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

• US 4,740,078 A
• Filed: 01/14/1985
• Issued: 04/26/1988
• Est. Priority Date: 01/24/1984
• Status: Expired due to Fees

• Current Assignee: Mettler-Toledo AG IM Langacher Greifensee Switzerland
• Original Assignee: Mettler Instrumente AG (Mettler-Toledo International Inc.)
• Inventors: Bertholds, Axel, Daendliker, Rene
• Primary Examiner(s): Rosenberger, R. A.
• Assistant Examiner(s): Cooper, Crystal D.
• Application Number:
  US06/690,997
• Time in Patent Office: 1,198 Days
• Field of Search: 356/73.1, 356/33, 356/35.5, 356/364, 356/365, 356/367, 356/366, 356/345, 250/225
• US Class Current: 356/35.5
• CPC Class Codes: G01L 1/243 using means for applying force perpendicular to the fibre axis

Abstract

The force measuring apparatus includes a single-mode optical fiber having a defined anisotropy, whereby coherent light suppled to one end of the optical fiber is guided as two waves having linear orthogonal polarizations, respectively. The optical fiber is arranged between a pair of parallel planar pressure plates, the orientation of the optical fiber being such that the aforementioned linear polarizations are parallel with and normal to the remote surfaces of the pressure plates, respectively. Upon the application of a force to be measured in the compressive direction normal to one remote surface of a plate, a phase shift is produced between the waves which is a function of the magnitude of the load to be measured, as determined by interferometer means arranged at the other end of the optical fiber. The optical fiber may be of a helical configuration having turns that are contained between successive pairs of pressure plates, respectively, arranged in a stack, thereby achieving a multiplication of the measuring effect with increased accuracy of the corresponding measurement.

Claims

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.

2. Force measuring apparatus, comprising

(a) an anisotropic single-mode optical fiber (1) of well-defined anisotropy having inlet and outlet ends;

(b) a stack of parallel spaced annular pressure plates (25-29), said optical fiber having a helical configuration defining turns (21-24) contained between, and in contiguous relation with, the adjacent faces of successive pairs of said plates, respectively;

(c) laser means (4) for introducing coherent light into one end of said optical fiber, thereby exciting in said optical fiber, due to birefringence caused by the optical anisotropy, a pair of linearly polarized light waves having orthogonal planes of oscillation, said optical fiber turns being so arranged relative to said plates that said planes of oscillation are parallel with and normal to the remote parallel surfaces of said pressure plate stack, respectively, whereby when a compressive force to be measured (F) is applied in a direction normal to one of said remote surfaces, an additional phase shift is produced between said light waves by 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.

3. Force measuring apparatus, comprising:

(a) a single-mode optical fiber (1) having inlet and outlet end portions, and an intermediate portion of non-circular cross-section 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 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 intermediate portion, respectively;

(c) laser means (4) for introducing coherent light into said optical fiber inlet end portion, thereby exciting in said optical fiber, due to the birefringence caused by the optical anisotropy, a pair of linearly polarized light waves having orthogonal planes of oscillation, said optical fiber being so arranged relative to said plates that said planes of oscillation are parallel with and normal to the remote parallel surfaces of 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 by the stress-induced birefingence; 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.

4. Apparatus as defined in claim 3, wherein at least a portion of said optical fiber intermediate portion between said pressure plates is curved and contained in a given plane for lateral compression by said pressure plates.