Tilt compensated interferometers
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.
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 INVENTIONIt 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
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
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.
In this embodiment the beamsplitter contains a parallel reflector assembly including mirror 40 as shown in
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
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.
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.
Type: Application
Filed: Nov 21, 2005
Publication Date: Oct 26, 2006
Inventor: Christopher Manning (Troy, ID)
Application Number: 11/284,983
International Classification: G01B 9/02 (20060101); G01J 3/45 (20060101);