Mold and method for preparing fiber bragg grating strain gauge

Abstract

The invention relates to the technical field of optical fiber sensors, and discloses a mold and a method for preparing an optical fiber grating strain gauge. The mold comprises an upper mold and a lower mold, a first rectangular groove is formed in the upper mold, and first arc-shaped grooves are connected to the two ends of the first rectangular groove; the lower die is provided with a second rectangular groove, and the two ends of the second rectangular groove are connected with second arc-shaped grooves. The upper die and the lower die are combined together to form a cavity matched with a fiber bragg grating strain gauge semi-finished product. The method can be used for manufacturing fiber grating strain gauges which are good in strain linearity, high in strain transmission efficiency, good in consistency, capable of being manufactured in batches, capable of bearing certain high temperature, small in size, provided with certain flexibility and capable of meeting different strain sensitivity requirements under different working conditions.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber sensors, and in particular to a mold and a method for preparing an optical fiber grating strain gauge. Background Art

[0002] Fiber Bragg grating sensor is a sensor based on the periodic structure inside the optical fiber. Compared with traditional resistance strain gauges, strain gauges and other sensors, fiber Bragg grating sensors have the advantages of fast response, small size, light weight, strong anti-interference, easy remote monitoring, etc., and are widely used in the field of structural health detection. At present, in actual engineering applications, fiber Bragg grating sensors are mostly surface-mounted. The basic working principle is: the strain generated by the object being measured is transmitted to the grating through the glue outside the grating, causing the change of the central wavelength of the grating, and then the strain generated on the surface of the object being measured is obtained.

[0003] CN 114739431 A discloses a method for encapsulating optical fiber sensors without adhesive and an optical fiber sensor. However, in the actual operation process, the amount of polyimide dripped in is not easy to control, which will affect the strain transfer coefficient of the grating strain gauge. The different strain transfer coefficients caused by the uncertainty of the dripping amount make it impossible to mass-produce and standardize the production. Due to the natural bending of polyimide, it may not be well attached to the base material, thereby affecting the strain transfer coefficient. When a polyamide solution is dripped in, bubbles or pores exist in the solution, which will affect the linear transmission of strain and the strain transfer coefficient after the polyimide film is solidified to form. The variable of the thickness between the base material and the optical fiber cannot be accurately controlled, resulting in the inability to repeatedly produce the sensor in batches.

[0004] CN 116972767 A discloses a fiber grating strain gauge and a manufacturing method thereof, which adopts an epoxy resin-based composite material as a base material and is packaged and prepared together with an optical fiber. In actual engineering tests, the epoxy resin-based composite material as a base material does not have a certain flexibility, so it cannot be well fitted on the surface of some objects with a slight curvature to obtain a relatively accurate strain size; the epoxy resin-based composite material will have a certain creep and aging in the long-term strain monitoring process, resulting in the inability to obtain accurate strain transfer results; the elastic modulus of the base material has an influence on the strain transfer coefficient of the grating strain gauge, and the epoxy resin-based composite material as a base material has no adjustable base, which cannot meet the sensitivity measurement requirements under the same test object; the manufactured strain gauge has a specification of 100mm*100mm, a large volume, and the strain measurement result is not accurate enough. Summary of the invention

[0005] The purpose of the present invention is to overcome the above-mentioned problems existing in the prior art, and to provide a mold and a method for preparing a fiber Bragg grating strain gauge, so as to solve the problems existing in the prior art surface mount packaging technology, such as the difficulty in ensuring the consistency and reliability of measurement parameters, the difficulty in ensuring the reliability and accuracy of measurement results, the insufficient mechanical strength, the large temperature influence, the complicated and difficult packaging, and the low production efficiency. The present invention can be used to produce fiber Bragg grating strain gauges with good strain linearity, high strain transfer efficiency, good consistency, batch production, ability to withstand a certain high temperature, small size, certain flexibility, and the ability to meet different strain sensitivity requirements under different working conditions.

[0006] In order to achieve the above object, the present invention provides a mold for preparing a fiber Bragg grating strain gauge, the mold comprising an upper mold and a lower mold, the upper mold is provided with a first rectangular groove, and the two ends of the first rectangular groove are connected with a first arc groove;

[0007] The lower die is provided with a second rectangular groove, and both ends of the second rectangular groove are connected with second arc grooves;

[0008] The upper mold and the lower mold are combined to form a cavity adapted to the semi-finished fiber grating strain gauge.

[0009] A second aspect of the present invention provides a method for preparing a fiber Bragg grating strain gauge, the method being implemented in the above-mentioned mold for preparing a fiber Bragg grating strain gauge, the method comprising the following steps:

[0010] S1, applying polyamic acid on the surface of the substrate, then placing the substrate with the polyamic acid on the surface in the second rectangular groove of the lower mold, then placing the optical fiber on the second arc groove, and making the grating of the optical fiber located at the second rectangular groove, and then placing packaging filling blocks on both sides of the grating, and the distance between the packaging filling blocks is greater than the diameter of the optical fiber;

[0011] S2, dripping polyamic acid into the gap between the encapsulation filling blocks, so that the polyamic acid wraps the optical fiber located at the second rectangular groove, and then closing the upper mold to perform a heat shrink polymerization reaction;

[0012] S3. After the heat shrink polymerization reaction is completed, the upper mold and the lower mold are separated to obtain a semi-finished fiber Bragg grating strain gauge, and then an adhesive is applied on the surface of the packaging filling block to form an adhesive layer, and a polyimide film is covered on the adhesive layer to obtain a fiber Bragg grating strain gauge.

[0013] Beneficial effects of the present invention:

[0014] 1. The consistency and reliability of the measurement parameters are guaranteed. The length, width and height of the package are fixed by using a mold. The polyamic acid solution used for the package is determined by type.

[0015] 2. The strain linearity is good and the strain transfer efficiency is high. The mold is used for fixing and curing, which avoids the bubbles and pores caused by the dripping of polyamide acid solution. It also avoids the natural bending of the polyimide film after preparation. The obtained fiber Bragg grating strain gauge has good linearity and high strain transfer efficiency.

[0016] 3. The measurement range is larger. After packaging, the gate area can withstand greater deformation, so the measurement range becomes larger.

[0017] 4. It can be produced on demand. According to different usage scenarios, different grating lengths and mold sizes can be selected to produce fiber Bragg grating strain gauges that meet the needs (generally speaking, since the grating length is generally 10mm or 5mm, the fiber Bragg grating strain gauges produced in this way are generally small in size).

[0018] 5. The installation is convenient and quick. When in actual use, you only need to stick it on the surface of the object to be measured to start using it, which greatly reduces the time for on-site installation.

[0019] 6. High mechanical strength, good overall performance, and the use of polyamic acid solution for packaging, which gets rid of the traditional surface-mount adhesive glue that is not resistant to high temperatures, is prone to aging and strength reduction due to long-term monitoring.

[0020] 7. Suitable for extremely changing environments. After packaging, the fiber grating and the base material can be packaged as a whole, and the thermal expansion coefficient is relatively close, which avoids the shedding and creep when the temperature changes.

[0021] 8. It has a certain flexibility and is encapsulated using polyamide acid solution. Due to its physical properties, it can be pasted on the surface of the object to be tested with a certain curvature.

[0022] 9. High production efficiency, batch production is possible, and mold production is used. A large number of identical strain gauges can be produced at one time, and the consistency and reliability of product performance are guaranteed.

[0023] 10. The appearance is beautiful and standardized. It is prepared by only using a mold for one time. Not only is the quality guaranteed, but the appearance is also consistent, which is more beautiful and standardized. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the upper mold structure of the mold of the present invention;

[0025] Figure 2 It is a schematic diagram of the lower mold structure of the mold of the present invention;

[0026] Figure 3 It is a flow chart of a method for preparing a fiber Bragg grating strain gauge using the mold of the present invention;

[0027] Figure 4is a top view of the packaging structure of the fiber Bragg grating strain gauge;

[0028] Figure 5 is a cross-sectional schematic diagram of the packaging structure of the fiber Bragg grating strain gauge;

[0029] Figure 6 It is accompanied by a tooling diagram.

[0030] Description of Reference Numerals

[0031] 1. Upper mold; 11. First rectangular groove; 12. First arc-shaped groove; 2. Lower mold; 21. Second rectangular groove; 22. Second arc-shaped groove; 3. Polyimide film; 31. Adhesive layer; 4. Substrate; 41. Adhesive layer; 5. Polyimide packaging block; 6. Packaging filling block; 7. Optical fiber; 71. Grating. DETAILED DESCRIPTION

[0032] The specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0033] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0034] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back...), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0035] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or suggesting their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In addition, the technical solutions between the various embodiments provided by the present invention can be combined with each other, but they must be based on the ability of ordinary technicians in the field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0036] The present invention will be described in detail below through examples.

[0037] Example 1

[0038] like Figure 1 and Figure 2 The mold for preparing the fiber Bragg grating strain gauge shown in FIG. Figure 1 The upper mold 1 shown and Figure 2 The lower die 2 shown (the upper die 1 and the lower die 2 are obtained by cutting and processing using a CNC machine tool) has a first rectangular groove 11 formed on the upper die 1, and first arc grooves 12 are connected to both ends of the first rectangular groove 11;

[0039] The lower die 2 is provided with a second rectangular groove 21, and both ends of the second rectangular groove 21 are connected with second arc grooves 22;

[0040] The upper mold 1 and the lower mold 2 are combined to form a cavity adapted to the semi-finished fiber Bragg grating strain gauge.

[0041] Furthermore, the bottom of the first arc-shaped groove 12 is flush with the bottom of the first rectangular groove 11 .

[0042] Furthermore, the bottom of the second arc-shaped groove 22 is spaced apart from the bottom of the first rectangular groove 21 (the bottom of the first rectangular groove 21 is lower than the bottom of the second arc-shaped groove 22), and the size of the spacing is equal to the thickness of the substrate 4 (the thickness of the substrate 4 is determined according to the transfer coefficient required by the specifically prepared fiber grating strain gauge).

[0043] In the present invention, the length of the first rectangular groove 11 is measured along the axial direction of the first arc-shaped groove 12, and the length of the second rectangular groove 21 is measured along the axial direction of the second arc-shaped groove 22; wherein the lengths of the first rectangular groove 11 and the second rectangular groove 21 are the same, and the widths of the first rectangular groove 11 and the second rectangular groove 21 are also the same.

[0044] In the present invention, the specific sizes of the second rectangular groove 21 and the first rectangular groove 11 are related to the size of the prepared fiber Bragg grating strain gauge, wherein the length and width of the second rectangular groove 21 and the first rectangular groove 11 are the same as the length and width of the packaging structure of the finally obtained fiber Bragg grating strain gauge, wherein the depth of the second rectangular groove 21 is equal to the sum of the radius of the optical fiber 7 and the thickness of the substrate 4, and the depth of the first rectangular groove 11 is equal to the radius of the optical fiber 7.

[0045] Furthermore, both the first arc-shaped groove 12 and the second arc-shaped groove 22 are semicircular grooves, and the diameters thereof are equal to the diameter of the optical fiber 7 .

[0046] Furthermore, screw holes are evenly arranged around the upper mold 1 and the lower mold 2. By inserting screws into the corresponding screw holes of the upper mold 1 and the lower mold 2, it is ensured that the upper mold 1 and the lower mold 2 can be tightly combined.

[0047] Example 2

[0048] like Figure 3 The method for preparing a fiber Bragg grating strain gauge shown in the embodiment is implemented in the mold for preparing a fiber Bragg grating strain gauge in Example 1, and comprises the following steps:

[0049] S1, polyamic acid is applied on the surface of the substrate 4, and then the substrate 4 with polyamic acid applied on the surface is placed in the second rectangular groove 21 of the lower mold 2 (the substrate 4 is a rectangular parallelepiped, the length of the substrate 4 is equal to the length of the second rectangular groove 21, and the width of the substrate 4 is equal to the width of the second rectangular groove 21), and then the optical fiber 7 is placed on the second arc groove 22, and the grating 71 of the optical fiber 7 is located at the second rectangular groove 21, and then the encapsulation filling blocks 6 are placed on both sides of the grating 71, and the distance between the encapsulation filling blocks 6 is greater than the diameter of the optical fiber 7;

[0050] S2, dripping polyamic acid into the gap between the encapsulation filling blocks 6, and making the polyamic acid wrap the optical fiber 7 located at the second rectangular groove 21, and then closing the upper mold 1 to perform a heat shrink polymerization reaction;

[0051] S3. After the heat shrink polymerization reaction is completed, the upper mold 1 and the lower mold 2 are separated to obtain a semi-finished fiber Bragg grating strain gauge. Then, adhesive is applied on the surface of the packaging filling block 6 to form an adhesive layer 31. The adhesive layer 31 is covered with a polyimide film 3 to obtain a fiber Bragg grating strain gauge.

[0052] In step S1 of the present invention, the material of the substrate 4 is selected from aluminum, aluminum alloy, stainless steel or copper (in this embodiment, the material of the substrate 4 is selected from stainless steel); since the elastic modulus of the material of the substrate 4 will affect the strain transfer coefficient, that is, the sufficiency of the strain transfer of the measured object, when the shear modulus of the material of the substrate 4 is less than 400Mpa, a small change in the shear modulus will have a greater impact on the strain transfer coefficient. Therefore, the material selected here avoids the influence of the shear modulus on strain transfer on the one hand, and different materials can be used to prepare fiber grating strain gauges with different strain transfer coefficients to meet the sensitivity requirements under different working conditions. Therefore, in the selection of materials, materials with large shear modulus are selected, and the thickness of the substrate 4 will affect the strain transfer coefficient. The thicker the substrate, the less sufficient the strain transfer. Then, the thinner the material that meets the requirements has a correspondingly higher preparation cost. Therefore, under the condition of meeting different working conditions, a substrate 4 of appropriate thickness can be selected.

[0053] Further in step S1 of the present invention, the purpose of applying polyamic acid on the surface of substrate 4 is to degrease and roughen the surface of substrate 4 (degreasing is to remove grease, dust, sweat and other substances on the surface of substrate 4, and roughening makes the surface of substrate 4 microscopically rough, which can ensure that the subsequently dropped polyamic acid has good bonding force with the surface of substrate 4) to ensure good wettability and adhesion. At the same time, after the subsequent heat shrinkage polymerization reaction, the polyamic acid on the surface of substrate 4 will solidify to form a corresponding bonding layer 41 (the material of bonding layer 41 is polyimide) on the surface of substrate 4, which further plays a bonding role.

[0054] In the present invention, the amount of polyamic acid applied on the surface of the substrate 4 is sufficient as long as it can play a role in degreasing and bonding. Therefore, the depth of the second rectangular groove 21 in the present invention is equal to the sum of the radius of the optical fiber 7, the thickness of the substrate 4 and the thickness of the bonding layer 41. However, in actual operation, the amount of polyamic acid applied on the surface of the substrate 4 is relatively small, so the thickness of the bonding layer 41 can be ignored.

[0055] In the present invention, the optical fiber 7 is placed on the second arc groove 22 at both ends of the second rectangular groove 21, and the optical fiber 7 located at the second arc groove 22 is fixed in turn with a high-temperature tape of polyimide, and at the same time, the grating 71 on the optical fiber 7 (the grating 71 is engraved in turn using an excimer laser and a phase mask, the optical fiber is placed behind the phase mask (the fiber stripping part faces the phase mask), and the laser is emitted by the excimer laser. The laser is irradiated to the phase mask after passing through the beam expander and the focusing lens, and after passing through the phase mask, a diffraction beam is formed, which is irradiated to the optical fiber core, and the fiber The hydrogen molecules in the illuminated part of the core immediately react chemically with germanium to form Ge-OH bonds and Ge-H bonds, thereby permanently increasing the refractive index of this part. The above steps are the grating writing process) is located at the second rectangular groove 21 (the coating layer at the grating 71 is stripped off. The coating layer is to protect the grating area and realize certain functions (such as the polyimide coating layer, which can make the optical fiber resistant to high temperatures). In the process of making the fiber grating strain gauge, since the grating 71 has been encapsulated inside, in order to reduce a transmission layer and make the result more accurate, the coating layer will be stripped off).

[0056] Furthermore, the length of the second rectangular groove 21 in the present invention is 2-3 times the length of the grating 71 . In this embodiment, the length of the second rectangular groove 21 is twice the length of the grating 71 , and the grating 71 is located in the middle of the second rectangular groove 21 .

[0057] In the present invention, the packaging filling block 6 used is in the shape of a rectangular parallelepiped, and the length of the packaging filling block 6 is equal to the length of the second rectangular groove 21, the height of the packaging filling block 6 is equal to the diameter of the optical fiber 7, the packaging filling blocks 6 are placed on both sides of the grating 71, and the side walls of the packaging filling blocks 6 are tightly fitted with the side walls of the second rectangular groove 21 (the packaging filling blocks 6 and the optical fiber 7 located at the second rectangular groove 21 are placed on a substrate 4 with polyamide acid coated on the surface), and the distance between the packaging filling blocks 6 on both sides is greater than the diameter of the optical fiber 7, that is, the width of the second rectangular groove 21 is equal to the sum of the width of the packaging filling blocks 6 on both sides and the distance between the packaging filling blocks 6 on both sides.

[0058] In the present invention, the encapsulation filling block 6 can limit the flow of polyamic acid, making the finished product more beautiful. At the same time, since the object to be measured is not necessarily a standard horizontal structure and may have a certain curvature, the encapsulation filling block 6 must have a certain elasticity. At the same time, the material selected for the encapsulation filling block 6 needs to have a certain flexibility, so that the finished product can have a certain flexibility. Therefore, in a preferred case, the material of the encapsulation filling block 6 used in the present invention is selected from polyimide, epoxy resin or silicone rubber, and further epoxy resin.

[0059] In the present invention, after the encapsulation filling blocks 6 are placed on both sides of the grating 71, polyamic acid is dripped into the gap between the encapsulation filling blocks 6, and the dripped polyamic acid fills the gap between the encapsulation filling blocks 6 and wraps the optical fiber 7 located at the second rectangular groove 21 (polyamic acid has low fluidity and is a viscous liquid at room temperature with a certain surface tension, so it can completely fill the gap between the encapsulation filling blocks 6), and then the upper mold 1 is closed, and the upper mold 1 is tightly combined with the lower mold 2 (so that the first rectangular groove 11 and the second rectangular groove 21 are matched to form a cavity adapted to the encapsulation structure of the semi-finished fiber grating strain gauge). The chamber, the first arc groove 12 and the second arc groove 22 correspond to each other to form a fiber long hole for placing the optical fiber 7), and is placed in a hot oven for heat shrinkage polymerization reaction (including two processes of dehydration and imidization). The conditions of the heat shrinkage polymerization reaction include: keeping warm at 80-100°C for 0.5-1.5h, then heating to 150-200°C for 0.5-1.5h, and finally heating to 200-220°C for 1-3h; in this embodiment, the conditions of the heat shrinkage polymerization reaction include: keeping warm at 100°C for 1h, then heating to 160°C for 1h, and finally heating to 220°C for 2h.

[0060] After the heat shrink polymerization reaction is completed in the present invention, the hot oven is closed and allowed to cool naturally. When the temperature drops to an appropriate temperature, the mold is taken out, and the upper mold 1 and the lower mold 2 are separated to obtain a semi-finished fiber Bragg grating strain gauge (at this time, the polyamic acid filled in the gap between the encapsulation filling blocks 6 undergoes a heat shrink polymerization reaction and solidifies to form a polyimide encapsulation block 5, the length of the polyimide encapsulation block 5 is equal to the length of the second rectangular groove 21, the height is equal to the diameter of the optical fiber 7, and the width is equal to the distance between the encapsulation filling blocks 6 on both sides; at the same time, the polyamic acid applied on the surface of the substrate 4 is solidified to form a corresponding bonding layer 41 on the upper surface of the substrate 4.

[0061] In the present invention, a semi-finished fiber Bragg grating strain gauge is taken, and adhesive is applied on the upper surfaces of the packaging filling block 6 and the polyimide packaging block 5 to form an adhesive layer 31, and then the polyimide film 3 is covered on the upper surface of the adhesive layer 31 (the polyimide film 3 is used here to cover the fiber Bragg grating strain gauge to a certain extent for packaging and beautification; the polyimide film 3 has high elasticity and low thermal expansion coefficient, and there will be no aging creep phenomenon in long-term use, which makes the structure of the grating strain gauge stable, and due to the physical properties of polyimide, the grating strain gauge can work normally in a certain high temperature environment), and finally the fiber Bragg grating strain gauge is obtained.

[0062] Furthermore, the amount of adhesive used is sufficient to form an adhesive layer 31 to adhere to the polyimide film 3. Furthermore, the length of the polyimide film 3 is equal to the length of the second rectangular groove 21, the width of the polyimide film 3 is equal to the width of the second rectangular groove 21, and the thickness is determined according to the transfer coefficient required by the specifically prepared fiber grating strain gauge.

[0063] In the present invention, the adhesive is a conventional adhesive type glue. Further, the adhesive is selected from epoxy resin adhesive and / or UV curing adhesive. In this embodiment, the adhesive is selected from UV curing adhesive.

[0064] The fiber Bragg grating strain gauge prepared in the present invention comprises an optical fiber 7, a section of the optical fiber 7 is provided with a packaging structure, and a top view of the packaging structure is shown in FIG. Figure 4 As shown, the cross section of the package structure is Figure 5As shown, the packaging structure includes a substrate 4 and an adhesive layer 41 arranged on the upper surface of the substrate 4, a polyimide packaging block 5 is arranged on the upper surface of the adhesive layer 41, and packaging filling blocks 6 are arranged on both sides of the polyimide packaging block 5. The polyimide packaging block 5 and the packaging filling block 6 cover the upper surface of the adhesive layer 41, the polyimide packaging block 5 is wrapped on the outside of the optical fiber 7, and the wrapped optical fiber 7 is provided with a grating 71; an adhesive layer 31 is arranged on the upper surfaces of the polyimide packaging block 5 and the packaging filling block 6, and the upper surface of the adhesive layer 31 is also covered with a polyimide film 3 (compared with the packaging structure of the optical fiber Bragg grating strain gauge semi-finished product, the packaging structure of the optical fiber Bragg grating strain gauge semi-finished product does not have the adhesive layer 31 and the polyimide film 3).

[0065] Furthermore, the unpackaged exposed optical fiber is still a bare fiber, and is still easy to break during actual use. Therefore, the exposed optical fiber and the ordinary bare fiber, about 6-12 strands, are further combined with the optical fiber jointing machine to perform jointing work, such as Figure 6 As shown, the non-grid area parts are protected by parallel strips, which can enhance the bending and tensile resistance of the non-grid area parts.

[0066] It should be understood that parts not elaborated in detail in this specification belong to the prior art.

[0067] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A mold for preparing a fiber Bragg grating strain gauge, characterized in that: The mold comprises an upper mold (1) and a lower mold (2); a first rectangular groove (11) is formed on the upper mold (1); and first arc grooves (12) are connected to both ends of the first rectangular groove (11); The lower die (2) is provided with a second rectangular groove (21), and the two ends of the second rectangular groove (21) are connected with second arc grooves (22); The upper mold (1) and the lower mold (2) are combined together to form a cavity that is compatible with the semi-finished fiber grating strain gauge.

2. The mold for preparing a fiber Bragg grating strain gauge according to claim 1, characterized in that: The groove bottom of the first arc-shaped groove (12) is flush with the groove bottom of the first rectangular groove (11).

3. The mold for preparing a fiber Bragg grating strain gauge according to claim 1, characterized in that: The groove bottom of the second arc-shaped groove (22) is spaced apart from the groove bottom of the first rectangular groove (21).

4. A method for preparing a fiber Bragg grating strain gauge, characterized in that: The method is implemented in a mold for preparing a fiber Bragg grating strain gauge according to any one of claims 1 to 3, and the method comprises the following steps: S1, applying polyamic acid on the surface of the substrate (4), then placing the substrate (4) with the surface applied with polyamic acid in the second rectangular groove (21) of the lower mold (2), then placing the optical fiber (7) on the second arc groove (22), and making the grating (71) of the optical fiber (7) located at the second rectangular groove (21), and then placing encapsulation filling blocks (6) on both sides of the grating (71), and the distance between the encapsulation filling blocks (6) is greater than the diameter of the optical fiber (7); S2, dripping polyamic acid into the gap between the encapsulation filling blocks (6), so that the polyamic acid wraps the optical fiber (7) located at the second rectangular groove (21), and then closing the upper mold (1) to perform a heat shrink polymerization reaction; S3. After the heat shrink polymerization reaction is completed, the upper mold (1) and the lower mold (2) are separated to obtain a semi-finished fiber Bragg grating strain gauge. Then, adhesive is applied on the surface of the packaging filling block (6) to form an adhesive layer (31), and a polyimide film (3) is covered on the adhesive layer (31) to obtain a fiber Bragg grating strain gauge.

5. The method for preparing a fiber Bragg grating strain gauge according to claim 4, characterized in that: The material of the substrate (4) is selected from aluminum, aluminum alloy, stainless steel or copper.

6. The method for preparing a fiber Bragg grating strain gauge according to claim 4 or 5, characterized in that: The packaging filling block (6) is in the shape of a cuboid.

7. The method for preparing a fiber Bragg grating strain gauge according to claim 4, characterized in that: The material of the packaging filling block (6) is selected from polyimide, epoxy resin or silicone rubber.

8. The method for preparing a fiber Bragg grating strain gauge according to claim 4, characterized in that: The length of the second rectangular groove (21) is 2-3 times the length of the grating (71).

9. The method for preparing a fiber Bragg grating strain gauge according to claim 4, characterized in that: The conditions of the heat shrink polymerization reaction include: keeping the temperature at 80-100° C. for 0.5-1.5 hours, then heating to 150-200° C. for 0.5-1.5 hours, and finally heating to 200-220° C. for 1-3 hours.

10. The method for preparing a fiber Bragg grating strain gauge according to claim 4, characterized in that: The adhesive is selected from epoxy resin adhesive and / or UV curing adhesive.

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