Narrowband etalon filters for stratospheric wind measurements

Proof of concept narrow-band etalon filters have been fabricated and characterized for the SWIFT instrument program. The Stratospheric Wind Interferometer For Transport studies is a limb viewing satellite instrument which is intended to measure stratospheric horizontal wind velocities in the altitude range of 20 to 40 km. In addition to providing the atmospheric research community with the first direct measurements of stratospheric dynamics on a global scale, continuous global SWIFT data is expected to improve long range weather forecasting in the troposphere. To isolate the single lines required for the Doppler measurement of the SWIFT instrument, two narrow-band germanium etalon filters centered near 9 micrometer and with 0.8 nm and 2.5 nm bandwidths were fabricated and tested. The SWIFT filter testbed consists of a cryogenic dewar and temperature controller for stabilizing and tuning the filters. The SWIFT filter requirements are discussed, as is the filter testbed design. The measured filter characteristics: transmittance as a function of wavelength, temperature and angle of incidence and tandem filter properties are discussed in the context of satellite instrument requirements.