Phase-sensitive optical time domain reflectometer based on parallel frequency division multiplexing sequence

Abstract

The invention relates to a phase-sensitive optical time domain reflectometer based on a parallel frequency division multiplexing sequence. Continuous laser is divided into a local path and a detection path through an optical coupler; the local path is input to the polarization coherent receiving module, the detection path is converted into a pulse laser sequence with intensity modulation through a light intensity modulator driven by a radio frequency signal provided by the signal generation circuit module, and the radio frequency signal is composed of two frequency division multiplexing sequences with difference; the pulse laser sequence enters the sensing link to be measured to generate Rayleigh backscattered light, and the Rayleigh backscattered light and local continuous laser are subjected to beat frequency coherent receiving through a circulator; and the signal acquisition circuit is used for sampling and analyzing the interference signal, and performing frequency domain comparison on an interleaving result of the two frequency division multiplexing pulse sequences to eliminate a harmonic frequency and retain a real strain signal frequency. Compared with the prior art, the non-harmonic strain signal measurement is realized, the high-frequency response performance and the high-precision strain measurement capability are excellent, and meanwhile, the long-distance coverage capability is also realized.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement and sensing technology, and in particular to a phase-sensitive optical time domain reflectometer based on a parallel frequency division multiplexing sequence. Background Art

[0002] A phase-sensitive optical time-domain reflectometer (Φ-OTDR) uses optical pulses to detect the optical fiber under test and uses the Rayleigh backscattered light generated during the transmission of the optical pulses to demodulate strain information at different locations on the fiber under test. Due to its long-distance sensing and high-spatial-resolution continuous distributed measurement capabilities, the Φ-OTDR system is widely used in vibration monitoring and safety warning for critical infrastructure such as bridges, tunnels, oil and gas pipelines, and power lines. Φ-OTDR distinguishes different locations on the optical fiber by the time it takes for the Rayleigh backscattered light to return. Therefore, the next optical pulse must be injected after a round trip on the fiber under test. In other words, the maximum detection frequency of an optical pulse is the inverse of the time it takes for light to travel back and forth in the fiber. According to the Nyquist sampling theorem, the maximum frequency response of a phase-sensitive optical time-domain reflectometer is half the detection frequency of the optical pulse. Therefore, the frequency response range of a phase-sensitive optical time-domain reflectometer and the length of the optical fiber are mutually constrained.

[0003] Frequency division multiplexing (FDM) is an effective method for significantly broadening the frequency response range of a sensing system without relying on prior information. Its core principle is to transmit multiple optical pulses of different frequencies in a timed sequence through an optical fiber, with the time interval between pulses of the same frequency being greater than the round-trip time of the optical signal in the fiber. At the receiving end, the system demodulates the Rayleigh scattered signals corresponding to the different frequencies and interweaves and fuses the multi-frequency demodulation results based on the pulse incidence sequence. This method enables efficient detection of shorter time intervals in the optical fiber under test, effectively improving the frequency response bandwidth of the phase-sensitive optical time-domain reflectometer.

[0004] However, the Rayleigh scattering signal at a specific location on an optical fiber is determined by the Rayleigh scattering signals generated by randomly distributed scattering points around it. Because light of different wavelengths produces different phases at the scattering points, even when the same strain is applied, the differential phase of light of different wavelengths can produce different results. In other words, the strain sensitivity coefficients of light of different wavelengths are inconsistent. A key step in frequency division multiplexing technology is to interleave the detection results of different frequencies. However, because the strain sensitivity coefficients of light of different frequencies are inconsistent, even when the applied strain signal is the same, the strain magnitudes detected by light pulses of different frequencies can differ. This results in additional harmonic frequency components in the power spectrum of the interleaved signal. Without prior knowledge, these harmonic frequency components cannot be distinguished from the true strain frequency components, ultimately affecting the practical use of optical time domain reflectometry.

[0005] Currently, the widespread problem of harmonics in the interleaved signal caused by the varying sensitivity coefficients of optical pulses of different frequencies significantly limits the effective application of high-frequency optical time-domain reflectometry based on frequency division multiplexing. Using machine learning to distinguish between true strain frequency components and harmonic components generally requires prior knowledge, and because harmonics are related to true strain signals, machine learning methods cannot guarantee their effectiveness. Furthermore, the randomness of Rayleigh scattering causes differences in the sensitivity coefficients of light of different frequencies, which is related to both the state of the optical fiber and the center frequency of the laser. If corrections are to be made by pre-measuring the sensitivity coefficients of light of different frequencies, it is necessary to ensure that the state of the optical fiber and the center frequency of the laser do not change significantly, which places stringent demands on the stability of the system.

[0006] For example, Chinese patent application CN116952358A discloses a phase-sensitive optical time-domain reflectometer based on a multi-frequency, non-uniform periodic pulse train. This achieves multi-frequency detection through non-uniform pulse spacing. However, the irregular pulse spacing requires high precision from the signal generator, making system implementation complex. Furthermore, non-uniform sampling can lead to spectral aliasing, limiting signal reconstructability and placing strict restrictions on the spectral width of the measured signal.

[0007] For example, Chinese patent application CN112697181A proposes a frequency-division multiplexing (FDM) Φ-OTDR system based on direct detection. This direct detection scheme is sensitive to variations in light intensity and is easily affected by environmental disturbances, leading to large phase demodulation errors. Furthermore, the direct input of the probe light into a photodetector for acquisition limits measurement sensitivity and signal-to-noise ratio.

[0008] In summary, existing technologies have certain limitations in detection accuracy and system stability: 1. They have narrowband requirements for the signal to be measured, which makes it difficult to meet the stable reconstruction of broadband signals; 2. The signal-to-noise ratio and sensitivity are low, and the measurement accuracy is limited; 3. None of them corrects the harmonic distortion problem that appears in the signal spectrum after frequency division multiplexing interleaving, which easily causes deviation in the measurement results. Summary of the Invention

[0009] The purpose of the present invention is to overcome the defects of the prior art and provide a phase-sensitive optical time domain reflectometer based on a parallel frequency division multiplexing sequence.

[0010] The purpose of the present invention can be achieved by the following technical solutions:

[0011] A phase-sensitive optical time-domain reflectometer based on a parallel frequency division multiplexing sequence, comprising a continuous laser, an optical coupler, a signal generation circuit module, an optical intensity modulator, an optical circulator, a sensing optical fiber to be measured, a polarization coherent receiving module, and a signal acquisition circuit;

[0012] The continuous laser generates a continuous laser which is divided into a local path and a detection path through an optical coupler; wherein the local path is input to a polarization coherent receiving module; and the detection path is incident to a light intensity modulator;

[0013] The signal generating circuit module generates a plurality of frequency division multiplexing sequence combinations of radio frequency signals and inputs them into the optical intensity modulator; in the plurality of frequency division multiplexing sequence combinations, the sampling interval of each frequency division multiplexing sequence is the same, the frequency division number within different groups and the repetition period of the same frequency within the group are different, and the multiple frequency division multiplexing sequences are staggered in time; the repetition interval of the same frequency pulses of each frequency division multiplexing sequence is greater than the round-trip time of light in the sensing optical fiber;

[0014] The optical intensity modulator modulates the continuous laser into a pulse laser sequence composed of multiple frequency division multiplexing sequences based on the radio frequency signal; the pulse laser sequence enters the sensing optical fiber to be tested through the first port of the optical circulator;

[0015] The Rayleigh backscattered light generated by the sensing optical fiber to be tested is received by the polarization coherent receiving module through the optical circulator and the local continuous laser, and the beat frequency result of the local continuous laser and the backscattered Rayleigh light is converted into an electrical signal and output to the signal acquisition circuit;

[0016] The signal acquisition circuit samples the backscattered Rayleigh signal, demodulates the signals of different frequency division multiplexing sequences respectively, and then interleaves them; by analyzing the spectrum of the interleaved signal, the harmonic frequency components and the real signal frequency components are distinguished, and the dynamic strain signal is restored.

[0017] As a preferred technical solution, the continuous laser is a narrow-linewidth single-frequency laser.

[0018] As an optimal technical solution, the optical coupler adopts a polarization-maintaining optical coupler, and most of the continuous laser is incident on the light intensity modulator as the detection light, and another small part of the continuous laser is incident on the input end of the polarization coherent receiving module as the local oscillator light, which is used to beat with the backscattered Rayleigh scattering signal.

[0019] As a preferred technical solution, the optical intensity modulator adopts a polarization-maintaining fiber acousto-optic modulator, and the input end and the output end of the optical intensity modulator are both polarization-maintaining jumpers.

[0020] As a preferred technical solution, an optical amplifier is provided between the optical intensity modulator and the optical circulator, and the optical amplifier is used to amplify the power of the pulse laser sequence and inject the amplified power into the optical circulator.

[0021] As a preferred technical solution, the frequency division multiplexing sequence generated by the signal generating circuit module includes two groups of different frequency division multiplexing sequences;

[0022] The frequency multiplexing number of the first frequency division multiplexing sequence is 2, the width of each pulse is 15μs, the sweep frequency ranges are 260-280MHz and 280-300MHz respectively, the repetition interval of pulses of the same frequency is 500μs, and the sampling interval between pulses is 250μs;

[0023] The frequency multiplexing number of the second frequency division multiplexing sequence is 3, the width of each pulse is 15 μs, the sweep frequency ranges are 300-320 MHz, 320-340 MHz, and 340-360 MHz, the repetition interval of pulses of the same frequency is 750 μs, and the sampling interval between pulses is 250 μs.

[0024] As a preferred technical solution, the optical circulator is a single-mode optical circulator;

[0025] The first port of the optical circulator is connected to an optical amplifier, guiding the pulse laser sequence to pass through the second port of the optical circulator into the sensing optical fiber to be measured;

[0026] The Rayleigh backscattered signal generated by the sensing optical fiber to be measured enters the polarization coherent receiving module through the third port of the optical circulator.

[0027] As an optimal technical solution, the polarization coherent receiving module adopts a polarization diversity coherent receiver, which divides the local oscillator path into two orthogonal polarization states and beats them with the Rayleigh backscattered signal returned from the third port of the optical circulator, converts the optical phase of the Rayleigh backscattered signal into an electrical signal and transmits it to the signal acquisition circuit.

[0028] As a preferred technical solution, the signal acquisition circuit includes a high-speed digital acquisition card and a computer; the high-speed acquisition card has a programmable function, and the computer is used to process the electrical signal.

[0029] As a preferred technical solution, the specific processing flow of the signal acquisition circuit to process the dynamic strain signal is as follows:

[0030] The signals of all frequency channels are demodulated independently. For each frequency sweep pulse, it is used as a matched filter to perform a cross-correlation operation with the collected electrical signal. The output of the cross-correlation operation is further processed by time difference and spatial difference to complete the demodulation of the vibration information:

[0031]

[0032] Among them, s k (t) is the kth frequency sweep pulse signal; h(t) is the impulse response of the optical fiber; is the correlation operation; T is the sampling interval of each group of frequency division multiplexing sequences;

[0033] Interleave multiple frequency division multiplexing sequences according to the order in which they are injected into the optical fiber at different frequencies, and perform Fourier transform on the interleaved time domain signals of the frequency division multiplexing sequences to obtain the spectrum result;

[0034] When the spectrum amplitudes of multiple groups of frequency division multiplexing sequences are inconsistent at a certain frequency f, the frequency is determined to be a harmonic position, and a group of frequency division multiplexing sequences with a smaller amplitude at frequency f is selected as the retained value.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] 1) A multi-frequency pulse sequence with multiple uniform periods but different frequency division ratios was designed and combined with a coherent detection structure to achieve high-sensitivity signal reception. Furthermore, a harmonic correction algorithm was introduced to effectively eliminate spectral distortion, targeting the spectral characteristics of the interleaved signal, thereby achieving high-precision, high-bandwidth vibration signal measurement. This system exhibits excellent high-frequency response performance and high-precision strain measurement capabilities, while also offering long-range coverage. It is suitable for applications requiring high-precision high-frequency strain measurement accuracy, such as bridge structural health monitoring and perimeter security, and has broad engineering application value.

[0037] 2) During the signal processing phase, the present invention independently demodulates the beat frequency results of local light and backscattered Rayleigh light across all frequency channels. The frequency-division multiplexed sequences are then interleaved according to the order in which they were injected into the optical fiber, and converted to the frequency domain for analysis. Comparative analysis of the interleaved results across multiple spectrum groups, based on the spectral characteristics of the interleaved signal, distinguishes between signal and harmonic frequencies, enabling effective identification and correction of harmonics, completely suppressing harmonic components, and improving the accuracy and signal-to-noise ratio of high-frequency signal measurements.

[0038] 3) The phase-sensitive optical time-domain reflectometer device proposed in the present invention can reduce the impact of transient effects of optical amplifiers and avoid nonlinear effects of polarization coherent receivers caused by excessive optical power by modulating the detection laser with an interleaved frequency division multiplexing sequence. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a schematic structural diagram of a phase-sensitive optical time domain reflectometer based on a parallel frequency division multiplexing sequence according to the present invention;

[0040] Figure 2 This is a schematic structural diagram of one embodiment of a phase-sensitive optical time-domain reflectometer based on a parallel frequency division multiplexing sequence according to the present invention;

[0041] Figure 3 Schematic diagram of a frequency division multiplexing pulse sequence generated by a signal generating circuit in an embodiment of the present invention;

[0042] Figure 4Graph showing actual test results of the distribution of demodulation phase standard deviation versus distance in an embodiment of the present invention;

[0043] Figure 5 This is a diagram showing the actual test results of the external strain sensing in an embodiment of the present invention;

[0044] The numbers in the figure are as follows: 1. Continuous laser, 2. Optical coupler, 3. Signal generation circuit module, 4. Light intensity modulator, 5. Optical amplifier, 6. Optical circulator, 7. Sensing fiber to be tested, 8. Polarization coherent receiving module, 9. Signal acquisition circuit. DETAILED DESCRIPTION

[0045] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0046] Example 1

[0047] The present invention proposes a phase-sensitive optical time domain reflectometer system based on parallel frequency division multiplexing sequence, such as Figure 1 As shown, the phase-sensitive optical time-domain reflectometer specifically includes a continuous laser 1, an optical coupler 2, a signal generating circuit module 3, an optical intensity modulator 4, an optical amplifier 5, an optical circulator 6, a sensing fiber to be measured 7, a polarization coherent receiving module 8 and a signal acquisition circuit 9. Among them, the continuous laser 1 generates laser light and is incident on the input end of the optical coupler 2; the optical coupler 2 divides the laser light into two branches, a local path and a detection path, wherein the local path is input to the input end of the polarization coherent receiving module 8, and the detection path is incident on the input end of the optical intensity modulator 4; the signal generating circuit module 3 generates a plurality of frequency division multiplexing sequence combinations of radio frequency signals and inputs them to the optical intensity modulator 4; the optical intensity modulator 4 modulates the continuous laser light into a plurality of frequency division multiplexing sequence combinations of pulsed laser light and is incident on the input end of the optical amplifier 5; the optical amplifier 5 The power of the pulsed laser sequence is amplified and incident on an optical circulator 6. This circulator then injects the pulsed laser sequence into the sensing fiber 7 under test. The Rayleigh backscattered light generated in the sensing fiber 7 passes through the optical circulator and is incident on the input of a polarization-coherent receiver module 8. The polarization-coherent receiver module 8 converts the beat frequency of the local light and the Rayleigh backscattered light into an electrical signal and outputs it to a signal acquisition circuit 9. The signal acquisition circuit 9 samples the Rayleigh backscattered signal, demodulates the signals of different frequency-division multiplexing sequences, and then interleaves them. By analyzing the spectrum of the interleaved signal, it is possible to effectively distinguish between harmonic frequency components and true signal frequency components, thereby restoring the dynamic strain signal at a high sampling rate.

[0048] The working principle of the phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence is as follows: first, a continuous laser 1 generates laser light; the continuous laser light is divided into a local path and a detection path after passing through an optical coupler 2; the local path is directly input into a polarization coherent receiving module 8; the detection path is converted into a pulse laser sequence with intensity modulation after passing through an optical intensity modulator 4. The optical intensity modulator 4 is driven by a radio frequency signal provided by a signal generating circuit module 3, and the radio frequency signal is composed of two different frequency division multiplexing sequences; the pulse laser sequence is power amplified by an optical amplifier 5 and then enters the sensing fiber 7 to be measured through an optical circulator 7; the Rayleigh backscattered light generated by the sensing fiber 7 to be measured passes through the circulator 6 and is beat-frequency coherently received by the local continuous laser light in the polarization coherent receiving module 8; the signal acquisition circuit 9 samples and analyzes the interference signal, and by performing frequency domain comparison on the interleaving results of the two frequency division multiplexing pulse sequences, the harmonic frequencies are identified and eliminated, and only the true strain signal frequency is retained, thereby realizing harmonic-free and high-sampling-rate strain signal measurement. The device provided by this invention uses multi-frequency laser pulses to measure optical fiber, acquiring more response data per unit time. This significantly increases the sampling rate and expands the system's measurable frequency range compared to traditional single-pulse solutions. By comparing and analyzing the results of dual-sequence interleaving, it effectively distinguishes between signal and harmonic frequencies, completely suppressing harmonic components and improving the accuracy and signal-to-noise ratio of high-frequency signal measurements. The system boasts excellent high-frequency response performance and high-precision strain measurement capabilities, while also offering long-distance coverage. It is suitable for applications requiring high-precision high-frequency strain measurement, such as bridge structural health monitoring and perimeter security, and has broad engineering applications.

[0049] like Figure 2 As shown, in this embodiment, the continuous laser 1 is a narrow-linewidth single-frequency laser with a nominal output frequency of 1550.12 nm, an output linewidth of 100 Hz, and an output power of 20 mW.

[0050] The optical coupler 2 uses a polarization-maintaining optical coupler with a nominal light splitting ratio of 90:10. The 90% end is incident on the light intensity modulator 4 as the detection light, and the 10% end is incident on the input end of the polarization coherent receiving module 8 as the local oscillator light, which is used to beat the backscattered Rayleigh scattered signal and amplify it.

[0051] The optical intensity modulator 4 uses a 1550nm polarization-maintaining fiber acousto-optic modulator, which has a nominal acousto-optic crystal frequency shift of 350MHz, an extinction ratio greater than 50dB, an insertion loss less than 4dB, and a bandwidth of 240-400MHz. Both its input and output ends are polarization-maintaining jumpers.

[0052] Signal generation circuit module 3 is an arbitrary waveform generator (AWG) with a nominal sampling rate of 1.25GSa / s, an analog bandwidth of 500MHz, and a storage capacity of 64MSa. It has two independent output channels and one synchronous trigger channel. One independent output channel of the AWG generates a frequency-division multiplexing sequence with multiple frequency references. To ensure the normal operation of the AOM, the RF signal frequency provided by the AWG must be consistent with the AOM's bandwidth.

[0053] The frequency division multiplexing sequence contains multiple groups of different frequency division multiplexing sequences. The sampling interval of each group of frequency division multiplexing sequences is the same, but the frequency division number within different groups and the repetition period of the same frequency within the group are different, and they are staggered in time. Since the sampling interval of each group of frequency division multiplexing sequences is the same, the spectrum range covered by different groups of sequences is consistent during the spectrum reconstruction process, that is, the position of the real vibration signal in each group of spectra should be consistent. However, the position of the occurrence of the harmonic frequency component is not only related to the frequency of the vibration signal itself, but also affected by the repetition period of the same frequency pulse in the sequence in which it is located. Since each group of frequency division multiplexing sequences in the present application has a different repetition period, even if it is the same vibration signal, the harmonic frequency components it induces in different groups of sequences will appear at different frequency positions. Based on this characteristic, if a certain frequency component shows consistent intensity and position in all spectra, it can be judged as a real vibration signal; if a certain frequency component has inconsistent intensity or misplaced position in different groups of spectra, it can be identified as a harmonic interference component. Through cross-comparison and consistency judgment between multiple sets of spectrum, the system can effectively identify and correct harmonics, significantly improving the accuracy and stability of signal demodulation.

[0054] The frequency division multiplexing sequence used in this embodiment is as follows Figure 3 As shown, it contains 2 groups of different frequency division multiplexing sequences. The frequency multiplexing number of the first group of frequency division multiplexing sequences is N1=2, the width of each pulse is 15μs, the sweep frequency ranges are 260-280MHz and 280-300MHz, the repetition interval of pulses with the same frequency is T1=500μs, and the sampling interval between pulses is T=250μs. The frequency multiplexing number of the second group of frequency division multiplexing sequences is N2=3, the width of each pulse is 15μs, the sweep frequency ranges are 300-320MHz, 320-340MHz, and 340-360MHz, the repetition interval of pulses with the same frequency is T2=750μs, and the sampling interval between pulses is T=250μs. The repetition interval of pulses with the same frequency is greater than the round-trip time of light in the sensing fiber. The complete pulse sequence period T global The RF output amplitude of the arbitrary waveform generator is 250mV.

[0055] Optical amplifier 5 is a pulse fiber erbium-doped fiber amplifier with a nominal operating wavelength of 1550.12 nm, a maximum input peak optical power of 1 dBm, an output peak optical power of 1000 mW, and a noise figure of 4.5 dB.

[0056] Optical circulator 6 uses a single-mode optical circulator with a nominal insertion loss of less than 1.0dB, an isolation exceeding 60dB, and a return loss exceeding 50dB. Port 1 of optical circulator 6 is connected to optical amplifier 5, which guides the pulsed laser sequence through port 2 and into the sensor fiber 7 to be tested.

[0057] The sensing fiber 7 to be tested uses G.652.D single-mode fiber, which has a nominal sensing distance of about 50.0km and a transmission loss of less than 0.2dB / km@1550nm. The pulse laser sequence excites Rayleigh scattering along the single-mode fiber, and part of the scattered light propagates in the opposite direction along the fiber to form a Rayleigh backscattering signal.

[0058] Polarization-coherent receiving module 8 utilizes a polarization-diversity coherent receiver with a nominal operating wavelength of 1510-1590 nm, a 3dB bandwidth of AC-400 MHz, a detector responsivity of 0.95 A / W at 1550 nm, and a transimpedance gain of 10 kV / A. This polarization-diversity coherent receiver splits the local oscillator into two orthogonal polarization states, which are then frequency-beated with the Rayleigh backscattered signal returning from port 3 of optical circulator 6. The optical phase of the Rayleigh backscattered signal is then converted into an electrical signal and transmitted to the high-speed digital acquisition card in signal acquisition circuit 9.

[0059] Signal acquisition circuit 9 consists of a high-speed digital acquisition card and a computer. The high-speed acquisition card is programmable, with a nominal analog bandwidth of 400 MHz, a sampling rate of 1 GSa / s, and a resolution of 16 bits. The acquisition card is set to single-record acquisition mode and runs continuously throughout the test, with a cumulative acquisition time of 0.6 seconds.

[0060] The computer completes subsequent data processing, including independent signal demodulation of the frequency division multiplexing sequence and identifying and eliminating harmonics by comparing the spectrum of the demodulated signals of each frequency division multiplexing sequence. The specific processing flow is: the matched filter corresponding to each sweep pulse is cross-correlated with the collected electrical signal to distinguish the aliased Rayleigh scattered signal. Assume that the kth sweep pulse signal is:

[0061]

[0062] Among them, A k is the amplitude of the kth sweep pulse, f k is the starting frequency of the kth sweep pulse, K is the sweep rate, τ pis the pulse width. Let h(t) be the impulse response of the optical fiber and T be the sampling interval of each frequency division multiplexing sequence. The aliased Rayleigh scattering signal can be expressed as:

[0063]

[0064] in, For related operations.

[0065] For a linear frequency swept pulse, the matched filter is itself, that is:

[0066]

[0067] For linear sweep pulses with different sweep ranges, they are orthogonal to each other, that is:

[0068]

[0069] In order to extract the Rayleigh scattering signal component corresponding to a specific frequency sweep pulse, the matched filter of the pulse can be used to perform a cross-correlation operation on the overall aliased signal, that is:

[0070]

[0071] The cross-correlation output of this channel is further processed using temporal and spatial differentiation to complete the demodulation of the vibration information. Using frequency-sweep pulses of different frequencies to construct corresponding matched filters, the signals of all frequency channels are independently demodulated, achieving a spatial resolution of 10m.

[0072] When the sensing fiber 7 to be tested is subjected to external strain disturbance, the phase of the corresponding Rayleigh scattering signal will change. The two frequency division multiplexing sequences are interleaved in the order in which they are injected into the optical fiber at different frequencies. Since the sampling interval T of the two frequency division multiplexing sequences is the same, the corresponding equivalent sampling rates are the same, that is:

[0073] FFT1(f),FFT2(f) 0≤f≤1 / T

[0074] Among them, FFT1(f) is the spectrum result obtained by Fourier transforming the time domain signal interleaved with the first set of frequency division multiplexing sequences, and FFT1(f) is the spectrum result obtained by Fourier transforming the time domain signal interleaved with the second set of frequency division multiplexing sequences. Since the two sets of sequences have the same sampling interval T, their spectrum ranges f are consistent. For a real vibration signal, its frequency component f v It should appear at the same position in both sets of spectra and have the same amplitude, that is:

[0075] |FFT1(f v )|=|FFT2(f v )|

[0076] Harmonics appear at |f v ±1 / T single | position, where T single is the repetition interval of the same frequency pulse in the frequency division multiplexing sequence group. Since the group repetition intervals T1 and T2 of the two frequency division multiplexing sequences are different, the harmonic positions on the spectrum of the interleaved signal will be located at |f v ±1 / T1| and |f v ±1 / T2|, therefore, when the amplitudes of two sets of spectra are inconsistent at a certain frequency f, it can be determined that the frequency is a harmonic position. To eliminate this interference, the set with the smaller amplitude at this frequency is strategically selected as the retained value, that is:

[0077] S(f)=min(|FFT1(f),FFT2(f)|)

[0078] Through the above method, the true vibration signal component can be effectively retained, while the harmonic interference caused by different frequency sensitivities and offset differences can be suppressed, achieving high-precision and high-response vibration signal measurement.

[0079] For the phase-sensitive optical time-domain reflectometer solution based on parallel frequency division multiplexing sequence in the embodiment of the present invention, Figure 4 The phase standard deviation obtained after demodulation of all frequency pulses varies with the distance of the 50km sensing fiber, verifying the effective detection capability of this method within a 50km range. To further verify the system's ability to sense external strain, we applied a 900-1100Hz swept frequency strain signal to the end of the sensing fiber. Figure 5 The figure shows the Fourier transform amplitude spectrum after direct interleaving and compensation, with the effective spectrum range of 0-2000 Hz and the symmetrical spectrum range of 2000-4000 Hz. Experimental results demonstrate that the proposed method can completely eliminate all harmonics generated by interleaving and stably and accurately reconstruct high-frequency dynamic strain signals, validating its applicability and effectiveness in high-fidelity sensing scenarios.

[0080] The above describes in detail the preferred embodiments of the present invention. It should be understood that those skilled in the art can make numerous modifications and variations based on the concepts of the present invention without inventive effort. Therefore, any technical solutions that can be derived by those skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art should be within the scope of protection defined by the claims.

Claims

1. A phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence, characterized in that: The phase-sensitive optical time-domain reflectometer comprises a continuous laser (1), an optical coupler (2), a signal generation circuit module (3), an optical intensity modulator (4), an optical circulator (6), a sensing optical fiber to be measured (7), a polarization coherent receiving module (8), and a signal acquisition circuit (9); The continuous laser (1) generates continuous laser light which is divided into a local path and a detection path through an optical coupler (2); wherein the local path is input to a polarization coherent receiving module (8); and the detection path is incident to a light intensity modulator (4); The signal generating circuit module (3) generates a plurality of frequency division multiplexing sequence combinations of radio frequency signals which are input to the optical intensity modulator (4); in the plurality of frequency division multiplexing sequence combinations, the sampling interval of each frequency division multiplexing sequence is the same, the frequency division numbers within different groups and the repetition period of the same frequency within the group are different, and the plurality of frequency division multiplexing sequences are staggered in time; the repetition interval of the same frequency pulses of each frequency division multiplexing sequence is greater than the round trip time of light in the sensing optical fiber; The optical intensity modulator (4) modulates the continuous laser into a pulse laser sequence composed of a plurality of frequency division multiplexing sequences based on the radio frequency signal; the pulse laser sequence enters the sensing optical fiber (7) to be tested through the first port of the optical circulator (6); The Rayleigh backscattered light generated by the sensing optical fiber to be measured (7) is received by the polarization coherent receiving module (8) through the optical circulator (6) and the local continuous laser for beat frequency coherence, and the beat frequency result of the local continuous laser and the backscattered Rayleigh light is converted into an electrical signal and output to the signal acquisition circuit (9); The signal acquisition circuit (9) samples the backscattered Rayleigh scattering signal, demodulates the signals of different frequency division multiplexing sequences respectively, and then interleaves them; and analyzes the spectrum of the interleaved signal to distinguish the harmonic frequency components from the real signal frequency components, thereby restoring the dynamic strain signal.

2. The phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence according to claim 1, characterized in that: The continuous laser (1) adopts a narrow linewidth single-frequency laser.

3. The phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence according to claim 1, characterized in that: The optical coupler (2) adopts a polarization-maintaining optical coupler, and most of the continuous laser light is incident on the light intensity modulator (8) as detection light, and another small part of the continuous laser light is incident on the input end of the polarization coherent receiving module (4) as local oscillator light, which is used to beat with the back Rayleigh scattering signal.

4. The phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence according to claim 1, characterized in that: The light intensity modulator (4) adopts a polarization-maintaining fiber acousto-optic modulator, and both the input end and the output end of the light intensity modulator (4) are polarization-maintaining jumpers.

5. The phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence according to claim 1, characterized in that: An optical amplifier (5) is provided between the optical intensity modulator (4) and the optical circulator (6), and the optical amplifier (5) is used to amplify the power of the pulse laser sequence and inject the amplified power into the optical circulator (6).

6. The phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence according to claim 1, characterized in that: The frequency division multiplexing sequence generated by the signal generating circuit module (3) includes two groups of different frequency division multiplexing sequences; The frequency multiplexing number of the first frequency division multiplexing sequence is 2, the width of each pulse is 15μs, the sweep frequency ranges are 260-280MHz and 280-300MHz respectively, the repetition interval of pulses of the same frequency is 500μs, and the sampling interval between pulses is 250μs; The frequency multiplexing number of the second frequency division multiplexing sequence is 3, the width of each pulse is 15 μs, the sweep frequency ranges are 300-320 MHz, 320-340 MHz, and 340-360 MHz, the repetition interval of pulses of the same frequency is 750 μs, and the sampling interval between pulses is 250 μs.

7. The phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence according to claim 1, characterized in that: The optical circulator (6) is a single-mode optical circulator; The first port of the optical circulator (6) is connected to the optical amplifier (5), guiding the pulse laser sequence to pass through the second port of the optical circulator (6) and enter the sensing optical fiber (7) to be tested; The Rayleigh backscattered signal generated by the sensing optical fiber to be measured (7) enters the polarization coherent receiving module (8) through the third port of the optical circulator (6).

8. The phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence according to claim 1, characterized in that: The polarization coherent receiving module (8) adopts a polarization diversity coherent receiver, divides the local oscillator path into two orthogonal polarization states, and respectively performs frequency beats with the Rayleigh backscattering signal returned from the third port of the optical circulator, converts the optical phase of the Rayleigh backscattering signal into an electrical signal, and transmits it to the signal acquisition circuit (9).

9. The phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence according to claim 1, characterized in that: The signal acquisition circuit (9) comprises a high-speed digital acquisition card and a computer; the high-speed acquisition card has a programmable function, and the computer is used for performing data processing on the electrical signal.

10. The phase-sensitive optical time-domain reflectometer based on parallel frequency division multiplexing sequence according to claim 1, characterized in that: The specific process of the signal acquisition circuit (9) acquiring the dynamic strain signal is as follows: The signals of all frequency channels are demodulated independently. For each frequency sweep pulse, it is used as the corresponding matched filter to perform cross-correlation operation with the collected electrical signal. The output of the cross-correlation operation is further processed by time difference and spatial difference to complete the demodulation of the vibration information: Among them, s k (t) is the kth frequency sweep pulse signal; h(t) is the impulse response of the optical fiber; is the correlation operation; T is the sampling interval of each group of frequency division multiplexing sequences; Use swept pulses of different frequencies to construct corresponding matched filters. Interleave multiple frequency division multiplexing sequences according to the order in which they are injected into the optical fiber at different frequencies, and perform Fourier transform on the interleaved time domain signals of the frequency division multiplexing sequences to obtain the spectrum result; When the spectrum amplitudes of multiple groups of frequency division multiplexing sequences are inconsistent at a certain frequency f, the frequency is determined to be a harmonic position, and a group of frequency division multiplexing sequences with a smaller amplitude at frequency f is selected as the retained value.

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