Fiber optic temperature sensor and sensing head structure

A fiber optic temperature sensor, comprising:

• a broad spectrum light source,
• a first fiber optic coupler,
• a spectrometer,
• a first sensing interferometer, and
• a second sensing interferometer;
• wherein the first sensing interferometer and the second sensing interferometer have opposite temperature responses;
• wherein a first free spectral range corresponding to the first sensing interferometer is not equal to a second free spectral range corresponding to the second sensing interferometer;
• the first sensing interferometer is configured to be a Sagnac interferometer comprising a second fiber optic coupler and a polarizing optical fiber, and the second sensing interferometer is configured to be an F-P interferometer formed by connecting a single-mode optical fiber and a polydimethylsiloxane (PDMS) cavity;
• and
• the fiber optic temperature sensor has a temperature sensitivity S12 as follows:
• S
• 1
• ⁢
• 2
• =
• M
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• (
• S
• 2
• -
• S
• 1
• )
• =
• {
• M
• 1
• ′
• ⁢
• S
• 1
• M
• 2
• ′
• ⁢
• S
• 2
• ;
• wherein
• ⁢
• ⁢
• {
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• M
• 1
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• =
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• [
• 1
• -
• B
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• (
• α
• +
• n
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• β
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• Δ
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• B
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• M
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• >
• ❘
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• M
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• M
• 2
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• =
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• [
• 1
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• B
• B
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• (
• α
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• n
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• M
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• >
• ❘
• "\[LeftBracketingBar]"
• M
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• ;
• M is an amplification factor of a conventional vernier effect;
• S1 represents a temperature sensitivity of the Sagnac interferometer, wherein
• S
• 1
• =
• Δ
• ⁢
• B
• B
• ,
• B represents a birefringence coefficient, ΔB represents a change in a refractive index of the polarizing optical fiber when temperature increases by 1° C., and ΔB<0;
• S2 represents a temperature sensitivity of the F-P interferometer, wherein
• S
• 2
• =
• λ
• m
• n
• ⁢
• (
• α
• +
• n
• ⁢
• β
• )
• ,
• α represents a thermo-optical coefficient of PDMS, λm represents a peak wavelength of an interference spectrum, n represents a refractive index of the PDMS, and β is a thermal expansion coefficient of the PDMS;
• the fiber optic temperature sensor further comprises a fiber optic isolator and an attenuator;
• an incident light outputted by the broad spectrum light source is divided into two paths after successively passing through the fiber optic isolator and the first fiber optic coupler, wherein one path enters the Sagnac interferometer, and the other path enters the F-P interferometer through the attenuator;
• a result of superimposing an interference spectrum returned by the Sagnac interferometer and an interference spectrum returned by the F-P interferometer is obtained by the spectrometer;
• a first terminal of the second fiber optic coupler is connected to the first fiber optic coupler;
• the polarizing optical fiber is connected between a second terminal and a third terminal of the second fiber optic coupler;
• the incident light entering the Sagnac interferometer is divided in the polarizing optical fiber into two light beams having polarization directions perpendicular to each other, an interference occurs between the two light beams after passing through the polarizing optical fiber, and an obtained interference light is outputted from the first terminal of the second fiber optic coupler and then received by the spectrometer through the first fiber optic coupler;
• a part of the incident light entering the F-P interferometer is reflected back to the single-mode optical fiber at an interface between the single-mode optical fiber and the PDMS cavity, and another part of the incident light is transmitted to enter the PDMS cavity and then partially reflected back to the single-mode optical fiber at an interface between the PDMS cavity and air, and is interfered with the part of the incident light reflected back to the single-mode optical fiber at the interface between the single-mode optical fiber and the PDMS cavity;
• and
• wherein a length of the polarizing optical fiber and an axial length of the PDMS cavity are configured to make the first free spectral range of the Sagnac interferometer not equal to the second free spectral range of the F-P interferometer.