INTERFEROMETRIC SYSTEM WITH REDUCED VIBRATION SENSITIVITY AND RELATED METHOD
A source module (12) generates mutually orthogonally polarized beams of light as emanating from two spatially separated point sources (Sv, Sw) for use in a phase shifting interferometer.
This application claims the priority of U.S. Provisional Application Ser. No. 60/429,669, filed Nov. 27, 2002, and U.S. Provisional Application Ser. No. 60/459,149, filed Mar. 31, 2003, the contents of both which are incorporated herein by reference.
FIELD OF THE INVENTIONThe instant invention is directed to an interferometric system and method, in particular, an interferometric system and related method for enabling measurements of a wavefront in the presence of vibration or other disturbances that impede accurate measurements.
BACKGROUND OF INVENTIONInterferometers have been known and used for a long time. They are used for many purposes, including measuring characteristics of gases, liquids, and materials, through the use of transmitted or reflected light. There exist many types of interferometers that are classified by their optical design. A few of the most widely used interferometer types include Fizeau, Twyman-Green, Michaelson, and Mach-Zender. Each of these optical designs produces interference patterns called interferograms which are generated by the optical interference of test and reference wavefronts. In a typical interferometer, test and reference beams are obtained by appropriately splitting an incoming source beam (“beams” and “wavefronts” used interchangeably herein, with a “wavefront” being understood by one of ordinary skill in the art as propagating along the optical axis and sweeping out a volume that defines the light beam). One of the beams interacts with an object under test (hence commonly referred to as the “test beam”) thus carrying information about the test object being measured, while the other interacts with a known reference object (hence, commonly referred to as the “reference beam”). Interfering or otherwise coherently superimposing these two wavefronts produces an interferogram.
Information about a measured object can be extracted from a single interferogram. This technique allows for fast data acquisition, however, it typically suffers from poor spatial resolution, time consuming and complex data processing and/or non-uniform data sampling. Thus, it is often desirable to use other techniques instead. The most common techniques use three or more phase-shifted interferograms (typically three to twelve). Using multiple phase-shifted interferograms provides additional information that can be used to greatly increase the accuracy of the analysis.
Phase-shifting is a method used to change the phase between the test and reference wavefronts in a controllable way. During the last 20 years, various methods have been used to practically implement phase shifting techniques, including mechanically moving the reference object small distances comparable to the wavelength of light, or placing photo-elastic modulators and crystal retarders in the beam path. Almost all of these methods use a sequential approach (serial in time) to generate phase-shifted interferograms, which is accomplished by introducing prescribed changes to the wavefront phase while a detector acquires a series of data images. For example, the sequence of acquiring temporal phase-shifted interferograms occurs as follows: acquire interferogram, then shift the phase, acquire interferogram, then shift the phase, and so on. However, these known time-dependent methods are sensitive to environmental conditions during the span of time in which series of interferograms are acquired. Environmental conditions that can introduce errors include vibration, airflow, temperature changes, object movements, etc. Vibration is usually the major cause of error. Elaborate mounts or expensive vibration isolation tables are commonly used to isolate temporal phase-shifted interferometers from the physical environment.
To enable interferometric measurements under normal environmental conditions, without special isolation equipment, instruments have been developed to acquire multiple phase-shifted interferograms simultaneously. This eliminates or greatly reduces the effect of these errors on measurements. However, such simultaneous phase shifting methods have to date been limited to particular types of interferometers, such as the Twyman-Green or Mach-Zender types discussed below.
U.S. Pat. No. 4,583,855 (issued to Barekat) entitled “Optical Phase Measuring Apparatus” relates to use of a polarization type Twyman-Green interferometer with quarter-waveplates and polarizers. (“Quarter-waveplates” and “half-waveplates” used herein are understood by one of ordinary skill in the art as equivalent to quarter-wave retarders and half-wave retarders, respectively). Koliopulos in a paper entitled “Simultaneous Phase Shift Interferometer”, Proc. SPIE Vol. 1531, p. 119 (1992), described the use of a polarization type Twyman-Green interferometer. A. Hettwer, J. Krantz and J. Schwider in a paper titled “Three Channel Phase-Shifting Interferometer Using Polarization Optics and A Diffraction Grating” Opt. Eng., 39(4) (April 2000) described a Twyman-Green interferometer. German Patent DE 196,52,113,A1 awarded to J. Schwider discloses the invention that is described in his above-cited paper, based on a Twyman-Green interferometer. U.S. Pat. No. 6,304,330 entitled “Method and Apparatus for Splitting, Imaging and Measuring Wavefronts in Interferometry” and U.S. Pat. No. 6,552,808 are directed to a modified polarization type Mach-Zender and Twyman-Green interferometers.
As intimated above, optical interferometers are typically constructed of optical components such as lenses, mirrors, beamsplitters, and waveplates. These components usually have slight imperfections or deviations from an ideal perfect component. From a practical standpoint, Twyman-Green type interferometers can suffer from a configuration having a reference arm and a test arm that are of separate paths. Because the interferogram generated by the interferometer is an image or pattern that registers differences between the test and reference wavefront, a separation of the test and reference path such as in a Twyman-Green type interferometer, can cause imperfections and aberrations in the optical components encountered in one path, but not in the other path, to register as measurement errors. That is, where the beam paths are separate, an error in one path not present in the other path can register in the final comparison result (the interferogram). Because the aforementioned interferometers have Twyman Green type configurations, they are susceptible to the disadvantages of separate paths between the test and reference beams.
A well recognized advantage of a Fizeau interferometer is the feature of a common path shared by the test and reference wavefronts throughout most of the interferometer. Where the test and reference wavefronts both travel through the same optical components, imperfections and aberrations in components are common to both wavefronts, and do not register as measurement errors in the interferogram. Thus imperfect components do not impart “difference errors” in the final comparison of the test object to the reference object. As such, the Fizeau configuration is significantly more tolerant and robust compared to other interferometry systems. Imperfect components in its construction have little or no effect on the accuracy and precision of the final measurement results. This and other typical features of the Fizeau, including an alignment mode, ability to measure large flat optics, zoom capabilities, and ease of use with corrective null optics, have made the Fizeau a very popular, if not the most popular, interferometer configuration for practical applications.
However, despite such advantages of the Fizeau-type interferometers, there has been little, if any, ability or method known to construct or use a Fizeau interferometer that is capable of simultaneous phase-shifting.
Accordingly, there is a desire for a Fizeau-type interferometer capable of simultaneous phase shifting, and, further, for a simultaneous phase shifting Fizeau-type interferometer that uses orthogonally polarized beams.
SUMMARY OF THE INVENTIONThe instant invention is directed to an interferometric system having a source module, an interferometry module and a simultaneous phase shifting module. In particular, the source module generates mutually orthogonally polarized beams of light that are received by the interferometry module for interaction with a reference object and a test object. The interferometry module is configured with various optical elements that define a common beam pathway so as to minimize the introduction of measurement errors. Test and reference beams exiting the interferometry module then enter the simultaneous phase shifting module where at least two phase shifted interferograms are generated substantially simultaneously.
More specifically, the present invention is directed to an interferometric system, having a source module with a source of polarized light, a polarization beamsplitter element configured to act on the polarized light to generate mutually orthogonally polarized beams of light, an interferometry module that includes a mechanism for overlapping a test beam and a reference beam, and a phase shifting module that generates at least two phase-shifted interferograms substantially simultaneously from overlapping test and reference beams.
The present invention may further provide a source module having a polarization beamsplitter element configured to generate mutually orthogonally polarized beams as emanating from two spatially separated point sources (either real or virtual). The present invention also contemplates an interferometry module having a test object and a reference, a beam splitter and a collimator, where the beamsplitter and the collimator define a substantially common path for the two orthogonally polarized beams, and the mechanism for overlapping permits a selection of a specific pair of mutually orthogonally polarized reference and test beams for processing by the simultaneous phase-shifting module.
The present invention specifically contemplates an interferometric system with a Fizeau or Fizeau-type front end assembly that processes orthogonally polarized test and reference wavefronts for input to a simultaneous phase-shifting module for purposes of generating two or more phase-shifted interferograms, where the phase shifting may be accomplished by a variety of simultaneous phase shifting methods. The simultaneous acquisition of multiple wavefronts results in robust measurements in the presence of vibration and other environmental conditions.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.
An interferometric system 10 of the present invention is shown in
In the embodiment of
The embodiment of the source 12 as shown in
In accordance with the present invention, the two mutually orthogonally polarized wavefronts V and W exiting the polarization beamsplitter element 38 are displaced with respect to each other as if they originated from two slightly spatially separated (virtual or real) sources Sv and Sw, respectively. With respect to the embodiment of
Entering the interferometry module 14, the two wavefronts V and W (mutually orthogonally polarized and emanating from spatially separated sources Sv and Sw, respectively) travel through various optics, including a non-polarizing beamsplitter 40, a quarter waveplate 42, a collimator 44 (whose focal plane defines the location of the virtual sources Sv and Sw), before they encounter a reference or known object R. There, a percentage of each of the two wavefronts V and W reflects off a surface Ra of the reference object R, while another percentage of the wavefronts V and W travels (to the left) toward a test object T. The percentage reflected off the surface Ra forms reference wavefronts Vr and Wr which (traveling to the right in
The other percentage of the two wavefronts V and W that transmitted completely through the reference object R continues to travel toward the test object T (to the left in
It is understood by one of ordinary skill in the art that the collimator 44 can be obviated from the module 14 where the reference object R is configured with appropriate surface curvature to direct or focus the wavefronts Vt, Wt (or Vt′ or Wt′) back along the same path traveled by the wavefronts V and W entering the object R. It is further understood by one of ordinary skill in the art that the quarter waveplate 42 is an optional component of the interferometry module 14 and is commonly used to produce circularly polarized light which is often preferred for measurements.
In the embodiment of
As shown in
In order to generate an interferogram purposeful for revealing information about the test object T, a test wavefront is to at least overlap a reference wavefront. Consequently, orthogonally polarized wavefronts are to overlap sufficiently at the input of the simultaneous phase-shifting module 20, in order for simultaneous phase-shifted interferograms to be generated. Accordingly, of the four polarized wavefront spots, either the orthogonal pair Vr and Wt are to overlap, or the orthogonal pair Wr and Vt are to overlap. To that end, the alignment camera 52 provides the user with a view of the relative positioning of the four wavefronts and any visible degree of overlap between them.
In the situation shown in
As shown in the
The wavefronts Wr and Vt are now appropriately positioned relative to each other as shown in
Because the diffuser 60 maintains the polarization, the overlapped orthogonal wavefronts Wr and Vt, which form a disc of light 62 on the diffuser, will remain orthogonally polarized as they propagate beyond the diffuser 60. It is understood by one of ordinary skill in the art that the diffuser 60 is optional and that it is used to reduce speckle in the resulting interferograms. That is, the diffuser 60 can be desirable, but is not a necessary component of the present invention for the purpose of simultaneously sets of phase-shifted interferograms. In any case, the wavefronts Wr and Vt forming the disc of light 62 on the diffuser 60 are then imaged or otherwise relayed by lenses 64 (e.g., zoom lenses) to the simultaneous phase-shifting apparatus 20, with their mutually orthogonal polarizations maintained in the state they were in on the surface of the diffuser 60. The wavefronts Wr and Vt can now be manipulated and processed by the module 20 to interfere and produce interferograms, of which two or more (preferably three to six) phase-shifted interferograms may be produced substantially simultaneously and used for final analysis.
In accordance with an aspect of the present invention, the polarization type beamsplitter element 38 functions to produce the two mutually orthogonally polarized beams V and W, and further to produce such beams as originating from spatially separated sources (virtual or real). As understood by one of ordinary skill in the art, the possible embodiments of the polarization beamsplitter element 38 with the aforementioned functions is not limited to the embodiments discussed in detail below.
Referring to
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The embodiment of
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The embodiment illustrated in
Referring to
Referring to
It is understood by one of ordinary skill in the art that for each of the fiber optic methods above of
The embodiments illustrated in
Referring to
In the embodiment of
The other percentage of the wavefront B that is transmitted completely through the reference object R continues to travel toward the quarter waveplate 45 where it is converted to circular polarization before reflecting off the test object T and forms test wavefront Bt (traveling to the right in
The wavefronts Br and Bt now carry characteristics or information about the test object T which were imparted to these wavefronts as they reflected off or otherwise interacted with the test object T, and are mutually orthogonally polarized before entering the simultaneous phase shifting module 20 for processing to produce interferograms suitable. Again, it is understood by one of ordinary skill in the art that depending on the optical properties of the test object T, the other percentage incidental on the test object T can also can transmit through the test object T and reflect off a second reference object R′ (to create Br′, not shown). In the latter event, the wavefront Br′ is treated by the system 10 in a fashion similar to that described herein for the wavefronts Vt and Wt.
It is understood by one of ordinary skill in the art that the interferometer described in the present invention can be used as a standard phase shifting Fizeau-type interferometer providing that a standard phase shifting mechanism is present. Additionally, because the system of the present invention produces and uses orthogonally polarized test and reference beams (which can be in the visible light spectrum or other regions of the electromagnetic spectrum with longer or shorter wavelengths), it is possible to use a variable phase retarder after the source 12 to induce phase shifts. This would normally alleviate the need in a standard Fizeau to phase shift by physically moving the reference element, which can be large for testing large optics.
Additionally, an important aspect of the present invention allows for a variable intensity ratio between reference and test beams by rotating the polarization of the source 24. This would normally allow for measurements of a variety of objects with different coefficients of reflection (or transmission) without the use of an attenuator. The polarization from the source 24 can be rotated by physically rotating the source, or by optically rotating the polarization of the source. Where the source is linearly polarized, the polarization can be rotated by inserting a half waveplate after the source 24 and adjusting its rotation. This would normally change the amount of intensity in the two orthogonally polarized beams W and V, making one brighter than the other. If the test object is relatively more reflective, then it would typically be advantageous to decrease the intensity delivered to the test object so the reflected beam's intensity is roughly equal to the beam reflected from the reference object. This produces fringes of higher contrast in the interferograms.
In yet another embodiment of the present invention (referring to
Optical components of interferometer front-end and back-end would be modified if necessary to provide achromatic properties. Phase-shifting module 20 would then be replaced with a phase-shifting module capable of processing multiple wavelengths for dual-wavelength interferometry. Among other applications, this would increase the dynamic range or height measuring capabilities of the invention, when measuring 3D profiles.
It is further understood by one of ordinary skill in the art that any simultaneous phase shifting apparatus that uses orthogonally polarized test and reference beams at its input, can be used in lieu of the module 20, to produce multiple interferograms.
It is also further understood by one of ordinary skill in the art that other types of interferometers, common path interferometers, and differential interferometers, can be adapted in a similar way (as the classical Fizeau-type was here), to be converted to a simultaneous phase shifting configuration.
It is understood by one of ordinary skill in the art that the scope of the invention is not limited to the embodiments described above. Many other modifications and variations will be apparent to those of ordinary skill in the art, and it is therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
Claims
1. (canceled)
2. An interferometric system of claim 35, wherein said interferometric module is configured to define a substantially common path for said beams between said source module and a reflective surface of said reference object.
3. (canceled)
4. An interferometric system of claim 35, wherein said reference beam emanated from one of said spatially separated sources and said test beam emanated from another of said spatially separated sources.
5. An interferometric system of claim 35, wherein said reference and test beams received by said simultaneous phase shifting module substantially overlap each other.
6. An interferometric system of claim 35, wherein the mutually orthogonally polarized beams are coherent.
7. An interferometric system of claim 35, wherein there are two of said spatially separated sources.
8. An interferometric system of claim 35, further
9. (canceled)
10. (canceled)
11. An interferometric system of claim 35, wherein said sources are virtual.
12. An interferometric system of claim 35, wherein said sources are real.
13. An interferometric system of claim 35, wherein the interferometry module further includes a nonpolarizing beamsplitter.
14. An interferometric system of claim 13, wherein the nonpolarizing beamsplitter is positioned substantially between the source module and the reference object.
15. An interferometric system of claim 35, wherein the interferometry module further includes a quarter waveplate positioned between the source module and the reference object.
16. An interferometric system of claim 15, wherein the quarter waveplate is positioned substantially between the nonpolarizing beamsplitter and a collimator.
17. An interferometric system of claim 35, wherein the interferometry module is of a Fizeau configuration.
18. An interferometric system of claim 8, wherein the alignment module is positioned to intercept the beams between the interferometry module and the simultaneous phase-shifting module.
19. An interferometric system of claim 9, wherein the imaging module is positioned to intercept the beams between the interferometry module and the simultaneous
20. (canceled)
21. An interferometric system of claim 35, wherein said polarization beamsplitter comprises a prism.
22. An interferometric system of claim 35, wherein said polarization beamsplitter comprises a calcite beam displacer.
23. An interferometric system of claim 35, wherein said polarization beamsplitter comprises two calcite beam displacers and a half waveplate.
24. An interferometric system of claim 35, wherein the polarization beamsplitter comprises two fiber optics and cube polarizing beamsplitter.
25. An interferometric system of claim 35, wherein the polarization beamsplitter comprises a polarizing lateral displacement beamsplitter.
26. An interferometric system of claim 35, wherein the polarization beamsplitter comprises a cube polarizing beamsplitter and mirror.
27. An interferometric system of claim 35, further comprising a filter to block said other portion of the beams from entering the simultaneous phase shifting module.
28. An interferometric system of claim 27, wherein said filter is configured with an aperture to permit passage of said portion of the beams received by the simultaneous phase shifting module.
29-34. (canceled)
35. An interferometric system, comprising: a source module having a source of polarized light and a polarization beamsplitter configured to act on said polarized light to generate mutually orthogonally polarized beams of light; an interferometry module receiving said orthogonally polarized beams from said source, having optical elements, a reference object and a test object, said interferometry module further comprising a mechanism for manipulating a test beam and a reference beam into an overlapping position; a phase shifting module receiving a portion of said beams from said interferometry module to generate at least two phase-shifted interferograms substantially simultaneously from said test and reference beams, and an alignment camera which provides a view of relative positioning of the wavelengths and degree of overlap between them.
36. An interferometric system of claim 35, wherein said polarized light from said source module is linearly polarized.
37. An interferometric system of claim 35, wherein the mechanism for manipulating comprises a tip-tilt mechanism.
38. An interferometric system, comprising: a source module having a source of linearly polarized light, and a polarization beamsplitter configured to generate mutually orthogonally polarized wavefronts as emanating from two spatially separated sources; an interferometry module receiving said orthogonally polarized wavefronts, said interferometry module having a test object and a reference, a beam splitter and a collimator, wherein orthogonally polarized reference wavefronts and orthogonally polarized test wavefronts exit the interferometry module; a tip-tilt mechanism for overlapping one of said orthogonally polarized reference wavefront with one of said orthogonally polarized test wavefronts; a simultaneous phase shifting module receiving said overlapping one reference wavefront and said one test wavefront from said interferometry module for generating at least two phase-shifted interferograms substantially simultaneously, wherein said wavefronts follow a substantially common path through said interferometric system.
39. An interferometric system of claim 38, wherein said portion of said beams comprises mutually orthogonally polarized reference and test beams.
40. An interferometric system of claim 39, wherein said reference beam emanated from one of said spatially separated sources and said test beam emanated from another of said spatially separated sources.
41. An interferometric system of claim 38, wherein the mutually orthogonally polarized beams are coherent.
42. An interferometric system of claim 38, wherein there are two of said spatially separated sources.
43. An interferometric system of claim 38, further comprising an alignment module.
44. An interferometric system of claim 38, further comprising an imaging module.
45. An interferometric system of claim 38, wherein the source module includes a linearly polarized light source and a polarization beamsplitter configured to split linearly polarized light into said two mutually orthogonally polarized beams.
46. An interferometric system of claim 38, wherein said sources are virtual.
47. An interferometric system of claim 38, wherein said sources are real.
48. An interferometric system of claim 38, wherein the interferometry module further includes a nonpolarizing beamsplitter.
49. An interferometric system of claim 48, wherein the nonpolarizing beamsplitter is positioned substantially between the source module and the reference object.
50. An interferometric system of claim 38, wherein the interferometry module further includes a quarter waveplate positioned between the source module and the reference object.
51. An interferometric system of claim 50, wherein the quarter waveplate is positioned substantially between the nonpolarizing beamsplitter and a collimator.
52. An interferometric system of claim 38, wherein the interferometry module is of a Fizeau configuration.
53. An interferometric system of claim 52, wherein the alignment module is positioned to intercept the beams between the interferometry module and the simultaneous phase-shifting module.
54. An interferometric system of claim 44, wherein the imaging module is positioned to intercept the beams between the interferometry module and the simultaneous phase shifting module.
55. An interferometric system of claim 38, wherein the source module includes a polarization beamsplitter configured to interact with a beam from a source to provide said mutually orthogonally polarized beams.
56. An interferometric system of claim 55, wherein said polarization beamsplitter comprises a prism.
57. An interferometric system of claim 55, wherein said polarization beamsplitter comprises a calcite beam displacer.
58. An interferometric system of claim 55, wherein said polarization beamsplitter comprises two calcite beam displacers and a half waveplate.
59. An interferometric system of claim 55, wherein the polarization beamsplitter comprises two fiber optics and cube polarizing beamsplitter.
60. An interferometric system of claim 55, wherein the polarization beamsplitter comprises a polarizing lateral displacement beamsplitter.
61. An interferometric system of claim 55, wherein the polarization beamsplitter comprises a cube polarizing beamsplitter and mirror.
62. An interferometric system of claim 38, further comprising a filter to block said other portion of the beams from entering the simultaneous phase shifting module.
63. An interferometric system of claim 62, wherein said filter is configured with an aperture to permit passage of said portion of the beams received by the simultaneous phase shifting module.
Type: Application
Filed: Aug 22, 2011
Publication Date: Feb 2, 2012
Inventors: Piotr Szwaykowski (Glendale, CA), Federick N. Bushroe (Tucson, AZ), Raymond J. Castonguay (Tucson, AZ)
Application Number: 13/215,071
International Classification: G01B 9/02 (20060101);