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Optical bistability and flip-flop operation in DFB laser diodes injected with a CW signal

All-optical flip-flops draw more and more attention as potential parts of all-optical packet or burst switching schemes. Recently several schemes for such all-optical flip-flops have been proposed, e.g. mutually coupled laser diodes, Mach-Zehnder interferometers with fed back output and ring lasers. All flip-flops are based on bistable behaviour and on the possible switching between the two stable states using short optical pulses. Previously, we have shown numerically that a DFB laser diode in which a CW signal is injected can exhibit a bistability in the laser output power and the amplification of the input power. A condition for this bistability was that the injected light is not reflected inside the laser diode. The wavelength of the injected light must therefore be not too close to the Bragg wavelength and the laser must be AR coated. In this contribution, we present additional modelling results as well as some experimental results for λ/4-shifted DFBs. We show numerically that for a certain bias current the bistability only occurs when the carrier lifetime and the series resistance of the laser diode are not too small, such that sufficient spatial hole burning can exist inside the laser. Experimental results show that bistability is observed in some lasers but not in others, something which may be related to the series resistance and carrier lifetime. We will also discuss the dynamic, all-optical flip-flop operation, which is possible by injecting short pulses on either facet of the laser.