Composite integrity monitoring

• US 4,983,034 A
• Filed: 02/13/1989
• Issued: 01/08/1991
• Est. Priority Date: 12/10/1987
• Status: Expired due to Term

• Current Assignee: Simmonds Precision Products, Inc. (Raytheon Technologies Corporation d/b/a RTX)
• Original Assignee: Simmonds Precision Products, Inc. (Raytheon Technologies Corporation d/b/a RTX)
• Inventors: Spillman, William B. Jr.
• Primary Examiner(s): McGraw, Vincent P.
• Application Number:
  US07/310,148
• Time in Patent Office: 694 Days
• Field of Search: 250/227, 250/225, 250/227.17, 356/32, 356/33, 356/34, 356/35.5, 356/73, 73/800, 73/802
• US Class Current: 356/32
• CPC Class Codes: G01L 1/242 the material being an optical fibre

Abstract

An optical frequency domain distributed strain sensor for determining the strain distribution along an optical fiber includes an optical source that provides a polarization controlled optical interrogation signal having a frequency that varies in a recurring linear manner. The interrogation signal is injected into a sensor fiber embedded within a composite structure that places the fiber under strain. A portion of the interrogation signal is backscattered from the sensing fiber as a consequence of the strain experienced by the fiber and is mixed with a reference signal to produce beat frequency signals. The frequency of the beat signals is directly related the to position of backscatter in the sensing fiber while the amplitude of each beat frequency signal is directly related to the integrated strain-induced birefringence up to the backscatter point. An in-line fiber polarizer and an associated controllable polarizer control the polarization state of the interrogation signal in the sensor fiber to provide zero point sensitivity compensation and controllable testing for ambiguous strain points.

Claims

1. A method of composite integrity monitoring by sensing strain distributed along an optical fiber, comprising the steps of:

providing a composite structure, and an optical fiber, said structure comprising composite material, said optical fiber being embedded in said composite material, said optical fiber being subjected to strain;

launching initial frequency varying modulated optical energy having a selectively controllable polarization state into said optical fiber;

providing a reference signal representative of the modulated optical energy;

mixing a portion of the optical energy backscattered from the fiber with a portion of the reference signal to produce a beat signal, the frequency of which is representative of strain at a location along the fiber;

sensing the polarization state of the backscattered optical energy; and

controlling the polarization state of said initial optical energy launched in response to said beat signal.

2. The method of claim 1, wherein said controlling step comprises controlling the polarization state of the optical energy launched in response to said beat signal, to reduce the effect of backscattering at locations other than said strain location.

3. The method of claim 1 wherein said structure is an aircraft.

4. The method of claim 1 wherein said structure is a spacecraft.

5. The method of claim 1 wherein said composite material comprises graphite.

6. A method of composite integrity monitoring by sensing strain at a selected location along an optical fiber, comprising the steps of:

providing a composite structure, and an optical fiber, said structure comprising composite material, said optical fiber being embedded in said composite material, said optical fiber being subjected to strain;

launching initial frequency varying optical energy having a selectively controllable polarization state into said optical fiber;

providing a reference signal representative of the modulated optical energy;

sensing the polarization state of the backscattered optical energy from the fiber below a selected frequency;

controlling the polarization state of said initial optical energy launched in response to said sensed polarization state of the backscattered energy to minimize backscatter; and

increasing the selected frequency to correspond to a selected location and mixing a portion of the optical energy backscattered from the fiber with a portion of the reference signal to produce a beat signal representative of strain at the selected location.

7. The method of claim 6 wherein said structure is an aircraft.

8. The method of claim 6 wherein said structure is a spacecraft.

9. The method of claim 6 wherein said composite material comprises graphite.

10. A method of composite integrity monitoring by sensing strain at successive selected locations along an optical fiber, comprising the steps of:

providing a composite structure, and an optical fiber, said structure comprising composite material, said optical fiber being embedded in said composite material, said optical fiber being subjected to strain;

launching initial frequency varying optical energy having a selectively controllable polarization state into said optical fiber;

providing a reference signal representative of the modulated optical energy;

sensing the polarization state of the backscattered optical energy from the fiber below a selected frequency;

controlling the polarization state of said initial optical energy launched in response to said sensed polarization state of the backscattered energy to minimize backscatter;

increasing the selected frequency to correspond to a selected location and mixing a portion of the optical energy backscattered from the fiber with a portion of the reference signal to produce a beat signal representative of strain at the selected location; and

repeating the second, third, and fourth steps at selectively incremented frequencies.

11. The method of claim 10 wherein said structure is an aircraft.

12. The method of claim 10 wherein said structure is a spacecraft.

13. The method of claim 10 wherein said composite material comprises graphite.