Wideband Optical MEMS Interferometer Enabled by Multimode Interference Waveguides

A novel optical MEMS interferometer is proposed based on spatial splitting and combining of optical beams using the imaging properties of multimode interference (MMI) waveguides. The light propagates in air, allowing operation over wide spectral range covering both the infrared and the visible ranges. The optical propagation in the structure is modeled and the interferometer is analyzed by incorporating the modal analysis technique for the waveguides and the angular spectrum approach for free-space propagation. The beam splitter and the overall interferometer are fabricated using deep reactive ion etching technology on silicon-on-insulator wafer. The MMI waveguide sidewalls are aluminum metalized to improve the insertion loss of the interferometer. The fabricated splitter and interferometer are characterized in the visible and near-infrared spectral ranges. The splitter output intensity profile is recorded to verify its wideband proper operation. The interferometer is characterized versus the wavelength and tested as a Fourier transform spectrometer, thanks to a monolithically integrated corner mirror driven by a comb-drive actuator. The spectral resolution obtained is 2.5 nm at 635-nm wavelength.