High Spectral Resolution Lidar

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Instrumentation

HSRL transmitter

Figure 1:

The HSRL transmitter. The HSRL transmitter employs an injection seeded, frequency doubled Nd:YAG laser. A polarizing cube is used to clean the residual cross polarized component from the laser output. A Pockels cell in the output rotates the polarization of the outgoing laser beam by 90 degrees for alternative laser pulses. The final alignement between the transmitter and receiver polarization axes is performed with a half-waveplate. Fiber optic delay lines inject two pulses of transmitted light directly into the receiver for calibration. The first pulse provides an unfiltered sample of the transmitted pulse. The second pulse is routed through an iodine absorption filter. It is used to frequency lock the transmitter to an iodine absorption line.

HSRL receiver

Figure 2: The HSRL receiver.

A polarizing prism at the output of the telescope separates orthogonally polarized signals between the wide field of view channel (PMT5) and the other channels. Since the polarization of the transmitted laser pulse is rotated by 90 degrees between laser pulses, each channel alternatively receives signals with polarization perpendicular and parallel to the transmitted polarization. The polarization component which is not transmitted to PMT5 is directed through a motorized aperture and then filtered with an interference filter followed an etalon.The 110

rad field of view for the Gieger Mode APD and PMT2 is defined by a hole in the mirror 1. The signal transmitted through the hole is directed to a beam splitter. The signal detected with PMT1 contains information on the total backscattering from aerosols and molecules. The aerosol signal directed through the iodine absorption cell and detected by PMT2 is highly attenuated while the spectral wings of the Doppler-broadened molecular signal are transmitted with little attenuation. Signals from PMT1 and PMT2 are used along with calibration data to derive separate aerosol and molecular lidar returns [1,2,3]. Multiply scattered photons, which have been deflected out of the 110

rad field of view, are reflected to another beam splitter and divided between PMT3 and PMT4 in the same manner as the 110

rad field of view signals. Signals from PMT3, PMT4 and PMT5 are used for particle size and multiple scattering measurements. PMT5 can't be used at the same time as PMT3 and PMT4 because our photon counting data system currently has only 4 channels. The refocusing lens in the telescope output is used to view clouds between altitudes 0.3 km and 2.5 km. Diffractive diffusers in the optical paths in front of PMT2 and PMT4 serve to compensate for the spatial non-uniformity of the high speed photo-tubes used for these channels. Notice that the secondary polarizing beam splitters shown in front of PMT5 and in the lower left hand corner of the figure are not depicted in their correct orientations--the one in front of PMT5 is rotated 90 degrees about the optical axis and the one in the corner directs light out of the page.

HSRL specifications (April 01, 1995)

Next: References Up: HSRL Contents Previous: HSRL measurement principles

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Paivi Piironen
Thu Dec 28 08:53:01 CST 1995