Coherent Lidar Measurement of Range-Resolved Tropospheric Water Vapor Concentration and Backscattered Signal Statistics

The sensitivity advantages of coherent detection can potentially be employed to make range-resolved differential absorption lidar (DIAL) measurements of atmospheric species even when transmit pulse energies are relatively low. By operating in a shot-noise-limited mode, coherent lidars often obtain average carrier-to-noise ratios (CNR's) as much as 30 dB higher than would be measured with equivalent direct-detection systems. This apparent advantage in average CNR is at least partially offset by the characteristically large fluctuation in the instantaneous received irradiance due to target speckle effects.1 To measure mean backscattered-signal irradiance accurately, the variance due to these fluctuations must be reduced to an acceptable level by averaging.