Tilt compensated interferometers

Abstract

A novel variation of Michelson'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

BACKGROUND AND SUMMARY 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. 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

FIG. 1 shows a diagram of a prior art Michelson interferometer.

FIG. 2 shows a diagram of a novel interferometer.

FIG. 3A shows a diagram of the beam footprints on the moving mirror.

FIG. 3B shows a diagram of an alternative arrangement of beam footprints on the moving mirror.

FIG. 4 shows a diagram of a section view of a beamsplitter and parallel reflector assembly.

FIG. 5 shows a diagram of the beamsplitter and parallel reflector assembly from an isometric perspective.

FIG. 6 shows a diagram of a section view of the beamsplitter cartridge assembly.

DETAILED DESCRIPTION

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.

At the ends of the two arms are mirrors 80A and 80B 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 21A that concentrates the radiation at a sampling point 23. The radiation from the sampling point may be collected by a mirror 21B and focused onto a detector 20. Many alternatives to the mirror combination 21A and 21B 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.

A disadvantage of the Michelson interferometer is that the two end mirrors 80A and 80B 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.

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.

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 60A and 60B. The cube-corner retroreflectors 60A and 60B 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 60A and 60B and the common terminal mirror 80.

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 60A and 60B 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.

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.

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 60A and 60B 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.

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 60A to be position somewhat closer to the rotating disk mirror 52 than is the cube corner reflector 60B.

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.

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.

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.

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 220B a clamp ring and the compensator plate 34B are affixed.

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 34A and the beamsplitter plate 34B, which function as a beamsplitter, are affixed to the two sides of the positioning ring 221 with retaining rings 220A and 220B. The ring 221 serves to hold each of the mirrors 34 A and 34B 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 220A and 220B include features to accommodate O-rings 222A and 222B which protect the mirrors from direct metal contact and serve to comply with thermal expansion mismatch of the components 221, 222A, 222B, 34A, 34B 220A and 220B.

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.

Claims

1. A spectrometer, comprising: a source of a primary beam of radiant energy; a beamsplitter with parallel reflector assembly, fixed in position relative to the primary beam of radiant energy, for dividing the primary beam of radiant energy into at least first and second energy beams which are parallel; a single moving mirror that receives the first and second energy beams from the beamsplitter and parallel reflector assembly, the moving mirror having a planar optical surface mounted so as to reflect the first and second energy beams, the single moving mirror also being mounted to move the planar optical surface relative to the primary beam of radiant energy; a retroreflector, fixed in position relative to the primary beam of radiant energy, positioned to receive the first energy beam after it is reflected by the optical surface of the moving mirror; a second retroreflector, fixed in position relative to the primary beam of radiant energy, positioned to receive the second energy beam after it is reflected by the optical surface of the moving mirror;

2. A spectrometer as claimed in claim 1, wherein the retroreflector is a cube-corner reflector.

3. A spectrometer as claimed in claim 1, wherein the retroreflector is a lateral-transfer retroreflector.

4. A spectrometer as claimed in claim 1, wherein a retroreflector inverts the energy beam from the moving mirror.

5. A spectrometer as claimed in claim 1, wherein the mounting of the moving mirror rotates it about an axis of rotation.

6. A spectrometer as claimed in claim 6, wherein the moving mirror has a disk shape and the optical surface is one side of the disk.

7. A spectrometer as claimed in claim 7, wherein the disk-shape of the moving mirror has a thickness that varies sinusoidally with angle about the axis of rotation.

8. A spectrometer as claimed in claim 7, wherein the side of the disk opposite the optical surface is contoured to compensate for deformation of the disk caused by rotation of the disk about the axis of rotation.

9. A spectrometer as claimed in claim 7, wherein the disk is balanced for rotation about the axis of rotation.

10. A spectrometer as claimed in claim 7, wherein the disk is compensated for stretching distortion to provide a flat surface when rotated about the axis of rotation.

11. A spectrometer as claimed in claim 1, wherein the mounting of the moving mirror pivots it about a pivot axis.

12. A spectrometer as claimed in claim 1, wherein the mounting of the moving disk mirror translates it along a translation axis where a portion of the translation is parallel to the optical surface of the parallel reflector.