502 CHAPTER 10. COHERENT LIGHTWAVE SYSTEMS
Figure 10.11: Schematic of a two-port balanced coherent receiver.The subtraction of the two currents provides the heterodyne signal. The dc term is
eliminated completely during the subtraction process when the two branches are bal-
anced in such a way that each branch receives equal signal and local-oscillator powers.
More importantly, the intensity noise associated with the dc term is also eliminated
during the subtraction process. The reason is related to the fact that the same local
oscillator provides power to each branch. As a result, intensity fluctuations in the two
branches are perfectly correlated and cancel out during subtraction of the photocur-
rentsI+andI−. It should be noted that intensity fluctuations associated with the ac
term are not canceled even in a balanced receiver. However, their impact is less severe
on the system performance because of the square-root dependence of the ac term on
the local-oscillator power.
Balanced receivers are commonly used while designing a coherent lightwave sys-
tem because of the two advantages offered by them. First, the intensity-noise problem
is nearly eliminated. Second, all of the signal and local-oscillator power is used effec-
tively. A single-port receiver such as that shown in Fig. 10.1 rejects half of the signal
powerPs(and half ofPLO) during the mixing process. This power loss is equivalent
to a 3-dB power penalty. Balanced receivers use all of the signal power and avoid
this power penalty. At the same time, all of the local-oscillator power is used by the
balanced receiver, making it easier to operate in the shot-noise limit.
10.5.3 Polarization Mismatch
The polarization state of the received optical signal plays no role in direct-detection
receivers simply because the photocurrent generated in such receivers depends only
on the number of incident photons. This is not the case for coherent receivers, whose
operation requires matching the state of polarization of the local oscillator to that of the
signal received. The polarization-matching requirement can be understood from the
analysis of Section 10.1, where the use of scalar fieldsEsandELOimplicitly assumed
the same polarization state for the two optical fields. If ˆesand ˆeLOrepresent the unit
vectors along the direction of polarization ofEsandELO, respectively, the interference
term in Eq. (10.1.3) contains an additional factor cosθ, whereθis the angle between
eˆsand ˆeLO. Since the interference term is used by the decision circuit to reconstruct the
transmitted bit stream, any change inθfrom its ideal value ofθ=0 reduces the signal