Fiber optic vibration sensor

• US 5,381,492 A
• Filed: 02/15/1994
• Issued: 01/10/1995
• Est. Priority Date: 02/15/1994
• Status: Expired due to Fees

• Current Assignee: Martin Marietta Energy Systems, Inc. (Martin Marietta Corporation)
• Original Assignee: Martin Marietta Energy Systems, Inc. (Martin Marietta Corporation)
• Inventors: Tobin, Kenneth W., Dooley, Joseph B., Muhs, Jeffrey D.
• Primary Examiner(s): Healy, Brian
• Application Number:
  US08/196,963
• Time in Patent Office: 329 Days
• Field of Search: 385/12, 385/13, 385/15, 385/27, 385/28, 385/30, 385/31, 385/32, 385/42, 385/43, 385/36, 385/46, 385/48, 385/49, 356/345, 250/227.11, 250/227.14, 250/227.16, 250/227.18, 250/227.19, 250/227.23, 73/800, 73/795
• US Class Current: 385/12
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125

Abstract

A fiber optic vibration sensor utilizes two single mode optical fibers supported by a housing with one optical fiber fixedly secured to the housing and providing a reference signal and the other optical fiber having a free span length subject to vibrational displacement thereof with respect to the housing and the first optical fiber for providing a signal indicative of a measurement of any perturbation of the sensor. Damping or tailoring of the sensor to be responsive to selected levels of perturbation is provided by altering the diameter of optical fibers or by immersing at least a portion of the free span length of the vibration sensing optical fiber into a liquid of a selected viscosity.

Claims

1. A fiber optic vibration sensor comprising a housing, first elongated optical fiber means having first and second oppositely disposed end regions and fixedly supported over substantially the full length thereof to said housing, second elongated optical fiber means having first and second oppositely disposed end regions fixedly supported by said housing and a central region spatially separated from said housing for relative movement thereof with respect to both the housing and the first optical fiber means, light source means for providing a beam of light at a selected frequency, light beam splitting means coupled the first end region of the first and second optical fiber means and adapted to receive the beam of light for splitting the beam of light into first and second light beams for the respective transmission thereof along optical paths through the first and second optical fiber means, light beam coupling means connected to the second end region of the first and second optical fiber means for receiving and combining the total amount of light transmitted through the first and second optical fiber means, light detecting means connected to the light beam coupling means for receiving the light therefrom and providing signals indicative of any changes in the frequency of light transmitted through the first and second fiber means as caused by a change in the optical path of light transmitted through the second optical fiber means upon said relative movement of the second optical fiber means, and fringe counting means connected to the light detecting means for receiving the signals therefrom and for providing a signal indicative of any change in the frequency of light transmitted through the first and second optical fiber means.

2. A fiber optic vibration sensor as claimed in claim 1, wherein the light source means is a laser.

3. A fiber optic vibration sensor as claimed in claim 1, wherein said first and second optical fiber means are single mode glass fibers.

4. A fiber optic vibration sensor as claimed in claim 3, wherein said single mode glass fibers are of a diameter in the range of about 50 to 500 micrometers.

5. A fiber optic vibration sensor as claimed in claim 1, wherein said housing comprises first and second elongated sections having first and second end regions and spatially separated from one another by a third elongated section disposed substantially perpendicular to the first and second sections and contacting the first end region of the first and second sections for defining an open space extending between the second end region of the first and second sections, wherein the first and second end regions of the second optical fiber means are supported by the second end region of the first and second elongated sections with said central region of the second optical fiber means being contained in said open space.

6. A fiber optic vibration sensor as claimed in claim 5, wherein said central region of the second optical fiber means contains at least one complete loop, and wherein the at least one loop contained in the central region of the optical fiber means is spatially separated from the housing for relative movement with respect the housing and the first optical fiber means fixedly supported by said housing.

7. A fiber optic vibration sensor as claimed in claim 6, wherein said first and second optical fiber means are single mode glass fibers, and wherein said light source means is a laser.

8. A fiber optic vibration sensor as claimed in claim 5, wherein the first optical fiber means is fixedly supported by the end sections and the central section of the housing, and wherein the first and second optical fiber means are disposed in a substantially common vertical plane with second optical fiber means overlying at least the central section of the housing.

9. A fiber optic vibration sensor as claimed in claim 5, wherein the end sections of the housing are substantially vertically oriented, and wherein the end sections and the central section of the housing define receptacle means for containing a pool of liquid, and wherein at least a portion of the central region of the second optical fiber means is adapted to be contained in the pool of liquid.

10. A fiber optic vibration sensor as claimed in claim 9, wherein the liquid in said pool of liquid is of a selected viscosity for selectively damping the sensitivity of the second optical fiber means to movement thereof with respect to the housing and the first optical fiber means.

11. A fiber optic vibration sensor as claimed in claim 9, wherein substantially the entire central region of the second optical fiber means is contained in the pool of liquid.

12. A fiber optic vibration sensor as claimed in claim 9, wherein said at least a portion of the central region of the second optical fiber means contains at least one complete loop, wherein said at least one loop is contained in the pool of liquid, and wherein the at least one loop contained in the central region of the optical fiber means is spatially separated from the housing for relative movement with respect the housing and the first optical fiber means fixedly supported by said housing.

13. A fiber optic vibration sensor as claimed in claim 5, wherein cover means are supported by the first and second end regions of the housing for defining an enclosed volume, wherein at least the central region of the second optical fiber means are contained in the enclosed volume, and wherein fill tube means communicate with the enclosed volume for admitting a gas or mixture of gases into the enclosed volume for pressurizing the latter to a selected pressure for selectively damping the sensitivity of at least the central region of the second optical fiber means to movement thereof with respect to the housing and the first optical fiber means.

14. A fiber optic vibration sensor as claimed in claim 1, wherein the fringe counting means is adapted to measure any change in the frequency of light transmitted through the first and second optical fiber means, and wherein said signal from the optical fiber means is indicative of said change in the frequency of light transmitted through the first and second optical fiber means.