Abstract

Due to the many potential applications of infrared lidar to measurements of gaseous and aerosol composition in various atmospheric regions, the influence of diffuse (optically rough) topographic targets on the return signal intensity fluctuations must be well characterized. Often topographic targets are the most convenient means of providing a backscattered signal when measurements are to be made over an atmospheric path. Yet the far-field speckle pattern which is produced when a rough surface is illuminated by coherent radiation produces intensity fluctuation effects which are not always well understood and are often difficult to separate from effects of atmospheric turbulence or transmitter beam jitter. The statistics of fully developed speckle patterns are well understood [1,2]. The reduction of the variance in detected intensity through aperture averaging is straightforward [1,3] and has been employed in many DIAL systems which use direct detection [4]. For coherent detection systems, aperture average is not an option, and one would normally expect to observe the effects of fully developed speckle when using topographic targets. However, when short-pulse laser transmitters are used, there are cases for which the coherence length of the radiation is not large compared to the depth variation of the ensemble of scatterers which provide the backscatter. In such cases, the statistical properties of the lidar signal change significantly.