A long-period fiber grating temperature sensor based on liquid periodic refractive index modulation

Abstract

The application discloses a long-period fiber grating temperature sensor based on liquid periodic refractive index modulation, and relates to the technical field of fiber grating sensing, wherein the sensor comprises a suspended core fiber arranged between two single-mode optical fibers; the suspended core fiber and the two single-mode optical fibers form a concave structure; the inner wall of the suspended core fiber is provided with an annular cladding; the annular cladding surrounds an air hole; the suspended core is suspended on the inner wall of the air hole; an air slot is arranged on the suspended core in an axial direction; the air hole and the air slot are filled with liquid; the liquid filled in the air slot of the suspended core forms periodic refractive index modulation in the axial direction of the suspended core, thereby forming a long-period fiber grating. Changes in external temperature cause changes in the refractive index of the filled liquid, which leads to changes in the refractive index modulation amount of the long-period fiber grating, thereby causing resonance peak drift. Temperature sensing is realized by detecting the resonance peak drift amount. Therefore, the sensor has high temperature sensitivity and overcomes the problem of low temperature sensitivity of an all-quartz long-period fiber grating.

Description

Technical Field

[0001] This invention relates to the field of fiber optic grating sensing technology, and in particular to a long-period fiber optic grating temperature sensor based on liquid periodic refractive index modulation. Background Technology

[0002] Fiber Bragg grating (FBG) sensors are crucial passive fiber optic devices, possessing advantages such as high sensitivity, compact size, corrosion resistance, and electromagnetic interference immunity, making them a long-standing research hotspot in the field of fiber optic sensing. Currently, long-period FBG sensors are widely used for measuring physical parameters such as refractive index, temperature, strain, and bending. Long-period FBG temperature sensors are mainly divided into two categories: those without temperature-sensitive materials and those with temperature-sensitive materials. Long-period FBGs without temperature-sensitive materials primarily rely on the inherent thermo-optical and thermal expansion properties of the fiber itself to achieve a temperature-responsive transmission characteristic. However, due to the extremely small thermo-optical and thermal expansion coefficients of silica fiber, the sensitivity is very low, typically on the order of tens of pm / ℃. To improve sensitivity, temperature-sensitive materials are coated onto the cladding of the silica fiber, or onto the exposed core after removing the cladding, forming temperature-sensitive material-assisted long-period FBGs. When the ambient temperature changes, the refractive index of the temperature-sensitive material changes significantly, causing a marked change in the effective refractive index of the cladding mode or core mode of the long-period FBG, thus significantly altering the transmission characteristics of the long-period FBG and improving its temperature sensitivity. However, since the temperature-sensitive material is only attached to the cladding or fiber core surface, its influence on the mode in the optical fiber is limited, so the improvement in temperature sensitivity is limited, usually only one to two orders of magnitude, which is still difficult to meet the needs of high-sensitivity temperature measurement.

[0003] Therefore, it is necessary to provide a long-period fiber grating temperature sensor based on liquid periodic refractive index modulation to solve the above problems. Summary of the Invention

[0004] The purpose of this invention is to provide a long-period fiber grating temperature sensor based on liquid periodic refractive index modulation. This is achieved by creating periodic slots on a suspended fiber core and filling them with liquid, thereby forming a periodic refractive index modulation axially on the suspended fiber core and ultimately creating a long-period fiber grating. Since the liquid becomes part of the fiber core, and considering its high thermo-optic coefficient, temperature changes can strongly influence the mode within the fiber core, significantly altering the transmission characteristics of the long-period fiber grating and greatly improving its temperature sensitivity.

[0005] To achieve the above objectives, the present invention provides a long-period fiber grating temperature sensor based on liquid periodic refractive index modulation. The sensor includes a suspended core fiber disposed between two single-mode fibers. The suspended core fiber and the two single-mode fibers form a concave structure. An annular cladding is disposed on the inner wall of the suspended core fiber, and an air hole is formed around the annular cladding. The suspended core fiber is suspended on the inner wall of the air hole. A slot is axially arranged on the suspended core fiber. Both the air hole and the slot are filled with liquid. Ultraviolet adhesive is disposed perpendicularly between the annular cladding and the outer side of the air hole below the single-mode fibers and the left and right single-mode fibers.

[0006] Preferably, the empty slots are prepared by femtosecond laser dry etching technology or by femtosecond laser direct writing technology combined with wet etching technology.

[0007] Preferably, the liquid filling the cavity forms a periodic refractive index modulation along the axial direction on the suspended fiber core; the cladding of the suspended fiber core consists of an annular cladding and liquid filling the air pores; the UV adhesive seals the liquid in the air pores, and the cladding diameter is 125 micrometers.

[0008] Preferably, the shape of the suspended fiber core is set to be circular or elliptical to support fundamental mode transmission. The distance between two adjacent slots on the suspended fiber core is set to be the same or gradually changing. The width of the slots is the same or gradually changing, and the width of the slots is set to 40-100 micrometers.

[0009] Preferably, if the spacing between adjacent slots is the same and the width of the slots is the same, the grating formed is a conventional long-period fiber grating; if the spacing between adjacent slots gradually changes or the width of the slots gradually changes, the grating formed is a chirped long-period fiber grating.

[0010] Preferably, the suspended core fiber and the single-mode fiber are fused together using core alignment technology, so that all the core energy of the single-mode fiber is coupled into the suspended core.

[0011] Preferably, the refractive index of the liquid is lower than that of the suspended fiber core. The liquid is a glycerol aqueous solution or a liquid with a high thermo-optical coefficient. The refractive index of the glycerol aqueous solution is between 1.334 and 1.336. When the temperature is between 20°C and 60°C, the refractive index of the glycerol aqueous solution changes with temperature; for every 1°C increase in temperature, the refractive index decreases by 2.9 × 10⁻⁶. -4 .

[0012] Preferably, changes in external temperature cause changes in the refractive index of the liquid, creating a strong periodic refractive index modulation on the suspended fiber core. The fundamental mode in the suspended fiber core couples with the co-propagating cladding, generating a resonance wavelength of λ in the transmission spectrum. res The resonance peak of the long-period fiber grating is given by the formula:

[0013] λ res =Λ·[n co-n cl ]

[0014] Where, n co n is the effective refractive index of the fiber core. cl λ is the effective refractive index of the cladding, Λ is the period of the long-period fiber grating, and λ is the effective refractive index of the cladding. res This is the resonant wavelength.

[0015] Preferably, the air hole is set to a large-size air hole with a diameter of 85 micrometers, the diameter of the suspension core is set to 11 micrometers, and the shape of the slot is the same as the shape of the suspension core.

[0016] Therefore, the present invention employs the above-mentioned long-period fiber grating temperature sensor based on liquid periodic refractive index modulation, which has the following beneficial effects:

[0017] (1) In this invention, the liquid is used as a component of the suspended fiber core. The transmission mode in the suspended fiber core is more sensitive to temperature, which can greatly improve the temperature sensitivity of long-period fiber gratings.

[0018] (2) When the fiber core is processed by femtosecond laser etching technology, the cladding is not affected, which enables the fiber structure of the fiber sensor to be intact and the mechanical strength to be high.

[0019] (3) The present invention relates to the principle that changes in external temperature cause changes in the refractive index of the filling liquid, which affects the transmission mode in the suspended fiber core and causes resonance peak shift. By monitoring the amount of resonance peak shift in the transmission spectrum, external temperature can be measured.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 This is a structural diagram of a long-period fiber grating temperature sensor based on liquid periodic refractive index modulation according to the present invention.

[0022] Figure 2 This is a schematic diagram of the long-period fiber grating temperature sensor based on liquid periodic refractive index modulation in this invention.

[0023] Figure 3 This is a diagram of the suspended core optical fiber structure in Embodiment 2 of the present invention;

[0024] Attached Figure Captions

[0025] 1. Single-mode fiber; 2. Suspended core fiber; 21. Ring cladding; 22. Suspended core; 23. Air hole; 24. UV adhesive; 25. Void; 26. Liquid; 3. Broadband light source; 4. Spectrometer. Detailed Implementation

[0026] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0027] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0028] The terms "comprising" or "including" as used in this invention mean that the element preceding the term encompasses the element listed after the term, and do not exclude the possibility of encompassing other elements. Terms such as "inner," "outer," "upper," and "lower" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. When the absolute position of the described object changes, the relative positional relationship may also change accordingly. In this invention, unless otherwise explicitly specified and limited, the term "attached" and similar terms should be interpreted broadly. For example, it can refer to a fixed connection, a detachable connection, or an integral part; it can refer to a direct connection or an indirect connection through an intermediate medium; it can refer to the internal communication of two elements or the interaction relationship between two elements. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0029] like Figure 1 As shown, this invention provides a long-period fiber grating temperature sensor based on liquid periodic refractive index modulation. The sensor includes a suspended core fiber 2 disposed between two single-mode fiber segments 1. The suspended core fiber 2 and the two single-mode fiber segments 1 form a concave structure. The suspended core fiber 2 and the single-mode fiber 1 are fused together using core alignment technology, so that all the core energy of the single-mode fiber 1 is coupled into the suspended core 22. An annular cladding 21 is provided on the inner wall of the suspended core fiber 2, forming an air hole 23. The suspended core 22 is suspended on the inner wall of the air hole 23. A slot 25 is axially arranged on the suspended core 22. Both the air hole 23 and the slot 25 are filled with liquid 26. Ultraviolet adhesive 24 is perpendicularly disposed between the outer side of the annular cladding 21 and the air hole 23 below the single-mode fiber 1 and the left and right single-mode fibers 1. The air hole 23 is a large-sized air hole with a diameter of 85 micrometers.

[0030] The empty groove 25 is prepared by femtosecond laser dry etching or by femtosecond laser direct writing combined with wet etching. The liquid 26 filling the empty groove 25 forms a periodic refractive index modulation along the axial direction on the suspended fiber core 22; the cladding of the suspended fiber core 22 consists of an annular cladding 21 and liquid 26 filling the air holes 23; the UV adhesive 24 seals the liquid 26 in the air holes, and the cladding diameter of the suspended fiber core 22 is 125 micrometers.

[0031] The suspended fiber core 22 is circular or elliptical in shape to support fundamental mode transmission. The distance between two adjacent slots 25 on the suspended fiber core 22 is the same or gradually changing. The width of the slots 25 is the same or gradually changing, and the width of the slots 25 is set to 40-100 micrometers. The diameter of the suspended fiber core 22 is set to 11 micrometers, and the shape of the slots 25 is the same as that of the suspended fiber core 22.

[0032] If the spacing between adjacent slots 25 is the same and the width of slots 25 is the same, the grating formed is a regular long-period fiber grating; if the spacing between adjacent slots 25 gradually changes or the width of slots 25 gradually changes, the grating formed is a chirped long-period fiber grating.

[0033] The refractive index of liquid 26 is less than that of the suspension fiber core 22. Liquid 26 is set as an aqueous solution of glycerol or a liquid 26 with a large thermo-optical coefficient. The refractive index of the aqueous solution of glycerol is between 1.334 and 1.336. When the temperature is between 20℃ and 60℃, the refractive index of the aqueous solution of glycerol changes with temperature. For every 1℃ increase in temperature, the refractive index decreases by 2.9 × 10⁻⁶. -4 .

[0034] External temperature changes cause changes in the refractive index of liquid 26, resulting in strong periodic refractive index modulation of the suspended fiber core 22. The fundamental mode in the suspended fiber core 22 couples with the co-propagating cladding, generating a resonance wavelength of λ in the transmission spectrum. res The resonance peak of the long-period fiber grating is given by the formula:

[0035] λ res =Λ·[n co -n cl ]

[0036] Where, n co n is the effective refractive index of the fiber core. cl λ is the effective refractive index of the cladding, Λ is the period of the long-period fiber grating, and λ is the effective refractive index of the cladding. res This is the resonant wavelength.

[0037] Example 1

[0038] A core-aligned fiber 2 was fused to two single-mode fibers 1 using a core alignment technique. Axially distributed slots 25 were fabricated on the core 22 using femtosecond laser dry etching. The width of the slots 25 was set to 60 micrometers, and their shape was identical to that of the core 22. The prepared sample was placed tightly on a glass slide, and under a microscope, the micropores were aligned upwards. A glycerol aqueous solution was dropped onto the solder joint between the core 2 and the single-mode fiber 1. Due to the siphon effect, the glycerol aqueous solution filled the air holes 23. Finally, it was sealed with UV adhesive 24. The fabricated long-period fiber grating device created a strong periodic refractive index modulation on the core 22. The long-period fiber grating device had a period of 600 micrometers, and the slots 25 filled with the glycerol aqueous solution had a width of 60 micrometers and a period of 25. The filling length of the liquid 26 in the air holes 23 was determined by the length of the core 22, thus resulting in better consistency and repeatability of the filling liquid length.

[0039] Example 2

[0040] The suspended core fiber 2 was fused to two single-mode fiber segments 1 using core alignment technology. Femtosecond laser direct writing technology was used to induce refractive index modification, fabricating the designed slot 25 model in the suspended core 22. The slot 25 had a width of 60 micrometers, a period of 600 micrometers, and a period number of 25. The induced slot 25 model was then chemically etched using an etchant, such as hydrofluoric acid or potassium hydroxide. The prepared sample was then firmly attached to a glass slide, and the siphon effect filled the air holes with a glycerol aqueous solution. Finally, it was sealed with UV adhesive 24. The fabricated long-period fiber grating device created a strong periodic refractive index modulation on the suspended core 22. The filling length of the liquid 26 in the air hole 23 was determined by the length of the suspended core 22, thus ensuring better consistency and repeatability of the filling liquid length.

[0041] like Figure 2 and Figure 3 The diagram shows a long-period fiber optic temperature sensor based on liquid periodic refractive index modulation. The working process and principle of the entire device are as follows:

[0042] The light emitted by the broadband light source 3 is input into the suspended core fiber 2 through the single-mode fiber 1. Because the suspended core 22 has a long-period fiber grating, when the modes in the suspended core 22 and the modes in the cladding meet the phase-matching condition at a specific wavelength, resonant coupling occurs between the two modes, resulting in a resonant coupling peak at that wavelength, which is the resonant wavelength. Another section of the single-mode fiber 1 is connected to the spectrometer 4 to display the transmission spectrum. When the external temperature changes, the refractive index of the liquid filling the air hole 23 changes, causing a change in the following formula, resulting in a resonant wavelength shift. Therefore, temperature sensing can be achieved by detecting the amount of resonant peak shift.

[0043] λres =Λ·[n co -n cl ]

[0044] Where, n co n is the effective refractive index of the fiber core. cl λ is the effective refractive index of the cladding, Λ is the period of the long-period fiber grating, and λ is the effective refractive index of the cladding. res This is the resonant wavelength.

[0045] Once the above steps are completed, the sensor probe is finished.

[0046] Therefore, this invention employs a long-period fiber grating temperature sensor based on liquid periodic refractive index modulation. Using femtosecond laser micro-nano fabrication technology, axially periodically distributed slots are created in the suspended fiber core. The suspended fiber core is completely ablated at the fabrication site, and the liquid filling the slots creates periodic refractive index modulation along the fiber core's axis, thus forming a long-period fiber grating. Changes in external temperature cause changes in the refractive index of the filling liquid, resulting in changes in the refractive index modulation of the long-period fiber grating and causing a shift in the resonance peak. The external temperature is measured based on the amount of resonance wavelength shift corresponding to the resonance peak.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A long-period fiber optic temperature sensor based on liquid periodic refractive index modulation, characterized in that: The sensor includes a suspended core fiber disposed between two single-mode optical fibers. The suspended core fiber and the two single-mode optical fibers form a concave structure. The inner wall of the suspended core fiber is provided with an annular cladding, which surrounds and forms an air hole. The suspended core fiber is suspended on the inner wall of the air hole. The suspended core fiber is axially provided with slots. Both the air hole and the slots are filled with liquid. The annular cladding and the outer side of the air hole, which are located below the single-mode optical fibers, are vertically disposed between the left and right single-mode optical fibers and ultraviolet adhesive. The liquid filling the cavity forms a periodic refractive index modulation along the axial direction on the suspended fiber core; the cladding of the suspended fiber core consists of an annular cladding and liquid filling the air pores; the UV adhesive seals the liquid in the air pores, and the cladding diameter is 125 micrometers. The shape of the suspended fiber core is set to be circular or elliptical to support fundamental mode transmission. The distance between two adjacent slots on the suspended fiber core is set to be the same or gradually changing. The width of the slots is the same or gradually changing, and the width of the slots is set to 40-100 micrometers. If the spacing between adjacent slots is the same and the width of the slots is the same, the resulting grating is a conventional long-period fiber grating; if the spacing between adjacent slots or the width of the slots gradually changes, the resulting grating is a chirped long-period fiber grating.

2. The long-period fiber optic temperature sensor based on liquid periodic refractive index modulation as described in claim 1, characterized in that: The empty slots are prepared by femtosecond laser dry etching technology or by femtosecond laser direct writing technology combined with wet etching technology.

3. The long-period fiber optic temperature sensor based on liquid periodic refractive index modulation as described in claim 1, characterized in that: The suspended core fiber and the single-mode fiber are fused together using core alignment technology, which couples all the core energy of the single-mode fiber into the suspended core.

4. The long-period fiber optic temperature sensor based on liquid periodic refractive index modulation as described in claim 1, characterized in that: The refractive index of the liquid is lower than that of the suspended fiber core. The liquid is set as an aqueous solution of glycerol or a liquid with a high thermo-optical coefficient. The refractive index of the aqueous solution of glycerol is between 1.334 and 1.

336. When the temperature is between 20°C and 60°C, the refractive index of the aqueous solution of glycerol changes with temperature; for every 1°C increase in temperature, the refractive index decreases by 2.9 × 10⁻⁶. -4 .

5. The long-period fiber optic temperature sensor based on liquid periodic refractive index modulation as described in claim 1, characterized in that: Changes in external temperature cause changes in the refractive index of the liquid, resulting in strong periodic refractive index modulation of the suspended fiber core. The fundamental mode in the suspended fiber core couples with the co-directionally propagating cladding, generating a resonance wavelength in the transmission spectrum. The resonance peak of the long-period fiber grating is given by the formula: ； in, The effective refractive index of the fiber core, The effective refractive index of the cladding, The period of a long-period fiber grating. This is the resonant wavelength.

6. The long-period fiber optic temperature sensor based on liquid periodic refractive index modulation as described in claim 1, characterized in that: The air pores are set to large-sized air pores with a diameter of 85 micrometers, the diameter of the suspension core is set to 11 micrometers, and the shape of the slot is the same as that of the suspension core.

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