Laser speckle effects on hard target differential absorption lidar

摘要

Reflection of laser light from a diffuse surface exhibits a complex interference pattern known as laser speckle. Measurement of the reflected intensity from remote targets, common to ''hard-target'' differential absorption lidar (DIAL) requires consideration of the statistical properties of the reflected light. The authors have explored the effects of laser speckle on the noise statistics for CO(sub 2) DIAL. For an ensemble of independent speckle patterns it is predicted that the variance for the measured intensity is inversely proportional to the number of speckle measured. They have used a rotating drum target to obtain a large number of independent speckle and have measured the predicted decrease in the variance after correlations due to system drifts were accounted for. Measurements have been made using both circular and linear polarized light. These measurements show a slight improvement in return signal statistics when circular polarization is used. The authors have conducted experiments at close range to isolate speckle phenomena from other phenomena, such as atmospheric turbulence and platform motion thus allowing them to gain a full understanding of speckle. They have also studied how to remove correlation in the data due to albedo inhomogeneities producing a more statistically independent ensemble of speckle patterns. They find that some types of correlation are difficult to remove from the data.