On-chip polarization-insensitive Fourier transform spectrometer.

Abstract

Chip-scale monolithic Fourier transform spectrometers (FTSs) offer great potential for inexpensive, high-resolution, and robust spectroscopic applications in a wide variety of scenarios. Having attracted considerable attention, spatial heterodyne FTSs (SH-FTSs) are featured with a simple and stable configuration composed of an array of Mach-Zehnder interferometers (MZIs) with linearly increasing optical path differences. Owing to the strong waveguide birefringence, MZIs on the popular silicon-on-insulator platform are polarization-sensitive, raising the challenge of polarization control of incident light. We propose and demonstrate a polarization-insensitive SH-FTS using a two-dimensional grating coupler to split an arbitrary state of polarization into two orthogonal polarization components that are both coupled into the TE mode but propagate in opposite directions in the arrayed MZIs. The two orthogonal polarization components are finally recombined in photodetection without polarization-dependent losses. An edge-coupling configuration using a polarization splitter-rotator is also proposed.