Single-ended inquiry forward sensing system and optical fiber vibration waveform reconstruction and positioning method

Abstract

The invention discloses a single-ended inquiry forward sensing system and an optical fiber vibration waveform reconstruction and positioning method, and belongs to the technical field of optical fiber sensing. In order to solve the problems of high vibration positioning cost and complex structure in the existing communication-sensing integrated optical fiber network, an optical signal delay module is configured at the far end of an optical fiber link, vibration detection, multi-path signal return and coherent receiving processing under a single-end inquiry architecture are realized, and vibration waveform reconstruction and positioning are completed in combination with digital signal processing. According to the invention, accurate vibration detection and positioning can be realized with low cost and low complexity on the basis of the existing communication optical fiber infrastructure, and the method is suitable for a smart city sensing integrated network.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber sensing, and particularly relates to a single-end interrogation forward sensing system based on a linear sweep probe signal and an optical fiber vibration waveform reconstruction and positioning method. BACKGROUND

[0002] Distributed acoustic sensing (DAS) technology, as a distributed monitoring method based on optical fiber, has been widely studied and applied in pipeline state monitoring, perimeter intrusion protection, railway track inspection, and other fields [Z. He et al., “Optical fiber distributed acoustic sensors: a review,” J. Light. Technol. 39(12), 3671–3686 (2021)]. In the construction of smart city sensing networks, to realize the large-scale deployment of DAS technology, integrated sensing and communication (ISAC) technology has become a key solution due to its high practicability and cost-effectiveness. The ISAC technology paradigm can rely on existing optical fiber infrastructure to simultaneously realize information transmission and environmental sensing functions, effectively converting the laid optical fiber resources into a distributed sensing network with wide coverage and high deployment density [E. Ip et al., “DAS over 1,007-km hybrid link with 10-Tb / s DP-16QAM co-propagation using frequency-diverse chirped pulses,” J. Light. Technol. 41, 1077–1086 (2023)]. Through this collaborative mechanism of communication and sensing, the deployment cost and cycle of DAS technology are significantly reduced, providing feasibility for the landing of large-scale city sensing networks. However, the existing traditional DAS technology based on optical time domain reflectometry (OTDR) has outstanding technical problems in compatibility with existing communication systems, specifically: on the one hand, the high peak power pulse detection signal used by OTDR-type DAS can cause cross-phase modulation (XPM) effect [A. Bononi et al., “Cross-phase modulation induced by ook channels on higher-rate dqpsk and coherent qpsk channels,” J. Light. Technol. 27, 3974–3983 (2009)], leading to the decline of transmission performance of the communication system; on the other hand, the sensing distance of OTDR-type DAS is limited by the weak intensity of Rayleigh backscattering (RBS) light, which has a fundamental distance limitation [Y. Rao et al., “Recent advances in phase-sensitive optical time domain reflectometry,” Photonic Sensors 11, 1–30 (2021)].

[0003] To solve the above-mentioned defects of the traditional DAS technology, the forward sensing technology is recognized as the most promising technical solution to replace the traditional DAS. Unlike the Rayleigh backscattering (RBS) based sensing mechanism, the forward sensing technology realizes detection through a continuous wave (CW) probe signal propagating forward along the optical fiber, which can naturally avoid the influence of the XPM effect and break through the distance limitation brought by the Rayleigh backscattering. However, the existing forward sensing schemes generally have the problems of system structure complication and deployment cost increase while realizing the vibration positioning function, which restricts their practical application in the ISAC system. Specifically, the double-end interrogation scheme in the existing forward sensing scheme has the following defects: a set of coherent transceiver is required to be configured at both ends of the optical fiber link, and the devices at both ends are required to maintain high-precision time synchronization [E. Ip et al., “Vibration detection and localization using modified digital coherent telecom transponders,” J. Light. Technol. 40, 1472-1482 (2022)]. Due to the problems of high cost of user-end devices, large system structure complexity and strict time synchronization requirement of the scheme, it is difficult to realize the fusion application with short-range networks such as passive optical network (PON) and radio access network (RAN). In contrast, the single-end interrogation scheme in the existing forward sensing scheme has certain advantages in architecture: only one set of coherent transceiver is required to be configured and no time synchronization is required [Y. Yan et al., “Forward transmission based ultra-long distributed vibration sensing with wide frequency response,” J. Light. Technol. 39, 2241-2249 (2021)], but it still has the core technical defect: an active frequency shifting device must be introduced in the sensing link, and such a device is a non-standard communication device, which will significantly increase the hardware cost and structure complexity of the system, and this defect is more prominent when the system is expanded to the point-to-multipoint (P2MP) network scenario. Therefore, there is an urgent need for a simplified forward sensing architecture in the existing technology to realize high compatibility with the existing communication infrastructure and fundamentally solve the technical problems of high cost and large complexity that have existed in the actual ISAC system for a long time. SUMMARY

[0004] The application aims to provide a low-cost and low-complexity distributed vibration positioning solution under the architecture of a sensing-integrated optical fiber access network.

[0005] To achieve the above-mentioned purpose, the technical solution adopted by the application is as follows.

[0006] A single-end interrogation forward sensing system comprises: a narrow linewidth laser for generating an optical source signal; an arbitrary waveform generator for generating a linear frequency modulation waveform; an IQ modulator for receiving the optical source signal and modulating the optical source signal to form a frequency modulation continuous wave detection signal under the driving of the linear frequency modulation waveform provided by the arbitrary waveform generator; an erbium-doped fiber amplifier for power amplifying the frequency modulation continuous wave detection signal; a narrowband optical filter for filtering the amplified detection signal to suppress amplified spontaneous emission noise; a first optical fiber coupler for dividing the filtered detection signal into a detection signal path and a local signal path; a first optical circulator for injecting the detection signal into a first standard single-mode optical fiber and receiving an interference signal back along the first standard single-mode optical fiber; a second optical circulator for connecting a far end of the first standard single-mode optical fiber with an optical signal delay module to realize beam splitting, time delay and beam combining of the detection signal and form the interference signal; an integrated coherent receiver for receiving the local signal and the interference signal transmitted by the first optical circulator and performing coherent mixing processing on the two signals; a digital oscilloscope for digitally collecting the mixed electric signal output by the integrated coherent receiver.

[0007] Further, the optical signal delay module comprises a 1xN optical splitter, an N-path optical fiber delay line array and a Nx1 optical combiner connected in sequence. The N-path optical fiber delay line array is a core functional unit for realizing unbalanced delay of the module, the input end of which is connected to the N-path output end of the 1xN optical splitter one by one, and the output end is connected to the N-path input end of the Nx1 optical combiner one by one. The N-path optical fiber delay line array comprises N (N is a positive integer greater than or equal to 2) optical fiber delay lines with different lengths, each of which corresponds to receive an optical signal after being split by the 1xN optical splitter, and the multiple optical signals after differential delay processing are transmitted to the Nx1 optical combiner for combining.

[0008] Further, a piezoelectric ceramic is arranged between the input end and the far end of the first standard single-mode optical fiber, and a second standard single-mode optical fiber is connected to the piezoelectric ceramic through a wound optical fiber, for simulating vibration.

[0009] Further, the erbium-doped fiber amplifier is a polarization maintaining erbium-doped fiber amplifier.

[0010] A fiber vibration waveform reconstruction and positioning method based on the above system, comprising the following steps: 1) A narrow linewidth laser is used as a light source, a linear sweep waveform is generated by an arbitrary waveform generator, and the waveform is modulated onto an optical carrier by a single polarization IQ modulator to form a linear sweep detection signal; 2) The linear sweep detection signal is divided into two paths by a fiber coupler, one of which is used as a detection signal and injected into a standard single-mode fiber link through an optical circulator, and the other is reserved locally as a local signal; 3) An optical signal delay module is configured at the far end of the fiber link, which comprises an N-path optical fiber delay line array, and the N optical paths of the N-path optical fiber delay line array comprise optical fiber delay lines with different lengths to realize different optical paths. The forward propagating detection signal enters the optical signal delay module, is split and transmitted through different optical paths, and then is combined to form a multipath signal, which is transmitted back along the original fiber link through an optical circulator; 4) The back-propagated multipath signal and the locally reserved local signal are mixed in an integrated coherent receiver to generate an electrical signal containing vibration information, and the original digital signal is obtained by digitizing the electrical signal collected by a digital oscilloscope; 5) Digital signal processing on the original digital signal, including: link parameter scanning on the actual value of the fiber link length and the length of each optical path delay of the N-path optical fiber delay line array; constructing a digital local oscillator signal for demodulation according to the determined link parameters, and respectively down-converting the multipath signals to baseband; obtaining a plurality of phase signals by phase extraction and unwinding; generating a reconstructed vibration waveform by differentiating any two of the plurality of phase signals; performing cross-correlation operation on the reconstructed vibration waveform to determine the signal time delay, and calculating the position of the vibration event on the fiber link according to the speed of light and the refractive index of the optical fiber.

[0011] Further, the construction of the digital local oscillator signal in step 5) includes generating a complex exponential form signal according to the effective time delay of two signals in the multipath signal to realize the down-conversion of different signals to baseband respectively.

[0012] Further, the cross-correlation operation in step 5) is used to obtain the time delay corresponding to the maximum cross-correlation value The position of the vibration event on the fiber link is calculated according to the following formula: Wherein, represents the propagation speed of light in vacuum, represents the effective refractive index of the optical fiber, , respectively represents the propagation time delay of the fiber link and the time delay of the N-path optical fiber delay line array.

[0013] Further, the link parameter scanning in step 5) only needs to be performed once on the fixed link to determine the actual fiber link length and the length of each optical path delay in the N-path optical fiber delay line array.

[0014] The present application has the following technical advantages.

[0015] 1) Single-ended interrogation architecture is adopted, and only one set of coherent transceiver is needed to complete the transmission and reception of sensing signals, without the need to deploy double sets of equipment at both ends of the fiber link, and the dependence on high-precision time synchronization mechanism is completely eliminated. Compared with the existing double-ended interrogation scheme, the hardware configuration complexity and deployment difficulty of the system can be significantly reduced.

[0016] 2) Linear sweep wave is used as a probe signal to replace the active frequency shifting device in the traditional scheme, realizing the all-passive design of the sensing link. The system can directly rely on standard single-mode optical fiber to complete the detection and positioning of vibration signals, without the need to introduce non-standard communication components, and can be compatible with existing short-distance communication links (including passive optical networks and wireless access networks).

[0017] 3) The linear sweep probe signal has the natural low backscattering light noise characteristics, compared with the traditional single frequency continuous wave signal, can effectively reduce the interference of noise on the sensing performance, so that the system obtains greater dynamic range, supports long distance distributed sensing, and further expands the sensing distance of the forward sensing scheme.

[0018] 4) The introduction of the delay array makes the system can synchronously detect the vibration information under different frequency responses; through the respective processing and combination of each frequency response detection signal, the vibration sensing of wide frequency response can be realized, and the consistency of positioning accuracy in the whole frequency response range can be ensured, which effectively breaks through the limitations of traditional double-path delay scheme in frequency fading and positioning accuracy. At the same time, thanks to the simplification of the linear sweep wave probe for the sensing system, without additional complex hardware, the traditional double-path delay architecture can be expanded to a multi-channel delay array at low cost, which further improves the system performance while controlling the deployment cost.

[0019] Compared with the prior art, the present application has the following positive effects.

[0020] 1) Relying on the single-end interrogation architecture, the present application can complete the whole process of transmitting and receiving sensing signals through only one set of coherent transceiver, without deploying redundant equipment at both ends of the optical fiber link, and completely eliminating the dependence on high-precision time synchronization mechanism. Compared with the traditional double-end interrogation scheme, the hardware configuration architecture can be significantly simplified, the implementation difficulty can be reduced, and the needs of low cost and low complexity of the sensing integrated optical fiber access network can be better met.

[0021] 2) The linear sweep wave is used as the probe signal to replace the active frequency shifting device relied on in the traditional scheme, realizing the all-passive architecture of the sensing link. The system can directly use standard single-mode optical fiber to complete the detection and positioning of vibration signals, realizing the positioning accuracy of about 0.9 meters in the 50 kHz high vibration frequency scene, and realizing the positioning accuracy of about 4.5 meters and 10.6 meters in the 20 kHz and 10 kHz vibration frequency scenes, respectively. Since non-standard communication components are not needed, the system can be compatible with existing short-distance communication links such as passive optical networks and wireless access networks, improving the adaptation ability in the existing network architecture.

[0022] 3) The linear sweep probe signal has the natural low backscattering light noise characteristics, compared with the traditional single frequency continuous wave signal, can effectively reduce the interference of noise on the sensing performance. Experimental data shows that, under the same detection signal power condition, the Rayleigh backscattering power is reduced by about 15 dB compared with the traditional single frequency continuous wave signal. This characteristic makes the system obtain greater dynamic range, supports long distance distributed sensing, and effectively expands the sensing boundary of the forward sensing scheme, which can extend the unrepeatered sensing distance to about 100 kilometers.

[0023] 4) The introduction of the optical fiber optical delay line array makes the system capable of synchronously detecting vibration information under different frequency responses; combined with the separate processing and combination of each frequency response detection signal, not only does it realize wide frequency response vibration sensing, solve the problem of periodic frequency response decline of traditional single-ended forward devices, but also effectively avoid the positioning deviation problem introduced by frequency decline, ensure the consistency of vibration sensing positioning accuracy in the full frequency response range, and significantly improve the practicality and reliability of the device in the vibration sensing scene. Simulation results show that when the number of branches N of the delay line array is 2, there is a serious frequency decline or a problem of limited low frequency response, and after adding at least one additional branch of the optical fiber delay line array, that is, N≥3, the frequency response of the system is improved in the low frequency part and remains flat in the range of 50 kHz to 250 kHz. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The schematic diagram of the single-ended interrogation forward sensing system based on the linear sweep probe signal.

[0025] Figure 2 The time-frequency analysis comparison diagram of the received signal and the digital local oscillator.

[0026] Figure 3 The example waveform diagram of 4 kHz vibration signal waveform reconstruction and positioning.

[0027] Figure 4 The positioning accuracy diagram under different vibration frequencies.

[0028] Figure 5 The positioning accuracy diagram under different received optical power.

[0029] Figure 6 The Rayleigh backscattering noise comparison diagram of the single frequency probe and the linear sweep probe.

[0030] Figure 7 The simulation result diagram of the number of branches of the delay line array on the frequency response of the system forward sensing system.

[0031] BRIEF DESCRIPTION OF DRAWINGS: 1-narrow linewidth laser, 2-arbitrary waveform generator, 3-IQ modulator, 4-erbium-doped fiber amplifier, 5-narrowband optical filter, 6-7:3 optical coupler, 7-first optical circulator 1, 8-integrated coherent receiver, 9-digital oscilloscope, 10-first standard single-mode optical fiber, 11-piezoelectric ceramic, 12-second standard single-mode optical fiber, 13-second optical circulator, 14-1xN optical splitter, 15-N-path optical fiber delay line array, 16-Nx1 optical combiner. DETAILED DESCRIPTION

[0032] In order to make the above technical solutions of the present application more obvious and easy to understand, the following embodiments are described in detail with reference to the accompanying drawings.

[0033] The embodiment specifically discloses a single-end interrogation forward sensing system, as shown in the figure, which comprises a narrow linewidth laser 1, an arbitrary waveform generator 2, an IQ modulator 3, a doped fiber amplifier 4, narrow-band optical filters 5, a 7:3 optical fiber coupler 6, a first optical circulator 7, an integrated coherent receiver 8, a digital oscilloscope 9, a first standard single-mode optical fiber 10, a piezoelectric ceramic 11, a second standard single-mode optical fiber 12, a second optical circulator 13, a 1xN optical splitter 14, an N-path optical fiber delay line array 15, and a Nx1 optical combiner 16. Figure 1 The narrow linewidth laser 1 is used as a light source, and the output light signal of the laser is injected into the single-polarization IQ modulator 3, wherein the IQ modulator is used for modulating the input light signal. The arbitrary waveform generator 2 generates a linear frequency modulation waveform, and the waveform drives the IQ modulator 3, so that the input light signal is modulated to form a frequency-modulated continuous wave probe signal, wherein the frequency range of the linear frequency modulation waveform is 50 MHz to 200 MHz, and the period is 4.16 μs.

[0034] The modulated probe signal is sequentially transmitted to the polarization-maintaining doped fiber amplifier 4 for power amplification, and then filtered by the narrow-band optical filter 5 to suppress the amplified spontaneous emission noise. The filtered light signal is divided into two paths by the 7:3 optical fiber coupler 6: one path is used as a probe signal and is injected into the first standard single-mode optical fiber 10 with a length of 15 km through the first optical circulator 7; the other path is used as a local signal and is directly transmitted to the integrated coherent receiver 8.

[0035] In order to simulate a vibration event, the piezoelectric ceramic 11 is arranged at a position 10 km away from the input end of the first standard single-mode optical fiber 10, and a 15-meter-long optical fiber is wound on the piezoelectric ceramic, which is used to connect the second standard single-mode optical fiber 12.

[0036] The far end of the first standard single-mode optical fiber 10 is connected to an optical signal delay module through the second optical circulator 13, and the optical signal delay module is sequentially connected by the 1xN optical splitter 14, the N-path optical fiber delay line array 15 and the Nx1 optical combiner 16. After the probe signal enters the optical signal delay module through the second optical circulator 13, it experiences beam splitting, time delay and beam combining processing in sequence to form an interference signal, and then is transmitted back along the first standard single-mode optical fiber 10 through the second optical circulator 13. In the embodiment, the number N of optical paths of the optical fiber delay line array is 2, and the optical signal delay module is composed of a 1x2 optical splitter, an optical fiber delay path with a length of 80 m and a 2x1 optical combiner.

[0037]

[0038] ​The returned interference signal is transmitted to the integrated coherent receiver 8 through the first optical circulator 7, and is coherently mixed with the local signal. The mixed electrical signal is digitized and collected by the digital oscilloscope 9.

[0039] The embodiment also discloses a fiber vibration waveform reconstruction and positioning method based on the system.

[0040] 1) Probe signal generation: a narrow linewidth laser is used as a light source, a linear sweep waveform with a period of and a sweep rate of is generated by an arbitrary waveform generator; the linear sweep waveform is modulated onto an optical carrier by using a single polarization IQ modulator to obtain a linear sweep probe signal.

[0041] 2) Signal branching and transmission: the linear sweep probe signal is branched into two paths by an optical fiber coupler, wherein the first path is used as a probe signal and injected into a single-mode fiber link with a length of through an optical circulator (the corresponding time delay is denoted as ), and the second path is used as a local signal and reserved locally.

[0042] 3) Remote interference structure configuration: an optical signal delay module is configured at the far end of the fiber link. For the case that the delay line array branch number N=2, the optical signal delay module has two branch optical paths with fixed optical path difference, one of which contains an optical fiber delay line with a length of (corresponding time delay denoted as ). After entering the module, the probe signal is branched into two beams, which are transmitted through different optical paths and then recombined to form a dual-path signal, and the dual-path signal is transmitted back along the original fiber link through an optical circulator. The dual-path signal contains signals transmitted back to the transmitting end from two different paths, and the path that returns to the transmitting end through the additional delay line is denoted as path , and the path that does not pass through the additional delay line is denoted as path .

[0043] 4) Coherent detection and digitization: the returned dual-path signal is mixed with the local signal in the integrated coherent receiver to generate an electrical signal containing vibration information; the electrical signal is sampled by a real-time digital oscilloscope to obtain an original digital signal.

[0044] 5) Digital signal processing and vibration waveform reconstruction: the original digital signal is processed to realize vibration waveform reconstruction and positioning, specifically including the following steps.

[0045] 5-1) Link parameter calibration: the total length of the fiber link and the relative delay line length of the two branches in the fiber delay line array are scanned around the nominal values to determine their accurate values (only need to be performed once for a fixed link).

[0046] 5-2) Digital LO generation: generate digital LOs matching the time-frequency characteristics of the received signals according to the link calibration parameters. Where the effective time delays of the dual-path signals with respect to the LO signals are and , respectively, where represents the period of the linear sweep waveform, represents the fiber link propagation time delay, represents the interferometer delay arm fiber time delay.

[0047] The digital LOs for signal demodulation can be constructed from the effective time delays: where represents the constructed digital LO signals, where the indices correspond to the digital LOs of path and path , respectively, represents time, represents the imaginary unit, represents the complex exponential function, represents path corresponding effective time delay.

[0048] 5-3) Signal demodulation and phase extraction: demodulate using the digital LO signals, which is achieved by multiplying the original digital signals with the complex conjugates of the digital LOs and , respectively. This operation can optionally down-convert the signal components from different paths and to baseband. Through phase extraction and unwrapping processing, the target phase information carried by the linear frequency modulation probe signals along path and path is obtained, denoted as and , respectively.

[0049] 5-4) Differential phase calculation: perform differential on the two-phase signals to obtain the reconstructed vibration waveforms and , which are and , respectively.

[0050] 5-5) Cross-correlation positioning: perform cross-correlation operation on the two differential phase signals to determine the time delay , where xcor represents the cross-correlation function.

[0051] From which the specific location of the vibration event on the fiber link can be calculated: in, This represents the speed of light in a vacuum. Indicates the effective refractive index of the optical fiber. , These represent the propagation delay of the fiber optic link and the delay of the fiber optic delay line array, respectively.

[0052] In an embodiment of a multi-channel fiber delay line array where N is greater than 2, differential calculations can be performed on any two of the multiple phase signals to generate a reconstructed vibration waveform. The magnitude of the delay difference between different optical paths determines the frequency response of the current calculation. By combining the results of different delay difference branches, the frequency response and positioning accuracy of the system can be improved.

[0053] Figure 2 A time-frequency analysis comparison chart of the received signal and the digital local oscillator is shown when the number of branches N=2 in the fiber optic delay line array. Among them, Figure 2 Figure (a) is the time-frequency spectrum of the received signal. Figure 2 Figure (b) shows the time-frequency spectrum of one of the two digital local oscillators. Successful demodulation requires the digital local oscillator to accurately reproduce the time-frequency characteristics of the target signal. The digital local oscillator consists of two time segments with different frequencies, namely... and ; Figure 2 The time-frequency characteristic of the digital local oscillator shown in Figure (b) is exactly the same as... Figure 2 The components on one of the paths of the received signal in Figure (a) are consistent, thus the phase signal on that path can be demodulated independently.

[0054] When the number of branches in the fiber delay line array is N=2, for a vibration event of 4 kHz, Figure 3 The subsequent signal processing steps are shown. Figure 3 Figure (a) shows the original phase signal, which exhibits significant noise before low-pass filtering and differential processing; Figure 3 Figure (b) shows the filtered differential phase signal, which presents a clear sine waveform with a period of 0.25ms, consistent with the 4kHz vibration frequency. Figure 3 Figure (c) shows the cross-correlation function waveform, which has a sharp peak at a time delay of 0.50987ms, corresponding to a calculated vibration position of 9898m, which is highly consistent with the preset position of 10km for the piezoelectric ceramic 11.

[0055] Figure 4The evaluation results of the positioning accuracy of the present application with N = 2 branches of the fiber delay line array in the experiment by changing the vibration frequency are shown. In the experiment, the average positioning reference of the vibration event is fixed at 10 kilometers, and the positioning accuracy is quantified by estimating the standard deviation (STD) of the vibration position relative to the average position. The vibration frequency is controlled by adjusting the driving signal of the piezoelectric ceramic 11, and the specific test frequencies include 50 kHz, 20 kHz, 10 kHz and 4 kHz; 30 data acquisitions are performed for each frequency, and the corresponding positioning accuracies are 0.9 m, 4.5 m, 10.6 m and 84.9 m, respectively, which correspond to Figure 4 Figs. (a) to (d) in the description. The experimental results show that the present application can achieve meter-level positioning accuracy at high vibration frequencies and ten-meter-level positioning accuracy at medium vibration frequencies; at the same time, it can be seen that the positioning accuracy decreases with the decrease of the vibration frequency, which is due to the cosine-type frequency response introduced by the delay subtraction operation, and the delay is directly related to the length of the interferometer delay line . In this verification experiment, a relatively short delay line with a length of 80 meters is used; in practical applications, increasing the length of the delay line can effectively improve the positioning accuracy at low vibration frequencies.

[0056] Figure 5 The evaluation results of the influence of the received optical power (ROP) on the positioning accuracy of the present application with N = 2 branches of the fiber delay line array in the experiment are shown. In the experiment, the received optical power is adjusted in the range of -15 dBm to -51 dBm by a variable optical attenuator, the vibration frequency is fixed at 20 kHz, and the piezoelectric ceramic transducer is set at the 10-kilometer position; 10 data acquisitions are performed for each received optical power level. In the figure, the triangle marks the estimated vibration position obtained by each acquisition, the circle represents the standard deviation (STD) of the estimated position at each received optical power level, and the dashed line is the polynomial fitting curve of the standard deviation value. The experimental results show that even when the received optical power is as low as -45 dBm, the system positioning accuracy remains stable at the ten-meter level, indicating that the present application has strong robustness to amplitude noise. This makes the system have a large dynamic range and can extend the relay-free sensing distance to 100 kilometers. The high sensitivity is due to the inherent low amplitude noise characteristic of the forward sensing system of the present application. The noise suppression effect is mainly due to the frequency-modulated continuous wave (FMCW) technology, which can effectively suppress the Rayleigh backscattering (RBS) noise in the detection signal. To verify this, the Rayleigh backscattering power of the frequency-modulated continuous wave detection signal and the traditional single-frequency continuous wave detection signal is compared under the same detection signal power, and the results are shown in Figure 6 . A significant reduction of about 15 dB is observed, which is due to the fact that the Rayleigh backscattering power of the frequency-modulated continuous wave signal is distributed in the entire sweep bandwidth; in contrast, the Rayleigh backscattering power of the single-frequency detection signal is concentrated on a single spectral component, resulting in much higher noise spectral density.

[0057] Figure 7 Simulation results demonstrate the impact of the number of branches N in the delay line array on the frequency response of the single-ended interrogation forward sensing system of this invention. The aim is to improve the system's sensing adaptability to vibration signals of different frequencies by optimizing the number of branches and the relative delay length. It should be noted that in the delay line array configuration of this system, "N" represents the total number of branches, and all configurations include one reference path, with the remaining branches being delay paths—that is, when N=2, it is "1 reference path + 1 delay path," and when N=3, it is "1 reference path + 2 delay paths." The "relative delay length" mentioned later refers to the "physical length corresponding to the delay between the delay path and the reference path," thus clarifying the definition benchmark of the delay parameter. Figure 7 Figure a shows the system frequency response characteristics when N=2 (i.e., 1 reference path + 1 delay path) and the relative delay length between the delay path and the reference path is 600m. As can be seen from the figure, the system does not exhibit frequency fading points within the 250kHz frequency range, demonstrating good high-frequency response stability. However, under this configuration, the system's 3dB response frequency in the low-frequency band is 50kHz, which means it cannot effectively sense low-frequency vibration signals below 50kHz, indicating a limitation in low-frequency sensing capability. Figure 7 Figure b shows the frequency response curves when N=2 (1 reference path + 1 delay path) and the relative delay length between the delay path and the reference path is increased to 2100m. Thanks to the longer relative delay length design, the system's low-frequency response performance is significantly improved, effectively expanding the low-frequency sensing coverage. However, this configuration also brings new problems—a periodic fading phenomenon in the frequency response, which leads to a decrease in the sensing accuracy of vibration signals in specific frequency ranges, affecting the overall sensing stability of the system. To address the shortcomings of the above-mentioned single dual-branch (1 reference + 1 delay) configuration and further optimize the system's frequency response, an improved scheme of N=3 (i.e., 1 reference path + 2 delay paths) is proposed: the relative delay lengths of the two delay paths and the reference path are set to 600m and 2100m respectively (and... Figure 7 In Figures a and b, the relative delay parameters of the "single delay path and the reference path" are consistent. Furthermore, based on the delay difference between these two delay paths and the reference path, the "relative delay length between the two delay paths" (2100m - 600m = 1500m) can be derived. The frequency response corresponding to this derived delay parameter is... Figure 7 The characteristics are shown in Figure c. By synergistically combining the above three frequency responses (corresponding to relative delays of 600m for "delay path 1 - reference path", 2100m for "delay path 2 - reference path", and 1500m for "delay path 2 - delay path 1"), comprehensive optimization of the system frequency response can be achieved (see Figure c). Figure 7The final frequency response curve not only greatly improves the low frequency amplitude frequency response performance (solves the low frequency sensing limitation of N=2, 600m relative delay configuration), but also completely eliminates the frequency fading phenomenon in the frequency range of 250 kHz (avoids the periodic fading problem of N=2, 2100m relative delay configuration), thereby providing key technical support for accurate sensing of vibration signals in the whole frequency band.

[0058] Although the present application has been disclosed with examples as above, it is not intended to limit the present application, and any appropriate modification or equivalent replacement of the technical solutions of the present application made by those skilled in the art shall be covered within the protection scope of the present application, and the protection scope of the present application is defined by the claims.

Claims

1. A single-ended interrogating forward sensing system, characterized by, The application relates to a kind of coherent optical vibration sensor, comprising: a narrow linewidth laser for generating an optical source signal; an arbitrary waveform generator for generating a linear frequency modulation waveform; an IQ modulator for receiving the optical source signal and modulating the optical source signal to form a frequency modulation continuous wave detection signal under the driving of the linear frequency modulation waveform provided by the arbitrary waveform generator; an erbium-doped fiber amplifier for power amplifying the frequency modulation continuous wave detection signal; a narrow-band optical filter for filtering the amplified detection signal to suppress amplified spontaneous emission noise; a first fiber coupler for splitting the filtered detection signal into a detection signal path and a local signal path; a first optical circulator for injecting the detection signal into a first standard single-mode fiber and receiving an interference signal back-propagated along the first standard single-mode fiber in the reverse direction; a second optical circulator for connecting a distal end of the first standard single-mode fiber with an optical signal delay module to split, delay and combine the detection signal and form the interference signal; an integrated coherent receiver for receiving the local signal and the interference signal transmitted by the first optical circulator and coherently mixing the two signals; a digital oscilloscope for digitally collecting the mixed electrical signal output by the integrated coherent receiver.

2. The single-ended interrogating forward sensing system of claim 1, wherein, The optical signal delay module comprises a 1xN optical splitter, an N-path fiber delay line array and a Nx1 optical combiner connected in sequence.

3. The single-ended interrogating forward sensing system of claim 1, wherein, A piezoelectric ceramic is arranged between the input end and the distal end of the first standard single-mode fiber, and a second standard single-mode fiber is connected to the piezoelectric ceramic through a wound optical fiber for simulating vibration.

4. The single-ended interrogating forward sensing system of claim 1, wherein, The erbium-doped fiber amplifier is a polarization-maintaining erbium-doped fiber amplifier.

5. A method for reconstructing and locating the vibration waveform of an optical fiber based on the system of any one of claims 1-4, characterized in that, The application also discloses a coherent optical vibration sensor system comprising the following steps: 1) using a narrow linewidth laser as an optical source, generating a linear sweep waveform through an arbitrary waveform generator, and modulating the waveform onto an optical carrier through a single-polarization IQ modulator to form a linear sweep detection signal; 2) splitting the linear sweep detection signal into two paths through a fiber coupler, one of which is used as a detection signal to be injected into a standard single-mode fiber link through an optical circulator, and the other of which is used as a local signal to be reserved locally; 3) configuring an optical signal delay module at the distal end of the fiber link, which comprises an N-path fiber delay line array, and the N paths of the N-path fiber delay line array contain optical fiber delay lines with different lengths to realize different optical paths, N is a positive integer greater than or equal to 2, and the forward-propagating detection signal enters the optical signal delay module, is split, transmitted through different optical paths, combined again to form a multi-path signal, and back-propagated along the original fiber link through an optical circulator; 4) mixing the back-propagated multi-path signal with the locally reserved local signal in an integrated coherent receiver to generate an electrical signal containing vibration information, and digitally collecting the electrical signal through a digital oscilloscope to obtain an original digital signal; 5) performing digital signal processing on the original digital signal, including: performing link parameter scanning to determine actual values for the length of the fiber link and the length of each optical path of the N-path fiber delay line array. According to the determined link parameters, a digital local oscillator signal for demodulation is constructed, and the multipath signals are respectively down-converted to baseband; a plurality of phase signals are obtained through phase extraction and unwrapping; any two of the plurality of phase signals are differentially processed to generate a reconstructed vibration waveform; a cross-correlation operation is performed on the reconstructed vibration waveform to determine a signal time delay, and the position of the vibration event on the optical fiber link is calculated according to the speed of light and the refractive index of the optical fiber.

6. The method of claim 5, wherein, The construction of the digital local oscillator signal in step 5) includes generating a complex exponential form signal according to the effective time delay of two signals in the multipath signal, so as to realize the down-conversion of different signals to the baseband.

7. The method of claim 5, wherein the step of determining the fiber vibration waveform comprises the steps of: determining the fiber vibration waveform by using the fiber vibration waveform reconstruction method of claim 1. The cross-correlation operation in step 5) is used to find the time delay corresponding to the maximum cross-correlation value the location of the vibration event on the fiber link is calculated according to the following formula: wherein, denotes the propagation speed of light in vacuum, denotes the effective refractive index of the optical fiber, , denote the optical fiber link propagation delay and the N-path optical fiber delay line array delay, respectively.

8. The method of claim 5, wherein, The link parameter scanning in step 5) only needs to be performed once for a fixed link, so as to determine the actual optical fiber link length and the optical path delay length of each optical path of the N-path optical fiber delay line array.

9. The method of claim 5, wherein, N is greater than or equal to 3.

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