An in-depth analysis of reflections in MMI couplers using optical low-coherence reflectometry: design optimization and performance evaluation

This paper describes a comprehensive report on the high potential of optica l low-coherence reflectometry (OLCR) measurements to assist the design opti mization and performance evaluation of symmetric MMI couplers. Using three sets of deep-ridge InGaAsP-InP couplers on InP and by performing OLCR measu rements simultaneously in reflection and transmission modes, the nature and origin of all reflections in MMI have first been identified with respect t o a deliberately chosen design parameter, namely, the coupler length. In ad dition, and in total agreement with the self-imaging principle and also the BPM simulations, the back reflections in combiner mode are shown to be pro minent even in fully optimized devices. Finally, a simple design artifact h as been proposed and demonstrated experimentally for the first time to supp ress back reflections.