Image Processing Reference
In-Depth Information
Chromatic Dispersion
Non Linearities
t
t
Attenuation
Noise
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t
t
t
Fig. 1. Optical fiber transmission impairments.
into two counter-propagating signals. The weaker partial pulse passes first through the
EDFA where it is amplified by about 20dB. It gains a significant phase shift due to self-phase
modulation (Stephan et al., 2009) in the highly non-linear fiber (HNLF). The initially stronger
pulse propagates through the fiber before it is amplified, so that the phase shift in the HNLF is
marginal. At the output coupler the strong partial pulse with almost unchanged phase and the
weak partial pulse with input-power-dependent phase shift interfere. The first, being much
stronger, determines the phase of the output signal and therefore ensures negligible phase
distortions.
Various investigations have been also been reported to examine the effect of optical link
design (Lin et al., 2010a; Randhawa et al., 2010; Tonello et al., 2006) on the compensation
of fiber impairments. However, the applications of all-optical methods are expensive, less
flexible and less adaptive to different configurations of transmission. On the other hand
with the development of proficient real time digital signal processing (DSP) techniques and
coherent receivers, finite impulse response (FIR) filters become popular and have emerged as
the promising techniques for long-haul optical data transmission. After coherent detection the
signals, known in amplitude and phase, can be sampled and processed by DSP to compensate
fiber transmission impairments.
DSP techniques are gaining increasing importance as they allow for robust long-haul
transmission with compensation of fiber impairments at the receiver (Li, 2009; Savory et al.,
2007). One major advantage of using DSP after sampling of the outputs from a phase-diversity
receiver is that hardware optical phase locking can be avoided and only digital phase-tracking
is needed (Noe, 2005; Taylor, 2004). DSP algorithms can also be used to compensate chromatic
dispersion (CD) and polarization-mode dispersion (PMD) (Winters, 1990). It is depicted that
for a symbol rate of
,a 2 tap delay finite impulse response (FIR) filter may be used to reverse
the effect of fiber chromatic dispersion (Savory et al., 2006). The number of FIR taps increases
linearly with increasing accumulated dispersion i.e the number of taps required to compensate
1280 ps/nm of dispersion is approximately 5.8 (Goldfarb et al., 2007). At long propagation
distances, the extra power consumption required for this task becomes significant. Moreover,
a longer FIR filter introduces a longer delay and requires more area on a DSP circuitry.
τ
 
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