Multi-parameter sensor based on butterfly-shaped photonic crystal fiber and decoupling method thereof

Abstract

The invention discloses a multi-parameter sensor based on a butterfly-shaped photonic crystal fiber and a decoupling method thereof, and belongs to the technical field of optical fiber sensing. The multi-parameter sensor based on the butterfly-shaped photonic crystal fiber comprises a section of photonic crystal fiber PCF with a high birefringence effect structure, two fiber bragg gratings with different central wavelengths, namely FBG-A and FBG-B, are written into adjacent or overlapped sections in the fiber through a laser writing method, and the FBG-A and the FBG-B are connected with the photonic crystal fiber PCF. Each FBG respectively forms a pair of reflection peaks in two polarization directions so as to generate four wavelength response signals; the FBG-B section is also provided with a sleeve structure made of glass, metal or polymer materials; the two sides of the butterfly-shaped photonic crystal fiber PCF fiber core are provided with asymmetric air hole structures. According to the invention, the dual-wavelength fiber bragg grating is written into the butterfly-shaped PCF, and the natural high birefringence characteristic of the dual-wavelength fiber bragg grating is combined with air hole filling regulation and control, so that high-sensitivity measurement and decoupling of three parameters of temperature, strain and pressure are realized, and the test precision is improved.

Description

Technical Field

[0001] This invention relates to a multi-parameter sensor based on a butterfly-shaped photonic crystal fiber and its decoupling method, belonging to the field of fiber optic sensing technology. Background Technology

[0002] Fiber Bragg Gratings (FBGs), as a novel type of fiber optic sensor, have been widely used in aerospace, energy equipment, civil engineering, and medical fields due to their advantages such as resistance to electromagnetic interference, high temperature resistance, small size, light weight, and the ability to be embedded within materials. In multiphysics monitoring scenarios, FBGs are commonly used to measure temperature and strain parameters, and related research is relatively mature.

[0003] However, traditional FBG sensors have certain limitations in sensing pressure parameters. Due to the limited lateral flexibility of standard communication optical fibers or ordinary high-birefringence optical fibers, their sensitivity to changes in external pressure is generally low, usually requiring indirect conversion of pressure signals through external diaphragms, sleeves, or special encapsulation structures. These methods not only increase the sensor's packaging size and complexity but may also introduce additional temperature / strain cross-sensitivity, limiting their application in high-precision multi-parameter measurements.

[0004] On the other hand, existing research on multi-parameter decoupling mainly focuses on temperature-strain dual-parameters, often achieved through dual-wavelength FBG, coating with thermosensitive materials, or dual-fiber combinations. However, in the decoupling of temperature-strain-pressure three-parameters, there is still a lack of a compact, highly sensitive, and mass-producible fiber optic sensing scheme.

[0005] Butterfly-shaped photonic crystal fiber (FBF) possesses naturally high birefringence due to the presence of asymmetric large air holes on both sides of its core. When a fiber grating is written into this type of fiber, the reflection spectrum exhibits significant splitting in both polarization directions. This polarization-dependent characteristic is not only sensitive to external strain and temperature changes but also demonstrates a strong response to external transverse pressure, providing a new structural basis for improving the pressure sensitivity of FBFs.

[0006] Therefore, how to fully utilize the structural characteristics of butterfly-shaped PCFs and construct a sensing method that can decouple the three parameters of temperature, strain, and pressure by rationally designing the grating writing scheme and the local air hole filling method has become an urgent problem to be solved in the field of fiber optic sensing. Summary of the Invention

[0007] To address the problems of low pressure sensing sensitivity and difficulty in decoupling three parameters in existing fiber Bragg grating sensors, the present invention aims to provide a multi-parameter sensor based on a butterfly-shaped photonic crystal fiber and its decoupling method. By writing a dual-wavelength fiber Bragg grating into the butterfly-shaped PCF and combining its natural high birefringence characteristics with air hole filling control, high-sensitivity measurement and decoupling of the three parameters of temperature, strain and pressure can be achieved.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] The multi-parameter sensor based on a butterfly-shaped photonic crystal fiber disclosed in this invention includes a section of photonic crystal fiber PCF with a high birefringence effect structure. Two fiber Bragg gratings with different center wavelengths, namely FBG-A and FBG-B, are written into adjacent or overlapping sections of the fiber using a laser writing method. Each FBG forms a pair of reflection peaks in two polarization directions, thereby generating four wavelength response signals. The FBG-B section is also provided with a sleeve structure of glass / metal / polymer material. Asymmetric air hole structures exist on both sides of the core of the butterfly-shaped photonic crystal fiber PCF.

[0010] The four wavelength response signals correspond to the reflection peak wavelengths of FBG-A in the x-polarization and y-polarization directions, and the reflection peak wavelengths of FBG-B in the x-polarization and y-polarization directions, respectively, denoted as λ. a1 , λ a2 , λ b1 , λ b2 .

[0011] The laser writing method involves placing the optical fiber in a hydrogen atmosphere to enhance the photosensitivity of the Ge-doped fiber core; then removing the fiber coating area; and finally writing using a laser.

[0012] The laser exposure parameters are as follows:

[0013] Source: KrF excimer laser (248nm), pulse width 20ns, repetition frequency 100Hz;

[0014] Phase mask: 1520nm / 1560nm design center wavelength;

[0015] Focusing system: cylindrical lens + precision five-dimensional displacement stage, control resolution ≤50nm;

[0016] Exposure energy density: (100–150) mJ / cm 2 ;

[0017] Simultaneously monitor the reflection spectrum and stop exposure once the target wavelength is reached.

[0018] The method to improve the photosensitivity of Ge-doped fiber core is to place the fiber in a high-purity hydrogen environment at 180 atm and 60°C for 72 hours to improve the photosensitivity of Ge-doped fiber core.

[0019] The center wavelength of FBG-A and the center wavelength of FBG-B are spaced at more than 40 nm to ensure that the reflection spectra do not overlap and to obtain independent sensitivity characteristics.

[0020] The preferred filling scheme for the air holes in the photonic crystal fiber section containing FBG-A / FBG-B is as follows:

[0021] 1) No material was filled in the FBG-A section to enhance strain response sensitivity;

[0022] 2) The air pores in the FBG-B section are filled with ultra-soft silicone. Because ultra-soft silicone has an extremely low Young's modulus, the pore size deforms significantly under external pressure, thereby amplifying the pressure sensitivity.

[0023] The ultra-soft silicone is a silicon-based elastomer with a Young's modulus of less than 0.5 MPa.

[0024] The metal material is aluminum, copper, or stainless steel; the sleeve thickness is 0.2mm to 1.5mm.

[0025] The wavelength of the reflection peak in the polarization direction is read in real time by a polarization-sensitive demodulator. The demodulator supports peak extraction of multi-peak reflection spectra and outputs λ. a1 , λ a2 , λ b1 , λ b2 The wavelength drift value.

[0026] The sensor system is suitable for aircraft, wind turbine blades, tunnels, storage tanks, composite material structures, or other engineering scenarios that require simultaneous monitoring of strain, pressure, and temperature.

[0027] The decoupling method for a multi-parameter sensor based on a butterfly-shaped photonic crystal fiber disclosed in this invention is implemented based on the aforementioned multi-parameter sensor based on a butterfly-shaped photonic crystal fiber. The decoupling method for the multi-parameter sensor based on a butterfly-shaped photonic crystal fiber includes the following steps:

[0028] Step 1: Control the variation range of temperature T, strain ε, and pressure P respectively, and perform sensor sensitivity calibration under single parameter variation conditions to obtain the temperature, strain, and pressure sensitivities of the four spectral lines FBG, and establish the sensitivity matrix K.

[0029] Step 2: When temperature T, strain ε and pressure P are measured simultaneously, a high-resolution demodulator is used to record the spectral changes of the two polarization reflection peaks of FBG-A and FBG-B respectively, and the changes of the center wavelength of the four spectral lines are obtained. Further, the wavelength change matrices of the four spectral lines corresponding to parameters T, ε and P are obtained as Δλ1, Δλ2, Δλ3 and Δλ4.

[0030] Step 3: According to the wavelength drift formula of FBG

[0031] Δλ i =k i,ε ·Δε+k i,p ·ΔP+k i,T ·ΔT+η i

[0032] Substituting the four-line FBG into the wavelength drift formula, an overdetermined system of equations is constructed, namely...

[0033]

[0034] Substituting the wavelength drift matrix and sensitivity matrix, the parameter vector is solved using the least squares method:

[0035] [ΔT Δε ΔP] T =(K T K) -1 K T [Δλ]

[0036] By minimizing the sum of squared residuals through redundant equations, the optimal solutions for temperature T, strain ε, and pressure P are obtained, thereby enabling independent decoupled measurement of the three parameters of strain, pressure, and temperature.

[0037] Beneficial effects:

[0038] 1. This invention discloses a multi-parameter sensor based on a butterfly-shaped photonic crystal fiber and its decoupling method. The butterfly-shaped PCF, due to the asymmetric air hole structure on both sides of the fiber core, possesses a large transverse birefringence effect. After writing FBG into this type of fiber, the reflection spectrum splits in two polarization directions, and the splitting interval has high sensitivity to changes in external transverse pressure, thus overcoming the problem of insufficient pressure response of traditional FBGs and improving pressure sensitivity.

[0039] 2. The multi-parameter sensor based on butterfly-shaped photonic crystal fiber and its decoupling method disclosed in this invention writes two FBG segments with different center wavelengths (such as 1520nm and 1560nm) at the same position of the butterfly-shaped PCF, so that they exhibit differentiated sensitivity responses under the same external conditions, forming complementary parametric sensitivity characteristics, providing more independent equations for three-parameter decoupling, and helping to realize dual-wavelength grating optimization.

[0040] 3. The multi-parameter sensor based on a butterfly-shaped photonic crystal fiber and its decoupling method disclosed in this invention, in a dual-wavelength FBG, leaves the air holes vacant in the FBG-A region to enhance its strain response; in the FBG-B region, the air holes are filled with a high-sensitivity polymer (e.g., Ecoflex 00-30 or equivalent ultra-soft RTV silicone). Through the differences in the mechanical and thermal properties of the materials, enhanced response to pressure and temperature is achieved. This differentiated design makes the sensitivity matrices of the two grating segments independent of each other in the three parameters, facilitating decoupling and thus enabling differentiated filling of the air holes.

[0041] 4. This invention discloses a multi-parameter sensor based on a butterfly-shaped photonic crystal fiber and its decoupling method. The dual-wavelength FBG generates reflection peaks in both polarization directions, thus obtaining four independent spectral lines. Based on the overdetermined equations constructed using these four spectral lines, multi-spectral line optimization is performed using the least squares method, which reduces noise interference and improves the accuracy of the three-parameter calculation. Theoretical results show that the four-spectral-line method reduces the mean square error by approximately 20% to 35% compared to the traditional three-spectral-line method.

[0042] 5. The multi-parameter sensor and its decoupling method based on butterfly-shaped photonic crystal fiber disclosed in this invention, based on the above four beneficial effects, achieves decoupling and high-precision measurement of the three parameters of temperature, strain, and pressure through the structural advantages of butterfly-shaped PCF, dual-wavelength FBG writing, differentiated filling of air holes, and multi-spectral line optimization calculation. This solves the technical problems of low pressure sensitivity and difficulty in decoupling the three parameters in traditional FBG sensors. In addition, this invention does not require additional external diaphragms or complex packaging, and can obtain high pressure sensitivity by relying on the structure of the butterfly-shaped PCF itself. Combined with laser grating writing process and local filling method, it can achieve miniaturization and mass production, and is suitable for embedding in composite material structures or for use in aerospace equipment. Attached Figure Description

[0043] Figure 1 A schematic diagram of the cross-section of a butterfly-shaped photonic crystal fiber;

[0044] Figure 2 This is a schematic diagram showing the writing positions of dual-wavelength FBG and the arrangement of different air hole filling materials. Detailed Implementation

[0045] To better illustrate the purpose and advantages of the present invention, the invention will be further described below in conjunction with the accompanying drawings and examples.

[0046] Example 1: Writing of Dual-Wavelength FBG

[0047] Using femtosecond lasers or phase masking, two fiber Bragg gratings (FBGs) with different center wavelengths (λ1 = 1520 nm, λ2 = 1560 nm) are written adjacent to each other in the same region of the core of the butterfly-shaped PCF, and named FBG-A and FBG-B.

[0048] Specifically, the length of the FBG is (3-5) mm, and the distance between the edges of the two FBGs is (1-2) mm.

[0049] Due to the difference in microstructure between the PCF core region and the air pores, the birefringence effect is enhanced, and each FBG produces two spectral lines, λx and λy. The total number of output reflection spectra is four, which is sufficient to construct an overdetermined system of equations.

[0050] Example 2: Differentiated filling of air holes in PCS

[0051] The FBG-A region retains its unfilled air pores to ensure direct transmission of axial strain (high strain sensitivity, weak pressure response). The FBG-B region has ultra-soft silicone (e.g., Ecoflex 00-30 or equivalent ultra-soft RTV silicone, Young's modulus approximately 0.05MPa–0.3MPa) filled into the corresponding air pores, and is externally fitted with a thin-walled metal tube (aluminum or stainless steel). The ultra-soft filler easily deforms under external pressure, transmitting pressure more effectively to the fiber core region, while the metal tube further alters the stress distribution and loading direction, significantly improving the pressure sensitivity of FBG-B. The difference in sensitivity direction between the two regions forms a good condition number sensitivity matrix K (4×3), which is solved using the least squares method, and redundant spectral lines are used to suppress noise and improve accuracy. The specific implementation method is as follows:

[0052] 1) Area isolation: Fill the air hole area between FBG-A and FBG-B with a small amount of low-modulus epoxy resin or a small amount of UV-cured adhesive to prevent the air hole filler in the FBG-B area from flowing into the air hole area of ​​FBG-A.

[0053] 2) Injection method: Place the optical fiber on the microscopic stage, place the ultra-soft silicone material on the capillary needle end at one end, and connect the other end to a small vacuum pump. Perform a slight vacuum and gently press the syringe. Utilize the capillary effect and pressure difference to introduce the ultra-soft silicone into the air hole of the designated FBG-B area. The coverage length is the FBG-B length + 1mm margin.

[0054] 3) Curing: Curing at room temperature for 24 hours or heat curing at 60℃ for 2 hours.

[0055] 4) End sealing: After curing, seal both ends of the filled area with a small amount of low-modulus epoxy / UV adhesive, then put on heat shrink tubing and shrink to fix it; if it is necessary to improve the structural reliability, a short section of thin-walled glass capillary (or metal or polymer capillary) can be put on the ultra-soft silicone filled area and fixed with a thin layer of epoxy.

[0056] 5) Cleaning: Remove excess material from the surface and measure the four FBG spectra again, recording the wavelength shift of the reflection peak (as an initial reference after packaging).

[0057] Example 3: Sensitivity Calibration

[0058] Equipment: Standard strain gauge (capable of applying axial strain), standard pressure gauge (capable of applying radial hydrostatic pressure), standard temperature gauge.

[0059] step:

[0060] 1) Fix the sensor sample, maintain the baseline temperature (e.g., 25°C), and the pressure at normal pressure. Ensure that the sensor grating is not under stress, and measure and record the wavelengths of the four baseline peaks.

[0061] 2) Strain calibration (at room temperature and pressure): Using a standard strain gauge, apply axial strain steps (e.g., 0, 100, 200, ..., 1000 με) to the sensor. After each step is stable, record 4 peaks Δλ. Repeat 3 times and take the average. Use linear regression to fit each spectrum line to obtain k_{i,ε}(pm / με).

[0062] 3) Pressure calibration (fixed temperature, no axial strain): Place the sensor segment into the pressure chamber, apply radial (external) pressure (range, for example, 0–10 MPa, step size 0.5 MPa), record Δλ and fit k_{i,P} (pm / MPa).

[0063] 4) Temperature calibration (fixed strain, pressure): Place in a constant temperature furnace (standard temperature device), and gradually change the temperature (e.g.,

[0064] (20℃~80℃, step size 10℃), record Δλ and fit k_{i,T}(pm / ℃).

[0065] 5) Construct the sensitivity matrix K: Write out K (4×3) in order. If the fitting is significantly nonlinear, a second-order term can be added to the model or an interpolation table + local linearization can be used.

[0066] 6) Cross-validation: Perform blind testing with several combined loads (simultaneously changing ε, P, and T), and compare the least squares solution results.

[0067] The results are compared with the actual loaded values, and the errors (RMSE, deviation, linear correlation) are calculated.

[0068] Example 4: Encapsulation and Pre-stretching

[0069] After filling and sealing, a pre-stretch (typically 50–150 με) is applied to the entire fiber optic cable, and then the outer fixing material (epoxy) is cured while maintaining the tension. This reduces long-term zero drift after release. If used in humid / seawater environments, a secondary waterproof seal (epoxy + heat shrink + waterproof adhesive) is required for the outer layer.

[0070] Example 5: Mathematical Model and Least Squares Solution

[0071] FBG wavelength drift formula:

[0072] Δλ i =k i,ε ·Δε+k i,p ·ΔP+k i,T ·ΔT+η i

[0073] in:

[0074] ·Δλ i : The center wavelength shift of the reflection peak of the i-th FBG spectral line, with a total of 4 independent FBG reflection peaks (2 wavelengths × 2 polarization splits);

[0075] ·k i,ε k i,P k i,T Sensitivity coefficient (requires experimental calibration);

[0076] • Δε, ΔP, ΔT: Parameters to be determined (strain, pressure, temperature change);

[0077] ·η i Measurement noise or model error.

[0078] Matrix form:

[0079] Δλ=Kx+η

[0080]

[0081] The optimal solution is obtained by minimizing the sum of squared residuals using redundant equations.

[0082] Example 6: Calibration Results and Decoupling Calculation

[0083] 1) Example of a typical sensitivity calibration result matrix

[0084] Spectral line (i) ki,ε (pm / με) ki,P (pm / MPa) ki,T (pm / °C) λ1520 - x (FBG1) 1.30 0.80 12 λ1520 - y (FBG1) 1.20 1.00 13 λ1560 - x (FBG2) 1.00 6.00 16 λ1560 - y (FBG2) 1.05 8.00 18

[0085] The three columns correspond to strain (ε), pressure (P), and temperature (T), respectively, and the four rows correspond to four spectral lines.

[0086] The system model is: Δλ=Kx+η

[0087] Where x = [Δε, ΔP, ΔT] T The noise η is independent and identically distributed Gaussian white noise.

[0088] The least squares method is used for three-parameter decoupling, and the estimated covariance is:

[0089]

[0090] In this embodiment, the standard deviation of spectral demodulation noise is assumed to be σ = 1 pm.

[0091] Calculate K T K:

[0092]

[0093] Find the inverse:

[0094]

[0095] Covariance matrix (σ = 1pm):

[0096]

[0097] Parameter uncertainty (standard deviation):

[0098] Strain resolution: Pressure resolution: Temperature resolution:

[0099] By employing differentiated structural designs (void vs. ultra-soft silicone filling + metal sleeve) in two different wavelength FBG regions, and combining polarization splitting to obtain four independent spectral lines, joint decoupling of three parameters can be achieved at a noise level σ = 1 pm, achieving a resolution of strain ±7.21 με, pressure ±0.85 MPa, and temperature ±0.78 ℃.

[0100] These results demonstrate that the proposed solution can significantly improve pressure sensitivity and decoupling accuracy, and meet the requirements of engineering applications. Furthermore, if the demodulation noise is further reduced (e.g., by coherent demodulation or signal averaging, σ is reduced to 0.1 pm), the joint decoupling of the three parameters can be further optimized, achieving a resolution of ±0.721 με for strain, ±0.085 MPa for pressure, and ±0.078 °C for temperature.

[0101] Example 7:

[0102] The decoupling method for a multi-parameter sensor based on a butterfly-shaped photonic crystal fiber disclosed in this embodiment is implemented in the following steps:

[0103] Step 1: Fiber Optic and Structural Design Stage.

[0104] Step 1.1: Fiber optic structure selection.

[0105] The butterfly-shaped photonic crystal fiber (PCF) is used, with an asymmetric distribution of air holes on both sides of the cross-section to introduce high polarization birefringence characteristics.

[0106] ο Fiber cladding diameter: 125μm;

[0107] Effective refractive index of the core region: 1.445;

[0108] Air hole spacing Λ: 3.5μm;

[0109] The difference in air aperture between the left and right wings is about 20%; this structure ensures a significant split between the strain-sensitive axis and the temperature-sensitive axis, forming a polarization dual-mode.

[0110] Step 1.2: Dual-zone layout design.

[0111] • Write two different wavelengths of FBG onto a continuous segment of the same butterfly-shaped PCF:

[0112] οFBG-A: Center wavelength 1520nm, unfilled air holes (maintained air state).

[0113] οFBG-B: Center wavelength 1560nm, air pore filled with ultra-soft silicone (Ecoflex 00-30 or equivalent RTV), Young's modulus (0.05–0.3)MPa.

[0114] • Each FBG is 5mm long and spaced (1-2)mm apart.

[0115] The silicone filler area of ​​FBG-B is approximately 8mm long, slightly longer than the grating area to ensure complete boundary stress transfer.

[0116] Step 2: Grating etching stage.

[0117] Step 2.1: Fiber preprocessing.

[0118] ο Hydrogen sensitization: Place in an environment of 180 atm and 60°C for 72 hours to enhance the photosensitivity of the Ge-doped fiber core.

[0119] ο Coating removal: The coating area of ​​the 15mm optical fiber is removed using CO2 laser or chemical stripping.

[0120] Step 2.2: Engraving settings.

[0121] Source: KrF excimer laser (248nm), pulse width 20ns, repetition frequency 100Hz;

[0122] Phase mask: 1520nm / 1560nm design center wavelength;

[0123] Focusing system: cylindrical lens + precision five-dimensional displacement stage, control resolution ≤50nm;

[0124] Exposure energy density: (100–150) mJ / cm 2 ;

[0125] The laser focus is adjusted to approximately 1 μm behind the fiber core to achieve a Type IIA grating structure, thereby enhancing reflection intensity and temperature stability.

[0126] Simultaneously monitor the reflection spectrum and stop exposure once the target wavelength is reached.

[0127] Step 3: Filling and Encapsulation Stage.

[0128] Step 3.1: Filling air holes (FBG-B area).

[0129] Using a vacuum-assisted injection device (negative pressure approximately -0.08 MPa), inject Ecoflex 00-30 silicone into the air pores using a needle.

[0130] Because the air holes of the butterfly-shaped PCF are arranged in a linear two-wing distribution, the optical fiber needs to be rotated slowly to fill it evenly.

[0131] Curing at 60°C for 2 hours helps the silicone maintain its elasticity and prevents residual air in the pores.

[0132] Step 3.2: Fixing and Encapsulation

[0133] The οFBG area is wrapped with a glass capillary tube with a diameter of 0.8 mm, and the outer layer is sealed with low modulus silicone rubber (E~1MPa);

[0134] The ends are fixed with fiber optic epoxy adhesive to prevent strain leakage;

[0135] After packaging, spectral verification is performed to ensure that the reflection spectrum remains stable within ±0.05nm.

[0136] Step 4: Sensor sensitivity calibration

[0137] The sensitivity of the three-parameter sensor (four-spectral line) was calibrated using a standard strain gauge (capable of applying axial strain), a standard pressure gauge (capable of applying radial hydrostatic pressure), and a standard temperature gauge.

[0138] When calibrating temperature, strain, and pressure sensitivity, each calibration should be performed by controlling the gradient change of a single parameter, while keeping the other two parameters constant during the calibration process.

[0139] The sensitivity matrix K is constructed using the obtained 4-line 3-parameter sensitivity.

[0140] Step 5: Three-parameter measurement and decoupling

[0141] Verify the accuracy of the solution results in this embodiment under noise conditions (σ = 1 pm):

[0142] 1) Select the sensitivity matrix K (simulation estimate)

[0143] Each row corresponds to a spectral line (λ1520-x, λ1520-y, λ1560-x, λ1560-y), and each column represents the sensitivity of the corresponding parameter (in order: strain ε, pressure P, temperature T), with units of pm / (με, MPa, ℃).

[0144]

[0145] (This matrix is ​​an example estimate / calibration value; the actual value should be replaced with the measured calibration matrix.)

[0146] 2) Model and Objective

[0147] Linear model:

[0148]

[0149] noise And take independent and identically distributed: Σ η =σ 2 I. In this embodiment, σ = 1pm.

[0150] Objective: To provide The estimated covariance and the uncertainty (standard deviation) of each component.

[0151] 3) Calculation steps (matrix operations: step by step)

[0152] (A) Calculate K T K

[0153] K T K is a 3×3 matrix whose elements are the dot product of column vectors (the values ​​have been calculated in advance):

[0154]

[0155] (Calculation method: for example, the first row and first column = 1.30) 2 +1.20 2 +1.00 2 +1.05 2 =5.2325, and the same logic applies to other terms.

[0156] (B) Inverse (K) T K)-1

[0157] Calculate the numerical inverse matrix (this can be verified using Matlab / Python / numerical calculator). The result is:

[0158]

[0159] (Numerical explanation: The elemental magnitude is related to the scale of K; the negative sign indicates a negative correlation between estimates.)

[0160] (C) Covariance matrix (least squares case)

[0161] When there is independent and identically distributed noise and σ = 1pm:

[0162]

[0163] Therefore, the covariance is the numerical matrix in the above equation:

[0164]

[0165] • The first row / column corresponds to the variance and covariance terms of the strain.

[0166] • The second row / column corresponds to the variance and covariance terms of the pressure.

[0167] • The variance and covariance terms for the temperature in the third row / column.

[0168] (D) Standard deviation (uncertainty) of each parameter

[0169] Pick Square root of the diagonal element:

[0170] ·Strain σ_ε=√52.0048≈7.21με

[0171] Pressure σ_P=√0.7236≈0.85MPa

[0172] Temperature σ_T=√0.6080≈0.78℃

[0173] Under measurement noise conditions of σ = 1 pm, the statistical uncertainty (1σ) of the three parameters is obtained by least squares from four spectral lines.

[0174] The K listed in this embodiment is an example estimated / calibrated value; a detailed calibration should be performed on the prepared sample to obtain the final K and calculate the final accuracy accordingly. To verify the data of the above embodiment, it is preferable to perform calibration tests on the sensor along the three axes of strain, pressure, and temperature after preparation (applying multi-point stepped loading and fitting respectively) to obtain the measured sensitivity matrix K, which will serve as the basis for decoupling inversion and accuracy evaluation.

[0175] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A multi-parameter sensor based on a butterfly-shaped photonic crystal fiber, characterized in that: The Butterfly PCF photonic crystal fiber includes a section with a butterfly-shaped air-hole arrangement structure exhibiting a high birefringence effect. Two fiber Bragg gratings (FBG-A and FBG-B) with different center wavelengths are written into the fiber core region via laser exposure writing in adjacent or overlapping sections. Each FBG forms a pair of reflection peaks in two polarization directions, thereby generating four wavelength response signals. The FBG-B section also features a sleeve structure made of glass, metal, or polymer material. Asymmetric air-hole structures exist on both sides of the butterfly-shaped PCF core. The four wavelength response signals correspond to the reflection peak wavelengths of FBG-A in the x-polarization and y-polarization directions, and the reflection peak wavelengths of FBG-B in the x-polarization and y-polarization directions, respectively, denoted as λ. a1 , λ a2 , λ b1 , λ b2 .

2. The sensor as described in claim 1, characterized in that: The laser writing method involves placing the optical fiber in a hydrogen atmosphere to enhance the photosensitivity of the Ge-doped fiber core; then removing the fiber coating area; and finally writing using a laser.

3. The sensor as described in claim 2, characterized in that: The laser exposure parameters are as follows: Source: 248nm KrF excimer laser, pulse width 20ns, repetition frequency 100Hz; Phase mask: 1520nm / 1560nm design center wavelength; Focusing system: cylindrical lens + precision five-dimensional displacement stage, control resolution ≤50nm; Exposure energy density: (100–150) mJ / cm 2 ; The laser focus is adjusted to about 1μm behind the fiber core to achieve the Type IIA grating structure, thereby enhancing the reflection intensity and temperature stability. Simultaneously monitor the reflectance spectrum and stop exposure once the target reflectance is reached.

4. The sensor as described in claim 2, characterized in that: The method to improve the photosensitivity of Ge-doped fiber core is to place the fiber in an environment of 180 atm and 60°C for 72 hours to improve the photosensitivity of Ge-doped fiber core.

5. The sensor as described in claim 2, characterized in that: The center wavelength of FBG-A and the center wavelength of FBG-B are spaced at more than 40 nm to ensure that the reflection spectra do not overlap and to obtain independent sensitivity characteristics. The filling scheme selected for the air holes in the photonic crystal fiber section where FBG-A / FBG-B are located is as follows: 1) Section FBG-A was not filled with any material; 2) The air holes in the FBG-B section are filled with ultra-soft silicone.

6. The sensor as described in claim 5, characterized in that: The ultra-soft silicone is a silicon-based elastomer with a Young's modulus of less than 0.5 MPa.

7. The sensor as described in claim 2, characterized in that: The sleeve structure is preferably a glass capillary tube with an outer diameter of 0.8 mm, an inner diameter of 0.7 mm, and a thin-walled structure.

8. A decoupling method for a multi-parameter sensor based on a butterfly-shaped photonic crystal fiber, implemented on the basis of the multi-parameter sensor based on a butterfly-shaped photonic crystal fiber as described in claims 1, 2, 3, 4, 5, 6 or 7, characterized in that: Includes the following steps, Step 1: Control the variation range of temperature T, strain ε and pressure P respectively, and perform sensor sensitivity calibration under single parameter variation conditions to obtain the temperature, strain and pressure sensitivities of the four spectral lines FBG, and establish the sensitivity matrix K. Step 2: When temperature T, strain ε and pressure P are measured simultaneously, a high-resolution demodulator is used to record the spectral changes of the two polarization reflection peaks of FBG-A and FBG-B respectively, and the changes of the center wavelength of the four spectral lines are obtained. Further, the wavelength change matrices of the four spectral lines corresponding to parameters T, ε and P are obtained as Δλ1, Δλ2, Δλ3 and Δλ4. Step 3: According to the wavelength drift formula of FBG Dl i =k i,ε ·No+k i,P ·ΔP+k i,T ·ΔT+η i Substituting the four-line FBG into the wavelength drift formula, an overdetermined system of equations is constructed, namely... Substituting the wavelength drift matrix and sensitivity matrix, the parameter vector is solved using the least squares method: [ΔT Not ΔP] T =(K T K) -1 K T [Dl] By minimizing the sum of squared residuals through redundant equations, the optimal solutions for temperature T, strain ε, and pressure P are obtained, thereby enabling independent decoupled measurement of the three parameters of strain, pressure, and temperature.

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