Recent developments in phase synchronization techniques for optical space communication PSK homodyne receivers

By allowing phase synchronization of the local oscillator and BPSK demodulation and by using only a simple "in-phase" demodulator front-end with a 180-degree hybrid the "Syncbit" method relocates the problem of optical complexity (90 deg. hybrid) into additional electronic circuitry. This has been advantageous, as long as the transmitted data rates are reasonable. In the optical ISL systems envisaged today, transmission rates of several Gigabit per second are used. The problems arising from the high data rates and from the implicit band limitations by electronic and electrooptic components initiated a further search for alternative and simple phase error detection schemes. All of these alternatives make use of the inherent nonlinear characteristic of the electrooptic phase modulator and in-phase demodulator combination.