Extremely small multimode interference couplers and ultrashort bends on InP by deep etching

We realized the smallest integrated optical multimode interference (MMI) couplers to date by etching through the quaternary guiding layer, maximizing the lateral index contrast, and thus obtaining a sufficient number of guided modes for proper operation despite the small size. Results include a 107 μm long 3 dB coupler with 0.9 dB excess loss and 0.2 dB unbalance, and a 216 μm long cross coupler with 2.0 dB excess loss and -28 dB crosstalk (TE). We also succeeded in making waveguide bends with a record bending radius of only 30 μm and negligible radiation loss.