Thermal Design, Test, and Lessons Learned for a Thermally-Tuned Optical Component for the ICESat-2 Mission

Abstract

This paper describes the thermal design and ETU thermal vacuum test performed for the ATLAS Optical Filtering Module (OFM), as well as lessons learned in the design of this temperature-sensitive optics application. The OFM is used to remove background solar radiation from the signal and to filter the desired 532nm wavelength of light to the detectors of NASA’s Advanced Topographic Laser Altimeter System (ATLAS), which will be flown as the sole instrument aboard the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2). ATLAS will be used to take measurements of topography and ice thickness for Arctic and Antarctic regions, providing crucial data used to predict future changes in worldwide sea levels. The set of identical OFM’s must filter the wavelength of received light with high precision, within +/-3 picometers of the desired wavelength. The OFM must also be tunable to other wavelengths; this is achieved through thermal tuning. The coating on the etalon optic in the middle of the OFM is temperature sensitive, where the index of refraction changes slightly with temperature, such that the wavelength of light that passes through can be determined by the temperature of the etalon. Because of this thermal tuning design requirement, the OFM must have tight temperature control, within +/0.2°C orbit average temperature at the thermal interface. In addition to discussing the results of the ETU test, in which etalon transmission was measured and performance verified as a function of temperature, this paper also describes the unique design challenges encountered in controlling optics under such tight thermal requirements.