Existing photonic couplers are limited by either large footprint or long interaction length. Here a highly efficient and ultracompact three-port trench-based coupler (TBC) is proposed on an InP platform in terms of the frustrated total internal reflection principle. A single slash-shaped narrow trench located at the "T" intersection of two InP/InGaAsP multiquantum well ridge waveguides forms the coupler. The finite-difference time-domain numerical method is utilized to optimize the parameterization of the couplers, such as splitting ratios and efficiency versus trench widths, lengths, locations, and angles. A single-mode 2 mu m wide ridge waveguide coupler having a high aspect ratio trench filled with Al2O3 was fabricated and characterized at 1.55 mu m wavelength. The "trench-first" fabrication process is optimized to reduce its inherent insertion loss (IL), showing the IL within a range of 0.3-0.5 dB. The devices can outperform the state-of-the-art trench couplers by >= similar to 24 times the figure of merit. These TBCs are thus promising candidates for applications in the next generation of photonic integrated circuits. (C) 2020 Optical Society of America
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