Compensating for time varying phase changes in interferometric measurements

1. A method for processing interference pattern data generated by an interferometer, where the interferometer provides a laser signal from a tunable laser along a given optical path having an associated path delay and to a reference optical path and combines light reflected from the given optical path and from the reference path thereby generating the interference pattern data, comprising:

estimating a first laser optical phase of the laser signal;

calculating an expected complex response for the given optical path based on the estimated laser optical phase;

multiplying the interference pattern data from the interferometer by the expected complex response to generate a product; and

filtering the product to extract interference pattern data associated with the given optical path from the interference pattern data generated by the interferometer.

2. The method in claim 1 , wherein the given optical path is associated with a device under test (DUT).

3. The method in claim 1 , wherein the step of calculating an expected complex response for the given optical path based on the estimated laser optical phase further comprises:

estimating a delayed version of the laser optical phase of the laser signal;

determining a difference phase between the delayed version of the estimated laser optical phase and the estimated first laser optical phase; and

calculating the cosine of the difference phase to form the real part of the expected complex response; and

calculating the sine of the difference phase to form the imaginary part of the expected complex response.

4. The method in claim 3 , further comprising:

low pass filtering and decimating the real and imaginary parts of the expected complex response to extract interference pattern data associated with the given optical path from the interference pattern data generated by the interferometer.

5. The method in claim 1 , wherein estimating the laser optical phase includes:

coupling a portion of the laser light to a second interferometer;

converting an interference fringe signal from the second interferometer into a digital signal corresponding to the interference pattern data, the digital signal being a sampled form of the interference fringe signal;

estimating the laser phase based upon the digital signal.

6. The method in claim 5 , further comprising:

estimating a first derivative of the laser optical phase based in the digital signal including:

Fourier transforming the digital signal;

windowing the transformed signal to identify a portion of the transformed signal that corresponds to the given optical path delay;

inverse Fourier transforming the windowed signal; and

computing the phase of the signal.

7. The method in claim 5 , further comprising:

estimating a second derivative of the laser optical phase based upon the digital signals by identifying zero crossings of the digital signal, and

counting a number of samples between the zero crossings of the digital signal.

8. The method in claim 7 , wherein the step of calculating an expected complex response for the given optical path based on the estimated laser optical phase further comprises:

estimating a delayed version of the second derivative of the laser optical phase;

calculating a running sum of the second derivative of the laser optical phase, where a length of the running sum is associated with a length of the given optical path delay;

accumulating the running sum;

calculating a sine of the accumulated sum to form the imaginary part of the expected complex response; and

calculating a cosine of the accumulated sum to form the real part of the expected complex response.

9. The method in claim 8 , further comprising:

low pass filtering and decimating the real and imaginary parts of the expected complex response to extract interference pattern data associated with the given optical path from the interference pattern data generated by the interferometer.

10. Apparatus for processing interference pattern data generated by an interferometer, where the interferometer provides a laser signal from a tunable laser along a given optical path having an associated path delay and to a reference optical path and combines light reflected from the given optical path and from the reference path thereby generating the interference pattern data, comprising:

means for estimating a first laser optical phase of the laser signal;

means for calculating an expected complex response for the given optical path based on the estimated laser optical phase;

means for multiplying the interference pattern data from the interferometer by the expected complex response to generate a product; and

means for filtering the product to extract interference pattern data associated with the given optical path from the interference pattern data generated by the interferometer.

11. The apparatus in claim 10 , wherein the given optical path is associated with a device under test (DUT).

12. The apparatus in claim 10 , wherein the means for calculating an expected complex response for the given optical path based on the estimated laser optical phase further comprises:

means for estimating a delayed version of the laser optical phase of the laser signal;

means for determining a difference phase between the delayed version of the estimated laser optical phase and the estimated first laser optical phase; and

means for calculating the cosine of the difference phase to form the real part of the expected complex response; and

means for calculating the sine of the difference phase to form the imaginary part of the expected complex response.

13. The apparatus in claim 12 , further comprising:

means for low pass filtering and decimating the real and imaginary parts of the expected complex response to extract interference pattern data associated with the given optical path from the interference pattern data generated by the interferometer.

14. The apparatus in claim 10 , wherein means for estimating the laser optical phase includes:

means for coupling a portion of the laser light to the interferometer;

means for converting an interference fringe signal from the interferometer into a digital signal corresponding to the interference pattern data, the digital signal being a sampled form of the interference fringe signal;

means for estimating the laser phase based upon the digital signal.

15. The apparatus in claim 14 , further comprising:

means for estimating a first derivative of the laser optical phase based in the digital signal including:

means for Fourier transforming the digital signal;

means for windowing the transformed signal to identify a portion of the transformed signal that corresponds to the given optical path delay;

means for inverse Fourier transforming the windowed signal; and

means for computing the phase of the signal.

16. The apparatus in claim 15 , further comprising:

means for estimating a second derivative of the laser optical phase based upon the digital signals by identifying zero crossings of the digital signal, and

means for counting a number of samples between the zero crossings of the digital signal.

17. The method in claim 16 , wherein the means for calculating an expected complex response for the given optical path based on the estimated laser optical phase further comprises:

means for estimating a delayed version of the second derivative of the laser optical phase;

means for calculating a running sum of the second derivative of the laser optical phase, where a length of the running sum is associated with a length of the given optical path delay;

means for accumulating the running sum;

means for calculating a sine of the accumulated sum to form the imaginary part of the expected complex response; and

means for calculating a cosine of the accumulated sum to form the real part of the expected complex response.

18. The apparatus in claim 17 , further comprising:

means for low pass filtering and decimating the real and imaginary parts of the expected complex response to extract interference pattern data associated with the given optical path from the interference pattern data generated by the interferometer.

19. The apparatus in claim 10 , wherein the means are implemented using a field programmable gate array.

20. The apparatus in claim 10 , wherein the means are implemented using a software programmed data processor.

21. Apparatus for processing interference pattern data generated by an interferometer, where the interferometer provides a laser signal from a tunable laser along a given optical path having an associated path delay and to a reference optical path and combines light reflected from the given optical path and from the reference path thereby generating the interference pattern data, comprising electronic circuitry configured to:

estimate a first laser optical phase of the laser signal;

calculate an expected complex response for the given optical path based on the estimated laser optical phase;

multiply the interference pattern data from the interferometer by the expected complex response to generate a product; and

filter the product to extract interference pattern data associated with the given optical path from the interference pattern data generated by the interferometer.

22. The apparatus in claim 21 , wherein the electronic circuitry is further configured to:

estimate a delayed version of the laser optical phase of the laser signal;

determine a difference phase between the delayed version of the estimated laser optical phase and the estimated first laser optical phase; and

calculate the cosine of the difference phase to form the real part of the expected complex response; and

calculate the sine of the difference phase to form the imaginary part of the expected complex response.

23. The apparatus in claim 22 , wherein the electronic circuitry is further configured to:

low pass filter and decimate the real and imaginary parts of the expected complex response to extract interference pattern data associated with the given optical path from the interference pattern data generated by the interferometer.