Fiber optic grating-based weighted, two-mode fiber sensors

• US 5,208,877 A
• Filed: 08/29/1991
• Issued: 05/04/1993
• Est. Priority Date: 08/29/1991
• Status: Expired due to Fees

• Current Assignee: Virginia Tech Intellectual Properties, Inc. (Virginia Polytechnic Institute and State University)
• Original Assignee: The Center For Innovative Technology, VPI & SU
• Inventors: Greene, Jonathan, Fogg, Brian, Vengsarkar, Ashish, Claus, Richard O., Murphy, Kent A.
• Primary Examiner(s): Ullah, Akm E.
• Application Number:
  US07/751,737
• Time in Patent Office: 614 Days
• Field of Search: 250/227.14-227.19, 385/1, 385/12, 385/13, 385/27, 385/28, 385/37
• US Class Current: 385/12
• CPC Class Codes: G01D 5/3538 using a particular type of fiber, e.g. fibre with several cores, PANDA fiber, fiber with an elliptic core or the like
• CPC Class Codes: G02B 6/02047 Dual mode fibre G02B6/105 takes precedence
• CPC Class Codes: G02B 6/02123 characterised by the method of manufacture of the grating photolithography G03F7/0005
• CPC Class Codes: G02B 6/14 Mode converters

Abstract

Two-mode, elliptical-core optic fibers with a permanent photo-induced index change are used as sensors with sensitivity varying as a function of length. The optic fiber sensors act as vibrational-mode filters thereby performing initial signal processing of the sensor signal. The sensors are based on photo-induced refractive index changes. These refractive index changes affect the differential phase modulation between the LP₀₁ and the LP₁₁^even modes. The change in beat-length is dependent on the amount of strain induced in the fiber while the grating is being formed. The pattern is thus varied along the length of the fiber by straining the fiber in a specific fashion while the grating is being written. This changes the sensitivity, of the sensor along its length. By choosing an appropriate weighting function in the manufacture of the sensor, it is possible to implement vibrational-mode analysis, vibrational-mode filtering and other functions that are critical in control system applications.

Claims

1. A two-mode, elliptical-core, vibration-modal optical fiber strain sensor with a permanent photo-induced index change forming a grating within the optical fiber characterized by a change in beat-length along the length of the optical fiber which is dependent on the amount of strain induced in the optical fiber while the grating is being formed, said grating being produced by differential phase modulation between LP₀₁ and LP₁₁^even modes of laser light propagating in said optical fiber while the fiber is under stress, said optical fiber sensor exhibiting a strain sensitivity which is a function of distance along the length of the optical fiber.

2. The optical fiber sensor recited in claim 1 wherein strain sensitivity of the optical fiber is tailored to a specific transducer application according to a weighting function for the application.

3. A method of manufacturing a two-mode, elliptical-core, vibration-modal optical fiber sensor with a permanent photo-induced index change forming a grating within the optical fiber comprising the steps of:

mounting an elliptical-core optical fiber to a beam; and

flexing said beam to induce a strain in the optical fiber while simultaneously exposing the fiber to a high power laser to form a grating within said optical fiber and producing a change in beat-length along the length of the optical fiber which is dependent on the amount of strain induced in the optical fiber while said grating is being formed.

4. The method recited in claim 3 further comprising the step of tailoring the sensor to a specific transducer application by selecting a weighting function for the application, said weighting function being a function of the strain induced in the optical fiber while said grating is being formed.