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

Abstract

The invention discloses a long-period fiber bragg grating temperature sensor based on liquid periodic refractive index modulation, and relates to the technical field of fiber bragg grating sensing, the sensor comprises a suspension core optical fiber arranged between two sections of single-mode optical fibers, and the suspension core optical fiber and the two sections of single-mode optical fibers form a concave structure. An annular cladding is arranged on the inner wall of the suspension core optical fiber and surrounds to form an air hole, the suspension fiber core is suspended on the inner wall of the air hole, an empty groove is axially formed in the suspension fiber core, the air hole and the empty groove are filled with liquid, and the liquid filled in the empty groove of the suspension fiber core forms periodic refractive index modulation in the axial direction of the suspension fiber core. And the long-period fiber grating is formed. The refractive index of the filling liquid is changed due to the change of external temperature, the refractive index modulation amount of the long-period fiber grating is changed, and formant drift is caused. And temperature sensing is realized by detecting the formant drift amount. Therefore, the sensor has high temperature sensitivity, and the problem of low temperature sensitivity of the all-quartz long-period fiber grating is solved.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber grating sensing, and in particular to a long-period optical fiber grating temperature sensor based on periodic refractive index modulation of liquid. Background Art

[0002] Fiber Bragg grating sensor is a very important fiber passive device with the advantages of high sensitivity, compact structure, corrosion resistance, and anti-electromagnetic interference. It has long been a research hotspot in the field of fiber optic sensing. At present, long-period fiber Bragg grating sensors have been widely used to measure physical parameters such as refractive index, temperature, strain, and bending. At present, long-period fiber Bragg grating temperature sensors are mainly divided into two categories: non-temperature sensitive materials and temperature sensitive materials. Long-period fiber Bragg grating without temperature sensitive materials mainly relies on the thermal light and thermal expansion properties of the optical fiber itself to make the long-period fiber Bragg grating respond to temperature in terms of transmission characteristics. However, since the thermal light and thermal expansion coefficients of the quartz optical fiber itself are very small, the sensitivity is very low, usually in the order of tens of pm / ℃. In order to improve the sensitivity, temperature sensitive materials are coated on the quartz optical fiber cladding, or on the exposed core after removing the cladding, to form a long-period fiber Bragg grating assisted by temperature sensitive materials. When the ambient temperature changes, the refractive index of the temperature sensitive material changes significantly, causing the effective refractive index of the cladding mode or core mode of the long-period fiber Bragg grating to change significantly, so that the transmission characteristics of the long-period fiber Bragg grating change significantly, thereby improving the temperature sensitivity. However, since the temperature-sensitive material is only attached to the cladding or core surface, its impact on the mode in the optical fiber is limited, so the improvement in temperature sensitivity is limited, usually only 1-2 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 periodic refractive index modulation of liquid to solve the above problems. Summary of the invention

[0004] The purpose of the present invention is to provide a long-period fiber Bragg grating temperature sensor based on periodic refractive index modulation of liquid, by preparing periodic empty grooves on the suspended fiber core and filling them with liquid, to form periodic refractive index modulation axially on the suspended fiber core, thereby forming a long-period fiber Bragg grating. The liquid becomes part of the fiber core, and the large thermo-optic coefficient of the liquid is used to make the temperature change have a strong influence on the mode in the fiber core, thereby significantly changing the transmission characteristics of the long-period fiber Bragg grating and greatly improving the temperature sensitivity.

[0005] To achieve the above-mentioned purpose, the present invention provides a long-period fiber grating temperature sensor based on periodic refractive index modulation of liquid, the sensor includes a suspended core fiber arranged between two sections of single-mode optical fibers, the suspended core fiber and the two sections of 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 to form an air hole, the suspended core is suspended on the inner wall of the air hole, empty grooves are axially arranged on the suspended core, the air hole and the empty grooves are filled with liquid, and ultraviolet glue is vertically arranged between the annular cladding arranged below the single-mode optical fiber and the outer side of the air hole and the left and right single-mode optical fibers.

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

[0007] Preferably, the liquid filled in the empty groove 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 filled in the air hole; UV glue seals the liquid in the air hole, and the cladding diameter is 125 microns.

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

[0009] Preferably, if the spacing between adjacent empty grooves is the same and the width of the empty grooves is the same, the formed grating is an ordinary long-period fiber grating; if the spacing between adjacent empty grooves is gradually changing or the width of the empty grooves is gradually changing, the formed grating is a chirped long-period fiber grating.

[0010] Preferably, the suspended core optical fiber and the single-mode optical fiber are fused by using a core alignment technology to couple all the core energy of the single-mode optical fiber into the suspended core.

[0011] Preferably, the refractive index of the liquid is less than the refractive index of the suspension core, and the liquid is set to be a glycerol aqueous solution or a liquid with a large 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 the temperature. When the temperature rises by 1°C, the refractive index decreases by 2.9×10 -4 .

[0012] Preferably, the change in external temperature causes the change in the refractive index of the liquid, forming a strong periodic refractive index modulation on the suspended fiber core, and the fundamental mode in the suspended fiber core is coupled with the cladding transmitting in the same direction, generating a resonance wavelength of λ in the transmission spectrum. res The resonance peak of the long-period fiber grating is:

[0013] λ res =Λ·[n co-n cl ]

[0014] Among them, n co is the effective refractive index of the fiber core, n cl is the effective refractive index of the cladding, Λ is the period of the long-period fiber grating, λ res is the resonance wavelength.

[0015] Preferably, the air holes are configured as large-sized air holes, the diameter of the large-sized air holes is 85 microns, the diameter of the suspension fiber core is configured as 11 microns, and the shape of the empty slot is the same as that of the suspension fiber core.

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

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

[0018] (2) The present invention uses femtosecond laser etching technology to process the fiber core while ensuring that the cladding is not affected, so that the fiber optic sensor fiber structure is complete and has high mechanical strength.

[0019] (3) In the present invention, the change of external temperature causes the refractive index of the filling liquid to change, which affects the transmission mode in the suspended fiber core and causes the resonance peak to drift. By monitoring the resonance peak drift in the transmission spectrum, the external temperature measurement can be achieved.

[0020] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0022] Figure 2 It is a schematic diagram of a long-period fiber Bragg grating temperature sensor based on periodic refractive index modulation of liquid in the present invention;

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

[0024] Figure Annotation

[0025] 1. Single-mode optical fiber; 2. Suspended core optical fiber; 21. Annular cladding; 22. Suspended core; 23. Air hole; 24. UV glue; 25. Empty slot; 26. Liquid; 3. Broadband light source; 4. Spectrometer. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0027] Unless otherwise defined, technical or scientific terms used in the present invention shall have the common meanings understood by one having ordinary skills in the field to which the present invention belongs.

[0028] The words "include" or "comprises" and the like used in the present invention mean that the elements before the word include the elements listed after the word, and do not exclude the possibility of also including other elements. The orientation or position relationship indicated by the terms "inside", "outside", "upper", "lower", etc. is based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it cannot be understood as a limitation of the present invention. When the absolute position of the described object changes, the relative position relationship may also change accordingly. In the present invention, unless otherwise clearly specified and limited, the terms "attachment" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral body; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0029] like Figure 1 As shown, the present invention provides a long-period fiber grating temperature sensor based on periodic refractive index modulation of liquid, the sensor comprises a suspension core fiber 2 arranged between two sections of single-mode optical fibers 1, the suspension core fiber 2 and the two sections of single-mode optical fibers 1 form a concave structure, the suspension core fiber 2 and the single-mode optical fibers 1 are fused by core alignment technology, so that the core energy of the single-mode optical fibers 1 is fully coupled to the suspension core 22. An annular cladding 21 is arranged on the inner wall of the suspension core optical fiber 2, the annular cladding 21 surrounds and forms an air hole 23, the suspension core 22 is suspended on the inner wall of the air hole 23, an empty groove 25 is axially arranged on the suspension core 22, the air hole 23 and the empty groove 25 are filled with liquid 26, and ultraviolet glue 24 is vertically arranged between the annular cladding 21 and the air hole 23 arranged below the single-mode optical fiber 1 and the outer side of the left and right single-mode optical fibers 1. The air hole 23 is set as a large-sized air hole, and the diameter of the large-sized air hole is 85 microns.

[0030] The empty groove 25 is prepared by femtosecond laser dry etching technology or processed by femtosecond laser direct writing technology combined with wet etching technology. The liquid 26 filled in the empty groove 25 forms a periodic refractive index modulation along the axial direction on the suspension core 22; the cladding of the suspension core 22 is composed of an annular cladding 21 and the liquid 26 filled in the air hole 23; the ultraviolet glue 24 seals the liquid 26 in the air hole, and the cladding diameter of the suspension core 22 is 125 microns.

[0031] The shape of the suspension core 22 is set to be circular or elliptical, supporting fundamental mode transmission, the distance between two adjacent empty slots 25 on the suspension core 22 is set to be the same or gradually changing, the width of the empty slots 25 is the same or gradually changing, and the width of the empty slots 25 is set to be 40-100 microns. The diameter of the suspension core 22 is set to 11 microns, and the shape of the empty slots 25 is the same as that of the suspension core 22.

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

[0033] The refractive index of the liquid 26 is less than the refractive index of the suspension core 22. The liquid 26 is set to be a glycerol aqueous solution or a liquid 26 with a large 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 the temperature. When the temperature rises by 1°C, the refractive index decreases by 2.9×10 -4 .

[0034] The change of external temperature causes the refractive index of the liquid 26 to change, forming a strong periodic refractive index modulation on the suspended fiber core 22. The fundamental mode in the suspended fiber core 22 couples with the cladding transmitting in the same direction, generating a resonance wavelength of λ in the transmission spectrum. res The resonance peak of the long-period fiber grating is:

[0035] λ res =Λ·[n co -n cl ]

[0036] Among them, n co is the effective refractive index of the fiber core, n cl is the effective refractive index of the cladding, Λ is the period of the long-period fiber grating, λ res is the resonance wavelength.

[0037] Embodiment 1

[0038] The core alignment technology is used to fuse the suspension core optical fiber 2 with two sections of single-mode optical fiber 1, and the femtosecond laser dry etching technology is used to prepare the empty groove 25 distributed along the axial direction on the suspension core 22. The width of the empty groove 25 is set to 60 microns, and the shape of the empty groove 25 is the same as that of the suspension core 22. The prepared sample is placed tightly on the glass slide, and the micropores are adjusted upward under a microscope. The glycerol aqueous solution is dripped onto the welding point at one end of the suspension core optical fiber 2 and the single-mode optical fiber 1. Due to the siphon effect, the glycerol aqueous solution will fill the air hole 23, and finally sealed with ultraviolet glue 24. The prepared long-period fiber grating device forms a strong periodic refractive index modulation on the suspension core 22. The period of the long-period fiber grating device is 600 microns, and the width of the empty groove 25 filled with the glycerol aqueous solution is set to 60 microns, and the number of periods is 25. The filling length of the liquid 26 in the air hole 23 is determined by the length of the suspension core 22, so the length of the filled liquid has better consistency and good repeatability.

[0039] Embodiment 2

[0040] The core alignment technology is used to fuse the suspended core optical fiber 2 with two sections of single-mode optical fiber 1, and the refractive index modification is induced by femtosecond laser direct writing technology to process the designed empty groove 25 model in the suspended core 22. The width of the empty groove 25 is set to 60 microns, the period is 600 microns, and the number of periods is 25. The induced empty groove 25 model is chemically corroded by an etchant, wherein hydrofluoric acid or potassium hydroxide can be used as an etchant. The prepared sample is tightly attached to a glass slide, and the siphon effect fills the air hole with a glycerol aqueous solution, and finally sealed with ultraviolet glue 24. The prepared long-period fiber grating device forms a strong periodic refractive index modulation on the suspended core 22. The filling length of the liquid 26 in the air hole 23 is determined by the length of the suspended core 22, so the length of the filled liquid has better consistency and good repeatability.

[0041] like Figure 2 and Figure 3 The figure shows a schematic diagram of a long-period fiber Bragg grating temperature sensor based on periodic refractive index modulation of liquid. 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 suspension core optical fiber 2 through the single-mode optical fiber 1. Since the suspension core 22 has a long-period fiber grating, when the mode in the suspension core 22 and the mode in the cladding meet the phase matching condition at a specific wavelength, resonant coupling occurs between the two modes, and a resonant coupling peak appears at this wavelength, which is the resonant wavelength. Another section of single-mode optical fiber 1 is connected to the spectrometer 4 to realize the display of the transmission spectrum. When the external temperature changes, the refractive index of the liquid filled in the air hole 23 changes, causing the change in the following formula, causing the resonant wavelength to drift. Therefore, temperature sensing can be achieved by detecting the resonance peak drift.

[0043] λres =Λ·[n co -n cl ]

[0044] Among them, n co is the effective refractive index of the fiber core, n cl is the effective refractive index of the cladding, Λ is the period of the long-period fiber grating, λ res is the resonance wavelength.

[0045] After the above production is completed, the sensor probe is completed.

[0046] Therefore, the present invention adopts the above-mentioned long-period fiber grating temperature sensor based on periodic refractive index modulation of liquid, and uses femtosecond laser micro-nano processing technology to prepare empty grooves periodically distributed along the axial direction in the suspension core. The suspension core is completely ablated at the processing site, and the liquid filled in the empty groove of the suspension core forms a periodic refractive index modulation in the axial direction of the suspension core, thereby forming a long-period fiber grating. Changes in external temperature cause changes in the refractive index of the filling liquid, thereby causing changes in the refractive index modulation amount of the long-period fiber grating, causing the resonance peak to drift, and the external temperature is measured according to the resonance wavelength drift amount corresponding to the resonance peak.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A long-period fiber grating temperature sensor based on periodic refractive index modulation of liquid, characterized in that: The sensor includes a suspended core optical fiber arranged between two sections of single-mode optical fibers, the suspended core optical fiber and the two sections of single-mode optical fibers form a concave structure, the inner wall of the suspended core optical fiber is provided with an annular cladding, the annular cladding surrounds an air hole, the suspended fiber core is suspended on the inner wall of the air hole, and empty grooves are axially arranged on the suspended fiber core, the air hole and the empty grooves are filled with liquid, and ultraviolet glue is vertically arranged between the annular cladding arranged below the single-mode optical fiber and the outer side of the air hole and the left and right single-mode optical fibers.

2. A long-period fiber grating temperature sensor based on periodic refractive index modulation of liquid according to claim 1, characterized in that: The empty groove is prepared by femtosecond laser dry etching technology or processed by femtosecond laser direct writing technology combined with wet etching technology.

3. A long-period fiber grating temperature sensor based on periodic refractive index modulation of liquid according to claim 1, characterized in that: The liquid filled in the empty groove forms a periodic refractive index modulation along the axial direction on the suspended fiber core; the cladding of the suspended fiber core is composed of an annular cladding and liquid filled in the air hole; UV glue seals the liquid in the air hole, and the cladding diameter is 125 microns.

4. A long-period fiber grating temperature sensor based on periodic refractive index modulation of liquid according to claim 4, characterized in that: The shape of the suspended fiber core is set to be circular or elliptical to support fundamental mode transmission. The distance between two adjacent empty slots on the suspended fiber core is set to be the same or gradually changing, the width of the empty slot is the same or gradually changing, and the width of the empty slot is set to 40-100 microns.

5. A long period fiber grating temperature sensor based on periodic refractive index modulation of liquid according to claim 4, characterized in that: If the spacing between adjacent empty slots is the same and the width of the empty slots is the same, the grating formed is an ordinary long-period fiber grating; if the spacing between adjacent empty slots changes gradually or the width of the empty slots changes gradually, the grating formed is a chirped long-period fiber grating.

6. A long-period fiber grating temperature sensor based on periodic refractive index modulation of liquid according to claim 1, characterized in that: The suspended core optical fiber and the single-mode optical fiber are fused by using the core alignment technology to couple all the core energy of the single-mode optical fiber into the suspended core.

7. A long-period fiber grating temperature sensor based on periodic refractive index modulation of liquid according to claim 1, characterized in that: The refractive index of the liquid is less than that of the suspended fiber core. The liquid is set to be a glycerol aqueous solution or a liquid with a large 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 the temperature. For every 1°C increase in temperature, the refractive index decreases by 2.9×10 -4 .

8. A long period fiber grating temperature sensor based on periodic refractive index modulation of liquid according to claim 5, characterized in that: The change of external temperature causes the refractive index of the liquid to change, forming a strong periodic refractive index modulation on the suspended fiber core. The fundamental mode in the suspended fiber core couples with the cladding transmitting in the same direction, generating a resonant wavelength of λ in the transmission spectrum. res The resonance peak of the long-period fiber grating is: l res =Λ·[n co -n cl ] Among them, n co is the effective refractive index of the fiber core, n cl is the effective refractive index of the cladding, Λ is the period of the long-period fiber grating, λ res is the resonance wavelength.

9. A long period fiber grating temperature sensor based on periodic refractive index modulation of liquid according to claim 1, characterized in that: The air hole is set to be a large-sized air hole, the diameter of the large-sized air hole is 85 microns, the diameter of the suspension fiber core is set to 11 microns, and the shape of the empty groove is the same as that of the suspension fiber core.

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