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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.

We propose and a novel InGaAs/InAlAs multiple quantum well (MQW) Mach–Zehnder (MZ) modulator with a single microring resonator, and a significant reduction of driving voltage is demonstrated for the first time. The modulator is driven by the quantum-confined Stark effect (QCSE) in the MQW and the driving voltage of the proposed modulator is expected to be significantly reduced by the phase-shift-enhancement effect in the microring. A waveguide structure was grown by solid-source molecular beam epitaxy and fabricated by inductively coupled plasma etching. A directional coupler with a shallow gap is employed to control the coupling parameters between a busline and the microring waveguide. An asymmetrical splitter was used as an input coupler to prevent the degradation of the extinction ratio of the MZ modulator. The extinction ratio of the fabricated microring MZ modulator was approximately 17.5 dB. The product of half-wave voltage and phase shifter length V π L was 2.0 Vmm in static modulation. This value was one-third that of a conventional MZ modulator with the same waveguide structure.