WO2007070565A2

WO2007070565A2 - Off-axis paraboloid interferometer with off-axis illumination

Info

Publication number: WO2007070565A2
Application number: PCT/US2006/047520
Authority: WO
Inventors: Alex Klooster; Carl Aleksoff
Original assignee: Coherix, Inc.
Priority date: 2005-12-12
Filing date: 2006-12-12
Publication date: 2007-06-21
Also published as: WO2007070566A2; WO2007070565A3; WO2007070566A3

Abstract

The present invention includes an interferometric imaging system including a paraboloid mirror defining an axis and an illumination source adapted to radiate light from an illumination point at an off-axis portion of the paraboloid mirror- wherein the illumination point is disposed remotely from the axis of the paraboloid mirror. The illumination point is disposed relative to the axis of the paraboloid mirror such that light diverges from the illumination point and proceeds to the paraboloid mirror. The paraboloid mirror reflects light from the illumination source into a substantially parallel beam of light for illumination of an object. The paraboloid mirror of the present invention functions as both a collimator of light from the illumination point as well as a collector of light reflected from the object under examination.

Description

OFF-AXIS PARABOLOID INTERFEROMETER WITH OFF-AXIS

ILLUMINATION

TECHNICAL FIELD

[0001] This invention relates generally to the optical field, and more specifically to an improved system optical system for interferometric imaging.

BACKGROUND

[0002] Interferometry has been used for over a century to measure the surface topography of objects, typically optical components, and distances and small changes in such distances. With the advent of lasers having long coherence lengths and high brightness, the field has expanded greatly. Interferometric comparison of objects with a known surface has been difficult to implement for very large objects with surfaces with steps or slopes greater than a half wavelength of light per resolution element of the imaging system, because the phase count is lost, and the height of the object surface is known only modulo λ / 2, where λ is the wavelength of light used for the interferometer.

[0003] If a series of imaging interferograms are recorded with different wavelengths λ 1, the ambiguity in the phase may be resolved, and the heights on the object surface relative to a particular location on the particle surface may be calculated, as is described in the in various prior art publications assigned to the assignee of the present invention. For example, U. S. Patent 5,907,404, U. S. Patent 5,926,277, U. S. Patent Application 10/893052, U. S. Patent Application 10/349651, U. S. Patent Application 11/181664, and U. S. Patent Application 11/194097, which all describe phase resolution and surface calculations and are incorporated in their entirety by this reference.

[0004] FIGURE 1 is a schematic diagram of a prior art interferometer. The particular interferometer shown in FIGURE 1 is conventionally called a Michelson interferometer, and has been used since the nineteenth century in optical experiments and measurements. A light source 10 produces light that is collimated by passing through a lens system 11 to produce a parallel beam of light 12 that passes to a beamsplitter 13. The beam of light 12 is partially reflected to a reference mirror 14 and partially transmitted to an object 15. Light reflected from the reference mirror 14 partially passes through the beamsplitter to an image receiver 16. Light reflected from the object is partially reflected from the beamsplitter 15 and is passed to the image receiver 16. The image receiver 16 may be film, or may be an electronic photodetector or a CCD or a CMOS array, or any other image receiver known in the art.

[0005] If both the reference mirror 14 and the object 15 are flat mirrors aligned perpendicular to the incoming light from beam 12, and the light path traversed by the light from the light source to the image receiver is identical, the light from both the reference mirror and the object mirror will be in phase, and the image receiver will show a uniformly bright image. Such devices were the bane of undergraduate optics students before the advent of lasers, since the distances had to be equal to within distances measured by the wavelength of the light and the mirrors had to be aligned within microradians. Even with the advent of lasers with very long coherence lengths, such devices are subject to vibration, thermal drift of dimensions, shocks, etc. [ooo6] The Michelson interferometer design of FIGURE l is useful to explain the many different types of interferometers known in the art. In particular, suppose the reference mirror 14 is moved back and forth in the direction of the arrow in FIGURE 1. As the reference mirror is moved, the phase of the light beam reflected from the reference mirror and measured at the image receiver 16 will change by 180 degrees with respect to the phase of the light reflected from the object 15 for every displacement of one quarter wavelength. The light from the two beams reflected from the object 15 and the reference mirror 14 will interfere constructively and destructively as the mirror moves through one-quarter wavelength intervals. If the intensity of both the reference and object beams are equal, the intensity at the image receiver will be zero when the mirrors are positioned for maximum destructive interference. Very tiny displacements of one of the mirrors 14 or 15 can thus be measured.

[0007] FIGURE 2 is a schematic diagram of a prior art imaging interferometer much like the interferometer of FIGURE 1, except that the light source does not use a lens to collimate the light into a parallel beam 12. Instead, an off-axis paraboloid mirror 24 is used to reflect the light output 26 of an optical fiber 20 into a parallel beam of light 12. Mirror 24 is a section having a reflecting surface that is part of a parabola of revolution about the axis 22. The end of the optical fiber 20 is placed on the axis 22 at or very near the focal point P of the paraboloid mirror, i.e., the point to which a parallel light beam parallel to light beam the axis 22 (which is the optical axis of the paraboloid mirror) coming in to and reflected from the mirror 24 would be focused.

[0008] The optical fiber 20 may incorporate a lens system (not shown) that appears to diverge the beam of light from the focal point P. An optical system (shown symbolically as lens 29) is shown for imaging the surface of the object 15 on to the image receiver 16. The optical system 29 and image receiver 15 can be integrated into a camera, where the image size of the object 15 on the image receiver may be much smaller than the size of the object 15.

[0009] The optical set up shown in FIGURE 2 is shown as a telecentric optical system, where diverging light rays 25 scattered from a point on the surface of the object 15 diverge until they pass through lens 29, then travel parallel to each other through an aperture 27, and are converged again to a point on the surface of the image receiver 16.

[0010] Both prior art versions of the Michelson interferometer incorporate far too many cumbersome and expensive optical elements. Notably, the prior art interferometers must include a reference mirror, a beam splitter, and an optical system or lens for collimating the light into the image receiver. Accordingly, there is a need in the art for interferometric system for investigating, imaging, and measuring the topography of the surfaces of large objects having lighter and/or less expensive optical elements. Moreover, there is a need in the art for an interferometric system having an easily variable ratio of object illumination intensity to reference beam intensity.

SUMMARY

[0011] Accordingly, the present invention includes an interferometric imaging system that reduces or eliminates the need for any of the cumbersome and expensive optical elements associated with the prior art. In particular, the system of the present invention includes a paraboloid mirror defining an axis and an illumination source adapted to radiate light from an illumination point at an off-axis portion of the paraboloid mirror, wherein the illumination point is disposed remotely from the axis of the paraboloid mirror.

[0012] As described in detail below, the illumination point is disposed relative to the axis of the paraboloid mirror such that light diverges from the illumination point and proceeds to the paraboloid mirror. The paraboloid mirror reflects light from the illumination source into a substantially parallel beam of light for illumination of an object. Unlike the prior art, however, the paraboloid mirror of the present invention functions as both a collimator of light from the illumination point as well as a collector of light reflected from the object under examination. [0013] These and further details and advantages of the present invention are detailed below in respect to the preferred embodiments of the present invention described with reference to the following FIGURES.

BRIEF DESCRIPTION OF THE FIGURES

[0014] FIGURES 1 and 2 are schematic drawings of prior art interferometric systems.

[0015] FIGURE 3 is a schematic diagram of an interferometric system in accordance with a preferred embodiment of the present invention. [0016] FIGURE 4 is a depiction of calculated distortions of divergent light at an object.

[0017] FIGURE 5 is a depiction of calculated distortions of divergent light at an image receiver.

[0018] FIGURE 6 is a schematic drawing of a variation of the preferred embodiment of the invention. [0019] FIGURE 7 is a schematic drawing of another variation of the preferred embodiment of the invention.

[0020] FIGURE 8 is a schematic drawing of another variation of the preferred embodiment of the invention.

[0021] FIGURE 9 is a schematic drawing of another variation of the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0022] The following description of preferred embodiments of the invention is not intended to limit the invention to these embodiments, but rather to enable any person skilled in the art to make and use this invention.

[0023] A preferred embodiment of the present invention includes an mterferometric imaging system that reduces or eliminates the need for any of the cumbersome and expensive optical elements associated with the prior art. In particular, the system of the preferred embodiment includes a paraboloid mirror defining an axis and an illumination source adapted to radiate light from an illumination point at an off-axis portion of the paraboloid mirror, wherein the illumination point is disposed remotely from the axis of the paraboloid mirror. [0024] The illumination point is disposed relative to the axis of the paraboloid mirror such that light diverges from the illumination point and proceeds to the paraboloid mirror. The paraboloid mirror reflects light from the illumination source into a substantially parallel beam of light for illumination of an object. Unlike the prior art, however, the paraboloid mirror of the present invention functions as both a collimator of light from the illumination point as well as a collector of light reflected from the object under examination. [0025] As shown in FIGURE 3, in the system of the preferred embodiment the large reference mirror, the large beam splitter, and the lens described in the prior art are no longer needed. Light 36 is radiated from an illumination source 30 and is shown diverging from an illumination point Pi disposed remotely from the focus point P of the paraboloid mirror 24 and thus remotely from an optical axis 22 of the paraboloid mirror 24. The light travels to the paraboloid mirror 24 and is reflected as a substantially parallel beam 37 that falls on the surface of an object 15. Since the illumination point Pi is apart from the focus point P of the paraboloid, a collimated light beam 37 is not parallel to the optical axis 22 of the paraboloid mirror. Object light 38 is shown as a parallel beam reflecting from a surface of the object 15, where the surface is substantially perpendicular to the optical axis 22. Object light 38 reflects again from the paraboloid mirror 24, and is then brought to a focus at a second point P₂ that is also disposed remotely from the focal point P and the optical axis 22 of the paraboloid mirror 24.

[0026] In a first variation of the preferred embodiment, the system includes an aperture 31 that limits the light scattered from the object surface 15. Additionally, the system of preferred embodiment can include a beamsplitter 33 such that inbound light 39 is combined with light from a reference light source 32 prior to receipt by an image receiver 34. The image receiver 34 captures the image of the surface of the object 15 and displays an interferometric phase image of the object surface.

[0027] A second variation of preferred embodiment of the system can also include a computer (not shown) connected to the image receiver 34. The computer functions to capture and display phase images of the surface of the object 15 at different relative phases between the reference source 32 and the illumination source 3O and different wavelengths of light from the reference source 32 and the illumination source 30. The computer further functions to construct synthetic phase images and holograms from the phase and wavelength data, functions that are known generally in the art of interferometry.

[0028] In a third variation of the system of the preferred embodiment, the illumination source 30 can be a laser light source, a diode laser light source, a light emitting diode (LED) light source, a gas discharge light source, an optical fiber laser, or an arc or incandescent light source.

[0029] In a fourth variation of the system of the preferred embodiment, the illumination source 30 can be optically coupled to an optical fiber. The optical fiber is adapted to direct light from the illumination source 30 to the illumination point Pi. In alternatives to this variation of the system of the preferred embodiment, the illumination source 30 can include for example a diode laser source, a light emitting diode, or an arc or incandescent light source that is connectable to the optical fiber. Alternatively, the illumination source 30 can be a fixed frequency light source, a tunable frequency light source, or any other type of light source that is either fixed or tunable with respect to frequency.

[0030] In order to use the paraboloid mirror 24 as both a collimating optical element and as an image gathering optical element, there must be room for the incoming and outgoing focused beams to pass each other without obstruction or physical occlusion of either beam. Otherwise, a beam splitter would have to be used at or near the intersection point of the incoming or outgoing beams, and only a quarter of the possible object illumination light would reach the camera. Accordingly, in another variation of the system of the preferred embodiment, the beam splitter 33 can transmit most of the inbound light 39, while reflecting only a small part of a reference light from a reference source 32.

[0031] The point Pi is defined as apart from the focus P of the paraboloid mirror 24 when the distortion introduced in the beam incident on the object 15 is greater than one wave distortion across the object 15 as long as the paraboloid mirror 24 is paraboloid to within a small part of a wavelength. The prior art system cited had the object illumination located so that the wavefront distortion at the object was less than one wave. The point Pi is defined as being near to the point P when the illumination light 36 passes closely to but is not blocked by the aperture 31, and when the illumination source 30 and associated optics do not occlude or block inbound light 39 or obstruct optical elements necessary to combine the inbound beam 39 with the reference light from the reference source.

[0032] In one example embodiment based on variations of the system of the preferred embodiment described herein, the inventors found that light diverging from a point that is not coincident with the focus of the paraboloid mirror 24 will have large aberrations or distortions when it reaches the object 15. FIGURE 4 shows that the calculated distortions at the object are eighteen waves when the paraboloid mirror 24 is a thirty centimeters square mirror and where the optical axis 22 is forty eight millimeters from the edge of the mirror, the free space length from Pi to the object 15 surface is three thousand five hundred twenty two millimeters, and the point Pi is displaced from the focus point P by two millimeters. However, when the light from a plane mirror replacing the object 15 is placed perpendicular to the optic axis 22 of the paraboloid mirror 24 is calculated, the inventors have found that the calculated aberrations cancel to the first order, and the distortion at the camera is only three hundred sixty five thousandths of a wave, as shown in FIGURE 5. 20

[0033] In a fifth variation of the system of the preferred embodiment, the system can include a phase changing element adapted to change the phase of the light directed by the optical fiber. As shown in FIGURE 6, a phase changing element 62 is connected to an optical fiber 60 that directs the illumination light 36 to the paraboloid mirror 24. In one alternative, the optical fiber 60 is stretchable such that a mechanical, electronic, electro-mechanical or thermal device can alter and/ or extend the length of the optical fiber 60 thereby providing a corresponding change in the phase of the light directed by the optical fiber 60. Correspondingly, the phase changing element 62 can include for example a mechanical, electronic, electromechanical or thermal device that is adapted to alter and/or extend the length of the optical fiber 60 thereby providing a corresponding change in the phase of the light directed by the optical fiber 60.

[0034] One alternative to the variation of the system of the preferred embodiment is shown in FIGURE 7. In this alternative, the phase changing element 62 includes a piezoelectric tube 70, a portion of which is attached to the optical fiber 60 by an adhesive 74, epoxy, or other similar bonding agent. Another portion of the tube 70 is joined to a base 72 that is fixed with respect to the paraboloid mirror 24. In operation, the application of a voltage to the piezoelectric tube 70 lengthens piezoelectric tube 70, thereby stretching the optical fiber 60 and changing the relative phase of the illumination light 36 by a predetermined number of wavelengths.

[0035] In a sixth variation of the system of the preferred embodiment, the system further includes a second illumination source adapted to radiate light from an illumination point at an off-axis portion of the paraboloid mirror 24 such that the second illumination point is disposed remotely from the axis 22 of the paraboloid mirror 24. As in the single illumination point variations described above, the second illumination source can be a laser light source, a diode laser light source, a light emitting diode (LED) light source, a gas discharge light source, an optical fiber laser, or an arc or incandescent light source.

[0036] Correspondingly, in an alternative to the sixth variation, the second illumination source can be optically coupled to a second optical fiber. The second optical fiber is adapted to direct light from the illumination source to the illumination point Pi. In alternatives to this variation of the system of the preferred embodiment, the second illumination source can include for example a diode laser source, a light emitting diode, or an arc or incandescent light source that is connectable to the optical fiber. Alternatively, the second illumination source can be a fixed frequency light source, a tunable frequency light source, or any other type of light source that is either fixed or tunable with respect to frequency. [0037] An example of this variation of the system of the preferred embodiment is shown in FIGURE 8, in which a first optical fiber 80 and a second optical fiber 82 are disposed within a predetermined range of each other thereby producing the illumination light 36. Each of the optical fibers 80, 82 can be placed so that the respective illumination points of the fibers are within a few hundred microns of each other. In one alternative to the sixth variation, the predetermined range between the first illumination point and the second illumination point is approximately 350 microns. In a second alternative to the sixth variation, the predetermined range between the first illumination point and the second illumination point is less than 350 microns. In a third alternative to the sixth variation, the predetermined range between the first illumination point and the second illumination point is between 100 and 350 microns. T/US2006/047520

[0038] Each of the first optical fiber 80 and the second optical fiber 82 provides a diverging beam of light that diverges from different points Pm. Accordingly, each of the first optical fiber 80 and the second optical fiber 82 can have individual phase changing elements 62 attached thereto, or each fiber may be attached to a single phase changing element 62. As noted above, a phase changing element 62 is connected to an optical fiber 80, 82 that directs the illumination light 36 to the paraboloid mirror 24. In one alternative, the optical fiber 80, 82 is stretchable such that a mechanical, electronic, electro-mechanical or thermal device can alter and/ or extend the length of the optical fiber 80, 82 thereby providing a corresponding change in the phase of the light directed by the optical fiber 80, 82. Correspondingly, the phase changing element 62 can include for example a mechanical, electronic, electro-mechanical or thermal device that is adapted to alter and/or extend the length of the optical fiber 80, 82 thereby providing a corresponding change in the phase of the light directed by the optical fiber 80, 82. [0039] Alternatively, the phase changing element 62 can include a piezoelectric tube 70 as shown in FIGURE 7, a portion of which is attached to the optical fiber 80, 82 by an adhesive 74, epoxy, or other similar bonding agent. Another portion of the tube 70 is joined to a base 72 that is fixed with respect to the paraboloid mirror 24. In operation, the application of a voltage to the piezoelectric tube 70 lengthens piezoelectric tube 70, thereby stretching the optical fiber 80, 82 and changing the relative phase of the illumination light 36 by a predetermined number of wavelengths.

[0040] In an seventh variation of the preferred embodiment, the system further includes a fiber optical beam combiner adapted to combine light directed by the first optical fiber and the second optical fiber. As shown in FIGURE 9, the fiber optical beam combiner 90 is adapted to combine light directed by multiple optical fibers 92, 94, and 96, wherein each of the optical fibers 92, 94, 96 direct light from distinct illumination sources 93, 95, and 97. Each of the multiple optical fibers 92, 94, 96 can be connected to a phase changing element as described above. Alternatively, a phase changing element can be connected to a separate optical fiber 60 that is adapted to direct a consolidated beam of light subsequent to the fiber optical beam combiner 90. Each of the illumination sources 93, 95, 97 can include for example a laser light source, a diode laser light source, a light emitting diode (LED) light source, a gas discharge light source, an optical fiber laser, an arc or incandescent light source, or any combination or permutation thereof. [0041] As a person skilled in the art will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention defined in the following claims.

Claims

CLAIMS We claim:

1. An interferometric imaging system comprising: a paraboloid mirror defining an axis, and an illumination source adapted to radiate light from an illumination point at an off-axis portion of the paraboloid mirror, the illumination point disposed remotely from the axis of the paraboloid mirror; wherein the illumination point is disposed relative to the axis of the paraboloid mirror such that light diverges from the illumination point and proceeds to the paraboloid mirror such that the paraboloid mirror reflects light from the illumination source into a substantially parallel beam of light for illumination of an object.

2. The system of claim i wherein the illumination source comprises a laser light source.

3. The system of claim 1 wherein the illumination source comprises a light emitting diode (LED) light source.

4. The system of claim 1 wherein the illumination source comprises a gas discharge light source.

5. The system of claim 1 wherein the illumination source comprises a tunable wavelength optical illumination source.

6. The system of claim l further comprising a first optical fiber adapted to direct light from the illumination source to the illumination point.

7. The system of claim 6 further comprising a phase changing element adapted to change the phase of the light directed by the first optical fiber.

8. The system of claim 7 wherein the first optical fiber is stretchable to change the phase of the light directed there through.

9. The system of claim 7 wherein the phase changing element is adapted to stretch the first optical fiber such that the phase of the light directed by the first optical fiber is changed in response thereto.

10. The system of claim 9 wherein the phase changing element comprises a piezoelectric tube having a first portion connected to the first optical fiber and a second portion fixed with respect to the paraboloid mirror, such that in response to an input the piezoelectric tube causes a change in the length of the first optical fiber.

11. The system of claim 6 further comprising a second illumination source adapted to radiate light from a second illumination point at an off-axis portion of the paraboloid mirror, the second illumination point disposed remotely from the axis of the paraboloid mirror.

12. The system of claim 11 further comprising a second optical fiber adapted to direct light from the second illumination source to the second illumination point.

13- The system of claim 11 wherein the second illumination source comprises a laser light source.

14. The system of claim 11 wherein the second illumination source comprises a light emitting diode (LED) light source.

15. The system of claim 11 wherein the second illumination source comprises a gas discharge light source.

16. The system of claim 11 wherein the second illumination source comprises a tunable wavelength optical illumination source.

17. The system of claim 11 further comprising a second optical fiber adapted to direct light from the second illumination source to the second illumination point.

18. The system of claim 16 further comprising a second phase changing element adapted to change the phase of the light directed by the second optical fiber.

19. The system of claim 17 wherein the second optical fiber is stretchable to change the phase of the light directed there through.

20. The system of claim 18 wherein the second phase changing element is adapted to stretch the second optical fiber such that the phase of the light directed by the second optical fiber is changed in response thereto. 47520

21. The system of claim 18 wherein the second phase changing element comprises a piezoelectric tube having a first portion connected to the second optical fiber and a second portion fixed with respect to the paraboloid mirror, such that in response to an input the piezoelectric tube causes a change in the length of the second optical fiber.

22. The system of claim 11 wherein the first illumination point is disposed within a predetermined range of the second illumination point.

23. The system of claim 22 wherein the predetermined range is approximately 350 microns.

24. The system of claim 22 wherein the predetermined range is less than 350 microns.

25. The system of claim 22 wherein the predetermined range is between 100 and 350 microns.

26. The system of claim 17 further comprising a fiber optical beam combiner adapted to combine light directed by the first optical fiber and the second optical fiber.