Optical Theory for the Advancement of Polarization Lidar

• Main Author: Hayman, Matthew
• Format: Text
• Language: unknown
• Published: CU Scholar 2011
• Subjects: Lidar; Optics; Polarization; Remote Sensing; Electrical and Computer Engineering; Greenland; Kangerlussuaq; Mueller; Summit Camp
• Online Access: https://scholar.colorado.edu/ecen_gradetds/25; https://scholar.colorado.edu/cgi/viewcontent.cgi?article=1024&context=ecen_gradetds

Determining the polarization scattering properties of sub micron size particles through interactions with transmitted visible wavelengths requires the capability to detect polarization effects on the order of a few percent. Such small changes in polarization can easily be overwhelmed by the intrinsic polarization properties of the instrument. When applied to lidar remote sensing techniques, additional environmental factors such as background noise, volume content of scatterers, range to the scatterer, and temporal variations in the scattering medium result in degradation of the instrument's SNR. Furthermore, present approaches in polarization lidar are often confined to measurement of a single parameter which provides no distinction between different scattering and instrumentation polarization effects, limiting the possible interpretations of the measurement. These issues confronting polarization lidar present an opportunity for a novel approach in lidar polarization studies through expansion of system measurement capabilities and instrument performance optimization. In this work, I discuss how these issues may be addressed for the purpose of characterizing particle properties through polarization. Instrument retarding effects are reduced by measuring the optical system Mueller matrix and implementing a hardware polarization compensator which also increases system SNR by improving rejection of the polarized sky noise component. We have developed a calibration algorithm which then removes residual phase shifts, depolarization, and misalignment of transmitter and receiver polarization planes. These techniques are proven through polarization data from atmospheric aerosols measured by the ARCLITE lidar in Kangerlussuaq, Greenland. By recognizing that a scattering phase matrix is a Mueller matrix, the polarization effects of scatterers can be decomposed and described as a combination of depolarizers, retarders, and diattenuators. Furthermore, the polarization attributes of scatterers can be directly related to their physical properties. While it is well established that depolarizing effects can distinguish between thermodynamic phase of tropospheric clouds, diattenuation can be used as an indicator for the presence of horizontally oriented ice crystals which are known to impact Earth's radiative budget. We have developed techniques for making this new and novel polarization measurement in the atmosphere. A NOAA lidar has been designed to detect diattenuation in the troposphere and has begun a campaign to detect oriented scatterers over Summit Camp, Greenland. The lidar was tilted by 11 degrees off zenith in late April 2011 and initial results of this campaign are shown. These results appear promising in demonstrating the lidar's ability to perform this novel measurement for detection of horizontally oriented ice crystals.