Correction for Nonlinear Photon Counting Effects in Lidar Systems

Abstract

Photomultiplier tubes (PMTs) employed in the photon counting (PC) mode of operation are widely used as detectors in lidar systems. Their use, however is restricted mainly to conditions where the signal levels are relatively low and the responses of the PMT and the pulse discrimination/counting system are linear. It is well known that in this low light level regime, where the signal-to-noise (S/N) levels approach unity, the inherent S/N for photon counting is better than that obtained from other detection techniques. A problem encountered with such systems however is the rapid degradation of performance as the signal levels increase. In such situations the photon counting response becomes non-linear and the output count value is no longer proportional to the incident light intensity. In our laboratory we have developed a versatile Nd:YAG lidar which is used for measurements of both the middle atmosphere and the troposphere. [1] With this system we encounter a very wide range of signal levels ranging from the extremely weak signals from the top of the mesosphere to the very strong returns from low level clouds.