Advanced SAPHIRA HgCdTe APD Arrays for NASA Space Lidar Applications

In its 2017 Earth Science Decadal Survey report, the National Academy of Sciences recommended that NASA conduct missions to acquire data for nine Explorer- and Incubation-class observables. A next generation lidar system being developed at Goddard Space Flight Center, the Concurrent Artificially-intelligent Spectrometry and Adaptive Lidar System (CASALS), will be able to acquire data for five of these nine: terrestrial ecosystem structure, ice elevation, snow depth and water equivalent, surface topography and vegetation structure, and the surface-atmosphere boundary layer. The CASALS lidar will have a fast wavelength scanning transmitter which can position the laser beam at 960 footprint locations scanned across a 5.8 km wide swath. The 960 footprint locations on the ground will be mapped to an HgCdTe APD array detector in the lidar receiver. At any time, the lidar can arbitrarily point at tens to hundreds of the 960 possible footprint locations by wavelength tuning. At the receiver, to cover all 960 locations, a large linear detector array with close to 100% fill factor is required. We are demonstrating the scanning concept using a linear APD array developed by DRS in collaboration with Goddard. However, its small number of pixels (30 in the current device) and small fill factor (14%) are insufficient for full implementation of the wide-swath mapping. The detector capability is CASALS's limiting performance factor. To overcome this limitation, we propose a planar linear array HgCdTe APD detector with 100% fill factor. This APD array will have a large array size (320 pixels), high APD gain (>300), high speed (~350MHz), low dark current (<1fA/pixel/rtHz) and high quantum efficiency (~70%). The final packaging of the device will have a matched transimpedance amplifier array, one per detector cell, inside the silicon fan-out underneath the HgCdTe APD chip. The amplifier will have bandwidth matching that of the HgCdTe APD with low noise (~1.5 pA/rtHz). The proposed device is based on the Leonardo SAPHIRA HgCdTe 320x256 planar APD imaging array developed for astronomy. Our initial test results under ESTO QRS-17 have demonstrated performance with very low dark currents, 1ns pulse response time and greater than 70% quantum efficiency in the near infrared spectral range. This detector will provide a significant performance advance for future NASA space lidar, including the CASALS 3D imaging lidar and CO2 and CH4 trace-gas lidar for Earth and planetary remote sensing, as well as hazard avoidance imaging lidar for NASA landers. We propose a three-year work program with the first two years concentrating on assessing, adapting and optimizing the Leonardo detector technology development. In Year 3, we will assess the performance of a delivered detector array at Goddard and demonstrate its performance with the CASALS instrument. Our entry TRL is estimated to be at 2 and we expect to exit at TRL 4.

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