Daytime photon counting lidar prototype in the Fraunhofer lines

摘要

Lidar detects atmospheric parameters by transmitting laser pulse to the atmosphere and receiving the backscattering signals from molecules and aerosol particles. Because of the small backscattering cross section, lidar usually uses the high sensitive photomultiplier and avalanche photodiode as detector and uses photon counting technology for collection of weak backscatter signals. Photon Counting enables the capturing of extremely weak lidar return from long distance throughout dark background by a long time accumulation. Because of the strong solar background, the signal-to-noise ratio of lidar in the daytime could be greatly restricted, especially for the lidar operating at visible band where solar background is prominent. Narrow band-pass filters must therefore be installed in order to isolate solar background noise at wavelengths close to that of the lidar receiving channel, whereas the background light in superposition with signal spectrum limits an effective margin for SNR improvement. This work propose a high spectral resolution lidar prototype operating at the Fraunhofer lines, the invisible band of solar spectrum, to achieve photon counting under intense solar background. The laser transmitter is an optical parametric oscillator pump by 355 run Nd:YAG laser and is tuned to the dark line by the Magnesium atom of the solar atmosphere by which nearly 90% of solar radiation is absorbed. The prototype provides some theoretical references for optimized design of photon counting lidar in visible band with the comparable SNR at night, which may contribute to the research on diurnal variation of atmosphere and to the operational observation of lidar.