Fiber devices and sensors based on multimode fiber Bragg gratings

• US 5,848,204 A
• Filed: 09/08/1995
• Issued: 12/08/1998
• Est. Priority Date: 09/08/1995
• Status: Expired due to Fees

• Current Assignee: Indiana University
• Original Assignee: Indiana University
• Inventors: Wanser, Keith H.
• Primary Examiner(s): Bovernick, Rodney B.
• Assistant Examiner(s): Wise, Robert E.
• Application Number:
  US08/525,967
• Time in Patent Office: 1,187 Days
• Field of Search: 385/12, 385/13
• US Class Current: 385/12
• CPC Class Codes: G01D 5/35316 using a Bragg gratings
• CPC Class Codes: G01L 1/246 using integrated gratings, e.g. Bragg gratings
• CPC Class Codes: G02B 6/02085 characterised by the grating profile, e.g. chirped, apodised, tilted, helical

Abstract

New sensing and diagnostic methods are implemented with multimode fiber Bragg gratings (MMFBG) and methods for sensor addressing and multiplexing are described. Real time structural strain and vibration monitoring using optical time domain reflectometry (OTDR) is discussed. The first all-mode, wave optics calculations of the reflectivity of Bragg gratings in highly multimode optical fibers are presented, including the effects of individual mode power changes due to microbending transducer loss upstream from the grating. Two categories of multimode fiber gratings are possible, independent mode and mode coupling, and the fabrication differences between these types of gratings are described. The reflectivity properties of independent mode multimode fiber gratings are examined and compared to single mode fiber gratings. In particular, multimode fiber gratings offer added flexibility in grating design and performance characteristics compared to single mode fiber gratings, because the reflectivity response may be tuned by the spatial periodicity, length, core size, numerical aperture, and mode coupling characteristics of the grating.

Claims

1. An apparatus for sensing the mechanical disturbance of an optical fiber comprising a length of multi-mode optical fiber having a Bragg grating at a selected location in said fiber for reflecting light within a selected plurality of contiguous bandwidths;

a source of light having emission within said selected plurality of bandwidths, said source being optically coupled to said fiber at an end thereof for transmitting said light into said fiber;

an optical coupler coupled to said fiber between said source and said grating for directing light reflected from said grating to a location external of said fiber;

at least two optical detectors positioned for receiving light from said optical coupler;

a filter for directing light in at least one of said selected plurality of bandwidths to a first one of said detectors and for directing light in at least one other of said selected plurality of bandwidths to a second one of said detectors;

a comparator for comparing the respective magnitudes of light detected by said first and second detectors, respectively, and generating an output proportional to the ratio of said magnitudes, said ratio being a function of the mechanical disturbance of said fiber, and a device for indicating said output.

2. The apparatus recited in claim 1 wherein said filter is a bandpass filter for transmitting light within a selected first bandwidth and for reflecting light not within said selected first bandwidth.

3. The apparatus recited in claim 1 wherein said source of light is a light emitting diode.

4. The apparatus recited in claim 1 further comprising a transducer for inducing a bending of said fiber.

5. An apparatus for using the bending of an optical fiber for sensing a parameter, comprising:

a length of multi-mode optical fiber having a Bragg grating at a selected location in said fiber for reflecting light within a selected plurality of contiguous bandwidths;

a source of broadband light optically coupled to said fiber for transmitting light in at least two of said bandwidths;

a plurality of detectors coupled to said fiber for measuring the respective power of reflected light in at least said two bandwidths;

a comparator for determining the relative light power in said at least two bandwidths in accordance with bending imparted to said fiber, said comparator generating an output dependent on said relative light power;

an indicator for generating an observable manifestation of said output for measuring said bending; and

further comprising a filter for directing reflected light in at least one said bandwidth to a first corresponding detector and for directing reflected light in at least another said bandwidth to a second corresponding detector.

6. An apparatus for using the bending of an optical fiber for sensing a parameter, comprising:

a length of multi-mode optical fiber having a Bragg grating at a selected location in said fiber for reflecting light within a selected plurality of contiguous bandwidths;

a source of broadband light optically coupled to said fiber for transmitting light in at least two of said bandwidths;

a plurality of detectors coupled to said fiber for measuring the respective power of reflected light in at least said two bandwidths;

a comparator for determining the relative light power in said at least two bandwidths in accordance with bending imparted to said fiber, said comparator generating an output dependent on said relative light power;

an indicator for generating an observable manifestation of said output for measuring said bending; and

further comprising means for determining a shift in the frequency of a peak reflected light magnitude resulting from the bending of said optical fiber.

7. A sensing apparatus comprising a length of multi-mode optical fiber having a spaced, serial array of Bragg gratings, each such grating configured for reflecting light over a different bandwidth;

a mode scrambler for each such grating beyond the first grating of said array for re-equalizing modal power distribution;

a source of broadband light coupled to said fiber;

an optical coupler coupled to said fiber for redirecting light reflected by said gratings;

a plurality of detectors, each such detector configured for receiving light within a selected bandwidth from said optical coupler for determining the modal reflected power distribution for each such grating; and

means for indicating the respective modal reflected power distribution of said detectors.

8. The apparatus recited in claim 7 further comprising a bulk grating positioned between said optical coupler and said detectors for separating said redirected reflected light by respective wavelengths.

9. The apparatus recited in claim 7 wherein said source of broadband light is a light emitting diode.

10. The apparatus recited in claim 7 comprising a plurality of fiber bending transducers, at least one such transducer being associated with each said Bragg grating for inducing a bending of said fiber.

11. A method for sensing the strain of an optical fiber;

the method comprising the steps of;

a) treating a multi-mode optical fiber to provide at least a pair of spaced-apart Bragg gratings to form an interferometric pattern of reflected light to detect phase-shifts in said optical fiber between said gratings;

b) coupling a broadband light source into the fiber;

c) determining the phase of the reflected light pattern in each of a plurality of bandwidths;

d) comparing the phase of the measured light patterns in at least two such bandwidths as a function of fiber strain; and

e) generating an indication of the relative phases of measured light patterns in said at least two bandwidths.

12. A sensor comprising:

a multimode optical fiber having at least one Bragg grating formed therein;

a broadband source of light coupled to said fiber;

at least one bandwidth limited detector for detecting reflected modal power in said fiber, whereby changes in modal power distribution within the bandwidth of said detector indicate a sensed parameter; and

means for indicating the detected reflected modal power in said fiber for measuring said sensed parameter.

13. The sensor recited in claim 12 wherein said detector is wavelength dependent.