阅读说明：本技术 面向燃气管网泄漏的分布式光纤拉曼传感系统和方法 (Distributed optical fiber Raman sensing system and method for gas pipe network leakage ) 是由 李健 许扬 周新新 张明江 尹子彤 王晨懿 冯凯 于 2020-12-01 设计创作，主要内容包括：本发明属于分布式拉曼光纤传感技术领域,公开了一种面向管网泄漏的分布式光纤拉曼传感系统和方法,装置包括脉冲激光器、波分复用器、传感光纤、光电探测器和数据采集系统,所述脉冲激光器输出的脉冲激光经波分复用器后入射至传感光纤,所述传感器中反射的反斯托克光经波分复用器后,被光电探测器探测,所述传感光纤的一部分设置在恒温槽中,所述恒温槽用于位于其中的传感光纤进行恒温控制；光电探测器探测的信号经数据采集系统进行采集和解调,得到传感光纤沿线的分布式温度场信息。本发明优化了系统分辨率,提高了测量精度。(The invention belongs to the technical field of distributed Raman fiber sensing, and discloses a distributed fiber Raman sensing system and a distributed fiber Raman sensing method facing to pipe network leakage, wherein the device comprises a pulse laser, a wavelength division multiplexer, a sensing fiber, a photoelectric detector and a data acquisition system, pulse laser output by the pulse laser is incident to the sensing fiber after passing through the wavelength division multiplexer, anti-Stokes light reflected in a sensor is detected by the photoelectric detector after passing through the wavelength division multiplexer, one part of the sensing fiber is arranged in a constant temperature bath, and the constant temperature bath is used for constant temperature control of the sensing fiber positioned in the constant temperature bath; and the signal detected by the photoelectric detector is acquired and demodulated by a data acquisition system to obtain the distributed temperature field information along the sensing optical fiber. The invention optimizes the system resolution and improves the measurement precision.)

1. A distributed optical fiber Raman sensing system facing pipe network leakage is characterized by comprising a pulse laser (1), a wavelength division multiplexer (2), a sensing optical fiber (3), a photoelectric detector (4) and a data acquisition system, wherein pulse laser output by the pulse laser (1) enters the sensing optical fiber (3) after passing through the wavelength division multiplexer (2), anti-Stokes light reflected in the sensor (3) is detected by the photoelectric detector (4) after passing through the wavelength division multiplexer (2), one part of the sensing optical fiber (3) is arranged in a constant temperature tank (8), and the constant temperature tank (8) is used for carrying out constant temperature control on the sensing optical fiber (3) positioned in the constant temperature tank; the signal detected by the photoelectric detector is collected and demodulated by a data acquisition system to obtain the distributed temperature field information along the sensing optical fiber (3).

2. The distributed fiber Raman sensing system facing pipe network leakage according to claim 1, wherein the data acquisition system comprises a data acquisition card (6) and a calculation unit (7).

3. The distributed fiber Raman sensing system facing pipe network leakage according to claim 2, further comprising an amplifier (5), wherein the output signal of the photodetector (4) is collected by the data collection card (6) after passing through the amplifier (5).

4. The distributed fiber Raman sensing system facing pipe network leakage according to claim 2, wherein the sampling rate of the data acquisition card (6) is 10GS/s, and the bandwidth is 10 GHz.

5. The distributed optical fiber Raman sensing system facing pipe network leakage of claim 1, wherein the wavelength of the pulse laser (1) is 1550nm, the pulse width is 10ns, and the repetition rate is 6 KHz; the bandwidth of the photoelectric detector (4) is 100MHz, and the spectral response range is 900-1700 nm; the sensing optical fiber (3) is a refractive index graded multi-mode optical fiber.

6. The distributed fiber Raman sensing system facing pipe network leakage of claim 1, wherein the temperature demodulation formula is as follows:

；

wherein T represents the temperature of the nth measurement interval of the sensing optical fiber obtained by demodulation,kis the boltzmann constant, and is,△νin order to be the raman shift frequency,his the constant of the Planck, and is,T _cfor measuring the set temperature, T, of the thermostatic bath during the phase_c0For calibrating the set temperature, T, of the thermostatic bath₀Sensing the ambient temperature of the fiber for calibration stage, I_al0Indicating the intensity of the backward Raman anti-Stokes scattered light, I, of the respective measurement interval in the thermostatic bath obtained in the calibration phase_alnIndicating the intensity of the backward Raman anti-Stokes scattered light of the nth measurement interval of the sensing fiber outside the thermostatic bath obtained in the calibration stage_ac0Indicating the intensity of the backward Raman anti-Stokes scattered light of each measurement interval in the thermostatic bath obtained during the measurement phase, I_acnAnd the light intensity of backward Raman anti-Stokes scattered light obtained in the nth measurement interval of the sensing optical fiber outside the constant temperature bath in the measurement stage is shown, and n is a positive integer greater than zero.

7. The distributed fiber Raman sensing system facing pipe network leakage of claim 6, wherein I is_alnAnd I_acnThe calculation formula of (2) is as follows:

wherein the content of the first and second substances,I _an ' andI _{a n-1（）}' indicating respectively the backward Raman anti-Stokes scattered light intensity data of the nth and the n-1 th sampling intervals of the fiber outside the thermostatic bath obtained in the calibration stage, I_{al（n- X）}Backward Raman of n-X measuring interval outside the thermostatic bath obtained in the calibration stageThe light intensity of the anti-stokes scattered light, X is equal to the ratio of the pulse width to the length of the sampling interval,I _a0 ^’indicating the light intensity of the backward Raman anti-Stokes scattered light of the last sampling interval in the constant temperature bath obtained in the calibration stage;I _an andI _{a n-1（）}respectively showing the backward Raman anti-Stokes scattered light intensity data of the nth and the n-1 th sampling intervals of the optical fiber outside the constant temperature bath obtained in the measuring stage, I_ac（n-X）Indicating the light intensity of the backward Raman anti-Stokes scattered light in the n-X measuring interval outside the constant temperature bath obtained in the measuring stage,I _a0and the light intensity of the backward Raman anti-Stokes scattered light of the last sampling interval in the constant temperature bath obtained in the measuring stage is shown.

8. A distributed optical fiber Raman sensing method facing to pipe network leakage is realized by adopting a system of any one of claims 1-7, and comprises the following steps:

s1, calibration stage: the temperature of the constant temperature bath is set toT _c0 The ambient temperature of the sensing fiber is set to T₀Setting the sampling period of the data acquisition system to be 1/X of the width of a single pulse, and acquiring the light intensity of all Raman anti-Stokes lights detected by the photoelectric detector (4) by using the data acquisition system, including the light intensity I of backward Raman anti-Stokes lights in each sampling interval of the sensing optical fiber in the thermostatic bath_a0 ^’And the light intensity I of the backward Raman anti-Stokes scattered light obtained in each sampling interval of the sensing optical fiber outside the constant temperature bath_an ^’；

S2, measurement stage: the temperature of the constant temperature bath is set toT _cSetting the same sampling period, and collecting all the Raman anti-Stokes light intensities detected by the photoelectric detector by using the data acquisition system, including the backward Raman anti-Stokes light intensity I of each sampling interval of the sensing optical fiber in the thermostatic bath_a0And the light intensity I of backward Raman anti-Stokes light obtained in each sampling interval of the sensing optical fiber outside the constant temperature bath_an；

And S3, calculating and demodulating the data obtained by measurement in the steps S1 and S2 to obtain the distributed temperature field information along the sensing optical fiber (3).

9. The distributed fiber Raman sensing method facing pipe network leakage according to claim 8, wherein the value of X is 100.

Technical Field

The invention belongs to the technical field of distributed Raman fiber sensing, and particularly relates to a distributed fiber Raman sensing system facing to pipe network leakage and considering both long sensing distance and high spatial resolution.

Background

Pipe network transportation has become the life line of modern industry and national economy. With the development of the gas transmission pipe network industry, the problem of safety monitoring of the gas transmission pipe network is increasingly aggravated. The gas transmission pipe network ages gradually along with the continuous increase of the service cycle, so that the gas transmission pipe network has the hidden danger of leakage, and major safety accidents can be caused due to the development of the gas transmission pipe network. However, the current single-point, multi-probe and single-parameter sensor cannot meet the requirement of large-range long-distance detection of pipe network transportation.

The distributed optical fiber Raman sensing system can continuously measure the distributed temperature characteristic information along the sensing optical fiber. In the distributed optical fiber Raman sensing system, backward Raman scattering light along the sensing optical fiber can be modulated by the temperature around the sensing optical fiber, and the temperature change condition of each point along the sensing optical fiber can be obtained after the system is demodulated.

In a distributed fiber raman sensing system, spatial resolution refers to the minimum length over which the temperature change of the fiber can be resolved. The improvement of the spatial resolution has important significance to the field of pipe network leakage safety monitoring. At present, a positioning method of a distributed optical fiber raman sensing system is an optical time domain reflection technology, but due to the limitation of the pulse width of a light source, the sensing distance and the spatial resolution cannot be considered at the same time, and the optimal spatial resolution is only 1 m. Because the temperature of the leakage position can be changed due to the leakage of the pipe network, the leakage of the pipe network can be positioned through distributed temperature measurement, but when the spatial resolution of the distributed Raman sensing system in the prior art is high, the temperature change in a tiny range caused by the leakage is difficult to detect.

Based on the distributed optical fiber Raman sensing system based on reconstruction and layered analysis and the temperature demodulation method, the problem that the space resolution and the sensing distance of the existing distributed optical fiber Raman sensing system cannot be considered at the same time can be solved, the space resolution of the system is expected to be increased to 1cm, and the requirement of gas pipe network leakage detection is further met.

Disclosure of Invention

The invention overcomes the defects of the prior art, and solves the technical problems that: a distributed optical fiber Raman sensing system facing to leakage of a pipe network is provided.

In order to solve the technical problems, the invention adopts the technical scheme that: a distributed optical fiber Raman sensing system facing to pipe network leakage comprises a pulse laser, a wavelength division multiplexer, a sensing optical fiber, a photoelectric detector and a data acquisition system, wherein pulse laser output by the pulse laser is incident to the sensing optical fiber after passing through the wavelength division multiplexer, anti-Stokes light reflected in a sensor is detected by the photoelectric detector after passing through the wavelength division multiplexer, one part of the sensing optical fiber is arranged in a constant temperature bath, and the constant temperature bath is used for carrying out constant temperature control on the sensing optical fiber positioned in the constant temperature bath; and the signal detected by the photoelectric detector is acquired and demodulated by a data acquisition system to obtain the distributed temperature field information along the sensing optical fiber.

The data acquisition system comprises a data acquisition card and a calculation unit.

The distributed optical fiber Raman sensing system facing to the leakage of the pipe network further comprises an amplifier, and output signals of the photoelectric detector are collected by the data acquisition card after passing through the amplifier.

The sampling rate of the data acquisition card is 10GS/s, and the bandwidth is 10 GHz.

The wavelength of the pulse laser is 1550nm, the pulse width is 10ns, and the repetition frequency is 6 KHz; the bandwidth of the photoelectric detector is 100MHz, and the spectral response range is 900-1700 nm; the sensing optical fiber is a refractive index graded multimode optical fiber.

The distributed optical fiber Raman sensing system facing to the leakage of the pipe network has the temperature demodulation formula as follows:

wherein T represents the temperature of the nth measurement interval of the sensing optical fiber obtained by demodulation, k is Boltzmann constant, Deltav is Raman frequency shift, h is Planckian constant, and T_cFor measuring the set temperature, T, of the thermostatic bath during the phase_c0For calibrating the set temperature, T, of the thermostatic bath₀Sensing the ambient temperature of the fiber for calibration stage, I_al0Indicating the intensity of the backward Raman anti-Stokes scattered light, I, of the respective measurement interval in the thermostatic bath obtained in the calibration phase_alnIndicating the intensity of the backward Raman anti-Stokes scattered light of the nth measurement interval of the sensing fiber outside the thermostatic bath obtained in the calibration stage_ac0Indicating the intensity of the backward Raman anti-Stokes scattered light of each measurement interval in the thermostatic bath obtained during the measurement phase, I_acnAnd the light intensity of backward Raman anti-Stokes scattered light obtained in the nth measurement interval of the sensing optical fiber outside the constant temperature bath in the measurement stage is shown, and n is a positive integer greater than zero.

I_alnAnd I_acnThe calculation formula of (2) is as follows:

wherein, I_an' and I_a(n-1)' indicating respectively the backward Raman anti-Stokes scattered light intensity data of the nth and the n-1 th sampling intervals of the fiber outside the thermostatic bath obtained in the calibration stage, I_al(n-X)Indicating the light intensity of the backward Raman anti-Stokes scattered light of the n-X measuring interval outside the constant temperature bath obtained in the calibration stage, wherein X is equal to the ratio of the pulse width to the length of the sampling interval, I_a0' represents the light intensity of the backward Raman anti-Stokes scattered light of the last sampling interval in the constant temperature bath obtained in the calibration stage; i is_anAnd I_a(n-1)Individual watchShowing the backward Raman anti-Stokes scattered light intensity data of the nth and the n-1 th sampling intervals of the optical fiber outside the constant temperature bath obtained in the measuring stage, I_ac(n-X)Indicating the intensity of the backward Raman anti-Stokes scattered light obtained in the measurement phase in the n-X measurement interval outside the thermostatic bath, I_a0And the light intensity of the backward Raman anti-Stokes scattered light of the last sampling interval in the constant temperature bath obtained in the measuring stage is shown.

In addition, the invention also provides a distributed optical fiber Raman sensing method facing to pipe network leakage, which is realized by adopting the system and comprises the following steps:

s1, calibration stage: setting the temperature of the thermostatic bath to T_c0The ambient temperature of the sensing fiber is set to T₀Setting the sampling period of the data acquisition system to be 1/X of the width of a single pulse, and acquiring the light intensity of all Raman anti-Stokes lights detected by the photoelectric detector by using the data acquisition system, including the light intensity I of backward Raman anti-Stokes lights in each sampling interval of the sensing optical fiber in the thermostatic bath_a0' and the light intensity I of the backward Raman anti-Stokes scattered light obtained in each sampling interval of the sensing fiber outside the constant temperature bath_an’；

S2, measurement stage: setting the temperature of the thermostatic bath to T_cSetting the same sampling period, and collecting all the Raman anti-Stokes light intensities detected by the photoelectric detector by using the data acquisition system, including the backward Raman anti-Stokes light intensity I of each sampling interval of the sensing optical fiber in the thermostatic bath_a0And the light intensity I of backward Raman anti-Stokes light obtained in each sampling interval of the sensing optical fiber outside the constant temperature bath_an；

And S3, calculating and demodulating the data obtained by the measurement in the steps S1 and S2 to obtain the distributed temperature field information along the sensing optical fiber.

The value of X is 100.

Compared with the prior art, the invention has the following beneficial effects: the invention provides a distributed optical fiber Raman sensing system and a distributed optical fiber Raman sensing method facing pipe network leakage, which break through the limitation of the light source pulse width of the traditional distributed optical fiber Raman temperature measuring system on the spatial resolution by carrying out reconstruction-based layered analysis on the collected Raman anti-Stokes optical signals, optimize the spatial resolution of the system while ensuring the sensing distance, and eliminate random noise in devices such as light source output and photoelectric detectors by arranging a constant temperature tank at the front end of the sensing optical fiber, thereby improving the measurement precision and being further suitable for the leakage detection of a gas pipe network.

Drawings

Fig. 1 is a schematic structural diagram of a distributed optical fiber raman sensing system facing to leakage of a gas pipe network according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of the measurement of the present invention;

in the figure: 1-pulse laser, 2-wavelength division multiplexer, 3-sensing optical fiber, 4-photoelectric detector, 5-amplifier, 6-data acquisition card, 7-computer and 8-thermostatic bath.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments; all other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

As shown in fig. 1, an embodiment of the present invention provides a distributed optical fiber raman sensing system facing to leakage of a gas pipe network, which is a distributed optical fiber raman sensing apparatus capable of realizing centimeter-level spatial resolution based on a reconstructed layered analytic signal. The device comprises a pulse laser 1, a wavelength division multiplexer 2, a sensing optical fiber 3, an avalanche photodetector 4, an amplifier 5, a high-speed data acquisition card 6, a computer 7 and a thermostatic bath 8; the peak output power of the pulse laser 1 is 5W, the output end of the pulse laser is connected with a first port a of a wavelength division multiplexer, a second port b of the wavelength division multiplexer 2 is connected with a first port a of a sensing optical fiber 3, the sensing optical fiber 3 and the second port b of the wavelength division multiplexer 2 are 5m away, a part of the sensing optical fiber 3 with the length of 1m is placed in a thermostatic bath 8, a third port c of the wavelength division multiplexer 2 is connected with the input end of an avalanche photodetector 4, the output end of the avalanche photodetector 4 is connected with the input end of an amplifier 5, the output end of the amplifier 5 is connected with the input end of a high-speed data acquisition card 6, and the output end of the high-speed data acquisition card 6 is connected with a computer 7. And the computer 7 is used for demodulating according to the acquired anti-stokes signals to obtain the distributed temperature field information along the sensing optical fiber 3.

Further, in this embodiment, the wavelength of the pulse laser 1 is 1550nm, the pulse width is 10ns, the repetition frequency is 6KHz, the bandwidth of the avalanche photodetector 4 is 100MHz, and the spectral response range is 900-1700 nm. The number of channels of the high-speed data acquisition card 6 is 1, the sampling rate is 10GS/s, and the bandwidth is 10 GHz. The sensing optical fiber 3 is a refractive index graded multi-mode optical fiber.

The following describes a measurement method and a measurement principle of a distributed optical fiber raman sensing system for gas pipe network leakage according to an embodiment of the present invention. The method mainly comprises the following steps:

the method comprises the following steps: and (3) building a distributed optical fiber Raman anti-Stokes light self-demodulation device.

The output end of the pulse laser 1 is connected with a first port a of a wavelength division multiplexer, a second port b of the wavelength division multiplexer 2 is connected with a first port a of a sensing optical fiber 3, the sensing optical fiber 3 and the second port b of the wavelength division multiplexer 2 are 5m away, a part of the sensing optical fiber 3 with the length of 1m is placed in a thermostatic bath 8, a third port c of the wavelength division multiplexer 2 is connected with the input end of an avalanche photodetector 4, the output end of the avalanche photodetector 4 is connected with the input end of an amplifier 5, the output end of the amplifier 5 is connected with the input end of a high-speed data acquisition card 6, and the output end of the high-speed data acquisition card 6 is connected with a computer 7. And the computer 7 is used for demodulating according to the acquired Raman anti-Stokes signals to obtain the distributed temperature field information along the sensing optical fiber 3.

Step two: and (5) performing light intensity processing on the Raman anti-Stokes signal.

In conventional temperature demodulation, the light intensity of the raman anti-stokes scattered signal excited at the location of the sensing fiber L is:

I_a(T, L) is the intensity of the backward Raman anti-Stokes scattered light, where M_aAmplification factor, K, of Raman anti-Stokes scattered light for a light amplification system_aCoefficient of scattering cross section, v, of Raman anti-Stokes scattered light_aThe optical frequency of the scattered light, I, for Raman anti-Stokes₀Is the intensity of incident light, alpha₀、α_aIs the attenuation coefficient of incident light and Raman anti-Stokes light in the optical fiber, L is the position of the optical fiber, T is the temperature of the position of L, wherein h is the Planckian constant. Δ ν is the raman shift, equal to 13.2THz, k is the boltzmann constant.

The distributed optical fiber Raman sensing system utilizes a pulse time flight method to carry out space positioning, and the influence of pulse width on the system positioning can be neglected in the traditional theoretical analysis. Because the pulse has a certain width, the information acquired by the high-speed data acquisition card at any time is not the light intensity information of one point at the position of the optical fiber L, but the superposition of Raman anti-Stokes light intensity information of the whole laser pulse in the optical fiber sensing distance equal to half pulse time scale. For example, when the pulse width of the detection signal is 10ns, the light intensity information of the backward raman anti-stokes signal acquired by the high-speed data acquisition card at the position corresponding to the optical fiber L actually includes superposition of 10ns pulses in a time scale (i.e. 5ns) where the optical fiber sensing distance is equal to half of a pulse time scale, the length in the corresponding sensing optical fiber is 1m, and the specific expression is as follows:

in the formula (I), the compound is shown in the specification,indicating that the Raman anti-Stokes signals collected by the high-speed data acquisition card at the position of the sensing optical fiber L are accumulated whenWhen the pulse width is 10ns, the accumulated length is [ L-1-L ]]I.e. 5ns, corresponding to a length of 1 m.

Step three: scaling stage Raman anti-Stokes signal processing

The pulse laser 1 emits laser pulses (1550nm) having a pulse width of 10ns, and the laser pulses are incident into the sensing fiber 3 via the wavelength division multiplexer 2. Backward raman scattering anti-stokes light (1450nm) generated in sensing light enters an avalanche photodetector 4 through a wavelength division multiplexer 2 to convert optical signals into electric signals, the electric signals are amplified and subjected to analog-to-digital conversion through an amplifier 5 and a high-speed data acquisition card 6 in sequence, and finally enter a computer 7, so that the position and light intensity information of the raman anti-stokes light is obtained.

Setting the environmental temperature of the whole sensing optical fiber to T in the calibration stage₀The light intensity of the backward raman anti-stokes scattered light of the sensing fiber 3 at the L position, which is acquired by the high-speed data acquisition card 6, can be expressed as:

in this embodiment, the sampling rate of the data acquisition card 6 is set to be 10GS/s, and the sampling period is 1% of the pulse width (corresponding to 1cm of the optical fiber length). Thus, 99% of the information collected for each discrete point's data value as represented in the previous data point is information representing the same location on the fiber. Because the temperature in the constant temperature bath is constant, the energy loss of the pulse laser in a short distance (1m) is ignored, and then the light intensity of the backward Raman anti-Stokes scattered light at each position in the constant temperature bath is considered to be equal, which can be expressed as:

wherein, T_c0Is the temperature of the thermostatic bath, d is the length of the corresponding optical fiber accumulated by the collected light intensity at the sensing optical fiber L, wherein I_al0＝1％*I_a0’。

Since 99% of the information collected for each discrete point is information representing the same location on the fiber as the data represented in the previous data point. Therefore, the light intensity of the backward Raman anti-Stokes scattered light of the first measurement interval (the length is 1% of the length of the sampling interval) of the sensing fiber positioned outside the constant temperature bath can be expressed as follows:

I_al1(T₀,L)＝I_a1'-I_a0'+0.01I_a0'； (5)

wherein, I_al1Indicating the intensity of the backscattered Raman anti-Stokes light obtained in the first measurement interval, I_a0' indicates data in which all the information on the last measured temperature is contained in the thermostatic bath, wherein I_a0’＝100I_al0，I_a1' denotes the first Raman anti-Stokes light intensity data measured in the sensing fiber outside the thermostatic bath, and so on, I_an' denotes nth raman anti-stokes light intensity data measured outside the thermostatic bath. Thus, it can be seen that:

after 100 calculations, I_a101' and I_a100' the temperature information contained is not already information about the optical fiber located in the thermostatic bath, so from I_al101Initially, the corresponding processed information, i.e. I, should be subtracted_ac1. When the pulse width is X times the length of the interval, expressions (5) and (6) can be written as:

equation (7) can be finally expressed as:

the value range of X depends on the performance of the signal acquisition equipment andthe noise suppression capability of the system. I is_alnRepresenting the intensity of the back-Raman anti-Stokes scattered light obtained in the nth measurement interval during the calibration phase, wherein I_an' and I_a(n-1)' indicating respectively the backward Raman anti-Stokes scattered light intensity data of the nth and the n-1 th sampling intervals of the fiber outside the thermostatic bath obtained in the calibration stage, I_al(n-X)Indicating the light intensity of the backward Raman anti-Stokes scattered light of the n-X measuring interval outside the constant temperature bath obtained in the calibration stage, wherein X is equal to the ratio of the pulse width to the length of the sampling interval, I_a0' data indicating the last sampling interval in the thermostatic bath obtained in the calibration phase, I_a0’＝X·I_al0. As can be seen from the above formula, the light intensity of the backward raman anti-stokes scattered light in each interval can be derived by formula (7).

As shown in fig. 2, in a conventional raman system, a light time domain reflection technique is used, and light intensity information acquired at each moment is superposition of temperature signals corresponding to a section of length in an optical fiber (i.e. I in fig. 2)_anOr Ia_(n-1)) And therefore limited spatial resolution, the embodiment subtracts the repeated parts of the two data by increasing the sampling rate to finally obtain I_alnThe range of information contained becomes smaller, I_alnThe corresponding information range is the measurement interval of the application, I_anThe corresponding information range is the sampling interval of the method, so that the method can solve the data of each cell by a difference-by-difference method, and the spatial resolution of the sensing system is improved.

Step four: measurement phase raman anti-stokes signal processing

The pulse laser 1 emits laser pulses with the pulse width of 10ns, the temperature and the position along the sensing optical fiber 3 are respectively represented by T and L, the high-speed data acquisition card 6 receives backward Raman anti-Stokes scattered light at each position of the sensing optical fiber 3, and the light intensity is represented as follows:

I_a(T, L) is the intensity of the back Raman anti-Stokes scattered light amplified by an electrical amplifier in relation to temperature and position, where M_aRespectively, the magnification factor, K, of the Raman anti-Stokes scattered light by the light amplification system_aCoefficient of scattering cross section, v, of Raman anti-Stokes scattered light_aThe optical frequency of the scattered light, I, for Raman anti-Stokes₀Is the intensity of incident light, alpha_aIs the attenuation coefficient of Raman anti-Stokes light in the optical fiber, L is the position of the optical fiber, T is the temperature of the L position, wherein h is the Planckian constant. Δ ν is the raman shift, equal to about 13.2 THz. k is the boltzmann constant.

The temperature of the thermostatic bath is set to be T in the measuring stage_cThe light intensity of the backward raman anti-stokes scattered light at each position in the thermostatic bath is equal, and can be expressed as:

wherein, T_cThe constant temperature is the temperature of the constant temperature bath, and d is the length of the corresponding optical fiber accumulated by the light intensity collected at the sensing optical fiber L. As with the calibration stage processing method, the light intensity of the backward raman anti-stokes scattered light in the first measurement interval (1% of the sampling interval) in which the sensing fiber is located outside the thermostatic bath can be expressed as:

I_ac1(T,L)＝I_a1-I_a0+0.01I_a0； (10)

in the scaling stage of step three, when the pulse width is X times the length of the interval, the expression can be written as:

where equation (11) can be written finally:

the value range of X depends on the performance of the signal acquisition equipment and the noise suppression capability of the system. Wherein, I_ac1Indicating the intensity of the backscattered Raman anti-Stokes light obtained in the first interval, I_a1Representing the first measured data relating to the fiber outside the oven; i is_anAnd I_a(n-1)Respectively showing the backward Raman anti-Stokes scattered light intensity data of the nth and the n-1 th sampling intervals of the optical fiber outside the constant temperature bath obtained in the measuring stage, I_ac(n-X)Indicating the intensity of the backward Raman anti-Stokes scattered light obtained in the measurement phase in the n-X measurement interval outside the thermostatic bath, I_a0Data representing the last sampling interval in the thermostatic bath obtained during the measurement phase, I_a0＝X·I_ac0(ii) a Wherein, I_acnIndicating the light intensity of the backward raman anti-stokes scattered light obtained in the nth interval. As can be seen from the above formula, the light intensity of the backward raman anti-stokes scattered light in the measurement region of each sensing fiber can be derived by formula (12).

Step five: and (4) demodulating the temperature.

In the measuring stage, the calculation formula of the Raman anti-Stokes light intensity of each measuring interval along the sensing optical fiber 3 is as follows:

in addition, the calculation formula of the raman anti-stokes light intensity of each measurement interval along the sensing fiber 3 in the calibration stage is as follows:

the vertical type (4), (9), (13) and (14) can obtain:

obtaining by solution:

in the formula, T is the temperature distribution along the optical fiber, T_cFor measuring the temperature value, T, of the thermostatic bath 8₀For calibrating the temperature value, T, of the optical fibre_c0Is the temperature value of the constant temperature groove 8 in calibration. I is_al0Indicating the intensity of the backward Raman anti-Stokes scattered light, I, of the respective measurement interval in the thermostatic bath obtained in the calibration phase_alnIndicating the intensity of the backward Raman anti-Stokes scattered light of the nth measurement interval of the sensing fiber outside the thermostatic bath obtained in the calibration stage_ac0Indicating the intensity of the backward Raman anti-Stokes scattered light of each measurement interval in the thermostatic bath obtained during the measurement phase, I_acnAnd the light intensity of backward Raman anti-Stokes scattered light obtained in the nth measurement interval of the sensing optical fiber outside the constant temperature bath in the measurement stage is shown, and n is a positive integer greater than zero.

In summary, the distributed optical fiber raman sensing system and method facing to pipe network leakage provided by the invention are expected to optimize the spatial resolution of the system to 1cm by performing reconstruction and layered analysis processing on the original signals superposed together, and random noise in equipment such as light source output and APD can be eliminated by arranging the constant temperature bath at the front end of the sensing optical fiber, so that the measurement precision is improved.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.