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Optical Fundamentals Russ Gyurek [email protected] © 2003, 2001, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 1 Agenda • Introduction • Safety • Optical propagation in Fibers • Attenuation & Dispersion • Non Linearity • SM Optical Fiber Types • Summary, Q&A © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 2 Modern Lightwave Eras 10000 Capacity (Gb/s) 1000 100 Optical networking Wavelength Switching Research Systems 10 Commercial Systems 1 Fiberization Digitization SONET rings and DWDM linear systems 0.1 1985 1990 1995 2000 Year © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 3 Some terminology: • Decibels (dB): unit of level (relative measure) X dB is 10-X/10 in linear dimension e.g. 3 dB Attenuation = 10-.3 = 0.501 Standard logarithmic unit for the ratio of two quantities. In optical fibers, the ratio is power and represents loss or gain. • Decibels-milliwatt (dBm) : Decibel referenced to a milliwatt X mW is 10log10(X) in dBm, Y dBm is 10Y/10 in mW. 0dBm=1mW, 17dBm = 50mW • Wavelength (): length of a wave in a particular medium. Common unit: nanometers, 10-9m (nm) 300nm (blue) to 700nm (red) is visible. In fiber optics primarily use 850, 1310, & 1550nm • Frequency (): the number of times that a wave is produced within a particular time period. Common unit: TeraHertz, 1012 cycles per second (Thz) Wavelength x frequency = Speed of light x = C © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 4 Some more terminology • Attenuation = Loss of power in dB/km The extent to which lighting intensity from the source is diminished as it passes through a given length of fiber-optic (FO) cable, tubing or light pipe. This specification determines how well a product transmits light and how much cable can be properly illuminated by a given light source. • Chromatic Dispersion = Spread of light pulse in ps/nm-km The separation of light into its different coloured rays. • ITU Grid = Standard set of wavelengths to be used in Fibre Optic communications. Unit Ghz, e.g. 400Ghz, 200Ghz, 100Ghz • Optical Signal to Noise Ration (OSNR) = Ratio of optical signal power to noise power for the receiver • Lambda = Name of Greek Letter used as Wavelength symbol () • Optical Supervisory Channel (OSC) = Management channel © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 5 dB versus dBm • dBm used for output power and receive sensitivity (Absolute Value) • dB used for power gain or loss (Relative Value) © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 6 ITU Wavelength Grid 1530.33 nm 0.80 nm 195.9 THz 100 GHz 1553.86 nm 193.0 THz • ITU-T grid is based on 191.7 THz + 100 GHz • It is a standard for the lasers in DWDM systems Freq (THz) 192.90 192.85 192.80 192.75 192.70 192.65 192.60 © 2003, Cisco Systems, Inc. All rights reserved. ITU Ch 29 28 27 26 Wave (nm) 1554.13 1554.54 1554.94 1555.34 1555.75 1556.15 1556.55 FTTH Conference October 2003 7 Bit Error Rate ( BER) • BER is a key objective of the Optical System Design • Goal is to get from Tx to Rx with a BER < BER threshold of the Rx • BER thresholds are on Data sheets • Typical minimum acceptable rate is 10 -12 © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 8 Optical Budget Basic Optical Budget = Output Power – Input Sensitivity Pout = +6 dBm R = -30 dBm Budget = 36 dB Optical Budget is affected by: Fiber attenuation Splices Patch Panels/Connectors Optical components (filters, amplifiers, etc) Bends in fiber Contamination (dirt/oil on connectors) © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 9 Power Budget with Power Penalties Fiber Loss + Splices + Connectors + Dispersion Penalties + Fiber Nonlinearities Penalty + Component Aging Penalties < Power Budget = Launch Power – Receiver Sensitivity © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 10 Glass Purity Fiber Optics Requires Very High Purity Glass Window Glass 1 inch (~3 cm) Optical Quality Glass 10 feet (~3 m) Fiber Optics 9 miles (~14 km) Propagation Distance Need to Reduce the Transmitted Light Power by 50% (3 dB) © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 11 Fiber Fundamentals Attenuation Dispersion Nonlinearity Distortion It May Be a Digital Signal, but It’s Analog Transmission Transmitted Data Waveform © 2003, Cisco Systems, Inc. All rights reserved. Waveform After 1000 Km FTTH Conference October 2003 12 Agenda • Introduction • Safety • Optical propagation in Fibers • Attenuation & Dispersion • Non Linearity • SM Optical Fiber Types • Summary, Q&A © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 13 A Few Words on Optical Safety Think Optical Safety at ALL Times Optical Power is INVISIBLE to the Human Eye NEVER stare at an Optical Connector Keep Optical Connectors Pointed AWAY FROM YOURSELF AND OTHERS Glass (Fiber Cable) Can CUT and PUNCTURE © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 14 Laser Classifications / Safety ICONS Class 1 Lasers that are incapable of causing damage when the beam is directed into the eye under normal operating conditions. These include helium-neon lasers operating at less than a few microwatts of radiant power. Class 2 SR and IR Optics, some LR Lasers that can cause harm if viewed directly for ¼ second or longer. This includes helium-neon lasers with an output up to 1 mW (milliwatt). Class 3A Many LR Optics, CWDM GBICS Lasers that have outputs less than 5 mW. These lasers can cause injury when the eye is exposed to either the beam or its reflections from mirrors or other shiny surfaces. As an example, laser pointers typically fall into this class. Class 3B Some LR Optics, Amplifier Outputs Lasers that have outputs of 5 to 500 mW. The argon lasers typically used in laser light shows are of this class. Higher power diode lasers (above 5 mW) from optical drives and high performance laser printers also fall into this class. Class 4 Lasers that have outputs exceeding 500 mW. These devices produce a beam that is hazardous directly or from reflection and can produce skin burn. Many ruby, carbon dioxide, and neodymium-glass lasers are class 4. © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 15 Protective Eyewear Available Protective goggles or glasses should be worn for all routine use of Class 3B and Class 4 lasers. Remember: Eyewear is wavelength specific, a pair of goggles that effectively blocks red laser light affords no protection for green laser light. Do not use worn or scratched glasses. Inspect at every use! Laser Safety Equipment can be investigated in greater detail at the following link: http://www.lasersafety.co.uk/frhome.html © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 16 Some Final Words on Optical Equipment Safety Remember: Optical cabling is constructed from strands of glass, about the size of a human hair. Never handle exposed fiber strands unless you have the proper training. Fiber fragments that become embedded under the skin can be extremely painful, and are very difficult (in some cases impossible) to remove. Remember: Optical equipment is typically powered from either 120vac or 48vdc. Always remove jewelry such as rings, watches, bracelets, etc…when working with energized equipment. Care should be taken to never come in contact with exposed electrical connections or buswork. Remember: Be careful not to damage equipment through ESD (Electrostatic Discharge). Ensure that all equipment is properly grounded, wrist straps are used when removing modules with active components and always handle equipment modules by their edges. **Note: Many companies have documented ESD guidelines specific to their environment. Please consult those documents for additional detail as appropriate**. © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 17 Agenda • Introduction • Safety • Optical propagation in Fibers • Attenuation & Dispersion • Non Linearity • SM Optical Fiber Types • Summary, Q&A © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 18 Analog Transmission Effects Attenuation: Reduces power level with distance Dispersion and Nonlinearities: Erodes clarity with distance and speed Signal detection and recovery is an analog problem © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 19 Fiber Geometry Core Cladding • An optical fiber is made of three sections: The core carries the light signals The cladding keeps the light in the core The coating protects the glass © 2003, Cisco Systems, Inc. All rights reserved. Coating FTTH Conference October 2003 20 Fiber Dimensions • Fiber dimensions are measured in µm 1 µm = 0.000001 meters (10-6) 1 human hair ~ 50 µm Cladding (125 µm) Coating (245 µm) • Refractive Index (n) n = c/v n ~ 1.46 n (core) > n (cladding) © 2003, Cisco Systems, Inc. All rights reserved. Core (8–62.5 µm) FTTH Conference October 2003 21 Geometrical Optics • Light is reflected/refracted at an interface q1 = Angle of incidence q1r = Angle of reflection q2 = Angle of refraction • Above qcritical=Sin-1(n2/n1), all light is totally internally reflected © 2003, Cisco Systems, Inc. All rights reserved. n2 n1 q2 q1 q1r Snell’s Law q1 = q1r n1Sin q1 = n2Sin q2 FTTH Conference October 2003 22 Propagation in Fiber n2 q0 n1 Cladding q1 Core Intensity Profile • Light propagates by total internal reflections at the core-cladding interface • Total internal reflections are lossless • Each allowed ray is a mode © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 23 Different Types of Fiber n2 Cladding • Multimode fiber Core diameter varies 50 mm for step index 62.5 mm for graded index Bit rate-distance product >500 MHz-km • Single-mode fiber Core diameter is about 9 mm Bit rate-distance product >100 THz-km © 2003, Cisco Systems, Inc. All rights reserved. n1 n2 n1 Core Cladding Core FTTH Conference October 2003 24 Agenda • Introduction • Safety • Optical propagation in Fibers • Attenuation & Dispersion • Non Linearity • SM Optical Fiber Types • Summary, Q&A © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 25 Optical Spectrum IR UV 125 GHz/nm Visible • Light Ultraviolet (UV) Visible Infrared (IR) 850 nm 980 nm 1310 nm 1480 nm 1550 nm • Communication wavelengths 850, 1310, 1550 nm Low-loss wavelengths • Specialty wavelengths 980, 1480, 1625 nm © 2003, Cisco Systems, Inc. All rights reserved. 1625 nm C = x (nanometers) Frequency: (terahertz) Wavelength: FTTH Conference October 2003 26 Optical Attenuation • Specified in loss per kilometer (dB/km) 0.40 dB/km at 1310 nm 0.25 dB/km at 1550 nm • Loss due to absorption by impurities 1310 Window 1550 Window 1400 nm peak due to OH ions • EDFA optical amplifiers available in 1550 window © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 27 Optical Attenuation • Pulse amplitude reduction limits “how far” • Attenuation in dB=10xLog(Pi/Po) • Power is measured in dBm: P(dBm)=10xlog(P mW/1 mW) Examples 10dBm 10 mW 0 dBM 1 mW -3 dBm 500 uW -10 dBm 100 uW -30 dBm 1 uW ) Pi P0 T © 2003, Cisco Systems, Inc. All rights reserved. T FTTH Conference October 2003 28 Types of Dispersion • Chromatic Dispersion Different wavelengths travel at different speeds Causes spreading of the light pulse • Polarization Mode Dispersion (PMD) Single-mode fiber supports two polarization states Fast and slow axes have different group velocities Causes spreading of the light pulse © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 29 Fiber Chromatic Dispersion Characteristics Dispersion ps/nm-km Normal Fiber (SMF-28 or Equivalent) Nondispersion Shifted Fiber (NDSF) >95% of Deployed Plant 20 Wavelength 0 1310 nm 1550nm Normal(ITU-T G.652) Dispersion Shifted Fiber (DSF) (ITU-T G.653) Nonzero Dispersion Shifted Fibers (NZDSF) (ITU-T G.655) © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 30 A Snapshot on Chromatic Dispersion Interference • Affects single channel and DWDM systems • A pulse spreads as it travels down the fiber • Inter-symbol Interference (ISI) leads to performance impairments • Degradation depends on: laser used (spectral width) bit-rate (temporal pulse separation) Different SM types © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 31 Fiber Chromatic Dispersion (CD) • The refractive index is wavelength dependent • Different frequency-components of the optical pulses travel at different speeds (the blue is faster than red for anomalous dispersion where D > 0) • As a result, we see pulse broadening and ISI Red z z z © 2003, Cisco Systems, Inc. All rights reserved. Blue z Transmission Fiber FTTH Conference October 2003 32 Limitations From Chromatic Dispersion • Dispersion causes pulse distortion, pulse "smearing" effects • Higher bit-rates and shorter pulses are less robust to Chromatic Dispersion • Limits "how fast“ and “how far” 10 Gbps 60 Km SMF-28 t 40 Gbps 4 Km SMF-28 © 2003, Cisco Systems, Inc. All rights reserved. t FTTH Conference October 2003 33 Combating Chromatic Dispersion • Use DSF and NZDSF fibers (G.653 & G.655) • Dispersion Compensating Fiber • Transmitters with narrow spectral width © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 34 Dispersion Compensating Fiber • Dispersion Compensating Fiber: By joining fibers with CD of opposite signs (polarity) and suitable lengths an average dispersion close to zero can be obtained; the compensating fiber can be several kilometers and the reel can be inserted at any point in the link, at the receiver or at the transmitter © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 35 Dispersion Compensation Cumulative Dispersion (ps/nm) Total Dispersion Controlled +100 0 -100 -200 -300 -400 -500 No Compensation With Compensation Distance from Transmitter (km) Dispersion Shifted Fiber Cable Transmitter Dispersion Compensators © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 36 Polarization Mode Dispersion (PMD) • Caused by ovality of core due to: –Manufacturing process –Internal stress (cabling) –External stress (trucks) • Only discovered in the 90s • Most older fiber is not characterized for PMD © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 37 Polarization Mode Dispersion (PMD) Ey nx Ex ny Pulse As It Enters the Fiber Spreaded Pulse As It Leaves the Fiber • The optical pulse tends to broaden as it travels down the fiber; this is a much weaker phenomenon than chromatic dispersion and it is of some relevance at bit rates of 10Gb/s or more © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 38 Combating PMD • Factors contributing to PMD Bit Rate Fiber core symmetry Environmental factors Bends/stress in fiber Imperfections in fiber • Solutions for PMD Improved fibers Regeneration Follow manufacturer’s recommended installation techniques for the fiber cable © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 39 Agenda • Introduction • Safety • Optical propagation in Fibers • Attenuation & Dispersion • Non Linearity • SM Optical Fiber Types • Summary, Q&A © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 40 From Linear to Non Linear Propagation • As long as optical power within an optical fiber is small, the fiber can be treated as a linear medium Loss and refractive index are independent of the signal power • When optical power levels gets fairly high, the fiber becomes a nonlinear medium Loss and refractive index depend on the optical power © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 41 Optical Fiber’s Nonlinear Index Optical Pulse Index of Nonlinear Light Refraction Coefficient Intensity Intensity n = n0 + N2 Fast Phase Velocity Slow Phase Velocity Time • Intensity of an optical pulse modulates the index of refraction • Nonlinearity scales as (channel power)2 © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 42 Effects of Nonlinearity • A single channel’s pulses interact as they travel Interference Multiple channels interact as they travel Interference © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 43 Types of Nonlinearities • Nonlinear index Four-wave mixing FWM Self-phase modulation Cross-phase modulation • Stimulated scattering Raman Raman Brillouin © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 44 Four-Wave Mixing 1 2 Into Fiber 21-2 1 2 22-1 Out of Fiber • Channels beat against each other to form intermodulation products • Creates in-band crosstalk that can not be filtered (optically or electrically) © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 45 FWM Example Output Spectrum after 25 km of Dispersion Shifted Fiber -5 • FWM effects increase geometrically with: Number of channels Spacing of channels Optical power level Power (dBm) -10 Input Power = 3 mw/ch -15 -20 -25 -30 -35 -40 1542 1543 1544 1545 1546 1547 1548 Wavelength (nm) © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 46 FWM and Dispersion Dispersion Washes Out FWM Effects FWM Efficiency (dB) 0 D=0 -10 -20 D=0.2 -30 D=2 -40 D=17 -50 0.0 0.5 1.0 1.5 2.0 2.5 Channel Spacing (nm) © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 47 Agenda • Introduction • Safety • Optical propagation in Fibers • Attenuation & Dispersion • Non Linearity • SM Optical Fiber Types • Summary, Q&A © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 48 Types of Single-Mode Fiber • SMF (standard, 1310 nm optimized, G.652) Most widely deployed so far, introduced in 1986, cheapest • DSF (Dispersion Shifted, G.653) Intended for single channel operation at 1550 nm • NZDSF (Non-Zero Dispersion Shifted, G.655) – LS For WDM operation in the 1550 nm region only – TrueWave, FreeLight, LEAF, TeraLight… Latest generation fibers developed in mid 90’s For better performance with high capacity DWDM systems –Low PMD ULH fibers © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 49 Fiber Dispersion Characteristics Normal fiber Non-dispersion shifted fiber (NDSF) G.652 ~95% of deployed plant Dispersion (in ps/nm- km) 25 20 DS NZDS+ NZDS- SMF 15 10 5 0 -5 -10 DSF G.653 NZDSF G.655 -15 -20 1350 1370 1390 1410 1430 1450 1470 1490 1510 1530 1550 1570 1590 1610 1630 1650 Wavelength (in nm) © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 50 Different Solutions for Different Fiber Types SMF •Good for TDM at 1310 nm (G.652) •OK for TDM at 1550 •OK for DWDM (With Dispersion Mgmt) DSF •OK for TDM at 1310 nm (G.653) •Good for TDM at 1550 nm •Bad for DWDM (C-Band) NZDSF •OK for TDM at 1310 nm (G.655) •Good for TDM at 1550 nm •Good for DWDM (C + L Bands) Extended Band •Good for TDM at 1310 nm (G.652.C) •OK for TDM at 1550 nm (suppressed attenuation in the traditional water peak region) •OK for DWDM (With Dispersion Mgmt •Good for CWDM (>8 wavelengths) The primary Difference is in the Chromatic Dispersion Characteristics © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 51 Agenda • Introduction • Safety • Optical propagation in Fibers • Attenuation & Dispersion • Non Linearity • SM Optical Fiber Types • Summary, Q&A © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 52 The 3 “R”s of Optical Networking A Light Pulse Propagating in a Fiber Experiences 3 Type of Degradations: Pulse as It Enters the Fiber Pulse as It Exits the Fiber Loss of Energy Shape Distortion Phase Variation Loss of Timing (Jitter) (From Various Sources) © 2003, Cisco Systems, Inc. All rights reserved. t ts Optimum Sampling Time t ts Optimum Sampling Time FTTH Conference October 2003 53 The 3 “R”s of Optical Networking (Cont.) The Options to Recover the Signal from Attenuation/Dispersion/Jitter Degradation Are: Pulse as It Enters the Fiber Pulse as It Exits the Fiber Re-Gen to Boost the Power Re-Shape DCU Phase Variation Phase Re-Alignment* O-E-O Re-Time t ts Optimum Sampling Time t ts Optimum Sampling Time Re-gen, Re-shape and ts Optimum Remove Optical Noise Sampling Time t *Simplification © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 54 F0_5585_c2 © 1999, Cisco Systems, Inc. 55 Back-up Slide(s) © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 56 How Far Can I Go w/o Dispersion Comp? Distance (Km) = Specification of Transponder (ps/nm) Coefficient of Dispersion of Fiber (ps/nm*km) A laser signal with dispersion tolerance of 3400 ps/nm is sent across a standard SMF fiber which has a Coefficient of Dispersion of 17 ps/nm*km. It will reach 200 Km at maximum bandwidth. Note that lower speeds will travel farther. © 2003, Cisco Systems, Inc. All rights reserved. FTTH Conference October 2003 57