REFRACTIVELY SCANNED INTERFEROMETER

Abstract: An interference spectrometer in which a wedge-shaped prism (32) is moved across one arm (22) of an interferometer for variation of the path difference, and where the beam splitter substrate (36) and prism (32) are optically compensated for by elements substantially identical to the substrate and prism (38, 40) in the other arm of the interferometer. In a first embodiment two refractive elements are in each arm of the interferometer (40, 36, 38, 32). In the preferred embodiment, only one prism is used in each arm, (46, 44) with the beam splitter surface (12) being located on a fixed prism (46), in one arm, adjacent a movable prism (44) in the other arm.

Description: REFRACTIVELY SCANNED INTERFEROMETER Background of the Invention This invention relates to the field of interferom- etry, and particularly to scanning interferometers intended for use in spectro etry. More specifically, its primary focus is on improving Michelson interferometers intended for use in infrared Fourier transform spectroscopy. The present invention is an improvement relating to the same general subject matter as two earlier filed applications of Walter M. Doyle, assigned to the assignee of this application. The first of the related Doyle applications is Serial No. 790,457, filed April 25, 1977, titled "Refractively Scanned Interferometer", and issued on February 26, 1980 as Patent No. 4,190,366. That patent discloses and claims an interferometer having stationary reflectors at the end of each interferometer "arm" combined with a wedge-shaped prism movable across one arm to cause scanning. The other of the related Doyle applications is Serial No. 808,951, filed June 22, 1977, titled "Refractively Scanned Interferometer", and issued on August 28, 1979 as Patent No. 4,165,938. That patent discloses and claims an interferometer of the same type as S.N. 790,457, wherein the orientation and direction of motion of the wedge-shaped scanning prism are such that displacement of the refracted optical beam by the prism is minimized or eliminated. The interferometers shown in the cited Doyle appli¬ cations have provided a significant development in the field of Fourier transform spectroscopy. However, certain performance limitations have become apparent in those interferometers, which are attributed to certain optical aberrations not fully compensated for in the Doyle inter¬ ferometers. There are three general types of optical aberration encountered in apparatus of this type: (a) chromatic aberration, (b) spherical aberration, and (c) astigmatic aberration. Chromatic aberration is partially compensated for in the Doyle apparatus by the use of corner reflectors. How¬ ever, the compensation is only partial, and, further, ther is an upper limit in such apparatus on the allowable dis¬ tance between the scanning wedge and the corner reflector in the same arm. This limit in some cases is not accept¬ able. Spherical aberration is minimized in the Doyle appa¬ ratus by matching the refractive thickness of the beam¬ splitter substrate to the refractive thickness of the scanning wedge in its centered position. In other words, the arrangement is such as to equalize the average optical path lengths in those two elements, which are located in different arms of the interferometer. While this arrange¬ ment, as predicted by Doyle, provides a reasonable solutio of the spherical aberration problem, it tends to preclude solution of the astigmatism problem, for reasons which wil be more fully discussed below. Astigmatic aberration (astigmatism) was not fully compensated for in the apparatus of the Doyle applications although it was later discovered that first order terms could be compensated for by a lateral displacement of one of the corner reflectors. Nevertheless, astigmatism has proved to be a limiting factor in the performance of the Doyle apparatus. The general purpose of the present invention is to retain the significant advantages of the Doyle apparatus, while adding thereto the benefits of maximum compensation for, or balancing of, the three types of optical aberratio Summary of the Invention The present invention provides compensation for astigmatism by balancing the refractive effect of the beamsplitter with an equal and angularly identical re- fractive effect in the other arm of the interferometer. Since this removes the Doyle solution to the spheri¬ cal aberration problem, the present invention provides a refractive element which compensates for, or balances, the refractive effect of the movable prism, and which is located in the interferometer arm other than the one having the movable prism. The balancing prism is stationary, and is preferably substantially identical in shape to the mov¬ able prism. With this arrangement, in addition to solving the spherical aberration problem, an improved solution of the chromatic aberration problem is provided. The net result of the foregoing changes is to provide interferometer arms which are fully symmetrical in centered position, i.e., which are so constituted as to be "mirror images" of one another. In an earlier developed version of the present inven¬ tion, the foregoing benefits are obtained by using a total of four optical refractive elements — a beamsplitter, a movable prism, a compensating element to balance the opti¬ cal refractive effect of the beamsplitter substrate, and a compensating element to balance the optical refractive effect of the movable prism. In the preferred version of the present invention, the same benefits are obtained by using a total of two optical refractive elements, one in each interferometer arm, which elements are substantially the same in their shapes and optical refractive characteristics. Each element is a wedge-shaped prism. One of them is movable for scanning purposes; the other is stationary and pre¬ ferably is the one having the beamsplitter surface. The movable prism is preferably very close to the fixed prism OMPI and moves along a line parallel to the adjacent surface of the fixed prism. Each interferometer "arm" is perpen¬ dicular to the prism surface nearest the reflector at the end of that arm, i.e., the radiation exiting and entering the surface of each prism nearest its reflector intersects that surface at right angles thereto, thus insuring that the motion of the moving prism will conform to the require ments explained in Doyle Patent No. 4,165,938. Brief Description of the Drawings Figure 1 is a diagrammatic plan view of an interfer¬ ometer incorporating the four-element version of the present invention; Figure 2 is a diagrammatic plan view of an interfer¬ ometer incorporating the preferred two-element version of the present invention; Figure 3 is a closeup of the wedge-shaped prisms of Figure 2, showing more clearly the changes in direction of the radiation paths; Figure 4 is a diagrammatic plan view of a modified version of the preferred form of the invention, in which the wedge-shaped prisms each have a third optical surface through which entering or exiting radiation passes; and Figure 5 is a particular example of the interferomete of Figure 4, in which the interferometer arms are perpen- dicular to one another. Detailed Description of the Specific Embodiments First, referring to Figure 1, the elements which are included in the Doyle applications will be described, and then a detailed analysis will be made of the aberration problems and solutions provided by the present invention. The Doyle apparatus includes essentially a light source 10, a beamsplitter 12 tilted at a 45° angle to the radiation beam 14, two stationary reflectors 16 and 18 located at the ends of the two interferometer "arms" to reflect the "split" radiation beams 20 and 22, respectively, a detector 24 which receives the recombined radiation 26, and a movable wedge-shaped prism 28 which moves across the path of beam 22 to cause refractive scanning, suitable driving means 29 being provided to cause the movement of prism 28 back and forth across the radiation path. The stationary reflectors 16 and 18 are retro-reflectors, because of significant functional benefits, even though some of their benefits are provided by the optical com- pensation features added by the present invention. The retro-reflectors 16 and 18 are shown as "cube corner" reflectors; but other types of retro-reflectors, i.e., "cats-eye" reflectors, or "rooftop" reflectors, may be used. For the reasons taught by Doyle Patent No. 4,165,938, it is important that the direction of scanning motion and the orientation of the prism 28 be such that the apparent deflection point of the optical beam passing through the prism remains at substantially the same position throughout scanning motion of the prism. The arrow in the Figure shows the appropriate direction of scanning motion of the prism 28 for the configuration shown. And the dashed lines illustrate one of the positions to which the prism moves as a result of its translatory motion. Because beam 22 enters prism 28 at right angles to front surface 30 of the prism, it is not angularly deflected until it emerges from the rear surface 32 of the prism. Since scanning motion is along the line defined by rear surface 32 of the prism, the apparent deflection point of beam 22, and the apparent deflection points of the parallel rays there¬ in, remain at the same point(s) as the prism 28 is moved for scanning purposes. Different shapes of prism 28 and different orientations with respect to the entering beam 22 would require different directions of translatory motion in order to meet the stated condition. With only the elements thus far described, my experi¬ ence indicates that astigmatic aberrations are produced which constitute a significant limiting factor in perform¬ ance of the apparatus. Astigmatic aberration is caused primarily by the angular orientation of the refractive portion of the beamsplitter 12. The partially reflective, partially transmitting surface of the beamsplitter is shown at 34. This is the functioning surface of the beamsplitter. Since it must, of necessity, be supported by a substrate, or plate, portion 36, which could be provided on either side of surface 34, the substrate 36 unavoidably injects a refractive factor into the apparatus. Because the beamsplitter, by its very nature, has to be oriented at an angle, generally 45°, to the radiation 14 from source 10, the substrate 36 constitutes a refractive element tilted at an angle to the direction of propagation, there¬ by introducing asymmetrical distortion, or astigmatism, into the optical system. In a conventional, or non-refractive, interferometer, adjusted for equal path lengths, the field-of-view (FOV) is in principle not limited by lateral coherence effects. As the length of one of the arms is varied away from this condition, the coherent FOV will be reduced due to the joint dependence of phase on both arm length and angle of propagation. The foregoing reference to conventional interferometers means those in which the length of the variable-length radiation path is changed by moving the reflector in one of the interferometer arms; whereas interferometers following the teachings of the Doyle applications may be conveniently referred to as refractive interferometers, because the length of the variable-length radiation path is changed by movement of the refractive element across one of the interferometer arms. Refractive interferometershave a similar FOV phenomenon, requiring the refractive path lengths in the two arms to be mutually compensating. In the earlier refractive inter¬ ferometers, second order effects of this type were com- pensated by making the optical path length in the beam¬ splitter equal to the average optical path length in the wedge. Recently, however, an analysis of lateral coher¬ ence effects has been performed which indicates that the prior approach is probably not adequate. In order to solve this problem, it is necessary to compensate for the beamsplitter substrate 36 with a similar element in the other arm of the interferometer. This compensating element should have the same refractive effect as substrate 36, and it should be so oriented with respect to the optical path as to provide the same 45° angle of intersection of the radiation path. Such a compensating element is shown in Figure 1 at 38. Although the overall refractive effect is what needs to be balanced by compen¬ sating element 38, it is preferable that it have the same refractive index and the same thickness as substrate 36, as well as having the same angular orientation. If desired, the beamsplitter surface 34 could be placed on the element 38 as a substrate, instead of the element 36. As shown in Figure 1, the elements 36 and 38 are parallel, and the angles α 1 and 2 are both 45°. The inclusion of the beamsplitter compensating ele¬ ment 38 in the variable-length arm of the interferometer creates an imbalance of the refractive path lengths in the two arms because of the presence of refractive scanning element 28. In order to meet the spherical aberration problem created by this imbalance, I propose the inclusion of a compensating element in the fixed-length interferom¬ eter arm, which, in centered position of the interferometer, will provide a refractive path equal to the radiation path length through the prism 28. While this solution of the spherical aberration problem could be accomplished by a - ΛJREA ? OMPI Λ._ WIPO plate having a uniform optical thickness equal to the average optical path length in prism 28, it is preferable to use a wedge-shaped prism 40, having the same shape as prism 28, and oriented toward the fixed-length optical path in the same way that prism 28 is oriented toward the variable-length optical path. Spherical aberration tends to create different curvatures of the radiation wavefronts at the detector, resulting in a distortion balanced around the system axis. Matching the optical path lengths in the two interferometer arms, in centered position, is the appropriate solution of the problem. The use of a compensating prism 40 which matches pris 28 has the additional benefit of providing complete balanc ing of chromatic dispersion in the two arms of the inter- ferometer, thereby providing a more complete solution of the chromatic aberration problem than would be provided solely by the use of retro-reflectors. Chromatic aberra¬ tion results from a non-flat wavefront, due to the wave¬ length-dependent refractive index which determines the amount of bending of the light rays as they pass through the prism 28. Balancing this effect with the matching prism 40, preferably located in the other arm of the interferometer from scanning prism 28, is the best solu¬ tion of the chromatic aberration problem. Compensating prism 40 is stationary, and it should be located at the same position with respect to its arm of the interferometer as the position of the prism 28 with respect to its arm of the interferometer. This means that the distance of compensating, or balancing, prism 40 from reflector 16 should be the same as the distance of movable prism 28 from reflector 18 when prism 28 is in its centere position. From the foregoing description, it can be seen that the best results, in terms of avoidance of spherical, astigmatic and chromatic aberrations, in an interferometer of the refractive type, can be obtained by providing com¬ pletely symmetrical interferometer arms, in which both the beamsplitter substrate and refractive wedge are fully compensated for, or balanced, in the other arm of the interferometer. With this fully symmetrical arrangement, the interferometer will have no limit on FOV when prism 28 is at centered position. Although the primary solution of the chromatic aber¬ ration problem is provided by the stationary balancing prism 40, it is still highly desirable, and in fact necessary, to use retro-reflectors 16 and 18. The retro- reflectors are considered necessary because the wedge-shaped prisms, if flat reflective mirrors were used, would create a dispersion of rays which, while identical in the two arms, would be too broadly dispersed at the detector 24. Addi¬ tionally, the retro-reflectors provide several advantages; and, because they are both stationary, there is no problem due to their relative bulk. For one thing, they tend to compensate for any chromatic aberration due to imperfect matching of the two wedge-shaped prisms 28 and 40. Use of the retro-reflectors also makes the optical system less vulnerable to problems due to slight misalignment of the wedge-shaped scanning prism 28. In other words, use of retro-reflectors reduces sensitivity to angular errors * of the wedge motion. Furthermore, the retro-reflectors tend to compensate for astigmatism in the system. And finally, the retro-reflectors provide a preferred approach to dual beam, i.e., dual detector, spectroscopy, as dis¬ cussed in Doyle Patent No. 4,190,366, and also in a Doyle application titled "Dual Beam Fourier Spectrometer", Serial No. 830,576, filed September 6, 1977, and issued on January 15, 1980 as Patent 4,183,669. It should also be noted that the use of a dual beam spectrometer of the type disclosed in Patent No. 4,183,669, is rendered more feasible by the increased FOV due to the full optical compensation provided by the present applica- tion. The interrelation of the various optical features of the present invention is significant in providing the de¬ sired accuracy of the complete apparatus. As explained in Doyle application S.N. 808,951, the orientation and direction of motion of the scanning wedge 28 are of par¬ ticular importance where retro-reflectors are used. And, as outlined above, the retro-reflectors contribute to solution of the aberration problems; and their use is made feasible by the fact that scanning is accomplished by wedge 28 and not by movement of one of the reflectors. Given the foregoing facts, it becomes desirable in balanc¬ ing out the aberration problems to compensate for both the beamsplitter and the scanning wedge, as detailed above. In order to provide the aberration-avoidance benefits of the structure of Figure 1, while reducing the number of arm-intersecting optical elements to two instead of four, it is necessary to make the movable scanning wedge and the beamsplitter substrate mutually compensating elements. Therefore, they are substantially identical, as shown in Figure 2. The beamsplitter substrate is shown at 42, and it constitutes a wedge-shaped member having the same shape and optical characteristics as the scanning wedge, or prism, 28. The beamsplitter 12 is provided by a coating on the inner surface of substrate element 42, i.e., the surface nearest scanning wedge 28. The movable wedge-shaped prism, or scanning wedge, 28 and the fixed wedge-shaped prism, or beamsplitter substrate, 32 are so arranged as to be "mirror images" of one another in the two arms of the interferometer. It is obvious that in Figure 2 the two arms of the interferometer, i.e., the optical paths 20 and 22, are not at right angles to one another, as is usually the case in Michelson interferometers. The only case in which the arms are at right angles is illustrated in Figure 5, which is discussed below. The major departures in the present invention from customary interferometer design are a resul of the fundamental functional tmiqueness of the interfer¬ ometer disclosed herein. ^-^^u TE OMP w p The orientation of interferometer arms 20 and 22 is dictated by the requirement that arm 22 be perpendicular to the nearest surface 44 of the movable wedge 28. This requirement is based on the direction of motion of the movable wedge, and on the fact, taught by Doyle Patent No. 4,165,938, that the apparent deflection point of the optical beam passing through the movable wedge should remain at the same position throughout the scanning motion. The direction of motion of the movable wedge 28 should 10 be in the direction shown for effective functioning of the interferometer. The space between the two optical wedges 42 and 28 should be as small as possible, consistent with the avoidance of mechanical contact between them as wedge 28 is moved. This closeness of wedges 42 and 28 is impor- -*--* * tant in order to minimize chromatic aberration. If they were not located very close, the lever arms between each wedge and its related corner reflector (16, 18) would be different, and differential wavelength-dependent lateral displacements at the reflectors would occur. Another way 20 of explaining the requirement for closeness of the two wedges is to point out that they must be close in order to balance the interferometer arms, which should be as nearly identical as possible. In other words, everything in the two arms should be symmetrical with respect to the 25 beamsplitter surface 12, which is the "center" of the interferometer. If the two wedges were in contact with one another, the interferometer arms would be identical. But such contact is not possible because of the scanning motion of wedge 28. 30 This situation is more clearly depicted in Figure 3, which is a closeup of the "center" of the interferometer. As shown in Figure 3, the optical path 14 from the source is deflected at surface 46 of wedge 42 to follow path 14a within the wedge, and is deflected again to follow path 15 35 • as it leaves the inner surface of wedge 42 at beamsplitter 12. Thus a deflected optical path 15 occurs in the air gap 48 between the two wedges. As optical path 15 inter sects inner surface 50 of movable wedge 28, deflected, and it then follows optical path 22a, which is that portion of arm 22 within wedge 28, and which is parallel to path 14a. The lateral deflection of path 22a from path 14a should be minimized in order to minimize chromatic aberration by keeping air gap 48 as small as possible. It is important that optical path 22a within movable wedge 28 and the remainder of arm 22 leading to reflector 18 be in direct alignment with one another, i.e., that they constitute a non-deflected straight line. And this will be true only if arm 22 is perpendicular to surface 44 of wedge 28. The direction of scanning motion of wedge 28, as shown by the arrow, must be parallel to the beamsplitter 12 because of the requirement for minimized space between the two wedges throughout the scan. At the same time, as taught in Doyle Patent No. 4,165,938, the wedge motion must be along the line defined by the apparent deflection points of the rays passing through the scanning wedge 28. Therefore, it is necessary to simultaneously meet these two requirements. In order to do so, the deflection point of the optical path 22a through wedge 28 must remain at the inner surface 50 of the wedge as scanning motion occur This will only be true if optical paths 22a and 22 are in direct alignment, thus causing inner surface 50 of wedge 28 to be the only deflecting surface for the optical path through scanning wedge 28. In other words, in order for the deflection of the optical path to take place at the inner surface 50 of the scanning wedge, the optical path must be perpendicular to the outer surface 44 of the scanning wedge. The design requirement that optical path 22 be per¬ pendicular to surface 44, combined with the parallelism requirement of surfaces 12 and 50, determines the directio of portion 14a of the optical path, and also determines th direction of portion 14, once the angle of the wedges has been determined. The decision as to the apex angle of scanning wedge 28 depends on the desired length of the scanning motion. A relatively narrow angle has the ad¬ vantage of minimizing the criticality of mechanical motion. A relatively wide angle has the advantage of providing a greater effective optical scanning distance, and hence improved spectral resolution for a given mechanical scan¬ ning distance. Once the apex angle of scanning wedge 28 has been determined, the apex angle of fixed wedge 42 is established, because the two wedges must be substantially identical in order to make the interferometer arms symmetrical, thereby minimizing optical aberrations. Because of the equal angles of wedges 28 and 42, the optical geometry results in a perpendicular relationship between optical path, or arm, 20 and the outer surface 46 of fixed wedge 42, i.e., optical path 20 is in direct alignment with the portion 20a thereof which passes through the fixed wedge. The radiation which recombines at the beamsplitter 12, coming from arm 22 reflected by mirror 18, and from arm 20 reflected by mirror 16, follows optical path 17 through air gap 48, and is deflected at surface 50 of wedge 28 to follow optical path 26a through the wedge. It is deflected again as it exits the wedge at surface 44, and thereafter follows optical path 26 to the detector 24. It will be noted that, as wedge 28 is moved for scan¬ ning purposes, the optical path 26 is laterally displaced, as shown by the arrow crossing path 26. This lateral dis¬ placement of the output beam 26 can be adjusted for at the detector 24 by increasing the size of the focusing optics. • While, theoretically, the beamsplitter coating 12 could be provided on the inner surface of movable wedge 28 instead of the inner surface of fixed wedge 42, it is much better to locate it on the fixed wedge because the alignment of the beamsplitter in the interferometer is critical, and movement of the beamsplitter would tend to cause misalignment. In the centered position of the interferometer, the length of the optical path through the movable wedge 28 should equal its length through the fixed wedge 42. Therefore, the initial adjustment of the position of movable wedge 28 to "center" it will include an adjustment to compensate for the slight lateral displacement of the optical path 14a - 22a due to air gap 48. It will be apparent that the three forms of optical aberration — astigmatic, spherical, and chromatic — are compensated for, or balanced, to substantially the same extent as in the embodiment of Figure 1. Astigmatic aberration is minimized by the fact that the optical path intersects the refractive surfaces within the interferometer at the same angle in each arm of the interferometer. Astigmatism is caused when light goes through a critical inner surface of the interferometer, such as the beamsplitter substrate, at a non-perpendicular angle. This is compensated for by the balancing effect due to the equal angle of intersection of the optical path with the inner surface of the other wedge, i.e., the scan¬ ning wedge. In other words, the angle of intersection of the optical path is the same at the inner surface 50 of scanning wedge 28 as it is at the inner surface of fixed wedge 42 which carries the beamsplitter coating 12. Thus the astigmatic effects created in one wedge are substan¬ tially "cancelled out" as the beam enters the other wedge. Spherical aberration is also substantially "cancelled out" because both the non-refractive and the refractive optical path lengths in the two interferometer arms are substantially identical in the centered position of the scanning wedge. Chromatic aberration is minimized by providing match¬ ing wedges having equal apex angles, thus balancing the effects of the chromatically dispersed wavefront in one interferometer arm with a similar chromatically dispersed wavefront in the other interferometer arm. OMPI Figure 4 shows an embodiment of the invention in which each wedge-shaped refractive element — the movable scanning wedge and the fixed compensating wedge — has three polished surfaces instead of two, and is so designed that (a) the optical path from the source intersects the fixed wedge at a different surface of the fixed wedge from the surface intersected by the optical path to the adjacent reflector, and (b) the optical path to the detector inter¬ sects the movable wedge on a different surface of the movable wedge from the surface intersected by the optical path to the adjacent reflector. This has certain func¬ tional advantages over the embodiment of Figures 2 and 3, but it is only applicable where a wider wedge angle is usable for scanning purposes. The design illustrated in Figures 2 and 3 works quite well for small wedge angles and is probably usable with wedge angles as large as 20 or even 30 degrees. However, as the wedge angle is increased, it becomes necessary to increase the lengths of the wedges so that all four of the optical beams — 14, 20, 22, and 26 — will enter and leave the wedges through the polished surfaces. In addition, the lateral displacement during scanning of beam 26 leading to the detector will become more severe as the wedge angle is increased. The use of a large wedge angle may be desirable in some cases, particularly when a large effective optical scanning distance is desired, as in high resolution spectroscopy. For such cases, the geometry of Figure 4 will be advantageous. As shown, the movable scanning wedge 28b has three optical (polished) surfaces — inner surface 50b adjacent beamsplitter 12b, surface 44b nearest reflector 18, and a third optical surface 52 which is intersected by the optical path 26b leading to detector 24. The fixed compensating wedge 42b also has three optical (polished) surfaces — the inner surface on which beamsplitter coating 12b is supported, surface 46b nearest -16- reflector 16, and a third optical surface 54 which is intersected by the optical path 14b leading from radiation source 10. Because of the presence in Figure 4 of signif¬ icant angles at both ends of the inner surfaces of both wedges, the wedge, or apex, angle of movable wedge 28b which determines the refractive changes during scanning is identified by the numeral 56; and the related, or balancing, angle of fixed wedge 42b is indicated by the numberal 58. As in the preceding embodiment, these angles 56 and 58 are equal in order to minimize optical aberra¬ tions. The size of angles 56 and 58 is determined by the desired scanning parameters. Also, for the reasons discussed in connection with Figures 2 and 3, it is necessary that optical path 22b leading to reflector 18 be substantially perpendicular to the surface 44b of scanning wedge 28b nearest reflector 18. Consequently, optical path 20b leading to reflector 16 will be substantially perpendicular to the surface 46b nearest reflector 16. Also it is necessary that the di- rection of motion of wedge 28b during scanning be substan¬ tially parallel to the beamsplitter 12b, as indicated by the arrow. A further advantage will result if surface 52 of scanning wedge 28b is made parallel to surface 46b of fixed wedge 42b. In this case, as shown, the detector beam 26b will automatically be perpendicular to the exit surface 52, and no displacement of this beam will occur during scanning, as demonstrated by the dashed line posi¬ tion of the scanning wedge. For this condition, the cross section of the scanning wedge 28b will be an isoceles triangle, i.e., its angle 60 will equal its angle 56. For symmetry, we may also make angle 62 of fixed wedge 42b equal to angles 56, 58 and 60, with all optical paths entering and leaving the two wedges at right angles. This latter condition has the additional advantage of eliminat¬ ing any chromatic dispersion of the incoming light beam 14b. s ( OM IP Figure 5 illustrates a species of Figure 4 which constitutes the only design in which the interferometer arms are perpendicular to one another. In Figure 5, the movable scanning wedge 28c and the fixed compensating wedge 42c constitute, geometrically, a split cube. The cross section of each wedge is a right-angled triangle, in which the angles 56c, 58c, 60c and 62c are each 45°. The arms of the interferometer — optical paths 20c and 22c, leading to the respective reflectors 16 and 18 — are perpendicular to one another.. The other relationships described in connection with Figure 4 are applicable, including the fact that optical path 26c to detector 24 is not laterally displaced during scanning, as demonstrated by the dashed line position of scanning wedge 28c. The large wedge angle in Figure 5, which determines the ratio between wedge motion and spectral resolution, has the disadvantage that it decreases the opto-mechanical scan amplification, but it has the advantage of providing a higher resolution. The optical structures of Figures 4 and 5 provide the same benefits as that of Figures 2 and 3, insofar as the balancing of, or compensating for, optical aberrations is concerned. The explanation of these aspects of the present invention has already been provided. As pointed out in the discussion of Figure 1, the best results, in terms of avoidance of spherical, astigmatic, and chromatic aberrations, in an interferometer of the refractive type, can be obtained by providing completely symmetrical interferometer arms, in which both the beam- splitter substrate and the scanning wedge are fully com¬ pensated for, or balanced, in the other arm of the inter¬ ferometer. In the preferred embodiment shown in Figures 2-5, only two optical elements are required, each balancing the other. With this fully symmetrical arrangement, the interferometer will have no limit on field of view when the scanning wedge is at centered position. In other words, the two-element embodiment of Figures 2-5 retains the aberration-compensating benefits of the embodiment of Figure 1, while eliminating the disadvantages of opti¬ cal losses and increased cost which are associated with the use of four separate refractive elements. The following claims are intended not only to cover specific embodiments disclosed, but also to cover the inventive concepts explained herein with the maximum breadth and comprehensiveness permitted by the prior art. OMPI &7

Claims: What is claimed is: 1. A spectral analysis interferometer, of the type having diverging arms along which radiation is directed, comprising: a beamsplitter for directing radiation from a source in optical paths along the two diverging arms of the inter¬ ferometer, said beamsplitter including a partially reflect¬ ing surface which divides radiation between the interferom¬ eter arms, and a supporting substrate for said surface which tends to create an optical imbalance between the arms of the interferometer; a first stationary reflector at the end of the first interferometer arm; a second stationary reflector at the end of the second interferometer arm; a scanning prism in one of the interferometer arms which is movable across that arm to vary the length of the optical paths in that arm; and compensating means for avoiding aberrations due to the refractive characteristics of both the beamsplitter substrate and the scanning prism, said compensating means providing refractive effects in the other arm of the inter¬ ferometer which substantially balance the refractive effects of the beamsplitter substrate and of the scanning prism in its centered position. 2. The spectral analysis interferometer of Claim 1 wherein the scanning prism is so oriented and has such a direction of movement across the optical path that the apparent deflection point of the optical beam passing through the scanning prism remains at substantially the same position throughout the motion of the scanning prism. 3. The spectral analysis interferometer of Claim 1 or Claim 2 wherein the compensating means includes a refractive element for balancing the refractive effect of the beamsplitter substrate, said element being in the othe arm of the interferometer from said substrate, and having the same refractive thickness as said substrate and the same angular orientation to its optical path as said sub¬ strate has to its optical path. 4. The spectral analysis interferometer of any of Claims 1, 2 or 3 wherein the compensating means includes a refractive element for balancing the refractive effect of the scanning prism, said element being in the other arm of the interferometer from said prism, and having the same shape as said prism and the same location in its arm of the interferometer as said prism has in its arm of the interferometer. 5. A spectral analysis interferometer, of the type having diverging arms along which radiation is directed, comprising: a first stationary reflector at the end of the first interferometer arm; a second stationary reflector at the end of the secon interferometer arm; a movable wedge-shaped prism in one of the interfer- ometer arms which is moved across the optical path in that arm to cause spectral scanning; and a stationary wedge-shaped prism in the other arm of the interferometer which balances the optical effect of the movable prism when the latter is in its centered posi- tion, thereby tending to avoid optical aberrations in the interferometer. 6. A spectral analysis interferometer, of the type having a first arm in which the optical path leads to a first reflector and a second arm in which the optical path leads to a second reflector, comprising: a movable scanning prism having a wedge angle to vary its refractive path length and movable across the optical path of the first interferometer arm; a fixed prism having a wedge angle substantially equal to that of the scanning prism and located in the second arm of the interferometer in such a way as to substantially balance the refractive effects in the two arms when the scanning wedge is in its centered position; and a beamsplitter surface supported on one of the prisms. 7. The spectral analysis interferometer of Claim 6 wherein the beamsplitter surface is supported on the fixed prism. 8. The spectral analysis interferometer of Claim 7 wherein the scanning prism is located as close as practic- able to the beamsplitter surface, and the direction of mo¬ tion of the scanning prism is parallel to the beamsplitter surface. 9. The spectral analysis interferometer of any of Claims 6, 7 or 8 wherein the optical path of the first arm intersects at right angles the surface of the scanning prism nearest the first reflector. 10. The spectral analysis interferometer of any of Claims 6, 7, 8 or 9 wherein the scanning prism is the only refractive element in the first arm, and the fixed prism is the only refractive element in the second arm. ^UR£ _OMPI_ IPO . 11. The spectral analysis interferometer of any of Claims 7, 8, 9 or 10 wherein the scanning prism has three optical surfaces, the first of which is parallel to the beamsplitter surface, the second of which is intersected by the first interferometer arm, and the third of which is intersected by the optical path exiting the interferometer

Inventors: CLARKE W, DOYLE W
Applicants: LASER PRECISION CORP
Application year: 1980
Main classification: G01B9
IPC codes: G01B9, G01J3, G01J9, G01N21, G02B27