Heterodyne Doppler 1-microm lidar measurement of reduced effective telescope aperture due to atmospheric turbulence.

We performed an experimental study on the effect of atmospheric turbulence on heterodyne and direct detection lidar at 1 microm, employing a pulsed Nd:YAG bistatic focused beam lidar that permitted simultaneous heterodyne and direct detection of the same lidar returns. The average carrier-to-noise ratio and statistical fluctuation level in the lidar return signals were measured in various experimental and atmospheric conditions. The results showed that atmospheric turbulence could reduce the effective receiver telescope diameter of the l-microm heterodyne lidar to <5cm at a relatively short range of approximately 450 m near the ground. The observed effective telescope aperture and heterodyne detection efficiency varied during the day as the atmospheric turbulence level changed. At this time, we are not able to compare our experimental lidar data to a rigorous atmospheric turbulence and lidar detection theory which includes independently variable transmitter, receiver, and detector geometry. It is interesting to note, however, that the observed limitation of the effective receiver aperture was similar in functional form with those predictions based on the heterodyne wavefront detection theory by D. L. Fried [Proc. IEEE 55, 57-67 (1967)] and the heterodyne lidar detection theory for a fixed monostatic system by S. F. Clifford and S. Wandzura [Appl. Opt. 20, 514-516 (1981)]. We have also applied such an effective receiver aperture limitation to predict the system performance for a heterodyne Ho lidar operating at 2 microm.