CN102997945A - Multipoint disturbance positioning method of fiber-distributed disturbance sensor - Google Patents

Abstract

The invention discloses a multipoint disturbance positioning method of a fiber-distributed disturbance sensor. The fiber-distributed disturbance sensor is a fiber-distributed disturbance sensor based on double Mach-Zehnder interferometers. The method includes the steps: S1, obtaining a first output signal and a second output signal respectively through the double Mach-Zehnder interferometers; S2, respectively preprocessing the first output signal and the second output signal to obtain phase information in the first output signal and the second output signal; S3, performing time domain cross-correlation processing for the processed first output signal and the processed second output signal; and S4, performing frequency domain spectral analysis processing for results after the time domain cross-correlation processing, and extracting position information of a multipoint disturbance signal. By the aid of the method, the fiber-distributed disturbance sensor which is simple in structure and low in hardware cost can be used for achieving multipoint disturbance positioning.

Description

Multipoint disturbance positioning method of optical fiber distributed disturbance sensor

Technical Field

The invention relates to the technical field of optical fiber disturbance signal monitoring, in particular to a multipoint disturbance positioning method of an optical fiber distributed disturbance sensor.

Background

The optical fiber distributed disturbance sensor can monitor the disturbance (time-varying signal) at any point on the sensing optical fiber to obtain the time domain waveform of the disturbance signal, judge according to the disturbance event property and give alarm information; and simultaneously gives information on the spatial position of the occurrence of the disturbance event.

According to different working principles, the optical fiber distributed sensor can be classified into sensing technologies of an interferometer type, an optical fiber grating type, an optical time domain reflectometer type, an optical frequency domain reflectometer type, an intensity modulation type, and the like. The interferometer type distributed sensor has the excellent characteristics of simple realization principle, high sensitivity, high response speed, low hardware cost, suitability for long-distance sensing and the like, and becomes a main technical scheme of the optical fiber distributed disturbance sensor.

At present, the theoretical schemes of the distributed optical fiber disturbance sensor of the interferometer type mainly include single sagnac type, double mach-zehnder type, double sagnac type, sagnac + michelson type, sagnac + mach-zehnder type, dual wavelength sagnac type, double modulation frequency sagnac type interferometer and the like. The optical path based on the double-Mach-Zehnder type optical fiber distributed sensor is simple in structure, low in hardware cost and free of limitation of a signal spectrum range, and single-point disturbance can be positioned through a related time delay algorithm. However, in the prior art, under the condition of multi-position (multi-point) simultaneous disturbance, accurate positioning of the disturbance position cannot be provided, and the practical performance of the sensor is reduced. Besides the dual-wavelength Sagnac type interferometer, the other interferometer type optical fiber distributed disturbance sensors in the prior art also have the problem that multi-point simultaneous disturbance positioning is difficult to realize.

Fig. 1 shows a schematic diagram of an optical path of an optical fiber distributed disturbance sensor based on a dual-wavelength sagnac interferometer. A broadband low coherence light source is wavelength divided into two optical wavelength bands using wavelength division multiplexing. Light propagates in two different sagnac interferometers at different wavelengths. The first interferometer is composed of a bidirectional light path consisting of a light source A, a light path B, an optical fiber delay coil light path C, a sensing optical fiber light path E, a piezoelectric phase modulator light path F, a light path H and a detector I; the second Sagnac interferometer consists of a light source A, a light path B, a piezoelectric phase modulator light path D, a sensing optical fiber light path E, an optical fiber delay coil light path G,The light path H and the detector I form a bidirectional light path. Each Sagnac interferometer receives a sinusoidal strain signal (frequency f)₁、f₂) Generating a phase offset of₁、f₂Is not uniform, and should satisfy | f₁-f₂I is greater than the disturbance frequency of the measured signal, i.e. greater than the width of the fundamental frequency of the output of the sensor. Since the phase offset provides amplitude modulated carrier signals at different frequencies to different interferometers, the two interferometers can share a single photodetector. The sensitivity of the interferometer can be improved by adjusting the operating point of the system to the high slope region corresponding to the sinusoidal phase. When there is a disturbance on the sagnac loop, the offset modulated odd harmonic signals can start to be detected. The signal received by the photoelectric detector is decomposed into output signals of the two Sagnac interferometers by a double-active homodyne demodulation technology. FIG. 2 is a schematic block diagram of a demodulation circuit of a fiber-optic distributed perturbation sensor of a dual-wavelength Sagnac-type interferometer, wherein clock is a clock, f₁、f₂、2f₁、2f₂Two offset modulation frequencies and twice the modulation frequency, respectively. The signal enters two lock-in amplifiers to demodulate D (f) respectively₁)、D(f₂)、D(2f₁)、D(2f₂) The position-dependent results can be simply processed. When the system keeps the optical phase modulation caused by the disturbance less than 0.1rad, the output result has little relation with the change rate and amplitude of the disturbance.

The optical fiber distributed disturbance sensor based on the dual-wavelength Sagnac interferometer has the following defects:

1) the premise of disturbing signal demodulation and positioning is that the system is assumed to be affected by small signals (namely, the phase difference caused by the disturbing signals is less than 0.1 rad); 2) the relative phase shift produced by the perturbation is a function of position on the sagnac loop, where the sensitivity of the sensor at the center of the loop is close to zero; 3) due to the adoption of devices such as a Wavelength Division Multiplexer (WDM), the hardware cost of the system and the complexity of the structure are increased; 4) due to the non-ideal type of the wavelength division multiplexer, a certain amount of optical serial interference exists, and the error of the output result of the system is caused, so that the false alarm rate of the system is improved, and the positioning accuracy of the system is reduced.

These above drawbacks seriously affect the reliability of the sensor in practical monitoring applications, limiting the practical implementation of this solution.

Disclosure of Invention

Technical problem to be solved

The technical problem to be solved by the invention is as follows: the multipoint disturbance positioning method of the optical fiber distributed disturbance sensor is provided, the multipoint disturbance positioning is realized based on the optical fiber distributed disturbance sensor with simple structure and low hardware cost, and the inherent defects of a dual-wavelength Sagnac interferometer do not exist.

(II) technical scheme

In order to solve the above problems, the present invention provides a multipoint disturbance positioning method for an optical fiber distributed disturbance sensor, wherein the optical fiber distributed disturbance sensor is an optical fiber distributed disturbance sensor based on a double mach-zehnder interferometer, and the method comprises the following steps:

s1: obtaining a first output signal and a second output signal respectively through two interferometers of the double Mach-Zehnder interferometer;

s2: respectively preprocessing the first output signal and the second output signal to obtain phase information in the first output signal and the second output signal;

s3: performing time domain cross-correlation processing on the preprocessed first and second output signals;

s4: performing frequency domain spectrum analysis processing on the result subjected to the time domain cross-correlation processing; and extracting the position information of the multi-point disturbance signal.

Preferably, the preprocessing of step S2 includes:

s21: respectively carrying out DC blocking processing on the first output signal and the second output signal, and filtering out a DC item and a low-frequency interference item;

s22: respectively obtaining the light intensity and the visibility information of the first output signal and the second output signal after the blocking treatment so as to eliminate the change of the visibility of the first output signal and the second output signal and obtain the light intensity and the visibility information of the first output signal and the second output signal;

s23: and respectively extracting the phase information of the first output signal and the second output signal processed in the step S22, and filtering out the interference signal with the slowly changed phase.

Preferably, the dc blocking process in step S21 is implemented by directly adding a capacitor to the circuit, or by means of active or passive high-pass filtering.

Preferably, in the step S22, the peak-to-peak value is obtained by a method of piecewise finding to eliminate the variation of the visibility of the first and second output signals, and obtain the light intensity and visibility information of the first and second output signals.

Preferably, in step S22, the variation of the visibility of the first and second output signals is eliminated by an anti-polarization fading technique and an optical power stability control technique, and the light intensity information of the first and second output signals is obtained by finding a peak-to-peak value.

Preferably, the step S23 obtains phase information of the first and second output signals through a phase extraction method or a PGC modulation method.

Preferably, the step S23 filters out the phase-slowly varying interference signal by high-pass filtering.

Preferably, the preprocessing of step S2 further includes adding a step of amplifying and filtering the first and second output signals for suppressing noise and interference between steps S21 and S22.

(III) advantageous effects

1) The invention reserves the optical path structure of the double Mach-Zehnder interferometer which has the most advantages and is widely applied at present, and has the advantages of simple optical path structure, low hardware cost and the like; 2) the invention eliminates the change of the visibility of the interference signal caused by factors such as optical power fluctuation, signal polarization induced fading and the like possibly suffered in the positioning calculation process by preprocessing the disturbance signal, thereby indirectly eliminating the problem of possible sensor positioning failure caused by the change of the visibility; 3) the invention filters out the phase drift signal by preprocessing the disturbance signal, thereby eliminating the influence of phase slow change; 4) according to the invention, the time domain correlation and the frequency domain spectrum analysis are carried out on the preprocessed signals, so that respective position information when different disturbances occur simultaneously is extracted. The multi-point disturbance positioning method only acts on the signal processing module, can be realized through software, and does not change the optical path structure, so that additional optical path errors and device cost are not introduced.

Drawings

FIG. 1 is a schematic diagram of an optical path of a fiber distributed disturbance sensor based on a dual-wavelength Sagnac interferometer in the prior art;

FIG. 2 is a schematic diagram of a demodulation circuit of a prior art based on a dual-wavelength Sagnac type optical fiber distributed disturbance sensor;

FIG. 3 is a schematic diagram of the optical path of the optical fiber distributed disturbance sensor based on the dual Mach-Zehnder interferometer of the present invention;

FIG. 4 is a flow chart of a multi-point disturbance positioning method according to the present invention;

FIG. 5 is a flow chart of a pre-treatment process according to the present invention.

Detailed Description

The present invention will be described in detail below with reference to the drawings and examples.

FIG. 3 is a schematic diagram of the optical path of the optical fiber distributed disturbance sensor based on the double Mach-Zehnder interferometer of the present invention. As shown in FIG. 3, the light wave emitted from the Laser passes through the first coupler C₁Light splitting: one path of light wave propagates along the counterclockwise direction and passes through the first optical fiber L_cTransmitting through the third coupler C₃Respectively injected into the sensing arms L after light splitting_aAnd a reference arm L_bAnd then on the second coupler C₂Is interfered by the first photoelectric detector PD₁Receiving, forming a first interferometer; the other light wave propagates along the clockwise direction and passes through a second coupler C₂Respectively injected into the sensing arms L after light splitting_aAnd a reference arm L_bAnd then again in the third coupler C₃Where interference occurs, through the second optical fiber L_dTransmitted by the second photodetector PD₂And receiving, forming a second interferometer.

In the present embodiment, the difference in arm length is ignored, and therefore the sensor arm L is provided_aReference arm L_bA first optical fiber L_cAnd a second optical fiber L_dAre all L.

The embodiment describes a multipoint disturbance positioning method based on the double-Mach-Zehnder interferometer type optical fiber distributed disturbance sensor. Fig. 4 is a flowchart of the multipoint disturbance positioning method of the present invention, as shown in fig. 4, the method includes the following steps:

s1: first photodetector PD of first interferometer of double Mach-Zehnder interferometer₁Obtaining a first output signal; while a second photodetector PD of a second interferometer of said dual Mach-Zehnder interferometer₂Obtaining a second output signal;

take two simultaneous perturbations as an example. When two-point disturbance f₁(t) and f₂(t) at the same time, the length and propagation constant of the fiber will change, causing a phase change in the interferometer. Let the two perturbation-induced phase changes be

And

wherein

Where L is the arm length of the interferometer, β is the propagation constant, Δ L₁And Δ L₂Is a change in length of the optical fiber of both arms, Delta beta₁And Δ β₂Is the change in the propagation constant of the two arms. According to the fiber sensing theory, the change of the output phase information is proportional to the disturbance signal, there

Where B is a scale factor corresponding to the phase of the perturbation.

The first detector PD is thus₁And a second detector PD₂The two received and output signals are respectively

In the formula, τ₁And τ₂From two disturbance points to the first detector PD₁Propagation time of τ₃And τ₄From two disturbance points to the second detector PD₂The propagation time of (c). K₁And K₂Is the visibility of the interferometer and is,

is the initial phase difference of the interferometer, I₀Depending on the optical power of the laser output. To simplify the analysis, the difference in length between the sensing fiber and the conducting fiber, L, is ignored₁And L₂Three couplers C of two disturbing distances respectively₃The travel times are respectively expressed as:

${\mathrm{\τ}}_1=\frac{n(L-L_1)}{c}---\left(7\right)$

${\mathrm{\τ}}_2=\frac{n(L-L_2)}{c}---\left(8\right)$

${\mathrm{\τ}}_3=\frac{n(L+L_1)}{c}---\left(9\right)$

${\mathrm{\τ}}_4=\frac{n(L+L_2)}{c}---\left(10\right)$

s2: respectively preprocessing the first output signal and the second output signal to obtain phase information in the first output signal and the second output signal;

the preprocessing of step S2 includes:

s21: respectively carrying out DC blocking processing on the first output signal and the second output signal, and filtering out a DC item and a low-frequency interference item; the blocking processing in step S21 is realized by directly adding a capacitor to the circuit, or by an active or passive high-pass filtering manner;

s22: respectively obtaining the light intensity and the visibility information of the first output signal and the second output signal after the blocking treatment so as to eliminate the change of the visibility of the first output signal and the second output signal and obtain the light intensity and the visibility information of the first output signal and the second output signal;

the interference signal generally has a cosine function form, and the visibility information can be obtained by obtaining a peak-to-peak value of the signal, but due to the influence of factors such as polarization fading, the visibility in different time periods can be changed, so that in the embodiment, the change of the visibility of the first and second output signals and the light intensity information of the first and second output signals are eliminated by obtaining the peak-to-peak value by segmenting the time, so as to reduce or eliminate the influence of the visibility, better achieve the purpose of obtaining the light intensity and the visibility information, and reduce the error; the first and second output signals after the processing of step S22 are:

in addition to the above method of finding peak and peak values in segments, the present embodiment may also adopt an anti-polarization fading technique and an optical power stability control technique to eliminate the visibility change of the first and second output signals, and obtain the light intensity and visibility information of the first and second output signals by finding peak and peak values;

s23: and respectively extracting the phase information of the first output signal and the second output signal processed in the step S22, and filtering out the interference signal with the slowly changed phase.

In the present embodiment, I is obtained by performing a cosine function phase extraction algorithm on equations (11) and (12)₁' (t) and I₂' (t) phase information, yielding:

wherein,

is a phase-slowly varying interference signal, which is filtered out by high-pass filtering in this embodiment, so that equations (13) and (14) become:

the purpose of extracting the phase information in the trigonometric function by the cosine function phase extraction algorithm in this embodiment is to provide basic preparation for subsequent signal positioning, and the phase information in the trigonometric function can also be extracted by other phase unwrapping algorithms or similar phase information extraction algorithms, such as PGC modulation (including PGC inner modulation and PGC outer modulation).

In other embodiments, a step of amplifying and filtering the first and second output signals for suppressing noise and interference may be added between steps S21 and S22.

S3: performing time domain cross-correlation processing on the preprocessed first and second output signals;

in general, assume two perturbation signals f₁(t) and f₂(t) are each independently

In the formula A_i，A′_j，ω_i，ω′_j，

And

respectively representing the amplitude, angular frequency and initial phase of the two perturbations, N and M being the number of frequency component components of the two perturbations, respectively.

According to the formulae (3) and (4), it is possible to obtain

In the formula, B_iAnd B_j' denotes the magnitude of the phase difference caused by the two disturbances, respectively.

By substituting formulae (19) and (20) for formulae (15) and (16), respectively

Derived from equations (21) and (22):

(23)

to I₃(t) and I₄(t) performing a cross-correlation operation to obtain

(24)

In the formula (24), B_k″，ω_k"and

respectively representing the amplitude, angular frequency and initial phase of the same frequency component in the two perturbations, P being the number of same frequency components. In equation (24), the same frequency component can be expressed as:

wherein,

$C\left(k\right)=\frac{{\mathrm{<figure-callout\; id="2"\; label="B\; k"\; filenames="HDA0000091766430000012.png"\; state="\{\{state\}\}">B</figure-callout>}}_k^{}}{{2\mathrm{\&omega;}}_k}\cos \left(\frac{{2\mathrm{\&omega;}}_k{\mathrm{nL}}_1}{c}\right)+\frac{{B_k}^{\&prime;2}}{{2\mathrm{\&omega;}}_k}\cos \left(\frac{{2\mathrm{\&omega;}}_k^{\&prime;}{\mathrm{nL}}_2}{c}\right)---\left(26\right)$

$D\left(k\right)=\frac{{\mathrm{<figure-callout\; id="2"\; label="B\; k"\; filenames="HDA0000091766430000012.png"\; state="\{\{state\}\}">B</figure-callout>}}_k^{}}{{2\mathrm{\&omega;}}_k}\sin \left(\frac{{2\mathrm{\&omega;}}_k{\mathrm{nL}}_1}{c}\right)+\frac{{B_k}^{\&prime;2}}{{2\mathrm{\&omega;}}_k}\sin \left(\frac{{2\mathrm{\&omega;}}_k^{\&prime;}{\mathrm{nL}}_2}{c}\right)---\left(27\right)$

$\tan \left[\mathrm{\&phi;}\left(k\right)\right]=\frac{D\left(k\right)}{C\left(k\right)}---\left(28\right)$

s4: performing frequency domain spectrum analysis processing on the result subjected to the time domain cross-correlation processing, and extracting the position information of the multi-point disturbance signal; that is, the phase spectrum analysis of equation (24) is performed using τ as a variable, and the positions L of two disturbances can be extracted₁And L₂. The positioning algorithm of two-point disturbance is popularized to three-point and more-than-three-point disturbance, and the multi-point simultaneous disturbance detection and positioning of the optical fiber distributed disturbance sensor based on the double Mach-Zehnder interferometer can be realized.

The above embodiments are only for illustrating the invention and are not to be construed as limiting the invention, and those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention, therefore, all equivalent technical solutions also belong to the scope of the invention, and the scope of the invention is defined by the claims.

Claims (8)

1. A multipoint disturbance positioning method of an optical fiber distributed disturbance sensor is characterized in that the optical fiber distributed disturbance sensor is an optical fiber distributed disturbance sensor based on a double Mach-Zehnder interferometer, and the method comprises the following steps:

s1: obtaining a first output signal and a second output signal respectively through two interferometers of the double Mach-Zehnder interferometer;

s2: respectively preprocessing the first output signal and the second output signal to obtain phase information in the first output signal and the second output signal;

s3: performing time domain cross-correlation processing on the preprocessed first and second output signals;

s4: performing frequency domain spectrum analysis processing on the result subjected to the time domain cross-correlation processing; and extracting the position information of the multi-point disturbance signal.

2. The method for multi-point disturbance positioning of the optical fiber distributed disturbance sensor according to claim 1, wherein the preprocessing of step S2 includes:

s21: respectively carrying out DC blocking processing on the first output signal and the second output signal, and filtering out a DC item and a low-frequency interference item;

s22: respectively obtaining the light intensity and the visibility information of the first output signal and the second output signal after the blocking treatment so as to eliminate the change of the visibility of the first output signal and the second output signal and obtain the light intensity and the visibility information of the first output signal and the second output signal;

s23: and respectively extracting the phase information of the first output signal and the second output signal processed in the step S22, and filtering out the interference signal with the slowly changed phase.

3. The method for multi-point disturbance positioning of the optical fiber distributed disturbance sensor according to claim 2, wherein the dc blocking process in step S21 is implemented by directly adding capacitance to a circuit or by an active or passive high-pass filtering manner.

4. The method for locating the multi-point disturbance of the optical fiber distributed disturbance sensor according to claim 2, wherein the step S22 is implemented by using a peak-to-peak method to eliminate the variation of the visibility of the first and second output signals and obtain the light intensity and visibility information of the first and second output signals.

5. The method for locating multi-point disturbance of an optical fiber distributed disturbance sensor according to claim 2, wherein in step S22, the variation of the visibility of the first and second output signals is eliminated by an anti-polarization fading technique and an optical power stability control technique, and the light intensity and visibility information of the first and second output signals is obtained by finding the peak-to-peak value.

6. The method for multi-point disturbance positioning of the optical fiber distributed disturbance sensor according to claim 2, wherein the step S23 is implemented by obtaining phase information of the first and second output signals through a phase extraction method or a PGC modulation method.

7. The method for multi-point disturbance positioning of the optical fiber distributed disturbance sensor according to claim 6, wherein the step S23 is performed by high-pass filtering to filter out phase-slowly varying disturbance signals.

8. The method for multi-point disturbance localization of a fiber optic distributed disturbance sensor according to claim 2, wherein the preprocessing of step S2 further comprises adding steps of amplifying and filtering conditioning the first and second output signals between steps S21 and S22 for suppressing noise and interference.

Images