CN109029770B - Distributed optical fiber Raman temperature and strain demodulation method based on loop demodulation - Google Patents
Distributed optical fiber Raman temperature and strain demodulation method based on loop demodulation Download PDFInfo
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Abstract
The invention discloses a distributed optical fiber Raman temperature and strain demodulation method based on loop demodulation, which comprises the steps of building a distributed optical fiber Raman temperature and strain detection system based on loop demodulation; the distributed optical fiber Raman temperature and strain detection system based on loop demodulation comprises an optical fiber backscattering signal acquisition instrument, a high-speed optical switch, a constant temperature tank, a temperature sensor and a sensing optical fiber; the optical fiber backscattering signal acquisition instrument comprises a pulse laser, a WDM, an APD, an LNA, a data acquisition card and a computer; wherein, the output end of the pulse laser is connected with the input end of the WDM; the output end of the WDM is connected with the input end of the APD; the output end of the APD is connected with the input end of the LNA; the output end of the LNA is connected with the input end of the data acquisition card; the output end of the data acquisition card is connected with the input end of the computer; the computer is connected with the temperature sensor in a bidirectional way. The detection method can simultaneously detect the temperature and the stress distribution along the optical fiber by using one sensing optical fiber.
Description
Technical Field
The invention relates to the technical field of temperature and strain demodulation in a distributed optical fiber sensing system, in particular to a distributed optical fiber Raman temperature and strain demodulation method based on loop demodulation.
Background
The distributed optical fiber sensing technology utilizes an optical fiber as a signal transmission medium and a sensing unit, so as to obtain the distribution condition of external physical quantity of the whole optical fiber link. The distributed optical fiber sensing system has high measurement accuracy, long sensing distance and better reliability, and is widely applied to health monitoring of infrastructures such as smart grids.
In the distributed optical fiber sensing technology, according to the backscattering type of the optical fiber, the distributed optical fiber sensing system based on rayleigh scattering, the distributed optical fiber sensing system based on brillouin scattering and the distributed optical fiber sensing system based on raman scattering can be classified. Distributed optical fiber sensing systems based on rayleigh scattering are mostly applied to fault point detection of optical fibers. The distributed optical fiber sensing technology based on Raman scattering is only applied to temperature monitoring along the optical fiber. In the conventional raman temperature demodulation method, in order to demodulate temperature information by eliminating fiber attenuation, the whole fiber to be measured must be subjected to calibration processing at a constant temperature before temperature measurement (if the fiber to be measured is replaced, the power of a laser is adjusted, or any system device is replaced, the calibration processing must be performed again), so that the operation is complicated, and the temperature measurement efficiency of the system is low.
The principle of the distributed optical fiber sensing technology is that strain, stress and temperature are respectively measured by changing the variable quantity of Brillouin frequency shift of the distributed optical fiber sensing technology based on Brillouin scattering, the Brillouin frequency shift is sensitive to tensile strain and temperature change at the same time, namely the frequency shift caused by the tensile strain and the frequency shift caused by the temperature change need to be distinguished in the temperature demodulation process, namely the temperature and strain conditions along the optical fiber cannot be measured at the same time at a single time, the Brillouin system device and the demodulation process are complex, the measurement time reaches the order of minutes, in addition, a pumping light source and a demodulation system required by a Brillouin distributed optical fiber sensing system are complex, and the development and application of the real-time performance and the face engineering of the Brillouin distributed optical fiber sensing system are greatly limited.
Based on the above, a brand-new strain and temperature demodulation method is needed to be invented to solve the problems of low temperature measurement accuracy and low temperature measurement efficiency of the system caused by the fact that the temperature and the strain in the existing distributed optical fiber sensing system are sensitive to each other in a crossed manner, the measurement time is long, and the calibration process is required before measurement.
Disclosure of Invention
The invention provides a method for simultaneously detecting distributed optical fiber Raman temperature and strain based on loop demodulation, and aims to solve the problems that the existing distributed optical fiber sensing systems are cross-sensitive in temperature and strain, long in measuring time and incapable of being applied to engineering and the process of calibration in the existing distributed optical fiber temperature measuring systems.
The invention is realized by adopting the following technical scheme:
a distributed optical fiber Raman temperature and strain demodulation method based on loop demodulation is a temperature demodulation scheme based on distributed optical fiber Raman temperature measurement of a loop demodulation technology and an optical fiber attenuation detection scheme along a line based on loop demodulation, and comprises the following specific steps:
firstly, building a distributed optical fiber Raman temperature and strain detection system based on loop demodulation;
the distributed optical fiber Raman temperature and strain detection system based on loop demodulation comprises an optical fiber backscattering signal acquisition instrument, a high-speed optical switch, a thermostatic bath, a sensing optical fiber and a temperature sensor;
the optical fiber backscattering signal acquisition instrument comprises a pulse laser, a WDM, an APD, an LNA, a data acquisition card and a computer; wherein, the output end of the pulse laser is connected with the input end of the WDM; the output end of the WDM is connected with the input end of the APD; the output end of the APD is connected with the input end of the LNA; the output end of the LNA is connected with the input end of the data acquisition card; the output end of the data acquisition card is connected with the input end of the computer; the computer is connected with the temperature sensor in a bidirectional way;
the input end of the high-speed optical switch is connected with the common end of the WDM2, the f output end of the high-speed optical switch is connected with the front end of the sensing optical fiber, and the b output end of the high-speed optical switch is connected with the rear end of the optical fiber to be measured; the front part of the sensing optical fiber is wound with a reference optical fiber, and the rear part of the sensing optical fiber is used as an optical fiber to be detected; the reference optical fiber is placed in the constant temperature groove; the temperature sensor is installed on the thermostatic bath.
Step two, setting the temperature value of the thermostatic bath as T1; then, starting the optical fiber backscattering signal acquisition instrument, and enabling a first laser pulse emitted by the pulse laser to enter the high-speed optical switch through the WDM and then enter the optical fiber to be detected through the f output end of the high-speed optical switch; the laser pulse generates spontaneous Raman scattering when propagating in the optical fiber to be detected, so that anti-Stokes light transmitted in a back direction is generated at each position of the optical fiber to be detected;
the second laser pulse emitted by the pulse laser is emitted to the high-speed optical switch through WDM and then is emitted to the optical fiber to be measured through the b output end of the high-speed optical switch; the laser pulse generates spontaneous Raman scattering when propagating in the optical fiber to be detected, and anti-Stokes light transmitted back to the optical fiber to be detected is generated at each position of the optical fiber to be detected.
And step three, when external larger stress and strain act on the sensing optical fiber, the stretching of the optical fiber will inevitably influence the area of the cross section in the sensing optical fiber and the bending loss of the optical fiber at the point, thereby influencing the attenuation coefficient in the optical time domain reflection curve of the Raman scattering signal at the point in the sensing optical fiber. The system detects the attenuation coefficient of each point along the optical fiber according to a time sequence signal based on a Raman scattering signal and an optical time domain reflection technology, and then measures the strain and stress change along the optical fiber according to the mathematical function relation between the attenuation coefficient and the external stress strain. Namely, the distributed optical fiber Raman temperature and strain system based on loop demodulation demodulates stress and strain data distributed along the optical fiber according to the attenuation of anti-Stokes light.
And fourthly, demodulating temperature data distributed along the optical fiber by the optical fiber back scattering signal acquisition instrument according to the light intensity data of the anti-Stokes light and the attenuation along the optical fiber.
And step five, the system computer simultaneously displays the change conditions of the temperature and the strain along the optical fiber according to the step three and the step four.
Compared with the existing distributed optical fiber sensing system, the distributed optical fiber Raman temperature and strain detection method based on loop demodulation has the following advantages: firstly, the detection method can simultaneously detect the temperature and the stress distribution along the optical fiber by using one sensing optical fiber. Secondly, the invention has simple structure and device, and the measuring time depends on the measuring speed of the data acquisition card, thereby greatly improving the measuring speed of the system and simultaneously reducing the cost of the system. Thirdly, the calibration processing is not needed before the temperature and the strain measurement, and the industrial process of the distributed optical fiber sensing system is accelerated.
Drawings
Fig. 1 shows a schematic diagram of a distributed optical fiber temperature and stress sensing device based on loop demodulation in the invention.
In the figure: 1-pulse laser, 2-WDM (wavelength division multiplexer), 3-APD (first avalanche photodiode), 4-LNA (low noise amplifier), 5-data acquisition card, 6-computer, 7-high speed optical switch, 8-reference optical fiber, 9-multimode sensing optical fiber, 10-temperature sensor and 11-thermostatic bath.
Detailed Description
The following provides a detailed description of specific embodiments of the present invention.
A distributed optical fiber Raman temperature and strain demodulation method based on loop demodulation comprises the following steps:
step one, building a distributed optical fiber temperature and stress sensing system based on loop demodulation;
the distributed optical fiber temperature and stress sensing system based on loop demodulation comprises an optical fiber backscattering signal acquisition instrument, a high-speed optical switch 7, a constant temperature tank 8, sensing optical fibers and a temperature sensor 10.
The optical fiber backscattering signal acquisition instrument comprises a pulse laser 1, a WDM2, an APD 3, an LNA 4, a data acquisition card 5 and a computer 6; wherein, the output end of the pulse laser 1 is connected with the input end of the WDM 2; the output end of the WDM2 is connected with the input end of the APD 3; the output end of the APD 3 is connected with the input end of the LNA 4; the output end of the LNA 4 is connected with the input end of the data acquisition card 5; the output end of the data acquisition card 5 is connected with the input end of the computer 6; the computer 6 is bidirectionally connected to the temperature sensor 10.
The input end of the high-speed optical switch 7 is connected with the common end of the WDM2, the f output end of the high-speed optical switch 7 is connected with the front end of the sensing optical fiber, and the b output end is connected with the rear end of the sensing optical fiber; the front part of the sensing optical fiber is wound with a reference optical fiber 8, and the rear part of the sensing optical fiber is used as an optical fiber 9 to be measured; the reference light 8 ring is placed in the thermostatic bath 11; the temperature sensor 10 is arranged on the high-precision thermostatic bath 8; the temperature sensor 10 is bidirectionally connected to the computer 6.
Step two, setting the temperature value of the thermostatic bath 8 as T1(ii) a Then, starting the optical fiber backscattering signal acquisition instrument, and enabling a first laser pulse emitted by the pulse laser 1 to enter the high-speed optical switch 7 through the WDM2 and then enter the optical fiber 9 to be detected through the f output end of the high-speed optical switch; when the laser pulse is transmitted in the optical fiber 9 to be detected, spontaneous Raman scattering occurs, so that anti-Stokes light transmitted in the back direction is generated at each position of the optical fiber 9 to be detected;
the anti-Stokes light sequentially enters the data acquisition card 5 through the WDM2, the APD 3 and the LNA 4, and the data acquisition card 5 performs analog-to-digital conversion on the anti-Stokes light, so that a light intensity curve of the first anti-Stokes light is obtained.
The second laser pulse emitted by the pulse laser 1 is emitted to the high-speed optical switch 7 through the WDM2 and then is emitted to the optical fiber 9 to be measured through the output end b of the high-speed optical switch; spontaneous Raman scattering occurs when the laser pulse is transmitted in the optical fiber 9 to be detected, so that anti-Stokes light transmitted in a back direction is generated at each position of the optical fiber 9 to be detected;
the anti-Stokes light sequentially enters the data acquisition card 5 through the WDM2, the APD 3 and the LNA 4, and the data acquisition card 5 performs analog-to-digital conversion on the anti-Stokes light, so that a light intensity curve of the secondary anti-Stokes light is obtained.
And thirdly, demodulating the stress distribution distributed along the optical fiber by the distributed optical fiber temperature and stress sensing system based on loop demodulation according to the acquired light intensity data of the anti-Stokes light.
The specific stress and strain demodulation formula is as follows:
in the formula: phi is aafThe backward scattering light intensity data collected when the incident pulse is output at the output end of the high-speed optical switch f is as follows:
φabthe backward scattering light intensity data collected when the incident pulse is output at the output end of the high-speed optical switch b is as follows:
φab＝KaVa 4φeSRa(T)exp[-(αo+αa)(L-l)] (3)
wherein, the temperature modulation function R of anti-Stokes lighta(T) is:
in the formula, KaIs a coefficient related to the cross section of the scattering end of the fiber, VaFrequency of anti-Stokes light, [ phi ]eIs the light intensity of incident light, S is the scattering cross section, h and K are Planck constant and Boltzmann constant, respectively, and Raman of Deltav optical fiberAmount of frequency shift, αoAnd alpha a is the attenuation coefficient of the incident light and the anti-Stokes light in the optical fiber in unit length respectively; t represents the temperature value of the position l of the optical fiber 9 to be measured; l represents the length of the optical fiber 9 to be measured; l represents the distance between the position and the front end of the optical fiber 9 to be measured; h represents the Planck constant; Δ v represents the amount of raman frequency shift of the optical fiber; k represents boltzmann's constant. The specific value of the parameter A can be obtained by a fitting curve of an actual stress value in an experiment and an attenuation value along the optical fiber before measurement.
In specific implementation, the wavelength of the pulse laser is 1550.1nm, the pulse width is 10ns, and the repetition frequency is 8 KHz. The wavelength of WDM operation is 1550nm/1450nm/1663 nm. The bandwidth of APD is 80MHz, and the spectral response range is 900-1700 nm. The bandwidth of the LNA is 100 MHz. The channel number of the data acquisition card is 4, the sampling rate is 100M/s, and the bandwidth is 100 MHz. The switching speed of the high speed optical switch is less than 10 ms. The sensing optical fiber is a common multimode optical fiber.
And fourthly, demodulating the temperature data distributed along the optical fiber by the distributed optical fiber temperature and stress sensing system based on loop demodulation according to the acquired light intensity data of the anti-Stokes light.
The specific temperature demodulation formula is as follows:
in the formula, T represents a temperature value of the l position of the optical fiber 9 to be measured; phi is aaRepresenting the light intensity value of anti-Stokes light generated at the l position; phi is aa1Representing the light intensity value of anti-Stokes light generated by the position of the reference optical fiber; l1Indicating the distance between the position of the reference fiber and the front end of the fiber 9 to be measured.
And step five, the system can simultaneously measure the temperature and strain along the optical fiber by using one optical fiber according to the formula (1) and the formula (5).
Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and not for limiting, and although the detailed description is made with reference to the embodiments of the present invention, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention and shall be covered by the claims of the present invention.
Claims (2)
1. A distributed optical fiber Raman temperature and strain demodulation method based on loop demodulation is characterized in that: the method comprises the following steps:
firstly, building a distributed optical fiber Raman temperature and strain detection system based on loop demodulation;
the distributed optical fiber Raman temperature and strain detection system based on loop demodulation comprises an optical fiber backscattering signal acquisition instrument, a high-speed optical switch (7), a thermostatic bath (11), a temperature sensor (10) and a sensing optical fiber;
the optical fiber backscattering signal acquisition instrument comprises a pulse laser (1), a WDM (2), an APD (3), an LNA (4), a data acquisition card (5) and a computer (6); wherein, the output end of the pulse laser (1) is connected with the input end of the WDM (2); the output end of the WDM (2) is connected with the input end of the APD (3); the output end of the APD (3) is connected with the input end of the LNA (4); the output end of the LNA (4) is connected with the input end of the data acquisition card (5); the output end of the data acquisition card (5) is connected with the input end of the computer (6); the computer (6) is connected with the temperature sensor (10) in a bidirectional way;
the input end of the high-speed optical switch (7) is connected with the common end of the WDM (2), the f output end of the high-speed optical switch (7) is connected with the front end of the sensing optical fiber, and the b output end of the high-speed optical switch (7) is connected with the rear end of the sensing optical fiber; the front part of the sensing optical fiber is wound with a reference optical fiber (8), the rear part of the sensing optical fiber is used as an optical fiber (9) to be measured, the reference optical fiber (8) is placed in a constant temperature bath (11), and a temperature sensor (10) is arranged on the constant temperature bath (11);
step two, setting the temperature value of the thermostatic bath (11) as T1(ii) a Then, starting the optical fiber backscattering signal acquisition instrument, and enabling a first laser pulse emitted by the pulse laser (1) to enter a high-speed optical switch (7) through the WDM (2) and then enter an optical fiber (9) to be detected through an f output end of the high-speed optical switch; when the laser pulse is transmitted in the optical fiber (9) to be detected, spontaneous Raman scattering occurs, so that anti-Stokes light transmitted back to the optical fiber (9) to be detected is generated at each position of the optical fiber (9);
the anti-Stokes light sequentially passes through the WDM (2), the APD (3) and the LNA (4) to enter a data acquisition card (5), and the data acquisition card (5) performs analog-to-digital conversion on the anti-Stokes light, so that a light intensity curve of the first anti-Stokes light is obtained;
a second laser pulse emitted by the pulse laser (1) is incident to the high-speed optical switch (7) through the WDM (2), and then is incident to the optical fiber (9) to be tested through the output end b of the high-speed optical switch; spontaneous Raman scattering occurs when the laser pulse is transmitted in the optical fiber (9) to be detected, and anti-Stokes light transmitted back to the optical fiber (9) to be detected is generated at each position;
the anti-Stokes light sequentially passes through the WDM (2), the APD (3) and the LNA (4) to enter a data acquisition card (5), and the data acquisition card (5) performs analog-to-digital conversion on the anti-Stokes light, so that a light intensity curve of the secondary anti-Stokes light is obtained;
thirdly, demodulating the stress distribution distributed along the optical fiber by the distributed optical fiber temperature and stress sensing system based on loop demodulation according to the acquired light intensity data of the anti-Stokes light;
the specific stress and strain demodulation formula is as follows:
in the formula: phi is aafThe backward scattering light intensity data collected when the incident pulse is output at the output end of the high-speed optical switch f is as follows:
φabthe backward scattering light intensity data collected when the incident pulse is output at the output end of the high-speed optical switch b is as follows:
wherein, the temperature modulation function R of anti-Stokes lighta(T) is:
in the formula, KaIs a coefficient related to the cross section of the scattering end of the fiber, VaFrequency of anti-Stokes light, [ phi ]eIs the light intensity of incident light, S is the scattering cross section, h and K are respectively Planck constant and Boltzmann constant, the Raman frequency shift of Deltav optical fiber, alphao、αaRespectively the attenuation coefficients of incident light and anti-Stokes light in the optical fiber under the unit length; t represents the temperature value of the position l of the optical fiber 9 to be measured; l represents the length of the optical fiber 9 to be measured; l represents the distance between the position and the front end of the optical fiber 9 to be measured; h represents the Planck constant; Δ v represents the amount of raman frequency shift of the optical fiber; k represents a boltzmann constant; the specific value of the parameter A can be obtained through a fitting curve of an actual stress value in an experiment and an attenuation value along the optical fiber before measurement;
fourthly, demodulating temperature data distributed along the optical fiber by the distributed optical fiber temperature and stress sensing system based on loop demodulation according to the acquired light intensity data of the anti-Stokes light;
the specific temperature demodulation formula is as follows:
in the formula, T represents the temperature value of the l position of the optical fiber (9) to be measured; phi is aaRepresenting the light intensity value of anti-Stokes light generated at the l position; phi is aa1Representing the light intensity value of anti-Stokes light generated by the position of the reference optical fiber; l1Representing the distance between the position of the reference fiber and the front end of the sensing fiber;
and step five, the system computer simultaneously displays the change conditions of the temperature and the strain along the optical fiber according to the step three and the step four.
2. The distributed fiber Raman temperature and strain demodulation method based on loop demodulation of claim 1, wherein: the wavelength of the pulse laser (1) is 1550.1nm, the pulse width is 10ns, and the repetition frequency is 8 KHz; the working wavelength of the WDM (2) is 1550nm/1450nm/1663 nm; the bandwidth of the APD (3) is 80MHz, and the spectral response range is 900-1700 nm; the bandwidth of the LNA (4) is 100 MHz; the number of channels of the data acquisition card is 4, the sampling rate is 100M/s, and the bandwidth is 100 MHz; the switching speed of the high-speed optical switch (7) is less than 10 ms; the sensing optical fiber is a common multimode optical fiber.
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| CN111638025B (en) * | 2020-05-19 | 2022-02-25 | 太原理工大学 | Distributed optical fiber Raman sensing device and method for monitoring tunnel leakage water |
| CN111896136B (en) * | 2020-06-29 | 2021-11-09 | 太原理工大学 | Dual-parameter distributed optical fiber sensing device and method with centimeter-level spatial resolution |
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