4.6. SENSITIVITY DEGRADATION 171
Figure 4.21: Power penalty versus the intensity noise parameterrI.than the extinction ratio, for which the power penalty remains finite for all values ofrex
such thatrex<1.
The preceding analysis assumes that the intensity noise at the receiver is the same
as at the transmitter. This is not typically the case when the optical signal propagates
through a fiber link. The intensity noise added by in-line optical amplifiers often be-
comes a limiting factor for most long-haul lightwave systems (see Chapter 5). When
a multimode semiconductor laser is used, fiber dispersion can lead to degradation of
the receiver sensitivity through the mode-partition noise. Another phenomenon that
can enhance intensity noise is optical feedback from parasitic reflections occurring all
along the fiber link. Such transmission-induced power-penalty mechanisms are con-
sidered in Chapter 7.
4.6.3 Timing Jitter
The calculation of receiver sensitivity in Section 4.5 is based on the assumption that
the signal is sampled at the peak of the voltage pulse. In practice, the decision instant
is determined by the clock-recovery circuit (see Fig. 4.11). Because of the noisy nature
of the input to the clock-recovery circuit, the sampling time fluctuates from bit to bit.
Such fluctuations are calledtiming jitter[92]–[95]. The SNR is degraded because
fluctuations in the sampling time lead to additional fluctuations in the signal. This
can be understood by noting that if the bit is not sampled at the bit center, the sampled
value is reduced by an amount that depends on the timing jitter∆t. Since∆tis a random