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CN-122029377-A - Dynamic interferometer illuminator

CN122029377ACN 122029377 ACN122029377 ACN 122029377ACN-122029377-A

Abstract

An illumination system for an interferometer is disclosed, including a system light source, a steering mirror assembly for receiving and reflecting system light in at least two orthogonal directions, a tracking mechanism for tracking an angular orientation of the steering mirror assembly in the two orthogonal directions and providing an electronic signal indicative of the angular orientation, a focusing lens assembly for focusing system light reflected off the steering mirror assembly onto a focused spot on a 2-dimensional plane corresponding to a source plane of the interferometer, and an electronic controller operatively coupled to the steering mirror assembly and configured to cause the focused spot on the source plane to follow a predetermined motion trajectory.

Inventors

  • L.L. Decker
  • J. A. Subitsky

Assignees

  • 齐戈股份有限公司

Dates

Publication Date
20260512
Application Date
20241007
Priority Date
20240830

Claims (20)

  1. 1. An illumination system for an interferometer, comprising: (a) A system light source; (b) A turning mirror assembly for receiving and reflecting the system light in at least two orthogonal directions; (c) A tracking mechanism for tracking the angular orientation of the steering mirror assembly in the two orthogonal directions and providing an electronic signal representative of the angular orientation; (d) A focusing lens assembly for focusing system light reflected off the turning mirror assembly onto a focused spot on a 2-dimensional plane corresponding to a source plane of the interferometer, and (E) An electronic controller is operably coupled to the steering mirror assembly and configured to cause the focused spot on the source plane to follow a predetermined motion profile.
  2. 2. The system of claim 1, wherein the steering mirror assembly comprises at least one mirror and a transducer element, and wherein the electronic controller is configured to operatively control an orientation of the at least one mirror with the transducer element.
  3. 3. The system of claim 2, wherein the at least one mirror comprises a single two-dimensional turning mirror.
  4. 4. The system of claim 2, wherein the at least one mirror comprises two one-dimensional mirrors configured to steer the light beam in two orthogonal directions.
  5. 5. The system of claim 1, wherein the focusing lens assembly is telecentric.
  6. 6. The system of claim 1, wherein the tracking mechanism comprises an electromechanical or optoelectronic sensor directly coupled with the steering mirror assembly to provide the electronic signal representative of the angular orientation.
  7. 7. The system of claim 1, wherein the tracking mechanism includes a position sensitive detector to provide the electronic signal representative of the angular orientation.
  8. 8. The system of claim 7, wherein the tracking mechanism further comprises a control beam source to illuminate at least one mirror in the steering mirror assembly with a control beam and subsequently detect the control beam with the position sensitive detector to provide an electronic signal representative of the angular orientation.
  9. 9. The system of claim 7, wherein the tracking mechanism includes optics to pick up a portion of the system light reflected by the turning mirror assembly and direct it to the position sensitive detector to provide the electronic signal representative of the angular orientation.
  10. 10. The system of claim 1, wherein the electronic controller is further operatively coupled to the tracking mechanism, and wherein during operation the electronic controller corrects the angular orientation of the steering mirror assembly based on a difference between a desired mirror orientation and a measured electronic signal of the mirror orientation provided by the tracking mechanism.
  11. 11. The system of claim 1, wherein the electronic controller stores calibration information for mapping the angular orientation of the steering mirror assembly to the position of the focused spot in the source plane of the interferometer.
  12. 12. The system of claim 1, wherein the electronic controller includes a user interface for receiving information defining the predetermined motion profile.
  13. 13. The system of claim 1, wherein the electronic controller includes a memory for storing information defining the predetermined motion profile.
  14. 14. The system of claim 1, wherein the predetermined motion profile comprises a plurality of arcs having different radii from an optical axis of the interferometer.
  15. 15. The system of claim 1, wherein the predetermined motion profile comprises a plurality of circles of different radii about an optical axis of the interferometer.
  16. 16. The system of claim 1, wherein the predetermined motion profile comprises at least one spiral around an optical axis of the interferometer.
  17. 17. The system of claim 1, wherein the system light source comprises a laser.
  18. 18. The system of claim 1, wherein the interferometer is a Michelson interferometer, a Twyman-Green interferometer, or a Fizeau interferometer.
  19. 19. The system of claim 18, wherein the interferometer is configured to illuminate the sample on an interferometer aperture with a wavefront defined by a position of a focused spot in a source plane of the interferometer.
  20. 20. The system of claim 19, wherein the focusing lens assembly defines a Numerical Aperture (NA) that provides a divergence of the focused spot sufficient to cover an aperture of the interferometer.

Description

Dynamic interferometer illuminator Cross Reference to Related Applications The present application claims priority from U.S. application Ser. No.18/820,383, filed 8/30 at 2024, which claims priority from provisional application Ser. No.63/544,007, filed 13 at 10 at 2023, the contents of which are incorporated herein in their entirety. Technical Field The present disclosure relates to optical interferometry. Background Interferometers are widely used tools for high precision characterization of engineered surfaces. Surface profiling is one such application, and interferometers are used to analyze surface topography over a wide range of spatial scales and resolutions. Interferometers designed for topography analysis of medium-sized (-10 mm to >1 m) optically smooth surfaces are employed because of their ease of use, high speed, high precision (nm scale) and measured non-contact properties. Often in combination with phase-shifting interferometry (PSI) techniques, surfaces can be measured to fractions of nanometers quickly and conveniently without risk of damaging high-value surfaces. These interferometers have various optical geometries that optimize different measurement characteristics. For example, michelson interferometers are illumination source friendly, accommodating widely varying source sizes and/or bandwidths, while the Twyman-Green geometry allows for convenient access to the reference path. The laser makes Fizeau geometry particularly popular because it is easy to set up and common path architecture, which reduces sensitivity to manufacturing errors in the tool optics. In combination with lasers Fizeau is ideally suited for long optical paths often encountered in characterizing large or complex optical components. Regardless of the architecture used, these tools typically have a similar illumination configuration, with the light source focused to a point (or spot) located in the Back Focal Plane (BFP) of the lens (collimator). The source spot is located in the BFP and produces a planar wavefront on the optical axis defined by the collimator that propagates parallel to the optical axis, covering the full aperture. The wavefront changes after reflection from the surface of interest are captured and analyzed by a camera to recover the surface topography, typically using PSI techniques. Additional refractive/diffractive elements behind the collimator provide alternative wave fronts to match different test surfaces, e.g. lenses for testing convex or concave spherical surfaces or Computer Generated Holograms (CGH) for aspherical surfaces. In all cases, the illumination source point should be stable to minimize errors in the measurement process and is typically spatially constrained to maximize wavefront quality and interference contrast, which directly affects the measurement signal-to-noise ratio (SNR). Static illumination source shapes other than single points are known to provide other advantages. For example, when coherent illumination is used, stationary spatially extended disks with diameters greater than the spatial resolution limit of the interferometer have been used to reduce coherent noise, but the technique is limited to cavity optical thicknesses less than the longitudinal coherence limit defined by the disk size and optical system. Other prior art methods disclose source shapes other than single dots or discs that provide advantages. One prior art shape, a thin fixed radius ring (see U.S. patent 6,643,024 and 6,804,011), whose thickness is diffraction limited in radial dimension and centered on the optical axis, provides coherent noise reduction without the disk cavity thickness limitations. These source shapes may be generated by static or dynamic means. For example, a static ring shape may be created with a holographic optical element or axicon. A static shape has the advantage of being physically stable and all points within the shape contribute to interference at the same time, but when used with coherent light it has drawbacks due to mutual interference between different points in the source shape. Thus, in the case of coherent illumination, the static shape typically requires additional optical elements to eliminate the additional interference created by the mutual interference, such as a rotating diffuser, which itself is a dynamic element. Dynamically creating source shapes using a moving point source that delineates the desired shape may be more efficient and versatile. In this way, arbitrary shapes can be produced with the same apparatus, and each interferogram contains contributions from each point in the shape without mutual interference effects. The prior art method used with loops is known as CARS (coherent artifact reduction system) and involves averaging phase maps acquired sequentially from a number of locations distributed around the loop. While effective, this pause gaze technique is slow. An alternative is also described in the prior art, namely to dynamically track the