Coherent Coupling in High-Mesa Semiconductor Directional Coupler

The effect of coherent coupling on the performance of a directional coupler is investigated for the first time. Coherent coupling is the interference between a guided mode in a core layer and an unguided mode in a cladding layer (cladding mode) in a waveguide. It was experimentally confirmed that the cladding mode exists in an actual high-mesa semiconductor waveguide and it can be represented by a single propagation constant. The effect of coherent coupling on coupling loss and coupling efficiency in a directional coupler is analyzed using a new technique based on a matrix method. The analyzed results reveal that a small periodic oscillation of coupling loss of the transmitted light that depends on coupler length occurs owing to the coherent coupling in a strong-coupled directional coupler, and its effect should be considered when we minimize propagation loss in high-mesa semiconductor directional couplers. It is also shown that it is possible to design a directional coupler with both the desired coupling efficiency and low loss on the basis of the periodic oscillation of coupling loss.