Abstract:
The present invention provides an economically feasible robust spatial heterodyne spectroscopy (SHS) interferometer. A first type prior art monolithic SHS interferometer is exceedingly expensive, whereas a second type of prior art SHS interferometer is extremely large and has many components, which need to be tuned. The present invention is much less expensive than the first type of prior art SHS interferometer and is much smaller that the second type of prior art SHS interferometer.

Description:
[0001]     This Application claims the benefit of U.S. Provisional Application No. 60/740,295, filed 11/18/2005, the entire disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     Spatial heterodyne spectroscopy (SHS) was conceived in the late 1980s by Prof. Fred Roesler and his graduate student John Harlander at the University of Wisconsin. SHS interferometers include a beamsplitter two gratings, and two (optional) field-widening prisms. A detailed description of SHS can be found in “Robust monolithic ultraviolet interferometer for the SHIMMER instrument on STPSat-1 ”, APPLIED OPTICS , vol. 42, Nol. 15, 20 May 2003, by J. M. Harlander et al. The optical components of SHS interferometers have to be mounted within tight. interferometeric tolerances to achieve high performance. There are generally two types of conventional SHS interferometer configurations, those with discrete optical elements and those with a monolithic optical element.  
         [0003]      FIG. 1  illustrates an exemplary convention SHS spectrometer having an interferometer of discrete optical elements. In the figure, the 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 .  
         [0004]     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 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 1  of collimated light λ 1  is transmitted through mirror  106  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  11   6 . 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 .  
         [0005]     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 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.  
         [0006]     The optical components of intertferometer  100  are individually mounted using commercial or custom-made mechanical mounting techniques like lens holders and three-point mounts. Due to the tight tolerances, the holders are typically adjustable, so the interferometer can be aligned after its assembly. In order to build a rugged interferometer, the holding fixtures have to be very stiff and the adjustable optics mounts get complicated. For example, all elements may typically be held individually in adjustable mounts within a steel fixture. The weight of such an interferometer (−7 kg) is dominated by the steel fixture that is necessary to keep the optical components in position. Alternatively a laboratory breadboard interferometer assembly may use commercial fixtures to hold the individual components. This set up is particularly sensitive to vibration since the commercial mounts are not optimized for stiffness. In any event such conventional discrete optical element types of SHS interferometers generally are relatively heavy as a result of the required mounting systems and have very time consuming adjustment procedures.  
         [0007]     The conventional discrete optical elements design of SHS is appropriate for laboratory investigations but the inherent lack of ruggedness excludes these designs from virtually all operational applications that are based on platforms like land vehicles, airplanes, unmanned aerial vehicles (UAVs), satellites, or even a handheld device which has to withstand rough environments to be reliable. Moreover, the realignment of a misaligned interferometer is not trivial and requires a trained person and appropriate equipment, thus further hampering the use of this assembly technique for commercial or military devices.  
         [0008]      FIG. 2  illustrates a conventional SHS interferometer comprising a monolithic design.  FIG. 2  does not include input optics, output optics or a detector as illustrated in  FIG. 1 . However, one of skill in the art would understand the operation of the configuration illustrated in  FIG. 2  Within a spectrometer.  
         [0009]     In the figure, an interferometer  200  includes a first leg portion  202 , a second leg portion  204  and an optical beam splitter  206  having a half mirror  208  therein. First leg portion  202  includes a reflective grating  210 , a spacer  212 , a prism  216  and a spacer  214 . Similarly, second leg portion  204  includes a reflective grating  218 , a spacer  220 , a prism  224  and a spacer  222 .  
         [0010]     In operation, input light  226  passes into beamsplitter  206  and a portion of which  240 , ultimately exits. Specifically, input light  226  is incident upon half mirror  208  and first portion  228  of input light  226  is reflected toward first leg portion  202  and a second portion  232  is transmitted toward second leg portion  204 . In a manner similar to the system illustrated in  FIG. 1 , portion  228  of the input light transmits through prism  216 , which is then refracted by an angle toward grating  210 . Grating  210  reflects the light back through prism  216  and toward beam splitter  206 , where the light is partially reflected at half mirror  208 , wherein portion  236  is transmitted to an output face of beamsplitter  206  and wherein portion  240  is transmitted to the input face of beamsplitter  206 . Similarly portion  232  of the input light transmits through prism  224 , which is then refracted by an angle toward grating  220 . Grating  220  reflects the light back through prism  224  and toward beam splitter  206  where the light is partially reflected at half mirror  208 , wherein portion  238  is transmitted to an output face of beamsplitter  206  and wherein portion  242  is transmitted to the input face of beamsplitter  206 . Output  244  is a combination of light portion  236  and light portion  238 , which eventually is detected as an interference pattern.  
         [0011]     The main difference between the system illustrated in  FIG. 1  and the device illustrated in  FIG. 2  is that interferometer  200  of  FIG. 2  includes spacers  212 ,  214 ,  220  and  222 , which enables interferometer  200  to be monolithic.  
         [0012]     The main driver for a monolithic SHS interferometer is its inherent ruggedness which was lacking in the system of  FIG. 1 . Spacers  212 ,  214 ,  220  and  222  maintain alignment of the remaining optical components. In such a conventional monolithic type device, the optical components are optically contacted with spacers to form a truly monolithic piece of glass. Specifically, spacer  214  is in optical contact with beamsplitter  206  and prism  216 , spacer  212  is in optical contact with prism  216  and grating  210 , spacer  222  is in optical contact with beamsplitter  206  and prism  224 , and spacer  220  is in optical contact with prism  224  and grating  218 .  
         [0013]     Optical contacting is a method where the interfacing surfaces of two components are polished to extremely high flatness (several nanometers) before they are contacted. The close proximity of the flat surfaces causes the van der Waals forces to form a strong bond between the components without any adhesives. The lack of a layer of adhesive between components is beneficial, mainly because the thickness of the adhesive layers does not have to be controlled during assembly, which simplifies the “self alignment” during the interferometer assembly, which is then provided by the spacers alone. In order to get a strong bond and to avoid stress due to unequal thermal expansion coefficients, the spacers are made from the same material as the optical components.  
         [0014]     Unfortunately, as a consequence of the strong bond between the surfaces, a monolithic interferometer cannot easily be disassembled without risking the destruction of the entire interferometer. The monolithic design provides lighter intrinsically aligned interferometers that are insensitive to vibration. However, polishing the interface surfaces to meet the precision and accuracy required for the optical contacting is very labor intensive, since it has to be performed partly by hand. Therefore, this production technique is very expensive and time consuming (over one hundred thousand USD per interferometer). A monolithic interferometer is appropriate e.g. for one of a kind satellite instruments, but again, it is not suitable for wide spread applications in the commercial or military sector due to its high cost.  
         [0015]     In summary, the conventional design and assembly techniques for SHS interferometers seriously impede the development of SHS spectrometers for wide spread applications in the commercial or military sector. The main reasons are either the sensitivity to vibration or the prohibitive cost per unit.  
         [0016]     What are needed are SHS interferometers that have the ruggedness of a monolithic device, while significantly reducing the cost per unit. These will finally allow the development of commercial and military SHS devices for application areas where SHS is superior to current spectroscopic techniques like Fourier transform spectroscopy or grating spectroscopy.  
         [0017]     There is an increased interest in spectroscopy, such as passive remote sensing for multiple purposes like intelligence gathering (monitoring of exhaust fumes), tactical battlefield applications (chemical threat identification), or tagging, tracking and locating.  
       BRIEF SUMMARY  
       [0018]     It is an object of the present invention to overcome the problems associated with conventional SHS interferometers.  
         [0019]     The present invention provides a technique that simplifies the production, assembly, and alignment of SHS interferometers, while preserving the robustness achieved by monolithic SHS interferometers.  
         [0020]     The present invention provides a way to fabricate rugged SHS interferometers in a faster, more flexible, and much more cost effective way by avoiding optical contacts. Avoiding optical contacts significantly relaxes the surface flatness requirements. The mechanical tolerances are then driven by the optical performance only, which yields especially relaxed tolerances for longer wavelengths, e.g. in the infrared. It also provides the option of using different spacer materials in the interferometer, which allows more flexibility in the temperature compensation of the interferometer if it is needed.  
         [0021]     The invention includes a type of SHS interferometer design and assembly technique wherein the optical components, e.g., the beamsplitter, gratings and optional prisms, are separated by properly dimensioned spacers. Further, the alignment of the interferometer components and spacers is achieved using alignment edges, points, and surfaces. The surface flatness of the components and spacers are only driven by the interferometer performance need, not the optical contacting. Unlike conventional systems, components in the present invention are held together as a group by externally applied compression forces, not adhesives or van der Waals forces. Once the compression forces are applied alignment edges/surfaces/points that do not support the compression can be removed. Still further, the spacers in accordance with the present invention can be made of materials other than the materials of the other optical elements.  
         [0022]     The invention combines the ruggedness, self-alignment, and a lightweight of the monolithic design with the flexibility and cost effectiveness of the discrete components design. Accordingly, the main obstacles of the conventional systems can be overcome, which have previously impeded the application of SHS for widespread commercial or military applications, namely the vibration sensitivity and unit cost.  
         [0023]     Advantages of the compression assembly of the present invention over discrete components include the compression assembly being more robust and being self-aligned. The specific advantages of the compression assembly of the present invention over an optically contacted, monolithic interferometer include: the surface flatness requirements being relaxed because they arc not driven by the optical contacting (i.e., there is a significant cost advantage); the compression assembly being easily disassembled and components therein being easily replaced; and there being no need to manufacture the spacers from the same material as the optical components.  
         [0024]     Additional objects, advantages and novel features of the invention are set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     
    
     BRIEF SUMMARY OF THE DRAWINGS  
       [0025]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate an exemplary embodiment of the present invention and together with the description, serve to explain the principles of the invention. It is noted that the exemplary embodiment is drawn to iris recognition. In the drawings:  
         [0026]      FIG. 1  illustrates a conventional spectrometer having a conventional SHS interferometer comprising discrete optical elements;  
         [0027]      FIG. 2  illustrates a conventional SHS interferometer comprising a monolithic design;  
         [0028]      FIG. 3  shows the fringe pattern from a monochromatic source obtained using a conventional spectrometer, having an SHS interferometer comprising a monolithic design;  
         [0029]      FIG. 4  illustrates a SHS cubic interferometer compression assembly in accordance with an exemplary embodiment of the invention; and  
         [0030]      FIG. 5  illustrates a SHS hexagonal interferometer compression assembly in accordance exemplary embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0031]      FIG. 4  illustrates a cubic interferometer SHS compression assembly for use in a SHS interferometer in accordance with an exemplary embodiment of the invention. As seen in the figure interferometer  400  includes an optical beamsplitter  406 , a first leg portion  402 , a second leg portion  404 , a First retaining portion  464 , and a second retaining portion  466 . Optical beamsplitter  406  includes an input face  456 , an output face  458 , a first processing face  460 , and a second processing face  462 . First leg portion  402  includes a first optical grating  410 , a first field-widening prism  416  and a first spacer system. Second leg portion  404  includes a second optical grating  418 , a second field-widening prism  424  and a second spacer system. First retaining portion  464  keeps first leg portion  402  in contact with optical beamsplitter  406  at first processing face  460 . Second retaining portion  466  keeps the second leg portion  404  in contact with optical beamsplitter  406  at second processing face  462 .  
         [0032]     In the exemplary embodiment as illustrated in the figure interferometer  400  additionally may include a first leg-restricting portion including items  426  and  428  and a second leg-restricting portion including items  430  and  432 . Items  426  and  428  of first leg-restricting portion prevent relative movement between first optical grating  410 , first field-widening prism  416  and the first spacer system. The first leg-restricting portion may comprise other devices or mechanisms that prevent relative movement between first optical grating  410 , first field-widening prism  416  and the first spacer system, a non-limiting example of which includes a unitary enveloping sleeve. Items  430  and  432  of second leg-restricting portion prevent relative movement between second optical grating  418 , second field-widening prism  424  and the second spacer system. The second leg-restricting portion may comprise other devices or mechanisms that prevent relative movement between second optical grating  418 , second field-widening prism  424  and the second spacer system, a non-limiting example of which includes a unitary enveloping sleeve.  
         [0033]     First retaining portion  464  may comprise a first compression force applicator, and second retaining portion  466  may comprise a second compression force applicator. The compression force may be applied by any known method, non-limiting examples of which include: a spring with reproducible compression force to assure an equal amount of compression force each time an interferometer is assembled; and lapped metal interface surface that avoids isolated pressure or stress points in the optical component. An optional counter force application piece  434  may be included, which provides a counter force against the compression force applied by first retaining portion  464  and the compression force applied by second retaining portion  466 . Of course optional counter force application piece  434  may not be included if for example, beamsplitter  406  is rendered immobile.  
         [0034]     In the exemplary embodiment as illustrated in the figure, first spacer system may include a first spacer  412  and a second spacer  414 , wherein first spacer  412  is disposed between first optical grating  410  and first field-widening prism  416 , and wherein first field-widening prism  416  is disposed between first spacer  412  and second spacer  414 . Second spacer system may include a third spacer  420  and a fourth spacer  422 , wherein third spacer  420  is disposed between second optical grating  418  and second field-widening prism  424 , and wherein second field-widening prism  424  is disposed between third spacer  420  and the fourth spacer  422 . Similar to the conventional SHS interferometers as discussed above, field-widening prisms  416  and  424  are optional and are used to compensate for non-paraxial rays within the interferometer to increase the interferometer throughput.  
         [0035]     In the exemplary embodiment as illustrated in the figure, optical beamsplitter  406  is a cubic optical beamsplitter, wherein input face  456  is opposite the second processing face  462 , and wherein output face  458  is opposite the first processing face  460 .  
         [0036]     In operation, input light  436  passes into beamsplitter  406  at input face  456  and a portion  450  of input light  436  ultimately exits at output face  458 . Specifically, input light  436  is incident upon half mirror  408  and first portion  438  of input light  436  is reflected toward first leg portion  402  and a second portion  442  is transmitted toward second leg portion  404 . Portion  438  transmits through prism  416 , which is then refracted by an angle toward grating  410 . Grating  410  reflects light  440  back through prism  416  and toward beam splitter  406 , where the light is partially reflected at half mirror  408 , wherein portion  446  is transmitted to output face  458  of beamsplitter  406  and wherein portion  452  is transmitted to input face  456  of beamsplitter  406 . Similarly, portion  442  transmits through prism  424 , which is then refracted by an angle toward grating  418 . Grating  418  reflects light  444  back through prism  424  and toward beam splitter  406 , where the light is partially reflected at half mirror  408 , wherein portion  448  is reflected to output face  458  of beamsplitter  406  and wherein portion  454  is transmitted to input face  456  of beamsplitter  406 . Output  450  is a combination of light portion  446  and light portion  448 , which eventually is detected as an interference pattern.  
         [0037]      FIG. 5  illustrates a hexagonal SHS compression assembly for use in a SHS interferometer in accordance with another exemplary embodiment of the invention. As seen in the figure, interferometer  500  includes an optical beamsplitter  506 , a first leg portion  502 , a second leg portion  504 , a first retaining portion  564 , and a second retaining portion  566 . Optical beamsplitter  506  includes an input face  556 , an output face  558 , a first processing face  560 , and a second processing face  562 . First leg portion  502  includes a first optical grating  510 , a first field-widening prism  516  and a first spacer system. Second leg portion  504  includes a second optical grating  518 , a second field-widening prism  524  and a second spacer system. First retaining portion  564  keeps first leg portion  502  in contact with optical beamsplitter  506  at first processing face  560 . Second retaining portion  566  keeps the second leg portion  504  in contact with optical beamsplitter  506  at second processing face  562 .  
         [0038]     In the exemplary embodiment as illustrated in the figure, interferometer  500  additionally may include a first leg-restricting portion including items  526  and  528  and a second leg-restricting portion including items  530  and  532 . Items  526  and  528  of first leg-restricting portion prevent relative movement between first optical grating  510 , first field-widening prism  516  and the first spacer system. The first leg-restricting portion may comprise other devices or mechanisms that prevent relative movement between first optical grating  510 , first field-widening prism  516  and the first spacer system, a non-limiting example of which includes a unitary enveloping sleeve. Items  530  and  532  of second leg-restricting portion prevent relative movement between second optical grating  518 , second field-widening prism  524  and the second spacer system. The second leg-restricting portion may comprise other devices or mechanisms that prevent relative movement between second optical grating  518 , second field-widening prism  524  and the second spacer system, a non-limiting example of which includes a unitary enveloping sleeve.  
         [0039]     First retaining portion  564  may comprise a first compression force applicator, and second retaining portion  566  may comprise a second compression force applicator. The compression force may be applied by any known method, non-limiting examples of which include: a spring with reproducible compression force to assure an equal amount of compression force each time an interferometer is assembled; and lapped metal interface surface that avoids isolated pressure or stress points in the optical component. An optional counter force application piece  534  may be included, which provides a counter force against the compression force applied by first retaining portion  564  and the compression force applied by second retaining portion  566 . Of course optional counter force application piece  534  may not be included if, for example, beamsplitter  506  is rendered immobile.  
         [0040]     In the exemplary embodiment as illustrated in the figure, first spacer system may include a first spacer  512  and a second spacer  514 , wherein first spacer  512  is disposed between first optical grating  510  and first field-widening prism  516 , and wherein first field-widening prism  516  is disposed between first spacer  512  and second spacer  514 . Second spacer system may include a third spacer  520  and a fourth spacer  522 , wherein third spacer  520  is disposed between second optical grating  518  and second field-widening prism  524 , and wherein second field-widening prism  524  is disposed between third spacer  520  and the fourth spacer  522 . Similar to the conventional SHS interferometers as discussed above, field-widening prisms  516  and  524  are optional and are used to increase the interferometer throughput.  
         [0041]     In the exemplary embodiment as illustrated in the figure, optical beamsplitter  506  is a hexagonal optical beamsplitter, wherein input face  556  is opposite second processing face  562 , and wherein output face  558  is opposite first processing face  560 .  
         [0042]     In operation, input light  536  passes into beamsplitter  506  at input face  556  and a portion  550  of input light  536  ultimately exits at output face  558 . Specifically, input light  536  is incident upon half mirror  508  and first portion  538  of input light  536  is reflected toward first leg portion  502  and a second portion  542  is transmitted toward second leg portion  504 . Portion  538  transmits through prism  516 , which is then refracted by an angle toward grating  510 . Grating  510  reflects light  540  back through prism  516  and toward beam splitter  506 , where the light is partially reflected at half mirror  508 , wherein portion  546  is transmitted to output face  558  of beamsplitter  506  and wherein portion  552  is transmitted to input face  556  of beamsplitter  506 . Similarly, portion  542  transmits through prism  524 , which is then refracted by an angle toward grating  518 . Grating  518  reflects light  544  back through prism  524  and toward beam splitter  506 , where the light is partially reflected at half mirror  508 , wherein portion  548  is reflected to output face  558  of beamsplitter  506  and wherein portion  554  is transmitted to input face  556  of beamsplitter  506 . Output  550  is a combination of light portion  546  and light portion  548 , which eventually is detected as an interference pattern.  
         [0043]     The present invention, for example as illustrated in  FIG. 4  or  FIG. 5  provides a way to fabricate rugged SHS interferometers in a faster, more flexible, and much more cost effective way by avoiding optical contacts. Specifically, the retaining portions of the present invention remove the requirement for optical contacts, and therefore significantly relax the surface flatness requirements. The mechanical tolerances for an SHS interferometer in accordance with the present invention are thereby driven by the optical performance only, which yields especially relaxed tolerances for longer wavelengths, e.g. in the infrared. Furthermore, without optical contacts, the present invention provides the option of using different spacer materials in the interferometer. This feature permits more flexibility in the temperature compensation of the interferometer if it is needed and further promotes easy switching of optical components for varying uses. Once the compression forces are applied, alignment edges/surfaces/points that do not support the compression can be removed. Still further, the spacers in accordance with the present invention can be made of materials other than the materials of the other optical elements.  
         [0044]     The foregoing description of various preferred embodiments of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiments, as described above, were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.