Abstract:
The present invention provides means for correcting interferometer alignment errors through the use of corrective elements. The corrective elements allow reduced accuracy in the assembly process. Residual alignment errors caused by imprecise mounting of permanently mounted components can be corrected using relatively low precision positioning of corrector components. The technique can be particularly applicable to the mass production of interferometers, for which achieving and maintaining the required assembly tolerances might otherwise be prohibitively expensive. Interferometers according to the present invention can be used, for example, in optical spectroscopy and in interferometers.

Description:
REFERENCES TO RELATED APPLICATIONS 
   1. Field of the Invention 
   The present invention generally relates to interferometers, and more specifically to the alignment thereof. The present invention provides corrective elements that, incorporated in interferometers, aid in achieving the precise alignment required by many applications. 
   2. Background of the Invention 
   One common interferometer, a Michelson interferometer, often comprises a beam splitter and two reflectors, one in each optical path created by the beam splitter. To this basic arrangement a compensator is often added of the same material, thickness, and angle of incidence as the beam splitter substrate. This balances the optical path length in both legs at all wavelengths. A complete spectrometer based on a Michelson interferometer further comprises a light source, a means of limiting the angular subtense of light traversing the interferometer, a means of placing a sample to be tested in the optical path, and some means of detecting the light after it has traversed the two legs of the interferometer and recombined. It also contains some means of varying the optical path length difference (OPD) between the two interferometer legs to produce an interferogram, and a means of measuring this OPD, often with a position encoder based on an auxiliary monochromatic light source. Since the advent of fast Fourier transform algorithms in conjunction with a digital computer the Michelson interferometer and numerous variants of it have been used to measure the spectrum of light sources, either directly or after passing through a material with properties that can be determined by the measurement of spectral absorbance. Several authors have provided detailed reviews of this type of spectrometer and its merits relative to other spectrometers for chemometric measurements. See, e.g., Griffiths and De Haseth,  Fourier Transform Infrared Spectroscopy,  Wiley Interscience, 1986. 
   The alignment of a Michelson interferometer can be critical to its performance. Various “self-compensating” designs have been used which involve a number of flat mirrors or mirrors in conjunction with refractive elements. In these designs, the optical arrangement is such that the precision required for maintaining the optical alignment is built separately into each piece or sub assembly; the precision does not rely on the relationship between subassemblies. See, e.g., U.S. patent application Ser. No. 09/415,600, Messerschmidt and Abbink, incorporated herein by reference, (the required precision is contained within the parallelism of two faces of two solid refractive components); European Patent no. 0 681 166 B1, Turner (1995) (the critical precision is built into two subassemblies consisting of flat components with bonded spacers to keep the subassembly components precisely parallel). A shortcoming common to these designs is that the optical path length through the instrument becomes larger than through the simple Michelson interferometer, often by a rather large factor. The result is that, for an extended source, increased vignetting cannot be avoided unless the clear apertures are made larger than they would need to be with an interferometer with short optical path length. 
   SUMMARY OF THE INVENTION 
   The present invention provides means for correcting interferometer alignment errors through the use of corrective elements. The corrective elements allow reduced accuracy in the assembly process. Residual alignment errors caused by imprecise mounting of permanently mounted components can be corrected using relatively low precision positioning of corrector components. The technique can be particularly applicable to the mass production of interferometers, for which achieving and maintaining the required assembly tolerances might otherwise be prohibitively expensive. Interferometers according to the present invention can be used, for example, in optical spectroscopy such as, as examples, those described in U.S. Pat. Nos. 6,441,388, 4,975,581, 6,073,037, 5,857,462, 6,152,876, 5,830,132, and U.S. patent application Ser. No. 09/832,585, each of which is incorporated herein by reference, and in interferometers such as those described in U.S. patent application Ser. No. 09/415,600, incorporated herein by reference. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the invention. 
       FIG. 1  is a schematic representation of an interferometer according to the present invention. 
     FIGS.  2 ( a,b ) are schematic representations of corrective elements according to the present invention. 
     FIGS.  3 ( a,b ) are schematic representations of corrective elements according to the present invention. 
     FIGS.  4 ( a,b,c ) are schematic representations of interferometers according to the present invention. 
       FIG. 5  is a schematic representation of an interferometer according to the present invention. 
     FIGS.  6 ( a,b,c,d,e ) are schematic representations of interferometers according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A daunting challenge to production of many interferometers is that one of several permanently mounted components must undergo a final adjustment in tip and tilt to tolerances in the small arc second range. The present invention provides a corrective element that changes the nature of the alignment question, easing the production of precise interferometers.  FIG. 1  is a schematic representation of an interferometer  100  according to the present invention. The representation in  FIG. 1  illustrates relationships among parts of the interferometer; it is not intended to depict actual geometries. A beam splitter  112  is in optical communication with a reflective subsystem  114  via first  118  and second  120  optical paths. First optical path  118  interacts with a corrective element  116  (shown in the figure as passing through in both directions; in various embodiments light can interact with corrective element along various portions of the path). In operation, beam splitter  112  directs input light  122  along the first  118  and second  120  optical paths. Light returns to beam splitter  112  from reflective subsystem  114  and becomes output light  124 . Proper operation of interferometer generally depends on a precise relationship between light in output light  124  that traversed first optical path  118  and light in output light  124  that traversed second optical path  120 . Corrective element  116  creates a relationship between light entering and light exiting it; proper selection of this relationship allows the precise relationship desired in output light  124  to be achieved. 
   As a specific application, alignment-critical elements in beam splitter  112  and reflective subsystem  114  can be fixedly mounted relative to each other, which, such mounting does not afford sufficiently precise alignment, can result in undesirable relationships in output light  124 . Corrective element  116  can be used to correct such undesirable relationships, allowing the precise alignment requirements to be addressed in the mounting of a single element. 
   Example Corrective Elements 
   FIGS.  2 ( a,b ) are sectional schematic representations of example corrective elements according to the present invention. In each, a sectional view depicts an optical path direction change in two dimensions; rotating the corrective element can accomplish a redirection in three dimensions. In  FIG. 2   a,  corrective element  216  comprises an optically refractive material. Corrective element  216  comprises first  217   a  and second  217   b  faces, oriented non-parallel to each other. Light incident on first face  217   a  at an incidence angle (perpendicular in the figure, though in operation can be any angle) enters the refractive material, after any refraction due to the angle of incidence and differences in refractive index between corrective element  116  and the input medium  202 . Light  223  within corrective element  216  encounters second face  217   b,  where it exits corrective element  216  after any refraction due to the angle of incidence and differences in refractive index between corrective element  116  and the output medium  203 . Output light  224  thus follows a path that is non-parallel to that of incident light  222 . Note that corrective element  216  generates this non-parallel relationship as long as at least one face represents a boundary between different refractive indices and is subject to light at non-perpendicular incidence. The angular relationship between the input  222  and output  224  light can be determined from knowledge of the refractive element (angle and index) and the surrounding media. Equivalently, an appropriate refractive element can be formed if the desired angular relationship is known; the inclination of the faces and index of refraction of the element can be selected to produce the desired relationship. The desired angular relationship can be determined, for example, to be that which will correct misalignment of fixedly mounted interferometer components. 
   In  FIG. 2   b,  corrective element  236  comprises first  236   a  and second  236   b  reflective elements, shown for simplicity as single surfaces although a variety of reflective elements can be suitable. First  236   a  and second  236   b  reflective elements are oriented non-parallel to each other. Light  252  incident on first reflective element  236   a  reflects therefrom toward second reflective element  236   b,  then exits corrective element  236  after reflecting from second reflective element  236   b.  The non-parallel relative orientation of first  236   a  and second  236   b  reflective elements causes output light  254  to follow a path that is non-parallel to that of input light  252 . The angular relationship between the input  252  and output  254  light can be determined from knowledge of the reflective elements (relative orientation). Equivalently, an appropriate corrective element can be formed if the desired angular relationship is known. The desired angular relationship can be determined, for example, to be that which will correct misalignment of fixedly mounted interferometer components. 
   FIGS.  3 ( a,b ) are sectional schematic views of a corrective element according to the present invention. Corrective element  316  comprises first  316   a  and second  316   b  refractive elements, each characterized by first and second faces inclined relative to each other. In  FIG. 3   a,  first  316   a  and second  316   b  refractive elements are oriented relative to each other such that the thinnest section of one aligns with the thickest section of the other, making the angle between the opposing faces  317   a,    317   b  of the entire element  316  at a minimum (the difference in inclination of the faces of first  316   a  and second  316   b  refractive elements). In  FIG. 3   b,  first and second refractive elements are oriented relative to each other such that their thinnest sections align with each other, making the angle between the opposing faces  317   a,    317   b  of the entire element  316  at a maximum (the sum of the inclinations of the first  316   a  and second  316   b  refractive elements). Intermediate angles can be obtained by changing the relative orientation of the first  316   a  and second  316   b  refractive elements. A corrective element with continuously adjustable correction angle can thus be formed from two refractive elements. The two refractive elements can be mounted such that their faces are in contact, or separated. They can also be mounted in separate optical paths; the effect on the overall alignment can still be obtained from the combination of the angles. 
   As a specific example, consider a pair of refractive elements comprising plates having slight inclinations of the opposing faces. For a small wedge angle, the deviation of a ray, in air, going through a wedged glass plate is approximately equal to N−1 times the wedge angle, where N is the refractive index of the glass. For example, for fused silica, with a refractive index of 1.45, a wedge angle of 1 arc minute will deviate a ray by about 27 arc seconds. This deviation angle is only a weak function of the angle of incidence of the ray on the plate and thus the tilt angle of the plate can be changed substantially without having a large effect on the deviation angle. A ray going through a pair of such plates can be deviated up to 54 arc seconds when the narrow ends of the plates are oriented in the same direction. As the plates are rotated about their axes relative to each other any deviation angle between zero and 54 arc seconds can be obtained, with zero occurring at a relative rotation angle of 180 degrees. A pair of plates can redirect a ray in three dimensions by rotating the plates relative to each other about their surface normals to set the magnitude of the deviation and by rotating the plates together about their surface normals to set the azimuth direction. In practice a truly zero deviation can be difficult to achieve using just two plates since the wedge angle match between the two plates would need to be perfect. For an application in which it is desired to allow for error corrections for all angles between zero and a maximum, at least one of the wedged plates can be divided into two wedged plates. The plates can then be made with relaxed tolerances and still be able to correct for any angle error between zero and the sum of the deviations of the plates. The only condition that must be met is that one pair of plates be able to be adjusted so that their combined deviation angle range includes the deviation angle produced by a third plate. To appreciate the advantage of using a pair of wedged corrector plates as in this example, consider that the deviation angle change is 54 arc seconds for a 180 degree azimuthal rotation of one corrector plate relative to the other. A 1 arc second change in the deviation angle is achieved by an average rotation angle change of 3.3 degrees. If a corrector plate 20 mm in diameter were rotated about its center, 3.3 degrees is equivalent to a tangential movement at the edge of the plate of about 580 microns. By contrast, if we consider the tilt movement of a 20 mm diameter mirror to achieve a 1 arc second tilt change, we find that a tilt movement of only 0.1 microns of one edge of the mirror relative to the other is required. Thus, we see that much less precision is required in the positioning of a refractive corrector element than by changing the tilt of a mirror in the reflective subsystem. Although angle correction, in this example, can be achieved by placing both wedged plates in one leg of the interferometer, chromatic errors (change in optical path difference with wavelength) can be minimized in some applications by placing one of the plates in each leg. 
   In the previous example, the corrective elements were used to redirect a ray in angle.  FIGS. 4   a  and  4   b  illustrate that a corrective element ( 412  in  FIG. 4   a,    414  in  FIG. 4   b ) can be used to correct for angle errors in a reflective subsystem in which a ray undergoes a specular reflection (angle of reflection equal to negative of angle of incidence) ( FIG. 4   a ) or for shear errors in a retroreflective subsystem ( FIG. 4   b ). Shear error is the lateral displacement between two parallel rays, one having traveled a first path through the interferometer and the other having traveled a second path. A retroreflective system is one in which a reflected ray always returns parallel to the incident ray. In these cases, the correction can be achieved by rotating the wedged corrective elements about an axis approximately normal to one of the element surfaces. For the retroreflective system the magnitude of correction can also be changed by translating the corrective element in a direction approximately normal to one of the element surfaces. In  FIGS. 4   a  and  4   b,  the uncorrected optical path is depicted as a dotted line; the corrected path is shown as a solid line. 
     FIG. 4   c  depicts another corrective element according to the present invention, which can be used to correct for shear errors in a retroreflective subsystem. It consists of one or more refractive plates  416  with the two surfaces approximately parallel. Shear correction in two dimensions can be obtained by tipping or tilting a plate about an axis approximately parallel to a plate surface. In the figure, the dotted line represents the uncorrected path; the solid line represents the corrected path. As an example, consider a fused silica plate 5 mm thick, nominally positioned with its surface normal parallel to the axis of propagation of a ray. Tilting the plate 1 degree will then deviate the ray by approximately 27 microns. To appreciate the advantage of using this type of corrector we note that for a plate 20 mm in diameter, the deviation of 27 microns is achieved by moving one edge of the plate 350 microns relative to the opposite edge. Thus, we see that shear errors in a reflective subsystem can be corrected using less precise tilt movements of the corrector plate than by direct lateral movement of a retroreflective element itself. Although a complete two axis shear correction can be obtained with a single plate, the use of two plates of equal thickness, one in each leg of the interferometer has the advantage, in some applications, that chromatic errors (i.e. a change in shear correction and optical path difference as a function of wavelength) which might arise from the insertion of a plate into only one leg of the interferometer can be eliminated by correcting for half the tilt error with the plate in one leg and half with the plate in the other leg. The choice of whether to use a parallel plate shear corrector or wedged plate pair shear corrector can be a function of other construction details. For example, it can be mechanically simpler or more stable to provide for rotation of a wedged plate about its normal axis than to tip or tilt a parallel plate. 
   Example Interferometers 
     FIG. 5  is a schematic representation of an interferometer  500  according to the present invention. A beam splitter  510  mounts relative to first  514  and second  512  reflective elements (e.g., mirrored surfaces or retroreflectors). Nominally, first  514  and second  512  reflective elements mount relative to each other such that a single ray incident on the beam splitter with return to the same place on the beam splitter after reflecting from the first reflective element as after reflecting from the second reflective element. The alignment precision required for some interferometer applications can be very difficult to achieve, however. Also, mounting, material, or other constraints can make exact alignment problematic. Misalignment can produce output rays that are not properly aligned, reducing the performance of the interferometer. 
   According to the present invention, first  516  and second  517  corrective elements can be added to interferometer  500  such that a first optical path  526  passes through first corrective element  516  and second optical path  524  passes through second corrective element  517 . Each optical path is bent by the corresponding corrective element. The shape and orientation of corrective elements  516 ,  517  is such that they direct the optical paths so as to bring the output light paths into the desired alignment (generally collinear in this interferometer geometry). Corrective elements such as those described above can be suitable. As a specific example, first  516  and second  517  corrective elements can be rotated, individually or in concert, to align the paths. 
   Method of Making an Interferometer 
   The following examples illustrate how an interferometer can be made according to the current invention. Consider the interferometer configuration  601  of  FIG. 6   a,  consisting of a beam splitter  602 , first  603  and second  604  flat end mirrors, and a compensator plate  606 . In this illustration the compensator plate is used to balance the air/glass distance in both legs of the interferometer. The OPD can be varied by mounting one of the mirrors on a precision carriage mechanism, for example a flexure, or by a number of methods allowing both end mirrors to be mounted in a fixed manner. See, e.g., U.S. Pat. No. 3,482,919 (the OPD is varied by rotating the compensator plate); W. H. Steel, “Interferometers for Fourier Spectroscopy,” Aspen International Conference on Fourier Spectroscopy, 1970, pp. 43-50 (AFCRL 71-0019, Special Report No. 114) (describing an arrangement wherein a wedged refractive plate is translated to provide a varying amount of glass in the path of one interferometer leg, thereby changing the OPD); European patent 0 681 166 B1 (rotating a two mirror subassembly to accomplish the OPD variation). The corrective methods of the current invention work with these and other techniques. 
   Assembly can be begun by permanently mounting beam splitter  602  and two end mirrors  603 ,  604 , onto a common structure that maintains them in a fixed angular relationship to each other once mounted. An example of this kind of mounting includes using a temporary alignment fixture to hold the components in place on a common base plate while an epoxy adhesive cures. Alignment at this stage of construction need not be of the arc second accuracy required for the final product but only good enough to provide a residual error small enough for a corrective element to remove.  FIGS. 6   b, c, d  and  e,  illustrate four different arrangements of corrective elements. In  FIG. 6   b,  an auxiliary means, such as an autocollimator instrument, can quantify the angle error between the two end mirrors  613 ,  614  as viewed through the beam splitter  612 . A single wedged corrector plate  618  can then be fabricated or chosen from a set of plates with known deviation angles. It can be inserted into one leg of the interferometer at the appropriate rotation angle to correct the error. Success can be verified by observing the signal modulation efficiency in the operating interferometer, a technique known to those skilled in the art. In  FIG. 6   c  two or more wedged corrector plates  628 ,  629  are used. The advantage of this arrangement is that a range of angle errors can be corrected by, for example, rotating the plates separately in azimuth about the direction of propagation. The step of post-measurement fabricating or choosing a plate with the right wedge angle can be avoided. In this method the error angle can optionally be directly corrected without making a quantitative measurement of the required correction. This can be done by observing the modulation efficiency of the operating interferometer while adjusting the rotation angles of the plates to maximize the signal. 
   As explained earlier, an optional third plate can be used to allow angular errors near zero to be corrected using plates not perfectly matched in angle. A way to avoid using a third plate is to build the temporary assembly and alignment fixturing such that a small error in one of the end mirror angles is built in. If this error is made larger than the deviation angle difference between two corrector plates then alignment correction can be accomplished using only two plates. Another method of avoiding correction angles near zero is to include the two corrector plates in the initial assembly, setting them so that the narrow end of one wedged plate is rotated about 90 degrees from the narrow edge of the other plate. This produces an angle deviation of about half of the total available. The temporary alignment fixture can then be adjusted to produce a coarse alignment, either by observing the interferogram produced by the operating interferometer or by observing the output of an autocollimator, as described above. This process sets the available correction angles to mid range, avoiding angles near zero. Permanent bonding of the beam splitter and end mirrors then needs to be performed only to an accuracy that can be corrected by a plus and minus deviation of half the available correction range. This technique has been used successfully to correct for angle changes caused by the shrinking of epoxy as it cures. 
     FIG. 6   d  illustrates an arrangement wherein wedged corrector plates  638 ,  639  are placed in both legs of the interferometer. This arrangement has the advantage that corrector plates can be added to the interferometer without changing the balance between air and glass in the two legs. This arrangement provides the same corrective action afforded by the two plates in one interferometer leg. 
   As a final example consider  FIG. 6   e.  In this case the compensator plate  646  can be wedged to correct error as described in relationship to  FIG. 6   b.  It can also be split into two wedged plates  648 ,  649  which can be rotated about their surface normals to affect the correction in a manner similar to that of  FIG. 6   c.  It is found that wedging a tilted component, such as the compensator works best if the compensator is not used as the OPD scanner although modest error corrections can also be achieved even in the case of a nutating compensator plate. The reason for caution is that the corrective action of the wedges varies slightly with angle of incidence. This effect can be negligible when the error to be corrected is small. 
   The wedged refractive corrector plates can also be used in the examples of  FIG. 6 , especially  FIG. 6   d,  to make small adjustments in the OPD. This can be useful, for example, when it is desired to adjust the zero path difference position (ZPD) to coincide with a specific position of the primary OPD generating mechanism. This utility takes advantage of the fact that for a corrector plate with a small wedge angle the OPD is increased as the corrector plate is tilted, whereas the angle correction is affected only weakly. Tilting a plate in one leg will move the ZPD position in one direction whereas tilting a plate in the other leg will move ZPD in the opposite direction. If necessary, tilting and rotating can be done iteratively to more accurately correct both angle errors and ZPD errors. 
   The angle correction schemes described herein using two or more corrective elements can also be used in situations where dynamic correction is needed. For example, a practice used in some interferometer designs has been to provide an auxiliary wavefront error sensor to control piezoelectric translators on one end mirror to dynamically correct angle errors produced by imperfections in a carriage moving the other end mirror. The current invention can be used in a similar manner to replace the piezoelectric translators and affect a high degree of correction using rotational servos of only moderate accuracy. Similarly, servos of moderate accuracy can be used to tilt a parallel plate corrective element in an interferometer using retroreflectors rather than flat mirrors. 
   Design Considerations 
   The following considerations can be useful in making embodiments of the present invention. They are not intended to be limiting, since specific requirements can vary. Materials for wedged refractive correctors generally should have good transmittance in the spectral region over which the interferometer is to be used. In addition, they should have a refractive index homogeneity, surface flatness, and construction rigidity that allow the wavefront quality within the interferometer to be maintained at an acceptable level for the intended application. For example, in the spectral region of 0.4 through 2.5 microns, fused silica has been successfully used for wedged corrector elements with a diameter of 25 mm and an element thickness of 4 mm. A surface flatness of 1/20 of the shortest wavelength of interest, peak-to-valley, on each of the surfaces was found adequate to maintain acceptable interferometer performance. 
   Those skilled in the art will recognize that the present invention can be manifested in a variety of forms other than the specific embodiments described and contemplated herein. Accordingly, departures in form and detail can be made without departing from the scope and spirit of the present invention as described in the appended claims.