Patent Publication Number: US-8115925-B1

Title: Polarization switching lidar device and method

Description:
This application is a divisional application of the U.S. patent application Ser. No. 12/693,172 filed on 25 Jan. 2010, now U.S. Pat. No. 8,054,464 which is incorporated here by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to fixed and scanning lidar systems, and in particular to a polarization switching lidar device for remote detection and characterization of airborne aggregation of particulates. 
     BACKGROUND OF THE INVENTION 
     Many diverse applications may benefit from an effective remote detection and characterization of airborne aggregations of particulates. For example, climate change studies have shown that cloud effects and aerosol-cloud interactions (i.e. aerosol, indirect effects) are among the largest uncertainties in simulations of climate change. Elastic backscatter lidars are highly sensitive instruments capable of providing profiles of clouds, aerosols, and other particulates aggregation structures within the atmosphere. An addition of polarization-sensitive detection provides information pertaining to the phase of cloud particulates and to the type of aerosol particulates. The U.S. DOE Atmospheric Radiation Measurements (ARM) Program has deployed eye-safe lidars for semi-autonomous operation at each of its climate research facilities for over a decade. Recently, polarization-sensitive lidar systems have been deployed by ARM through straightforward modifications of pre-existing designs and equipment. 
     Remote detection and stand-off characterization of chemical/biological agents may be a decisive factor in early warning chemical/biological systems allowing for improved survivability of personnel in the battlefield and/or other targeted or associated areas. One exemplary system incorporating a pulsed lidar operating using visible light is described by Lee, et al, “Micro Pulse lidar for Aerosol &amp; Cloud Measurement”, Advances in Atmospheric Remote Sensing with lidar, pp. 7-10, A. Ansmann, Ed., Springer Verlag, Berlin, 1997, while a near IR, is described, for example, by Condatore, et al, “U.S. Army Soldier and Biological Chemical Command Counter Proliferation Long Range—Biological Standoff Detection System (CP LR BSDS)”, Proceedings of SPIE, Vol. 3707, 1999. the entire contents of which are incorporated herein by reference, have demonstrated the high sensitivity and long-range (up to 50 km) capability to detect aerosol clouds. Consequently, an aerosol lidar is a demonstrated technique for long-range detection and characterization of bio-warfare aerosols. Furthermore, similar lidar systems may be used for remote sensing and stand-off detection of air polluting aggregations of particulates generated by intentional commercial activities or accidentally released particulate aggregations. 
     The polarization switching lidar devices for remote detection and characterization of airborne aggregation of particulates in accordance with the present invention are essentially sensitive to the polarization relative to a predetermined plane of polarization. Therefore, any phase retardation that contributes to the same relative angle of polarization with respect to the predetermined plain of polarization cannot be resolved and are considered identical. More particularly, all phase retardation states having phase differences (“retardations”) Δφ=nπ radians (n=0, ±1, ±2, ±3 . . . ) are considered substantially equal and inclusively designated as a “zero retardation state”, while all phase retardation states having phase differences (“retardations”) Δφ=mπ/2 radians (m=±1, ±3, ±5 . . . ) are considered substantially equal and inclusively designated as a “quarter-wave retardation state” for the purposes of the further recitations. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a polarization switching lidar device for remote detection and characterization of at least one airborne aggregation of particulates including a source of a polarized pulsed laser light beam; a direction controlling mirror arranged to reflect the polarized pulsed laser light beam and redirect the reflected polarized pulsed laser light beam; a polarizing beam splitter arranged to intersect the reflected polarized pulsed laser light beam and to redirect a portion of the reflected polarized pulsed laser light beam; an actively controlled retarder arranged to intersect the redirected portion of the reflected polarized pulsed laser light beam, and to be controllably alternated between a zero retardation state and a quarter-wave retardation state such that the transmitted portion of the polarized pulsed laser light beam exiting the actively controlled retarder is linearly polarized in a predetermined direction when the actively controlled retarder is in the zero retardation state, while the transmitted portion of the polarized pulsed laser light beam exiting the actively controlled retarder is circularly polarized in a predetermined rotational sense when the actively controlled retarder is in the quarter-wave retardation state; a directable (i.e. arranged to be Manually or automatically specially oriented in at least one direction of interest in order to observe a predetermined sets of space angles) telescoping assembly arranged to intersect the transmitted portion of the polarized pulsed laser light beam exiting the actively controlled retarder and to controllably redirect the intersected polarized pulsed laser light beam into a predetermined space angle while collecting at least a portion of depolarized backscattered photons from the scanned polarized pulse laser light beam backscattered by the at least one airborne aggregations of particulates, and to redirect the collected portion of depolarized backscattered photons onto the polarizing beam splitter; an optical matcher arranged to collect a fraction of backscattered photons exiting the polarizing beam splitter and focus the collected fraction of depolarized backscattered photons onto a photodetector arranged to generate at least one electronic signal proportional to the collected portion of depolarized backscattered photons. 
     In addition, another apparatus embodying the present invention incorporates a polarization switching lidar device for remote detection and characterization of at least one airborne aggregation of particulates including a source of a polarized pulsed laser light beam; a first actively controlled retarder arranged to intersect the polarized pulsed laser light beam, to transmit a portion of the polarized pulsed laser light beam, and to be controllably alternated between a zero retardation state and a quarter-wave retardation state such that the transmitted portion of the polarized pulsed laser light beam exiting the first actively controlled retarder  150  is linearly polarized in a predetermined direction when the actively controlled retarder is in the zero retardation state, while the transmitted portion of the polarized pulsed laser light beam exiting the first actively controlled retarder is circularly polarized in a predetermined rotational sense when the first actively controlled retarder is in the quarter-wave retardation state; a first directable telescoping assembly arranged to intersect the transmitted portion of the polarized pulsed laser light beam exiting the first actively controlled retarder and to controllably redirect the intersected polarized pulsed laser light beam into a predetermined space angle; a second directable telescoping assembly arranged to collect at least a portion of backscattered photons from the scanned polarized pulse laser light beam backscattered by the at least one airborne aggregations of particulates, and to redirect the collected portion of backscattered photons along a detection optical path; a second actively controlled retarder arranged along the detection optical path to intersect the collected portion of backscattered photons, to transmit a fraction of the collected portion of backscattered photons, and to be controllably alternated between a zero retardation state and a quarter-wave retardation state such that the transmitted fraction of the collected portion of backscattered photons exiting the second actively controlled retarder is linearly polarized when the first actively controlled retarder is in the zero retardation state, while the transmitted fraction of the collected portion of backscattered photons exiting the second actively controlled retarder is linearly polarized in a direction perpendicular to the predetermined direction when the first actively controlled retarder is in the quarter-wave retardation state; a polarizer arranged to intersect the backscattered photons exiting the second actively controlled retarder and to selectively transmit only a part of the backscattered photons exiting the second actively controlled retarder which is linearly polarized in the direction perpendicular to the predetermined direction; an optical matcher arranged to collect the transmitted part of the backscattered photons exiting the polarizer and to focus the transmitted part of the backscattered photons exiting the polarizer onto a photodetector arranged to generate at least one electronic signal proportional to the collected part of the backscattered photons exiting the polarizer which is linearly polarized in the direction perpendicular to the predetermined direction. 
     Furthermore, a method embodying the present invention includes steps of generating a linearly polarized pulsed laser light beam having a predetermined direction of linear polarization; using at least one the actively controlled retarder, sequentially controllably switching a polarization state of the polarized pulsed laser light beam between a circularly polarized state polarized into a predetermined rotational sense, when the at least one actively controlled retarder is controllably switched into a quarter-wave retardation state, and into a linearly polarized state linearly polarized in a direction substantially equivalent to the predefined direction of linear polarization when the at least one actively controlled retarder is controllably switched into a zero retardation state such that a transmitted portion of polarized pulsed laser light beam exiting the at least one actively controlled retarder is linearly polarized in the predetermined polarization direction when the at least one actively controlled retarder is in the zero retardation state while the transmitted portion of polarized pulsed laser light beam exiting the at least one actively controlled retarder is circularly polarized having the predetermined rotational sense when the at least one actively controlled retarder is in the quarter-wave retardation state; scanning the transmitted portion of polarized pulsed laser light beam exiting the actively controlled retarder by at least one telescoping assembly arranged to intersect the transmitted portion of polarized pulsed laser light beam exiting the actively controlled retarder and controllably redirect the intersected polarized pulsed laser light beam into a predetermined space angle toward at least one airborne aggregation of particulates; backscattering the scanned transmitted portion of the polarized pulsed laser light beam sequentially polarized into the circularly polarized state having the predetermined rotational sense and into the linearly polarized state having the predetermined polarization direction so that a fraction of photons scatters back from at least one airborne aggregation of Particulates such that the photons in circularly polarized state having the predetermined rotational sense acquire an opposite rotational sense of circular polarization, while at least a fraction of photons polarized in the predetermined linearly polarized state, when scattered back, acquires a linear polarization state polarized in an orthogonal direction relative to the predetermined direction of linear polarization; collecting photons from the predetermined space angle and redirecting the collected photons onto the actively controlled retarder by the at least one telescoping assembly; sequentially converting by the actively controlled retarder in the quarter-wave retardation state the collected backscattered photons having the circularly polarized state polarized in the opposite rotational sense relative to the rotational sense of the predetermined sense of circular polarization into the linearly polarized state having the polarization state polarized in an orthogonal direction relative to the predetermined direction of linear polarization, while transmitting the scattered back photons polarized in the linear polarization state having orthogonal direction of linear polarization relative to the predetermined direction of linear polarization when the actively controlled retarder in the zero retardation state, and redirecting the converted photons onto the polarizing beam splitter; selectively separating the converted photons collected sequentially onto the polarizing beam splitter by transmitting only the linearly polarized photons having orthogonal direction of linear polarization relative to the predetermined-direction of linear polarization; optically matching the transmitted linearly polarized photons having orthogonal direction of linear polarization relative to the predetermined direction of linear polarization using an optical matcher arranged to collect at least a fraction of the transmitted linearly polarized photons having orthogonal direction of linear polarization relative to the predetermined direction of linear polarization and focus the transmitted linearly polarized photons having orthogonal direction of linear polarization relative to the predetermined direction of linear polarization onto a photodetector arranged to generate at least two electronic signals proportional to the transmitted linearly polarized photons having orthogonal direction of linear polarization relative to the predetermined direction of linear polarization; and separating at least one electrical signal generated during the quarter-wave retardation state of the from the at least two electrical signals from the at least another electrical signal generated during the zero retardation state of the actively controlled retarder and storing the separated signals into at least two dedicated memory sections. 
    
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
       The above and other embodiments, features, and aspects of the present invention are considered in more detail in relation to the following description of embodiments shown in the accompanying drawings, in which: 
         FIG. 1  is an illustration of an exemplary embodiment of the polarization switching lidar device according to the present invention. 
         FIG. 2  is an illustration of an exemplary measurement results from an embodiment of the polarization switching lidar device according to the present invention. 
         FIG. 3  is an illustration of a different exemplary embodiment of the polarization switching lidar device according to the present invention. 
         FIG. 4  is an illustration of a flow chart diagram of operation of an exemplary embodiment of the polarization switching lidar device according to the present invention. 
         FIG. 5  is an illustration of additional exemplary embodiments of the polarization switching lidar devices according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention summarized above may be better understood by referring to the following description, which should be read in conjunction with the accompanying drawings. This description of an embodiment, set out below to enable one to build and use an implementation of the invention, is not intended to limit the invention, but to serve as a particular example thereof. Those skilled in the art should appreciate that they may readily use the conception and specific embodiments disclosed as a basis for modifying or designing other methods and systems for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent assemblies do not depart from the spirit and scope of the invention in its broadest form. 
     One exemplary embodiment of the polarization switching lidar device  100  for remote detection and characterization of atmospheric aggregation of particulates in accordance with the present invention is represented schematically in  FIG. 1 . The exemplary polarization switching lidar device includes a source  110  of a polarized pulsed laser light beam  120  linearly polarized in a predetermined direction of polarization  130  (for the illustrated example in  FIG. 1 , having the direction of polarization  130  perpendicular to the plane of the  FIG. 1 ). The source  110  of the polarized pulsed laser light beam  120  may include a laser head  112  and a polarizer  116  arranged and oriented to define with sufficient accuracy the predetermined direction of polarization  130 . The laser head  112  of the illustrated embodiment incorporates commercial Photonics Industries International, Inc. Nd:YVO 4  laser head arranged to a pulse rate ranging from 1 kHz to 150 kHz. It should be noted that a plethora of commercial or experimental lasers including, but not limited to: laser diodes, fiber lasers, diode-pumped solid-state lasers, and lamp-pumped solid-state lasers, are known to the practitioners and may be arranged in various pulse modes of operation (some exhibiting pulse length shorter than 50 ns) and used in different embodiments of the present invention. 
     The embodiment illustrated in  FIG. 1  utilizes a direction controlling mirror  135  arranged to reflect the polarized pulsed laser light beam  120  and to redirect the reflected polarized pulsed laser light beam  125  in the direction of a polarizing beam splitter  140  arranged to intersect the reflected polarized pulsed laser light beam  125  and to redirect a portion of the reflected polarized pulsed laser light beam  125  in the direction of an actively controlled retarder  150 . More particularly, the polarizing beam splitter  140  of the illustrated embodiment is oriented such that the reflected polarized pulsed laser light beam  125  linearly polarized in predetermined direction of polarization  130  is redirected while any component of the light beam  125  exhibiting linear polarization substantially different from the predetermined direction of polarization  130  is effectively absorbed in the polarizing beam splitter  140  and/or associated supporting structures. 
     The actively controlled retarder  150  is arranged to intersect the redirected portion  145  of the reflected polarized pulsed laser light beam  125 , and to be controllably alternated between a zero retardation state and a quarter-wave retardation state by application of an appropriate control signal customarily characterized by at least two distinct voltage levels. 
     In particular, an actively controlled retarder  150  of the illustrated embodiment may be implemented so that during a predetermined time period when the “lower level” voltage signal is applied to a control input pin, no significant phase retardation is added to the light traversing an active medium of the actively controlled retarder  150 , while when the “high level” voltage signal is applied to the control input pin the actively controlled retarder  150  behaves essentially as a quarter-wave plate causing a quarter-wave (Δφ=π/2) retardation of the phase of the appropriately polarized traversing the active medium of the actively controlled retarder  150 . Consequently, for the example illustrated schematically in  FIG. 1 , when the active medium of the actively controlled retarder  150  is in the zero retardation state (Δφ=0), the polarized pulsed laser light beam exiting the actively controlled retarder  150  is linearly polarized in the predetermined direction  130 , while the transmitted portion  155  exiting the actively controlled retarder  150  of the polarized pulsed laser light beam  145  is circularly polarized in a predetermined rotational sense  160  when the actively controlled retarder  150  is in the quarter-wave retardation state and arranged to have an optical axis oriented at 45° relative to the predetermine direction of liner polarization  130 . 
     Many actively controlled retarders  150  (such as ones based on Pockles cells technology) are known to change retardation states during time intervals in order of Ins (e.g. KD*P Pockels Cells available from Cleveland Crystals) and can be arranged to support pulse operations having pulse length shorter than 50 ns. 
     Therefore, in the exemplary embodiment illustrated in  FIG. 1  the polarized pulsed laser light beam  145  is essentially polarized in the predetermine direction of liner polarization  130 , such that the transmitted portion  155  of the polarized pulsed laser light beam  145  exiting the actively, controlled retarder  150  remains linearly polarized in the predetermine direction of liner polarization  130  when the actively controlled retarder  150  is in zero retardation state, while the transmitted portion  155  of the polarized pulsed laser light beam  145  exiting the actively controlled retarder  150  becomes circularly polarized in a predetermined rotational sense  160  when the actively controlled retarder  150  is in the quarter-wave retardation state. 
     The exemplary embodiment of the polarization switching lidar device in  FIG. 1  includes a directable telescoping assembly  162  arranged to intersect the transmitted portion  155  of the polarized pulsed laser light beam  145  exiting the actively controlled retarder  150  and to be controllably directable (i.e. to be manually or automatically specially oriented in a direction of interest in order to observe a predetermined sets of space angles) so as to redirect the intersected transmitted portion  155  of the polarized pulsed laser light beam  145  into the predetermined set of space angles of interest, while collecting portions  163  or  164  of depolarized backscattered photons from the scanned polarized pulse laser light beam  165  backscattered by the at least one airborne aggregation of particulates  166 . The directable telescoping assembly  162  is also arranged to redirect the collected portions  163  or  164  of depolarized backscattered photons onto the actively controlled retarder  150  and further onto the polarizing the beam splitter  140 . 
     The directable telescoping assembly  162  of the particular exemplary embodiment illustrated in  FIG. 1  includes a scanning telescope incorporating an internal scanner positioned between the telescope principle optical elements is described in the co-pending co-owned U.S. patent application Ser. No. 12/547,237, entitled: TELESCOPE WITH A WIDE FIELD OF VIEW INTERNAL OPTICAL SCANNER, which is here incorporated by reference in its entirety. As disclosed in more details in the incorporated U.S. patent application Ser. No. 12/547,237, the directable telescope assembly  162  of the exemplary embodiment is based on a classic Maksutov-Cassegrain telescope reflector configuration (more particularly, Questar FR7 model commercially obtained for example from Company. Seven Astro-Optics Division of Montpelier, Md., and modified by the Sigma Space Corporation to conform with the components of the disclosed polarization switching lidar device in accordance to standard optical engineering principles and practices.) It may be noted that other telescope systems including for example Galilean, Keplerian, Newtonian, Cassegrain, Maksutov-Cassegrain, Argunov-Cassegrain, Ritchey-Chratien, Dall-Kirkham, Gregorian, Hershelian, Schiefspiegler, and Yolo, telescope configuration or any combination of the listed telescope configurations may be integrated and used in various embodiments of the present invention. 
     In addition, it may be discerned that different scanning telescope devices incorporating a telescope and an external scanner may be used in other embodiments of the polarization switching lidar device in accordance with the present invention. For example, a scanning telescope device having an external scanner is disclosed in more details in the co-pending and co-owned U.S. patent application Ser. No. 11/683,549, entitled: SCANNER/OPTICAL SYSTEM FOR THREE-DIMENSIONAL LIDAR IMAGING AND POLARIMETRY, here also incorporated by reference in its entirety. 
     It may be also noted that propagation of the scanned polarized pulse laser light beam  165  through the atmosphere containing negligible amount of scatterers results in substantially no backscattered photons and no significant depolarization of the scanned polarized pulse laser light beam  165  which remains either linearly polarized in the predetermined direction of polarization  130  or in the predetermined rotational sense  160 , as disclosed above. In contrast, when the scanned polarized pulse laser light beam  165  intersects at least one airborne aggregation of particulates  166 , the resulting interaction may increase probabilities of backscatter. More particularly, it is known that, when the airborne aggregation of particulates  166  includes a significant concentrations of symmetric constituents (like droplets of water or other liquids like acid or salts solutions or suspensions) elastic backscattering processes may predominate resulting in a geometric inversion of the circular polarization of the backscattered photons from the predetermined rotational sense  160  of circular polarization into a circular polarization having an opposite rotational sense  167  relative to the predetermined rotational sense  160 . Conversely, when the airborne aggregation of particulates  166  includes a significant concentrations of irregularly shaped solid particulates (like ice crystals or particulates of sooth, smoke, industrially or naturally generated dust particulates, solid particulates incorporating carbon, solid particulates incorporating salt, mixtures and combinations of above particulates etc.) the backscattering may result in an enhanced depolarization of the scanned polarized pulse laser light beam  165  from the linearly, polarized in the predetermined direction of polarization  130  into a linearly polarized in the direction of polarization  168  which is perpendicular to the predetermined direction of polarization  130 . 
     Consequently, as the actively controlled retarder  150  of polarization switching lidar device of the present invention rapidly alternates between the zero retardation state Δφ=0 and the quarter-wave retardation state Δφ=π/2 (as controlled by a preprogrammed controller  170 ) causing alternations of polarization states of collected backscattered photons between the linearly polarized state  168  and circularly polarized state  167 . As the directable telescoping assembly  162  redirects the collected portions  163  or  164  of depolarized backscattered photons onto the actively controlled retarder  150  and further onto the polarizing the beam splitter  140 , the collected portion  164  traverses the actively controlled retarder  162  as being in the quarter-wave retardation state, while the collected portion  163  traverses the actively controlled retarder  162  as being in the zero retardation state. Therefore, the polarization states of both collected portions  163  and  164  of interest are arranged to be in the state of the linearly polarized in the direction of polarization  168  which is perpendicular to the predetermined direction of polarization  130 , and thus arranged to traverse the polarization beam splitter  140  with minimal loss. In opposition, a significant portion of undesirable “stray light”, diffusively reflected or scattered by impurities and imperfections of the constituent parts of the polarization switching lidar device, remain polarized predominantly in the predetermined direction of polarization  130  and, ipso facto, filtered out by the polarization beam splitter  140 . 
     The exemplary embodiment illustrated in  FIG. 1  features an optical matcher  175  arranged to collect a fraction of the transmitted portion  155  of backscattered photons exiting the polarizing beam splitter and focus the collected fraction of depolarized backscattered photons onto a photodetector  180  arranged to generate at least one electronic signal proportional to the collected portion of depolarized backscattered photons. The optical matcher  175  of the illustrated exemplary embodiment is based on a narrowband filter  177  arranged to narrowly transmit photons of selected wavelength (such is the narrowband filter having central wavelength at 532.07 nm and FWHM no greater than 0.15 nm, available as a custom product from Barr Associates, Inc. of Westford, Mass.) The incoming photons may be collimated using an collimating lens  178  (e.g. Plano-Convex Lens 6.0 mm Dia.×9.0 mm focal length commercially available from Edmund Optics of Barrington, N.J.) to ensure narrowband performance of the narrowband filter  177 , while an additional lens  176  (e.g. Mounted Geltech Aspheric Lens, AR-Coated: 350-700 nm, part number C230TME-A, from Thorlabs of Newton, N.J.) may be used to match the filtered photons to the photodetector  180  (e.g. Single Photon Counting Module SPCM-AQR-14-FC commercially available from PerkinElmer&#39;s Marketing and Marketing Communications of Salem, Mass.) 
     The photodetector  180  is arranged to generate at least one electronic signal proportional to the collected portion of depolarized backscattered photons which can be digitized and stored into a dedicated memory  190 . In the embodiment represented in  FIG. 1  the memory  190  is controlled by the controller  170  so as to generate at least two separate digital records of which at least one is generated and stored during the time period when the actively controlled retarder  150  is in the zero retardation state (Δφ=0) and at least another digital record is generated and stored during the quarter-wave retardation state (Δφ=π/2) of the actively controlled retarder  150  (as controlled by a preprogrammed controller  170 ). Therefore, at least one digital record may be predominantly sensitive to the backscattering from the aggregations irregularly shaped particulates (when Δφ=0), while at least another digital record may be predominantly sensitive to the backscattering on the aggregations of predominantly symmetric particulates (when Δφ=π/2). Thus, the polarization switching lidar embodiment illustrated in  FIG. 1  may exhibit a fundamental simplicity of a “single beam” (“single channel”) scattering device, while the resulting digital records may enjoy enhanced sensitivities usually associated with significantly more complex multi-beam (“multichannel”) lidar devices. 
     In a framework of a more concise theoretical consideration of the lidar measuring sequence as described in the above recitation, lidar backscattered signal can be considered to be essentially incoherent and may be represented sufficiently accurately as a 4-component Stokes vector and analyzed using Mueller matrix calculus. Therefore, the lidar measurement sequence may be symbolically represented as a sequence of Mueller operators acting on a initial polarization vector {right arrow over (P)} S  as:
 
 {right arrow over (P)}   final   =M   LPH   M   LCR (φ,−45) M   atm   M   LCR (φ,+45) M   LPV   {right arrow over (P)}   S   (Eq. 1)
 
where M LPV  stands for the PBS acting as a linear polarizer with axis aligned to the vertical, M LCR (φ,+45) stands for the actively controlled retarder with retardation φ aligned with fast axis at +45° to vertical, M atm  represents the interaction with the atmosphere, M LCR (φ,−45) is again the actively controlled retarder but now with fast axis aligned at −45° to vertical, and M LPH  is the PBS now acting as a linear polarizer with axis aligned horizontally. Note that the angles are defined as positive clockwise while facing in the direction of propagation. When the direction of propagation is reversed for the returning light, the angles are also reversed. Mueller matrices do not represent optical components so much as optical interactions explaining why different Mueller matrices are used to represent the same optical component. With the exception of M atm , the other operators represent the actions of elementary optical elements with known form.
 
     The Mueller matrix for the atmosphere is, as well understood in standard practice, a changing quantity and is a subject of intense study [9-10]. For a common simple case of single scattering on particles having a plane of symmetry or random orientation (which includes spheres, randomly oriented ice crystals, and horizontal plates) we benefit from substantial cancellation of matrix elements based on symmetry arguments to obtain 
                       M   atm     =     a   ⁡     [         1       0       0       0           0         1   -   d         0       0           0       0         d   -   1         0           0       0       0           2   ⁢   d     -   1           ]         ,           (     Eq   .           ⁢   2     )               
where a is proportional to the magnitude of the return signal and d is indicative of the degree to which the return signal is depolarized. For d=0, M atm  is identical to Mueller matrix for normal incidence on a perfect mirror.
 
     Starting with {right arrow over (P)} S  taken as linearly polarized vertically, the equation Eq. 1 together with the equation Eq. 2 yields final polarization vectors {right arrow over (P)} ⊥ , {right arrow over (P)} □ , {right arrow over (P)} RH , and {right arrow over (P)} LH  as 
     
       
         
           
             
               
                 
                   
                     
                       
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                     = 
                     
                       ( 
                       
                         
                           
                             
                               d 
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                     . 
                     
                         
                     
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                     3 
                   
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     Several relevant depolarization ratios can be defined as: 
     
       
         
           
             
               
                 
                   
                     
                       δ 
                       linear 
                     
                     = 
                     
                       
                         
                           
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                           ⊥ 
                         
                         ⁡ 
                         
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                           • 
                         
                         ⁡ 
                         
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                       circ 
                     
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                             ⁢ 
                             
                                 
                             
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                         ⁡ 
                         
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                             R 
                             ⁢ 
                             
                                 
                             
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                   , 
                   
                     
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                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         δ 
                         MPL 
                       
                     
                     = 
                     
                       
                         
                           
                             
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                   ( 
                   
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                     . 
                     
                         
                     
                     ⁢ 
                     4 
                   
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     Combining Eq. 3 and Eq. 4 results in: 
     
       
         
           
             
               
                 
                   
                     
                       δ 
                       linear 
                     
                     = 
                     
                       d 
                       
                         2 
                         - 
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                   , 
                   
                     
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                       circ 
                     
                     ⁢ 
                     
                       d 
                       
                         1 
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                       ⁢ 
                       
                           
                       
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                         δ 
                         MPL 
                       
                     
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                         d 
                         
                           2 
                           ⁢ 
                           
                             ( 
                             
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                       . 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
     Equation Eq. 5 in combination with Eq. 3 leads to following relationships: 
     
       
         
           
             
               
                 
                   
                     
                       
                         δ 
                         linear 
                       
                       = 
                       
                         
                           
                             
                               δ 
                               MPL 
                             
                             
                               
                                 δ 
                                 MPL 
                               
                               + 
                               1 
                             
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           and 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             δ 
                             circ 
                           
                         
                         = 
                         
                           2 
                           × 
                           
                             δ 
                             MPL 
                           
                         
                       
                     
                     , 
                     or 
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       
                         δ 
                         MPL 
                       
                       = 
                       
                           
                       
                       ⁢ 
                       
                         
                           
                             δ 
                             circ 
                           
                           2 
                         
                         = 
                         
                           
                             δ 
                             linear 
                           
                           
                             ( 
                             
                               1 
                               - 
                               
                                 δ 
                                 linear 
                               
                             
                             ) 
                           
                         
                       
                     
                     , 
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ) 
                 
               
             
           
         
       
     
     which can be interpreted as consequences of lidar signal power conservation, as may be expected. 
     One example of the results of measurements of the {right arrow over (P)} ⊥  (Δφ=π/2) and the {right arrow over (P)} ⊥ (Δφ=0) signals versus range obtained from the exemplary embodiment of  FIG. 1  is illustrated in  FIG. 2 . On the horizontal axis range up to 15 km is shown. The vertical axis shows range-square corrected signals, i.e. (total backscatter signal−background signal)*range 2 . The units of the range-square corrected signal are counts/microsecond*km 2 . The solid trace  210  indicates the range-square corrected signal resulting predominantly from the backscattering on the airborne aggregation  166  of symmetric particulates corresponding to the {right arrow over (P)} ⊥ (Δφ=π/2), while the dashed trace  220  indicates the range-Square corrected signal resulting predominantly from the backscattering on the airborne aggregation  166  of particulates generally lacking the spherical symmetry corresponding to the {right arrow over (P)} ⊥  (Δφ=0) signal. 
     The particular measurements yielding results illustrated in  FIG. 2  are made for 60 seconds with the retardation state of the actively controlled retarder alternating between the zero and the quarter-wave retardation states. The range-square corrected signals  210  and  220  are accumulated in separate locations in the memory  190 . The laser  112  of this exemplary embodiment has a pulse repetition rate of 2500 Hz. Effectively, 30 seconds or backscattered signals from 75,000 laser pulses are integrated in each range-square corrected signal and displayed. Typical pulse energy for this type of measurements is 6 microjoules. The range resolution of 30 m is estimated. 
     As is seen in the range-square corrected signal  210  given in a solid trace, a large backscatter is obtained from the primarily spherical aggregates present in the lower atmosphere. The {right arrow over (P)} ⊥ (Δφ=0) backscatter, proportional to the dashed trace  220 , remains low at these ranges. At the 4.5 and 8 km ranges, two cloud layers may be observed in both signals  210  and  220 . The backscatter from clouds is a strong because of a relative increase of density of the scatterers, and exhibits depolarization that is observed as the {right arrow over (P)} ⊥  (Δφ=0) backscatter due to the presence of non-spherical ice crystals present in these clouds. 
     The signals  210  and  220  in the 0-2 km range exhibit increases with distance before reaching a peak and subsequently falling off with range. This may be related to an instrument function of geometric overlap, known to be a characteristic of many lidars. The instrument overlap function normally corresponds to the design features of the particular telescopes including size, axis-to-axis distance between transmitter and receiver telescopes (in cases of lidars having separate transmitter and receiver units), the field of view the telescoping assembly designs, etc. 
     An exemplary embodiment of a polarization switching lidar device having aforementioned separate transmitter and receiver telescope assemblies is shown schematically in  FIG. 3 . The exemplary embodiments in  FIG. 1  and  FIG. 3  share several parts arranged to performing corresponding operations as described above regarding  FIG. 1 . Those shared parts are indicated in  FIG. 3  using same corresponding reference numerals as introduced in  FIG. 1 . For example, the controller  170  in  FIG. 3  is arranged to simultaneously control and synchronize a transmitter actively controlled retarder  350  and a receiver actively controlled retarder  355  both having the arrangement and the functions analogues to those of the actively controlled retarder  150 . Similarly, the controller  170  is arranged to control the transmitting directable telescoping assembly  362  and the transmitting directable telescoping assembly  365  to have essentially overlapping fields of view in order to obtain backscattering signals from predetermined volumes of the airborne aggregation of particulates  166 . 
     One notable difference between embodiments in  FIGS. 1 and 3  is the replacement of the polarization beam splitter  140  with an additional polarizer  340 . As the function, of combining/separating of the optical paths of transmitted and received laser beams performed by the polarization beam splitter  140  is, by definition, absent from the embodiment having separated transmitted and received beam paths in FIG.  3 , a simpler polarizer  340 , generally similar to the polarizer  116  but rotated by π/2 (relative to the polarizer  116 ) may be sufficient for suppression of the “stray light” discussed above regarding  FIG. 1 . 
     A method for remote detection and characterization of at least one airborne aggregation of particulates utilizing a polarization switching lidar device of the present invention is generally related to the arrangements of the disclosed embodiments schematically given in  FIGS. 1 and 3 . More particularly, the principle steps of the method ere given in the flowchart in  FIG. 4 , which closely corresponds to the exemplary embodiment in  FIG. 1 . The method corresponding to the embodiment in  FIG. 3 , generally differs only in details from the flowchart in  FIG. 4 . For example, the steps  410  and  420  encompasses transmitting/receiving functions using the single directable telescoping assembly,  162  and using the transmitting directable telescoping assembly  362  and the receiving directable telescoping assembly  365 , but the step  430  generally pertains to the embodiment illustrated in  FIG. 1  as the embodiment illustrated in  FIG. 3  may include the polarizer  340  instead of the polarization beam splitter  140 . 
     Several embodiments of applications of the polarization switching lidar device  100  are illustrated in  FIG. 5 . For example, the lidar device  100  may be arranged on an flying vehicle  510  and having field of view  520  arranged to detect clouds  566  and/or atmospheric aggregations of particulates  166  at altitudes equal or below the flight altitude (including the height h v  relative to a predetermined reference level  570  and the absolute altitude of the reference surface  570 ) of the (over)flying vehicle  510 . 
     In another embodiment illustrated in  FIG. 5 , the polarization switching lidar device  100  is arranged on a surface  530  erected on a stationary or semi-stationary (transportable) structure having altitude h relative to the reference surface  570 . Such an installation can accommodate the field of view  520  which may include clouds  566  at altitudes above the altitude of the surface  530  or low-altitude particulate aggregations  556 , characteristic of altitudes below the average altitude of the surface  530 . In addition, such an embodiment can be arranged to detect time-dependent characteristics of the aggregations  166 ,  556 , and/or  566  allowing for positive identification of sources  580  of the particulates of interest and determination of habitudes of the characterized sources  580 . 
     The present invention has been described with references to the exemplary embodiments arranged for different applications. While specific values, relationships, materials and components have been set forth for purposes of describing concepts of the invention, it will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the basic concepts and operating principles of the invention as broadly described. It should be recognized that, in the light of the above teachings, those skilled in the art can modify those specifics without departing from the invention taught herein. Having now fully set forth the preferred embodiments and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to: those skilled in the art upon becoming familiar with such underlying concept. It is intended to include all such modifications, alternatives and other embodiments insofar as they come within the scope of the appended claims or equivalents thereof. It should be understood, therefore, that the invention may be practiced otherwise than as specifically set forth herein. Consequently, the present embodiments are to be considered in all respects as illustrative and not restrictive.