30 GHz sub-clock recovery using an opto-electronic phase-locked loop based on four-wave mixing in a semiconductor optical amplifier

Clock recovery is an important function of any digital communications system, critical for receiving and possibly regenerating the signal. For higher bit rates, it has been proposed to use an opto-electronic PLL, in which the up-front mixer or phase comparator is replaced by a semiconductor optical amplifier (SOA): the nonlinearities of this device, e.g. four-wave mixing (FWM), act as a mixer, not only in the optical-frequency domain but also in the modulation-frequency domain; and its time-response characteristics act as a NRZ-to-pseudo-RZ converter. Such a system, akin to a simple PLL in which FWM in a SOA replaces the high-frequency mixer, had been demonstrated for 10 Gbps RZ and NRZ signals.