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The HSRL incorporates a continuously-pumped, injection-seeded, Q-switched, frequency doubled Nd:YAG laser, operating at 4 kHz, to direct pulse of coherent light in the near-zenith direction. The beam is offset 4 degrees from the vertical to prevent specular reflection from preferentially oriented crystals in cirrus ice clouds. A Pockels cell rotates successive pulses by 90 degrees for polarization measurements.
The output frequency is locked to an iodine absorption line, which helps to prevent seed-laser drift and provides an output pulse that is spectrally tuned to the bandpass of the detection system. The pulse energy is monitored on a shot-to-shot basis to normalize the return signal.
Proper alignment is required to ensure that the transmitter and receiver field of views are coincident. A pair of servo-motors in the surface plane control the transmitted pulse direction, such that the transmitter and receiver are properly aligned before data acquisition. This is achieved by maximizing the return signal as a function of transmitter direction in the x- and y-planes.