10.5. SENSITIVITY DEGRADATION 499
Figure 10.8: Receiver sensitivityN ̄pversus∆ν/Bfor asynchronous ASK and FSK heterodyne
receivers. The dashed line shows the sensitivity degradation for a synchronous PSK heterodyne
receiver. (After Ref. [49];©c1988 IEEE; reprinted with permission.)
ate in a single longitudinal mode with a narrow linewidth and whose wavelength can
be tuned (at least over a few nanometers) to match the carrier frequencyω 0 and the
local-oscillator frequencyωLOeither exactly (homodyne detection) or to the required
intermediate frequency. Multisection DFB lasers have been developed to meet these
requirements (see Section 3.4.3). Narrow linewidth can also be obtained using a MQW
design for the active region of a single-section DFB laser. Values as small as 0.1 MHz
have been realized using strained MQW lasers [56].
An alternative approach solves the phase-noise problem by designing special re-
ceivers known asphase-diversity receivers[57]–[61]. Such receivers use two or more
photodetectors whose outputs are combined to produce a signal that is independent of
the phase differenceφIF=φs−φLO. The technique works quite well for ASK, FSK,
and DPSK formats. Figure 10.9 shows schematically a multiport phase-diversity re-
ceiver. An optical component known as anoptical hybridcombines the signal and
local-oscillator inputs and provides its output through several ports with appropriate
phase shifts introduced into different branches. The output from each port is processed
electronically and combined to provide a current that is independent ofφIF. In the case
of a two-port homodyne receiver, the two output branches have a relative phase shift
of 90◦, so that the currents in the two branches vary asIpcosφIFandIpsinφIF. When
the two currents are squared and added, the signal becomes independent ofφIF. In the
case of three-port receivers, the three branches have relative phase shifts of 0, 120◦, and
240 ◦. Again, when the currents are added and squared, the signal becomes independent
ofφIF.
The preceding concept can be extended to design receivers with four or more
branches. However, the receiver design becomes increasingly complex as more branches