Synchronous frequency-shift mechanism in fizeau interferometer

1. An optical device for characterizing a test object opposite to a reference surface in a Fizeau optical cavity, comprising:

a polarization frequency-shifting element operating on an input beam to generate two orthogonally polarized beams having respective frequencies differing by a first frequency shift;

means for projecting said two beams toward said Fizeau optical cavity to produce a pair of test beams and a pair of reference beams, wherein said beams in each pair have orthogonal polarization states and have frequencies differing by said first frequency shift;

means for producing a second frequency shift on either one of said pairs of test and reference beams;

means for combining said pairs of test and reference beams to produce an interferogram after said second frequency shift has been introduced; and

means for detecting and spatially sampling said interferogram;

wherein said first frequency shift and said second frequency shift are substantially equal in magnitude and are produced by a transducer component operating simultaneously on both the Fizeau optical cavity and the polarization frequency-shifting element.

2. The device of claim 1 , wherein said transducer component includes an acousto-optic modulator.

3. The device of claim 1 , wherein said transducer component includes a rotating radial diffraction grating.

4. The device of claim 1 , wherein said transducer component includes an electro-optical modulator.

5. The device of claim 1 , wherein said transducer component includes a fiber-optic modulator.

6. The device of claim 1 , wherein said transducer component includes a piezo-electric element.

7. The device of claim 1 , wherein said polarization frequency-shifting element comprises a polarization beamsplitter and two quarter-wave plates to produce said two orthogonally polarized beams and a moving mirror to impart said first frequency shift between the orthogonally polarized beams.

8. The device of claim 7 , wherein said input beam is characterized by a periodic coherence function having a repeat length; said polarization frequency-shifting element further includes a second, normally stationary mirror and means for translating said moving and normally stationary mirrors relative to one another; and an optical path difference between said orthogonally polarized beams is adjusted to optimize fringe contrast.

9. The device of claim 7 , wherein said polarization frequency-shifting element further includes a phase shifting device to introduce phase offsets between said orthogonally polarized beams.

10. The device of claim 1 , wherein said second frequency shift is a Doppler shift produced by said transducer component on either one of said pairs of test and reference beams in a direction substantially parallel to the pairs of test and reference beams.

11. The device of claim 1 , wherein said means for detecting and spatially sampling the interferogram comprises a pixelated phase-mask, a camera sensor optically aligned with the pixelated phase-mask, and a quarter-wave plate optically aligned with the pixelated phase-mask.

12. The device of claim 1 , wherein said means for detecting and spatially sampling said interferogram comprises a Wollaston prism, a polarizer and a camera sensor.

13. The device of claim 1 , wherein said means for detecting and spatially sampling is operated such as to substantially meet a condition whereby

ω 0 T=n 2π,

where T is integration time of a camera sensor and ω 0 is said frequency shift.

14. The device of claim 1 , wherein said polarization frequency-shifting element further includes a blocking element to block one of said orthogonally polarized beams.

15. The device of claim 1 , wherein said polarization frequency-shifting element comprises a polarization beamsplitter and two quarter-wave plates to produce said two orthogonally polarized beams and a moving mirror to impart said first frequency shift between the orthogonally polarized beams; and said means for detecting and spatially sampling the interferogram comprises a pixelated phase-mask, a camera sensor optically aligned with the pixelated phase-mask, and a quarter-wave plate optically aligned with the pixelated phase-mask.

16. An optical device for characterizing a test object opposite to a reference surface in a Fizeau optical cavity, comprising, in combination:

a light source projecting a light beam into said Fizeau optical cavity to produce a test beam and a reference beam;

means for producing a first frequency shift between said test and reference beams;

a polarization frequency-shifting element operating on said test and reference beams to generate two pairs of beams, said pairs being orthogonally polarized, and to produce a second frequency shift on either one of said pairs of beams;

means for combining said two pairs of beams to produce an interferogram after said second frequency shift has been introduced; and

means for detecting and spatially sampling said interferogram;

wherein said first frequency shift and said second frequency shift are substantially equal in magnitude and are produced by a transducer component operating simultaneously on both the Fizeau optical cavity and the polarization frequency-shifting element.

17. The device of claim 16 , wherein said means for producing a first frequency shift between said test and reference beams is a transducer component operating on either one of the test and reference beams in a direction substantially parallel to the test and reference beams.

18. The device of claim 16 , wherein said polarization frequency-shifting element comprises a polarization beamsplitter and two quarter-wave plates to produce said two pairs of beams and a moving mirror to impart said second frequency shift on either one of said pairs of beams.

19. The device of claim 18 , wherein said light source is characterized by a periodic coherence function having a repeat length; said polarization frequency-shifting element further includes a second, normally stationary mirror and means for translating said moving and normally stationary mirrors relative to one another; and an optical path difference between said orthogonally polarized beams is adjusted to optimize fringe contrast.

20. The device of claim 18 , wherein said polarization frequency-shifting element further includes a phase shifting device to introduce phase offsets between said orthogonally polarized beams.

21. The device of claim 16 , wherein said means for detecting and spatially sampling said interferograms comprises a pixelated phase-mask, a camera sensor optically aligned with the pixelated phase-mask, and a quarter-wave plate optically aligned with the pixelated phase-mask.

22. The device of claim 16 , wherein said means for detecting and spatially sampling said interferogram comprises a Wollaston prism, a polarizer and a camera sensor.

23. The device of claim 16 , wherein said means for detecting and spatially sampling is operated such as to substantially meet a condition whereby

ω 0 T=n 2π,

wherein T is integration time of a camera sensor and ω 0 is said frequency shift.

24. The device of claim 16 , wherein said polarization frequency-shifting element further includes a blocking element to block one of said pairs of orthogonally polarized beams.

25. The device of claim 16 , wherein said polarization frequency-shifting element comprises a polarization beamsplitter and two quarter-wave plates to produce said two pairs of beams and a moving mirror to impart said second frequency shift on either one of said pairs of beams; said means for detecting and spatially sampling said interferograms comprises a pixelated phase-mask, a camera sensor optically aligned with the pixelated phase-mask, and a quarter-wave plate optically aligned with the pixelated phase-mask.

26. An optical device for characterizing a test object opposite to a reference surface in a Fizeau optical cavity, comprising:

a combination including

a Fizeau optical cavity comprising means for producing a frequency shift in a beam emitted by the cavity; and

a polarization frequency-shifting element capable of generating orthogonally polarized beams having respective frequencies differing substantially by said frequency shift;

said optical cavity and polarization frequency-shifting element being optically aligned so as to convert an input beam into a first pair of orthogonally polarized beams with substantially no relative frequency shift and into a second pair of orthogonally polarized beams with a relative frequency shift substantially equal to twice said frequency shift, thereby producing an output including four beams;

means for combining said output to produce an interferogram; and

means for detecting and spatially sampling said interferogram;

wherein said frequency shift is produced in the Fizeau optical cavity and the polarization frequency-shifting element by a transducer component operating simultaneously on both the Fizeau optical cavity and the polarization frequency-shifting element.

27. The device of claim 26 , wherein said transducer component includes an acousto-optic modulator.

28. The device of claim 26 , wherein said transducer component includes a rotating radial diffraction grating.

29. The device of claim 26 , wherein said transducer component includes an electro-optical modulator.

30. The device of claim 26 , wherein said transducer component includes a fiber-optic modulator.

31. The device of claim 26 , wherein said means for producing a frequency shift is a transducer driving a reflective surface in the Fizeau optical cavity.

32. The device of claim 26 , wherein said polarization frequency-shifting element comprises a polarization beamsplitter, two quarter-wave plates and a moving mirror to produce said orthogonally polarized beams having respective frequencies differing substantially by said frequency shift.

33. The device of claim 26 , wherein said means for detecting and spatially sampling said interferogram comprises a pixelated phase-mask, a camera sensor optically aligned with the pixelated phase-mask, and a quarter-wave plate optically aligned with the pixelated phase-mask.

34. The device of claim 26 , wherein said means for detecting and spatially sampling said interferogram comprises a Wollaston prism, a polarizer and a camera sensor.

35. The device of claim 26 , wherein said means for detecting and spatially sampling is operated such as to substantially meet a condition whereby

ω 0 T=n 2π,

wherein T is integration time of a camera sensor and ω 0 is said frequency shift.

36. A method of characterizing a test object with a Fizeau interferometer comprising the following steps:

combining a Fizeau optical cavity that includes means for producing a frequency shift in a beam emitted by the cavity with a polarization frequency-shifting element capable of generating orthogonally polarized beams having respective frequencies differing by substantially said frequency shift;

aligning said optical cavity and polarization frequency-shifting element so as to convert an input beam into a first pair of orthogonally polarized beams with substantially no relative frequency shift and into a second pair of orthogonally polarized beams with a relative frequency shift substantially equal to twice said frequency shift, thereby producing an output including four beams;

combining said output to produce an interferogram; and

detecting and spatially sampling said interferogram;

wherein said frequency shift is produced in the Fizeau optical cavity and the polarization frequency-shifting element by a transducer component operating simultaneously on both the Fizeau optical cavity and the polarization frequency-shifting element.

37. The method of claim 36 , wherein said frequency shift is produced by a transducer driving a reflective surface in the Fizeau optical cavity.

38. The method of claim 36 , wherein said polarization frequency-shifting element comprises a polarization beamsplitter, two quarter-wave plates and a moving mirror for producing said orthogonally polarized beams having respective frequencies differing by substantially said frequency shift.

39. The method of claim 36 , wherein said input beam is characterized by a periodic coherence function having a repeat length; said polarization frequency-shifting element further includes a second, normally stationary mirror and means for translating said moving and normally stationary mirrors relative to one another; and further including the step of adjusting an optical path difference between said orthogonally polarized beams to optimize fringe contrast.

40. The method of claim 36 , wherein said polarization frequency-shifting element further includes a phase shifting device to introduce phase offsets between said orthogonally polarized beams; and further including the steps of making multiple measurements with corresponding phase offsets, and of averaging said measurements to reduce measurement errors.

41. The method of claim 36 , wherein said step of detecting and spatially sampling said interferogram is carried out with a pixelated phase-mask, a camera sensor optically aligned with the pixelated phase-mask, and a quarter-wave plate optically aligned with the pixelated phase-mask.

42. The method of claim 36 , wherein said step of detecting and spatially sampling said interferogram is carried out with a Wollaston prism, a polarizer and a camera sensor.

43. The method of claim 36 , wherein said detecting and spatially sampling step is carried out such as to substantially meet a condition whereby

ω 0 T=n 2π,

wherein T is integration time of a camera sensor and ω 0 is said frequency shift.