CN115290176A - Quasi-distributed vibration sensing system and method based on low-coherence light source - Google Patents
Quasi-distributed vibration sensing system and method based on low-coherence light source Download PDFInfo
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- G01—MEASURING; TESTING
- G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
- G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
- G01H9/004—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors
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- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
- G01B11/161—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by interferometric means
Abstract
The invention provides a quasi-distributed vibration sensing system and a method based on a low-coherence light source, which comprises the following steps: the device comprises a low-coherence light source, a balanced Mach-Zehnder interferometer unit, a weak reflection point array, a photoelectric detector and a signal processing unit; after an optical signal emitted by the low-coherence light source reaches a weak reflection point array through the unbalanced Mach-Zehnder interferometer unit, the optical signal enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer unit again, and the signal processing unit acquires a voltage signal output by the photoelectric detector and demodulates phase information of an interference signal generated by CW light and CCW light reflected by the same reflection point to output an external vibration signal.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a quasi-distributed vibration sensing system and a method based on a low-coherence light source.
Background
The quasi-distributed vibration sensing system is a sensing system which can detect and position strain signals generated between any reflection points by using a weak optical fiber grating array or a weak reflection point array in an optical fiber as a sensing probe and linearly obtain a strain signal waveform. Most of the distributed vibration sensing systems widely used at present are based on an optical time domain reflectometer or an optical frequency domain reflectometer structure, and optical fiber strain information caused by environmental vibration is obtained by measuring phase change of optical signals reflected by adjacent reflection points. In these systems, a high coherence light source with a line width in the kilohertz level is required, and the high coherence light source is very expensive, so that the cost of the sensing system is high, and the wide application of the quasi-distributed acoustic wave sensing system is limited.
Patent document CN207036249U (application number: 201621320115. X) discloses a distributed optical fiber vibration sensing system with high sensitivity. The utility model discloses the acquisition system adopts high coherent narrow linewidth laser as the light source, be two the tunnel through the coupler beam split, wherein be the light pulse sequence through the pulse modulator modulation all the way, use optical amplifier to enlarge the back and inject into vibration sensing optical fiber through the circulator, backward rayleigh scattered light in the sensing optical fiber is transmitted back through the circulator, carry out the heterodyne beat frequency through the local oscillator light of frequency shift ware frequency shift Δ f with another way, beat frequency optical signal is the signal of telecommunication through the detector conversion, carry out the collection and the processing of data afterwards.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a quasi-distributed vibration sensing system and a method based on a low-coherence light source.
According to the invention, the quasi-distributed vibration sensing system based on the low-coherence light source comprises: the device comprises a low-coherence light source, a non-equilibrium Mach-Zehnder interferometer unit, a weak reflection point array, a photoelectric detector and a signal processing unit;
after an optical signal emitted by the low-coherence light source reaches a weak reflection point array through the unbalanced Mach-Zehnder interferometer unit, the optical signal enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer unit again, the signal processing unit collects a voltage signal output by the photoelectric detector, demodulates phase information of an interference signal generated by CW light and CCW light reflected by the same reflection point and outputs an external vibration signal, and obtains a strain signal.
Preferably, the unbalanced mach-zehnder interferometer unit includes: the device comprises a first coupler, a second coupler, an acousto-optic modulator, a delay optical fiber and an optional polarization controller;
when incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CW light sequentially passes through the first coupler, the acousto-optic modulator, the delay optical fiber, the second coupler and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the polarization controller and the first coupler and enters the photoelectric detector;
when incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CCW light sequentially passes through the first coupler, the polarization controller, the second coupler and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the delay optical fiber, the acousto-optic modulator and the first coupler and enters the photoelectric detector.
Preferably, the first coupler and the second coupler are each a 50.
Preferably, the acousto-optic modulator further comprises an electric signal generator, wherein the electric signal generator generates a periodic linear frequency sweep pulse signal, or generates a combination of a linear frequency sweep pulse signal and a single frequency pulse signal periodically to drive the acousto-optic modulator, so as to apply a modulation signal to the CW light and the CCW light.
Preferably, when the electrical signal generator generates a periodic linear sweep pulse signal to drive the acousto-optic modulator, the method comprises the following steps:
a module M1: the signal processing unit collects the electric signal I (t) output by the photoelectric detector and performs Fourier transform on the collected electric signal I (t);
a module M2: the positions of the reflection points correspond to the frequency information after Fourier transform one by one, and the corresponding relation is
Wherein L is a Representing the total length of the interferometer upper arm; l is b Represents the total length of the lower arm of the interferometer; l is a radical of an alcohol i Representing the length of the optical fiber from the ith reflection point to the starting end of the reflection point array; v represents the propagation velocity of light in the fiber; kappa represents the frequency sweeping speed of the acousto-optic modulator;
a module M3: extracting phase information theta at peak position of reflection point i (t), i represents a reflection point number;
a module M4: will be adjacent to each otherThe phase signals at the peak position extracted by the shooting point are differentiated to obtain the differentiated phase signals delta theta i (t); the triggering of modules M1 to M4 is repeated for the signals in each cycle, using the resulting differentiated phase signal Delta theta i (t) calculating a strain signal;
wherein, tau vib Representing the repetition period of the linear sweep signal; tau represents the time delay of the current quasi-distributed vibration sensing system; τ = (L) a -L b )/υ；L a Representing the total length of the upper arm of the interferometer; l is b Represents the total length of the lower arm of the interferometer; v is the propagation velocity of light in the fiber; l is i-1,i Represents the reflection point L i-1 And L i The length of the optical fiber in between.
Preferably, in each detection period, the electric signal generator generates a linear frequency sweep pulse signal and a single-frequency pulse signal in turn to drive the acousto-optic modulator;
CW light is first modulated into a linear sweep pulse signal by an acousto-optic modulator, wherein the sweep start frequency is f l Sweep rate is κ and pulse duration is t p After reaching the weak reflection point array, reflected light enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer again;
CCW light is reflected by the weak reflection point array and then passes through the acousto-optic modulator, and the driving signal of the acousto-optic modulator is a single-frequency signal f s 。
Preferably, when the electrical signal generator periodically generates a combination of a linear swept-frequency pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator, the acousto-optic modulator comprises:
a module N1: the signal processing unit collects the electric signal I (t) output by the photoelectric detector and convolutes the collected electric signal I (t) with the matched filter u (-t) on a digital domain; the expression of the matched filter is u (t) = rect (t/t) p )exp{j2π(f l -f s )t+jπκt 2 Where rect represents the momentA shape window function; t is t p Represents the pulse width; f. of l Representing the sweep frequency starting frequency of the linear sweep frequency pulse signal; f. of s A frequency representing a single frequency signal;
a module N2: and the positions of the reflecting points correspond to the time of the time domain signals after matched filtering one by one, and the corresponding relation is as follows:
wherein L is a Representing the total length of the interferometer upper arm; l is b The total length of the lower arm of the interferometer is shown; l is i Representing the length of the optical fiber from the ith reflection point to the starting end of the reflection point array; v represents the propagation velocity of light in the optical fiber; i represents a reflection point serial number;
a module N3: extracting phase information theta at peak position of reflection point i (t), i represents a reflection point number;
and a module N4: the signals extracted from adjacent reflection points are differentiated to obtain a differentiated phase signal delta theta i (t); repeatedly triggering the modules N1 to N4 for signals in each period, and utilizing the obtained differential phase signals delta theta i (t) calculating a strain signal;
wherein, tau vib A pulse emission interval for the light source; τ is the system delay; τ = (L) a -L b )/υ。
The invention provides a quasi-distributed vibration sensing method based on a low-coherence light source, which comprises the following steps:
after an optical signal emitted by the low-coherence light source reaches a weak reflection point array through the unbalanced Mach-Zehnder interferometer unit, the optical signal enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer unit again, the signal processing unit acquires a voltage signal output by the photoelectric detector, demodulates phase information of an interference signal generated by CW light and CCW light reflected by the same reflection point, and outputs an external vibration signal to obtain a strain signal.
Preferably, the unbalanced mach-zehnder interferometer unit includes: the device comprises a first coupler, a second coupler, an acousto-optic modulator, a delay optical fiber and an optional polarization controller;
when incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CW light sequentially passes through the first coupler, the acousto-optic modulator, the delay optical fiber, the second coupler and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the polarization controller and the first coupler and enters the photoelectric detector;
when incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CCW light sequentially passes through the first coupler, the polarization controller, the second coupler and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the delay optical fiber, the acousto-optic modulator and the first coupler and enters the photoelectric detector.
Preferably, the acousto-optic modulator further comprises an electrical signal generator, wherein the electrical signal generator generates a periodic linear frequency sweep pulse signal, or generates a combination of a linear frequency sweep pulse signal and a single-frequency pulse signal periodically to drive the acousto-optic modulator, so as to apply a modulation signal to the CW light and the CCW light;
when the electric signal generator generates a periodic linear sweep pulse signal to drive the acousto-optic modulator, the method comprises the following steps:
step S11: the signal processing unit collects the electric signal I (t) output by the photoelectric detector and performs Fourier transform on the collected electric signal I (t);
step S12: the positions of the reflection points correspond to the frequency information after Fourier transform one by one, and the corresponding relation is
Wherein L is a Representing the total length of the upper arm of the interferometer; l is b Represents the total length of the lower arm of the interferometer; l is i The length of the optical fiber from the ith reflection point to the starting end of the reflection point array is shown; v represents the propagation velocity of light in the fiber; kappa represents the frequency sweeping speed of the acousto-optic modulator;
step S13: extracting phase information theta at peak position of reflection point i (t), i represents the number of reflection points；
Step S14: the phase signals at the peak positions extracted from the adjacent reflection points are differentiated to obtain the differentiated phase signals delta theta i (t); triggering steps S11 to S14 are repeated for signals in each period, and the obtained differential phase signal delta theta is used i (t) calculating a strain signal;
wherein, tau vib Representing the repetition period of the linear sweep signal; tau represents the time delay of the current quasi-distributed vibration sensing system; τ = (L) a -L b )/υ；L a Representing the total length of the upper arm of the interferometer; l is b Represents the total length of the lower arm of the interferometer; upsilon is the propagation speed of light in the optical fiber; l is a radical of an alcohol i-1,i Represents the reflection point L i-1 And L i A length of optical fiber in between;
in each detection period, the electric signal generator sequentially generates a linear frequency sweep pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator;
CW light is first modulated into a linear sweep pulse signal by an acousto-optic modulator, wherein the sweep start frequency is f l Sweep rate is κ and pulse duration is t p After reaching the weak reflection point array, reflected light enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer again;
CCW light is reflected by the weak reflection point array and then passes through the acousto-optic modulator, and the driving signal of the acousto-optic modulator is a single-frequency signal f s ；
When the electric signal generator periodically generates a combination of a linear sweep frequency pulse signal and a single frequency pulse signal to drive the acousto-optic modulator, the acousto-optic modulator comprises the following components:
step S21: the signal processing unit collects the electric signal I (t) output by the photoelectric detector and convolutes the collected electric signal I (t) with the matched filter u (-t) on a digital domain; the expression of the matched filter is u (t) = rect (t/t) p )exp{j2π(f l -f s )t+jπκt 2 }, wherein rect represents a rectangular window function; t is t p Represents the pulse width; f. of l Representing the sweep frequency starting frequency of the linear sweep frequency pulse signal; f. of s A frequency representing a single frequency signal;
step S22: and the positions of the reflecting points correspond to the time of the time domain signals after matched filtering one by one, and the corresponding relation is as follows:
wherein L is a Representing the total length of the interferometer upper arm; l is b Represents the total length of the lower arm of the interferometer; l is i Representing the length of the optical fiber from the ith reflection point to the starting end of the reflection point array; v represents the propagation velocity of light in the fiber; i represents a reflection point serial number;
step S23: extracting phase information theta at peak position of reflection point i (t), i represents a reflection point number;
step S24: the signals extracted from adjacent reflection points are differentiated to obtain a differentiated phase signal delta theta i (t); repeating the triggering steps S21 to S24 for the signals in each period, and using the obtained differential phase signal Delta theta i (t) calculating a strain signal;
wherein, tau vib A pulse emission interval for the light source; τ is the system delay; τ = (L) a -L b )/υ。
Compared with the prior art, the invention has the following beneficial effects:
1. according to the invention, by combining the unequal-arm interferometer and the weak reflection point array, a modulation device is introduced, the positions of the weak reflection points are mapped to a frequency domain or a time domain through different modulation schemes, and finally the phase change of CW light and CCW light in a time difference is measured, so that the positioning and the demodulation of vibration are realized;
2. compared with the existing distributed and quasi-distributed acoustic wave sensing systems which need high-coherence laser light sources and are expensive in cost, the invention can realize dynamic strain measurement by using low-coherence light sources with very low cost, simple devices and light paths, and obviously reduces the cost and the complexity of the system.
3. In the aspect of measurement accuracy, the invention can realize the measurement accuracy of the same magnitude as that of the existing acoustic wave sensing system based on high-coherent light. In addition, the acoustic wave sensing system based on the high-coherence light source has the problems that the system precision is limited by the laser line width and the like, and the precision of the invention is not limited by the laser line width and the drift of the laser wavelength.
Drawings
Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:
fig. 1 is a block diagram of a low-coherence light source-based quasi-distributed sensing system.
Fig. 2 is a flowchart of a demodulation method according to embodiment 2.
Fig. 3 is a distance-reflectivity graph of the first 5 weak reflection points obtained by fourier transform of the data acquired in example 2.
Fig. 4 shows the power spectral densities of the strain signals on the two channels after demodulation in example 2.
FIG. 5 shows the strain accuracy at 1kHz for each channel when the system of example 2 is at rest.
Fig. 6 is a flowchart of a demodulation method of embodiment 3.
Fig. 7 is a distance-reflectivity graph of the first 500m weak reflection point obtained by fourier transform of the primary acquired data in example 3.
Fig. 8 shows the power spectral densities of the strain signals on the two channels after demodulation in example 3.
FIG. 9 shows the strain accuracy at 1kHz for each channel when the system of example 3 is at rest.
The device comprises a low-coherence light source 1, a first coupler 2, an acousto-optic modulator 3, an electric signal generator 4, a delay optical fiber 5, a second coupler 6, a weak reflection point array 7, a polarization controller 8, a photoelectric detector 9 and a signal processing unit 10.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications can be made by persons skilled in the art without departing from the concept of the invention. All falling within the scope of the present invention.
The invention provides a quasi-distributed vibration sensing system and a method based on a low-coherence light source, which can reduce the cost of the sensing system while maintaining high strain resolution.
The invention relates to a quasi-distributed acoustic wave sensing system based on a low-coherence light source, which is used for measuring interference signals of two beams of light reflected by the same reflecting point and detecting vibration signals applied to a reflecting point array through phase change of the interference signals. The low-coherence pulse light enters the reflection point array after passing through the unbalanced Mach-Zehnder interferometer unit, and enters the photoelectric detector after passing through the unbalanced Mach-Zehnder interferometer unit again after being reflected, wherein optical signals of the following two paths are used for sensing: clockwise (CW) light: the light source is transmitted along the upper arm of the interferometer, reaches the reflection point array, is reflected to the lower arm of the interferometer, and is output to the photoelectric detector; counter Clockwise (CCW) light: the light is transmitted from the light source along the lower arm of the interferometer, reflected and then enters the upper arm of the interferometer, and finally output to the photoelectric detector. The CW light and the CCW light travel in different directions, travel the same optical path to the photodetector, and interfere at the detector. Because of the different positions of each reflection point, the CW light and the CCW light reflected by different reflection points are transmitted to the detector at different times, t i ＝(L a +L b +2L i ) V, wherein L a Total length of upper arm, L, of interferometer b Total length of lower arm of interferometer, L i Is the length of the optical fiber from the ith reflection point to the initial end of the reflection point array, and upsilon is the propagation speed of light in the optical fiber. By means of the transmission time, the light reflected back by different reflection points can be distinguished in the time domain. Because the lengths of the upper arm and the lower arm of the interferometer are different, the CW light and the CCW light reach the ithThe time of the reflection point is different, and a time delay tau = (L) exists a -L b ) And/upsilon. When strain epsilon (t) exists between the i-1 th reflection point and the ith reflection point, due to the time delay tau, when the CW light reaches the ith reflection point, the length from the reflection point to the initial section of the optical fiber is changed by deltaL when the CW light reaches the ith reflection point, and therefore, the phase of the interference signal of the light generated by the CW light CCW light from the ith reflection point will be changed. According to the photoelastic effect, the phase change amount is
Wherein ω is c Is the central angular frequency, L, of the light source i-1,i Is the length of the optical fiber between the i-1 st reflection point and the i-th reflection point, and gamma is the photoelastic coefficient. By detecting Δ θ i The vibration signal epsilon (t) can be linearly recovered. Since the length of the optical fiber from each reflection point to the starting end of the reflection point array is changed after the ith reflection point, the phases extracted from the adjacent reflection points need to be differentiated.
Example 1
According to the invention, the quasi-distributed vibration sensing system based on the low-coherence light source comprises: the device comprises a low-coherence light source, a non-equilibrium Mach-Zehnder interferometer unit, a weak reflection point array, a photoelectric detector and a signal processing unit;
after an optical signal emitted by the low-coherence light source reaches a weak reflection point array through the unbalanced Mach-Zehnder interferometer unit, the optical signal enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer unit again, light passing through the interferometer in a clockwise direction (CW) and light passing through the interferometer in a counterclockwise direction (CCW) simultaneously reach the photoelectric detector and interfere with each other, the phase difference of the CW light and the CCW light is modulated by vibration to be detected due to different time when the CW light and the CCW light reach the same weak reflection point, the signal processing unit acquires a voltage signal output by the photoelectric detector, phase information of interference signals generated by the CW light and the CCW light reflected by the same reflection point is demodulated through a signal processing technology, and an external vibration signal is accurately output;
specifically, the signal processing unit is configured to demodulate an electrical signal output by the photodetector to obtain a vibration signal to be measured.
Specifically, the unbalanced mach-zehnder interferometer unit includes: the device comprises a first coupler, a second coupler, an acousto-optic modulator, a time delay optical fiber and an optional polarization controller;
when incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CW light sequentially passes through the first coupler, the acousto-optic modulator, the delay optical fiber, the second coupler and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the polarization controller and the first coupler and enters the photoelectric detector;
when incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CCW light sequentially passes through the first coupler, the polarization controller, the second coupler and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the delay optical fiber, the acousto-optic modulator and the first coupler and enters the photoelectric detector.
Specifically, the unbalanced mach-zehnder interferometer is composed of two couplers to form a basic structure, an acousto-optic modulator and a time delay optical fiber are added to an upper arm, and a polarization controller is added to a lower arm, wherein the splitting ratios of the two couplers are both 50.
Specifically, the device further comprises an electric signal generator, wherein the electric signal generator generates a periodic linear frequency sweep pulse signal, or the periodic linear frequency sweep pulse signal and a single-frequency pulse signal are generated to drive the acousto-optic modulator in a combined mode, so that a modulation signal is applied to the CW light and the CCW light.
When the acousto-optic modulator is added on the unbalanced Mach-Zehnder interferometer unit, and a signal for driving the acousto-optic modulator is a periodic linear sweep frequency pulse signal. In one period, after the CW light and the CCW light enter the unbalanced Mach-Zehnder interference, because the transmission paths are opposite, the CW light and the CCW light will arrive at the acousto-optic modulator at different time and be adjusted to be linear sweep light. At the photoelectric detector, the CW light reflected by the same reflection point interferes with the CCW light to form a beat frequency signal, and finally the photoelectric detector outputs an electric signal I (t). And the electric signal I (t) enters a signal processing unit, and after a signal processing flow, strain information epsilon (t) applied to the sensing array is output.
Specifically, when the electrical signal generator generates a periodic linear sweep pulse signal to drive the acousto-optic modulator, the method includes:
a module M1: the signal processing unit collects the electric signal I (t) output by the photoelectric detector and performs Fourier transform on the collected electric signal I (t);
a module M2: the positions of the reflection points are in one-to-one correspondence with the frequency information after Fourier transform, and the correspondence is
Wherein L is a Representing the total length of the interferometer upper arm; l is b Represents the total length of the lower arm of the interferometer; l is a radical of an alcohol i Representing the length of the optical fiber from the ith reflection point to the starting end of the reflection point array; v represents the propagation velocity of light in the fiber; kappa represents the frequency sweeping speed of the acousto-optic modulator; obtaining the specific position of a reflection point through frequency information;
a module M3: extracting phase information theta at peak position of reflection point i (t), i represents a reflection point number;
a module M4: the phase signals at the peak positions extracted from the adjacent reflection points are differentiated to obtain the differentiated phase signals delta theta i (t); repeatedly triggering the modules M1 to M4 for signals in each period, and utilizing the obtained differential phase signals delta theta i (t) calculating a strain signal;
wherein, tau vib Representing the repetition period of the linear sweep signal; tau represents the time delay of the current quasi-distributed vibration sensing system; τ = (L) a -L b )/υ；L a Representing the total length of the interferometer upper arm; l is a radical of an alcohol b The total length of the lower arm of the interferometer is shown; v is the propagation velocity of light in the fiber; l is i-1,i Represents the reflection point L i-1 And L i The length of the optical fiber in between.
The upper arm and the blind ratio of the interferometer respectively refer to the upper side light path and the lower side light path in the attached drawing 1, the upper arm and the blind ratio of the interferometer are both common single-mode optical fibers, and the most basic interferometer can be obtained after the first coupler is connected with the second coupler.
Specifically, an acousto-optic modulator is added on a non-equilibrium Mach-Zehnder interferometer unit, and a signal for driving the acousto-optic modulator is divided into two parts, wherein one part is a linear sweep pulse signal, and the other part is a single-frequency pulse signal. After CW light and CCW light enter a non-balanced Mach-Zehnder interferometer unit, the CW light is modulated into a linear frequency sweep pulse signal through an acousto-optic modulator, wherein the frequency sweep starting frequency is f l Sweep rate is κ and pulse width is t p And after reaching the weak reflection point array, the reflected light enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer again. CCW light is reflected by the weak reflection point array and then passes through the acousto-optic modulator, and the driving signal of the acousto-optic modulator is a single-frequency signal f s . At the photoelectric detector, CW light reflected by the same reflection point interferes with CCW light to form a beat frequency signal, and finally the photoelectric detector outputs an electric signal I (t). And the electric signal I (t) enters a data processing unit, and after a signal processing flow, the strain information epsilon (t) applied to the sensing array is output.
Specifically, when the electrical signal generator periodically generates a combination of a linear sweep pulse signal and a single frequency pulse signal to drive the acousto-optic modulator, the method comprises the following steps:
a module N1: the signal processing unit collects the electric signal I (t) output by the photoelectric detector and convolutes the collected electric signal I (t) with the matched filter u (-t) on a digital domain; the expression of the matched filter is u (t) = rect (t/t) p )exp{j2π(f l -f s )t+jπκt 2 }, wherein rect represents a rectangular window function; t is t p Represents the pulse width; f. of l Representing the sweep frequency starting frequency of the linear sweep frequency pulse signal; f. of s A frequency representing a single frequency signal;
a module N2: and the positions of the reflecting points correspond to the time of the time domain signals after matched filtering one by one, and the corresponding relation is as follows:
wherein L is a Representing the total length of the interferometer upper arm; l is a radical of an alcohol b The total length of the lower arm of the interferometer is shown; l is i The length of the optical fiber from the ith reflection point to the starting end of the reflection point array is shown; v represents the propagation velocity of light in the fiber; i represents a reflection point serial number;
a module N3: extracting phase information theta at peak position of reflection point i (t), i represents a reflection point number;
and a module N4: the signals extracted from adjacent reflection points are differentiated to obtain a differentiated phase signal delta theta i (t); repeatedly triggering the modules N1 to N4 for signals in each period, and utilizing the obtained differential phase signals delta theta i (t) calculating a strain signal;
wherein, tau vib A pulse emission interval for the light source; τ is the system delay; τ = (L) a -L b )/υ。
The quasi-distributed vibration sensing system based on the low-coherence light source can be realized through the step flow in the quasi-distributed vibration sensing method based on the low-coherence light source. The quasi-distributed vibration sensing method based on the low-coherent light source can be understood as a preferable example of the quasi-distributed vibration sensing system based on the low-coherent light source by those skilled in the art.
Compared with the prior art, the invention can realize high-precision dynamic strain measurement by using very simple devices and light paths, has simple demodulation process and obviously reduces the system cost and complexity.
Example 2
Example 2 is a preferred example of example 1
As shown in fig. 1, the present embodiment relates to a quasi-distributed acoustic wave sensing system based on a low coherent light source for implementing the above method, which includes: the system comprises a low-coherence light source 1, a first coupler 2, an acousto-optic modulator 3, an electric signal generator 4, a delay optical fiber 5, a second coupler 6, a weak reflection point array 7, a polarization controller 8, a photoelectric detector 9 and a signal processing unit 10;
wherein: the first coupler 2, the acousto-optic modulator 3, the delay optical fiber 5, the second coupler 6 and the polarization controller 8 form an unbalanced interferometer structure, a port b of the first coupler 2 is sequentially connected with the acousto-optic modulator 3, ports a of the delay optical fiber 5 and the second coupler 6 are sequentially connected, and a port d of the first coupler 2 is sequentially connected with ports c of the polarization controller 8 and the second coupler 6. The b port of the second coupler 6 is connected to the weak reflection point array, and the c port of the first coupler is connected to the photodetector 9. Pulse optical signals emitted by the low-coherence broadband light source 1 are output to a port a of the first coupler 2, and enter the unbalanced bar interferometer structure through a port b and a port d of the first coupler 2 after passing through the first coupler 2. The optical signal passing through the port b of the first coupler is output to the port a of the second coupler through the acousto-optic modulator 3 and the delay optical fiber 5, enters the weak reflection point array through the port b of the second coupler, is reflected by reflection points, then enters the photoelectric detector 9 through the port c of the second coupler 6 and the polarization controller 8 and the port c of the first coupler 2 to form an optical signal transmitted in the clockwise direction; the optical signal passing through the d port of the first coupler passes through the unbalanced interferometer structure in the opposite direction and finally enters the photodetector 9, forming an optical signal transmitted in the counterclockwise direction. The signal processing unit 10 collects the input voltage signal of the photodetector 9 and performs synchronous demodulation.
In this embodiment, the signal for driving the acousto-optic modulator is a linear swept-frequency pulse signal, and the phase of the CW light reflected by the ith reflection point reaching the photodetector is:
the CCW light reflected by the ith reflection point reaches the photodetector with a phase:
whereinΩ 0 The initial angular frequency of the acousto-optic modulator, and k is the sweep rate.
The phase difference between the CW light and the CCW light is:
Δ L is the change of the position of the reflection point when the CW light and the CCW light reach the ith reflection point, where τ = (L) a -L b )/υ，L a Is the total length of the upper arm, L, of the interferometer b The total length of the lower arm of the interferometer. The first term in brackets relates to the position of the reflection point and can be used for positioning the reflection point; Δ L tends to be in the order of magnitude of nano-strain, so the second term is small and can be ignored; the third term and the optical path caused by strain are transformed into a linear relation, and can be used for demodulating a strain signal; the fourth term is a constant term and does not affect the demodulation of the strain. The ac portion of the photocurrent signal detected at the photodetector is: i is AC ∝r i cos(2πf i t+θ i +C i ) Wherein r is i Is the reflectivity of the ith reflection point,
θ i ＝2ΔL(ω c +Ω 0 )，C i ＝(L a +L b +2L i )Ω 0 . After the received electric signals are subjected to Fourier transform, fi can be obtained at the same time i And theta i The value of (c). Because the optical path of each reflection point is changed, the phases extracted from adjacent reflection points need to be differentiated to obtain the phase difference delta theta between the i-1 th reflection point and the i-th reflection point i 。
The low-coherence light source is an erbium-doped superfluorescent optical fiber light source, the center wavelength is 1550nm, the spectral bandwidth is 35nm, the output optical power is set to be 80mW, and the width of a light pulse generated by the light source is 200us;
the light source can be replaced by low-cost light sources such as LEDs;
the driving signal of the acousto-optic modulator is a linear sweep frequency pulse signal with the frequency of 150MHz to 250MHz, the pulse width is 200us, the pulse emission interval is 250us, and the sweep frequency rate is 500GHz/s.
The length of the delay optical fiber is 2.1km of the common single-mode optical fiber, and the time delay tau of the CW and the CCW is 10.5us. The time delay optical fiber can be replaced by common single-mode optical fiber with other lengths;
the first optical fiber coupler is a 50;
the weak reflection point array is a weak reflection point array which focuses femtosecond laser at the fiber core position and is written point by point with lower reflectivity, and the reflectivity of each reflection point to incident light with different wavelengths is almost consistent;
in the weak reflection point array, the distance between reflection points is 20m, the average reflectivity is-40 dB, and 40 reflection points are provided;
the weak reflection point array can be replaced by a weak fiber grating array and a fiber grating array with non-overlapping reflection spectrums;
the specific steps of this embodiment are as follows, as shown in fig. 2:
step one, outputting a pulse light source with the width of 200us and the period of 250us to the input end of the unbalanced interferometer, and adjusting the time of a radio frequency signal generated by an electric signal generator to align the electric signal applied to the acousto-optic modulator with an optical pulse signal in a time domain. At the moment, the signal generated by the electric signal generator is a linear sweep frequency pulse signal with the frequency from 150MHz to 250MHz, and the pulse width and the period are consistent with those of the optical pulse signal;
collecting an electric signal I (t) generated by the primary photoelectric detector, and carrying out Fourier transform on the electric signal;
step three, performing peak searching on the signals after Fourier transform, and finding out the position of each weak reflection, as shown in FIG. 3; the converted signal is a complex number, and a phase signal at the peak value of each weak reflection point can be extracted; subtracting the phase signals extracted from the adjacent channels to obtain the phase difference delta theta between the i-1 th reflection point and the i reflection points i 。
Step four, repeating the step two and the step threeWill measure Δ θ at different times i Is formed into a time domain signal delta theta i (t) converting the obtained Δ θ i (t) integration in the time domain multiplied by the sampling interval τ of the system for the vibrations vib The ratio of τ to τ is specifically:
in this example τ vib Tau is 10.52us for a pulse emission interval of 250 us.
Step five, applying a 500Hz sinusoidal strain signal between the 1 st reflection point and the 2 nd reflection point, applying a 700Hz sinusoidal strain signal between the 2 nd reflection point and the 3 rd reflection point, and repeating the steps two to four to obtain the power spectral densities of the demodulated strain signals on the two channels, as shown in FIG. 4; the two to four steps are repeated under the condition of not applying a strain signal, the strain precision of each channel at 1kHz in the figure 5 can be obtained, and the precision of the first ten channels can be read as
In the present embodiment, the reflection points are located in a manner that positions of different reflection points are mapped to different frequencies of the beat signal, so that the frequencies after fourier transform correspond to the positions of the reflection points one to one.
The spatial resolution of this example is determined by the distance between the reflection points, and after fourier transform, the full width at half maximum of the main lobe at the reflection points is determined by the pulse frequency sweep range, i.e. Δ z = v/(2B), where B is the frequency sweep range, and in this embodiment, B is 100MHz, and therefore the main lobe width is 1 meter.
The vibration frequency response bandwidth of the embodiment is detected by the emission time interval tau of the optical pulse of the frequency sweep vib Determined as 1/2 τ vib In the present embodiment,. Tau. vib 250us, and thus the vibration frequency response bandwidth is 2kHz.
Compared with the prior art, the embodiment is based on a low-coherence light source laser, can linearly recover a strain signal, and has the strain precision of
And the system thereofThe complexity and the cost are obviously reduced, and the method has good practical value.
Example 3
Example 3 is a preferred example of example 1
As shown in fig. 1, the second embodiment relates to a quasi-distributed acoustic wave sensing system based on a low coherent light source for implementing the above method, and the system configuration is the same as that of the first embodiment.
In this embodiment, the signal for driving the acousto-optic modulator is composed of two segments of signals in one period, the former segment is a linear sweep pulse, so that the CW-direction pulse light can be modulated into the linear sweep pulse after passing through the acousto-optic modulator, and the latter segment is a single-frequency pulse signal, so that the CCW light can be applied with a fixed frequency shift when passing through the acousto-optic modulator. The electric field signal of the CW light reflected by the ith reflection point and reaching the photodetector is
Where ai is the loss of CW light during transmission, ri is the reflectivity of the ith reflection point, rect is a rectangular window function,
for CW light transmission duration, t p Is the pulse width, omega c Is the central angular frequency, f, of the light source l The start frequency of the linear sweep pulse, κ the sweep rate. The electric field signal of the CCW light reaching the photodetector after being reflected by the ith reflection point is
Wherein f is s Is the frequency shift that the acousto-optic modulator modulates on the CCW light as it passes through the acousto-optic modulator. When the strain epsilon (t) is generated between the ith reflection point and the ith reflection point, the time for light to pass through the region is changed due to the change of the length of the strain section optical fiber, and the change time is
The time for transmitting the CW light and the CCW light to the photoelectric detector after being reflected by the ith reflection point is t l ＝(L b +L i ) V and t s ＝(L a +L i ) V, wherein L a And L b The lengths of the upper arm and the lower arm of the interferometer respectively. At this time, the phase difference between the CW light and the CCW light reaches the photodetector:
where τ (t) is the time variation caused by the vibration, the value is very small and is therefore in
τ (t) in the second and third terms can be ignored. The current signals generated by the CW light and the CCW light of the ith reflection point at the photodetector are:
wherein theta is i (t)＝exp{jω c [τ(t-t s ) - τ t-tl, ut = retttpephph 2 π fl-fst + j π kt2. The second term of Iit, the AC term, contains phase information, and θ therein i (t) contains information about strain for extracting theta i (t) the acquired electrical signal needs to be convolved in the digital domain with a matched filter u (-t). Will I i After the alternating term in (t) is convolved with u (-t), a convolved signal R can be obtained i (t) a specific expression is
Wherein
For convolution operations, the conjugate sign. Rit is in the shape of sinc functionA complex signal of several pulses, where the peak indicates that theta can be extracted i (t) of (d). Because the optical path of each reflection point is changed, the phases extracted from adjacent reflection points need to be differentiated to obtain the phase difference delta theta between the i-1 th reflection point and the i-th reflection point i (t)。
The low-coherence light source is an erbium-doped superfluorescent optical fiber light source, the center wavelength is 1550nm, the spectral bandwidth is 35nm, the peak power of the output light pulse is set to be 500mW, the width of the light pulse generated by the light source is 2us, and the pulse emission interval is 100us;
the light source can be replaced by low-cost light sources such as LEDs;
the period of the driving signal of the acousto-optic modulator is 100us, the first half of the sweep frequency signal is a sweep frequency signal with the width of 2us, the sweep frequency range is 190MHz to 240MHz, the second half of the sweep frequency signal is a single frequency signal with the frequency of 170MHz, the duration length is at least the time of covering the CCW light reflected by all the reflection points, and in the example, the time is 20us, as shown in fig. 6.
The length of the delay fiber is 3.906km of the common single-mode fiber, and the time delay tau of CW and CCW is 19.53us. The time delay optical fiber can be replaced by common single-mode optical fiber with other lengths;
the first optical fiber coupler is a 50;
the weak reflection point array is a weak reflection point array which focuses femtosecond laser at a fiber core position and is written point by point with lower reflectivity, and is characterized in that each reflection point almost has the same reflectivity to incident light with different wavelengths;
in the weak reflection point array, the distance between reflection points is 20m, the average reflectivity is-40 dB, and 40 reflection points are provided;
the weak reflection point array can be replaced by a weak fiber grating array and a fiber grating array with non-overlapping reflection spectrums;
the specific steps of this example are as follows:
step one, outputting a pulse light source with the width of 2us and the period of 100us to the input end of the unbalanced interferometer, and adjusting the time of a radio frequency signal generated by an electric signal generator to align a first half section of linear sweep frequency electric signal applied to the acousto-optic modulator with a light pulse signal on a time domain. At the moment, the signal generated by the electric signal generator is a linear sweep frequency pulse signal with the frequency from 190MHz to 240MHz, the pulse width and the period are consistent with those of the optical pulse signal and are both 2us, and the rear half section of the signal is a single-frequency signal with the frequency of 170 MHz;
step two, collecting an electric signal I (t) generated by a primary photoelectric detector, and convolving the electric signal with a matched filter, wherein the matched filter is a linear sweep frequency pulse signal with the length of 2us and the sweep frequency range of 20MHz to 70 MHz;
step three, performing peak searching on the convolved signals, and finding out the position of each weak reflection, as shown in fig. 7; the converted signal is a complex number, and a phase signal at the peak value of each weak reflection point can be extracted; subtracting the phase signals extracted from the adjacent channels to obtain the phase difference delta theta between the i-1 th reflection point and the i reflection points i 。
Step four, repeating the step two and the step three for many times, and measuring delta theta at different moments i Is formed into a time domain signal delta theta i (t), converting the obtained Δ θ i (t) integration in the time domain multiplied by the sampling interval τ of the system for the vibrations vib The ratio of τ to τ is specifically:
in this example τ vib Tau is 19.53us for a pulse emission interval of 100 us.
Step five, applying a 1kHz sinusoidal strain signal between the 1 st reflection point and the 2 nd reflection point, applying a 2kHz sinusoidal strain signal between the 2 nd reflection point and the 3 rd reflection point, and repeating the steps two to four to obtain the power spectral densities of the strain signals on the two demodulated channels, as shown in FIG. 8; the strain accuracy of each channel in the graph of FIG. 9 at 1kHz can be obtained by repeating the steps two to four under the condition of not applying a strain signal, and the accuracy of measuring thirty-five channels can be read as
The present embodiment locates the reflection point by mapping the positions of different reflection points to different transmission times.
The spatial resolution of this example is determined by the distance between the reflection points, and after matched filtering, the full width at half maximum of the main lobe at the reflection points is determined by the pulse sweep range, i.e. Δ z = v/(2B), where B is the sweep range, and in this embodiment, B is 50MHz, and thus the main lobe width is 2 meters.
The vibration frequency response bandwidth of the embodiment is detected by the emission time interval tau of the optical pulse of the frequency sweep vib Determined as 1/2 τ vib In the present embodiment,. Tau. vib 100us, and therefore the vibration frequency response bandwidth is 5kHz.
Compared with the prior art, the embodiment is based on a low-coherence light source laser, can linearly recover a strain signal, and has the strain precision of
And the system complexity and the cost are obviously reduced, and the method has good practical value.
Those skilled in the art will appreciate that, in addition to implementing the systems, apparatus, and various modules thereof provided by the present invention in purely computer readable program code, the same procedures can be implemented entirely by logically programming method steps such that the systems, apparatus, and various modules thereof are provided in the form of logic gates, switches, application specific integrated circuits, programmable logic controllers, embedded microcontrollers and the like. Therefore, the system, the device and the modules thereof provided by the present invention can be considered as a hardware component, and the modules included in the system, the device and the modules thereof for implementing various programs can also be considered as structures in the hardware component; modules for performing various functions may also be considered to be both software programs for performing the methods and structures within hardware components.
The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes or modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention. The embodiments and features of the embodiments of the present application may be combined with each other arbitrarily without conflict.
Claims (10)
1. A quasi-distributed vibration sensing system based on a low coherence light source, comprising: the device comprises a low-coherence light source, a non-equilibrium Mach-Zehnder interferometer unit, a weak reflection point array, a photoelectric detector and a signal processing unit;
after an optical signal emitted by the low-coherence light source reaches a weak reflection point array through the unbalanced Mach-Zehnder interferometer unit, the optical signal enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer unit again, the signal processing unit collects a voltage signal output by the photoelectric detector, demodulates phase information of an interference signal generated by CW light and CCW light reflected by the same reflection point and outputs an external vibration signal, and obtains a strain signal.
2. The low coherence light source based quasi-distributed vibration sensing system of claim 1, wherein the unbalanced mach-zehnder interferometer unit comprises: the device comprises a first coupler, a second coupler, an acousto-optic modulator, a time delay optical fiber and an optional polarization controller;
when incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CW light sequentially passes through the first coupler, the acousto-optic modulator, the delay optical fiber, the second coupler and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the polarization controller and the first coupler and enters the photoelectric detector;
when incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CCW light sequentially passes through the first coupler, the polarization controller, the second coupler and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the delay optical fiber, the acousto-optic modulator and the first coupler and enters the photoelectric detector.
3. The low coherence light source based quasi-distributed vibration sensing system of claim 1, wherein the first coupler and the second coupler are each 50:50 couplers.
4. The quasi-distributed vibration sensing system according to claim 1, further comprising an electrical signal generator, wherein the electrical signal generator generates a periodic linear frequency-sweeping pulse signal, or a combination of the periodic linear frequency-sweeping pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator, and applies a modulation signal to the CW light and the CCW light.
5. The quasi-distributed vibration sensing system according to claim 4, wherein when the acousto-optic modulator is driven by a periodic linear swept pulse signal generated by the electrical signal generator, the quasi-distributed vibration sensing system comprises:
a module M1: the signal processing unit collects the electric signal I (t) output by the photoelectric detector and performs Fourier transform on the collected electric signal I (t);
a module M2: the positions of the reflection points correspond to the frequency information after Fourier transform one by one, and the corresponding relation is
Wherein L is a Representing the total length of the upper arm of the interferometer; l is b The total length of the lower arm of the interferometer is shown; l is i The length of the optical fiber from the ith reflection point to the starting end of the reflection point array is shown; v represents the propagation velocity of light in the optical fiber; k represents the frequency sweeping speed of the acousto-optic modulator;
a module M3: extracting phase information theta at peak position of reflection point i (t), i represents a reflection point number;
a module M4: differentiating the phase signals at the peak positions extracted from the adjacent reflection points to obtain differential phase signals delta theta i (t); the triggering of modules M1 to M4 is repeated for the signals in each cycle, using the resulting differentiated phase signal Delta theta i (t) calculating a strain signal;
wherein, tau vib Representing the repetition period of the linear sweep signal; tau represents the time delay of the current quasi-distributed vibration sensing system; τ = (L) a -L b )/υ；L a Representing the total length of the interferometer upper arm; l is b Represents the total length of the lower arm of the interferometer; v is the propagation velocity of light in the fiber; l is i-1，， Represents the reflection point L i-1 And L i The length of optical fiber in between.
6. The quasi-distributed vibration sensing system based on the low coherent light source according to claim 4, wherein in each detection period, the electrical signal generator sequentially generates a linear frequency sweep pulse signal and a single frequency pulse signal to drive the acousto-optic modulator;
CW light is first modulated into a linear sweep pulse signal by an acousto-optic modulator, wherein the sweep start frequency is f l Sweep rate is κ and pulse duration is t p After reaching the weak reflection point array, reflected light enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer again;
CCW light is reflected by the weak reflection point array and then passes through the acousto-optic modulator, and the driving signal of the acousto-optic modulator is a single-frequency signal f s 。
7. The quasi-distributed vibration sensing system based on low coherence light source according to claim 6, wherein when the acousto-optic modulator is driven by the combination of the linear sweep pulse signal and the single frequency pulse signal generated periodically by the electric signal generator, it comprises:
a module N1: the signal processing unit collects the electric signal I (t) output by the photoelectric detector and convolutes the collected electric signal I (t) with the matched filter u (-t) on a digital domain; the expression of the matched filter is u (t) = rect (t/t) p )exp{j2π(f l -f s )t+jπκt 2 }, wherein rect represents a rectangular window function; t is t p Represents the pulse width; f. of l Representing the sweep frequency starting frequency of the linear sweep frequency pulse signal; f. of s A frequency representing a single frequency signal;
a module N2: and the positions of the reflecting points correspond to the time of the time domain signals after matched filtering one by one, and the corresponding relation is as follows:
wherein L is a Representing the total length of the interferometer upper arm; l is b The total length of the lower arm of the interferometer is shown; l is i The length of the optical fiber from the ith reflection point to the starting end of the reflection point array is shown; v represents the propagation velocity of light in the optical fiber; i represents a reflection point serial number;
a module N3: extracting phase information theta at the peak position of the reflection point i (t), i represents a reflection point number;
a module N4: the signals extracted from adjacent reflection points are differentiated to obtain a differentiated phase signal delta theta i (t); repeatedly triggering the modules N1 to N4 for signals in each period, and utilizing the obtained differential phase signals delta theta i (t) calculating a strain signal;
wherein, tau vib A pulse emission interval for the light source; tau is the system delay; τ = (L) a -L b )/υ。
8. A quasi-distributed vibration sensing method based on a low-coherence light source is characterized by comprising the following steps:
after an optical signal emitted by the low-coherence light source reaches a weak reflection point array through the unbalanced Mach-Zehnder interferometer unit, the optical signal enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer unit again, the signal processing unit acquires a voltage signal output by the photoelectric detector, demodulates phase information of an interference signal generated by CW light and CCW light reflected by the same reflection point, and outputs an external vibration signal to obtain a strain signal.
9. The method of claim 8, wherein the unbalanced mach-zehnder interferometer unit comprises: the device comprises a first coupler, a second coupler, an acousto-optic modulator, a delay optical fiber and an optional polarization controller;
when incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CW light sequentially passes through the first coupler, the acousto-optic modulator, the delay optical fiber, the second coupler and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the polarization controller and the first coupler and enters the photoelectric detector;
when incoherent light enters the unbalanced Mach-Zehnder interferometer unit, the CCW light sequentially passes through the first coupler, the polarization controller, the second coupler and the weak reflection point array to obtain reflected light; the reflected light sequentially passes through the second coupler, the delay optical fiber, the acousto-optic modulator and the first coupler and enters the photoelectric detector.
10. The quasi-distributed vibration sensing method based on low coherent light source according to claim 8, further comprising an electrical signal generator, wherein the electrical signal generator generates a periodic linear frequency sweep pulse signal, or periodically generates a combination of a linear frequency sweep pulse signal and a single frequency pulse signal to drive the acousto-optic modulator, and applies a modulation signal to CW light and CCW light;
when the electric signal generator generates a periodic linear sweep frequency pulse signal to drive the acousto-optic modulator, the acousto-optic modulator comprises:
step S11: the signal processing unit collects the electric signal I (t) output by the photoelectric detector and performs Fourier transform on the collected electric signal I (t);
step S12: the positions of the reflection points correspond to the frequency information after Fourier transform one by one, and the corresponding relation is
Wherein L is a Representing the total length of the interferometer upper arm; l is a radical of an alcohol b The total length of the lower arm of the interferometer is shown; l is i Indicating the ith reflection point to the start of the arrayA length of the optical fiber; v represents the propagation velocity of light in the fiber; k represents the frequency sweeping speed of the acousto-optic modulator;
step S13: extracting phase information theta at peak position of reflection point i (t), i represents a reflection point number;
step S14: differentiating the phase signals at the peak positions extracted by the adjacent reflection points to obtain a differentiated phase signal delta theta i (t); triggering steps S11 to S14 are repeated for each signal in each cycle, and the obtained differential phase signal delta theta is used i (t) calculating a strain signal;
wherein, tau vib Representing the repetition period of the linear sweep signal; tau represents the time delay of the current quasi-distributed vibration sensing system; τ = (L) a -L b )/υ；L a Representing the total length of the upper arm of the interferometer; l is a radical of an alcohol b Represents the total length of the lower arm of the interferometer; v is the propagation velocity of light in the fiber; l is i-1，i Represents the reflection point L i-1 And L i A length of optical fiber in between;
in each detection period, the electric signal generator sequentially generates a linear frequency sweep pulse signal and a single-frequency pulse signal to drive the acousto-optic modulator;
CW light is first modulated into a linear sweep pulse signal by an acousto-optic modulator, wherein the sweep start frequency is f l Sweep rate is κ and pulse duration is t p After reaching the weak reflection point array, reflected light enters the photoelectric detector through the unbalanced Mach-Zehnder interferometer again;
CCW light is reflected by the weak reflection point array and then passes through the acousto-optic modulator, and the driving signal of the acousto-optic modulator is a single-frequency signal f s ；
When the electric signal generator periodically generates a combination of a linear sweep frequency pulse signal and a single frequency pulse signal to drive the acousto-optic modulator, the acousto-optic modulator comprises the following components:
step S21: the signal processing sheetCollecting an electric signal I (t) output by the photoelectric detector, and convolving the collected electric signal I (t) with a matched filter u (-t) on a digital domain; the expression of the matched filter is u (t) = rect (t/t) p )exp{j2π(f l -f s )t+jπκt 2 }, wherein rect represents a rectangular window function; t is t p Represents the pulse width; f. of l Representing the sweep frequency starting frequency of the linear sweep frequency pulse signal; f. of s A frequency representing a single frequency signal;
step S22: and the positions of the reflecting points correspond to the time of the time domain signals after matched filtering one by one, and the corresponding relation is as follows:
wherein L is a Representing the total length of the upper arm of the interferometer; l is a radical of an alcohol b Represents the total length of the lower arm of the interferometer; l is i Representing the length of the optical fiber from the ith reflection point to the starting end of the reflection point array; v represents the propagation velocity of light in the fiber; i represents a reflection point serial number;
step S23: extracting phase information theta at the peak position of the reflection point i (t), i represents a reflection point number;
step S24: the signals extracted from adjacent reflection points are differentiated to obtain a differentiated phase signal delta theta i (t); repeating the triggering steps S21 to S24 for the signals in each period, and using the obtained differential phase signal delta theta i (t) calculating a strain signal;
wherein, tau vib A pulse emission interval for the light source; τ is the system delay; τ = (L) a -L b )/υ。
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