Fiber Bragg grating ultrasonic sensor based on cladding replacement

Abstract

The invention discloses a fiber bragg grating ultrasonic sensor based on cladding replacement, which relates to the field of ultrasonic detection and comprises a cladding, a fiber core, a fiber bragg grating and a replacement cladding with a specific Poisson ratio. A fiber bragg grating is engraved at a set position in the fiber core; the replacement cladding with a specific Poisson ratio wraps the surface of the fiber core engraved with the fiber bragg grating; and the cladding layer wraps the surface of the fiber core which is not engraved with the fiber bragg grating. According to the invention, the optical fiber cladding outside the FBG is replaced, and the Poisson's ratio of the cladding material is specifically designed and replaced, so that the sensitivity enhancement of ultrasonic sensing in all directions of the FBG can be realized, and especially the detection capability of the FBG on ultrasonic waves in the direction vertical to the axial direction of the optical fiber is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of ultrasonic detection, and particularly to an optical fiber Bragg grating ultrasonic sensor based on cladding replacement. BACKGROUND

[0002] Ultrasonic detection is a classic non-destructive testing technology, which has many advantages such as wide detection range, high detection precision, and strong material applicability. The traditional and mainstream ultrasonic detection technology uses a piezoelectric probe as a sensing element, and realizes ultrasonic detection based on the positive piezoelectric effect. Due to the inherent properties of piezoelectric materials, the volume of the piezoelectric probe reaches more than one centimeter, which greatly limits its application in limited space arrangement and embedding in materials or structures. At the same time, piezoelectric materials are easily affected by electromagnetic interference, which may cause distortion of the received ultrasonic signal and introduce environmental noise. In addition, since the piezoelectric crystal needs to transmit electrical signals through a circuit, it is difficult for the piezoelectric probe to work stably in a high-humidity environment for a long time. In view of the above problems, scholars have proposed the concept of optical fiber ultrasonic sensor. Ultrasonic waves affect the physical properties of optical fibers (such as refractive index, length, phase, etc.), thereby changing the transmission characteristics of optical signals. By demodulating the changes in the optical signal, ultrasonic detection can be realized. Optical fiber ultrasonic sensors have the advantages of high sensitivity, wide bandwidth, bending, immunity to electromagnetic interference, and corrosion resistance, and have been preliminarily applied in industrial detection, medical imaging, structural health monitoring, and other fields.

[0003] Fiber Bragg Grating (FBG) is a commonly used optical fiber ultrasonic sensor. FBG is a region of periodic refractive index modulation in the core of the optical fiber. Ultrasonic waves can cause changes in the period or refractive index of FBG, resulting in a shift in the center wavelength of the FBG reflection spectrum. Further, combined with narrowband laser and sideband filtering technology, the dynamic change of the center wavelength of FBG caused by ultrasonic waves is modulated as the change of reflected light intensity, and then the light intensity signal is converted into an electrical signal by a photodetector, which can effectively detect ultrasonic waves in the range of Hz to MHz. However, since FBG is a unique structure along the axis direction of the optical fiber, it makes FBG ultrasonic detection have significant directional characteristics. That is, FBG is very sensitive to ultrasonic waves along the axis direction of the optical fiber (i.e. parallel to FBG), while the sensitivity to ultrasonic waves perpendicular to the axis direction of the optical fiber (i.e. perpendicular to FBG) is very low, which greatly limits the all-directional ultrasonic detection capability of FBG. For this problem, the commonly used solution is to use a cross-shaped FBG arrangement, that is, to arrange two perpendicular FBGs at the detection position, so as to realize the detection of ultrasonic waves in multiple directions. However, this method requires two optical fibers at each measurement point, and each optical fiber needs to be equipped with an optical signal demodulation module separately, which greatly increases the complexity of the optical path and the cost of the equipment, and therefore is difficult to be widely applied. In summary, it is urgent to develop an intrinsic FBG to realize all-directional ultrasonic detection and improve the ultrasonic detection capability. SUMMARY

[0004] The purpose of the present application is to provide a fiber Bragg grating ultrasonic sensor based on cladding replacement, which can realize all-directional ultrasonic detection and improve the ultrasonic detection capability.

[0005] To achieve the above-mentioned purpose, the present application provides the following solutions:

[0006] In a first aspect, the present application provides a fiber Bragg grating ultrasonic sensor based on cladding replacement, comprising: a cladding, a core, a fiber Bragg grating, and a replacement cladding with a specific Poisson's ratio.

[0007] The fiber Bragg grating is inscribed at a set position in the core; the replacement cladding with a specific Poisson's ratio is wrapped on the surface of the core where the fiber Bragg grating is inscribed; and the cladding is wrapped on the surface of the core where the fiber Bragg grating is not inscribed.

[0008] In an embodiment, the fiber Bragg grating is inscribed at the set position of the core by ultraviolet laser interference method or phase mask method.

[0009] In an embodiment, the core is doped with a set amount of germanium oxide or fluorine.

[0010] In an embodiment, the replacement cladding with the specific Poisson ratio has a Poisson ratio ranging from [-1, 0.5).

[0011] In an embodiment, the cladding at a set position of the core is removed by mechanical grinding or hydrofluoric acid etching, and then the replacement cladding with the specific Poisson ratio is coated on the surface of the core where the fiber Bragg grating is inscribed.

[0012] According to the specific embodiments provided in the present application, the following technical effects are achieved.

[0013] The present application provides an optical fiber Bragg grating ultrasonic sensor based on cladding replacement, comprising: a cladding, a core, a fiber Bragg grating, and a replacement cladding with a specific Poisson ratio; a fiber Bragg grating is inscribed at a set position in the core; the replacement cladding with the specific Poisson ratio is wrapped on the surface of the core where the fiber Bragg grating is inscribed; and the cladding is wrapped on the surface of the core where the fiber Bragg grating is not inscribed. By replacing the optical fiber cladding outside the FBG, the Poisson ratio of the replacement cladding material is designed, the sensitivity of the FBG to ultrasonic sensing in all directions can be improved, and the detection capability of the FBG to ultrasonic waves in the direction perpendicular to the optical fiber axis can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0015] Figure 1 A structure schematic diagram of an optical fiber Bragg grating ultrasonic sensor based on cladding replacement is provided for an embodiment of the present application.

[0016] Figure 2 A schematic diagram of an FBG under the action of an ultrasonic wave in the X-Z plane or the X-Y plane;

[0017] Figure 3 A spatial distribution schematic diagram of the absolute value of the total shift coefficient of the center wavelength of an FBG with a replacement cladding with different Poisson ratios;

[0018] Figure 4 A schematic diagram of an ultrasonic detection implementation process of a cladding replacement FBG fixed on the surface of a substrate;

[0019] Figure 5 A schematic diagram of an optical signal demodulation scheme based on FBG sideband filtering;

[0020] Figure 6A schematic diagram of an experimental arrangement for FBG to detect the directionality of ultrasonic waves;

[0021] Figure 7 A graph of the time-domain response of ultrasonic waves in the vertical direction detected by FBGs with different cladding materials;

[0022] Figure 8 A schematic diagram of the spatial distribution of the peak-to-peak values of ultrasonic waves in multiple directions detected by FBGs. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application.

[0024] The above objects, features and advantages of the present application will be more apparent from the following further detailed description in conjunction with the accompanying drawings and specific embodiments.

[0025] The present application relates to a method for improving the omnidirectional ultrasonic detection capability of a fiber Bragg grating (FBG), and in particular to solving the problem of weak ultrasonic response capability of the FBG in the direction perpendicular to the fiber axis. The FBG omnidirectional ultrasonic detection capability is enhanced by designing the Poisson's ratio of the outer fiber cladding material of the FBG. The present application belongs to the technical field of ultrasonic detection and is suitable for the field of nondestructive testing.

[0026] In an exemplary embodiment, as shown in Figure 1 A fiber Bragg grating based on cladding replacement is provided, including a cladding 4, a core 5, a fiber Bragg grating 6, and a replacement cladding 7 with a specific Poisson's ratio.

[0027] The fiber Bragg grating 6 is inscribed at a set position of the core 5; the replacement cladding 7 with a specific Poisson's ratio is wrapped on the surface of the core 5 where the fiber Bragg grating 6 is inscribed; and the cladding 4 is wrapped on the surface of the core 5 where the fiber Bragg grating 6 is not inscribed.

[0028] As an optional implementation, the fiber Bragg grating 6 is inscribed at the set position of the core 5 by ultraviolet laser interference or phase mask method.

[0029] As an optional implementation, the core 5 is doped with a set amount of germanium oxide or fluorine.

[0030] As an optional implementation, the replacement cladding 7 with the specific Poisson ratio has a Poisson ratio range of [-1, 0.5).

[0031] As an optional implementation, the cladding at a set position of the core 5 is removed by mechanical grinding or hydrofluoric acid etching, and then the replacement cladding 7 with the specific Poisson ratio is coated on the surface of the core 5 where the fiber Bragg grating is inscribed.

[0032] The present application realizes the sensitivity enhancement of the full-directional ultrasonic sensing of the fiber Bragg grating FBG by designing the Poisson ratio of the fiber cladding material, and especially improves the detection capability of the fiber Bragg grating FBG for the ultrasonic wave in the direction perpendicular to the fiber axis. The fiber Bragg grating FBG has obvious ultrasonic detection direction characteristics, and has excellent sensitivity for the ultrasonic wave in the direction of the fiber axis, and poor sensitivity for the ultrasonic wave in the direction perpendicular to the fiber axis.

[0033] Figure 1 A fiber Bragg grating structure based on cladding replacement is shown, which includes a cladding 4, a core 5, a fiber Bragg grating 6, and a replacement cladding 7 with a specific Poisson ratio. The material of the fiber is silica, which includes two parts of the core 5 and the cladding 4, and both are coaxial cylindrical structures. For a single-mode fiber, the diameter of the core 5 is within 10 μm, and the diameter of the fiber is 125 μm, so the thickness of the cladding 4 is about 57.5 μm. Since a small amount of germanium oxide or fluorine elements are added to the core 5, the refractive index thereof is slightly increased compared with the cladding 4, so that the light in the fiber Bragg grating 6 satisfies the total reflection condition, and can be transmitted along the core 5 without leaking to the cladding 4. The fiber Bragg grating 6 is a special structure with periodic refractive index modulation inscribed in the core by ultraviolet laser interference method or phase mask method, and selectively reflects a part of light and allows other wavelengths of light to pass through. Further combined with the FBG sideband filtering technology, effective detection of ultrasonic waves can be realized.

[0034] The original cladding 4 at the position of the FBG 6 is removed by mechanical grinding or hydrofluoric acid etching, and then a replacement cladding 7 with a specific Poisson's ratio is coated on the surface of the fiber core 5, so as to realize the replacement of the outer fiber cladding of the FBG. The FBG 6 is inscribed in the fiber core 5, so that removing the cladding 4 outside the FBG 6 will not have any effect on the FBG 6. The material of the replacement cladding 7 with a specific Poisson's ratio is selected according to the actual application requirements of ultrasonic detection. When the FBG 6 needs to maintain a balanced detection effect on ultrasonic waves from all directions, a negative Poisson's ratio material can be selected as the replacement cladding 7 with a specific Poisson's ratio, such as a porous negative Poisson's ratio material, a negative Poisson's ratio composite material and a molecular negative Poisson's ratio material. When the ultrasonic wave mainly comes from the direction of the fiber axis or the direction perpendicular to the fiber axis, a material with a Poisson's ratio close to 0.5 can be selected as the replacement cladding 7 with a specific Poisson's ratio, such as rubber and polydimethylsiloxane (PDMS).

[0035] The directionality and sensitivity enhancement strategy of the FBG ultrasonic detection are illustrated by establishing a theoretical model of the ultrasonic wave acting on the FBG.

[0036] The FBG is a structure with a periodic refractive index modulation in the fiber core. The FBG selectively reflects a part of light and allows other wavelengths of light to pass through. The center wavelength of the reflected light is called the Bragg wavelength λ B (nm), and the center wavelength, the refractive index and the grating period satisfy the following relationship:

[0037] λ B =2n0Λ0(1)

[0038] where n0 represents the initial refractive index and Λ0 represents the grating period (nm). As shown in the formula, the strain acting on the fiber can be decomposed into a combination along the X, Y and Z directions. It is assumed that the fiber is in free space, and the ultrasonic wave in the surrounding medium can be perfectly coupled to the FBG. When the wavelength of the ultrasonic wave is much larger than the grating length, n and Λ are affected by the dynamic strain as follows: Figure 2

[0039]

[0040] Λ=(1+ε z )Λ0(3)

[0041] where ε x , ε y and ε z represent the strain components along the X, Y and Z axes respectively, and P 11 and P 12 are strain optical tensors. Two special cases of the ultrasonic wave acting on the FBG are considered:

[0042] ​

[0043] ε x =ε y =-νε z (4)

[0044] Substituting formula (4) into formula (1) - formula (3), the shift amount Δλ B of the Bragg wavelength is:

[0045] Δλ B =η z λ B ε z (5)

[0046]

[0047] where η z is the shift coefficient of the FBG central wavelength.

[0048] ②Considering that the optical fiber only has transverse strain along the X-axis or Y-axis under the action of ultrasonic waves, since the optical fiber is rotationally symmetric in the X-axis and Y-axis and is isotropic material, the relationship of the three directions strain is as follows:

[0049] -νε x =ε y =ε z orε x =-νε y =ε z (7)

[0050] Substituting formula (7) into formula (1) - formula (3), the shift amount Δλ B of the Bragg wavelength is:

[0051] Δλ B =η i λ B ε i (7)

[0052]

[0053] where i is X or Y, and η i is the shift coefficient of the FBG central wavelength. The larger η i is, the more significant the FBG spectrum change caused by the ultrasonic wave is, and the better the recognition effect of the FBG on the ultrasonic wave is. For ordinary single-mode optical fiber, n0 is 1.4453, P 11 is 0.12, P 12 is 0.275, and v is 0.17. Therefore, η x ​=η y = -0.2924, η z = 0.7829. The absolute value of the FBG center wavelength offset coefficient satisfies η z >η x And η z >η y The relationship indicates that FBG is more effective at detecting ultrasonic waves along the fiber optic axis than in the direction perpendicular to the fiber optic axis.

[0054] Furthermore, such as Figure 2 As shown, this study investigates the special case of ultrasound acting on a fiber optic grating (FBG) in the XZ or XY plane, where 1 represents the first ultrasound source, 2 represents the first optical fiber, and 3 represents the second fiber Bragg grating. The ultrasound generated by the first ultrasound source 1 propagates in the XZ or YZ plane to the second fiber Bragg grating 3 within the core of the first optical fiber 2, causing the second fiber Bragg grating 3 to vibrate. A spatial coordinate system is established with the center of the second fiber Bragg grating 3 as the origin, and the angle between the direction of ultrasound propagation and the fiber axis is α (0-90°). The wavelength shift at the center of the second fiber Bragg grating 3 caused by the strain induced by the ultrasound is (taking the XZ plane as an example):

[0055] Δλ B =η x λ B ε x +η z λ B ε z =(η x sinα+η z cosα)λ B ε(10)

[0056] η = η x sinα+η z cosα(11)

[0057] Where η represents the total offset coefficient of the FBG center wavelength. The larger the absolute value of η, the greater the influence of the strain induced by the ultrasound on the reflection spectrum of the FBG, and the stronger the FBG's ability to detect ultrasound in this direction.

[0058] This study investigates the effect of ultrasonic waves generated by the first ultrasonic source 1 at different positions within the XZ plane on the FBG3. The total offset coefficient η is calculated using parameters of a standard single-mode fiber. Due to the in-plane symmetry of the FBG3, the total offset coefficient η exhibits axisymmetric distribution with respect to both the X (Y) and Z axes. Using the material Poisson's ratio v as a variable, and considering a range of v from -1 to 0.5, the spatial distribution of the absolute value of the total offset coefficient η at the FBG center wavelength is presented in polar coordinates for Poisson's ratios v of -1, -0.5, 0, 0.17, and 0.5. The results are as follows:Figure 3 It can be seen that when the Poisson's ratio v is close to -1, the absolute values of the total displacement coefficients η in each direction are close to each other, and are all close to 0.4, indicating that the FBG has basically the same effect on the ultrasonic detection in each direction. As the Poisson's ratio v gradually increases, the absolute value of the total displacement coefficient η in the direction parallel to the FBG continuously increases, that is, the ultrasonic detection capability of the FBG in the axial direction of the fiber gradually increases. As the Poisson's ratio v gradually increases, the absolute value of the total displacement coefficient η in the direction perpendicular to the FBG first decreases and then increases, that is, the ultrasonic detection capability of the FBG in the perpendicular direction first decreases and then increases. When the Poisson's ratio is close to 0.5, the absolute value of the total displacement coefficient η exceeds 0.4, reaching the maximum absolute value of η in theory, that is, the ultrasonic detection capability of the FBG in the perpendicular direction reaches the best.

[0059] In summary, the Poisson's ratio of the fiber material has a significant influence on the ultrasonic detection effect of the fiber Bragg grating. Therefore, a scheme of replacing the cladding material of the fiber is proposed to improve the ultrasonic detection direction characteristics of the fiber Bragg grating FBG, especially to improve the ultrasonic detection capability in the perpendicular direction.

[0060] Figure 4 The specific implementation process of the FBG for detecting ultrasonic waves by replacing the cladding is shown, wherein 8 is a narrow-band laser, 9 is a fiber circulator, 10 is a second optical fiber, 11 is a base material, 12 is a second ultrasonic source, 13 is a FBG with cladding replacement, 14 is a photodetector, 15 is a preamplifier, and 16 is an oscilloscope.

[0061] The continuous laser emitted by the narrow-band laser 8 enters one port of the fiber circulator 9 and is emitted from the other port into the second optical fiber 10. The entire process is connected and conducts the laser through the optical fiber.

[0062] After the laser passes through the FBG 13 with cladding replacement, the light of a specific wavelength is reflected back to the fiber circulator 9 from the other port and is emitted into the photodetector 14 through the optical fiber.

[0063] The optical signal is converted into an electrical signal by the photodetector 14, and then transmitted to the preamplifier 15 through the wire to amplify the electrical signal. Finally, it is connected to the oscilloscope 16 through the wire.

[0064] The second optical fiber 10 with the FBG 13 with cladding replacement is fixed on the surface of the base material 11 to detect the ultrasonic waves generated by the second ultrasonic source 12.

[0065] Figure 5 The demodulation principle of the ultrasonic wave-induced optical signal based on the FBG sideband filtering is shown. As shown in FIG. 6, the ultrasonic wave-induced optical signal is modulated by the FBG with cladding replacement, and the modulated signal is filtered by the FBG with cladding replacement. Figure 5As shown, the laser wavelength emitted by the narrowband laser is modulated to the wavelength corresponding to the -3dB power of the FBG reflection spectrum. The reflection spectrum near this wavelength exhibits a nearly linear change, with the steepest slope in the linear region, making it most sensitive to changes in the FBG reflected light power. Ultrasound waves cause vibrations in the FBG, manifesting as a left-right shift in the reflection spectrum. Due to the extremely narrow bandwidth and constant wavelength of the laser emitted by the narrowband laser, ultrasound waves cause significant changes in the intensity of the reflected light. By demodulating the optical signal reflected by the FBG and converting it into an electrical signal using a photodetector, effective observation of ultrasound waves can be achieved.

[0066] This application enhances the ultrasonic sensing sensitivity of the FBG in all directions by replacing the fiber cladding outside the fiber optic cable and specifically designing the Poisson's ratio of the replacement cladding material. This is particularly effective in improving the FBG's ability to detect ultrasonic waves perpendicular to the fiber optic axis. Current related technologies achieve multi-directional ultrasonic detection by arranging two perpendicularly intersecting FBG-clad optical fibers at the test location. In contrast, this application does not increase the number of optical fibers and allows for flexible fiber placement at the test location, significantly reducing the required number of sensing elements and associated signal acquisition equipment, resulting in a significant cost advantage.

[0067] The following section uses polydimethylsiloxane (PDMS) as an example, employing it as a replacement cladding material for FBG, and demonstrates its effect on improving ultrasonic detection capabilities through experiments. Figure 1 The original fiber cladding outside the FBG is removed, and PDMS is then coated onto the fiber core as a replacement cladding. The Poisson's ratio of PDMS is close to 0.5. According to... Figure 4 As shown, an FBG with a PDMS-replaced cladding was attached to a 1mm thick aluminum plate. A commercial piezoelectric probe was used as the ultrasonic source to excite ultrasonic waves on the aluminum plate to test the FBG's ability to recognize ultrasonic waves in different directions. The specific experimental setup is as follows. Figure 6 As shown in the diagram. 17 is the third optical fiber, 18 is the FBG (Built-in-Glass) of the PDMS replacement cladding, 19 is an aluminum plate, and 20 is the third ultrasonic source. The third optical fiber 17 of the FBG 18 of the PDMS replacement cladding is attached to the surface of the aluminum plate 19. Seven third ultrasonic sources 20 are arranged within the aluminum plate 19, with angles of 0°, 30°, 60°, 90°, 120°, 150°, and 180° relative to the axis of the third optical fiber 17, respectively. All third ultrasonic sources 20 are 5 cm away from the FBG 18 of the PDMS replacement cladding. A 50 kHz continuous sine wave is excited at each of the third ultrasonic sources 20 using a commercially available piezoelectric probe with a center frequency of 50 kHz. The ultrasonic waves are then applied using... Figure 5 The sideband filtering method shown demodulates the ultrasonic signal detected by FBG18 with PDMS-replaced cladding.

[0068] The FBG with conventional cladding (SiO2) and the FBG with PDMS replacing the cladding are tested for the detection of ultrasonic waves in different directions. First, Figure 7 The time-domain responses of the ultrasonic waves in the direction perpendicular to the optical fiber axis detected by the two FBGs are shown, in which SiO2 and PDMS represent the two FBGs with different cladding materials, respectively. A commercial piezoelectric probe is used to excite ultrasonic waves at the ultrasonic source, and the ultrasonic energy input into the aluminum plate is kept constant in the test. It can be seen that both the two FBGs can effectively detect the sine wave propagating on the aluminum plate. The peak-to-peak values of the ultrasonic waves detected by the conventional FBG and the FBG with PDMS replacing the cladding are 0.2607 V and 0.9256 V, respectively. The greater the peak-to-peak value is, the more energy the received ultrasonic signal contains. Therefore, by increasing the cladding Poisson's ratio, the detection capability of the FBG for the ultrasonic waves in the vertical direction can be significantly improved. Second, Figure 8 The peak-to-peak values of the ultrasonic waves generated by the ultrasonic sources in different directions detected by the two FBGs are shown. It can be seen that the peak-to-peak values of the ultrasonic waves in different directions detected by the FBG with PDMS cladding are greater than those of the conventional FBG with SiO2 cladding, indicating that replacing the cladding effectively improves the ultrasonic detection capability of the FBG.

[0069] In summary, by reasonably designing the cladding Poisson's ratio of the external optical fiber of the FBG, the directional characteristics of the ultrasonic detection of the FBG can be greatly improved, the detection capability of the FBG for the ultrasonic waves in all directions can be comprehensively improved, and then the high-sensitivity and large-range ultrasonic detection can be achieved.

[0070] The technical features of the above embodiments can be combined in any manner. To make the description concise, not all possible combinations of the technical features in the above embodiments are described, but as long as the combinations of the technical features do not exist contradictions, they should be considered as the scope of the present application.

[0071] The principles and implementation modes of the present application are described by using specific examples in this paper, and the above examples are only used to help understand the method and its core idea of the present application; at the same time, for those skilled in the art, according to the idea of the present application, the specific implementation mode and application range will be changed. In summary, the content of the specification should not be understood as a limitation of the present application.

Claims

1. A cladding-replaced fiber Bragg grating ultrasonic sensor, characterized in that, The application relates to an optical fiber with a fiber Bragg grating and a cladding with a specific Poisson ratio. The fiber Bragg grating is inscribed at a set position in the core; the cladding with the specific Poisson ratio is coated on the surface of the core where the fiber Bragg grating is inscribed; and the cladding is coated on the surface of the core where the fiber Bragg grating is not inscribed. The fiber Bragg grating is inscribed at a set position in the core by means of ultraviolet laser interference or phase mask method.

2. The cladding-replacement-based fiber Bragg grating ultrasonic sensor of claim 1, wherein, The core is doped with a set amount of germanium oxide or fluorine.

3. The cladding-replacement-based fiber Bragg grating ultrasonic sensor of claim 1, wherein, The Poisson ratio of the cladding with the specific Poisson ratio ranges from -1 to 0.

5.

4. The cladding-replacement-based fiber Bragg grating ultrasonic sensor of claim 1, wherein, The cladding at the set position of the core is removed by mechanical grinding or hydrofluoric acid etching, and then the cladding with the specific Poisson ratio is coated on the surface of the core where the fiber Bragg grating is inscribed.

5. The cladding-replacement-based fiber Bragg grating ultrasonic sensor of claim 1, wherein, ​

Images