Dual F-shaped Fiber Bragg Grating Temperature Sensor

Abstract

The invention discloses a double F-shaped fiber Bragg grating temperature sensor, comprising two F-shaped structural members and optical fibers, wherein the thermal expansion coefficients of the two F-shaped structural members are different, the two F-shaped structural members are arranged in a central symmetrical manner, and the middle parts of the two F-shaped structural members are fixedly connected, a gap is formed between the head end cross beam of each F-shaped structural member and the tail end of another F-shaped structural member, the optical fiber is provided with a first fiber Bragg grating and a second fiber Bragg grating, the first fiber Bragg grating and the second fiber Bragg grating are respectively located at the two gaps, and the optical fibers on both sides of the fiber Bragg grating are respectively fixedly connected to the head end cross beam of one F-shaped structural member and the tail end of another F-shaped structural member. The two F-shaped structural members have a large difference in thermal expansion coefficients, and the relative displacement changes of the two F-shaped structural members caused by temperature are converted into strain changes of the fiber Bragg grating, thereby improving the sensitivity of the temperature sensor; when the temperature rises and falls, the central wavelength change of each fiber Bragg grating is used for detection, and the measurement range is doubled.

Description

Technical Field

[0001] The invention relates to the technical field of optical sensing, in particular to a double F-shaped optical fiber grating temperature sensor. Background Art

[0002] At present, the most commonly used temperature sensors in temperature measurement are electrical temperature sensors, such as thermocouples and thermistor temperature sensors, but these sensors are susceptible to electromagnetic interference, have poor measurement accuracy, short life, cannot be transmitted over long distances, and cannot be connected in series. Fiber Bragg grating temperature sensors have attracted widespread attention and great interest due to their many advantages such as anti-electromagnetic interference, corrosion resistance, high sensitivity, strong multiplexing ability, small size, light weight, and easy embedding into materials.

[0003] Fiber Bragg gratings have temperature-sensitive properties. Without any packaging and sensitization, their temperature sensitivity is about 10pm / ℃. Limited by the wavelength detection accuracy of the current signal demodulation instrument, the temperature measurement sensitivity of the temperature sensor based on the bare fiber Bragg grating is relatively low. To this end, people continue to study methods to improve the temperature measurement sensitivity of fiber Bragg gratings. Currently, the commonly used fiber Bragg grating temperature sensor sensitization methods include single metal substrate packaging, polymer packaging, and double metal substrate packaging. Single metal substrate packaging is to fix all or two points of the fiber Bragg grating on the surface of a metal substrate with a high thermal expansion coefficient, but its sensitization effect is limited. The fiber Bragg grating temperature sensor made is about 3 times that of the bare fiber Bragg grating. Polymer packaging has a larger thermal expansion coefficient, so compared with metal substrate packaging, the fiber Bragg grating temperature sensor made by it has a higher sensitivity, but most polymer materials have a glass transition temperature. When the temperature is higher than the glass transition temperature, its thermal expansion coefficient, elastic modulus, etc. will be greatly changed, which greatly limits the scope of use of fiber Bragg grating temperature sensors. In order to further improve the sensitivity of fiber Bragg grating temperature sensors, researchers have proposed bimetallic substrate packaging methods in recent years, such as the Chinese invention patents CN202010954557.4, CN201910713117.7 and CN200710178947.1, which are based on bimetallic strips as core components and achieve highly sensitive temperature measurement through different structural designs. However, these proposed bimetallic substrate packaging methods either have high sensitivity but a temperature measurement range of only a few degrees Celsius; or have a large range but low sensitivity and are prone to cause fiber Bragg grating chirp. Summary of the invention

[0004] The purpose of the present invention is to provide a double F-shaped fiber Bragg grating temperature sensor with simple structure, large range and high sensitivity to solve the problem that the fiber Bragg grating temperature sensor in the prior art is difficult to achieve both large range and high sensitivity.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A double F-shaped fiber Bragg grating temperature sensor comprises two F-shaped structural members and optical fibers, wherein the thermal expansion coefficients of the two F-shaped structural members are different, the two F-shaped structural members are arranged in a central symmetrical manner, and the middle parts of the two F-shaped structural members are fixedly connected, a gap is formed between the head end crossbeam of each F-shaped structural member and the tail end of the other F-shaped structural member, the optical fiber is provided with a first fiber Bragg grating and a second fiber Bragg grating, the first fiber Bragg grating and the second fiber Bragg grating are respectively located at the two gaps, the optical fibers on both sides of the first fiber Bragg grating are respectively fixedly connected to the head end crossbeam of one F-shaped structural member and the tail end of the other F-shaped structural member, and the optical fibers on both sides of the second fiber Bragg grating are respectively fixedly connected to the tail end of one F-shaped structural member and the head end crossbeam of the other F-shaped structural member.

[0007] The two F-shaped structural members of the present invention have a large difference in thermal expansion coefficients, and the relative displacement change of the two F-shaped structural members caused by temperature is converted into the strain change of the fiber grating packaged thereon, thereby greatly improving the sensitivity of the fiber grating temperature sensor. When the temperature rises and falls, the central wavelength change of each fiber grating is used for detection, so that the measurement range of the temperature sensor is doubled compared with that of conventional temperature sensors.

[0008] As a preferred solution of the present invention, the lengths of the two gaps are equal, and the grating area lengths of the first fiber grating and the second fiber grating are the same and smaller than the length of the gap, that is, the first fiber grating and the second fiber grating are in a suspended state.

[0009] As a preferred solution of the present invention, the grating region length of the first fiber grating and the second fiber grating is less than 5 mm, and has the characteristics of short grating region and high tensile strength.

[0010] As a preferred solution of the present invention, the center wavelengths of the first fiber grating and the second fiber grating differ by at least 5 nm.

[0011] As a preferred solution of the present invention, the first fiber grating and the second fiber grating are written on the optical fiber by a femtosecond laser without stripping off the coating layer.

[0012] As a preferred solution of the present invention, the optical fiber is fixed to the F-shaped structural member by adhesive bonding or low-temperature glass welding after being prestressed and tightened.

[0013] As a preferred solution of the present invention, the middle cross beams of the two F-shaped structural members are fixedly connected by a threaded fastener.

[0014] As a preferred solution of the present invention, the two F-shaped structural members are identical in shape and size.

[0015] As a preferred solution of the present invention, one of the two F-shaped structural members is an Invar structural member, and the other is an aluminum alloy structural member or a stainless steel structural member. Of course, the two F-shaped structural members can also be made of other materials with a larger difference in thermal expansion coefficient.

[0016] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0017] 1. The two F-shaped structural members of the present invention have a large difference in thermal expansion coefficients, and the relative displacement change of the two F-shaped structural members caused by temperature is converted into the strain change of the fiber Bragg grating encapsulated thereon, thereby greatly improving the sensitivity of the fiber Bragg grating temperature sensor. When the temperature rises and falls, the central wavelength change of each fiber Bragg grating is used for detection, so that the measurement range of the temperature sensor is doubled compared with that of conventional temperature sensors.

[0018] 2. The present invention uses fiber Bragg gratings fixed at different positions of the same structure to measure heating and cooling respectively, and no matter heating or cooling, the central wavelength of a fiber Bragg grating in working state always increases, that is, the fiber Bragg grating is elongated. The present invention adopts a two-point fixing method to effectively avoid the chirp problem of the fully-adhesive fiber Bragg grating temperature sensor.

[0019] 3. The temperature sensor of the present invention has a simple structure, is easy to process and is convenient to package. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic structural diagram of the double F-shaped fiber Bragg grating temperature sensor of the present invention.

[0021] Figure 2 It is a top view of the double F-shaped fiber Bragg grating temperature sensor of the present invention.

[0022] Figure 3 It is a structural schematic diagram of the F-shaped structural member described in the present invention.

[0023] Icon 1-F-shaped structural member, 11-head end beam, 12-tail end, 13-middle beam, 2-optical fiber, 3-notch, 4-first optical fiber Bragg grating, 5-second optical fiber Bragg grating, 6-threaded fastener. DETAILED DESCRIPTION

[0024] The present invention will be described in detail below in conjunction with the accompanying drawings.

[0025] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0026] Example 1

[0027] like Figure 1-Figure 3 As shown, the first fiber Bragg grating 4 and the second fiber Bragg grating 5 written on the same optical fiber 2 are slightly prestressed and respectively packaged between two adjacent pairs of end points of two F-shaped structural members 1 to form a double F-shaped fiber Bragg grating temperature sensor. The thermal expansion coefficients of the two F-shaped structural members 1 are quite different. The two F-shaped structural members 1 are arranged in a central symmetrical manner, and the middle parts of the two F-shaped structural members 1 are fixedly connected. A gap 3 is formed between the head end crossbeam 11 of one F-shaped structural member 1 and the tail end 12 of the other F-shaped structural member 1, and the first fiber Bragg grating 4 and the second fiber Bragg grating 5 are respectively suspended at the gap 3.

[0028] When the ambient temperature of the sensor changes, on the one hand, the thermal expansion effect and thermo-optical effect of the optical fiber material will cause a small change in the central wavelengths of the two fiber gratings. On the other hand, due to the large difference in the thermal expansion coefficients of the two F-shaped structural members 1, the distance between the two adjacent pairs of endpoints will become longer or shorter, thereby tightening or loosening the fiber grating fixed therebetween, and the stress will cause a large change in the central wavelengths of the two fiber gratings. Compared with conventional fiber grating temperature sensors, the unique design of the double F structure concentrates the displacement changes caused by thermal expansion and contraction on the shorter fiber grating. Therefore, whether the temperature rises or falls, it will inevitably cause a significant change in the wavelength of one of the two fiber gratings, which greatly improves the sensitivity of the sensor. At the same time, the increase and decrease in temperature are measured independently by two fiber gratings, so the range of the sensor is also doubled compared to the conventional one.

[0029] The working principle of the present invention is described in detail below.

[0030] When the temperature of the environment where the sensor is located increases, the first fiber grating 4 is in working state. On the one hand, due to the thermal expansion effect and thermo-optical effect of the optical fiber material, the relative wavelength shift of the center wavelengths of the first fiber grating 4 and the second fiber grating 5 is

[0031]

[0032] In the formula, λ B is the Bragg center wavelength of the fiber Bragg grating in the free state, Δλ B-T is the drift of the central wavelength of the fiber Bragg grating due to a temperature increase of ΔT, ξ is the thermo-optical coefficient of the fiber material, α is the thermal expansion coefficient of the fiber material, and ΔT is the temperature increase relative to the initial state.

[0033] On the other hand, since the thermal expansion coefficients of the two F-shaped structural members 1 are quite different (it is assumed here that the F-shaped structural member 1 on the left is made of a material with a larger thermal expansion coefficient, and the F-shaped structural member 1 on the right is made of a material with a lower thermal expansion coefficient), as Figure 1-2 As shown, the F-shaped structural member 1 on the left will have a larger elongation after the temperature rises, while the F-shaped structural member 1 on the right will have a smaller elongation. Since the middle parts of the two F-shaped structural members 1 are fixedly connected. Then, Figure 1 The distance between the left end of the F-shaped structural member 1 on the left and the left end of the F-shaped structural member 1 on the right will become larger, and its value ΔL is

[0034] ΔL=(L h ×α h -L l ×α l )×ΔT

[0035] Where, L h and L l are the effective lengths of the F-shaped structural member 1 on the left and the F-shaped structural member 1 on the right, respectively, h and α l are the thermal expansion coefficients of the materials of the F-shaped structural member 1 on the left and the F-shaped structural member 1 on the right, respectively.

[0036] The increase in the distance between the two will stretch the first fiber Bragg grating 4 fixed between the two, causing additional strain inside it. According to the fiber Bragg grating sensing principle for strain, the relative wavelength drift of the first fiber Bragg grating 4 caused by this axial strain is

[0037]

[0038] In the formula, Δλ B-ε is the drift of the fiber Bragg grating center wavelength caused by the relative displacement of the two F-shaped structural members 1, P eff is the effective elastic-optic coefficient of the optical fiber, ε is the strain of the fiber Bragg grating 4, and L is the initial distance of the notch 3 (ie, the effective length of the fiber Bragg grating).

[0039] The relative wavelength drift of the first fiber Bragg grating 4 due to the increase in ambient temperature is the sum of equations (1) and (2):

[0040]

[0041] In this way, by optimizing the size and material of the F-shaped structural member 1 and shortening the initial minimum distance L of the gap 3 between two F-shaped structural members 1, the response sensitivity of the fiber Bragg grating to temperature can be greatly improved.

[0042] For the second fiber Bragg grating 5, when the temperature rises, the central wavelength of the fiber Bragg grating caused by the temperature increases. However, at this time, the distance between the right end of the F-shaped structural member 1 on the left and the right end of the F-shaped structural member 1 on the right will be reduced, so that the prestress of the second fiber Bragg grating 5 fixed thereon is released. When the temperature further rises to a certain value, the prestress of the second fiber Bragg grating 5 will be completely released. In this process, the double F structure has no sensitization effect on it, and it is only a simple temperature response sensitivity.

[0043] On the contrary, when the ambient temperature decreases, the second fiber Bragg grating 5 is in working state. In addition to the normal temperature response, the second fiber Bragg grating 5 will also be subjected to additional strain due to the double F structure. However, at this time, the center wavelength of the second fiber Bragg grating 5 caused by temperature will become smaller, while the strain imposed by the double F structure will increase, that is, the center wavelength of the second fiber Bragg grating 5 caused by strain will increase. Therefore, the relative wavelength drift of the second fiber Bragg grating 5 caused by the decrease in ambient temperature is the difference between (1) and (2)

[0044]

[0045] Since the strain response of the double F structure applied to the second fiber Bragg grating 5 by the temperature drop is about one order of magnitude higher than the simple temperature response, the overall response sensitivity of the second fiber Bragg grating 5 to temperature is still greatly improved. Similarly, at this time, the first fiber Bragg grating 4 is only a simple temperature response sensitivity, and the double F structure has no sensitization effect on it.

[0046] The temperature decrease and increase are measured respectively by two fiber Bragg gratings. Therefore, when the tensile strength of the fiber Bragg gratings is consistent, the measuring range of the double F-shaped fiber Bragg grating temperature sensor is doubled compared with the conventional one.

[0047] The double F-shaped large-range and high-sensitivity fiber Bragg grating temperature sensor of the present invention has a simple structure, mainly comprising two fiber Bragg gratings and a double F-shaped structure, and matching wiring terminals and a protective housing can be added according to actual installation needs.

[0048] Example 2

[0049] This embodiment further describes the structure of the embodiment 1 in detail. Specifically, in this embodiment, the double F-shaped fiber Bragg grating temperature sensor includes two F-shaped structural components 1, an optical fiber 2, a first fiber Bragg grating 4, a second fiber Bragg grating 5 and a threaded fastener 6.

[0050] like Figure 3As shown, the two F-shaped structural members 1 are both F-shaped structures with exactly the same size, facing each other end to end, but made of different materials. Each F-shaped structural member 1 includes a head end cross beam 11, a tail end 12, and an intermediate cross beam 13, wherein the tail end refers to the tail end point of the vertical rod connecting the head end cross beam 11 and the intermediate cross beam 13. A notch 3 is formed between the head end cross beam 11 of one F-shaped structural member 1 and the tail end 12 of another F-shaped structural member 1, respectively, and the lengths of the two notches 3 are equal. The intermediate cross beams 13 of the two F-shaped structural members 1 are fixedly connected by a threaded fastener 6.

[0051] The first fiber grating 4 and the second fiber grating 5 are inscribed on the optical fiber 2 by a femtosecond laser without stripping the coating layer, and the central wavelengths of the two fiber gratings differ by at least 5nm. The grating area lengths of the first fiber grating 4 and the second fiber grating 5 are the same and smaller than the length of the notch 3, and the distance between the two fiber gratings is greater than the distance between the two notches 3. The first fiber grating 4 and the second fiber grating 5 are fixed between the tail end 13 of an F-shaped structural member 1 and the head end cross beam 11 of another F-shaped structural member 1 by adhesive bonding or low-temperature glass welding after being slightly prestressed and tightened, and the grating area is located at the notch 3 and is in a suspended state.

[0052] The F-shaped structural member 1 on the left side of this embodiment is made of aluminum alloy, and its thermal expansion coefficient is 2.2×10 -5 / ℃, the F-shaped structural member 1 on the right is made of Invar alloy, and its thermal expansion coefficient is 1.2×10 -6 / ℃. Figure 2 As shown, the total width of the two F-shaped structural members 1 after being fixedly connected is equal to the maximum width of each F-shaped structural member 1, that is, the length of the head end cross beam 11 in the F-shaped structural member 1. The length of the head end cross beam 11 is twice the length of the middle cross beam 13. The thickness of the F-shaped structural member 1 is 2mm, the length of the head end cross beam 11 is 6mm, the width is 5mm, the length of the middle cross beam 13 is 3mm, the width is 5mm, and the center distance between the head end cross beam 11 and the middle cross beam 13 is 20mm. The total length of the vertical rod of the F-shaped structural member 1 is 35mm, and the width is 2mm. From the cross section, the vertical rod is square. According to the above dimensions, the width of the gap 3 formed between the head and tail after the two F-shaped structures 1 are fixedly connected is 5mm.

[0053] In this embodiment, the central wavelengths of the second fiber grating 5 and the first fiber grating 4 are 1555 nm and 1548 nm respectively, and the thermo-optical coefficient of the optical fiber 2 is 6.34×10 -6 / ℃, thermal expansion coefficient is 0.55×10 -6 / ℃, the effective elastic coefficient is 0.22, then according to the above parameters, when the temperature rises, the sensitivity of the double F-shaped fiber Bragg grating temperature sensor is about 90pm / ℃; when the temperature drops, the sensitivity of the double F-shaped fiber Bragg grating temperature sensor is about 40pm / ℃. If the working wavelength range of the second fiber Bragg grating 5 and the first fiber Bragg grating 4 is conservatively set to 5nm, the double F-shaped fiber Bragg grating temperature sensor can cover a measurement range of about 170℃.

[0054] The distance between the head end cross beam 11 and the middle cross beam 13 and the length of the vertical rod can be changed according to the material and temperature sensitivity requirements to achieve the adjustment of sensitivity and measurement range.

[0055] Example 3

[0056] In this embodiment, the F-shaped structural member 1 on the left is made of stainless steel, and the central wavelengths of the second fiber grating 5 and the first fiber grating 4 are 1550nm and 1544nm respectively. Other components and their connection relationships are the same as those in the second embodiment.

[0057] Example 4

[0058] In this embodiment, the distance between the head end cross beam 11 and the middle cross beam 13 of the two F-shaped structural members 1 is 20 mm, the total length of the vertical rod is 40 mm, and the other components and the connection relationship of the components are the same as those in the second embodiment.

[0059] Example 5

[0060] In this embodiment, the F-shaped structural member 1 on the left is made of Invar alloy, and the F-shaped structural member 1 on the right is made of aluminum alloy. Other components and their connection relationships are the same as those in the fourth embodiment.

[0061] Example 6

[0062] The thickness of the two F-shaped structural members 1 is 2.5 mm. Other components and their connection relationships are the same as those in the second embodiment.

[0063] Example 7

[0064] The thickness of the two F-shaped structural members 1 is 3 mm. Other components and their connection relationships are the same as those in the second embodiment.

[0065] Example 8

[0066] The thickness of the two F-shaped structural members 1 is 2.5 mm. Other components and their connection relationships are the same as those in the fourth embodiment.

[0067] Example 9

[0068] The thickness of the two F-shaped structural members 1 is 3 mm. Other components and their connection relationships are the same as those in the fourth embodiment.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A double F-shaped fiber Bragg grating temperature sensor, characterized in that: The invention comprises two F-shaped structural members (1) and an optical fiber (2), wherein the two F-shaped structural members (1) have different thermal expansion coefficients, the two F-shaped structural members (1) are centrally symmetrically arranged, and the middle parts of the two F-shaped structural members (1) are fixedly connected, a gap (3) is formed between a head end cross beam (11) of each F-shaped structural member (1) and a tail end (12) of another F-shaped structural member (1), and the optical fiber (2) is provided with a first optical fiber grating (4) and a second optical fiber grating (5), and the first optical fiber grating (5) is The optical fiber grating (4) and the second optical fiber Bragg grating (5) are respectively located at the two notches (3); the optical fibers (2) on both sides of the first optical fiber Bragg grating (4) are respectively fixedly connected to the head end cross beam (11) of one of the F-shaped structural components (1) and the tail end (12) of the other F-shaped structural component (1); the optical fibers (2) on both sides of the second optical fiber Bragg grating (5) are respectively fixedly connected to the tail end (12) of one of the F-shaped structural components (1) and the head end cross beam (11) of the other F-shaped structural component (1); and the middle cross beams (13) of the two F-shaped structural components (1) are fixedly connected via two threaded fasteners (6).

2. The double F-shaped fiber Bragg grating temperature sensor according to claim 1, characterized in that: The two gaps (3) are of equal length, and the grating area lengths of the first optical fiber grating (4) and the second optical fiber grating (5) are the same and smaller than the length of the gap (3).

3. The double F-shaped fiber Bragg grating temperature sensor according to claim 1, characterized in that: The grating area lengths of the first optical fiber grating (4) and the second optical fiber grating (5) are less than 5 mm.

4. The double F-shaped fiber Bragg grating temperature sensor according to claim 1, characterized in that: The center wavelengths of the first optical fiber grating (4) and the second optical fiber grating (5) differ by at least 5 nm.

5. The double F-shaped fiber Bragg grating temperature sensor according to claim 1, characterized in that: The optical fiber (2) is inscribed with the first optical fiber grating (4) and the second optical fiber grating (5) by using a femtosecond laser without stripping off the coating layer.

6. The double F-shaped fiber Bragg grating temperature sensor according to claim 1, characterized in that: The optical fiber (2) is fixed to the F-shaped structural component (1) by means of adhesive bonding or low-temperature glass welding after being prestressed and tightened.

7. The double F-shaped fiber Bragg grating temperature sensor according to claim 1, characterized in that: The two F-shaped structural members (1) have the same shape and size.

8. The double F-shaped fiber Bragg grating temperature sensor according to any one of claims 1 to 7, characterized in that: One of the two F-shaped structural parts (1) is an Invar structural part, and the other is an aluminum alloy structural part or a stainless steel structural part.

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