Abstract:
Optical instruments having, inter alia, optics to process wavelengths of electromagnetic radiation to produce an interferogram. The instruments include at least one optical path and optical elements positioned along this path for splitting and recombining the wavelengths which interfere with each other to produce a plurality of different fringes of different wavelengths. In one group, the optics include matched gratings which are positioned along the optical path outside of the interferometer optics to produce first and second sets of spectrally dispersed beams. The interferometer optics also includes a beam splitter and first and second mirrors. The gratings may be positioned in a variety of locations along the optical path. In another group, the optics include a beam splitter having a plurality of surfaces, wherein each of the surfaces is either 100% reflective, 100% transmissive or 50% reflective and 50% transmissive. In a third group, the optics includes the beam splitter having a plurality of reflective and transmissive surfaces and matched gratings. The instruments can all include a detector for detecting the interferogram and means for processing the detected interferogram to produce spectral information.

Description:
GOVERNMENTAL RIGHTS CLAUSE 
   This invention is made with U.S. Government support under F 29601-96-C-0096 and F29601-98-0204 awarded by the U.S. Air Force, and N00178-02-3096 awarded by the U.S. Navy. The U.S. Government has certain rights in the invention. 

   FIELD OF THE INVENTION 
   The present invention relates to optical instruments which process wavelengths of electromagnetic radiation to produce an interferogram. More particularly, the present invention relates to instruments (e.g., Fourier transform spectrometers) that produce interferograms of a scene, which instruments include an optical system which both splits the incoming wavelengths and spectrally disperses them to produce two sets of spectrally dispersed beams. The dispersion is achieved by a matched pair of gratings positioned outside the interferometer optics. U.S. Pat. No. 6,687,007 B1 discloses embodiments wherein the matched pair of gratings is positioned inside the interferometer optics. 
   The present invention also relates to a new beam splitter which eliminates the 50% light loss inherent in the Sagnac (or common path) interferometer. 
   These instruments are useful in analyzing individual chemical species in absorption, emission, or reflected spectroscopy where there is a need to image a time and spatially varying scene. This could be, for example, imaging an emission plume for a jet or rocket engine or a smoke-stack, environmental observations, non invasive blood monitoring, and other medical observations. 
   BACKGROUND OF THE INVENTION 
   Imaging spectrometers are, broadly speaking, optical instruments which process the electromagnetic radiation from a source into its fundamental components. For instance, an interferometric based spectrometer divides light from a source and interferes it to produce a fringe pattern of interfering light (i.e., an interferogram). The interference pattern can be captured on film or by, for instance, an electronic detector, for example, a semi-conductor array detector (e.g., a charged coupled device (CCD)). 
   There are numerous optical interferometer designs. The basic form of the Sagnac (or common path) interferometer is illustrated in  FIG. 1 . It is also illustrated in U.S. Pat. No. 4,976,542 to Smith. Other designs include the Mach-Zender interferometer, the Michelson interferometer and Twyman-Green interferometer (See W. L. Wolfe, Introduction to Imaging Spectrometers, SPIE Optical Engineering Press, pp. 60–64, 1997), the Fabry-Perot interferometer (see Wolfe, p. 70–73), the Lloyd&#39;s mirror interferometer (see the Smith patent) and, a variation of the common path interferometer (Sagnac) sometimes referred to as the Barnes interferometer (see T. S. Turner Jr., et al., A Ruggedized Portable Fourier Transform Spectrometer for Hyperspectral Imaging Applications, SPIE Vol. 2585 pp 222–232.) There are also dispersive spectrometers such as prism spectrometers and grating spectrometers. (See Wolfe, pp. 50–52 and 55–57). 
   In a non-imaging Fourier transform spectrometer the point source of radiation is split into two virtual points a fixed distance apart to yield a fringe pattern at the detector. If one wants to attain a fine spectral resolution, the distance between the two virtual points should be large; for a course spectral resolution, it should be short. This distance may be controlled by shifting one of the mirrors (typically referred to as lateral shear) of, for instance, the common path interferometer. With this arrangement, a wide spectral range measurement loses resolution, while a high resolution measurement reduces the effective spectral range. In an imaging spectrometer, the point source is imaged with a set of imaging optics and a slit is inserted giving the instrument the capability of one-dimensional imaging in the direction perpendicular to the shear. 
   Shear, both lateral and angular, is discussed in Turner, Jr. et al. (supra). For the Sagnac, translation of either mirror in the plane of  FIG. 1  produces lateral shear. Mirror tilt about an axis perpendicular to the drawing plane also produces lateral shear. Conversely, in the Barnes interferometer only angular shear is possible and is produced only by mirror tilt. See  FIGS. 2 and 3  of Turner, Jr., et al. 
   U.S. Pat. No. 4,976,542 to W. H. Smith discloses a Fourier transform spectrometer which incorporates the common path (or Sagnac) interferometer and in which a charge-coupled device (CCD) is placed in the image plane instead of film. The CCD has pixels aligned along two dimensions to provide both spectral resolution and spatial resolution. The CCD is characterized by greater dynamic range, lower pixel response variation, and is photon nose limited, all of which enhances its use as a detector for a spectrometer. See also Digital Array Scanned Interferometers for Astronomy, W. H. Smith, et al., Experimental Astronomy 1: 389–405, 1991. In these devices, the interferometer introduces lateral shear in one direction and a two dimensional camera is aligned so a row of pixels is parallel to this geometric plane. In the perpendicular direction, a set of cylindrical lenses is used to provide an imaging capability along the columns of pixels. A row plot from the detector is an interferogram similar to the interferogram collected in a temporally modulated Michelson interferometer. 
   In a paper published in 1985, T. Okamoto et al. describe a method for optically improving the resolving power of the photodiode array of a Fourier transform spectrometer by modulating the spatial frequency of the interferogram with a dispersing element. With the use of a dispersing element, particularly an optical element with parallel surfaces, the distance between the two virtual sources varies with the wave number (the inverse of wavelength) of the source. Thus, as illustrated in  FIG. 2  of this reference, by placing their optical dispersive element into the optical path of a common path interferometer, the distance between the virtual source becomes a function of the wave number (i.e., the optical dispersive element refracts the blue beam more than the red beam, yielding a wide distance between S 1 blue and S 2 blue and a narrower distance between S 1 red and S 2 red). The authors claim that use of the optical parallel greatly enhances the resolution. In principle, the spectrometer can be designed to examine any wavelength band of interest by careful choice of the type of dispersive glass utilized and the thickness of the glass. See “Optical Method for Resolution Enhancement in Photodiode Array Fourier Transform Spectroscopy,” T. Okamoto et al, Applied Optics Vol. 24, No. 23, pp 4221–4225, 1 Dec. 1985. 
   The approach of Okamoto et al. has a number of drawbacks. First, because of the use of the dispersive block, the system no longer operates with constant wave number increments. This is in contrast with conventional Fourier transform spectrometers, which are constant wave number devices and are inherently spectrally calibrated. Thus, with Okamoto et al., blue wavelengths have a much smaller spectral resolution than red wavelengths, and the spectral calibration of the instrument becomes a major issue. Another drawback is that the spectral dispersion, while it enhances spectral resolution, adversely affects spatial resolution. Thus, the dispersive element would greatly increase the complexity of an imaging Okamoto et al. spectrometer. Another disadvantage of this technique is that its dependence on a dispersive material restricts its use to wavelengths that can be effectively transmitted through a dispersive element. Finally, the limited glass types that are available restrict the range of spectral enhancements available. While it is theoretically possible to use any dispersive glass and increase the size of the block to achieve the desired spectral enhancement, in practice the size of the block may become so large that the instrument is no longer practical. Also, since the enhancement depends on the glass type and size, the instrument designer has a limited number of parameters to use to optimize the spectrometer design and may not be able to arbitrarily set the lower and upper limits of the spectral region of interest. 
   In “Spatial Heterdoyne Spectroscopy: A Novel Interferometric Technique for the FUV,” J. Harlander et al., SPIE Vol. 1344, pp. 120–131 (1990), the authors describe an improved interference spectrometer which has no moving parts, can be field widened, and can be built in an all reflection configuration for UV applications, particularly FUV applications. Harlander et al. are addressing a different problem from that addressed in Okamoto et al. and approach their solution in a different manner (e.g., the use of angular shear instead of the lateral shear required by Okamato et al.). The basic concept (illustrated in  FIG. 1  of this reference) is based on a Michelson type interferometer in which the return mirrors are replaced by diffractive gratings. These gratings, which disperse the radiation, produce Fizeau fringes (i.e., interferograms) which are recorded by a detector positioned in the image plane. The Fourier transform of the fringe pattern recovers the spectrum. An all reflection version of the foregoing utilizes a collimator, a diffraction grating and two mirrors. Light from the collimator is split into two beams by the first half of the diffraction grating, which travel in different directions until they are recombined by the second half of the same grating and focused onto the detector by a mirror. This is illustrated in  FIG. 2  of this reference. See also, “Spatial Hetrodyne Spectroscopy for the Exploration of Diffuse Interstellar Emission Lines at For-Ultraviolet Wavelengths,” J. Harlander et al., The Astrophysical Journal, 396: 730–740, 1992 Sep. 10, and U.S. Pat. No. 5,059,027 to Roesler et al. All the designs suggested/disclosed require the use of collimated light and angular shear. 
   There are a number of drawbacks/limitations associated with the designs suggested/disclosed in the above referenced Harlander et al. publications and Roesler, et al. patent (collectively “Harlander et al.”). First of all, Harlander et al. do not disclose the concept of imaging a spatially varying scene. Their invention is discussed in the context of imaging a star or some other type of point source. They implicitly assume that the light coming into their optical system is homogenous and report a single spectra. In many cases this may not be true, and proper measurement of the scene would require spectra for each spatial element in the scene. Secondly, all of the Harlander et al. designs require collimating the input beam. Such designs are inherently more complicated than designs which do not require collimated light. Third, the Michelson design on which their designs are based is inherently less mechanically stable than the common path design, since the interferometer is not self-compensating for motions in the elements of the interferometer. It is also not clear if the concept of Harlander et al. is applicable to instruments which utilize lateral shear, as opposed to angular shear. Fourth, although not explicitly stated, all the designs of Harlander et al. require a re-imaging lens to image the virtual sources at infinity. Finally, Harlander et al. require a complex method for separating wavelengths below the central wavelength from those above the central wavelength. That is, a detected fringe pattern could have two different interpretations, it could be from a source a below the central wavelength or Δλ above. Harlander, et al., discusses methods for determining the true wavelength. 
   OBJECTS OF THE INVENTION 
   It is an object of the present invention to provide Fourier transform spectrometer which has all the advantages of the spectrometers disclosed in U.S. Pat. No. 6,687,007 B1, but which is: (a) easier to construct; (b) works in all wavelengths, including infrared and, particularly, long wave infrared (approximately 8–12 microns); and (c) has an increased optical throughput. 
   It is another object of the present invention to provide a Fourier transform spectrometer which both splits and spectrally disperses incoming wavelengths to produce two sets of spectrally dispersed beams in which the long wavelengths within the range of wavelengths of interest do not overlap. 
   It is yet another object of the present invention to provide a Fourier transform spectrometer which utilizes a matched pair of gratings to spectrally disperse the incoming beam of wavelengths, which pair of gratings are located outside that portion of the spectrometer&#39;s optical system, the interferometer optics portion, which splits the incoming beam into two paths. 
   It is still yet another object of the present invention to place the grating pair in front of the interferometer optics. 
   It is still yet another object of the present invention to place the grating pair between the interferometer optics and the detector of the Fourier transform spectrometer. 
   It is a further object of the present invention to provide for an improved beam splitter which effectively eliminates the light loss inherent in prior beam splitters utilized in common path interferometers. 
   It is still another object of the present invention to provide an improved beam splitter which can be used in a common path interferometer without the use of dispersive gratings. 
   The foregoing and other objects will be apparent from the drawings and the description set forth herein. 
   SUMMARY OF THE INVENTION 
   This invention relates to optical instruments having, inter alia, optics to process wavelengths of electromagnetic radiation to produce an interferogram. The instruments include at least one optical path and optical elements positioned along this path for splitting the wavelengths and spectrally dispersing them to produce first and second sets of spectrally dispersed beams which are subsequently interfered with each other to produce a plurality of different fringes of different wavelengths. The optics for dispersing the wavelengths may include at least one matched pair of gratings. The optics also includes a beam splitter, positioned along the optical path, for splitting the optical path, and first and second reflecting surfaces. The beam splitter and first and second reflecting surfaces constitute the interferometer optics. The gratings may be positioned along the optical path either in front of or after the interferometer optics. 
   Finally, the interferometer optics includes a novel beam splitter including an optically transmissive element having first and second surfaces. The first surface is divided into first, second and third zones. In each of these zones, the percentage of light that is either reflected or transmitted is described as substantially for the reason that no optics can be perfect. While there will be an extremely minimal amount of loss inherent to the optics, high quality parts can reduce this loss to, in some cases less than 0.1% of the total wavelengths incident. The first zone has a first coating which, for the wavelengths being split, is substantially 100% reflective. The second zone has a second coating which is allows for substantially 50% of the wavelengths to be reflected and 50% to be transmitted. The third zone is substantially 100% transmissive. The second zone is between the first and the third zone. The second surface may have an anti-reflective coating. The first and second surfaces are parallel. The novel beam splitter design can be used with or without the dispersive gratings of, for instance, the present invention. 
   The instruments further include an aperture positioned along the optical path to define one spatial dimension, a detector for detecting the interferogram positioned along the optical path, and optics for focusing the aperture on the detector to create a one dimension spectral image, and means for processing the detected interferogram to produce spectral information. 
   The invention also includes the method of spectrally dispersing the wavelengths to produce first and second sets of spectrally dispersed beams which interfere with each other to produce a plurality of different fringes of different wavelengths with the described instrumentation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an optical schematic of a prior art spectrometer incorporating a common path or Sagnac interferometer. 
       FIG. 2  is an optical schematic of the first embodiment of the present invention incorporated in a Sagnac interferometer in which the grating pair is placed in front of the interferometer. 
       FIG. 3  is an optical schematic of the second embodiment of the present invention, in which the grating pair is positioned along the optical path of the interferometer after the interferometer optics. 
       FIG. 4  is an additional partial optical schematic of the first embodiment of the present invention, particularly illustrating the novel beam splitter of the present invention and the reflected portion of the beam. 
       FIG. 5  is an additional partial optical schematic of the first embodiment of the present invention, particularly illustrating the novel beam splitter of the present invention and the transmitted portion of the beam. 
       FIG. 6  is a partial optical schematic of the first embodiment of the present invention illustrating, inter alia, the convergence of the transmitted and reflected portions of the beam. 
       FIG. 7  is an optical schematic of the second embodiment of the present invention in which the prior art beam splitter is replaced by the beam splitter of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to  FIG. 1 , Fourier transform spectrometer  11  processes an incident light source  13  through an aperture  15 , to a beam splitter  17 , where source  13  is divided into a reflected beam (represented by central ray path  19 ) and a transmitted beam (represented by central ray path  21 ). The portion of source  13  represented by path  19  is reflected from the front surface a first mirror  23  to the front surface a second mirror  25 , and then back to beam splitter  17 . The second, transmitted portion of source  13  is reflected off second mirror  25 , back to first mirror  23  and through beam splitter  17 . Thus, beam splitter  17 , together with mirrors  23  and  25 , serve to split incident source  13  into two portions. Spectrometer  11  also includes a detector  27  which is placed at the back focal plane of spherical (Fourier) lens  29 . (Aperture  15  is in the front focal plane.) A second, cylindrical lens  31  is interposed between detector  27  and spherical lens  29 , which images spatial locations from  15  onto detector  27 . As is well known in the art, spherical lens  29  and cylindrical lens  31  may be assembled from one of several optical elements in various sequences to minimize optical aberrations. As is well understood in the art, this basic arrangement produces an interfering light pattern or fringe pattern at the focus of spherical lens  29 , commonly referred to as an interferogram. Cylindrical lens  31  images the relative physical distribution of source  13  as selected by aperture  15  such that as it impinges on detector  27  it represents the relative spacing for the various sources and their locations in source  13 . The detector  27  is a charge-coupled device, or CCD. Alternatively, any photon counting array noise limited device, or other means of recording the optical signature, may be used. For an electric device, the output of detector  27  is processed by a computer  33  using Fourier transform techniques as is well known in the art to resolve the spectrum detected by detector  27 . 
   To correct the problems inherent in the interferometer of Okamoto et al., and achieve the objectives set forth above, a pair of matched gratings is incorporated into the interferometer of the present invention. Thus, with reference to  FIGS. 2 and 3 , matched gratings have been integrated into the conventional interferometer design.  FIG. 2 , spectrometer  41 , includes an aperture  43 , first and second matched gratings  45  and  47 , beam splitter  49 , first mirror  51 , second mirror  53 , lens system  55  and detector  57 . For grating pair  45 ,  47 , the choice of grating pitch, grating order, etc., is determined in the manner set forth with regard to, for instance, the embodiments of FIGS. 2 and 3 of U.S. Pat. No. 6,687,007 B1. Beam splitter  49 , is, in this embodiment, of conventional design. Mirrors  51  and  53  are typically first surface mirrors. Beam splitter  49 , together with mirrors  51  and  53  is sometimes referred to as interferometer optics  59 . Lens system  55  is of the conventional Fourier lens/cylindrical lens combination used to image spatial locations from aperture  43  onto detector  57 , as discussed above with regard to  FIG. 1 . Further, as with the prior embodiment, the output of detector  57  is processed by a computer (not shown) using well known Fourier transform technique to resolve the spectrum detected. 
   In operation, incident light, represented by central ray path  61 , passes through aperture  43  and onto first and second gratings  45  and  47  where it is dispersed to produce the desired amount of lateral wavelength dependant spectral spreading of the light. This is illustrated in FIG. 2 of U.S. Pat. No. 6,687,007 B1. As with, for instance, the embodiments of FIGS. 2 and 3 of U.S. Pat. No. 6,687,007 B1, in addition to producing the desired lateral wavelength dependant spectral spreading, the use of grating pair  45  and  47  allows for an easy adjustment of the amount of lateral spectral spread in the system, without introducing any optical aberrations, because the gratings diffract light only in a plane perpendicular to the grooves. 
   After being dispersed by gratings  45  and  47 , the light is divided into a reflected beam, represented by central ray path  63  and a transmitted beam, represented by central ray path  65 . As those skilled in the art appreciate, ray paths  63  and  65  are recombined by lens system  55  and focused onto detector  57 . This invention has the advantage over the prior art of allowing spectral mapping to be conducted at selectable wavelengths, defined by the choice of grating specifications. 
     FIG. 3 , spectrometer  71  illustrates an alternate embodiment of the present invention in which the grating pair is positioned after the interferometer optics. Spectrometer  71  includes the aperture  43 , beam splitter  49 , first mirror  51  and second mirror  53  of interferometer optics  59 , and detector  57 , which are identical to those depicted in  FIG. 2  and function to split incident radiation into two separate beams.  FIG. 3  also depicts lens system  85  which is the functional equivalent of lens system  55  of  FIG. 2 . As is well known in the art, lens system  85  includes a spherical (Fourier) lens  87  and a cylindrical lens  89  and functions to recombine the split beam paths onto detector  57 . As is well known in the art, spherical lens  87  and cylindrical lens  89  may be assembled from one of several optical elements in various sequences to minimize optical aberrations.  FIG. 3  further depicts the incorporation of first pair of gratings  73  and  75 , and second pair of gratings  77  and  79 . For grating pairs  73 ,  75  and  77 ,  79 , the choice of grating pitch, grating order etc. is determined in the manner set forth with regard to, for instance, the embodiments of FIGS. 2 and 3 of U.S. Pat. No. 6,687,007 B1. The embodiment of  FIG. 3  of the current application differs from the invention of the prior application in that the reflected beam and transmitted beam are dispersed by separate and distinct pair of gratings. This arrangement is necessary as the beam of light is dispersed subsequent to being split. This arrangement, as in the previous embodiment, has the benefit of allowing spectral mapping to be conducted over selectable wavelengths based on the grating parameters. 
   In operation, with reference to  FIG. 3 , incident light, represented by central ray path  61  is divided into a reflected beam, represented by central ray path  81 , and a transmitted beam (represented by central ray path  83 ) by interferometer optics  59 . Path  81  is directed onto a first pair of gratings  73  and  75 . The gratings, as is well known in the art, function to spectrally disperse the incoming beam path to produce the required amount of lateral spread in the wavelengths. The dispersed beam is depicted in  FIG. 3  as three beam paths collectively referred to as dispersed beam path  91 . Beam path  91  is then focused by lens system  85  onto detector  57 . Path  83  is spectrally dispersed in the same manner by a second pair of gratings  77  and  79  to produce dispersed beam path  93 , which is depicted in  FIG. 3  as three beam paths. Gratings  73  and  75  are symmetrically opposed and identical in all other respects to gratings  77  and  79 . Dispersed paths  91  and  93 , as those skilled in the art appreciate, are recombined by lens system  85  and focused onto detector  57 . 
   The beam splitter  49  (and ultimately interferometer  59  which is commonly referred to as a Sagnac or common path interferometer) of  FIGS. 1 ,  2  and  3  is of conventional design and is well known in the art. One inherent disadvantage of this conventional beam splitter design is that it ultimately loses 50% of the incident light during the beam splitting process as those skilled in the art will appreciate. This deficiency is evident from  FIGS. 1 ,  2  and  3 . In  FIG. 3 , for example, incoming beam path  61  is split into central ray paths  81  and  83 , each representing 50% of the of the original light. Path  81  is reflected by mirrors  51  and  53  and directed back toward beam splitter  49 . Beam splitter  49  functions in exactly the same manner as described above in that it reflects 50% of the incident radiation light and transmits 50%. The 50% that is transmitted is not available to be directed to the detector, representing a loss in optical throughput. Thus the remaining light represents only 25% of the total incident light. The total light of path  83  is similarly reduced with the difference that after being reflected by mirrors  51  and  53  onto beam splitter  49 , the reflected portion of the incident light is lost from the system. Thus, the total throughput of the spectrometer is reduced by 50% relative to the light which is inputted into the system. 
   To solve this inherent problem of the Sagnac (or common path) interferometer, beam splitter  103  (as referenced on  FIGS. 4 ,  5 , and  6 ) is incorporated into the interferometer optics  105  of spectrometer  101 , replacing beam splitter  49 .  FIG. 4  is a partial optical schematic that illustrates the use of beam splitter  103  to reflect a portion of the incident light.  FIG. 5  is a partial optical schematic that illustrates the use of beam splitter  103  to transmit a portion of the incident light.  FIG. 6  represents the combination of reflected light path of  FIG. 4  and the transmitted light path of  FIG. 5 . Spectrometer  101  further includes lens system  121  which is functionally equivalent to lens system  55  of  FIG. 2 , comprised of a cylindrical lens and a Fourier lens combined to focus the radiation on detector  57 . 
   Beam splitter  103  is constructed with an optically transmissive and reflective material. Beam splitter  103  includes first surface  107  and second surface  109 . First surface  107  is divided into first zone  111 , second zone  113 , and third zone  115 . In one embodiment all three zones of equal lengths along first surface  107 . In other embodiments, first zone  111  and third zone  115  are of equal lengths along first surface  107  with second zone  113  occupying the balance of length on first surface  107 . Third zone  115  (on surface  107 ) is coated to reflect substantially 100% of the incident light in a spectral bandwidth compatible with the desired operating optical bandwidth of the system. Second zone  113  is coated to partially reflect and partially transmit the incident light, normally 50% each. First zone  111  is coated with an anti-reflective (or transmissive) coating to transmit substantially 100% of the incident light. 
   In operation, with reference to  FIGS. 4 ,  5  and  6 , incident light, represented by ray path  61  is partially reflected and partially transmitted by the coating of zone  113  on surface  107  of beam splitter  103 .  FIG. 4  depicts the path of the reflected light as two paths as it is naturally dispersed, collectively referred to as path  117 .  FIG. 5  depicts the path of the transmitted light as two paths as it is naturally dispersed, collectively referred to as path  119 .  FIG. 4  further illustrates that reflected beam path  117  is reflected by mirrors  51  and  53  and is directed toward third zone  115  of beam splitter  103 . Path  117  is incident upon second surface  109  of beam splitter  103  and is refracted toward the third zone  115  of first surface  107  where path  117  is reflected back through beam splitter  103  toward second surface  109 . Beam  117  is refracted upon exiting beam splitter  103  toward lens system  121  where it is focused upon detector  57 . The angles of refraction, as is well known in the art, can be calculated by standard methodology. 
     FIG. 5  further illustrates the transmitted beam path  119  of spectrometer  101 . Transmitted beam path  119  is reflected by mirrors  51  and  53  and is directed toward first zone  111  of beam splitter  103  where it is further transmitted and refracted through first zone  111  of beam splitter  103  toward lens system  121 . 
     FIG. 6  shows the combination of beam paths  117  and  119  of spectrometer  101 .  FIG. 6  further illustrates beam paths being recombined by lens system  121  and focused on detector  57 . 
   In the spectrometer  131  of  FIG. 7 , beam splitter  103  is incorporated into the invention as depicted in  FIG. 3 . Beam splitter  103  takes the place of the conventionally designed beam splitter  49 , and the disclosure of  FIGS. 4 ,  5  and  6  is incorporated into the description of  FIG. 7 . The embodiment of the invention as depicted in  FIG. 7  has multiple advantages over the prior art. First, it incorporates the benefits of gratings being placed after the beam splitter apparatus as described with reference to  FIG. 3  above. Second, the new beam splitter increases the throughput of the interferometer by approximately 100% as compared to the conventional interferometer design. Grating pairs  73 ,  75  and  77  and  79  function in the same manner as is disclosed with reference to  FIG. 3  above. The choice of grating pitch, grating order, etc., is determined in the manner set forth with regard to, for instance, the embodiments of FIGS. 2 and 3 of U.S. Pat. No. 6,687,007 B1, and will not be affected by the index of refraction of the substrate material of beam splitter  103 .  FIG. 7  also depicts lens system  133  which is the functional equivalent of lens system  85  of  FIG. 3 . As is well known in the art, lens system  133  includes a spherical (Fourier) lens  135  and a cylindrical lens  137  and functions to recombine the split beam paths onto detector  57 . 
   Whereas the drawings and accompanying description have shown and described the preferred embodiment, it should be apparent to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.