Terahertz photon counting: Large-format SPAD arrays for lidar remote sensing of the atmosphere and ocean from space

Following on the success of the NASA-CNES CALIPSO/CALIOP space-based cloud-aerosol lidar, which is now approaching its 14th year of continuous operation, NASA Langley Research Center has been advancing space-based lidar mission concepts and related technologies to address future cloud, aerosol and ocean science objectives identified in the 2017 Decadal Survey for Earth Science and Applications from Space. Recently, scientists at NASA Langley have been able to use data from CALIOP to retrieve global ocean biomass, which is a measurement that was never originally envisioned for this instrument, and represents a new scientific frontier for space-based lidar. CALIOP's measurements complement ocean color records, extending ocean retrievals into nighttime and to high latitudes, and creating a more complete picture of global ocean biomass. Due to detector bandwidth limitations, however, CALIOP, as with ocean color, only obtains column-averaged measurements that are biased towards the surface. To address this limitation, Politecnico di Milano is developing a lidar detector based on a single photon avalanche diode (SPAD) array. This detector is expected to achieve a depth resolution of <1 m in the ocean, and have a detection efficiency and dynamic range that far exceeds that of the analog-readout, PMT-based receiver on CALIOP. Naturally, these same characteristics will also benefit cloud and aerosol retrievals, making SPAD arrays an excellent detector candidate for future space-based atmosphere and ocean lidar missions. Here, we describe potential advantages of fast (~THz) photon counting SPAD arrays, and how they are being developed for this application.