1. Modulation formats for digital fiber transmission
Eric Tell

2. 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

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

4. Chromatic dispersion Distance limit ~1/(bit rate)²
Example: Single mode
chromatic dispersion: 17ps/km-nm
dispersion limited distance:
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!

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

6. 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)

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

8. 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)

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

10. 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

11. 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

12. 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

13. 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

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

15. 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:

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

17. 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

18. 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

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

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

21. 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

22. 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)

23. 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

24. 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!

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

26. Creating an SSB signal Two ways Use a filter (half the energy is lost)
Use the Hilbert transform
known as a Hartley modulator

27. Hartley modulator
SSB signal:
Baseband signal:

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

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

30. More practical issues…
ASK
Nees more power => non-linearities limiting
Duo-binary
Needs extra filtering
Optical dispersion compensation could be an alternative
nm has been reached

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

32. 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

33. 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

34. 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