4.6. SENSITIVITY DEGRADATION 169
Figure 4.20: Power penalty versus the extinction ratiorex.This equation shows thatP ̄recincreases whenrex=0. The power penalty is defined
as the ratioδex=P ̄rec(rex)/P ̄rec( 0 ). It is commonly expressed in decibel (dB) units by
using
δex=10 log 10( ̄
Prec(rex)
P ̄rec( 0 ))
=10 log 10(
1 +rex
1 −rex)
. (4.6.4)
Figure 4.20 shows how the power penalty increases withrex. A 1-dB penalty occurs
forrex= 0 .12 and increases to 4.8 dB forrex= 0 .5. In practice, for lasers biased below
threshold,rexis typically below 0.05, and the corresponding power penalty (<0.4 dB)
is negligible. Nonetheless, it can become significant if the semiconductor laser is biased
above threshold. An expression forP ̄rec(rex)can be obtained [3] for APD receivers by
including the APD gain and the shot-noise contribution toσ 0 andσ 1 in Eq. (4.6.2). The
optimum APD gain is lower than that in Eq. (4.5.19) whenrex=0. The sensitivity is
also reduced because of the lower optimum gain. Normally, the power penalty for an
APD receiver is larger by about a factor of 2 for the same value ofrex.
4.6.2 Intensity Noise
The noise analysis of Section 4.4 is based on the assumption that the optical power
incident on the receiver does not fluctuate. In practice, light emitted by any transmitter
exhibits power fluctuations. Such fluctuations, called intensity noise, were discussed
in Section 3.3.8 in the context of semiconductor lasers. The optical receiver converts
power fluctuations into current fluctuations which add to those resulting from shot noise
and thermal noise. As a result, the receiver SNR is degraded and is lower than that
given by Eq. (4.4.19). An exact analysis is complicated, as it involves the calculation