An optical PSK heterodyne communications receiver is investigated. The receiver is based on the decision-directed phase-locked loop. The performance of the phase-locked loop subsystem is analyzed taking into account both shot noise and laser phase noise. It is shown that for reliable phase locking (rms phase error less than 10°), heterodyne second-order loops require at least 6771 electrons/s per volt every hertz of the laser linewidth. This number corresponds to the limit when the loop dumping factor η is infinitely large; if \eta = 0.7 , then the loop needs 10 157 electrons/(s . Hz). If the detector has a unity quantum efficiency and \lambda = 1.5 \mu m, the above quoted numberers give 0.9 pW/ kHz for \eta \rightarrow \infty and 1.35 pW/kHz for \nu = 0.7 . The loop bandwidth required is also evaluated and found to be 155 \Delta\nu , where \Delta\nu is the laser linewidth. Finally, the linewidth permitted for PSK heterodyne recievers is evaluated and found to be 2.26 \cdot 10^{-3} R_{b} where R b is the system bit rate. For R_{b}=100 Mbit/s, this leads to \Delta\nu kHz. Such and better linewidths have been demonstrated with laboratory external cavity lasers. For comparison, ASK and FSK heterodyne receivers are much more tolerant to phase noise,-they can tolerate \Delta\nu up to 0.09 R b . At the same time, homodyne receivers impose much more stringent requirements on the laser linewidth ( \Delta\nu ).