Integrated InP optical unitary converter with compact half-integer multimode interferometers

Abstract

Integrated optical unitary converters (OUCs) are vital devices for various emerging applications such as mode-multiplexed optical communication, optical neural networks, and quantum computing. In order to realize large-scale OUCs in a limited footprint, the number of elements, as well as the size of each element, is important. In this work, we present a novel type of OUC using half-integer multimode interferometers (MMIs) based on the multi-plane light conversion (MPLC) concept. A half-integer MMI enables unitary coupling among the multiple input and output ports, while requiring only half the length of a conventional uniform MMI. Although the splitting ratio is not uniform across the ports, we show both numerically and experimentally that arbitrary unitary operation can still be achieved with comparable performance. We fabricate 4×4 OUC with half-integer MMIs on the monolithic InP platform and experimentally demonstrate reconfigurable 4-mode sorting and switching with a significantly reduced footprint compared with the conventional OUCs using uniform MMIs.