Design and characterization of spot size converters for integration in InP-based photonic integrated circuits

We report on design and measurement of a series of spot-size converters (SSCs) for adapting the optical mode size in an InP-based photonic integrated circuit. SSCs are designed to transfer the mode of a 0.6mm-thick high-contrast waveguide to that of a 3mm-thick low-contrast waveguide below the core layer using a vertical taper. Fabrication of the taper is compatible with the standard processing and coupling loss values below 1dB have been achieved. Based on the experimental results we have designed an improved structure which enables adiabatic vertical coupling for taper lengths below 1mm. The simulation results predict losses below 0.5dB