Advanced Silicon Avalanche Photodiodes on NASA’s Global Ecosystem Dynamics Investigation (GEDI) Mission

摘要

Silicon Avalanche Photodiodes (APDs) are used in NASA’s Global Ecosystem Dynamics Investigation (GEDI) that waslaunched in December 2018 and is currently measuring the Earth’s vegetation vertical structure from the InternationalSpace Station. The APDs were specially made for space lidar with a much lower hole-to-electron ionization coefficientratio (k-factor ～0.008) than that of commercially available silicon APDs in order to reduce the APD excess noise. A siliconheater resistor was used under the APD chip to heat the device to 70°C to improve its quantum efficiency at the1064-nm laser wavelength while maintaining a low dark current such that the overall signal to noise ratio is optimized.Special APD protection circuits were included to raise the overload damage threshold to prevent device damage fromstrong laser returns from specular surfaces, such as still water bodies, and space radiation events. The APD and a hybridtransimpedance amplifier circuit were hermetically sealed in a TO-8 type metal package with a sufficiently low leak rateto ensure a multi-year operation lifetime in space. The detector assemblies underwent a series of pre-launch tests perNASA Goddard Environmental Verification Standard for space qualification. The APDs have performed exactly asexpected in space. A detailed description of the GEDI detector design, signal and test results are presented in this paper.