High-Temperature Sensor Based on Twisted Long-Period Fiber Bragg Grating and Its Fabrication Method

Abstract

This invention relates to the field of fiber optic temperature measurement technology, specifically to a high-temperature sensor based on a torsion-type long-period fiber grating and its fabrication method. The fabrication process includes: fixing an optical fiber to a rotating fixture and rotating it by a preset angle to prepare an optical fiber with a preset torsional stress field; controlling a laser and a displacement platform to alternately perform fixed-point pulse irradiation and step-by-step movement to induce thermal relaxation of the stress field to prepare the grating; collecting transmission spectral data and controlling the processing equipment based on power loss values; and calculating the temperature sensitivity coefficient. In this invention, by pre-setting a torsional stress field and inducing periodic thermal relaxation of stress, dependence on photosensitive doping materials is eliminated, mechanical strength is maintained, and efficient refractive index modulation is achieved, resulting in excellent thermal stability. Combined with real-time spectral acquisition and loss analysis, a closed-loop feedback is constructed to precisely adjust irradiation parameters, improving temperature sensitivity and linearity, and solving the problems of low yield and poor long-term stability associated with traditional processes.

Description

Technical Field

[0001] This invention relates to the field of fiber optic temperature measurement technology, and in particular to a high-temperature sensor based on a torsion-type long-period fiber grating and its fabrication method. Background Technology

[0002] The field of fiber optic temperature measurement technology mainly encompasses the use of optical fibers as sensing elements to measure temperature by detecting changes in characteristic parameters such as light intensity, wavelength, or phase caused by changes in ambient temperature during light wave propagation within the fiber. Traditional high-temperature sensor fabrication methods involve selecting a section of doped optical fiber and stripping off its acrylate coating. A laser beam emitted from an ultraviolet laser is then used to expose the fiber core at specific points through a phase mask, or a high-frequency carbon dioxide laser is used to scan and write the fiber's side surface point by point. This introduces an axially distributed periodic refractive index modulation structure within the fiber core to form a grating. The written fiber device is then placed in a tubular furnace for annealing to eliminate residual internal stress.

[0003] Existing technologies mainly rely on ultraviolet exposure combined with phase masks or high-frequency heat sources for point-by-point scanning. This method relies excessively on the photosensitive properties of fiber materials, thus limiting substrate selection. It often requires the removal of coating layers, which damages the integrity of the fiber surface, resulting in a significant decrease in mechanical strength. Complex mask alignment processes are easily affected by environmental vibrations. The thermal effects generated by high-frequency continuous scanning are difficult to control precisely, and non-uniform residual stress is easily introduced into the fiber core, causing instability in the refractive index modulation depth. The lack of a real-time feedback mechanism leads to a large deviation in the resonant wavelength of the finished product. Gratings formed solely by photoinduced refractive index changes are prone to thermal decay at high temperatures, leading to sensor signal drift. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the existing technology and to propose a high-temperature sensor based on a torsion-type long-period fiber grating and its fabrication method.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A high-temperature sensor based on a twisted long-period fiber grating, wherein the materials used to fabricate the high-temperature sensor include: Single-mode quartz optical fiber and high-temperature resistant ceramic encapsulation tube; The single-mode silica optical fiber consists of a germanium-doped silica core located at the center and a pure silica cladding covering the outside of the germanium-doped silica core. The high-temperature resistant ceramic encapsulation tube is made of high-purity alumina ceramic. The high-temperature resistant ceramic encapsulation tube is sleeved on the outside of the single-mode quartz optical fiber.

[0006] As a further aspect of the present invention, the diameter of the germanium-doped silica fiber core ranges from 8 micrometers to 10 micrometers, the diameter of the pure silica cladding ranges from 124 micrometers to 126 micrometers, and the numerical aperture parameter of the single-mode silica optical fiber ranges from 0.13 to 0.15.

[0007] As a further aspect of the present invention, the alumina content of the high-temperature resistant ceramic encapsulation tube is greater than 99.5%, and the difference between the inner diameter of the high-temperature resistant ceramic encapsulation tube and the outer diameter of the pure silica cladding is less than 5 micrometers.

[0008] A method for fabricating a high-temperature sensor based on a twisted long-period fiber grating, the method comprising the following steps: S1: Fix both ends of a single-mode silica fiber to a coaxially aligned rotating clamp, control the rotating clamp to rotate around the single-mode silica fiber by a preset torsion angle, and prepare the single-mode silica fiber with a preset torsion stress field in a torsion-locked state. S2: Set the parameters of the carbon dioxide laser, control the two-dimensional electric displacement platform to work in coordination with the carbon dioxide laser, and alternately perform fixed-point laser pulse irradiation and stepping movement along the axis of the single-mode quartz fiber to induce thermal relaxation of the preset torsional stress field and prepare a torsional long-period fiber grating. S3: The transmission spectrum data of the torsion long-period fiber grating is acquired by a spectrum analyzer, the power loss value at the resonant wavelength of the transmission spectrum data is extracted, and the carbon dioxide laser and the two-dimensional electric displacement platform are controlled based on the difference between the power loss value and the target attenuation threshold. S4: Obtain the resonant wavelength drift value of the torsion long-period fiber grating after constant temperature annealing in a high-temperature tubular heating furnace at discrete calibration temperature points, substitute the resonant wavelength drift value and the discrete calibration temperature points into the least squares model, and calculate the temperature sensitivity coefficient of the torsion long-period fiber grating.

[0009] As a further aspect of the present invention, the process of establishing the pre-set torsional stress field includes: S11: Use mechanical stripping pliers to remove the polymer coating layer of a predetermined length in the middle section of the single-mode quartz fiber, exposing the cladding structure of the single-mode quartz fiber, and use anhydrous ethanol solution to ultrasonically clean the exposed cladding structure surface. S12: Place the cleaned single-mode silica fiber horizontally into the rotating clamp, and adjust the left and right chucks of the rotating clamp to ensure that the axis of the single-mode silica fiber coincides with the axis of rotation of the rotating clamp. S13: Lock the left end chuck of the rotating fixture, drive the right end chuck of the rotating fixture to rotate in a preset direction until the preset torsion angle is reached, and lock the position of the right end chuck to establish a stable preset torsional stress field inside the single-mode quartz optical fiber.

[0010] As a further aspect of the present invention, the calculation process of the preset torsion angle in S13 includes: The shear modulus, cladding radius, and preset torsion length of the single-mode silica fiber are obtained. Substitute the shear modulus, the cladding radius, and the torsion length into the torsion angle calculation model to calculate and generate the preset torsion angle; The expression for the torsion angle calculation model is as follows: in, Represents the preset torsion angle, Represents the pre-torsional compensation coefficient. This represents the target shear stress value applied to the surface of the single-mode quartz fiber. Represents the torsion length, Represents the shear modulus. This represents the cladding radius.

[0011] As a further aspect of the present invention, the fabrication process of the twisted long-period fiber grating includes: S21: Configure the peak pulse power, pulse repetition frequency, and pulse width parameters of the carbon dioxide laser, and focus the laser beam emitted by the carbon dioxide laser through a cylindrical lens to form a linear spot; S22: Adjust the height axis of the two-dimensional electric displacement platform so that the focal point of the linear light spot falls precisely on the core position of the single-mode quartz fiber. S23: Control the two-dimensional electric displacement platform to perform stepping motion along the axis of the single-mode silica fiber at a preset grating period length, and trigger the carbon dioxide laser to emit a quantitative laser pulse at each step stop point to periodically perform local thermal relaxation modulation on the preset torsional stress field until the preset number of grating periods are completed to obtain the torsional long-period fiber grating.

[0012] As a further aspect of the present invention, the process for determining the single-point irradiation energy density of the laser pulse in S23 includes: The peak pulse power of the carbon dioxide laser, the effective area of ​​the linear spot, and the pulse width are obtained. The optical power density is calculated based on the pulse peak power and the effective area, and the single-point irradiation energy density is generated by combining the pulse width. The formula for calculating the single-point irradiation energy density is as follows: in, This represents the single-point irradiation energy density. Represents the peak power of the pulse. Represents the pulse width. This represents the projected area of ​​the linear light spot on the surface of the single-mode silica fiber.

[0013] As a further aspect of the present invention, the control process of the carbon dioxide laser and the two-dimensional electric displacement platform includes: S31: Connect the input end of the twisted long-period fiber grating to the broadband light source and connect its output end to the spectrometer to start the real-time spectral scanning mode. S32: Identify the center wavelength position of the resonance peak in the transmission spectrum data, and calculate the power loss value of the transmission power at that position relative to the baseline power in real time; S33: Determine whether the power loss value has reached the target attenuation threshold; If the target is not met, the carbon dioxide laser is controlled to increase the number of laser pulses or the two-dimensional electric displacement platform is controlled to reset for repeated scanning and writing. If the target is reached, the output of the carbon dioxide laser is stopped and the torsional locking state of the rotary clamp is released.

[0014] As a further aspect of the present invention, the calculation process of the temperature sensitivity coefficient includes: S41: The torsion long-period fiber grating is placed in the high-temperature tubular heating furnace, heated to the preset annealing temperature at a preset heating rate and kept at a constant temperature, and then naturally cooled to room temperature after eliminating residual processing stress. S42: Control the high-temperature tubular furnace to gradually increase the temperature according to the preset temperature gradient, and after holding the temperature at each discrete calibration temperature point for a preset time, record the resonant wavelength drift value. S43: Establish a linear regression dataset with the discrete calibration temperature points as independent variables and the resonant wavelength drift value as dependent variable, calculate the slope of the regression line using the least squares model, and determine the slope as the temperature sensitivity coefficient.

[0015] Compared with the prior art, the advantages and positive effects of the present invention are as follows: In this invention, by pre-setting a torsional stress field on the optical fiber and inducing periodic thermal relaxation of the stress using a pulsed heat source, the dependence on photosensitive doping materials is eliminated. While maintaining mechanical strength, efficient modulation of the refractive index is achieved. The spiral structure constructed using the torsional stress release mechanism has excellent thermal stability, avoiding grating degradation under high temperature conditions. By combining real-time spectral acquisition and loss analysis to construct a closed-loop feedback, the irradiation parameters of the heat source are precisely adjusted to ensure accurate positioning of the resonant wavelength and loss peak, significantly improving the temperature sensitivity and linearity of the grating, and solving the problems of low yield and poor long-term stability of traditional processes. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating the overall process of fabricating a high-temperature sensor based on a torsion-type long-period fiber grating according to the present invention. Figure 2 This is a flowchart illustrating the establishment of the pre-set torsional stress field in this invention. Figure 3 This is a flowchart of the laser fabrication process for the torsional long-period fiber grating of the present invention; Figure 4 This is a flowchart illustrating the feedback control process of the carbon dioxide laser and the two-dimensional electric displacement platform of the present invention. Figure 5 This is a flowchart illustrating the calculation of the temperature sensitivity coefficient of the torsional long-period fiber grating according to the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining the technical solutions of this invention and do not constitute a limitation on the scope of protection.

[0018] In the description of this invention, the process flow relationships or material and energy transfer paths indicated by terms such as "unit," "step," "equipment," "pipeline," "material flow," and "process parameters" are defined based on the process flow diagram or equipment structure diagram corresponding to the embodiments. This way of expression is only used to clearly illustrate the logical relationship between the elements in the technical solution, and not to limit the specific equipment connection method or physical layout. The term "multiple" includes two or more technical units, including but not limited to multiple reactors, pumps, valves, separation units, or detection instruments and other expandable elements. The specific number is determined according to specific process requirements or production scale and needs to be specifically stated.

[0019] Example 1 In this embodiment, all key process parameters involving numerical ranges are set using the minimum value of that range. Specifically, the diameter of the germanium-doped silica core of the single-mode silica fiber is set to 8 micrometers, the diameter of the pure silica cladding is set to 124 micrometers, and the numerical aperture is set to 0.13. The alumina content of the high-temperature resistant ceramic encapsulation tube is set to 99.5%, and the difference between the inner diameter and the outer diameter of the cladding is set to 1 micrometer.

[0020] The cladding radius of the single-mode silica fiber used It is 62 micrometers, shear modulus 31 GPa; preset torsional length The value is 0.01 meters; the target shear stress value is... Set to 0.03 GPa; peak pulse power of the carbon dioxide laser Set to 0.5 watts, pulse width Set to 5 microseconds; the projected area of ​​the linear light spot on the fiber surface for Square meters; grating period length set to 400 micrometers; target attenuation threshold set to 10 dB; annealing temperature set to 300 degrees Celsius.

[0021] Please see Figure 1 This invention provides a technical solution: a high-temperature sensor based on a twisted long-period fiber grating, wherein the materials used to fabricate the high-temperature sensor based on the twisted long-period fiber grating include: Single-mode quartz optical fiber and high-temperature resistant ceramic encapsulation tube; Single-mode silica optical fiber consists of a germanium-doped silica core at the center and a pure silica cladding covering the outside of the germanium-doped silica core. The diameter of the germanium-doped silica fiber core ranges from 8 micrometers to 10 micrometers, the diameter of the pure silica cladding ranges from 124 micrometers to 126 micrometers, and the numerical aperture parameter of the single-mode silica fiber ranges from 0.13 to 0.15. The high-temperature resistant ceramic encapsulation tube is made of high-purity alumina ceramic; The alumina content of the high-temperature resistant ceramic encapsulation tube is greater than 99.5%, and the difference between the inner diameter of the high-temperature resistant ceramic encapsulation tube and the outer diameter of the pure silica cladding is less than 5 micrometers. A high-temperature resistant ceramic encapsulation tube is fitted over the outside of a single-mode quartz optical fiber.

[0022] Please see Figure 1 and Figure 2 A method for fabricating a high-temperature sensor based on a torsion long-period fiber grating, wherein the method is performed based on the aforementioned high-temperature sensor based on a torsion long-period fiber grating, and includes the following steps: S1: Fix both ends of a single-mode silica fiber to a coaxially aligned rotating fixture, control the rotating fixture to rotate around the single-mode silica fiber by a preset torsion angle, and prepare a single-mode silica fiber with a preset torsion stress field under torsion locking state. The process of establishing a pre-set torsional stress field includes: S11: Use mechanical stripping pliers to remove the polymer coating layer of a predetermined length in the middle section of the single-mode silica fiber, revealing the cladding structure of the single-mode silica fiber, and use anhydrous ethanol solution to ultrasonically clean the surface of the exposed cladding structure. S12: Place the cleaned single-mode silica fiber horizontally into the rotating clamp, and adjust the left and right chucks of the rotating clamp to ensure that the axis of the single-mode silica fiber coincides with the axis of rotation of the rotating clamp. S13: Lock the left end chuck of the rotating fixture, drive the right end chuck of the rotating fixture to rotate in the preset direction until the preset torsion angle is reached, and lock the position of the right end chuck to establish a stable preset torsional stress field inside the single-mode silica fiber. The calculation process for the preset torsion angle in S13 includes: Obtain the shear modulus, cladding radius, and preset torsion length of the single-mode silica fiber; Substitute the shear modulus, cladding radius, and torsion length into the torsion angle calculation model to calculate and generate the preset torsion angle; The expression for the torsion angle calculation model is as follows: in, Represents the preset torsion angle. Represents the pre-torsional compensation coefficient. This represents the target shear stress value applied to the surface of a single-mode quartz fiber. Represents the torsion length. Represents shear modulus, Represents the cladding radius.

[0023] First, pretreatment and stress field construction of the single-mode silica fiber are performed. The operator selects a single-mode silica fiber that meets the above parameter limits and uses a mechanical stripping pliers with an accuracy of 0.01 mm to strip a 20 mm long acrylic polymer coating layer at the middle section of the fiber, exposing a pure silica cladding structure with a diameter of 124 micrometers.

[0024] The aforementioned mechanical stripping pliers are a type of manual tool specifically designed for optical fiber processing. They cut and strip the polymer protective layer on the surface of the optical fiber through a pre-set aperture blade, without damaging the internal quartz cladding structure.

[0025] The stripped optical fiber was immersed in an ultrasonic cleaning tank containing a 99.9% anhydrous ethanol solution. The ultrasonic frequency was set to 40 kHz, the cleaning temperature to 25 degrees Celsius, and the cleaning time to 10 minutes. The ultrasonic cavitation effect was used to remove residual coating debris and oil stains from the cladding surface.

[0026] The ultrasonic cleaning machine mentioned above utilizes the cavitation, acceleration, and direct flow effects of ultrasound in liquids to cause direct and indirect shearing and destruction of the liquid and contaminants, thereby dispersing, emulsifying, and peeling off the contaminant layer to achieve the cleaning purpose.

[0027] After removing the optical fiber, wipe it dry with lint-free paper. Then, place the clean optical fiber horizontally in the left and right V-grooves of the high-precision electric rotary clamp system. Using a microscope-assisted observation system, adjust the XY-axis displacement stages of the left and right chucks until the deviation between the optical fiber axis and the rotation center of the rotary clamp spindle is less than 0.1 micrometers. Tighten the locking nut of the left chuck to fix one end of the optical fiber. Start the rotation control system to drive the right chuck to rotate slowly in a clockwise direction.

[0028] In this process, the preset torsion angle needs to be accurately calculated and executed. Based on the aforementioned torsion angle calculation model:

[0029] in, This represents the preset torsion angle, in radians. The pre-torsional compensation coefficient is a dimensionless constant. This represents the target shear stress value applied to the surface of a single-mode quartz fiber, expressed in Pascals. Represents the length of torsion, in meters; The shear modulus of single-mode quartz optical fiber material, expressed in Pascals. This represents the cladding radius of a single-mode silica optical fiber, measured in meters.

[0030] In this embodiment, parameters Values Pascal; The value is 0.01 meters; Take constant Pascal; Converted from a diameter of 124 micrometers, it is... In actual high-energy laser processing, transient softening of local quartz glass inevitably leads to stress loss. To ensure that the effective modulation stress in the grating micro-region reaches the expected level after final cooling, excessive mechanical pre-torsion is required. Experiments have verified that the reasonable range of the pre-torsion compensation coefficient k is 1.5 to 2.5, and in this embodiment, k=2 is preferred.

[0031] Substitute the numerical values ​​into the formula to perform the calculation:

[0032] Calculated preset torsion angle Approximately 0.3122 radians; the control system converts this radian value into degrees, i.e. The servo locking function is immediately activated after the right chuck rotates to the 17.89 degree position to keep the optical fiber in this torsional state, thereby establishing a preset torsional stress field inside the optical fiber. This stress field changes the refractive index distribution inside the optical fiber, providing anisotropic initial conditions for subsequent grating writing.

[0033] Please see Figure 1 and Figure 3 S2: Set the parameters of the carbon dioxide laser, control the two-dimensional electric displacement platform to work in coordination with the carbon dioxide laser, and alternately perform fixed-point laser pulse irradiation and stepping movement on the axis of the single-mode quartz fiber to induce thermal relaxation of the pre-set torsional stress field and prepare a torsional long-period fiber grating. The fabrication process of a twisted long-period fiber grating includes: S21: Configure the peak pulse power, pulse repetition frequency, and pulse width parameters of the carbon dioxide laser, and focus the laser beam emitted by the carbon dioxide laser through a cylindrical lens to form a linear spot; S22: Adjust the height axis of the two-dimensional electric displacement platform so that the focal point of the linear light spot falls precisely on the core position of the single-mode silica fiber. S23: Control the two-dimensional electric displacement platform to perform stepping motion along the axis of the single-mode silica fiber at a preset grating period length, and trigger the carbon dioxide laser to emit a quantitative laser pulse at each step stop point to perform periodic local thermal relaxation modulation on the preset torsional stress field until the preset number of grating periods are completed to obtain a torsional long-period fiber grating. The process of determining the single-point irradiance energy density of the laser pulse in S23 includes: Obtain the peak pulse power, effective area of ​​the linear spot, and pulse width of the carbon dioxide laser; The optical power density is calculated based on the pulse peak power and the effective area, and the single-point irradiation energy density is generated by combining the pulse width. The formula for calculating the energy density of a single-point irradiation is as follows: in, Represents the single-point irradiation energy density. Represents the peak power of the pulse. Represents pulse width. This represents the projected area of ​​the linear light spot on the surface of a single-mode silica fiber.

[0034] Next, carbon dioxide laser pulse irradiation was performed to fabricate a torsional long-period fiber grating. A radio frequency-excited carbon dioxide laser with a wavelength of 10.6 micrometers was activated. A zinc selenide cylindrical lens with a focal length of 50 millimeters was configured in the optical path system to shape the Gaussian beam into a linear spot with its major axis perpendicular to the fiber axis. The Z-axis of a two-dimensional electric displacement platform was raised and lowered, and the energy center of the linear spot was focused onto the fiber core by observing the spot image fed back by a CCD camera. The displacement platform was controlled to move along the fiber axis in steps of 400 micrometers (i.e., the grating period). After each step, the displacement platform stopped, and the laser trigger output a single pulse sequence to locally heat the fiber.

[0035] In this step, the single-point irradiation energy density needs to be precisely set to induce appropriate thermal relaxation. The formula for calculating the single-point irradiation energy density is as follows: ; in, This represents the energy density of a single point of irradiation, expressed in joules per square meter. The peak pulse power of a carbon dioxide laser, measured in watts. Represents pulse width, in seconds; This represents the projected area of ​​the linear light spot on the surface of a single-mode silica fiber, expressed in square meters.

[0036] In this embodiment, It is 0.5 watts; for Second; The light spot analyzer measured as follows: square meters.

[0037] Substitute the numerical values ​​into the formula to perform the calculation:

[0038] Calculation results show that the single-point irradiation energy density The energy density is 500 joules per square meter (i.e., 0.05 joules per square centimeter). This energy density acts on the twisted optical fiber, softening the quartz glass in the irradiated area. The pre-set torsional shear stress is released instantaneously in the micro-region (thermal relaxation), causing the refractive index of the region to modulate relative to the unirradiated region. The displacement platform repeats the above "move-stop-irradiate" action for a total of 50 cycles, forming a periodic refractive index modulation structure along the optical fiber axis.

[0039] The aforementioned thermal relaxation refers to the microstructural adjustment process that occurs within a material as it transitions from a non-equilibrium state (such as pre-set mechanical stress) to an equilibrium state. In this embodiment, it is manifested as the stress release in the laser-heated region.

[0040] Please see Figure 1 and Figure 4S3: The transmission spectrum data of the torsion long-period fiber grating is collected by the spectral analyzer, the power loss value at the resonant wavelength of the transmission spectrum data is extracted, and the carbon dioxide laser and the two-dimensional electric displacement platform are controlled based on the difference between the power loss value and the target attenuation threshold. The control process of the carbon dioxide laser and the two-dimensional electric displacement platform includes: S31: Connect the input end of the twisted long-period fiber grating to the broadband light source, connect its output end to the spectrometer, and start the real-time spectral scanning mode. S32: Identify the center wavelength position of the resonance peak in the transmission spectrum data and calculate the power loss value of the transmission power at that position relative to the baseline power in real time. S33: Determine whether the power loss value has reached the target attenuation threshold; If the target is not met, the carbon dioxide laser is controlled to increase the number of laser pulses or the two-dimensional electric displacement platform is controlled to reset for repeated scanning and writing. If the target is reached, the output of the carbon dioxide laser will be stopped and the torsional lock of the rotary clamp will be released.

[0041] Subsequently, real-time monitoring and feedback control of the transmission spectrum were performed. The fiber optic input end of the writing area was fused to a broadband light source with a wavelength range of 1250 nm to 1650 nm, and the output end was connected to a spectrometer. The continuous scanning mode of the spectrometer was activated, and the resolution was set to 0.02 nm. The spectrometer acquired the transmission spectrum data after grating modulation in real time. The system automatically tracked the depth of the valley at the resonant wavelength (e.g., around 1550 nm) and calculated the current power loss value. This refers to the difference between the baseline power and the power at the lowest point of the trough; the system sets the target attenuation threshold to 10 dB.

[0042] The aforementioned broadband light source refers to a light source device whose output spectrum covers a wide wavelength range. In this embodiment, an ASE (amplified spontaneous emission) light source is used to provide the continuous spectral signal required for fiber optic grating testing.

[0043] If monitored in real time The reading was 8.5 dB, which did not reach the 10 dB threshold. The control program instructed the two-dimensional electric displacement platform to reset to the starting coordinates and, while maintaining the fiber torsion lock, instructed the carbon dioxide laser to perform a second round of scanning and writing. At this time, the number of laser pulses increased by 1. When the cumulative writing scan reached the third time, the resonant peak loss value was detected to reach 10.2 dB, exceeding the set target attenuation threshold. The control system immediately cut off the laser power supply and sent a command to the rotating fixture to release the servo lock of the right end chuck, release the remaining macroscopic torsional torque of the fiber, and complete the preliminary fabrication of the torsion long-period fiber grating.

[0044] Please see Figure 1and Figure 5 S4: Obtain the resonant wavelength drift value of the torsional long-period fiber grating after constant temperature annealing in a high-temperature tubular furnace at discrete calibration temperature points. Substitute the resonant wavelength drift value and the discrete calibration temperature points into the least squares model to calculate the temperature sensitivity coefficient of the torsional long-period fiber grating.

[0045] The calculation process for the temperature sensitivity coefficient includes: S41: Place the torsion long-period fiber grating into a high-temperature tubular furnace, heat it to the preset annealing temperature at a preset heating rate and keep it constant, eliminate residual processing stress and then cool it naturally to room temperature. S42: Control the high-temperature tubular furnace to gradually increase the temperature according to the preset temperature gradient, and record the resonant wavelength drift value after holding the temperature at each discrete calibration temperature point for a preset time. S43: Establish a linear regression dataset with discrete calibration temperature points as independent variables and resonant wavelength drift values ​​as dependent variables. Calculate the slope of the regression line using the least squares model and determine the slope as the temperature sensitivity coefficient.

[0046] Finally, high-temperature annealing and temperature sensitivity coefficient calibration were performed. The prepared grating was inserted into a high-purity alumina ceramic encapsulation tube (125 micrometers inner diameter, 1 micrometer gap with the fiber cladding) and placed in the isothermal zone of a high-temperature tubular furnace. The heating rate was set to 5 degrees Celsius / minute, and the temperature was raised to 300 degrees Celsius and held for 2 hours to eliminate residual thermal stress introduced by laser processing and stabilize the grating structure. It was then allowed to cool naturally to room temperature of 25 degrees Celsius.

[0047] During the calibration phase, the heating furnace starts at 50 degrees Celsius and sets discrete calibration temperature points in increments of 50 degrees Celsius, up to a maximum of 300 degrees Celsius. At each temperature point, the temperature is held constant for 30 minutes to ensure thermal equilibrium, and then the center position of the resonant wavelength on the spectrometer is recorded.

[0048] As shown in Table 1, the resonant wavelength data of this embodiment at different temperature points are recorded.

[0049] Table 1. Data Recording Table for Temperature Calibration Experiment in Example 1

[0050] Referring to Table 1, the resonant wavelength redshifts with increasing temperature. The temperature sensitivity coefficient is calculated using a least squares model. A linear regression equation is then established. ,in The change in temperature (i.e.) ), This is the wavelength shift. The slope This is the intercept.

[0051] The aforementioned least squares model is a mathematical optimization technique that finds the best function match for the data by minimizing the sum of squared errors. In this embodiment, it is used to determine the linear relationship between temperature and wavelength drift.

[0052] Substitute the data points into the calculation:

[0053] The slope is calculated using software. Nanometers per degree Celsius, or 79 picometers per 100 degrees Celsius; this slope This is confirmed as the temperature sensitivity coefficient of the torsional long-period fiber grating in this embodiment in the low-temperature range (50-300 degrees Celsius).

[0054] Table 2 Comparison of Product Performance in Example 1

[0055] Example 2 In this embodiment, all key process parameters involving numerical ranges are set using the maximum value within that range. Specifically, the diameter of the germanium-doped silica core of the single-mode silica fiber is set to 10 micrometers, the diameter of the pure silica cladding is set to 126 micrometers, and the numerical aperture is set to 0.15. The alumina content of the high-temperature resistant ceramic encapsulation tube is set to 99.9%, and the difference between the inner diameter and the outer diameter of the cladding is set to 4.9 micrometers.

[0056] The cladding radius of the single-mode silica fiber used It is 63 micrometers, shear modulus 31 GPa; preset torsional length The value is 0.05 meters; the target shear stress value is... Set to 0.10 GPa; peak pulse power of the carbon dioxide laser Set to 5.0 watts, pulse width Set to 100 microseconds; the projected area of ​​the linear light spot on the fiber surface for Square meters; grating period length set to 600 micrometers; target attenuation threshold set to 25 dB; annealing temperature set to 800 degrees Celsius.

[0057] Please see Figure 1 This invention provides a technical solution: a high-temperature sensor based on a twisted long-period fiber grating, wherein the materials used to fabricate the high-temperature sensor based on the twisted long-period fiber grating include: Single-mode quartz optical fiber and high-temperature resistant ceramic encapsulation tube; Single-mode silica optical fiber consists of a germanium-doped silica core at the center and a pure silica cladding covering the outside of the germanium-doped silica core. The diameter of the germanium-doped silica fiber core ranges from 8 micrometers to 10 micrometers, the diameter of the pure silica cladding ranges from 124 micrometers to 126 micrometers, and the numerical aperture parameter of the single-mode silica fiber ranges from 0.13 to 0.15. The high-temperature resistant ceramic encapsulation tube is made of high-purity alumina ceramic; The alumina content of the high-temperature resistant ceramic encapsulation tube is greater than 99.5%, and the difference between the inner diameter of the high-temperature resistant ceramic encapsulation tube and the outer diameter of the pure silica cladding is less than 5 micrometers. A high-temperature resistant ceramic encapsulation tube is fitted over the outside of a single-mode quartz optical fiber.

[0058] Please see Figure 1 and Figure 2 A method for fabricating a high-temperature sensor based on a torsion long-period fiber grating, wherein the method is performed based on the aforementioned high-temperature sensor based on a torsion long-period fiber grating, and includes the following steps: S1: Fix both ends of a single-mode silica fiber to a coaxially aligned rotating fixture, control the rotating fixture to rotate around the single-mode silica fiber by a preset torsion angle, and prepare a single-mode silica fiber with a preset torsion stress field under torsion locking state. The process of establishing a pre-set torsional stress field includes: S11: Use mechanical stripping pliers to remove the polymer coating layer of a predetermined length in the middle section of the single-mode silica fiber, revealing the cladding structure of the single-mode silica fiber, and use anhydrous ethanol solution to ultrasonically clean the surface of the exposed cladding structure. S12: Place the cleaned single-mode silica fiber horizontally into the rotating clamp, and adjust the left and right chucks of the rotating clamp to ensure that the axis of the single-mode silica fiber coincides with the axis of rotation of the rotating clamp. S13: Lock the left end chuck of the rotating fixture, drive the right end chuck of the rotating fixture to rotate in the preset direction until the preset torsion angle is reached, and lock the position of the right end chuck to establish a stable preset torsional stress field inside the single-mode silica fiber. The calculation process for the preset torsion angle in S13 includes: Obtain the shear modulus, cladding radius, and preset torsion length of the single-mode silica fiber; Substitute the shear modulus, cladding radius, and torsion length into the torsion angle calculation model to calculate and generate the preset torsion angle; The expression for the torsion angle calculation model is as follows: in, Represents the preset torsion angle. Represents the pre-torsional compensation coefficient. This represents the target shear stress value applied to the surface of a single-mode quartz fiber. Represents the torsion length. Represents shear modulus, Represents the cladding radius.

[0059] First, pretreatment and stress field construction of the single-mode silica fiber are performed. The operator selects an optical fiber with a core diameter of 10 micrometers and a cladding diameter of 126 micrometers, and increases the stripping length to 50 millimeters to accommodate a longer torsion area; ultrasonic cleaning with high-purity ethanol at 25 degrees Celsius for 15 minutes is used to ensure the cleanliness of the large exposed area; the optical fiber is then loaded into a rotating fixture, and the axis alignment is calibrated.

[0060] Based on this, the preset torsion angle under high stress is calculated. According to the torsion angle calculation model:

[0061] in, This represents the preset torsion angle, in radians. The pre-torsional compensation coefficient is a dimensionless constant. This represents the target shear stress value applied to the surface of a single-mode quartz fiber, expressed in Pascals. Represents the length of torsion, in meters; The shear modulus of single-mode quartz optical fiber material, expressed in Pascals. This represents the cladding radius of a single-mode silica optical fiber, measured in meters.

[0062] This embodiment aims to introduce a stronger pre-stress field, and the parameters are assigned the following values: Set to high stress value Pascal; The torsional length is 0.05 meters; Maintain 31 GPa (i.e. Pascal's remains unchanged; for To counteract the more severe stress thermal relaxation phenomenon during high-power laser (5.0 watts) writing and to ensure that the optical fiber does not break under high stress, this embodiment, based on previous process optimization experience, still selects the optimal balance point 2 for the pre-torsion compensation coefficient k.

[0063] Substitute the numerical values ​​into the formula to perform the calculation:

[0064] Calculated preset torsion angle Approximately 5.1203 radians; converted to degrees, The right-end chuck of the control fixture rotates nearly a full circle (approximately 293.37 degrees) and locks in this large-angle torsion state; this high torsion introduces extremely strong shear stress distribution on the cross-section of the optical fiber, laying the physical basis for forming a grating with deep modulation depth.

[0065] Please see Figure 1 and Figure 3 S2: Set the parameters of the carbon dioxide laser, control the two-dimensional electric displacement platform to work in coordination with the carbon dioxide laser, and alternately perform fixed-point laser pulse irradiation and stepping movement on the axis of the single-mode quartz fiber to induce thermal relaxation of the pre-set torsional stress field and prepare a torsional long-period fiber grating. The fabrication process of a twisted long-period fiber grating includes: S21: Configure the peak pulse power, pulse repetition frequency, and pulse width parameters of the carbon dioxide laser, and focus the laser beam emitted by the carbon dioxide laser through a cylindrical lens to form a linear spot; S22: Adjust the height axis of the two-dimensional electric displacement platform so that the focal point of the linear light spot falls precisely on the core position of the single-mode silica fiber. S23: Control the two-dimensional electric displacement platform to perform stepping motion along the axis of the single-mode silica fiber at a preset grating period length, and trigger the carbon dioxide laser to emit a quantitative laser pulse at each step stop point to perform periodic local thermal relaxation modulation on the preset torsional stress field until the preset number of grating periods are completed to obtain a torsional long-period fiber grating. The process of determining the single-point irradiance energy density of the laser pulse in S23 includes: Obtain the peak pulse power, effective area of ​​the linear spot, and pulse width of the carbon dioxide laser; The optical power density is calculated based on the pulse peak power and the effective area, and the single-point irradiation energy density is generated by combining the pulse width. The formula for calculating the energy density of a single-point irradiation is as follows: in, Represents the single-point irradiation energy density. Represents the peak power of the pulse. Represents pulse width. This represents the projected area of ​​the linear light spot on the surface of a single-mode silica fiber.

[0066] Laser writing was performed using high-power parameters. A carbon dioxide laser was configured, and the focal length of the cylindrical lens was adjusted to adapt the laser spot to the coarser 126-micron cladding. Z-axis focusing ensured that the energy was concentrated in the 10-micron coarse fiber core region. The displacement platform step size was set to 600 microns to match the phase matching conditions required by the long-period grating in high-temperature sensing.

[0067] Laser parameters are determined based on the formula for calculating single-point irradiation energy density:

[0068] in, This represents the energy density of a single point of irradiation, expressed in joules per square meter. The peak pulse power of a carbon dioxide laser, measured in watts. Represents pulse width, in seconds; This represents the projected area of ​​the linear light spot on the surface of a single-mode silica fiber, expressed in square meters.

[0069] In this embodiment, the upper limit power is used. 5.0 watts; pulse width Increase to Second; Due to beam widening adjustment, the projected area is square meters.

[0070] Substitute into the calculation:

[0071] Calculation results show that the single-point irradiation energy density Up to 62,500 joules per square meter (i.e., 6.25 joules per square centimeter); this high-energy-density pulse acts instantaneously on the optical fiber, inducing not only stress thermal relaxation, but also slight physical densification of the fiber core glass, thereby producing a huge refractive index modulation depth; this process is repeated along a 50-millimeter torsion length until a grating structure of about 80 cycles is written.

[0072] The aforementioned physical densification refers to the physical phenomenon in which the glass network structure collapses or rearranges under the action of high-energy lasers, resulting in a local increase in material density and an increase in refractive index.

[0073] Please see Figure 1 and Figure 4 S3: The transmission spectrum data of the torsion long-period fiber grating is collected by the spectral analyzer, the power loss value at the resonant wavelength of the transmission spectrum data is extracted, and the carbon dioxide laser and the two-dimensional electric displacement platform are controlled based on the difference between the power loss value and the target attenuation threshold. The control process of the carbon dioxide laser and the two-dimensional electric displacement platform includes: S31: Connect the input end of the twisted long-period fiber grating to the broadband light source, connect its output end to the spectrometer, and start the real-time spectral scanning mode. S32: Identify the center wavelength position of the resonance peak in the transmission spectrum data and calculate the power loss value of the transmission power at that position relative to the baseline power in real time. S33: Determine whether the power loss value has reached the target attenuation threshold; If the target is not met, the carbon dioxide laser is controlled to increase the number of laser pulses or the two-dimensional electric displacement platform is controlled to reset for repeated scanning and writing. If the target is reached, the output of the carbon dioxide laser will be stopped and the torsional lock of the rotary clamp will be released.

[0074] High-threshold spectral feedback control is implemented. A broadband light source is connected to a high-precision spectrometer. Due to the use of strong torsion and high-energy laser writing in this embodiment, the expected resonance peak depth is relatively deep, so the target attenuation threshold is set to 25 dB. The system monitors the transmission spectrum in real time. The initial scan shows that the resonance peak loss is 18 dB. The control system determines that the target is not met and instructs the laser to increase the number of pulses in place, performing multiple cumulative exposures on each grating period point (e.g., 5 pulses per point). As the accumulated energy increases, the stress release is more thorough, and the refractive index modulation is enhanced. When the resonance peak loss reaches 25.5 dB, the system determines that the target (greater than 25 dB) has been reached, and then stops the laser output and unlocks the rotating fixture. At the moment of unlocking, due to the release of huge elastic potential energy, the spectral characteristics of the grating will be fine-tuned and eventually stabilized at the design wavelength.

[0075] Please see Figure 1 and Figure 5 S4: Obtain the resonant wavelength drift value of the torsional long-period fiber grating after constant temperature annealing in a high-temperature tubular furnace at discrete calibration temperature points. Substitute the resonant wavelength drift value and the discrete calibration temperature points into the least squares model to calculate the temperature sensitivity coefficient of the torsional long-period fiber grating.

[0076] The calculation process for the temperature sensitivity coefficient includes: S41: Place the torsion long-period fiber grating into a high-temperature tubular furnace, heat it to the preset annealing temperature at a preset heating rate and keep it constant, eliminate residual processing stress and then cool it naturally to room temperature. S42: Control the high-temperature tubular furnace to gradually increase the temperature according to the preset temperature gradient, and record the resonant wavelength drift value after holding the temperature at each discrete calibration temperature point for a preset time. S43: Establish a linear regression dataset with discrete calibration temperature points as independent variables and resonant wavelength drift values ​​as dependent variables. Calculate the slope of the regression line using the least squares model and determine the slope as the temperature sensitivity coefficient.

[0077] High-temperature annealing and full-temperature sensitivity calibration were performed. The optical fiber was encapsulated in a high-purity (99.9%) alumina tube with a 4.9-micron gap to allow the fiber to expand freely at high temperatures without being compressed by the tube wall. The assembly was placed in a tube furnace, heated to 800 degrees Celsius, and held at that temperature for 4 hours. This high-temperature annealing was designed to completely eliminate deep structural defects introduced by laser energy up to 6.25 J / cm², enabling the sensor to operate for extended periods at 800 degrees Celsius.

[0078] After cooling, calibration was performed, with temperature points covering 100°C to 800°C, in 100°C increments. Table 3 shows the test data for this embodiment over a wide temperature range.

[0079] Table 3. Data Recording Table for Temperature Calibration Experiment in Example 2

[0080] Referring to Table 3, the sensor exhibits good linear response in the high-temperature range. Data is processed using the least squares method to establish... Model. Input dataset The slope of the regression line was obtained through numerical calculation. Nanometers per degree Celsius, or 106 picometers per degree Celsius; this value indicates that the grating prepared by high-stress torsion and high-energy writing has an extremely high sensitivity coefficient in the high-temperature region.

[0081] Table 4 Comparison of Product Performance in Example 2

[0082] The above embodiments illustrate preferred embodiments of the present invention. Any equivalent adjustments to the technical solution based on chemical engineering methods are within the scope of protection, including but not limited to: using different chemical reaction processes to achieve technical effects, optimizing the production process flow, adjusting the raw material ratio scheme, improving reactor design, and improving energy efficiency. Any implementation scheme derived from reasonable modifications to the production process, raw material utilization, equipment configuration, or system integration level without departing from the core technology of the present invention should be considered within the scope of protection defined by the claims of the present invention.

Claims

1. High temperature sensor based on a torsional long period fiber grating, characterized in that, The materials used to fabricate the high-temperature sensor based on a torsion-type long-period fiber grating include: Single-mode quartz optical fiber and high-temperature resistant ceramic encapsulation tube; The single-mode silica optical fiber consists of a germanium-doped silica core located at the center and a pure silica cladding covering the outside of the germanium-doped silica core. The high-temperature resistant ceramic encapsulation tube is made of high-purity alumina ceramic. The high-temperature resistant ceramic encapsulation tube is sleeved on the outside of the single-mode quartz optical fiber.

2. The high temperature sensor based on a torsion type long period fiber grating according to claim 1, characterized in that, The diameter of the germanium-doped silica fiber core ranges from 8 micrometers to 10 micrometers, the diameter of the pure silica cladding ranges from 124 micrometers to 126 micrometers, and the numerical aperture parameter of the single-mode silica fiber ranges from 0.13 to 0.

15.

3. The high temperature sensor based on a torsion type long period fiber grating according to claim 1, characterized in that, The high-temperature resistant ceramic encapsulation tube has an alumina content greater than 99.5%, and the difference between the inner diameter of the high-temperature resistant ceramic encapsulation tube and the outer diameter of the pure silica cladding is less than 5 micrometers.

4. A method for manufacturing a high-temperature sensor based on a torsion-type long-period fiber grating, characterized by, The method is used to prepare the high-temperature sensor based on a twisted long-period fiber grating as described in any one of claims 1-3, and includes the following steps: S1: Fix both ends of a single-mode silica fiber to a coaxially aligned rotating clamp, control the rotating clamp to rotate around the single-mode silica fiber by a preset torsion angle, and prepare the single-mode silica fiber with a preset torsion stress field in a torsion-locked state. S2: Set the parameters of the carbon dioxide laser, control the two-dimensional electric displacement platform to work in coordination with the carbon dioxide laser, and alternately perform fixed-point laser pulse irradiation and stepping movement along the axis of the single-mode quartz fiber to induce thermal relaxation of the preset torsional stress field and prepare a torsional long-period fiber grating. S3: The transmission spectrum data of the torsion long-period fiber grating is acquired by a spectrum analyzer, the power loss value at the resonant wavelength of the transmission spectrum data is extracted, and the carbon dioxide laser and the two-dimensional electric displacement platform are controlled based on the difference between the power loss value and the target attenuation threshold. S4: Obtain the resonant wavelength drift value of the torsion long-period fiber grating after constant temperature annealing in a high-temperature tubular heating furnace at discrete calibration temperature points, substitute the resonant wavelength drift value and the discrete calibration temperature points into the least squares model, and calculate the temperature sensitivity coefficient of the torsion long-period fiber grating.

5. The method for fabricating a high-temperature sensor based on a twisted long-period fiber grating according to claim 4, characterized in that, The process of establishing the pre-set torsional stress field includes: S11: Use mechanical stripping pliers to remove the polymer coating layer of a predetermined length in the middle section of the single-mode quartz fiber, exposing the cladding structure of the single-mode quartz fiber, and use anhydrous ethanol solution to ultrasonically clean the exposed cladding structure surface. S12: Place the cleaned single-mode silica fiber horizontally into the rotating clamp, and adjust the left and right chucks of the rotating clamp to ensure that the axis of the single-mode silica fiber coincides with the axis of rotation of the rotating clamp. S13: Lock the left end chuck of the rotating fixture, drive the right end chuck of the rotating fixture to rotate in a preset direction until the preset torsion angle is reached, and lock the position of the right end chuck to establish a stable preset torsional stress field inside the single-mode quartz optical fiber.

6. The method for fabricating a high-temperature sensor based on a twisted long-period fiber grating according to claim 5, characterized in that, The calculation process for the preset torsion angle mentioned in S13 includes: The shear modulus, cladding radius, and preset torsion length of the single-mode silica fiber are obtained. Substitute the shear modulus, the cladding radius, and the torsion length into the torsion angle calculation model to calculate and generate the preset torsion angle; The expression for the torsion angle calculation model is as follows: in, Represents the preset torsion angle, Represents the pre-torsional compensation coefficient. This represents the target shear stress value applied to the surface of the single-mode quartz fiber. Represents the torsion length, Represents the shear modulus. This represents the cladding radius.

7. The method for fabricating a high-temperature sensor based on a twisted long-period fiber grating according to claim 6, characterized in that, The fabrication process of the twisted long-period fiber grating includes: S21: Configure the peak pulse power, pulse repetition frequency, and pulse width parameters of the carbon dioxide laser, and focus the laser beam emitted by the carbon dioxide laser through a cylindrical lens to form a linear spot; S22: Adjust the height axis of the two-dimensional electric displacement platform so that the focal point of the linear light spot falls precisely on the core position of the single-mode quartz fiber. S23: Control the two-dimensional electric displacement platform to perform stepping motion along the axis of the single-mode silica fiber at a preset grating period length, and trigger the carbon dioxide laser to emit a quantitative laser pulse at each step stop point to periodically perform local thermal relaxation modulation on the preset torsional stress field until the preset number of grating periods are completed to obtain the torsional long-period fiber grating.

8. The method for fabricating a high-temperature sensor based on a twisted long-period fiber grating according to claim 7, characterized in that, The process for determining the single-point irradiation energy density of the laser pulse described in S23 includes: The peak pulse power of the carbon dioxide laser, the effective area of ​​the linear spot, and the pulse width are obtained. The optical power density is calculated based on the pulse peak power and the effective area, and the single-point irradiation energy density is generated by combining the pulse width. The formula for calculating the single-point irradiation energy density is as follows: in, This represents the single-point irradiation energy density. Represents the peak power of the pulse. Represents the pulse width. This represents the projected area of ​​the linear light spot on the surface of the single-mode silica fiber.

9. The method for fabricating a high-temperature sensor based on a twisted long-period fiber grating according to claim 4, characterized in that, The control process for the carbon dioxide laser and the two-dimensional electric displacement platform includes: S31: Connect the input end of the twisted long-period fiber grating to the broadband light source and connect its output end to the spectrometer to start the real-time spectral scanning mode. S32: Identify the center wavelength position of the resonance peak in the transmission spectrum data, and calculate the power loss value of the transmission power at that position relative to the baseline power in real time; S33: Determine whether the power loss value has reached the target attenuation threshold; If the target is not met, the carbon dioxide laser is controlled to increase the number of laser pulses or the two-dimensional electric displacement platform is controlled to reset for repeated scanning and writing. If the target is reached, the output of the carbon dioxide laser is stopped and the torsional locking state of the rotary clamp is released.

10. The method for fabricating a high-temperature sensor based on a twisted long-period fiber grating according to claim 4, characterized in that, The calculation process for the temperature sensitivity coefficient includes: S41: The torsion long-period fiber grating is placed in the high-temperature tubular heating furnace, heated to the preset annealing temperature at a preset heating rate and kept at a constant temperature, and then naturally cooled to room temperature after eliminating residual processing stress. S42: Control the high-temperature tubular furnace to gradually increase the temperature according to the preset temperature gradient, and after holding the temperature at each discrete calibration temperature point for a preset time, record the resonant wavelength drift value. S43: Establish a linear regression dataset with the discrete calibration temperature points as independent variables and the resonant wavelength drift value as dependent variable, calculate the slope of the regression line using the least squares model, and determine the slope as the temperature sensitivity coefficient.

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