A method for demodulating an FBG sensor array based on microwave photon phase inversion

Abstract

The application discloses a FBG sensor array demodulation method based on microwave photon phase inversion, which comprises the following steps: a wide-spectrum light source outputs a wide-spectrum continuous light signal, the light signal is modulated by a swept-frequency microwave signal intensity output by a vector network analyzer through an electro-optic modulator to generate an optical carrier microwave signal, and the optical carrier microwave signal is injected into a FBG sensor array; reflected light is converted into a microwave signal by a photoelectric detector after wavelength-time mapping delay by a dispersion module, and the microwave signal is input into a vector network analyzer to obtain a system wideband frequency response; inverse Fourier transform, time-domain gating interception and Fourier transform are sequentially performed on the frequency response to obtain local microwave phase responses of the FBGs; and wavelength drift is calculated based on the local microwave phase shifts. The application replaces discrete pulse sampling with continuous sweeping to eliminate time-domain blind areas, realizes anti-interference high-precision measurement by phase demodulation, is compatible with wavelength division and all-same FBG arrays, and is suitable for synchronous demodulation of dynamic and static parameters of large-scale quasi-distributed sensing networks.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a demodulation method for an FBG sensor array based on microwave photonic phase inversion. Background Technology

[0002] Fiber Bragg gratings (FBGs) have become core components in fiber optic sensing technology due to their advantages such as compact structure, resistance to electromagnetic interference, and ability to measure various physical parameters including temperature, stress, and vibration. By cascading multiple gratings onto a single optical fiber to form an FBG array, long-distance quasi-distributed sensing can be effectively achieved. FBGs are widely used in structural health monitoring of infrastructure such as bridges, pipelines, and power grids, as well as in condition monitoring in fields such as automated production, aerospace, and precision manufacturing.

[0003] Traditional FBG sensors typically rely on spectrometers or tunable lasers to directly track Bragg wavelength shifts. However, these optical instruments are not only expensive but also struggle to achieve both high resolution and high sampling rates. In FBG array demodulation schemes, wavelength division multiplexing (WDM) solutions face bottlenecks due to low energy efficiency and limited node capacity. Under this mechanism, the multiplexing capacity of high-reflectivity gratings is limited by the total bandwidth of the light source; while weak-reflectivity gratings, although having low reflectivity, cannot utilize the residual transmitted light energy to increase multiplexing capacity because WDM schemes rely on wavelength differences to distinguish positions. This not only leads to low system energy efficiency but also limits the maximum number of sensors that can be multiplexed due to the limited total spectral bandwidth of the light source, making large-scale sensor networking difficult. In contrast, time-division multiplexing (TDM) based demodulation schemes have stronger multiplexing capabilities. However, the spatial resolution of optical temporal reflectance (OTDR) technology is limited by the width of the probe light pulse. This results in a technical deficiency in existing technologies where it is difficult to balance resolution and signal-to-noise ratio (SNR). Improving resolution requires shortening the pulse width, but a reduced pulse width leads to decreased echo energy, thus degrading the SNR. Furthermore, to avoid echo crosstalk in fiber optic links, the system needs to strictly limit the pulse repetition frequency. Existing discrete pulse sampling mechanisms have long time-domain blind spots, making it difficult to effectively capture high-frequency or transient perturbation signals and achieve continuous dynamic sensing of environmental parameters in real time. Although optical frequency domain reflection technology has high spatial resolution, its core relies on expensive high-coherence linear sweep lasers, and the system performance is inevitably limited by the inherent sweep rate bottleneck and nonlinear error of the light source. At the same time, the demodulation mechanism based on full-link optical beat frequency generates a huge amount of data, and the cumbersome signal processing process significantly prolongs the demodulation time, making it difficult to meet the real-time measurement requirements of high-frequency dynamic parameters and restricting its networking and multiplexing capabilities for large-scale grating arrays.

[0004] In practical applications, appropriate wavelength division multiplexing (WDM) or identical FBG arrays need to be deployed according to the scenario requirements. However, the aforementioned demodulation schemes have limitations in their applicability to various scenarios. For WDM schemes, the main reliance on differences in Bragg wavelengths to distinguish spatial locations makes it difficult to demodulate identical FBG arrays with overlapping reflected wavelengths, limiting the number of multiplexed sensor nodes and the flexibility of network deployment. For time division multiplexing (TDM) schemes, when demodulating WDM FBG arrays, the spectral range of the probe pulses must completely cover the reflected wavelengths of all WDM FBGs. Broad-spectrum pulses will undergo severe broadening due to group velocity dispersion when transmitted in fiber optic links. This not only causes overlap and crosstalk between adjacent grating time-domain signals, increasing demodulation difficulty and making errors more likely, but also reduces the spatial resolution of the system. Wavelength-time mapping-based microwave photonic sensing technology can directly map the wavelength shift of an optical fiber optic array (FBG) into the time-domain delay variation of the optical microwave signal. This effectively overcomes the limitations of traditional demodulation schemes in terms of applicable scenarios. It provides compatible and robust multiplexing demodulation for both wavelength division multiplexing (WDM) and identical FBG arrays, meeting the deployment requirements of complex scenarios. Conventional optical microwave amplitude response demodulation methods are susceptible to fluctuations in light source output power and various environmental noise fluctuations in the fiber optic link, resulting in poor anti-interference capabilities and low overall demodulation measurement accuracy.

[0005] Therefore, there is an urgent need for a sensing demodulation method that can overcome interference from light source fluctuations and environmental noise, has fast and high measurement accuracy, and is widely compatible with wavelength division multiplexing and all-type FBG arrays. Summary of the Invention

[0006] To achieve the above technical objectives, this invention provides a demodulation method for FBG sensor arrays based on microwave photonic phase inversion, the detailed technical solution of which is as follows: A demodulation method for an FBG sensor array based on microwave photonic phase inversion includes the following steps: S1. Uses a broadband light source to output a broadband continuous light signal; S2. A vector network analyzer outputs a swept-frequency microwave signal, and an electro-optic modulator is used to convert the swept-frequency microwave signal into a frequency-sweeping microwave signal. The intensity of the signal is modulated onto the broadband continuous optical signal to generate an optical microwave signal; S3. The optical microwave signal is injected into an FBG sensor array composed of N cascaded FBGs via an optical circulator, where N≥2. The reflected light signal of each FBG in the FBG sensor array carries wavelength shift information caused by the change of the parameter of the target under test. S4. After the reflected light signal is delayed by wavelength-time mapping by the dispersion module, it is converted into a microwave signal by the photodetector and input into the vector network analyzer to obtain the system broadband frequency response containing all the sensing information of the FBG sensor array. S5. Perform inverse Fourier transform, time-domain gating and truncation and Fourier transform on the broadband frequency response of the system in sequence. Specifically, first obtain the time-domain response through inverse Fourier transform, use time-domain gating to accurately locate, separate and truncate the time-domain echo signal of a specific FBG, and then obtain the local microwave phase response of the FBG through Fourier transform. S6. The wavelength drift of each FBG is calculated based on the inverted local microwave phase shift, realizing multi-node quasi-distributed demodulation of the FBG sensor array.

[0007] In some embodiments, in step S4, the system broadband frequency response H(Ω) is expressed as: , Where Ω is the angular frequency of the microwave signal output by the vector network analyzer, N is the total number of FBGs cascaded in the FBG sensor array, and a i Let τ be the reflection amplitude of the i-th FBG; i Let be the total group delay of the i-th FBG in the detection system.

[0008] In some embodiments, after performing an inverse Fourier transform on the system's broadband frequency response in step S5, the system's time-domain response h B (t) is represented by the superposition of N sinc pulse signals: , Wherein, B is the sweep bandwidth of the vector network analyzer.

[0009] In some embodiments, step S5 specifically involves: for the i-th FBG to be demodulated, applying a time-domain gating signal with a modulus of τ to its complex time-domain response signal. i The real-valued window function w centered on i (t), separate and extract the local time-domain echo signal h corresponding to the specific FBG. g,i (t): , The truncated local time-domain echo signal h g,i (t) Perform a Fourier transform to reconstruct the local complex frequency response of the specific FBG and extract its local microwave phase response.

[0010] In some embodiments, in step S6, the offset Δφ(Ω) of the local microwave phase and the wavelength drift Δλ i The relationship is: , Where D(λ) i) represents the dispersion coefficient of the dispersion module at the corresponding Bragg wavelength; the wavelength drift can be calculated by quantifying the microwave phase shift in the local frequency response of a specific FBG.

[0011] In some embodiments, the FBG sensor array is any one of a wavelength division FBG array, an identical FBG array, or a wavelength division / identical hybrid FBG array. When the FBG sensor array is a wavelength division FBG array, each FBG has a different Bragg wavelength. The sinc pulses of each FBG are staggered in the time domain by relying on the group velocity dispersion characteristics of the dispersion module to achieve time-domain gated interception. When the FBG sensor array is an identical FBG array, each FBG has the same Bragg wavelength. The intrinsic round-trip optical path difference formed by the physical spatial interval between each FBG allows the sinc pulse of each FBG to be independently separated in the time domain to achieve time-domain gated interception. For the wavelength division / identical hybrid configuration FBG array, wavelength division FBGs are distinguished by dispersion delay, and identical FBGs are distinguished by physical spatial spacing. Wavelength division FBGs and identical FBGs are separated in the time domain according to their respective delay mechanisms.

[0012] In some embodiments, the FBG sensor array demodulation method of the present invention further includes a synchronous measurement step of dynamic vibration parameters: When a specific FBG is subjected to external high-frequency dynamic vibration perturbation, in the time domain response obtained after performing an inverse Fourier transform on the system frequency response, vibration-induced sideband signals are symmetrically generated on both sides of the main reflection pulse peak; the time domain delay difference between the sideband signal and the main reflection pulse peak has a linear mapping relationship with the vibration frequency, and the vibration frequency of the external dynamic perturbation is calculated based on this time domain delay difference. Frequency-domain local microwave phase extraction is performed on the specific FBG. By eliminating the dependence of microwave phase on radio frequency, the time-domain frequency-independent phase shift is obtained. The vibration amplitude is then inverted based on the fluctuation amplitude of the frequency-independent phase shift.

[0013] In some embodiments, the slope S of the time-domain delay difference in response to the vibration frequency satisfies: , Among them, T s N represents the system's single frequency sweep time. p Where IF is the number of sampling points for frequency response measurement, IF is the intermediate frequency bandwidth of the vector network analyzer, and B is the sweep bandwidth. The maximum measurable vibration frequency f supported by the system v,max for: , Where, τadjacent This represents the intrinsic time-domain delay between adjacent FBGs.

[0014] In some embodiments, the broadband light source is an amplified spontaneous emission source operating in the C-band with a wavelength range of 1530 nm to 1565 nm; the vector network analyzer has a spectrum measurement range of 0–6 GHz, an intermediate frequency bandwidth of 1 kHz, and a sweep frequency of 5001 points; the dispersion module is a dispersion-compensating fiber or a chirped fiber grating, and its group velocity dispersion is set to make the reflected signals of adjacent FBGs resolvable in the time domain; the bandwidth of the electro-optic modulator and photodetector covers the sweep frequency range of the vector network analyzer.

[0015] In some embodiments, the target parameter to be measured is any one or more of temperature, stress, pressure, refractive index, displacement, or acceleration; the fiber optic sensing element in the FBG sensor array is a fiber Bragg grating or a tilted fiber Bragg grating.

[0016] The technical effects of this invention are as follows: 1. By using broadband continuous light in conjunction with frequency-sweeping microwave continuous modulation, and replacing discrete pulse sampling with continuous frequency sweeping, the time-domain blind zone is fundamentally eliminated, and all-time continuous sensing of external environmental parameters is achieved.

[0017] 2. By using the signal processing chain of "inverse Fourier transform → time-domain gating truncation → Fourier transform → local microwave phase extraction", wavelength drift measurement is transformed from time-domain peak tracking to microwave phase offset quantization. No interpolation calculation is required, and the sensitivity of phase measurement to noise is lower than that of time-domain peak tracking, thus realizing fast and high-precision wavelength drift measurement.

[0018] 3. Using microwave phase response instead of amplitude response as the demodulation basis, the local microwave phase shift is naturally immune to light source power fluctuations and link strength noise, fundamentally overcoming the anti-interference defects of amplitude demodulation method.

[0019] 4. By combining dispersive wavelength-time mapping with time-domain gating, demodulation of wavelength division array and identical array can be supported simultaneously under the same signal processing framework without switching system configurations, thus breaking through the limitations of traditional solutions in terms of applicable scenarios. Attached Figure Description

[0020] Figure 1 This is a system schematic diagram of the FBG sensor array demodulation method based on microwave photonic phase inversion according to an embodiment of the present invention; Figure 2 This is a schematic diagram of a quasi-distributed wavelength division multiplexing (FBG) array according to an embodiment of the present invention; Figure 3 This is a schematic diagram of a quasi-distributed identical FBG array according to an embodiment of the present invention; Figure 4 The figure shows the experimental test results of static strain sensing of FBG array based on microwave photonic phase inversion according to an embodiment of the present invention. Figure 5 The figure shows the experimental test results of FBG dynamic vibration sensing based on microwave photonic phase inversion in an embodiment of the present invention. Detailed Implementation

[0021] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be described in detail below with reference to the accompanying drawings. The description in this part is only exemplary and explanatory, and should not be used to limit the scope of protection of the present invention in any way.

[0022] To overcome the limitations of traditional demodulation methods based on optical microwave amplitude response, such as poor anti-interference capability, low measurement accuracy, and slow response speed, and to achieve fast and accurate demodulation of large-scale grating arrays, this invention provides a demodulation method for FBG sensor arrays based on microwave photon phase inversion.

[0023] Its demodulation system schematic diagram is as follows Figure 1 As shown, it includes: a broadband light source 101, a single-mode optical fiber 102, an electro-optic modulator 103, an optical circulator 104, a fiber Bragg grating array 105, a dispersion module 106, a photodetector 107, a coaxial cable 108, and a vector network analyzer 109.

[0024] During operation, the broadband continuous optical signal output by the broadband light source 101 is transmitted to the electro-optic modulator 103 via the single-mode fiber 102. It is then modulated by the swept-frequency microwave signal output by the vector network analyzer 109 to generate an optical microwave signal. This optical microwave signal is injected into the FBG array 105 via the optical circulator 104. The optical signal reflected from the array undergoes wavelength-time mapping delay through the dispersion module 106, or time-domain differentiation is introduced based on different physical spatial delays. It is then transmitted to the photodetector 107 via the optical circulator 104 and converted into a microwave signal, which is input to the vector network analyzer 109 via the coaxial cable 108.

[0025] The frequency response of the system is acquired by broadband frequency scanning, thereby mapping the minute drift in the wavelength domain to the group delay and phase change of the microwave signal. When the Bragg wavelength of a specific FBG drifts due to changes in the parameters of the target under test, causing a corresponding change in the group delay, the acquired system frequency response is sequentially subjected to inverse Fourier transform, time-domain gated truncation, and Fourier transform. This allows for precise separation of the echo signal of the specific FBG in the time domain and inversion of its microwave phase response. Finally, the wavelength drift is accurately calculated based on the extracted local microwave phase change, thereby achieving quasi-distributed fast demodulation of the FBG array.

[0026] In this embodiment, Figure 1The FBG array 105 demonstrated in the paper can be a wavelength division multiplexing (WDM) FBG array, an identical FBG array, or a WDM / identical hybrid FBG array.

[0027] It should be understood that, for the purpose of explaining the principle of the present invention, this embodiment uses wavelength division multiplexing (WDM) FBG array and identical FBG array as examples, but it is not limited to these in specific applications. A WDM / identical FBG array can also be used.

[0028] In this embodiment, Figure 1 When the quasi-distributed FBG array demonstrated in the image is the wavelength division FBG, its detailed structure is as follows: Figure 2 As shown, it includes: a first wavelength division multiplexing (WDM) fiber Bragg grating 201, a second wavelength division multiplexing (WDM) fiber Bragg grating 202, and an Nth wavelength division multiplexing (WDM) fiber Bragg grating 203; as... Figure 2 As shown, N FBGs with different Bragg wavelengths are cascaded sequentially along a single optical fiber to form an array.

[0029] In this embodiment, Figure 1 When the quasi-distributed FBG array demonstrated in the paper is the identical FBG, its detailed structure is as follows: Figure 3 As shown, it includes: a first identical fiber Bragg grating 301, a second identical fiber Bragg grating 302, and an Nth identical fiber Bragg grating 303. Figure 3 As shown, N FBGs with the same Bragg wavelength are cascaded sequentially along a single optical fiber to form an array.

[0030] To facilitate the explanation of the working principle of this invention, a mathematical model of the FBG array system based on optically carried microwaves is established below. The system frequency response H(Ω) can be expressed as: , Where Ω is the angular frequency of the microwave signal output by the vector network analyzer, N is the total number of FBGs cascaded in the FBG sensor array, and a i Let τ be the reflection amplitude of the i-th FBG; i Let be the total group delay of the i-th FBG in the detection system.

[0031] For the acquired system broadband frequency response, an inverse Fourier transform is first performed to map it to the time domain. Since the sweep bandwidth B of the vector network analyzer 109 is limited, the time domain response h of the mapped system is... B (t) is represented by the superposition of N sinc pulse signals, which can be expressed as: , Wherein, B is the sweep bandwidth of the vector network analyzer.

[0032] In the above formula, as long as the sinc pulses do not overlap, the echo signal of each FBG can be distinguished in the time domain. However, directly tracking the peak delay of the pulse in the time domain requires performing tedious interpolation calculations, which not only greatly increases the computational complexity but also makes the measurement accuracy susceptible to noise.

[0033] To overcome the shortcomings of traditional time-domain peak-finding demodulation, this invention, based on time-domain gating technology, applies a time-domain gating parameter τ to the complex time-domain response signal of a specific FBG to be demodulated. i The central real-valued window function w i (t), the local time-domain echo signal h corresponding to the specific FBG is separated and accurately extracted using this window function. g,i (t), this process can be represented as: , Furthermore, the truncated local time-domain echo signal h g,i (t) Perform a Fourier transform to convert it back to the microwave frequency domain to reconstruct the local complex frequency response of the specific FBG and extract its local microwave phase response.

[0034] When the parameter of the target under test changes, it causes a change of Δλ in the Bragg wavelength of the i-th FBG. i When there is a small drift, the wavelength drift is converted into a change in the corresponding group delay, which is then reflected in the phase change of the reconstructed local frequency response. The relationship between the local microwave phase offset Δφ(Ω) and the wavelength drift can be expressed as: , Where D(λ) i ) represents the dispersion coefficient of the dispersion module 106 at the corresponding Bragg wavelength; based on the above phase inversion mechanism, wavelength drift can be quickly monitored simply by quantifying the microwave phase shift in the local frequency response of the specific FBG.

[0035] Based on the microwave phase shift mechanism described above, the strain measurement sensitivity of this specific FBG follows the equation below: , Where f is the modulation frequency of the microwave signal, λ B λ is the Bragg center wavelength, and ε is the strain applied to the FBG.

[0036] In this embodiment, to demonstrate the versatility of the demodulation method of the present invention, which combines the characteristics of wavelength division multiplexing and time division multiplexing, when using... Figure 2 When using the quasi-distributed wavelength division multiplexing (FBG) array shown, each FBG in the array reflects a different center wavelength. The dispersion delay provided by the dispersion module 106 mainly relies on the time-domain gating of the sinc pulses of different FBGs to achieve time-domain gating. Figure 3 In the quasi-distributed identical FBG array shown, each FBG reflects the same center wavelength. Traditional wavelength division multiplexing schemes fail because they cannot distinguish the same wavelength. This invention mainly relies on the round-trip spatial delay difference provided by the physical spacing of the optical fiber to separate the sinc pulses of the same wavelength independently in the time domain. At this time, the wavelength drift can still be accurately calculated based on the intercepted and inverted local microwave phase shift.

[0037] In this embodiment, for ease of implementation, the spectrum measurement range of the vector network analyzer 109 is set to 0-6GHz, the intermediate frequency bandwidth is set to 1 kHz, and the number of sweep points is 5001.

[0038] For ease of implementation, the broadband light source 101 is selected as an amplified spontaneous emission source operating in the C-band, with a wavelength range of 1530 nm to 1565 nm to provide a sufficiently wide spectral coverage. The dispersion module 106 can be selected as a dispersion-compensating fiber or a chirped fiber grating, and its group velocity dispersion should be such that the reflection signals of adjacent FBGs are distinguishable in the time domain. The bandwidth of the electro-optic modulator 103 and the photodetector 107 should cover the sweep frequency range of the vector network analyzer 109.

[0039] like Figure 4 As shown, the static strain sensing performance of the present invention was verified through the following preferred embodiments.

[0040] As a preferred embodiment, the following is adopted: Figure 2 In the implementation of the quasi-distributed wavelength division multiplexing (FBG) array shown, eight FBGs with reflectivities exceeding 80% and different Bragg wavelengths are cascaded along a single fiber, with a physical spacing of 10 cm between adjacent FBGs. Without applying strain, an inverse Fourier transform is performed on the acquired complex frequency response, yielding the following result: Figure 4 The initial time-domain response of the system is shown in (a). Thanks to the wavelength-time mapping induced by the dispersive module 106, the echo signals of each wavelength division multiplexing (FBG) are clearly and equally separated in the time domain, verifying the system's efficient time-domain discrimination capability for the WDM array. Subsequently, tensile strain was gradually applied to a portion of the gratings in the WDM array in steps of 100 με, and its microwave phase response was reconstructed by time-domain gating and inversion. Figure 4 The left subplot (c) shows that the phase shift gradually increases with increasing strain. Further extraction of the phase shift at 2 GHz and linear fitting with the strain yields the following results: Figure 4 As shown in the right subfigure of (c), a wavelength sensitivity of 0.0011 nm / με and a phase sensitivity of 0.0139 rad / με were obtained. The experimental results verify the high-precision measurement capability of the present invention for static strain under wavelength division multiplexing configuration.

[0041] As a preferred embodiment, the following is adopted: Figure 3 The quasi-distributed identical FBG array shown is implemented by connecting 10 low-reflectivity FBGs with a Bragg wavelength of 1549.4 nm and a reflectivity of only 1% in series along a single optical fiber, and the physical spacing between adjacent FBGs is also maintained at 10 cm. Figure 4 Figure (b) shows the time-domain response signal of the identical FBG array without applied strain. It is evident that, without relying on the difference in reflection wavelengths, multiple independent and non-overlapping main reflection pulse peaks can be obtained in the time domain solely by the intrinsic round-trip optical path difference introduced by the physical spatial spacing, demonstrating the high spatial resolution of this invention in the time-division multiplexing dimension. When tensile strain is gradually applied to a portion of the gratings in the identical array in steps of 20 με, the evolution of the local phase response and the linear fitting relationship between the phase shift at 2 GHz and the strain are as follows: Figure 4 As shown in (d) in the figure. Figure 4 The left subplot (d) shows that even with smaller strain perturbations, the phase curve still maintains clear following and regular shift; the linear fitting results of the right subplot are equally ideal, yielding a wavelength sensitivity of 0.0011 nm / με and a phase sensitivity of 0.0138 rad / με. Experimental results show that the present invention can also provide high-precision static strain demodulation for identical FBG arrays.

[0042] like Figure 5 As shown, the present invention verifies the dynamic vibration sensing performance through the following preferred embodiments.

[0043] In this embodiment, in addition to the static sensing applications described above, the present invention also supports real-time sensing of high-frequency dynamic parameters.

[0044] It should be noted that when a specific FBG is subjected to external high-frequency dynamic vibration perturbation, due to the dynamic strain modulation during the system's frequency sweep, the time-domain response obtained after performing an inverse Fourier transform on the system's frequency response not only includes the main reflection pulse peak corresponding to that specific FBG, but also generates vibration-induced sideband signals symmetrically on both sides of the main reflection pulse peak; for example... Figure 5 As shown in (a), under vibration excitation at different frequencies, the time-domain response not only exhibits a sharp main reflection peak, but also clearly derives corresponding sideband peaks on both sides, and the position of the sideband signal in the time domain shifts significantly with the change of vibration frequency.

[0045] Furthermore, the time-domain delay difference between the aforementioned sideband signal and the main reflected pulse peak has a linear mapping relationship with the applied vibration frequency. The slope S of this time-domain delay difference in response to the vibration frequency can be expressed as: , Among them, T sN represents the system's single frequency sweep time. p Where IF is the number of sampling points for frequency response measurement, IF is the intermediate frequency bandwidth of the vector network analyzer, and B is the sweep bandwidth.

[0046] Based on this time-domain delay difference, and combined with the ratio of the system's single frequency sweep time to the frequency sweep bandwidth, the vibration frequency of external dynamic disturbances can be calculated directly and accurately.

[0047] Based on this, frequency-domain local microwave phase extraction is performed on this specific FBG, such as... Figure 5 As shown in (b), the inverted frequency response phase shift exhibits chirping characteristics that are synchronized with the external vibration. To further quantify the vibration amplitude, the frequency-independent phase shift in the time domain is obtained by eliminating the dependence of the microwave phase on the radio frequency, as shown in Figure [Figure number missing]. Figure 5 As shown in (c), under different excitation amplitudes, the recovered phase time domain response curves exhibit smooth and regular sinusoidal waveforms, and the fluctuation amplitude of phase shift has a strict monotonic linear relationship with the dynamic peak strain caused by vibration. Thus, the amplitude of vibration can be accurately inverted using the pre-calibrated phase sensitivity.

[0048] It should be noted that when performing dynamic sensing on an FBG array containing cascaded gratings, it is necessary to avoid temporal overlap and crosstalk between the vibration sideband signal of one node and the main reflected pulse peak of adjacent FBG nodes. Under this constraint, the maximum measurable vibration frequency that the system can support is jointly determined by the intrinsic time-domain delay between adjacent FBGs, the number of system sampling points, the intermediate frequency bandwidth, and the sweep bandwidth, and its expression is: , Where, τ adjacent The intrinsic time-domain delay between adjacent FBGs is given by IF, where IF is the intermediate frequency bandwidth of the vector network analyzer 109, B is the sweep bandwidth, and N is the frequency bandwidth. p This represents the number of sampling points for frequency response measurement.

[0049] In specific deployment, by co-optimizing the parameters of the vector network analyzer 109 and reasonably setting the physical fiber spacing between adjacent FBGs, peak overlap can be effectively avoided while meeting the response bandwidth requirements, thereby achieving high-precision synchronous demodulation of large-scale array static strain and high-frequency dynamic vibration in a single scan.

[0050] It should be understood that, for the purpose of explaining the principle of the present invention, this embodiment mainly uses a conventional fiber Bragg grating (FBG) array as the sensing core for description. However, in specific applications, it is not limited to this. The fiber sensing element can also be replaced by other microstructure fiber devices with wavelength drift characteristics, such as a tilted fiber Bragg grating (TFBG).

[0051] It should be understood that the target parameters to be measured in this invention are not limited to temperature and stress. Any physical parameter that can cause the FBG Bragg wavelength to drift, including but not limited to pressure, refractive index, displacement, acceleration, etc., are all within the protection scope of this invention.

[0052] This invention uses specific examples to illustrate the principles and implementation methods of the invention. The examples described above are merely to aid in understanding the method and core ideas of this invention. The above descriptions are only preferred embodiments of the invention. It should be noted that due to the limitations of written expression, there are objectively infinite specific structures. For those skilled in the art, several improvements, modifications, or variations can be made without departing from the principles of this invention, and the above technical features can be combined in an appropriate manner. These improvements, modifications, variations, or combinations, or the direct application of the inventive concept and technical solution to other situations without modification, should all be considered within the scope of protection of this invention.

Claims

1. A demodulation method for an FBG sensor array based on microwave photonic phase inversion, characterized in that, It includes the following steps: S1. Uses a broadband light source to output a broadband continuous light signal; S2. A vector network analyzer is used to output a swept microwave signal, and the intensity of the swept microwave signal is modulated onto the broadband continuous optical signal through an electro-optic modulator to generate an optical microwave signal. S3. The optical microwave signal is injected into an FBG sensor array composed of N cascaded FBGs via an optical circulator, where N≥2. The reflected light signal of each FBG in the FBG sensor array carries wavelength shift information caused by the change of the parameter of the target under test. S4. After the reflected light signal is delayed by wavelength-time mapping by the dispersion module, it is converted into a microwave signal by the photodetector and input into the vector network analyzer to obtain the system broadband frequency response containing all the sensing information of the FBG sensor array. S5. Perform inverse Fourier transform, time-domain gating, and Fourier transform on the broadband frequency response of the system in sequence. Specifically, first, obtain the time-domain response through inverse Fourier transform, then use time-domain gating to accurately locate, separate, and extract the time-domain echo signal of a specific FBG, and finally obtain the local microwave phase response of the FBG through Fourier transform. S6. The wavelength drift of each FBG is calculated based on the inverted local microwave phase shift, realizing multi-node quasi-distributed demodulation of the FBG sensor array.

2. The FBG sensor array demodulation method based on microwave photonic phase inversion as described in claim 1, characterized in that, In step S4, the system's broadband frequency response H(Ω) is expressed as: ， Where Ω is the angular frequency of the microwave signal output by the vector network analyzer, N is the total number of FBGs cascaded in the FBG sensor array, and a i Let τ be the reflection amplitude of the i-th FBG; i Let be the total group delay of the i-th FBG in the detection system.

3. The FBG sensor array demodulation method based on microwave photonic phase inversion as described in claim 2, characterized in that, In step S5, after performing an inverse Fourier transform on the system's broadband frequency response, the system's time-domain response h B (t) is represented by the superposition of N sinc pulse signals: ， Wherein, B is the sweep bandwidth of the vector network analyzer.

4. The FBG sensor array demodulation method based on microwave photonic phase inversion as described in claim 2, characterized in that, In step S5, the time-domain gating interception specifically involves: for the i-th FBG to be demodulated, applying a gate with a value of τ to its complex time-domain response signal. i The real-valued window function w centered on i (t), separate and extract the local time-domain echo signal h corresponding to the specific FBG. g,i (t): ， The truncated local time-domain echo signal h g,i (t) Perform a Fourier transform to reconstruct the local complex frequency response of the specific FBG and extract its local microwave phase response.

5. The FBG sensor array demodulation method based on microwave photonic phase inversion as described in claim 2, characterized in that, In step S6, the local microwave phase offset Δφ (Ω) and the wavelength drift Δλ i The relationship is: ， Where D(λ) i ) represents the dispersion coefficient of the dispersion module at the corresponding Bragg wavelength; the wavelength drift can be calculated by quantifying the microwave phase shift in the local frequency response of a specific FBG.

6. The FBG sensor array demodulation method based on microwave photonic phase inversion as described in claim 1, characterized in that, The FBG sensor array is any one of wavelength division FBG array, identical FBG array, or wavelength division / identical hybrid FBG array. When the FBG sensor array is a wavelength division FBG array, each FBG has a different Bragg wavelength. The sinc pulses of each FBG are staggered in the time domain by relying on the group velocity dispersion characteristics of the dispersion module to achieve time-domain gated interception. When the FBG sensor array is an identical FBG array, each FBG has the same Bragg wavelength. The intrinsic round-trip optical path difference formed by the physical spatial interval between each FBG allows the sinc pulse of each FBG to be independently separated in the time domain to achieve time-domain gated interception. For the wavelength division / identical hybrid configuration FBG array, wavelength division FBGs are distinguished by dispersion delay, and identical FBGs are distinguished by physical spatial spacing. Wavelength division FBGs and identical FBGs are separated in the time domain according to their respective delay mechanisms.

7. The FBG sensor array demodulation method based on microwave photonic phase inversion as described in claim 1, characterized in that, It also includes the synchronous measurement steps of dynamic vibration parameters: When a specific FBG is subjected to external high-frequency dynamic vibration perturbation, in the time domain response obtained after performing an inverse Fourier transform on the system frequency response, vibration-induced sideband signals are symmetrically generated on both sides of the main reflection pulse peak; the time domain delay difference between the sideband signal and the main reflection pulse peak has a linear mapping relationship with the vibration frequency, and the vibration frequency of the external dynamic perturbation is calculated based on this time domain delay difference. Frequency-domain local microwave phase extraction is performed on the specific FBG. By eliminating the dependence of microwave phase on radio frequency, the time-domain frequency-independent phase shift is obtained. The vibration amplitude is then inverted based on the fluctuation amplitude of the frequency-independent phase shift.

8. The FBG sensor array demodulation method based on microwave photonic phase inversion as described in claim 7, characterized in that, The slope S of the response of the time-domain delay difference to the vibration frequency satisfies: , Among them, T s N represents the system's single frequency sweep time. p Where IF is the number of sampling points for frequency response measurement, IF is the intermediate frequency bandwidth of the vector network analyzer, and B is the sweep bandwidth. The maximum measurable vibration frequency f supported by the system v,max for: ， Where, τ adjacent This represents the intrinsic time-domain delay between adjacent FBGs.

9. The FBG sensor array demodulation method based on microwave photonic phase inversion as described in claim 1, characterized in that, The broadband light source is an amplified spontaneous emission source operating in the C-band, with a wavelength range of 1530 nm to 1565 nm; the vector network analyzer has a spectrum measurement range of 0–6 GHz, an intermediate frequency bandwidth of 1 kHz, and a sweep frequency of 5001 points; the dispersion module is a dispersion-compensating fiber or a chirped fiber grating, and its group velocity dispersion is set to make the reflected signals of adjacent FBGs resolvable in the time domain; the bandwidth of the electro-optic modulator and photodetector covers the sweep frequency range of the vector network analyzer.

10. The FBG sensor array demodulation method based on microwave photonic phase inversion as described in claim 1, characterized in that, The target parameter to be measured is any one or more of temperature, stress, pressure, refractive index, displacement, or acceleration; the fiber optic sensing element in the FBG sensor array is a fiber Bragg grating or a tilted fiber Bragg grating.

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