Sensing apparatus and method for detecting strain between fiber bragg grating sensors inscribed into an optical fiber

• US 5,748,312 A
• Filed: 09/19/1995
• Issued: 05/05/1998
• Est. Priority Date: 09/19/1995
• Status: Expired due to Term

• Current Assignee: The United States of America As Represented By The Secretary of Agriculture, the united states of america as represented by the secretary of the navy
• Original Assignee: The United States of America As Represented By The Secretary of Agriculture
• Inventors: Kersey, Alan D., Davis, Michael A.
• Primary Examiner(s): Turner, Samuel A.
• Application Number:
  US08/530,044
• Time in Patent Office: 959 Days
• Field of Search: 356/345, 356/352, 356/32, 356/33, 356/35.5, 250/227.13, 250/227.18, 250/227.23, 73/800
• US Class Current: 356/478
• CPC Class Codes: G01D 5/35303 using a reference fibre, e.g. interferometric devices
• CPC Class Codes: G01D 5/35316 using a Bragg gratings
• CPC Class Codes: G01D 5/35354 Sensor working in reflection
• CPC Class Codes: G01L 1/246 using integrated gratings, e.g. Bragg gratings

Abstract

A sensing apparatus and related method for sensing strain between fiber Bragg grating sensors inscribed in an optical fiber. The fiber is inscribed with a first fiber Bragg grating sensor and a second fiber Bragg grating sensor. A strain detection unit detects strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor based on the optical propagation time of light projected into the fiber and reflected by the first fiber Bragg grating sensor and the optical propagation time of light projected into the fiber and reflected by the second fiber Bragg grating sensor. More specifically, light projected into the fiber is modulated with an RF signal. A phase change of the RF signal is determined from the light projected into the fiber and reflected by the first fiber Bragg grating sensor and from the light projected into the fiber and reflected by the second fiber Bragg grating sensor. The phase change of the RF signal is related to the optical propagation time of light projected into the fiber and reflected by the first and second fiber Bragg grating sensors. The strain between the first and second fiber Bragg grating sensors is then determined from the detected phase changes.

Claims

1. A sensing apparatus comprising:

a fiber inscribed with a first fiber Bragg grating sensor and a second fiber Bragg grating sensor; and

a strain detection unit detecting the strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor;

wherein the strain detection unit detects the strain on the first fiber Bragg grating sensor and the strain on the second fiber Bragg grating sensor in addition to the strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor.

2. A sensing apparatus comprising:

a fiber inscribed with a first fiber Bragg grating sensor and a second fiber Bragg grating sensor; and

a strain detection unit detecting the strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor;

wherein the first fiber Bragg grating sensor and the second fiber Bragg grating sensor each have a corresponding Bragg wavelength and reflect light which is at approximately the corresponding Bragg wavelength, the strain detection unit adapted for;

projecting light into the fiber at approximately the Bragg wavelength corresponding to the first fiber Bragg grating sensor, effective to cause the light at approximately the Bragg wavelength of the first fiber Bragg grating sensor to be reflected by the first fiber Bragg grating sensor,

projecting light into the fiber at approximately the Bragg wavelength corresponding to the second fiber Bragg grating sensor, effective to cause the light at approximately the Bragg wavelength of the second fiber Bragg grating sensor to be reflected by the second fiber Bragg grating sensor, and

determining the strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor based on the optical propagation time of the light projected into the fiber and reflected by the first fiber Bragg grating sensor and the optical propagation time of the light projected into the fiber and reflected by the second fiber Bragg grating sensor.

3. A sensing apparatus as in claim 2, wherein the strain detection unit comprises a light source for projecting light into the fiber, the light source being one of a tunable laser and a broadband light source.

4. A sensing apparatus as in claim 2, wherein the strain detection unit further comprises:

a modulator modulating the light projected into the fiber at approximately the Bragg wavelength corresponding to the first fiber Bragg grating sensor and the light projected into the fiber at approximately the Bragg wavelength corresponding to the second fiber Bragg grating sensor, with an RF signal;

a phase detector receiving the RF signal as a reference signal and adapted for

detecting, with respect to the reference signal, the phase of the RF signal modulated on the light projected into the fiber at approximately the Bragg wavelength corresponding to the first fiber Bragg grating sensor and reflected by the first fiber Bragg grating sensor, and

detecting, with respect to the reference signal, the phase of the RF signal modulated on the light projected into the fiber at approximately the Bragg wavelength corresponding to the second fiber Bragg grating sensor and reflected by the second fiber Bragg grating sensor; and

a strain computation unit computing the strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor from the detected phase of the RF signal modulated on the light projected into the fiber and reflected by the first fiber Bragg grating sensor and the detected phase of the RF signal modulated on the light projected into the fiber and reflected by the second fiber Bragg grating sensor.

5. A sensing apparatus as in claim 4, wherein the strain detection unit further comprises:

a tunable laser for projecting light into the fiber and being selectively tunable to respective frequencies; and

a laser controller detecting the wavelength of the light reflected by the first fiber Bragg grating sensor and using the detected wavelength to selectively tune the tunable laser to the Bragg wavelength of the second fiber Bragg grating sensor.

6. A sensing apparatus as in claim 5, wherein the laser controller comprises:

a peak detector detecting the intensity of the light reflected by the first fiber Bragg grating sensor; and

a wavelength controller tuning the tunable laser in accordance with the intensity of the light reflected by the first fiber Bragg grating sensor and detected by the peak detector.

7. A sensing apparatus comprising:

a fiber inscribed with a first fiber Bragg grating sensor and a second fiber Bragg grating sensor; and

a strain detection unit detecting the strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor;

wherein the fiber is inscribed with a plurality of sequentially arranged fiber Bragg grating sensors and the strain detection unit detects the strain between respective adjacent fiber Bragg grating sensors of the plurality of sequentially arranged fiber Bragg grating sensors.

8. A sensing apparatus comprising:

a fiber inscribed with

a first fiber Bragg grating sensor having a corresponding Bragg wavelength and which is effective to reflect light at approximately the corresponding Bragg wavelength, and

a second fiber Bragg grating sensor having a corresponding Bragg wavelength and which is effective to reflect light at approximately the corresponding Bragg wavelength;

a light source adapted for

projecting light into the fiber at approximately the Bragg wavelength corresponding to the first fiber Bragg grating sensor, effective to cause the light at approximately the Bragg wavelength of the first fiber Bragg grating sensor to be reflected by the first fiber Bragg grating sensor, and

projecting light into the fiber at approximately the Bragg wavelength corresponding to the second fiber Bragg grating sensor, effective to cause the light at approximately the Bragg wavelength of the second fiber Bragg grating sensor to be reflected by the second fiber Bragg grating sensor;

a modulator modulating the light projected into the fiber at approximately the Bragg wavelength corresponding to the first fiber Bragg grating sensor and the light projected into the fiber at approximately the Bragg wavelength corresponding to the second fiber Bragg grating sensor, with an RF signal;

a phase detector receiving the RF signal as a reference signal and adapted for

detecting, with respect to the reference signal, the phase of the RF signal modulated on the light projected into the fiber at approximately the Bragg wavelength corresponding to the first fiber Bragg grating sensor and reflected by the first fiber Bragg grating sensor, and

detecting, with respect to the reference signal, the phase of the RF signal modulated on the light projected into the fiber at approximately the Bragg wavelength corresponding to the second fiber Bragg grating sensor and reflected by the second fiber Bragg grating sensor; and

a strain computation unit computing the strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor from the detected phase of the RF signal modulated on the light projected into the fiber and reflected by the first fiber Bragg grating sensor and the detected phase of the RF signal modulated on the light projected into the fiber and reflected by the second fiber Bragg grating sensor.

9. A method for sensing strain on a fiber, comprising:

providing a fiber inscribed with a first fiber Bragg grating sensor and a second fiber Bragg grating sensor; and

detecting the strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor;

wherein the first fiber Bragg grating sensor and the second fiber Bragg grating sensor each have a corresponding Bragg wavelength and reflect light which is at approximately the corresponding Bragg wavelength, the method further comprising;

projecting light into the fiber at approximately the Bragg wavelength corresponding to the first fiber Bragg grating sensor, effective to cause the light at approximately the Bragg wavelength of the first fiber Bragg grating sensor to be reflected by the first fiber Bragg grating sensor;

projecting light into the fiber at approximately the Bragg wavelength corresponding to the second fiber Bragg grating sensor, effective to cause the light at approximately the Bragg wavelength of the second fiber Bragg grating sensor to be reflected by the second fiber Bragg grating sensor; and

determining the strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor based on the optical propagation time of the light projected into the fiber and reflected by the first fiber Bragg grating sensor and the optical propagation time of the light projected into the fiber and reflected by the second fiber Bragg grating sensor.

10. A method as in claim 9, further comprising:

modulating the light projected into the fiber at approximately the Bragg wavelength corresponding to the first fiber Bragg grating sensor and the light projected into the fiber at approximately the Bragg wavelength corresponding to the second fiber Bragg grating sensor, with an RF signal;

detecting, with respect to the RF signal as a reference signal, the phase of the RF signal modulated on the light projected into the fiber at approximately the Bragg wavelength corresponding to the first fiber Bragg grating sensor and reflected by the first fiber Bragg grating sensor;

detecting, with respect to the RF signal as a reference signal, the phase of the RF signal modulated on the light projected into the fiber at approximately the Bragg wavelength corresponding to the second fiber Bragg grating sensor and reflected by the second fiber Bragg grating sensor; and

computing the strain between the first fiber Bragg grating sensor and the second fiber Bragg grating sensor from the detected phase of the RF signal modulated on the light projected into the fiber and reflected by the first fiber Bragg grating sensor and the detected phase of the RF signal modulated on the light projected into the fiber and reflected by the second fiber Bragg grating sensor.

11. A method as in claim 10, wherein a light source is selectively tunable to project light into the fiber at different frequencies, the method further comprising:

detecting the wavelength of the light reflected by the first fiber Bragg grating sensor; and

using the detected wavelength to selectively tune the light source to the Bragg wavelength of the second fiber Bragg grating sensor.

12. A method as in claim 10, further comprising:

detecting the intensity of the light reflected by the first fiber Bragg grating sensor; and

tuning the light source in accordance with the detected intensity.