Abstract:
Transmission spectrometers require low levels of background light so that the signal to noise ratio is increased, and also require stable performance over wide temperature ranges. Light reflected by the transmission grating can result in increased background levels. One approach to reducing the background level is to orient the transmissive diffraction grating so that light reflected by the grating is reflected out of the diffraction plane. The temperature-induced wavelength drift of a transmission spectrometer can be due to the frame upon which the transmission grating is mounted. The wavelength drift is reduced by allowing the thermal expansion of the grating to be independent of the frame.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is directed generally to optical systems, and more particularly to an optical spectrometer that uses a transmissive diffraction grating.  
       BACKGROUND  
       [0002]     Optical spectrometers are useful devices for analyzing the spectral content of a beam of light. A spectrometer uses a dispersing element, for example a diffraction grating or a prism to spatially separate different wavelengths of light. A spatially sensitive detector arrangement is then used to detect the dispersed light. One type of spatially sensitive detector arrangement uses a detector array to detect the dispersed light.  
         [0003]     For increased sensitivity, it is important to be able to increase the signal to noise ratio. In many applications, particularly where the amount of light fed into the spectrometer is limited, the signal to noise ratio may be increased by reducing the amount of background noise. This may be achieved, for example, by reducing the amount of unwanted light reaching the detector.  
         [0004]     The temperature-related behavior of the spectrometer can be an important characteristic to control. For example, in some spectrometers the apparent wavelength of the detected light drifts with temperature, and so these spectrometers have to be operated at constant temperature in order to maintain the accuracy of the measurement. This severely limits the use of the spectrometer in certain applications where the temperature is not easily controlled, or is expensive to control.  
       SUMMARY OF THE INVENTION  
       [0005]     One type of scattered light that it is particularly important to reduce is light that is scattered within the spectrometer from the diffraction element. For a transmissive diffracting element, such as a diffraction grating, a portion of the incident light is reflectively diffracted. This light may propagate within the spectrometer and be incident on the detector if not properly controlled. One approach to reducing the effect of reflectively diffracted light in the spectrometer is to orient the diffraction grating so that the light is reflected out of the plane of the detected light.  
         [0006]     Another aspect of good performance of a transmission spectrometer is that it behaves uniformly over a range of temperatures. The diffraction element, when attached to a frame, often warps or stretches when the temperature changes. This results in a shift and/or defocusing of the light at the detector, thus leading to a degradation of the spectral performance of the spectrometer. One approach to reduce the grating&#39;s temperature-dependent behavior is to mount it in a manner that permits independent thermal expansion and contraction of the grating and the frame when the temperature changes. This reduces the temperature-related stresses that occur when the temperature changes, and so the associated drift with temperature is reduced. As a result, the temperature range over which the spectrometer can operate with high accuracy is increased.  
         [0007]     Consequently, one embodiment of the invention is directed to an optical spectrometer having an input port, an optical detector and at least a first transmissive diffraction grating disposed to diffract light received from the input port to the optical detector. Light from the input port is diffracted parallel to a diffraction plane. The first transmissive diffraction grating is oriented so that light reflected by the first transmissive diffraction grating is reflected in a direction non-parallel to the diffraction plane. A first focusing unit is disposed between the first transmissive diffraction grating and the optical detector. The first focusing unit focuses light from the first transmissive diffraction grating to the optical detector.  
         [0008]     Another embodiment of the invention is directed to an optical spectrometer that comprises an input port, an optical detector defining an active aperture and at least a first transmissive diffraction grating disposed to diffract light received from the input port to the optical detector. A first focusing unit is disposed between the first transmissive diffraction grating and the optical detector. The first focusing unit focuses light from the first transmissive diffraction grating to the optical detector. The first transmissive diffraction grating is oriented so that light, reflected from the transmissive diffraction grating and reflected back through the transmissive diffraction grating, reaches a focal plane of the first focusing unit outside the active aperture.  
         [0009]     Another embodiment of the invention is directed to a method of aligning a spectrometer having at least a first transmissive diffraction grating. The method includes passing light from an input port to the first transmissive diffraction grating and diffracting the light in a diffraction plane by the first transmissive diffraction grating. The light diffracted by the first transmissive diffraction grating is focused to a detector defining an active aperture. The first transmissive diffraction grating is oriented so that light reflected by the first transmissive diffraction grating is reflected out of the diffraction plane.  
         [0010]     Another embodiment of the invention is directed to a spectrometer that comprises an input port, an optical detector and at least a first transmissive diffraction grating unit disposed to diffract light received from the input port to the optical detector. Light from the input port is diffracted parallel to a diffraction plane. The first transmissive diffraction grating unit comprises a transmissive diffraction grating attached to a frame using a mounting, the mounting permitting independent thermal expansion and contraction of the grating and the frame under conditions of changing temperature. A first focusing unit is disposed between the optical detector and the first transmissive diffraction grating unit. The first focusing unit focuses diffracted light from the first transmissive diffraction grating unit to the optical detector.  
         [0011]     Another embodiment of the invention is directed to a method of mounting a transmissive diffraction grating to a frame. The method comprises attaching the transmissive diffraction grating to the frame while permitting independent thermal expansion and contraction of the transmissive diffraction grating and the frame under conditions of changing temperature.  
         [0012]     Another embodiment of the invention is directed to a spectrometer that comprises an input port, an optical detector and at least a first transmissive diffraction grating unit disposed to diffract light received from the input port to the optical detector. The first transmissive diffraction grating unit comprises a transmissive diffraction grating attached to a frame using a mounting. A first focusing unit is disposed between the optical detector and one or more transmissive diffraction grating units of the at least one transmissive diffraction grating unit. The first focusing unit focuses diffracted light to the optical detector. The at least one diffraction grating unit, the first focusing unit and the optical detector are arranged to operate at light wavelengths in excess of 100 nm, and the temperature dependent wavelength shift of diffracted light at the optical detector is no more than 0.01 nm/K.  
         [0013]     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
         [0015]      FIG. 1  schematically illustrates the optical layout of an embodiment of spectrometer that employs a transmissive diffractive element according to principles of the present invention;  
         [0016]      FIGS. 2A and 2B  schematically illustrate an embodiment of a spectrometer using a transmissive diffraction element;  
         [0017]      FIG. 3A  schematically illustrates a perpendicular view of the spectrometer of  FIGS. 2A and 2B .  
         [0018]      FIG. 3B  schematically illustrates a perpendicular view of an embodiment of a spectrometer using a transmissive diffraction element according to principles of the present invention;  
         [0019]      FIG. 4A  schematically illustrates a perspective view of an embodiment of a grating unit;  
         [0020]      FIG. 4B  schematically illustrates a side view of the grating unit of  FIG. 4A  warping under differential thermal expansion;  
         [0021]      FIGS. 5A and 5B  schematically illustrate a side view of an embodiment of a grating unit according to principles of the present invention;  
         [0022]      FIGS. 6A and 6B  schematically illustrate a side view of another embodiment of a grating unit according to principles of the present invention  
         [0023]      FIG. 7  schematically illustrates a side view of another embodiment of a grating unit according to principles of the present invention;  
         [0024]      FIG. 8 . schematically illustrates a side view of another embodiment of a grating unit according to principles of the present invention; and  
         [0025]      FIG. 9  schematically illustrates an embodiment of a dual-grating transmission spectrometer according to principles of the present invention. 
     
    
       [0026]     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0027]     An embodiment of a spectrometer  100  is schematically illustrated in  FIG. 1 . The spectrometer  100  has an input port  102  through which light  104  is introduced to the spectrometer  100 . The input port  102  may include a slit  106 , or an optical fiber  108 , or a combination of both. An important function of the input port  102  is to provide some spatial restriction to the light  104  entering the spectrometer  100  so as to increase the resolution at the detector, since the input port  102  is typically imaged to the detector.  
         [0028]     The light  104  passes from the input port  102  to a transmissive diffraction element  110 , for example a diffraction grating. The divergence of the light  104  may be reduced by a first collimating unit  112 . The first collimating unit  112  may comprise one or more lenses in combination. The first collimating unit  112  may collimate the light  104  so that the light  114  incident on the grating  110  is parallel. It is advantageous, although not necessary, that the first collimating unit  112  be achromatic, so that the divergence of the light  114  incident on the grating is uniform over a range of wavelengths. The first collimating unit  112  may also include one or more aspherical surfaces.  
         [0029]     Light transmitted through the grating is  110  is diffracted, the angle of diffraction being dependent on the wavelength of the light  114  incident on the grating  110 . The illustration shows light at two different wavelengths, λ 1  and λ 2 , in the spectrometer  100 . Further, for ease of understanding, light that propagates as a combination of wavelengths is shown with solid lines, light at the wavelength λ 1  is shown with a dashed line and light at λ 2  is shown with a dotted line. Light  116  at λ 1  is diffracted by the grating  110  to a first angle, and passes to the detector  118 . Light  120  at λ 2  is diffracted by the grating  110  to a second angle and passes to the detector  118 . The transmissive diffraction grating  110  diffracts light in a direction parallel to a diffraction plane, parallel to the y-z plane. Note that, for a transmissive diffraction grating, the diffraction plane is defined as being the plane of diffraction for transmitted light.  
         [0030]     A focusing unit  122  is used to focus the light from the grating  110  to the detector  118 . The spectrometer  100  may be enclosed within a housing  124  to reduce the amount of stray light incident on the detector  118 . The housing may also provide the support (not shown) for the various optical elements within the spectrometer.  
         [0031]     The detector  118  may be connected to an analyzer  126  for analyzing and/or displaying the signals detected by the detector. The analyzer  126  may be, for example, an optical spectrum analyzer or may be a computer with appropriate software for analyzing and displaying the detected signals.  
         [0032]     In the illustrated embodiment, the detector  118  is an array detector, for example an array of photodiodes. Other types of detector may be used, for example a single detector that travels with a slit. In another embodiment, not illustrated, a single detector is fixed and the grating  110  rotates to change the wavelength of light incident on the detector. The detector  118  is typically positioned at the focal plane of the focusing unit  122 .  
         [0033]     The focusing unit  122  may be a single lens or a combination of two or more lenses. In the illustrated embodiment, the focusing unit  122  is a single, bi-convex lens. Where a single lens is used, the single lens may have one or both sides aspheric.  
         [0034]     The light  114  is incident on the grating  110  at or around the Bragg angle in order to use the first order diffraction to separate different wavelengths. There exist reflected beams for each wavelength component present, corresponding to reflected diffraction orders. This is illustrated in  FIG. 2A , which shows the light  114  incident on the grating  110  and the first order diffractively reflected light  202  and  204  for the wavelengths λ 1  and λ 2  respectively. It should be noted that, in  FIG. 2A , the light transmitted through the grating  110  has been omitted.  
         [0035]     The first order diffractively reflected light  202  and  204  passes through the first collimating unit  112  and is focused to the input port  102 . The size of the focal spot produced by the first collimating unit  112  may depend on the angle at which the light travels through the first collimating unit  112 . This is particularly important, for example, where the first collimating unit  112  includes an aspheric lens surface. Accordingly, the reflected light  202  and  204  may be defocused on reaching the plane of the input port  102 .  
         [0036]     The reflected light  202  and  204  may be further reflected, back towards the grating  110  from the input port  102 , as illustrated in  FIG. 2B . This figure only shows the path of light reflected from the input port  102  and, for clarity, does not show the light entering the spectrometer  100  through the input port  102 . It will be appreciated that it need not only be the first order diffractively reflected light that is directed back to the input port: diffractively reflected from other diffraction orders may be directed back to the input port, depending on the wavelength of the light incident on the grating  110  and the angle at which the light  114  is incident on the grating  110 .  
         [0037]     The light  212  reflected from the input port  102  at λ 1  passes back through the first collimating unit  112  to the diffraction grating  110 . The light  212  is then diffracted, and focused by the focusing unit  122  towards the detector  118 . Likewise, the light  214  reflected from the input port  102  at λ 2  passes back through the first collimating unit  112  to the diffraction grating  110 . The light  214  is then diffracted and focused towards the detector  118  by the focusing unit  122 . The light  212  and  214  at the different wavelengths, λ 1  and λ 2 , is directed to substantially the same position at the focal plane of the focusing unit  122 , and so the light at λ 1  and λ 2  overlaps at the detector  118  to form a spot at position  126 .  
         [0038]     The light at the spot  126  is not dispersed, in other words, is independent of wavelength. The light at the spot  126  contributes to an undesirable background signal in the spectrometer. Various approaches may be used to reduce the intensity of the non-dispersed light at spot  126 . For example, the surface of the housing  124  around the input port  102  may be made to absorb light, scatter light, or both. However, a residual non-dispersed signal may still remain even after the reflecting surface has been treated to reduce the amplitude of the non-dispersed light at the spot  126 .  
         [0039]     Another approach to reducing the amount of light in the non-dispersed spot  126  that is detected by the detector  118  is now described with reference to  FIGS. 3A and 3B . It should be noted that the x and z axes shown in  FIGS. 3A and 3B , and the y and z axes shown in  FIGS. 1, 2A  and  2 B, correspond to a Cartesian co-ordinate system having x, y, and z as orthogonal axes.  
         [0040]      FIG. 3A  schematically illustrates a view of the spectrometer perpendicular to that illustrated in  FIG. 2B . This figure shows light  214  at λ 2  reflected from the input port  102  being collimated in the first collimating unit  112  and being transmitted through the diffraction grating  110 . Light at λ 1  is omitted from the figure for purposes of clarity. The focusing unit  122  focuses the light to the detector  118 . The detector is shown as an array detector having a number of individual detector elements  318 , having an active aperture with a transverse width, d. The active aperture corresponds to that width of the detector in which incident light causes a detection signal. The extent of the active aperture is normally set by the extent of the detector elements  318  themselves, in order to produce the largest signal. The extent of the active aperture may also be limited by covers overlapping the edges of the detector elements  318 .  
         [0041]     In  FIG. 3A , the diffraction grating  110  is aligned so that the grooves  310  are perpendicular to the light incident from the input port  102 . As a result, the point  302   a  from which the light  214  is reflected lies in the same plane as the light entering the input port  102 , and so the reflected light  214  propagates in a direction parallel to the spectrometer axis  320 . The plane of diffraction is a plane that coincides with the axis  320 , but is perpendicular to the plane of the figure illustrated in  FIG. 3A . The axis  320  is parallel to the z-axis.  
         [0042]     In  FIG. 3B , the light  104  from the input port  102  is not perpendicularly incident on the grooves  310  of the diffraction grating  110 , because the grating has been turned through an angle a relative to the orientation shown in  FIG. 3A . The light  316 , from the input port  102  that is transmitted through the grating  110 , is focused by the focusing unit  122  to the detector elements  318  of the detector  118 . The light that is reflected from the grating, however, follows a different path. In the illustration, only the light  204  reflected at λ 2  is shown, although it will be appreciated that the following description is also applicable to light reflected at other wavelengths. Due to the tilt of the grating  110 , the reflected light  204  is reflected off the axis  320 , in a direction non-parallel to the plane of diffraction. Consequently, the light  204  is reflected at a point  302   b  that is off the axis  320 , with the result that the light  204  reaches the detector  118  at a position  326  that is also off the axis  320 . If the angle α is sufficiently large, then the light  204  reaches the detector  118  at a position that is outside the detector elements  318 , and so the reflected light  204  is not detected by the detector  118 .  
         [0043]     Thus, orienting the transmission diffraction grating  110  so that it reflects first order light in a direction non-parallel to the diffraction plane results in the non-dispersed light at spot  126  being moved off the detector, if the rotation is sufficiently large. The sufficiency of the rotation depends on various factors including, but not limited to, the physical size of the spectrometer  100 ; the width, d, of the detector elements  318 ; and the focusing powers of the first collimating unit  112  and the focusing unit  122 . The rotation of the grating  110  may be only a few degrees.  
         [0044]     In some situations, the reflected light  212  and  214  is reflected by the surroundings of the input port  102 . For example, where the input port is a slit in a material, for example a metal, the light  212  and  214  may be reflected by the material surrounding the slit. Where the input port  102  is a fiber, the light  212  and  214  may be reflected at the fiber face, or may result from a reflection of light that has passed back down the fiber from the grating  110 . It is important to understand that it is not important from where the light  212  and  214  is reflected back to the grating  110 , but that the light  212  and  214  originated by reflection off the grating  110 . It should also be appreciated that this technique may be used to counteract the deleterious effects of light diffusely reflected from the input plane and/or its surroundings, in addition to light specularly reflected.  
         [0045]     In one particular embodiment of the spectrometer  100 , the input port  102  is formed by an input slit, for example having a slit size 70 μm×500 μm. The first collimating unit  112  may be any suitable lens unit, for example a Melles Griot, model no. LA0019 achromat having a focal length of 25 mm. The collimating unit  112  may also comprise a reflective surface.  
         [0046]     The grating may be formed by etching any type of transmissive material, such as glass, for example BK7 glass, quartz, fused silica, or the like. In one particular embodiment, the transmissive diffraction grating is formed by etching a set of lines at a pitch of 621.855 lines/mm. One suitable type of focusing unit  122  is a bi-aspheric lens. In one particular embodiment, the bi-asphere has the following characteristics: decentered 2.2 mm, and center thickness of 12.3 mm. The first surface of the bi-asphere has a radius of curvature of 18.462 mm and a conic constant of −1.015629. The second surface of the bi-asphere has a radius of curvature of −19.7 mm and a conic constant of −4.307554. All aspheric coefficients of the first and second surfaces of the bi-asphere are zero. It will be appreciated that this is simply an example of one suitable lens for use in the focusing unit  122 , and is not meant to limit the invention in any way. In addition, the focusing unit  122  may comprise a reflecting surface.  
         [0047]     In one embodiment, the detector  118  includes a photodiode array having elements  318  that have a width, d, of 500 μm, and a center-to-center spacing of 25 μm. The photodiode array elements may be formed from any suitable type of semiconductor material for detecting the light analyzed in the spectrometer. For example, where the light is visible or in the near-infrared, the photodiode array elements may be formed from silicon, gallium arsenide or a gallium arsenide alloy. It will be appreciated that other types of detector material may also be used.  
         [0048]     The spectrometer may also include various baffles, screens and the like to reduce stray light within the housing. For example, there may be one or more baffles along the optical path between the input port and the detector. Furthermore, there may be absorbing screens or baffles to deflect and/or absorb light in unwanted diffraction orders from the diffraction grating. For example, there may be an absorbing screen positioned to absorb zero order light transmitted through the diffraction grating. Absorbing baffles and screens may be made from any suitable absorbing material: black anodized aluminum is a useful material for this purpose, although other materials may also be used.  
         [0049]     The spectrometer may operate in a wide variety of wavelength ranges. For example, the spectrometer may be an ultraviolet spectrometer, measuring light over the range of 200 nm-400 nm or into the visible region. The spectrometer may also cover parts of the visible or the infrared region of the spectrum. For example, the spectrometer may be a near infrared spectrometer with a range of 900 nm-1700 nm. In many cases, the minimum wavelength range of the spectrometer is higher than 100 nm.  
         [0050]     It is important in a transmission spectrometer that the temperature-related spatial movement of the spectrally separated and focused light at the focal plane be reduced, so that detected signal is not significantly temperature dependent.  
         [0051]     One important factor in controlling the temperature dependence of the detected signal is the thermal expansion coefficient of the diffraction grating. Expansion and contraction of the grating directly results in a change in the grating period, which results in a change in the positions of different wavelengths of light at the detector  118 . This effect may be reduced by forming the grating in a material having relatively low thermal expansion coefficient, such as fused silica. The thermal expansion coefficient of fused silica is low, at about 0.55×10 −6  K −1 . For example, where the grating pitch is 621.855 lines/mm, the pitch decreases to 621.828 lines/mm if the temperature is increased by 80° C.  
         [0052]     The grating is normally mounted on a frame in the housing. If the frame is made from a material having the same or similarly low thermal expansion coefficient, then problems associated with the relative thermal expansion between the grating and the frame are reduced. It is difficult and/or expensive, however, to manufacture the frame from fused silica or another low thermal expansion material, such as INVAR. It is often more convenient, and less expensive, to manufacture the frame from a metal, such as titanium, stainless steel or aluminum. These metals, however, have a thermal expansion coefficient that is many times higher than fused silica. For example, titanium has a thermal expansion coefficient of about 9×10 6 K −6 , stainless steel has a thermal expansion coefficient of about 16×10 −6 K −1 , while aluminum has a thermal expansion coefficient of about 25×10 −6 K −1 .  
         [0053]     A conventional approach for making a grating unit  400  is schematically illustrated in  FIG. 4A . The grating unit  400  includes a grating  402  mounted to a frame  404  with a line of adhesive  406  along its edges. When the temperature is changed, the frame  404  and the grating  402  expand or contract at different rates, with the result that the grating  402  becomes warped or stretched, for example as shown in  FIG. 4B . In the illustration, the frame  404  has expanded by more than the grating  402 , with the result that the grating  402  becomes distorted: the adhesive is typically not very elastic, and rigidly binds the grating  402  to the frame  404 . When this happens, the period of the grating  402  changes by an amount controlled more by the thermal expansion coefficient of the frame material than the grating material.  
         [0054]     One approach for overcoming this temperature dependent behavior is now discussed with reference to  FIG. 5A . A grating unit  500  has a grating  502  fixedly attached to the frame  504  at only one position along the frame  504 . For example, the grating  502  may be attached to the frame  504  using adhesive  506  applied at only one position along the frame  504 . The point of attachment may be anywhere along the length of the frame  504 , for example at the end of the frame  504  (illustrated) or somewhere in the middle portion of the frame  504  (not illustrated. The adhesive  506  may be applied in a notch  508  in the frame  504  to permit the lower surface  512  of the grating  502  to sit on flat on the upper side  510  of the frame  504 .  
         [0055]     This approach permits independent expansion and contraction of the grating  502  and the frame  504 , without warping or stretching the grating  502 . An example of the independent expansion is schematically illustrated in  FIG. 5B , which shows the grating unit  500  under a temperature that is higher than the temperature associated with  FIG. 5A . The thermal expansion of the frame  504  is assumed to be greater than for the grating  502 . The right hand end of the frame  504  has extended beyond the end of the grating  502  under higher temperatures, but the grating  502  has not distorted or stretched because the expansions of the grating  502  and the frame  504  are independent. It will be appreciated that the size of the thermal expansion under realistic temperature changes is significantly smaller than the scale of expansion illustrated in  FIG. 5B . The diffraction grating mounted so as to expand and contract independently of the frame may be a transmissive grating or may be a reflective grating.  
         [0056]     It is noted that, in order to reduce warping or stretching of the grating, and thus reduce temperature dependent changes in the effective grating period, it is important to provide independent thermal expansion and contraction in a direction across the grooves, shown as the x-direction in  FIG. 4 . It is less important, however, to provide for independent thermal expansion and contraction in the direction parallel to the grooves, shown in  FIG. 4  as the y-direction (the co-ordinate system shown in  FIG. 4  does not correspond to the co-ordinate system shown in  FIGS. 1-3 ). Warping or stretching in the direction parallel to the grooves does not result in a change in the effective pitch of the grooves, and so the effects of warping or stretching in the direction parallel to the grooves is less deleterious to the spectrometer&#39;s performance than for warping or stretching in the direction across the grooves.  
         [0057]     Other approaches to achieving independent thermal expansion and contraction may also be used, one of which is illustrated in  FIG. 6A . The grating unit  600  may have the grating  602  held against the frame  604  using a method that prevents movement of the grating  602  relative to the frame  604  under normal operating conditions at constant temperature, but which permits the grating  602  to slip relative to the frame  604  under changing temperature conditions. One such method is to use a clip  606 , or, multiple clips  606 , to hold the grating  602  to the frame  604 . It is important that the clips hold the grating against the upper surface  610  of the frame without significantly distorting the grating  602 .  
         [0058]     When the temperature rises, the frame  604  experiences a larger thermal expansion than the grating  602 . This situation is schematically illustrated in  FIG. 6B —note that the gaps between the clips  606  and the grating  602  are larger at the higher temperature. The clips  606  are designed to be able to move on the upper surface  611  of the grating  602  without significantly distorting or stretching the grating  602 . To achieve this, the contact points  612  of the clips may be formed of a deformable material, such as a polymer. The contact points  612  may, in addition, be formed of a material having a low coefficient of friction, for example Teflon.  
         [0059]     Another approach to permitting independent thermal expansion contraction of the grating and the frame is schematically illustrated in  FIG. 7 , which shows a grating unit  700  with a grating  702  attached to a frame  704  via a layer  706  of an elastic adhesive. The elastic adhesive  706  may be, for example, an elastomer, or a flexible epoxy such as EPO-TEK® 310, available from Epoxy Technology, Billerica, Mass. The elastic adhesive  706  is preferably made sufficiently thick that, over the anticipated operating temperature range, the thermal expansion of the grating  702  is close to a value that would be calculated from the thermal expansion coefficient of the grating material alone. For example, over a temperature range of no more than 80K, the thermal expansion of the grating  702  may be within a factor of three of that of the grating material alone, and in fact may be approximately equal to that of the grating material alone. In illustration, the effective thermal expansion of a fused silica grating mounted on the frame is less than 1.65×10 −6 K −1  over a temperature range of no more than 80 K.  
         [0060]     It will be appreciated that other approaches to maintaining independent thermal expansion of the grating and the frame may be used. Furthermore, different combinations of such approaches may also be employed. For example, as is schematically illustrated in  FIG. 8 , a grating unit  800  may use a combination of adhesive and clips to hold the grating  802  to the frame  804 . In the illustrated embodiment, the grating  802  is attached to the frame  804  via a short portion of adhesive  806 , preferably positioned within a notch  808 . One or more clips  810  may also be used to the grating  802  to the frame  804 . This arrangement may be particularly useful where there is at least one clip  810  holding the end  812  of the grating  802  that is free from the adhesive  806 .  
         [0061]     It will be appreciated that other arrangements may be possible, for example the adhesive  806  may be positioned mid-way along the frame  804 , with clips  810  positioned at each end of the grating  802 .  
         [0062]     Permitting the grating and the grating frame to thermally expand independently of each other allows for a very low thermal dependence on the wavelength shift at the detector. For example, by implementing the invention described herein, the wavelength detected at the detector may shift by 0.01 nm K −1 , or as little as 0.005 nm K 31 1 , thus enabling accurate operation over a wide temperature range.  
         [0063]     It will be further appreciated that the inventions described herein may be implemented in different configurations of transmissive spectrometers. For example, the spectrometer may use more than one transmissive grating, an example of such spectrometer  900  being schematically illustrated in  FIG. 9 . Light  904  is input to the spectrometer  900  through an input port  902 , illustrated in this particular embodiment as an optical fiber. The light  904  is collimated using in a first lens unit  912 . The collimated light  914  is diffracted by a first transmissive diffracting element  910   a  and then by a second diffracting element  910   b . The doubly diffracted light  916  is focused by a focusing unit  922 , in this case a mirror, to a detector unit  918 . The focusing unit  922  may be a spherical mirror, or may be an aspherical mirror. The detector unit  918  may be a detector array having a number of detector elements  918   a .  
         [0064]     Each of the transmissive diffracting elements  910   a  and  910   b  may reflect light backwards towards the input port  902 . Therefore, in a manner like that described earlier with regards to  FIG. 3B , either or both of the transmissive diffracting elements  910   a  and  910   b  may oriented to reflect light out of the diffraction plane, equivalently out of the y-z plane. Consequently, the reflected light, upon being reflected back through the transmissive diffracting elements  910   a  and  910   b , is diverted away from the detector unit  918  and so the effects of the non-dispersed light on the detector signal is reduced.  
         [0065]     Furthermore, one or both of the transmissive diffracting elements  910   a  and  910   b  may be mounted on respective grating frames in a manner as described above to permit independent thermal expansion of the diffractive element and the frame. It will further be appreciated that a spectrometer may also use more than two diffracting elements.  
         [0066]     The present invention is applicable to spectrometers that use a transmissive diffraction grating, and is believed to be particularly useful for reducing background noise in the detected signal and for increasing the ability of the spectrometer to operate over a wide range of temperatures. The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.