Method and system for solving phase rotation angle in φ-otdr based on superposition of rotation vector

Abstract

The present application relates to a kind of based on φ-OTDR in phase rotation angle solving method and system of vector superposition of rotation.Its method part mainly includes: according to pulse time interval, signal at a position of optical fiber is sampled n times, and the Rayleigh scattering light intensity and phase information at m different center frequencies are obtained each time sampling;For the Rayleigh scattering light intensity and phase information under two different center frequencies at the same time, intensity product and phase angle difference are obtained;If the maximum of intensity product is greater than the preset threshold, then the phase angle difference corresponding to the maximum is used as the phase rotation angle under the corresponding frequency, if the maximum of intensity product is not greater than the preset threshold, then the vector summation of intensity product and phase angle difference at all times is carried out, and the phase angle difference obtained by summation is used as the phase rotation angle under the corresponding frequency.The present application greatly improves the phase rotation angle solving precision, and optimizes the interference fading suppression effect.

Description

Technical Field

[0001] This invention relates to the field of distributed optical fiber sensing technology, and in particular to a method and system for solving the phase rotation angle in a φ-OTDR based on the superposition of rotating vectors. Background Technology

[0002] φ-OTDR is a distributed fiber optic sensing technology capable of monitoring vibration signals along optical fibers. It is currently widely used in fields such as oil pipeline inspection, perimeter security, and smart grid monitoring. φ-OTDR is based on Rayleigh scattering signals in optical fibers. When there is an external disturbance, it causes a change in the refractive index at that point in the fiber, resulting in changes in the phase and intensity of the Rayleigh scattering light. By measuring the backscattered Rayleigh light information generated by the optical pulse and using the time difference between signal reception and pulse emission, φ-OTDR can locate the area where the disturbance occurred.

[0003] From the perspective of signal demodulation methods, φ-OTDRs can be divided into two categories: intensity demodulation and phase demodulation. Intensity demodulation φ-OTDRs can only locate the vibration occurrence position, while phase demodulation φ-OTDRs can quantitatively reconstruct the external vibration signal. Therefore, phase demodulation φ-OTDRs have a broader application prospect. However, affected by measurement noise, the phase demodulation accuracy is directly related to the Rayleigh scattering light intensity: when the Rayleigh scattering light intensity is large, it is less affected by noise, and the phase demodulation accuracy is high; when the Rayleigh scattering light intensity is small, the signal is submerged in noise, and the phase demodulation accuracy will be significantly reduced, leading to false alarms and other events. Since φ-OTDRs usually use narrow linewidth light sources, the measured signal is the result of coherent superposition of Rayleigh scattering light within the pulse width, causing the Rayleigh scattering light intensity to fluctuate randomly along the fiber length. At some locations, due to destructive interference during superposition, the Rayleigh scattering light intensity approaches zero, which is called the interference fading phenomenon. In the interference fading region, the phase demodulation accuracy will be significantly reduced.

[0004] In recent years, various technical solutions have been proposed to address the interference fading problem, one of which is frequency division multiplexing (FDM). In φ-OTDR based on FDM, multi-frequency optical pulses are used for measurement. Since the interference fading regions differ for different optical frequencies, superimposing the measurement results obtained from demodulation at different optical frequencies can significantly reduce the probability of interference fading. However, since the demodulation result of φ-OTDR is a vector, its magnitude and phase represent the Rayleigh scattering intensity and phase at that point, respectively. When the magnitudes of two optical frequency measurement results are close and the angle between their phases is close to 180 degrees, direct vector addition will actually decrease the magnitude of the superimposed result, exacerbating the interference fading problem. To avoid this issue, a rotating vector superposition method is typically used. This involves rotating the phases of the measurement results at different optical frequencies by a fixed angle, making the angle between the superimposed vectors approach zero, thus ensuring the maximum magnitude is obtained after superposition. The rotating vector superposition method can effectively alleviate the interference fading problem while preserving phase changes caused by vibrations; however, the interference fading suppression effect is significantly affected by the rotation angle of the phases at different optical frequencies. In conclusion, existing technologies cannot achieve a good effect in suppressing interference fading.

[0005] In view of this, how to overcome the shortcomings of the existing technology and solve the technical problem of poor interference fading suppression effect is a difficult problem to be solved in this technical field. Summary of the Invention

[0006] To address the deficiencies or improvement needs in existing technologies, this invention provides a method and system for solving the phase rotation angle in φ-OTDR based on the superposition of rotating vectors. By analyzing the intensity product and phase angle difference between measurement results of different optical frequencies at different times, a more accurate phase rotation angle for different optical frequencies can be obtained, thereby ensuring the best interference fading suppression effect after the superposition of rotating vectors.

[0007] The embodiments of the present invention adopt the following technical solutions:

[0008] In a first aspect, the present invention provides a method for solving the phase rotation angle in φ-OTDR based on the superposition of rotation vectors, including:

[0009] The signal at a certain location in the optical fiber is sampled n times according to the pulse time interval, and each sampling acquires the intensity and phase information of Rayleigh scattered light at m different center frequencies.

[0010] For the intensity and phase information of Rayleigh scattered light at two different center frequencies at the same time, calculate their intensity product and phase angle difference, and determine whether the maximum value of the intensity product at all times is greater than a preset threshold.

[0011] If the maximum value of the intensity product is greater than a preset threshold, the phase angle difference corresponding to the maximum value is taken as the phase rotation angle at the corresponding frequency. If the maximum value of the intensity product is not greater than the preset threshold, the intensity product and phase angle difference at all times are vector-summed, and the phase angle difference obtained by the summation is taken as the phase rotation angle at the corresponding frequency.

[0012] Furthermore, the step of sampling the signal at a certain location in the optical fiber n times according to the pulse time interval, and acquiring Rayleigh scattering light intensity and phase information at m different center frequencies in each sampling, specifically includes:

[0013] Multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm are acquired, and the Rayleigh scattering light signal is obtained by measuring the multi-frequency optical pulses.

[0014] After the measurement signal passes through a digital filter with center frequencies of fa and fb, and then undergoes IQ demodulation, the measurement results at the optical frequencies of fa and fb can be obtained, where fa and fb ∈ {f1, f2, ..., fm}.

[0015] The optical fiber is sampled n times at a certain position z according to the pulse time interval to form the times t1, t2, ..., tn, where the time of the kth sampling is time tk, and tk∈{t1, t2, ..., tn};

[0016] At a certain position z in the optical fiber, the Rayleigh scattered light signals at frequencies fa and fb at time tk are respectively... and It can be represented as: Among them, ra k and Φ ak The intensity and phase of Rayleigh scattered light at frequency fa at time tk are respectively, r bk and Φ bk denoted as the intensity and phase of the Rayleigh scattered light at frequency fb at time tk, respectively; j indicates that it is a complex number.

[0017] Furthermore, the calculation of the intensity product and phase angle difference of Rayleigh scattered light at the same time and two different center frequencies specifically includes:

[0018] Calculate time tk and The product of their intensities: A(tk) = r ak ·r bk ;

[0019] Calculate time tk and The phase angle difference between them: ΔΦ(tk)=Φ ak -Φ bk .

[0020] Furthermore, if the maximum value of the intensity product is greater than a preset threshold, the phase angle difference corresponding to the maximum value is used as the phase rotation angle at the corresponding frequency. If the maximum value of the intensity product is not greater than the preset threshold, the intensity product and phase angle difference at all times are vector-summed, and the phase angle difference obtained by the summation is used as the phase rotation angle at the corresponding frequency. Specifically, this includes:

[0021] Find the maximum value A(tm) of the intensity product at all times, where tm∈{t1, t2, ..., tn};

[0022] If A(tm) is greater than the preset threshold A(th), then the phase angle difference ΔΦ(tm) at time tm is taken as the phase rotation angle ΔΦb at frequency fb.

[0023] If A(tm) is less than or equal to the preset threshold A(th), then a vector summation is performed over all time points: A(t1)e j ΔΦ(t1) +A(t2)e jΔΦ(t2) +…+A(tn)e jΔΦ(tn) =Ae jΔΦ At this point, the summation ΔΦ is taken as the phase rotation angle ΔΦb at frequency fb.

[0024] Furthermore, it also includes: multiplying the measurement results of other optical frequencies besides f1 by their corresponding phase rotation angles, and then performing vector superposition, thereby achieving rotational vector superposition of different optical frequencies at the z position of the fiber. For time tx, tx∈{t1, t2, ..., tn}, the superposition result is expressed as:

[0025] Furthermore, the acquisition of multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm specifically includes:

[0026] Multiple frequency sideband signals are generated, and then a fixed frequency shift is introduced and pulse chopping is performed to finally obtain multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm.

[0027] Furthermore, when acquiring multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm, m ≥ 3; when sampling a certain position z of the optical fiber n times according to the pulse time interval, the value of n ranges from 50 to 500.

[0028] Furthermore, the preset threshold A(th) is set based on the average intensity of Rayleigh scattering light at that location in the optical fiber. If the average intensity of Rayleigh scattering light at that location is a, then the preset threshold A(th) is set to 0.6a. 2 .

[0029] Secondly, the present invention provides a phase rotation angle solution system for φ-OTDR based on rotation vector superposition, used to implement the phase rotation angle solution method for φ-OTDR based on rotation vector superposition as described in the first aspect. The system includes a narrow linewidth light source, an electro-optic modulator, an acousto-optic modulator, a signal generator, an optical amplifier, a circulator, a sensing fiber, a balanced detector, and a data acquisition card, wherein:

[0030] The signal generator is used to drive the electro-optic modulator to generate sideband signals of multiple frequencies. Then, a fixed frequency shift is introduced through the acousto-optic modulator and pulse chopping is performed to finally obtain multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm and pulse repetition frequency of Fp. The multi-frequency optical pulses are amplified by the optical amplifier and injected into the sensing fiber through the circulator for measurement. Rayleigh scattered light is detected by the balanced detector and then acquired by the data acquisition card.

[0031] After the data acquisition card acquires the measurement signal, it passes the signal through a digital filter with center frequencies of fa and fb, and then undergoes IQ demodulation to obtain the measurement results at the fa and fb optical frequencies. The Rayleigh scattering light intensity and phase information are then calculated, and their intensity product and phase angle difference are obtained. It is then determined whether the maximum value of the intensity product at all times is greater than a preset threshold. If the maximum value of the intensity product is greater than the preset threshold, the phase angle difference corresponding to that maximum value is used as the phase rotation angle at the corresponding frequency. If the maximum value of the intensity product is not greater than the preset threshold, the intensity product and phase angle difference at all times are vector-summed, and the summed phase angle difference is used as the phase rotation angle at the corresponding frequency.

[0032] Furthermore, the linewidth of the narrow linewidth light source is less than 3kHz; the sideband frequency interval of the sideband signal generated by the electro-optic modulator is 20-200MHz; the frequency shift of the acousto-optic modulator is 40-200MHz, the chopping pulse width is 50-500ns, and the pulse repetition frequency Fp is 200Hz-10kHz; the bandwidth of the digital filter is less than half of the adjacent frequency interval.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] (1) This invention analyzes the intensity product and phase angle difference between measurement results at different times and different optical frequencies, which greatly improves the accuracy of phase rotation angle calculation and optimizes the interference fading suppression effect.

[0035] (2) This invention uses the maximum value of the product of different optical frequency intensities as a reference: when the intensity product is large, the phase angle difference is considered to have high accuracy, and the phase angle difference at that moment is directly used as the phase rotation angle; when the intensity product is small, the phase angle differences at different moments are summed according to the intensity product, thereby improving the accuracy of the phase rotation angle calculation. The above method effectively reduces the algorithm complexity while ensuring the accuracy of the calculation.

[0036] (3) The algorithm of this invention has high reliability and can be applied to the superposition of rotation vectors of any number of optical frequencies, with a wide range of application scenarios. Attached Figure Description

[0037] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments of the present invention will be briefly described below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0038] Figure 1 A flowchart of a method for solving the phase rotation angle in φ-OTDR based on the superposition of rotation vectors provided in Embodiment 1 of the present invention;

[0039] Figure 2 This is an architecture diagram of a phase rotation angle solution system based on rotation vector superposition in φ-OTDR provided in Embodiment 2 of the present invention;

[0040] Figure 3 This is a schematic diagram of the multi-frequency pulse spectrum provided in Embodiment 3 of the present invention;

[0041] Figure 4 This is a schematic diagram of the Rayleigh scattering light intensity at different optical frequencies at 1-2 km along the sensing fiber, provided in Embodiment 3 of the present invention.

[0042] Figure 5 This is a schematic diagram of the product of Rayleigh scattering light intensity at 50MHz and 80MHz optical frequencies in the vibration region of the optical fiber end provided in Embodiment 3 of the present invention;

[0043] Figure 6 This is a schematic diagram of the phase angle difference of Rayleigh scattered light at 50MHz and 80MHz optical frequencies in the vibration region of the optical fiber end provided in Embodiment 3 of the present invention.

[0044] Figure 7 This is a phase diagram showing the result of vector rotation of the 80MHz and 110MHz optical frequency measurement results and superposition with the 50MHz optical frequency measurement results provided in Embodiment 3 of the present invention. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0046] This invention is an architecture of a specific functional system. Therefore, the specific embodiments mainly describe the functional logic relationship of each structural module, and do not limit the specific software and hardware implementation methods.

[0047] Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other, and the order of the steps can be changed as long as they are logical and do not conflict. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0048] Example 1:

[0049] This invention provides a method for solving the phase rotation angle in φ-OTDR based on the superposition of rotating vectors. By analyzing the intensity product and phase angle difference between measurement results of different optical frequencies at different times, a more accurate phase rotation angle is obtained to ensure optimal interference fading suppression after the superposition of rotating vectors. Figure 1 As shown, the method includes the following steps.

[0050] Step 100: Sample the signal at a certain location in the optical fiber n times according to the pulse time interval, and obtain the Rayleigh scattering light intensity and phase information at m different center frequencies for each sampling.

[0051] Step 200: For the intensity and phase information of Rayleigh scattered light at the same time and two different center frequencies, calculate their intensity product and phase angle difference, and determine whether the maximum value of the intensity product at all times is greater than a preset threshold.

[0052] Step 300: If the maximum value of the intensity product is greater than the preset threshold, the phase angle difference corresponding to the maximum value is taken as the phase rotation angle at the corresponding frequency. If the maximum value of the intensity product is not greater than the preset threshold, the intensity product and phase angle difference at all times are vector summed, and the phase angle difference obtained by the summation is taken as the phase rotation angle at the corresponding frequency.

[0053] Specifically, step 100 of this embodiment, "sampling the signal at a certain location in the optical fiber n times according to the pulse time interval, and acquiring Rayleigh scattering light intensity and phase information at m different center frequencies in each sampling," specifically includes the following process: acquiring multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm, and using the multi-frequency optical pulses for measurement to obtain Rayleigh scattering light signals; after the measurement signals pass through digital filters with center frequencies of fa and fb, and then undergo IQ demodulation, the measurement results at the fa and fb optical frequencies can be obtained, where fa, fb ∈ {f1, f2, ..., fm}. It should be noted that in this embodiment, ... Using fa as the reference and fixed at f1 (or any other optical frequency as the reference), fb is tested at frequencies f2, ..., fm (other than the reference) to obtain the intensity product and phase angle difference of each optical frequency relative to the f1 optical frequency (the reference frequency), thus obtaining the phase rotation angle at each optical frequency. At a certain position z in the optical fiber, n samples are taken according to the pulse time interval to form times t1, t2, ..., tn, where the time of the k-th sample is time tk, tk ∈ {t1, t2, ..., tn}. At a certain position z in the optical fiber, the Rayleigh scattered light signals at frequencies fa and fb at time tk are respectively... and It can be represented as: Among them, ra k and Φ ak The intensity and phase of Rayleigh scattered light at frequency fa at time tk are respectively, r bk and Φ bk denoted as the intensity and phase of the Rayleigh scattered light at frequency fb at time tk, respectively; j indicates that it is a complex number.

[0054] In the extended process of step 100 in this embodiment, obtaining multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm specifically includes: generating sideband signals of multiple frequencies, introducing a fixed frequency shift and performing pulse chopping to finally obtain multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm. Furthermore, when obtaining multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm, m ≥ 3; and when sampling a certain position z in the optical fiber n times according to the pulse time interval, the value of n ranges from 50 to 500.

[0055] In step 200 of this embodiment, "for the intensity and phase information of Rayleigh scattered light at the same time and two different center frequencies, calculate their intensity product and phase angle difference," the specific process includes the following steps: Calculate time tk. and The product of their intensities: A(tk) = r ak ·r bk ; Calculate time tk and The phase angle difference between them: ΔΦ(tk)=Φ ak -Φ bk .

[0056] In step 300 of this embodiment, "if the maximum value of the intensity product is greater than a preset threshold, then the phase angle difference corresponding to the maximum value is used as the phase rotation angle at the corresponding frequency; if the maximum value of the intensity product is not greater than the preset threshold, then the intensity product and phase angle difference at all times are vector-summed, and the phase angle difference obtained by the summation is used as the phase rotation angle at the corresponding frequency," specifically includes: calculating the maximum value A(tm) of the intensity product at all times, tm∈{t1, t2, ..., tn}; if A(tm) is greater than the preset threshold A(th), then the phase angle difference ΔΦ(tm) at time tm is used as the phase rotation angle ΔΦb at frequency fb; if A(tm) is less than or equal to the preset threshold A(th), then the vector summation is performed at all times: A(t1)e jΔΦ(t1) +A(t2)e jΔΦ(t2) +…+A(tn)e jΔΦ(tn) =Ae jΔΦ At this point, the summation ΔΦ is taken as the phase rotation angle ΔΦb at frequency fb.

[0057] In the extended process of step 300 in this embodiment, the preset threshold A(th) is set according to the average intensity of Rayleigh scattering light at that location in the optical fiber. If the average intensity of Rayleigh scattering light at that location is a, then the preset threshold A(th) is set to 0.6a. 2 .

[0058] After completing steps 100-300 above, this embodiment further includes: multiplying the measurement results of other optical frequencies besides f1 by their corresponding phase rotation angles, and then performing vector superposition, thereby realizing the rotational vector superposition of different optical frequencies at the z position of the optical fiber. For time tx, tx∈{t1, t2, ..., tn}, the superposition result is expressed as:

[0059] In summary, this invention analyzes the intensity product and phase angle difference between measurement results at different times and optical frequencies, significantly improving the accuracy of phase rotation angle calculation and optimizing the interference fading suppression effect. This invention uses the maximum value of the intensity product of different optical frequencies as a reference: when the intensity product is large, the phase angle difference is considered to have high accuracy, and the phase angle difference at that moment is directly used as the phase rotation angle; when the intensity product is small, the phase angle differences at different times are summed according to the intensity product, thereby improving the accuracy of the phase rotation angle calculation. This method effectively reduces algorithm complexity while ensuring calculation accuracy. The algorithm of this invention has high reliability, is applicable to the superposition of rotation vectors of any number of optical frequencies, and has a wide range of applications.

[0060] Example 2:

[0061] Based on the phase rotation angle solution method in φ-OTDR based on the superposition of rotating vectors provided in Embodiment 1, this Embodiment 2 provides a phase rotation angle solution system in φ-OTDR based on the superposition of rotating vectors to implement the method of Embodiment 1.

[0062] refer to Figure 2 As shown, the system in this embodiment includes a narrow linewidth light source, an electro-optic modulator, an acousto-optic modulator, a signal generator, an optical amplifier, a circulator, a sensing fiber, a balanced detector, and a data acquisition card. The signal generator drives the electro-optic modulator to generate sideband signals of multiple frequencies. These signals are then introduced with a fixed frequency shift by the acousto-optic modulator and pulsed, ultimately obtaining multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm, and a pulse repetition frequency of Fp. The multi-frequency optical pulses are amplified by the optical amplifier and injected into the sensing fiber through the circulator for measurement. Rayleigh scattering light is detected by the balanced detector and then acquired by the data acquisition card. After acquiring the measurement signals, the data acquisition card processes them according to the pulse time interval (...). The measurement signal is passed through digital filters with center frequencies of fa and fb (pulse time interval 1 / Fp) and then demodulated by IQ to obtain the measurement results at the fa and fb optical frequencies. Then, the intensity and phase information of the Rayleigh scattered light are calculated, and their intensity product and phase angle difference are obtained. It is then determined whether the maximum value of the intensity product at all times is greater than a preset threshold. If the maximum value of the intensity product is greater than the preset threshold, the phase angle difference corresponding to the maximum value is taken as the phase rotation angle at the corresponding frequency. If the maximum value of the intensity product is not greater than the preset threshold, the intensity product and phase angle difference at all times are vector-summed, and the phase angle difference obtained by the summation is taken as the phase rotation angle at the corresponding frequency.

[0063] Specifically, the operation process after the data acquisition card acquires the measurement signal can be expanded to the following steps.

[0064] Step 1: After the measurement signal passes through digital filters with center frequencies f1 and f2, and then undergoes IQ demodulation, the measurement results at optical frequencies f1 and f2 can be obtained. At a certain position z in the optical fiber, the Rayleigh scattering signals at frequencies f1 and f2 at time t1 are respectively... and It can be represented as:

[0065]

[0066]

[0067] Where r11 and Φ11 are the intensity and phase of Rayleigh scattered light at frequency f1 at time t1, respectively, and r21 and Φ21 are the intensity and phase of Rayleigh scattered light at frequency f2 at time t1, respectively. Calculate the intensity and phase of Rayleigh scattered light at time t1. and The product of their intensities A(t1) = r 11 ·r 21 And the phase angle difference ΔΦ(t1)=Φ 11 -Φ 21 .

[0068] Step 2: Sample the signal at position z of the fiber n times according to the pulse time interval. For times t2, t3, ..., tn, calculate the intensity products A(t2), A(t3), ..., A(tn) and the phase angle differences ΔΦ(t2), ΔΦ(t3), ..., ΔΦ(tn) respectively according to Step 1, where:

[0069] A(tk)=r 1k ·r 2k

[0070] ΔΦ(tk)=Φ 1k -Φ 2k

[0071] r 1k and r 2k The Rayleigh scattering intensities at frequencies f1 and f2 at time tk are Φ, respectively. 1k and Φ 2k These are the Rayleigh scattering phases at frequencies f1 and f2 at time tk, respectively.

[0072] Step 3: Calculate the maximum value A(tm) in the intensity product. If A(tm) is greater than the preset threshold A(th), then the vector angle difference ΔΦ(tm) at time tm is taken as the phase rotation angle ΔΦ2 at frequency f2. If A(tm) is less than or equal to the preset threshold A(th), then the vector summation is calculated according to the following formula:

[0073] A(t1)e jΔΦ(t1) +A(t2)e jΔΦ(t2) +…+A(tn)e jΔΦ(tn) =Ae jΔΦ At this point, the summation ΔΦ is taken as the phase rotation angle ΔΦ2 at frequency f2.

[0074] Step four: For other optical frequencies, calculate the vector rotation angle (i.e., phase rotation angle) according to steps one through three, obtaining the vector rotation angles ΔΦ3, ..., ΔΦm for f3, ..., fm. Multiply the measurement results of other optical frequencies (except f1) by their corresponding phase rotation angles, and then perform vector superposition to obtain the superimposed result. Taking time tx as an example, the superimposed result can be expressed as:

[0075] This enables the superposition of rotational vectors of different optical frequencies at the z-position of the optical fiber. The method for superimposing rotational vectors of different optical frequencies at other positions of the optical fiber is the same as that at the z-position.

[0076] In this preferred embodiment, the narrow linewidth of the light source is typically less than 3kHz. The electro-optic modulator can be replaced by a phase modulator, etc., its main purpose being to generate multiple frequency sidebands. The sideband frequency spacing of the sideband signals generated by the electro-optic modulator or phase modulator, etc., is typically 20-200MHz. The acousto-optic modulator typically has a frequency shift of 40-200MHz, a chopping pulse width of 50-500ns, and a pulse repetition frequency Fp of 200Hz-10kHz. Furthermore, in the above process, m≥3, the digital filter bandwidth in step one is less than half the adjacent frequency interval, the value of n in step two ranges from 50-500, and the preset threshold A(th) in step three can be set according to the average intensity of Rayleigh scattering light at that location in the optical fiber. For example, if the average intensity of Rayleigh scattering light at that location is a, then the preset threshold A(th) can be set to 0.6a. 2 .

[0077] In summary, this invention analyzes the intensity product and phase angle difference between measurement results at different times and optical frequencies, significantly improving the accuracy of phase rotation angle calculation and optimizing the interference fading suppression effect. This invention uses the maximum value of the intensity product of different optical frequencies as a reference: when the intensity product is large, the phase angle difference is considered to have high accuracy, and the phase angle difference at that moment is directly used as the phase rotation angle; when the intensity product is small, the phase angle differences at different times are summed according to the intensity product, thereby improving the accuracy of the phase rotation angle calculation. This method effectively reduces algorithm complexity while ensuring calculation accuracy. The algorithm of this invention has high reliability, is applicable to the superposition of rotation vectors of any number of optical frequencies, and has a wide range of applications.

[0078] Example 3:

[0079] Based on the phase rotation angle solution method and system in φ-OTDR based on rotation vector superposition provided in Embodiments 1 and 2, this Embodiment 3 provides a specific example to illustrate the invention in detail.

[0080] refer to Figure 2As shown in the system diagram, in this embodiment, the light source is a 1550nm narrow-linewidth laser with a linewidth of 2kHz. After passing through coupler 1 with a splitting ratio of 90:10, 90% of the light enters the electro-optic modulator, where double-sideband modulation generates two sidebands with a frequency interval of 30MHz near the center frequency of the optical carrier. Subsequently, the continuous light is modulated into pulses with a pulse width of 200ns by an acousto-optic modulator, and an 80MHz fixed frequency is introduced for up-shifting, resulting in a pulse repetition frequency of 2kHz. A schematic diagram of the multi-frequency pulse spectrum is shown below. Figure 3 As shown, there are three main frequency components: 50MHz, 80MHz, and 110MHz. The multi-frequency pulses are then amplified by an erbium-doped fiber amplifier (...). Figure 2 (EDFA) amplification, through circulator ( Figure 2 The 1, 2, and 3 ring structures between the EDFA and the sensing fiber are injected into a 5km sensing fiber. The backscattered Rayleigh light returning from the fiber is coherently coupled with the local oscillator light, and the resulting beat frequency signal is received by a balanced detector. The data acquisition card collects the output of the balanced detector.

[0081] The acquired signals were passed through digital bandpass filters with center frequencies of 50MHz, 80MHz, and 110MHz, respectively, with a filter bandwidth of 5MHz. The filtered signals were then demodulated using IQ modulation to obtain Rayleigh scattering intensity and phase information. The Rayleigh scattering intensity at different optical frequencies at 1km-2km along the fiber is shown below. Figure 4 As shown, the interference fading regions are different for different optical frequencies. By superimposing the measurement results of different optical frequencies, it may be possible to eliminate the interference fading phenomenon.

[0082] A piezoelectric ceramic was placed at 4950m at the end of the sensing fiber to generate a 50Hz sinusoidal vibration signal. The sampling rate of the data acquisition card was 500MSa / s, and a total of 100 sets of pulse signal measurement results were collected. It should be noted that, compared with Example 1, m in this example is 3, meaning 50MHz, 80MHz, and 110MHz correspond to f1, f2, and f3 in Example 1, respectively. f1 = 50MHz is the reference optical frequency used for calculation in this example. Based on this, for the vibration region at the end of the fiber, the product of the Rayleigh scattering light intensity at 50MHz and 80MHz optical frequencies was calculated, and the measurement results are as follows: Figure 5 As shown, the intensity of Rayleigh scattered light in the vibration region also fluctuates. The intensity product A(t11) of the 11th measurement has a maximum value of 0.0077, while the intensity product A(t57) of the 57th measurement is smaller, only 0.00072.

[0083] The phase angle difference between 50MHz and 80MHz optical frequencies is as follows Figure 6As shown in the figure, since the frequency difference between the 50MHz and 80MHz optical frequencies is small, the phase changes caused by artificial vibrations are almost the same. Therefore, theoretically, the phase angle difference between the two optical frequencies should remain constant. In reality, due to the influence of noise such as Gaussian white noise, the phase angle difference between the 50MHz and 80MHz optical frequencies constantly changes. At time t11, which is the moment when the intensity product is at its maximum, the phase angle difference ΔΦ(t11) is 0.063 rad, while at time t57, which is the moment when the intensity product is smaller, the phase angle difference is 0.893 rad. From the distribution of the phase angle difference in 100 measurements, it can be seen that when the intensity product is small, the phase angle difference deviation is usually large, while when the intensity product is large, the phase angle difference remains basically the same. This result shows that the phase angle difference is directly related to the intensity product; the larger the intensity product, the higher the accuracy of the phase angle difference calculation. Using the intensity product as a reference, analyzing the phase angle difference at different times can obtain a more accurate vector rotation angle (phase rotation angle).

[0084] Assumption: By averaging the Rayleigh scattering intensity at this point (4950m from the end of the sensing fiber) over 1 minute, the average Rayleigh scattering intensity at this point is found to be 0.14. Therefore, the preset threshold A(th) is set to 0.0118. Since the maximum value of the intensity product A(t11) is 0.0077, which is less than the preset threshold A(th), the summation is calculated according to the following formula:

[0085] A(t1)e jΔΦ(t1) +A(t2)e jΔΦ(t2) +…+A(tn)e jΔΦ(tn) =Ae jΔΦ

[0086] The calculated ΔΦ is 0.0582 rad. This 0.0582 rad is taken as the phase rotation angle at the 80 MHz optical frequency (if the maximum value of the intensity product is greater than a preset threshold, the phase angle difference corresponding to the maximum intensity product is directly taken as the phase rotation angle at the corresponding frequency). For the 110 MHz optical frequency, the same steps are followed as for the 80 MHz optical frequency to calculate the phase rotation angle, which is -0.4307 rad (that is, calculating the product of Rayleigh scattering light intensity and the phase angle difference at 50 MHz and 110 MHz optical frequencies to obtain the phase rotation angle of the 110 MHz optical frequency relative to the 50 MHz optical frequency). The measurement results at different times for the 80 MHz and 110 MHz optical frequencies are multiplied by the phase rotation angle, and then vector-superimposed with the measurement results for the 50 MHz optical frequency. The superimposed vector phase is as follows. Figure 7 As shown in the figure, the phase obtained by the solution matches the vibration signal well. Both are sinusoidal vibration signals with a vibration frequency of 50Hz, which proves that the proposed algorithm can obtain a more accurate vector rotation angle, thereby better suppressing the interference fading phenomenon.

[0087] In summary, this invention analyzes the intensity product and phase angle difference between measurement results at different times and optical frequencies, significantly improving the accuracy of phase rotation angle calculation and optimizing the interference fading suppression effect. This invention uses the maximum value of the intensity product of different optical frequencies as a reference: when the intensity product is large, the phase angle difference is considered to have high accuracy, and the phase angle difference at that moment is directly used as the phase rotation angle; when the intensity product is small, the phase angle differences at different times are summed according to the intensity product, thereby improving the accuracy of the phase rotation angle calculation. This method effectively reduces algorithm complexity while ensuring calculation accuracy. The algorithm of this invention has high reliability, is applicable to the superposition of rotation vectors of any number of optical frequencies, and has a wide range of applications.

[0088] Those skilled in the art will understand that all or part of the steps in the various methods of the embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, which may include: read-only memory (ROM), random access memory (RAM), disk or optical disk, etc.

[0089] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A method for solving phase rotation angle in φ-OTDR based on superposition of rotation vectors, characterized in that, The method comprises the following steps: n times of sampling of the signal at a position of the optical fiber are performed at pulse time intervals, and m different center frequency Rayleigh scattering light intensity and phase information are obtained each time; the method comprises the following steps: obtaining multi-frequency light pulses with frequency shifts of f1, f2, …, and fm, and performing measurement using the multi-frequency light pulses to obtain Rayleigh scattering light signals; after the measurement signals pass through digital filters with center frequencies of fa, fb, and then pass through demodulation, the measurement results at the light frequencies of fa and fb can be obtained, wherein fa∈{f1, f2, …, fm}, fb∈{f1, f2, …, fm}; for the Rayleigh scattering light intensity and phase information at two different center frequencies at the same time, the intensity product and the phase angle difference are calculated, and it is determined whether the maximum value of the intensity product at all times is greater than a preset threshold value; fa is defined as the reference light frequency, and one of f1, f2, …, and fm is selected as the reference light frequency, and fb is selected as the light frequency other than the reference light frequency to perform testing, so as to obtain the intensity product and the phase angle difference of each light frequency relative to the reference light frequency; if the maximum value of the intensity product is greater than the preset threshold value, the phase angle difference corresponding to the maximum value is taken as the phase rotation angle at the corresponding frequency, and if the maximum value of the intensity product is not greater than the preset threshold value, the vector sum of the intensity product and the phase angle difference at all times is calculated, and the phase angle difference obtained by the sum is taken as the phase rotation angle at the corresponding frequency.

2. The method of claim 1, wherein the phase rotation angle is calculated by: ###0001### where φ is the phase rotation angle, φ0 is the initial phase rotation angle, φ1 is the phase rotation angle at the first time, φ2 is the phase rotation angle at the second time, and T is the time interval between the first time and the second time. The n times of sampling of the signal at a position of the optical fiber at pulse time intervals, and m different center frequency Rayleigh scattering light intensity and phase information are obtained each time, specifically comprises the following steps: n times of sampling are performed on the position z of the optical fiber at pulse time intervals to form t1, t2, …, and tn, wherein the time of the kth sampling is tk, and tk∈{t1, t2, …, tn}; At a position z of the optical fiber, the Rayleigh scattering light signals at frequencies faand fb at time tk are respectively and , which can be expressed as: , ; wherein, r k and Φ ak are the Rayleigh scattering light intensity and phase at frequency faat time tk, r bk and Φ bk are the Rayleigh scattering light intensity and phase at frequency fb at time tk; j represents a complex number.

3. The method of claim 2, wherein the phase rotation angle is calculated by: ###0001### where φ is the phase rotation angle, φ0 is the initial phase rotation angle, φ1 is the phase rotation angle at the first time, φ2 is the phase rotation angle at the second time, and T is the time interval between the first time and the second time. The calculation of the intensity product and the phase angle difference for the Rayleigh scattering light intensity and phase information at two different center frequencies at the same time specifically comprises the following steps: At the time tk The intensity product between ; and ; The phase angle difference between the time instants tk and tk-1 is: The phase angle difference between the time instants tk and tk-1 is: The phase angle difference between the time instants tk and tk-1 is: The 4. The method of claim 3, wherein the phase rotation angle is calculated by: ###0002### where φ is the phase rotation angle, φ0 is the initial phase rotation angle, φ1 is the phase rotation angle at the first time, φ2 is the phase rotation angle at the second time, and T is the time interval between the first time and the second time. if the maximum value of the intensity product is greater than the preset threshold value, the phase angle difference corresponding to the maximum value is taken as the phase rotation angle at the corresponding frequency, and if the maximum value of the intensity product is not greater than the preset threshold value, the vector sum of the intensity product and the phase angle difference at all times is calculated, and the phase angle difference obtained by the sum is taken as the phase rotation angle at the corresponding frequency, specifically comprising the following steps: calculating the maximum value A(tm) of the intensity product at all times, tm∈{t1, t2, …, tn}; if A(tm) is greater than a preset threshold value A(th), the phase angle difference ΔΦ(tm) at tm is taken as the phase rotation angle ΔΦb at the frequency of fb; If A(tm) is less than or equal to a preset threshold A(th), vector summation is calculated for all time points: At this time, ΔΦ obtained by the summation is taken as a phase rotation angle ΔΦb at the fb frequency.

5. The method of claim 4, wherein the phase rotation angle is calculated by The method further comprises the following steps: The other optical frequency measurement results except f1 are multiplied by their corresponding phase rotation angles respectively, and then vector superposition is performed, so as to realize rotation vector superposition of different optical frequencies at the z position of the optical fiber. For the tx moment, tx∈{t1, t2, …, tn}, the superposition result is expressed as: .

6. The method for solving the phase rotation angle in φ-OTDR based on superposition of rotation vectors according to any one of claims 2-5, characterized in that, The obtaining of the multi-frequency light pulses with frequency shifts of f1, f2, …, and fm specifically comprises the following steps: a plurality of frequency sideband signals are generated, a fixed size frequency shift is introduced, and pulse chopping is performed, and finally the multi-frequency light pulses with frequency shifts of f1, f2, …, and fm are obtained.

7. The method for solving the phase rotation angle in φ-OTDR based on the superposition of the rotation vector according to any one of claims 2-5, characterized in that, When acquiring multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm, m ≥ 3; when sampling a certain position z of the optical fiber n times according to the pulse time interval, the value of n ranges from 50 to 500.

8. The method for solving the phase rotation angle in φ-OTDR based on the superposition of the rotation vector according to any one of claims 4-5, characterized in that, The preset threshold A(th) is set according to the average intensity of Rayleigh scattering light at the position of the optical fiber. If the average intensity of Rayleigh scattering light at the position is a, the preset threshold A(th) is set as 0.6a 2 . 9.A system for solving phase rotation angle in φ-OTDR based on superposition of rotation vectors, characterized in that, This includes a narrow linewidth light source, an electro-optic modulator, an acousto-optic modulator, a signal generator, an optical amplifier, a circulator, sensing optical fibers, a balanced detector, and a data acquisition card, among which: The signal generator is used to drive the electro-optic modulator to generate sideband signals of multiple frequencies. Then, a fixed frequency shift is introduced through the acousto-optic modulator and pulse chopping is performed to finally obtain multi-frequency optical pulses with frequency shifts of f1, f2, ..., fm and pulse repetition frequency of Fp. The multi-frequency optical pulses are amplified by the optical amplifier and injected into the sensing fiber through the circulator for measurement. Rayleigh scattered light is detected by the balanced detector and then acquired by the data acquisition card. After the data acquisition card acquires the measurement signal, the signal is passed through a digital filter with center frequencies of fa and fb, and then demodulated by IQ to obtain the measurement results at the optical frequencies of fa and fb, where fa∈{f1, f2, ..., fm} and fb∈{f1, f2, ..., fm}. The Rayleigh scattering intensity and phase information are then calculated, and their intensity product and phase angle difference are obtained. fa is designated as the reference optical frequency, and one of the optical frequencies f1, f2, ..., fm is randomly selected as the reference optical frequency. For fb, optical frequencies other than the reference optical frequency are selected for testing to obtain the intensity product and phase angle difference of each optical frequency relative to the reference optical frequency. It is then determined whether the maximum value of the intensity product at all times is greater than a preset threshold. If the maximum value of the intensity product is greater than the preset threshold, the phase angle difference corresponding to the maximum value is used as the phase rotation angle at the corresponding frequency. If the maximum value of the intensity product is not greater than the preset threshold, the intensity product and phase angle difference at all times are vector-summed, and the summed phase angle difference is used as the phase rotation angle at the corresponding frequency.

10. The system for solving the phase rotation angle in φ-OTDR based on the superposition of the rotation vector according to claim 9, wherein, The linewidth of the narrow-linewidth light source is less than 3kHz; the sideband frequency interval of the sideband signal generated by the electro-optic modulator is 20-200MHz; the frequency shift of the acousto-optic modulator is 40-200MHz, the chopping pulse width is 50-500ns, and the pulse repetition frequency Fp is 200Hz-10kHz; the bandwidth of the digital filter is less than half of the adjacent frequency interval.

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