Longwave infrared snapshot spectrometer for chemical vapor identification

Snapshot spectrometers are a class of optical sensors in which the spectral resolution element is coupled to a camera focal plane detector array, and all the information required for the measurement of the spectrum is acquired in a single camera image. This work presents the development and characterization of a longwave infrared (LWIR) snapshot spectrometer that has been applied to the identification of chemical vapors. The spectral resolution of the sensor is provided by a 24×32 element array of Fabry-Perot filters. Complementary metal oxide semiconductor (CMOS) fabrication methods were used to create the filter array. Gray scale lithography was used to pattern a germanium layer to form Fabry-Perot filters with different cavity spacings that transmit different wavelength bands. The fabricated filter had an operational wavelength range from 7.8 to 11.2 µm. The process window for the wafer fabrication achieved 315 filter elements at 63 different center wavelengths. Details of the fabrication and characterization of the snapshot filter are presented. The application of the snapshot spectrometer for the identification and discrimination of chemical vapors is demonstrated for 1,1-difluoroethane, methanol, and sulfur hexafluoride. The concentration-path length limit of detection for these gases was 1.7, 3.1, and 0.4 mg/m², respectively.