Optical sensing device containing fiber bragg gratings

• US 20020041722A1
• Filed: 07/31/2001
• Published: 04/11/2002
• Est. Priority Date: 08/01/2000
• Status: Active Grant

• Current Assignee: 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 the navy
• Inventors: Chang, Chia-Chen, Althouse, Bryan L., Todd, Michael D., Johnson, Gregg A.
• Granted Patent: US 6,674,928 B2
• Time in Patent Office: Days
• US Class Current: 385/12
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• CPC Class Codes: G01K 11/3206 at discrete locations in the fibre, e.g. using Bragg scattering
• CPC Class Codes: G01L 1/243 using means for applying force perpendicular to the fibre axis
• CPC Class Codes: G01L 1/246 using integrated gratings, e.g. Bragg gratings

Abstract

A new optical sensing device containing fiber Bragg gratings, a scanning bandpass filter, an interferometer and multiple photodetectors is disclosed. The present invention also describes a new system and method for fibre Bragg grating (FBG) sensor interrogation and multiplexing. The new system combines a scanning Fabry-Perot (SFP) bandpass filter used to wavelength-multiplex multiple gratings in a single fiber, and an unbalanced Mach-Zehnder fibre interferometer made with a 3×3 coupler to detect strain-induced wavelength shifts. A passive technique for interferometer drift compensation using non-sensing FBGs is included in the system. A complete prototype system interrogates four gratings in a single fiber at a Nyquist sampling rate up to 10 kHz, with a noise floor measured near 4 nε Hz^−½ above 0.1 Hz. The inclusion of the interferometer drift compensation technique is shown to make quasi-static measurements feasible.

Claims

1. A device for measuring a change in an environment comprising:

a first optical fiber having a plurality of fiber Bragg gratings, wherein each of the fiber Bragg gratings has a different predetermined reflection wavelength;

a broadband light source optically coupled to the optical fiber;

a scanning filter optically coupled to optical fiber;

an interferometer optically coupled to the scanning filter;

a first photodetector generating a first output, wherein the first photodetector is optically coupled to the interferometer;

a second photodetector generating a second output, wherein the second photodetector is optically coupled to the interferometer; and

a third photodetector generating a third output, wherein the third photodetector is optically coupled to the interferometer.

2. A device as claimed in claim 1 further comprising:

a demodulation unit connected to the first, second and third photodetectors for processing the first output from the first photodetector and the second output from the second photodetector and generating a measurement of the change in the environment.

3. A device as claimed in claim 2, wherein the demodulation unit further comprises

means for computing a tangent of a phase of a phase signal using the outputs from the interrogation system;

means for computing an arctangent of the tangent of the phase of the phase signal; and

means for phase-unwrapping the arctangent.

4. A device as claimed in claim 1 wherein the broadband light source and the scanning filter are optically coupled to the first optical fiber via a first 2×

2 coupler.

5. A device as claimed in claim 4 further comprising a second optical fiber having at least one unattached fiber Bragg grating, wherein the second optical fiber is optically coupled to the broadband light source and the scanning filter via the first 2×

2 coupler, wherein the at least one unattached fiber Bragg grating is isolated from the environment, and wherein the at least one unattached fiber Bragg grating experiences a temperature variation which is the same as the fiber Bragg gratings of the first optical fiber.

6. A device as claimed in claim 1 further comprising a fourth photodetector, wherein the interferometer is optically coupled to the scanning filter through a splitter and a second 2×

2 coupler, wherein the splitter and the second 2×

2 coupler are optically coupled, and wherein the fourth photodetector is optically coupled to the splitter and generates a fourth output.

7. A device as claimed in claim 6 further comprising:

a first peak detector connected to the first photodetector and generating a first peak signal upon receiving the first output from the first photodetector;

a second peak detector connected to the second photodetector and generating a second peak signal upon receiving the second output from the second photodetector; and

a third peak detector connected to the third photodetector and generating a third peak signal upon receiving the third output from the third photodetector;

an A/D board connected to the first, second and third peak detectors and receiving the first, second, and third peak signal from the first, second and third peak detectors respectively;

a first comparator which receives the fourth output from the fourth photodetector and a predetermined voltage, wherein the first comparator is connected to the A/D board and a one-shot circuit which is further connected to the first, second and third peak detectors, whereby upon the fourth output of the fourth photodetector falling below the predetermined voltage, the first comparator triggers the one-shot circuit to reset the first, second and third peak detectors and allows the A/D board to start converting the first, second and third peak signals into digital signals with a built-in delay.

8. A device as claimed in claim 1, wherein the scanning filter is a scanning Fabry-Perot filter having a passband wavelength, the passband wavelength being controlled by a triangular voltage wave.

9. A device as claimed in claim 1, wherein the interferometer is a Mach-Zehnder interferometer comprising a first optical path and a second optical path.

10. A device as claimed in claim 1, wherein the interferometer is optically coupled to the first, second and third photodetectors through a 3×

3 coupler.

11. An interrogating system for a fiber optical sensor having fiber Bragg gratings comprising:

a scanning filter optically coupled to the fiber optical sensor;

an interferometer optically coupled to the output port of the scanning filter;

a first photodetector optically coupled to the interferometer;

a second photodetector optically coupled to the interferometer; and

a third photodetector optically coupled to the interferometer.

12. An interrogation system as claimed in claim 11 further comprising a 3×

3 coupler, wherein the interferometer is optically coupled to the first, second and third photodetectors through the 3×

3 coupler.

13. An interrogation system as claimed in claim 11, wherein the interferometer comprises two optical paths having different optical lengths.

14. An interrogation system as claimed in claim 13, wherein a difference in optical length between the two optical paths is about 2.75 mm.

15. An interrogation system as claimed in claim 11, wherein the scanning filter is a tunable Fabry-Perot filter having a passband wavelength which is regulated through a triangular voltage wave.

17. A method as claimed in claim 16, wherein the scanning filter is a tunable Fabry-Perot filter having a passband wavelength regulated by a triangular voltage wave.

18. A method as claimed in claim 16, wherein the interferometer comprises two optical paths with different optical lengths.

19. A method as claimed in claim 16, wherein the demodulating step is carried out using a demodulator comprising:

means for computing a tangent of a phase of the transmitted light using the voltage signals;

means for computing an arctangent of the tangent of the phase of the transmitted light; and

means for phase-unwrapping the arctangent to obtain the phase of the transmitted light.

16. A method for measuring a change in an environment comprising the steps of:

irradiating a fiber having a plurality of fiber Bragg gratings with different center wavelengths with a broadband light to generate a reflected light from each of the Bragg gratings, wherein each of the reflected lights is reflected from a specific fiber Bragg grating and has a wavelength which is the sum of the center wavelength characteristic of the specific fiber Bragg grating and a wavelength shift based on the change in the environment;

filtering one of the reflected lights through the scanning filter at a time;

encoding the wavelength of the reflected light from the scanning filter into a phase signal embedded in at least three interference lights using an interferometer;

converting the at least three interference lights into at least three voltage signals using at least three photodetectors;

demodulating the at least three voltage signals to obtain the phase of the phase signal;

calculating the wavelength shift of the transmitted light from the phase of the phase signal; and

determining the change in the environment from the wavelength shift.

20. A demodulating unit for processing outputs from an interrogation system for an optical sensing device having fiber Bragg gratings comprising:

means for determining a tangent of a phase of a reflected light from one of the fiber Bragg gratings from the reflected light passing through the interrogation system;

means for determining an arctangent of the tangent of the phase of the reflected light; and

means for phase-unwrapping the arctangent to obtain the phase of the transmitted light.

21. A demodulating unit as claimed in claim 20, wherein the means for determining the tangent, the means for determining the arctangent and the means for phase-unwrapping are encoded on a programmable chip.

22. A demodulating unit as claimed in claim 20, wherein the means for determining the tangent, the means for determining the arctangent and the means for phase-unwrapping are programs residing in a computer.

23. A demodulating unit as claimed in claim 20, wherein each of the means for determining the tangent, the means for determining the arctangent and the means for phase-unwrapping is an encoded chip.

25. A method as claimed in claim 24, where the steps of determining the tangent, determining the arctangent and phase-unwrapping the arctangent are carried out by at least one chip encoded with at least one of the steps.

26. A method as claimed in claim 24, where the steps of determining the tangent, determining the arctangent and phase-unwrapping the arctangent are carried out by a computer containing at least one program encoded with at least one of the steps.

24. A method for processing outputs from an interrogation system for an optical sensing device having fiber Bragg gratings comprising the steps of:

determining a tangent of a phase of a phase signal using the outputs from the interrogating system;

determining an arctangent of the tangent of the phase of the phase signal; and

phase-unwrapping the arctangent.