.7 dB 10 dB 2.2 dB Capacity x Reach = const. DP-16QAM DP-8QAM PS-16QAM DP-QPSK 100G DP-QPSK 37 09 – Optical Transmission Systems Outlook: Software-Defined Optics Multi-level modulation Client signal Symbol Mapping Client signal Symbol Mapping Multi-carrier modulation f Maintain symbol rate 38 Client signal SubCarrier Mapping Optical OFDM f Client signal SubCarrier Mapping Optical OFDM f Increase number of subcarriers Constant clock rate Change optical bandwidth Constant bandwidth Requires flexible wavelength grid Change modulation format Change subcarrier modulation format Information content per symbol Information content per symbol Noise and impairment tolerance Noise and impairment tolerance 09 – Optical Transmission Systems Wrap Up What you should recall from this chapter: • • • • • • • • 39 What system components are typically deployed in an optical transmission system? How does a optical transponder module look like internally? What optical modulation formats are typically used? Describe the setup of an optical transmitter/receiver for coherent and polarization multiplexed transmission What is a delay-line interferometer? How can it be used at the RX side? How does signal coding for duobinary transmission work? Which steps need to be taken for digital signal processing of a coherently received signal? Which impairments need to be corrected by the DSP on the receiver side? 09 – Optical Transmission Systems Lecture Optical Communications Simulation Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke Outline General Concept Nonlinear Schrödinger Equation Split-Step Fourier Method Step Size Distribution Noise Representation Simulation of Polarization Mode Dispersion 2 10 – Simulation Complex Baseband Representation 3 10 – Simulation Signal Representation Instead of sampling the entire waveform (including the ultra high frequency optical carrier) only the envelope of the signal (blue line) is sampled. The number of samples must be a power of two (to enable use of FFT). The complex baseband representation requires the definition of a center frequency (zero frequency in the baseband). Frequency shifts (e.g. of other WDM channels) lead to phase shifts (in time domain) in the complex baseband. 4 10 – Simulation Nonlinear Schrödinger Equation (NLSE) Can be used to model propagation of light through optical fiber. There exist several different forms of the NLSE, which shall be outlined in the following. NLSE for total field representation (TF) NLSE for separated channels representation (SC) TF equation including terms for coupling of polarization axes Various other versions exist making more or less simplifications (e.g. low or high birefringence in the fiber, inclusion of Raman effects, …) more in information in Lecture on „Numerische Simulation analoger und digitaler Nachrichtensysteme“ by J. Leibrich 5 10 – Simulation Total Field Representation + Suitable model for all linear and nonlinear effects occurring within a fiber - Frequencies between WDM channels are simulated Waste of simulation bandwidth for CWDM systems Slow for these systems 6 10 – Simulation Separated Channels Representation + Simulation bandwidth is used more efficiently + Nonlinear effects between different WDM channels can be evaluated independently - Linear crosstalk between different WDM channels cannot be simulated 7 10 – Simulation Nonlinear Schrödinger Equation (Linear part) j 2 1 3 A z, t j 0 1 2 2 3 3 A z, t 0 z t 2 t 6 t 2 Taylor series expansion of the propagation constant (truncated after third order term) Derivaties with respect to time can be replaced by multiplications with j in the frequency domain. Important: fiber attenuation constant is given here in linear units: Neper km dB km 8 ln10 dB km 0.23. 10 10 – Simulation Propagation Constant d 1 d 2 0 0 d 0 2 d 2 0 0 2 1 d 3 6 d 3 0 3 ... 0 Usually derivaties to the angular frequency are abbreviated by: d n d n n . 0 The first element 0 of the Taylor series expansion induces a frequency-independent phase response. The second term 1 is usually eleminated in simulations by a moving coordinate system (with the group velocity of the center frequency). Nota bene: There may exist a difference in 0 values between both polarization axes (and of course also of 1). 9 10 – Simulation Dispersion Parameters For completeness: The dispersion parameter D and dispersion slope parameter S are related to the propagation constant derivatives in the following way: 2 dD 2 c 1 1 S d 3 3 c 2 d 1 2 c D 2 2 d vg d 1 d 1 2 2 D d d vg 2 c 10 10 – Simulation d 2 3 3 S 2D 2 d 2 c Fiber Birefringence In single mode fibers generally two fundamental modes, orthogonal to each other, are guided. The difference of the propagation constants for both modes can be written as: 1 1, x 1, y and 0 1 1 c This leads to the propagation equation for a linear, birefringent fiber: Ax z, t j 0 z Ay z, t 2 2 0 0 1 1 0 j 2 1 3 Ax z, t 0. 0 2 0 1 t 2 2 t 2 6 3 t 3 Ay z, t More on the simulation of random varying birefringence (PMD) will follow later. 11 10 – Simulation Nonlinear Schrödinger Equation (Nonlinear Part) Dependence of the refractive index on the optical power described by the Kerr effect: n(t ) nlinear n2 A(t ) 2 n2 0 c0 Aeff An optical fiber with large birefringence (beat length lB >> l) 2 lB . x y can be modelled by 0 j Ax 1 1 2 1 3 Ax 3 2 z Ay 2 2 0 1 t 2 t 2 6 t 3 Ay 2 2 2 A A 0 y x Ax 3 Total Field j 2 2 Representation 2 Ay 0 A A y x 3 12 10 – Simulation Nonlinear Schrödinger Equation (Separated Channels) In separated channels representation the electric fields Ai of the individual WDM channels are separated. The different nonlinear contributions are included by coupling terms: An An 2 An 1 3 An j An 1,n 1,ref 2,n 3,n 2 3 z 2 t 2 t 6 t Attenuation Time delay Dispersion Dispersion slope N 2 2 j An An 2 Ai j Ai Aj Ak exp j kz i 1,i n n i j k ;i , j k SPM XPM FWM N n 1 n gR gR 2 2 Ai Ai An 2 K A 2 K A i 1 i n 1 i eff eff SRS 13 10 – Simulation Split-Step Fourier Method 14 10 – Simulation Solution of the NLSE by the Split-Step Fourier Method Split nonlinear Schrödinger equation in linear and nonlinear parts Separate solution of linear and nonlinear parts Solution of the linear part in the frequency domain and of the nonlinear part in time domain (acceptable for small step-sizes) Transformation between time and frequency domains by FFT (significant speedup can be expected from parallization) 15 10 – Simulation 15 Fast Fourier Transform (FFT) M 1 2 j k M k with wM exp V DFT v k v k wM k , 0,..., M 1 k 0 The idea of the FFT is to split up the input sequence into two sequences with even and odd arguments M /2 1 V M / 2 1 v 2l w 2 l M l 0 v 2l 1 w 2 l 1 M , 0,..., M 1 l 0 2l l With the help of the relation wM wM this can be rewritten M /2 1 V M / 2 1 v 2l w 2 l M wM l 0 v 2l 1 w , l M l 0 V1 wM V2 . 16 10 – Simulation 0,..., M 1 FFT (cont.) Finally, M / 2 DFTs of length 2 remain, which can be calculated by: 1 k G g k w2 , k 0 G 0 g 0 g 1 0,1 G 1 g 0 g 1 The Cooley-Tukey algorithms has a complexity of approximately 5n log2 n operations for an FFT length of 2n. 17 10 – Simulation Four-Step-FFT Algorithm Multiply with M twiddle factors and transpose the matrix FFT implementation of length M = 16. 8 FFTs of length 2 along the lines and 2 FFTs of length 8 along the lines. 1. Calculate M2 DFTs of length M1 along the lines 2. Multiply the data with the complex twiddle factors wMk2 µ1 exp 2 j k M 3. Transpose the complex matrix M1 M2 yielding M2 M1 4. Calculate M1 DFTs of length M2 along the lines 18 10 – Simulation 18 1 2 Criteria to Limit Step-Size Maximum nonlinear phase shift between two split-steps Maximum amount of artifical four-wave mixing Maximum walk-off between two WDM channels As the signal is attenuated along the fiber longer split-steps can be tolerated towards the end of the fiber as the nonlinear fiber effects are reduced. 19 10 – Simulation Nonlinear Phase Shift The maximum nonlinear phase shift is defined as: 2 φNL,max Amax z. As nonlinearity (nonlinear part of NLSE) and dispersion (linear part of NLSE) interact by transfering phase into amplitude fluctuations, the nonlinear phase shift must be limited. A good choice is a maximum nonlinear phase shift of ~0.1 mrad. 20 10 – Simulation Artifical Four-Wave Mixing (FWM) Maximum admissible step size: 1 1 n z ln 2 1 n 1 n 1...K 1 exp 2 l / K 3 K P leff 10 4 21 PartificialFWM 10 2 10 – Simulation G. Bosco et al, „Suppression of spurious tones in fiber system simulations based on the Split-Step method“, LEOS conference, 1999. Maximum Walk-Off between Channels Bound for step-size due to walk-off z Tmin max walkoff D (Linear) walk-off results from different propagation velocities of the WDM channels due to dispersion. If the maximum walk-off is chosen too large, it is possible that bits in the outermost WDM channels completely move across each other and crosstalk (e.g. XPM) cannot be determined correctly anymore. Thus the maximum admissible walk-off should only be a fraction of a bit period. 22 10 – Simulation Modelling of Noise 23 10 – Simulation Noise Representation by Noise Bins (Analytic Noise Representation) In analytic noise representation separate bins are used to represent the stochastic properties of noise. These bins have a certain coarse bandwidth (~4 GHz). A high spectral entire noise bandwidth should be simulated to ensure accurate results (~8 THz). 24 10 – Simulation Noise Representation by Noise Process (Numeric Representation) In numeric noise representation noise is included into the sampled signal. The simulation bandwidth has to be chosen accordingly. Numeric noise representation is the only way to accurately model the nonlinear interaction between noise and fiber effects (GordonMollenauer effect). Also receiver properties are modelled more accurately by numeric noise representation. 25 10 – Simulation Analytic BER Estimation The following noise terms can be included (and can be deactivated separately): ASE-shot noise ASE-ASE noise ASE-channel beat noise Channel shot noise Thermal noise 26 10 – Simulation BER Calculation (Analytic Noise Representation) P1 P0 12 i1 02 ith i0 Probability density ith i0 i1 ith 1 1 BER P1 P0 1 P0 P 1 0 P0 1 P 1 0 erfc erfc 2 2 1 2 0 2 The variance of the zero and one levels of the signal is derived from the analytical noise representation. 27 10 – Simulation BER Estimation (Numeric Noise Representation) Determination of the BER: • Monte-Carlo simulations - Very time consuming for a low BER + Very accurate • Tail extrapolation - Only valid for Gaussian distributed mark and space values + Very fast 28 10 – Simulation Tail Extrapolation for Estimation of Low BER Levels Deliberately vary the decision threshold to determine between zeros and ones of a binary signal. Count errors for these cases and plot them into a graph. Extrapolation of these BER curves yields the BER. 29 10 – Simulation Simulation of PMD 30 Simulation of Polarization Mode Dispersion (Wave Plate Model) Discretize fiber into small waveplates (e.g. of 1 km length), randomly turn polarization in front of waveplate. Waveplates are assumed to have a birefringence of 1 1,x - 1,y The mode coupling between the different elements is determined by the two angles and . The matrices for the transformation of the polarization state are referred to as Jones matrices. 31 10 – Simulation Transmission Function of a Waveplate H ( ) D 1 B( ) C D 1 exp z 0 1 i 2 B( ) 1 0 exp 1zi 2 exp j 0 2 C 0 exp j 2 cos sin D sin cos 32 10 – Simulation Click To Edit Master Title Style Wrap Up What you should recall from this chapter: • • • • • • • • • • 33 Describe the linear and nonlinear parts of the NLSE. What is the difference between total field and separated channels representation? How can the Split-Step Fourier Method be used to solve the NLSE? How can the step-size be determined? How does the artifical FWM process limit the maximum admissible step-size? What different approaches can be taken to model noise effects? What are the advantages and disadvantages of these approaches? How can PMD be modelled? What is a Jones matrix? How can the transmission function of a wave-plate be calculated? 10 - Simulation Lecture Optical Communications Outlook Prof. Dr.-Ing. Dipl.-Wirt.-Ing. Stephan Pachnicke Overview • Next generation access networks • Functional and structural convergence of fixed and mobile networks • Conclusion 2 11 - Trends Traffic Growth in the Internet DE-CIX GlobalPeer • Exponential growth of the bandwidth with approx. 30% p.a. • Drivers are new services such as IP-TV, Cloud, … 3 11 - Trends Access Network Technologies Fiber-to-the-Home 10G-EPON, XG-PON Fiber-to-the-Curb ADSL Copper ISDN Year • DSL-based technology cannot be scaled to higher data rates • • VDSL2 ~100 Mbit/s with reach of ~550-850 m G.fast (Vectoring) ~150 Mbit/s, ~250 m reach Fiber-to-the-Curb • Solution: fiber based systems, passive optical networks (PON) 4 11 - Trends System Architecture (FTTx) Point-to-point CO GPON TDMA-PON Passive splitter CO NG-PON2 WDM-PON CO Acronyms: WDM Mux/Demux + Physical layer p2p connection + High reach (approx. 80 km) + Aggregate bit rate/fiber = end user bit rate – N fibers in the CO – N transceivers in the CO + 1 fiber in the CO + 1 transceiver in the CO – Shared multipoint connection – Lower reach (approx. 10 km) – Aggregate bit rate/fiber = N x end user bit rate + Physical layer p2p connection + Medium reach (approx. 40 km) + 1 fiber in the CO + 1 transceiver in the CO + Aggregate bit rate/wavelength = end user bit rate CO=Central office; TDMA=Time Division Multiple Access; WDM=Wavelength Division Multiplex GPON=Gigabit capable passive optical networks; NG-PON2=Next Generation–PON2 5 11 - Trends WDM-PON Challenges Customer Premises Central Office Challenges Solutions • Integration of multi-channel transmitters and receivers • Photonic integrated circuit (PIC) (& driver array) • Low-cost tunable laser • Development of a novel (wavelength) tunable tranceiver module with centralized control SFP+ 6 11 - Trends Transmission System Setup OLT ONU AWG (G.698.3) SFP (L-Band) 1GBE . . . L-Band (RX) C-Band (TX) RN L-Band C-Band VOA CPE (with T-SFP+) . . . Fiber AWG (G.698.3) 90% SFP (L-Band) 1GBE C-Band AWG 10% Wavelength Controller L/C Splitter CPE (with T-SFP+) OLT: optical line terminal; RN: remote node; ONU: optical network unit; VOA: Variable Optical Attenuator; AWG: arrayed waveguide grating CPE: Customer Premises Equipment 7 11 - Trends Field Trial WDM-PON (2014) • Two different fiber routes (looped back to central office) • 15 km – red (approx. 7 dB attenuation) – Gmunden to Olsdorf • 25 km – orange (approx. 11 dB attenuation) – Gmunden to Laakirchen 8 11 - Trends Results S. Pachnicke, et al, OFC, Los Angeles, 2015. 9 11 - Trends Automatic Wavelength Control 194,9 Frequency [THz] 194,8 194,7 194,6 194,5 194,4 0 10 20 30 Phase 1 (Power Feedback) 40 50 Time [s] 60 70 80 90 100 Phase 2 (Wavelength & Power Feedback) On turn-on the ONU automatically tunes to the wavelength, which is equivalent to the AWG port it is attached to (considering the wavelength and power feedback it receives from the centralized controller) 10 11 - Trends Long Term Stability Long-term wavelength stability is achieved by the closed-loop wavelength control provided by the central office 11 11 - Trends Convergence of Fixed and Mobile Networks 12 11 - Trends Increase of Mobile Traffic Quelle: Cisco VNI Mobile, 2015 • Global mobile traffic will reach 24.3 EB/month in 2019 • Bandwidth growth of approx. 57% p.a. 13 11 - Trends Fixed-Mobile Convergence (FMC) NID / RGW Universal Access Gateway UAG Carrier Ethernet Switch / Aggregator U E eNB Access- and aggregation network NFV Server WiFi AP Low-Latency Cross-Connect eNB ONU Small Cell RRU ONU RRU ONU RRU ONU WDM-PON OLT Mobile Fronthaul Shared use of the infrastructure by fixed and mobile networks 14 11 - Trends Advantages Use of a shared infrastructure for fixed and mobile networks: • Improved QoE for the end user (e.g. seamless handover between WiFi/LTE) • Reduced costs and lower energy consumption Differentiation into • • 15 Functional convergence wrt. required functions in fixed and mobile networks • Enhanced control (improved offloading, seamless handover between fixed and mobile networks) • Use of virtualized functions on a shared NFV server (e.g. for universal authentication functionality, universal data path management, …) Structural convergence wrt. the infrastructure • Shared use of network infrastructure (e.g. PON network for FTTH & dedicated wavelengths for mobile fronthaul) • Consolidation of network nodes 11 - Trends Functional Convergence Management / Control SDN UAG / OpenFlow GbE / 10GbE Carrier Ethernet Switch / Aggregator eNB SDN / OpenFlow VNF 1 NFV VNF 2 VNF 3 Server Access and Aggregation WiFi AP IP / MPLS Network SDN / OpenFlow Backbone LER BBUH Low-Latency Cross-Connect Sync Timing WDM-PON OLT RRU ONU Mobile RRU ONU RRU ONU Fronthaul Content Cache Mobile EPC uAAA Perform. Interface Monitor Select Enhanced control & virtualization functions (SDN/OpenFlow Controller, NFV-Server) 16 11 - Trends NFV & SDN Software Defined Networking (SDN) Network Function Virtualization (NFV) Cache Firewall Router App App App API’s Rework of network applications to run on arbitrary hardware, independent of custom networking hardware 17 Forwarding plane is programmable in real time by independently developed software 11 - Trends Network Function Virtualization (NFV) Traditional Network Appliances Message Router DPI CDN Firewall Virtualized Network Functions Session Border WAN Controller Acceleration Carrier Grade NAT Standard, high-volume servers Standard, high-volume storage Tester/QoE Monitor Standard, high-volume switches SGSN/GGSN PE Router BRAS RAN Controller Use of commodity hardware (e.g. blade server), software-based network functions, faster innovation cycle 18 11 - Trends Examples of NFV Implementations Mobile Networks • Virtual EPC (vEPC), Centralized RAN (C-RAN) Residential Customers • Virtual Home Gateway (vHG) Business Customers • Virtual Customer Premises Equipment (vCPE) First use in mobile networks then for residential and business customers 19 11 - Trends Advantages of NFV Physical Network Hybrid Network Virtual Network Scope Functions tied to hardware Selective functions virtualized with gradual introduction of orchestration Orchestration across virtualized functions Operations Slow, manual service creation Gradual transition, benefits expand as scope of virtualization grows Automated real-time service creation Cost CAPEX-heavy, over-provisioning Initial impact may be limited, because functions are virtualized in isolation OPEX-heavy, licensebased model Innovation Cycle Innovation tied to both hardware and software Impact depends on which functions are virtualized Software and hardware become independent Source: It is crucial to find the right way to a fully virtualized network 20 11 - Trends Example: Double Attachment SDN Controller Network Assistance for Access Selection and Utilization UE WiFi AP Interface Selection WiFi AP Double Attachment Interface Changing (Inter-system Handover) SDN-API Northbound uAAA U E WiFi AP Changing (Inter-AP Handover) Control Plane U E Access- and aggregation network UAG LTE-EPC Performance Monitoring U E Content Cache SDN: Software-defined networking, uAAA: universal Authentication, Authorization and Accounting, UAG: universal access gateway, EPC: evolved packet core; UE: user equipment • User Equipment (UE) authenticates via uAAA in mobile and WiFi networks • UE can be attached via different paths at the same time (double attachment) • Centralized network control steers traffic (offloading) 21 11 - Trends Example: Centralized-RAN Management / Control SDN UAG / OpenFlow GbE / 10GbE Carrier Ethernet RRU ONU RRU ONU RRU ONU Switch / Aggregator SDN / OpenFlow VNF 1 NFV VNF 2 VNF 3 Server Access and Aggregation WiFi AP IP / MPLS Network SDN / OpenFlow Backbone LER BBUH Low-Latency Cross-Connect Sync Timing WDM-PON OLT RRU ONU Mobile RRU ONU RRU ONU Fronthaul Content Cache Mobile EPC uAAA Use of BBU hoteling. Advantages by • Sharing of the same BBU for serving a business area in day time • and a residential area during night 22 11 - Trends Perform. Interface Monitor Select Conclusions • Bit rates in access networks will increase exponentially in the next years • Fiber-to-the-Home systems will be the only long-term solution • Especially WDM-PON systems are promising due to their high bandwidth and low latency • Fixed- and mobile networks will converge in the future • NFV & SDN will allow flexible reconfiguration of the network and realization of network functions in software 23 11 - Trends
