Grating interference distributed vibration sensing system and method without interferometer structure

Abstract

The invention proposes a grating interference distributed vibration sensing system and method without an interferometer structure, and relates to the technical field of optical fiber sensing, and the system comprises a dual-frequency modulation unit, a first power amplification unit, a circulator, an identical weak grating array, a second power amplification unit, a photoelectric detector, and a signal collection device. The signal acquisition equipment is connected with the first power amplification unit and the signal acquisition equipment respectively; the first power amplification unit is connected with the circulator; the circulator is respectively connected with the identical weak grating array and the second power amplification unit, and the identical weak grating array is used for receiving the amplified double-frequency monopulse signals and enabling the double-frequency monopulse signals to be overlapped in a time domain so as to obtain beat frequency signals; and the photoelectric detector is respectively connected with the second power amplification unit and the signal acquisition equipment. According to the invention, the stability and the signal-to-noise ratio of the grating interference distributed vibration sensing system can be improved.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a grating interferometer-free distributed vibration sensing system and method. Background Technology

[0002] In traditional grating-matched interferometric distributed vibration sensing systems, phase-sensitive optical time-domain reflectometry (Φ-OTDR) technology serves as the core technology. It achieves path matching between adjacent gratings through an interferometer structure, generating two-beam interference. The vibration information of the target object is then reconstructed by demodulating the phase information changes generated by the interference. It is evident that the core of achieving path matching between adjacent gratings lies in the interferometer structure within the sensing system. Because the interferometer structure has non-equidistant arm lengths, the length matching with the spacing between adjacent gratings can be achieved by adjusting the difference in the interferometer arm lengths. This ensures that the optical path lengths of the reflected pulse signals from adjacent gratings are equal, allowing the two pulse light signals from adjacent gratings to meet and interfere in the time domain.

[0003] Chinese Patent CN113654641B discloses a distributed fiber optic vibration sensing system and demodulation method. The system includes a direct-detection Φ-OTDR section, an interferometric section, and a signal acquisition section. The direct-detection Φ-OTDR section locates vibration events, while the interferometric section enhances the system's frequency response. The combination of these two components enables the system to locate vibration events and measure wide-spectrum signals. The demodulation algorithm improves the spatial positioning accuracy of vibration events and faithfully reconstructs the spectral characteristics of the vibration signal, resulting in higher accuracy in identifying vibration events. However, the above solution is extremely sensitive to mechanical vibrations and easily generates additional environmental phase noise, requiring active vibration isolation measures. Otherwise, the signal-to-noise ratio and measurement accuracy will decrease. Therefore, it is essential to provide a grating interferometric distributed vibration sensing system and method without an interferometer structure, which helps improve the stability and signal-to-noise ratio of the grating interferometric distributed vibration sensing system. Summary of the Invention

[0004] In view of this, the present invention proposes a grating interferometric distributed vibration sensing system and method without an interferometer structure. The grating interferometric distributed vibration sensing system does not require precision interferometer arms and phase matching elements, eliminating the problems of optical path rebalancing and interference phase drift caused by vibration. The sensing system has strong resistance to environmental disturbances at the demodulation instrument end, and significantly improves the signal-to-noise ratio after photoelectric detection.

[0005] This invention provides a grating interferometer-free distributed vibration sensing system, comprising a dual-frequency modulation unit, a first power amplification unit, a circulator, an identical weak grating array, a second power amplification unit, a photodetector, and a signal acquisition device, wherein... The dual-frequency modulation unit is connected to the first power amplifier unit and the signal acquisition device respectively. The dual-frequency modulation unit generates a continuous optical signal and modulates the continuous optical signal into a dual-frequency single-pulse signal with different carrier frequencies. The first power amplification unit is connected to the circulator, and the first power amplification unit is used to sequentially amplify the peak power and bandpass filter the dual-frequency single-pulse signal; The circulator is connected to the identical weak grating array and the second power amplifier unit respectively. The identical weak grating array is used to receive the amplified dual-frequency single pulse signal and make the dual-frequency single pulse signal overlap in the time domain to obtain the beat frequency signal. The photodetector is connected to the second power amplification unit and the signal acquisition device respectively. The photodetector is used to receive the beat frequency signal amplified by the second power amplification unit and perform photoelectric conversion on the beat frequency signal to obtain a wide pulse beat frequency electrical signal. The signal acquisition device is used to demodulate the phase of the wide pulse beat frequency electrical signal to restore the vibration signal.

[0006] Based on the above technical solutions, preferably, the dual-frequency modulation unit includes a narrow-linewidth laser, an acousto-optic modulator, a power amplifier, and an arbitrary waveform generator, wherein, The narrow linewidth laser is connected to the acousto-optic modulator, and the narrow linewidth laser is used to generate continuous optical signals; The acousto-optic modulator is connected to the power amplifier and the first power amplifier unit respectively, and the acousto-optic modulator is used to modulate the continuous optical signal into a dual-frequency single-pulse signal. The power amplifier is connected to the arbitrary waveform generator, and the power amplifier is used to amplify the electrical signal transmitted by the arbitrary waveform generator. The arbitrary waveform generator is connected to the signal acquisition device, and the arbitrary waveform generator is used to generate two modulated electrical signals with different carrier frequencies and the same pulse width.

[0007] Based on the above technical solutions, preferably, the first power amplification unit includes a first fiber amplifier and a first bandpass filter. The first fiber amplifier is connected to the first bandpass filter and the dual-frequency modulation unit respectively. The first bandpass filter is connected to the first end of the circulator. The first fiber amplifier is used to amplify the peak power of the dual-frequency single-pulse signal, and the first bandpass filter is used to perform bandpass filtering on the dual-frequency single-pulse signal after peak power amplification.

[0008] More preferably, the second power amplification unit includes a second fiber amplifier and a second bandpass filter. The second fiber amplifier is connected to the second bandpass filter and the third end of the circulator, respectively. The second bandpass filter is connected to the photodetector. The second fiber amplifier is used to amplify the peak power of the beat frequency signal, and the second bandpass filter is used to bandpass filter the beat frequency signal after peak power amplification.

[0009] More preferably, the dual-frequency single-pulse signal includes a first optical pulse and a second optical pulse. When the dual-frequency single-pulse signal meets the preset beat frequency condition, the identical weak grating array outputs a beat frequency signal, wherein the pulse width of the first optical pulse and the second optical pulse are the same.

[0010] More preferably, the preset beat frequency conditions include: The pulse width of the dual-frequency single-pulse signal is greater than the time delay corresponding to one round trip of the optical signal between adjacent gratings in the identical weak grating array, and the pulse width of any one of the first optical pulses or the second optical pulse is less than or equal to the time delay corresponding to one round trip of the optical signal between adjacent gratings in the identical weak grating array.

[0011] More preferably, the signal acquisition device uses a heterodyne IQ phase demodulation algorithm to perform phase demodulation on the beat frequency signal in the effective interference region.

[0012] More preferably, both the first fiber amplifier and the second fiber amplifier are erbium-doped fiber amplifiers, and the first bandpass filter and the second bandpass filter have the same bandwidth.

[0013] More preferably, the center frequency of the acousto-optic modulator is the same as the frequency shift amount of the continuous optical signal by the dual-frequency modulation unit.

[0014] A second aspect of this application provides a grating interferometric distributed vibration sensing method without an interferometer structure, the method comprising: A continuous optical signal is generated by a dual-frequency modulation unit, and the continuous optical signal is modulated into a dual-frequency single-pulse signal with different carrier frequencies, and the dual-frequency single-pulse signal is input into a first power amplifier unit; The dual-frequency single-pulse signal is sequentially amplified by peak power and filtered by bandpass by the first power amplification unit, and then input into the identical weak grating array through the circulator, so that the dual-frequency single-pulse signal overlaps in the time domain to obtain the beat frequency signal, and the beat frequency signal is input to the second power amplification unit through the circulator. The beat frequency signal amplified by the second power amplification unit is then photoelectrically converted to obtain a wide-pulse beat frequency electrical signal. This pulse beat frequency electrical signal is then input into a signal acquisition device for demodulation to restore the vibration signal.

[0015] The grating interferometer-free distributed vibration sensing system and method provided by this invention have the following advantages over the prior art: (1) No precision interferometer arm and phase matching element are required, eliminating the problem of interference phase drift caused by optical path rebalancing and vibration. Furthermore, the grating interference distributed vibration sensing system has fewer components and lower optical alignment requirements. The sensing system instrument has strong resistance to environmental disturbances. The dual-frequency modulation unit directly outputs the same dual-frequency single pulse signal, which is time-domain superimposed in the weak light grating array to obtain a robust beat frequency signal. This avoids the complexity of external cavity or reference arm lasers. The first-stage power amplification unit and the second-stage power amplification unit can effectively suppress the modulation sub-frequency and background noise, significantly improving the signal-to-noise ratio after photoelectric detection. At the same time, the identical weak light grating array serves as a distributed reflection point, which can obtain beat frequency signals in parallel at multiple points, supporting large-scale, multi-channel vibration distribution monitoring. The weak reflection of the grating avoids crosstalk, and the spatial resolution and optical pulse width can be adjusted in a coordinated manner.

[0016] (2) When the overall pulse width of the dual-frequency single-pulse signal is greater than the round-trip delay between adjacent gratings, the two frequency components can continuously overlap, interfere and accumulate phase difference within the entire weak grating array, generating a stable and concentrated beat frequency signal, ensuring that the entire grating array participates in the dual-frequency beat frequency, improving the signal-to-noise ratio and spectral purity of the beat frequency output, and by limiting the pulse width of any single-frequency optical pulse to within the round-trip delay between adjacent gratings, it is ensured that the subsequent reflection of the pulse will not overlap with the next excitation pulse or its own echo, effectively suppressing cross-interference and ghost pulses between different grating reflections, improving spatial resolution and measurement accuracy, while retaining multiple equally spaced and ordered beat frequency interference structures in the array, so that the echo of each grating carries clear beat frequency phase information, combined with the high SNR and low crosstalk dual-frequency beat frequency signal, the center wavelength drift of each grating can be accurately demodulated, realizing high-precision, multi-point real-time monitoring of the distributed fiber optic sensing array. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1A schematic diagram of a grating interferometer-free distributed vibration sensing system provided by the present invention; Figure 2 The schematic diagram of intra-pulse dual-frequency modulation grating matching interference provided by the present invention.

[0019] Explanation of reference numerals in the attached figures: 1. Dual-frequency modulation unit; 11. Narrow linewidth laser; 12. Acousto-optic modulator; 13. Power amplifier; 14. Arbitrary waveform generator; 2. First power amplifier unit; 21. First fiber optic amplifier; 22. First bandpass filter; 3. Circulator; 4. Identical weak grating array; 5. Second power amplifier unit; 51. Second fiber optic amplifier; 52. Second bandpass filter; 6. Photodetector; 7. Signal acquisition device. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, the terms "an" or "a" and similar terms do not indicate a quantity limitation, but rather indicate the presence of at least one. The terms "connected" or "linked" and similar terms are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. "Up," "down," "left," "right," etc., are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship also changes accordingly.

[0022] refer to Figure 1 This invention provides a grating interferometer-free distributed vibration sensing system, comprising a dual-frequency modulation unit 1, a first power amplification unit 2, a circulator 3, an identical weak grating array 4, a second power amplification unit 5, a photodetector 6, and a signal acquisition device 7, wherein... The dual-frequency modulation unit 1 is connected to the first power amplifier unit 2 and the signal acquisition device 7 respectively. The dual-frequency modulation unit 1 generates a continuous optical signal and modulates the continuous optical signal into a dual-frequency single-pulse signal with different carrier frequencies.

[0023] In this embodiment, the dual-frequency modulation unit 1 includes a narrow-linewidth laser 11, an acousto-optic modulator 12, a power amplifier 13, and an arbitrary waveform generator 14. The narrow-linewidth laser 11 is connected to the acousto-optic modulator 12 and is used to generate a continuous optical signal. The acousto-optic modulator 12 is connected to both the power amplifier 13 and the first power amplification unit 2, and is used to modulate the continuous optical signal into a dual-frequency single-pulse signal. The power amplifier 13 is connected to the arbitrary waveform generator 14 and is used to amplify the electrical signal emitted by the arbitrary waveform generator 14. The arbitrary waveform generator 14 is connected to the signal acquisition device 7 and is used to generate two modulated electrical signals with different carrier frequencies but the same pulse width. The center frequency of the acousto-optic modulator 12 is the same as the frequency shift amount of the continuous optical signal by the dual-frequency modulation unit 1.

[0024] The first power amplifier unit 2 is connected to the circulator 3. The first power amplifier unit 2 is used to sequentially amplify the peak power and bandpass filter the dual-frequency single-pulse signal. The first power amplifier unit 2 includes a first fiber optic amplifier 21 and a first bandpass filter 22. The first fiber optic amplifier 21 is connected to the first bandpass filter 22 and the dual-frequency modulation unit 1, respectively. The first bandpass filter 22 is connected to the first end of the circulator 3. The first fiber optic amplifier 21 is used to amplify the peak power of the dual-frequency single-pulse signal, and the first bandpass filter 22 is used to bandpass filter the dual-frequency single-pulse signal after peak power amplification.

[0025] Furthermore, a dual-frequency carrier signal is generated by an arbitrary waveform generator 14 and amplified by a power amplifier 13 to obtain the driving signal for the acousto-optic modulator 12. The dual-frequency single-pulse signal is amplified by a first erbium-doped fiber amplifier and filtered by a first bandpass filter 22. The first filter has a bandwidth of 50 GHz to reduce the influence of broadband spontaneous emission noise caused by the first erbium-doped fiber amplifier.

[0026] The circulator 3 is connected to the identical weak grating array 4 and the second power amplifier unit 5 respectively. The identical weak grating array 4 is used to receive the amplified dual-frequency single pulse signal and make the dual-frequency single pulse signal overlap in the time domain to obtain the beat frequency signal.

[0027] The second power amplification unit 5 includes a second fiber amplifier 51 and a second bandpass filter 52. The second fiber amplifier 51 is connected to the second bandpass filter 52 and the third terminal of the circulator 3, respectively. The second bandpass filter 52 is connected to the photodetector 6. The second fiber amplifier 51 is used to amplify the peak power of the beat frequency signal, and the second bandpass filter 52 is used to perform bandpass filtering on the amplified beat frequency signal. Both the first fiber amplifier 21 and the second fiber amplifier 51 are erbium-doped fiber amplifiers, and the first bandpass filter 22 and the second bandpass filter 52 have the same bandwidth.

[0028] Furthermore, the dual-frequency single-pulse signal includes a first optical pulse and a second optical pulse. When the dual-frequency single-pulse signal meets the preset beat frequency condition, the identical weak grating array 4 outputs a beat frequency signal, wherein the pulse width of the first optical pulse and the second optical pulse are the same.

[0029] The preset beat frequency conditions include: the pulse width of the dual-frequency single-pulse signal is greater than the time delay corresponding to one round trip of the optical signal between adjacent gratings in the identical weak grating array 4, and the pulse width of any one of the first or second optical pulses is less than or equal to the time delay corresponding to one round trip of the optical signal between adjacent gratings in the identical weak grating array 4.

[0030] In one example, the grating interferometer distributed vibration sensing system modulates the output of a signal with a certain frequency difference Δ. f A wide pulse signal, where Δ f=f 2- f 1. The single-pulse signal is divided into two parts; the frequency of the first half of the optical pulse is... f 1. The frequency of the light pulse in the latter half is f 2, of which f The fundamental frequency of the local oscillator is Δ, and the pulse width of both parts is Δ. T Therefore, the single pulse width is 2Δ T Assuming the grating spacing is... L The optical path traveled by the optical signal in one round trip between adjacent gratings is 2. L The corresponding time delay is expressed as:

[0031] in, n For the refractive index of the sensing fiber, c The speed of light in a vacuum. When Δ T Satisfy 2Δ T >Δ T At time 1, the reflected signals from adjacent gratings meet in the time domain, thus obtaining the beat frequency signal. Simultaneously, to avoid three-pulse or multi-pulse interference effects in the sensing fiber and to introduce phase change information from other measurement areas, the single-frequency pulse width needs to satisfy Δ... T ≤Δ T 1. At this time, the effective time length of the sensing area is 2Δ. T -Δ T 1, referred to as the effective interference region. The heterodyne IQ phase demodulation algorithm is used to demodulate the phase of the beat frequency signal within the effective interference region. Therefore, the success of recovering phase information from the effective interference region depends on the quality of the beat frequency signal. The frequency of the beat frequency signal is Δ. fThe larger the frequency difference, the smaller the minimum effective interference region length, and the smaller the possible value of the single pulse width. Simultaneously, to ensure the system can accurately determine the location of sensor disturbances and thus further pinpoint the location, it is necessary to clearly distinguish the interference beat frequency signals generated by the corresponding sensor region in the time domain. Therefore, the single pulse width must satisfy: Δ T 1<2Δ T <2Δ T 1.

[0032] The single-pulse dual-frequency modulation signal is generated by the acousto-optic modulator 12 (AOM). The AOM can modulate the continuous light output from the laser with a narrow pulse, and also introduces a fixed frequency shift to change the frequency of the optical signal. The AOM achieves optical wave manipulation through the acousto-optic medium, and its energy conversion process is completed by a piezoelectric transducer. An external carrier drive electrical signal drives the piezoelectric transducer attached to the acousto-optic crystal, and the piezoelectric transducer simultaneously generates an ultrasonic signal with the same frequency as the carrier. The ultrasonic signal is introduced into the acousto-optic medium, and due to the photoelastic effect, the ultrasonic waves acting on the acousto-optic medium cause its refractive index to change periodically. When the laser output light enters the AOM, they interact and change the propagation direction of the input light, producing Bragg diffraction.

[0033] Most AOMs operate under the Bragg mechanism, with the first-order diffracted light exhibiting the highest efficiency. By using a carrier drive signal to transfer all incident light energy to the direction of the first-order diffracted light, acousto-optic frequency shift can be achieved. The shift amount corresponds to the center frequency of the AOM. By adjusting the frequency of the diffracted light, i.e., the carrier modulation signal frequency, the AOM can output different frequency shift amounts, thereby achieving intra-pulse dual-frequency modulation. Therefore, by using an arbitrary waveform generator 14 (AWG), modulation pulses with the same width and carrier frequency can be generated. f 1 and f 2. Modulate the pulse driving electrical signal and drive the AOM, thereby obtaining a signal with the same pulse width as the modulated pulse driving electrical signal, but with different frequency shifts. f 1 and f 2. Dual-frequency single-pulse signal. By injecting the dual-frequency modulated probe light pulse signal into the grating array sensor, the corresponding time-domain beat frequency signal after reflection by the grating array can be acquired by the detection device without the need for an interferometer structure, which effectively improves the anti-environment interference capability of the grating interferometric distributed vibration sensing system.

[0034] In this embodiment, when the overall pulse width of the dual-frequency single-pulse signal is greater than the round-trip delay between adjacent gratings, the two frequency components can continuously overlap, interfere, and accumulate phase difference within the entire weak grating array, generating a stable and concentrated beat frequency signal. This ensures that the entire grating array participates in the dual-frequency beat frequency, improving the signal-to-noise ratio and spectral purity of the beat frequency output. Furthermore, by limiting the pulse width of any single-frequency optical pulse to within the round-trip delay between adjacent gratings, it ensures that the subsequent reflection of the pulse will not overlap with the next excitation pulse or its own echo, effectively suppressing cross-interference and ghost pulses between reflections from different gratings, improving spatial resolution and measurement accuracy. Simultaneously, multiple equally spaced and ordered beat frequency interference structures are retained in the array, ensuring that the echo of each grating carries clear beat frequency phase information. Combined with the high SNR and low crosstalk dual-frequency beat frequency signal, precise demodulation of the center wavelength drift of each grating can be achieved, enabling high-precision, multi-point real-time monitoring of the distributed fiber optic sensing array. The overall excitation with a long pulse width enhances the robustness of the beat frequency signal to external power fluctuations and phase noise. The short pulse width single-channel excitation prevents phase disorder caused by the accumulation of multiple internal reflections, thereby further reducing the background noise of the measurement system.

[0035] The photodetector 6 is connected to the second power amplifier unit 5 and the signal acquisition device 7 respectively. The photodetector 6 is used to receive the beat frequency signal amplified by the second power amplifier unit 5 and perform photoelectric conversion on the beat frequency signal to obtain a wide pulse beat frequency electrical signal. The signal acquisition device 7 is used to demodulate the phase of the wide pulse beat frequency electrical signal to restore the vibration signal.

[0036] In this embodiment, the signal acquisition device 7 uses a heterodyne IQ phase demodulation algorithm to demodulate the beat frequency signal in the effective interference region. To extract the phase information to be measured contained in the beat frequency signal, the heterodyne IQ phase demodulation algorithm is used to demodulate the phase information of the beat frequency signal. In the above technical solution, the optical field information of the dual-frequency single-pulse signal obtained after reflection from the first Bragg grating in the identical weak grating array 4 can be expressed as:

[0037] Similarly, the optical field information of the dual-frequency single-pulse signal after reflection from the first Bragg grating in the identical weak grating array 4 can be expressed as:

[0038] in E 1,first This represents the light field intensity in the first half of the first optical pulse in a dual-frequency single-pulse signal. E 1,second This represents the light field intensity in the latter half of the first optical pulse in a dual-frequency single-pulse signal. E 2,firstThis represents the light field intensity in the first half of the second optical pulse in a dual-frequency single-pulse signal. E 2,second This represents the light field intensity in the latter half of the second optical pulse in a dual-frequency single-pulse signal. A 1,first This represents the amplitude of the light wave corresponding to the first half of the first optical pulse in a dual-frequency single-pulse signal. A 1,second This represents the amplitude of the light wave corresponding to the latter half of the first optical pulse in a dual-frequency single-pulse signal. A 2,first This indicates the amplitude of the light wave corresponding to the first half of the second light pulse in a dual-frequency single-pulse signal. A 2,second This indicates the amplitude of the light wave corresponding to the latter half of the second optical pulse in a dual-frequency single-pulse signal. t 0 indicates the start time of the light pulse. f 0, first This indicates the initial phase of the optical signal before the first optical pulse is injected into the identical weak grating array 4 in the dual-frequency single-pulse signal. f 0,second This indicates the initial phase of the optical signal before the second optical pulse is injected into the identical weak grating array 4 in the dual-frequency single-pulse signal. f 1 represents the phase difference generated when the optical signal travels back and forth once between adjacent gratings. f 1 is a constant. Δ f This indicates the phase information to be measured in the sensing area. The beat frequency signal is received by photodetector 6. Due to the linear gain characteristic of the photodetector, the photocurrent of the collected optical signal is proportional to the optical power, thus the photocurrent expression is obtained as follows:

[0039] Where, Δ f 0= f 0, first + f 1- f 0,second As a constant, the beat frequency signal is divided into four frequency components, the first three of which are 2( f + f 1), 2( f + f 2) and ( f + f 1+ f 2). However, since the laser's output optical frequency is close to the THz level, these three AC signals cannot be received by photodetector 6 (with a bandwidth in the MHz range), and the corresponding AC components will be filtered out. Therefore, the AC and DC signals output after the beat frequency signal is received by photodetector 6 are represented as follows:

[0040] in, A For DC term amplitude, B The amplitude of the AC term is given. The beat frequency signal intensity is proportional to the amplitude of the pulsed light signal, and its phase information includes the difference frequency term, the fixed phase difference of the reflected pulse signals between adjacent gratings, and the phase to be measured in the sensing area. By combining the heterodyne IQ phase demodulation algorithm, the beat frequency signal is further demodulated and processed to obtain the intensity and phase information of the target signal.

[0041] like Figure 2 As shown, the four Bragg gratings (FBG#1, FBG#2, FBG#3, and FBG#4) sequentially distributed on the identical weak grating array 4, along with the temporal width relationship of the two pulses and the positions of the echoes from each grating, are illustrated. The beat frequency signal is obtained after the dual-frequency single-pulse signal is reflected by the grating array sensor, avoiding environmental noise interference introduced by the interferometer structure, achieving accurate acquisition of external vibration signals, and improving the system's noise immunity.

[0042] In this embodiment, no precision interferometer arm or phase matching element is required, eliminating the interference phase drift problem caused by optical path rebalancing and vibration. Furthermore, the grating interferometric distributed vibration sensing system has fewer components, lower optical alignment requirements, and strong resistance to environmental disturbances at the sensing system's instrument end. The dual-frequency modulation unit 1 directly outputs the same dual-frequency single-pulse signal, which is time-domain superimposed in the weak-light grating array to obtain a robust beat frequency signal, avoiding the complexity of external cavity or reference arm lasers. The first and second stage power amplification units effectively suppress modulation sub-frequency and background noise, significantly improving the signal-to-noise ratio after photoelectric detection. Simultaneously, the identical weak-light grating array, as a distributed reflection point, can obtain beat frequency signals in parallel at multiple points, supporting large-scale, multi-channel vibration distribution monitoring. The weak reflection of the grating avoids crosstalk, and the spatial resolution and pulse width can be adjusted synergistically. After photoelectric conversion, a wide-pulse beat frequency electrical signal is obtained, with a bandwidth smaller than the optical modulation bandwidth, facilitating acquisition and processing by backend electronic equipment. Wide pulse plus phase demodulation can balance high sensitivity and large amplitude response, significantly improving the dynamic range. Through digital phase demodulation technology, minute phase changes can be directly extracted from beat frequency electrical signals, and vibration detection sensitivity can reach the nanometer or even picometer level.

[0043] Based on the above-mentioned grating interferometric distributed vibration sensing system, this application also discloses a grating interferometric distributed vibration sensing method without an interferometer structure, the method comprising: A continuous optical signal is generated by the dual-frequency modulation unit 1, and the continuous optical signal is modulated into a dual-frequency single-pulse signal with different carrier frequencies, and the dual-frequency single-pulse signal is input into the first power amplifier unit 2; The dual-frequency single-pulse signal is sequentially amplified by peak power and filtered by bandpass through the first power amplification unit 2, and then input into the identical weak grating array 4 through the circulator 3, so that the dual-frequency single-pulse signal overlaps in the time domain to obtain the beat frequency signal, and the beat frequency signal is input to the second power amplification unit 5 through the circulator 3. The beat frequency signal amplified by the second power amplification unit 5 is photoelectrically converted to obtain a wide-pulse beat frequency electrical signal, which is then input into the signal acquisition device 7 for demodulation to restore the vibration signal.

[0044] In this embodiment, when the total pulse width of the dual-frequency single-pulse signal is greater than the round-trip delay of adjacent gratings, the two frequency components continuously overlap and interfere within the entire weak grating array, accumulating a steady-state beat frequency signal with concentrated intensity and high spectral purity, significantly improving the signal-to-noise ratio. Ensuring that the pulse width of any single-frequency pulse is less than or equal to the round-trip delay of adjacent gratings ensures that its echo does not overlap with subsequent excitation pulses or its own reflection, effectively eliminating cross-interference and ghost pulses between reflections of different gratings, and significantly improving spatial resolution and measurement accuracy. Retaining multiple equally spaced and ordered beat frequency interference structures in the array ensures that each grating echo carries clear beat frequency phase information. Combined with a high SNR and low crosstalk signal, precise demodulation of the center wavelength drift at each point can be achieved, realizing high-precision, multi-point real-time monitoring of the distributed fiber optic sensing array. Long-pulse-width overall excitation enhances robustness to external power fluctuations and phase noise; short-pulse-width local excitation prevents phase misalignment caused by multiple internal reflections, further reducing background noise and ensuring long-term stability and reliability of the measurement system.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A grating interferometric distributed vibration sensing system without an interferometer structure, characterized in that, It includes a dual-frequency modulation unit (1), a first power amplifier unit (2), a circulator (3), an identical weak grating array (4), a second power amplifier unit (5), a photodetector (6), and a signal acquisition device (7), wherein, The dual-frequency modulation unit (1) is connected to the first power amplification unit (2) and the signal acquisition device (7) respectively. The dual-frequency modulation unit (1) generates a continuous optical signal and modulates the continuous optical signal into a dual-frequency single-pulse signal with different carrier frequencies. The first power amplification unit (2) is connected to the circulator (3), and the first power amplification unit (2) is used to sequentially amplify the peak power and bandpass filter the dual-frequency single pulse signal; The circulator (3) is connected to the identical weak grating array (4) and the second power amplifier unit (5) respectively. The identical weak grating array (4) is used to receive the amplified dual-frequency single pulse signal and make the dual-frequency single pulse signal overlap in the time domain to obtain the beat frequency signal. The photodetector (6) is connected to the second power amplification unit (5) and the signal acquisition device (7) respectively. The photodetector (6) is used to receive the beat frequency signal amplified by the second power amplification unit (5) and perform photoelectric conversion on the beat frequency signal to obtain a wide pulse beat frequency electrical signal. The signal acquisition device (7) is used to perform phase demodulation on the wide pulse beat frequency electrical signal to restore the vibration signal.

2. The grating interferometer-free distributed vibration sensing system as described in claim 1, characterized in that, The dual-frequency modulation unit (1) includes a narrow-linewidth laser (11), an acousto-optic modulator (12), a power amplifier (13), and an arbitrary waveform generator (14), wherein, The narrow linewidth laser (11) is connected to the acousto-optic modulator (12), and the narrow linewidth laser (11) is used to generate a continuous optical signal; The acousto-optic modulator (12) is connected to the power amplifier (13) and the first power amplifier unit (2) respectively. The acousto-optic modulator (12) is used to modulate the continuous optical signal into a dual-frequency single-pulse signal. The power amplifier (13) is connected to the arbitrary waveform generator (14), and the power amplifier (13) is used to amplify the electrical signal transmitted by the arbitrary waveform generator (14); The arbitrary waveform generator (14) is connected to the signal acquisition device (7). The arbitrary waveform generator (14) is used to generate two modulated electrical signals with different carrier frequencies and the same pulse width.

3. The grating interferometer-free distributed vibration sensing system as described in claim 1, characterized in that, The first power amplification unit (2) includes a first fiber amplifier (21) and a first bandpass filter (22). The first fiber amplifier (21) is connected to the first bandpass filter (22) and the dual-frequency modulation unit (1) respectively. The first bandpass filter (22) is connected to the first end of the circulator (3). The first fiber amplifier (21) is used to amplify the peak power of the dual-frequency single-pulse signal, and the first bandpass filter (22) is used to bandpass filter the dual-frequency single-pulse signal after peak power amplification.

4. The grating interferometer-free distributed vibration sensing system as described in claim 3, characterized in that, The second power amplification unit (5) includes a second fiber amplifier (51) and a second bandpass filter (52). The second fiber amplifier (51) is connected to the second bandpass filter (52) and the third end of the circulator (3) respectively. The second bandpass filter (52) is connected to the photodetector (6). The second fiber amplifier (51) is used to amplify the peak power of the beat frequency signal, and the second bandpass filter (52) is used to perform bandpass filtering on the beat frequency signal after peak power amplification.

5. The grating interferometer-free distributed vibration sensing system as described in claim 1, characterized in that, The dual-frequency single-pulse signal includes a first optical pulse and a second optical pulse. When the dual-frequency single-pulse signal meets the preset beat frequency condition, the identical weak grating array (4) outputs a beat frequency signal, wherein the pulse width of the first optical pulse and the second optical pulse are the same.

6. The grating interferometer-free distributed vibration sensing system as described in claim 5, characterized in that, The preset beat frequency conditions include: The pulse width of the dual-frequency single-pulse signal is greater than the time delay corresponding to one round trip of the optical signal between adjacent gratings in the identical weak grating array (4), and the pulse width of any one of the first optical pulses or the second optical pulse is less than or equal to the time delay corresponding to one round trip of the optical signal between adjacent gratings in the identical weak grating array (4).

7. The grating interferometer-free distributed vibration sensing system as described in claim 1, characterized in that, The signal acquisition device (7) uses the heterodyne IQ phase demodulation algorithm to perform phase demodulation on the beat frequency signal in the effective interference region.

8. The grating interferometer-free distributed vibration sensing system as described in claim 4, characterized in that, Both the first fiber amplifier (21) and the second fiber amplifier (51) are erbium-doped fiber amplifiers, and the first bandpass filter (22) and the second bandpass filter (52) have the same bandwidth.

9. The grating interferometer-free distributed vibration sensing system as described in claim 2, characterized in that, The center frequency of the acousto-optic modulator (12) is the same as the frequency shift of the continuous optical signal by the dual-frequency modulation unit (1).

10. A grating interferometric distributed vibration sensing method without an interferometer structure, characterized in that, The method includes: A continuous optical signal is generated by a dual-frequency modulation unit (1), and the continuous optical signal is modulated into a dual-frequency single-pulse signal with different carrier frequencies, and the dual-frequency single-pulse signal is input into a first power amplifier unit (2); The dual-frequency single-pulse signal is sequentially amplified by the first power amplification unit (2) and then filtered by the bandpass filter before being input into the identical weak grating array (4) through the circulator (3), so that the dual-frequency single-pulse signal overlaps in the time domain to obtain the beat frequency signal, and the beat frequency signal is input to the second power amplification unit (5) through the circulator (3). The beat frequency signal amplified by the second power amplification unit (5) is photoelectrically converted to obtain a wide pulse beat frequency electrical signal, and the pulse beat frequency electrical signal is input to the signal acquisition device (7) for demodulation to restore the vibration signal.

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