Abstract
One of the most severe impairments that affect coherent optical systems
employing high-order modulation formats is phase noise due to transmit and
receive lasers. This is especially detrimental in uncompensated links, where
an ideal compensator for channel distortions and laser phase noise should
first eliminate receive phase noise, then equalize channel distortions, and
only later compensate for transmit phase noise. Unfortunately, the
simultaneous presence of transmit and receive phase noise makes very
difficult to discriminate between them, even in the presence of a pilot
tone. Moreover, the picture is different for optical systems using
single-carrier or orthogonal frequency division multiplexing, where transmit
and receive phase noise components may have a different impact. All these
aspects are analyzed and discussed in this paper. A novel digital coherence
enhancement (DCE) technique, able to significantly reduce the phase noise of
transmit or receive lasers by using an interferometric device plus a very
simple electronic processing, is also described. The performance of this
technique and the statistical properties of the residual phase noise are
analytically derived and verified by simulations, showing a high increase of
the maximum bit-rate-distance product. The practical implementation of DCE
is finally discussed and some alternative implementation schemes are
presented.
© 2011 IEEE
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