CN105806465A - Novel [phi]-OTDR detection device based on fixed reflection points and detection method thereof - Google Patents

Abstract

The invention discloses a novel Φ‑OTDR detection device based on a fixed reflection point, comprising 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 An optical fiber, a polarization controller, a second coupler, an avalanche photodetector and a signal acquisition card; the optical fiber detection unit includes several optical fibers connected in sequence, and fixed reflection points are formed at the joints at both ends of the optical fiber. The invention also discloses a detection method based on a new type of Φ‑OTDR detection device based on a fixed reflection point. The invention uses an optical fiber detection unit to measure the surrounding vibration, and performs three-port phase demodulation through a look-up table method to realize vibration detection. Real-time monitoring of position, frequency and amplitude; the optical fiber detection unit is used to divide the optical fiber into several sections, avoiding the interference of a position vibration event on the entire section of optical fiber, and the system of this structure is easy to disassemble and maintain.

Description

A Novel Φ-OTDR Detection Device Based on Fixed Reflection Point and Its Detection Method

technical field

The invention relates to the technical field of optical fiber sensing, in particular to a novel Φ-OTDR detection device based on fixed reflection points and a detection method thereof.

Background technique

Phase-sensitive optical time domain reflectometer (Φ-OTDR) vibration measurement has developed rapidly in the past ten years, and it has important application value in security monitoring of important places and health monitoring of major engineering structures. For example, the security monitoring of prison fences or national borders, and bridges, roads and buildings will inevitably be damaged with the increase in service life. The distributed vibration measurement system can be installed in security areas or on the surface of buildings for monitoring by Vibration caused by special events, so as to facilitate real-time monitoring or early detection of hidden dangers that may cause building accidents.

Φ-OTDR can detect the position and frequency domain information of the external disturbance on the sensing fiber through the coherent fading phenomenon of the Rayleigh backscattered light in the fiber. Like other optical time domain reflectometers, Φ-OTDR is also a sensing system for single-ended measurement, but in order to obtain a usable time-domain curve of back Rayleigh scattered light, a laser with narrow linewidth and stable frequency is required. Short narrow-band pulses are sent into the sensing fiber, and the returned Rayleigh scattering waveform is modulated into a sawtooth shape due to the coherence of different scattering centers. By analyzing the change of the saw-tooth Rayleigh waveform, the refractive index changes in the sensing fiber due to external disturbances can be monitored, and weak refractive index changes can be strengthened by the coherence effect between scattering elements.

Φ-OTDR has two obvious advantages of fast response speed and high sensitivity. Compared with Brillouin scattered light and Raman scattered light to measure vibration, Rayleigh scattered light has stronger scattered light power, so the signal detected by the photodetector is also stronger, and there is no need for multiple accumulation and averaging, so Φ- OTDR has a fast response speed and can be used to detect fast-changing dynamic disturbances, such as vibration signals, and then the time domain and frequency domain information of vibration signals can be obtained through appropriate data processing methods. In Φ-OTDR, the external disturbance information is obtained by sensing the Rayleigh coherent fading phenomenon of Rayleigh scattered light backward in the optical fiber, and the Rayleigh coherent fading is affected by the phase of light in the optical fiber, so as long as the external disturbance If the phase of the detection light in the optical fiber is affected, then Φ-OTDR has the ability to detect external disturbance events, so Φ-OTDR has high sensitivity and can detect small external disturbances.

However, the currently commonly used Φ-OTDR also has obvious shortcomings. Most of them can only detect the location of the strain and extract its frequency domain information, and cannot quantitatively measure the strain value. The main reason is that there is no definite, one-to-one correspondence between the optical signal obtained by the receiving end and the strain value loaded on the optical fiber, so the strain value loaded on the optical fiber cannot be calculated through the demodulation algorithm. A Masoudi and M Belal of the University of Southampton in the UK wrote "A distributed optical fiber dynamic strain sensor based on phase-OTDR" using a 3×3 coupler and a time-delay fiber for phase demodulation to achieve amplitude detection, but A positional vibration event is likely to cause interference on the entire section of optical fiber. If the optical fiber is damaged, it will be complicated to disassemble and repair the optical fiber; Zhu Fan and Zhang Yixin from the Optical Communication Engineering Research Center of Nanjing University et al. based on Ultra-weak Fiber Bragg Grating Array" uses Bragg grating reflection signals for phase demodulation, but it needs to control the frequency of the laser, the phase solution algorithm is relatively complicated, and the spatial resolution is low because of the need to use wide pulses; and Zhang Yixin , Zhang Xuping and others improved the method in the paper of Zhu Fan and Zhang Yixin in the patent "Phase-sensitive optical time domain reflection device and method based on fiber Bragg grating array" (patent number: CN201510389252.2), which can be used on the basis of quantitative analysis High spatial resolution is achieved on the surface, but in order to achieve high spatial resolution, the gratings used are relatively dense, which will increase the cost of detection, the complex process of installation and repair after damage is complicated, and the requirements for operators are high. Promote apps.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a novel Φ-OTDR detection device and detection method based on fixed reflection points. The present invention is based on the existing Φ-OTDR sensing system, The surrounding vibration is measured by introducing an optical fiber detection unit, and the three-port phase demodulation is performed by the look-up table method to realize real-time monitoring of the vibration position, frequency and amplitude.

The present invention adopts the following technical solutions for solving the problems of the technologies described above:

A kind of novel Φ-OTDR detecting device based on fixed reflection point proposed according to the present invention, comprises delay generator, laser, pulse modulator, erbium-doped fiber amplifier, optical circulator, optical fiber detection unit, first coupler, delay optical fiber, a polarization controller, a second coupler, an avalanche photodetector and a signal acquisition card; the optical fiber detection unit includes several sequentially connected optical fibers, and fixed reflection points are formed at the joints at both ends of the optical fiber; wherein,

The delay generator is used to generate modulation pulses and trigger pulses; the modulation pulses are input to the pulse modulator, and the trigger pulses are input to the signal acquisition card;

a laser for generating continuous-mode narrow-linewidth laser light and outputting it to a pulse modulator;

The pulse modulator is used to convert the continuous-mode narrow-linewidth laser into pulsed light and output it to the erbium-doped fiber amplifier according to the received modulated pulse;

An erbium-doped fiber amplifier, which is used to amplify the pulsed light into detection light and then output it to the optical circulator;

The optical circulator is used to input the detection light through its first port and inject it into the optical fiber detection unit through its second port;

The optical fiber detection unit is used to output the generated back Rayleigh scattered light and reflected light to the second port of the optical circulator, and output it to the first coupler through the third port of the optical circulator;

The first coupler is used to divide the reflected light and back Rayleigh scattered light into two paths: the first path of light is output to the delay fiber, and the second path of light is output to the polarization controller;

Delay optical fiber, used to delay the first path of light, and output the delayed first path of light to the second coupler;

a polarization controller, configured to control the polarization state of the second light path so that its polarization state matches the polarization of the first light path, and the polarized second light path is output to the second coupler;

The second coupler is used to coherently beat the delayed first light and the polarized second light, so that a phase difference is generated between the three output lights, and the three lights are output to the avalanche photodetector ;

The avalanche photodetector is used to convert the three-way light into an electrical signal and output it to the signal acquisition card;

The signal acquisition card is used to convert the electrical signal into a digital signal for subsequent processing according to the trigger pulse.

As a further optimization scheme of the novel Φ-OTDR detection device based on fixed reflection points according to the present invention, the periods of the modulation pulse and the trigger pulse are both synchronized with the period of the detection light.

As a further optimization scheme of a novel Φ-OTDR detection device based on fixed reflection points according to the present invention, the optical fiber detection unit is several optical fiber jumpers connected in sequence.

As a further optimization scheme of the novel Φ-OTDR detection device based on fixed reflection points in the present invention, the time difference between the first light path and the delayed first light path is a reflection interval length.

As a further optimization scheme of a novel Φ-OTDR detection device based on fixed reflection points according to the present invention, the first coupler is a 50:50 coupler.

As a further optimization scheme of a novel Φ-OTDR detection device based on fixed reflection points in the present invention, the second coupler is a 3×3 coupler, and the phase difference of the three paths of light is 120°.

A detection method based on a novel Φ-OTDR detection device based on fixed reflection points, comprising the following steps,

Step 1: Use a laser to generate a continuous-mode narrow-linewidth laser, convert the continuous-mode narrow-linewidth laser into pulsed light, amplify the pulsed light into probe light, and output it to several sequentially connected optical fibers. Fixed reflection points are formed at the end connections;

Step 2, the detection light generates back Rayleigh scattered light in the optical fiber, and generates reflected light at the reflection point at the junction of the two ends of the optical fiber; the back Rayleigh scattered light is used to detect the vibration position, and the reflected light is used to detect the vibration frequency and amplitude;

Step 3: Divide the reflected light and Rayleigh backscattered light into two paths: the first path of light and the second path of light;

Step 4. Delay the first light, and the time difference between the delayed first light and the first light is a reflection interval length;

Step 5, controlling the polarization state of the second light path so that its polarization state matches the polarization state of the first light path;

Step 6. Coherently beat the delayed first light and the polarized second light to output three light beams, and a phase difference is generated between the three light beams;

Step 7. According to the intensity of the three-way light, demodulate the phase information, so as to realize the real-time monitoring of the vibration position, frequency and amplitude; (-π, π) changes at a certain interval, and calculates the corresponding three-way light intensity value to form a table. According to the actually detected three-way light intensity, the corresponding phase information is obtained by looking up the table.

Compared with the prior art, the present invention adopts the above technical scheme and has the following technical effects:

(1) the present invention measures the vibration around by adopting optical fiber detection unit, carries out three-port phase demodulation by look-up table method and can realize the real-time monitoring to the position, frequency and amplitude of vibration;

(2) Use the optical fiber detection unit to divide the optical fiber into several sections to avoid the interference caused by a position vibration event on the entire section of optical fiber. At the same time, the system with this structure is easy to disassemble and maintain.

Description of drawings

Fig. 1 is a system structure diagram of the present invention.

Figure 2 is a schematic diagram of coherent signal measurement.

Fig. 3 is a schematic diagram of demodulation by the look-up table method.

detailed description

Below in conjunction with accompanying drawing, technical scheme of the present invention is described in further detail:

As shown in Figure 1, it is a system structure diagram of the present invention, a novel Φ-OTDR detection device based on a fixed reflection point, including a delay generator, a laser, a pulse modulator, an erbium-doped fiber amplifier, an optical circulator, and 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; Fixed reflection points; where,

The delay generator is used to generate modulation pulses and trigger pulses; the modulation pulses are input to the pulse modulator, and the trigger pulses are input to the signal acquisition card;

a laser for generating continuous-mode narrow-linewidth laser light and outputting it to a pulse modulator;

The pulse modulator is used to convert the continuous-mode narrow-linewidth laser into pulsed light and output it to the erbium-doped fiber amplifier according to the received modulated pulse;

An erbium-doped fiber amplifier, which is used to amplify the pulsed light into detection light and then output it to the optical circulator;

The optical circulator is used to input the detection light through its first port and inject it into the optical fiber detection unit through its second port;

The optical fiber detection unit is used to output the generated back Rayleigh scattered light and reflected light to the second port of the optical circulator, and output it to the first coupler through the third port of the optical circulator;

The first coupler is used to divide the reflected light and back Rayleigh scattered light into two paths: the first path of light is output to the delay fiber, and the second path of light is output to the polarization controller;

Delay optical fiber, used to delay the first path of light, and output the delayed first path of light to the second coupler;

a polarization controller, configured to control the polarization state of the second light path so that its polarization state matches the polarization of the first light path, and the polarized second light path is output to the second coupler;

The second coupler is used to coherently beat the delayed first light and the polarized second light, so that a phase difference is generated between the three output lights, and the three lights are output to the avalanche photodetector ;

The avalanche photodetector is used to convert the three-way light into an electrical signal and output it to the signal acquisition card;

The signal acquisition card is used to convert the electrical signal into a digital signal for subsequent processing according to the trigger pulse.

The periods of the modulation pulse and the trigger pulse are both synchronized with the period of the detection light.

The performance of the device used in the laboratory: the laser model is RIO laser, the laser wavelength is 1550nm, the line width is 10kHz, and the output optical power is 13dBm; the pulse modulator is an electro-optic modulator, which can generate a minimum 20ns optical pulse; BA series amplifiers have a center frequency of 1550nm, an operating wavelength of 20nm, and a constant power gain of 17dBm. The first coupler is a 50:50 coupler and the second coupler is a 3×3 coupler.

Specific experimental conditions: In this scheme, an optical fiber jumper is used as the detection optical fiber, and the end face of the connector produces reflection. When installing and connecting the jumper, the flange should be tightened to the tightest. There is no need to manually adjust its tightness to control the reflection of the fiber end face during measurement, and if one of the fibers changes its length due to external vibrations, it will not affect the adjacent Transmission of light in patch cords. After comparing and analyzing different combinations of optical fiber joints, the system chooses the connection mode of FC/PC—FC/PC. In this connection mode, the reflection generated by the fiber end face is roughly -50dBm, and the Rayleigh scattered light in the fiber is between -70dBm and -60dBm, so that the reflected light intensity can be significantly stronger than the Rayleigh scattered light. The length of the sensing fiber used in the experiment is L=5km, so the required pulse period should be greater than 50μs. 5 segments of fiber jumpers are connected to the end of the 5km optical fiber, the length of each segment of fiber jumper is d=20m, and the length of the delay fiber is 40m.

The specific steps for combining experimental parameters are as follows:

Step 1: The laser generates continuous-mode narrow-linewidth laser, and converts the continuous-mode narrow-linewidth laser into pulsed light with a pulse width of 20ns-1us; then it is amplified by EDFA and becomes detection light, which enters the optical fiber detection unit after passing through the optical circulator.

Step 2: As shown in Figure 2, the detection light is transmitted in the optical fiber detection unit, and reflected back when encountering a reflection point, and the reflected original signal is obtained. The original signal is split by the first coupler, and 50% of the first part of the light is delayed The delayed signal is obtained after the optical fiber, and the difference between the signal and the original signal is a reflection interval length; the other part 50% passes through the polarization controller to keep its polarization state matched with the delayed signal to suppress polarization noise, and then the two signals pass through The second coupler generates coherence and outputs to the three APDs to be converted into electrical signals to be collected and processed by the acquisition card. If a vibration event occurs, the phases of the original signal and the delayed signal will change, and the intensity of the coherent signal will change significantly.

Step 3: The principle of three-port demodulation is to divide the signal to be demodulated into three signals with a phase difference of 2π/3, perform summation, differentiation, integration and other operations to restore the phase term in the original signal, that is, by the formula get the process of. The following is a quantitative explanation of the principle of three-port demodulation:

Due to the characteristics of the device, the phase difference of the three-way light obtained after the reflected light of the system passes through the three-port coupler is 2π/3, which can be expressed as:

where D is the average value of the reflected signal strength, E is the peak power compared to this average, is the optical phase to be demodulated, k=1,2,3.

When the above three signals are summed, the result of adding the AC term is zero, thus obtaining the DC term D in the signal. After subtracting 1/3 of the summation result from each signal, a signal containing only the AC term is obtained:

Differentiating this signal gives Do the following operations: and sum the obtained results to get

pass easy to get Finally, by integral operation, we get That is, the phase of the signal.

Therefore, the method of directly performing three-port demodulation is complex and time-consuming, and cannot meet the requirements of real-time calculation. Therefore, the table look-up method is adopted, that is, the optical phase values corresponding to the intensities measured by the three detectors are pre-calculated. In actual measurement, it is necessary to match the measured intensity values with the existing values to find the corresponding phase values. As shown in Figure 3, a three-dimensional space table is generated, the coordinate axis is the light intensity I obtained by the three detectors, and the phase can be obtained by using the value obtained by the detector as the coordinate index

The above content is a further detailed description of the present invention in conjunction with specific preferred embodiments, and it cannot be assumed that the specific implementation of the present invention is limited to these descriptions. For those of ordinary skill in the technical field of the present invention, without departing from the concept of the present invention, some simple deductions or substitutions can be made, which should be regarded as belonging to the protection scope of the present 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.