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A fiber optic sensor, which includes an interference energy analyzer, is used to measure strain and temperature distribution along a test fiber. The sensor includes the following: a plurality of double-Bragg grating elements positioned along a test fiber, a broadband light source which produces a br

A fiber optic sensor, which includes an interference energy analyzer, is used to measure strain and temperature distribution along a test fiber. The sensor includes the following: a plurality of double-Bragg grating elements positioned along a test fiber, a broadband light source which produces a broadband spectral profile that propagates along the test fiber, an optical filter that is able to change the parameters of the broadband spectral profile, an optical reflection detector, a fiber optic beamsplitter, and an interference energy analyzer. Each double-Bragg grating element consists of two weak Bragg gratings, separated by a distance unique to each element. The interference energy analyzer calculates the energies of the interference patterns, which are created by beams reflected from double-Bragg grating elements. The energy of the interference signal changes when the gratings in one element non-uniformly change its parameters due to non-equal temperature or strain influence on two gratings.

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1. A differential fiber optical sensor, comprising:a test fiber with a plurality of double-fiber grating elements positioned along the test fiber, all fiber gratings having overlapping reflection spectra, and each said double-fiber grating element consist of two fiber gratings, separated by a unique

1. A differential fiber optical sensor, comprising:a test fiber with a plurality of double-fiber grating elements positioned along the test fiber, all fiber gratings having overlapping reflection spectra, and each said double-fiber grating element consist of two fiber gratings, separated by a unique distance inherent for this element only,a light source to radiate light inside a measurement wavelength range, said light source producing a beam of light propagating along test fiber;an optical filter connected to the light source having possibility to change its central transmission wavelength inside said measurement wavelength range, the output radiation of said optical filter having a coherence length, which exceeds distance between Bragg gratings in any one element;a fiber optic beamsplitter having a first port connected to the optical filter, a fourth port connected to te test fiber, so as beam of light emitted by broadband light source and passed through optical filter is launched in test fiber, and a second and a third ports;an optical reflection detector to receive the light flux, connected to the second port of optic beamsplitter so as to detect the reflected from the test fiber light flux, the said reflection detector being operable to sense changes in the intensity of the reflected from the test fiber light flux inside the measurement wavelength range,an interference energy analyzer connected to reflection detector, said interference energy analyzer calculate the energies of the interference patterns separately for each double-Bragg grating element, interference patterns are created by beams reflected from double-Bragg grating elements. 2. The sensor as defined in claim 1 wherein, said light source and said optical filter together comprises a tunable laser which operate at said central transmission wavelength, and output radiation of said tunable laser having a coherence length, which exceeds distance between Bragg gratings in any one element; said tunable laser connected to the first port of beamsplitter. 3. The sensor as defined in claim 1 wherein, said optical filter connected to the second port of beamsplitter and to the reflection detector. 4. The sensor as defined in claim 1 wherein, said optical filter connected to the fourth port of beamsplitter and to the test fiber. 5. The sensor as defined in claim 1 wherein, said optical filter comprises an unbalanced interferometer, having possibility to change the value of optical path difference between its two arms from the value which is less than minimum optical distance between two gratings in one said double-Bragg grating element to the value which is more than maximum one, and said interference energy analyzer calculate the energies of the interference patterns, separately for each double-Bragg grating element, said interference patterns are created by beams reflected from double-Bragg grating element with the change of said optical length difference between two arms of said unbalanced interferometer. 6. The sensor as defined in claim 1 wherein, in order to linearize the sensor response and overcome the sign ambiguity of measurand the Bragg wavelength of one grating from each double-Bragg grating elements is shifted in respect to another one, so as the start point of the measurement is placed in the center of linear length of the dependence of energy of the interference pattern on measurand. 7. The sensor as defined in claim 1 wherein, in order to measure absolute value of temperature one Bragg grating from each said double-Bragg grating elements is mechanically coupled to a bulk sample with thermal expansion coefficient which is differ from the thermal expansion coefficient of the test fiber. 8. The sensor as defined in claim 1 wherein, in order to measure absolute value of temperature two Bragg gratings from each said double-Bragg grating elements are mechanically coupled to a two bulk samples with different thermal expansion coefficients, correspondingly. 9. The sensor as defined in claim 1 wherein, in order to measure absolute value of strain one Bragg grating from each said double-Bragg grating elements is mechanically coupled to a bulk sample, the strain-response of said bulk sample under applied longitudinal force is differ from the strain-response of the test fiber. 10. The sensor as defined in claim 1 wherein in order to measure absolute value of strain two Bragg gratings from each said double-Bragg grating elements are mechanically coupled to a two bulk samples with different strain-response under applied longitudinal force, correspondingly. 11. The sensor as defined in claim 1 wherein, in order to detect the presence of a chemical agent, one Bragg gratings from each said double-Bragg grating elements is mechanically coupled to a absorber/expander member, which is swelling with absorption of said chemical agent. 12. The sensor as defined in claim 1 wherein, in order to detect the presence of a chemical agent, two Bragg gratings from each said double-Bragg grating elements are mechanically coupled to a two absorber/expanders members having different swelling efficiency with absorption of said chemical agent, correspondingly. 13. The sensor as defined in claim 1 wherein in order to increase the spatial resolution by means of increasing of the number of double-Bragg grating elements each double-Bragg grating element with an unique distance, which is inherent for this element comprises series of double-Bragg grating elements positioned along the test fiber with different Bragg wavelengths and nonoverlaping spectra for different elements in this series. 14. The sensor as defined in claim 3 wherein, said optical filter and said reflection detector together comprises a spectroanalyzer. 15. The sensor as defined in claim 5 wherein in order to measure a temperature free from the strain influence, said two bulk samples having the same strain-response under applied longitudinal force. 16. The sensor as defined in claim 7 wherein in order to measure a strain free from the temperature influence, said bulk sample had made from the material with the thermal expansion coefficients equal to the thermal expansion coefficient of the test fiber. 17. The sensor as defined in claim 8 wherein in order to measure a strain free from the temperature influence, said two bulk samples had made from the materials with the same thermal expansion coefficients. 18. The system as in claim 1:Where at least one of the double-fiber grating sensors are written onto birefringent optical fiber. 19. The system as in claim 1:Where at least one of the double-fiber gratings are written onto birefringent fiber and one of said fiber gratings being mounted to minimize strain induced effects and the other end mounted to enhance strain induced effects. 20. The fiber sensor as defined in claim 19: where one mounting end is configured to measure transverse strain.