Polarization Ratios

1. DIFFERENTIAL POLARIZATION REFLECTIVITY AT 1.574 µm EYE-SAFE BACKSCATTER LIDAR • Javier Fochesatto, Kenneth Sassen and Richard L. Collins Atmospheric Sciences Group. Geophysical Institute University of Alaska Fairbanks Abstract A preliminary analysis of differential polarization reflectivity lidar signals is reported using a novel Eye-Safe Backscatter Lidar. This lidar has polarization diverse at 1.574 µm. The instrument is based on a previously designed, single-channel Compact Eye-Safe Backscatter Lidar (CESBL) assembled in a compact optical bench. The instrument was upgraded for polarization selectivity in the laser emission. A full Stokes analysis will be performed by adding linear polarization discrimination in the receiver unit. The instrument has computer controlled polarization selectivity in the laser emission and is able to perform sequential measurements in different polarization modes including both linear and circular polarization. The lidar design focuses on high accuracy polarization analysis, better than 60 dB, in polarization emission purity and polarization discrimination accuracy over 50 dB in the receiver. These features allow depolarization ratios and polarimetric parameters retrieval of ~0.1%. In this article we present the application of this instrument to perform differential polarization reflectivity analysis. Differential Polarization Analysis • Experimental Setup: • Laser Emission is setup sequentially in two linear polarization modes Vertically and Horizontal Polarization accuracy in the emission is ~ 60 db • Single channel receiver with non-polarization discrimination • PV & PH are backscatter power for the emission at both linear polarization states • ZDR I & II are the Polarization Differential Reflectivity based in PV and PH backscatter Fields Laser Emission Backscatter Signal Note: No polarization discrimination is indicated in the reception Polarization Diversity Lidar & Polarization Ratios - Background • Sij contain information about the size distribution, shape and refractive index of the scatters • For an anisotropic distribution of aerosols, the backscattering matrix is simplified. For spherical particles, only 4 independent matrix elements are needed, because spherically symmetrical particles (e.g., cloud droplets) do not produce any change in the backscattered polarization state in single scattering • Scattering by randomly oriented aerosols Sij simplifies into 10 unknown coefficients for each scattering angle. • Scattering matrix simplification is generally considered in well known cases, but potentially the 16 elements are necessary to investigate certain environments involving non-randomly oriented non-spherical particles. Scattering Matrix Differential Polarization Reflectivity Polarization Differential Reflectivity's ZDRII & ZDRI Backscatter Power for Vertical Polarization Emission Polarization Ratios Full Scattering Matrix S (4x4) defined by the optical properties of the atmospheric layer under interrogation by the laser Linear Polarization (0, 90, 45, -45) and Circular (right and left) in the Laser Emission Mueller Matrix Backscatter Power for Horizontal Polarization Emission L (Linear Polarization), CR( Circular Right), CL (Circular Left) K (instrumental constant) L,L (Linear Emission, Linear Reception) • Summary • Differential Reflectivity allows calculation of polarization ratios to investigate the scattering matrices related to microphysical aerosol and cloud processes • In this particular experiment a unique channel is used in the receiver, therefore no instrumental constant is needed to process the signal • Extinction correction at 1574 nm is very low when compares with visible and UV wavelengths • Improvements towards full Stokes polarization diversity analysis are on-going • Comparison with visible and IR polarization lidars are on-going activities • Further investigations will be conduced to study the application of polarization diversity for aerosol and cloud assessment. • Despite having the same Stokes parameters relationship (Δ and δ-Linear Depolarization Ratio); Δexhibits higher values than linear depolarization measurements at visible wavelength (if compares with AFARS-Ruby collocated data) GI/UAF EYE-SAFE LIDAR FOR AEROSOL ASSESSMENT Prototype Eye-Safe Lidar working at the Laser Research Laboratory, Poker Flat Research Range, December 2004 Compact Eye-Safe Backscatter Lidar Geophysical Institute/UAF campus June, 2005. Eye–Safe Polarization Diversity Lidar February 2006 Acknowledgements We gratefully acknowledge the financial support of Johns Hopkins University, Applied Physics Laboratory, UPOS program and the NSF-MASINT program.