Abstract:
Systems and methods for processing light from multiple fields ( 48, 54, 55 ) of view without excessive machinery for scanning optical elements. In an exemplary embodiment of the invention, multiple holographic optical elements ( 41, 42, 43, 44, 45 ), integrated on a common film ( 4 ), diffract and project light from respective fields of view.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This Application claims the benefit of application Ser. No. 60/142,527 of Geary Karl Schwemmer filed Jul. 7, 1999 for SHARED APERTURE MULTIPLEXED HOLOGRAPHIC SCANNING TELESCOPE, the contents of which are herein incorporated by reference. 
    
    
     ORIGIN OF THE INVENTION 
     The invention described herein was made by an employee of the United States Government, and may be manufactured and used by or for the Government for governmental purposes without the payment of royalties thereon or therefor. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates generally to optical observations and, more particularly, to methods and systems for collecting data from multiple fields of view. 
     2. Background Art 
     Telescopes typically are required to operate through multiple fields of view. This requirement may result from the area of interest being larger than a single field of view of the telescope. For example, in terrain mapping applications, the terrain of interest may be larger then a single field of view of the telescope. 
     Operating through multiple fields of view may also be desirable in applications, such as wind monitoring, that benefit from multiple viewing angles. 
     STATEMENT OF THE INVENTION 
     It is an object of the present invention to provide relatively efficient methods and systems for collecting data from multiple fields of view. 
     To achieve this and other objects of the present invention, a method comprises receiving light from a first field; responsive to a wavelength received in the previous step, projecting from an optical assembly to a first location; detecting light projected in the previous step; receiving light from a second field; responsive to the wavelength received in the previous step, projecting from the optical assembly; detecting light projected in the previous step; and processing a result of the detecting steps, wherein the optical assembly is stationary relative to the first location. 
     According to another aspect of the present invention, a system comprises a first projector that projects light received from a first field to a first location; a detector that generates electrical signal responsive to light from the first projector; a second projector that projects light received from a second field; and a processor that receives and processes electrical signals representing light from first and second projectors, wherein the first and second projectors are stationary relative to the detector. 
     According to yet another aspect of the present invention, a system comprises means for receiving light from a first field; means responsive to a wavelength received by the previous means, for projecting from an optical assembly to a first location; means for detecting light projected by the previous mean s; means for receiving light from a second field; means responsive to the wavelength received in the previous step, for projecting from the optical assembly; means detecting light projected by the previous means; and means for processing signals from the detecting means, wherein the optical assembly is stationary relative to the detecting means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a satellite having multiple fields of view according to a first preferred embodiment of the invention. 
     FIG. 2 is a diagram showing another aspect of a satellite and fields of view of FIG.  1 . 
     FIG. 3 is a diagram of a satellite having multiple fields of view according to a second preferred embodiment of the invention. 
     FIG. 4 is a diagram showing another aspect of a satellite and fields of view of FIG.  3 . 
     FIG. 5 is a diagram of another satellite having multiple fields of view according to a second preferred embodiment of the invention. 
     FIG. 6 is a diagram for describing how make to a component of a preferred embodiment of the invention. 
     The accompanying drawings which are incorporated in and which constitute a part of this specification, illustrate embodiments of the invention and, together with the description, explain the principles and advantages of the invention. Throughout the drawings, corresponding parts are labeled with corresponding reference numbers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows system  100  including satellite  2  in orbit over ground  12  in accordance with a first preferred embodiment of the present invention. Satellite  2  operates in a wind profiling mode by collecting data about winds  14  and transmitting the data to ground station  16  via transmitter  47  on satellite  2 . 
     Satellite  2  includes light box  30  defining an aperture  6  through which light from outside of satellite  2  impinges on holographic unit  4 . Satellite  2  includes a transparent glass (not shown) in aperture  6 . 
     Light box  30  also defines  5  other windows through which light may travel from holographic unit  4  to respective light detectors. FIG. 1 shows  2  of the  5  other windows: field stop aperture  23  through which light may travel from holographic unit  4  to focal plane optics  32  to light detector  22 ; and field stop aperture  25  through which light may travel from holographic unit  4  to focal plane optics  34  to light detector  24 . 
     Holographic unit  4  acts to selectively diffract received light depending on angle and wavelength. Holographic unit  4  transmits the remainder of the received light undiffracted. 
     Holographic unit  4  includes holographic optical elements (HOEs)  41 ,  42 ,  43 ,  44 , and  45 . Each of HOEs  41 ,  42 ,  43 ,  44 , and  45  is a volume phase hologram having optical focusing power. Each of HOEs  41 ,  42 ,  43 ,  44 , and  45  is embedded in a common holographic film by multiple exposure such that each HOE acts on a different set of light rays corresponding to a specific wavelength and angle of incidence. Provided: there is sufficient angular separation between the planes of diffraction of the HOEs, the impact of each HOE on the performance of the others will be acceptable. 
     All 5 HOEs share the same full physical aperture  6  for the collection of light rays. Each hologram has a moderately wide field-of-view, and acts as a separate telescope looking in its own direction, with its own focus. Thus, holographic unit  4  has an effective wide-field spectral imaging capability. 
     Processor  40  includes circuitry to determine a wind profile. In this Disclosure, the term circuitry encompasses both dedicated hardware and programmable hardware, such as a CPU or reconfigurable logic array, in combination with programming data, such as sequentially fetched CPU instructions or programming data for a reconfigurable logic array. 
     Processor  40  includes circuitry to control laser  26 , causing laser  26  to emit laser beam  28  towards mirror  29 . Laser light reflected from mirror  29  travels to holographic unit  4 . Holographic unit  4  diffracts and collimates the light from mirror  29  to illuminate field of view  48 . 
     Laser light  28  back scattered from field of view  48  is depicted by solid lines  46  in FIG.  1 . Back scattered laser light  46  travels from field of view  48  to holographic unit  4 . Holographic unit  4  diffracts and focuses light  46  through field stop aperture  23 , to detector  22  via focal plane optics  32 . 
     Processor  40  includes circuitry to control laser  31 , causing laser  31  to emit laser beam  33  towards mirror  36 . Laser light reflected from mirror  36  travels to holographic unit  4 . Holographic unit  4  diffracts and collimates the light from mirror  36  to illuminate field of view  54 . 
     Laser light  33  back scattered from field of view  54  is depicted by dotted lines  52  in FIG.  1 . Back scattered laser light  52  travels from field of view  54  to holographic unit  4 . Holographic unit  4  diffracts and focuses light  52  through field stop aperture  25 , to detector  24  via focal plane optics  34 . 
     Processor  40  includes circuitry to process the detected signal from detector  22  and the detected signal from detector  24 , to send a signal to transmitter  47 . Responsive to the signal from processor  40 , transmitter  47  sends a signal to ground station  16 . Ground station  16  includes circuitry that receives and processes the signal transmitted by transmitter  47 . 
     FIG. 2 emphasizes another aspect of system  100 , including field of view  48 , field of view  54 , field of view  55 , field of view  56 , and field of view  57 . HOE  41  diffracts light from field  48  and focuses the diffracted light toward detector  22 . HOE  42  diffracts light from field of view  54  and focuses the diffracted light toward detector  24 . HOE  43  diffracts light from field of view  55  and focuses the diffracted light toward detector  37 , as shown by rays  58  in FIG.  2 . HOE  44  diffracts light from field of view  56  and focuses the diffracted light toward detector  38 . HOE  45  diffracts light from field of view  57  and focuses the diffracted light toward detector  39 . 
     In summary, HOE  41  of unit  4  acts to receive light from field  48  and, responsive to a wavelength from field  48 , HOE  41  projects to optics  32 . Detector  22  detects light from optics  32 . HOE  42  acts to receive light from fields  54  and, responsive to wavelength the from field  54 , HOE  42  projects to optics  34 . Detector  24  detects light form optics  34 . Processor  40  includes circuitry to process electrical signals generated by detectors  22  and  24 . Holographic unit  4  is stationary relative to optics  32 , detector  22 , optics  34 , and detector  24 . 
     In other words HOE  41  acts as a projector and HOE  42  acts as another projector. 
     FIG. 3 shows system  101  including satellite  102  in orbit over ground  12  in accordance with a second preferred embodiment of the present invention. Satellite  102  operates in a wind profiling mode by collecting data about winds  14  and transmitting the data to ground station  16  via transmitter  47  on satellite  102 . 
     Satellite  102  includes light box  130  defining an aperture  6  through which light from outside of satellite  102  impinges on holographic unit  104 . Satellite  102  includes a transparent glass (not shown) in aperture  6 . 
     Light box  130  also defines field stop aperture  123  through which light may travel from holographic unit  104  to focal plane optics  35  to light detector  11 . 
     Holographic unit  104  acts to selectively diffract received light depending on angle and wavelength. Holographic unit  104  transmits the remainder of the received light through the film undiffracted. Holographic unit  104  includes HOEs  141 ,  142 ,  143 ,  144 , and  145 . Each of HOEs  141 ,  142 ,  143 ,  144 , and  145  is a volume phase hologram having optical focusing power. Each of HOEs  141 ,  142 ,  143 ,  144 , and  145  is embedded in a common holographic film by multiple exposure such that each HOE acts on a different set of light rays corresponding to a specific wavelength and angle of incidence. 
     All 5 holograms share the same full physical aperture  6  for the collection of light rays. Each hologram has a moderately wide field-of-view, and acts as a separate telescope looking in its own direction, with its own focus. Thus, holographic unit  104  has an effective wide-field spectral imaging capability. 
     Processor  140  includes circuitry to control laser  26 , causing laser  26  to emit laser beam  28  towards mirror  29 . Laser light reflected from mirror  29  travels to holographic unit  104 . Holographic unit  104  diffracts and collimates the light from mirror  29  to illuminate field of view  48 . 
     Laser light  28  back scattered from field of view  48  is depicted by solid lines  46  in FIG.  3 . Back scattered laser light  46  travels from field of view  48  to holographic unit  104 . Holographic unit  104  diffracts and focuses light  46  through field stop aperture  123 , to detector  11  via focal plane optics  35 . 
     Processor  140  includes circuitry to control laser  31 , causing laser  31  to emit laser beam  33  towards mirror  36 . Laser light reflected from mirror  36  travels to holographic unit  104 . Holographic unit  4  diffracts and collimates the light from mirror  36  to illuminate field of view  54 . 
     Laser light  33  back scattered from field of view  54  is depicted by dotted lines  52  in FIG.  3 . Back scattered laser light  52  travels from field of view  54  to holographic unit  104 . Holographic unit  104  diffracts and focuses light  52  through field stop aperture  123 , to detector  11  via focal plane optics  35 . 
     Processor  40  includes circuitry to process the detected signals from detector  11 , to send a signal to transmitter  47 . Responsive to the signal from processor  140 , transmitter  47  sends a signal to ground station  16 . 
     FIG. 4 emphasizes another aspect of system  101 , including field of view  48 , field of view  54 , field of view  55 , field of view  56 , and field of view  57 . HOE  141  diffracts light from field  48  and focuses the diffracted light toward detector  11 . HOE  142  diffracts light from field of view  54  and focuses the diffracted light toward detector  11 . HOE  143  diffracts light from field of view  55  and focuses the diffracted light toward detector  11 ., as shown by rays  68  in FIG.  4 . HOE  144  diffracts light from field of view  56  and focuses the diffracted light toward detector  11 . HOE  145  diffracts light from field of view  57  and focuses the diffracted light toward detector  11 . 
     FIG. 5 shows system  103  including satellite  3  operating in a terrain mapping mode, collecting data from multiple ground footprints, in accordance with a third preferred embodiment for the present invention. Processor  141  includes circuitry to control lasers  26  and  31  and collect terrain map data. Points on ground  12  correspond to fixed points in the image planes of holographic unit  204 . 
     Holographic unit  204 , in satellite  3 , is a 5-exposure holographic element for the 1064 nm fundamental wavelength of the most common altimetry laser, Nd:YAG. Holographic unit  204  includes HOEs  241 ,  242 ,  243 ,  244 , and  245 . Each of HOEs  241 ,  242 ,  243 ,  244 , and  245  is a volume phase hologram having optical focusing power. Each of HOEs  241 ,  242 ,  243 ,  244 , and  245  is embedded in a common holographic film by multiple exposure such that each HOE acts on a different set of light rays corresponding to a specific wavelength and angle of incidence. Holographic unit  204  has a diameter of approximately 25-30 cm. Each HOE is configured to image an extended target spanning a field of view of 9.5 degrees wide with at least 1 milliradian resolution over the entire field. A linear array of laser pulses across each field of view maps a 9.5 degree wide swath as the satellite  3  orbits overhead. 
     The five field of views of the second embodiment are laid out such that the ground-track swaths are contiguous with each other, creating a total 46 degree wide swath with a small amount of overlap between field of views. 
     Processor  141  includes circuitry to control laser  26 , causing laser  26  to emit laser beam  28  towards mirror  29 . Laser light reflected from mirror  29  travels to holographic unit  204 . Holographic unit  204  diffracts and collimates the light from mirror  29  to illuminate the ground within field of view  148 . 
     Laser light  28  back scattered from the ground within field of view  148  is depicted by solid lines  46  in FIG.  5 . Back scattered laser light  46  travels from the ground within field of view  148  to holographic unit  204 . Holographic unit  204  diffracts and focuses light  46  through field stop aperture  23 , to detector  22  via focal plane optics  32 . 
     Processor  141  includes circuitry to control laser  31 , causing laser  31  to emit laser beam  33  towards mirror  36 . Laser light reflected from mirror  36  travels to holographic unit  204 . Holographic unit  204  diffracts and collimates the light from mirror  36  to illuminate the ground within field of view  154 . 
     Laser light  33  back scattered from the ground within field of view  154  is depicted by dotted lines  52  in FIG.  5 . Back scattered laser light  52  travels from the ground within field of view  154  to holographic unit  204 . Holographic unit  204  diffracts and focuses light  52  through field stop aperture  25 , to detector  24  via focal plane optics  34 . 
     Processor  141  includes circuitry to process the detected signal from detector  22  and the detected signal from detector  24 , to send a signal to transmitter  47 . Responsive to the signal from processor  141 , transmitter  47  sends a signal to ground station  16 . 
     Variations on the basic concept include a single laser steered using acousto-optical Bragg deflectors or other means to consecutively select holographic optical elements, instead of multiple lasers to address the various holographic optical elements. 
     The relevant holographic optical element performance parameters are efficiency, blur circle, focal ratios, scattered light levels, cross-talk, and background light rejection. Diffraction efficiency requirements may limit the number of holographic optical elements that can be multiplexed, and can be traded for an increased number holographic of optical elements and corresponding field of views. 
     In other alternate embodiments, the transmitter optics used to introduce the transmitted laser beam into the receiver optic axis can be a separate unit for transmitting only. This could be several single hologram, of the same type of multiplexed hologram as the receiver, or other suitabut the focal spots from which the lasers emanate would be offset from the receiver foci. The receiver foci may now be superimposed if desired, so that a single detector can be used for all of the receivers, provided the transmitted pulses from the various lasers are sufficiently separated in time so as not to cause the lidar return signals to overlap. 
     Preferred embodiments of the invention allow a smaller aperture size for a given flight altitude and laser size. Alternatively, since the technology is scalable to very large (1 meter) sizes, it allows a smaller laser system for a given aircraft altitude or higher altitudes with corresponding wider swath widths on the ground. 
     Detectors that convert light signals to electrical signals may be located behind the field stops or may instead be spatially displaced from the field stops and light passing through the field stops could travel to the detectors via fiberoptic cables. 
     FIG. 6 is a diagram for describing how to make holographic unit  4 . Deposit a film emulsion  61  on an optically transparent substrate. Illuminate emulsion  61  with two mutually coherent laser beams  62  and  63 , to create an interference pattern. Beam  62 , generally referred to as the object beam, contains spherical wavefronts emanating from a point source  65 . Beam  63 , generally referred to as a collimated beam or reference beam, contains plane wavefronts. The angle between a line, from the center of emulsion  61  to point source  65 , and the plane of emulsion  61 , during the manufacturing process, corresponds to the angle between holographic unit  4  and detector  22  during the operation of satellite  2 . 
     In addition, illuminate emulsion  61  with two mutually coherent laser beams  67  and  68 , to create an interference pattern. Beam  67  contains spherical wavefronts emanating from a point source  70 . Beam  68  contains plane wavefronts. The angle between a line, from the center of emulsion  61  to point source  70 , and the plane of emulsion  61 , during the manufacturing process, corresponds to the angle between holographic unit  4  and detector  24  during the operation of satellite  2 . 
     After exposure to the beams, wash the photo-sensitive material, typically ammonium dichromate, from the emulsion and fix the film&#39;s interference pattern by chemical processing commensurate with the type of film emulsion used. In dichromated gelatin, for example, the photo-exposed regions produce molecular cross-links that increase the gelatin hardness and refractive index in those regions. 
     Although the illustrated embodiments show holograms, residing on a common film, acting as light diffractors, the invention in its broadest sense may be practiced with other types of diffractive optical elements, such as separate holographic films stacked in layers, such as surface holograms, or binary (digital) optical elements. 
     Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or the scope of Applicants&#39; general inventive concept. The invention is defined in the following claims.