Frequency multiplexed, multiple channel heterodyne interferometer

• US 7,405,814 B2
• Filed: 12/19/2006
• Issued: 07/29/2008
• Est. Priority Date: 12/19/2006
• Status: Active Grant

• Current Assignee: The Boeing Co.
• Original Assignee: The Boeing Co.
• Inventors: Muenter, Steven E.
• Primary Examiner(s): Tarcza; Thomas H.
• Assistant Examiner(s): Ratcliffe; Luke D
• Application Number:
  US11/613,028
• Publication Number:
  US 20080144041A1
• Time in Patent Office: 588 Days
• Field of Search: 356 301- 315, 356 401- 41, 356 501- 515, 356 6- 22, 356/28, 356/28.5
• US Class Current: 356/28.5
• CPC Class Codes: G01B 9/02003 using beat frequencies
• CPC Class Codes: G01B 9/02045 using the Doppler effect
• CPC Class Codes: G01H 9/00 Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means

Abstract

A novel technique using an acousto-optic modulator (AOM) as part of a heterodyne interferometer which measures optical path differences between a test signal and a reference signal is disclosed. An array of distinct frequencies are used to drive the AOM, yielding a spatially dispersed array of frequency-shifted subaperture beams of the test signal which are interfered with the wavefront to be measured and then combined with the dispersed reference signal. The frequency shifting of the AOM allows a single detector to collect the beams for signal processing to determine a measurement of the wavefront.

Claims

1. An apparatus for capturing measurement information, comprising:

a beam splitter for splitting a laser beam into a reference beam and a test beam;

an acousto-optic modulator (AOM) for spatially separating and modulating the test beam into a subaperture array of distinct frequency-shifted beams, each obtaining a unique frequency shift, for applying across a measured element, the AOM driven by a drive frequency array of signal inputs;

a beam expander for spatially dispersing the reference beam to at least match the subaperture array of the distinct frequency-shifted beams;

a beam combiner for interfering the subaperture array of the distinct frequency-shifted beams applied to the measured element with the spatially dispersed reference beam;

optics for directing the interfered subaperture array of the distinct frequency-shifted beams;

a photodetector for receiving the interfered subaperture array of the distinct frequency-shifted beams to produce an electrical signal from the photodetector carrying the measurement information of the measured element; and

a processor for comparing an array of reference optical path differences derived from the array of signal inputs with the electrical signal from the photodetector to determine the measurement information of the measured element.

2. The apparatus of claim 1, wherein the drive frequency array comprises a plurality of distinct frequencies.

3. The apparatus of claim 2, wherein the drive frequency array comprises a first range of drive frequencies and a second range of drive frequencies.

4. The apparatus of claim 3, wherein each of the plurality of distinct frequencies comprises a sum of a first frequency from the first range and a second frequency from the second range.

5. The apparatus of claim 3, wherein the first range of drive frequencies are driven by a first signal source and the second range of drive frequencies are driven by a second signal source.

6. The apparatus of claim 1, the processor for determining the measurement information of the measured element comprises calculating a phase shift of each of the distinct frequency-shifted beams of the subaperture array and determining a corresponding optical path difference and determining in turn the measurement information of the measured element corresponding to a combination of the corresponding optical path difference across the subaperture array.

7. The apparatus of claim 1, wherein the measured element comprises a wavefront that the subaperture array of distinct frequency-shifted beams is directed across before being directed to the beam combiner.

8. The apparatus of claim 7, wherein the measurement information of the wavefront comprises a profile of the wavefront.

9. The apparatus of claim 1, wherein the measured element comprises a vibrating object that the subaperture array of distinct frequency-shifted beams are reflected off before being directed to the beam combiner.

10. The apparatus of claim 9, wherein the measurement information of the vibrating object comprises velocity, displacement, and vibration history across a surface of the vibrating object.

11. A method for capturing measurement information comprising the steps of:

splitting a laser beam into a reference beam and a test beam;

spatially separating and modulating the test beam into a subaperture array of distinct frequency-shifted beams, each obtaining a unique frequency shift, driven by a drive frequency array of signal inputs;

applying the subaperture array of the distinct frequency-shifted beams across a measured element;

spatially dispersing the reference beam to at least match the subaperture array of the distinct frequency-shifted beams;

interfering the subaperture array of the distinct frequency-shifted beams with the spatially dispersed reference beam;

directing the interfered subaperture array of the distinct frequency-shifted beams onto a photodetector to produce an electrical signal from the photodetector carrying the measurement information of the measured element; and

comparing an array of reference optical path differences derived from the array of signal inputs with the electrical signal from the photodetector to determine the measurement information of the measured element.

12. The method of claim 11, wherein the drive frequency array comprises a plurality of distinct frequencies.

13. The method of claim 12, wherein the drive frequency array comprises a first range of drive frequencies and a second range of drive frequencies.

14. The method of claim 13, wherein each of the plurality of distinct frequencies comprises a sum of a first frequency from the first range and a second frequency from the second range.

15. The method of claim 13, wherein the first range of drive frequencies are driven by a first signal source and the second range of drive frequencies are driven by a second signal source.

16. The method of claim 11, the processor for determining the measurement information of the measured element comprises calculating a phase shift of each of the distinct frequency-shifted beams of the subaperture array and determining a corresponding optical path difference and determining in turn the measurement information of the measured element corresponding to a combination of the corresponding optical path difference across the subaperture array.

17. The method of claim 11, wherein the measured element comprises a wavefront that the subaperture array of distinct frequency-shifted beams is directed across before being directed to the beam combiner.

18. The method of claim 17, wherein the measurement information of the wavefront comprises a profile of the wavefront.

19. The method of claim 11, wherein the measured element comprises a vibrating object that the subaperture array of distinct frequency-shifted beams are reflected off before being directed to the beam combiner.

20. The method of claim 19, wherein the measurement information of the vibrating object comprises velocity, displacement, and vibration history across a surface of the vibrating object.

21. An apparatus for capturing measurement information comprising:

a beam splitting means for splitting a laser beam into a reference beam and a test beam;

a modulating means for spatially separating and modulating the test beam into a subaperture array of distinct frequency-shifted beams, each obtaining a unique frequency shift, applied across a measured element;

a dispersing means for spatially dispersing the reference beam to at least match the subaperture array of the distinct frequency-shifted beams;

a beam combiner means for interfering the subaperture array of the distinct frequency-shifted beams with the spatially dispersed reference beam;

an optical means for directing the interfered subaperture array of the distinct frequency-shifted beams;

a photodetecting means for receiving the interfered subaperture array of the distinct frequency-shifted beams to produce an electrical signal from the photodetecting means carrying the measurement information of the measured element; and

a processor means for comparing an array of reference optical path differences derived from the array of signal inputs with the electrical signal from the photodetecting means to determine the measurement information of the measured element.