Optical Heterodyning Versus Optical Homodyning: A Comparison

Progress in semiconductor laser technology stimulated a revived interest in optical heterodyne and homodyne techniques. The main purpose of this paper is a tutorial comparison of optical heterodyning with optical homodyning. Based on idealized device models, we show that optical homodyning can provide a 3 dB performance improvement with respect to optical heterodyning. The foregoing statement is valid for both ASK and PSK modulation formats. FSK modulation format cannot be used in conjunction with single-detector optical homodyning. Heterodyne FSK systems have the same theoretical performance as heterodyne ASK systems in terms of the average received signal power. Further, we compare the impact of real-world device characteristics on homodyne and heterodyne systems. The impact of optical phase and frequency instability is much more pronounc ed for homodyne systems than for heterodyne systems. On the other hand, homodyne systems can be built with much slower photodetectors than heterodyne systems. A peak in the laser phase noise spectrum can limit the maximum information transmission rate for both homodyne and heterodyne receivers. However, the maximum data rate limit for homodyne systems is 5 to 6 times larger than that for heterodyne systems.