Design and fabrication of short and middle wavelength infrared dual band-pass filter at cryogenic temperature

Dual band-pass filter can simultaneously form two spectral channels to transmit at any position of the element, so as to realize simultaneous detection of dual spectral channels. In this paper, an infrared dual band-pass filter used at 100 K temperature was developed. Sapphire (Al₂O₃) was used as substrate, and Ge and SiO were used as high (H) and low (L) refractive index thin films respectively. An infrared dual band-pass filter combined with a shorter wavelength channel (2.60-2.85 μm) and a longer wavelength channel (4.10-4.40 μm) was designed and fabricated. Based on Fabre-Perot (F-P) filter structure, Ge and SiO thin films were deposited by electron beam evaporation and resistance thermal evaporation on the two sides of the substrate. At the working temperature (100 K), the filter transmittance of shorter channel is 91.2%, and the top ripple amplitude is 2.1%; the average transmittance of longer channel is 87.7%, and the top ripple amplitude is 3.8%. Between the two channels (wavelength 3.00-3.95 μm), the cut-off depth is less than 0.1%. The optical performance of the infrared dual band-pass filter can meet the spectral requirements and contribute to more accurate infrared remote sensing and detection.