Microlens reflector for out-of-plane optical coupling of a waveguide to a buried silicon photodiode

A photoresist microlens is shown to be an extremely efficient reflective element for out-of-plane coupling of light from a negative epoxy-based photoresist (SU8) optical waveguide into a buried silicon pn photodetector. Experimentally determined coupling efficiencies are given for integrated waveguide∕photodetector and waveguide∕microlens∕photodetector structures. The highest measured coupling efficiency in a structure with a 40μm diameter, 13μm high microlens is about 74%, versus an average of about 11% in structures without a microlens. The measured microlens-mediated coupling efficiency is in satisfactory agreement with a simple ray tracing simulation that provides a calculated value of 77.5% for the coupling efficiency.