Phase-sensitive optical time domain reflectometer function optimization method based on self-phase modulation

Abstract

The invention belongs to the technical field of optical time domain reflectometer, and particularly relates to a phase-sensitive optical time domain reflectometer performance optimization method based on self-phase modulation. The method comprises the following steps: when a self-phase modulation effect occurs due to the fact that a high-power pulse reaches a nonlinear threshold value, or the pulse power of an input optical fiber is adjusted to exceed the nonlinear threshold value, the spectral range of a detected scattered signal can be expanded; based on short-time Fourier transform, extracting the characteristic that the frequency spectrum of the signal changes along with the transmission distance, designing a short-time segmentation filter adaptive to the change of the transmission distance, and carrying out phase shift transformation processing on the signal; processing and synthesizing a multi-frequency component caused by the self-phase modulation effect by using an analysis and synthesis algorithm to generate a complex analysis signal matrix; through IQ demodulation, phase difference calculation and unwrapping processes, high-precision demodulation phase information is obtained. According to the invention, the sensing distance can be obviously prolonged, the coherent fading effect is effectively inhibited, and the detection sensitivity and stability of the system are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of optical time domain reflectometry, and in particular relates to a method for optimizing the function of a phase-sensitive optical time domain reflectometer based on self-phase modulation. Background Art

[0002] Phase-sensitive optical time-domain reflectometry (Φ-OTDR), as the core of distributed fiber optic sensing technology, is widely used in oil and gas pipeline monitoring, earthquake monitoring, perimeter security and other fields. This technology uses Rayleigh scattering signals in optical fibers to detect external disturbances with high sensitivity, enabling real-time, continuous, long-distance monitoring and quantitative analysis of environmental changes. The advantage of Φ-OTDR technology is that it does not require traditional sensor nodes, can cover a large area and efficiently locate disturbance sources.

[0003] However, Φ-OTDR technology faces great challenges in long-distance monitoring. As the optical pulse propagates in the optical fiber, the signal will gradually attenuate, and the scattered light will also be attenuated when it returns to the receiving end, resulting in a significant decrease in the intensity of the scattered signal at the tail end under long-distance transmission. This makes the tail end signal too weak or even impossible to demodulate when the nonlinear effect does not reach the threshold, which seriously affects the monitoring accuracy and sensing distance, and limits the application scope of Φ-OTDR technology.

[0004] In addition, under high-power pulsed lasers, the self-phase modulation effect in the optical fiber will cause the signal spectrum to be broadened, generating multiple frequency components and causing signal distortion. This nonlinear effect further exacerbates the coherent fading of the signal, which is particularly evident in long-distance monitoring. The spectrum broadening and coherent fading phenomena make it impossible for Φ-OTDR to effectively suppress signal interference during long-distance sensing, thereby affecting the sensitivity and stability of the system. Therefore, how to overcome these nonlinear effects and extend the sensing distance without sacrificing signal quality has become a key problem in the development of Φ-OTDR technology. Previous studies have mainly focused on improving the signal strength below the nonlinear threshold of the optical fiber through Raman distributed optical amplification, tail optical amplification, and remote pumping, thereby enhancing the sensing distance of the Φ-OTDR system. These methods have solved the signal attenuation problem to a certain extent, but since the signal is still limited by the long-distance attenuation and nonlinear effects of the optical fiber, its performance is still affected during extremely long-distance monitoring. At the same time, in order to solve the problem of coherent fading, traditional fading suppression methods mostly use multi-frequency light sources or swept-frequency light source technology to alleviate the fading effect by introducing multi-frequency components. However, these methods are complex and costly, and in some cases they still cannot completely eliminate the fading phenomenon.

[0005] The present invention proposes for the first time to use the self-phase modulation effect in optical fiber as the core technology, breaking through the limitations of traditional methods, and compared with the existing technology, this method does not require additional optoelectronic devices. Self-phase modulation is a nonlinear effect that can introduce spectrum broadening in optical fiber through high-power pulse excitation, thereby effectively expanding the spectrum range of the scattered signal. By accurately adjusting the pulse power and utilizing the new frequency components generated by the self-phase modulation effect, not only the intensity of the sensing signal is significantly improved, but also the coherent fading problem caused by the nonlinear effect is effectively suppressed.

[0006] The innovation of this patent is that it uses the self-phase modulation effect to optimize the performance of the Φ-OTDR system, achieving a significant increase in sensing distance and effective suppression of coherent fading. Unlike traditional multi-frequency or swept-frequency light source technology, this method does not require the addition of additional optoelectronic devices, such as optical amplifiers or complex light source systems, greatly simplifying the design and implementation of the system and providing a new solution for long-distance, high-precision fiber optic sensing. Summary of the invention

[0007] The purpose of the present invention is to provide a method for optimizing the performance of a phase-sensitive optical time domain reflectometer (Φ-OTDR) based on self-phase modulation, so as to significantly increase the sensing distance and effectively suppress coherent fading.

[0008] The phase-sensitive optical time-domain reflectometer of the present invention has a basic structure as shown in Figure 1 As shown, it includes: a narrow linewidth laser (light source module), two fiber couplers, an adjustable attenuator, a circulator, an arbitrary waveform generator, an acousto-optic modulator, an erbium-doped fiber amplifier, a balanced detector, a data acquisition card, a polarization controller, a first sensing fiber and a second sensing fiber, piezoelectric ceramics, etc.;

[0009] The performance optimization method of the phase-sensitive optical time-domain reflectometer based on self-phase modulation provided by the present invention is based on the self-phase modulation (SPM) effect generated by high-power pulses in the optical fiber. Regardless of whether the effect is triggered by actively adjusting the input optical fiber pulse power, as long as the self-phase modulation effect occurs, the demodulation performance of the phase-sensitive optical time-domain reflectometer can be optimized. Therefore, the present invention utilizes the self-phase modulation effect combined with the digital domain processing technology of short-time segmented overlapping filter and phase shift transformation to achieve performance optimization. Specifically:

[0010] First, when the self-phase modulation effect occurs due to high-power pulses reaching the nonlinear threshold, or by adjusting the pulse power of the input fiber to exceed the nonlinear threshold, the spectrum range of the detected scattered signal can be extended;

[0011] Then, based on the short-time Fourier transform (STFT), the characteristics of the signal spectrum changing with the transmission distance are extracted, and a short-time segmented filter that adapts to the change of transmission distance is designed to perform phase shift transformation on the signal. In this process, in order to ensure the continuity of the data, the overlapping data length is reasonably selected;

[0012] Subsequently, using an analytical and synthetic algorithm (including but not limited to a combination of Hilbert transform and rotation vector method or other applicable algorithms), the multi-frequency components caused by the self-phase modulation effect are processed and synthesized to generate a complex analytical signal matrix;

[0013] Finally, high-precision demodulation phase information is obtained through IQ demodulation, phase difference calculation and unwinding process. The present invention effectively solves the problem of limited sensing distance of traditional phase-sensitive optical time-domain reflectometer due to self-phase modulation limitation. While significantly extending the sensing distance, the new frequency component generated by the self-phase modulation effect is used to suppress the coherent fading effect, thereby improving the detection sensitivity and stability of the system.

[0014] The present invention adopts self-phase modulation technology, expands the spectral range of the scattered signal by adjusting the pulse power, and optimizes signal processing by combining short-time Fourier transform and short-time segmented overlapping filter, thereby solving the problems of limited sensing distance and severe coherent fading in traditional methods, and effectively improving the detection sensitivity and stability of the system. The specific steps are as follows:

[0015] Step 1: When the high-power pulse reaches or exceeds the nonlinear threshold of self-phase modulation by adjusting the pulse power of the input optical fiber, the spectrum range of the scattered signal is expanded.

[0016] Step 2: Use short-time Fourier transform to perform time-frequency domain analysis on the received scattered signal, and process the signal in segments to obtain the instantaneous spectrum characteristics at each transmission position.

[0017] Step 3: Design and apply short-time overlapping segmented filters that vary with the transmission distance. According to the pulse spectrum characteristics obtained in step 2, design and apply multiple short-time segmented overlapping filters. The parameters of the filter (such as bandwidth and center frequency) are dynamically adjusted according to the transmission distance of the signal. Use these filters to perform phase shift transformation processing on the signal.

[0018] Step 4: Use a signal analysis and synthesis algorithm (including but not limited to a combination of Hilbert transform and rotation vector method or other applicable algorithms) to process and synthesize the multi-frequency components caused by the self-phase modulation effect to generate a complex analysis signal matrix.

[0019] Step 5: Obtain high-precision phase information through IQ demodulation, phase difference calculation and phase unwrapping, achieve high-fidelity reconstruction of external disturbance events, and improve the detection sensitivity and stability of the system.

[0020] Furthermore, the self-phase modulation nonlinear threshold mentioned in step 1 refers to its significance threshold, which is defined as P th =1 / (γL eff ), γ is called the nonlinear coefficient of the optical fiber, L eff =(1-e -αL ) / α represents the total equivalent nonlinear action length determined by the fiber loss and total length, which is called the effective length of the fiber, where α represents the fiber attenuation coefficient and L represents the total fiber length. When the input power P0 exceeds this threshold, the self-phase modulation effect is significantly enhanced, resulting in spectrum broadening and nonlinear phase accumulation.

[0021] Furthermore, the step 1 is specifically as follows:

[0022] Step 1-1: Use a high-power pulsed laser and adjust the pulse width and repetition rate to match the nonlinear characteristics of the optical fiber and system requirements.

[0023] Step 1-2: gradually increase the input pulse power by using an adjustable optical attenuator, directly adjusting the output power of the laser, or increasing the amplification current of the erbium-doped fiber amplifier (EDFA).

[0024] Step 1-3: Use a spectrum analyzer to detect the output spectrum to confirm whether spectrum broadening occurs, or use short-time Fourier transform to perform time-frequency domain analysis on the beat frequency signal detected by heterodyne. According to the spectrum broadening effect, adjust the pulse power to the optimal level to ensure that the spectrum expansion meets the signal detection requirements.

[0025] Furthermore, the input optical pulse in step 1 is a pulse with a smooth rising edge and falling edge, such as a Gaussian pulse or a super Gaussian pulse. Most of the pulses actually modulated have these characteristics, which can better control the new frequency components generated by the self-phase modulation effect, thereby effectively enhancing the spectrum range of the signal. The expression of the super Gaussian pulse is as follows:

[0026]

[0027] Among them, U(z,T) represents the signal pulse form obtained after the transmission loss is reduced to the normalized pulse amplitude, T0 represents the pulse width, and m reflects the sharpness of the pulse edge. When m=1, the above formula represents a Gaussian pulse, and the time scale is defined as:

[0028]

[0029] Among them, t represents time, z represents the transmission distance, and v g Represents the group velocity in the optical fiber.

[0030] Furthermore, the step 3 is specifically as follows:

[0031] Step 3-1: Based on the pulse spectrum characteristics obtained in step 2, dynamically adjust the bandwidth and center frequency parameters of the filter according to the spectrum characteristics of the signal at different transmission distances, where the center frequency of the filter is located at the high and low frequency positions generated by the self-phase modulation effect and the initial shift frequency position. Each filter segment corresponds to a specific transmission distance interval, ensuring that noise can be effectively filtered out and useful signals can be retained in each interval.

[0032] Step 3-2: Process the signal in segments, and keep a certain overlap length between each segment to ensure the continuity and integrity of the data. The overlap length is selected based on the time-frequency characteristics of the signal, which prevents the filter from generating transient effects at the segment boundaries and ensures a smooth transition between adjacent segments, avoiding the loss or distortion of amplitude and phase information, thereby ensuring the data continuity of the segment filter during the final data processing.

[0033] Step 3-3: Apply the designed short-time segmented overlapping filters to the received signal in sequence and perform phase shift transformation processing. Through the phase shift effect of the filter, the phase information of the signal is adjusted to suppress the coherent fading effect.

[0034] Furthermore, in step 3-1: the phase shift characteristic of the designed short-time segmented overlapping filter should minimize the correlation between the output amplitude curve and the reference signal. The design principle of the phase shift characteristic can be selected to use only π phase shift, or combine π and π / 2 phase shift to optimize the phase transformation effect.

[0035] Furthermore, in step 3-1, the center frequency of the short-time segmented overlapping filters of different frequency bands increases exponentially along the fiber transmission distance. This is because the self-phase modulation effect in the ultra-Gaussian pulse fiber will cause the frequency offset to increase continuously with the increase of the transmission distance, and the frequency offset is linearly related to the transmission distance, an exponential saturation nonlinear relationship, and the specific formula is derived as follows:

[0036]

[0037] Among them, δω(T) is the variation curve of frequency chirp in the whole pulse width, L NL =γP0 is called the nonlinear characteristic length Z of the optical fiber eff =(1-e -αz ) / α represents the equivalent nonlinear action length that varies with distance;

[0038] Let the time derivative of δω(T) be 0, and we can get the maximum value of δω(T)

[0039]

[0040] Where f is a constant related to the input pulse shape. It can be seen that the frequency chirp of the super-Gaussian pulse only appears near the rising and falling edges of the pulse, and its change is not linear. In other words, the frequency at the center of the pulse will maintain the value of the initial frequency shift. Therefore, it is necessary to design three sets of short-time segmented overlapping filters, whose center frequencies are located at the original center frequency and the frequency offsets generated along the rising and falling edges of the pulse due to the self-phase modulation effect, and the center frequencies of these offsets show an exponential saturation nonlinear relationship with the increase of the transmission distance z.

[0041] Furthermore, in step 3-3, the phase shift transformation operation of the short-time segmented overlapping filter can also be implemented in the frequency domain through short-time Fourier transform, thereby further optimizing the phase adjustment process of the signal.

[0042] Furthermore, in step 1, the method for generating the self-phase modulation effect can also be implemented by distributed Raman amplification or other amplification methods. However, it should be noted that this method will cause the signal spectrum curve obtained in step 2 to change. Therefore, in step 3, the parameters of the short-time segmented overlapping filter must be adjusted in real time according to the changes in the spectrum curve to ensure the accuracy and effectiveness of signal processing.

[0043] Furthermore, the step 4 is specifically as follows:

[0044] Step 4-1: For each overlapping data segment processed in step 3, use a signal analysis method (including but not limited to the Hilbert transform method, Fourier integration method, sine-cosine demodulation method or other applicable analysis methods) to convert the real signal into a complex analytical signal, generate the real part and imaginary part of the signal, and form a complex representation of each data segment.

[0045] Step 4-2: Through the rotating vector method or other superposition methods (including direct vector addition method, amplitude preferred vector addition method, etc.), the phases of the frequency components generated by different phase shift filters at the same transmission distance are uniformly adjusted to the reference phase. From the perspective of improving the continuity of the demodulated data along the optical fiber distance, the smooth transition of the signals processed by different phase shift filters is ensured, thereby effectively synthesizing multi-frequency information at the same distance.

[0046] Step 4-3: All the complex analytical signal segments after the above processing are superimposed and synthesized to form a complete complex analytical signal matrix, fully integrating the multi-frequency components generated by the self-phase modulation effect to ensure effective synthesis of the signal.

[0047] Furthermore, the performance optimization method of phase-sensitive optical time domain reflectometer based on self-phase modulation proposed in the present invention is applicable to phase-sensitive optical time domain reflectometer with heterodyne detection, and is first used in non-diversity systems; when applied to polarization diversity systems, it can further effectively suppress polarization fading.

[0048] Technical features and beneficial effects of the present invention:

[0049] (1) It breaks through the limitations of traditional phase-sensitive optical time-domain reflectometer (Φ-OTDR) in long-distance sensing. By adjusting the pulse power to induce the self-phase modulation effect, the spectrum range of the scattered signal is expanded, thereby effectively improving the detection distance, and without the need for additional optoelectronic devices, the coherent fading caused by nonlinear effects is suppressed.

[0050] (2) Combining the digital domain processing technology of short-time Fourier transform and short-time segmented overlapping filter, dynamic analysis and optimization of the spectrum are realized, which can accurately capture the frequency characteristics of the signal at different transmission positions and suppress coherent fading through the phase shift transformation of the short-time segmented overlapping filter, effectively enhancing the continuity and stability of the signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a schematic diagram of the structure of the sensing system of the phase-sensitive optical time-domain reflectometer provided in an embodiment of the present invention.

[0052] Figure 2 It is a flow chart of the performance optimization method of the phase-sensitive optical time-domain reflectometer based on self-phase modulation of the present invention.

[0053] Figure 3 The short-time Fourier transform spectrum of the detected original beat frequency signal. (a) is the spectrum when there is no self-phase modulation effect; (b) is the spectrum after the intensity of the self-phase modulation effect is adjusted to a more appropriate value.

[0054] Figure 4 This is the demodulation result when there is no self-phase modulation effect: It shows the demodulation result of the signal after filtering and phase shift transformation without the influence of self-phase modulation effect, showing the initial state of the signal and the response of the system under normal working conditions.

[0055] Figure 5 The demodulation results when the sensing light undergoes self-phase modulation effect. (a) is the demodulation result after corresponding filtering and phase shift transformation when the sensing light undergoes self-phase modulation effect, showing the spectrum change and signal attenuation caused by the self-phase modulation effect. (b) is the demodulation result obtained by the method proposed in the present invention, showing that the system effectively suppresses the coherent fading caused by the self-phase modulation effect through this method, restores the integrity of the signal and improves the signal quality.

[0056] Figure 6The disturbance time domain and frequency domain demodulation results obtained by the method of the present invention. Among them, (a) shows that the system restores the time domain information of the external disturbance in the optical fiber link, and can accurately capture and present the occurrence and change of the disturbance event. (b) shows the frequency domain demodulation result of the disturbance, indicating that the system can restore the spectrum information of the disturbance through high-fidelity signal processing, showing the high sensitivity detection capability of the system.

[0057] Numbers in the figure: 1-narrow linewidth laser, 2-fiber coupler, 3-arbitrary waveform generator, 4-acoustic-optic modulator, 5-erbium-doped fiber amplifier, 6-adjustable optical attenuator, 7-circulator, 8-fiber coupler, 9-balanced detector, 10-data acquisition device, 11 polarization controller, 12-first sensing fiber, 13-piezoelectric ceramic, 14-second sensing fiber. DETAILED DESCRIPTION

[0058] The present invention is further described below by way of embodiments in conjunction with the accompanying drawings.

[0059] The phase-sensitive optical time-domain reflectometer provided by the embodiment of the present invention has a structure as shown in Figure 1 As shown, it includes: a narrow linewidth laser (light source module) 1, fiber couplers 2, 8, an adjustable attenuator 6, a circulator 7, an arbitrary waveform generator 3, an acousto-optic modulator 4, an erbium-doped fiber amplifier 5, a balanced detector 9, a data acquisition card 10, a polarization controller 11, a first sensing fiber 12, a second sensing fiber 14, and a piezoelectric ceramic 13; wherein:

[0060] The laser emitted by the narrow linewidth laser (light source module) 1 has a central wavelength of 1550.12nm and a linewidth of less than 100Hz. The laser is divided into two light outputs through the first fiber coupler 2, one of which enters the polarization controller 11 as the local oscillator light, and the other enters the acousto-optic modulator 4, and modulates the ultra-Gaussian detection pulse with a pulse width of 100ns, a frequency shift of 80MHz, and a repetition frequency of 1kHz under the drive of the arbitrary waveform generator 3. The ultra-Gaussian detection pulse is connected to the port 1 of the circulator 7 through the erbium-doped fiber amplifier 5 and the adjustable attenuator 6, and the port 2 of the circulator 6 is connected to the first sensing fiber optic cable with a length of 45km. 12 is connected, the piezoelectric ceramic 13 generates external disturbance, the frequency is 120Hz, the vibration coverage range is about 10m, and it is connected to the second sensing optical fiber 14 with a length of 2km. The backward Rayleigh scattered light returns through the port three of the circulator 6 and beats with the local oscillation light at the 50:50 second optical fiber coupler 8, and then is detected by a balanced detector 9 with a response bandwidth of 350Mhz, and is collected by a 400Mhz data acquisition card 10 to obtain the intermediate frequency original signal; the microwave output end of the arbitrary waveform generator 3 is respectively connected to the microwave loading end of the acousto-optic modulator 4 and the trigger signal of the data acquisition card 10.

[0061] For example Figure 1The optical time domain reflectometer shown in FIG. 1 performs the following steps: Figure 2 As shown:

[0062] Step 1: First, adjust the output power of the laser to 20mw, set the pulse width to 100ns, and the pulse repetition frequency to 1khz. Then, adjust the erbium-doped fiber amplifier and attenuator until the pulse light propagates in the optical fiber to stimulate nonlinear effects, and the spectrum range of the scattered signal is expanded. During the adjustment process, the short-time Fourier transform diagram can be observed in real time to ensure the stability of the signal quality during the optical fiber transmission process.

[0063] Step 2. This embodiment selects 2048 sampling points as the window size of the short-time Fourier transform, and the sampling rate is 400MHz, so the time length of each window is 5.12 microseconds. When the signal is processed in segments, each segment contains 2048 sampling points, and some sampling points are overlapped between adjacent segments. This setting ensures the continuity of the data and the smooth transition between adjacent segments. Apply short-time Fourier transform to each segment to obtain the spectral characteristics of the signal at different time points, and then extract the instantaneous spectral characteristics of each transmission position, providing a basis for subsequent signal analysis and processing. The results are as follows: Figure 2 As shown, the time-frequency spectrum after short-time Fourier transform processing is displayed, which allows us to intuitively observe the distribution of the signal in the time and frequency domains, as well as the spectral characteristics of the signal changing over time.

[0064] like Figure 3 As shown in the figure, the changes in the signal spectrum before and after the self-phase modulation effect is excited are shown. In Figure (a), the spectrum is relatively concentrated, no obvious spectrum broadening occurs, and the signal bandwidth is narrow; while in Figure (b), after adjusting the attenuator and exciting the self-phase modulation effect, the spectrum is significantly broadened, more frequency components appear, and the bandwidth is significantly expanded, indicating that the intensity of the self-phase modulation effect has been adjusted to an appropriate level. The energy of the signal is concentrated in three main areas, corresponding to the center frequency of the original acousto-optic modulator, and the positive and negative frequency shifts excited by the rising and falling edges. After spectrum expansion, the spectrum of the signal expands to the frequency areas of the rising and falling edges based on the center frequency, showing an obvious spectrum broadening phenomenon.

[0065] Step 3. This embodiment designs three groups of short-time segmented overlapping filters. The window size of each group of filters is 4096 sampling points. The sampling points overlap. The filter adopts FIR filter and uses Hamming window to reduce spectrum leakage. The center frequency of the filter is respectively located at the center frequency of the original signal and the center of the frequency offset caused by the rising and falling edges of the pulse under the effect of self-phase modulation. The center frequencies of these offsets are saturated nonlinear transformations with the increase of transmission distance. However, in this embodiment, due to the small attenuation of optical fiber and transmission distance, the change of center frequency can be fitted into a linear relationship for processing. The basis of this fitting step can be obtained from Figure 3 (b) From the results of the short-time Fourier transform, it can be seen that the change characteristics of the center frequency are approximately linear. However, when the fiber length is further increased, the change of the center frequency must be fitted into a saturated nonlinear relationship to more accurately reflect the frequency offset characteristics. Specifically, at 47 km, the center of the frequency offset caused by the rising edge is about 40 MHz, and the center of the frequency offset caused by the falling edge is about 120 MHz. The center frequency of each segmented filter is set to f c , the spectrum width corresponding to the pulse width is set to f w , the bandwidth of the original filter is set to 4f w , the frequency phase shift transformation is performed by using the π phase shift method. The selection of the phase shift frequency band follows the principle of reducing the correlation of the amplitude curve generated after the transformation. The frequency band ranges of the phase shift transformation are: [f c -2f w ,f c ]、[f c -0.22f w ,f c +0.22f w ]、[f c -0.38f w ,f c +0.54f w ],[f c -0.18f w ,f c +0.54f w ]. These frequency band selections ensure that key frequency components in the signal can be effectively extracted under the self-phase modulation effect, while minimizing the correlation between the frequency bands, thereby improving the accuracy and robustness of signal processing.

[0066] Step 4: For each overlapping data segment processed in step 3, apply Hilbert transform to convert the real signal into a complex analytical signal, generate the real and imaginary parts of the signal, and form a complex representation of each data segment. Then, through the rotating vector method, the phase of each frequency component generated by different phase shift filters at the same transmission distance is uniformly adjusted to the reference phase to ensure smooth transition of the signals processed by different phase shift filters and effectively synthesize the information generated by different phase shift filters at the same distance. Finally, all the complex analytical signal segments processed by Hilbert transform and rotating vector method are superimposed and synthesized to form a complete complex analytical signal matrix, which fully integrates the multi-frequency components generated by the self-phase modulation effect to ensure the effective synthesis of the signal.

[0067] Step 5: Convert the signal into I / Q components through the IQ demodulation method to obtain the phase information of each signal segment. Then, calculate the phase difference at each transmission position, and use the phase difference calculation method with a fixed differential distance to obtain the phase change between different positions. Next, use the phase unwrapping algorithm to eliminate phase jumps and restore continuous phase changes. Finally, the processed phase information is used to reconstruct external disturbance events, thereby achieving high-fidelity reconstruction of external disturbances and improving the detection sensitivity and stability of the system.

[0068] Figure 4 It shows the situation when the sensing light does not undergo self-phase modulation, such as Figure 3 In the line condition shown in (a), the original AOM frequency of 80MHz is used as the center frequency, and the bandwidth of the original filter is set to 40MHz. In this case, the demodulation result after four phase shift transformations is performed.

[0069] Figure 5 (a) shows the situation when the sensing light undergoes self-phase modulation, such as Figure 3 In the line condition shown in (a), the frequency of the original AOM is used as the center frequency, and the bandwidth of the original filter is set to 40MHz. In this case, the frequency phase shift transformation is performed using the π phase shift method, and the demodulation result after four phase shift transformations is shown. Figure 5 (b) shows the demodulation result obtained by the method proposed in the present invention. It can be seen from the figure that in the 47km optical fiber link, coherent fading is completely suppressed. By comparison Figure 4 (a) and Figure 5 (a), it can be found that when the self-phase modulation effect occurs, the frequency energy near the center frequency of the acousto-optic modulation is reduced, resulting in a poor demodulation result, thereby limiting the performance of the Φ-OTDR system. However, using the method of the present invention, the new frequency components generated by the rising and falling edges are fully utilized during the demodulation process, which can not only effectively suppress coherent fading, but also improve the performance of the system while increasing the sensing distance.

[0070] Figure 6 (a) shows the demodulation result obtained according to the method proposed in the present invention, showing the time domain information of the restored disturbance at the position of 44.7km to 45.4km. Figure 6 (b) shows the frequency domain information of the restored disturbance, where the frequency domain signal-to-noise ratio is about 35.6 dB, indicating that the system can restore the signal of phase change in the external disturbance event with high fidelity. This shows that the method of the present invention can effectively improve the detection accuracy of the Φ-OTDR system and accurately capture and restore the disturbance events occurring in the sensing optical fiber.

Claims

1. A method for optimizing the performance of a phase-sensitive optical time-domain reflectometer based on self-phase modulation, the phase-sensitive optical time-domain reflectometer comprising: The invention comprises a narrow linewidth laser, namely a light source module, two optical fiber couplers, an adjustable attenuator, a circulator, an arbitrary waveform generator, an acousto-optic modulator, an erbium-doped optical fiber amplifier, a balanced detector, a data acquisition card, a polarization controller, a first sensing optical fiber and a second sensing optical fiber, and a piezoelectric ceramic. The invention is characterized in that the performance of a phase-sensitive optical time domain reflectometer is optimized by using a self-phase modulation effect combined with a short-time segmented overlapping filter and a digital domain processing technology of phase shift transformation. Specifically: First, when the self-phase modulation effect occurs due to high-power pulses reaching the nonlinear threshold, or by adjusting the pulse power of the input fiber to exceed the nonlinear threshold, the spectrum range of the detected scattered signal is expanded; Next, based on the short-time Fourier transform, the characteristics of the signal spectrum changing with the transmission distance are extracted, and a short-time segmented filter that adapts to the change of the transmission distance is designed to perform phase shift transformation on the signal. In this process, in order to ensure the continuity of the data, the overlapping data length is reasonably selected; Subsequently, the multi-frequency components caused by the self-phase modulation effect are processed and synthesized using an analysis and synthesis algorithm to generate a complex analysis signal matrix; Finally, through IQ demodulation, phase difference calculation and unwrapping process, high-precision demodulation phase information is obtained; The performance optimization includes significantly extending the sensing distance and effectively suppressing the coherent fading effect.

2. The method for optimizing the performance of a phase-sensitive optical time-domain reflectometer based on self-phase modulation according to claim 1, characterized in that: The specific steps are as follows: Step 1, when the high-power pulse reaches or exceeds the nonlinear threshold of self-phase modulation by adjusting the pulse power of the input optical fiber, the spectrum range of the scattered signal is expanded; Step 2: Use short-time Fourier transform to perform time-frequency domain analysis on the received scattered signal, and process the signal in segments to obtain the instantaneous spectrum characteristics at each transmission position; Step 3: design and apply a short-time overlapping segmented filter that varies with the transmission distance. According to the pulse spectrum characteristics obtained in step 2, design and apply multiple short-time segmented overlapping filters. The parameters of the filters are dynamically adjusted according to the transmission distance of the signal. Use these filters to perform phase shift transformation processing on the signal. Step 4: Use a signal analysis and synthesis algorithm to process and synthesize the multi-frequency components caused by the self-phase modulation effect to generate a complex analysis signal matrix; Step 5: Through IQ demodulation, phase difference calculation and phase unwrapping, high-precision phase information is obtained to achieve high-fidelity reconstruction of external disturbance events.

3. The method for optimizing the performance of a phase-sensitive optical time-domain reflectometer based on self-phase modulation according to claim 2, characterized in that: The self-phase modulation nonlinearity threshold mentioned in step 1 is its significance threshold, defined as P th =1 / (γL eff ), γ is called the nonlinear coefficient of the optical fiber, L eff =(1-e -αL ) / α represents the total equivalent nonlinear action length determined by the fiber loss and the total length, which is called the effective length of the fiber, where α represents the fiber attenuation coefficient and L represents the total fiber length. When the input power P0 exceeds this threshold, the self-phase modulation effect is significantly enhanced, resulting in spectrum broadening and nonlinear phase accumulation.

4. The method for optimizing the performance of a phase-sensitive optical time-domain reflectometer based on self-phase modulation according to claim 2, characterized in that: The specific process of step 1 is as follows: Step 1-1: Use a high-power pulsed laser to adjust the pulse width and repetition frequency to match the nonlinear characteristics of the optical fiber and system requirements; Step 1-2, gradually increase the input pulse power by directly adjusting the output power of the laser or increasing the amplification current of the erbium-doped fiber amplifier through an adjustable optical attenuator; Step 1-3: Use a spectrum analyzer to detect the output spectrum to confirm whether spectrum broadening occurs, or use short-time Fourier transform to perform time-frequency domain analysis on the beat frequency signal detected by heterodyne. According to the spectrum broadening effect, adjust the pulse power to the optimal level to ensure that the spectrum expansion meets the signal detection requirements.

5. The method for optimizing the performance of a phase-sensitive optical time-domain reflectometer based on self-phase modulation according to claim 2, characterized in that: In step 1, the input optical pulse is a pulse with a gentle rising edge and falling edge, including a Gaussian pulse or a super Gaussian pulse. The expression of the super Gaussian pulse is as follows: Among them, U(z,T) represents the signal pulse form obtained after the transmission loss is reduced to the normalized pulse amplitude, T0 represents the pulse width, and m reflects the sharpness of the pulse edge. When m=1, the above formula represents a Gaussian pulse, and the time scale is defined as: Among them, t represents time, z represents the transmission distance, and v g Represents the group velocity in the optical fiber.

6. The method for optimizing the performance of a phase-sensitive optical time-domain reflectometer based on self-phase modulation according to claim 2, characterized in that: The specific process of step 3 is as follows: Step 3-1, based on the pulse spectrum characteristics obtained in step 2, dynamically adjust the bandwidth and center frequency parameters of the filter according to the spectrum characteristics of the signal at different transmission distances, wherein the center frequency of the filter is located at the high and low frequency positions generated by the self-phase modulation effect and the initial shift frequency position; each filter segment corresponds to a specific transmission distance interval, ensuring that noise can be effectively filtered out and useful signals can be retained in each interval; Step 3-2, process the signal in segments, and keep a certain overlap length between each segment to ensure the continuity and integrity of the data; the overlap length is selected based on the time-frequency characteristics of the signal, which not only prevents the filter from generating transient effects at the segment boundaries, but also ensures a smooth transition between adjacent segments, avoiding the loss or distortion of amplitude and phase information, thereby ensuring the data continuity of the segment filter during the final data processing; Step 3-3, apply the designed short-time segmented overlapping filters to the received signal in sequence, and perform phase shift transformation processing; through the phase shift effect of the filter, adjust the phase information of the signal and suppress the coherent fading effect.

7. The method for optimizing the performance of a phase-sensitive optical time-domain reflectometer based on self-phase modulation according to claim 6, characterized in that: In step 3-1, the phase shift characteristic of the designed short-time segmented overlapping filter should minimize the correlation between the output amplitude curve and the reference signal; the design principle of the phase shift characteristic is to use only π phase shift, or combine π and π / 2 phase shift to optimize the phase transformation effect; In step 3-1, the center frequency of the short-time segmented overlapping filters of different frequency bands designed increases exponentially along the fiber transmission distance in a saturated nonlinear manner. The specific formula is derived as follows: Among them, δω(T) is the variation curve of frequency chirp in the whole pulse width, L NL =γP0 is called the nonlinear characteristic length Z of the optical fiber eff =(1-e -αz ) / α represents the equivalent nonlinear action length that varies with distance; Let the time derivative of δω(T) be 0, and the maximum value of δω(T) is: Among them, f is a constant related to the input pulse shape; the frequency chirp of the super-Gaussian pulse only appears near the rising and falling edges of the pulse, and its change is not linear; that is, the frequency at the center of the pulse will maintain the value of the initial frequency shift; therefore, three groups of short-time segmented overlapping filters are designed, and their center frequencies are located at the original center frequencies respectively; due to the self-phase modulation effect, frequency offsets are generated along the rising and falling edges of the pulse, and the center frequencies of these offsets show an exponential saturated nonlinear relationship with the increase of the transmission distance z.

8. The method for optimizing the performance of a phase-sensitive optical time-domain reflectometer based on self-phase modulation according to claim 6, characterized in that: In step 3-3, the phase shift transformation operation of the short-time segmented overlapping filter can also be implemented in the frequency domain through short-time Fourier transform, thereby further optimizing the phase adjustment process of the signal.

9. The method for optimizing the performance of a phase-sensitive optical time-domain reflectometer based on self-phase modulation according to claim 2, characterized in that: The specific process of step 4 is as follows: Step 4-1, for each overlapping data segment processed in step 3, using a signal analysis method, converting a real signal into a complex analytical signal, generating a real part and an imaginary part of the signal, and forming a complex representation of each data segment; Step 4-2: By using the rotating vector method or the superposition method, the phases of the frequency components generated by different phase shift filters at the same transmission distance are uniformly adjusted to the reference phase, so as to improve the continuity of the demodulated data along the optical fiber distance, ensure the smooth transition of the signals processed by different phase shift filters, and effectively synthesize the multi-frequency information at the same distance; Step 4-3: All the complex analytical signal segments after the above processing are superimposed and synthesized to form a complete complex analytical signal matrix, fully integrating the multi-frequency components generated by the self-phase modulation effect to ensure effective synthesis of the signal.

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