Phase-shifted fiber bragg grating and demodulation method, preparation method and preparation system thereof

Abstract

This invention discloses a phase-shifting fiber Bragg grating and its demodulation method, fabrication method, and fabrication system. The phase-shifting fiber Bragg grating includes a fiber core, a cladding covering the fiber core, and a grating region formed on the fiber core. A V-groove is formed on the sidewall of the cladding corresponding to the grating region. The cross-section of the V-groove along the fiber axis is V-shaped, and the bottom of the V-groove is located within the cladding and does not contact the fiber core. The V-groove design causes the fiber grating spectrum to split from a single-peak or single-valley structure (unstressed) into a double-peak or double-valley structure when the fiber is subjected to axial tension. By analyzing the characteristic parameters of this spectral splitting structure, the magnitude of the axial stress can be determined. Combined with the overall wavelength shift of the centroid of the double-peak or double-valley structure, temperature changes can be further demodulated, thereby achieving temperature / stress decoupling on a single grating.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing, and more specifically, to a phase-shifted fiber Bragg grating and its demodulation method, fabrication method, and fabrication system. Background Technology

[0002] Fiber Bragg gratings (FBGs) are a mature fiber optic sensor device. Stress and temperature can be detected by the movement of their peaks, but the movement of a single peak cannot determine the cause of the movement.

[0003] Therefore, during the sensing process, the detection at the same point requires the introduction of a free, stress-free FBG to measure the temperature separately, and then corrections are made for changes in stress. This process not only increases costs, but more importantly, it wastes the bandwidth of the fiber optic sensing, effectively halving the number of monitorable points for the same fiber optic modem.

[0004] Current FBG demodulation capabilities are sufficient for precise wavelength demodulation, typically achieving picometer-level accuracy. Given the typical 0.1 nanometer bandwidth of FBGs, this demodulation capability is adequate. Current research has revealed that altering the fiber diameter (i.e., diameter reduction) through etching, femtosecond etching, and discharge taper can create locally deficient fiber Bragg gratings. This generates splits within the original reflection peaks, forming a double-peak structure (splits within the transmission valleys of the transmission spectrum, creating a double-valley structure). By observing the different stress responses of the embedded peak positions (embedded valley positions in the transmission spectrum), independent stress-sensing data demodulation can be achieved.

[0005] However, the proposed solution has the following problems: First, the fabrication process is complex: the proposed diameter reduction schemes all aim to maintain the original circular symmetry of the optical fiber. Etching requires complex masks and etching. The proposed femtosecond laser processing requires a complex rotation and positioning system. Furthermore, femtosecond etching diameter reduction may damage the FBG gate region. Other schemes are also difficult to continuously and efficiently produce diameter reduction effects on the same optical fiber, making them incompatible with the current ultra-weak FBG sensing network system. Secondly, the cost remains high, with excessively high equipment and operating costs associated with the variable diameter process. Femtosecond lasers are very expensive, corrosion makes high-speed continuous operation difficult, and they involve hazardous chemicals. While discharge tapers, which can be used for continuous operation, consume a lot of electrodes, requiring electrode replacement and equipment recalibration after approximately 5,000 discharges, which is not conducive to mass production. Third, the demodulation process requires simultaneous monitoring of spectral peaks and valleys. For weak gratings, there will always be a situation where the signal strength is too low, which is not conducive to stable demodulation. Summary of the Invention

[0006] The purpose of this invention is to propose a phase-shifted fiber Bragg grating and its demodulation method, fabrication method and fabrication system, so that the characteristic parameters of the phase-shifted grating are only related to stress and not to temperature, while being compatible with temperature measurement, expanding the bandwidth of the original modem, and reducing the requirements for signal-to-noise ratio through peak height comparison and other methods.

[0007] To achieve the above objectives, in a first aspect, the present invention proposes a phase-shifting fiber Bragg grating, comprising: a fiber core, a cladding covering the fiber core, and a grating region formed on the fiber core; A V-groove is formed on the cladding sidewall corresponding to the grating region. The cross-section of the V-groove along the fiber axis is V-shaped, and the bottom of the V-groove is located inside the cladding and does not contact the fiber core.

[0008] Secondly, this invention proposes a method for fabricating a phase-shifted fiber Bragg grating, comprising: Using ordinary optical fiber consisting of a core and cladding as the substrate, a grating region is prepared in a predetermined section of the core; Using a V-groove processing device, a V-groove is processed on the cladding sidewall corresponding to the grating region, and the bottom of the V-groove is made close to but not in contact with the fiber core; during the processing, the stress demodulation range of the phase-shifting fiber Bragg grating is controlled by adjusting the depth of the V-groove.

[0009] Thirdly, the present invention proposes a stress demodulation method for demodulating the stress on the phase-shifted fiber Bragg grating described in the first aspect, comprising: The reflection or transmission spectrum of the fiber grating structure when it is not subjected to axial tension is obtained to obtain a single reflection peak or a single transmission valley and its resonant bandwidth. The reflection spectrum or transmission spectrum of the fiber grating structure under axial tension is obtained. In the reflection spectrum, the original single reflection peak is split into a double peak structure; in the transmission spectrum, the original single transmission valley is split into a double valley structure. The magnitude of the axial stress applied to the optical fiber is determined based on the characteristic parameters of the bimodal or bivalve structure.

[0010] Fourthly, the present invention proposes a fabrication system for a phase-shifting fiber Bragg grating, used to fabricate the phase-shifting fiber Bragg grating described in the first aspect, the system comprising: Fiber optic transmission device for continuously winding optical fibers; A continuous fiber Bragg grating writing system is used to sequentially write fiber Bragg gratings at predetermined positions on an optical fiber, wherein the fiber grating includes an ultra-weak fiber Bragg grating. The V-groove processing device is integrated on the same production line as the ultra-weak fiber Bragg grating writing system. It is used to continuously process V-grooves on the fiber Bragg grating portion of the fiber during the fiber Bragg grating writing process.

[0011] The beneficial effects of this invention are as follows: 1. Achieving temperature / stress decoupling: By creating grooves in the fiber cladding, axial stress information is modulated into characteristic parameters (such as peak height (valley) ratio) of the single-peak (single-valley) splitting phenomenon and double-peak (double-valley) structure of the spectrum, while temperature changes only cause an overall shift in the spectrum. The characterization methods of the two physical effects are independent of each other, thus enabling simultaneous and independent measurement of axial stress and temperature on a single fiber grating, solving the problem of cross-sensitivity.

[0012] 2. Improved system capacity and economy: Since there is no need to deploy additional temperature compensation gratings, the present invention saves the cost of sensor components and effectively improves the channel capacity of the same demodulation system, thereby improving the bandwidth utilization and economic benefits of the sensor network.

[0013] 3. Strong demodulation robustness: Stress demodulation is performed using characteristic parameters such as the ratio of double peak height (or double valley depth). This ratio is not sensitive to absolute fluctuations in optical power and can effectively resist interference from factors such as light source power jitter and changes in optical path connection loss. This reduces the requirements for signal-to-noise ratio and improves the stability and reliability of measurement. It is especially suitable for ultra-weak fiber optic grating sensing networks with weak signals.

[0014] 4. Simple fabrication process and mass production capability: The grooves are processed on the outer surface of the optical fiber, which can be achieved using mature methods such as laser processing or mechanical grinding. The process is simple and low-cost. This fabrication method is easy to integrate with existing fiber Bragg grating writing processes, enabling automated continuous production or mass production.

[0015] The present invention has other features and advantages, which will be apparent from or will be set forth in detail in the accompanying drawings and the following detailed description, which together serve to explain the particular principles of the invention. Attached Figure Description

[0016] The above and other objects, features and advantages of the present invention will become more apparent from the accompanying drawings, in which like reference numerals generally denote like parts.

[0017] Figure 1 A schematic diagram of a phase-shifted fiber Bragg grating according to an embodiment of the present invention is shown.

[0018] Figure 2 The diagram shows a comparison of the transmission spectra of a phase-shifted fiber Bragg grating according to an embodiment of the present invention under no axial tension (dashed line) and with axial tension (solid line). Detailed Implementation

[0019] The invention will now be described in more detail with reference to the accompanying drawings. While preferred embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0020] Example 1

[0021] like Figure 1 As shown, this embodiment provides a phase-shifting fiber Bragg grating, including: a fiber core 1, a cladding 2 covering the fiber core 1, and a grating region 3 formed on the fiber core 1; A V-groove 4 is formed on the sidewall of the cladding 2 corresponding to the grating region 3. The cross-section of the V-groove 4 along the fiber axis is V-shaped. The extension direction of the V-groove 4 is perpendicular to the fiber axis. The bottom of the V-groove 4 is located inside the cladding 2 and does not contact the fiber core 1.

[0022] Optionally, the V-groove 4 is made by mechanical grinding, laser cutting, or thermal evaporation.

[0023] Specifically, a V-groove 4 is fabricated on a cross-section in the middle of a standard fiber Bragg grating using mechanical grinding, laser cutting, or thermal evaporation. The bottom of the V-groove 4 must not contact the light-guiding portion of the fiber core 1. The depth of the V-groove 4 can be controlled by adjusting the distance between the grinding tool and the fiber, or by adjusting the number of scans, power, and distance between the laser and the laser marking machine's focal point and the fiber. A carbon dioxide laser marking machine is preferred, and the process is performed on the fiber Bragg grating using a transverse scanning method. The bottom of the V-groove 4 always retains a certain thickness of the cladding 2, without damaging the light-guiding area of ​​the fiber core 1 (i.e., the grating area), ensuring the fiber's light-guiding performance and mechanical strength. Preferably, the V-groove 4 is located on the cladding 2 corresponding to the middle of the grating area 3. The V-groove 4 being located in the middle of the fiber Bragg grating area 3 allows for a more uniform local stress distribution, a more symmetrical bimodal shape, and improved detection accuracy.

[0024] The V-groove 4 is located inside the cladding 2 and does not contact the light-guiding area of ​​the fiber core 1. This structure breaks the technical bias that traditional diameter reduction schemes must maintain circular symmetry and reduces processing difficulty. The structural defect formed by the V-groove 4 can cause local stress concentration in the optical fiber under axial stress, generating a non-uniform phase difference through photoelastic effect. The original transmission spectrum of the fiber Bragg grating is split into a double-valley structure (the reflection spectrum is split into a double-peak structure). The shape of this split double-valley (or double-peak) structure and the ratio of valley depth (or peak height) only change with stress. Temperature change only drives the overall translation of the fiber Bragg grating spectrum without splitting, thus achieving decoupled sensing of stress and temperature. Therefore, this invention can achieve decoupled measurement of stress and temperature with a single grating without the need to introduce an additional free and stress-free reference FBG, which nearly doubles the number of monitorable points of the same modem compared to traditional schemes, significantly saving fiber sensing bandwidth resources.

[0025] Furthermore, this phase-shifted fiber Bragg grating structure is compatible with ultra-weak fiber Bragg grating sensor networks and can be simultaneously deployed at multiple ultra-weak FBG points on a single long fiber, making it suitable for long-distance, large-scale sensor networks.

[0026] Example 2

[0027] This embodiment provides a method for fabricating a phase-shifted fiber Bragg grating as described in Embodiment 1, comprising: S1: Using ordinary optical fiber consisting of a core and cladding as the substrate, a grating region is prepared in a predetermined section of the core; Specifically, using ordinary single-mode optical fiber with a core and cladding structure as the substrate, fiber Bragg grating regions are written into a designated section of the core to complete the fabrication of the basic grating; S2: Using a V-groove processing device, a V-groove 4 is processed on the cladding sidewall corresponding to the grating region, and the bottom of the V-groove 4 is close to but does not contact the fiber core; during the processing, the stress demodulation range of the phase-shifting fiber Bragg grating is controlled by adjusting the depth of the V-groove.

[0028] The V-groove processing device is a mechanical grinder, a laser, or a laser marking machine; the V-groove is formed by mechanical grinding or laser cutting and thermal evaporation. The depth of the V-groove is controlled by adjusting the distance between the V-groove processing device and the optical fiber, and the stress demodulation range of the phase-shifting fiber Bragg grating is adjusted by adjusting the depth of the V-groove.

[0029] For scenarios where multiple fiber gratings are continuously processed at different positions on a single long optical fiber, multiple V-grooves are sequentially prepared at corresponding points of each fiber grating on the optical fiber by continuously winding the optical fiber and synchronously driving the V-groove processing device. In scenarios where multiple fiber gratings are processed simultaneously on multiple optical fibers, the multiple optical fibers are arranged side by side on the same horizontal plane. After calibration using a reference optical fiber, the V-groove processing device is used to process V-grooves on the multiple optical fibers simultaneously.

[0030] Specifically, continuous processing of fibers at different positions on a single long optical fiber can be achieved by controlling the continuous winding position of the fiber and the light output of the laser marking machine to continuously control the fabrication of V-grooves at the fiber positions. This corresponds to the continuous processing of ultra-weak fiber gratings (FBGs) on a single optical fiber. The laser marking machine can be integrated with an ultra-weak FBG writing system, directly writing V-grooves during the ultra-weak FBG writing process, thus adapting to the continuous processing requirements of long-distance ultra-weak FBG networking.

[0031] For cases involving the continuous processing of multiple fiber Bragg gratings (FBGs) on non-single optical fibers, V-groove batch processing can be performed using lasers or laser marking machines. This allows multiple FBGs to be processed side-by-side on the same horizontal plane in the same batch, improving processing efficiency. During processing, ordinary optical fibers can be used for calibration to ensure horizontal alignment and guarantee the uniformity of the entire batch of processed fibers.

[0032] This preparation method uses mechanical grinding, laser cutting, or thermal evaporation to process V-grooves on the fiber cladding. Compared with existing technologies such as etching (requiring complex masks), femtosecond laser (requiring a rotation positioning system), and discharge taper (electrode lifespan of only 5000 cycles), it has significant advantages such as low equipment cost, simple operation, no electrode consumption, continuous operation, and batch processing. At the same time, V-groove processing devices such as laser marking machines can be integrated with ultra-weak fiber grating writing systems on the same production line, directly processing V-grooves during the grating writing process, realizing integrated mass production of "writing-variable diameter", and filling the gap in the existing ultra-weak FBG sensing networks that are difficult to be compatible with variable diameter structures.

[0033] Example 3

[0034] This embodiment provides a stress demodulation method for demodulating the stress on the phase-shifted fiber Bragg grating described in Embodiment 1. This method aims to accurately determine the axial stress applied to the fiber using the spectral splitting phenomenon. The method includes: First obtain as follows Figure 1 The reflection or transmission spectra of the fiber Bragg grating shown are obtained when it is not subjected to axial tension. The single reflection peak in the reflection spectrum or the single transmission valley (transmission dip) in the transmission spectrum and its resonant bandwidth are obtained as a benchmark for subsequent comparison.

[0035] Next, in actual measurements, the spectral data (reflection or transmission spectra) of the fiber Bragg grating structure under unknown axial tension are acquired in real time. The system then determines whether a spectral splitting structure caused by the axial tension appears in the spectrum. In the reflection spectrum, the original single reflection peak splits into a double-peak structure under phase shift; in the transmission spectrum, the original single transmission valley splits into a double-valley structure under phase shift. If a double-peak (or double-valley) structure exists, the process proceeds to the next step; otherwise, it is determined that the current axial stress is zero or below the sensor's response threshold. Finally, based on the characteristic parameters of the double-peak (or double-valley) structure, the magnitude of the axial stress applied to the fiber Bragg grating is determined.

[0036] In a preferred embodiment, the characteristic parameters used to determine the stress magnitude can be selected from multiple options to adapt to different application scenarios and demodulation equipment requirements. For example, if stress demodulation uses the reflection spectrum of a fiber optic grating, the magnitude of the axial stress applied to the fiber can be determined based on the peak height ratio of the two peaks in the double-peak structure of the reflection spectrum, or the relative position of the double-peak structure within the original single-reflection peak resonant bandwidth. If stress demodulation uses the transmission spectrum, the magnitude of the axial stress applied to the fiber can be determined based on the depth ratio of the two valleys in the double-valley structure of the transmission spectrum, or the relative position of the double-valley structure within the original single-transmission valley resonant bandwidth.

[0037] Demodulation using the ratio of peak height (or valley depth) of two peaks has the advantage of being insensitive to absolute fluctuations in optical power and exhibiting strong robustness. Demodulation using relative positions, such as monitoring the difference between the center wavelength of the split double peak and the center wavelength of the original single peak, can also reflect the magnitude of stress, providing another feasible technical approach for demodulation.

[0038] It should be noted that the splitting characteristics in both reflection and transmission spectra correspond to each other in wavelength position (i.e., the wavelength positions of the double peaks and double valleys are the same), and the spacing between the double peaks and the spacing between the double valleys are equal. In terms of intensity characteristics, they are complementary (the higher the reflection peak, the deeper the corresponding transmission valley). However, both the peak height ratio of the double peaks and the valley depth ratio of the double valleys can be used to characterize the phase shift, thereby demodulating stress. This splitting phenomenon relies on the combined effect of the V-groove's geometrically variable diameter structure and axial tension: the V-groove is the structural basis for generating non-uniform stress, and the axial tension is the external force condition for exciting non-uniform phase differences.

[0039] Specifically, such as Figure 1The working principle of the phase-shifting fiber Bragg grating (FBG) structure with a V-groove shown is as follows: When an axial tension is applied to the fiber, the cross-sectional area of ​​the fiber is smaller in the narrower section of the V-groove, resulting in greater stress and a non-uniform optical phase difference generated by the photoelastic effect. The ratio of this phase difference to the fiber grating period under the current stress changes with the magnitude of the axial tension, forming a stress-responsive tunable phase-shifting grating. Taking the measurement of the transmission spectrum of this FBG as an example, when stress exists, i.e., when a phase difference exists, the original single transmission valley of the FBG transmission spectrum (such as...) Figure 2 As shown by the dashed line (without axial stress), an upward transmission peak will appear in the middle (see...). Figure 2 The relative position (i.e., its location within the transmission valley) and depth of this transmission peak change with variations in tension. The presence of this transmission peak causes the original single transmission valley to split into a double-valley structure (e.g., ...). Figure 2 In the FBG (Full-Layered Fiber Growth) spectrum, the axial tensile force is 1.013 Newtons, and the solid-line spectrum exhibits two transmission valleys (a and b), with a transmission peak between them. The valley depth ratio of the double-valve structure changes with the axial tensile force. When the temperature changes without axial tensile force, thermal expansion and contraction only occur in the FBG core, independent of the structural shape. That is, temperature only causes the thermo-optical effect of the core material and the thermal expansion of the grating period. Since there is no stress inhomogeneity caused by geometric structure, temperature changes only alter the overall position of the FBG transmission valleys (i.e., shift them), but do not produce a double-valve structure. The principle of using reflectance spectroscopy is similar and will not be elaborated here.

[0040] Furthermore, during the V-groove fabrication process, the stress measurement range can be adjusted within the fiber fracture strength range by regulating the depth of the V-groove. Specifically, the depth of the V-groove determines the remaining cladding thickness of the fiber at that location, thus affecting the cross-sectional area. Under the same axial tensile force, the smaller the cross-sectional area, the greater the local stress, and the greater the phase difference generated by the photoelastic effect. Therefore, the deeper the V-groove (closer to the fiber core), the higher the stress sensitivity of the fiber grating; a smaller tensile force can produce obvious double-peak splitting, suitable for high-precision measurements over a small stress range. A shallower V-groove requires a larger tensile force to produce distinguishable double-peak or double-valley shifts, suitable for measurements over a large stress range. By designing different groove depths, the stress response curve of the phase-shifting fiber Bragg grating can be customized, thereby obtaining different stress demodulation ranges (i.e., the range from the minimum to the maximum stress value that can be accurately measured). Within the allowable robustness range, reducing the diameter of the double V-groove (without touching the fiber core) can achieve a lower stress response threshold.

[0041] Therefore, when Figure 1When the FBG shown is applied to sensing, although temperature and stress will cause changes in the peak or valley positions of the FBG, the uneven stress generated by tension will make the FBG an adjustable phase-shifting grating. This will create a double-peak or double-valley structure in the FBG's reflection peak or transmission valley that varies with tension, while temperature changes alone will not have a corresponding effect. The modulator / demodulator can demodulate the signal using either the ratio of the peak height (or valley depth) of the two peaks (or valleys) in the double-peak or double-valley structure, or the relative position of the double-peak (or double-valley) structure within the resonant bandwidth of the original single-peak (single-valley) structure. Demodulation using the peak height (or valley depth) ratio can effectively reduce the signal-to-noise ratio requirement, making it particularly suitable for applications involving ultra-weak fiber optic gratings. The above structure does not affect the original FBG temperature measurement process. The temperature scale can be recalibrated using the ratio and position of the double peaks (or valleys).

[0042] In a specific example, such as Figure 1 During the operation of the FBG shown, its transmission spectrum was first measured without applying any external force to the optical fiber (i.e., the axial tension was 0N). Figure 2 As shown by the dashed line, a standard, symmetrical single transmission valley can be observed near 1550 nm, with a 3 dB bandwidth of approximately 0.2 nm. Then, an axial tension of 1.013 N was applied to both ends of the fiber using a precision displacement stage, and its transmission spectrum was measured again. Figure 2 As shown by the solid line, two significant changes occurred in the spectrum at this point: First, the entire spectrum shifted by approximately 1.5 nm towards longer wavelengths, a result of the combined effects of stress and temperature. Second, the original single downward peak disappeared, and the spectrum split into two independent transmission valleys, transmission valley a and transmission valley b, around 1551.5 nm. This phenomenon indicates that axial tension can induce spectral splitting. At this point, the valley bottom power values ​​of valleys a and b were recorded, and their valley depth ratio was calculated. For example, the valley depth can be defined as the difference between the transmission spectrum valley bottom power and the background power, calculated using the following formula: R=Ha / Hb Where R is the valley depth ratio, Ha is the valley depth of transmission valley a, and Hb is the valley depth of transmission valley b. Using a pre-calibrated "valley depth ratio-stress" relationship curve (which is typically monotonic), the currently applied axial stress of 1.013 N can be demodulated based on the calculated R value. Simultaneously, by tracking the overall drift (1.5 nm) of the centroid of the double-valley structure and subtracting the known drift component caused by the 1.013 N stress, the change in ambient temperature can be demodulated. This embodiment enables simultaneous and independent measurement of axial stress and temperature at a single point, achieving temperature / stress decoupling. The measurement process is intuitive and reliable, verifying the feasibility of the technical solution of this invention.

[0043] Example 4

[0044] This embodiment provides a phase-shifting fiber Bragg grating fabrication system for fabricating the phase-shifting fiber Bragg grating described in Embodiment 1. The system includes: Fiber optic transmission device for continuously winding optical fibers; A continuous fiber Bragg grating writing system is used to sequentially write fiber Bragg gratings, especially ultra-weak fiber Bragg gratings, at predetermined positions on an optical fiber. The V-groove processing device is integrated on the same production line as the ultra-weak fiber grating writing system. It is used to process V-grooves 4 on the fiber grating part of the fiber during the process of writing ultra-weak fiber gratings.

[0045] Among them, the ultra-weak fiber grating writing system is an existing mature system, and the V-groove processing device can be a mechanical grinder, a laser or a laser marking machine, preferably a laser marking machine.

[0046] This embodiment integrates an optical fiber transmission device, an ultra-weak FBG writing system, and a CO2 laser marking machine onto the same production line, all synchronously controlled by a control system. This system achieves full automation from grating writing to V-groove processing, and is suitable for large-scale production of stress-sensitive phase-shifting gratings. It can be widely applied in fields requiring dual-parameter monitoring of stress and temperature, such as structural health monitoring, petrochemicals, aerospace, and bridge and tunnel engineering.

[0047] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A phase-shifting fiber Bragg grating, characterized in that, include: The fiber core, the cladding covering the fiber core, and the grating region formed on the fiber core; A V-groove is formed on the cladding sidewall corresponding to the grating region. The cross-section of the V-groove along the fiber axis is V-shaped, and the bottom of the V-groove is located inside the cladding and does not contact the fiber core.

2. The phase-shifting fiber Bragg grating according to claim 1, characterized in that, The V-groove is located on the cladding layer corresponding to the middle of the grating region.

3. The phase-shifting fiber Bragg grating according to claim 1, characterized in that, The V-groove is made by mechanical grinding, laser cutting, or thermal evaporation.

4. A method for fabricating a phase-shifting fiber Bragg grating as described in any one of claims 1 to 3, characterized in that, include: Using ordinary optical fiber consisting of a core and cladding as the substrate, a grating region is prepared in a predetermined section of the core; Using a V-groove processing device, a V-groove is processed on the cladding sidewall corresponding to the grating region, and the bottom of the V-groove is made close to but not in contact with the fiber core; during the processing, the stress demodulation range of the phase-shifting fiber Bragg grating is controlled by adjusting the depth of the V-groove.

5. The preparation method according to claim 4, characterized in that, The V-groove processing device is a mechanical grinder, a laser, or a laser marking machine; The V-groove is produced by mechanical grinding, laser cutting, or thermal evaporation.

6. The preparation method according to claim 5, characterized in that, The depth of the V-groove is controlled by adjusting the distance between the mechanical grinding tool and the optical fiber, or by controlling the number of scans, power, and focal point of the laser or laser marking machine, as well as the distance between the laser and the optical fiber.

7. The preparation method according to claim 4, characterized in that, For scenarios where multiple fiber gratings are continuously processed at different positions on a single long optical fiber, multiple V-grooves are continuously prepared at corresponding points of each fiber grating on the optical fiber by continuously winding the optical fiber and synchronously driving the V-groove processing device. In scenarios where multiple fiber gratings are processed simultaneously on multiple optical fibers, the multiple optical fibers are arranged side by side on the same horizontal plane. After calibration using a reference optical fiber, the V-groove processing device is used to process V-grooves on the multiple optical fibers simultaneously.

8. A stress demodulation method for demodulating the stress on a phase-shifted fiber Bragg grating as described in any one of claims 1 to 3, characterized in that, include: The reflection or transmission spectrum of the fiber grating structure when it is not subjected to axial tension is obtained to obtain a single reflection peak or a single transmission valley and its resonant bandwidth. The reflection spectrum or transmission spectrum of the fiber grating structure under axial tension is obtained. In the reflection spectrum, the original single reflection peak is split into a double peak structure under the phase shift. In the transmission spectrum, the original single transmission valley is split into a double valley structure under the phase shift. The magnitude of the axial stress applied to the optical fiber is determined based on the characteristic parameters of the bimodal structure or the bivalve structure.

9. The stress demodulation method according to claim 1, characterized in that, Determining the magnitude of the axial stress applied to the optical fiber based on the characteristic parameters of the bimodal structure or the bivalve structure includes: The magnitude of the axial stress applied to the optical fiber is determined based on the peak height ratio of the two peaks in the double-peak structure, or the relative position of the double-peak structure within the original single-reflection peak resonant bandwidth. or, The magnitude of the axial stress applied to the optical fiber is determined based on the depth ratio of the two valleys in the dual-valley structure, or the relative position of the dual-valley structure within the original single-transmission valley resonant bandwidth.

10. A phase-shifting fiber Bragg grating fabrication system, used to fabricate the phase-shifting fiber Bragg grating as described in any one of claims 1-3, characterized in that, The system includes: Fiber optic transmission device for continuously winding optical fibers; A continuous fiber Bragg grating writing system is used to sequentially write fiber Bragg gratings at different predetermined positions on an optical fiber, wherein the fiber grating includes an ultra-weak fiber Bragg grating. The V-groove processing device is integrated on the same production line as the ultra-weak fiber Bragg grating writing system. It is used to process V-grooves on the fiber Bragg grating part of the fiber during the process of writing fiber Bragg gratings.

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