Improved fibre coupling to GaAs/AlGaAs rib waveguides using a regrowth-free mode `drive-down´ technique

An integrated optical mode expander is reported which reduces measured coupling losses between high-NA rib-loaded GaAs/AlGaAs waveguides and standard fibre to 3 dB at 1.3 μm wavelength. By adding a low refractive index buffer layer to an existing waveguide geometry, and laterally tapering a new etched rib of carefully selected depth, an adiabatic and reciprocal `drive-down´ transition to a large spot-size mode is achieved. The wet-etched device does not require semiconductor regrowth or submicrometre patterned features