Two channel receiver design and implementation for a ground based micro-pulse differential absorption LIDAR (DIAL) instrument

Abstract

Current standard water vapor measurement techniques lack the required temporal and spatial resolution needed to further our understanding of the role of water vapor in the Earth's atmosphere. This thesis reports on the continued efforts to bring a cost-effective, autonomous, eye-safe, ground based, micro-pulse differential absorption lidar for the continuous measurement of water vapor into fruition. More specifically, the receiver for this instrument needs a dynamic range of measurement spanning from as close to the Earth's surface as possible, up through the troposphere. Previous reports on this system provide accurate backscatter measurement down to 2 km above the surface. A newly designed receiver has been modeled with the help of Zemax optical design software. It implements a 10% pickoff of the total received light into a second detection channel with a wider field of view. This channel utilizes a free space avalanche photodiode (APD) and has a full angle field of view of ~1 mrad. This channel (the near field) provides accurate backscatter measurement between 600 meters and 5 km in theory. The 90% channel utilizes a fiber coupled APD with a full angle field of view of ~200 microradians. This channel (the far field) has been shown to provide accurate backscatter measurements between 2 km and 12 km. While the near field channel has shown improvement to the overall system, the measured results appear to be accurate down to ~1 km. The results show continuous and autonomous operation with water vapor measurements in close agreement from both detection channels. Further comparisons with radio-sondes provide validation of the water vapor DIAL data product.