Applications of GaAs quantum well infrared photoconductors at the NASA/Goddard Space Flight Center

Prior to 1990 large format, hybridized GaAs quantum well infrared photoconductors (QWIP) arrays had not been fabricated. In a unique collaboration between the Goddard Space Flight Center, ATT/Bell Labs and the Rockwell Science Center we designed and built a 128 x 128 element GaAs/AlGaAs QWIP array, developed a continuous flow liquid helium cooler/camera system and successfully performed airborne imaging test flights over the Wallops Flight Facility. As a result of work performed at Lockheed Martin of Dallas, Texas a 256 x 256 QWIP was developed and Goddard obtained a unit for a second-generation camera system. A more compact imaging system was developed which could operate with a liquid nitrogen dewar. This instrument was used as part of the joint NASA/Southern African Fire-Atmosphere Research Initiative campaign to gather airborne terrestrial science information. This system was also used to evaluate the emission/reflectance properties of commercially available pyrgeometers and pyranometers. These radiometers rely on constants for the filter dome and the detector which are key in making absolute measurements. In an effort to improve the optical coupling efficiency and also to develop QWIPs with sensitivity beyond 11 mum a partnership has been established between Goddard and the Army Research Laboratory (ARL). By corrugating the surface of the QWIP array, normal incidence radiation is deflected almost horizontally allowing for absorption in the quantum wells. Most recently, a joint Goddard/Jet Propulsion Laboratory/ARL proposal to develop a four band, hyperspectral, 512 x 640 QWIP array has been funded by the Earth Science Technology Office of NASA. This three year effort will focus on expanding the integrated QWIP spectral imaging capability, utilizing a new Sunpower, Inc. cryocooler and developing a linear variable etalon to produce the hyperspectral image from 3 to 15.4 mum. The four IR bands are (1) 3-5 mum, (2) 8.5-10 mum, (3) 10-12 mum and (4) 14-15.4 mum. These four bands are spectrally subdivided into approximately 200 bands. This paper will discuss these efforts in general with an emphasis on the system performance. (C) 2001 Published by Elsevier Science B.V.