Optical clock recovery for high-speed optical networks

A novel inline optical clock recovery circuit, based on the periodic oscillations that result from the astable behavior of a bistable optical device with delayed feedback, is described. Clock extraction from an intensity modulated 13.6-Mb/s return-to-zero (RZ) optical data stream is demonstrated. Operation at much higher speed can be achieved by monolithic integration. The circuit is based on an integrated-optic modulator with delayed feedback. The incoming optical data signal is added to a constant optical bias before entering the modulator. Part of the light, transmitted through the modulator, is photodetected; the resulting electrical signal is amplified and then fed back to the electrical input of the modulator after an electrical bias has been added to it. With only the constant optical input, the output signal is a continuous square-wave oscillation whose period is determined by the time delay τ introduced by the feedback loop. This time-dependent behavior of the output intensity, even though the input is steady, arises from the periodic instabilities in bistable systems caused by a finite feedback time delay. The characteristics and performance of the experimental circuit are presented.