Modulation formats for digital fiber transmission Eric Tell 050329

Outline Fiber performance limitations WDM Optical vs. radio communication Optical modulators Modulation formats Amplitude shift keying Duo-binary signalling Optical single sideband signalling Simulation/experimental results Summary

Fiber performance limitations Fiber Loss Chromatic dispersion different refractive index for different wavelengths Fiber non-linearities

Chromatic dispersion Distance limit ~1/(bit rate)² Example: Single mode fiber @1550nm chromatic dispersion: 17ps/km-nm dispersion limited distance: ~100km @10Gbit/s comparable to loss limit EDFA => increased loss-limited distance Chromatic dispersion becomes the limiting factor in single mode long-haul fibers! We want to decrease the bandwidth for a given datarate!

Wave Division Multiplexing Decreased channel spacing leads to interchannel interference and makes it difficult to compensate for fiber nonlinearities Narrower subchannels would be nice...

WDM (cont'd) In a high capacity link the whole EDFA spectrum is filled with subchannels The bandwidth of each subchannel is proportional to its bit rate Total 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'd More GB/s per channel does not increase total bandwith, however It results in fewer channels to manage Increased channel spacing decreases some non-linear distortions BUT 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 signaling Optical Single Sideband (OSSB)

Comparison to radio systems Much of the same theory can be applied, except Carrier frequency is different 1550 nm => 194 Thz The available components are different no coherent detection (no PLLs) The channel is different

Component imperfections Modulators are nonlinear difficult to achieve pure AM PIN photo detectors responds to optical power rather than electrical field amplitude (“square envelope”) Dispersion introduces a frequency dependent phase shift “intensity-modulated” approaches are used

Optical Modulators Direct modulation Absorbtion modulation directly modulate the drive current of a semiconductor laser Absorbtion modulation Modulate the absorption spectrum of reverse-biased diod placed in front of the laser Faster and more linear than direct modulation (60 GHz) The Mach-Zender (MZ) modulator modulation my adding phase shifted signals

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

The MZ modulator contacts V1(t) waveguide LiNbO3 Ein/2 Ein Eout γEin/2

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

MZ modulator biasing “Normal bias”: “Bias at extinction”:

MZ modulators - observations These modulators are only linear in a small region A problem for other than RZ/NRZ signaling There must normally be an unmodulated carrier in order to use non-coherent detection

M-ASK Less bandwidth More power needed for a given BER non-linearities become limiting in long-haul DWDM systems More complicated (analog and digital) electrical circuits Possibly useful in multi-mode dispersion limited systems e.g. 10 Gbit/s Ethernet levels bandwidth 2 ±B 4 ±B/2 8 ±B/3 16 ±B/4 32 ±B/5 64 ±B/6

Duo-binary signaling Introduce correlation between consecutive symbols A special case of partial response signaling:

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

AM-PSK Duo-binary Problem: Normally impractical to handle three levels Solution: Use 0,E,-E The detector will detect two levels 0 and E² By precoding these two levels will correspond to 0 and 1 a.k.a Amplitude Modulated Phase Shift Keying (AM-PSK) duo-binary signaling

AM-PSK duo-binary system 0,0,1,0,1 1,-1,-1,1,1 map 1 xor 1,1,0,1,0 0,1,1,0,0 0,0,-2,0,2 Precoder MZ modulator biased at extinction 0,0,-E,0,E 0,0,E2,0,E2 |x|2 Photo detector (fiber)

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

Subcarrier OSSB In conventional subcarrier modulation the subcarrier appears on both sides of the optical carrier Dispersion causes a phase shift between the two signals, which depends on the distance At 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 signal Two ways Use a filter (half the energy is lost) Use the Hilbert transform known as a Hartley modulator

Hartley modulator SSB signal: Baseband signal:

Optical SSB modulator “Approximation” of SSB signal: Hilbert a(t) transform a(t) â(t) Optical carrier MZ Amplitude modulator Phase modulator OSSB signal

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

More practical issues… ASK Nees more power => non-linearities limiting Duo-binary Needs extra filtering Optical dispersion compensation could be an alternative 225 km @10Gbit/s 1550 nm has been reached

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

DWDM “Normal” NRZ Duo-binary AM-PSK OSSB 40% spectral efficiency over 150 km Duo-binary AM-PSK 100% over 100 km OSSB 66% over 300 km

Summary Distance between repeaters is limited by either of Fiber loss Chromatic dispersion Fiber non-linearities With the advent of EDFA chromatic dispersion has become the limiting factor in long-haul systems

Summary (cont’d) We want to limit the bandwidth in order too Reduce the effects of chromatic dispersion Reach higher spectral efficiency in DWDM systems Two potential methods: Duo-binary signaling Optical single sideband Both methods could potentially halve the bandwidth None of the methods are currently used in commercial systems, but there are some promising experimental results