CN105806465A - Novel [phi]-OTDR detection device based on fixed reflection points and detection method thereof - Google Patents
Novel [phi]-OTDR detection device based on fixed reflection points and detection method thereof Download PDFInfo
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- G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
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Abstract
The invention discloses a novel [phi]-OTDR detection device based on fixed reflection points. The novel [phi]-OTDR detection device comprises a time delay generator, a laser, a pulse modulator, an erbium-doped optical fiber amplifier, an optical circulator, an optical fiber detection unit, a first coupler, a time delay optical fiber, a polarization controller, a second coupler, an avalanche photodetector and a signal collection card. The optical fiber detection unit comprises a plurality of optical fibers which are connected in sequence, and the fixed reflection points are respectively formed at the connection parts on two ends of each optical fiber. The invention further discloses a detection method of the novel [phi]-OTDR detection device based on the fixed reflection points. According to the invention, the vibration around the optical fiber detection unit is measured by the optical fiber detection unit, and the vibration position, the frequency and the amplitude are monitored in real time by means of three-port phase demodulation through a look-up table method; in addition, the optical fiber is divided into a plurality of segments by the optical fiber detection unit, the interference of the vibration event on one position to the whole optical fiber is avoided, and the system of the structure is easy to dismount and maintain.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a novel phi-OTDR detection device based on a fixed reflection point and a detection method thereof.
Background
The phase-sensitive optical time domain reflectometer (phi-OTDR) vibration measurement is rapidly developed in the past more than ten years, and has important application value in security monitoring of important places and health monitoring of major engineering structures. For example, for security monitoring of prison fences or border lines, and bridges, roads and buildings will inevitably be damaged as the service life increases, distributed vibration measuring systems may be installed in security areas or on the surface of buildings for monitoring vibrations caused by special events, so as to facilitate real-time monitoring or early discovery of potential hazards that may cause building accidents.
The phi-OTDR can detect the position of external disturbance and frequency domain information on the sensing fiber through the coherent fading phenomenon of backward Rayleigh scattering light in the fiber. As with other optical time domain reflectometers, Φ -OTDR is also a single-ended measurement sensing system, but in order to obtain a usable time domain profile of the backward rayleigh scattered light, a narrow linewidth, frequency stabilized laser is required. A narrow-band short pulse is launched into the sensing fiber and the waveform of the returning rayleigh scattering is modulated into a sawtooth shape due to the coherent action of different scattering centers. By analyzing the change of the sawtooth-shaped Rayleigh waveform, the refractive index change caused by external disturbance in the sensing optical fiber can be monitored, and the weak refractive index change can be enhanced by the coherent effect among scattering elements.
The phi-OTDR has two obvious advantages of high response speed and high sensitivity. Compared with Brillouin scattering light and Raman scattering light measurement vibration, Rayleigh scattering light has stronger scattering light power, so that signals detected by a photoelectric detector are stronger, multiple accumulation averaging is not needed, the response speed of the phi-OTDR is high, the phi-OTDR can be used for detecting rapidly-changing dynamic disturbance such as vibration signals, and time domain and frequency domain information of the vibration signals can be obtained through a proper data processing method. In the phi-OTDR, external disturbance information is obtained by sensing the Rayleigh coherent fading phenomenon of backward Rayleigh scattered light in an optical fiber, and the Rayleigh coherent fading is the phase of light in the optical fiber, so that as long as the external disturbance influences the phase of detection light in the optical fiber, the phi-OTDR can detect external disturbance events, and the phi-OTDR has high sensitivity and can detect tiny external disturbance.
However, the currently used Φ -OTDR also has an obvious disadvantage, and most of them can only detect the occurrence position of strain and extract the frequency domain information, and cannot quantitatively measure the strain value. The main reason is that the optical signal acquired by the receiving end and the strain value loaded on the optical fiber are not in a definite one-to-one correspondence relationship, so that the strain value loaded on the optical fiber cannot be calculated through a demodulation algorithm. The detection of amplitude can be realized by performing phase demodulation on A discrete parametric fiber sensor based phase-OTDR by A Masoudi, M Belal and the like of the university of south Ampton in England by using a 3X 3 coupler and a delay optical fiber, but a position vibration event is easy to cause interference on the whole section of optical fiber, and the optical fiber is complex to disassemble, assemble and repair if the optical fiber is damaged; in advanced phi-OTDR Sensing System for Distributed structured measurement based on Ultra-weak Fiber Bragg Grating reflection signals, the Zhufan and Zhang YiXin of the research center of optical communication engineering of Nanjing university use Bragg Grating reflection signals to perform phase demodulation, but the laser needs to be subjected to frequency sweep control, the phase demodulation algorithm is relatively complex, and meanwhile, the space resolution is lower due to the adoption of wide pulses; while Zygui, Zhang Xue and so on in the patent of phase-sensitive optical time domain reflection device and method based on fiber Bragg grating array (patent number: CN201510389252.2), the method in the Jufan and Zygui treatise is improved, high spatial resolution can be realized on the basis of quantitative analysis, but in order to realize high spatial resolution, the used grating is dense, so that the detection cost is increased, the complex process of repairing after installation and damage is complex, the requirement on operators is high, and the popularization and application in engineering are difficult.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a novel phi-OTDR detection device based on a fixed reflection point and a detection method thereof.
The invention adopts the following technical scheme for solving the technical problems:
the invention provides a novel phi-OTDR detection device based on a fixed reflection point, which comprises a time delay generator, a laser, a pulse modulator, an erbium-doped fiber amplifier, an optical circulator, an optical fiber detection unit, a first coupler, a time delay fiber, a polarization controller, a second coupler, an avalanche photodetector and a signal acquisition card; the optical fiber detection unit comprises a plurality of optical fibers which are connected in sequence, and fixed reflection points are formed at the joints of the two ends of each optical fiber; wherein,
the time delay generator is used for generating modulation pulses and trigger pulses; modulating pulse is input to a pulse modulator, and triggering pulse is input to a signal acquisition card;
the laser is used for generating continuous mode narrow linewidth laser and outputting the laser to the pulse modulator;
the pulse modulator is used for converting the continuous mode narrow linewidth laser into pulse light according to the received modulation pulse and outputting the pulse light to the erbium-doped fiber amplifier;
the erbium-doped optical fiber amplifier is used for amplifying the pulse light into detection light and outputting the detection light to the optical circulator;
the optical circulator is used for inputting the detection light from the 1 st port and injecting the detection light into the optical fiber detection unit from the 2 nd port;
the optical fiber detection unit is used for outputting the generated back Rayleigh scattered light and the reflected light to the 2 nd port of the optical circulator and outputting the back Rayleigh scattered light and the reflected light to the first coupler from the 3 rd port of the optical circulator;
a first coupler for splitting the reflected light and the back rayleigh scattered light into two paths: the first path of light is output to the delay optical fiber, and the second path of light is output to the polarization controller;
the delay optical fiber is used for delaying the first path of light and outputting the delayed first path of light to the second coupler;
the polarization controller is used for controlling the polarization state of the second path of light to enable the polarization state of the second path of light to be matched with the polarization of the first path of light, and the polarized second path of light is output to the second coupler;
the second coupler is used for carrying out coherent beat frequency on the delayed first path of light and the polarized second path of light, so that phase difference is generated between the three paths of output light, and the three paths of output light are output to the avalanche photodetector;
the avalanche photodetector is used for converting the three paths of light into electric signals and outputting the electric signals to the signal acquisition card;
and the signal acquisition card is used for converting the electric signal into a digital signal for subsequent processing according to the trigger pulse.
As a further optimization scheme of the novel phi-OTDR detection device based on the fixed reflection point, the periods of the modulation pulse and the trigger pulse are synchronous with the period of the detection light.
As a further optimization scheme of the novel phi-OTDR detection device based on the fixed reflection point, the optical fiber detection unit is a plurality of optical fiber jumpers connected in sequence.
As a further optimization scheme of the novel phi-OTDR detection device based on the fixed reflection point, the first path of light and the delayed first path of light are different by the time of one reflection interval length.
As a further optimization scheme of the novel phi-OTDR detection device based on the fixed reflection point, the first coupler is a 50:50 coupler.
As a further optimization scheme of the novel phi-OTDR detection device based on the fixed reflection point, the second coupler is a 3 multiplied by 3 coupler, and the phase difference of three paths of light is 120 degrees.
A detection method based on a novel phi-OTDR detection device based on a fixed reflection point comprises the following steps,
the method comprises the following steps that firstly, a laser is adopted to generate continuous mode narrow linewidth laser, the continuous mode narrow linewidth laser is converted into pulse light, the pulse light is amplified into probe light and then output to a plurality of optical fibers which are connected in sequence, and fixed reflection points are formed at the joints of the two ends of each optical fiber;
secondly, the detection light generates backward Rayleigh scattering light in the optical fiber and generates reflected light at the reflection points at the connection positions of the two ends of the optical fiber; the back Rayleigh scattering light is used for detecting the vibration position, and the reflected light is used for detecting the vibration frequency and the vibration amplitude;
step three, dividing the reflected light and the back Rayleigh scattered light into two paths: a first path of light and a second path of light;
step four, delaying the first path of light, wherein the delayed first path of light and the first path of light are different by a reflection interval length;
fifthly, controlling the polarization state of the second path of light to enable the polarization state of the second path of light to be matched with the polarization of the first path of light;
step six, carrying out coherent beat frequency on the delayed first path of light and the polarized second path of light, and outputting three paths of light, wherein phase differences are generated among the three paths of light;
step seven, demodulating phase information according to the intensity of the three paths of light, thereby realizing real-time monitoring on the position, frequency and amplitude of vibration; the demodulation method adopts a lookup table, the lookup table is obtained by pre-calculation, the phase changes within (-pi, pi) at certain intervals, the corresponding intensity value of the three paths of light is calculated to form a table, and the corresponding phase information is obtained by lookup through the lookup table according to the actually detected intensity of the three paths of light.
Compared with the prior art, the invention adopting the technical scheme has the following technical effects:
(1) the invention measures the surrounding vibration by adopting the optical fiber detection unit, and can realize the real-time monitoring of the position, the frequency and the amplitude of the vibration by carrying out three-port phase demodulation by a table look-up method;
(2) the optical fiber is divided into a plurality of sections by using the optical fiber detection unit, so that the interference of a position vibration event on the whole section of the optical fiber is avoided, and meanwhile, the system with the structure is easy to disassemble, assemble and maintain.
Drawings
FIG. 1 is a system block diagram of the present invention.
Fig. 2 is a schematic diagram of coherent signal measurement.
Fig. 3 is a schematic diagram of a look-up table demodulation.
Detailed Description
The technical scheme of the invention is further explained in detail by combining the attached drawings:
fig. 1 shows a system structure diagram of the present invention, a novel fixed reflection point based Φ -OTDR detection apparatus, which includes a delay generator, a laser, a pulse modulator, an erbium-doped fiber amplifier, an optical circulator, an optical fiber detection unit, a first coupler, a delay fiber, a polarization controller, a second coupler, an avalanche photodetector and a signal acquisition card; the optical fiber detection unit comprises a plurality of optical fibers which are connected in sequence, and fixed reflection points are formed at the joints of the two ends of each optical fiber; wherein,
the time delay generator is used for generating modulation pulses and trigger pulses; modulating pulse is input to a pulse modulator, and triggering pulse is input to a signal acquisition card;
the laser is used for generating continuous mode narrow linewidth laser and outputting the laser to the pulse modulator;
the pulse modulator is used for converting the continuous mode narrow linewidth laser into pulse light according to the received modulation pulse and outputting the pulse light to the erbium-doped fiber amplifier;
the erbium-doped optical fiber amplifier is used for amplifying the pulse light into detection light and outputting the detection light to the optical circulator;
the optical circulator is used for inputting the detection light from the 1 st port and injecting the detection light into the optical fiber detection unit from the 2 nd port;
the optical fiber detection unit is used for outputting the generated back Rayleigh scattered light and the reflected light to the 2 nd port of the optical circulator and outputting the back Rayleigh scattered light and the reflected light to the first coupler from the 3 rd port of the optical circulator;
a first coupler for splitting the reflected light and the back rayleigh scattered light into two paths: the first path of light is output to the delay optical fiber, and the second path of light is output to the polarization controller;
the delay optical fiber is used for delaying the first path of light and outputting the delayed first path of light to the second coupler;
the polarization controller is used for controlling the polarization state of the second path of light to enable the polarization state of the second path of light to be matched with the polarization of the first path of light, and the polarized second path of light is output to the second coupler;
the second coupler is used for carrying out coherent beat frequency on the delayed first path of light and the polarized second path of light, so that phase difference is generated between the three paths of output light, and the three paths of output light are output to the avalanche photodetector;
the avalanche photodetector is used for converting the three paths of light into electric signals and outputting the electric signals to the signal acquisition card;
and the signal acquisition card is used for converting the electric signal into a digital signal for subsequent processing according to the trigger pulse.
The periods of the modulation pulse and the trigger pulse are synchronous with the period of the detection light.
Device performance in the laboratory: the type of the laser is RIO laser, the wavelength of the laser is 1550nm, the line width is 10kHz, and the output optical power is 13 dBm; the pulse modulator is an electro-optical modulator and can generate light pulses of 20ns at least; the EDFA selects a Zhongxing BA series amplifier, the center frequency is 1550nm, the working wavelength is 20nm, and the constant power gain can reach 17 dBm. The first coupler is a 50:50 coupler and the second coupler is a 3 x 3 coupler.
The specific experimental conditions are as follows: adopt the optic fibre wire jumper as detecting optic fibre in this scheme, the terminal surface of joint produces the reflection. The flange is arranged to be the tightest during installation and connection of the jumper, the tightness degree of the flange does not need to be manually adjusted during measurement to control the reflection of the end face of the optical fiber, and if one section of the optical fiber changes the length due to the influence of external vibration, the transmission of light in the adjacent jumper cannot be influenced. Through comparative analysis of different optical fiber joint combination schemes, the system adopts an FC/PC-FC/PC connection mode. In the connection mode, the reflection generated on the end face of the optical fiber is approximately-50 dBm, and the Rayleigh scattering light in the optical fiber is between-70 dBm and-60 dBm, so that the reflected light intensity can be obviously stronger than a Rayleigh scattering signal. The length of sensing fiber used in the experiment is L-5 km, and the required pulse period should be more than 50 μ s. And 5 sections of optical fiber jumpers are connected to the tail ends of the 5km optical fibers, the length of each section of optical fiber jumper is 20m, and the length of each delay optical fiber is 40 m.
The specific procedure for binding the experimental parameters was as follows:
the method comprises the following steps: the laser generates continuous mode narrow linewidth laser, converts the continuous mode narrow linewidth laser into pulse light, and the pulse width is 20 ns-1 us; then amplified by EDFA to be probe light, and enters the optical fiber detection unit after passing through the optical circulator.
Step two: as shown in fig. 2, the detection light is transmitted in the optical fiber detection unit, and is reflected back when encountering a reflection point to obtain a reflected original signal, the original signal is split by the first coupler, a first part of 50% of the light passes through the delay optical fiber to obtain a delayed signal, and the delayed signal and the original signal have a difference of a reflection interval length; and the other part of the signals is 50% of the signals which are kept in a polarization state by a polarization controller to be matched with the delay signals so as to inhibit polarization noise, and then the two paths of signals are coherent by a second coupler and output to three APDs to be converted into electric signals to be collected and processed by an acquisition card. If a vibration event occurs, the phases of the original signal and the delayed signal are changed, and the light intensity of the coherent signal is obviously changed.
Step three: the principle of three-port demodulation is that the signal to be demodulated is divided into three signals with phase difference of 2 pi/3, and the phase terms in the original signal are restored after the operations of summation, differentiation, integration and the like, namely, the phase terms are expressed by a formulaTo obtainThe process of (1). The following is a quantitative explanation of the principle of three-port demodulation:
due to the characteristics of the device, the phase difference between three paths of light obtained after reflected light of the system passes through the three-port coupler is respectively 2 pi/3, and the phase difference can be expressed as follows:
where D is the average of the reflected signal strength, E is the peak power compared to this average,is the optical phase that needs to be demodulated, k is 1,2, 3.
And summing the three signals, wherein the result of the addition of the alternating current terms is zero, and thus the direct current term D in the signals is obtained. Each signal is subtracted by 1/3 of the summation result to obtain a signal containing only alternating current terms:
differentiating the signal to obtainThe following operations were performed:and summing the resultsTo obtain
By passingIs easy to obtainFinally, the integral operation is carried out to obtainI.e. the phase of the signal.
Therefore, the method of directly performing three-port demodulation is complex in calculation, consumes much time, and cannot meet the requirement of real-time calculation. Therefore, a table look-up mode is adopted, that is, the optical phase values corresponding to the intensities measured by the three detectors are calculated in advance, and the measured intensity values need to be matched with the existing values in the actual measurement median values to find the corresponding phase values. As shown in FIG. 3, a three-dimensional space table is generated, the coordinate axis is the light intensity I obtained by three detectors, and the phase can be obtained by using the value obtained by the detectors as the coordinate index
The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all should be considered as belonging to the protection scope of the invention.
Claims (7)
1. A novel phi-OTDR detection device based on a fixed reflection point comprises a time delay generator, a laser, a pulse modulator, an erbium-doped fiber amplifier, an optical circulator, an optical fiber detection unit, a first coupler, a time delay fiber, a polarization controller, a second coupler, an avalanche photodetector and a signal acquisition card; the optical fiber detection unit is characterized by comprising a plurality of optical fibers which are sequentially connected, and fixed reflection points are formed at the joints of the two ends of each optical fiber; wherein,
the time delay generator is used for generating modulation pulses and trigger pulses; modulating pulse is input to a pulse modulator, and triggering pulse is input to a signal acquisition card;
the laser is used for generating continuous mode narrow linewidth laser and outputting the laser to the pulse modulator;
the pulse modulator is used for converting the continuous mode narrow linewidth laser into pulse light according to the received modulation pulse and outputting the pulse light to the erbium-doped fiber amplifier;
the erbium-doped optical fiber amplifier is used for amplifying the pulse light into detection light and outputting the detection light to the optical circulator;
the optical circulator is used for inputting the detection light from the 1 st port and injecting the detection light into the optical fiber detection unit from the 2 nd port;
the optical fiber detection unit is used for outputting the generated back Rayleigh scattered light and the reflected light to the 2 nd port of the optical circulator and outputting the back Rayleigh scattered light and the reflected light to the first coupler from the 3 rd port of the optical circulator;
a first coupler for splitting the reflected light and the back rayleigh scattered light into two paths: the first path of light is output to the delay optical fiber, and the second path of light is output to the polarization controller;
the delay optical fiber is used for delaying the first path of light and outputting the delayed first path of light to the second coupler;
the polarization controller is used for controlling the polarization state of the second path of light to enable the polarization state of the second path of light to be matched with the polarization of the first path of light, and the polarized second path of light is output to the second coupler;
the second coupler is used for carrying out coherent beat frequency on the delayed first path of light and the polarized second path of light, so that phase difference is generated between the three paths of output light, and the three paths of output light are output to the avalanche photodetector;
the avalanche photodetector is used for converting the three paths of light into electric signals and outputting the electric signals to the signal acquisition card;
and the signal acquisition card is used for converting the electric signal into a digital signal for subsequent processing according to the trigger pulse.
2. A novel fixed reflection point based Φ -OTDR probe according to claim 1, characterized by that the period of both said modulation pulse and trigger pulse is synchronized with the period of the probe light.
3. A novel fixed reflection point based Φ -OTDR detection device according to claim 1, characterized by that, the fiber detection unit is several fiber jumpers connected in sequence.
4. A novel fixed reflection point based Φ -OTDR probe according to claim 1, characterized by that, the first path of light differs from the delayed first path of light by a time of one reflection interval length.
5. A novel fixed reflection point based Φ -OTDR detection device according to claim 1, characterized by that said first coupler is a 50:50 coupler.
6. A novel fixed reflection point based Φ -OTDR probe according to claim 1, characterized by that, said second coupler is a 3 x 3 coupler, and the phases of the three lights differ by 120 °.
7. A detection method of a novel fixed reflection point based phi-OTDR detection device based on any of claims 1-6, characterized by comprising the following steps,
the method comprises the following steps that firstly, a laser is adopted to generate continuous mode narrow linewidth laser, the continuous mode narrow linewidth laser is converted into pulse light, the pulse light is amplified into probe light and then output to a plurality of optical fibers which are connected in sequence, and fixed reflection points are formed at the joints of the two ends of each optical fiber;
secondly, the detection light generates backward Rayleigh scattering light in the optical fiber and generates reflected light at the reflection points at the connection positions of the two ends of the optical fiber; the back Rayleigh scattering light is used for detecting the vibration position, and the reflected light is used for detecting the vibration frequency and the vibration amplitude;
step three, dividing the reflected light and the back Rayleigh scattered light into two paths: a first path of light and a second path of light;
step four, delaying the first path of light, wherein the delayed first path of light and the first path of light are different by a reflection interval length;
fifthly, controlling the polarization state of the second path of light to enable the polarization state of the second path of light to be matched with the polarization of the first path of light;
step six, carrying out coherent beat frequency on the delayed first path of light and the polarized second path of light, and outputting three paths of light, wherein phase differences are generated among the three paths of light;
step seven, demodulating phase information according to the intensity of the three paths of light, thereby realizing real-time monitoring on the position, frequency and amplitude of vibration; the demodulation method adopts a lookup table, the lookup table is obtained by pre-calculation, the phase changes within (-pi, pi) at certain intervals, the corresponding intensity value of the three paths of light is calculated to form a table, and the corresponding phase information is obtained by lookup through the lookup table according to the actually detected intensity of the three paths of light.
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