All-optical clock extraction using two-contact devices

Abstract

A comprehensive summary of the operation of two-contact semiconductor self-pulsating laser diode (SP—LD) in both return-to-zero (RZ) and non-return-to-zero (NRZ) optical transmission systems is presented. Results demonstrate that this type of device has great potential as the basis for all-optical clock recovery circuits at multi-Gbits rates for switching applications. Results describe the basic device behaviour showing how zinc doping the shorter (absorber) region of the device enables repeatable and controlled GHz pulsations, within the range ∼ 0.6 to > 5.5 GHz and tunability (via the DC gain current) over many GHz, to be achieved. Experimental results show that the SP-LD can be locked with μW of incident power to produce a locked oscillator with a linewidth of < 10 Hz at 5 GHz and with 20 dB power gain across the device. New results, addressing the pattern dependence, demonstrate that long breaks (up to ∼ 30 ‘zeros’) in the clock can be accommodated without significant degradation of the locked clock purity; the length of break being dependent on the initial state of locking. Other new results show that the lock-up time for such circuits is of the order of 100 clock cycles. System performance is investigated using these devices within a 20 Gbit/s (4 × 5 Gbit/s) optical-time-division-multiplexed demonstrator; the results showing no significant degradation of the bit-error-ratio performance. Other system results at 3.2 Gbit/s show that this technique can be applied to NRZ systems when also utilising a nonlinear effect within a similar device biased below threshold, and identifying the differences from RZ operation. These results show that such an approach could provide major benefits in developing the next generation of telecommunications networks.