Abstract:
The tilt-compensated interferometers of the present invention are novel variations of Michelson&#39;s interferometer that use tilt- and shear-compensation to provide excellent photometric accuracy even when there are imperfections in the scanning motion used to produce variation of path difference. The tilt-compensation mechanism of the present invention consists of antiparallel reflections from a beamsplitter element and a roof reflector element, which elements are held rigidly in alignment. Several particularly useful embodiments of the invention are described. Other advantages of the present invention include photometric stability and reduced cost because manual alignment is not required. This interferometer has applications in spectrometry, spectral imaging and metrology.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a CONTINUATION of prior application Ser. No. 09/299,022, filed on Apr. 26, 1999, entitled TILT-COMPENSATED INTERFEROMETERS, which will issue as on Oct. 22, 2002 as U.S. Pat. No. 6,469,790. The application Ser. No. 09/299,022 claimed priority under 35 U.S.C. § 119(e) from Provisional Patent Application No. 60/120,736, filed Feb. 18, 1999. The application Ser. No. 09/299,022 was a continuation of application Ser. No. 08/959,030, filed Oct. 28, 1997, which issued as U.S. Pat. No. 5,898,495. The application Ser. No. 09/299,022 and 60/120,736 and 08/959,030 applications and U.S. Pat. Nos. 5,898,495 and 6,469,790 are hereby incorporated herein by reference for the entirety of their disclosures. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   It is an object of the present invention to provide new interferometers, which are better than prior art in respect to stability, scan speed and cost of manufacture. It is an object of the present inventions to improve the state-of-the-art in photometric accuracy of interferometric measurements. 
   2. Background Information 
   The book by Griffiths and deHaseth, “Fourier Transform Infrared Spectrometry,” ISBN 0-471-09902-3, is included by reference for the entirety of its content. This book describes much of the prior art and practice in great detail. 
   Michelson interferometers can be used for many purposes, including spectrometry and metrology. The principle of operation is that a beam of electromagnetic radiation or energy is divided into two portions, which propagate in first and second paths; the two portions are delayed and recombined, leading to interference that is a function of the path difference between the first and second paths, as well as frequency content of the radiation. This prior art is illustrated by  FIG. 1. A  source of radiation is indicated by  10  and the radiation may be collimated by a parabolic mirror  11  into a primary beam of energy. The primary beam is divided at the beamsplitter  30  into two portions, which propagate in first 15 and second 16 paths. These paths  15  and  16  are often called the arms of the interferometer, and are typically oriented at 90 degrees to each other, as shown in FIG.  1 . 
   At the ends of the two arms or paths  15  and  16  are mirrors  80 A and  80 B from which the two beams are reflected back toward the beamsplitter, retracing more or less exactly paths  15  and  16 . At the beamsplitter, each of the two returning beams are split again resulting in two recombined beams. One recombined beam propagates back toward the source, generally being lost from use, and the second (hereafter, “the recombined beam”) propagates out of the interferometer at an angle to the input beam, and follows a path to a detector. The second recombined beam may propagate to a parabolic focusing mirror  21 A that concentrates the radiation at a sampling point  23 . The radiation from the sampling point may be collected by a mirror  21 B and focused on a detector  20 . Many alternatives to the mirror combination  21 A and  21 B are known in the art. Radiation from a second source  12 , generally having a precisely known wavelength, may be used as an internal standard of distance for the interferometer. Such a source  12  is often a helium-neon laser, but may be instead a diode laser, stabilized diode laser or gas discharge lamp. The radiation from the second source may be observed simultaneously with a second detector  21 . External focusing optics generally are not required for a reference laser such as  12  because the beam is already tightly collimated. The interferometer usually is operated by moving one of the mirrors; the most common method for driving the mirror is a voice coil linear motor, but many other approaches are possible and some are known. The mirror may be moved at constant velocity and reciprocated, or it may be moved incrementally and stopped. It is understood that the signals from the detectors  12  and  22  are generally digitized and input to a computer. The data are processed by known means to generate and display spectra of the source radiation as modified by interaction with a sample. 
   A disadvantage of the Michelson interferometer and many related designs is that the two end mirrors  80 A and  80 B are susceptible to misalignment with each other and with the beamsplitter  30 . Further, the alignment must be preserved, to interferometric tolerances, during and between motion of one or both of the mirrors  80 A and  80 B. Expensive, high-quality bearings are often required to provide such precise rectilinear motion. In some instruments, airbearings are used with the aforementioned voice coil linear motors to provide such motion. Various other solutions to this problem have been proposed in the literature and prior art. For example, Jamin (1856), Solomon (U.S. Pat. No. 5,675,412), Turner and Mould (U.S. Pat. No. 5,808,739), Spanner (U.S. Pat. No. 6,369,951), Frosch (U.S. Pat. No. 4,278,351), Woodruff (U.S. Pat. No. 4,391,525), and related designs provide interferometers in which slight misalignments of the optical components are compensated with respect to interferometric alignment. Slight residual misalignment may result in a beam of radiation not reaching exactly the intended detection location. The sensitivity to the resulting misalignment of the source image on the detector is roughly 100 times smaller than the sensitivity to interferometric misalignment. In short, there is a substantial advantage to optical tilt compensation. However, a disadvantage of the prior art is the long optical paths required for optical tilt compensation. For example, the approaches of Frosch, Woodruff, Solomon, Turner, and others, impose an undesirable limit on throughput, which is the product of solid angle and aperture area. 
   Larsson has described in U.S. Pat. No. 5,650,848 a method for tilt-compensating the scanning mirrors of an interferometer. This prior art extends ideas described by Steel in the 1960&#39;s (see for example, “Interferometry,” Cambridge University Press, 1967 W. H. Steel). Larsson&#39;s approach employs three mutually perpendicular facets on a common assembly, but the radiation reaches them in a substantially different order than in the present invention. The radiation first reaches a beamsplitter. After being divided into two beams, in Larsson&#39;s interferometer design, each of the two beams reaches a single reflector surface rigidly and perpendicularly mounted to the beamsplitter. Because the single reflectors are mutually perpendicular, the two beams from the beamsplitter are both folded by 90 degrees (in different planes), rendering them antiparallel. Two plane mirrors held in opposition on a common carriage may be used to return the beams to the beamsplitter. One substantial improvement, relative to Larsson, of the present invention described herein, is that two reflector facets are placed in one arm of an interferometer. Further, the facets are combined into a roof reflector, which makes the assembly of the present invention more compact, more rigid and provides a rigid mounting surface for the beamsplitter and associated components. 
   Bleier et al. describe what they term “monolithic optical assemblies” in two U.S. patent, U.S. Pat. No. 5,949,543 (1999), and U.S. Pat. No. 6,141,101 (2000). Their system might also be called a bilithic interferometer, in that two assemblies are juxtapositioned to form a variation of Michelson&#39;s design. The two assemblies are a hollow cube corner retroreflector and the monolithic assembly containing a beamsplitter and two reflecting surfaces. The two reflecting surfaces are interferometer end mirrors analogous to mirrors  80 A and  80 B of FIG.  1 . Bleier&#39;s two reflecting surfaces, like  80 A and  80 B of  FIG. 1 , are mutually perpendicular with each other, but not with the beamsplitter; they are oriented at 45 degree angles relative to the beamsplitter. Hence, Bleier&#39;s monolithic interferometer is not tilt-compensated to the full extent of the present invention. However, the construction techniques taught by Bleier for producing monolithic assemblies can be applied to the interferometer designs of the present invention. This is not presently the preferred approach to construction, because the resulting assemblies are more fragile and thought to be more expensive than the construction of the preferred embodiments described below. 
   SUMMARY OF THE INVENTION 
   The present invention described herein substantially increases throughput relative to prior art tilt-compensated interferometers, while enabling very compact designs. Tilt compensation can improve photometric accuracy and also improve very rapid scan operation of a reciprocating mirror interferometer. The present invention provides a related series of novel tilt-compensated interferometer designs comprised of a beamsplitter rigidly mounted to a roof reflector, combined with various other optical components. By “roof reflector”, what is meant is a pair of abutted mirror facets that are perpendicular to interferometric tolerances, as is known in various optical arts. The present invention is illustrated with several embodiments in which the advantages are employed to achieve a series of related ends. Interferometric alignment means an accuracy on the order of the wavelength of the radiation being used in the interferometer. In the context of angles, interferometric alignment means that the displacement of a beam by angular deviation is on the order or less than a wavelength of electromagnetic radiation over the length of travel. Angles this small are generally about 1 arcsecond or a few microradians. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a diagram of a prior art Michelson interferometer. 
       FIG. 2  shows a diagram of a first interferometer embodiment. 
       FIG. 3A  shows a diagram of second interferometer embodiment. 
       FIG. 3B  shows a diagram of second interferometer embodiment. 
       FIG. 4  shows a diagram of third interferometer embodiment. 
       FIG. 5A  shows a diagram of a beamsplitter/roof reflector assembly. 
       FIG. 5B  shows a diagram of a beamsplitter/roof reflector assembly. 
       FIG. 6A  shows details of the beamsplitter and compensator mounting. 
       FIG. 6B  shows details of the beamsplitter and compensator mounting. 
       FIG. 7A  shows a diagram of a scanning assembly. 
       FIG. 7B  shows a diagram of some components of a scanning assembly. 
       FIG. 7C  shows a diagram of some components of a scanning assembly. 
       FIG. 8  shows a diagram of alternative scanning assembly. 
       FIG. 9A  shows a diagram of a very-rapid-scan drive mechanism. 
       FIG. 9B  shows a diagram of a very-rapid-scan drive mechanism. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The operation of the various interferometer embodiments of the invention described below is analogous to the operation of the prior art Michelson interferometer. The same external components such as source, laser, detector and laser detector may be used with the designs described below to achieve ends that are either known or as-yet undiscovered. Electronics and computers are preferably used to process the data according to known means. Many of the known methods of operation are detailed in Griffiths and deHaseths&#39; book, cited above. 
     FIG. 2  shows a plan view of a permanently aligned interferometer for which errors of the motion of the scanning assembly  81  are compensated. The assembly  81  may be supported by a variety of mechanisms including ball bearing slides, roller bearings, flex pivots, or airbearings to provide smooth, essentially rectilinear motion. The beamsplitter is indicated by  30  and roof reflector/beamsplitter mounting monolith is indicated by  46 . Two mirrors that form the terminal ends of the interferometer are indicated by  80 A and  80 B. These mirrors are rigidly affixed to the moving assembly  81  in direct opposition. The parallelism of their faces  80 A and  80 B is preferably good to interferometric tolerances. In the preferred embodiment, these mirrors may be manufactured on aluminum substrates using single-point diamond turning, then glued to the moving assembly with a highly-filled epoxy such as Epotek H77 (Billerica, Mass.). If the moving assembly also is fabricated from aluminum, which is a light and stiff metal, then the coefficient of expansion of the mirror, the epoxy and the moving assembly will be closely matched. Alternatively, the mirrors may be formed as an integral component of the scanning carriage. Consequently, changes in the temperature of the system will generate only minimal distortions of the alignment. A mirror  40  may be used to fold the second recombined beam out of the interferometer to the detector focusing mirror  21 . This mirror is optional and the detector focusing mirror  21  may be located in the same place as mirror  40 , or at another location. The angle of the folding mirror  40  is not critical, so long as the beam accurately reaches the detector  20 . 
   The primary beam of radiation is split at the beamsplitter coating  32 . A reflected portion enters the first optical path  15  where it propagates to a first reflector  80 A. The incidence of the first energy beam on the reflector  80 A is substantially perpendicular to the surface. Thus, to interferometric tolerances the radiation beam reflected from the mirror  80 A to become the first returning beam exactly retraces the path  15  taken from the beamsplitter coating  32 . When the first returning beam reaches the beamsplitter coating  32 , a portion is transmitted to become part of the recombined beam ultimately reaching the detector  20 . 
   A transmitted portion of the primary radiation beam enters the second optical path  16 . It interacts with the facets  47  and  48  of the roof reflector (detailed in FIG.  5 B). The beam is inverted about a horizontal plane and turned 90 degrees by the roof reflector  46 . The beam continues in the second optical path  16  to the second reflector  80 B. 
   A second returning beam is generated by the reflection from the second reflector  80 B. The second returning beam proceeds along the second optical path  16  in the opposite direction it took from the beamsplitter coating  32 . It reaches the roof reflector facets  47 ,  48  where it is again inverted about a horizontal plane. The second returning beam then propagates to the beamsplitter coating  32  where a portion of the energy is reflected into the path of the recombined beam. It mixes with the energy in the first returning beam that was transmitted by the beamsplitter coating  32 . The recombined beam propagates via mirrors  40  and  21  to the detector  20 . 
   The operation of the other embodiments described relative to  FIGS. 3A ,  3 B, and  4  are essentially the same, with the exception that additional reflectors are interposed in the first and second paths. Any number of such reflections may be interposed in the first and second paths, so long as the tilt and shear of the first and second energy beams are not disturbed. The use of such additional reflecting surfaces is one thrust described by U.S. patent application Ser. No. 09/299,030 by Manning (U.S. Pat. No. 5,898,495). 
   The advantages of the interferometer illustrated in  FIG. 2  are manifold. First, it is a tilt-compensated design providing improved photometric accuracy. Second, the throughput is very high relative to prior art tilt-compensated interferometers. Third, this design is relatively inexpensive to manufacture. Fourth, this interferometer provides compensation for imperfections of scan motion. Fifth, this design is particularly compact. Sixth, this design provides a large change in path difference for each unit of motion of the scanning carriage  81 . 
   FIG.  3 A and  FIG. 3B  illustrate another embodiment of the invention in which the antiparallel beams reflected from the beamsplitter  30 , coating  32  and roof reflector  46  are both folded by 180 degrees to reach opposite sides  80 A and  80 B of the same mirror  80 . The two faces  80 A,  80 B of mirror  80  are preferably parallel to interferometric tolerances. Methods for producing such mirrors are known in the art. If the mirror  80  is made of a lightweight material such as beryllium, it may be reciprocated very rapidly to effect scanning. Path difference scanning also may be provided by scanning either or both of the cube corner retroreflectors  60 A and  60 B in a manner well known in the prior art. The cube corner retroreflectors may be replaced by cat&#39;s eye retroreflectors. Either type may be used interchangeably. Any inadvertent motions, particularly tilt and shear, of the elements  60 A,  60 B, or  80  do not compromise the interferometric alignment. Any inadvertent tilting of the mirror  80  affects equally the angle of the beams in the first and second paths of the interferometer, thus providing optical cancellation of the effect of tilt on interferometric alignment. As before, the primary beam of energy from source  10  and collimating mirror  11  is split at element  30  by coating  32 . It should be noted that this diagram shows a different beamsplitter configuration, akin to those described in U.S. patent application Ser. No. 09/299,030. The beamsplitter geometries are essentially interchangeable, and for different applications one may be preferred over another. 
   The beamsplitter/compensator/mounting system described below relative to  FIG. 5  may be used in place of the ones described relative to  FIGS. 2 ,  3 A and  3 B, and  4 . The two antiparallel beams reach roof reflectors  60 A and  60 B where they are inverted and folded by 180 degrees. By positioning the cube corner retroreflectors  60 A and  60 B appropriately, the beams also are sheared downward to clear the beamsplitter/roof reflector assembly  30  and  46 . The three-dimensional nature of the system is shown clearly by the side view of FIG.  3 B. The dotted lines indicate that a beam is in a different plane relative to the paper. For clarity, the view of  FIG. 3B  omits source  10 , source collimating mirror  11 , detector focusing mirror  21  and detector  20 . The beamsplitter  30  shown in this  FIG. 2  has no compensator, to demonstrate that a variety of beamsplitter geometries are acceptable as long as the angle between the roof reflector facets and beamsplitter coating is accurately maintained perpendicular. 
   The advantages of this embodiment are also manifold. The throughput advantage described relative to  FIG. 2  is generally preserved. The main advantage of this design is that the light weight of mirror  80  allows very rapid-scan operation, while preserving optical tilt compensation for the moving element. A further advantage of this design is that it is compact. Yet another advantage of this design is that it provides a large change in optical path difference for each unit of motion of elements  60 A,  60 B and  80 . 
   Jennings has reported a related design (D. E. Jennings, Applied Optics Vol. 27, page 4605) that shares the potential advantage of very-rapid-scan operation, but does not provide the full degree of tilt-compensation that is possible with the present invention described herein. 
   The embodiment illustrated in  FIG. 4  combines the very-rapid-scan capability provided by the tilt-compensation techniques and disk mirrors described in U.S. Pat. No. 5,898,495 to Manning with the advantages of a tilt-compensating beamsplitter/roof reflector assembly  30  and  46 . The embodiment shown in  FIG. 4  is a variation of the design illustrated in  FIGS. 3A and 3B , modified by the insertion of disk mirrors  42  and  52  into the first and second paths. These paths are folded by the cube corner reflectors  60 A and  60 B to make tilt-compensating complementary reflections from the disk mirrors  42  and  52 . The tilt-compensation of the reflections from disk mirrors (and other plane mirrors) is the essence of U.S. Pat. No. 5,898,495 to Manning. It should be understood that optical path difference scanning can be achieved with only one disk mirror; therefore the second disk mirror is optional. The second disk mirror may be replaced by a flat mirror to maintain the folded geometry without the expense of a motor and disk mirror. The addition of a second disk mirror and motor into the system creates the possibilities described in U.S. patent application Ser. No. 09/299,030 (U.S. Pat. No. 5,898,495) for operating the disk mirrors with different angular velocities and different phase relationships. 
   The preferred motors  100 A and  100 B for this type of operation are brushless DC motors. Encoders may be used to track the phase of such motors very accurately. The motors may employ steel ball bearings, ceramic ball bearings, magnetic levitation or air bearing support for the armature. Such motors are controlled routinely for industrial applications using known art. It is generally preferred that the current in the brushless motor be controlled with a linear amplifier so that electromagnetic interference with the detectors  20  and  22  (e.g.,  FIG. 1 ) is minimized. This type of interferometer, when operated at speeds above about 100 scans per second, is best operated in a rigid vacuum housing for several reasons. First, the disk mirrors store substantial kinetic energy that can be inadvertently released, e.g. by fracture. A vacuum housing is generally strong enough to contain the disk or fragments. Second, the airflow around the disks can be quite loud; in vacuo, the operation is nearly silent. Third, it is possible that turbulence of the air on the disk surface creates fluctuations in the path differences experienced by different portions of the beams. The technology for vacuum interferometers is well known in the art. The operation of the interferometer of the present invention as shown in  FIG. 4  follows the discussion relative to FIG.  3 A and  FIG. 3B , with the variation that a reflection from the disk mirrors  42  and  52  occurs both before and after reflection in the two retroreflectors  60 A and  60 B. 
   FIG.  5 A and  FIG. 5B  show two views of a beamsplitter/roof reflector assembly;  FIG. 5A  is a section view through the assembly and  5 B is a perspective. The beamsplitter is held such that it is mutually perpendicular with two facets  47 ,  48  of a roof reflector  46 , which are themselves perpendicular. The key result of this arrangement is that a beam impinging on the beamsplitter  30  will be divided by the coating  32  into two beams that are made exactly antiparallel. Any motion of the assembly  32  and  46  with respect to the incident beam will be imposed exactly equally on the two antiparallel beams, thus maintaining them exactly antiparallel. The beams are antiparallel because the matrix transform for the reflection from a single plane (the beamsplitter coating  32 ) is the opposite of the matrix transform for the double reflection from the roof reflector facets  47  and  48 . 
     FIG. 5A  shows the detail of the beamsplitter mounting on the roof reflector substrate or monolith  46 . There are a variety of approaches that might be taken to hold a beamsplitter substrate  31  in alignment with the facets  47  and  48  of the roof reflector  46 . The illustrated embodiment is preferred because it allows precise adjustment of the angle between the beamsplitter substrate  31  (and consequently coating  32 ) and the roof reflector facets  47  and  48 , as well as providing a secure mounting for the optional compensator plate  33 . The plates  31  and  33  may be held by a sheet metal spring  37  against a lip  38  that is an integral component of the ring  35 . The ring  35  may be machined from aluminum alloy. The ring  35  is drilled and tapped such that screws  36  may push against the monolith  46  providing adjustment of the angle between the ring  35  and the facets  47 ,  48 . The ring  35  is restrained to the monolith by springs  39 .  FIG. 5B  shows a perspective view of the assembly looking into the aperture of the hollow cube retroreflector that is formed by the two facets  47  and  48  of the roof reflector and the facet comprised of the reflective coating  32  of the beamsplitter. Tapped holes  49  may be provided in the bottom of the monolith  46  to allow mounting of the assembly to an optical bench. Alternatively, the assembly may be held by adhesive to, or fabricated as part of, a larger system. 
   Two preferred methods for fabricating the roof reflector monolith are described. These techniques are generally known in the art and also may be applied to the manufacture of other components such as end mirrors. These techniques are electron-discharge machining (EDM), particularly wire EDM, and ceramic molding, either process followed by replication or polishing and metallization. It is thought that by the choice of substrate  46  metals that have very uniform coefficients of expansion, together with the use of wire EDM, that excellent precision substrates can be prepared. Preferred metals are aluminum, invar, and stainless steel. EDM uses electric arcing to erode metal from a part in a dielectric bath. The electric arcing produces very little damage to the part and particularly does not leave residual stresses that can cause adverse thermal distortion. Wire EDM can produce surface finishes that are excellent for replication or subsequent polishing. Replication is a known method for producing high-quality optical surfaces. Another preferred approach for preparation of roof reflector substrates is slip-casting of ceramic mixtures and molding of ceramic pastes or clays. By judicious choice of ceramic composition, the thermal expansion properties of the ceramic can be tailored to be isotropic (i.e., uniform in all directions), and also small. In particular, the addition of ceramic microspheres to the casting composition is preferred because the resulting ceramic bodies will tend to have isotropic properties. Some ceramic compositions have particles that have non-uniform properties. If the particles are shaped such that they align preferentially (e.g., flakes or rods), then the macroscopic properties of the resulting masses may not be isotropic, which can lead to thermal distortion when temperature changes occur. After casting or forming, it is understood that the resulting ceramic pieces are fired, then used as substrates for replication, or polished and metallized. 
     FIGS. 6A and 6B  show detail of the beamsplitter mounting hardware introduced relative to FIG.  5 A and FIG.  5 B. In particular, the spring fingers  37  can be clearly seen in  FIG. 6A  to be in contact with the beamsplitter substrate  31 . The detailed section of ring  35  can be seen in FIG.  5 B. The ring  35  is preferably turned on a lathe to produce a lip  38  of known dimensions. By using a computer-numerically-controlled lathe for this operation, the thickness of the lip  38  may be varied around its periphery to provide a wedge angle for the air gap between the beamsplitter substrate  31  and the compensator plate  33 . This practice is known in the art, as is the practice of wedging the beamsplitter substrate  31  and compensator plate  33 . The spring finger  37  may be manufactured by a variety of known manufacturing methods, including wire EDM, fine blanking, laser cutting and punching. A variety of other methods for mounting beamsplitters are known in the art. 
     FIG. 7  shows details of a preferred scanning carriage  81  introduced in FIG.  2 . This embodiment is preferred for prototyping and small production runs where casting is not economical. The scanning carriage may be manufactured as a monolith, for example, by milling a solid block of aluminum alloy. However, this is not the most economical method for constructing such a carriage. By manufacturing the carriage from extruded rods  83 , which may be aluminum alloy, and rolled plates  82 A and  82 B, which may also be aluminum alloy, the cost can be reduced. The parts are preferably stress relieved after machining by heating to 350 F for 4 hours. The methods of assembly must be chosen judiciously to avoid problems with mismatch of thermal expansion coefficients. 
   The preferred method for assembling this carriage from machined parts employs threads  86  cut in the ends of the rods  83  where they penetrate the plates  82 . Spring washers  88  are placed on the threaded ends  86  of the rods  83  and are secured with nuts  87 . The nuts  87  and the spring washers  88  may be steel. It is intended that the thermal expansion of the aluminum alloy plates  82 A and  82 B will match the thermal expansion of the rods  83 . Thus, the springs  88  compressed by the nuts  87  will provide essentially constant tension on the plates  82 A and  82 B, driving them against shoulders cut in the rods, and thereby maintaining constant alignment between the mirror surfaces  80 A and  80 B. The rods may be secured together by cross-members to provide additional stiffness for the scanning carriage  81 . The cross-members can take a variety of forms, including welded struts or sheet metal straps. The three-dimensional nature of the scanning carriage is obscured in the two-dimensional drawing of FIG.  7 A. It should be understood that at least a third rod of the type  83  is obscured beneath one of the rods  83  shown in the drawing. 
     FIGS. 7B and 7C  show details of the end plates  82 A and  82 B of the scanning carriage. The mirror facets  80 A and  80 B are shown in dashed outline in  FIG. 7C , which is the back view of one of the plates. The nuts  87  and ends of the rods  83  are shown in this view. The face of the plate  82 A is shown in FIG.  7 B and the replicated or polished mirror facet area  80 A is also shown. 
     FIG. 8  shows an alternative embodiment of the scanning carriage in which the mirror  80 A is mounted adjustably. The other mirror  80 B may be mounted in a fixed manner, preferably using a thermal coefficient of expansion-matched material. It may also be formed as an integral component of the scanning carriage  81 . For an aluminum mirror mounted to an aluminum carriage, Epotek H77 epoxy provides a thermal coefficient of expansion (TCE) match within about 6 parts per million per degree C. Springs  39  pull the mirror backing plate  89  against the tips of the adjustment screws  36 . The adjustment screws may be of the differential type known in the art. Thus, by turning the screws  36 , the tilt of the mirrors may be adjusted very precisely. The usual criterion for this adjustment is maximizing the strength of the detector signals. Alternatively, laser interferometry may be used as feedback for aligning the two mirrors  80 A and  80 B to precise parallelism. The carriage  81  shown in this diagram is indicated to be of monolithic construction. Such an assembly can be inexpensively manufactured by a variety of approaches; one that is particularly favored is casting of aluminum alloy. For aerospace applications, the favored material for this frame is beryllium alloy or carbon fiber composite. Zerodur mirrors may provide a close match to the thermal expansion coefficient of some carbon fiber composite materials. 
     FIG. 9  illustrates an ultra-rapid-scanning mechanism for driving the double-sided mirror  80  described relative to FIG.  3 A and FIG.  3 B. In one preferred embodiment, piezoelectric elements  90 A and  90 B are arranged symmetrically around the mirror  80 .  FIG. 9  indicates that the transducers  90 A and  90 B are a pair, which is particularly convenient. The mirror  80  shown in this figure has been stiffened by the addition of a ring  85  around the periphery. This ring helps the mirror resist distortion during the intense acceleration provided by the piezoelectric elements  90 A and  90 B. The piezoelectric elements may bear against counterweights  91 A and  91 B that absorb the reaction forces generated by the piezoelements. This prevents the optical bench (not shown) supporting the other optical components, such as  10 ,  11 ,  20 ,  21 ,  30 ,  46 ,  60 A and  60 B of  FIG. 3A , from being vibrated.  FIG. 9A  shows a top-down view of the scanning mechanism. The mounts  92 A and  92 B may be a hard rubber composition, or metal with a rubber insert around the piezoelectric transducer body  92 A and  92 B. The point of contact between the transducer  90 A and  90 B may be a rubber bushing to allow small motions of the piezoelement caused by imperfect balance between the counter-masses  91 A and  91 B and the mirror  80  and  85  mass. The piezoelectric elements  90 A and  90 B may be bonded to the mirror with epoxy  108  and to the counterweights  91 A and  91 B with epoxy  108 . The piezoelectric transducer must be provided with relatively high voltage and alternating current waveforms from power supplies generally indicated by  93 A and  93 B. These supplies are preferably under the control of a computer, using known technology.  FIG. 9B  shows an end view of the piezoelectric drive assembly. The hidden lines in mount  92 A and  92 B indicate holes that may be used to secure it to an optical bench or other assembly for incorporation in an interferometer of the type described relative to FIG.  3 A and FIG.  3 B. It should be understood that other types of transducers may be substituted for the piezoelectric devices  90 A and  90 B. For example, magnetostrictive transducers may be more suitable for generating larger forces and displacements in a given volume, but might require cooling. Many other types of transducers can also be substituted. It should be appreciated that the particular advantage of this geometry is that the tilt-compensation relaxes the requirement for strictly rectilinear motion of mirror facets  80 A and  80 B, allowing for lower precision and therefore lower cost control of the motion at high accelerations. 
   The principles, embodiments and modes of operation of the present invention have been set forth in the foregoing specification. The embodiments disclosed herein should be interpreted as illustrating the present invention and not as restricting it. The foregoing disclosure is not intended to limit the range of equivalent structure available to a person of ordinary skill in the art in any way, but rather to expand the range of equivalent structures in ways not previously contemplated. Numerous variations and changes can be made to the foregoing illustrative embodiments without departing from the scope and spirit of the present invention.