CN117009709A - Method for extracting equal time division dual-frequency optical phi-OTDR phase signal by bidirectional differential motion - Google Patents
Method for extracting equal time division dual-frequency optical phi-OTDR phase signal by bidirectional differential motion Download PDFInfo
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Abstract
The invention discloses a method for extracting equal time division dual-frequency optical phi-OTDR phase signals by bidirectional difference, which comprises the steps of sequentially processing data by measures of simple arrangement, band-pass separation, solving of the product of a modulus value and an initial phase, calculating of the product of a distance and a value, calculating of the product of a minimum value, confirming of a reference position, phase calculation before unwrapping, solving of the phase signals and the like aiming at a coherent detection curve. The invention provides a simple and easy method for selecting the reference position in the equal time division dual-frequency optical phase optical time domain reflectometer, which not only eliminates the inconsistency of the primary phase along the length direction of the optical fiber, but also avoids noise introduced by the reference phase. The method not only breaks through the limit of a phase unwrapping algorithm on pi value in the single-frequency optical phase optical time domain reflectometer, but also realizes accurate reconstruction of phase signals in the equal-time-division dual-frequency optical phase optical time domain reflectometer.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a method for extracting equal time division dual-frequency optical phi-OTDR phase signals by bidirectional differential.
Background
With the progress of related technologies such as optical fiber manufacturing and electronic communication, distributed optical fiber sensing technology has been increasingly applied in various fields such as roads, bridges, buildings, dams, power grids, submarine optical cables and the like. The mechanism according to the distributed optical fiber sensing technology can be classified into a rayleigh scattering type distributed optical fiber sensing technology, a brillouin scattering type distributed optical fiber sensing technology and a raman scattering type distributed optical fiber sensing technology. The distributed optical fiber sensing technology based on Rayleigh scattering has the highest return power and the best signal-to-noise ratio, and an optical time domain reflectometer, a coherent optical time domain reflectometer, a phase optical time domain reflectometer and a polarized optical time domain reflectometer are sequentially developed in the aspect of time domain reflection. The optical time domain reflectometer can measure events such as break points, has a simple structure and low manufacturing cost, but has a relatively short sensing distance. And the coherent optical time domain reflectometer can just make up for the defect of short detection distance of the optical time domain reflectometer. In order to have a sufficient coherence length, a high coherence light source is used in a coherent optical time domain reflectometer, and the time domain curve received by the photodetector is a coherent rayleigh curve that undulates along the optical fiber. In the coherent optical time domain reflectometer, researchers use various methods such as multiple averages, scrambling and the like to eliminate the influence of such high and low fluctuation so as to improve the ability of identifying static events. However, the coherent rayleigh curve with the high and low fluctuation can be used by researchers to solve the phase by using the curve or the curve after the direct current is eliminated. When the optical fiber senses an external disturbance event, the phase changes. In turn, one can measure the external disturbance event by a change in phase. Such distributed fiber optic sensing devices capable of measuring dynamic events are known as phase optical time domain reflectometers. Initially, phase optical time domain reflectometry was used primarily to measure whether or not there was a disturbance event along the fiber, so that dynamic events along the fiber, such as walking, digging, etc., could be monitored. With the deep research, both theory and practice prove that the phase change amount of the phase optical time domain reflectometer is equal proportion to the external disturbance event. Thus, phase optical time domain reflectometry has also gradually evolved from qualitative to quantitative measurements.
However, the application of phase optical time domain reflectometry to quantitative measurements also suffers from a number of problems. One of them is: the coherent rayleigh curve is an amplitude or intensity curve from which phase information is solved for, subject to an arctangent operation. Although quadrants can be considered in the arctangent operation, it is only possible to extend the result of the arctangent operation to the range of [ -pi, pi ]. When the amplitude of the external disturbance event is too large, the actual value of the phase change amount exceeds the range, and then the result is necessarily wrapped in the range when the arctangent operation is performed. A general phase unwrapping algorithm can unwrap the wrapped phases. However, there is a limit to this algorithm: the absolute difference of the phases of two adjacent points cannot be greater than pi. Reducing the conversion coefficient and increasing the sampling rate are two main technical routes to solve the problem. Reducing the conversion factor, while it may adjust the amount of change to meet the requirements of the phase unwrapping algorithm, typically reduces the signal-to-noise ratio. Increasing the sampling rate may be achieved by increasing the frequency of the emission of the light pulses. However, limited by the length of the fiber, increasing the frequency of emission of the light pulses is also limited. In order to break through the limitation of the optical fiber length, pulse light with different frequencies is introduced into the phase optical time domain reflectometer. However, introducing the pulsed light of different frequencies is different from increasing the frequency of emission of the light pulses, even if the light pulses of different frequencies are emitted at equal intervals, the actual physical phases corresponding to the correctly preliminarily solved phase values are not at equal intervals. This is because the initial phase distribution along the length of the fiber is not uniform for light pulses of different frequencies. Therefore, to achieve accurate measurement of phase information in a phase optical time domain reflectometer that introduces optical pulses of different frequencies, the problem of inconsistent initial phase distribution in the length direction of the optical fiber is to be solved. In addition, in order to accurately solve the phase information of the phase optical time domain reflectometer, a reference point is usually selected before a disturbance event, and then the phase difference between the phase after the reference point and the phase of the reference point is used to eliminate the initial phase inconsistency between different pulse sequences caused by clock jitter and other reasons, so as to convert the phase difference value relative to the transmitting end point of the laser into the phase difference value between two points of a section of optical fiber relative to the reference point. However, when the phase optical time domain reflectometer introduces optical pulses of different frequencies, the reference point selects a curve to satisfy the generation of two pulse lights of different frequencies at the same time due to inconsistent positions of polarization fading and coherent rayleigh fading, which increases difficulty in selection. Therefore, the invention respectively carries out bidirectional difference making based on the static reference point and the static reference phase in the length direction of the optical fiber and the pulse sequence direction, and provides a method for extracting the phase signal of the equal time division dual-frequency optical phi-OTDR by the bidirectional difference making so as to realize accurate measurement of the phase signal of the equal time division dual-frequency optical phase optical time domain reflectometer.
Disclosure of Invention
In the dual-frequency optical phase optical time domain reflectometer, due to the difference of the frequency of the detection pulse light, even under the condition of static non-disturbance event, the difference of distribution of the initial phases of the solved single pulse along the length direction of the optical fiber is solved, and the difference between the initial phases corresponding to the detection pulse light with different frequencies can be further generated. Moreover, the positions of the polarization attenuation points and the coherent attenuation points corresponding to the detection pulse light with different frequencies are not consistent. This makes it impossible to uniformly sample the probe pulse light of two different frequencies of the dual-frequency optical phase optical time domain reflectometer even if the probe pulse light is completely time-divided at equal intervals. Because the sampled values are not as expected to occur, although equally spaced from the time interval alone, are uniform, they are in fact not uniformly sampled and are randomly sampled. In order to realize true uniform sampling and accurately extract the phase signal of an equal time division dual-frequency optical phase optical time domain reflectometer, the invention provides a method for extracting the phase signal of the equal time division dual-frequency optical phi-OTDR by bidirectional difference:
in a coherent detection type phi-OTDR system, the reference light has a frequency f 0 Is a high-frequency stable continuous laser, and the frequency of the detection pulse light is f respectively from the frequency phase difference delta f 1 And f 2 Is formed by equally-spaced and crossed combination of two high-frequency-stabilization laser pulse trains, and the coherent Rayleigh curve output by the system, namely the original data, is I D The data is processed by the steps of:
step one, simple arrangement: coherent rayleigh curve I to be acquired D Conversion into a two-dimensional array E i (M, N), wherein M is the number of times of light pulse emission, the total number of times is M, the value range of M is recorded as 1.ltoreq.m.ltoreq.m, N is the number of points of optical fiber sampling, the total number of points is N, and the value range of N is recorded as 1.ltoreq.mN is less than or equal to N, i=1 corresponds to the first optical frequency f 1 I=2 corresponds to the second optical frequency f 2 ；
Step two, band-pass separation: using center frequencies of |f respectively 1 -f 0 I and/f 2 -f 0 The two bandpass filters of I will be coherent rayleigh curve data I D Separated into a first optical frequency f 1 And a first optical frequency f 2 First amplitude component A respectively corresponding to 1 And a second amplitude component A 2 ；
Step three, solving a modulus value and an initial phase: for the first amplitude component A respectively using quadrature demodulation method 1 And a second amplitude component A 2 Obtaining a modulus value H 1 (m, n) and H 2 (m, n) and simultaneously obtaining the initial phaseAnd->
Step four, calculating the product of the distance and the value: respectively to the modulus value H 1 (m, n) and H 2 (m, n) solving the modulus H at each fiber sampling point number n 1 (m, n) and H 2 Distance sum value b of (m, n) 1 (n) and b 2 (n) and b is counted on each fiber sample point n 1 (n) and b 2 (n) multiplying and normalizing the result, and recording the result as a distance and value multiplication product b (n);
step five, calculating the product of the minimum values: respectively to the modulus value H 1 (m, n) and H 2 (m, n) solving the modulus H at each fiber sampling point number n 1 (m, n) and H 2 Minimum value h of (m, n) 1 (n) and h 2 (n) and adding h to the number of samples n of each fiber 1 (n) and h 2 (n) multiplying and normalizing the result, and recording the result as a minimum product h (n);
step six, confirming a reference position: determining the location area [ n ] of an event from the distance and value product b (n) x ,n y ]Will be at n x The number n of optical fiber sampling points where the maximum value of the left minimum value product h (n) is located c Is determined as a reference bitPlacing; step seven, calculating the phase before unwrapping: respectively make the primary phasesAnd->Phase ∈>And->Difference is made to obtain the differential phase beta without winding 1 (m,n i ) And beta 2 (m,n i ) Wherein n is i ＝n-n c Selecting the differential phase beta of the un-wound during a static non-disturbance event 1 (m c ,n i ) And beta 2 (m c ,n i ) As the first optical frequency f 1 And a first optical frequency f 2 Corresponding reference phase, differential phase beta without unwinding 1 (m,n i ) And beta 2 (m,n i ) Respectively with reference phase beta 1 (m c ,n i ) And beta 2 (m c ,n i ) Difference is made to obtain the phase change alpha without unwinding 1 (m,n i ) And alpha 2 (m,n i )；
Step eight, unwrapping and solving phase signals: phase change alpha without unwinding 1 (m,n i ) And alpha 2 (m,n i ) Press-down type recombination
Then at each optical fiber sampling point n i Unwrapping using a conventional phase unwrapping algorithm, resulting in an unwrapped phase change γ (j, n) i ) And then gamma (j, n) i ) The phase change delta (j, n) after the difference is calculated according to the following formula i )
According to the phase change delta (j, n i ) Linear profile along the length of the fiber, n is selected y Right-hand noiseless rest position n s And delta (j, n) s ) And is recorded as the final solved phase signal.
Further, the components forming the equal-time-division dual-frequency optical phi-OTDR system comprise a laser, a coupler A, a coupler B, an acousto-optic modulator A, an acousto-optic modulator B, a coupler C, an erbium-doped optical fiber amplifier, a circulator, a test optical fiber, a coupler D, a balance detector, an amplifier, a computer, a signal generator, a driver A, a driver B and a pulse generator, wherein the mutual connection relation is as follows:
the laser is connected to the coupler A;
the coupler A is connected with the coupler B and the coupler D;
the coupler B is connected with the acousto-optic modulator A and the acousto-optic modulator B;
the acousto-optic modulator A and the acousto-optic modulator B are connected to the coupler C;
the coupler C is connected with the erbium-doped fiber amplifier;
the erbium-doped fiber amplifier is connected to the circulator;
the circulator is connected with the test optical fiber and the coupler D, and the test optical fiber comprises wound piezoelectric ceramics;
the coupler D is connected to the balance detector;
the balance detector is connected with the amplifier;
the amplifier is connected with the computer, and a data acquisition card is embedded in the computer;
the signal generator is connected with a computer and acts on the piezoelectric ceramics wound by the optical fiber;
the pulse generator is connected with the driver A and the driver B;
the driver A is connected to the acousto-optic modulator A;
the driver B is connected to the acousto-optic modulator B.
Further, the coupler B and the coupler D are 3dB couplers.
Further, the fluctuation of the light source frequency of the phi-OTDR system is less than 50kHz.
The method of the invention provides a simple and easy method for selecting the reference position in the equal time division dual-frequency optical phase optical time domain reflectometer on the basis of breaking through the limit of the traditional phase unwrapping algorithm to pi value in the single-frequency optical phase optical time domain reflectometer, and the secondary difference of the reference phase not only eliminates the inconsistency of the primary phase along the length direction of the optical fiber, but also avoids the noise introduced by the reference phase. By implementing the method, the phase signal of the equal time division dual-frequency optical phase optical time domain reflectometer can be truly and uniformly sampled and accurately reconstructed.
Drawings
FIG. 1 is a schematic diagram of a process for accurately solving phase signals of an equitime-division dual-frequency optical phase optical time domain reflectometer;
FIG. 2 is a schematic diagram of a system configuration of an equitime-division dual-frequency optical phase optical time domain reflectometer;
FIG. 3 is a coherent Rayleigh plot of the origin of 40MHz frequency shifted light;
FIG. 4 is a coherent Rayleigh plot of the onset of 80MHz frequency shifted light;
FIG. 5 is an amplitude plot of 40MHz frequency shifted light;
FIG. 6 is an amplitude plot of 80MHz frequency shifted light;
FIG. 7 is a plot of the modulus of 40MHz frequency shifted light;
FIG. 8 is a plot of the modulus of 80MHz frequency shifted light;
FIG. 9 is a statistical phase of 40MHz frequency shifted light;
FIG. 10 is a statistical phase of 80MHz frequency shifted light;
FIG. 11 is a model of two frequency components;
FIG. 12 is a product of distance and;
FIG. 13 is a product of minima;
fig. 14 is a phase change of a reference phase primary difference calculation;
FIG. 15 is a phase change of a reference phase quadratic difference calculation;
FIG. 16 is a phase signal of a two-way difference determination;
fig. 17 is a phase signal simply obtained.
Detailed Description
The technical scheme of the invention is further described in detail below with reference to the accompanying drawings:
it will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The single-frequency optical phase optical time domain reflectometer is limited by the length of the optical fiber and cannot increase the emission frequency of the optical pulse without limit, so that a method of increasing the sampling rate by increasing the emission frequency of the optical pulse to meet the pi limitation condition of the traditional phase unwrapping algorithm is challenged. The use of equal time division dual frequency probe pulses in phase optical time domain reflectometers can formally double the sampling frequency of the signal and still appear to be uniformly sampled. However, such uniform sampling is not an accurate sampling, and is not truly uniform sampling in fact. For single-frequency detection pulse light, the initial phase of the single-frequency detection pulse light along the length direction of the optical fiber is inconsistent due to the non-uniformity of the refractive index distribution of the optical fiber. For the detection pulse light with different frequencies, even under the condition of full static state, the initial phases corresponding to the detection pulse light with different frequencies are mostly inconsistent at the same optical fiber sampling position, which directly results in that the untreated phases are extremely inaccurate for uniform sampling of the dual-frequency light. If these initial phase values are also wrapped and cannot be properly unwrapped, even if the phase change values are calculated, the linear distribution of the phase change along the length of the fiber will be lost. If the external measured event is an unknown disturbance signal, the double-frequency optical phase optical time domain reflectometer cannot accurately identify and reconstruct the disturbance signal. Thus, the inconsistency of the initial phase along the length of the fiber must be compensated, and the present invention uses the phase of a certain pulse light in a static state, i.e., without disturbance of an external event, as a reference phase. For a dual frequency optical phase optical time domain reflectometer, each frequency probe pulse has one such reference phase. The dual-frequency detection pulse light not only can bring about the change of initial phase distribution, but also can bring about the increase of the number of points of polarization fading and coherent Rayleigh fading, which increases the difficulty in selecting the reference position. Therefore, the invention provides the data processing scheme shown in fig. 1, and the data processing scheme sequentially undergoes the operation processes of simple arrangement, band-pass separation, solution of the product of a modulus value and an initial phase, calculation of the product of a distance and a value, calculation of the product of a minimum value, confirmation of a reference position, phase calculation before unwrapping, solution of a phase signal and the like. This process is explained below in connection with the example shown in fig. 2.
In fig. 2, the laser generates a narrow linewidth continuous laser light having a frequency drift of less than 50kHz during an observation time, which is split into two paths of continuous light after passing through the coupler a, and the upper path laser light is used as signal light, and the lower path light carrying most of the energy and a small portion of the energy is used as reference light. The signal light is divided into two paths of continuous light with equal energy after passing through the coupler B, and then is modulated into two paths of detection pulse light with the pulse width of 200ns by the acousto-optic modulator A and the acousto-optic modulator B respectively. The pulse width, the pulse frequency and the pulse interval are all controlled by the pulse generator through the driver A or the driver B to the acousto-optic modulator A or the acousto-optic modulator B. The pulse emission frequency of the single frequency light was 50kHz and the time interval between the light pulses of different frequencies was 10us. The acousto-optic modulator A and the acousto-optic modulator B are modulators with frequency shifts of 40MHz and 80MHz respectively, so that the frequencies of two paths of detection pulse light entering the coupler C are different by 40MHz. The two paths of detection pulses pass through the coupler C to synthesize one path of detection pulse light. The detection pulse light sequence is amplified by an erbium-doped fiber amplifier and then injected into a test fiber by a circulator. Due to the Rayleigh scattering effect, as the probe pulse light is transmitted, the fiber produces Rayleigh scattered light along the line, a portion of which returns to the fiber injection port, known as back Rayleigh scattered light. The back rayleigh scattered light is injected into coupler D via the circulator along with the reference light, 50: the combination of the coupler D and balanced detector of 50 is a classical optical path which can improve the signal to noise ratio. The balance detector outputs AC component which is amplified by 10 times by the amplifier and then sent to a high-speed data acquisition card in a computer. The signal generator applies a Burst driving signal to the piezoelectric ceramics PZT while informing the computer of the time to start data acquisition. The piezoceramic PZT has a test fiber wound directly around it of about 10 m. In this embodiment, the fiber wrapped around the piezoceramic PZT is part of the test fiber and the signal generator drives the piezoceramic PZT with a Burst signal to simulate a disturbance event. The following procedure was followed for a piece of data collected using the experimental setup, as shown in fig. 1.
Step one, simple arrangement. Although the detection pulse light is cross-combined according to different frequencies, the photoelectric signal output by the balance detector contains two frequency components at the same time. On the other hand, for the data acquisition card built in the computer, the acquisition of the data is one-dimensional continuous. However, for the phase optical time domain reflectometer, when implementing quantitative measurement, it is necessary to confirm at which optical fiber sampling position in the optical fiber length direction external disturbance is sensed, and quantitatively display external disturbance in the signal time direction based on the optical fiber sampling position. Therefore, the one-dimensional data acquired by the data acquisition card, namely, the original data, needs to be arranged into two-dimensional data in two directions of 'optical fiber length' and 'signal time'. The "fiber length" actually records time information of the back rayleigh scattered light according to the relationship between the time of the back rayleigh scattered light return and the fiber length, and thus the coordinate axis on which the "fiber length" is located is also referred to as a "fast time axis". The axis in which the "signal time" is located records the time series information of the external disturbance signal, and on this axis, the sampling rate of the external disturbance signal is actually the frequency of occurrence of the light pulse, and this axis is also called "slow time axis", that is, the direction in which the pulse sequence changes. For dual-frequency optical phase optical time domain reflectometers, two frequency components need to be separated from one-dimensional data, and then the two frequency components are combined together in a crossing manner. The time interval of the pulse light with two different frequencies is different from the half period of the transmitting pulse of the single-frequency optical phase optical time domain reflectometer. Thus, the two-dimensional data of the two different frequency pulsed lights are not identical in starting point in the one-dimensional acquisition data. Fig. 3 is a diagram showing the data collected by starting recording when the 40MHz frequency-shifted light pulse is triggered, and a total of 1000 coherent rayleigh curves corresponding to the trigger pulses are superimposed. Fig. 4 is a diagram showing data acquired by starting recording when triggering with an 80MHz frequency-shifted light pulse, and a total of 1000 coherent rayleigh curves corresponding to the triggering pulses are superimposed.
Step two, band-pass separation. The data in fig. 3, which is data that starts recording when the 40MHz frequency-shifted light pulse is triggered, contains not only information of the back rayleigh scattered light generated by the 40MHz frequency-shifted light but also information of the back rayleigh scattered light generated by the 80MHz frequency-shifted light. Thus, the information of the backward Rayleigh scattered light generated by the 80MHz frequency-shifted light is stripped off by a band-pass filter with a center frequency of 40MHz, and at the same time, the amplitude curve generated by the band-pass filtered 40MHz frequency-shifted light is subjected to a de-noising process in fact, so as to obtain the curve shown in FIG. 5. The same method is used for fig. 4 to obtain an amplitude curve generated by only 80MHz frequency shifted light, fig. 6.
And thirdly, solving a modulus value and an initial phase. The in-phase component and the vertical component are found for the amplitude curve in fig. 5 using a method of digital quadrature demodulation, and then the modulus value and the initial phase are found based on the two components, respectively. The modulus is the square root of the in-phase and vertical components, and the initial phase takes into account the quadrants in which the two components are located when performing an arctangent operation with the in-phase and vertical components, so that its derived phase value is distributed over the range of [ -pi, pi ] instead of just [ -pi/2, pi/2 ]. Fig. 7 and 8 show the modulus values of the 40MHz frequency-shifted light component and the 80MHz frequency-shifted light component calculated from the square roots of the in-phase component and the vertical component, respectively. There is a region on both figures greater than about 1km where the visibility is different from the other, i.e., where the PZT is loaded. Fig. 9 and 10 show the initial phases, i.e., the unwrapped statistical phases, of the 40MHz frequency-shifted light component and the 80MHz frequency-shifted light component calculated from the arctangent of the in-phase component and the vertical component, respectively. It can be seen from fig. 9 and 10 that even in the completely undisturbed static region, the initial phases are not completely equal in the direction of the ordinate. This illustrates the existence of clock jitter and other factors in the system, which is why a reference position is selected along the length of the fiber.
And step four, calculating the product of the distance and the value. Fig. 11 is a plot of 40MHz frequency shifted light and 80MHz frequency shifted light together, and the visibility of the mode values is observed along the length of the fiber, again as a disturbance event can be found to exist at about 1 km. In order to further confirm the area where the disturbance event is located, solving the distance sum value of the mode values among different pulses at each optical fiber sampling position aiming at the mode value of the 40MHz frequency-shifted light, and solving the distance sum value of the mode values among different pulses at each optical fiber sampling position aiming at the mode value of the 80MHz frequency-shifted light at the same time, wherein the two distances sum values at each optical fiber sampling position are multiplied to obtain the product of the distance sum value, and the product is still called as the product of the distance sum value after normalization for the convenience of expression. FIG. 12 illustrates the product of the distance and value of the region near the disturbance event. From this figure it can be seen that the disturbance zone of the event is located in the region 1036 meters, 1102 meters. It should be noted that the regions given by different event localization methods may be slightly different. In the method, the area between two position points where the bottom just starts to rise and completely fall to the bottom of the product of the distance and the value is considered as the area where the disturbance event is located.
And step five, calculating the minimum value product. The minimum value is found at each optical fiber sampling position for the mode value of the 40MHz frequency-shifted light, and the minimum value is found at each optical fiber sampling position for the mode value of the 80MHz frequency-shifted light, the two minimum values at each optical fiber sampling position are multiplied, the product of the minimum values is called as the product of the minimum values after normalization, and the result of the product in the area near the disturbance event is shown in fig. 13.
And step six, confirming the reference position. For fig. 13, the location of the maxima is necessarily not the location of the polarization fading and the coherent rayleigh fading. On the other hand, the phase is solved from the amplitude curve, and the higher the modulus, the lower the amplitude curve is affected by noise, and the lower the effect of noise on the phase correspondingly solved. For the mode values generated by the pulse light with two different frequencies, the distribution of the respective maximum values is different, so the calculation method in the fifth step comprehensively considers the situation of two frequency components. The leftmost side of the disturbance event area sees a maximum at the fiber sample position 1015.68m at 1036 meters, looking leftwards from the 1036 meter position on the "minimum product" curve, thus confirming the fiber sample position 1015.68m as the reference position. It is noted that the maximum value immediately to the left of the fiber sample position 1015.68m, although not shown in fig. 13, is smaller than the value of the fiber sample position 1015.68m, otherwise the maximum value immediately to the left of the fiber sample position 1015.68m is selected as the reference position, and in summary, the fiber sample position where the value of the first-order maximum value on the left is no longer reduced is selected as the reference position.
And step seven, calculating the phase before unwrapping. The un-unwrapped statistical phase of each pulse in fig. 9 and 10 is differenced from the un-unwrapped statistical phase of the corresponding frequency component at the reference location, resulting in an un-unwrapped differential phase. The primary phase inconsistency of the pulse sequence direction is eliminated by making a difference, and then the primary phase inconsistency of the optical fiber length direction is eliminated by making a difference. The reference phase that is differenced is first the differential phase taken from the unwrapped, non-disturbance event time. In this experimental illustration, the exact first half of the captured disturbance signal is the zero level moment of the Burst signal, which corresponds to the moment when no disturbance event is loaded on the fiber, while the second half of the disturbance signal is the sinusoidal part of the Burst signal. Thus, only one pulse instant is selected in the first half of the Burst signal, and then the differential phase of the pulse instant that is not unwound is taken as the reference phase. In the operation process, each frequency component has 1000 pulses, each of the unreeled differential phases of the two frequency components takes the unreeled differential phase corresponding to the 490 th pulse as a reference phase, and then the value corresponding to each pulse in the unreeled differential phases of the two frequency components is respectively differenced from the reference phase of the respective frequency component, and the difference results in the unreeled phase change of the respective frequency component.
And step eight, unwrapping and solving the phase signals. The non-unwrapped phase changes of the two frequency components are cross-combined in pulse sequence, with the combined odd pulse sequence from the non-unwrapped differential phase of the 40MHz frequency shifted light component and the even pulse sequence from the non-unwrapped differential phase of the 80MHz frequency shifted light component. The combined values are then unwrapped at each fiber sampling location. The value after the unwrapping is the phase change as shown in fig. 14. Since the starting point of the phase unwrapping algorithm is not the pulse time at which the reference phase is located, but the time at which the first pulse of the 40MHz frequency-shifted light is emitted, the unwrapped differential phase of the pulse time and the reference phase corresponding to the 40MHz frequency-shifted light are different due to noise and the like, so that zero values at sampling positions of part of the optical fibers deviate, and a local concave-convex variation of the phase variation along the length direction of the optical fibers except for peaks, burrs and noise substrates is generated as shown in fig. 14. At this time, the phase change of the odd pulse sequence is extracted to form a new array based on the phase change after the unwrapping, and the phase change after the unwrapping of the even pulse sequence is also left to become a new array. The phase change of the two new arrays is different from the phase change of the 490 th pulse sequence of each array, and for convenience of description, the phase change after unwrapping corresponding to the 490 th pulse of each frequency component involved in the difference is still referred to as a reference phase, and the result of the difference is still referred to as a phase change. Then, the phase changes of the two arrays are combined again according to the previous rule of pulse sequence intersection, and the phase change obtained by combination is the phase change amount accurately solved, as shown in fig. 15. The phase at 490 pulse is subtracted twice, effectively suppressing noise that a single reference curve might carry in. The phase change at fiber sample position 1096m near the right of the reference position is taken as the final resolved phase signal, as shown in fig. 16. The data for the sinusoidal portion of the Burst signal in fig. 16 was fitted with a once-banded sine function, resulting in a chi-square coefficient of 0.9998 and a root mean square error of 0.3872rad. Meanwhile, the maximum value of the absolute difference of the phases of two adjacent points of each signal sampling point in fig. 16 is 4.5105rad. The example illustrates that the equal time division dual-frequency optical phase optical time domain reflectometer breaks through the limitation that the phase cannot be accurately solved due to pi limiting conditions in the traditional phase unwrapping algorithm in the single-frequency optical phase optical time domain reflectometer, and the accurate reconstruction of the disturbance signal is carried out, so that the real uniform sampling is realized on the basis. For the sake of enhanced contrast, FIG. 17 is the result of solving directly at fiber sample position 1096m based on reference position and reference phase without employing the present invention. Obviously, the result of the solution is not ideal.
In summary, in the coherent detection phase optical time domain reflectometer, the reference position is required to eliminate the inconsistency of the initial phase in the signal time direction caused by factors such as clock jitter, and after the introduction of equal time division dual-frequency light, the positions of polarization fading and coherent fading are different due to the difference of the pulse frequency of the detection light, so that the difficulty of selecting the reference position is increased; while non-uniformity in the refractive index profile results in a random initial phase along the length of the fiber. Therefore, measures such as simple arrangement, band-pass separation, solving of the product of a modulus value and an initial phase, calculating of the product of a distance and a value, calculating of the product of a minimum value, confirming of a reference position, phase calculation before unwrapping, solving of a phase signal and the like are sequentially used for the coherent detection curve to process data. The method breaks through the limit of a phase unwrapping algorithm in the single-frequency optical phase optical time domain reflectometer on pi value, and realizes accurate reconstruction of phase signals in the time-division dual-frequency optical phase optical time domain reflectometer.
The above description is only of the preferred embodiments of the present invention, and is not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (4)
1. The method for extracting equitime-division dual-frequency optical phi-OTDR phase signal by bidirectional difference is characterized by that in coherent detection type phi-OTDR system the reference light is frequency f 0 Is a high-frequency stable continuous laser, and the frequency of the detection pulse light is f respectively from the frequency phase difference delta f 1 And f 2 Is formed by equally-spaced and crossed combination of two high-frequency-stabilization laser pulse trains, and the coherent Rayleigh curve output by the system, namely the original data, is I D The data is processed by the steps of:
step one, simple arrangement: coherent rayleigh curve I to be acquired D Conversion into a two-dimensional array E i (M, N), wherein M is the number of times of light pulse emission, the total number of times is M, the value range of M is recorded as 1.ltoreq.m.ltoreq.m, N is the number of points of optical fiber sampling, the total number of points is N, the value range of N is recorded as 1.ltoreq.n, i=1 corresponds to the first light frequency f 1 I=2 corresponds to the second optical frequency f 2 ；
Step two, band-pass separation: using center frequencies of |f respectively 1 -f 0 I and/f 2 -f 0 The two bandpass filters of I will be coherent rayleigh curve data I D Separated into a first optical frequency f 1 And a second optical frequency f 2 First amplitude component A respectively corresponding to 1 And a second amplitude component A 2 ；
Step three, solving a modulus value and an initial phase: for the first amplitude component A respectively using quadrature demodulation method 1 And a second amplitude component A 2 Obtaining a modulus value H 1 (m, n) and H 2 (m, n) and simultaneously obtaining the initial phaseAnd->
Step four, calculating the product of the distance and the value: respectively to the modulus value H 1 (m, n) and H 2 (m, n) solving the modulus H at each fiber sampling point number n 1 (m, n) and H 2 Distance sum of (m, n)Value b 1 (n) and b 2 (n) and b is counted on each fiber sample point n 1 (n) and b 2 (n) multiplying and normalizing the result, and recording the result as a distance and value multiplication product b (n);
step five, calculating the product of the minimum values: respectively to the modulus value H 1 (m, n) and H 2 (m, n) solving the modulus H at each fiber sampling point number n 1 (m, n) and H 2 Minimum value h of (m, n) 1 (n) and h 2 (n) and adding h to the number of samples n of each fiber 1 (n) and h 2 (n) multiplying and normalizing the result, and recording the result as a minimum product h (n);
step six, confirming a reference position: determining the location area [ n ] of an event from the distance and value product b (n) x ,n y ]Will be at n x The number n of optical fiber sampling points where the maximum value of the left minimum value product h (n) is located c Determining as a reference position;
step seven, calculating the phase before unwrapping: respectively make the primary phasesAnd->Phase with reference positionAnd->Difference is made to obtain the differential phase beta without winding 1 (m,n i ) And beta 2 (m,n i ) Wherein n is i ＝n-n c Selecting the differential phase beta of the un-wound during a static non-disturbance event 1 (m c ,n i ) And beta 2 (m c ,n i ) As the first optical frequency f 1 And a second optical frequency f 2 Corresponding reference phase, differential phase beta without unwinding 1 (m,n i ) And beta 2 (m,n i ) Respectively are provided withWith reference phase beta 1 (m c ,n i ) And beta 2 (m c ,n i ) Difference is made to obtain the phase change alpha without unwinding 1 (m,n i ) And alpha 2 (m,n i )；
Step eight, unwrapping and solving phase signals: phase change alpha without unwinding 1 (m,n i ) And alpha 2 (m,n i ) Recombination is performed as follows:
then at each optical fiber sampling point n i Unwrapping using a conventional phase unwrapping algorithm, resulting in an unwrapped phase change γ (j, n) i ) And then gamma (j, n) i ) The phase change delta (j, n) after the difference is calculated according to the following formula i )
According to the phase change delta (j, n i ) Linear profile along the length of the fiber, n is selected y Right-hand noiseless rest position n s And delta (j, n) s ) And is recorded as the final solved phase signal.
2. The method for extracting equal time division dual-frequency optical Φ -OTDR phase signal according to claim 1, wherein the components forming the equal time division dual-frequency optical Φ -OTDR system include a laser, a coupler a, a coupler B, an acousto-optic modulator a, an acousto-optic modulator B, a coupler C, an erbium-doped fiber amplifier, a circulator, a test fiber, a coupler D, a balanced detector, an amplifier, a computer, a signal generator, a driver a, a driver B, a pulse generator, and the connection relationship among them is:
the laser is connected to the coupler A;
the coupler A is connected with the coupler B and the coupler D;
the coupler B is connected with the acousto-optic modulator A and the acousto-optic modulator B;
the acousto-optic modulator A and the acousto-optic modulator B are connected to the coupler C;
the coupler C is connected with the erbium-doped fiber amplifier;
the erbium-doped fiber amplifier is connected to the circulator;
the circulator is connected with the test optical fiber and the coupler D, and the test optical fiber comprises wound piezoelectric ceramics;
the coupler D is connected to the balance detector;
the balance detector is connected with the amplifier;
the amplifier is connected with the computer, and a data acquisition card is embedded in the computer;
the signal generator is connected with a computer and acts on the piezoelectric ceramics wound by the optical fiber;
the pulse generator is connected with the driver A and the driver B;
the driver A is connected to the acousto-optic modulator A;
the driver B is connected to the acousto-optic modulator B.
3. The method for extracting equal time division dual frequency optical Φ -OTDR phase signal according to claim 2 wherein said coupler B and said coupler D are 3dB couplers.
4. A method of bi-directionally differencing an equitime-division dual-frequency optical Φ -OTDR phase signal according to claim 1 or 2 wherein the optical source frequency of the Φ -OTDR system fluctuates by less than 50kHz.
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