Design of a photonic crystal tapered coupler with different section lengths based on multimode interference and mode matching

Abstract

Step tapered couplers, which act as mode converters, are usually used between photonic crystal (PC) waveguides and other optical devices for efficient coupling. In this study, the multimode interference (MMI) phenomena in multiple-line-defect PC waveguides are observed. It is found that the optimal length of each section in a PC step tapered coupler is related to the imaging length of MMI. Therefore, a PC step tapered coupler with different section lengths can be designed. Because the modes are matched between two adjacent sections and the scattering losses occurring at the corners of abrupt steps are reduced, it can provide an excellent transmission between PC waveguides and other optical devices. This study also reveals the reason why in some cases, the transmission of PC step tapered couplers decreases counterintuitively when the taper length is increased.