Force measuring apparatus and method using stress-induced birefringence in a single-mode optical fiber

Claim 1

Force measuring apparatus, comprising: (a) a single-mode optical fiber (1) having inlet and outlet end portions, and a curved intermediate portion contained solely in a given plane, the curvature of said intermediate portion being such as to produce an inherent well-defined controlled anisotropy extending along the length of said intermediate portion, thereby to cause the resultant birefringence produced by the lateral compression of said intermediate portion to have the same orientation along the entire length thereof; (b) force-responsive means for laterally compressing said curved optical fiber intermediate portion, said force-responsive means including at least two parallel spaced pressure plates (2,3) having planar adjacent surfaces on opposite sides of, and in engagement with, said optical fiber curved portion, respectively; (c) laser means (4) for introducing coherent light into said optical fiber inlet end portion, thereby to excite in said optical fiber, due to the birefringence caused by the inherent optical anisotropy thereof, a pair of linearly polarized light waves having orthogonally-arranged planes of oscillation, said optical fiber being so arranged relative to said plates that said planes of oscillation are parallel with and normal to said adjacent faces of said pressure plates, respectively, whereby when a compressive force to be measured (F) is applied in a direction normal to said pressure plate adjacent faces, a phase shift is produced between said light waves as a function of the degree of change in the stress-induced birefringence; and (d) means including interferometer means (5-8) for measuring the magnitude of the applied force as a function of the phase shift between said light waves.