Patent Publication Number: US-2009231592-A1

Title: Refractive spatial heterodyne spectrometer

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a spatial heterodyne spectrometer. More particularly, the invention is directed to a refractive spatial heterodyne spectrometer that employs a mirror and a dispersing prism in lieu of a diffraction grating in each arm. 
     Passive remote sensing is increasingly useful in myriad applications, including industrial, scientific, and military. Military applications include intelligence gathering, e.g. monitoring exhaust fumes to infer the nature and scope of industrial processes , tactical battlefield applications such as chemical threat identification, and tagging, tracking, and location efforts. 
     Spatial heterodyne spectroscopy (SHS), e.g. as described in U.S. Pat. No. 5,059,027, Roesler et al., issued Oct. 22, 1991, and incorporated herein by reference, has primarily been used for ultraviolet applications that require very high spectral resolution and a narrow passband. Recently, SHS has also been considered for applications that require moderate resolution, e.g. the SHIM-Fire breadboard instrument that has a passband in the near infrared (700 nm-900 nm) with a spectral resolution of 0.7 nm. In the future, SHS instruments with even lower resolution (resolving power of a few hundred) are planned e.g. for the remote detection of atmospheric gasses. The two main methods that are currently used for these moderate resolution applications are diffraction grating spectroscopy and conventional Fourier transform spectroscopy (FTS). Depending on the specific requirements of the application, SHS can be superior to the other methods. For instance, if the target is rapidly changing, FTS (but not SHS) is forced to scan rapidly in order not to confuse spectral and temporal information. 
     SHS is similar to a Michelson interferometer but the mirrors terminating the interferometer arms are replaced by fixed, tilted diffraction gratings. A basic SHS configuration 10 is illustrated in  FIG. 1 . An SHS spectrometer  100  includes input optics, an interferometer and output optics. The input optics include an input aperture  102 , and collimating lens  104 . The interferometer includes a beam splitter  106 , prism  108 , prism  110 , grating  112 , and grating  114 . The output optics include focusing lens  116 , collimating lens  118  and detector  120 . 
     In operation, input light passes through input aperture  102  and diverges to collimating lens  104 . Collimated light λ 1  includes an incident wave front  122 . Collimated light λ 1  is then incident upon beam splitter  106 . A first portion of collimated light λ 2  is reflected in a first arm  123  of spectrometer  100  toward prism  108 , which is then refracted by an angle  124  toward grating  112 . Grating  112  reflects light λ 3  back through prism  108  and toward beam splitter  106 , where light λ 3  is partially reflected toward lens  104  and partially transmitted toward lens  116 . The output optics portion is designed to image the grating planes  112  and  114  onto the detector  120 . Here, the partially transmitted light λ 6  includes a wave front  128  and is focused by lens  116  to a point  134 . The light λ 6  then diverges toward lens  118  to be imaged on detector  120 . A second portion λ 4  of collimated light λ 1  is transmitted through beam splitter  106  in a second arm  129  of spectrometer  100  toward prism  110 , which is then refracted by an angle  126  toward grating  114 . Grating  114  reflects light λ 5  back through prism  110  and toward beam splitter  106 , where light λ 1  is partially transmitted toward lens  104  and partially reflected toward lens  116 . In the output optics portion, the partially reflected light λ 7  includes a wave front  130  and is focused by lens  116  to a point  134 . The light λ 7  then diverges toward lens  118  to be imaged on detector  120 . 
     Wave front  128  constructively and destructively interferes with wave front  130 , such that that image detected by detector  120  is an interference pattern. An example of such an interference pattern for a monochromatic source is illustrated in  FIG. 3 . The characteristics of the pattern are based on the wavelength of the light λ 1  and the angle  132  between wave front  128  and wave front  130 . Angle  132  is mainly based on the frequency of the input light λ 1  and the structure and angle of gratings  112  and  114 . Field-widening prisms  106  and  108  are optional, and merely compensate for non-paraxial rays within the interferometer, in order to increase the throughput. 
     One limitation of moderate resolution SHS interferometers is the order overlap. In that case, the angular region covered by the signal within the passband and one order of grating diffraction overlaps with the angular region covered by an adjacent order. Once the orders overlap, the relation between the wavelength and angle g is not unique any longer and the unwanted orders will contaminate the resulting interferogram and spectrum, resulting in spurious fringes and/or increased noise. It is therefore desirable to provide an SHS interferometer without such limitations. 
     BRIEF SUMMARY OF THE INVENTION 
     A refractive spatial heterodyne spectrometer includes an input aperture for receiving an input light; a collimating lens for collimating the input light into a collimated light; and a beamsplitter for reflecting one part of the collimated light into a first arm and transmitting another part of the collimated light into a second arm. The first arm includes a first dispersing prism for receiving and refracting the first part of the collimated light, and a first mirror positioned to reflect the refracted first collimated light back through the first dispersing prism and to the beamsplitter as a first light wavefront. The second arm includes a second dispersing prism for receiving and refracting the other part of the collimated light, and a second mirror positioned to reflect this refracted light back through the second dispersing prism and to the beamsplitter as a second light wavefront. The beamsplitter transmits a portion of the first light wavefront and reflects a portion of the second light wavefront into an output optics section to inferometrically combine into an interference image, and a detector receives the interference image and outputs an interference image pattern. 
     For moderate resolution applications, the throughput, and therefore the sensitivity, of prior SHS interferometer designs are limited by contamination from unwanted grating orders within the instrument passband. The invention avoids this limitation by employing refractive prisms in lieu of diffraction gratings since refractive prisms do not produce multiple orders. As a result the refractive SHS can achieve larger throughput and a larger spectral range in moderate or low resolution applications. The invention enables a smaller, lighter spectrometer, which is important for applications requiring minimal weight loadings, such as Unmanned Aerial Vehicles or other applications where the equipment has to be transported, e.g. by a warfighter for use in a battlefield environment. Its increased throughput provides higher sensitivity that provides faster threat recognition with lower false alarm rates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a schematic diagram of a prior art SHS spectrometer; 
         FIG. 2  is a schematic diagram of an SHS spectrometer according to the invention; and 
         FIG. 3  is an interferogram of a monochromatic source measured by the near infrared SHIM-Fire SHS instrument. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Definitions: As used herein, the term “field-widening prism” means a wedged, refractive elements whose purpose is to increase the throughput of the system by reducing the path difference change between on and off-axis rays. Exemplary field-widening prisms include prisms typically manufactured from low-dispersion glass. As used herein, the term “dispersing prism” means a wedged, refractive element whose purpose is to make the angle of deviation of the beam a function of wavelength. Exemplary dispersing prisms include prisms typically manufactured from a high-dispersion glass. 
     The invention is similar to the conventional SHS shown in  FIG. 1  except that the diffraction gratings in each arm are replaced by a mirror and a dispersing prism. Additionally in order to obtain a large interferometer field of view a second low-dispersion field widening prism is preferably inserted in each arm. The combination of two prisms in each arm results in a system that has no contamination from unwanted grating orders while at the same time retaining the advantages of conventional SHS (small size, extremely sensitive and robust). Referring now to  FIG. 2 , an SHS spectrometer  200  according to the invention receives input light through input aperture  202  that diverges to collimating lens  204 . Collimated light λ 1  is then incident upon beamsplitter  206  that in a first arm  203  reflects a first portion λ 2  to an optional low-dispersion field widening prism  208  at an angle γ F  with respect to the normal of its leading surface  209 , which refracts it to a dispersing prism  210  at an angle γ D  with respect to the normal of its leading surface  211  that then refracts it to a mirror  212  positioned perpendicular to light of a particular wavelength incident on it. Mirror  212  is thereby positioned to reflect light λ 3  back through prisms  210  and  208  to beamsplitter  206 . 
     In a second arm  205 , a second portion λ 4  of collimated light λ 1  is transmitted through beamsplitter  206  toward a second optional low-dispersion field widening prism  214  also positioned at angle γ F  with respect to the normal of its leading surface  215 , which refracts it to a second dispersing prism  216  also at angle γ D  with respect to the normal of its leading surface  217  that then refracts to a second mirror  218  positioned perpendicular to light of a particular wavelength incident on it. Mirror  218  is thereby positioned to reflect light λ 5  back through prisms  216  and  214  to beamsplitter  206 . 
     Light λ 3  is partially reflected by beamsplitter  206  toward lens  204  and partially transmitted as λ 6  having a wavefront  221  into the output optics portion  219 , and likewise light λ 5  is partially transmitted toward lens  204  and partially reflected as λ 7  having a wavefront  223  into the output optics portion where the wavefronts of λ 6  and λ 7  combine and are focused by lens  220  to a point  222 . The light then diverges toward lens  224  to be imaged on detector  226 , e.g. a CCD-based sensing system. 
     The output optics portion is designed to image the dispersing prism/mirror sections onto the detector  226 . The two wave fronts λ 6  and λ 7  constructively and destructively interfere such that that image detected by detector  226  is an interference pattern. An example of such an interference pattern is illustrated in  FIG. 3 . The characteristics of the pattern are based on the wavelength of the light λ 1  and the angle  227  between the wave fronts. Angle  227  is mainly based on the frequency of the input light λ 1  and the optical properties and angle of the two prisms/mirrors. As discussed, the field-widening prisms  208  and  214  are optional, and merely compensate for non-paraxial rays within the interferometer, in order to increase the throughput. 
     Both the dispersing (P D ) and field-widening (P F ) prisms shown in  FIG. 2  are oriented for minimum deviation at the Littrow wavelength. In this geometry the optical axis enters and leaves the prism symmetrically at angles γ D  and γ F , respectively. The relationship between the angles of incidence and the prism apex angles (α D  and α F ) are: 
       n D  sin(α D /2)=sin γ D   
       n F  sin(α F /2)=sin γ F   
     Where subscript D refers to the high-dispersing, or simply dispersing, as the term is employed herein, prism, and F refers to the low-dispersing field-widening prism. The relationship determining the resolving power (R 0 =λ/dλ where dλ is the minimum resolvable wavelength interval) of the all-refractive SHS is given by: 
     
       
         
           
             
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     Where W is the width of the beam in air.
 
The system achieves a wide field when the high and low dispersion prisms are oppositely oriented, as shown in  FIG. 2  and the prism angles γ D  and γ F  are given by:
 
     
       
         
           
             
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     It is also important that the system achieve a large field of view over a moderately large wavelength range. This can be accomplished using the condition: 
     
       
         
           
             
               
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     Where n D  and n F  are the refractive indices of the respective prism materials. 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that the scope of the invention should be determined by referring to the following appended claims.