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S-72.227 Digital Communication Systems Overview into Fiber Optic Communications Overview into Fiber Optic Communications Capacity of telecommunication networks Advantages of optical systems Optical fibers – single mode – multimode Modules of fiber optic link Optical repeaters - EDFA Dispersion in fibers – inter-modal and intra-modal dispersion Fiber bandwidth and bitrate Optical sources: LEDs and lasers Optical sinks: PIN and APD photodiodes Design of optical links Timo O. Korhonen, HUT Communication Laboratory Capacity of telecommunication networks Telecommunications systems – tend to increase in capacity – have increasingly higher rates Increase in capacity and rate requires higher carriers Optical system offers – very high bandwidths – repeater spacing up to hundreds of km – versatile modulation methods Optical communications is especially applicable in – ATM links – Local area networks (high rates/demanding environments) Timo O. Korhonen, HUT Communication Laboratory MESSAGE BANDWIDTH 1 GHz-> 10 MHz 100 kHz 4 kHz Summarizing advantages of optical systems Enormous capacity: 1.3 mm ... 1.55 mm allocates bandwidth of 37 THz!! Low transmission loss – Optical fiber loss 0.2 dB/km, Coaxial cable loss 10 … 300 dB/km ! Cables and equipment have small size and weight – aircrafts, satellites, ships Immunity to interference – nuclear power plants, hospitals, EMP (Electromagnetic pulse) resistive systems (installations for defense) Electrical isolation – electrical hazardous environments – negligible crosstalk Signal security – banking, computer networks, military systems Fibers have abundant raw material Timo O. Korhonen, HUT Communication Laboratory Optical fibers Two windows available, namely at – 1.3 mm and 1.55 mm The lower window is used with Si and GaAlAs and the upper window with InGaAsP compounds There are single and monomode fibers that have step or graded refraction index profile Propagation in optical fibers is influenced by – attenuation – scattering – absorption to a fiber – dispersion Link manufacturer's page! Timo O. Korhonen, HUT Communication Laboratory Characterizing optical fibers Optical fiber consist of (a) core, (b) cladding, (c) mechanical protection layer Refraction index of core n1 is slightly larger causing total internal refraction at the interface of core and cladding n1 1.48 0.01 n2 n1 (1 ) n1 cos1 n2 cos 2 core n1 1 1 n2 2 n1 n2 Fibers can be divided into four classes: property connection of light BW losses price Timo O. Korhonen, HUT Communication Laboratory multimode fibers single mode fibers step index graded index step index graded index ++ -++ + +- ++ ++ -++ +- Single mode and multimode fibers Timo O. Korhonen, HUT Communication Laboratory Fiber modes Electromagnetic field propagating in fiber can be described by Maxwell’s equations whose solution yields number of modes M for step index profile as 2 2 a 2 2 M V 2 / 2, where V 2 n1 n2 where a is the core radius and V is the mode parameter, or normalized frequency of the fiber n2 k k2 k1 n1k , Depending on fiber k 2 / 0 parameters, number of different propagating modes appear For single mode fibers V 2.405 Single mode fibers do not have mode dispersion Timo O. Korhonen, HUT Communication Laboratory Inter-modal (mode) dispersion Multimode fibers exhibit modal dispersion that is caused by different propagation modes taking cladding different paths: n mod Tmax Tmin v s / t v c / n (n1 n2 ) / n1 n1 cos1 n2 1 Path 1 1 Path 2 s L n2 n1 (1 ) n1 cos1 n2 cos 2 cos1 n2 / n1 1 L / s s L / cos1 L /(1 ) L Tmin Tmax s / v L / (1 )c / n1 c / n1 mod Tmax Tmin Ln1 / c(1 ) Ln1 / c mod Timo O. Korhonen, HUT Communication Laboratory 1 Ln1 Ln1 c 1 c 1 core cladding Chromatic dispersion Chromatic dispersion (or material dispersion) is produced when different frequencies of light propagate using different velocities in fiber Therefore chromatic dispersion is larger the wider source bandwidth is. Thus it is largest for LEDs (Light Emitting Diode) and smallest for LASERs (Light Amplification by Stimulated Emission of Radiation) diodes LED BW about 5% of 0 , Laser BW about 0.1 % of 0 Optical fibers have dispersion minimum at 1.3 mm but their attenuation minimum is at 1.55 mm. Therefore dispersion shifted fibers were developed. Example: GaAlAs LED is used at 0=1 mm. This source has spectral width of 40 nm and its material dispersion is Dmat(1 mm)=40 ps/(nm x km). How much is its pulse spreading in 25 km distance? mat Timo O. Korhonen, HUT Communication Laboratory ps 40nm 40 25km=40ns nm km Chromatic and waveguide dispersion In addition to chromatic dispersion, there exist also waveguide dispersion that is significant for single mode fibers in long wavelengths Chromatic and waveguide Chromatic and waveguide dispersion cancel each other dispersion are denoted as Chromatic intra- modal dispersion and their effects cancel each other at a certain wavelength This cancellation is used in dispersion shifted fibers Fiber total dispersion is determined as the geometric sum effect of intramodal and inter-modal (or mode) dispersion with net pulse spreading: 2 2 tot intermod intramod Timo O. Korhonen, HUT Communication Laboratory Dispersion due to different mode velocities waveguide+chromatic dispersion Fiber dispersion, bit rate and bandwidth Usually fiber systems apply amplitude modulation by pulses whose width is determined by – linewidth of the optical source – rise time of the optical source – dispersion properties of the fiber – rise time of the detector unit Assume optical power emerging from the fiber has Gaussian shape g (t ) exp t 2 / 2 2 / 2 G ( ) exp 2 2 / 2 / 2 From time-domain expression the time required for pulse to reach its half-maximum, e.g the time to have g(t 1/2)=g(0)/2 is t1/ 2 (2ln 2)1/ 2 t FWHM / 2 where tFWHM is the “full-width-half-maximum”-value Relationship between fiber risetime and its bandwidth is (next slide) 0.44 f3dB B3dB tFWHM Timo O. Korhonen, HUT Communication Laboratory Using MathCad to derive connection between fiber bandwidth and rise time exp g( t ) t 2 2 2 G( f ) 2 2 2 exp 2 f 2 g( 0) 2 1 2 2 th exp 2 2 2 2 ln ( 2 ) th 2 ln ( 2 ) th 2 ln ( 2 ) 1 G( 0 ) 1 2 2 1 ( 2( ) ) f 3dB exp 2 f 3dB 2 2 2 4 2 2 ln ( 2 ) substitute ln ( 2 ) t h Timo O. Korhonen, HUT Communication Laboratory 1 2 th 0 f 3dB yeilds 2 ln ( 2 ) ln ( 2 ) 0.221 t FWHM 2 t h ( 2( ) ) 2 ln ( 2 ) 1 2 ln ( 2 ) ( 2( ) ) 1 t ln ( 2 ) h 2 1 2 4 0 System rise-time Total system rise time can be expressed as 1/ 2 tsys 2 q 2 2 440 L 350 2 2 2 ttx Dmat L B B 0 rx mode dispersion transmitter rise-time intra-modal dispersion receiver rise-time where L is the fiber length [km] and q is the exponent characterizing bandwidth. Fiber bandwidth is therefore also B0 BM ( L) q L Bandwidths are expressed here in [MHz] and wavelengths in [nm] Here the receiver rise time (10-to-90-% BW) is derived based 1. order lowpass filter amplitude from gLP(t)=0.1 to gLP(t)= 0.9 where g LP (t ) 1 exp 2 Brxt u (t ) Timo O. Korhonen, HUT Communication Laboratory Example Calculate the total rise time for a system using LED and a driver causing transmitter rise time of 15 ns. Assume that the led bandwidth is 40 nm. The receiver has 25 MHz bandwidth. The fiber has 400MHz km bandwidth distance product with q=0.7. Therefore 1/ 2 tsys tsys 2 q 2 2 440 L 350 2 2 2 ttx Dmat L B B 0 rx 1/ 2 (15ns) 2 (21ns) 2 (3.9ns) 2 (14ns) 2 tsys 30ns Note that this means that the electrical signal bandwidth is B 350/ tot [ns] 11.7 MHz For raised cosine shaped pulses thus over 20Mb/signaling rate can be achieved Timo O. Korhonen, HUT Communication Laboratory Optical amplifiers Direct amplification without conversion to electrical signals Three major types: – Erbium-doped fiber amplifier at 1.55 mm (EDFA and EDFFA) – Praseodymium-doped fiber amplifier at 1.3 mm (PDFA) – semiconductor optical amplifier - switches and wavelength converters (SOA) Optical amplifiers versus opto-electrical regenerators: – large bandwidth and gain – easy usage with wavelength division multiplexing (WDM) – easy upgrading – insensitivity to bitrate and signal formats All based on stimulated emission of radiation - as lasers (in contrast to spontaneous emission) Stimulated emission yields coherent radiation - emitted photons are perfect clones Timo O. Korhonen, HUT Communication Laboratory Erbium-doped fiber amplifier (EDFA) Erbium fiber Signal in (1550 nm) Isolator Isolator Pump Signal out Residual pump 980 or 1480 nm Amplification (stimulated emission) happens in fiber Isolators and couplers prevent resonance in fiber (prevents device to become a laser) Popularity due to – availability of compact high-power pump lasers – all-fiber device: polarization independent – amplifies all WDM signals simultaneusly Timo O. Korhonen, HUT Communication Laboratory EDFA - energy level diagram Fluoride class level (EDFFA) E4 excited state absorption 980 nm E3 32 1m s Er3+ levels E2 1530 nm 980 nm 1480 nm 21 10 ms E1 Pump power injected at 980 nm causes spontaneous emission from E1 to E3 and there back to E2 Due to the indicated spontaneous emission lifetimes population inversion (PI) obtained between E1 and E2 The higher the PI to lower the amplified spontaneous emission (ASE) Thermalization (distribution of Er3+ atoms) and Stark splitting cause each level to be splitted in class (not a crystal substance) -> a wide band of amplified wavelengths Practical amplification range 1525 nm - 1570 nm, peak around 1530 nm Timo O. Korhonen, HUT Communication Laboratory LEDs and LASER-diodes Light Emitting Diode (LED) is a simple pn-structure where recombining electron-hole pairs convert current into light In fiber-optic communications light source should meet the following requirements: – Physical compatibility with fiber – Sufficient power output – Capability of various types of modulation – Fast rise-time – High efficiency – Long life-time – Reasonably low cost Timo O. Korhonen, HUT Communication Laboratory Modern GaAlAs light emitter Timo O. Korhonen, HUT Communication Laboratory Light generating structures In LEDs light is generated by spontaneous emission In LDs light is generated by stimulated emission Efficient LD and LED structures – guide the light in recombination area – guide the electrons and holes in recombination area – guide the generated light out of the structure Timo O. Korhonen, HUT Communication Laboratory LED types Surface emitting LEDs: (SLED) 100 nm – light collected from the other surface, other attached to a heat sink – no waveguiding – easy connection into multimode fibers Edge emitting LEDs: (ELED) 60 80 nm – like stripe geometry lasers but no optical feedback – easy coupling into multimode and single mode fibers Superluminescent LEDs: (SLD) 30 40 nm – spectra formed partially by stimulated emission – higher optical output than with ELEDs or SLEDs For modulation ELEDs provides the best linearity but SLD provides the highest light output Timo O. Korhonen, HUT Communication Laboratory FWHM width Lasers Timo O. Korhonen, HUT Communication Laboratory Lasing effect means that stimulated emission is the major for of producing light in the structure. This requires – intense charge density – direct band-gap material->enough light produced – stimulated emission Connecting optical power Numerical aperture (NA): n2 n1 (1 ) n1 cos1 n2 cos 2 Minimum (critical) angle supporting internal reflection sin C n2 / n1 n sin 0,min n1 sin C (n12 n22 )1/ 2 NA n1 2 Connection efficiency is defined by Pfibre / Psource Additional factors of connection efficiency: fiber refraction index profile and core radius, source intensity, radiation pattern, how precisely fiber is aligned to the source, surface quality Timo O. Korhonen, HUT Communication Laboratory Modulating lasers Timo O. Korhonen, HUT Communication Laboratory Example: LD distortion coefficients Let us assume that an LD transfer curve distortion can be described by y (t ) a1 x(t ) a2 x 2 (t ) a3 x 3 (t ) where x(t) is the modulation current and y(t) is the optical power n:the order harmonic distortion is described by the distortion coefficient A H n 20log10 n A1 and y(t ) A0 A1 cost A2 cos 2t A3 cos3t... x(t ) cos t 2a2 the applied signal we assume and therefore A2 aFor x ( t ) a cos t 1 1 H 2 20log10 A 20log10 3a 4a 3 1 1 a2 x 2 (t ) cos 2 (t ) (a2 / 2)(1 cos 2 t ) a3 H 20log A3 20log a3 x3 (t ) (a3 / 4)(3cos t cos3 t ) 2 10 3 A 3 a 4 a 1 3 1 a a 3a a y (t ) 2 3 a1 cos t 2 cos 2 t 3 cos3 t 2 4 2 4 Timo O. Korhonen, HUT Communication Laboratory A1 A2 Optical photodetectors (PDs) PDs work vice versa to LEDs and LDs Two photodiode types – PIN – APD For a photodiode it is required that it is – sensitive at the used – small noise – long life span – small rise-time (large BW, small capacitance) – low temperature sensitivity – quality/price ratio Timo O. Korhonen, HUT Communication Laboratory Optical communication link Timo O. Korhonen, HUT Communication Laboratory Link calculations In order to determine repeater spacing on should calculate – power budget – rise-time budget Optical power loss due to junctions, connectors and fiber One should also estimate required margins with respect of temperature, aging and stability For rise-time budget one should take into account all the rise times in the link (tx, fiber, rx) If the link does not fit into specifications – more repeaters – change components – change specifications Often several design iteration turns are required Timo O. Korhonen, HUT Communication Laboratory Link calculations (cont.) Specifications: transmission distance, data rate (BW), BER Objectives is then to select FIBER: – Multimode or single mode fiber: core size, refractive index profile, bandwidth or dispersion, attenuation, numerical aperture or modefield diameter SOURCE: – LED or laser diode optical source: emission wavelength, spectral line width, output power, effective radiating area, emission pattern, number of emitting modes DETECTOR/RECEIVER: – PIN or avalanche photodiode: responsivity, operating wavelength, rise time, sensitivity Timo O. Korhonen, HUT Communication Laboratory The bitrate-transmission length grid 1-10 m <10 Kb/s 10-100 Kb/s 100-1000 Kb/s 1-10 Mb/s 10-50 Mb/s 50-500 Mb/s 500-1000 Mb/s >1 Gb/s 10-100 m 100-1000 m 1-3 km 3-10 km 10-50 km 50-100 km >100 km VII V I V II III IV VI I Region: BL 100 Mb/s SLED with SI MMF II Region: 100 Mb/s BL 5 Gb/s LED or LD with SI or GI MMF III Region: IV Region: BL 100 Mb/s ELED or LD with SI 5 Mb/s BL 4 Gb/s ELED or LD with V Region: 10 Mb/s BL 1 Gb/s VI Region: 100 Mb/s BL 100 Gb/s VII Region: 5 Mb/s BL 100 Mb/s LD with LD with MMF GI MMF GI MMF SMF LD with SI or GI MMF SI: step index, GI: graded index, MMF: multimode fiber, SMF: single mode fiber Timo O. Korhonen, HUT Communication Laboratory