CN112880711B - 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: obtaining continuous light; modulating the continuous light into a periodically-changing double-pulse sequence through an acousto-optic modulator (2) for amplification treatment to obtain first high-power detection light; the first high-power detection light enters 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 the backward Rayleigh scattering light which interfere with each other, and the backward Rayleigh light which interferes with each other carries disturbance information of the sensing optical fiber; amplifying and coupling the backward Rayleigh scattered light to obtain second high-power detection light; and obtaining disturbance information on the sensing optical fiber by using the second high-power detection light through photoelectric conversion. The invention solves the technical problems that the prior art can not simplify the sensing device and reduce the complexity of the demodulation method, and can ensure enough sensing distance and 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 utilizing optical fiber as sensing sensor and signal transmission medium. The principle of the distributed optical fiber sensing system is that an optical fiber is used as a sensing sensitive element and a transmission signal medium, and an advanced OTDR technology is adopted to detect the temperature and strain changes at different positions along the optical fiber, so that the 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 distributed field value of the physical quantity to be tested can be realized, and the sensing element is only an optical fiber. The distributed optical fiber sensing system has a large application range, can be used for a full-automatic safety monitoring system, and provides a convenient line tampering or interference warning for the existing optical cable communication line. The intrusion of the closed area and the high-safety area of the enclosing wall can be monitored in a full-automatic all-weather mode, 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 also be realized. Therefore, the research of the distributed optical fiber sensing system has positive significance for safety monitoring, production and life
The existing optical fiber sensing technology is mainly divided into a distributed optical fiber sensor based on an optical time domain reflectometer and an interferometer structure. The optical time domain reflectometer based on Rayleigh scattering is divided into a polarized optical time domain reflectometer (P-OTDR) and a phase sensitive optical time domain reflectometer (phi-OTDR) according to different detection methods. But the P-OTDR technology requires the adoption of polarization maintaining optical fibers, the positioning precision is high, but the sensing distance is relatively short; and the phi-OTDR has the advantages of high sensitivity, high positioning precision, simple data processing and the like.
The current phi-OTDR phase demodulation method mainly comprises a phase generation carrier method (homodyne method), a 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 influence of random phase drift, phase fading and the like, but is easy to limit by frequency and sensing distance, so that the monitoring frequency range of the system is reduced, an interferometer structure is required to be introduced, the structure is complex, and the system is easy to influence by environment; heterodyne methods generally require that local light and signal light interfere with each other, but such methods are expensive in terms of device cost, require high coherence of the light source, require small frequency offset, and require high detector bandwidth, which is disadvantageous for large-scale use.
In view of the above problems, no effective solution has been proposed at present.
Disclosure of Invention
The embodiment of the invention provides a distributed optical fiber sensing method and a distributed optical fiber sensing system based on double pulse modulation, which at least solve the technical problems that the prior art cannot simplify a sensing device, reduce the complexity of a demodulation method and ensure enough sensing distance and demodulation precision.
According to an aspect of an embodiment of the present invention, there is provided a distributed optical fiber sensing method based on double pulse modulation, including: obtaining continuous light;
modulating the continuous light into a periodically-changing double-pulse sequence through an acousto-optic modulator (2) for amplification treatment to obtain first high-power detection light;
the first high-power detection light enters 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 the backward Rayleigh scattering light which interfere with each other, and the backward Rayleigh light which interferes with each other carries disturbance information of the sensing optical fiber;
amplifying and coupling the backward Rayleigh scattered light to obtain second high-power detection light;
and obtaining disturbance information on the sensing optical fiber by using the second high-power detection light through photoelectric conversion.
Optionally, the step of obtaining the disturbance information on the sensing optical fiber by using the second high-power probe light through photoelectric conversion includes:
and transmitting the coupled second high-power detection light to a photoelectric detector (9) for photoelectric conversion, and collecting data through a data collecting card (10).
Optionally, the step of 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) includes:
preprocessing the second high-power detection light, and acquiring an intensity signal value at a preset position;
fitting and summing the intensity signal values to obtain a summation value;
and performing differential cross summation operation according to the summation value.
Optionally, after the differential cross-summation 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 probe light is coupled through a 3×3 coupler of a Sagnac interferometer (8).
Optionally, in the step of coupling the second high-power probe light through the 3×3 coupler of the Sagnac interferometer (8), the second high-power probe light enters the Sagnac interferometer (8) and is respectively transmitted counterclockwise along d to f ports and clockwise along f to d ports of the 3×3 coupler (82) of the Sagnac interferometer (8), and finally is output 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 following components:
the device comprises an acousto-optic modulator (2), a first erbium-doped optical fiber amplifier (31), a second erbium-doped optical fiber amplifier (32), a circulator (5), a sensing optical fiber (6) and a Sagnac interferometer (8);
the Sagnac interferometer (8) comprises: a short optical fiber (81) and a 3 x 3 coupler (82);
the acousto-optic modulator (2) is connected with the laser (1), 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 scattered light generated in the sensing optical fiber (6) by front and back pulses of the first high-power detection light interfere 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 a port of the circulator (5) connected with the second erbium-doped optical fiber amplifier (31), and the 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), is respectively transmitted anticlockwise along d to f ports and clockwise along f to d ports of the 3X 3 coupler (82), and is finally output from b and c ports of the 3X 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 photoelectric detector (9) connected to the Sagnac interferometer (8), and a data acquisition card (10);
the detection light containing disturbance information is transmitted to a photoelectric detector (9) for photoelectric conversion; after data acquisition is carried out by the data acquisition card (10), two light intensity values in the time domain can be obtained, and the light intensity values are demodulated by a demodulation algorithm to obtain disturbance information on the sensing optical fiber.
Optionally, the circulator (5) is preceded by a first filter (41) after the first erbium doped fiber amplifier (31), and the Sagnac interferometer (8) is preceded by a second filter (42) after the second erbium doped fiber amplifier (32).
Optionally, an optical isolator (7) is also arranged 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 generated by direct modulation, and scattered light between front and rear pulses of the double pulse sequence is utilized for interference, so that a two-path interferometer structure and a coherent detection structure are not needed, a carrier is not needed, and the positioning and the phase demodulation can be performed by only two photoelectric detectors, thereby simplifying the system configuration and reducing the cost. The demodulation method is simple, complex calculation is not needed, and the demodulation speed and the demodulation precision are further improved.
In an alternative embodiment, the Sagnac interferometer structure is used for collecting signals, so that the signals are insensitive to environmental disturbance, the stability of the system is improved, and the signal to noise ratio is improved.
In an alternative embodiment, the phase demodulation algorithm in the invention can demodulate the two paths of light intensity signals without generating 120 DEG phase difference. The method solves the technical problems that the prior art can not simplify the sensing device and reduce the complexity of the demodulation method, and can ensure enough sensing distance and demodulation precision.
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 embodiments of the invention and together with the description serve to explain the invention and do not constitute a limitation on the invention. In the drawings:
FIG. 1 is a flow chart of a distributed fiber sensing method based on double pulse modulation in accordance with an embodiment of the present invention;
FIG. 2 is a schematic diagram of a distributed optical fiber sensing system based on double pulse modulation according to an embodiment of the present invention;
FIG. 3 is a phase demodulation flow chart of a distributed fiber optic sensor based on double pulse modulation according to an embodiment of the present invention;
in the figure, a 1-laser light source, a 2-acousto-optic modulator, a 31/32-erbium-doped fiber amplifier, a 41/42-filter, a 5-circulator, a 6-sensing fiber, a 7-optical isolator, an 8-Sagnac interferometer, an 81-short fiber, an 82-3X 3 coupler, a 9-photoelectric detector and a 10-data acquisition card are arranged.
Detailed Description
In order that those skilled in the art will better understand the present invention, a technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in which it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present invention without making any inventive effort, shall fall within the scope of the present invention.
It should be noted that the terms "first," "second," and the like in the description and the claims of the present invention and the above figures are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used may be interchanged where appropriate such that the embodiments of the invention described herein may be implemented in sequences other than those illustrated or otherwise 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 art, the 3×3 coupler phase demodulation method has a complex system structure and a complex demodulation process, and most importantly, the 3×3 coupler phase demodulation structure requires that an absolute 120-degree phase difference is generated between output signals of the 3×3 coupler, namely, the 3×3 coupler has a light splitting ratio of 1:1:1, which is difficult to achieve in practice, and the instability of the light splitting ratio and the phase difference affects demodulation results; in addition, the 3×3 coupler needs to obtain intensity signals of three channels simultaneously, so the method needs three channels of synchronous data acquisition and interferometer structures of three photodetectors with the same characteristics, and the complexity of the system is greatly increased. Therefore, how to simplify the sensing device and reduce the complexity of the demodulation method while ensuring sufficient sensing distance and demodulation accuracy is a problem that needs to be solved in the current distributed optical fiber sensing system.
In accordance with an embodiment of the present invention, there is provided a method embodiment of a distributed optical fiber sensing method based on double pulse modulation, it being noted that the steps shown in the flowchart of the figures may be performed in a computer system, such as a set of computer executable instructions, and that, although a logical order is shown in the flowchart, in some cases, the steps shown or described may be performed in an order other than that shown herein.
Example 1
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 distributed optical fiber sensing system based on double pulse modulation according to an embodiment of the present invention, as shown in fig. 1 and 2, the method includes the following steps:
step S102, continuous light is acquired. Step S104, modulating the continuous light into a periodically-changing double-pulse sequence through an acousto-optic modulator (2) for amplification treatment to obtain first high-power detection light;
step S106, the first high-power detection light enters 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 the backward Rayleigh scattering light which is interfered with each other, and the backward Rayleigh light which is interfered with each other carries disturbance information of the sensing optical fiber;
step S108, amplifying and coupling the backward Rayleigh scattered light to obtain second high-power detection light;
and 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 present invention emits continuous laser light through the laser. The light source provided by the embodiment of the invention is not limited to a laser, and can also comprise other types of light sources.
In step S104, the continuous light emitted by the laser 1 is modulated into a periodically varying double pulse sequence by the acousto-optic modulator 2, and then 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 linewidth, and in order to satisfy the coherent detection principle of the phi-OTDR, the optical pulse number amplified by the first erbium-doped fiber amplifier 31 also needs to be optically filtered by the first optical fiber filter 41, so that the high-power detection light with a narrow linewidth and high coherence required by the phi-OTDR is obtained and is recorded as the first high-power detection light. It should be noted that the first optical fiber filter 41 is an optional element, and it is obvious to those skilled in the art that the technical scheme and the technical effect of the present invention can be achieved if the first optical fiber filter is not provided.
When light propagates in the optical fiber in step S106, due to the non-uniformity of the refractive index, rayleigh scattering will occur in the optical fiber, and the rayleigh scattering has a power distribution in the whole space, where there is axial back scattering along the optical fiber, which we call the axial back scattering rayleigh back scattering (or back scattering).
The amplified and filtered double pulse light enters the sensing optical fiber 6 through the circulator 5. The rayleigh backscattering excited by the light pulse along the whole path reaches the receiver to form a continuous time-series distributed intensity signal, the intensity of each time signal uniquely corresponds to the rayleigh scattering intensity of the 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 strain generated by the disturbance will deform the optical fiber, so that the refractive index in the optical fiber is changed, the Rayleigh scattering signal intensity of the optical fiber is changed, and the disturbance point can be positioned by analyzing the Rayleigh scattering intensity received subsequently. Subsequently, backward rayleigh scattered light generated in the sensing optical fiber 6 by the front and back pulses in the double pulse sequence in the embodiment of the present invention interfere with each other, so that disturbance information on the sensing optical fiber 6 will be carried.
In step S108, the backward rayleigh scattering light carrying the disturbance information is returned to the circulator 5 and is output from the 3 port of the circulator 5, and the rayleigh scattering signal is weaker at this time, and the signal to noise ratio is not high, so that the technical effect of filtering noise in other frequency bands while amplifying the signal can be achieved through the second erbium-doped optical fiber amplifier 32 and the second optical fiber filter 42.
Optionally, in step S108, the second high power probe light may be coupled by a Sagnac interferometer 8, in particular, the Sagnac interferometer 8 comprises a short optical fiber 81 and a 3×3 coupler 82; and coupling the second high-power detection light through a 3×3 coupler, and obtaining a result for subsequent electrical signal analysis.
In step S110, obtaining the disturbance information on the sensing optical fiber by using the second high-power probe light through photoelectric conversion may include: and performing photoelectric conversion on the coupled second high-power detection light, and performing data acquisition to obtain the disturbance information.
Optionally, the probe light containing disturbance information includes disturbance location information for calculating 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, is respectively transmitted anticlockwise along the d-to-f-port and the f-to-d-port of the 3×3 coupler 82, is finally transmitted from the b-to-c-port of the 3×3 coupler 82 to the photodetector 9 for photoelectric conversion, and is finally subjected to data acquisition by the data acquisition card 10, so as to obtain two light intensity values in the time domain, and the light intensity values are demodulated by a subsequent algorithm, so as to obtain the disturbance information on the sensing optical fiber.
Optionally, in step S110, the step of obtaining the disturbance information on the sensing optical fiber by using the second high-power probe light through photoelectric conversion, 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 disturbance information, and acquiring an intensity signal value at a preset position; fitting and summing the intensity signal values to obtain a summation value; and performing differential cross summation operation according to the summation value.
Optionally, after the differential cross-summation operation 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 phase adjustment flow chart of a distributed optical fiber sensing based on double pulse modulation according to an embodiment of the present invention, in which:
wherein A is i ＝E Ai 2 +E Bi 2 ,B i ＝2E Ai E Bi i＝1,2；
And->
For the initial phases of the b, c ports, phi (t) is the phase change caused by external disturbances.
It can be seen that the phase change Φ (t) of the external disturbance is proportional to the demodulation result S (t).
The correct demodulation result can be obtained by utilizing the algorithm to simulate the sine signal, the triangular wave signal, the cosine sweep frequency signal, the AM amplitude modulation signal and the like, and the demodulation result of the cosine sweep frequency signal shows that the demodulation waveform is very fit with the original waveform.
By the embodiment, the technical problems that the prior art cannot simplify a sensing device and reduce the complexity of a demodulation method, and meanwhile, enough sensing distance and demodulation precision can be ensured are solved.
Example two
The second embodiment of the present invention further proposes a distributed optical fiber sensing system based on double pulse modulation, as shown in fig. 2, the distributed optical fiber sensing system includes: 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 short optical fiber 81, a 3×3 coupler 82; the acousto-optic modulator 2 is connected to the laser 1, and continuous light emitted by the laser 1 is modulated into a periodically-changing double pulse sequence by the acousto-optic modulator 2, and the double pulse sequence is amplified by 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 scattered light generated in the sensing optical fiber 6 by front and back pulses of the first high-power detection light interfere 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 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 the Sagnac interferometer 8, is respectively transmitted anticlockwise along the d-to-f ports and the f-to-d ports of the 3×3 coupler 82, is finally output from the b-to-c ports of the 3×3 coupler 82, and forms 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 photoelectric detector 9 connected to the Sagnac interferometer 8 and a data acquisition card 10;
the detection light containing disturbance information is transmitted to a photoelectric detector 9 for photoelectric conversion; after data acquisition is performed by the data acquisition card 10, two light intensity values in the time domain 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, the circulator 5 is preceded by a first filter 41 after the first erbium doped fiber amplifier 31, and a second filter 42 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 aforementioned distributed optical fiber sensing system based on double pulse modulation, the continuous light emitted by the laser 1 is modulated into a periodically varying double pulse sequence by the acousto-optic modulator 2, and the double pulse sequence is amplified by the first erbium-doped optical fiber amplifier 31; and 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 in the sensing optical fiber 6 by front and back pulses of the high-power detection light interfere 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 and 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 the Sagnac interferometer 8, is transmitted anticlockwise along the d-to-f ports and the f-to-d ports of the 3×3 coupler 82 respectively, is transmitted clockwise, and is finally output from the b-to-c ports of the 3×3 coupler 82, so as to form coupled probe light.
Transmitting the detection light containing disturbance information to a photoelectric detector 9 for photoelectric conversion; after data acquisition is performed by the data acquisition card 10, two light intensity values in the time domain can be obtained, and disturbance information on the sensing optical fiber can be obtained by demodulating the light intensity values by a subsequent algorithm.
Specifically, when implementing this embodiment, the continuous light emitted by the laser 1 should be modulated into a periodically varying 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, where the pump light source inside the first erbium-doped fiber amplifier 31 has a very wide linewidth, so that, to satisfy the coherent detection principle of Φ -OTDR, the optical pulse signal amplified by the first erbium-doped fiber amplifier 31 may also be filtered by the first optical fiber filter 41, so as to obtain the high-power detection light with a narrow linewidth and high coherence required by Φ -OTDR, which is denoted as the first high-power detection light.
The amplified and filtered double pulse first high power probe light is then passed through circulator 5 into sensing fiber 6. The rayleigh backscattering excited by the light pulse along the whole path reaches the receiver to form a continuous time-series distributed intensity signal, the intensity of each time signal uniquely corresponds to the rayleigh scattering intensity of the 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 strain generated by the disturbance causes the optical fiber to deform, so that the refractive index in the optical fiber is changed, the Rayleigh scattering signal intensity of the optical fiber is changed, and the disturbance point can be positioned by analyzing the Rayleigh scattering intensity received subsequently, wherein backward Rayleigh scattering light generated by front and rear pulses in the sensing optical fiber 6 in the double pulse sequence interferes with each other, so that disturbance information on the sensing optical fiber 6 is carried; the backward Rayleigh scattering light carrying disturbance information is returned to the circulator 5 and is output from the 3 port of the circulator 5, at the moment, the Rayleigh scattering signal is weak, the signal to noise ratio is low, and the signal is amplified while other frequency band noise is filtered out by the second erbium-doped optical fiber amplifier 32 and the second optical fiber filter 42, so that a second high-power detection light is formed.
Finally, the second high-power detection light enters the Sagnac interferometer 8, is respectively transmitted anticlockwise along the d-to-f ports and the f-to-d ports of the 3×3 coupler 82, is finally transmitted from the b-to-c ports of the 3×3 coupler 82 to the photoelectric detector 9 for photoelectric conversion, is finally subjected to data acquisition through the data acquisition card 10 to obtain two light intensity values in the time domain, 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 to use a Faraday rotating mirror, so that the structure is simplified, an unbalanced optical path is not required to be generated, and the stability of the whole system is improved; compared with the traditional coherent detection structure, the structure does not need to introduce local light to interfere with signal light, so that a direct detection system is realized, the system configuration is simplified, and the cost is reduced. In addition, the 3 multiplied by 3 coupler in the experimental structure can demodulate disturbance waveforms well without ensuring strict light splitting ratio, so that the requirements of the experimental device are reduced.
In the embodiment of the invention, a simple system structure is adopted, the double pulse sequence is generated by direct modulation, and scattered light between front and rear pulses of the double pulse sequence is utilized for interference, so that a two-path interferometer structure and a coherent detection structure are not needed, a carrier is not needed, and the positioning and the phase demodulation can be performed by only two photoelectric detectors, thereby simplifying the system configuration and reducing the cost. The demodulation method is simple, complex calculation is not needed, and the demodulation speed and the demodulation precision are further improved.
In an alternative embodiment, the Sagnac interferometer structure is used for collecting signals, so that the signals are insensitive to environmental disturbance, the stability of the system is improved, and the signal to noise ratio is improved.
In an alternative embodiment, the phase demodulation algorithm in the invention can demodulate the two paths of light intensity signals without generating 120 DEG phase difference. The method solves the technical problems that the prior art can not simplify the sensing device and reduce the complexity of the demodulation method, and can ensure enough sensing distance and demodulation precision.
The foregoing is merely a preferred embodiment of the present invention and it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present invention, which are intended to be comprehended within the scope of the present invention.
Claims (9)
1. The distributed optical fiber sensing method based on the double pulse modulation is characterized by being applied to a distributed optical fiber sensing system based on the double pulse modulation, wherein the distributed optical fiber sensing system based on the double pulse modulation comprises an acousto-optic modulator (2), a first erbium-doped optical fiber amplifier (31), a second erbium-doped optical fiber amplifier (32), a circulator (5), a sensing optical fiber (6) and a Sagnac interferometer (8); the Sagnac interferometer (8) comprises: a short optical fiber (81) and a 3 x 3 coupler (82); the acousto-optic modulator (2) is connected with the laser (1), 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 scattered light generated in the sensing optical fiber (6) by front and back pulses of the first high-power detection light interfere 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 a port of the circulator (5) connected with the second erbium-doped optical fiber amplifier (31), and the 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), is respectively transmitted anticlockwise along d to f ports of the 3X 3 coupler (82) and is transmitted clockwise along f to d ports, and finally is output from b and c ports of the 3X 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;
the distributed optical fiber sensing method based on double pulse modulation comprises the following steps:
obtaining continuous light;
modulating the continuous light into a periodically-changing double-pulse sequence through an acousto-optic modulator (2) for amplification treatment to obtain first high-power detection light;
the first high-power detection light enters 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 the backward Rayleigh scattering light which interfere with each other, and the backward Rayleigh light which interferes with each other carries disturbance information of the sensing optical fiber;
amplifying and coupling the backward Rayleigh scattered light to obtain second high-power detection light;
obtaining disturbance information on the sensing optical fiber through photoelectric conversion by using the second high-power detection light;
in the step of amplifying and coupling the backward Rayleigh scattered light, the second high-power detection light is coupled through a 3×3 coupler of a Sagnac interferometer (8).
2. The method of claim 1, wherein the step of obtaining disturbance information on the sensing fiber via photoelectric conversion using the second high power probe light comprises:
and transmitting the coupled second high-power detection light to a photoelectric detector (9) for photoelectric conversion, and collecting data through a data collecting card (10).
3. The method according to claim 2, wherein the step of transmitting the coupled second high power probe light to a photodetector (9) for photoelectric conversion and for 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 and summing the intensity signal values to obtain a summation value;
and performing differential cross summation operation according to the summation value.
4. A method according to claim 3, wherein after said differential cross-summing operation from 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 according to claim 1, wherein in the step of coupling the second high power probe light through a 3 x 3 coupler of a Sagnac interferometer (8), the second high power probe light enters the Sagnac interferometer (8) and is transmitted counter-clockwise along d-to-f ports and f-to-d ports of the 3 x 3 coupler (82) of the Sagnac interferometer (8), respectively, and is finally output from b-to-c ports of the 3 x 3 coupler (82).
6. A distributed optical fiber sensing system based on double pulse modulation, comprising:
the device comprises an acousto-optic modulator (2), a first erbium-doped optical fiber amplifier (31), a second erbium-doped optical fiber amplifier (32), a circulator (5), a sensing optical fiber (6) and a Sagnac interferometer (8);
the Sagnac interferometer (8) comprises: a short optical fiber (81) and a 3 x 3 coupler (82);
the acousto-optic modulator (2) is connected with the laser (1), 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 scattered light generated in the sensing optical fiber (6) by front and back pulses of the first high-power detection light interfere 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 a port of the circulator (5) connected with the second erbium-doped optical fiber amplifier (31), and the 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), is respectively transmitted anticlockwise along d to f ports of the 3X 3 coupler (82) and is transmitted clockwise along f to d ports, and finally is output from b and c ports of the 3X 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.
7. The distributed optical fiber sensing system based on double pulse modulation according to claim 6, further comprising a photodetector (9) connected to the Sagnac interferometer (8), a data acquisition card (10);
the detection light containing disturbance information is transmitted to a photoelectric detector (9) for photoelectric conversion; after data acquisition is carried out by the data acquisition card (10), two light intensity values in the time domain can be obtained, and the light intensity values are demodulated by a demodulation algorithm to obtain disturbance information on the sensing optical fiber.
8. The distributed optical fiber sensing system based on double pulse modulation according to claim 7, wherein after the first erbium doped fiber amplifier (31), the circulator (5) is preceded by a first filter (41), and after the second erbium doped fiber amplifier (32), the Sagnac interferometer (8) is preceded by a second filter (42).
9. The distributed optical fiber sensing system based on double pulse modulation according to claim 8, wherein an optical isolator (7) is further provided between the second filter (42) and the Sagnac interferometer (8).
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