A large-diameter fiber grating and a method for manufacturing the same

Abstract

The application relates to the technical field of fiber grating, in particular to a large-diameter fiber grating and a preparation method thereof. In order to solve the problem that the fiber grating in the prior art cannot be applied to strong acid and high-temperature extreme environments, the application provides the large-diameter fiber grating and the preparation method thereof, the fiber has a single-mode transmission characteristic and the outer diameter of the fiber is 0.7-1.5 mm. The application fundamentally overcomes the technical barriers that the existing fiber is easily broken and fails in a high-temperature and strong-acid compound extreme and harsh environment, the cost is high, and the performance is difficult to consider, and has a wide application prospect in harsh engineering fields such as petroleum chemical industry, deep land and deep sea exploration and metallurgical refining.

Description

Technical Field

[0001] This invention relates to the field of fiber grating technology, specifically to a large-diameter fiber grating and its fabrication method. Background Technology

[0002] Currently, fiber Bragg gratings (FBGs), as core devices for fiber optic sensing and communication, are widely used in strong acid and high temperature fields such as chemical smelting, energy refining, and extreme environment monitoring. However, traditional fiber Bragg gratings mostly use quartz-based single-mode fiber (such as the commonly used 9 / 125 µm standard fiber), which faces severe limitations in both thermal and chemical stability under extreme operating conditions. On the one hand, its long-term operating temperature is limited by the thermal stability of the quartz material and the thermal degradation of the grating refractive index modulation mechanism. In environments above 800℃, the glass structure of the quartz optical fiber gradually loosens, resulting in a decrease in the intensity of the reflection peak and a severe shift in the center wavelength. On the other hand, traditional thin-diameter optical fibers rely heavily on polymer coatings (such as polyimide) for mechanical protection. However, when these polymer materials come into contact with corrosive media such as high-concentration strong acids (such as 98% concentrated sulfuric acid), they will rapidly swell, degrade, or even completely carbonize and peel off. The 125 µm bare fiber, which has lost the protection of the coating, is directly exposed to strong acid. The micro-defects on its surface will rapidly expand under the erosion of the acid, resulting in a precipitous drop in mechanical properties and making it extremely prone to brittle fracture.

[0003] To overcome the limitations of silica fiber applications in extreme environments, researchers have explored using sapphire fiber as a sensing medium. Sapphire fiber possesses a high melting point (approximately 2045°C) and excellent chemical inertness, maintaining its optical transmission properties even at temperatures above 1000°C and in corrosive environments. Existing literature (such as He et al., Optics Letters, 2024; Tan et al., Optics Express, 2025) has reported femtosecond laser-written sapphire fiber gratings (SFBGs). However, the fabrication process of sapphire fiber is extremely complex, its core-cladding structure is difficult to maintain strict concentricity, and the controllability of single-mode transmission is poor. More critically, sapphire material is extremely intrinsically brittle and hard, easily fracturing under the dynamic scouring of strong acid fluids or in-situ encapsulation stress, making its implementation in complex engineering environments virtually impossible.

[0004] On the other hand, researchers have also proposed various high-temperature grating fabrication techniques for silica optical fibers, such as Type II gratings and femtosecond laser-written gratings, which have locally increased the operating temperature to the 1000–1100℃ range. However, these fibers still cannot escape the original 125 µm cladding structure. Especially under the dual extreme coupling conditions of "high temperature + strong acid," even if the grating itself is heat-resistant, the tensile fatigue parameters of the overall structure of conventional thin-diameter bare fibers will drop sharply. The residual tensile strength is far from meeting the stringent acceptance standards for mechanical stress in harsh industrial environments, and it is extremely prone to bending and microcrack failure under thermomechanical shock or fluid disturbance.

[0005] To improve the mechanical reliability of optical fibers at high temperatures, the existing technology proposes a "gold-clad fiber" approach, which involves coating the outer layer of the fiber with gold to enhance high-temperature protection and strength. However, this approach is extremely costly, and the metal coating preparation process is complex. It not only significantly increases the price of the fiber and introduces optical loss and thermal stress mismatch issues, but also poses a risk of damage to the metal layer in certain complex and corrosive acid media. Therefore, this approach is difficult to promote on a large scale in engineering applications.

[0006] In summary, the existing technology has the following shortcomings:

[0007] (1) The problem that the polyimide coating peels off and fails in high temperature or strong acid environment, causing the fine bare fiber to break easily due to the propagation of microcracks, has not yet been solved;

[0008] (2) The gold coating solution is expensive and has poor thermal stress mismatch and chemical compatibility;

[0009] (3) In the combined extreme environments of "high temperature + strong acid", an optical fiber structure that takes into account high mechanical strength, corrosion resistance, optical single-mode properties and economic feasibility has not yet been established. Summary of the Invention

[0010] In order to solve the problem that existing fiber gratings cannot be used in extreme environments such as strong acids and high temperatures, this invention provides a large-diameter fiber grating and its fabrication method.

[0011] This invention is achieved using the following technical solution:

[0012] A large-diameter fiber grating, wherein the outer diameter of the fiber 1 is 0.7–1.5 mm and the fiber has single-mode transmission characteristics (in specific implementation, the fiber transmission needs to meet the normalized frequency requirement). To ensure that it is a single-mode transmission, in the formula Where λ is the fiber core radius and λ is the operating wavelength. The refractive index of the fiber core, (Refractive index of the cladding). By significantly increasing the outer diameter of the optical fiber from the conventional 125µm to over 700µm, the cladding thickness is increased by nearly ten times, giving the device extremely excellent self-supporting structural rigidity, fundamentally eliminating the need for an external protective layer, and at the same time improving the overall bending strength and tensile performance of optical fiber 1.

[0013] Furthermore, the optical fiber cladding material is high-purity quartz to enhance its high-temperature mechanical strength and oxidation resistance.

[0014] A method for fabricating a large-diameter fiber grating as described above is disclosed. The large-diameter fiber grating is fabricated using a femtosecond laser writing process. The optical fiber is fixed on a stage. During fabrication, a femtosecond laser is used to write multiple refractive index modulation regions of equal length and equidistant spacing along the fiber core region at a constant laser power. The length direction of each grating line is parallel to the fiber axis. The writing interval between adjacent refractive index modulation regions is determined by the grating period, forming a fiber Bragg grating. Femtosecond lasers possess ultra-short pulse widths (typically on the order of 290 fs) and high peak power, allowing optical energy to be locally deposited in the fiber core in a very short time, thereby inducing a permanent refractive index change and avoiding the thermal damage and thermal diffusion problems caused by traditional ultraviolet exposure processes.

[0015] Furthermore, a U-shaped positioning groove is provided in the middle of the stage. The depth of the positioning groove is adapted to the outer diameter of the optical fiber. The positioning groove is filled with a matching liquid to reduce optical reflection and scattering. An optical glass cover is placed over the upper opening of the positioning groove for optical isolation and mechanical support. The preparation method described in this invention innovatively adopts "optical glass cover + matching liquid" to assist in refractive index compensation, combined with a femtosecond laser point-by-point direct second-order writing process. This method effectively eliminates aberrations, enables laser energy to be precisely focused deep into the fiber core, avoids thermal diffusion effects, and results in a grating with extremely high reflectivity, low insertion loss, and excellent spectral symmetry.

[0016] Furthermore, the refractive index of the matching fluid filling the positioning groove is 1.518.

[0017] Furthermore, the optical glass cover has a thickness of 0.16 mm and a refractive index of 1.50.

[0018] Furthermore, the femtosecond laser pulse width is 100fs to 500fs, the repetition frequency is 1kHz to 500kHz, and the focused spot diameter is less than 2μm to ensure the accuracy and stability of grating writing.

[0019] The beneficial effects of this invention are as follows: By employing a large-diameter pure quartz substrate in the device's physical structure and innovatively using refractive index-compensated femtosecond laser direct writing technology in the writing process, this invention achieves for the first time in the industry a large-diameter fiber grating that combines high structural rigidity, resistance to strong acids and corrosion, short-term resistance to extreme high temperatures of 1400℃, and single-mode transmission characteristics. Furthermore, compared to expensive gold-coated fibers or extremely difficult-to-process sapphire fibers, the material cost of this invention is low. Although the overall outer diameter of the fiber is significantly increased to 900µm, the central core still maintains a strictly 9µm structure. A reasonable core-cladding refractive index design ensures perfect single-mode transmission characteristics, allowing for direct compatibility with existing fiber drawing processes and seamless integration into standard fiber optic sensing and communication systems. In addition, this invention fundamentally overcomes the technical barriers of existing fibers, such as easy breakage and failure in extreme environments of "high temperature + strong acid," high cost, and difficulty in balancing performance. It has irreplaceable and broad application prospects in demanding engineering fields such as petrochemicals, deep-sea exploration, and metallurgical refining. Attached Figure Description

[0020] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the spectrum of the large-diameter fiber grating described in this invention;

[0023] Figure 2 The curves showing the variation of the center wavelength and peak value of the spectrum of the SMF-28e fiber grating (fiber diameter of 125 μm) and the large-diameter fiber grating described in this invention (specifically, fiber diameter of 900 μm) are shown.

[0024] Figure 3 This is a schematic diagram of a sulfuric acid thermometry experiment.

[0025] Figure 4 This is a schematic diagram of the three repeatability fitting curves of a large-diameter fiber grating under a strong acid environment.

[0026] Figure 5 A schematic diagram showing the temperature calibration curves of a large-diameter fiber grating compared to a thermocouple.

[0027] Figure 6This is a schematic diagram of the test curve for the center wavelength stability of a large-diameter fiber grating during the constant temperature insulation stage at 270℃.

[0028] Figure 7 A schematic diagram of the fabrication of a large-diameter fiber grating;

[0029] Figure 8 An axial view of a large-diameter fiber grating placed on a stage.

[0030] In the diagram: 1-optical fiber, 2-optical glass cover, 3-positioning groove. Detailed Implementation

[0031] To better understand the above-mentioned objectives, features, and advantages of the present invention, the solutions of the present invention will be further described below. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0032] Many specific details are set forth in the following description in order to provide a full understanding of the invention, but the invention may also be practiced in other ways different from those described herein; obviously, the embodiments in the specification are only some embodiments of the invention, and not all embodiments.

[0033] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0034] A large-diameter fiber grating, wherein the outer diameter of fiber 1 is 0.7–1.5 mm and fiber 1 has single-mode transmission characteristics (in specific implementation, fiber transmission needs to meet the normalized frequency requirement). To ensure that it is a single-mode transmission, in the formula Where λ is the fiber core radius and λ is the operating wavelength. The refractive index of the fiber core, (Refractive index of the cladding). By significantly increasing the outer diameter of fiber 1 from the conventional 125µm to over 700µm, the cladding thickness is increased by nearly ten times, giving the device extremely excellent self-supporting structural rigidity, fundamentally eliminating the need for an external protective layer, and at the same time improving the overall bending strength and tensile performance of fiber 1.

[0035] In practice, the cladding material of optical fiber 1 is high-purity quartz to enhance its high-temperature mechanical strength and oxidation resistance.

[0036] A method for fabricating a large-diameter fiber grating as described above is disclosed. The large-diameter fiber grating is fabricated using a femtosecond laser writing process. An optical fiber 1 is fixed on a stage. During fabrication, a femtosecond laser is used to write multiple refractive index modulation regions of equal length and equidistant spacing along the axial direction of the fiber 1 at a constant laser power. The length direction of each grating line is parallel to the axis of the fiber 1. The writing interval between adjacent refractive index modulation regions is determined by the grating period, forming a fiber Bragg grating. The femtosecond laser possesses ultra-short pulse widths (typically on the order of 290 fs) and high peak power, allowing optical energy to be locally deposited in the fiber core in a very short time, thereby inducing a permanent refractive index change and avoiding the thermal damage and thermal diffusion problems caused by traditional ultraviolet exposure processes.

[0037] In specific implementation, a U-shaped positioning groove 3 is provided in the middle of the stage. The depth of the positioning groove 3 is adapted to the outer diameter of the optical fiber 1. The positioning groove 3 is filled with a matching liquid to reduce optical reflection and scattering. An optical glass cover 2 is placed over the upper opening of the positioning groove 3 for optical isolation and mechanical support. The preparation method described in this invention innovatively adopts "optical glass cover 2 + matching liquid" to assist in refractive index compensation, combined with a femtosecond laser point-by-point direct second-order writing process. This method effectively eliminates aberrations, enables laser energy to be precisely focused deep into the fiber core, avoids thermal diffusion effects, and the resulting grating has extremely high reflectivity, low insertion loss, and excellent spectral symmetry.

[0038] In practice, the refractive index of the matching liquid filling the positioning groove 3 is 1.518.

[0039] In practice, the thickness of the optical glass cover 2 is 0.16 mm and the refractive index is 1.50.

[0040] In practice, the femtosecond laser pulse width is 100fs to 500fs, the repetition frequency is 1kHz to 500kHz, and the focused spot diameter is less than 2μm to ensure the accuracy and stability of grating writing.

[0041] The performance of the large-diameter fiber grating described in this invention is verified below:

[0042] Figure 1 This is a schematic diagram of the spectrum of the large-diameter fiber grating described in this invention. Figure 1 It can be seen that its signal-to-noise ratio is 39.3dB, reflectivity is 88%, and -3dB bandwidth is 0.34nm, indicating that the femtosecond laser direct-writing large-diameter fiber grating has excellent spectral characteristics of high signal-to-noise ratio, high reflectivity, and narrow linewidth.

[0043] Figure 2The spectral center wavelength and peak value variation curves of the SMF-28e fiber grating (fiber 1 diameter is 125um) and the large-diameter fiber grating described in this invention (specifically, fiber 1 diameter is 900um) are presented. Figure 2 It is known that, unlike ordinary single-mode fiber gratings which rapidly become brittle and break at extremely high temperatures, the device of this invention can still maintain the integrity of its physical form and optical continuity after being heated to an extreme limit temperature of up to 1400°C, which is far superior to the upper limit of use of existing polyimide-coated fiber 1 (usually below 800°C) and conventional large-diameter doped fiber 1.

[0044] Figure 3 This is a schematic diagram of a sulfuric acid thermometry experiment. Three repeatable experiments were conducted on a large-diameter fiber grating under a strong acid environment. The fitted curves are shown below. Figure 4 ,Depend on Figure 4 It can be seen that the spectral signal is stable and has no obvious attenuation. Moreover, the repeatability error of the three repeatability fitting curves is 1.42%, with extremely small measurement error and excellent repeatability. This indicates that after long-term exposure to extremely high temperature annealing and strong acid environment, the large-diameter optical fiber 1 can still maintain excellent physical integrity. This completely eliminates the risk that conventional small-diameter bare fibers will easily break due to small deformation or physical impact from acidic fluids after losing coating protection.

[0045] To verify the accuracy of its temperature measurement, such as Figure 5 As shown, temperature was measured using both a thermocouple and the large-diameter fiber optic grating described in this invention. The large-diameter fiber optic grating was calibrated using a thermocouple. After comparing the gradient heating curves, the temperature error between the two was 0.4℃, indicating that the large-diameter fiber optic grating described in this invention has good temperature measurement performance and accuracy.

[0046] like Figure 6 The figure shows the test curve of the center wavelength stability of a large-diameter fiber grating under the constant temperature insulation stage of 270℃. The change of the center wavelength of the grating was monitored for 12 minutes continuously. The maximum wavelength error of the grating over the entire period was measured to be 6 pm, indicating that the grating has good wavelength stability under the high temperature insulation condition of 270℃.

[0047] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the present invention. Although detailed descriptions have been provided with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments, and they should all be covered within the protection scope of the claims.

Claims

1. A large-diameter fiber grating, characterized in that, The outer diameter of the optical fiber (1) is 0.7 to 1.5 mm and the optical fiber (1) has single-mode transmission characteristics.

2. The large-diameter fiber grating according to claim 1, characterized in that, The cladding material of the optical fiber (1) is high-purity quartz.

3. A method for fabricating a large-diameter fiber grating as described in claim 1 or 2, characterized in that, The large-diameter fiber grating is fabricated by femtosecond laser writing process. The fiber (1) is fixed on the stage. During the fabrication process, a femtosecond laser is used to write multiple refractive index modulation regions of the same length and equidistant spacing along the fiber (1) axis with constant laser power. The length direction of each grating line is parallel to the axis of the fiber (1). The writing interval between adjacent refractive index modulation regions is determined by the grating period to form a fiber Bragg grating.

4. The method for fabricating a large-diameter fiber grating according to claim 3, characterized in that, The stage has a U-shaped positioning groove (3) in the middle. The depth of the positioning groove (3) is adapted to the outer diameter of the optical fiber (1). The positioning groove (3) is filled with matching liquid.

5. The method for fabricating a large-diameter fiber grating according to claim 4, characterized in that, The refractive index of the matching liquid filling the positioning groove (3) is 1.

518.

6. The method for fabricating a large-diameter fiber grating according to claim 5, characterized in that, The optical glass cover (2) has a thickness of 0.16 mm and a refractive index of 1.50.

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