CN110487389B - Coherent fading suppression method based on optimal position tracking - Google Patents
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- 238000000253 optical time-domain reflectometry Methods 0.000 abstract description 2
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- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
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- G01D5/35338—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using other arrangements than interferometer arrangements
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Abstract
The invention discloses a coherent fading suppression method based on optimal position tracking, which comprises the following steps: s1: performing intermediate frequency filtering on a beat frequency signal output after backward Rayleigh scattering light generated by the detection pulse light and reference light are mixed and coherent, and extracting an intermediate frequency component; s2: IQ demodulation is carried out on the intermediate frequency component to obtain the phase of the intermediate frequency component; s3: selecting k groups of reference areas which are in a stable state before and after the vibration action area, and reconstructing the phase of the intermediate frequency component at two reference positions of each group of reference areas; s4: calculating the signal-to-noise ratio of the intermediate frequency component in two reference positions of each group of reference areas, and selecting the minimum signal-to-noise ratio; s5: and selecting the maximum signal-to-noise ratio at any moment, and taking the reconstructed signal corresponding to the maximum signal-to-noise ratio as a final reconstructed signal. The invention realizes the high fidelity reconstruction of the external vibration signal and obviously reduces the false alarm rate on the premise of only using the common single mode sensing optical fiber and not changing the structure of the traditional phi-OTDR system.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a coherent fading suppression method based on optimal position tracking.
Background
The phase-sensitive optical time domain reflectometer (phi-OTDR) has the characteristics of distributed sensing, high response speed, high sensitivity and the like, and shows great application potential in the fields of structural health monitoring, perimeter intrusion monitoring, earthquake monitoring and the like. The phi-OTDR generally selects two sections of optical fibers in a stable state before and after a vibration action area as reference areas, and realizes quantitative measurement of external vibration by extracting the change of the phase difference of backward Rayleigh scattering light (RBS) of the two reference areas along with time. Because the narrow linewidth light source used by the phi-OTDR has extremely long coherent length, RBS generated by the detection pulse light interferes in the pulse, so that the intensity of RBS received by a receiving end is randomly fluctuated, and the phenomenon is called coherent fading. Coherent fading causes the RBS intensity to approach zero in certain areas of the fiber where the phase demodulation results are susceptible to a sharp degradation of the signal-to-noise ratio. Even if the selected reference area has a high RBS intensity at the beginning, the RBS intensity of the area may approach zero due to the frequency drift of the light source as time passes, so that the reconstructed external vibration signal is distorted, and frequent false alarms are caused, which brings great inconvenience to the application of the Φ -OTDR in engineering.
For suppressing the influence of coherent fading, Zhoujun et al, published in the journal of Chinese laser, comprehensive discrimination based on multiple frequencies
The phase demodulation technology of interference fading false signals in the system provides that the phase demodulation is carried out on three beat frequency signals with different frequencies at the same time, and then the amplitude is used as the basis for judging whether the phase demodulation is correct or not, thereby identifying false alarm caused by the interference fading. AFading-Dis published in the IEEE PHOTONICS TECHNOLOGY LETTERS journal by numerous flicking et alcrimination Method for Distributed Vibration Sensor Using Coherent Detection of
It is pointed out that unlike coherent fading which only affects the phase at the phase alarm point, external vibration can also affect the phase after the phase alarm point, the phase alarm point caused by coherent fading can be eliminated by using the phase difference between the two points before and after the phase alarm point, and the accuracy of phi-OTDR positioning is improved. The method can eliminate false alarms, but does not fundamentally improve the fidelity of the reconstructed signal. Masude et al propose that amplitude is used as a judgment basis in A Continuous coding efficiency Suppression Method for phi-OTDR Systems Using optimal-Tracking over Multiple Probe Frequencies published in JOURNAL OF LIGHT TWAVE TECHNOLOGY JOURNAL, and always select the most accurate intermediate frequency component reconstruction signal at any time for connection, thereby realizing high-fidelity reconstruction OF external vibration signals. In the patent of signal processing method for reducing probability of detection dead zone in phase-sensitive optical time domain reflectometer, zhou tong et al propose that amplitude is taken as a judgment basis, and a most accurate reconstruction signal of a reference region is always selected at any time for connection, so that high-fidelity reconstruction of an external vibration signal is realized. The method is based on the premise that the amplitude is used as a judgment basis and the noise intensity of each intermediate frequency component and each group of reference areas is the same, and is unreliable to a certain extent.
Disclosure of Invention
The purpose of the invention is as follows: the invention provides a coherent fading suppression method based on optimal position tracking, which aims at solving the problem that the high fidelity reconstruction of an external vibration signal can be realized but the false alarm rate is high on the premise of only using a common single-mode sensing optical fiber and not changing the structure of a traditional phi-OTDR system.
The technical scheme is as follows: in order to realize the purpose of the invention, the technical scheme adopted by the invention is as follows:
a coherent fading suppression method based on optimal position tracking specifically comprises the following steps:
s1: performing intermediate frequency filtering on a beat frequency signal output after backward Rayleigh scattering light generated by the detection pulse light and reference light are mixed and coherent, and extracting an intermediate frequency component from the beat frequency signal;
s2: performing IQ demodulation on the intermediate frequency component to acquire the phase of the intermediate frequency component;
s3: selecting k groups of reference regions [ ai, bi ] i which are in a stable state before and after a vibration action region from the optical fiber, wherein: ai and bi are two different reference positions in the reference area, and i is the group number of the reference area;
simultaneously reconstructing the phases of the intermediate frequency components at two reference positions of each group of reference areas to obtain a reconstructed signal of each group of reference areas;
s4: calculating the signal-to-noise ratio of the intermediate frequency component in two reference positions of each group of reference areas, and selecting the corresponding minimum signal-to-noise ratio in the reference areas from the two signal-to-noise ratios;
s5: and selecting the maximum signal-to-noise ratio from the selected minimum signal-to-noise ratio at any moment according to the relation between the signal-to-noise ratio and the reconstructed signal, and taking the reconstructed signal corresponding to the maximum signal-to-noise ratio as a final reconstructed signal.
Further, in the k sets of reference regions [ ai, bi ], a distance between reference positions ai in two adjacent reference regions, a distance between reference positions bi in two adjacent reference regions are not less than 1/4 of the spatial resolution.
Further, in step S4, the signal-to-noise ratios of the intermediate frequency component in the two reference positions of each group of reference areas are specifically:
taking the reference position in the reference area as a center, taking the intermediate frequency component of the spatial resolution as a length, performing Fourier transform on the intermediate frequency component of the spatial resolution, and calculating the ratio of the intermediate frequency band power after the Fourier transform to the total power of other frequency bands.
Has the advantages that: compared with the prior art, the technical scheme of the invention has the following beneficial technical effects:
the coherent fading suppression method of the invention carries out phase reconstruction on a plurality of groups of reference areas which are in a stable state before and after the vibration action area, and takes the intermediate frequency component signal-to-noise ratio of each group of reference areas as a judgment basis, and always selects the most accurate reconstruction signal for connection at any moment, thereby realizing high fidelity reconstruction of external vibration signals and obviously reducing the false alarm rate on the premise of only using common single-mode sensing optical fibers and not changing the structure of the traditional phi-OTDR system.
Drawings
FIG. 1 is a flow chart of the coherent fading suppression method of the present invention;
FIG. 2 is a reconstructed signal of sets of reference regions of the present invention;
FIG. 3 is a diagram illustrating the determination of the accuracy of the reconstructed signals for each set of reference regions according to the present invention;
FIG. 4 is a graph of reconstructed signals after optimal position tracking of the present invention;
fig. 5 is a comparison of phase discrimination results of the conventional method and the present invention;
fig. 6 is a comparison of the false alarm rates of the phase detection results of the conventional method and the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. The described embodiments are a subset of the embodiments of the invention and are not all embodiments of the invention. Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention.
Example 1
Referring to fig. 1, the present embodiment provides a coherent fading suppression method based on optimal position tracking, which specifically includes the following steps:
step S1: the method comprises the steps of outputting a beat frequency signal after backward Rayleigh scattered light and reference light generated by detection pulse light are mixed and coherent, wherein the output beat frequency signal needs to be subjected to intermediate frequency filtering, and an intermediate frequency component is extracted from the beat frequency signal. In the present embodiment, the frequency of the beat signal is selected to be 150MHz, and specifically, the beat signal is subjected to intermediate frequency filtering with 150MHz as the center of the pass band, from which the intermediate frequency component Δ ω is extracted.
Step S2: IQ demodulation is performed on the intermediate frequency component Δ ω extracted in step S1, and the phase Φ of the intermediate frequency component Δ ω is obtained. The phase Φ is formed by a matrix of time rows, each of which represents the distribution of the phase over the distance at a fixed time, and distance columns, each of which represents the change of the phase over the time at a fixed position.
Step S3: according to the vibration action region in the optical fiber, k groups of reference regions [ ai, bi ] i in a stable state are selected from the front and the back of the vibration action region, wherein: ai and bi are two different reference positions within the reference area, and i is the group number where the reference area is located.
In the process of selecting k sets of reference regions [ ai, bi ], the following selection rules need to be noted:
the distance between the reference positions ai in the two adjacent reference regions, the distance between the reference positions bi in the two adjacent reference regions are not less than 1/4 for the spatial resolution. Wherein the spatial resolution is half of the width of the detection pulse light in the optical fiber.
In this embodiment, the vibration action area in the optical fiber is 460m to 490m, and three groups of reference positions are selected from the vibration action area, which are respectively: (435m,505m), (440m,510m), and (445m,515 m).
Referring to fig. 2, the phase Φ of the intermediate frequency component Δ ω obtained in step S2 is reconstructed at two reference positions of the three groups of reference regions, so as to obtain a reconstructed signal Φ of each group of reference regionsab_i. In the ideal case, the reconstructed signal Φab_iAre consistent with the external vibration signal, namely the 25Hz sinusoidal excitation is consistent.
In the present embodiment, a reconstructed signal Φ for each group of reference regions is obtainedab_iThe method specifically comprises the following steps: and subtracting the phases at the two reference positions to obtain a phase difference, and performing phase unwrapping on the phase difference at different moments.
Step S4: calculating the signal-to-noise ratio of the intermediate frequency component in two reference positions of each group of reference areas, specifically:
taking the reference position in the reference area as the center and the intermediate frequency component of the spatial resolution as the length, carrying out Fourier transform on the intermediate frequency component of the spatial resolution, and then calculating the ratio between the intermediate frequency band power after the Fourier transform and the total power of other frequency bands.
After the signal-to-noise ratios in the two reference positions of each group of reference regions are obtained, the two signal-to-noise ratios are compared, and the minimum signal-to-noise ratio A corresponding to each group of reference regions is selected from the two signal-to-noise ratiosab_i。
Step S5: from the relation between the signal-to-noise ratio and the reconstructed signal, i.e. with reference to fig. 3, the signal-to-noise ratio a can be foundab_iThe larger the reconstructed signal phiab_iThe more accurate.
According to the three minimum signal-to-noise ratios A obtained in the step S4ab_iFor the reconstructed signal Φ obtained in step S3.3ab_iAnd (6) selecting. I.e. from the selected minimum signal-to-noise ratio a at any timeab_iSelecting the maximum signal-to-noise ratio and reconstructing a signal phi corresponding to the maximum signal-to-noise ratioab_iAs the final reconstructed signal phiab. Referring to fig. 4, it can be found that: the final reconstructed signal phiabCompared to the reconstructed signal phiab_iWith higher fidelity.
Referring to fig. 5 and fig. 6, it can be seen that the coherent fading suppression method achieves high fidelity reconstruction of the external vibration signal and significantly reduces the false alarm rate compared to the conventional method.
The present invention and its embodiments have been described in an illustrative manner, and are not to be considered limiting, as illustrated in the accompanying drawings, which are merely exemplary embodiments of the invention and not limiting of the actual constructions and methods. Therefore, if the person skilled in the art receives the teaching, the structural modes and embodiments similar to the technical solutions are not creatively designed without departing from the spirit of the invention, and all of them belong to the protection scope of the invention.
Claims (2)
1. A coherent fading suppression method based on optimal position tracking is characterized by comprising the following steps:
s1: performing intermediate frequency filtering on a beat frequency signal output after backward Rayleigh scattering light generated by the detection pulse light and reference light are mixed and coherent, and extracting an intermediate frequency component from the beat frequency signal;
s2: performing IQ demodulation on the intermediate frequency component to acquire the phase of the intermediate frequency component;
s3: k groups of reference regions [ ai, bi ] are selected from the optical fiber, wherein the k groups of reference regions [ ai, bi ] are in a stable state before and after the vibration action region, i is 1,2, … k, and the reference regions are as follows: ai and bi are two different reference positions in the reference area, and i is the group number of the reference area;
simultaneously reconstructing the phases of the intermediate frequency components at two reference positions of each group of reference areas to obtain a reconstructed signal of each group of reference areas;
s4: calculating the signal-to-noise ratio of the intermediate frequency component in two reference positions of each group of reference areas, and selecting the corresponding minimum signal-to-noise ratio in the reference areas from the two signal-to-noise ratios;
s5: selecting a maximum signal-to-noise ratio from the selected minimum signal-to-noise ratios at any time according to the relation between the signal-to-noise ratio and the reconstructed signal, and taking the reconstructed signal corresponding to the maximum signal-to-noise ratio as a final reconstructed signal;
in step S4, the signal-to-noise ratios of the intermediate frequency component in the two reference positions of each group of reference areas are specifically:
taking the reference position in the reference area as a center and the intermediate frequency component of the spatial resolution as a length, performing Fourier transform on the intermediate frequency component of the spatial resolution, and calculating the ratio of the intermediate frequency band power after the Fourier transform to the total power of other frequency bands.
2. The method according to claim 1, wherein in the k sets of reference regions [ ai, bi ], the distance between the reference positions ai in two adjacent reference regions and the distance between the reference positions bi in two adjacent reference regions are not less than 1/4 of the spatial resolution.
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