NCAR'S new Raman-shifted eye-safe aerosol lidar (real)

The design features of, and first observations from, a new eye-safe elastic backscatter lidar system operating at a wavelength of 1.54 μm are presented. Unlike previous lidar transmitters using stimulated Raman scattering in methane, the pump beam is not focused and the cell is injection seeded to improve conversion efficiency and beam quality. The receiver uses custom focusing optics and a 200-μm diameter InGaAs avalanche photodiode (APD). An important achievement was reducing the transmit beam divergence so that it was smaller than the field-of-view (FOV) subtended by the receiver. The first results were obtained by operating the system in a non-eye-safe dual-wavelength mode (1.064 μm and 1.543 μm simultaneously). Single-wavelength eye-safe operation is achieved by the use of a prism to separate and block the 1.064 μm beam before transmitting into the atmosphere. We are currently developing depolarization ratio measurement capability for use when pointing vertical and a beam steering unit for applications requiring scanning. The system is capable of transmitting over 200 mJ/pulse at 10 Hz at 1.543 μm. Examples of backscatter data from vertical and horizontal pointing periods are shown.