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