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– 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

V1(t)

V2(t)

Ein

Ein/2

γEin/2

Eout

LiNbO3

waveguidecontacts

VtvjVtvjintjtjinout ee

Eee

EE /)(/)()()( 2121

22

MZ modulator transfer function VtvjVtvj

in

out eeE

tEtvtv /)(/)(

2121

2

1)())(),((

VtvtvjetvtvV

tvtv 2/))()((2121

21))()((2

cos())(),((

))(cos( 1 tvV

EE inout

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:

))((cos 12 tv

VPP inout

MZ modulator biasing)()( 12 tvVtv dc

VVjdctdc

dceVvV

Vtv 2/)(11 )2(

2cos()),((

2/VVdc

VVdc

)(1

2

1

4

1)(cos)2/),(( 1

1 tvVV

tvVta

)()(sin2

1)(cos)),(( 11

1 tvV

tvVV

tvVta

“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

-2,0,2,0MZ modulator

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

1,1,0,1,0

MZ modulator biased at extinction

xor map

|x|2

0,1,1,0,0

1,-1,-1,1,1

0,0,-2,0,2 0,0,-E,0,E

1

0,0,1,0,1

0,0,E2,0,E2

(fiber)

Photo detector

Precoder

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)2sin()(ˆ)2cos()()( tftatftats ccsSSB

)(ˆ)()( tajtats sSSB SSB signal:

Baseband signal:

Optical SSB modulator)(ˆ)()( tajetatg “Approximation” of SSB signal:

MZAmplitudemodulator

Hilberttransform

Phasemodulator

a(t)

Optical carrier

â(t)

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– 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