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. Also, sub-clock recovery is required for time-domain demultiplexing. Both are traditionally performed by an electronic phase-locked loop (PLL); however, such devices are limited in bit rate to a few tens of Gbps. For higher bit rates, it has been proposed [1, 2] 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 [2]. The same scheme, described figure 1. is capable of extracting a 10 GHz sub-clock from a 30 GHz sinusoidal signal, which is generated by overmodulating a CW optical signal with a lithium niobate Mach-Zehnder modulator driven by a 15 GHz sine wave. Figure 2 shows the spectrum of the recovered sub-clock, and the corresponding signal from a sampling oscilloscope using the resulting clock as trigger. Depending on the experimental parameters (signal input power and loop gain), the lock bandwidth (w.r.t. the 30 GHz signal) ranged 36-614kHz, and the tracking bandwidth 44- 1416kHz. Experiments are underway to increase the bit rate and to explore the system's dynamics in greater detail.