Abstract:
A novel variation of Michelson&#39;s interferometer uses tilt- and shear-compensation optics together with a beamsplitter and parallel reflector assembly to allow various mirror motions to produce variation of path difference. The tilt-compensation mechanism consists of two complementary reflections from a single plane mirror to produce a beam having a constant angle of propagation, typically the same as the input beam. Using a retroreflector to invert the image of the single plane mirror before the second reflection produces the complementary reflections. A particularly efficient embodiment of the present invention uses a balanced disk-shaped mirror to effect very rapid variation of path difference by nutation or precession. Other advantages of tilt-compensation include photometric stability. This interferometer has applications in spectrometry, spectral imaging and metrology.

Description:
RELATED APPLICATIONS  
       [0001]     This is a CONTINUATION of pending prior application Ser. No. 10/277,439, filed on Oct. 21, 2002, entitled TILT-COMPENSATED INTERFEROMETERS, which will issue on Nov. 22, 2005 as U.S. Pat. No. 6,967,722. The Ser. No. 10/277,439 Application claimed priority under 35 U.S.C. sctn 119(e) from Provisional patent application Ser. No. 10/277,439. The Application was a continuation of application Ser. No. 09/299,022, which issued as U.S. Pat. No. 6,469,790. The Ser. No. 09/299,022 Application, which issued as U.S. Pat. No. 6,469,790, was a continuation of application Ser. No. 08/959,030, which issued as U.S. Pat. No. 5,898,495. The Ser. Nos. 10/277,439 and 09/299,022 and 08/959,030 Applications and U.S. Pat. Nos. 6,967,722 and 6,469,790 and 5,898,495 are hereby incorporated herein by reference for the entirety of their disclosures. 
     
    
     BACKGROUND AND SUMMARY OF THE INVENTION  
       [0002]     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. The present invention described herein enables very compact designs. Tilt compensation can improve photometric accuracy and also improve very rapid scan operation of an interferometer. The present invention provides a novel tilt-compensated design comprised of a parallel reflector assembly including mirror, combined with various other optical components.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]      FIG. 1  shows a diagram of a prior art Michelson interferometer.  
         [0004]      FIG. 2  shows a diagram of a novel interferometer.  
         [0005]      FIG. 3A  shows a diagram of the beam footprints on the moving mirror.  
         [0006]      FIG. 3B  shows a diagram of an alternative arrangement of beam footprints on the moving mirror.  
         [0007]      FIG. 4  shows a diagram of a section view of a beamsplitter and parallel reflector assembly.  
         [0008]      FIG. 5  shows a diagram of the beamsplitter and parallel reflector assembly from an isometric perspective.  
         [0009]      FIG. 6  shows a diagram of a section view of the beamsplitter cartridge assembly. 
     
    
     DETAILED DESCRIPTION  
       [0010]     Michelson interferometers can be used for many purposes, including spectrometry and metrology. The principle of operation is that a beam of electromagnetic radiation is divided into two portions; the two portions are then delayed and recombined, leading to interference that is controlled by the path difference between the two portions of radiation. This prior art is illustrated by  FIG. 1 . Radiation from source  10  may be collimated by a parabolic mirror  11 . The source beam  15  is divided at the beamsplitter  30  into two portions which propagate in first and second paths as the reflected beam  16  and the transmitted beam  17 . These paths are often called the arms of the interferometer, and typically are oriented at 90 degrees to each other as shown in  FIG. 1 .  
         [0011]     At the ends of the two arms are mirrors  80 A and  80 B from which the two beams are reflected back toward the beamsplitter. 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 by way of mirror  11 , generally being lost from use, and the second recombined beam  18  propagates out of the interferometer at an angle to the input beam. The second recombined beam  18  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 onto 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  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 or stabilized diode laser. The radiation from the second source may be observed simultaneously with a second detector  22 . External focusing optics generally are not required for a reference laser such as  12  because the beam is already tightly collimated. The interferometer is usually 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. The mirror drive is not shown here, the usual approaches being known in the art.  
         [0012]     A disadvantage of the Michelson interferometer is that the two end mirrors  80 A and  80 B in the arms are susceptible to misalignment with each other and with the beamsplitter  30 . Further, the alignment must be preserved, to interferometric tolerances, during motion between one or both of the mirrors. Expensive, high-quality bearings generally are required to provide precise rectilinear motion. In many 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), Frosch (U.S. Pat. No. 4,278,351), Woodruff (U.S. Pat. No. 4,391,525) and related designs provide an interferometer framework in which slight misalignments of the optical components are compensated with respect to interferometric alignment. Slight residual misalignment may result in the recombined beam  18  of radiation not reaching exactly the intended detection location, but generally the sensitivity to such misalignment is 100 times smaller than the sensitivity to interferometric misalignment. In short, there is a substantial advantage to optical tilt-compensation.  
         [0013]     The new invention provides a series of related novel tilt-compensated interferometer designs comprised of a beamsplitter rigidly mounted to a reflector, combined with various other optical components. The invention is illustrated by one embodiment in which the advantages are employed to achieve a series of related ends. Interferometric alignment requires an accuracy on the order of the wavelength of the radiation being used in the interferometer. In the context of angles, interferometric alignment requires that the angular deviation causes a beam to be displaced by a distance that is a fraction of the wavelength of electromagnetic radiation over the length of travel. Such angles are generally on the order of arcseconds and microradians.  
         [0014]      FIG. 2  is the embodiment of the present invention. A source  10  of radiation provides a beam  15  that interacts with a beamsplitter  30 . One beam is transmitted by the beamsplitter  30  to a parallel reflector  40  held rigidly in alignment to the beamsplitter  30 . The beam  16  reflected by the beamsplitter  30  is exactly parallel to the beam  17  reflected by the parallel reflector  40  such that both beams reach the moving mirror  52 , which may be a rotating disk mirror driven by a motor  100 , and supported by a shaft  65 . Preferably the two beams are disposed generally on sides of the moving mirror  52  opposite the center. The two beams  16  and  17  reflect off of the disk mirror  52  to two respective cube-corner retroreflectors  60 A and  60 B. The cube-corner retroreflectors  60 A and  60 B invert the beams returning them to the disk mirror. Generally the returned beams are disposed on the same sides of the disk center as their respective first reflections as will be seen (vide infra). The beams are reflected by the disk mirror  52  a second time propagating to a common end mirror  80 , to which they are precisely perpendicular. The beams are thus reflected back through the same arrangement, to arrive at the beamsplitter  30  where they are both split again to form a new beam  18  that propagates to the detector  20 . This embodiment simplifies alignment since no component requires precise alignment to another, with the exception of mirror  40  and beamsplitter  30  which are rigidly coupled by an assembly  223 . This embodiment is intrinsically insensitive to small shifts of alignment of the moving disk mirror  52 , the retroreflectors  60 A and  60 B and the common terminal mirror  80 .  
         [0015]     In this embodiment the beamsplitter contains a parallel reflector assembly including mirror  40  as shown in  FIG. 4A . Beamsplitter/reflector assemblies of this general type are known in the literature (see for example, Solomon, U.S. Pat. No. 5,675,412, and Turner, U.S. Pat. No. 5,808,739). The first and second energy beams  16  and  17  are very accurately parallel as a result of one reflecting from the mirror  40  which is parallel to the beamspliter  30 . Any small changes in alignment of the beamsplitter  30  are identical at parallel reflector  40 , thereby maintaining parallelism between the two elements  30  and  40 . The present invention may be applied as shown in  FIG. 2  to scan the optical path difference while maintaining the intrinsic tilt-compensation of the beamsplitter  30  afforded by the beamsplitter/parallel reflector assemblies taught by Solomon and Turner. In  FIG. 2 , each energy beam makes two complementary reflections at the moving disk mirror  52  such that the first energy beam  16  propagating to end mirror  80  is exactly parallel to the second energy beam  17  propagating to end mirror  80  via the cube-corner retroreflectors  60 A and  60 B respectively. As before, tilt of all components in the system is compensated. The optical path difference can be scanned by a variety of motions of moving mirror  52 , although rotation is generally preferred. Such rotation can be driven very rapidly by a brushless motor indicated by  100 .  
         [0016]     Rotating the moving mirror  52  can vary the path length of the first and second energy beams simultaneously. The moving disk mirror  52  can be rotated about an axis of rotation by driving it with a motor  100  according to any method already known in the art. The moving mirror  52  is rigidly attached to the motor shaft  65  that defines the axis of rotation. The speed of rotation is controlled by varying the current and voltage applied to the motor windings according to known means. Rotation of the moving disk mirror  52  body produces precession or nutation of the surface of moving disk mirror  52 .  
         [0017]     It is possible to adjust the path difference of the interferometer between zero and the maximum allowed by a given tilt angle, by sliding the moving disk mirror  52  relative to the beam footprints  206 ,  207 ,  216  and  217  shown in  FIG. 3A  and  FIG. 3B . The path length in each arm of the embodiment shown in  FIG. 2  must be relatively long to accommodate the beam folding. The cube-corner retroreflectors  60 A and  60 B and the moving disk mirror  52  should preferably have apertures at least twice as large as the beam diameter and interferometer aperture. The resolution may be a function of beam motion on the optical surfaces, as well as of the tilt angle. Throughput may be a function of the amount of the divergence of the beam that can be accommodated without the beams being clipped at the edges of the components such as mirror  80  in the optical system.  
         [0018]     Because the path of the beam reflected from the parallel reflector  40  is longer than the beam that is reflected first by the beamsplitter  30 , it is necessary for the cube corner reflector  60 A to be position somewhat closer to the rotating disk mirror  52  than is the cube corner reflector  60 B.  
         [0019]      FIG. 3A  illustrates one placement of beam footprints  206 ,  207 ,  216 ,  217  on the moving mirror  52 . It can be discerned from  FIG. 3A  that there is a relationship between the disk diameter and the maximum beam diameter that can be accommodated. In  FIG. 3A , the disk diameter must be somewhat larger than four times the beam diameter, which governs the beam footprints  206 ,  207 ,  216  and  217 . The tilt angle as well as the beam divergence and the hub  202  diameter govern the exact relationship of dimensions. The center of the beam footprints indicated by line  205  are aligned with the axis of rotation in the center of hub  202 . Other beam footprint locations on the disk are possible.  
         [0020]      FIG. 3B  illustrates an alternative placement of beam footprints  206 ,  207 ,  216 , and  217  on the moving mirror  52 . The moving disk mirror  52  in this embodiment may be smaller, with a diameter slightly larger than three times the beam diameter.  
         [0021]      FIG. 4  shows a sectioned view of a beamsplitter mounting block generally indicated by  223  with support for a beamsplitter assembly generally indicated by  30 . The block may be fabricated from solid aluminum alloy, such as 6061-T6. After machining operations to create passages in the block, the assembly may be stress relieved by heating it to a temperature such as 350 F or 400 F according to practices that are well known in the various arts. The passages in the block allows beams  17  and  18  to pass through, with beam  17  reaching the parallel reflector mirror  40 , which is rigidly mounted to the block. The mirror  40  may be formed as an integral component of the block by using a technique known as single point diamond turning.  
         [0022]      FIG. 5  shows an isometric view of the beamsplitter parallel reflector mounting block generally indicated by  223 . Mounted within the block  223  is a beamsplitter assembly generally indicated by  30 , which is itself comprised a ring  221  to which the other components such as  220 B a clamp ring and the compensator plate  34 B are affixed.  
         [0023]      FIG. 6  illustrates details of the beamsplitter cartridge assembly  30  of  FIG. 5 . At the center of the cartridge is a robust mounting ring  221 . The ring may be machined from aluminum and stress relieved. The compensator plate  34 A and the beamsplitter plate  34 B, which function as a beamsplitter, are affixed to the two sides of the positioning ring  221  with retaining rings  220 A and  220 B. The ring  221  serves to hold each of the mirrors  34  A and  34 B aligned to one another and maintain a necessary spacing. The positioning ring  221  also functions as a rigid connector to the interferometer mounting block  223  as shown in  FIG. 4  and  FIG. 5 . Retaining rings  220 A and  220 B include features to accommodate O-rings  222 A and  222 B which protect the mirrors from direct metal contact and serve to comply with thermal expansion mismatch of the components  221 ,  222 A,  222 B,  34 A,  34 B  220 A and  220 B.  
         [0024]     The principles, embodiments and modes of operation of the present inventions have been set forth in the foregoing provisional 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 available to a person of ordinary skill in the art in any way, but rather to expand the range in ways not previously considered. Numerous variations and changes can be made to the foregoing illustrative embodiments without departing from the scope and spirit of the present inventions. In particular, these facets of the invention or inventions may be combined in new and useful ways.