Dual-wavelength fiber grating sensor and demodulation system thereof

Abstract

This application discloses a dual-wavelength fiber Bragg grating sensor and its demodulation system. The dual-wavelength fiber Bragg grating sensor includes a fiber core, a cladding, and two gratings with different center wavelengths etched at the same position along the axial direction of the fiber core. The first and second gratings are spatially intersected, with their intersection point located on the fiber core axis and at the midpoint of the two grating segments. The intersection angle between the two gratings is 1° to 5°. The plane containing the two gratings is located in the radial section of the fiber core axis. By coaxially etching two adjacent gratings on the same fiber, this application ensures that they are in identical measurement positions, structurally avoiding errors caused by grating spacing, thereby significantly improving the demodulation accuracy and making the measurement results more precise. This application can simultaneously measure temperature and strain using only a single fiber, simplifying the sensor layout and reducing material and installation costs.

Description

Technical Field

[0001] This application belongs to the field of fiber Bragg grating sensing technology, and more specifically, relates to a dual-wavelength fiber Bragg grating sensor and its demodulation system. Background Technology

[0002] In fiber Bragg grating (FBG) sensing technology, a key challenge is the cross-sensitivity between temperature and strain: the wavelength change of a single grating may be caused by either temperature or strain, making it impossible to directly distinguish between these two physical quantities.

[0003] The most common solution currently is to use the dual-grating compensation method. For example, a long-period grating and a Bragg grating can be used together, or two FBGs can be placed near the same location: one senses both temperature and strain, and the other senses only temperature, and then the two can be separated by calculation.

[0004] However, these methods also have drawbacks: the grating arrangement is relatively complex, and there is usually a distance of several millimeters between two gratings, making it difficult to ensure that they are in the same temperature or strain environment, thus introducing errors. In addition, the corresponding detection system is also relatively complex. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this application is to provide a dual-wavelength fiber optic grating sensor and its demodulation system, aiming to solve the problem of large measurement error caused by the calculation error due to the spacing between the two gratings in existing dual-grating compensation schemes.

[0006] This application provides a dual-wavelength fiber optic grating sensor, comprising: a fiber core, a cladding, and two first gratings and a second grating with different center wavelengths inscribed at the same position along the axial direction of the fiber core; the first grating and the second grating are spatially intersected, and the intersection of the first grating and the second grating is located on the fiber core axis and the intersection is the midpoint of the two grating segments.

[0007] The first and second gratings are spatially intersected at an angle of 1° to 5°.

[0008] Furthermore, the plane containing the first grating and the second grating is located in the radial section of the fiber core axis, and the first grating and the second grating are symmetrically distributed about the fiber core axis.

[0009] Furthermore, the difference in center wavelength between the first grating and the second grating is greater than 500 nm; the spacing between the center wavelengths of the first grating and the second grating is much greater than the sum of their respective 3 dB bandwidths.

[0010] More preferably, the reflectivity of the grating with a larger wavelength is 50% to 70%, and the 3dB bandwidth is 0.3 nm to 0.5 nm; the reflectivity of the grating with a smaller wavelength is 20% to 40%, and the 3dB bandwidth is 0.1 nm to 0.3 nm.

[0011] The fiber has a cutoff wavelength of less than 820 nm, a core diameter of 4 μm to 6 μm, and a core radius of... a With numerical aperture NA a NA < 0.325 μm. Fiber attenuation @1550 nm ≤ 1.0 dB / km, @850 nm ≤ 4.0 dB / km.

[0012] This application inscribes two intersecting gratings of different wavelengths at the same position along the axis of a single optical fiber, achieving micron-level consistency in physical space and ensuring the consistency of the measurement field; in addition, the different wavelengths can guarantee different sensitivity coefficients of the two optical fibers.

[0013] This application also provides a demodulation system based on the above-mentioned dual-wavelength fiber Bragg grating sensor, comprising: a dual fiber Bragg grating sensor, a WDM device array, a first demodulation unit, and a second demodulation unit; one end of the dual-wavelength fiber Bragg grating sensor is connected to the multiplexing end of the WDM device; the dual fiber Bragg grating sensor is used to sense changes in physical quantities of the external environment and feed back the corresponding changes in optical signals; the wavelength division end of the WDM device is connected to the first demodulation unit and the second demodulation unit by a multi-stage coupler, and the WDM device is used to achieve simultaneous transmission of different wavelengths while avoiding demodulation interference between different wavelengths; the first demodulation unit and the second demodulation unit are respectively used to realize the transmission and demodulation of signal light of different wavelengths, thereby realizing the demodulation of the measured physical quantity.

[0014] In this configuration, the wavelength division terminal of the WDM device with a larger wavelength is connected to the corresponding demodulation unit with a larger wavelength, and the wavelength division terminal of the WDM device with a smaller wavelength is connected to the corresponding demodulation unit with a smaller wavelength.

[0015] Furthermore, the first demodulation unit and the second demodulation unit have the same structure. The first demodulation unit includes a first circulator, a first light source, and a first demodulation module. The second demodulation unit includes a second circulator, a second light source, and a second demodulation module. The first port of the first circulator is connected to the first light source, the third port of the first circulator is connected to the first demodulation module, and the second port of the first circulator is used to connect to one wavelength division end of the WDM device. The first port of the second circulator is connected to the second light source, the third port of the second circulator is connected to the second demodulation module, and the second port of the second circulator is used to connect to the other wavelength division end of the WDM device. The first circulator is used to receive the optical signal emitted by the first light source and output it to the matching input end of the WDM device, and output the received wavelength-matched feedback optical signal filtered by the WDM device to the first demodulation module. The second circulator is used to receive the optical signal emitted by the second light source and output it to the matching input end of the WDM device, and output the received wavelength-matched feedback optical signal filtered by the WDM device to the second demodulation module. The first demodulation module and the second demodulation module are used to realize the conversion of optical signals carrying test physical quantities into electrical signals.

[0016] During operation, the light signal emitted by the light source enters the first port of the circulator and is output from the second port to the input terminal of the WDM device that matches it. After passing through the multiplexing terminal of the WDM device, it enters the dual fiber grating sensor. The reflected light signal is filtered by the WDM device, and each demodulation unit only accepts the light signal with the wavelength that matches it. Each feedback light signal enters the circulator through the second port of the circulator and enters the demodulation module through the third port of the circulator.

[0017] Among them, WDM devices can be connected to each demodulation unit through a multi-stage coupler array, so that signals from multiple sensing channels can be simultaneously accessed by the demodulation unit.

[0018] This application employs an optical path constructed using a WDM array and multi-stage couplers, with the multiplexing end displaying a composite spectrum and each demodulation unit displaying a single-wavelength spectrum; this avoids crosstalk between signals of different wavelengths. It achieves efficient screening of composite spectra and multi-channel parallel demodulation, thus enabling simultaneous temperature and strain measurements based on a single optical fiber.

[0019] Compared with the prior art, the technical solution conceived in this application utilizes the differences in the response characteristics of different wavelengths to temperature and strain, and solves a system of two equations to simultaneously measure and distinguish temperature and strain, effectively solving the problem of cross-sensitivity; it has the following technical advantages:

[0020] (1) High measurement accuracy: This application ensures that two adjacent gratings are in the same measurement position by coaxially writing two adjacent gratings on the same optical fiber, thus avoiding errors caused by the grating spacing from the structure, thereby greatly improving the calculation accuracy and making the measurement results more accurate.

[0021] (2) Low cost: This application can simultaneously measure temperature and strain using only one optical fiber, which simplifies the sensor layout structure and reduces material and installation costs.

[0022] (3) More flexible deployment and easy expansion: The demodulation system of this application adopts a modular design, which can easily expand the number of channels through WDM arrays and multi-level couplers. The expensive core demodulation equipment can be flexibly reused in multiple locations, which is particularly suitable for inspection applications in multiple key sites such as the power industry, greatly improving equipment utilization and deployment efficiency. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the axial cross-section of the dual-wavelength fiber Bragg grating sensor provided in this application;

[0024] Figure 2 This is a radial cross-sectional schematic diagram of the structure of the dual-wavelength fiber Bragg grating sensor provided in this application;

[0025] Figure 3 This is a schematic diagram of the optical path structure in the demodulation system provided in this application;

[0026] Figure 4 This is a schematic diagram of the demodulation system structure provided in this application;

[0027] Figure 5(a) shows the curve of the center wavelength of sample 1# in the 850 band changing with time in the example of this application; Figure 5(b) shows the curve of the center wavelength of sample 1# in the 1550 band changing with time in the example of this application.

[0028] Figure 6(a) shows the curve of the center wavelength of sample #2 in the 850 band changing with time in the example of this application; Figure 6(b) shows the curve of the center wavelength of sample #2 in the 1550 band changing with time in the example of this application. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0030] This application proposes a novel coaxial dual-wavelength fiber Bragg grating sensor and demodulation system. The core of this approach lies in employing two sensors with different wavelengths, both simultaneously affected by temperature and strain at the same location. Due to their different wavelengths, their sensitivity coefficients to temperature and strain also differ. By constructing and solving the sensitivity coefficient matrix, the cross-sensitivity problem can be fundamentally solved, enabling simultaneous and accurate measurement of temperature and strain.

[0031] This application is based on the following sensing principle: when two gratings of different wavelengths are under the same temperature and strain field, the relationship between their wavelength shift and the change of physical quantities can be described by the following set of equations:

[0032] Δλ1=K_T1*ΔT+K_ε1*Δε

[0033] Δλ2=K_T2*ΔT+K_ε2*Δε

[0034] Where: Δλ1 and Δλ2 are the measured changes (e.g., wavelength drift) at two wavelengths; K_T1 and K_T2 are the sensitivity coefficients of wavelengths λ1 and λ2 to temperature; K_ε1 and K_ε2 are the sensitivity coefficients of wavelengths λ1 and λ2 to strain; ΔT is the temperature change; and Δε is the strain change.

[0035] Figure 1 The axial cross-section of the dual-wavelength fiber optic grating sensor provided in this application is shown. Figure 2 The radial cross-section of the dual-wavelength fiber Bragg grating sensor provided in this application is shown; it is now described in detail below with reference to the accompanying drawings:

[0036] The dual-wavelength fiber optic grating sensor 5 provided in this application includes a fiber core 1, a cladding 2, and two first gratings 3 and second gratings 4 with different center wavelengths etched at the same position along the axial direction of the fiber core 1; the first gratings 3 and second gratings 4 are spatially intersected, and the intersection of the first gratings 3 and second gratings 4 is located on the fiber core axis, and the intersection is the midpoint of the two grating segments.

[0037] The optical fiber used in this application is a single-mode fiber with a cutoff wavelength <820nm, suitable for typical operating wavelengths such as 850nm, 980nm, and 1550nm. Based on the formula for the normalized frequency V value of single-mode transmission in optical fibers, the core diameter ranges from 4μm to 6μm, and the core radius... a (um) and numerical aperture NA satisfy a NA < 0.325um. Fiber attenuation @1550nm ≤ 1.0dB / km, @850nm ≤ 4.0dB / km. This fiber provides a stable transmission channel for multiple wavelengths of light while avoiding the inability to transmit light of a certain wavelength due to excessive attenuation.

[0038] The optical fiber used in this application is suitable for multiple typical operating wavelengths such as 850nm, 980nm, and 1550nm, avoiding signal crosstalk and range limitation caused by capacity issues in the same wavelength range.

[0039] The difference in center wavelength between the first grating 3 and the second grating 4 is greater than 500 nm. The center wavelength interval between the two gratings is much greater than the sum of their respective 3 dB bandwidths, effectively avoiding spectral signal crosstalk and providing each grating with an independent and wide wavelength drift range, thus ensuring that the sensor has a large measurement range. At the same time, different wavelengths correspond to different sensitivity coefficients; the greater the difference in sensitivity coefficients, the higher the test accuracy.

[0040] The first grating 3 and the second grating 4 are two fiber optic gratings with different center wavelengths, etched at the same position along the fiber axis. In practical applications, when sensors are subjected to stress, it is often not a single direction, but rather multiple forces acting in different directions. If the two gratings are arranged in parallel and subjected to stress perpendicular to the grating axis and parallel to the plane containing the two gratings, when the fiber is bent by external force, the gratings on the outer side, farther from the bending center, experience tension, resulting in a larger period, while the inner gratings experience compression, resulting in a smaller period. The trends of these two changes are opposite, and the superposition of forces from other directions leads to a decrease in overall measurement accuracy. Therefore, the two gratings are spatially intersected. Due to the limitations of the fiber core diameter and the conditions for fiber optic grating reflection, if the intersection angle is too small, the processing area will spatially overlap during processing, and the periodicity of the grating area will be disrupted. If the intersection angle is too large, on the one hand, light will enter the cladding, resulting in ineffective reflection; on the other hand, the limitation of the fiber core diameter will result in the grating area being too short, also failing to form effective signal reflection. To ensure accurate measurements while considering the feasibility and economy of fiber Bragg grating fabrication, the intersection angle between the two gratings is set from 1° to 5° depending on the fiber diameter. The larger the fiber core diameter, the larger the angle between the two gratings; the smaller the fiber core diameter, the smaller the angle. This allows the two gratings to physically "brush past" each other, minimizing optical coupling and maintaining the purity and independence of their respective reflection spectra. During light transmission, the intersection point of the two gratings lies on the fiber core axis and is the midpoint of the two grating segments. The plane containing the two gratings is located in a radial section passing through the fiber core axis, ensuring that the two gratings meet micrometer-level spatial consistency at the physical level, sensing identical temperature and strain fields. The small-angle intersection arrangement means that the grating closer to the outside is under tension, while the one closer to the inside is under compression. Both gratings chirp simultaneously, with a generally consistent trend, minimizing the impact on measurement accuracy.

[0041] The reflectivity of the gratings with larger wavelengths is 50%–70%, and the 3dB bandwidth is 0.3nm–0.5nm; the reflectivity of the gratings with smaller wavelengths is 20%–40%, and the 3dB bandwidth is 0.1nm–0.3nm. The different reflectivities of each wavelength are to adapt to the detection range of the demodulation module for each wavelength. Each grating uses a narrow bandwidth to ensure that the reflection spectrum has a sufficiently sharp peak for high-precision wavelength demodulation.

[0042] At the mathematical level, the responses of the first grating 3 and the second grating 4 to temperature and strain are "unique," meaning their sensitivity coefficients satisfy a non-proportional relationship: K_T1 / K_T2 ≠ K_ε1 / K_ε2. This guarantees that the coefficient matrix of the subsequent solution is full rank, and the equation has a unique solution.

[0043] Figure 3 The optical path structure of the demodulation system provided in this application is shown. For ease of explanation, only the parts relevant to this application are shown, and are described in detail below:

[0044] The demodulation system includes a first demodulation unit 7, a second demodulation unit 8, a WDM (Wavelength Division Multiplexing) device 6, and a dual-wavelength fiber Bragg grating sensor 5. The first demodulation unit 7 includes a first light source 10, a first demodulation module 11, and a first circulator 9. The second demodulation unit 8 includes a second light source 13, a second demodulation module 14, and a second circulator 12. The dual-wavelength fiber Bragg grating sensor 5 senses changes in the physical quantities of the external environment and feeds back the corresponding changes in the optical signal to the WDM device 6. The WDM divides the optical signal of each wavelength and transmits it to each demodulation unit through the circulator. One end of the dual-wavelength fiber Bragg grating sensor 5 is connected to the multiplexing end of the WDM, and the other end can be left floating or connected to other sensors. The multiplexing end of the WDM device 6 is connected to the dual-wavelength fiber Bragg grating sensor 5. The wavelength splitting ends are connected to couplers of different wavelengths and then to the demodulation unit. The wavelength splitting end of the larger wavelength is connected to the corresponding demodulation unit of the larger wavelength, and the wavelength splitting end of the smaller wavelength is connected to the corresponding demodulation unit of the smaller wavelength. This mainly realizes the merging of the input optical signal of the dual-wavelength fiber Bragg grating sensor 5 and the physical separation of the feedback optical signal, so as to achieve simultaneous transmission of different wavelengths and avoid demodulation interference between different wavelengths. The first light source 10 is connected to the first port of the first circulator 9, the first demodulation module 11 is connected to the third port of the first circulator 9, and the second port of the first circulator 9 is connected to one wavelength splitting end of the WDM device 6. The second light source 13 is connected to the first port of the second circulator 12, the second demodulation module 14 is connected to the third port of the second circulator 12, and the second port of the second circulator 12 is connected to the other wavelength splitting end of the WDM device 6. The entire demodulation unit realizes the transmission and demodulation of signal light, and realizes the demodulation of the measured physical quantity.

[0045] The working process of the demodulation system based on the above-mentioned dual-wavelength fiber Bragg grating sensor provided in this application is as follows:

[0046] The optical signal emitted by the first light source 10 enters the first port of the first circulator 9, and then enters the matching input terminal of the WDM device 6 through the second port of the first circulator 9; the optical signal emitted by the second light source 13 enters the first port of the second circulator 12, and then enters the matching input terminal of the WDM device 6 through the second port of the second circulator 12; the two optical signals emitted by the first light source 10 and the second light source 13 enter the dual-wavelength fiber Bragg grating sensor 5 simultaneously through the multiplexing terminal of the WDM device 6.

[0047] The dual-wavelength fiber optic grating sensor 5 uses two gratings to reflect multi-wavelength optical signals carrying physical quantity information that meet the center wavelength of the gratings. These signals are then reflected into the WDM device 6 through the input terminal. After selection by the WDM device 6, single-wavelength optical signals carrying physical quantity information are input to the second port of the corresponding circulator, and output to the corresponding demodulation unit through the third port of the circulator.

[0048] Light reflected from the dual-wavelength fiber Bragg grating sensor 5 into the first demodulation unit enters through the second port of the first circulator 9 and is transmitted to the first demodulation module 11 from the third port of the first circulator 9. Light reflected from the dual-wavelength fiber Bragg grating sensor 5 into the second demodulation unit enters through the second port of the second circulator 12 and is transmitted to the second demodulation module 14 from the third port of the second circulator 12. Changes in the external environment cause changes in the center wavelength of the dual gratings in the dual-wavelength fiber Bragg grating sensor 5, which in turn causes a change in the center wavelength of the reflected light signal, thus enabling the measurement of physical quantities.

[0049] The demodulation software in the computer system communicates synchronously with the first demodulation module 11 and the second demodulation module 14 to acquire data in a loop. The frequency range of the acquired data is 1 to 1000 Hz. The demodulation software analyzes the center wavelength of the spectral signal in each frame of data through a peak-finding algorithm. The application software acquires the center wavelength data of the demodulation software and various coefficients of the calibration software in real time. After substituting the data into the calculation, the results are displayed as the actual value of the measured physical quantity through the interface of the application software.

[0050] The key to this optical path lies in the use of wavelength division multiplexing (WDM) technology. A WDM device matches the signal side of the multi-wavelength composite optical signal carrying the actual physical signal—the optical signal reflected by the dual-wavelength fiber Bragg grating sensor 5—with the demodulation end, which demodulates the single-wavelength optical signal carrying the actual physical signal. The signal side represents a multi-wavelength composite spectrum, while each part of the demodulation end only accepts the matched single-wavelength optical signal. Different wavelength components in the reflected composite spectrum are precisely separated into different physical channels and processed by independent demodulation units. This physical isolation design avoids mutual interference between different wavelengths of light within the same device, while protecting optical components sensitive to specific wavelengths, achieving parallel, lossless, and high-precision signal processing.

[0051] The WDM device uses fiber with multiple typical operating wavelengths at the wavelength combining end, which is the same fiber as the dual-wavelength fiber grating sensor 5. The wavelength splitting end uses single-mode transmission fiber with matched wavelengths, and the optical path of each demodulation unit is a single-mode fiber with matched demodulation wavelength.

[0052] Figure 4 The diagram illustrates the entire demodulation system's structure, which utilizes a WDM array to expand the demodulation channels and employs multi-stage couplers in series to simultaneously connect signals from multiple sensor channels to the demodulation module. In practical applications, multi-channel measurement is required. Traditional optical switches are limited by the demodulation frequency; the more channels are expanded, the lower the demodulation frequency becomes.

[0053] To achieve multi-channel real-time monitoring and ensure frequency synchronization, this scheme designs a channel extension system based on hierarchical tree topology and wavelength multiplexing. The system employs multi-stage directional couplers for structured extension. Each stage coupler corresponds to a specific monitoring wavelength and follows a grouping principle of larger and smaller wavelengths. Couplers in different wavelength layers are independent and non-cascading. The specific connection architecture is as follows: the input port of the last stage coupler is connected to the second port of the corresponding circulator to inject the monitoring optical signal; the output ports of this stage coupler are connected to the input ports of the corresponding wavelength couplers of the next stage (i.e., the previous stage), while its other output port is connected to the input of another coupler in the same stage, forming a cascade. In this way, the input port of the second stage coupler receives the output signal from the corresponding wavelength coupler of the third stage, while its other output port is also extended to other couplers in the same stage; the input port of the first stage coupler is connected to the output port of the corresponding wavelength coupler of the second stage. In the first stage, the output ports of the larger wavelength couplers are connected to the wavelength splitters of the larger wavelengths in the WDM array, and the output ports of the smaller wavelength couplers are connected to the wavelength splitters of the smaller wavelengths in the WDM array, thereby achieving wavelength-based separation and convergence detection of the monitoring signal. Through the hierarchical tree structure described above, the system achieves efficient channel expansion in the optical domain and ensures that the detection frequency of all monitored channels is strictly consistent.

[0054] Each demodulation unit directly connects its data to the computer system. The software layer of the computer system specifically includes demodulation software, calibration software, and application software. The demodulation software utilizes the SDK function interface of the demodulation module to acquire spectral data carrying the measured physical quantity information in real time, and uses a peak-finding algorithm to acquire the wavelength values ​​of different wavelengths of optical signals from multiple channels in real time. The calibration software performs calculations, fitting, and other data processing by inputting the actual measured physical quantities from the laboratory environment and the wavelength values ​​of the optical signals calculated by the demodulation software, ultimately calibrating the temperature sensitivity coefficient (K_T1, K_T2) and strain sensitivity coefficient (K_ε1, K_ε2) of each dual-wavelength fiber Bragg grating sensor 5. These coefficients are bound to the dual-wavelength fiber Bragg grating sensor 5 and permanently stored in the computer system. The application software acquires the wavelength data of each dual-wavelength fiber Bragg grating sensor 5 from the demodulation software and the various coefficients of each dual-wavelength fiber Bragg grating sensor 5 from the calibration software in real time, and establishes a correspondence between the actual channel index and the sequence number of each wavelength of the optical signal arranged from smallest to largest. After calculation, the measured physical quantity data of each channel is displayed on the temporary interface after conversion, and alarm functions such as out-of-range data and missing data are provided.

[0055] The specific operating steps are as follows:

[0056] Step 1: Start the software in the computer system, turn on the light sources, and the emitted light signals enter the corresponding last-stage coupler through the second port of each circulator and then enter the next-stage coupler. After being split by multiple couplers, the light enters the wavelength division end of each WDM device in the WDM array, and then passes through the multiplexing end of each WDM to reach the dual-wavelength fiber optic grating sensor of each channel (two gratings with center wavelengths of 1550nm and 850nm are written at the same position along the axis on a single fiber).

[0057] Step 2: The dual-wavelength fiber Bragg grating sensors of each channel reflect back composite light carrying the measured physical quantity information. The reflected light enters the multiplexing terminal of the corresponding WDM device in the WDM array. After filtering, it enters the output port of the first-stage coupler through the wavelength division terminal of the WDM device, enters the output port of the next-stage coupler through the input port of this stage coupler, and finally enters the second port of the circulator through the input terminal of the last stage coupler, and enters the corresponding demodulation unit through the third port of the circulator.

[0058] Step 3: Establish a mapping relationship between the center wavelengths of different demodulated bands and the actual number of channels. Then, the demodulation software obtains the real-time wavelength values ​​λ1 and λ2 of each channel corresponding to each demodulation unit, and reads the coefficients of the physical quantities measured by the dual-wavelength fiber optic grating sensor 5 corresponding to each channel calibrated by the calibration software.

[0059] Step 4: The application software calculates the differences Δλ1 and Δλ between the real-time value and the initial wavelength, and then substitutes them into the inverse matrix of the sensitivity matrix to calculate the temperature change ΔT and strain change Δε. The measured physical quantity range is displayed in real time through the interface, and the alarm function is realized by setting the threshold range.

[0060] To further illustrate this application, a temperature test experiment using dual-wavelength fiber gratings is conducted to explain the demodulation principle. Two dual-wavelength fiber grating samples were fabricated using a surface-mount method. Sample #1 had center wavelengths of 1534nm / 832nm, and Sample #2 had center wavelengths of 1551nm / 841nm. The grating with a wavelength of 1550nm had a reflectivity of 50% and a bandwidth of 0.35nm, while the grating with a wavelength of 850nm had a reflectivity of 25% and a bandwidth of 0.15nm. The samples were simultaneously placed in the same temperature chamber and heated at 40℃~80℃ for 10 minutes at 10℃ intervals, followed by a 40-minute holding period before calibration. The center wavelength data were recorded in real time, as shown in Figures 5(a), 5(b), 6(a), and 6(b). Calibration was performed by calculating the average wavelength of the flat data segment.

[0061] Based on Δλ1=K_T1*ΔT+K_ε1*Δε and Δλ2=K_T2*ΔT+K_ε2*Δε, the dual fiber gratings in this example are in a free state in the temperature chamber and are not subject to external forces. Therefore, the formula in this example can be transformed into:

[0062] Δλ1 = K_T1 * ΔT; Δλ2 = K_T2 * ΔT; Define the initial wavelengths of the two optical fibers as λ10 and λ20, and the real-time wavelengths as Δλ1t and λ2t, then we have Δλ1 = λ1t - λ10; Δλ2 = λ2t - λ20; Combining these equations, we get:

[0063] (λ1t-λ10)-(λ2t-λ20)=K_T1*ΔT-K_T2*ΔT; available

[0064] (λ1t-λ2t)=(K_T1-K_T2)*ΔT+(λ10-λ20).

[0065] The temperature change of the optical fiber can be obtained by measuring the difference in the center wavelengths of the two gratings. The linear equation between the wavelength difference and temperature for the calibrated sample is as follows:

[0066] Sample #1: Δλ1 = 0.1049 * T + 701.73 (R² = 0.9998)

[0067] Sample #2: Δλ2 = 0.0799 * T + 710.07 (R² = 0.9998)

[0068] Repeated experimental tests showed that after changing the initial wavelength of the two optical fibers by external force, the temperature fluctuation range within the range of 40℃ to 80℃ was less than 2℃.

[0069] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A dual-wavelength fiber Bragg grating sensor, characterized in that, include: The fiber core (1), the cladding (2), and two gratings (3) and a second grating (4) with different center wavelengths inscribed at the same position along the axial direction of the fiber core (1). The first grating (3) and the second grating (4) are spatially intersected, and the intersection of the first grating (3) and the second grating (4) is located on the fiber core axis and the intersection is the midpoint of the two grating segments; The first grating (3) and the second grating (4) are spatially intersected at an angle of 1° to 5°. The plane containing the first grating (3) and the second grating (4) is a radial section passing through the axis of the fiber core (1), and the first grating (3) and the second grating (4) are symmetrically distributed about the axis of the fiber core (1).

2. The dual-wavelength fiber Bragg grating sensor as described in claim 1, characterized in that, The difference in center wavelength between the first grating (3) and the second grating (4) is greater than 500 nm.

3. The dual-wavelength fiber Bragg grating sensor as described in claim 2, characterized in that, The reflectivity of gratings with longer wavelengths is 50%–70%, and the 3dB bandwidth is 0.3nm–0.5nm; the reflectivity of gratings with shorter wavelengths is 20%–40%, and the 3dB bandwidth is 0.1nm–0.3nm.

4. The dual-wavelength fiber Bragg grating sensor as described in claim 1, characterized in that, The cutoff wavelength of the optical fiber is less than 820 nm, the core diameter is 4 μm to 6 μm, and the core radius is... a With numerical aperture NA a NA < 0.325 μm.

5. A demodulation system based on the dual-wavelength fiber Bragg grating sensor according to any one of claims 1-4, characterized in that, include: Dual fiber Bragg grating sensor (5), WDM device (6), first demodulation unit (7), and second demodulation unit (8); One end of the dual fiber grating sensor (5) is connected to the multiplexing end of the WDM device (6); the dual fiber grating sensor (5) is used to sense changes in physical quantities of the external environment and feed back the corresponding changes in optical signals. The wavelength division terminal of the WDM device (6) is connected to the first demodulation unit (7) and the second demodulation unit (8) respectively. The WDM device (6) is used to achieve simultaneous transmission of different wavelengths while avoiding demodulation interference between different wavelengths. The first demodulation unit (7) and the second demodulation unit (8) are respectively used to realize the emission and demodulation of signal light of different wavelengths, and realize the conversion of measured physical quantities into photoelectric signals.

6. The demodulation system as described in claim 5, characterized in that, The larger wavelength segmentation end of the WDM device (6) is connected to the corresponding larger wavelength demodulation unit, and the smaller wavelength segmentation end of the WDM device (6) is connected to the corresponding smaller wavelength demodulation unit.

7. The demodulation system as described in claim 5, characterized in that, The first demodulation unit (7) and the second demodulation unit (8) have the same structure. The first demodulation unit (7) includes a first circulator (9), a first light source (10) and a first demodulation module (11); the second demodulation unit (8) includes a second circulator (12), a second light source (13) and a second demodulation module (14). The first port of the first circulator (9) is connected to the first light source (10), the third port of the first circulator (9) is connected to the first demodulation module (11), and the second port of the first circulator (9) is used to connect to a wavelength division terminal of the WDM device (6). The first port of the second circulator (12) is connected to the second light source (13), the third port of the second circulator (12) is connected to the second demodulation module (14), and the second port of the second circulator (12) is used to connect to another wavelength division terminal of the WDM device (6); The first circulator (9) is used to receive the light signal emitted by the first light source (10) and output it to the input terminal of the WDM device that matches it, and output the received feedback light signal with wavelength matching after being filtered by the WDM device to the first demodulation module (11). The second circulator (12) is used to receive the optical signal emitted by the second light source (13) and output it to the input terminal of the WDM device that matches it, and output the received feedback optical signal with wavelength matching after being filtered by the WDM device to the second demodulation module (14). The first demodulation module (11) and the second demodulation module (14) are used to convert optical signals carrying test physical quantities into electrical signals.

8. The demodulation system according to any one of claims 5-7, characterized in that, Multiple WDM devices (6) are connected to each demodulation unit through a multi-level coupler array, so that signals from multiple sensing channels can be simultaneously accessed by the demodulation unit.

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