CN114812855A - Brillouin optical time domain scattering system based on optical flight time and self-calibration method - Google Patents
Brillouin optical time domain scattering system based on optical flight time and self-calibration method Download PDFInfo
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Abstract
The invention relates to a Brillouin optical time domain scattering system based on optical flight time and a self-calibration method. The invention fully utilizes the measuring process by adding a small amount of hardware, corrects the corresponding relation between the propagation delay and the optical fiber space position, ensures that two subtracted values correspond to the same section of optical fiber when the maximum gain frequency deviation variation is calculated, avoids additional errors, and continuously corrects the problem of inconsistent delay between measured data and reference data by using a calibration means, thereby effectively reducing the reading error of the BOTDA system and the reading drift after long-term operation, and having important significance for the long-term operation stability of the long-distance BOTDA sensing system.
Description
Technical Field
The invention belongs to the field of optical fiber sensing, and particularly relates to a Brillouin optical time domain scattering system based on optical flight time and a self-calibration method.
Background
A distributed optical fiber sensing system (hereinafter referred to as BOTDA) based on brillouin optical time domain analysis is a widely used distributed optical fiber sensing system. The system injects pulse pump light and continuous probe light at two ends of the optical fiber respectively. When the frequency offset of the probe light relative to the pump light is in a specific interval and the power is large enough, the stimulated brillouin scattering effect existing in the optical fiber transfers the pump light energy to the probe light, thereby amplifying the probe light. The probe optical gain varies with frequency offset, with the point of maximum gain being affected by the temperature and strain of the fiber. The BOTDA has long detection distance and high sensitivity, can realize full-distribution strain and temperature measurement, and has wide application in the fields of safety monitoring, state alarm and the like of tunnels, submarine cables, large buildings and the like.
In the prior art, the invention patent 201811580430.X in china discloses a long-gauge-length distributed optical fiber brillouin sensing-demodulating system and a strain measuring method, which relate to a high-precision, dynamic and non-uniform strain measuring technology for engineering measurement, but the existence of the strain measuring technology can increase the number of steps of frequency sweep detection, cause the increase of the measuring time of the distributed optical fiber brillouin sensing system, and cannot meet the requirements of dynamic measurement. At present, in order to track frequency offset change, before a BOTDA system starts measurement, maximum gain frequency offset under different delays after one-time pulse pump light emission is read as reference frequency offset, then the delays after the pulse pump light enters an optical fiber to be measured are mapped to spatial positions through the light speed in the optical fiber to obtain the reference frequency offset of different positions, and during measurement, the actually measured maximum gain frequency offset under the same delay is directly differed from the reference frequency offset to obtain frequency offset change. However, the light speed in the optical fiber is affected by environmental factors such as temperature, etc. to some extent, the measurement and the reference frequency offset at the same time delay are measured at different times, which may correspond to the optical fibers at different positions, and due to slight inconsistency of the optical fiber material, the reference frequency offset value at each position is different, and when the difference calculation frequency offset changes, an extra error will be introduced, which affects the measurement accuracy.
Disclosure of Invention
In view of the defects in the existing BOTDA system, the invention provides a Brillouin optical time domain scattering system based on optical flight time. The Brillouin optical time domain scattering system based on the optical flight time can fully utilize the measurement process, correct the corresponding relation between the propagation delay and the optical fiber space position and ensure that the problem of inconsistent delay between the measurement data and the reference data is continuously corrected when the maximum gain frequency offset variation is calculated.
In order to realize the purpose, the invention adopts the technical scheme that the method comprises the following steps:
a Brillouin optical time domain scattering system based on optical flight time comprises a laser light source, a first coupler, an electro-optic modulator, an acousto-optic modulator, a first optical fiber amplifier, a second optical fiber amplifier, an arbitrary waveform generator, a sensing optical fiber, a second coupler, a first photoelectric detection unit, a demodulation unit, a third coupler and a second photoelectric detection unit, wherein the sensing optical fiber comprises a first port of the sensing optical fiber and a second port of the sensing optical fiber;
the laser light source emits laser light to the first coupler to be split into detection light and pump light, the pump light is modulated into pulse pump light through the acousto-optic modulator, the pulse pump light is amplified through the optical fiber first amplifier and then reaches the first port of the sensing optical fiber through the second coupler, the pulse pump light enters the sensing optical fiber through the first port of the sensing optical fiber and is transmitted to the second port of the sensing optical fiber to be output in the sensing optical fiber, the output pulse pump light is received by the third coupler and then is output to the second photoelectric detection unit, and the second photoelectric detection unit outputs signals to the demodulation unit;
the detection light is modulated into continuous detection light after being subjected to frequency shift by the electro-optical modulator, the continuous detection light is amplified by the second optical fiber amplifier and then reaches the second port of the sensing optical fiber through the third coupler, the continuous detection light entering the sensing optical fiber is transmitted to the first port of the sensing optical fiber to be output, the output continuous detection light is received by the second coupler and then output to the first electro-optical detection unit, and the first electro-optical detection unit outputs an electric signal to the demodulation unit; the arbitrary waveform generator is respectively connected with the electro-optic modulator and the acousto-optic modulator, the arbitrary waveform generator provides two paths of electric signal outputs, one path of electric signal outputs a single-frequency rectangular signal to modulate the electro-optic modulator, and the other path of electric signal outputs a rectangular pulse signal to modulate the acousto-optic modulator.
The invention also relates to a self-calibration method of the Brillouin optical time domain scattering system based on the optical flight time, which comprises the following steps of setting a third coupler and a second photoelectric detection unit to measure the flight time of light traversing the whole section of optical fiber to be measured, realizing the time delay of calibrating the maximum gain frequency offset, corresponding data of the corrected current maximum gain frequency offset and the corrected reference maximum gain frequency offset correspond to the same optical fiber position, and realizing the self-calibration based on the Brillouin optical time domain analysis, wherein the method comprises the following steps:
step 21: before measurement starts, acquiring the maximum gain frequency offset in the detection light Brillouin system as reference maximum gain frequency offset, and acquiring the flight time of reference pulse pump light of the optical fiber to be measured;
step 22: during measurement, acquiring current maximum gain frequency offset, adjusting the frequency of a modulation signal applied to an electro-optic modulator by an arbitrary waveform generator in a step length of 1MHz between 9 and 13GHz, wherein the frequency of the modulation signal is the frequency offset of probe light relative to pump light, collecting Brillouin signals under each modulation frequency one by one to obtain Brillouin gain spectrums of each position of a sensing optical fiber, and recording the maximum gain point of the Brillouin gain spectrums as the maximum gain frequency offset;
step 23: measuring the flight time of the current pulse pump light, and recording the time delay of the pulse pump light which is transmitted through the whole section of optical fiber to reach the second photoelectric detection unit while carrying out frequency deviation measurement and emitting pulse light, namely the flight time of the current pulse pump light;
step 24: calibrating the time delay of the maximum gain frequency offset by using the flight time of the optical fiber to be measured;
step 25: corresponding data of the corrected current maximum gain frequency offset and the corrected reference maximum gain frequency offset correspond to the same optical fiber position, and calculating the change of the current maximum gain frequency offset;
step 26: and calculating the temperature measurement result of each position of the current optical fiber, demodulating the corrected signal and finally obtaining the temperature information sensed by the optical fiber.
In the self-calibration method of the brillouin optical time domain scattering system based on optical time of flight of the present invention, in the step 21, before the first measurement, the reference maximum gain frequency offset and the pulse pump optical time of flight start to be collected, and the step of collecting the reference maximum gain frequency offset is: adjusting the frequency of a modulation signal applied to an electro-optic modulator by an arbitrary waveform generator in a step length of 1MHz between 9 and 13GHz, wherein the frequency of the modulation signal is the frequency offset of probe light relative to pump light, collecting Brillouin signals under each modulation frequency one by one to obtain a Brillouin gain spectrum of each position of a sensing optical fiber, and recording the maximum gain point of the Brillouin gain spectrum as the reference maximum gain frequency offset; the process of the flight time of the pulse pump light is as follows: and recording the time delay of the pulse pump light which is transmitted through the whole section of optical fiber to reach the second photoelectric detection unit while performing frequency offset measurement and transmitting pulse light, namely the flight time of the reference pulse pump light.
In the self-calibration method of the Brillouin optical time domain scattering system based on the optical flight time, the delay after each pulse emission in the current measurement step 22 is collected and averaged to reduce errors.
In the self-calibration method of the Brillouin optical time domain scattering system based on the optical flight time, for the step 24, the direct current component of the flight time change is used for correcting the time delay, and the time delay of a certain currently measured data point is recorded as t N Then, the corresponding delay of the reference data is:
that is, it is considered that the reference data acquisition time t 0 And the current measurement time t N Corresponding to the signal of the same section of optical fiber.
In the self-calibration method of the Brillouin optical time domain scattering system based on the optical flight time, for the step 25, the actually measured maximum gain frequency deviation and the reference maximum gain frequency deviation are subtracted to obtain the current maximum gain frequency deviation change, the reference data time domain sampling is discrete, and if there is no corresponding delay of a sampling point, two or more data points which are most adjacent before and after the moment are selected and are approximately obtained through an interpolation method.
In the self-calibration method of the brillouin optical time domain scattering system based on the optical flight time of the present invention, for the step 26, the temperature measurement result is set to Δ T N (t) the amount of temperature change when the currently measured temperature is measured from the reference. By the formula Δ T N ＝Δf N and/A, obtaining the temperature measurement result of each position of the current optical fiber. Wherein the proportionality coefficient a is determined by the fiber material and is a known invariant constant before measurement. For example, in G652D standard single mode fiber manufactured by Corning corporation, A is 1.05 MHz/DEG C.
Based on the technical scheme, compared with the prior art, the long-distance Brillouin optical time domain analysis distributed optical fiber sensing system has the following technical advantages:
1. the invention fully utilizes the measuring process by adding a small amount of hardware, corrects the corresponding relation between the propagation delay and the optical fiber space position, ensures that two subtracted values correspond to the same section of optical fiber when the maximum gain frequency deviation variation is calculated, avoids additional errors, and continuously corrects the problem of inconsistent delay between measured data and reference data by using a calibration means, thereby effectively reducing the reading error of the BOTDA system and the reading drift after long-term operation, and having important significance for the long-term operation stability of the long-distance BOTDA sensing system.
2. The invention continuously corrects the problem of inconsistent time delay between the measured data and the reference data by using a self-calibration means, and minimizes the additional hardware of the system while increasing the calibration function, thereby ensuring controllable cost, obviously improving the performance and keeping good competitiveness.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a schematic structural diagram of an optical flight time-based brillouin optical time domain scattering system.
FIG. 2 is a measurement flow chart of the self-calibration method of the Brillouin optical time domain scattering system based on optical flight time.
The reference numbers in the figures denote:
1-laser light source, 2-first coupler, 3 electro-optic modulator, 4-first optical fiber amplifier, 5-acousto-optic modulator, 6-second optical fiber amplifier, 7-arbitrary waveform generator, 8-sensing optical fiber, 9-second coupler, 10-first photoelectric detection unit, 11-third coupler, 12-second photoelectric detection unit, 13-demodulation unit, 101-first port of sensing optical fiber, 1, 102-second port of sensing optical fiber
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without inventive effort based on the embodiments of the present invention, are within the scope of the present invention.
As shown in fig. 1, the present invention is a brillouin optical time domain scattering system based on optical time of flight, the system structure includes a laser light source 1, a first coupler 2, an electro-optic modulator 3, an acousto-optic modulator 5, a first optical fiber amplifier 4, a second optical fiber amplifier 6, an arbitrary waveform generator 7, a sensing optical fiber 8, a second coupler 9, a first photoelectric detection unit 10, a demodulation unit 13, a third coupler 11, and a second photoelectric detection unit 12, the sensing optical fiber 8 includes a sensing optical fiber first port 101 and a sensing optical fiber second port 102;
the laser light source 1 emits laser light to the first coupler 2 to be split into detection light and pump light, the pump light is modulated into pulse pump light through the acousto-optic modulator 5, the pulse pump light is amplified through the optical fiber first amplifier 6 and then reaches the first port 101 of the sensing optical fiber through the second coupler 9, the pulse pump light enters the sensing optical fiber 8 through the first port 101 of the sensing optical fiber, the pulse pump light is transmitted to the second port 102 of the sensing optical fiber through the sensing optical fiber 8 to be output, the output pulse pump light is received through the third coupler 11 and then is output to the second photoelectric detection unit 12, and the second photoelectric detection unit 12 outputs signals to the demodulation unit 13.
The detection light is modulated into continuous detection light after being subjected to frequency shift by the electro-optical modulator 3, the continuous detection light reaches the second port 102 of the sensing optical fiber through the third coupler 11 after being amplified by the second optical fiber amplifier 4, the continuous detection light entering the sensing optical fiber 8 is transmitted to the first port 101 of the sensing optical fiber to be output, the output continuous detection light is received by the second coupler 9 and then is output to the first electro-optical detection unit 10, and the first electro-optical detection unit 10 outputs an electric signal to the demodulation unit 13; the arbitrary waveform generator 7 is respectively connected with the electro-optical modulator 3 and the acousto-optical modulator 5, the arbitrary waveform generator 7 provides two paths of electric signal output, one path of electric signal outputs a single-frequency rectangular signal to modulate the electro-optical modulator 3, and the other path of electric signal outputs a rectangular pulse signal to modulate the acousto-optical modulator 5.
In the Brillouin optical time domain scattering system based on the optical flight time, a laser light source 1 provides continuous signal light; the first coupler 2 splits the light of the laser light source 1 into upper probe light and lower pump light; the electro-optical modulator 3 shifts the frequency of the detection light by a value between 9 and 13 GHz; the first fiber amplifier 4 is an erbium-doped fiber amplifier, and functions to amplify the probe light after frequency shift.
The acousto-optic modulator 5 generates pulse pump light with the pulse width of 20-100ns, and the second optical fiber amplifier 6 amplifies the pulse pump light; the arbitrary waveform generator 7 provides two paths of electric signal outputs, the upper path is a single-frequency sinusoidal signal modulation electro-optic modulator 3, the lower path is a rectangular pulse signal modulation acousto-optic modulator 5, and the repetition period of the rectangular pulse is determined by a user according to an applicable scene.
The total length of the sensing optical fiber 8 is 100km, and the sensing optical fiber is provided with a first sensing optical fiber port 101 and a second sensing optical fiber port 102, wherein pulse pumping light enters from the first sensing optical fiber port 101, continuous probe light enters from the second sensing optical fiber port 102, and Brillouin signals are generated by interaction in the transmission of the pulse pumping light and the continuous probe light; the second coupler 9 couples the pulse pumping light into the first port 101 of the sensing optical fiber, and simultaneously separates the detection light emitted from the first port 101 of the sensing optical fiber from the brillouin signal;
the first photoelectric detection unit 10 outputs an electrical signal with a voltage proportional to the intensity of the detection light, and the third coupler 11 couples the continuous detection light into the second port 102 of the sensing fiber and receives and emits the continuous detection light. The second photoelectric detection unit 12 outputs an electrical signal whose voltage is proportional to the intensity of the pump pulse light and the arrival time of the pulse, the demodulation unit 13 calculates the propagation delay of the pulse light in the whole section of the optical fiber, separates the voltage corresponding to the brillouin signal from the output of the first photoelectric detection unit 10, analyzes and calculates the maximum gain frequency offset under each delay, performs delay self-correction, calculates the change of the maximum gain frequency offset, and finally obtains the temperature information of each part of the optical fiber.
For a section of optical fiber in space, firstly, the pulsed pump light is transmitted from the first port of the sensing optical fiber to the section of sensing optical fiber 8, then, the pulsed pump light and the continuous probe light are overlapped in the section of sensing optical fiber 8, the stimulated brillouin scattering amplifies the light intensity of the probe light, and finally, the amplified probe light on the section of sensing optical fiber 8 is transmitted to the first port 101 of the sensing optical fiber and is received by the first photoelectric detector 10. The receiving delay is defined as the lag of the time when the first photo-detection unit 10 or the second photo-detection unit 12 receives the signal relative to the time when the pulsed pump light is incident on the first port 101 of the sensing fiber. Depending on the light propagation and interaction process, the brillouin signals from different fiber positions can be distinguished by the receive delay.
Compared with a typical BOTDA system, the system provided by the invention has the advantages that the coupler and the photoelectric detector are added before the probe light is injected to receive the pulse pumping light which propagates the whole section of the optical fiber to be detected, and the system is calibrated by acquiring the flight time of the light which propagates the whole section of the optical fiber to be detected. Generally, a BOTDA system records signals with different delays quickly, and converts the delays into distance information through the speed of light in an optical fiber; when the maximum gain frequency deviation is calculated, the current maximum gain frequency deviation and the reference maximum gain frequency deviation with the same time delay are directly subtracted. However, since the optical fiber is also subjected to environmental disturbance by physical quantities such as optical speed, the reference maximum gain frequency offset corresponds to the optical fiber section a, and the current maximum gain frequency offset corresponds to the other optical fiber section b. Since the BOTDA system usually has a long sensing distance of more than 10km, even if the relative delay variation is small, a large absolute value variation and positioning error may occur at the far end, and the reference maximum gain frequency offsets of a and b may be greatly different. Therefore, errors with different reference maximum gains between the first and the second are introduced when the maximum gain frequency deviation is calculated. For the entire length of fiber under test, the delay effect can be divided into a mean value of the delay variation per unit length (i.e., the "dc" component of the delay variation), and fluctuations of the delay variation above and below this mean value (i.e., the "ac" component of the delay variation), where the latter fluctuates spatially. The absolute value of the delay variation at the distal fiber will be larger because of the cumulative effect, but the accumulation of the ac component over longer distances (or converted to time in terms of light propagation velocity) tends to cancel out, contributing much less to the total delay variation than the "dc" component. Therefore, the total delay variation of the current whole section of the optical fiber can be measured, and the direct current component can be calculated, so that the delay of each point measured at present can be corrected to a great extent, and the error of calculating the maximum gain frequency offset can be minimized.
Fig. 2 is shown as a general procedure for measurement and signal calibration in an optical time-of-flight based brillouin optical time domain scattering system of the present invention. Wherein the "temperature measurement result" shows the difference between the temperature of each point of the optical fiber at the current measurement and the temperature at the reference measurement. The "current measurement" is not set as the nth measurement.
Step 21: before measurement starts, the maximum gain frequency offset in the detection light Brillouin system is obtained to be used as reference maximum gain frequency offset, and the flight time of reference pulse pump light of the optical fiber to be measured is obtained, wherein the two parameters, namely the flight time of the reference pulse pump light and the reference maximum gain frequency offset, are obtained by the same method as the steps 22 and 23, but the acquisition needs to be carried out before the first measurement starts.
Step 22: and acquiring the current maximum gain frequency offset during measurement. The specific operation mode is as follows: in the range of 9-13GHz, the frequency of the modulation signal (i.e. the frequency offset of the probe light relative to the pump light) applied to the first point electro-optical modulator 3 by the arbitrary waveform generator 7 is adjusted by a step of 1MHz, and the brillouin signals at each modulation frequency are collected one by one, so that the brillouin gain spectrum of each position of the sensing optical fiber 9 can be obtained. Taking the maximum gain point as f (t), and recording the maximum gain frequency deviation f of the optical fiber N (t) of (d). In particular, before all measurements are started, the reference maximum gain frequency offset f is recorded 0 (t), the maximum gain frequency offset of the reference section should be terminated as one of the two parameter sources of step 21.
Step 23: and measuring frequency deviation and emitting pulse light according to the step 22, and simultaneously recording the time delay of the pulse pump light which is transmitted through the whole section of optical fiber and reaches the second photoelectric detection unit 12, namely the current flight time of the pulse pump light. To reduce errors, the delay after each pulse transmission of the current measurement step 22 may be collected and averaged. Setting the measured delay of the pulse light to propagate the whole section of optical fiber as t N，max . In particular, the reference pulsed pump light time of flight t is obtained in the same way before all measurements are started 0，max As one of two parameter sources for step 21.
Step 24: the time delay is corrected using the "dc" component of the time of flight variation. Note that the delay of a certain data point is currently measured as t N Then, the corresponding delay of the reference data is:
that is, t at the time of reference data acquisition can be considered 0 And t at the current measurement N Corresponding to the signal of the same section of optical fiber.
Step 25: the measured maximum gain frequency deviation f N (T N ) And reference maximum gain frequency offset
Subtracting to obtain the current maximum gain frequency deviation change delta f N (t N ) That is to say that
If the time domain sampling of the reference data is discrete, no sampling point corresponds to
Delaying, selecting two or more data points which are most adjacent before and after the moment, and approximately obtaining the data points by an interpolation method and a linear interpolation method of maximum gain frequency deviation of the two most adjacent points
Step 26: temperature measurement set to Δ T N (t), i.e. the change value of the current measured temperature compared to the reference temperature. Delta T according to the properties of Brillouin scattering N ＝Δf N Based on the temperature measurement result delta T of each position of the current optical fiber N (t) of (d). Wherein the proportionality coefficient A is determined by the fiber material and is a known invariant constant before measurement. For example, in G652D standard single mode fiber manufactured by corning corporation, a is 1.05 MHz/DEG C.
Although the present invention has been described in detail with reference to the embodiments, it should be understood by those skilled in the art that modifications or improvements based on the disclosure of the present invention can be made without departing from the spirit and scope of the present invention, and these modifications and improvements are within the spirit and scope of the present invention.
Claims (8)
1. A Brillouin optical time domain scattering system based on optical flight time is characterized in that the system structure comprises a laser light source (1), a first coupler (2), an electro-optic modulator (3), an acousto-optic modulator (5), a first optical fiber amplifier (4), a second optical fiber amplifier (6), an arbitrary waveform generator (7), a sensing optical fiber (8), a second coupler (9), a first photoelectric detection unit (10), a demodulation unit (13), a third coupler (11) and a second photoelectric detection unit (12), wherein the sensing optical fiber (8) comprises a sensing optical fiber first port (101) and a sensing optical fiber second port (102);
the laser light source (1) emits laser to the first coupler (2) to be split into detection light and pump light, the pump light is modulated into pulse pump light through the acousto-optic modulator (5), the pulse pump light is amplified through the optical fiber first amplifier (6) and then reaches the first port (101) of the sensing optical fiber through the second coupler (9), the pulse pump light enters the sensing optical fiber (8) through the first port (101) of the sensing optical fiber and is transmitted to the second port (102) of the sensing optical fiber for output, the output pulse pump light is received through the third coupler (11) and then is output to the second photoelectric detection unit (12), and the second photoelectric detection unit (12) outputs signals to the demodulation unit (13);
the detection light is modulated into continuous detection light after frequency shift by the electro-optic modulator (3), the continuous detection light is amplified by the second optical fiber amplifier (4) and then reaches the second port (102) of the sensing optical fiber through the third coupler (11), the continuous detection light entering the sensing optical fiber (8) is transmitted to the first port (101) of the sensing optical fiber for output, the output continuous detection light is output to the first photoelectric detection unit (10) after being received by the second coupler (9), the first photoelectric detection unit (10) outputs an electric signal to the demodulation unit (13), the arbitrary waveform generator (7) is respectively connected with the electro-optic modulator (3) and the electro-optic modulator (5), the arbitrary waveform generator (7) provides two paths of electric signal output, and one path outputs a single-frequency rectangular signal to modulate the electro-optic modulator (3), the other path outputs a rectangular pulse signal to modulate the acousto-optic modulator (5).
2. A self-calibration method of a Brillouin optical time domain scattering system based on optical flight time is characterized in that a third coupler (11) and a second photoelectric detection unit (12) are arranged in the Brillouin optical time domain scattering system to measure the flight time of light traversing the whole section of an optical fiber to be tested, so that the delay of calibrating the maximum gain frequency offset is realized, the corresponding data of the corrected current maximum gain frequency offset and the corrected reference maximum gain frequency offset correspond to the same optical fiber position, and the self-calibration of the system is realized based on the Brillouin optical time domain analysis, and the method comprises the following steps:
step 21: before measurement starts, acquiring the maximum gain frequency offset in the detection light Brillouin system as reference maximum gain frequency offset, and acquiring the flight time of reference pulse pump light of the optical fiber to be measured;
step 22: during measurement, acquiring the current maximum gain frequency offset;
step 23: measuring the flight time of the current pulse pump light;
step 24: calibrating the time delay of the maximum gain frequency offset by using the flight time of the optical fiber to be measured in light propagation;
step 25: corresponding data of the corrected current maximum gain frequency offset and the corrected reference maximum gain frequency offset correspond to the same optical fiber position, and calculating the change of the current maximum gain frequency offset;
step 26: and calculating the temperature measurement result of each position of the current optical fiber, demodulating the corrected signal and finally obtaining the temperature information sensed by the optical fiber.
3. The self-calibration method of the optical time-of-flight based brillouin optical time domain scattering system according to claim 2, wherein in the step 21, before the first measurement, the reference maximum gain frequency offset and the pulse pump optical time-of-flight are collected, and the step of collecting the reference maximum gain frequency offset is: adjusting the frequency of a modulation signal applied to an electro-optical modulator (3) by an arbitrary waveform generator (7) in a step length of 1MHz between 9 and 13GHz, wherein the frequency of the modulation signal is the frequency offset of probe light relative to pump light, collecting Brillouin signals under each modulation frequency one by one to obtain a Brillouin gain spectrum of each position of a sensing optical fiber (8), and recording the maximum gain point of the Brillouin gain spectrum as the reference maximum gain frequency offset; the process of the flight time of the pulse pump light is as follows: and recording the time delay of the pulse pump light which is transmitted through the whole section of optical fiber to reach the second photoelectric detection unit (12) while carrying out frequency offset measurement and emitting pulse light, namely the flight time of the reference pulse pump light.
4. The self-calibration method of an optical time-of-flight based brillouin optical time domain scattering system according to claim 2, wherein in step 23, the delay after each pulse emission of the current measurement step 22 is collected and averaged to reduce the error.
5. The self-calibration method of the optical time-of-flight-based brillouin optical time-domain scattering system according to claim 2, wherein in the step 24, the time delay is corrected by using the "dc" component of the time-of-flight variation, and the time delay of a currently measured data point is recorded as t N Then, the corresponding delay of the reference data is:
that is, it is considered that the reference data acquisition time t 0 And the current measurement time t N Corresponding to the signal of the same section of optical fiber.
6. The self-calibration method of the brillouin optical time domain scattering system according to claim 2, wherein in step 25, the difference between the actually measured maximum gain frequency deviation and the reference maximum gain frequency deviation is subtracted to obtain the change Δ f of the current maximum gain frequency deviation N 。
7. The self-calibration method of the Brillouin optical time domain scattering system based on the optical flight time of claim 6, wherein if the time domain sampling of the reference data is discrete and there is no corresponding delay of sampling points, two or more data points which are nearest to each other before and after the time are selected and obtained approximately by an interpolation method.
8. The self-calibration method of an optical time-of-flight based brillouin optical time domain scattering system according to claim 2, wherein in said step 26, the temperature measurement result is set to Δ T N (T), i.e. the amount of change in temperature when the currently measured temperature is compared to the reference measurement, by the equation Δ T N ＝Δf N A for determining the current position of the optical fibreThe temperature measurement, in which the proportionality coefficient a is determined by the fiber material, is a known invariant constant prior to measurement.
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