We describe the design and characterization of a compact dual-polarization lidar that uses a liquid crystal variable retarder (LCVR) to discriminate between backscattered polarization states on alternate laser pulses (at 30 Hz). Measurements of the polarization discrimination of the system, including the liquid crystal and a Schmidt-Cassegrain receiver telescope, show that depolarization ratios can be determined with an additive error of less than 0.4%. The source is a Nd:YAG laser with a wavelength of 532 nm, pulse energy of 118 mJ, and pulse-repetition frequency of 30 Hz. The normal operating range is 15 km, with a 1.5-m range resolution. The full-angle receiver field of view is variable up to 8.8 mrad. Sample data from atmospheric clouds demonstrate the use of lidar depolarization measurements for distinguishing between ice and liquid water in thin clouds with low multiple scattering (with cloud phase verified using radiosonde profiles of atmospheric temperature and humidity). Also shown is a lidar observation of a depolarizing layer over Bozeman, Montana, identified as subvisual cirrus, aerosols transported from in or near China, or a combination thereof.
A polarization-sensitive lidar was used to detect honeybees trained to locate buried landmines by smell. Lidar measurements of bee location agree reasonably well with maps of chemical plume strength and bee density determined by visual and video counts, indicating that the bees are preferentially located near the explosives and that the lidar identifies the locations of higher bee concentration. The co-polarized lidar backscatter signal is more effective than the cross-polarized signal for bee detection. Laboratory measurements show that the depolarization ratio of scattered light is near zero for bee wings and up to approximately thirty percent for bee bodies.
Information on local cloud coverage, with high spatial and temporal resolution, is useful for studying how the radiative properties of clouds affect the climate. The resolution of a lidar allows for detection of subvisual cloud and aerosol layers, and for determining particle sizes of the scatterers. A cloud lidar sensitive to polarization can distinguish between ice and water in clouds, since ice crystals are more depolarizing than water droplets. Cloud lidars complement either ground-based or space-based cloud imagers by supplying the missing vertical dimension. This paper describes the design and characterization of a lidar system for the direct detection of clouds, using a liquid crystal to discriminate between backscattered polarization states on alternate laser pulses (at 30 Hz). The source is a Nd:YAG laser at a wavelength of 532 nm and with pulse energies of 118 mJ. The system is designed to be compact and robust enough for transport and deployment. Data presented show the lidar system is capable of detecting clouds up to 9.5 km above ground level (the normal operating range is 15 km) with a 1.5 m range resolution. The receiver field of view is conveniently variable up to 8.8 mrad. Daytime operation is possible, thanks to laser-line interference filters and a gated photomultiplier tube. Polarization discrimination is sufficient to measure depolarization ratios with an additive error of less than 0.4%.