Optical fiber strain sensor

1. A method of strain sensing comprising the steps of:

providing an optical fiber having a fiber Bragg grating (FBG) formed therein;

subjecting the optical fiber to a strain inducing force such that a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends; and

optically interrogating the FBG to determine a measure of a change in bandwidth of the FBG as a result of the compression and extension of the grating periods in the first and second portion respectively,

whereby the measure of the change in the bandwidth is representative of the strain induced, and

wherein the optical fiber is subjected to the strain inducing force such that a grating period in a third portion of the FBG remains unchanged.

2. The method as claimed in claim 1 , wherein the FBG, in a quiescent state, has a uniform period across the first and second portions.

3. The method as claimed in claim 1 , comprising providing an optical fiber having a plurality of FBGs formed therein and spaced apart along a length of the optical fiber;

subjecting the optical fiber to a plurality of strain inducing forces such that, for each FBG, a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends; and

optically interrogating the FBGs to determine a measure of changes in bandwidth of the respective FBGs as a result of the compression and extension of the grating periods in the first and second portion respectively,

whereby the measure of the changes in the bandwidth is representative of the strains induced.

4. The method as claimed in claim 1 , comprising measuring a reflected optical power from the FGB as the measure for the change in the bandwidth.

5. An optical fiber strain sensor comprising:

an optical fiber;

an FBG formed in the optical fiber;

a packaging structure embedding the optical fiber such that, if the optical fiber is subjected to a strain inducing force, a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends; and

an interrogation system coupled to the optical fiber for optically interrogating the FBG to determine a measure of a change in bandwidth of the FBG as a result of the compression and extension of the grating periods in the first and second portion respectively,

whereby the measure of the change in the bandwidth is representative of the strain induced, and

wherein the packaging structure embeds the optical fiber such that, if the optical fiber is subjected to the strain inducing force, a grating period in a third portion of the FBG remains unchanged.

6. The sensor as claimed in claim 5 , wherein the FBG, in a quiescent state, has a uniform period across the first and second portions.

7. The sensor as claimed in claim 5 , wherein the packaging structure comprises a composite laminate structure.

8. The sensor as claimed in claim 7 , wherein the composite laminate structure comprises fiber-reinforced carbon composite material prepregs.

9. The sensor as claimed in claim 7 , wherein the composite laminate structure is asymmetric with respect to the FBG.

10. The sensor as claimed in claim 5 , wherein the optical fiber has a plurality of FBGs formed therein and spaced apart along a length of the optical fiber; and the packaging structure embeds the optical fiber such that, if the optical fiber is subjected to a plurality of strain inducing forces, for each FBG a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends; and

the interrogating system optically interrogates the FBGs to determine a measure of changes in bandwidth of the respective FBGs as a result of the compression and extension of the grating periods in the first and second portion respectively,

whereby the measure of the changes in the bandwidth is representative of the strains induced.

11. The sensor as claimed in claim 5 , further comprising a photo detector for measuring a reflected optical power from the FBG as the measure for the change in the bandwidth.

12. A method of fabricating an optical fiber strain sensor, the method comprising the steps of:

providing an optical fiber;

forming an FBG formed in the optical fiber;

embedding the optical fiber in a packaging structure such that, if the optical fiber is subjected to a strain inducing force, a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends,

wherein the packaging structure embeds the optical fiber such that, if the optical fiber is subjected to the strain inducing force, a grating period in a third portion of the FBG remains unchanged.

13. The method as claimed in claim 12 , further comprising coupling an interrogation system to the optical fiber for optically interrogating the FBG to determine a measure of a change in bandwidth of the FBG as a result of the compression and extension of the grating periods in the first and second portion respectively, whereby the measure of the change in the bandwidth is representative of the strain induced.

14. The method as claimed in claim 12 or 13 , wherein the FBG, in a quiescent state, has a uniform period across the first and second portions.

15. The method as claimed in claim 12 , wherein the packaging structure comprises a composite laminate structure.

16. The method as claimed in claim 15 , wherein the composite laminate structure comprises fiber-reinforced carbon composite material prepregs.

17. The method as claimed in claim 15 , wherein the composite laminate structure is asymmetric with respect to the FBG.

18. The method as claimed in claim 12 , comprising forming a plurality of FBGs in the optical fiber and spaced apart along a length of the optical fiber; and embedding portions of the optical fiber in respective packaging structures such that, if the optical fiber is subjected to a plurality of strain inducing forces, for each FBG a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends.

19. The method as claimed in claim 18 , wherein the interrogating system optically interrogates the FBGs to determine a measure of changes in bandwidth of the respective FBGs as a result of the compression and extension of the grating periods in the first and second portion respectively; whereby the measure of the changes in the bandwidth is representative of the strains induced.

20. The method as claimed in claim 12 , wherein the interrogation system comprises a photo detector for measuring a reflected optical power from the FBG as the measure for the change in the bandwidth.

21. A method of strain sensing comprising the steps of:

providing an optical fiber having a fiber Bragg grating (FBG) formed therein;

subjecting the optical fiber to a strain inducing force such that a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends; and

optically interrogating the FBG to determine a measure of a change in bandwidth of the FBG as a result of the compression and extension of the grating periods in the first and second portion respectively,

whereby the measure of the change in the bandwidth is representative of the strain induced,

wherein the method further comprises the steps of:

providing an optical fiber having a plurality of FBGs formed therein and spaced apart along a length of the optical fiber;

subjecting the optical fiber to a plurality of strain inducing forces such that, for each FBG, a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends; and

optically interrogating the FBGs to determine a measure of changes in bandwidth of the respective FBGs as a result of the compression and extension of the grating periods in the first and second portion respectively,

whereby the measure of the changes in the bandwidth is representative of the strains induced.

22. An optical fiber strain sensor comprising:

an optical fiber;

an FBG formed in the optical fiber;

a packaging structure embedding the optical fiber such that, if the optical fiber is subjected to a strain inducing force, a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends; and

an interrogation system coupled to the optical fiber for optically interrogating the FBG to determine a measure of a change in bandwidth of the FBG as a result of the compression and extension of the grating periods in the first and second portion respectively,

whereby the measure of the change in the bandwidth is representative of the strain induced,

wherein the optical fiber has a plurality of FBGs formed therein and spaced apart along a length of the optical fiber; and the packaging structure embeds the optical fiber such that, if the optical fiber is subjected to a plurality of strain inducing forces, for each FBG a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends, and

the interrogating system optically interrogates the FBGs to determine a measure of changes in bandwidth of the respective FBGs as a result of the compression and extension of the grating periods in the first and second portion respectively,

whereby the measure of the changes in the bandwidth is representative of the strains induced.

23. A method of fabricating an optical fiber strain sensor, the method comprising the steps of:

providing an optical fiber;

forming an FBG formed in the optical fiber;

embedding the optical fiber in a packaging structure such that, if the optical fiber is subjected to a strain inducing force, a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends; and

forming a plurality of FBGs in the optical fiber and spaced apart along a length of the optical fiber; and embedding portions of the optical fiber in respective packaging structures such that, if the optical fiber is subjected to a plurality of strain inducing forces, for each FBG a grating period in a first portion of the FBG compresses and a grating period in a second portion of the FBG extends.

24. The method as claimed in claim 23 , wherein the interrogating system optically interrogates the FBGs to determine a measure of changes in bandwidth of the respective FBGs as a result of the compression and extension of the grating periods in the first and second portion respectively, whereby the measure of the changes in the bandwidth is representative of the strains induced.