Lidar Measurements Of Clouds And Aerosols Using AlGaAs Lasers Modulated With Pseudorandom Codes

AlGaAs lasers provide an optical source for lidar transmitters which is small, lightweight, and efficient. When compared to monopulse lidar transmitters, they are limited in peak power, but are capable of high average transmitted power. Atmospheric lidar profiles have been measured using a quasi-CW transmitted signal by means of pseudorandom-noise ( F" ) amplitude modulation [1,2]. A PN code modulated laser beam is transmitted through the atmosphere and a small fraction, less than one photon per bit, of the transmitted siganl is backscattered to the receiver. The receiver detects these photons and accumulates a photoelectron count over many repetitions of the code. This accumulated count is a histogram of counts versus time delay (expressed as number of bits). On average, the histogram represents the convolution of the atmospheric backscatter function with the transmitted code. The correlation properties of the PN code permit recovery of the atmospheric response by correlation of the histogram with the transmitted bit sequence.