CN112697180B - Fusion type distributed optical fiber sensing system and method for simultaneously measuring temperature and vibration - Google Patents
Fusion type distributed optical fiber sensing system and method for simultaneously measuring temperature and vibration Download PDFInfo
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- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
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- G01D5/35354—Sensor working in reflection
- G01D5/35358—Sensor working in reflection using backscattering to detect the measured quantity
- G01D5/35364—Sensor working in reflection using backscattering to detect the measured quantity using inelastic backscattering to detect the measured quantity, e.g. using Brillouin or Raman backscattering
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- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
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- G01D5/35354—Sensor working in reflection
- G01D5/35358—Sensor working in reflection using backscattering to detect the measured quantity
- G01D5/35361—Sensor working in reflection using backscattering to detect the measured quantity using elastic backscattering to detect the measured quantity, e.g. using Rayleigh backscattering
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Abstract
The invention discloses a fusion type distributed optical fiber sensing system and method for simultaneously measuring temperature and vibration. The invention also discloses a method for realizing the fusion type distributed optical fiber sensing system for simultaneously measuring the temperature and the vibration. The invention realizes the simultaneous acquisition of long-distance and distributed dynamic and static information, and effectively avoids the possible false alarm and missing report in engineering application.
Description
Technical Field
The invention belongs to the field of distributed optical fiber sensing, and particularly relates to a fusion type distributed optical fiber sensing system and method for simultaneously measuring temperature and vibration.
Background
The optical fiber sensing technology developed in the 70's of the 20 th century, and is a sensing technology which uses optical fiber as a medium and light as a carrier to sense and transmit external signals. When light waves are transmitted in the optical fibers, various characteristics of the transmitted light waves are changed correspondingly due to changes of external environment factors, and information of the external environment of the optical fibers can be obtained by monitoring changes of light wave parameters, so that sensing is realized. The distributed optical fiber sensing technology is more unique, fully exerts the characteristic of continuous optical fiber space distribution, can realize full-distributed measurement of temperature, strain, vibration and sound waves which are hundreds of kilometers long, has irreplaceable advantages, plays an important role in monitoring in the fields of petroleum, traffic, structures, geology and the like in recent years, and gets more and more attention.
Based on different light scattering mechanisms and nonlinear effects in optical fibers, a plurality of different distributed optical fiber sensing technologies are developed, but any distributed optical fiber sensing technology can only measure limited parameters. However, in actual monitoring, an external event is often caused by the combined action of multiple factors such as temperature, vibration, strain and the like, if a single parameter is used for monitoring the event, only one factor can be reflected, high false alarm rate and high false missing report rate are often caused, the limitation is large, and the problem of cross sensitivity often exists among multiple parameters. The method not only is difficult to realize the alarm of potential accidents, but also brings difficulty to the engineering application and popularization. Therefore, the comprehensive perception and analysis of different factors of the event can realize more accurate and effective identification, thereby performing system evaluation on the monitored unit in both dynamic and static aspects.
The existing fusion type distributed optical fiber sensing system includes: brillouin Optical Time Domain Reflectometer (BOTDR) and Raman Optical Time Domain Reflectometer (ROTDR) fusion system[1]The device is mainly proposed by the M.N. Alahbabi subject group of the university of south Anpnton, and realizes the simultaneous measurement of temperature and strain; fusion system of phase-sensitive optical time domain reflectometer (phi-OTDR) and polarization type optical time domain reflectometer (POTDR)[2]The method is mainly provided by the research group of the Rongyunjiang of the university of electronic technology, and realizes the simultaneous acquisition of polarization and phase information; and POTDR and BOTDR fusion system[3]The simultaneous measurement of strain and vibration was achieved, mainly proposed by the zhang xu apple topic group of Nanjing university. However, different distributed optical fiber sensing systems are often based on different light scattering mechanisms, and the sensing principles are different and have different system structures. How to correctly fuse a plurality of independent systems together to realize the measurement of a plurality of parameters and avoid the mutual interference of the independent systems under the condition of multiplexing more devices as much as possible, thereby reducing the cost while ensuring the performance of the fused system, which is a difficult problem to be solved at present.
Disclosure of Invention
The purpose of the invention is as follows: in order to solve the technical problems of the background art, the invention provides a fusion type distributed optical fiber sensing system and a method capable of simultaneously measuring temperature and vibration.
The technical scheme is as follows: in order to realize the purpose of the invention, the technical scheme adopted by the invention is as follows: a fusion type distributed optical fiber sensing system capable of simultaneously measuring temperature and vibration comprises a laser, a coupler, a circulator, a sensing optical fiber, a pulse modulation module, a heterodyne detection module and a data acquisition card, as shown in figure 1. The laser generates continuous mode ultra-narrow linewidth laser, the laser is divided into two paths through a coupler, one path of laser is output to a pulse modulation module, and the other path of laser is output to a heterodyne detection module as local oscillation light.
The pulse modulation module modulates input continuous laser into detection pulse light, the detection pulse light enters the sensing optical fiber through the circulator, the sensing optical fiber generates backward Rayleigh scattering light and backward Brillouin scattering light along the line, and the detection pulse light is input through the second port of the circulator and output to the heterodyne detection module from the third port.
Further, the heterodyne detection module receives the local oscillator light, the backward rayleigh scattering light and the brillouin scattering light at the same time, generates a coherent signal through frequency mixing, and receives the coherent signal by the data acquisition card.
A method for implementing simultaneous measurement of temperature and vibration, schematically shown in fig. 2, comprising the steps of:
step one, for the heterodyne detection module, dividing local oscillation light into two parts through a first coupler, and outputting the two parts to a third coupler and a fourth coupler respectively;
step two, the scattered light is divided into two parts by a second coupler and respectively output to a third coupler and a fourth coupler;
thirdly, mixing the local oscillator light and the back Brillouin scattering light in a third coupler, and inputting the generated coherent Brillouin scattering light to a first photoelectric detector; mixing the other path of local oscillator light and the backward Rayleigh scattering light in a fourth coupler, and inputting the generated coherent Rayleigh scattering light to a second photoelectric detector; sending the electric signals output by the first photoelectric detector and the second photoelectric detector into the data acquisition card for AD conversion;
fourthly, carrying out phase demodulation on the collected coherent Rayleigh scattering signals by utilizing an IQ demodulation technology to obtain initial phase signals;
fifthly, performing phase unwrapping on the phase signal through a phase unwrapping algorithm to obtain a phase unwrapped signal;
sixthly, carrying out Fourier transform on the phase unwrapping signals corresponding to each position of the optical fiber to obtain amplitude and frequency information of vibration of each position;
processing the coherent Brillouin scattering signals by using a Lorentz fitting algorithm to obtain Brillouin frequency shift of each position of the optical fiber;
step eight, calculating the variation of the external temperature according to the variation of the Brillouin frequency shift:
△νb＝νB-νB0＝CT*△T
wherein, DeltavbIs the variation of Brillouin frequency shift, vBV and vB0Brillouin frequency shift, C, before and after the external temperature change along the optical fiberTIs the temperature-frequency shift coefficient (about 1 MHz/DEG C) and DeltaT is the amount of change in temperature.
Preferably, the coupling ratio of the first coupler is 90:10, 90% of the light is output to the third coupler as the local oscillator light for brillouin heterodyne detection, and 10% of the light is output to the fourth coupler as the local oscillator light for rayleigh heterodyne detection, so that the threshold values of the first photodetector and the second photodetector are just reached. It has to be noted that the choice of the coupling ratio depends on the threshold value of the photo detector actually used.
Preferably, the coupling ratio of the second coupler is 90:10, 90% of the scattered light is output to the third coupler as the brillouin heterodyne detection, and 10% of the scattered light is output to the fourth coupler as the rayleigh heterodyne detection. It has to be noted that the choice of the coupling ratio depends on the actual rayleigh scattered light, the optical intensity of the brillouin scattered light and the signal-to-noise ratio of the system.
Preferably, the coupling ratio of the third coupler and the fourth coupler are both 50:50, so as to optimize the result of the mixing.
The beneficial technical effects are as follows: after the technical scheme is adopted, the invention has the following beneficial technical effects:
1. the system integrates the traditional BOTDR and phi-OTDR systems, solves the problem that a single sensing system is single in measuring parameter, realizes the simultaneous demodulation of two parameters of temperature and vibration, can simultaneously acquire long-distance and distributed dynamic and static information, and effectively avoids the false alarm and the missing alarm which possibly exist in engineering application. The same pulse modulation module is multiplexed on the system structure, and only the heterodyne detection module is optimized, so that the system complexity and the cost are reduced;
2. the method realizes effective separation of the back Rayleigh scattering signal and the back Brillouin scattering signal through a double heterodyne detection structure, simultaneously guarantees the signal-to-noise ratio of the two signals, and simultaneously avoids mutual influence between the two signals, thereby implementing independent and simultaneous signal demodulation.
Drawings
FIG. 1 is a schematic diagram of a solution system of the present invention;
FIG. 2 is a schematic view of a process according to the present invention;
FIG. 3 is a diagram of a practical system architecture of the present invention;
FIG. 4 is a diagram of a distribution of sensing fibers used in the experiments of the present invention;
FIG. 5 is a Brillouin frequency shift profile measured in accordance with the present invention;
FIG. 6 is a graph of the temperature change measured according to the present invention;
FIG. 7 is a graph of the phase unwrapped signal measured in accordance with the present invention;
FIG. 8 is a measured vibration signal spectrum of the present invention.
Detailed Description
The technical scheme of the invention is further explained by combining the attached drawings.
FIG. 3 is a diagram of a system structure according to the present invention, which includes a light source, a first coupler, a second coupler, a third coupler, a fourth coupler, a fifth coupler, an electro-optic modulator, an optical fiber amplifier, a circulator, a sensing optical fiber, a polarization scrambler, an acousto-optic modulator, a first detector, a second detector, a mixer, a microwave source, a low noise amplifier, a band pass filter, and a data acquisition card; wherein the content of the first and second substances,
a light source for generating a continuous narrow linewidth laser to the first coupler;
the first coupler is used for dividing the narrow-linewidth laser into two paths, wherein one path is used as initial detection light to be output to the electro-optical modulator, and the other path is used as initial local oscillation light to be output to the second coupler;
the electro-optical modulator is used for modulating the continuous narrow linewidth laser into detection pulse light and outputting the detection pulse light to the optical fiber amplifier;
the optical fiber amplifier is used for amplifying the power of the detection pulse light and outputting the detection pulse light to the first port of the circulator;
the circulator is used for inputting the detection pulse light from the first port, injecting the detection pulse light into the sensing optical fiber from the second port, and outputting the back scattering light to the third coupler from the third port;
the second coupler is used for dividing the initial local oscillator light into two paths: one path of the local oscillator light is output to a polarization scrambler as the local oscillator light participating in the backward Brillouin scattering heterodyne detection, and the other path of the local oscillator light is output to an acousto-optic modulator as the local oscillator light participating in the backward Rayleigh scattering heterodyne detection;
the third coupler is used for dividing the back scattering light output by the third port of the circulator into two paths: one path of the light is output to the fourth coupler as the Brillouin scattering light of heterodyne detection, and the other path of the light is output to the fifth coupler as the Rayleigh scattering light of heterodyne detection;
the polarization scrambler is used for scrambling the polarization state of the local oscillator light and outputting the polarization state to the fourth coupler;
the acousto-optic modulator is used for introducing frequency shift to the local oscillator light and outputting the frequency shift to the fifth coupler;
the fourth coupler is used for mixing the local oscillator output by the polarization scrambler with the back brillouin scattering light output by the first path of the third coupler and then outputting coherent brillouin scattering light to the first detector;
the fifth coupler is used for mixing the local oscillator light output by the acousto-optic modulator with the backward Rayleigh scattering light output by the second path of the third coupler and then outputting the mixed light to the second detector;
the first detector is used for converting the coherent Brillouin scattering light signal into an electric signal and outputting the electric signal to the mixer;
the second detector is used for converting the coherent Rayleigh scattered light signals into electric signals and outputting the electric signals to the data acquisition card;
the microwave source is used for generating a microwave signal with a specific scanning frequency and outputting the microwave signal to the mixer;
the mixer is used for mixing the frequency sweeping signal generated by the microwave source with the coherent Brillouin scattering electric signal and outputting the frequency sweeping signal and the coherent Brillouin scattering electric signal to the low-noise amplifier;
the low-noise amplifier is used for amplifying the power of the electric signal output by the frequency mixer and outputting the electric signal to the band-pass filter;
the band-pass filter is used for intercepting signals of a specific frequency band, filtering other parts and outputting the signals to the data acquisition card;
and the data acquisition card is used for simultaneously receiving the coherent Rayleigh scattering electric signal and the processed coherent Brillouin scattering electric signal and carrying out subsequent data processing.
Based on the experimental system, in this embodiment, the implementation steps are as follows:
step one, a light source generates continuous narrow-linewidth laser with the wavelength of 1550nm and the output power of 40mW, and the laser is divided into two paths through a first coupler (1 multiplied by 2 coupler): outputting 90% of the one path as initial detection light to an electro-optical modulator, and outputting 10% of the one path as initial local oscillation light to a second coupler;
step two, setting an external modulation signal for driving, and modulating continuous narrow linewidth laser into pulse light with the pulse width of 200ns and the repetition frequency of 1.8kHz by an electro-optical modulator, wherein the extinction ratio is 30 dB;
thirdly, the pulsed light output by the electro-optical modulator is further amplified by an optical fiber amplifier, and enters a sensing optical fiber with a length of 49.5km through a first port of the circulator after being amplified to a peak power of 22dB (as shown in fig. 4, the sensing optical fiber consists of five parts, wherein a sinusoidal vibration signal of 900Hz is applied to a 10m long optical fiber in the range of the tail end of the optical fiber, and a 20m long optical fiber is heated in a water bath at 20 ℃), and generates back scattered light to return to a second port of the circulator;
step four, dividing the initial local oscillation light output by the first coupler into two paths through a second coupler (1 multiplied by 2 coupler): outputting 90% of the one path of local oscillator light participating in back Brillouin scattering heterodyne detection to a polarization scrambler, and outputting 10% of the one path of local oscillator light participating in back Rayleigh scattering heterodyne detection to an acousto-optic modulator;
the polarization state of the input local oscillation light is disturbed by the polarization scrambler, the influence of polarization-related noise on temperature measurement is reduced, and 50% of the polarization-related noise is output to a fourth coupler (2 multiplied by 1 coupler);
introducing a frequency shift of 200MHz to the input local oscillator light by the acousto-optic modulator, and outputting to 50% of the path of the fifth coupler (2 multiplied by 2 coupler);
seventhly, dividing the back scattering light output by the third port of the circulator into two paths through a third coupler (1 multiplied by 2 coupler): mixing 90% of the path of light with a local oscillator light output by the polarization scrambler in a fourth coupler, and receiving the generated coherent Brillouin scattering light in the range from 10.6GHz to 11.0GHz by a first detector with the bandwidth of 12GHz and converting the coherent Brillouin scattering light into an electric signal; mixing 10% of the path of light with a local oscillator light output by the acousto-optic modulator in a fifth coupler, receiving the generated 200MHz coherent Rayleigh scattering light by a second detector with the bandwidth of 300MHz, and converting the coherent Rayleigh scattering light into an electric signal;
step seven, adjusting a microwave source to generate a microwave signal of 10.6GHz to 11.0GHz, enabling the frequency sweep to be separated by 5MHz, enabling the microwave signal and a Brillouin scattering electric signal output by a first detector to jointly enter a mixer to be mixed, enabling a generated intermediate frequency signal to be amplified with 30dB power through a low noise amplifier, filtering through a band-pass filter (3dB bandwidth 10MHz), outputting the intermediate frequency signal to a data acquisition card (sampling rate 1GSa/s), performing continuous 30000-time acquisition in the data acquisition card, accumulating the acquired intermediate frequency signal first, and then calculating an average value of the acquired intermediate frequency signal, so as to improve the signal-to-noise ratio, and finally obtaining original data corresponding to the Brillouin scattering signal;
step eight, the coherent Raman scattering signals output by the second detector are also sent into the data acquisition card to obtain original data corresponding to the Raman scattering signals;
and step nine, obtaining Brillouin frequency shift of each position corresponding to the optical fiber line by using a Lorentz fitting algorithm on the original data obtained in the step seven, as shown in FIG. 5. Enlarging the position in the dashed box of fig. 5, as shown in fig. 6, a 20MHz frequency shift change of 20m length from 49.12km to 49.14km is observed, and the temperature change of 20 degrees celsius is matched with the actual situation according to the temperature-frequency shift coefficient;
tenth, performing IQ demodulation on the original data obtained in the eighth step to obtain an initial phase signal, and performing phase unwrapping on the initial phase signal through a phase unwrapping algorithm to obtain phase unwrapped signals of each position of the optical fiber, as shown in fig. 7; then, the phase unwrapped signal is subjected to Fourier transform to obtain a vibration signal frequency spectrum in fig. 8, and a 900Hz measurement result is consistent with the actual situation.
According to the steps, the technical scheme provided by the invention is utilized to realize the simultaneous measurement of the temperature and the vibration signal.
While the foregoing is directed to embodiments of the present invention and test results, other and further modifications may be devised by those skilled in the art without departing from the principles of the invention and the scope thereof is determined by the appended claims.
Claims (5)
1. A method for achieving simultaneous measurement of temperature and vibration, the method comprising the steps of:
step one, dividing laser into two paths through a first coupler: one path is used as initial detection light and output to the electro-optical modulator, and the other path is used as initial local oscillation light and output to the second coupler;
step two, by setting external modulation signal drive, the electro-optic modulator modulates laser and then realizes power amplification by the optical fiber amplifier, and the amplified laser enters the sensing optical fiber through the first port of the circulator and generates back scattered light to return to the second port of the circulator;
step three, dividing the initial local oscillation light output by the first coupler into two paths through a second coupler: one path of local oscillator light is output to a polarization scrambler as local oscillator light participating in backward Brillouin scattering heterodyne detection, and the other path of local oscillator light is output to an acousto-optic modulator as local oscillator light participating in backward Rayleigh scattering heterodyne detection;
disturbing the polarization state of the input local oscillator light by the polarization scrambler, reducing the influence of polarization-related noise on temperature measurement, and outputting to 50% of the first path of the fourth coupler;
introducing a preset frequency shift to the input local oscillation light path by the acousto-optic modulator, and outputting the preset frequency shift to 50% of the path of the fifth coupler;
seventhly, dividing the back scattering light output by the third port of the circulator into two paths through a third coupler: one path of local oscillation light output by the polarization scrambler is mixed in the fourth coupler, and the generated coherent Brillouin scattering light is received by the first detector and converted into an electric signal; one path of local oscillation light output by the acousto-optic modulator is mixed in a fifth coupler, and the generated coherent Rayleigh scattering light is received by the second detector and converted into an electric signal;
step seven, adjusting a microwave source to generate a microwave signal, entering the microwave signal and the Brillouin scattering electric signal output by the first detector into a mixer together to generate a medium-frequency signal, realizing power amplification by a low-noise amplifier, filtering by a band-pass filter, and outputting the signal to a data acquisition card to obtain original data corresponding to the Brillouin scattering signal;
step eight, the coherent Raman scattering signals output by the second detector are also sent into the data acquisition card to obtain original data corresponding to the Raman scattering signals;
step nine, accumulating and averaging the original data obtained in the step seven, then obtaining Brillouin frequency shift of each position corresponding to the optical fiber line by using a Lorentz fitting algorithm, and calculating the variation of the external temperature according to the variation of the Brillouin frequency shift;
step ten, performing IQ demodulation on the original data obtained in the step eight to obtain an initial phase signal, and performing phase unwrapping on the initial phase signal through a phase unwrapping algorithm to obtain phase unwrapped signals of each position of the optical fiber; and then Fourier transform is carried out on the phase expansion signal to obtain the amplitude and frequency information of the vibration of each position.
2. The method for realizing simultaneous measurement of temperature and vibration according to claim 1, wherein the calculation method of the external stable variation in the ninth step is as follows:
△νb＝νB-νB0＝CT*△T
wherein, DeltavbIs the variation of Brillouin frequency shift, vBV and vB0Brillouin frequency shift, C, before and after the external temperature change along the optical fiberTIs the temperature-frequency shift coefficient, and Δ T is the amount of change in temperature.
3. The method for realizing simultaneous measurement of temperature and vibration according to claim 1 or 2, wherein the coupling ratio of the first coupler is 90:10, 90% of the first coupler is output as the local oscillator light for brillouin heterodyne detection, and 10% of the first coupler is output as the local oscillator light for rayleigh heterodyne detection to the fourth coupler.
4. The method for realizing simultaneous measurement of temperature and vibration according to claim 1 or 2, wherein the coupling ratio of the second coupler is 90:10, 90% of the scattered light is output to the third coupler as the light detected by Brillouin heterodyne, and 10% of the scattered light is output to the fourth coupler as the light detected by Rayleigh heterodyne.
5. A method of achieving simultaneous temperature and vibration measurement according to claim 1 or 2, wherein the coupling ratio of the third coupler and the fourth coupler are both 50: 50.
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