Design and analysis of 2 x 2 half-wave waveguide couplers

Conventional couplers including the 2 x 2 directional coupler and the self-imaging multimode interference coupler always produce a 90 degrees coupling phase to yield complementary power transfer functions when the relative phase of the two input fields changes. For applications in coupled-cavity lasers, a 2 x 2 half-wave optical coupler with a 180 degrees coupling phase is required in order to produce synchronous output power for achieving high single-mode selectivity. We present the designs and optimization of 2 x 2 half-wave couplers, including a three-waveguide directional coupler and a nonimaging multimode interference coupler.