10.5. SENSITIVITY DEGRADATION 501
Figure 10.10: Power penalty versus RIN for several values of the local-oscillator power.Figure 10.10 showsδIas a function of RIN for several values ofPLOusingη= 0. 8
andhν= 0 .8 eV for 1.55-μm coherent receivers. The power penalty exceeds 2 dB
whenPLO=1 mW even for a local oscillator with a RIN of−160 dB/Hz, a value
difficult to realize for DFB semiconductor lasers. For a local oscillator with a RIN of
−150 dB/Hz,PLOshould be less than 0.1 mW to keep the power penalty below 2 dB.
The power penalty can be made negligible at a RIN of−150 dB/Hz if only 10μW
of local-oscillator power is used. However, Eq. (10.1.13) is unlikely to be satisfied for
such small values ofPLO, and receiver performance would be limited by thermal noise.
Sensitivity degradation from local-oscillator intensity noise was observed in 1987 in
a two-port ASK homodyne receiver [63]. The power penalty is reduced for three-
port receivers but intensity noise remains a limiting factor forPLO> 0 .1 mW [61]. It
should be stressed that the derivation of Eq. (10.5.3) is based on the assumption that
the receiver noise is Gaussian. A numerical approach is necessary for a more accurate
analysis of the intensity noise [65]–[67].
A solution to the intensity-noise problem is offered by thebalanced coherent re-
ceiver[68] made with two photodetectors [69]–[71]. Figure 10.11 shows the receiver
design schematically. A 3-dB fiber coupler mixes the optical signal with the local os-
cillator and splits the combined optical signal into two equal parts with a 90◦relative
phase shift. The operation of a balanced receiver can be understood by considering the
photocurrentsI+andI−generated in each branch. Using the transfer matrix of a 3-dB
coupler, the currentsI+andI−are given by
I+=^12 R(Ps+PLO)+R√
PsPLOcos(ωIFt+φIF), (10.5.4)
I−=^12 R(Ps+PLO)−R√
PsPLOcos(ωIFt+φIF), (10.5.5)whereφIF=φs−φLO+π/2.