All‐Silicon Double‐Cavity Fourier‐Transform Infrared Spectrometer On‐Chip

Abstract It is expected that miniaturized infrared spectrometers will enable spectral sensing anywhere. Their applications are almost infinite, including in smart devices, monitoring of air and water quality, healthcare wearables, among many others. Free‐space on‐chip, or photonic MEMS, interferometers are an attractive solution since they provide the required miniaturization while achieving the optical throughput needed for device sensitivity. A Fourier‐transform spectrometer on‐chip is presented based on cascading silicon broadband photonic MEMS cavities. The double‐cavity configuration shrinks the size of the optical engine dramatically with respect to the Michelson configuration, while overcoming the need for bringing the micromirrors in physical contact as in case of a single cavity. The all‐silicon interfaces provide low enough reflectivity such that the response of the cavity can be approximated by a single harmonic allowing Fourier transform operation of the spectrometer. The spectrometer is fabricated using deep etching of a silicon‐on‐insulator substrate allowing release of movable elements. The optical engine size is 300 µm × 400 µm. The device is tested in the wavelength range of 1.1–2.1 µm and spectroscopy measurement of different materials with 7.5 nm spectral resolution is demonstrated and compared to a reference spectrometer.