500 CHAPTER 10. COHERENT LIGHTWAVE SYSTEMS
Figure 10.9: Schematic of a multiport phase-diversity receiver.are added. Moreover, high-power local oscillators are needed to supply enough power
to each branch. For these reasons, most phase-diversity receivers use two or three
ports. Several system experiments have shown that the linewidth can approach the bit
rate without introducing a significant power penalty even for homodyne receivers [58]–
[61]. Numerical simulations of phase-diversity receivers show that the noise is far from
being Gaussian [62]. In general, the BER is affected not only by the laser linewidth but
also by other factors, such as the the BPF bandwidth.
10.5.2 Intensity Noise
The effect of intensity noise on the performance of direct-detection receivers was dis-
cussed in Section 4.6.2 and found to be negligible in most cases of practical interest.
This is not the case for coherent receivers [63]–[67]. To understand why intensity noise
plays such an important role in coherent receivers, we follow the analysis of Section
4.6.2 and write the current variance as
σ^2 =σs^2 +σT^2 +σI^2 , (10.5.1)whereσI=RPLOrIandrIis related to therelative intensity noise(RIN) of the local
oscillator as defined in Eq. (4.6.7). If the RIN spectrum is flat up to the receiver band-
width∆f,rI^2 can be approximated by 2(RIN)∆f. The SNR is obtained by using Eq.
(10.5.1) in Eq. (10.1.11) and is given by
SNR=
2 R^2 P ̄sPLO
2 q(RPLO+Id)∆f+σT^2 + 2 R^2 PLO^2 (RIN)∆f. (10.5.2)
The local-oscillator powerPLOshould be large enough to satisfy Eq. (10.1.12) if
the receiver were to operate in the shot-noise limit. However, an increase inPLOin-
creases the contribution of intensity noise quadratically as seen from Eq. (10.5.2). If
the intensity-noise contribution becomes comparable to shot noise, the SNR would de-
crease unless the signal powerP ̄sis increased to offset the increase in receiver noise.
This increase inP ̄sis just the power penaltyδIresulting from the local-oscillator inten-
sity noise. If we neglectIdandσT^2 in Eq. (10.5.2) for a receiver designed to operate in
the shot-noise limit, the power penalty (in dB) is given by the simple expression
δI=10 log 10 [ 1 +(η/hν)PLO(RIN)]. (10.5.3)