Chromatic compensation in Fizeau interferometer

• US 20070296978A1
• Filed: 06/22/2007
• Published: 12/27/2007
• Est. Priority Date: 06/23/2006
• Status: Active Grant

• Current Assignee: Onto Innovation Inc.
• Original Assignee: 4D Technology Corporation (Onto Innovation Inc.)
• Inventors: Hayes, John
• Granted Patent: US 7,580,135 B2
• Time in Patent Office: Days
• US Class Current: 356/513
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 9/02039 by matching the wavefront with a particular object surface shape
• CPC Class Codes: G01B 9/02057 by using common path configuration, i.e. reference and object path almost entirely overlapping
• CPC Class Codes: G01B 9/02063 by particular alignment of focus position, e.g. dynamic focussing in optical coherence tomography

Abstract

The focus of the collimating lens in the optical train of a Fizeau interferometer is adjusted to change the power of the test beam illuminating the transmission sphere. As a result, the rays can be made sufficiently perpendicular to the reference surface to eliminate the chromatic focus shift and non-common path errors produced by a light source of wavelength different from the design wavelength of the transmission sphere. By making the position of the collimating lens relative to the beam expander adjustable along the optical axis over some small range, illumination sources of various wavelength can be used in the same interferometer.

Claims

1. In a Fizeau interferometer including a light source producing an illumination beam of predetermined wavelength in an optical axis toward a reference surface in a transmission sphere, the improvement comprising:

a beam expander;

a collimator; and

means for varying a distance between the beam expander and the collimator so as to affect a focal point position of a reference beam reflected by the reference surface.

2. The improvement of claim 1, wherein said means is operating by translating the collimator.

3. The improvement of claim 1, wherein said means is operating by translating the beam expander.

4. The improvement of claim 3, wherein said means is operating by translating an eyepiece lens of the beam expander.

5. A method of correcting for chromatic focus shift in a Fizeau interferometer including a light source of predetermined wavelength, a beam expander and a collimator along an optical axis toward a reference surface in a transmission sphere, the method comprising varying a distance between the beam expander and the collimator so as to affect a focal point position of a reference beam reflected by the reference surface.

6. The method of claim 5, wherein said varying step is carried out by translating the collimator.

7. The method of claim 5, wherein said varying step is carried out by translating the beam expander.

8. The method of claim 7, wherein said varying step is carried out by translating an eyepiece lens of the beam expander.

9. A method for adjusting an optical power of an illumination beam to match a design specification of a transmission sphere of a Fizeau interferometer, said interferometer including a light source of predetermined wavelength, a beam expander and a collimator along an optical axis toward a reference surface in the transmission sphere, the method comprising varying a distance between the beam expander and the collimator so as to affect a focal point position of a reference beam reflected by the reference surface.

10. The method of claim 9, wherein said varying step is carried out by translating the collimator.

11. The method of claim 9, wherein said varying step is carried out by translating the beam expander.

12. The method of claim 11, wherein said varying step is carried out by translating an eyepiece lens of the beam expander.