A Novel Working Scheme for Robust Optical Coupler Unit for Programable Unitary Matrices

Programmable optical unitary matrices (POUMs) in photonic integrated circuits (PICs) technology play a critical role in modern information technology. With the expansion of their scale and integration density, POUMs have their performance limited by fabrication errors and tuning phase errors of their building blocks, i.e., tunable 2 × 2 optical couplers. Here, we provide an in-depth theoretical analysis of a prior robust equivalent 2 × 2 optical coupler and demonstrate a general requirement of its passive beam splitters' split ratios, under which an arbitrary tunable transmission ratio can always be achieved. More importantly, we analyze the impact of the phase shifters' tuning phase errors which is always caused by the limited voltage/current resolution of the electronic driving interface or thermal fluctuation. By the analyzed result, we propose a novel working scheme for the first time by discovering an unconventional configuration with large beam split ratio errors, under which robust POUMs against fabrication and tuning phase errors can be achieved.