Multi-mode optical fiber sensor and method

• US 4,863,270 A
• Filed: 08/31/1988
• Issued: 09/05/1989
• Est. Priority Date: 08/31/1988
• Status: Expired due to Term

• Current Assignee: Simmonds Precision Products Incorporated Hercules Plaza Wilmington de 19894
• Original Assignee: Simmonds Precision Products, Inc. (Rtx Corporation)
• Inventors: Spillman, William B. Jr.
• Primary Examiner(s): Willis, Davis L.
• Assistant Examiner(s): Koren, Matthew W.
• Application Number:
  US07/238,668
• Time in Patent Office: 370 Days
• Field of Search: 356/345, 356/354, 250/227
• US Class Current: 356/477
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01D 5/35351 using other means to detect the measured quantity

Abstract

A multi-mode fiber optic sensor and method for optically sensing a physical perturbation includes a multi-mode optical fiber segment that accepts coherent monochromatic radiation from a suitable source. As the radiation is propagated in the fiber the various modes form a complex interference pattern that changes in response to a physical perturbation of the fiber. A detector provides an output signal to a signal processor that analyzes the signal as a function of the change in intensity to provide an information signal that is functionally related to the perturbation.

Claims

1. A multi-mode fiber optical sensor sensitive to a physical perturbation comprising:

a multi-mode optical fiber for transmitting optical energy between an input and an output in at least two modes to form an interference pattern between the at least two modes;

a single mode optical fiber coupled to the input of said multi-mode optical fiber;

means for providing optical energy to an input of said single-mode optical fiber for coupling to said multi-mode optical fiber, the optical energy sufficient to propagate in at least two modes along the multi-mode fiber, the interference pattern changing in response to a physical perturbation of the multi-mode fiber; and

means for detecting an interference pattern from the output of said multi-mode optical fiber and for generating a signal functionally related to the perturbation and in response to an intensity variation of at least a part of the interference pattern.

2. A multi-mode fiber optic sensor as recited in claim 1, wherein said detecting means comprises:

means for photodetecting a subset of the complex interference pattern from the output of said optical fiber segment, said photodetecting means generating said signal in response to a variation in the intensity of the subset of the complex interference pattern; and

means for filtering disposed between the output and said photodetector means for providing the subset of the complex interference pattern to said photodetector means.

3. A multi-mode fiber optic sensor as recited in claim 2, wherein said photodetecting means comprises processing means for determining a difference in the intensity of two time-separated subsets of the complex interference pattern.

4. A multi-mode fiber optic sensor as recited in claim 3, wherein said filtering means comprises an opaque surface having at least one aperture for passing a subset of the complex interference pattern.

5. A multi-mode fiber optic sensor as recited in claim 4, wherein said filtering means comprises an opaque surface having at least one circular hole.

6. A multi-mode fiber optic sensor as recited in claim 1, wherein said detecting means comprises a CCD array having a plurality of pixels, each said pixel being smaller than an average speckle of said complex interference pattern.

7. A multi-mode fiber optic sensor as recited in claim 6, wherein said detecting means determines a difference in the intensity of selected speckles of the complex interference pattern over time and sums the absolute value of the difference thereof.

8. A multi-mode fiber optic sensor as recited in claim 1, wherein said means for detecting determines a difference in intensity of part of the interference pattern, the difference corresponding to physical perturbation.

9. A multi-mode fiber optic sensor as recited in claim 8, further comprising:

filtering means for filtering the interference pattern prior to detection by said detecting means.

10. A multi-mode fiber optical sensor as recited in claim 9, wherein said filtering means comprises an opaque surface having at least one aperture for passing a subset of the complex interference pattern.