Optical fiber strain sensor with improved linearity range

• US 5,381,493 A
• Filed: 12/21/1992
• Issued: 01/10/1995
• Est. Priority Date: 12/21/1992
• Status: Expired due to Fees

• Current Assignee: The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Rogowski, Robert S., Egalon, Claudio O.
• Primary Examiner(s): FONT, FRANK G
• Application Number:
  US08/223,931
• Time in Patent Office: 750 Days
• Field of Search: 385/13, 385/3, 385/12, 385/122, 385/123, 385/28, 250/227.14, 250/227.16
• US Class Current: 385/13
• CPC Class Codes: G01D 5/35351 using other means to detect the measured quantity
• CPC Class Codes: G01L 1/242 the material being an optical fibre
• CPC Class Codes: G02B 6/02047 Dual mode fibre G02B6/105 takes precedence

Abstract

A strain sensor is constructed from a two mode optical fiber. When the optical fiber is surface mounted in a straight line and the object to which the optical fiber is mounted is subjected to strain within a predetermined range, the light intensity of any point at the output of the optical fiber will have a linear relationship to strain, provided the following equation is less than 0.17 radians ##EQU1## where n_co represents the refractive index of the core, k represents the wavenumber of the light, L represents the length of the optical fiber, S₁ represents axial strain, V is ##EQU2## U is a solution to the eigenvalue equation of the optical fiber, v_f represents the Poisson ratio, P_ef represents the effective strain-optic coefficient of the optical fiber and n_ln represents ##EQU3## where K₁ is the modified Bessel function of second kind of order/with argument given by

Claims

1. A strain sensor, comprising:

a two-mode optical fiber constructed of materials producing an intermodal phase difference δ

φ

₀₁ -δ

φ

₁₁ of less than 0.17 radians when subjected to stress within a measurable range of the strain sensor;

light supply means for supplying light to a first end of said optical fiber; and

light intensity detection means for detecting light intensity of the point at a second end of said optical fiber.

2. A strain sensor as recited in claim 1, wherein said light intensity detection means comprises:

conversion means for converting the light intensity to an electrical signal; and

means for transporting light emerging from the point at the second end of said optical fiber to said conversion means.

3. A strain sensor as recited in claim 1, wherein said optical fiber is constructed of material which when subjected to strain within a predetermined range has characteristic parameters for δ

φ

₀₁ -δ

φ

₁₁ to be less than 0.17 radians when defined as ##EQU10## where n_co represents the refractive index of the core, k represents the wavenumber of the light, L represents the length of said optical fiber, S₁ represents axial strain, V is ##EQU11## U is a solution to the eigenvalue equation of said optical fiber, v_f represents the Poisson ratio, P_ef represents the effective strain-optic coefficient of said optical fiber and n_ln represents ##EQU12## where K_l is the modified Bessel function of second kind of order/with argument given by ##EQU13##

4. A strain sensor, comprising:

a single mode optical fiber constructed of materials producing a phase of a fundamental mode δ

φ

₀₁ of less than 0.17 radians when subjected to stress within a measurable range of the strain sensor;

light supply means for supplying light to a first end of said optical fiber; and

light intensity detection means for detecting light intensity of the point at a second end of said optical fiber.