Large-Area MEMS Tunable Fabry–Perot Filters for Multi/Hyperspectral Infrared Imaging

This paper reports on a MEMS tunable Fabry-Perot filter technology capable of achieving nanometer-scale optical flatness across a large mirror area of up to square centimeters without any extraneous stress management techniques. The device employs a single-layer tensile silicon or germanium membrane for the suspended top mirror. Optical characterization of the fabricated single-membrane-based tunable filters for the SWIR, MWIR, and LWIR is presented. The fabricated 1000-μm dimension Si-membrane-based SWIR and MWIR filters are demonstrated with a wavelength tuning range of 1.77-2.42 and 4.1-4.9 μm, respectively, while the fabricated 200-μm-dimension Ge-membrane-based LWIR filter is demonstrated with a wavelength tuning range of 8.5-11.46 μm. All these filters are shown to achieve transmission characteristics that exceed the optical requirements for multispectral imaging applications. A large-area 1-cm dimension Si membrane-based SWIR tunable Fabry-Perot filter for multispectral imaging is demonstrated as a proof-of-concept, showing an excellent surface flatness in the order of 25 nm and an excellent optical uniformity with transmission peak wavelength variability less than 3% across the entire 1-cm dimension optical imaging area. In addition, the optical transmission behavior of the Fabry-Perot filters based on three-layer Si or Ge-based air-spaced DBRs for SWIR, MWIR, and LWIR is modeled, demonstrating that these filters can achieve a fine spectral resolution of several tens of nanometers suitable for hyperspectral imaging applications.