CN-122029398-A - Interferometry for measuring optical distance

Abstract

A method and corresponding apparatus for determining an absolute optical distance between two reflective surfaces comprising an interferometric cavity, the method comprising moving an illumination spot in a back focal plane of an interferometer along a trajectory having a non-zero radial component relative to an optical axis to cause different radial positions of the illumination spot along the trajectory while simultaneously acquiring images of an interferogram of the interferometric cavity, and analyzing the images using one or more electronic processors to determine a functional dependence of an optical path length difference (OPD) of the interferometric cavity on a radial position of the illumination spot, and extracting the absolute optical distance between the two reflective surfaces based on the determined functional dependence.

Inventors

• L.L. Decker

Assignees

• 齐戈股份有限公司

Dates

Publication Date

20260512

Application Date

20241007

Priority Date

20240830

Claims (20)

1. 1. A system for determining an absolute optical distance between two reflective surfaces comprising an interferometric cavity, wherein the system comprises: (a) An interferometer having an optical axis and configured to define the interference cavity with a test component; (b) A dynamic illumination system configured to move an illumination spot in a back focal plane of the interferometer along a trajectory having a non-zero radial component relative to the optical axis to cause different radial positions of the illumination spot along the trajectory; (c) A detector configured to acquire an image of an interferogram of the interferometric cavity as the dynamic illumination system moves an illumination spot in a back focal plane of the interferometer along the trajectory having the non-zero radial component relative to the optical axis, and (D) An electronic controller coupled to the dynamic illumination system and the detector and configured to analyze the interferograms to determine a functional origin of an optical path length difference (OPD) of the interferometric cavity to a radial position of an optical spot, the absolute optical distance between the two reflective surfaces being extracted based on the functional origin.
2. 2. The system of claim 1, wherein the test component comprises the two reflective surfaces that comprise the interferometric cavity.
3. 3. The system of claim 1, wherein the interferometer comprises a reference surface defining one of the two reflective surfaces, and wherein the test component defines the other of the two reflective surfaces that comprise the interferometric cavity.
4. 4. A system according to claim 3, wherein the reference surface is a reference plane surface.
5. 5. A system according to claim 3, wherein the reference surface is a surface of a transmissive sphere.
6. 6. The system of claim 3, wherein the interferometer comprises a phase shifter mechanically coupled to the reference surface.
7. 7. The system of claim 1, wherein the interferometer comprises collimation optics for directing light to the interference cavity, and wherein the collimation optics define a back focal plane of the interferometer.
8. 8. The system of claim 7, wherein the interferometer further comprises a beam splitter and imaging optics for imaging the interference image of the interference cavity onto the detector.
9. 9. The system of claim 1, wherein the interferometer is a Michelson interferometer, a Twyman-Green interferometer, or a Fizeau interferometer.
10. 10. The system of claim 1, wherein the dynamic illumination system comprises a system light source and a turning mirror assembly to receive and reflect the system light in at least two orthogonal directions.
11. 11. The system of claim 10, wherein the electronic controller stores calibration information for mapping the angular orientation of the steering mirror assembly to the position of the illumination spot in the source plane of the interferometer.
12. 12. The system of claim 10, wherein the electronic controller comprises a memory for storing information defining a predetermined movement of the trajectory of the illumination spot provided by the dynamic illumination system.
13. 13. The system of claim 12, wherein the trajectory of the predetermined motion comprises a linear trajectory through the optical axis.
14. 14. The system of claim 10, wherein the system light source comprises a laser.
15. 15. The system of claim 1, further comprising a variable focusing mechanism to focus the test surface onto the detector.
16. 16. The system of claim 1, wherein the electronic controller is configured to cause the detector to continuously acquire the images of the interferogram as the illumination spot moves along a predefined trajectory in the back focal plane.
17. 17. The system of claim 1, wherein the electronic controller is configured to cause the detector to acquire an image of the interferogram for each of a plurality of phase shifts provided by a phase shifter in the interferometer at each of a plurality of positions of the locus of the illumination spot in the back focal plane.
18. 18. The system of claim 1, wherein each interferogram image comprises a spatially dependent intensity value having a sinusoidal dependence on the optical path length difference (OPD), and wherein the optical path length dependence is representable by a sum of a spatially independent term that depends on a radial position of the illumination spot in the back focal plane and a spatially dependent offset term that does not depend on a radial position of the illumination spot in the back focal plane.
19. 19. The system of claim 18, wherein the electronic controller is configured to extract the spatially dependent offset term based on a phase shift interferogram produced by a phase shifter in the interferometer.
20. 20. The system of claim 18, wherein the electronic controller is configured to extract an average optical path length difference (OPD) for each different radial position of the illumination spot by averaging spatially dependent offset terms that are independent of the radial position of the illumination spot in the back focal plane.

Description

Interferometry for measuring optical distance Cross Reference to Related Applications The present application claims priority from U.S. application Ser. No.18/820,386, filed 8/30 at 2024, which claims priority from provisional application Ser. No.63/544,019, filed 10/13 at 2023, and also claims priority from U.S. application Ser. No.18/820,383, filed 8/30 at 2024, the contents of which are incorporated herein in their entirety. Technical Field The present disclosure relates to optical interferometry. Background Laser interferometers are widely used tools for high precision measurement of engineered surfaces. Often in combination with phase-shifting interferometry (PSI) techniques, surface properties can be measured quickly and easily to fractions of a nanometer. The laser interferometer has a convenient long-term coherence so that high-contrast interference is easily observed even with a large unequal optical path between the interference surfaces. Fizeau geometry is particularly popular because of its ease of setup and common path architecture. In combination with lasers Fizeau is ideally suited for long optical paths often encountered in characterizing large or complex optical components. The surface profile interferometer is an imaging system so that performance is optimized when the surface being measured is in focus. Us patent 10,890,428 teaches a method and apparatus designed to measure the optical distance between two surfaces in an interferometer equipped with a wavelength tunable laser. If one of those surfaces is a reference surface, then the autofocus method is also enabled if the position is known relative to the reference surface of the imaging system. An additional feature achieved by wavelength tunable lasers is the ability to measure the absolute optical distance between the interference surfaces. However, many interferometers use fixed wavelength lasers due to their simplicity and low cost. It would be useful to achieve auto-focusing and absolute optical distance capabilities without the need for a wavelength tunable laser. Disclosure of Invention A method and apparatus for determining the absolute optical distance between two reflective surfaces comprising an interferometric cavity is disclosed herein. The apparatus comprises an interferometer equipped with a dynamic illumination system, means for moving the illumination spot in the back focal plane of the interferometer along a trajectory having a non-zero radial component with respect to the optical axis while simultaneously acquiring cavity interferograms, means for analyzing the functional dependence of the optical path length difference ("OPD") of the interferograms on the radial position of the spot, and means for extracting the absolute optical distance of the cavity between the cavity surfaces from the measured functional dependence. Embodiments may further comprise any of wherein the spot trajectory is a linear trajectory, wherein the spot trajectory is a non-linear trajectory, wherein the spot trajectory passes through the optical axis, wherein the spot trajectory is a smooth continuous motion, wherein an interferogram is continuously acquired during the motion, wherein the spot trajectory is a series of stop and gaze positions along the trajectory and PSI measurements are made during the rest period, and calculating an optical distance between cavity boundary surfaces using a regression fit of interference phase piston variations to spot radial positions to obtain a theoretical correlation of optical distance to free parameters. In general, in one aspect, a system for determining an absolute optical distance between two reflective surfaces comprising an interference cavity is disclosed, wherein the system comprises a) an interferometer having an optical axis and configured to define an interference cavity with a test component, b) a dynamic illumination system configured to move an illumination spot in a back focal plane of the interferometer along a trajectory having a non-zero radial component relative to the optical axis to cause different radial positions of the illumination spot along the trajectory, c) a detector configured to acquire an image of an interference pattern of the interference cavity as the dynamic illumination system moves the illumination spot in the back focal plane of the interferometer along the trajectory having the non-zero radial component relative to the optical axis, and d) an electronic controller coupled to the dynamic illumination system and the detector and configured to analyze the interference pattern to determine a functional dependence of an optical path length difference (OPD) of the interference cavity on a spot radial position, and to extract the absolute optical distance between the two reflective surfaces based on the functional dependence. In general, in another aspect, a method for determining an absolute optical distance between two reflective surfaces comprising an interferometric ca