Performance analysis and laser linewidth requirements for optical PSK heterodyne communications systems

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 , then the loop needs 10 157 electrons/(s . Hz). If the detector has a unity quantum efficiency and m, the above quoted numberers give 0.9 pW/ kHz for and 1.35 pW/kHz for . The loop bandwidth required is also evaluated and found to be , where is the laser linewidth. Finally, the linewidth permitted for PSK heterodyne recievers is evaluated and found to be where Rbis the system bit rate. For Mbit/s, this leads to 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 up to 0.09 Rb. At the same time, homodyne receivers impose much more stringent requirements on the laser linewidth ( ).