CN102147236A - Fully distributed optical fiber strain and vibration sensing method and sensor - Google Patents

Abstract

Fully distributed optical fiber strain and vibration sensor, including laser (1), first coupler (2), pulse modulation module (3), optical amplifier (4), circulator (5), sensing optical fiber (6), optical fiber A grating (7), a polarization scrambler (8), a second coupler, a balanced photodetector, a polarizer, a photodetector (12), and a signal processing unit. The continuous light output by the laser (1) is divided into two paths after passing through the first coupler (2): the first path is used as a reference light, and is connected to the second path of the second coupler (9) after passing through the scrambler (8). One input end; the second path passes through the pulse modulation module (3) and the optical amplifier (4) and injects the detection pulse light into the first port of the circulator (5). The present invention uses Brillouin Optical Time Domain Reflectometry (BOTDR) and polarization Optical Time Domain Reflectometry (POTDR) performs fully distributed measurement of strain and vibration on a single optical fiber, which overcomes the shortcomings of a single BOTDR or POTDR system, and reduces the false alarm and false alarm rate of the system.

Description

A fully distributed optical fiber strain and vibration sensing method and sensor

technical field

The invention is an optical fiber sensing device for fully distributed monitoring of strain and vibration, especially a combination of Brillouin Optical Time Domain Reflectometry (BOTDR) technology and Polarized Optical Time Domain Reflectometry (POTDR) technology that can simultaneously measure strain And fiber optic sensing equipment for fully distributed monitoring of vibration.

Background technique

When the optical fiber is affected by the external environment (such as temperature, pressure, vibration, etc.), the parameters such as the intensity, phase, frequency, and polarization state of the transmitted light in the optical fiber will change accordingly. By measuring these parameters of the transmitted light, it can be obtained Corresponding physical quantity, this technology is called fiber optic sensing technology.

Distributed optical fiber sensing technology uses optical fiber as the sensing element, which can obtain the distribution information of events changing with time and space in the sensing fiber area, so the distributed optical fiber sensing technology can be used to detect the strain and vibration in the sensing fiber area. Perform fully distributed measurements.

Compared with traditional electrical sensors, optical fiber sensors have the advantages of high sensitivity, anti-electromagnetic interference, small size, low price, and long-distance distributed measurement. Therefore, since the late 1970s, optical fiber sensing technology has been widely used. The development of fully distributed optical fiber sensing technology based on Rayleigh scattering, Brillouin scattering, Raman scattering, etc., among which Brillouin Optical Time Domain Reflectometry (BOTDR) technology and Polarized Optical Time Domain Reflectometry (POTDR) technology They are two more common fully distributed fiber optic sensing technologies.

1) When the optical fiber is affected by the strain, the frequency of the Brillouin scattered light generated by the light wave in it will shift, which is called the Brillouin frequency shift. The magnitude of the frequency shift is proportional to the magnitude of the strain on the fiber. The Brillouin Optical Time Domain Reflectometry (BOTDR) technology injects pulsed light into the optical fiber, and measures the Brillouin frequency shift of the Brillouin scattered light continuously generated by the pulsed light during the propagation of the optical fiber, and then determines the Brillouin frequency shift of the Brillouin scattered light along the optical fiber. The strain information for the location. BOTDR technology is by far the most important fully distributed optical fiber sensing technology that can accurately measure the strain in optical fiber. However, BOTDR technology has weak measurement capability for vibration. Because although theoretically the optical fiber will generate strain at the same time when it is vibrated by the external influence, on the one hand, the weak strain caused by the tiny vibration has little influence on the Brillouin frequency shift, and on the other hand, the measurement of strain by BOTDR technology The speed is slow, usually more than ten seconds, so BOTDR technology is difficult to use to measure vibration.

2) The Polarized Optical Time Domain Reflectometry (POTDR) technology also injects pulsed light into the optical fiber. However, it determines the external events at various positions along the fiber by measuring the change of the polarization state of the scattered light returned by the pulsed light along the fiber, so as to perform a fully distributed measurement. Since changes in the polarization state of light waves in the fiber are very sensitive to external events, they can be used to measure weak external events. At the same time, because the POTDR technology judges the change of the polarization state of scattered light through the light intensity signal, and has a short response time, it can be used to measure vibrations in a large frequency range. Vibration within 10KHz can usually be measured. However, since the strain on the optical fiber is not in a one-to-one correspondence with the change of the polarization state, and the POTDR technology mostly uses the method of comparing with the previous measurement to judge the state of the optical fiber, it is difficult for the POTDR technology to accurately measure the static strain and the larger strain. detection.

Combining the BOTDR system and the POTDR system can simultaneously monitor strain and vibration on the same sensing fiber, and the overall cost is much smaller than the separate superposition of the two systems. In addition, compared with the single BOTDR system and POTDR system, when judging external events, the two systems work at the same time, and the chance of system misreporting and omission will be smaller.

Contents of the invention

The purpose of the present invention is to provide a fully distributed optical fiber sensor that can simultaneously measure strain and vibration changes.

The technical solution of the present invention is: in order to achieve the above purpose, the present invention provides a fully distributed optical fiber strain and vibration sensing method and sensor. The sensor includes a laser (1), a first coupler (2), a pulse modulation module (3), an optical amplifier (4), a circulator (5), a sensing fiber (6), a fiber grating (7), a scrambler A polarizer (8), a second coupler (9), a balanced photodetector (10), a polarizer (11), a photodetector (12), and a signal processing unit (13).

The continuous light output by the laser (1) is divided into two paths after passing through the first coupler (2): one of them is used as a reference light, which is connected to the first input of the second coupler (9) after passing through the scrambler (8) end; the second path is injected into the first port of the circulator (5) as detection pulse light after passing through the pulse modulation module (3) and the optical amplifier (4). The detection pulse light is incident into the sensing fiber (6) from the second port of the circulator (5). The Rayleigh scattered light and Brillouin scattered light generated by the detection pulse light in the sensing fiber (6) return to the second port of the circulator (5), and are connected to the fiber grating (7) through the third port of the circulator (5). ) to transmit the Brillouin scattered light while reflecting the Rayleigh scattered light; the transmitted Brillouin scattered light enters the second input end of the second coupler (9) and the reference light is in the second coupler (9) The formed mixed signal is input to the balanced photodetector (10) through the two output ends of the second coupler (9) to be converted into an electrical signal, and then enters the signal processing unit (13) for processing.

The Rayleigh scattered light reflected by the fiber grating (7) enters the signal processing unit (13) after being connected to the analyzer (11) and the photodetector (12) through the output port of the circulator (5).

The present invention can simultaneously process the reflected Rayleigh scattered light and the transmitted Brillouin scattered light, using Brillouin scattered light and Rayleigh scattered light, respectively using Brillouin Optical Time Domain Reflectometry (BOTDR) technology and polarized light Time Domain Reflectometry (POTDR) technology for fully distributed sensing of strain and vibration.

By using the Brillouin Optical Time Domain Reflectometry (BOTDR) technology to measure the frequency shift of the Brillouin light transmitted through the fiber grating (7), the strain is fully distributed; at the same time, the Polarized Optical Time Domain Reflectometry (POTDR) technology is used The change of the polarization state of the Rayleigh scattered light reflected by the fiber grating (7) is measured, and the vibration is fully distributed.

A polarization scrambler is connected in the first path after the first coupler (2), and there is no polarization scrambler in the second path.

The circulator (5) can be a four-port circulator, or two three-port circulators connected (the third port of the first three-port circulator is connected to the first port of the second three-port circulator ).

The selected laser (1) has a line width of no more than 10MHz, and its preferred working band is the optical fiber communication band within the range of 800nm to 1700nm. .

为传感光纤（6）的折射率，

为传感光纤（6）中的声速，

为激光器（1）的中心波长，为真空中的光速。The center wavelength of the selected fiber grating (7) is the center wavelength of the laser (1), and the 3dB bandwidth is smaller than the width determined by , which can reflect Rayleigh scattered light and transmit Brillouin scattered light. in,

is the refractive index of the sensing fiber (6),

is the speed of sound in the sensing fiber (6),

is the central wavelength of the laser (1), is the speed of light in vacuum.

In the fully distributed optical fiber strain and vibration sensing method, the Rayleigh signal reflected by the fiber grating (7) enters the signal processing unit (13) after passing through the analyzer (11) and photodetector (12), and then passes through the After processing, the polarization state change information of the Rayleigh scattered light is obtained, and the fully distributed monitoring of the vibration is performed using the polarized optical time domain reflectometry (POTDR) method. The Brillouin signal light and reference light transmitted through the fiber grating (7) Enter the balanced photodetector (10) through the second coupler (9) for coherent detection, and then obtain the frequency shift information of the Brillouin scattered light through the signal processing unit (13), and use the Brillouin optical time domain Reflection (BOTDR) method for fully distributed monitoring of strain.

In the detection method of the fully distributed optical fiber strain and vibration sensor, the Brillouin signal light and reference light transmitted through the fiber grating (7) enter the balanced photodetector (10) through the second coupler (9) for coherence Detection, and then the frequency shift information of the Brillouin scattered light is obtained through the signal processing unit (13), so as to determine the external strain event, which uses the Brillouin Optical Time Domain Reflectometry (BOTDR) technology;

The beneficial effects of the present invention are: the present invention combines BOTDR technology and POTDR technology, and the present invention utilizes Brillouin optical time domain reflection (BOTDR) and polarized light time domain reflection (POTDR) at the same time to respectively respond to strain and vibration on a single optical fiber For fully distributed measurement, only one sensing fiber can be used to measure strain events and vibration events at the same time, which overcomes the shortcomings of the single function of the single BOTDR system and POTDR system, and greatly reduces the false alarm and leakage of the system. rate. The measurement function and application range of the fully distributed optical fiber sensor are greatly improved, and the overall cost is much smaller than the separate superposition of the two systems.

Description of drawings

Fig. 1 is a structural diagram of a fully distributed optical fiber strain and vibration sensor provided by the present invention;

Figure 2(a) is a structural diagram of a common four-port circulator;

Figure 2(b) is a structural diagram of a four-port circulator composed of two three-port circulators; ①-④ are ports.

Detailed ways

The structure of a fully distributed strain and vibration sensor is shown in Figure 1, and its specific implementation steps for measuring strain and vibration are as follows:

1) The continuous light output by the laser (1) is divided into two paths after passing through the first coupler (2);

2) One of them is used as a reference light, which is connected to the first input end of the second coupler (9) after passing through the scrambler (8);

3) The second path is modulated into pulsed light by the pulse modulation module (3), and injected into the sensing fiber (6) as detection pulsed light through the circulator (5) after passing through the optical amplifier (4).

4) The Rayleigh scattered light and Brillouin scattered light in the sensing fiber (6) return and exit from the third port of the circulator (5).

5) The fiber grating (7) transmits the Brillouin scattered light and reflects the Rayleigh scattered light.

6) The transmitted Brillouin scattered light enters the second input end of the second coupler (9), and the mixed signal formed in the second coupler (9) with the reference light passes through the two ends of the second coupler (9). The first output terminal is input into the balanced photodetector (10) and converted into an electrical signal, and then enters the signal processing unit (13) for processing. After processing by the signal processing unit (13), the Brillouin frequency shift is obtained, thereby realizing Fully distributed sensing of strain over the area of the sensing fiber.

7) The reflected Rayleigh scattered light is output from the fourth port of the circulator (5), and enters the signal processing unit (13) after passing through the analyzer (11) and the photodetector (12). After being processed by the signal processing unit (13), the change of the polarization state of the Rayleigh scattered light is obtained, so as to realize fully distributed sensing of vibration within the area of the sensing fiber.

为1550nm，线宽为1MHz。它发出的激光通过耦合器（2）分成了两路，其中一路经电光调制器调制和掺铒光纤放大器放大后，作为探测脉冲光进入到了传感光纤。传感光纤使用的是普通的通信光纤，其折射率,光纤中的声速。脉冲光在传感光纤光纤中会产生瑞利散射光和布里渊散射光，其中瑞利散射光的频率与激光器的频率一致，布里渊散射光的频率会产生偏移，其布里渊频移为

。传感光纤受到应变影响时，会使布里渊散射光的布里渊频移发生改变，受到振动影响时，会使瑞利散射光和布里渊散射光的偏振态发生变化。光纤光栅的中心波长与激光器的波长一致，为1550nm，3dB带宽为

，其中为真空中的光速。布里渊散射光和瑞利散射光沿光纤返回经光纤光栅后，布里渊散射光会透过光纤光栅，与耦合器（2）中的另一路光信号一起进入耦合器（9）。它们的混合信号经响应频率为11.2GHz左右的平衡光电探测器（10）检测后转换为电信号，再通过信号处理单元得到布里渊频移的大小，便可实现对应变的全分布式测量。瑞利散射光被光纤光栅反射后，经过检偏器，输出检偏器的光信号强度会随瑞利散射光偏振态的变化而变化。这路信号由响应频率为0~50MHz光电探测器（12）转换为电信号后，通过信号处理单元处理后便可得到光纤中偏振态的变化情况，实现对振动的全分布式测量。As a specific implementation example, let the working wavelength of the laser be

It is 1550nm, and the line width is 1MHz. The laser light it emits is divided into two paths through a coupler (2), one of which is modulated by an electro-optic modulator and amplified by an erbium-doped fiber amplifier, and enters the sensing fiber as a detection pulse light. The sensing fiber uses common communication fiber, its refractive index, and the speed of sound in the fiber. The pulsed light will produce Rayleigh scattered light and Brillouin scattered light in the sensing fiber optic fiber. The frequency of the Rayleigh scattered light is consistent with the frequency of the laser, and the frequency of the Brillouin scattered light will shift. The Brillouin frequency moved to

. When the sensing fiber is affected by strain, the Brillouin frequency shift of Brillouin scattered light will be changed, and when it is affected by vibration, the polarization state of Rayleigh scattered light and Brillouin scattered light will be changed. The central wavelength of the fiber grating is consistent with the wavelength of the laser, which is 1550nm, and the 3dB bandwidth is

, where is the speed of light in vacuum. After the Brillouin scattered light and Rayleigh scattered light return along the optical fiber and pass through the fiber grating, the Brillouin scattered light will pass through the fiber grating and enter the coupler (9) together with another optical signal in the coupler (2). Their mixed signals are detected by a balanced photodetector (10) with a response frequency of about 11.2 GHz and converted into electrical signals, and then the Brillouin frequency shift is obtained through the signal processing unit, so that the fully distributed measurement of the strain can be realized . After the Rayleigh scattered light is reflected by the fiber grating, it passes through the analyzer, and the output optical signal intensity of the analyzer will change with the change of the polarization state of the Rayleigh scattered light. After the signal is converted into an electrical signal by the photodetector (12) with a response frequency of 0-50MHz, the polarization state change in the optical fiber can be obtained after being processed by the signal processing unit, realizing fully distributed measurement of vibration.

Claims (6)

1. A fully distributed optical fiber strain and vibration sensor, characterized in that it includes a laser (1), a first coupler (2), a pulse modulation module (3), an optical amplifier (4), a circulator (5), a transmission Sensing fiber (6), fiber grating (7), polarization scrambler (8), second coupler (9), balanced photodetector (10), analyzer (11), photodetector (12), signal Processing unit (13); the continuous light output by the laser (1) is divided into two paths after passing through the first coupler (2): the first path is used as a reference light, and is connected to the second coupling after passing through the scrambler (8) The first input terminal of the circulator (9); the second path is injected into the first port of the circulator (5) as the detection pulse light after passing through the pulse modulation module (3) and the optical amplifier (4); the detection pulse light is sent by the circulator (5) The two ports are incident into the sensing fiber (6); the Rayleigh scattered light and Brillouin scattered light generated by the probe pulse light in the sensing fiber (6) return to the second port of the circulator (5), and are transmitted by the circulator (5) The third port is connected to the fiber grating (7) to transmit the Brillouin scattered light and reflect the Rayleigh scattered light at the same time; the transmitted Brillouin scattered light enters the second input end of the second coupler (9) The mixed signal formed in the second coupler (9) with the reference light is input to the balanced photodetector (10) through the two output terminals of the second coupler (9) to be converted into an electrical signal, and then enters the signal processing unit (13) for processing; the Rayleigh scattered light reflected by the fiber grating (7) enters the signal processing unit (13) after being connected to the analyzer (11) and the photodetector (12) through the output port of the circulator (5); Fully distributed sensing of strain and vibration via transmitted Brillouin scattered light signal over reflected Rayleigh scattered light. 2. The fully distributed optical fiber strain and vibration sensor according to claim 1, characterized in that a polarization scrambler is connected to the first path after the first coupler (2), and there is no polarization scrambler in the second path. 3. The fully distributed optical fiber strain and vibration sensor according to claim 1, characterized in that the circulator (5) is a four-port circulator, or two three-port circulators are connected, wherein the first three-port The third port of the port circulator is connected to the first port of the second three-port circulator. 4. The fully distributed optical fiber strain and vibration sensor according to claim 1, characterized in that the line width of the selected laser (1) does not exceed 10MHz, and its working band is the optical fiber communication band within the range of 800nm to 1700nm. 5. The fully distributed optical fiber strain and vibration sensor according to claim 1, characterized in that the center wavelength of the selected fiber grating (7) is the center wavelength of the laser (1), the 3dB bandwidth is less than the width determined by , and can reflect Rayleigh scatters light and transmits Brillouin scattered light; where,

is the refractive index of the sensing fiber (6),

is the speed of sound in the sensing fiber (6), is the central wavelength of the laser (1),

is the speed of light in vacuum. 6. The fully distributed optical fiber strain and vibration sensing method according to claim 1, characterized in that the Rayleigh signal reflected by the fiber grating (7) passes through the analyzer (11) and the photodetector (12) After that, it enters the signal processing unit (13). After processing, the polarization state change information of Rayleigh scattered light is obtained, and the vibration is fully distributed by using the polarized optical time domain reflection (POTDR) method. ) transmitted Brillouin signal light and reference light enter the balanced photodetector (10) through the second coupler (9) for coherent detection, and then obtain the Brillouin scattered light through the signal processing unit (13) For the frequency shift information, the Brillouin optical time domain reflectometry (BOTDR) method is used to monitor the strain in a fully distributed manner.

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