CN103630229B - A kind of differential coherence time domain scatter-type distributed optical fiber vibration sensing method and system - Google Patents

Abstract

A differential coherent time-domain scattering distributed optical fiber vibration sensing method and system, comprising: a 1×2 optical splitter, a laser is connected to the beam combining port of the 1×2 optical splitter, and a beam splitting port Connect a delay fiber and the No. 1 port of the first 3-port circulator in sequence. The No. 2 port of the 3-port circulator is connected to an optical fiber in the sensing optical cable, and the No. 3 port is connected to a polarization controller and a 2×1 optical combiner in turn. One of the splitter ports; the other splitter port is directly connected to the No. 1 port of the second 3-port circulator, and the No. 2 port of this 3-port circulator is connected to another optical fiber in the sensing optical cable, and the No. 3 port is connected to another optical fiber in turn. Delay fiber, phase modulator and another splitting port of 2×1 optical combiner. The beam combining port of the 2×1 optical combiner is connected with a photodetector. The backscattered light signals formed by the pulsed light emitted by the laser in the two optical fibers of the sensing cable are coherent at the combining port of the 2×1 optical combiner.

Description

A Differential Coherent Time Domain Scattering Type Distributed Optical Fiber Vibration Sensing Method and System

technical field

The invention relates to a differential coherent time-domain scattering type distributed optical fiber vibration sensing method and system, and relates to the technical field of distributed optical fiber sensing and the field of optical coherent detection.

Background technique

Optical fiber sensing technology is a technology that developed rapidly with the development of optical fiber communication technology in the 1970s and 1980s. It uses laser as the communication carrier and optical fiber as the communication medium to perceive and transmit external measured signals. Optical fiber sensors have many advantages such as high measurement sensitivity, anti-electromagnetic interference, anti-radiation, high-voltage resistance, corrosion resistance, small size, light weight, and adaptability to harsh environments, and the optical fiber element itself is both a detection element and a transmission element. Connect many optical fiber sensing units to form a large-scale remote sensing system for distributed monitoring and measurement.

Distributed optical fiber sensing technology can continuously measure the physical quantities distributed along the optical fiber, and can obtain the spatial distribution status of these quantities. Most distributed sensing technologies are mainly used to measure some static quantities or slow variables, and they have achieved very high indicators so far. However, in applications such as gas-liquid pipeline monitoring, fire alarm, and perimeter security, sensors are required that can detect and locate time-varying disturbances such as sound and vibration. At present, distributed optical fiber vibration sensing technologies mainly include long-distance interference technology and coherent optical time domain reflection technology.

The distributed vibration sensing system based on long-distance interferometry mainly realizes the distributed measurement of vibration through the correlation between the frequency response of the interferometer and the location of the disturbance, and it is difficult to realize the detection and location of multi-point vibration events. The coherent time-domain scattering distributed optical fiber vibration sensing system perceives external vibration and positioning by coherently measuring the phase change of backscattered light generated by optical pulses in the sensing fiber. It has a simple positioning algorithm and can realize multi-point vibration events. Simultaneous detection and localization. There are two main types of coherent measurement techniques: ordinary coherence and differential coherence. Using ordinary coherent measurement methods, slow changes in ambient temperature and pressure have a greater impact on vibration detection. The method of differential coherent measurement is not sensitive to slow changes in ambient temperature and pressure, but sensitive to emergencies, and is more suitable for perimeter intrusion detection and monitoring of gas-liquid pipelines.

At present, the differential coherent time-domain scattering distributed optical fiber vibration sensing system has the disadvantages of large power loss, multiple blind areas, and short sensing distance. In engineering construction, it is necessary to add an optical fiber disk to the sensing optical cable to avoid blind areas, which is difficult to construct and greatly affects the stability of the system. These shortcomings limit the market application of differential coherent time-domain scattering distributed optical fiber vibration sensing systems with characteristics of low environmental noise, high vibration events, and sensing sensitivity.

Contents of the invention

The present invention provides a differential coherent time-domain scattering type distributed optical fiber vibration sensing method and system. The present invention forms a dual-pulse light-backward-facing The linear differential interference structure of scattered light realizes the differential coherent detection of the backscattered light wave modulated by the phase of the external vibration signal in the sensing optical cable. It has high sensitivity to sudden vibration events and is insensitive to slowly changing environmental parameters. Compared with the traditional differential coherent time-domain scattering distributed optical fiber vibration sensing system, it has the advantages of small optical path loss, high sensitivity, no sensing blind zone, and continuous large sensing dynamics.

The present invention adopts following technical scheme:

A differential coherent time-domain scattering distributed optical fiber vibration sensing method, the pulsed light output by the laser is divided into two pulsed lights by a 1×2 optical splitter,

A pulsed light passes through a delay fiber, is transmitted to port 1 of the first 3-port circulator, passes through port 2 of the first 3-port circulator, and is transmitted to a sensing fiber in the sensing optical cable, and then Scattering occurs in the above-mentioned one sensing fiber, and the backscattered light is generated, and the back-scattered light is transmitted to the first The No. 2 port of a 3-port circulator is transmitted to the 2×1 optical combiner through the No. 3 port of the first 3-port circulator and the polarization controller in sequence;

Another pulsed light is directly transmitted to the No. 1 port of the second 3-port circulator, and then transmitted to another sensing optical fiber in the sensing optical cable through the No. 2 port of the second 3-port circulator, and Scattering occurs in the other sensing fiber to generate backscattered light, and the backscattering light is transmitted in a direction opposite to that of another pulsed light in the other sensing fiber. To the No. 2 port of the second 3-port circulator, through the No. 3 port of the second 3-port circulator, another delay fiber and a phase modulator in turn, and transmit to the 2×1 optical combiner;

The two beams of backscattered light are transmitted to the photodetector after interference at the beam combining port of the 2×1 optical combiner. The core refractive index of the one delay fiber (31) and the other delay fiber (32) are the same, and the length difference ΔL and the linewidth Δf of the laser (1) satisfy the following relationship:

$\mathrm{<span\; class="notranslate">\; \&Delta;\; L</span>}<span\; class="notranslate">\; <</span>\frac{\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;\; f</span>}}$

n is the core refractive index of the one delay fiber (31) and the other delay fiber (32), C is the speed of light propagation in vacuum, and at the same time, the length difference ΔL is less than 10m.

A differential coherent time-domain scattering distributed optical fiber vibration sensing system, including: 1×2 optical splitter, sensing optical cable and 2×1 optical combiner, on the beam combining port of the 1×2 optical splitter connected to the laser,

A delay fiber is connected to a splitting port of the 1×2 optical splitter, and a splitting port of the 1×2 optical splitter is connected to one end of the delay fiber, and the one The other end of the delay fiber is connected to the first 3-port circulator, and the other end of the delay fiber is connected to the No. 1 port of the first 3-port circulator, and the No. 2 port of the first 3-port circulator It is connected to one end of a sensing fiber in the sensing optical cable, the No. 3 port of the first 3-port circulator is connected to a polarization controller and is connected to one end of the polarization controller, and the other end of the polarization controller Connect with a splitting port of 2×1 optical combiner;

A second 3-port circulator is connected to the other splitting port of the 1×2 optical splitter and is connected to the No. 1 port of the second 3-port circulator, and the No. 2 port of the second 3-port circulator It is connected with one end of another sensing fiber in the sensing optical cable, and the No. 3 port of the second 3-port circulator is connected with another delay fiber and is connected with a port of the other delay fiber, the Another port of the other delay fiber is connected to a phase modulator and is connected to a port of the phase modulator, and another port of the phase modulator is connected to another splitting port of the 2×1 optical combiner;

The beam combining port of the 2×1 optical combiner is connected with a photodetector. The core refractive index of the one delay fiber is the same as that of the other delay fiber, and the length difference ΔL and the linewidth Δf of the laser satisfy the following relationship:

$\mathrm{<span\; class="notranslate">\; \&Delta;\; L</span>}<span\; class="notranslate">\; <</span>\frac{\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;f</span>}}$

n is the core refractive index of the one delay fiber and the other delay fiber, C is the speed of light propagation in vacuum, and at the same time, the length difference ΔL is less than 10m.

Compared with prior art, the present invention has following advantage:

The invention realizes double pulsed light-backscattered light beam linear differential interference through the combined use of dual delay optical fibers, 3-port circulators, and dual-core optical fiber sensing optical cables. The pulsed light is divided into two beams of pulsed light by a 1×2 optical splitter. One beam of pulsed light passes through a delay fiber first, and then is coupled to a sensor optical cable by the No. 1 and No. 2 ports of the first 3-port circulator. In the sensing fiber, the backscattered light generated in the sensing fiber is coupled to the 2×1 optical combiner from the circulator through the No. The No. 1 and No. 2 ports of the circulator are coupled to another sensing optical fiber in the sensing optical cable, and the backscattered light generated in the sensing optical fiber is coupled to another delaying optical fiber from its No. 3 port by the circulator. It is transmitted to the 2×1 optical combiner through the phase modulator. Since the lengths of the two delay fibers are approximately equal and the difference in length is less than the coherence length of the laser, the two beams of light interfere. The two sensing optical fibers are in the same optical cable, the deformation of the optical fiber caused by external vibration is approximately the same, and the phase changes of the two beams of backscattered light modulated by the same vibration are consistent. Since one beam of backscattered light is delayed first and then modulated, and the other beam of backscattered light is modulated first and then delayed, there is a phase modulation time difference between the two beams of backscattered light. Equation (1) gives the The expression for the relationship between the power of the backscattered signal received by the photodetector and the position where the scattering occurs in the sensing fiber is:

In formula (1), S represents the backscattering capture coefficient, a represents the fiber loss coefficient (in Km-1), a _s represents the fiber scattering coefficient (in Km-1), and w represents the optical pulse width (in Km) , L ₁ represents the length of the delay fiber, P ₀ represents the output power of the light source, v _g represents the propagation speed of light in the fiber core, Indicates the phase change of the light wave propagating in the fiber caused by external vibration, Indicates the constant optical path difference between the two backscattered lights due to the different lengths of the two delay fibers and the introduction of the phase modulator.

It can be seen from formula (1) that the interference is differential interference, and for slowly changing environmental parameters, in formula (1-1) The optical power output by the combining port of the optical combiner will not change with changes in these physical quantities, thereby removing the limitations of these physical quantities on system performance and stability.

Compared with the coherent time-domain scattering distributed optical fiber vibration sensing system using ultra-narrow linewidth laser, external modulation and common interference, the performance of the system will not be limited by slowly changing environmental parameters, and the stability and reliability will be improved. And it does not need to use an ultra-narrow linewidth laser, and the cost is low.

Compared with the traditional differential coherent time-domain scattering type distributed optical fiber vibration sensing system based on the loop structure of the coupler combined wave coupling, due to the double sensing fiber used in the present invention, the double pulsed light with independent delay - the backscattered light line Differential interference, the light beam does not need to be coupled into the same sensing fiber through a coupler, there will be no sensing blind zone problem of the traditional differential coherent time-domain scattering distributed optical fiber vibration sensing system, and the optical path loss is 1 of the optical path loss of the traditional system /4. Therefore, the sensing range and sensitivity of the differential coherent time-domain scattering distributed optical fiber vibration sensing system proposed by the present invention are superior to the traditional traditional differential coherent time domain scattering distributed optical fiber vibration sensing system. Since there is no sensing blind area, there is no need to use optical fiber discs to avoid the blind areas in the sensing range, and there is no stability problem caused by the use of these optical fiber discs. The engineering construction is simple, and the system is stable and reliable.

Description of drawings

Fig. 1 is the structural diagram of the DC-OTDR type distributed optical fiber vibration sensing system proposed by the present invention;

Fig. 2 is that there is no vibration signal in the outside world, the relationship between the backscattered light signal received by the photodetector and the position it produces in the sensing fiber in the DC-OTDR type distributed optical fiber vibration sensing system proposed by the present invention;

Fig. 3 is the frequency response characteristic curve of the DC-OTDR distributed optical fiber vibration sensing system proposed by the present invention to the external vibration signal when there is a vibration signal from the outside.

Detailed ways

Example 1

A differential coherent time-domain scattering distributed optical fiber vibration sensing method, the pulsed light output by the laser 1 is divided into two pulsed lights by a 1×2 optical splitter 21,

A pulsed light passes through a delay fiber 31, is transmitted to the No. 1 port 4101 of the first 3-port circulator 41, passes through the No. 2 port 4102 of the first 3-port circulator 41, and is transmitted to a sensor in the sensing optical cable 8. In the optical fiber 81, and scattering occurs in the one sensing optical fiber 81, the backscattered light generated, the backscattered light travels in the same direction as the pulsed light in the one sensing optical fiber 81 The opposite direction is transmitted to the No. 2 port 4102 of the first 3-port circulator 41, and then transmitted to the 2×1 optical combiner through the No. 3 port 4103 of the first 3-port circulator 41 and the polarization controller 5 device 22;

Another pulsed light is directly transmitted to the No. 1 port of the second 3-port circulator 42, and then transmitted to another sensor in the sensing optical cable 8 through the No. 2 port 4202 of the second 3-port circulator 42. In the optical fiber 82, and scattering occurs in the other sensing optical fiber 82, the backscattered light generated, the backscattered light is pulsed with another pulse in the other sensing optical fiber 82 The direction of light transmission is opposite to the No. 2 port 4202 of the second 3-port circulator 42, and then passes through the No. 3 port 4203 of the second 3-port circulator 42, another delay fiber 32 and phase modulation Device 6, transmitted to 2×1 optical combiner 22;

The two beams of backscattered light are transmitted to the photodetector 7 after being interfered at the beam combining port of the 2×1 optical combiner 22 . The core refractive index of the one delay fiber 31 and the other delay fiber 32 are the same, and the length difference ΔL and the linewidth Δf of the laser 1 satisfy the following relationship:

$\mathrm{<span\; class="notranslate">\; \&Delta;L</span>}<span\; class="notranslate">\; <</span>\frac{\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;f</span>}}$

n is the core refractive index of the one delay fiber 31 and the other delay fiber 32, C is the speed of light propagation in vacuum, and the length difference ΔL is less than 10m.

Example 2

A differential coherent time-domain scattering distributed optical fiber vibration sensing system, comprising: 1×2 optical splitter 21, sensing optical cable 8 and 2×1 optical combiner 22, in the 1×2 optical splitter 21 A laser 1 is connected to the beam combining port,

A delay fiber 31 is connected to a splitting port of the 1×2 optical splitter 21, and a splitting port of the 1×2 optical splitter 21 is connected to one end of the delay fiber 31, The other end of the one delay fiber 31 is connected to the first 3-port circulator 41, and the other end of the one delay fiber 31 is connected to the No. 1 port 4101 of the first 3-port circulator 41, and the first 3-port circulator 41 is connected to the other end. The No. 2 port 4102 of a 3-port circulator 41 is connected with one end of a sensing fiber 81 in the sensing optical cable 8, and the No. 3 port 4103 of the first 3-port circulator 41 is connected with a polarization controller 5 and is connected with One end of the polarization controller 5 is connected, and the other end of the polarization controller 5 is connected to a beam splitting port of the 2×1 optical combiner 22;

A second 3-port circulator 42 is connected to another beam splitting port of the 1×2 optical splitter 21 and is connected to the No. 1 port 4201 of the second 3-port circulator 42, the second 3-port circulator No. 2 port 4202 of 42 is connected with another sensing fiber 82 end in the sensing optical cable 8, and No. 3 port 4203 of the second 3-port circulator 42 is connected with another delay fiber 32 and is connected with the A port of another delay fiber 32 is connected, and another port of the other delay fiber 32 is connected with a phase modulator 6 and is connected with a port of the phase modulator 6, and another port of the phase modulator 6 Connect with another splitting port of the 2×1 optical combiner 22;

The beam combining port of the 2×1 optical combiner 22 is connected with a photodetector 7 . The core refractive index of the one delay fiber 31 and the other delay fiber 32 are the same, and the length difference ΔL and the linewidth Δf of the laser 1 satisfy the following relationship:

$\mathrm{<span\; class="notranslate">\; \&Delta;L</span>}<span\; class="notranslate">\; <</span>\frac{\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;f</span>}}$

n is the core refractive index of the one delay fiber 31 and the other delay fiber 32, C is the speed of light propagation in vacuum, and the length difference ΔL is less than 10m.

In this implementation case, the length of one delay fiber 31 is 20145m, the length of the other delay fiber 32 is 20146m, the line width of the laser 1 is 2MHz, and the length of the sensing optical cable is 15Km, and the system is built. After testing, the loss of the optical path is 3.3dB, and the loss of the traditional differential coherent time-domain scattering type distributed optical fiber vibration sensing system based on the loop structure of the coupler combined wave coupling is 9.7dB. The optical path loss of the differential coherent time domain scattering type distributed optical fiber vibration sensing system proposed by the present invention is about 1/4 of the optical path loss of the traditional differential coherent time domain scattering type distributed optical fiber vibration sensing system. Fig. 2 shows the backscattered light signal received by the photodetector when the DC-OTDR type distributed optical fiber vibration sensing system proposed by the present invention is not vibrating outside. It can be seen from the figure that when there is no external vibration, the backscattered light signal is continuous, which indicates that the sensing optical cable can continuously measure external vibration signals without blind spots. Figure 3 shows the backscattered light signal received by the photodetector in the DC-OTDR distributed optical fiber vibration sensing system proposed by the present invention when mechanical vibration is applied at 5Km, 10Km and 12.5Km of the sensing optical cable. It can be clearly seen from Figure 3 that at 5Km, 10Km and 12.5Km, the power of the backscattered light signal received by the photodetector changes suddenly. Based on this, the detection and location of vibration events can be realized.

Claims (4)

1. A differential coherent time-domain scattering type distributed optical fiber vibration sensing method is characterized in that the pulsed light output by the laser (1) is divided into two pulsed light by a 1 * 2 optical splitter (21), A pulsed light passes through a delay fiber (31), is transmitted to the No. 1 port (4101) of the first 3-port circulator (41), and is transmitted to the No. 2 port (4102) of the first 3-port circulator (41) through the first 3-port circulator (41). In a sensing optical fiber (81) in the sensing optical cable (8), and scattering takes place in the described sensing optical fiber (81), produces backscattered light, and described backscattered light is with the said backscattered light A pulsed light in the above-mentioned one sensing fiber (81) is transmitted to the No. 2 port (4102) of the first 3-port circulator (41) in the opposite direction to the transmission direction, and passes through the first 3 ports in turn. The No. 3 port (4103) of the port circulator (41) and the polarization controller (5) are transmitted to the 2×1 optical combiner (22); Another pulsed light is directly transmitted to the No. 1 port of the second 3-port circulator (42), and then transmitted to the sensing optical cable (8) through the No. 2 port (4202) of the second 3-port circulator (42). ) in another sensing fiber (82), and scattering occurs in said another sensing fiber (82), producing backscattered light, said backscattering light is in the same way as said another sensing fiber (82). Another pulsed light transmission direction in a sensing fiber (82) is transmitted to the No. 2 port (4202) of the second 3-port circulator (42) in the opposite direction, and passes through the second 3 ports in turn Port No. 3 (4203) of the circulator (42), another delay fiber (32) and a phase modulator (6), are transmitted to the 2×1 optical combiner (22); The two beams of backscattered light are transmitted to the photodetector (7) after interference at the beam combining port of the 2×1 optical combiner (22). 2. a kind of differential coherent time-domain scattering type distributed optical fiber vibration sensing method according to claim 1, is characterized in that, the core refraction of described one delay optical fiber (31) and another delay optical fiber (32) The rate is the same, the length difference ΔL between the one delay fiber (31) and the other delay fiber (32) and the linewidth Δf of the laser (1) satisfy the following relationship: $\mathrm{<span\; class="notranslate">\; \&Delta;\; L</span>}<span\; class="notranslate">\; <</span>\frac{\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;\; f</span>}}$ n is the core refractive index of the one delay fiber (31) and the other delay fiber (32), C is the speed of light propagation in vacuum, and at the same time, the length difference ΔL is less than 10m. 3. A differential coherent time-domain scattering type distributed optical fiber vibration sensing system, characterized in that it comprises: 1 × 2 optical splitter (21), sensing optical cable (8) and 2 × 1 optical combiner (22 ), a laser (1) is connected to the beam combining port of the 1×2 optical splitter (21), A delay fiber (31) is connected to a splitting port of the 1×2 optical splitter (21), and a splitting port of the 1×2 optical splitter (21) is connected with the described one One end of the delay fiber (31) is connected, the other end of the one delay fiber (31) is connected with the first 3-port circulator (41), and the other end of the delay fiber (31) is connected to the first 3-port The No. 1 port (4101) of the circulator (41) is connected, and the No. 2 port (4102) of the first 3-port circulator (41) is connected to a sensing optical fiber (81) in the sensing optical cable (8) One end is connected, the No. 3 port (4103) of the first 3-port circulator (41) is connected with a polarization controller (5) and connected with one end of the polarization controller (5), and the polarization controller (5 ) is connected with a splitting port of the 2×1 optical combiner (22); A second 3-port circulator (42) is connected to another beam splitting port of the 1×2 optical splitter (21) and is connected to the No. 1 port (4201) of the second 3-port circulator (42), so Port No. 2 (4202) of the second 3-port circulator (42) is connected to one end of another sensing fiber (82) in the sensing optical cable (8), and the second 3-port circulator (42 ) port No. 3 (4203) is connected with another delay fiber (32) and is connected with a port of the other delay fiber (32), and another port of the delay fiber (32) is connected with The phase modulator (6) is connected to a port of the phase modulator (6), and another port of the phase modulator (6) is connected to another beam splitting port of the 2×1 optical combiner (22); The beam combining port of the 2×1 optical combiner (22) is connected with a photodetector (7). 4. A kind of differential coherent time-domain scattering type distributed fiber optic vibration sensing system according to claim 3, characterized in that, the core refraction of the one delay fiber (31) and another delay fiber (32) The rate is the same, the length difference ΔL between the one delay fiber (31) and the other delay fiber (32) and the linewidth Δf of the laser (1) satisfy the following relationship: $\mathrm{<span\; class="notranslate">\; \&Delta;L</span>}<span\; class="notranslate">\; <</span>\frac{\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;\; f</span>}}$ n is the core refractive index of the one delay fiber (31) and the other delay fiber (32), C is the speed of light propagation in vacuum, and at the same time, the length difference ΔL is less than 10m.