Fiber devices and sensors based on multimode fiber Bragg gratings

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.

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.

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.

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.

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.