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Chico PHYS 231 - REAL: 1.5 MICRON WAVELENGTH SCANNING POLARIZATION LIDAR

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REAL: 1.5 MICRON WAVELENGTH SCANNING POLARIZATION LIDARShane D. Mayor, Scott M. Spuler, Bruce M. Morley, Eric Loew,Tammy M. Weckwerth, Stephan De Wekker, and Daniel J. KirshbaumNational Center for Atmospheric ResearchEarth Observing LaboratoryP.O. Box 3000, Boulder, Colorado, 80307-3000, USAABSTRACTSince the last ILRC, NCAR’s Raman-shifted Eye-safeAerosol Lidar (REAL) has been upgraded to featurebackscatter polarization sensitivity and it has been de-ployed in several field experiments. This paper describesthe hardware improvements implemented to enable thepolarization capability and observations of various typesof aerosol plumes released on a military test range. TheREAL was also deployed as part of the NSF sponsoredT-REX experiment in March of 2006 in order to visu-alize the atmospheric flow in the Owens Valley of Cal-ifornia. Examples of data from that experiment will beshown. Lastly, we will briefly describe a second genera-tion REAL (“v2”) that was created by ITT Industries forcontinuous and unattended operation.1. MOTIVATIONREAL[1; 2; 3] operates at 1.54 microns wavelength tocapitalize on the maximum eye-safety in this wavelengthregion. The ability to safely transmit high energy pulsesallows one to generate strong signal-to-noise ratio (SNR)backscatter from long ranges from a single, or very smallnumber, of pulses. Eye-safety facilitates unattended op-eration and use of the system in urban areas or near air-craft and airports. The system is intended for use in ap-plications where high-resolution spatial-imagery of theaerosol distribution is desired. Time-lapse animations ofthe scans can be used to visualize atmospheric motion.2. POLARIZATION CAPABILITYDepolarization measurements with lidar systems are wellknown and have been used to identify particle shapes orphases of water in clouds over the past 30 years.[4] Forexample, it has been demonstrated that spherical parti-cles (i.e. droplets) backscatter linearly polarized laserlight in the same polarization plane.[5; 6] As a particle’sshape deviates from spherical, some of the incident lightis backscattered in other polarization planes. For exam-ple, crystals are highly depolarizing. Therefore, by ex-amining the depolarization of a condensed water cloud,Figure 1. The Raman-shifted Eye-safe Aerosol Lidar(REAL “v1”) resides in a 20-foot shipping container.Beam ReducerDiode Injection SeedTelescope1064Beam Dump AtmosphereAmplifierCollimationLensNeutral DensityFilter1/2 Wave PlateInterference FilterPolarization Beam Splitter CubeFocusingLensAPDAPDAmplifierPerpendicular ChannelParallelChannelDigitizer and Data Acquisition ComputerBeam expanderNd:YAG LaserIsolatorBeam DumpPellin Broca PrismCoating-Free Raman CellBeam Steering UnitFigure 2. System schematic of the Raman-shifted Eye-safe Aerosol Lidar (REAL) using the coating-free Ramancell and the two-channel depolarization receiver.23rd International Laser Radar Conference, Nara, Japan, 24-28 July 2006one may be able to infer its phase (i.e. liquid or ice). Ourintention was to extend this capability to optically thinaerosols. Because REAL is eye-safe and can scan in alldirections, this capability should be of use in the stand-off identification and tracking of small aerosol plumes,especially in urban areas.Obtaining absolute depolarization with a scanning de-vice, like REAL, takes special precautions.[7] Reflectionby the scanning mirrors has the effect of rotating the po-larization vector of the outgoing laser pulse. Upon prop-agation back to the receiver, the polarization vector iscounter-rotated by the same amount. However, if theaerosols have a preferred orientation, or the mirrors havedifferent reflection coefficients, the absolute measureddepolarization will be erroneous. In addition, mirrors im-part a small phase difference between the reflected elec-tric field components. This phase difference transformslinear polarization to elliptical which is not restored backto linear by back-propagation. These angle-dependent er-rors can be minimized with careful attention to the mirrorcoatings. Currently, protected gold coatings are used onthe BSU mirrors. These metallic coatings are inexpen-sive and considered insensitive to polarization, at least toa first order. The above issue must be addressed to makeabsolute depolarization measurements.To implement backscatter depolarization capability onREAL, the polarization purity of the transmit beam wasimproved by placing an optical isolator at the exit of theNd:YAG laser. The optical isolator consists of a Fara-day rotator and two polarizers (each polarizer is a pair ofdouble dielectric Brewsters plates each with an extinctionratio of 10−4.) This reduces the pump energy enteringthe Raman cell by about 20 percent to about 615 mJ perpulse. In addition to improving the polarization purity,the isolator protects the pump laser from potential opti-cal feedback. The improved polarization purity is alsorequired for efficient use of the second generation Ramancell which employs six optical elements at the Brewsterangle. The polarization purity of the beam transmitted tothe atmosphere is well in excess of 10000:1.The lidar receiver was modified to provide two detectionchannels: parallel and perpendicular polarization. Thefirst element of the receiver past the telescope is a focus-ing lens designed to collimate the light for transmissionthrough several subsequent optics. These include a neu-tral density filter, an interference filter, a 1/2 wave plate,and a calcite air-gap Glan Taylor polarization beam split-ter cube. The neutral density filter is part of a filter wheelthat allows the backscatter signal to be attenuated to pre-vent saturation and reduce the risk of damaging the pho-todetectors from hard target reflections.The polarization beam splitter cube has a 25 mm clearaperture. Backscattered light returned from the atmo-sphere in the same polarization plane that was transmit-ted passes through the cube and is focused on the InGaAsAPD on the right side of the diagram (referred to as theparallel channel). The orthogonal polarization state of thebackscattered light is reflected out the side of the polar-ization beam splitter cube at an angle of 109.9 degreesrelative to the axis of the parallel beam. This light isfocused on to a second InGaAs APD at the center andbottom of the diagram (referred to as the perpendicularchannel). The beamsplitter cube has an extinction ratioof 10−6for the transmitted beam. The reflected beam(the


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