CN115931167A - Distributed optical fiber Raman temperature sensing device and method based on multi-pulse measurement - Google Patents
Distributed optical fiber Raman temperature sensing device and method based on multi-pulse measurement Download PDFInfo
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Abstract
The invention relates to the technical field of distributed optical fiber sensing, and discloses a distributed optical fiber Raman temperature sensing device based on multi-pulse measurement, which comprises n lasers; the optical output ends of the n lasers are respectively connected with one wavelength division multiplexer, the wavelength division multiplexers are used for transmitting pulse light emitted by the corresponding lasers to the third coupler and respectively transmitting Stokes light and anti-Stokes light generated in the sensing optical fiber transmitted by the third coupler to the first photoelectric detector and the second photoelectric detector through the first coupler and the second coupler, and the output ends of the first photoelectric detector and the second photoelectric detector are connected with the computer; the wavelengths of the pulsed light emitted from the respective lasers are different. The invention improves the accuracy of distributed optical fiber Raman temperature sensing.
Description
Technical Field
The invention relates to the technical field of distributed optical fiber sensing, in particular to a distributed optical fiber Raman temperature sensing device with high precision based on multi-pulse measurement.
Background
The distributed optical fiber sensor has the advantages of being distributed, electrically insulated, corrosion resistant, electromagnetic resistant and the like, and is widely applied to safety monitoring in the field of linear infrastructures such as urban pipelines, oil/gas pipelines, bridges, rail transit, reservoir dams and the like. The temperature demodulation calculation of the distributed optical fiber Raman temperature sensing system depends on the optical time domain reflection principle. When laser pulses emitted by the laser are transmitted in the optical fiber, photons and optical fiber medium molecules act to generate Stokes and anti-Stokes scattering, and the position of a scattering point on the optical fiber can be positioned based on the optical time domain reflection principle.
The response time of the distributed optical fiber sensing system is the time required from the temperature change to the temperature change detection of the system, and is determined by the pulse repetition frequency, the optical fiber length and the accumulation times. The Raman anti-Stokes backscattering signals generated when the pulse light is transmitted in the sensing optical fiber are the superposition of scattered light intensity signals within half pulse width, and if the pulse light is injected into the sensing optical fiber within a time interval smaller than the pulse repetition period, the backscattering signals generated by the front pulse light and the rear pulse light interfere with each other, so that the temperature information along the optical fiber cannot be normally demodulated. Therefore, only one pulse light can exist in the sensing optical fiber at the same time. Such that the pulse repetition frequency is limited to the length of the sensing fiber. Thereby limiting the response time of the distributed fiber sensing system.
Based on this, the existing distributed optical fiber sensing technology needs to be improved to solve the technical problems of limited pulse repetition frequency and slow response time in the existing distributed optical fiber sensing system.
Disclosure of Invention
In order to solve the technical problems that the pulse repetition frequency is limited by a light time domain reflection principle and the response time is slow in the conventional distributed optical fiber sensing system, the invention provides a high-precision distributed optical fiber Raman temperature sensing device and method based on multi-pulse measurement, so as to improve the response speed required for sensing and positioning the temperature along the optical fiber.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows: a distributed optical fiber Raman temperature sensing device based on multi-pulse measurement comprises n lasers, n-channel optical switches, n wavelength division multiplexers, a first coupler, a second coupler, a first photoelectric detector, a second photoelectric detector and a computer, wherein the n wavelength division multiplexers are arranged on the optical switches; a third coupler and a sensing fiber; n is a positive integer greater than or equal to 2;
the optical output ends of the n lasers are respectively connected with the port a of one wavelength division multiplexer through the n-channel optical switch, the port b of each wavelength division multiplexer is respectively connected with one input end of a third coupler, and the output end of the third coupler is connected with one end of the sensing optical fiber; the c port of each wavelength division multiplexer is respectively connected with one input end of the first coupler, the d port of each wavelength division multiplexer is respectively connected with one input end of the second coupler, and the output ends of the first coupler and the second coupler are respectively connected with the first photoelectric detector and the second photoelectric detector;
the wavelength of the pulsed light emitted by each laser is different, and the wavelength division multiplexer is used for transmitting the pulsed light emitted by the corresponding laser to the third coupler and is also used for respectively transmitting stokes light and anti-stokes light generated in the sensing optical fiber transmitted by the third coupler to the first photoelectric detector and the second photoelectric detector through the first coupler and the second coupler;
and the output ends of the first photoelectric detector and the second photoelectric detector are connected with the computer.
The wavelength division multiplexer has a light-passing wavelength of a port a, a light-passing wavelength of a port b and a light-passing wavelength of λ a/λ c/λ d, a light-passing wavelength of a port c and a light-passing wavelength of a port d, wherein λ a is equal to the output wavelength of the corresponding laser, λ c is equal to the wavelength of stokes light corresponding to the light with the wavelength λ a, and λ d is the wavelength of anti-stokes light generated in the sensing optical fiber by the light with the wavelength λ a.
The first coupler, the second coupler and the third coupler are all n x 1 optical fiber couplers.
The wavelengths of the pulse light emitted by the n lasers are in an arithmetic progression.
The distributed optical fiber Raman temperature sensing device based on multi-pulse measurement further comprises a data acquisition card, and the output ends of the first photoelectric detector and the second photoelectric detector are connected with the computer through the data acquisition card.
The computer is used for calculating the temperature along the sensing optical fiber, and the calculation formula is as follows:
wherein T represents the temperature to be measured, T 0 The temperature of the sensing optical fiber in the calibration stage is represented, k represents a Boltzmann constant, h represents a Planckian constant, and Δ ν represents a Raman frequency shift, A1 represents the intensity ratio of Stokes light and anti-Stokes light at L in the sensing optical fiber collected in the calibration stage, and A2 represents the intensity ratio of Stokes light and anti-Stokes light at L in the sensing optical fiber collected in the measurement stage.
In addition, the invention also provides a distributed optical fiber Raman temperature sensing method based on multi-pulse measurement, which is realized based on the device and comprises the following steps:
s1, calibration stage: controlling pulse light output by one of the lasers to be incident into the sensing optical fiber, and collecting the intensity ratio A1 of Stokes light to anti-Stokes light at each position in the sensing optical fiber;
s2, a measurement stage: controlling each laser to respectively output pulse light to be incident into the optical fiber at different moments in a pulse period, and acquiring the intensity ratio A2 of Stokes light and anti-Stokes light at each position in the sensing optical fiber in the pulse period;
s3, calculating distributed temperature information along the optical fiber, wherein the calculation formula is as follows:
wherein T represents the temperature to be measured, T 0 The temperature of the sensing fiber in the calibration stage is represented, k represents a Boltzmann constant, h is a Planckian constant, Δ ν is a Raman frequency shift, A1 represents the intensity ratio of Stokes light to anti-Stokes light at L in the sensing fiber collected in the calibration stage, and A2 represents the intensity ratio of Stokes light to anti-Stokes light at L in the sensing fiber collected in the measurement stage.
In step S2, the sampling period is equal to the pulse period of the pulsed light, and the sampling time Tc > Tz, where Tz represents a time difference between a pulse end of the last laser and a pulse start of the first laser in the same sampling period.
Compared with the existing distributed optical fiber sensing device, the high-precision distributed optical fiber Raman temperature sensing device based on multi-pulse measurement provided by the invention has the advantages that: the multiple lasers are used for emitting pulses with different wavelengths in different time sequences in the same acquisition period, the fixed time interval is smaller than the repetition period of the pulses, multiple pulses exist in the sensing optical fiber within the same sampling time, interference cannot be generated due to different pulse wavelengths, the pulse repetition frequency of the whole system is improved, and multiple backscattering signals carrying temperature information are generated in the process of transmission. The frequency shift amount of the Raman anti-Stokes backscattered light and the Stokes backscattered light to the input pulse light is fixed, so that the wavelength of Raman backscattered signals can be calculated through the wavelength of the incident pulse light, the backscattered signals with different wavelengths are distinguished through corresponding wavelength division multiplexers, and finally, the collected multiple groups of Raman anti-Stokes backscattered signals are subjected to temperature demodulation processing to obtain a temperature curve of a temperature change area. Because a plurality of scattering signals which carry information and can be mutually separated exist in the optical fiber, the invention can realize the high-efficiency collection of light intensity information, improve the pulse repetition frequency and reduce the response time.
Drawings
Fig. 1 is a schematic structural diagram of a high-precision distributed optical fiber raman temperature sensing device based on multi-pulse measurement according to an embodiment of the present invention;
fig. 2 is a timing chart of pulses in a multi-pulse measurement-based high-precision distributed fiber raman temperature sensing method according to a second embodiment of the present invention.
In the figure, 1: a laser; 2. a wavelength division multiplexer; 3: an optical switch; 6: a coupler; 7: a coupler; 8: a first photodetector; 9: a second photodetector; 10: a data acquisition card; 11: a computer; 12: a coupler; 13: and a sensing optical fiber.
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.
Example one
As shown in fig. 1, an embodiment of the present invention provides a high-precision distributed fiber raman temperature sensing device based on multi-pulse measurement, including an optical switch 3 with n lasers 1, n channels, n wavelength division multiplexers 2, a first coupler 6, a second coupler 7, a first photodetector 8, a second photodetector 9, and a computer 11; a third coupler 12 and a sensing fiber 13; and n is a positive integer greater than or equal to 2.
The optical output ends of the n lasers 1 are respectively connected with the port a of one of the wavelength division multiplexers 2 through the n-channel optical switch 3, the port b of each wavelength division multiplexer 2 is respectively connected with one input end of a third coupler 12, and the output end of the third coupler 12 is connected with one end of the sensing optical fiber 13; the c port of each wavelength division multiplexer 2 is connected with one input end of the first coupler 6, the d port of each wavelength division multiplexer is connected with one input end of the second coupler 7, and the output ends of the first coupler 6 and the second coupler 7 are connected with the first photoelectric detector 8 and the second photoelectric detector 9 respectively.
The wavelength of the pulsed light emitted by each laser 1 is different, and the wavelength division multiplexer 2 is configured to transmit the pulsed light emitted by the corresponding laser 1 to a third coupler 12, and is further configured to transmit stokes light and anti-stokes light generated in a sensing fiber 13 transmitted by the third coupler 12 to the first photodetector 8 and the second photodetector 9 through the first coupler 6 and the second coupler 7, respectively; the output ends of the first photodetector 8 and the second photodetector 9 are connected with the computer 11.
Specifically, in this embodiment, the light passing wavelength of the a port of the wavelength division multiplexer 2 is λ a, the light passing wavelength of the b port is λ a/λ c/λ d, the light passing wavelength of the c port is λ c, and the light passing wavelength of the d port is λ d, where λ a is equal to the output wavelength of the corresponding laser 1, λ c is equal to the wavelength of the stokes light corresponding to the light with the wavelength λ a, and λ d is the wavelength of the anti-stokes light generated in the sensing fiber by the light with the wavelength λ a.
Specifically, in this embodiment, the first coupler 6, the second coupler 7, and the third coupler 12 are all n × 1 fiber couplers.
Specifically, in this embodiment, the wavelengths of the pulse lights emitted from the n lasers 1 are an arithmetic progression. For example, the first laser may have a center wavelength of 1550nm, the second laser may have a center wavelength of 1549nm, and so on, with the nth laser having a wavelength of (1550-n + 1) nm. Correspondingly, in the embodiment, the wavelengths of the stokes light and the anti-stokes light generated by the pulse light output by the first laser are 1450nm and 1650nm respectively, and the wavelengths of the stokes light and the anti-stokes light generated by the pulse light output by the second laser are 1449nm and 1649nm respectively, so that the stokes light and the anti-stokes light generated by the pulses output by different lasers can be separated by different wavelength division multiplexers. Then, in the same sampling period, stokes light generated by pulses output by each laser is detected by the first photodetector 8 after passing through the first coupler 6, and anti-stokes light generated by pulses output by each laser is detected by the second photodetector 9 after passing through the first coupler 6.
Further, the high-precision distributed optical fiber raman temperature sensing device based on multi-pulse measurement according to the present embodiment further includes a data acquisition card 10, output ends of the first photodetector 8 and the second photodetector 9 are connected to the computer 11 through the data acquisition card 10, and the calculating unit is configured to calculate temperature distribution information along the sensing optical fiber according to a light intensity ratio of the collected stokes light and the anti-stokes light.
The measurement principle of the embodiment of the present invention is described below.
1. And (5) a calibration stage.
The method comprises the following steps that pulsed light with the wavelength of 1550nm emitted by a first laser is used as a pulse light source, and the intensity of Raman anti-Stokes signals collected by a collecting card is as follows:
the intensity of the raman stokes signal is:
in the formula, phi asc (L) represents the anti-Stokes signal distributed light intensity information phi acquired by the data acquisition card 10 at the position L of the optical fiber in the calibration stage sc And (L) represents Stokes signal distributed light intensity information acquired by the acquisition card 10 at the position L of the optical fiber in the calibration stage. P is the incident power of the laser, K as 、K s Respectively representing coefficients relating to backscattering cross-sections of the Raman anti-Stokes signal and the Raman Stokes signal, S being a backscattering factor of the optical fiber, v as 、ν s Respectively represent the frequencies of the Raman anti-Stokes scattering signal and the Raman Stokes scattering signal, phi e Showing the light flux of the pulse laser coupled into the optical fiber, deltav is Raman frequency shift, h is Planckian constant, k is Boltzmann constant, T0 is the environmental temperature of the whole sensing optical fiber in the calibration stage, and alpha 0 、α as 、α s The loss coefficients of incident light, anti-Stokes light and Stokes light in the temperature measuring optical fiber are on the unit length respectively.
Then at the position L, the ratio A1 of the anti-stokes scattering signal intensity to the stokes scattering signal intensity is:
the calibration can eliminate the influence of other factors such as fiber loss on the raman intensity information, and in this embodiment, only one laser may be used to calibrate the system in the calibration stage.
Step two: measuring phase
Firstly, a first laser 1 is used for emitting pulse light with the wavelength of 1550nm to be output to a sensing optical fiber, an optical switch is switched to a second laser after an interval time delta t so that the pulse light with the wavelength of 1549nm enters the optical fiber, and then the optical switch is switched to a next laser with different wavelengths after the interval time delta t. The intensity of a Raman anti-Stokes signal generated in the sensing optical fiber by pulsed light emitted by the ith laser separated by the wavelength division multiplexer at L is as follows:
the intensity of a Raman Stokes signal generated in the sensing optical fiber by pulse light emitted by the ith laser separated by the wavelength division multiplexer at the position L is as follows:
wherein T is the temperature along the sensing optical fiber. Phi is a unit of asi (L) represents the light intensity information of the anti-Stokes signal separated by the ith wavelength division multiplexer at the L position of the optical fiber in the measuring stage, phi si (L) represents the Stokes signal of the fiber at the L position separated by the ith wavelength division multiplexer in the measurement phaseLight intensity information.
Since the anti-stokes signals acquired by the acquisition card 10 are the sum of the anti-stokes signals filtered by the 1 st to nth wavelength division multiplexers, and the acquired stokes signals are the sum of the stokes signals filtered by the 1 st to nth wavelength division multiplexers, the intensity of the anti-stokes signals acquired by the optical fiber at the L position acquisition card 10 in the measurement stage is as follows:
the intensity of the raman stokes signal is:
the ratio A2 of the anti-Stokes scattering signal intensity to the Stokes scattering signal intensity is:
and (3) calculating formulas (3) and (8) to obtain distributed temperature information along the optical fiber:
a1 represents the intensity ratio of Stokes light to anti-Stokes light at L in the sensing fiber collected in the calibration phase, and A2 represents the intensity ratio of Stokes light to anti-Stokes light at L in the sensing fiber collected in the measurement phase. Therefore, the computer 11 can calculate the temperature along the sensing fiber by equation (9).
Example two
The embodiment II of the invention provides a high-precision distributed optical fiber Raman temperature sensing method based on multi-pulse measurement, which is realized based on the device in the embodiment I and comprises the following steps:
s1, calibration stage: controlling pulse light output by one of the lasers 1 to be incident into the sensing optical fiber 13, and acquiring the intensity ratio A1 of Stokes light to anti-Stokes light at each position in the sensing optical fiber 13;
s2, a measuring stage: controlling each laser 1 to respectively output pulsed light to be incident into the optical fiber 13 at different moments in a pulse period, and acquiring the intensity ratio A2 of Stokes light and anti-Stokes light at each position in the sensing optical fiber 13 in the pulse period;
and S3, calculating distributed temperature information along the optical fiber.
As shown in fig. 2, in step S2 of this embodiment, the sampling period is equal to the pulse period of the pulsed light, and the sampling time Tc > Tz, where Tz represents the time difference between the pulse end of the last laser and the pulse start of the first laser in the same sampling period.
In the distributed optical fiber Raman temperature sensing device, under the condition that the length of an optical fiber is determined, the maximum value of the pulse repetition frequency under the condition of a single pulse in the system can be calculated according to an optical time domain reflection principle formula. In this embodiment, the maximum value of the pulse repetition frequency may determine the minimum pulse period of a single pulse. Satisfies the following conditions: the minimum pulse period of the single pulse is not more than n x (pulse width + interval time delta t), not more than the sampling time and not more than the pulse period, which is reasonable. For example: when the length of the optical fiber is 3km, the maximum value of the corresponding pulse repetition frequency is 5000Hz, the minimum pulse period of a single pulse is 200 mus, the sampling period is 200 mus, and the maximum 1.81 multiplied by 10 can be accommodated by taking the pulse width of 10ns and the interval time of 1ns as examples 4 Pulses when the pulse repetition rate is raised to M = 1/(11 ns) =9.1 × 10 7 Hz。
Therefore, in this embodiment, in a sampling period, the action moments of the lasers emitted by the lasers in the sensing fiber are different, and no interference occurs between the lasers, and since the sampling time covers the pulse emission time of each laser in the sampling period, the light intensities of the stokes light and the anti-stokes light generated by each laser in the sensing fiber are increased, so that efficient light intensity collection is realized, the pulse repetition frequency can be increased, and the response time of the system is reduced.
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.
Claims (8)
1. A distributed optical fiber Raman temperature sensing device based on multi-pulse measurement is characterized by comprising n lasers (1), n channels of optical switches (3), n wavelength division multiplexers (2), a first coupler (6), a second coupler (7), a first photoelectric detector (8), a second photoelectric detector (9) and a computer (11); a third coupler (12) and a sensing fiber (13); n is a positive integer greater than or equal to 2;
the optical output ends of the n lasers (1) are respectively connected with the port a of one wavelength division multiplexer (2) through the n-channel optical switch (3), the port b of each wavelength division multiplexer (2) is respectively connected with one input end of a third coupler (12), and the output end of the third coupler (12) is connected with one end of the sensing optical fiber (13); the port c of each wavelength division multiplexer (2) is respectively connected with one input end of a first coupler (6), the port d is respectively connected with one input end of a second coupler (7), and the output ends of the first coupler (6) and the second coupler (7) are respectively connected with a first photoelectric detector (8) and a second photoelectric detector (9);
the wavelength of the pulse light emitted by each laser (1) is different, and the wavelength division multiplexer (2) is used for transmitting the pulse light emitted by the corresponding laser (1) to a third coupler (12) and is also used for transmitting stokes light and anti-stokes light generated in a sensing optical fiber (13) transmitted by the third coupler (12) to the first photoelectric detector (8) and the second photoelectric detector (9) through a first coupler (6) and a second coupler (7) respectively;
the output ends of the first photoelectric detector (8) and the second photoelectric detector (9) are connected with the computer (11).
2. The distributed fiber Raman temperature sensing device based on multi-pulse measurement according to claim 1, wherein the wavelength division multiplexer (2) has an a port with a light passing wavelength λ a, a b port with a light passing wavelength λ a/λ c/λ d, a c port with a light passing wavelength λ c, and a d port with a light passing wavelength λ d, wherein λ a is equal to the output wavelength of the corresponding laser (1), λ c is equal to the wavelength of Stokes light corresponding to the light with the wavelength λ a, and λ d is the wavelength of anti-Stokes light generated in the sensing fiber by the light with the wavelength λ a.
3. The distributed fiber Raman temperature sensing device based on multi-pulse measurement according to claim 1, wherein the first coupler (6), the second coupler (7) and the third coupler (12) are all n x 1 fiber couplers.
4. The distributed fiber Raman temperature sensing device based on multi-pulse measurement according to claim 1, wherein wavelengths of the pulse light emitted by the n lasers (1) are in an arithmetic progression.
5. The distributed fiber Raman temperature sensing device based on multi-pulse measurement according to claim 1, further comprising a data acquisition card (10), wherein the output ends of the first photodetector (8) and the second photodetector (9) are connected to the computer (11) through the data acquisition card (10).
6. The distributed fiber Raman temperature sensing device based on multi-pulse measurement according to claim 1, wherein the computer is configured to calculate the temperature along the sensing fiber according to the following formula:
wherein T represents the temperature to be measured, T 0 The temperature of the sensing optical fiber (13) in the calibration stage is represented, k represents a Boltzmann constant, h represents a Planckian constant, and Deltav represents a Raman frequency shift, A1 represents the intensity ratio of Stokes light and anti-Stokes light at L in the sensing optical fiber collected in the calibration stage, and A2 represents the intensity ratio of Stokes light and anti-Stokes light at L in the sensing optical fiber collected in the measurement stage.
7. A distributed fiber raman temperature sensing method based on multi-pulse measurement, which is realized based on the device of any one of claims 1 to 5, and comprises the following steps:
s1, calibration stage: controlling pulsed light output by one of the lasers (1) to be incident into the sensing optical fiber (13), and acquiring the intensity ratio A1 of Stokes light to anti-Stokes light at each position in the sensing optical fiber (13);
s2, a measuring stage: controlling each laser (1) to respectively output pulsed light to be incident into the optical fiber (13) at different moments in a pulse period, and acquiring the intensity ratio A2 of Stokes light and anti-Stokes light at each position in the sensing optical fiber (13) in one pulse period;
s3, calculating distributed temperature information along the optical fiber, wherein the calculation formula is as follows:
wherein T represents the temperature to be measured, T 0 The temperature of the sensing fiber (13) in the calibration stage is shown, k is a Boltzmann constant, h is a Planckian constant, and Deltav is a Raman frequency shift, A1 is the intensity ratio of Stokes light and anti-Stokes light at L in the sensing fiber collected in the calibration stage, and A2 is the intensity ratio of Stokes light and anti-Stokes light at L in the sensing fiber collected in the measurement stage.
8. The distributed fiber Raman temperature sensing method based on multi-pulse measurement according to claim 7, wherein in step S2, a sampling period is equal to a pulse period of the pulsed light, and a sampling time Tc > Tz, where Tz represents a time difference between a pulse end of a last laser and a pulse start of a first laser in the same sampling period.
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