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 mum long 3 dB coupler with 0.9 dB excess loss and 0.2 dB unbalance, and a 216 mum 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 mum and negligible radiation loss.