Signal Processing for High-Speed Links - Signal Processing Systems - page 340

proportional to the received optical power, if a coherent receiver were used, then
the received electrical signal would be proportional the electromagnetic field in the
fiber. As a result, CD and PMD might appear as linear distortions, enabling linear
equalization techniques to substantially outperform their counterparts when used
after direct (square-law) detection. Four photo detectors are presently required to
recover the in-phase and quadrature (I and Q) components of the electromagnetic
field from both polarizations of the received optical signal. If the length of fiber
to be compensated were known in advance, techniques for coherent modulation
combined with linear pre-equalization [ 32 ] can be used. One advantage of electronic
pre-equalization is that direct detection can still be used at the receiver, in exchange
for substantial signal processing followed by digital to analog conversion at the
modulator.
In addition, techniques that exploit polarization diversity [ 19 ] and modal diver-
sity [ 37 , 38 ] are being explored, whereby PMD and modal dispersion are viewed as
opportunities for enhancing date rates, rather than impediments. This is analogous
to what is being done in wireless systems today. Thus, as was the case for back-
plane links, there exists a wide range of unexplored signal processing techniques in
the context of optical links.

6 Concluding Remarks

Signal processing techniques have found their way into the design of high-speed
links, such as back-plane and optical links, today. Thanks to the tremendous
advances in semiconductor technology, equalization in back-plane links and EDC-
based optical products are making their way into commercial systems. We hope
that this chapter has provided an overview of the general concepts, and will enable
further exploration into this fascinating and challenging topic.

Acknowledgements The authors thank their colleagues at the University of Illinois at Urbana-
Champaign, and Finisar Corporation, Inc., for their contributions.

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