Spatial Gain Overlap-Controlled Hysteresis in Multimode Interference-Based Bistable Laser Diodes

Multimode interferometer bistable laser diodes (MMI-BLDs) appeared recently as a potential candidate for implementing all-optical flip-flops. It was reported that in such MMI-BLDs, cross-gain saturation effect and saturable absorber together can control the hysteresis window characteristics; however, there was no clear description on how to analyze and estimate the effect of cross-gain saturation on hysteresis characteristics. In this paper, we present a model that can explain the experimental results reported for MMI-BLDs and predict the hysteresis behavior for such devices. The proposed simulation model can work for both symmetric and asymmetric configurations of MMI-BLDs as it considers the spatial gain overlap between the co-propagating modes inside the MMI-BLDs. This spatial overlap accounts for the discrepancies in hysteresis characteristics, i.e., hysteresis width and threshold currents, between both configurations. With the proposed model, as the asymmetric configuration provides variable spatial cross-gain overlap between the fundamental and first-order modes depending on device dimensions, hysteresis window characteristics vary accordingly. To confirm the spatial gain overlap role, we present the simulation results obtained by the proposed model for several asymmetric MMI-BLDs of different spatial gain overlap with fixed saturable absorber effect. The results obtained show that the higher the spatial gain overlap is, the lower the threshold currents of the hysteresis window are for the same hysteresis window width, which is determined mainly by the saturable absorption effect. The obtained simulation results match with the experimental results reported for the asymmetric MMI-BLD configuration.