Modulation formats for digital fiber transmissionEric Tell 050329

OutlineFiber performance limitationsWDMOptical vs. radio communicationOptical modulatorsModulation formatsAmplitude shift keyingDuo-binary signallingOptical single sideband signallingSimulation/experimental resultsSummary

Fiber performance limitationsFiber LossChromatic dispersiondifferent refractive index for different wavelengthsFiber non-linearities

Chromatic dispersionDistance limit ~1/(bit rate)Example: Single mode fiber @1550nmchromatic dispersion: 17ps/km-nmdispersion limited distance: ~100km @10Gbit/scomparable to loss limitEDFA => increased loss-limited distanceChromatic dispersion becomes the limiting factor in single mode long-haul fibers!We want to decrease the bandwidth for a given datarate!

Wave Division MultiplexingDecreased channel spacing leads to interchannel interference and makes it difficult to compensate for fiber nonlinearitiesNarrower subchannels would be nice...

WDM (cont'd)In a high capacity link the whole EDFA spectrum is filled with subchannelsThe bandwidth of each subchannel is proportional to its bit rateTotal fiber capacity is given by the spectral efficency: (bitrate per channel)/(channel spacing)

WDM (cont'd)In a practical case using NRZ a spectral efficiency of 40% can be reached

Power spectral density of NRZ

WDM (cont'dMore GB/s per channel does not increase total bandwith, howeverIt results in fewer channels to manageIncreased channel spacing decreases some non-linear distortionsBUT to reach higher spectral efficiency a format with narrower spectrum for a given bandwidth is needed (while at the same time not increasing other impairments)

How can this be achieved?M-ary Amplitude Shift Keying (ASK)Duo-binary signalingOptical Single Sideband (OSSB)

Comparison to radio systemsMuch of the same theory can be applied, exceptCarrier frequency is different1550 nm => 194 ThzThe available components are differentno coherent detection (no PLLs)The channel is different

Component imperfectionsModulators are nonlineardifficult to achieve pure AMPIN photo detectors responds to optical power rather than electrical field amplitude (square envelope)Dispersion introduces a frequency dependent phase shiftintensity-modulated approaches are used

Optical ModulatorsDirect modulationdirectly modulate the drive current of a semiconductor laserAbsorbtion modulationModulate the absorption spectrum of reverse-biased diod placed in front of the laserFaster and more linear than direct modulation (60 GHz) The Mach-Zender (MZ) modulatormodulation my adding phase shifted signals

Optical modulators (cont'd)Direct modulators and absorption modulators directly modulates the optical power, but will also generate phase modulationThe MZ modulator is more flexible and can generate different kinds if modulation other than NRZ/RZ/ASK

The MZ modulatorV1(t)V2(t)EinEin/2Ein/2EoutLiNbO3waveguidecontacts

MZ modulator transfer functionWith =1 this can be rewritten as:Amplitude modulationPhase modulation (chirp)With v1(t)=-v2(t) we remove the phase modulation and get:

MZ modulator biasingNormal bias:Bias at extinction:

MZ modulators - observationsThese modulators are only linear in a small regionA problem for other than RZ/NRZ signalingThere must normally be an unmodulated carrier in order to use non-coherent detection

M-ASKLess bandwidthMore power needed for a given BERnon-linearities become limiting in long-haul DWDM systemsMore complicated (analog and digital) electrical circuitsPossibly useful in multi-mode dispersion limited systems e.g. 10 Gbit/s Ethernet

Duo-binary signalingIntroduce correlation between consecutive symbolsA special case of partial response signaling:

Duo-binary signalingAdd consecutive symbols => three signal levels-1,1,1,-1-2,0,2,0MZ modulator

AM-PSK Duo-binaryProblem: Normally impractical to handle three levelsSolution: Use 0,E,-EThe detector will detect two levels 0 and EBy precoding these two levels will correspond to 0 and 1a.k.a Amplitude Modulated Phase Shift Keying (AM-PSK) duo-binary signaling

AM-PSK duo-binary system1,1,0,1,0MZ modulator biased at extinctionxormap|x|20,1,1,0,01,-1,-1,1,10,0,-2,0,20,0,-E,0,E10,0,1,0,10,0,E2,0,E2(fiber)Photo detectorPrecoder

Optical Single Sideband (OSSB)Observation: The frequency spectrum is symmetricalImplication: Half of it can be filtered out to save bandwidth => Single Sideband Transmission!Used e.g. in TV

Subcarrier OSSBIn conventional subcarrier modulation the subcarrier appears on both sides of the optical carrierDispersion causes a phase shift between the two signals, which depends on the distanceAt certain points the entire signal is canceled out!

Subcarrier OSSB (cont'd)(decided to skip the equations: Optical fiber communications IVB, eq.16.30-16.36)

Creating an SSB signalTwo waysUse a filter (half the energy is lost)Use the Hilbert transformknown as a Hartley modulator

Hartley modulatorSSB signal:

Baseband signal:

Optical SSB modulatorApproximation of SSB signal:MZAmplitudemodulatorHilberttransformPhasemodulatora(t)Optical carrier(t)OSSB signal

Simulation results: ASK/duo-binaryDispersion induced receiver sensitivity degradation for Gbit/s signalling

More practical issuesASKNees more power => non-linearities limitingDuo-binaryNeeds extra filteringOptical dispersion compensation could be an alternative225 km @10Gbit/s 1550 nm has been reached

Experimental results: OSSBExperimental receiver sensitivity degradation vs. fiber length @ 10Gbit/s, BER=10-9

DWDMNormal NRZ40% spectral efficiency over 150 kmDuo-binary AM-PSK100% over 100 kmOSSB66% over 300 km

SummaryDistance between repeaters is limited by either ofFiber lossChromatic dispersionFiber non-linearities With the advent of EDFA chromatic dispersion has become the limiting factor in long-haul systems

Summary (contd)We want to limit the bandwidth in order tooReduce the effects of chromatic dispersionReach higher spectral efficiency in DWDM systemsTwo potential methods:Duo-binary signalingOptical single sidebandBoth methods could potentially halve the bandwidthNone of the methods are currently used in commercial systems, but there are some promising experimental results