A phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding

Abstract

This invention relates to a phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing (OFDM) pulse coding. A continuous laser beam is split into a local path and a probe path via an optical coupler. The probe path, driven by a radio frequency signal, generates multiple frequency-orthogonal intensity-coded pulse sequences through an optical intensity modulator. These sequences are arranged sequentially in time, possessing the same symbol information but different chip sequences. The Rayleigh backscattered light generated by the pulse sequences entering the sensing fiber under test undergoes coherent interference with the local light in an optical bridge and polarization diversity module. The signal acquisition circuit utilizes an unmatched filter to achieve pulse compression and Rayleigh scattering signal separation, and synthesizes the results of the pulse coding sequences at each frequency to obtain the strain signal. Compared with existing technologies, this invention achieves pulse compression by designing an unmatched filter, improving the signal-to-noise ratio while maintaining spatial resolution. Furthermore, by synthesizing the results of orthogonal frequency pulse sequences, coherent fading is eliminated, achieving fading-free, high signal-to-noise ratio strain measurement.

Description

Technical Field

[0001] This invention relates to the field of optical measurement and sensing technology, and provides a phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding. Background Technology

[0002] Phase-sensitive optical time-domain reflectometry (Φ-OTDR) utilizes optical pulses to probe the fiber under test, and demodulates strain information at different locations on the fiber using Rayleigh backscattered light generated during pulse transmission. In traditional Φ-OTDR systems using narrow pulses, the pulse propagates through the fiber and interacts with minute refractive index inhomogeneities, producing continuous Rayleigh backscattered light. By detecting this backscattered light and demodulating its phase information, minute strain changes at various locations along the fiber can be obtained, enabling monitoring of structural health or perimeter safety. However, traditional single-frequency narrow-pulse Φ-OTDR systems are often limited in long-distance sensing by problems such as low input energy, significant coherent fading, and insufficient signal-to-noise ratio, making it difficult to simultaneously meet the requirements of high spatial resolution and long-distance measurement.

[0003] Pulse coding technology has been widely studied to improve the detection performance of phase-sensitive optical time-domain reflectometers (OTDRs). Its basic principle is to modulate a continuous laser beam into a pulse sequence with a specific intensity code, and then compress the pulse at the receiver to increase the input power and signal-to-noise ratio while maintaining spatial resolution. Intensity coding modulates the incident light power using a specific sequence of amplitudes, giving the Rayleigh backscattered signal generated by the probe pulse during fiber transmission a recognizable coded characteristic. This allows for signal compression and demodulation at the receiver using signal processing methods such as matched filtering. Compared to traditional single-pulse schemes, intensity coding can significantly increase the average input power while maintaining spatial resolution, improving long-distance detection capabilities. It has become one of the important directions for improving the performance of phase-sensitive OTDs in recent years. However, existing intensity coding methods mostly rely on specially designed pseudo-random sequences or coding schemes with specific autocorrelation characteristics, such as Golay codes and Simplex codes. These require precise design and optimization for different application scenarios or system parameters to achieve optimal pulse compression results, limiting the system's flexibility and versatility.

[0004] Meanwhile, in phase-sensitive optical time-domain reflectometry (OTDR) systems, Rayleigh scattering is a random superposition of scattering caused by minute refractive index fluctuations in the optical fiber. The phase of the scattered light is random, easily leading to coherent fading during coherent superposition. Coherent fading refers to the random spatial or temporal fluctuations in the intensity of the interference signal generated by coherent superposition. When the scattered light phases cancel each other out, a local signal fading region is formed, severely affecting the sensing results at that location. In traditional phase-sensitive OTDs, single-frequency narrow-pulse detection is particularly sensitive to coherent fading, often causing a sharp drop in the local measurement signal-to-noise ratio, limiting its application in long-distance, high-precision strain detection. Therefore, effectively suppressing coherent fading and improving the stability and reliability of the detection signal has always been one of the important research directions in phase-sensitive OTD technology.

[0005] Chinese patent application CN116576955A proposes a Φ-OTDR sensing system based on composite modulation pulse coding. This scheme improves spatial resolution while maintaining a high signal-to-noise ratio by introducing a composite modulation mechanism of inter-symbol phase modulation and intra-symbol linear frequency modulation. However, this scheme has the following drawbacks: First, the system complexity is high, as its modulation structure relies on high-precision synchronous control between the FPGA, DDS module, and voltage-controlled oscillator, making system integration and real-time control difficult. Second, the superposition of phase modulation and linear frequency modulation may introduce phase aliasing in the frequency domain, leading to spectral broadening and potential misjudgments or distortions during demodulation, affecting positioning and quantitative accuracy.

[0006] Chinese patent application CN114061736A proposes a Rayleigh distributed phase demodulation fiber optic sensing method and system based on frequency and intensity encoding and decoding. This scheme employs joint frequency and intensity encoding, constructing a decoding matrix and combining it with a cross-correlation algorithm to achieve phase demodulation of the Rayleigh scattering signal in the fiber. Its advantage lies in its relatively simple system structure and low linewidth requirements for the light source. However, this scheme relies on a cyclic encoding matrix, which, while reducing the time overhead of the modulation sequence, still limits the overall modulation accuracy due to the laser's frequency control capability, especially exhibiting a performance bottleneck in multi-frequency modulation scenarios. Furthermore, this scheme uses a direct detection method without introducing a coherent detection mechanism, resulting in a limited signal-to-noise ratio in complex interference environments, thus affecting the strain demodulation accuracy.

[0007] In summary, the two existing technologies mentioned above still have certain limitations in terms of improving detection accuracy and controlling system complexity. These limitations are mainly reflected in the following aspects: (1) They require the use of lasers to achieve frequency modulation or the introduction of additional modulation units to simultaneously achieve phase modulation and linear frequency modulation, resulting in complex optical paths and high control difficulty; (2) They have high requirements for the coding modulation at the detection end, and the stability and preset characteristics of the modulation structure must be guaranteed; (3) Neither of them has proposed an effective suppression strategy for coherent fading, which may lead to the risk of signal distortion and measurement errors in practical applications. The overall anti-interference performance still needs to be improved. Summary of the Invention

[0008] The purpose of this invention is to solve the problems of complex optical paths, high control difficulty, high requirements for coding and modulation at the detection end, and coherent fading in existing technical solutions, and to provide a phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding.

[0009] The objective of this invention can be achieved through the following technical solutions:

[0010] A phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding, the phase-sensitive optical time-domain reflectometer includes a continuous laser, an optical coupler, a signal generation circuit module, an optical intensity modulator, an optical circulator, a sensor link under test, an optical bridge and a polarization diversity module, a photodetector, and a signal acquisition circuit;

[0011] The continuous laser generates continuous laser light, which is split into a local path and a probe path by an optical coupler; the local path is input to the input of the optical bridge and the polarization diversity module, and the probe path is incident on the input of the optical intensity modulator.

[0012] The signal generation circuit module generates an orthogonal frequency division multiplexing intensity-coded radio frequency signal, which is input to the optical intensity modulator. The orthogonal frequency division multiplexing intensity-coded radio frequency signal contains multiple sets of intensity-coded sequences with mutually orthogonal frequencies. The multiple sets of intensity-coded sequences are arranged sequentially in time. The symbol information of the multiple sets of intensity-coded sequences is the same, but the chips of the modulation symbols are different.

[0013] The optical intensity modulator modulates continuous laser light into multiple frequency-orthogonal intensity-coded pulse sequences based on radio frequency signals and then incident them onto the optical fiber of the sensing fiber under test via an optical circulator.

[0014] The Rayleigh backscattered light generated in the optical fiber under test is coherently interfered with the local continuous laser in the optical bridge and polarization diversity module through a circulator, and the beat frequency signal is received by a photodetector.

[0015] The signal acquisition circuit samples and analyzes the beat frequency signal output by the photodetector. By using an unmatched filter obtained by chip modulation of the matched symbol sequence, the acquired electrical signal is pulse-compressed and the intensity-coded Rayleigh scattering signals corresponding to different frequency components are separated. The detection results of each frequency intensity-coded sequence are vector-synthesized to obtain the strain signal.

[0016] As a preferred technical solution, the continuous laser is a narrow linewidth single-frequency laser.

[0017] As a preferred technical solution, the optical coupler adopts a polarization-maintaining optical coupler, in which most of the continuous laser is incident on the optical intensity modulator as the probe light, and a small portion of the continuous laser is incident on the input end of the optical bridge and polarization diversity module as the local oscillator light, which is used to beat the back Rayleigh scattering signal and amplify it.

[0018] As a preferred technical solution, the signal generation circuit module employs an arbitrary waveform generator. The process by which the signal generation circuit module generates the orthogonal frequency division multiplexing (OFDM) intensity-coded sequence is as follows:

[0019] Generate a binary strength-coded symbol sequence c, and modulate the symbol sequence c with multiple mutually orthogonal chips to obtain the coded pulse sequence s. i (t):

[0020]

[0021] In the formula, K is the length of the strength-coded symbol sequence; c k Indicates the k-th code element; w i T represents i mutually orthogonal chips; c The pulse width of the chip.

[0022] As a preferred technical solution, the input end of the light intensity modulator adopts a polarization-maintaining jumper, and the output end adopts a single-mode jumper.

[0023] As a preferred technical solution, the optical circulator is a single-mode optical circulator; the first port of the optical circulator is connected to the optical intensity modulator, the second port is connected to the optical fiber under test, and the third port is connected to the input of the optical bridge and polarization diversity module.

[0024] As a preferred technical solution, the optical bridge and polarization diversity module adopts an optical mixer with an integrated polarization beam splitter and a 90° optical bridge. The phase angle between the I-path and Q-path is 85° to 95°. The optical mixer is used to split the local oscillator light and the signal light and perform optical interference with phase differences of 0° and 90° respectively. At the same time, it performs orthogonal polarization beam splitting on the input light so that signals with different polarization states are interfered and superimposed with the local oscillator light. The generated I-path and Q-path interference signals are output to the back-end balanced photodetector.

[0025] As a preferred technical solution, the photodetector is a balanced photodetector, which converts the coherently superimposed interference light field into an electrical signal. The signal is differentially amplified to suppress local oscillator DC noise and common-mode noise, while enhancing the beat frequency signal.

[0026] As a preferred technical solution, the signal acquisition circuit includes a high-speed digital acquisition card and a computer, and the electrical signal output by the photodetector is processed as follows:

[0027] The unmatched filter corresponding to each different orthogonal frequency intensity coding sequence is convolved with the acquired electrical signal to achieve pulse compression and separate the intensity-coded Rayleigh scattering signal corresponding to different frequency components.

[0028] After the scattering signal is separated, the compression results of the intensity encoded pulse sequences of different orthogonal frequencies are subjected to time difference processing. Each record is multiplied by the first record using conjugate, and the time difference results of multiple orthogonal frequencies are vector-superimposed.

[0029] The vector superposition result is processed by spatial difference and moving average to obtain the final demodulation result.

[0030] As a preferred technical solution, the unmatched filter is obtained by modulating the matched symbol sequence u with chips; the matched symbol sequence u is obtained by solving the least squares problem min||Cu-b|| 2 The analytical solution obtained:

[0031] u=(C T C) -1 C T b

[0032] In the formula, C is the encoding matrix of the Toeplitz structure constructed based on the symbol sequence c; b is the ideal output;

[0033] The unmatched filter h is obtained by modulating the matched symbol sequence u with chips. i (t):

[0034]

[0035] In the formula, M is the length of the unmatched filter; u m For the m-th matching symbol sequence; w i For the mutually orthogonal chips set; T c The pulse width of the chip.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1) The phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing (OFDM) pulse coding proposed in this invention addresses both system simplification and performance enhancement. It achieves simultaneous intensity coding and OFDM using a single intensity modulator, enabling the measurement of Rayleigh scattering responses under different coherent interference conditions using only a few fixed frequencies. Furthermore, it designs unmatched filters for arbitrary intensity-coded sequences through signal post-processing algorithms, achieving pulse compression and coherent fading suppression without increasing system cost. This design significantly alleviates the conflict between signal-to-noise ratio and spatial resolution, reduces system complexity, and ultimately achieves high-precision, high-resolution measurement of strain signals in long-distance, high-interference environments, possessing broad engineering application value.

[0038] 2) The proposed solution uses only an intensity modulator to generate an intensity-coded pulse sequence composed of multiple orthogonal frequencies. An optical bridge and polarization diversity module are introduced to achieve coherent detection, thereby effectively preserving the phase information in Rayleigh scattering. In the signal processing stage, an unmatched filter is designed to compress the intensity-coded sequence, significantly improving the signal-to-noise ratio. Simultaneously, the scattering responses obtained from multiple orthogonal frequency channels under different coherent fading modes are synthesized to effectively suppress fading interference, achieving high-precision and robust strain measurement.

[0039] 3) The present invention proposes a pulse coding sequence with mutually orthogonal frequencies. Since different coding sequences do not interfere with each other, they can be demodulated independently, effectively avoiding detection crosstalk; the Rayleigh scattering signals obtained from coding sequences of different frequencies complement each other in terms of coherent fading performance, and multiple sets of detection results can be vector synthesized, thereby effectively suppressing coherent fading; furthermore, unmatched filters can be flexibly designed for coding sequences of arbitrary intensity without the need for special design of intensity coding sequences, significantly improving the applicability and flexibility of the system. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the structure of a phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding according to the present invention.

[0041] Figure 2 This is a schematic diagram of one embodiment of a phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding according to the present invention.

[0042] Figure 3 This is a schematic diagram of the symbol sequence used and the designed matching symbol sequence in the embodiments of the present invention.

[0043] Figure 4 This is an actual test diagram showing the distribution of demodulation phase standard deviation with distance in an embodiment of the present invention.

[0044] Figure 5This is a test diagram showing the actual effect of external strain sensing in an embodiment of the present invention.

[0045] The numbers in the diagram indicate: 1. Continuous laser, 2. Optical coupler, 3. Signal generation circuit module, 4. Optical intensity modulator, 5. Optical circulator, 6. Fiber optic sensor under test, 7. Optical bridge and polarization diversity module, 8. Photodetector, 9. Signal acquisition circuit. Detailed Implementation

[0046] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0047] Example 1

[0048] This invention proposes a phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding, such as... Figure 1 As shown, this embodiment of the invention includes a continuous laser 1, an optical coupler 2, a signal generation circuit module 3, an optical intensity modulator 4, an optical circulator 5, a sensor link under test 6, an optical bridge and polarization diversity module 7, a photodetector 8, and a signal acquisition circuit 9. The continuous laser 1 generates laser light that is incident on the input of the optical coupler 2; the optical coupler 2 splits the laser light into two branches: a local path and a probe path. The local path is incident on the input of the optical bridge and polarization diversity module 7, and the probe path is incident on the input of the optical intensity modulator 4; the signal generation circuit module 3 generates an orthogonal frequency division multiplexing intensity-coded radio frequency signal and inputs it to the optical intensity modulator 4; the optical intensity modulator 4 modulates the continuous laser light into multiple orthogonal intensity-coded pulse sequences and incident them on the optical circulator 5; the optical circulator 5 incident the pulsed laser sequence onto the fiber optic link under test; and generates a signal in the fiber optic link under test. Rayleigh backscattered light is incident on the input of the optical bridge and polarization diversity module 7 through the optical circulator 5; the optical bridge and polarization diversity module 7 beat the light fields of the local light and the backscattered Rayleigh light, and the beat frequency result is incident on the input of the photodetector 8. The photodetector 8 converts the beat frequency result into an electrical signal and outputs it to the signal acquisition circuit 9; the signal acquisition circuit 9 samples the backscattered Rayleigh signal and performs pulse compression on the Rayleigh scattering signal by designing an unmatched filter; by vector synthesis of the results of pulse compression of Rayleigh scattering signals of different frequency components, coherent fading can be eliminated.

[0049] The working principle of the phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding proposed in this invention is as follows: First, a continuous laser is generated; after passing through an optical coupler, the continuous laser is divided into a local path and a probe path; the local path is directly input to the optical bridge and polarization diversity module; the probe path, after passing through an intensity modulator, is converted into a pulsed laser sequence with intensity modulation. The intensity modulator is driven by an RF signal provided by a signal generation circuit module, and the RF signal consists of an intensity-coded sequence with mutually orthogonal frequencies; the modulated pulsed laser sequence enters the sensor link under test through an optical circulator 5; the Rayleigh backscattered light generated by the sensor link under test undergoes coherent interference with the local continuous laser through the circulator at the optical bridge and polarization diversity module, and the beat frequency signal is received by a photodetector; the signal acquisition circuit samples and analyzes the beat frequency signal output by the photodetector. By designing a specific unmatched filter to pulse compress the intensity-coded sequence, the signal-to-noise ratio is improved, and the detection results of the mutually orthogonal frequency-coded sequences are synthesized to eliminate coherent fading, achieving fading-free, high signal-to-noise ratio strain signal measurement. This device, through the design of an unmatched filter, enables pulse compression of coded sequences of arbitrary intensity, significantly improving the energy injected into the optical fiber compared to traditional single-pulse schemes while maintaining the same spatial resolution. It acquires Rayleigh scattering signals under various coherent fading modes through orthogonal frequency division multiplexing and eliminates coherent fading through synthesis. This system possesses high signal-to-noise ratio and high-precision strain measurement capabilities, making it widely applicable in scenarios with high requirements for signal-to-noise ratio and strain measurement accuracy, such as tunnel deformation monitoring, long-distance pipeline safety monitoring, and health diagnosis of large civil structures, thus possessing significant engineering application value.

[0050] 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 1550.12nm, an output linewidth of 100Hz, and an output power of 20mW.

[0051] Optical coupler 2 is a polarization-maintaining optical coupler with a nominal splitting ratio of 90:10. 90% of the light is incident on the light intensity modulator 4 as the probe light, and 10% of the light is incident on the input of the optical bridge and polarization diversity module 7 as the local oscillator light, which is used to beat the back Rayleigh scattering signal and amplify it.

[0052] Signal generation circuit module 3 is an arbitrary waveform generator with a nominal sampling rate of 1.25 GSa / s, an analog bandwidth of 500 MHz, and a storage capacity of 64 MSa. It has two independent output channels and one synchronous trigger channel. One independent output channel of the arbitrary waveform generator will generate an orthogonal frequency division multiplexing (OFDM) intensity coding sequence. The intensity coding sequence used in this embodiment includes three sets of frequency-orthogonal intensity coding sequences. These three sets of sequences are arranged sequentially in time, and their symbol information is the same. The difference lies in the different chips of the modulation symbols in the three sets of sequences.

[0053] The symbol information of all three sequences is a 64-bit strength-coded sequence c = [c1, c2, c3, ..., c k ], where c k ∈{+1,-1}, k=1,2,…,K, this sequence can be generated by a pseudo-random binary sequence, or predefined as a ±1 sequence of arbitrary length. For any given binary symbol sequence c, the encoding matrix C of its Toeplitz structure is constructed.

[0054]

[0055] Where L = K + M - 1, K is the length of the intensity-coded sequence, and M is the length of the unmatched filter. Let the ideal output be the unit impulse response b, where b = [0,…,0,1,0,…,0]. T The value of b is 1 in the middle and 0 in the rest. The solution is to solve the least squares problem min||Cu-b|| 2 Its analytical solution is u = (C T C) -1 C T b, where u is the matching symbol sequence of symbol sequence c, using ( Pulse compression of the symbol sequence c can be achieved by performing a convolution operation. In this embodiment, the symbol sequence c and the matched symbol sequence u are as follows: Figure 3 As shown

[0056] For the same symbol information, this invention selects three mutually orthogonal chips to modulate the symbol sequence c to obtain the coded pulse sequence s. i (t), the three selected orthogonal chips are T c = 100MHz, 200MHz, and 300MHz sine waves with an 80ns pulse width:

[0057] w i (t)=sin(2πf i t), t∈[0,T c ]

[0058] f i ∈{100MHz,200MHz,300MHz},i=1,2,3

[0059]

[0060] In the formula, K is the length of the strength-coded sequence; c k Indicates the k-th code element; w i For the mutually orthogonal chips set, a sinusoidal signal is used in this embodiment; f iThis represents the frequency of the three orthogonal code chips.

[0061] Similarly, this invention modulates the designed matched symbol sequence u into chips, serving as an unmatched filter h for different coded pulse sequences. i (t):

[0062]

[0063] In the formula, M is the length of the unmatched filter; u m For the m-th matching symbol sequence; T c The pulse width of the chip.

[0064] The light intensity modulator 4 is a 1550nm intensity modulator with an extinction ratio greater than 25dB, an insertion loss less than 5dB, and an operating wavelength of 1530-1610nm. Its input is a polarization-maintaining jumper and its output is a single-mode jumper.

[0065] The optical circulator 5 is a single-mode optical circulator with a nominal insertion loss of less than 1.0 dB, an isolation of more than 60 dB, and a return loss of more than 50 dB. The first port of the optical circulator is connected to the optical intensity modulator 4, which guides the pulsed laser sequence through the second port into the optical fiber under test 6. The optical fiber under test 6 then outputs the signal through the third port to the optical bridge and polarization diversity module 7.

[0066] The sensing fiber 6 under test is a G.652.D single-mode fiber with a nominal sensing distance of about 10.1km and a transmission loss of less than 0.2dB / km@1550nm. The pulsed laser sequence excites Rayleigh scattering along the path in the single-mode fiber, and part of the scattered light propagates in the opposite direction along the fiber to form a Rayleigh backscattering signal.

[0067] The optical bridge and polarization diversity module 7 uses an optical mixer with an integrated polarization beamsplitter and a 90° optical bridge. Its nominal operating wavelength is 1520–1625 nm, the insertion loss of the signal path is 7.0 dB, the insertion loss of the local path is 10.0 dB, and the phase angle between the I and Q paths is 85°–95°. The optical mixer is used to separate the local oscillator light from the signal light, performing optical interference with phase differences of 0° and 90° respectively. Simultaneously, it performs orthogonal polarization beam splitting on the input light, ensuring that signals with different polarization states can interfere and superimpose with the local oscillator light. The generated I and Q path interference signals are output to the back-end balanced photodetector 8 to achieve efficient coherent detection and polarization diversity.

[0068] The photodetector 8 is a balanced photodetector with a nominal operating wavelength of 1200–1700 nm, a 3 dB bandwidth of DC-400 MHz, a detector responsivity of 0.85 A / W at 1300 nm, and a transimpedance gain of 10 kV / A. The balanced photodetector converts the coherently superimposed interference light field into an electrical signal, effectively suppressing local oscillator DC noise and common-mode noise through differential amplification, while simultaneously enhancing the beat frequency signal to achieve high-sensitivity coherent detection.

[0069] The 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 400MHz, a sampling rate of 1GSa / 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. Subsequent data processing is handled by the host computer. The acquisition card is set to single-record acquisition mode and runs continuously throughout the test, with a cumulative acquisition time of 20 milliseconds. The computer performs subsequent data processing, including signal pulse compression and demodulation, including suppression of coherent fading. The specific processing flow is as follows: First, the unmatched filter h corresponding to each different orthogonal frequency intensity encoded pulse sequence is... i The signal y(t) is convolved with the acquired electrical signal x(t) to achieve pulse compression, ensuring that the spatial resolution of the encoded pulse sequence remains consistent with that of a single pulse. Simultaneously, it separates the intensity-coded Rayleigh scattering signals corresponding to different frequency components. The pulse-compressed signal y(t) is:

[0070]

[0071] Suppose there are a total of N records, and the record of the αth pulse compression is written as... After separating the scattered signals, time difference processing is performed on the compression results of coded pulses of different frequencies. Each record is multiplied conjugately with the first record, and the time difference results of the three orthogonal frequencies are then processed. Vector superposition is performed to effectively suppress coherent fading:

[0072]

[0073] Finally, by using spatial difference and moving average processing, a demodulation result with high signal-to-noise ratio and no coherent fading is obtained.

[0074] For the phase-sensitive optical time-domain reflectometer system scheme based on orthogonal frequency division multiplexing intensity coding in the embodiments of the present invention... Figure 4The changes in phase standard deviation, obtained by demodulating each frequency intensity individually and synthesizing the demodulated results of the three frequencies, with a 10km sensing fiber distance are shown. The results are also magnified to demonstrate the situation at the end of the fiber (9km–10km), verifying the effectiveness of this method in eliminating coherent fading and improving strain resolution. To further verify the system's ability to detect external strain, a 400Hz sinusoidal strain signal was applied to the end of the sensing fiber. Figure 5 The figure shows the time-domain waveform and its corresponding frequency-domain power spectral density obtained by demodulation in the vibration region. Experimental results show that the method of the present invention can accurately reconstruct the time-domain strain waveform, exhibits a low noise floor in the power spectral density, and shows a significant peak at 400 Hz, further demonstrating its applicability and effectiveness in high-fidelity strain sensing scenarios.

[0075] The phase-sensitive optical time-domain reflectometer proposed in this invention, based on orthogonal frequency division multiplexing intensity coding, breaks through the constraint relationship between detection signal-to-noise ratio and spatial resolution in traditional phase-sensitive optical time-domain reflectometers. By synthesizing multiple sets of orthogonal coding results, it effectively suppresses coherent fading caused by Rayleigh scattering, thereby achieving high-precision and high-stability distributed strain measurement.

[0076] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this invention, or the part that contributes to the prior art, or a part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0077] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A phase sensitive optical time domain reflectometer based on orthogonal frequency division multiplexing pulse coding, characterized in that, The phase-sensitive optical time-domain reflectometer includes a continuous laser (1), an optical coupler (2), a signal generation circuit module (3), an optical intensity modulator (4), an optical circulator (5), a sensing fiber under test (6), an optical bridge and polarization diversity module (7), a photodetector (8), and a signal acquisition circuit (9). The continuous laser (1) generates continuous laser light, which is divided into a local path and a probe path by an optical coupler (2); the local path is input to the input end of the optical bridge and polarization diversity module (7), and the probe path is incident on the input end of the light intensity modulator (4); The signal generation circuit module (3) generates an orthogonal frequency division multiplexing intensity-coded radio frequency signal and inputs it to the optical intensity modulator (4); the orthogonal frequency division multiplexing intensity-coded radio frequency signal contains multiple sets of intensity-coded sequences with mutually orthogonal frequencies, the multiple sets of intensity-coded sequences are arranged sequentially in time, the symbol information of the multiple sets of intensity-coded sequences is the same, and the chips of the modulation symbols are different; The light intensity modulator (4) modulates the continuous laser into multiple frequency orthogonal intensity-coded pulse sequences based on the radio frequency signal and then incident them onto the optical fiber (6) of the sensing fiber under test via the optical circulator (5). The Rayleigh backscattered light generated in the optical fiber (6) under test is coherently interfered with the local continuous laser in the optical bridge and polarization diversity module (7) through the circulator (5), and the beat frequency result is converted into an electrical signal by the photodetector (8). The signal acquisition circuit (9) samples and analyzes the electrical signal output by the photodetector (8), and uses an unmatched filter obtained by chip modulation of the matched code sequence to perform pulse compression on the acquired electrical signal and separate the Rayleigh scattering signals corresponding to the pulse codes of different frequency components. The detection results of each frequency pulse code sequence are vector synthesized to obtain the strain signal. The signal acquisition circuit (9) includes a high-speed digital acquisition card and a computer. The electrical signal output by the photodetector (8) is processed as follows: The unmatched filter corresponding to each different orthogonal frequency intensity coding sequence is convolved with the acquired electrical signal to achieve pulse compression and separate the intensity-coded Rayleigh scattering signal corresponding to different frequency components. After the scattering signal is separated, the compression results of the intensity encoded pulse sequences of different orthogonal frequencies are subjected to time difference processing. Each record is multiplied by the first record using conjugate, and the time difference results of multiple orthogonal frequencies are vector-superimposed. The vector superposition result is then subjected to spatial difference and moving average processing to obtain the final demodulation result; The non-matched filter is obtained by modulating a sequence of matched symbols by a chip; the sequence of matched symbols is obtained by solving a least squares problem with an analytical solution: wherein is a code matrix of Toeplitz structure constructed based on the symbol sequence c is a code matrix of Toeplitz structure constructed based on the symbol sequence is an ideal output By modulating the chip with the matched symbol sequence a non-matched filter is obtained : wherein is the non-matched filter length; is the th m matched symbol sequence; is the set of mutually orthogonal chips; is the chip pulse width.

2. The phase sensitive optical time domain reflectometer based on orthogonal frequency division multiplexing pulse coding according to claim 1, characterized in that, The continuous laser (1) is a narrow linewidth single-frequency laser.

3. The phase sensitive optical time domain reflectometer based on orthogonal frequency division multiplexing pulse coding according to claim 1, characterized in that, The optical coupler (2) adopts a polarization-maintaining optical coupler, in which most of the continuous laser is incident on the optical intensity modulator (4) as the probe light, and a small portion of the continuous laser is incident on the input end of the optical bridge and polarization diversity module (7) as the local oscillator light, which is used to beat the back Rayleigh scattering signal and amplify it.

4. The phase sensitive optical time domain reflectometer based on orthogonal frequency division multiplexing pulse coding according to claim 1, characterized in that, The signal generation circuit module (3) uses an arbitrary waveform generator. The process by which the signal generation circuit module (3) generates the intensity coding sequence for orthogonal frequency division multiplexing is as follows: Generate a binary strength-coded symbol sequence by using multiple mutually orthogonal chip pairs of the symbol sequence. Modulation is performed to obtain a coded pulse sequence : In the formula, The length of the code symbol sequence for strength encoding; Indicates the first k Each code element; express i A series of mutually orthogonal code chips; The pulse width of the chip.

5. A phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding according to claim 4, characterized in that, The input of the light intensity modulator (4) uses a polarization-maintaining jumper, and the output uses a single-mode jumper.

6. A phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding according to claim 1, characterized in that, The optical circulator (5) is a single-mode optical circulator; the first port of the optical circulator (5) is connected to the optical intensity modulator (4), the second port is connected to the optical fiber under test (6), and the third port is connected to the input end of the optical bridge and polarization diversity module (7).

7. A phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding according to claim 1, characterized in that, The optical bridge and polarization diversity module (7) adopts an optical mixer with an integrated polarization beam splitter and a 90° optical bridge. The phase angle between the I and Q paths is 85°~95°. The optical mixer is used to split the local oscillator light and the signal light and perform optical interference with phase differences of 0° and 90° respectively. At the same time, it performs orthogonal polarization beam splitting on the input light so that the signals with different polarization states are superimposed with the local oscillator light through interference. The generated I and Q path interference signals are output to the back-end balanced photodetector (8).

8. A phase-sensitive optical time-domain reflectometer based on orthogonal frequency division multiplexing pulse coding according to claim 1, characterized in that, The photodetector (8) is a balanced photodetector, which converts the coherent superposition of the interference light field into an electrical signal. The signal is differentially amplified to suppress local oscillator DC noise and common-mode noise, while enhancing the beat frequency signal.

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