CN110579178B - Method and device for eliminating line width dependence of slope type Brillouin dynamic sensing system - Google Patents

Method and device for eliminating line width dependence of slope type Brillouin dynamic sensing system Download PDF

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CN110579178B
CN110579178B CN201911012881.8A CN201911012881A CN110579178B CN 110579178 B CN110579178 B CN 110579178B CN 201911012881 A CN201911012881 A CN 201911012881A CN 110579178 B CN110579178 B CN 110579178B
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李永倩
王磊
刘艳蕊
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North China Electric Power University
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Abstract

The invention discloses a method and a device for eliminating the line width dependence of a slope type Brillouin dynamic strain sensing system, which adopt an arbitrary waveform generator and an electro-optical modulator to modulate continuous light output by a narrow line width laser into four periodic frequency sections which are sequentially connected, wherein the frequency difference between two frequencies and pumping light is positioned at two different points at one side of a Brillouin gain slope, the other two frequencies are positioned at two points at the other side of the Brillouin gain slope and are symmetrical with the other two points, the modulated four-frequency continuous detection light enters a sensing optical fiber to generate stimulated Brillouin scattering effect with each corresponding pumping light which is transmitted in opposite directions, and the detection light carrying sensing information with different frequencies after the stimulated Brillouin scattering effect is detected by a photoelectric detector and is acquired by a data acquisition card and then is transmitted to a computer for processing and demodulation. The method can reduce the influence of the power fluctuation of the pump light and the detection light on the system performance, reduce the measurement error, effectively inhibit the fluctuation of the measurement signal amplitude caused by the line width change in the measurement process and improve the measurement precision of the system.

Description

Method and device for eliminating line width dependence of slope type Brillouin dynamic sensing system
Technical Field
The invention relates to the technical field of measurement, in particular to a method and a device for eliminating line width dependence of a slope type Brillouin dynamic strain sensing system.
Background
Brillouin Optical Time Domain Analysis (BOTDA) sensing technology is taken as a novel distributed sensing technology, has become a research hotspot at home and abroad in the field of current Optical fiber sensing application by virtue of the advantages of high detection signal intensity, high measurement precision, wide dynamic range, long sensing distance and the like, and is widely applied to the fields of equipment fault detection and positioning, oil and gas pipeline safety condition monitoring, large-scale structure health detection, geological disaster monitoring and early warning and the like.
The conventional BOTDA measurement principle is to inject pulsed pump light and continuous probe light into two ends of an optical fiber respectively, the two beams of light propagate in the optical fiber in opposite directions, and when the two beams of light meet and the frequency difference of the two beams of light is in the Brillouin gain spectrum range, the pulsed pump light transfers energy to the continuous probe light through Stimulated Brillouin Scattering (SBS). By scanning the frequency of the continuous probe light, the probe light power at different optical fiber positions changing along with the frequency can be obtained at the pumping end, so that a reconstructed Brillouin gain spectrum can be obtained, and the distribution of Brillouin frequency shift along the optical fiber can be obtained through Lorentz fitting. And the distributed sensing of the temperature or the strain is realized by utilizing the linear relation between the Brillouin shift frequency and the environmental temperature and the strain. However, in order to obtain a complete brillouin gain spectrum, a large temperature/strain measurement range and accurate brillouin frequency shift, the conventional swept-frequency BOTDA system usually needs a large swept-frequency range and a small swept-frequency interval, so that more swept frequencies are needed, and the whole measurement time is long, so that the system is only suitable for static or slowly-changing strain measurement, and the application of the system in the field of dynamic strain measurement is limited.
For this reason, in 2009 beiinii et al, proposed a ramp type BOTDA sensing system, which uses one side of the brillouin gain spectrum to approximate a linear ramp for dynamic measurement, and sets the frequency difference between the pump light and the probe light at the center of the ramp, so that when the strain at a certain position of the optical fiber dynamically changes with time, the probe light gain at the position changes linearly with the brillouin frequency shift, and the collected probe light gain is substituted into a function curve which is measured, calibrated and fit-analyzed in advance, so as to demodulate the dynamic strain. The system has a simple structure, does not need frequency sweeping, and solves the problems that the traditional frequency sweeping type BOTDA has long measuring time and can not realize dynamic strain measurement. However, the slope in the slope type brillouin dynamic strain sensing system has line width dependence, and in order to realize high-precision accurate measurement, the stability of the slope needs to be maintained, that is, the line width does not change in the measurement process. However, the linewidth of the brillouin gain spectrum changes due to the power fluctuation of the probe light and the pump light and the change of strain in the measurement process, so that the stability of the slope is damaged, and errors and even errors occur in the demodulation result. And at present, an effective method for effectively solving the problem of the line width dependence of the slope type Brillouin dynamic strain sensing system does not exist, so that a method and a device capable of effectively eliminating the line width dependence of the slope type Brillouin dynamic strain sensing system, reducing the measurement error and improving the measurement accuracy are urgently needed.
Disclosure of Invention
In order to solve the above problems, the present invention provides a method and an apparatus for eliminating line width dependency of a slope type brillouin dynamic strain sensing system, so as to suppress fluctuation of measurement signal amplitude caused by line width variation during measurement, reduce influence of the fluctuation on system performance, and improve accuracy and stability of measurement.
Therefore, the invention provides the following technical scheme:
a method for eliminating line width dependency of slope type Brillouin dynamic strain sensing system adopts arbitrary waveform generator and electro-optical modulator to modulate continuous light output by narrow line width laser into four periodic frequency sections connected in sequence, wherein the frequency difference between two frequencies and pumping light is located at two different points at one side of Brillouin gain slope, the other two are located at two symmetrical points at the other side of Brillouin gain slope, and the duration time of each frequency is equal to the repetition period of pulse signal light, the modulated four-frequency continuous detection light is incident into sensing optical fiber to generate stimulated Brillouin scattering effect with each corresponding pumping light transmitted in opposite direction, the detection light carrying sensing information with different frequencies after the effect is detected by photoelectric detector and collected by data collection card and then transmitted into computer to be processed and demodulated, the computer first pre-processes the collected data, and finally, demodulating by using a corresponding relation curve of different strain processing results and corresponding strains, which are stored in a computer and are measured, calibrated and fit-analyzed in advance, to obtain a final dynamic strain value.
Further, in the method for eliminating the line width dependency of the slope type brillouin dynamic strain sensing system, the computer preprocessing includes traditional data extraction, superposition average denoising, wavelet transformation and the like, and particularly means that logarithm is taken after acquired detection optical power under SBS action is divided by previously measured detection optical power without pump light action to obtain logarithmic gain and eliminate the influence of detection optical power fluctuation.
Further, in the method for eliminating the line width dependency of the slope type brillouin dynamic strain sensing system, the dual slope four-frequency difference division ratio method is to take reciprocal values of two probe light gains with different frequencies on each slope after pretreatment, subtract the reciprocal values to eliminate the influence of the line width parameter in the denominator of the brillouin gain spectrum expression, divide the obtained two data results to eliminate the influence of the line width parameter in the numerator of the brillouin gain spectrum expression and the influence of the pumping light power fluctuation, and finally take logarithm of the obtained results to facilitate fitting and demodulation.
Further, in the method for eliminating the line width dependency of the slope type brillouin dynamic strain sensing system, the corresponding relation curve of different strain processing results and corresponding strains, which is stored in the computer and is measured, calibrated, fitted and analyzed in advance, means that different degrees of strains are applied to the system before measurement, the obtained results are processed according to the processing procedures, each strain value corresponds to a corresponding processing result, each processing result and the corresponding strain value are fitted to obtain the corresponding relation curve and stored in the computer, and when demodulation is required, the measured results are substituted to obtain the corresponding dynamic strain values.
The computer preprocesses the collected data including traditional data extraction, superposition average denoising, wavelet transformation and the like, especially dividing the collected detection light power under SBS action and the detection light power which is measured in advance and does not act with the pump light, then taking logarithm to obtain logarithmic gain and eliminating the influence of the detection light power fluctuation, and the logarithmic gain data of one period frequency band after preprocessing is expressed as
Figure BDA0002244730080000041
In the formula, GnLogarithmic gain, I, for different frequencies of probe light after pretreatment along the fiber distribution over timenDetecting the power of light for different frequencies, GSBSIs the Brillouin amplitude gain, gBIs the peak of the Brillouin gain, IPFor pump light power, Δ z is the interaction length of the fiber, Δ vBFor the linewidth of the modulated Brillouin gain spectrum, DeltavnThe frequency detuning parameter is the gain response of different frequencies, z is the propagation distance of the optical signal, and epsilon is the dynamic strain signal.
Then, after the pretreatment, formal treatment can be carried out by utilizing a double-slope four-frequency difference ratio division methodThe dual-slope four-frequency difference division method is characterized in that two preprocessed detection light gains with different frequencies on each slope are subjected to reciprocal subtraction to eliminate the influence of line width parameters in a Brillouin gain spectrum expression denominator, and two processed differential gains GΔ1And GΔ2Comprises the following steps:
Figure BDA0002244730080000042
Figure BDA0002244730080000043
dividing the obtained two data results by eliminating the influence of line width parameters and the influence of pumping light power fluctuation in Brillouin gain spectrum expression molecules, and processing the ratio gain GRComprises the following steps:
Figure BDA0002244730080000044
and finally, logarithm is taken for the gain of the comparison value to obtain a final processing result R:
R(z,t,ε)＝lg[GR(z,t,ε)]
and then, substituting the obtained final processing result into a near-linear region of a pre-measured calibration and fitting analysis curve for demodulation to obtain the final dynamic strain response.
The invention also provides a device for eliminating the line width dependence of the slope type Brillouin dynamic strain sensing system, which comprises a narrow line width laser, a polarization maintaining coupler, a pulse signal source, a first electro-optical modulator, a first erbium-doped optical fiber amplifier, a first grating filter, a four-frequency period frequency band modulation module (which can be composed of an arbitrary waveform generator, a second electro-optical modulator, a pulse signal source, a second erbium-doped optical fiber amplifier and a second grating filter), a polarization scrambler, an isolator, an optical circulator, a sensing optical fiber, an electro-optical detector, a data acquisition card and a computer. The narrow linewidth laser outputs two paths of continuous light through a polarization maintaining coupler, the upper-branch continuous light is sequentially connected with a first optical port of an optical circulator through a first electro-optical modulator, a first erbium-doped optical fiber amplifier and a first grating filter which are driven by a pulse signal source, the lower-branch continuous light is sequentially connected with a second optical port of the optical circulator through a second electro-optical modulator, a second erbium-doped optical fiber amplifier, a second grating filter, a polarization scrambler and an isolator which are driven by an arbitrary waveform generator, and a third optical port of the optical circulator is connected with a computer through a photoelectric detector and a data acquisition card.
Preferably, the device for eliminating the line width dependency of the slope type brillouin dynamic strain sensing system, wherein the four-frequency periodic frequency band modulation module is composed of an arbitrary waveform generator, a second electro-optical modulator, a pulse signal source, a second erbium-doped fiber amplifier, and a second grating filter, the second electro-optical modulator driven by the arbitrary waveform generator performs four-frequency periodic frequency band modulation on the lower branch continuous optical signal output by the polarization maintaining coupler, the driving signal of the arbitrary waveform generator is four periodic frequency bands connected in sequence, and the duration time t of each frequency islengthThe repetition period of the pulse signal light is the same as that of the pulse signal light, the pulse signal source is used for synchronously triggering the random waveform generator to ensure the synchronization of the pumping light and the detection light, and the repetition period of the frequency segment is 4tlengthI.e. sampling period, the drive signal of the arbitrary waveform generator performs double sideband modulation of the suppressed carrier on the continuous optical signal entering the second electro-optical modulator with an output frequency v0±v1、v0±v2、v0±v3、v0±v4Of a four-frequency periodic frequency segment of (v) where v0Is the center frequency, v, of a narrow linewidth laser1、v2For two different frequency points on one side of the brillouin gain ramp, v3、v4Two frequency points that are symmetrical to the brillouin gain ramp on the other side of the brillouin gain ramp. The modulated detection light is amplified by the second erbium-doped fiber amplifier, and enters the sensing fiber after spontaneous radiation noise and an upper sideband signal are filtered by the second grating filter.
The method has the advantages that by means of the technical scheme, the double-slope four-frequency period frequency band modulation technology is introduced into the slope type Brillouin dynamic strain sensing system, and the influence of the line width dependence of the slope type Brillouin dynamic strain sensing system on the system performance is eliminated by using four symmetrical frequency points of the slopes on two sides generated by modulation and a corresponding double-slope four-frequency difference value-dividing method. The method can reduce the influence of the power fluctuation of the pump light and the detection light on the system performance, reduce the measurement error, effectively inhibit the fluctuation of the measurement signal amplitude caused by the line width change in the measurement process and improve the measurement precision of the system.
Drawings
FIG. 1 is a schematic diagram of the composition of the measuring apparatus of the present invention;
FIG. 2 is a schematic diagram of the drive signals for an arbitrary waveform generator;
FIG. 3 is a schematic representation of SBS functioning in an optical fiber;
fig. 4 is a schematic diagram of the demodulation principle of dynamic strain.
Wherein:
LD, narrow linewidth laser PCO, polarization maintaining coupler
PSG, pulse signal source EOM1, first electro-optical modulator
EDFA1, first erbium-doped fiber amplifier GF1, and first grating filter
AWG, arbitrary waveform generator EOM2, second electro-optical modulator
EDFA2, a second erbium-doped fiber amplifier GF2, and a second grating filter
PS, bias scrambler ISO, isolator
OC, optical circulator FUT, sensing optical fiber
PD, photoelectric detector DAQ, data acquisition card
COM and a computer.
Detailed Description
The technical scheme of the invention is further specifically described by the following embodiments and the accompanying drawings.
The symbols used herein are illustrated as follows:
v0is the output frequency, v, of a narrow linewidth laser1、v2For two different frequency points on one side of the brillouin gain ramp, v3、v4Two frequency points, t, which are symmetrical with the Brillouin gain slope at the other side of the Brillouin gain slopelengthFor the duration of each frequency, the same as the repetition period of the pulse signal light, E (t) detecting the intensity of the light with time for a frequency segment of four frequency cycles, EnFor detecting the amplitude of the light at different frequencies, t is the propagation time of the light pulse, J1(C) Is a first order bessel function, C is a modulation index,
Figure BDA0002244730080000071
for phases corresponding to different frequencies, n being the ordinal number of different frequencies, GnLogarithmic gain, I, for different frequencies of probe light after pretreatment along the fiber distribution over timenDetecting the power of light for different frequencies, GSBSIs the Brillouin amplitude gain, gBIs the peak of the Brillouin gain, IPFor pump light power, Δ z is the interaction length of the fiber, Δ vBFor the linewidth of the modulated Brillouin gain spectrum, DeltavnFrequency detuning parameter for different frequency gain response, z being the distance traveled by the optical signal, ε being the dynamic strain signal, GΔ1And GΔ2For the two differential gains after processing, GRFor the processed ratio gain, R is the final result of the processing, and Δ v is the frequency detuning parameter due to strain.
The invention is further illustrated in the following examples, which are given by way of illustration only and are not intended to limit the scope of the invention.
The invention uses the dual-slope four-frequency cycle frequency section as the modulation signal of the Brillouin dynamic strain sensing system, the electro-optic modulator driven by the arbitrary waveform generator respectively modulates the frequency of continuous probe light and the frequency of pump light to four symmetrical frequency points of the bilateral slopes of the Brillouin gain spectrum, the probe light with different frequencies and each corresponding pulse pump light have SBS action, the probe light with different frequencies after SBS action is collected and processed by the dual-slope four-frequency difference ratio method to inhibit the fluctuation of the measurement signal amplitude caused by the line width change in the measurement process and reduce the influence of the measurement signal amplitude on the system performance, thereby realizing the high-precision high-accuracy dynamic strain measurement.
Please refer to fig. 1, which is a schematic diagram of the measuring apparatus of the present invention. The system of the invention comprises the following components and working principles:
the narrow linewidth laser LD outputs two paths of continuous light through a polarization maintaining coupler PCO, wherein an upper branch is subjected to pulse modulation by a first electro-optical modulator EOM1 driven by a pulse signal source PSG to generate a frequency v0The pulse signal light is amplified by a first erbium-doped fiber amplifier EDFA1, spontaneous radiation noise is filtered by a first grating filter GF1, and then the pulse signal light enters an optical circulator OC; the second electro-optical modulator EOM2 driven by AWG in the lower branch carries out four-frequency periodic frequency band modulation, the driving signal of the arbitrary waveform generator is four periodic frequency bands connected in sequence, and the duration t of each frequencylengthThe repetition period of the frequency segment is 4t, which is the same as the repetition period of the pulse signal lightlengthThe pulse signal source is used for synchronously triggering the arbitrary waveform generator to ensure the synchronization of the pumping light and the detection light, the synchronous triggering time is the repetition period of the pulse signal light, the driving signal of the arbitrary waveform generator carries out double-sideband modulation of the suppressed carrier on the continuous light of the second electro-optical modulator, and the output frequency is v0±v1、v0±v2、v0±v3、v0±v4Of a four-frequency periodic frequency segment of (v) where v0Is the center frequency, v, of a narrow linewidth laser1、v2For two different frequency points on one side of the brillouin gain ramp, v3、v4The modulated probe light is amplified by the second erbium-doped fiber amplifier, and the spontaneous radiation noise and the upper sideband signal are filtered by the second grating filter, so that the output frequency is v0-v1、v0-v2、v0-v3、v0-v4The four-frequency periodic frequency band detection light enters the sensing optical fiber through the polarization scrambler PS and the isolator ISO and is transmitted to the pulse pump opposite to the upper branchThe light has SBS function, the detecting light with different frequency and carrying the sensing information after the function is detected directly by the photoelectric detector PD and then data is collected by the data collecting card DAQ, and the collected data is input into the computer COM for processing and demodulation.
The drive signal generated by the arbitrary waveform generator is shown in fig. 2, which performs double-sideband modulation of the suppressed carrier on the second electro-optical modulator, and the output optical field of one sampling period can be represented as
Figure BDA0002244730080000091
Wherein E (t) is the intensity of the probe light in the four-frequency cycle frequency band with time, EnFor detecting the amplitude of the light at different frequencies, t is the propagation time of the light pulse, J1(C) Is a first order bessel function, C is a modulation index,
Figure BDA0002244730080000092
the phases corresponding to different frequencies, n is the ordinal number of different frequencies.
In order to ensure the measurement accuracy, the frequency detuning amount of the four frequencies should be as large as possible.
The deterioration of the modulation performance of the first electro-optical modulator EOM1 and the second electro-optical modulator EOM2 changes the state of the system, and further degrades the system performance, so the electro-optical modulator used is a high extinction ratio and high stability modulator. The AWG needs accurate and high-bandwidth signals to drive the electro-optical modulator so as to ensure the accurate and stable four-frequency periodic frequency band modulation effect, so the used AWG is an AWG with high bandwidth, high accuracy and high stability.
The polarization scrambler PS is used for scrambling the polarization state of the detection light and reducing the polarization noise and polarization dependent fading of the system.
The four-frequency continuous probe light modulated by the four-frequency periodic frequency segment enters the sensing optical fiber to generate stimulated Brillouin scattering effect with each corresponding pump light transmitted in opposite directions, the schematic diagram of the action principle is shown in FIG. 3, and the probe light carrying sensing information with different frequencies after SBS effect is detected by the photoelectric detector PD and collected by the data acquisition card DAQ and then sent to the computer for processing and demodulation.
The computer firstly preprocesses the collected data, including traditional data extraction, superposition average denoising, wavelet transformation and the like, especially refers to dividing the collected detection optical power after SBS action and the detected optical power which is measured in advance and not acted with pumping light, then taking logarithm to obtain logarithm gain and eliminating the influence of the detection optical power fluctuation, the logarithm gain data of one period frequency band after preprocessing can be expressed as
Figure BDA0002244730080000101
In the formula, GnLogarithmic gain, I, for different frequencies of probe light after pretreatment along the fiber distribution over timenDetecting the power of light for different frequencies, GSBSIs the Brillouin amplitude gain, gBIs the peak of the Brillouin gain, IPFor pump light power, Δ z is the interaction length of the fiber, Δ vBFor the linewidth of the modulated Brillouin gain spectrum, DeltavnThe frequency detuning parameter is the gain response of different frequencies, z is the propagation distance of the optical signal, and epsilon is the dynamic strain signal.
Then, a double-slope four-frequency difference division method is used for formal processing, the double-slope four-frequency difference division method is used for subtracting two pretreated probe light gains with different frequencies on each slope after taking the reciprocal to eliminate the influence of a line width parameter in a Brillouin gain spectrum expression denominator, and the two processed differential gains GΔ1And GΔ2Comprises the following steps:
Figure BDA0002244730080000102
Figure BDA0002244730080000103
dividing the obtained two data results by eliminating the influence of line width parameters and the influence of pumping light power fluctuation in Brillouin gain spectrum expression molecules, and processing the ratio gain GRComprises the following steps:
Figure BDA0002244730080000104
and finally, logarithm is taken for the gain of the comparison value to obtain a final processing result R:
R(z,t,ε)＝lg[GR(z,t,ε)]
the schematic diagram of the demodulation principle of the dynamic strain is shown in fig. 4, and it can be seen from fig. 4 that the calibration curve can generate nonlinearity when the detuning generated by the strain is large, so that the demodulation generates an error, therefore, the linear part of the calibration curve defines the maximum value of the detuning amount, and the final processing result is substituted into the near-linear region of the calibration and fitting analysis curve measured in advance for demodulation, so that the final dynamic strain response can be obtained.
Although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the spirit and scope of the invention.

Claims (8)

1. A method for eliminating the line width dependency of a slope type Brillouin dynamic strain sensing system is characterized in that an arbitrary waveform generator and an electro-optical modulator are adopted to modulate continuous light output by a narrow line width laser into four periodic frequency sections which are sequentially connected, wherein the frequency difference between two frequencies and pumping light is positioned at two different points on one side of a Brillouin gain slope, the other two frequencies are positioned at two points on the other side of the Brillouin gain slope and symmetrical with the other two points, the duration time of each frequency is equal to the repetition period of pulse signal light, the modulated four-frequency continuous detection light enters a sensing optical fiber to generate stimulated Brillouin scattering effect with each corresponding pumping light which is transmitted oppositely, the detection light carrying sensing information with different frequencies after the stimulated Brillouin scattering effect is detected by a photoelectric detector and acquired by a data acquisition card and then transmitted to a computer for processing and demodulation, the computer firstly preprocesses the acquired data, then carries out formal processing by using a double-slope four-frequency difference division ratio method, and finally demodulates the corresponding relation curve of different strain processing results and corresponding strains, which are stored in the computer and are measured, calibrated, fitted and analyzed in advance, to obtain a final dynamic strain value.
2. The method according to claim 1, wherein the computer preprocessing is to divide the detected optical power after SBS action and the detected optical power without pump action measured in advance and then take logarithm to obtain a logarithmic gain and eliminate the influence of the detected optical power fluctuation.
3. The method according to claim 1, wherein the dual slope four-frequency difference division method is to subtract two reciprocal probe optical gains of different frequencies on each slope after pre-processing to eliminate the influence of the line width parameter in the denominator of the brillouin gain spectrum expression, divide the two obtained data results to eliminate the influence of the line width parameter in the numerator of the brillouin gain spectrum expression and the influence of the pump optical power fluctuation, and finally take the logarithm of the obtained results to facilitate fitting and demodulation.
4. The method for eliminating the line width dependence of the slope type brillouin dynamic strain sensing system according to claim 1, characterized in that the corresponding relationship curves of different strain processing results and corresponding strains, which are measured, calibrated, fitted and analyzed in advance, stored in the computer refer to that different degrees of strains are applied to the system before measurement, the obtained results are processed according to the above processing procedures, each strain value corresponds to a corresponding processing result, each processing result and the corresponding strain value are fitted to obtain the corresponding relationship curve and stored in the computer, and when demodulation is required, the corresponding dynamic strain value can be obtained by substituting the measured results.
5. The method according to claim 1, wherein the computer preprocesses the collected data by dividing the collected probe optical power after SBS action by the pre-measured probe optical power without pump action to obtain logarithmic gain and eliminate the influence of probe optical power fluctuation, and the logarithmic gain data of one cycle frequency segment after preprocessing is represented as
Figure FDA0002766356060000021
In the formula, GnLogarithmic gain, I, for different frequencies of probe light after pretreatment along the fiber distribution over timenDetecting the power of light for different frequencies, GSBSIs the Brillouin amplitude gain, gBIs the peak of the Brillouin gain, IPFor pump light power, Δ z is the interaction length of the fiber, Δ vBFor the linewidth of the modulated Brillouin gain spectrum, DeltavnThe frequency detuning parameter is the gain response of different frequencies, z is the propagation distance of the optical signal, and epsilon is the dynamic strain signal.
6. The method according to claim 1, wherein the formal processing by the dual-slope four-frequency difference division method is to subtract the reciprocal of two probe optical gains with different frequencies on each slope after the preprocessing to eliminate the influence of the line width parameter in the denominator of the brillouin gain spectrum expression, and the two differential gains G after the processing are divided into twoΔ1And GΔ2Comprises the following steps:
Figure FDA0002766356060000022
Figure FDA0002766356060000023
dividing the obtained two data results by eliminating the influence of line width parameters and the influence of pumping light power fluctuation in Brillouin gain spectrum expression molecules, and processing the ratio gain GRComprises the following steps:
Figure FDA0002766356060000031
and finally, logarithm is taken for the gain of the comparison value to obtain a final processing result R:
R(z,t,ε)＝lg[GR(z,t,ε)]
and then, substituting the obtained final processing result into a near-linear region of a pre-measured calibration and fitting analysis curve for demodulation to obtain the final dynamic strain response.
7. A device for eliminating the line width dependence of a slope type Brillouin dynamic strain sensing system is characterized by mainly comprising a narrow line width laser, a polarization maintaining coupler, a pulse signal source, a first electro-optical modulator, a first erbium-doped optical fiber amplifier, a first grating filter, a polarization scrambler, an isolator, an optical circulator, a sensing optical fiber, a photoelectric detector, a data acquisition card, a computer and a four-frequency periodic frequency band modulation module, wherein the four-frequency periodic frequency band modulation module consists of an arbitrary waveform generator, a second electro-optical modulator, a pulse signal source, a second erbium-doped optical fiber amplifier and a second grating filter; continuous light emitted by the narrow linewidth laser is output in two paths through the polarization-maintaining coupler, wherein the continuous light in an upper branch path sequentially passes through a first electro-optical modulator, a first erbium-doped optical fiber amplifier and a first grating filter driven by a pulse signal source and then is connected with a first optical port of the optical circulator, the continuous light in a lower branch path sequentially passes through a second electro-optical modulator, a second erbium-doped optical fiber amplifier, a second grating filter, a polarization scrambler and an isolator driven by an arbitrary waveform generator and then is connected with a second optical port of the optical circulator, and a third optical port of the optical circulator is connected into a computer through a photoelectric detector and a data acquisition card.
8. The apparatus according to claim 7, wherein the second electro-optical modulator driven by the arbitrary waveform generator performs four-frequency periodic frequency segment modulation on the lower branch continuous optical signal output by the polarization maintaining coupler, the driving signal of the arbitrary waveform generator is four sequentially connected periodic frequency segments, and the duration t of each frequency is tlengthThe repetition period of the pulse signal light is the same as that of the pulse signal light, the pulse signal source is used for synchronously triggering the random waveform generator to ensure the synchronization of the pumping light and the detection light, and the repetition period of the frequency segment is 4tlengthI.e. sampling period, the drive signal of the arbitrary waveform generator performs double sideband modulation of the suppressed carrier on the continuous optical signal entering the second electro-optical modulator with an output frequency v0±v1、v0±v2、v0±v3、v0±v4Of a four-frequency periodic frequency segment of (v) where v0Is the center frequency, v, of a narrow linewidth laser1、v2For two different frequency points on one side of the brillouin gain ramp, v3、v4The modulated probe light is amplified by the second erbium-doped fiber amplifier and enters the sensing fiber after spontaneous radiation noise and an upper sideband signal are filtered by the second grating filter for two frequency points which are positioned on the other side of the Brillouin gain slope and are symmetrical with the Brillouin gain slope.
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