Abstract:
In accordance with yet another aspect of the present invention, an active imaging system is provided for imaging a target of interest. An imaging assembly includes a light source and an optical assembly comprising a plurality of passive optical components. The optical assembly divides received light into a first beam, having a first polarization and a second beam, having a second, orthogonal polarization, directs the first and second beam along respective first and second optical paths within the optical assembly, and recombines the first and second beams into a combined beam. A sensor detects the combined beam.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to optical systems, and more specifically to efficiently recapture and processing of a reflected illumination beam in an active imaging system. 
       BACKGROUND OF THE INVENTION 
       [0002]    A laser weapon system can include a tracking illuminator laser for locating and tracking a moving object, such as a missile or vehicle, a beacon illuminator laser for determining the wavefront aberrations encountered along the optical path to the object, and a high energy laser for detonating or disabling the tracked object. The laser wavelengths can be selected to be slightly different from each other in order to use a shared aperture element to combine the laser trains for outgoing beams delivery. When the illuminator lasers engage a target, the reflected laser light is depolarized depending on laser polarization, target surface roughness, angle of incidence and atmospheric turbulence. The reflected track and beacon laser signals are captured by on board sensors for target tracking and wavefront correction. To simplify boresighting between the several beams and eliminate anisoplanatic effects, it is desirable that the outgoing tracking and beacon illuminator lasers and their respective return signals share the common optical path. Unfortunately, this can complicate the separation of the incoming signals from outgoing beams. 
       SUMMARY OF THE INVENTION 
       [0003]    In accordance with one aspect of the present invention, an apparatus is provided for detecting a beam of light reflected from a region of interest. A light source produces a light beam having a first polarization state. A first polarization beam splitter directs the light beam along a first optical path and allows light having a second polarization state, orthogonal to the first polarization state, to pass. An optical polarization modulator, located along the first optical path, applies a net rotation of ninety degrees to the polarization of light passing through the optical polarization modulator in a first direction and applies a net rotation of zero degrees to the polarization of light passing through the optical polarization modulator in a second direction. The first polarization beam splitter directs the light beam to the optical polarization modulator. A second polarization beam splitter, positioned along the first optical path such that the optical polarization modulator is between the first polarization beam splitter and the second polarization beam splitter, allows incident light having the second polarization state to pass onto the optical polarization modulator and directs incident light having the first polarization state along a second optical path. 
         [0004]    In accordance with another aspect of the present invention, a method is provided for imaging a region of interest. Light reflected from a moving object is received at an aperture. The received light is split according to its polarization state as to direct a first portion of the received light having a first polarization state to a first optical path and to direct a second portion of the received light having a second polarization state to a second optical path. The polarization state of the first portion of the received light is rotated from a first polarization state to a second polarization state. The first portion of the received light and the second portion of the received light are recombined at a sensor. 
         [0005]    In accordance with yet another aspect of the present invention, an active imaging system is provided for imaging a target of interest. An imaging assembly includes a light source and an optical assembly comprising a plurality of passive optical components. The optical assembly divides received light into a first beam, having a first polarization and a second beam, having a second, orthogonal polarization, directs the first and second beam along respective first and second optical paths within the optical assembly, and recombines the first and second beams into a combined beam. A sensor detects the combined beam. 
         [0006]    In accordance with still another aspect of the present invention, a method is provided for imaging a region of interest. A light beam is produced having a first polarization state on a first optical path. The polarization state of the light beam is rotated from a first polarization state to a second polarization state. The light beam is projected at a moving object through an aperture. Light reflected from the moving object is received at the aperture. The received light is split according to its polarization state as to direct a first portion of the received light having the second polarization state to the first optical path and to direct a second portion of the received light having the first polarization state to a second optical path. The first portion of the received light and the second portion of the received light are recombined at a sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0007]      FIG. 1  illustrates a functional block diagram of an active illumination system in accordance with an aspect of the present invention. 
           [0008]      FIG. 2  is a chart illustrating a relative power of a signal, represented on a vertical axis, as a function of the rotational angle of a half-wave plate, represented on a horizontal axis in degrees, used to apply polarization to the signal as part of an experimental arrangement. 
           [0009]      FIG. 3  illustrates one implementation of an imaging system in accordance with an aspect of the present invention. 
           [0010]      FIG. 4  illustrates a second implementation of an imaging system in accordance with an aspect of the present invention. 
           [0011]      FIG. 5  illustrates an object tracking system in accordance with an aspect of the present invention. 
           [0012]      FIG. 6  illustrates a first method for imaging a region of interest in accordance with an aspect of the present invention. 
           [0013]      FIG. 7  illustrates a second method for imaging a region of interest in accordance with an aspect of the present invention 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0014]    The present invention relates to systems and methods for efficiently recapturing and processing a reflected illumination beam in an active imaging system. An optical assembly, in accordance with an aspect of the present invention, includes a plurality of passive optical components that separate incoming light into two orthogonally polarized components and routes one of the components along the same optical path as a transmitted signal. The optical assembly finds particular application to high energy laser weapon systems, although it will be appreciated that it can be used in any active imaging system, sensor, or optical communications system. Since the device used only passive optical components to separate and convert light polarizations, it is possible to achieve integration of outgoing laser trains and return light collection with high efficiency. The surface Fresnel reflections of optical components are the only losses experienced within the optical assembly, and even this loss can be significantly mitigated by anti-reflection coatings. This efficiency allows for the use of illuminator lasers with significantly less power than prior art systems to obtain the same signal-to-noise ratio at the sensor. Further, the use of passive optical components allows the device to be both inexpensive and highly reliable. 
         [0015]      FIG. 1  illustrates an optical assembly  10  for an active illumination system in accordance with an aspect of the present invention. The illustrated assembly  10  provides means for illuminating a target region of interest and efficiently receiving return light from the illuminated target region. The assembly  10  comprises a light source  12  that produces a polarized light beam to illuminate a region of interest. For example, the light source  12  can comprise a laser or other appropriate device for producing light of a single polarization. 
         [0016]    The polarized light beam can be directed to a first polarization beam splitter  14  that reflects light having a first polarization state and allows light having a second polarization state to pass freely. For the purposes of the illustrated assembly  10 , the first polarization beam splitter  14  can be conceptualized as a router that routes incident light along one of two different possible paths according to its polarization. The relative orientation of the light source  12  and the first polarization beam splitter  14  as well as the polarization state of the polarized light beam can be selected such that the polarized light from the light source  12  is routed onto a first optical path  16 . 
         [0017]    The first optical path  16  can include an optical polarization modulator  18  that provides non-reciprocal rotation of polarized light. The optical polarization modulator  18  applies a net rotation of ninety degrees to the polarization of light passing through the optical polarization modulator in a first direction and applies a net rotation of zero degrees to the polarization of light passing through the optical polarization modulator in a second direction. It will be appreciated that the orientation of the optical polarization modulator  18  within the system can vary to either rotate light transmitted from the assembly  10  through the first optical path  16  or to rotate light received at the assembly through the first optical path. For the purpose of illustration, the following discussion will assume that the optical polarization modulator  18  is configured to rotate light transmitted through the optical path, so the light beam reflected from the first polarization beam splitter  14  is rotated during passage through the optical polarization modulator, but it will be appreciated that other configurations (See, e.g.,  FIG. 4 ) are possible. Accordingly, the polarized light beam reflected from the first polarization beam splitter  14  is rotated at the optical polarization modulator  18 , converting the polarized light beam from the first polarization state to a second polarization state. 
         [0018]    After passing through the optical polarization modulator  18 , the polarized light is directed onto a second polarization beam splitter  20 . The second polarization beam splitter  20  reflects light having a first polarization state and allows light having a second polarization state to pass freely. The second polarization beam splitter  20  can be oriented such that the polarized light from the light source  12  passes through the beam splitter into an aperture  22  that is oriented toward the region of interest to illuminate the region of interest with polarized light. The polarized light is reflected from the region of interest, and received at the aperture  22 . 
         [0019]    Depending on the nature of the region of interest, the reflected light can be significantly depolarized. Specifically, uneven texture of the target surface, atmospheric turbulence, and non-normal angles of incidence to the target can contribute to depolarization of the reflected light. Accordingly, the return light received at the aperture can comprise a mix of the first and second polarization states or even a random polarization. The returned light at the aperture  22  is directed to the second polarization beam splitter  20 , where it is split into a first component, having the first polarization state, and a second component, having the second polarization state. The first component is reflected to a second optical path, while the second component passes through the second polarization beam splitter  20  to the first optical path  16 . The second polarization component is directed onto the optical polarization modulator and passes through the optical polarization modulator power unchanged to the first polarization beam splitter  14 . Since the output of the optical polarization modulator is in the second polarization state, it passes through the first polarization beam splitter  14  to continue along the first optical path. 
         [0020]      FIG. 2  is a chart  50  illustrating a relative power of a signal, represented on a vertical axis  52 , as a function of the rotational angle of a half-wave plate, represented on a horizontal axis  54  in degrees, used to apply polarization to the signal as part of an experimental arrangement. A first plot line  56  represents the power of a vertically polarized portion of the signal after passage through a second optical path. A second plot line  58  represents the power of a horizontally polarized portion of the signal after passage through a first optical path. It will be appreciated that the power of the two polarized components varies significantly across the various angles of rotation of the half-wave plate. A total power received by an optical assembly in accordance with an aspect of the present invention is illustrated as a third plot line  60 . It will be appreciated that despite the large variance in the power of the two polarized components  56  and  58 , the total power  60  varies only slightly, remaining within approximately five percent of an average value. As this chart demonstrates, the optical assembly is effective in controlling variance in return power regardless of the relative power of the various components of a reflected, depolarized signal. 
         [0021]      FIG. 3  illustrates one implementation of an imaging system  100  in accordance with an aspect of the present invention. The illustrated system  100  allows the return light and the transmitted light to utilize the substantially the same optical path while preserving both the vertically polarized and horizontally polarized components in the return light. For ease of illustration, the polarization state of the light at each point in the system  100  is denoted by a capital “S” or “P”, representing vertical and horizontal polarization states respectively, underlined capitals representing returned light, and unmodified capitals representing transmitted light. 
         [0022]    In the illustrated system, an illumination laser  102  provides a vertically polarized light beam to a first polarization beam splitter  104 . The first polarization beam splitter  104  is configured to reflect vertically polarized light while remaining substantially transparent to horizontally polarized light, and the first polarization beam splitter can be configured to reflect the light beam to an optical polarization modulator  106 . The optical polarization modulator  106  comprises a Faraday rotator  108  that applies a forty-five degree rotation in a first direction to the transmitted light beam. A half-wave plate  110  is aligned with an appropriate optical axis such that it provides a second forty-five degree rotation in the first direction, such that the output of the optical polarization modulator  106  is a horizontally polarized beam. 
         [0023]    The horizontally polarized beam output from the optical polarization modulator  106  is provided to a second polarization beam splitter  112  that is configured to reflect vertically polarized light while remaining substantially transparent to horizontally polarized light. The horizontally polarized beam passes through the second polarization beam splitter  112  and can be directed toward a target location of interest. The polarized light reflects from the target location, illuminating the target, but characteristics of the target surface, such as motion of the surface and uneven texture, can cause depolarization of the reflected light. Accordingly, the reflected light can be expected to comprise a mix of both horizontally and vertically polarized light. 
         [0024]    Light reflected from the target location is received at the second polarization beam splitter  112  and split into a horizontally polarized component that is returned along the first optical path  116  and a vertically polarized component that is directed along a second optical path  120 . The horizontally polarized component is transmitted through the second polarization beam splitter  112  to the optical polarization modulator  106 . As before, the half-wave plate  110  provides a forty-five degree rotation in a first direction to the horizontally polarized component, but the Faraday rotator  108  is a non-reciprocal device, and applies a forty-five degree rotation in a second, opposing direction to the transmitted light beam, such that the output of the polarization modulator  106  remains a horizontally polarized beam. 
         [0025]    The horizontally polarized beam passes through the first polarization beam splitter to reflect from a first mirror  128 , aligning the beam with a photodetector array  130 . The beam is directed to a static phase compensator  132  that compensates for possible wavefront distortion in the outgoing beam caused by the Faraday rotator  108 . The beam is then directed to a third polarization beam splitter  134  that that is configured to reflect vertically polarized light while remaining effectively transparent to horizontally polarized light. The horizontally polarized beam passes through the third polarization beam splitter  134 . 
         [0026]    The vertically polarized component of the light reflected from the target location is directed to an optical path compensator  136  that adjusts the phase of the vertically polarized component to account for differences in the path lengths of the first optical path  116  and the second optical path  120 . The path adjusted beam is then reflected from a second mirror  138  to align the beam with the third polarization beam splitter  134 . Since the beam is vertically polarized, it reflects from the third polarization beam splitter  134  and is recombined with the horizontally polarized component. The recombined beam is then received at the detector  130 , where one or more characteristics of the target location or the intervening atmosphere can be determined from the received light. 
         [0027]      FIG. 4  illustrates a second implementation of an imaging system  150  in accordance with an aspect of the present invention. The illustrated system  150  allows the return light and the transmitted light to utilize the substantially the same optical path while preserving both the vertically polarized and horizontally polarized components in the return light. For ease of illustration, the polarization state of the light at each point in the system  150  is demoted by a capital “S” or “P”, representing vertical and horizontal polarization states respectively, underlined capitals representing returned light, and unmodified capitals representing transmitted light. 
         [0028]    In the illustrated system, an illumination laser  152  provides a horizontally polarized light beam to a first polarization beam splitter  154 . The first polarization beam splitter  154  is configured to reflect vertically polarized light while remaining substantially transparent to horizontally polarized light, and the first polarization beam splitter can be configured to allow the light beam to pass through onto an optical polarization modulator  156 . The optical polarization modulator  156  comprises a Faraday rotator  158  that applies a forty-five degree rotation in a first direction to the horizontally polarized component and a first half-wave plate  160  that is aligned with an appropriate optical axis to provide a forty-five degree rotation in a second, opposing direction to the transmitted light beam such that the output of the optical polarization modulator  156  remains a horizontally polarized beam. 
         [0029]    The horizontally polarized beam output from the optical polarization modulator  156  is provided to a second polarization beam splitter  162  that is configured to reflect vertically polarized light while remaining effectively transparent to horizontally polarized light. The horizontally polarized beam passes through the second polarization beam splitter  162  and can be directed toward a target location of interest. The polarized light reflects from the target location, illuminating the target, but characteristics of the target surface, such as motion, uneven texture, etc. can cause depolarization of the reflected light. Accordingly, the reflected light can be expected to comprise a mix of both horizontally and vertically polarized light. 
         [0030]    Light reflected from the target location is received at the second polarization beam splitter  162  and split into a horizontally polarized component that is returned along the first optical path  166  and a vertically polarized component that is directed along a second optical path  170 . The horizontally polarized component is transmitted through the second polarization beam splitter  162  to the optical polarization modulator  156 . The first half-wave plate  160  provides a forty-five degree rotation in a first direction, while the Faraday rotator  158 , which is a non-reciprocal rotator, applies a forty-five degree rotation in the same direction, such that the output of the polarization modulator  156  is a vertically polarized beam. 
         [0031]    The vertically polarized beam reflects from the first polarization beam splitter  154  and is directed to a second half-wave plate  178 . The second half wave plate  178  provides a full ninety degree rotation to the vertically polarized beam to provide a horizontally polarized beam. The horizontally polarized beam is then provided to a static phase compensator  182  that compensates for possible wavefront distortion in the outgoing beam caused by the Faraday rotator  158 . The beam is then directed to a third polarization beam splitter  184  that that is configured to reflect vertically polarized light while remaining substantially transparent to horizontally polarized light. The horizontally polarized beam passes through the third polarization beam splitter  184 . 
         [0032]    The vertically polarized component of the light reflected from the target location is directed to an optical path compensator  186  that adjusts the phase of the vertically polarized component to account for differences in the path lengths of the first optical path  166  and the second optical path  170 . The path adjusted beam is then reflected from a second mirror  188  to align the beam with the third polarization beam splitter  184 . Since the beam is vertically polarized, it reflects from the third polarization beam splitter  184  and is recombined with the horizontally polarized component. The recombined beam is then received at a detector  190 , where one or more characteristics of the target location or the intervening atmosphere can be determined from the received light. 
         [0033]      FIG. 5  illustrates an object illumination system  200  in accordance with an aspect of the present invention. The illustrated object illumination system  200  locates and tracks an object of interest  202  at a tracking assembly  210 , and estimates aberrations induced along the optical path to the target (e.g., due to atmospheric conditions and optical components) at a beacon assembly  220 . In the illustrated implementation, the illumination system  200  is part of a laser weapon system that tracks the object of interest, determines a degree of aberrations in the optical path as part of an adaptive optics arrangement, and corrects the wavefront error by a deformable mirror and directs a high power laser (not shown) at the object of interest  202  to disable the tracked object. In accordance with an aspect of the present invention, the tracking assembly  210  and the beacon assembly  220  utilize light of two distinct wavelengths to allow for the use of a shared aperture  204 . Specifically, a dichroic plate  206  can be used that is substantially transparent to light of a first wavelength and reflects light of a second wavelength. The dichroic plate  206  positioned behind the aperture in such a way as to separate light incident on the aperture by wavelength, allowing light of a first wavelength to pass through to the beacon assembly  220 , and reflecting light of a second wavelength into the tracking assembly  210 . 
         [0034]    The tracking assembly  210  includes a tracking laser  212  that projects light of the second wavelength into a first optical assembly  214  in accordance with an aspect of the present invention. The first optical assembly  214  selectively polarizes the transmitted light, and directs the polarized light to the dichroic plate  206  via passive optical components. In the illustrated implementation, the first optical assembly  214  can comprise an assembly similar to that illustrated in  FIG. 3 . The projected light, having the second wavelength, reflected by the dichroic plate  206  into the aperture, where it is reflected from the object of interest  202 . 
         [0035]    The beacon assembly  220  includes an illuminator beacon laser  222  that projects light of the first wavelength into a second optical assembly  224  in accordance with an aspect of the present invention. The second optical assembly  224  selectively polarizes the transmitted light, and directs the polarized light to the dichroic plate  206  via passive optical components. In the illustrated implementation, the second optical assembly  224  can comprise an assembly similar to that illustrated in  FIG. 4 . The projected light, having the first wavelength, transmitted through the dichroic plate  206  into the aperture, where it is reflected from the object of interest  202 . 
         [0036]    Light from the tracking laser  212  and the illuminator beacon laser  222  reflected from object of interest  202  is received at the aperture  204  and directed to the dichroic plate  206 . The reflected light from the tracking laser  212 , having the second wavelength, reflects by the dichroic plate  206  into the first optical assembly  214 . The first optical assembly  214  separates the light into horizontally and vertically polarized components and routes the polarized light separately as to avoid interference with the transmitted light from the tracking laser  212 . The horizontally and vertically polarized light are then recombined and provided to a tracking camera  216 . The tracking camera  216  can have associated image processing elements (not shown) that determine a position of the object of interest  202  from the image received at the tracking camera  216 . 
         [0037]    The transmitted light from the illuminator beacon laser  222 , having the first wavelength, passes through the dichroic plate  206  into the second optical assembly  224 . The second optical assembly  224  separates the light into horizontally and vertically polarized components and routes the polarized light separately as to avoid interference with the transmitted light from the illuminator beacon laser  222 . The horizontally and vertically polarized light are then recombined and provided to a wavefront sensor  226 . The wavefront sensor  226  determines optical aberrations experienced by the transmitted beacon light along the optical path to the object of interest  202 . The determined aberrations can be used as part of an adaptive optics arrangement to precompensate for the aberrations during operation of the high power laser, increasing its effectiveness. 
         [0038]    In view of the examples shown and described above, methodologies in accordance with the present invention will be better appreciated with reference to the flow diagrams of  FIGS. 6 and 7 . While, for purposes of simplicity of explanation, the methodologies are shown and described as executing serially, it is to be understood and appreciated that the present invention is not limited by the order shown, as some aspects may, in accordance with the present invention, occur in different orders and/or concurrently from that shown and described herein. Moreover, not all features shown or described may be needed to implement a methodology in accordance with the present invention. Additionally, such methodologies can be implemented in hardware (e.g., one or more integrated circuits), software (e.g., running on a DSP or ASIC) or a combination of hardware and software. 
         [0039]      FIG. 6  illustrates a method  300  that might be practiced, for example, with the system illustrated in  FIG. 3 , for imaging a region of interest in accordance with an aspect of the present invention. At  302 , a light beam having a first polarization state is generated along a first optical path. At  304 , the light beam is rotated at an optical polarization modulator and projected at the region of interest through an aperture. This is accomplished by passing the transmitted light through an optical polarization modulator, such that the polarization state of light transmitted through the first optical path is rotated to an orthogonal polarization state, while the polarization state of light received along the first optical path is unchanged. At  306 , light reflected from the region of interest is received at the aperture. 
         [0040]    The received light is split according to its polarization state at  308 , as to direct a first portion of the received light having a second polarization state to the first optical path and to direct a second portion of the received light having a first polarization state to a second optical path. For example, the first polarization state can be a vertical polarization state and the second polarization can be a horizontal polarization state. At  310 , one of the first portion of the received light and the second portion of the received light are compensated for differences in path length between the first optical path and the second optical path. The first portion of the received light and the second portion of the received light are then recombined at  312  and the combined light directed on a sensor. 
         [0041]      FIG. 7  illustrates a method  350  that might be practiced, for example, with the system illustrated in  FIG. 4 , for imaging a region of interest in accordance with an aspect of the present invention. At  352 , a light beam having a first polarization state is generated along a first optical path and projected at the region of interest through an aperture. At  354 , light reflected from the region of interest is received at the aperture. The received light is split according to its polarization state at  356 , as to direct a first portion of the received light having a first polarization state to the first optical path and to direct a second portion of the received light having a second polarization state to a second optical path. For example, the first polarization state can be a horizontal polarization state and the second polarization can be a vertical polarization state. 
         [0042]    At  358 , the polarization state of the first portion of the received light is rotated from a first polarization state to a second polarization state. In one implementation, this is accomplished by passing the received light through an optical polarization modulator, such that the polarization state of light transmitted through the first optical path is not changed, while the polarization state of light received along the first optical path is rotated to an orthogonal polarization state. At  360 , one of the first portion of the received light and the second portion of the received light are compensated for differences in path length between the first optical path and the second optical path. The first portion of the received light and the second portion of the received light are then recombined at  362  and the combined light directed on a sensor. 
         [0043]    What has been described above includes exemplary implementations of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.