EFFECT OF SPECKLE ON LIDAR PULSE-PAIR RATIO STATISTICS

The ratio of temporally adjacent lidar pulse returns is commonly used in differential absorption Lidar (DIAL) to reduce correlated noise. Th ese pulses typically are generated at different wavelengths with the a ssumption that the dominant noise is common to hath. This is not the c ase when the mean number of laser speckle integrated per pulse by the lidar receiver is small (namely, less than 10 speckles at each wavelen gth). In this case a large increase in the standard deviation of the r atio data results. We demonstrate this effect both theoretically and e xperimentally. The theoretical value for the expected standard deviati on of the pulse-pair ratio data compares well with the measured values that used a dual CO2 laser-based lidar with a hard target. Pulse aver aging statistics of the pulse-pair data obey the expected sigma(1)/roo t N reduction in the standard deviation, sigma(N), for N-pulse average s. We consider the ratio before average, average before ratio, and log of the ratio before average methods for noise reduction in the lidar equation. The implications of our results are discussed in the context of dual-laser versus single-laser lidar configurations. (C) 1997 Opti cal Society of America.