CN112880711A - Distributed optical fiber sensing method and system based on double-pulse modulation - Google Patents
Distributed optical fiber sensing method and system based on double-pulse modulation Download PDFInfo
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Abstract
The invention discloses a distributed optical fiber sensing method and system based on double-pulse modulation. Wherein, the method comprises the following steps: acquiring continuous light; modulating the continuous light into a periodically-changing double-pulse sequence through an acousto-optic modulator (2) for amplification processing to obtain first high-power detection light; enabling the first high-power detection light to enter a sensing optical fiber through a circulator to obtain backward Rayleigh scattering light, wherein a front pulse sequence and a rear pulse sequence of the first high-power detection light form mutually interfered backward Rayleigh scattering light, and the mutually interfered backward Rayleigh light carries disturbance information of the sensing optical fiber; amplifying and coupling the backward Rayleigh scattering light to obtain second high-power detection light; and obtaining disturbance information on the sensing optical fiber by photoelectric conversion by using the second high-power detection light. The invention solves the technical problems that the prior art can not simplify the sensing device, reduce the complexity of the demodulation method and simultaneously ensure the sufficient sensing distance and the demodulation precision.
Description
Technical Field
The invention relates to the field of light propagation, in particular to a distributed optical fiber sensing method and system based on double-pulse modulation.
Background
The distributed optical fiber sensing system is one sensing system with optical fiber as sensing element and signal transmitting medium. The principle of the distributed optical fiber sensing system is that optical fibers are simultaneously used as sensing sensitive elements and transmission signal media, and the advanced OTDR technology is adopted to detect the changes of temperature and strain at different positions along the optical fibers, so that real distributed measurement is realized.
The technology has the greatest advantages that the number of channels required by test data acquisition equipment is reduced, so that the test cost is reduced, the measurement of the distribution field value of the physical quantity to be measured can be realized, and the sensing element is only an optical fiber. The distributed optical fiber sensing system has a wide application range, can be used as a fully automatic safety monitoring system, and provides convenient line tampering or interference warning for the existing optical cable communication line. The intrusion of a closed area and a high-safety area of the enclosure can be automatically monitored all weather, and the areas mainly comprise military bases, national borders, nuclear facilities, prisons and the like. And the safety monitoring of pipeline monitoring, civil facility historical buildings and the like can be realized. Therefore, the research of the distributed optical fiber sensing system has positive significance on safety monitoring, production and life
The existing optical fiber sensing technology is mainly divided into distributed optical fiber sensors based on optical time domain reflectometry and interferometer structures. The optical time domain reflectometer based on Rayleigh scattering is divided into a polarization optical time domain reflectometer (P-OTDR) and a phase sensitive optical time domain reflectometer (phi-OTDR) according to different detection methods. However, the P-OTDR technology requires the adoption of polarization maintaining optical fiber, so that the positioning accuracy is high, but the sensing distance is relatively short; the phi-OTDR has the advantages of high sensitivity, high positioning precision, simple data processing and the like.
At present, phi-OTDR phase demodulation methods mainly comprise a phase generation carrier method (homodyne method), an heterodyne method, a 3 multiplied by 3 coupler phase demodulation method and the like. The phase generation carrier method is generally used in an interference type optical fiber sensor for eliminating the influences of random phase drift, phase fading and the like, but is easily limited by frequency and sensing distance, so that the monitoring frequency range of a system is reduced, and an interferometer structure is required to be introduced, so that the structure is complex and is easily influenced by the environment; the heterodyne method usually requires mutual interference between local light and signal light, but the method has high device cost, requires high coherence of a light source, small frequency offset and high bandwidth of a detector, and is not favorable for wide-range use.
In view of the above problems, no effective solution has been proposed.
Disclosure of Invention
The embodiment of the invention provides a distributed optical fiber sensing method and system based on double-pulse modulation, which at least solve the technical problems that the prior art can not simplify a sensing device, reduce the complexity of a demodulation method and simultaneously ensure enough sensing distance and demodulation precision.
According to an aspect of the embodiments of the present invention, there is provided a distributed optical fiber sensing method based on double pulse modulation, including: acquiring continuous light;
modulating the continuous light into a periodically-changing double-pulse sequence through an acousto-optic modulator (2) for amplification processing to obtain first high-power detection light;
enabling the first high-power detection light to enter a sensing optical fiber through a circulator to obtain backward Rayleigh scattering light, wherein a front pulse sequence and a rear pulse sequence of the first high-power detection light form mutually interfered backward Rayleigh scattering light, and the mutually interfered backward Rayleigh light carries disturbance information of the sensing optical fiber;
amplifying and coupling the backward Rayleigh scattering light to obtain second high-power detection light;
and obtaining disturbance information on the sensing optical fiber by photoelectric conversion by using the second high-power detection light.
Optionally, the step of obtaining disturbance information on the sensing optical fiber through photoelectric conversion by using the second high-power detection light includes:
and transmitting the coupled second high-power detection light to a photoelectric detector (9) for photoelectric conversion, and performing data acquisition through a data acquisition card (10).
Optionally, the step of transmitting the coupled second high-power detection light to a photodetector (9) for photoelectric conversion, and performing data acquisition by a data acquisition card (10) includes:
preprocessing the second high-power detection light, and acquiring an intensity signal value at a preset position;
fitting the intensity signal values and summing to obtain a sum value;
and performing a differential cross-summing operation based on the summed values.
Optionally, after the differential cross-summing operation according to the summation value, the method further comprises:
and integrating the summation value after differential cross summation to obtain a phase demodulation result.
Optionally, in the step of amplifying and coupling the backward rayleigh scattered light, the second high power detection light is coupled by a 3 × 3 coupler of a Sagnac interferometer (8).
Optionally, in the step of coupling the second high-power probe light through a 3 × 3 coupler of the Sagnac interferometer (8), the second high-power probe light enters the Sagnac interferometer (8) and travels counterclockwise along a d-to-f port and a f-to-d port of the 3 × 3 coupler (82) of the Sagnac interferometer (8), and travels clockwise along the f-to-d port, respectively, and finally exits from b and c ports of the 3 × 3 coupler (82).
The embodiment of the invention also provides a distributed optical fiber sensing system based on double-pulse modulation, which comprises:
the device comprises an acousto-optic modulator (2), a first erbium-doped fiber amplifier (31), a second erbium-doped fiber amplifier (32), a circulator (5), a sensing fiber (6) and a Sagnac interferometer (8);
the Sagnac interferometer (8) comprises: a short optical fiber (81), a 3 × 3 coupler (82);
the acousto-optic modulator (2) is connected to the laser (1), continuous light emitted by the laser (1) is modulated into a periodically-changed double-pulse sequence through the acousto-optic modulator (2), and the double-pulse sequence is amplified through the first erbium-doped fiber amplifier (31) to form first high-power detection light;
the first high-power detection light enters the sensing optical fiber (6) through the circulator (5), backward Rayleigh scattering light generated by front and back pulses of the first high-power detection light in the sensing optical fiber (6) interferes with each other, so that disturbance information on the sensing optical fiber (6) is carried, the backward Rayleigh scattering light carrying the disturbance information returns to the circulator (5), is output from a port of the circulator (5) connected with the second erbium-doped fiber amplifier (31), and forms second high-power detection light through the second erbium-doped fiber amplifier (32);
the amplified second high-power detection light enters a Sagnac interferometer (8), and is transmitted along a port d to a port f of the 3 x 3 coupler (82) in a counterclockwise mode and a port f to the port d in a clockwise mode respectively, and finally is output from ports b and c of the 3 x 3 coupler (82) to form coupled detection light;
the coupled detection light can be used for acquiring disturbance information on the sensing optical fiber after photoelectric conversion.
Optionally, the distributed optical fiber sensing system further comprises a photodetector (9) connected to the Sagnac interferometer (8), and a data acquisition card (10);
the detection light containing the disturbance information is transmitted to a photoelectric detector (9) for photoelectric conversion; after the data acquisition card (10) acquires data, light intensity values in two time domains can be obtained, and the light intensity values are demodulated by using a demodulation algorithm to obtain disturbance information on the sensing optical fiber.
Optionally, a first filter (41) is arranged after the first erbium-doped fiber amplifier (31) and before the circulator (5), and a second filter (42) is arranged after the second erbium-doped fiber amplifier (32) and before the Sagnac interferometer (8).
Optionally, an optical isolator (7) is further disposed between the second filter (42) and the Sagnac interferometer (8).
In the embodiment of the invention, a simple system structure is adopted, the double-pulse sequence is directly generated by modulation, and the scattered light between the front pulse and the rear pulse of the double-pulse sequence is used for interference, so that two-path interferometer structures, coherent detection structures and carrier waves are not needed, and only two photoelectric detectors are needed for disturbance positioning and phase demodulation, thereby simplifying the system configuration and reducing the cost. The adopted demodulation method is simple, complex calculation is not needed, and the demodulation speed and the demodulation precision are further improved.
In an optional embodiment, a Sagnac interferometer structure is used for collecting signals, so that the Sagnac interferometer structure is insensitive to environmental disturbance, the stability of a system is improved, and the signal-to-noise ratio is improved.
In an optional embodiment, the two paths of light intensity signals can be demodulated by using the phase demodulation algorithm of the invention without generating a phase difference of 120 degrees. The technical problem that the prior art cannot simplify a sensing device, reduce the complexity of a demodulation method and simultaneously ensure enough sensing distance and demodulation precision is solved.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention without limiting the invention. In the drawings:
FIG. 1 is a flow chart of a distributed fiber sensing method based on double pulse modulation according to an embodiment of the invention;
FIG. 2 is a schematic diagram of the structural principle of a distributed optical fiber sensing system based on double-pulse modulation according to an embodiment of the present invention;
FIG. 3 is a flow chart of phase demodulation for distributed fiber sensing based on double pulse modulation according to an embodiment of the present invention;
in the figure, 1-laser light source, 2-acousto-optic modulator, 31\ 32-erbium-doped fiber amplifier, 41\ 42-filter, 5-circulator, 6-sensing fiber, 7-optical isolator, 8-Sagnac interferometer, 81-short fiber, 82-3X 3 coupler, 9-photoelectric detector and 10-data acquisition card.
Detailed Description
In order to make the technical solutions of the present invention better understood, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be 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.
It should be noted that the terms "first," "second," and the like in the description and claims of the present invention and in the drawings described above are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used is interchangeable under appropriate circumstances such that the embodiments of the invention described herein are capable of operation in sequences other than those illustrated or described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
In the related technology, the phase demodulation method of the 3 × 3 coupler has a complex system structure and a complicated demodulation process, and most importantly, the phase demodulation structure of the 3 × 3 coupler requires that an absolute 120-degree phase difference is generated between output signals of the 3 × 3 coupler, that is, the splitting ratio of the 3 × 3 coupler is required to be 1:1:1, which is difficult to achieve in practice, and the instability of the splitting ratio and the phase difference affects the demodulation result; in addition, the 3 × 3 coupler needs to obtain intensity signals of three channels at the same time, so the method needs three photodetectors with the same characteristics for three-channel synchronous data acquisition and an interferometer structure, and the complexity of the system is greatly increased. Therefore, how to ensure sufficient sensing distance and demodulation accuracy while simplifying the sensing device and reducing the complexity of the demodulation method is a problem to be mainly solved in the existing distributed optical fiber sensing system.
In accordance with an embodiment of the present invention, there is provided a method embodiment of a distributed fiber optic sensing method based on double pulse modulation, it is noted that the steps illustrated in the flowchart of the accompanying drawings may be performed in a computer system such as a set of computer executable instructions, and that while a logical order is illustrated in the flowchart, in some cases the steps illustrated or described may be performed in an order different than here.
Example one
Fig. 1 is a flowchart of a distributed optical fiber sensing method based on double pulse modulation according to an embodiment of the present invention, and fig. 2 is a schematic diagram of a structural principle of a distributed optical fiber sensing system based on double pulse modulation according to an embodiment of the present invention, as shown in fig. 1 and fig. 2, the method includes the following steps:
step S102, acquiring continuous light. Step S104, modulating the continuous light into a periodically-changed double-pulse sequence through an acousto-optic modulator (2) for amplification processing to obtain first high-power detection light;
step S106, enabling the first high-power detection light to enter a sensing optical fiber through a circulator to obtain backward Rayleigh scattering light, wherein the backward Rayleigh scattering light which is interfered with each other is formed by a front pulse sequence and a back pulse sequence of the first high-power detection light, and the backward Rayleigh scattering light which is interfered with each other carries disturbance information of the sensing optical fiber;
step S108, amplifying and coupling the backward Rayleigh scattering light to obtain second high-power detection light;
and step S110, obtaining disturbance information on the sensing optical fiber by using the second high-power detection light through photoelectric conversion.
Specifically, in step S102, the embodiment of the invention emits a continuous laser beam through the laser. The light source provided by the embodiment of the invention is not limited to a laser, and may also include other types of light sources.
In step S104, the continuous light emitted from the laser 1 is modulated into a periodically changing double pulse sequence by the acousto-optic modulator 2, and then the double pulse sequence is amplified by the first erbium-doped fiber amplifier 31, the pump light source inside the first erbium-doped fiber amplifier 31 has a very wide line width, and in order to satisfy the coherent detection principle of the phi-OTDR, the light pulse number amplified by the first erbium-doped fiber amplifier 31 needs to be optically filtered by the first optical fiber filter 41, so that the narrow-line-width high-coherence high-power detection light required by the phi-OTDR is obtained and recorded as the first high-power detection light. It should be noted that the first optical fiber filter 41 is an optional component, and it is clear to those skilled in the art that the technical solution and the technical effect of the present invention can be achieved if the first optical fiber filter is not provided.
When the light propagates through the optical fiber in step S106, due to the non-uniform refractive index, rayleigh scattering occurs in the optical fiber, and the rayleigh scattering has a power distribution in the whole space, wherein there is a scattering backward along the axial direction of the optical fiber, which is called rayleigh backscattering (or backscattering).
The amplified and filtered dual pulse light enters the sensing fiber 6 through the circulator 5. The optical pulse reaches the receiver along the Rayleigh backscattering excited by the whole path to form a continuous intensity signal distributed according to time sequence, the intensity of each time signal uniquely corresponds to the Rayleigh scattering intensity of a corresponding position, and the OTDR obtains the information of each point on the length of the optical fiber according to the point. If there is disturbance on the sensing optical fiber 6, the strain generated by the disturbance will deform the optical fiber, so that the refractive index in the optical fiber is changed, the strength of the rayleigh scattering signal at the point of the optical fiber is changed, and the disturbance point can be positioned by analyzing the subsequently received rayleigh scattering strength. Then, the backward rayleigh scattered light generated in the sensing fiber 6 by the front and back pulses in the double pulse sequence of the embodiment of the present invention interfere with each other, thereby carrying the disturbance information on the sensing fiber 6.
In step S108, the backward rayleigh scattered light carrying the disturbance information is returned to the circulator 5 and output from the 3-port of the circulator 5, and at this time, the rayleigh scattered signal is weak and the signal-to-noise ratio is not high, so that the technical effect of amplifying the signal and filtering other band noise can be achieved through the second erbium-doped fiber amplifier 32 and the second fiber filter 42.
Optionally, in step S108, the second high-power detection light may be coupled through Sagnac interferometer 8, specifically, Sagnac interferometer 8 includes short optical fiber 81 and 3 × 3 coupler 82; and coupling the second high-power detection light through a 3 x 3 coupler, and using the obtained result for subsequent electrical signal analysis.
In step S110, obtaining disturbance information on the sensing optical fiber through photoelectric conversion by using the second high-power probe light, where the step may include: and carrying out photoelectric conversion on the coupled second high-power detection light, and carrying out data acquisition to obtain the disturbance information.
Optionally, the detection light containing the disturbance information includes disturbance positioning information for calculating the disturbance information and light intensity information.
Specifically, in the embodiment of the present invention, the second high-power detection optical signal enters the Sagnac interferometer 8, and is transmitted counterclockwise and clockwise along ports d to f of the 3 × 3 coupler 82, and is finally transmitted to the photodetector 9 from ports b and c of the 3 × 3 coupler 82 for photoelectric conversion, and is finally acquired by the data acquisition card 10 to obtain the light intensity values in two time domains, and the light intensity values are demodulated by the subsequent algorithm to obtain the disturbance information on the sensing optical fiber.
Optionally, in step S110, obtaining disturbance information on the sensing optical fiber by performing photoelectric conversion on the second high-power probe light, where the step of performing photoelectric conversion on the coupled second high-power probe light and performing data acquisition to obtain the disturbance information includes: preprocessing the detection light containing the disturbance information, and acquiring an intensity signal value at a preset position; fitting the intensity signal values and summing to obtain a sum value; and performing a differential cross-summing operation based on the summed values.
Optionally, after the differential cross-summing operation is performed according to the summation value, the method further includes: and integrating the summation value after differential cross summation to obtain a phase demodulation result.
Fig. 3 is a flow chart of phase adjustment of a distributed fiber sensing based on double pulse modulation according to an embodiment of the present invention, in which:
wherein A isi＝EAi 2+EBi 2,Bi＝2EAiEBi i＝1,2；
And
initial phase of ports b and c, and phase change caused by external disturbance phi (t).
It can be seen that the phase change Φ (t) of the external disturbance is proportional to the demodulation result s (t).
The algorithm is utilized to simulate sine signals, triangular wave signals, cosine frequency sweep signals, AM amplitude modulation signals and the like to obtain correct demodulation results, and the cosine frequency sweep signal demodulation results show that the demodulation waveforms are very fit with the original waveforms.
Through the embodiment, the technical problem that the prior art cannot simplify a sensing device, reduce the complexity of a demodulation method and simultaneously ensure sufficient sensing distance and demodulation precision is solved.
Example two
The second embodiment of the present invention further provides a distributed optical fiber sensing system based on double-pulse modulation, as shown in fig. 2, the distributed optical fiber sensing system includes: the device comprises an acousto-optic modulator 2, a first erbium-doped fiber amplifier 31, a second erbium-doped fiber amplifier 32, a circulator 5, a sensing fiber 6 and a Sagnac interferometer 8.
The Sagnac interferometer 8 includes: a stub fiber 81, a 3 × 3 coupler 82; the acousto-optic modulator 2 is connected to the laser 1, the continuous light emitted by the laser 1 is modulated into a periodically-changing double-pulse sequence through the acousto-optic modulator 2, and the double-pulse sequence is amplified through the first erbium-doped fiber amplifier 31 to form first high-power detection light; the first high-power detection light enters the sensing optical fiber 6 through the circulator 5, backward rayleigh scattering light generated in the sensing optical fiber 6 by front and back pulses of the first high-power detection light interferes with each other, so as to carry disturbance information on the sensing optical fiber 6, the backward rayleigh scattering light carrying the disturbance information returns to the circulator 5, and is output from a port of the circulator 5 connected with the second erbium-doped optical fiber amplifier 31, and second high-power detection light is formed through the second erbium-doped optical fiber amplifier 32; the amplified second high-power detection light enters a Sagnac interferometer 8, and is transmitted along a port d to a port f of the 3 × 3 coupler 82 in a counterclockwise manner and a port f to the port d in a clockwise manner, and is finally output from ports b and c of the 3 × 3 coupler 82 to form coupled detection light; the coupled detection light can be used for acquiring disturbance information on the sensing optical fiber after photoelectric conversion.
Optionally, the distributed optical fiber sensing system further includes a photodetector 9 connected to the Sagnac interferometer 8, and a data acquisition card 10;
the detection light containing the disturbance information is transmitted to a photoelectric detector 9 for photoelectric conversion; after data acquisition is performed by the data acquisition card 10, light intensity values in two time domains can be obtained, and the light intensity values are demodulated by using a demodulation algorithm to obtain disturbance information on the sensing optical fiber.
Optionally, a first filter 41 is disposed after the first erbium-doped fiber amplifier 31 and before the circulator 5, and a second filter 42 is disposed after the second erbium-doped fiber amplifier 32 and before the Sagnac interferometer 8.
Optionally, an optical isolator 7 is further disposed between the second filter 42 and the Sagnac interferometer 8.
In the distributed optical fiber sensing system based on double-pulse modulation, continuous light emitted by the laser 1 is modulated into a double-pulse sequence with periodic change through the acousto-optic modulator 2, and the double-pulse sequence is amplified through the first erbium-doped optical fiber amplifier 31; and is optically filtered by the first filter 41 to form a first high power probe light.
The first high-power detection light enters the sensing optical fiber 6 through the circulator 5, backward Rayleigh scattering light generated by front and back pulses of the high-power detection light in the sensing optical fiber 6 interferes with each other, so that disturbance information on the sensing optical fiber 6 is carried, the backward Rayleigh scattering light carrying the disturbance information returns to the circulator 5, the backward Rayleigh scattering light is output from a port of the circulator 5 connected with the second erbium-doped optical fiber amplifier 31, and other frequency band noise is filtered while signals are amplified through the second erbium-doped optical fiber amplifier 32 and the second filter 42, so that second high-power detection light is formed.
The second high-power probe light enters Sagnac interferometer 8, and is transmitted along the port d to the port f of the 3 × 3 coupler 82 counterclockwise and the port f to the port d clockwise, and finally is output from the ports b and c of the 3 × 3 coupler 82, so as to form coupled probe light.
Transmitting the detection light containing the disturbance information to a photoelectric detector 9 for photoelectric conversion; after data acquisition is performed by the data acquisition card 10, light intensity values in two time domains can be obtained, and disturbance information on the sensing optical fiber can be obtained by demodulating the light intensity values through a subsequent algorithm.
Specifically, when the embodiment is implemented, continuous light emitted from the laser 1 is modulated into a periodically-changing double-pulse sequence through the acousto-optic modulator 2, and then the double-pulse sequence is amplified through the first erbium-doped fiber amplifier 31, a pumping light source inside the first erbium-doped fiber amplifier 31 has a very wide line width, and in order to satisfy the coherent detection principle of the phi-OTDR, an optical pulse signal amplified by the first erbium-doped fiber amplifier 31 may also pass through optical filtering of the first optical fiber filter 41, so that high-power detection light with a narrow line width and high coherence required by the phi-OTDR is obtained and recorded as the first high-power detection light.
The amplified and filtered double-pulse first high-power probe light then enters the sensing fiber 6 through the circulator 5. The optical pulse reaches the receiver along the Rayleigh backscattering excited by the whole path to form a continuous intensity signal distributed according to time sequence, the intensity of each time signal uniquely corresponds to the Rayleigh scattering intensity of a corresponding position, and the OTDR obtains the information of each point on the length of the optical fiber according to the point.
If the sensing optical fiber 6 is disturbed, the optical fiber is deformed by the strain generated by the disturbance, so that the refractive index in the optical fiber is changed, the strength of the Rayleigh scattering signal at the point of the optical fiber is changed, and the disturbance point can be positioned by analyzing the subsequently received Rayleigh scattering strength, wherein backward Rayleigh scattering light generated by front and back pulses in a double-pulse sequence in the sensing optical fiber 6 interferes with each other, so that disturbance information on the sensing optical fiber 6 is carried; the backward rayleigh scattered light carrying the disturbance information returns to the circulator 5 and is output from the port 3 of the circulator 5, at this time, the rayleigh scattered signal is weak, the signal-to-noise ratio is not high, and the signal needs to be amplified by the second erbium-doped fiber amplifier 32 and the second fiber filter 42, and other frequency band noise is filtered while the signal is amplified, so that second high-power detection light is formed.
And finally, the second high-power detection light enters the Sagnac interferometer 8, is respectively transmitted along the port d to the port f of the 3 x 3 coupler 82 in the anticlockwise direction, is transmitted along the port f to the port d in the clockwise direction, is finally transmitted to the photoelectric detector 9 from the port b and the port c of the 3 x 3 coupler 82 in the clockwise direction, is subjected to photoelectric conversion, is subjected to data acquisition through the data acquisition card 10 to obtain light intensity values in two time domains, and is demodulated through a subsequent algorithm to obtain disturbance information on the sensing optical fiber.
Compared with the traditional MI interferometer optical system structure, the distributed optical fiber sensing system structure based on double-pulse modulation does not need a Faraday rotating mirror, simplifies the structure, does not need to generate an unbalanced light path, and is favorable for improving the stability of the whole system; compared with the traditional coherent detection structure, the structure does not need to introduce local light to interfere with signal light, realizes a direct detection system, simplifies system configuration and reduces cost. In addition, the 3 x 3 coupler in the experimental structure does not need to guarantee strict splitting ratio, and can also demodulate the disturbance waveform well, thereby reducing the requirements of the experimental device.
In the embodiment of the invention, a simple system structure is adopted, the double-pulse sequence is directly generated by modulation, and the scattered light between the front pulse and the rear pulse of the double-pulse sequence is used for interference, so that two-path interferometer structures, coherent detection structures and carrier waves are not needed, and only two photoelectric detectors are needed for disturbance positioning and phase demodulation, thereby simplifying the system configuration and reducing the cost. The adopted demodulation method is simple, complex calculation is not needed, and the demodulation speed and the demodulation precision are further improved.
In an optional embodiment, a Sagnac interferometer structure is used for collecting signals, so that the Sagnac interferometer structure is insensitive to environmental disturbance, the stability of a system is improved, and the signal-to-noise ratio is improved.
In an optional embodiment, the two paths of light intensity signals can be demodulated by using the phase demodulation algorithm of the invention without generating a phase difference of 120 degrees. The technical problem that the prior art cannot simplify a sensing device, reduce the complexity of a demodulation method and simultaneously ensure enough sensing distance and demodulation precision is solved.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (10)
1. A distributed optical fiber sensing method based on double-pulse modulation is characterized by comprising the following steps:
acquiring continuous light;
modulating the continuous light into a periodically-changing double-pulse sequence through an acousto-optic modulator (2) for amplification processing to obtain first high-power detection light;
enabling the first high-power detection light to enter a sensing optical fiber through a circulator to obtain backward Rayleigh scattering light, wherein a front pulse sequence and a rear pulse sequence of the first high-power detection light form mutually interfered backward Rayleigh scattering light, and the mutually interfered backward Rayleigh light carries disturbance information of the sensing optical fiber;
amplifying and coupling the backward Rayleigh scattering light to obtain second high-power detection light;
and obtaining disturbance information on the sensing optical fiber by photoelectric conversion by using the second high-power detection light.
2. The method of claim 1, wherein the step of obtaining the disturbance information on the sensing fiber through photoelectric conversion by using the second high-power detection light comprises:
and transmitting the coupled second high-power detection light to a photoelectric detector (9) for photoelectric conversion, and performing data acquisition through a data acquisition card (10).
3. The method according to claim 2, wherein the step of transmitting the coupled second high-power detection light to a photodetector (9) for photoelectric conversion and performing data acquisition via a data acquisition card (10) comprises:
preprocessing the second high-power detection light, and acquiring an intensity signal value at a preset position;
fitting the intensity signal values and summing to obtain a sum value;
and performing a differential cross-summing operation based on the summed values.
4. The method of claim 3, wherein after the differential cross-summing operation based on the summed values, the method further comprises:
and integrating the summation value after differential cross summation to obtain a phase demodulation result.
5. The method of claim 1, wherein the step of amplifying and coupling the backward rayleigh scattered light is by coupling the second high power probe light through a 3 x 3 coupler of a Sagnac interferometer (8).
6. The method according to claim 5, wherein in the step of coupling the second high power probe light through the 3 x 3 coupler of the Sagnac interferometer (8), the second high power probe light enters the Sagnac interferometer (8) and travels counterclockwise along the d-to-f port and clockwise along the f-to-d port of the 3 x 3 coupler (82) of the Sagnac interferometer (8), respectively, and finally exits from the b-and c-ports of the 3 x 3 coupler (82).
7. A distributed fiber optic sensing system based on dual pulse modulation, comprising:
the device comprises an acousto-optic modulator (2), a first erbium-doped fiber amplifier (31), a second erbium-doped fiber amplifier (32), a circulator (5), a sensing fiber (6) and a Sagnac interferometer (8);
the Sagnac interferometer (8) comprises: a short optical fiber (81), a 3 × 3 coupler (82);
the acousto-optic modulator (2) is connected to the laser (1), continuous light emitted by the laser (1) is modulated into a periodically-changed double-pulse sequence through the acousto-optic modulator (2), and the double-pulse sequence is amplified through the first erbium-doped fiber amplifier (31) to form first high-power detection light;
the first high-power detection light enters the sensing optical fiber (6) through the circulator (5), backward Rayleigh scattering light generated by front and back pulses of the first high-power detection light in the sensing optical fiber (6) interferes with each other, so that disturbance information on the sensing optical fiber (6) is carried, the backward Rayleigh scattering light carrying the disturbance information returns to the circulator (5), is output from a port of the circulator (5) connected with the second erbium-doped fiber amplifier (31), and forms second high-power detection light through the second erbium-doped fiber amplifier (32);
the amplified second high-power detection light enters a Sagnac interferometer (8), and is transmitted along a port d to a port f of the 3 x 3 coupler (82) in a counterclockwise mode and a port f to the port d in a clockwise mode respectively, and finally is output from ports b and c of the 3 x 3 coupler (82) to form coupled detection light;
the coupled detection light can be used for acquiring disturbance information on the sensing optical fiber after photoelectric conversion.
8. The distributed fiber optic sensing system based on dipulse modulation according to claim 7, further comprising a photodetector (9) connected to said Sagnac interferometer (8), a data acquisition card (10);
the detection light containing the disturbance information is transmitted to a photoelectric detector (9) for photoelectric conversion; after the data acquisition card (10) acquires data, light intensity values in two time domains can be obtained, and the light intensity values are demodulated by using a demodulation algorithm to obtain disturbance information on the sensing optical fiber.
9. The dipulse modulation-based distributed fiber optic sensing system of claim 8, wherein a first filter (41) is positioned after the first erbium-doped fiber amplifier (31) and before the circulator (5), and a second filter (42) is positioned after the second erbium-doped fiber amplifier (32) and before the Sagnac interferometer (8).
10. A distributed fibre optic sensing system based on dipulse modulation as claimed in claim 9, characterized in that an optical isolator (7) is further arranged between said second filter (42) and said Sagnac interferometer (8).
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