System and method for measuring optical distance

• US 20030112444A1
• Filed: 12/18/2001
• Published: 06/19/2003
• Est. Priority Date: 12/18/2001
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Wax, Adam, Yang, Changhuei, Dasari, Ramachandra R., Feld, Michael S.
• Granted Patent: US 6,934,035 B2
• Time in Patent Office: Days
• US Class Current: 356/486
• CPC Class Codes: G01B 2290/45 Multiple detectors for detecting interferometer signals
• CPC Class Codes: G01B 9/02002 using two or more frequencies
• CPC Class Codes: G01B 9/02007 Two or more frequencies or sources used for interferometric measurement using only beat G01B9/02003
• CPC Class Codes: G01B 9/02078 Caused by ambiguity
• CPC Class Codes: G01B 9/0209 Low-coherence interferometers
• CPC Class Codes: G01J 9/04 by beating two waves of a same source but of different frequency and measuring the phase shift of the lower frequency obtained
• CPC Class Codes: G01N 21/45 using interferometric methods; using Schlieren methods

Abstract

The methods of the present invention are directed at an accurate phase-based technique for measuring arbitrarily long optical distances with sub-nanometer precision. A preferred embodiment of the present invention method employs a interferometer, for example, a Michelson interferometer, with a pair of harmonically related light sources, one continuous wave (CW) and a second source having low coherence. By slightly adjusting the center wavelength of the low coherence source between scans of the target sample, the phase relationship between the heterodyne signals of the CW and low coherence light is used to measure the separation between reflecting interfaces with sub-nanometer precision. As the preferred embodiment of this method is completely free of 2π ambiguity, an issue that plagues most phase-based techniques, it can be used to measure arbitrarily long optical distances without loss of precision.

Claims

1. A method for measuring an optical distance comprising the steps of:

providing a first wavelength and a second wavelength of light;

directing light of the first wavelength and the second wavelength along both a first optical path and a second optical path, the first optical path extending onto a medium to be measured and the second path undergoing a change in path length;

detecting light from the medium and light from the second optical path to measure a first change in phase of light interacting with the medium;

adjusting the first wavelength of light to generate a third wavelength of light;

directing light of the third wavelength and the second wavelength along both the first optical path and the second optical path, the first optical path extending onto the medium to be measured and the second path undergoing a change in path length;

detecting light from the medium and light from the second optical path to re-measure a second change in phase of light interacting with the medium;

superposing the first change in phase and the second change in phase to determine at least two phase crossing points; and

determining the optical distance by counting the number of continuous interference fringes between the at least two phase crossing points.

2. The method of claim 1 wherein the medium comprises biological tissue.

3. The method of claim 1 wherein the medium comprises a semiconductor material.

4. The method of claim 1 further comprising the step of refining the optical distance by measuring the difference phase at the at least two phase crossing points.

5. The method of claim 1 wherein the step of changing the first wavelength of light comprises adjusting a center wavelength by approximately 2 nm.

6. The method of claim 1 further comprising providing a light source that emits the first wavelength and a second wavelength that are harmonically related.

7. The method of claim 1 further comprising providing a first low coherence light source and a second continuous wave light source.

8. A method for measuring an optical distance, comprising the steps of;

providing a first signal and a second signal generated by a first light source and a third signal generated by a second light source, the first light source being harmonically related to the second light source;

determining a first heterodyne signal from the first and the third signal and a second heterodyne signal from the second and third signal; and

determining the phase relationship between the first and second heterodyne signals to obtain the optical distance.

9. The method of claim 8 wherein the first signal and second signal are low coherence signals.

10. The method of claim 8 wherein the third signal is a continuous wave signal.

11. The method of claim 8 wherein providing the first and the second signals are generated by a broadband light source.

12. A system for measuring an optical distance, comprising the steps of;

a first light source that generates a first signal and a second signal;

a second light source that generates a third signal, the first light source being harmonically related to the second light source;

a detector system that measures a first heterodyne signal from the first and the third signal and a second heterodyne signal from the second and third signal; and

a processor that determines a phase relationship between the first and second heterodyne signals to obtain the optical distance.

13. The system of claim 12 wherein the first signal and second signal are low coherence signals.

14. The system of claim 12 wherein the third signal is a continuous wave signal.

15. The system of claim 12 wherein the first and the second signals are generated by a broadband light source.

16. The system of claim 12 further comprising an optical pathway including an optical fiber.

17. The system of claim 12 further comprising a low coherence signal having a bandwidth of at least 5 nm.

18. The system of claim 12 wherein the system comprises an interferometer.

19. The system of claim 12 further comprising a mirror and scanner that scans the mirror from a first position to a second position.

20. The system of claim 12 wherein the detector system comprises a first detector that detects a first signal and a second detector that detects a second signal harmonically related to the first signal.