Extrinsic fiber optic displacement sensors and displacement sensing systems

A quadrature phase-shifted extrinsic fiber optic sensor for the detection of the amplitude and the relative polarity of dynamically varying displacement comprising:

• first and second optical fibers inserted into one end of at least one hollow silica tube;
• a reflector, ends of said first and second optical fibers and said reflector within said at least one hollow silica tube forming first and second air gaps that act as cavities;
• a light source supplying light to said first and second optical fibers, said first and second optical fibers being used as input/output fibers, a Fresnel reflection from a glass/air interface at a front of each air gap serving as reference reflections and a reflection from an air/glass interface at a far end of each air gap serving as sensing reflections, said reference and sensing reflections interfering in the first and second fibers;
• first and second coupler means respectively connected to said first and second optical fibers for coupling reflections to respective first and second detectors, changes in said first and second air gap lengths causing changes in phase differences between respective reference reflections and sensing reflections and changes intensities of the light respectively monitored by said first and second detectors, gap separations for said first and second optical fibers being initially adjusted by moving the optical fibers in and out of said at least one silica tube until a 90.degree. phase shift is achieved at outputs of said first and second detectors;
• and
• means connected to said first and second detectors for monitoring both amplitude and relative polarity of dynamically varying strain by measuring amplitudes of signals from said first and second detectors and tracking lead-lag reversals of said signals.

A system for sensing displacements at a plurality of locations in a mechanical structure comprising:

• a plurality of extrinsic fiber optic sensors mounted or embedded in said mechanical structure for the detection of the amplitude of dynamically varying displacement, each said plurality of extrinsic fiber optic sensors comprising:
• an optical fiber inserted into one end of a hollow silica tube;
• a reflector inserted in an opposite end of said hollow silica tube, and end of said optical fiber and said reflector within said at least one hollow silica tube forming an air gap, said optical fiber being axially displaced relative to said reflector by a dynamically varying displacement and said air gap varying in axial length due to the relative displacement of said optical fiber, said optical fiber being used as an input/output fiber, a Fresnel reflection from a glass/air interface at a front of said air gap serving as a reference reflection and a reflection from an air/glass interface at a far end of each air gap serving as a sensing reflection, said reference and sensing reflections interfering in said optical fiber;
• means for coupling said plurality of extrinsic fiber optic sensors in cascade;
• multiplexing means for addressing and sensing an output of each of said plurality of extrinsic fiber optic sensors;
• a light source controlled by said multiplexing means for supplying light to said optical fiber of each of said plurality of extrinsic optical sensors;
• and
• coupler means connected to said optical fiber for coupling reflections to a detector, changes in said air gap length of a sensor causing changes in phase differences between said reference reflection and sensing reflection and changes the intensity of light monitored by said detector, said detector providing an output to said multiplexing means.