CN102937416B - A kind of fully distributed fiber switched based on orthogonal polarisation state strains and vibration sensing method and device - Google Patents

Abstract

A fully distributed optical fiber strain and vibration sensing device based on orthogonal polarization switching, including a laser (1), a polarization maintaining coupler (2), a polarization maintaining fiber, a pulse modulation module (3), and an optical amplifier (4) , circulator (5), sensing fiber (6), polarization switching device (7), coupler (8), balanced photodetector (9), microwave source (10), mixer (11), band Pass filter (12), signal acquisition and processing unit (13); the continuous light output by the laser is divided into two paths after passing through the polarization maintaining coupler (2): the first path output by the polarization maintaining coupler (2) All connecting fibers are polarization-maintaining fibers, which are input to the first input end of the coupler (8) after passing through the polarization state switching device; the polarization state switching device has two states, which respectively make the polarization direction of the output linearly polarized light Corresponding to the fast axis and the slow axis of the polarization maintaining fiber; the output end of the bandpass filter is connected to the signal acquisition and processing unit.

Description

A fully distributed optical fiber strain and vibration sensing method and device based on orthogonal polarization state switching

technical field

The invention relates to 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, and adopts polarization Fully distributed optical fiber strain and vibration sensing technology with state switching device and coherent heterodyne detection.

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.

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. has emerged. These optical fiber sensing technologies use different physical parameters of light waves in optical fibers for sensing, and can measure different sensing parameters corresponding to physical quantities on optical fibers.

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 Brillouin 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 during the propagation of the pulsed light in the optical fiber, so as to determine the 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 is weak in measuring vibration events. Because although theoretically the optical fiber will produce strain changes at the same time when it is affected by external vibrations. However, on the one hand, the weak strain caused by tiny vibrations has little effect on the Brillouin frequency shift, and on the other hand, the BOTDR technology measures the strain at a slow speed, which usually takes more than ten seconds. Therefore, the BOTDR technique 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 fast disturbances or vibrations. For a sensing distance of 1km, the maximum vibration frequency that POTDR can measure is within 100KHz. However, since the polarization state of light waves in ordinary single-mode optical fibers is difficult to keep constant for a long time, and the strain on the optical fiber is not in one-to-one correspondence with the change of polarization state, it is difficult for POTDR technology to detect quasi-static strain and large strain .

Contents of the invention

The purpose of the present invention is to provide a fully distributed optical fiber sensor that can measure both strain and rapid vibration changes. In particular, a fully distributed optical fiber strain and vibration sensing method and device based on orthogonal polarization state switching. Both strain events and vibration events can be measured, which greatly improves the measurement function and application range of the fully distributed optical fiber sensor, and ensures measurement accuracy.

The technical solution of the present invention is a fully distributed optical fiber strain and vibration sensing device based on orthogonal polarization switching, which is characterized in that it includes a laser (1), a polarization maintaining coupler (2), a polarization maintaining optical fiber, and a pulse modulation Module (3), optical amplifier (4), circulator (5), sensing fiber (6), polarization switching device (7), coupler (8), balanced photodetector (9), microwave source (10 ), a mixer (11), a bandpass filter (12), a signal acquisition and processing unit (13); the output continuous light of the laser (1) is divided into two paths after passing through the polarization maintaining coupler (2): All the connecting fibers in the first path output by the polarization-maintaining coupler are polarization-maintaining fibers, and the light waves in it are used as reference light, and then input to the first input end of the coupler (8) after passing through the polarization state switching device (7) ; The polarization state switching device (7) has two states, respectively making the polarization direction of the output linearly polarized light correspond to the fast axis and the slow axis of the polarization-maintaining fiber;

The second path output by the polarization-maintaining coupler passes through the pulse modulation module (3) and the optical amplifier (4), and then injects the detection pulse light into the sensing fiber (6) through the circulator (5); the Brillouin scattering in the sensing fiber The light returns through the circulator (5) and enters the second input end of the coupler (8); the mixed signal formed by the Brillouin scattered light and the reference light in the coupler (8) passes through the two outputs of the coupler (8) input to the balanced photodetector (9) to be converted into an electrical signal;

The output end of the balanced photodetector (9) is connected to the first input end of the mixer (11), the microwave source (10) is connected to the second input end of the mixer (11), and the mixer (11) The output end is connected to the band-pass filter (12), and the output end of the band-pass filter (12) is connected to the signal acquisition and processing unit (13).

When the polarization state switching device (7) is in any single state, coherent heterodyne detection is performed by the Brillouin scattered light and the reference light to obtain the change information of the polarization state of the scattered light, so as to determine the external vibration event information.

The polarization state switching device (7) can be any device or instrument that can switch the polarization state of linearly polarized light in two orthogonal polarization directions, such as a polarization switch, a polarization controller, or a polarization analyzer. The polarizations in the two orthogonal directions respectively correspond to the first and second working states of the polarization state switching device (7).

According to a fully distributed optical fiber sensor that can measure both strain and rapid vibration changes provided by the present invention for measuring optical fiber sensing method, when the polarization state switching device (7) is in any working state, the sensing optical fiber The obtained signal is the result of the coherent heterodyne detection of the scattered light signal in the sensing fiber and the linearly polarized light whose polarization direction is along the fast axis or slow axis of the polarization maintaining fiber through the coupler (8) and the balanced photodetector (9). ; When a certain position of the sensing fiber (6) is affected by strain, the magnitude of the Brillouin frequency shift of the Brillouin scattered light generated at this position changes; when affected by vibration, the polarization of the detected pulsed light at this position When the state changes, the polarization state of the Brillouin scattered light in the sensing fiber (6) changes; therefore, by measuring the center frequency of the Brillouin scattered light, the strain of the sensing fiber (6) is sensed, The vibration of the sensing fiber (6) is sensed by measuring the polarization state change of the Brillouin scattered light.

Further, the signal obtained by the sensing fiber (6) contains the information of the polarization state change of the Brillouin scattered light, which is measured by the principle of polarized light time-domain reflectometry (POTDR), and the vibration is transmitted by using the information of the change of the polarization state of the scattered light. feel.

Furthermore, when the polarization state switching device (7) works for equal time in the first and second working states respectively, and then accumulates the heterodyne signal power obtained in these two working states, the sensing can be eliminated Effects of changes in the polarization state of light waves in an optical fiber. At this time, the principle of Brillouin Optical Time Domain Reflectometry (BOTDR) can be used for measurement, and the frequency information of Brillouin scattered light can be used to sense the strain in the optical fiber.

When the polarization state switching device (7) is in any working state, since the polarization states of the pulsed light at different positions in the fiber are different, the pulsed light obtained after coherent heterodyne detection by the balanced photodetector (9) is in the fiber The Brillouin signal generated along the line has corresponding fluctuations. After measuring the Brillouin signal in the optical fiber several times at different times, by comparing the fluctuations between them or analyzing the frequency spectrum of the fluctuations at each position of the optical fiber by using the method of time-frequency transformation, the pulsed light in the optical fiber can be obtained. The change in polarization state at a location.

When the pulsed light is transmitted in the sensing fiber, it will continuously generate Brillouin scattered light along the fiber, and its center frequency has a certain offset from the center frequency of the detection light. This offset is called the Brillouin frequency shift v _B ;Set the frequency of the microwave source (10) to v _M (v _M is relatively close to v _B ), then after the high-frequency signal passes through the mixer (11), its center frequency will be converted to |v _B -v _M |; By scanning the frequency of the microwave source (10) to adjust the frequency of the output signal of the mixer (11), when the signal power through the band-pass filter (12) reaches the maximum, the Brillouin scattered light can be obtained Brillouin frequency shift, whose magnitude is v _B =v _M -f _BPF , where f _BPF is the center frequency of the band-pass filter (12). The scattered light used by the sensing device when measuring vibration and strain is Brillouin scattered light.

Beneficial effects of the present invention: the present invention combines BOTDR technology and POTDR technology, which can measure both strain events and vibration events, which greatly improves the measurement function and application range of fully distributed optical fiber sensors, and can greatly reduce system omissions Rate. The invention only uses the Brillouin scattered light signal to sense the strain and the vibration, the system device is simple, the overall cost and the number of devices are much smaller than the separate superposition of the two systems. While using POTDR technology for vibration measurement, by switching the working state of the polarization state switching device, the measurement of strain using BOTDR technology can be realized, so the measurement of vibration and strain can be carried out at the same time, and the measurement time is shorter than that of using BOTDR technology and respectively. POTDR technology greatly shortens the measurement time of strain and vibration, and can realize uninterrupted measurement of strain and vibration.

Description of drawings

FIG. 1 is a structural diagram of a fully distributed optical fiber strain and vibration sensing device based on polarization switching provided by the present invention.

Detailed ways

As shown in Figure 1, the structure of a fully distributed optical fiber strain and vibration sensing device based on orthogonal polarization switching, including a laser (1), a polarization maintaining coupler (2), and a polarization maintaining fiber (2-1) , pulse modulation module (3), optical amplifier (4), circulator (5), sensing fiber (6), polarization switching device (7), coupler (8), balanced photodetector (9), microwave source (10), mixer (11), band-pass filter (12), signal acquisition and processing unit (13). The continuous light output by the laser (1) is divided into two paths after passing through the polarization maintaining coupler (2): all the connecting fibers in the first path output by the polarization maintaining coupler are polarization maintaining fibers, and the light waves in it are used as reference light , after passing through the polarization state switching device (7), it is input to the first input end of the coupler (8); the polarization state switching device (7) has two states, respectively making the polarization direction of the output linearly polarized light correspond to The fast axis and the slow axis of the polarizing fiber; the second path output by the polarization maintaining coupler passes through the pulse modulation module (3) and the optical amplifier (4), and then injects the detection pulse light into the sensing fiber (6) through the circulator (5); The Brillouin scattered light in the sensing fiber returns through the circulator (5) and then enters the second input end of the coupler (8); the mixed signal formed by the Brillouin scattered light and the reference light in the coupler (8) passes through The two output terminals of the coupler (8) are input into the balanced photodetector (9) and converted into electrical signals.

The output end of the balanced photodetector (9) is connected to the first input end of the mixer (11), the microwave source (10) is connected to the second input end of the mixer (11), and the mixer (11) The output end is connected to the band-pass filter (12), and the output end of the band-pass filter (12) is connected to the signal acquisition and processing unit (13).

The connecting optical fiber of the reference optical path is a polarization maintaining optical fiber, and the optical fiber of the detection optical path can be a single-mode optical fiber or a polarization maintaining optical fiber, and the effect of the polarization maintaining optical fiber is better.

The polarization state switching device (7) can be any device or instrument that can switch the polarization state of linearly polarized light in two orthogonal polarization directions, such as a polarization switch, a polarization controller, or a polarization analyzer. The polarizations in the two orthogonal directions respectively correspond to the first and second working states of the polarization state switching device (7).

The pulse modulation module (3) is located in the detection optical path, and the polarization state switching device (7) is located in the reference optical path. The pulse modulation module is used to modulate light waves into pulsed light. The width of the pulsed light is determined by the spatial resolution required for measurement, generally between 10 ns and 10 μs.

The scattered light used by the sensing device when measuring vibration and strain is Brillouin scattered light.

As an example of measuring strain and vibration, the specific implementation steps are as follows:

The continuous light emitted by the laser (1) is linearly polarized light with a central wavelength of 1550nm, a line width of 2MHz, and a power of 10dBm. The light wave is divided into two paths after passing through the polarization maintaining coupler (2), and the first path is used as a reference light. The first input of a coupler (8) with a coupling ratio of 50:50. The second light is modulated by the pulse modulation module (3) (electro-optic modulator) into pulsed light with a pulse width of 20 ns, and then amplified by the optical amplifier (4) (erbium-doped fiber amplifier), and then passed through the circulator ( 5) Enter the sensing fiber (6). The sensing fiber (6) is an ordinary single-mode fiber, its refractive index n=1.46, and the sound velocity v _a in the fiber is 5945m/s. When the pulsed light is transmitted in the sensing fiber, it will continuously generate Brillouin scattered light along the fiber, and its center frequency has a certain offset from the center frequency of the detection light. This offset is called the Brillouin frequency shift v _B , and its size v _B =2nv _a /λ=11.2GHz. When a certain position of the sensing fiber is affected by strain, the magnitude of the Brillouin frequency shift of the Brillouin scattered light generated at this position will change, and when it is affected by vibration, the polarization of the detected pulse light at this position will be changed. The state changes, which in turn causes the polarization state of the Brillouin scattered light generated in the fiber to change. Therefore, by measuring the center frequency of the Brillouin scattered light, the strain of the optical fiber can be sensed, and by measuring the polarization state change of the Brillouin scattered light, the vibration of the optical fiber can be sensed.

After the Brillouin scattered light returns along the optical fiber, it enters the coupler (8) together with the reference light in the first path. Their mixed signals are converted into electrical signals after being received by a balanced photodetector (9) whose response frequency can cover about 11.2 GHz. The frequency of this electrical signal is the frequency difference between the Brillouin scattered light and the reference light, and its center frequency is v _B . Set the frequency of the microwave source (10) to v _B (v _M is closer to v _B ), then the high-frequency signal will be converted to |v _B -v _M | . By scanning the frequency of the microwave source (10) to adjust the frequency of the output signal of the mixer (11), when the signal power passing through the band-pass filter (12) reaches the maximum, the Brillouin scattered light can be obtained Deep frequency shift, its magnitude is v _B =v _M -f _BPF , where f _BPF is the center frequency of the band-pass filter (12).

When the detection pulse light is transmitted in the sensing fiber (6), it will continuously generate backward Brillouin scattered light. In order to avoid the mutual interference of the Brillouin scattered light generated by different detection pulse lights in the sensing fiber (6), When collecting signals, only one detection pulse light is allowed to exist in the sensing optical fiber (6) at the same time. The pulse modulation module (3) is thus synchronized with the signal acquisition and processing unit (13). When receiving the Brillouin scattered light, start timing from the moment when the detection pulse light is incident on the sensing fiber (6) until the Brillouin scattered light generated by the detection pulse light at the end of the sensing fiber (6) is received. The pulse modulation module (3) generates the next detection pulse light. The position l where Brillouin scattered light is generated in the sensing fiber (6) can be obtained from the speed of light in the fiber and the time difference, that is, l=cΔt/2n, where c≈3×10 ⁸ m/s is the speed of light in vacuum, Δt is the elapsed time from the start of timing to receiving the Brillouin scattered light at the desired location. For the sensing fiber (6) with a length of L, the time interval ΔT between two detection pulsed lights should be greater than 2nL/c, which is the time required for the pulsed light to go back and forth in the fiber once. If the length of the sensing fiber (6) is 10km, then 2nL/c ≈100μs.

Every time the microwave source changes to a new frequency, the polarization state switching device (7) first works in the first working state, so that the polarization direction of the reference light is along the slow axis of the polarization-maintaining fiber in the first path. Since the polarization states of the Brillouin scattered light generated from different positions of the optical fiber are different, the Brillouin scattered light generated from different positions of the optical fiber is mixed with the reference light and obtained after coherent heterodyne detection by the balanced photodetector (9) The signal will produce corresponding fluctuations, which will affect the accuracy of the Brillouin frequency shift measurement, but it also contains information about the polarization state of the light wave along the fiber. Since the energy of Brillouin scattered light is very small, in order to obtain a higher signal-to-noise ratio, each time the microwave source changes to a new frequency, it is necessary to accumulate and average the obtained signal N times. When the polarization state switching device (7) is in the first working state, the signal is repeatedly measured N/2 times, so the required time is NΔT/2. If the length of the sensing fiber (6) is 10km, the time interval between two pulsed lights ΔT=120μs, and the average number of times N=16384 times, the time NΔT/ 2≈1s. Then, by comparing the changes of the signals obtained by these N/2 measurements at different positions of the optical fiber or analyzing the frequency spectrum of these signals at different positions of the optical fiber by using the method of time-frequency transformation, the change of the polarization state of the light wave along the optical fiber can be obtained, and then it can be realized. Sensing of vibrations along optical fibers. Then make the polarization state switching device (7) work in the second working state, so that the polarization direction of the reference light is along the fast axis of the polarization-maintaining fiber in the first path. The signal obtained at this time also contains information about the polarization state of the light wave along the fiber. In this state, the signal is repeatedly measured N/2 times, and the change of the polarization state of the light wave along the fiber can also be obtained through analysis, thereby ensuring uninterrupted sensing and measurement of the vibration along the fiber. Accumulate and average the signals measured N/2 times by the polarization state switching device (7) in two working states, which can eliminate the fluctuation of the coherent heterodyne signal caused by the polarization state of the Brillouin scattered light along the optical fiber. The ups and downs are also equivalent to averaging the Brillouin signal N times, thereby improving the accuracy of the power measurement of the Brillouin signal. Then change the frequency of the microwave source again, and repeat the above-mentioned measurement steps to finally obtain the Brillouin frequency shift, and then realize the measurement of the strain in the optical fiber.

Claims (5)

1. A fully distributed optical fiber strain and vibration sensing device based on orthogonal polarization switching, which is characterized by including a laser (1), a polarization maintaining coupler (2), a polarization maintaining fiber (2-1), a pulse modulation Module (3), optical amplifier (4), circulator (5), sensing fiber (6), polarization switching device (7), coupler (8), balanced photodetector (9), microwave source (10 ), a mixer (11), a bandpass filter (12), a signal acquisition and processing unit (13); the output continuous light of the laser (1) is divided into two paths after passing through the polarization maintaining coupler (2): All the connecting fibers in the first path output by the polarization-maintaining coupler (2) are polarization-maintaining fibers, and the light wave in it is used as a reference light. An input end; the polarization state switching device (7) has two states, respectively making the polarization direction of the output linearly polarized light correspond to the fast axis and the slow axis of the polarization-maintaining fiber; The second path output by the polarization-maintaining coupler (2) passes through the pulse modulation module (3) and the optical amplifier (4), and then injects the detection pulse light into the sensing fiber (6) through the circulator (5); the sensing fiber (6) The Brillouin scattered light in the circulator (5) returns to the second input end of the coupler (8); the mixed signal formed by the Brillouin scattered light and the reference light in the coupler (8) passes through the coupler ( The two output terminals of 8) are input into the balanced photodetector (9) and converted into electrical signals; The output end of the balanced photodetector (9) is connected to the first input end of the mixer (11), the microwave source (10) is connected to the second input end of the mixer (11), and the mixer (11) The output end is connected to the band-pass filter (12), and the output end of the band-pass filter (12) is connected to the signal acquisition and processing unit (13); the polarization state of the linearly polarized light is switched in two orthogonal polarization directions The polarization state switching device (7) is a polarization switch, a polarization controller or a polarization analyzer, and the polarizations of linearly polarized light in two orthogonal directions that are mutually orthogonal correspond to the first and second polarization state switching devices (7) respectively. Two working states; the pulse modulation module is located in the detection optical path, and the polarization state switching device is located in the reference optical path. 2. The fully distributed optical fiber strain and vibration sensing device based on orthogonal polarization state switching according to claim 1, characterized in that when the polarization state switching device (7) is in any working state, the sensing fiber ( 6) The obtained signal is the scattered light signal in the sensing fiber (6) and the linearly polarized light whose polarization direction is along the fast axis or slow axis of the polarization-maintaining fiber, through the coupler (8) and the balanced photodetector (9). The result after differential detection; when a certain position of the sensing fiber (6) is affected by strain, the magnitude of the Brillouin frequency shift of the Brillouin scattered light generated at this position changes; when affected by vibration, the The polarization state of the detection pulse light changes, and then the polarization state of the Brillouin scattered light in the sensing fiber (6) changes; therefore, by measuring the center frequency of the Brillouin scattered light, the sensing fiber (6) The strain is sensed, and the vibration of the sensing fiber (6) is sensed by measuring the polarization state change of the Brillouin scattered light; The signal obtained by the sensing fiber (6) contains the information on the change of the polarization state of the Brillouin scattered light, which is measured by the polarized light time domain reflection method, and the vibration is sensed by using the information on the change of the polarization state of the scattered light. 3. The fully distributed optical fiber strain and vibration sensing device based on orthogonal polarization switching according to claim 2, characterized in that when the polarization switching device (7) is in the first and second working states respectively Working for a long time, adding the heterodyne signal power obtained under these two working conditions can eliminate the influence of the change of the polarization state of the light wave in the sensing fiber; at this time, the Brillouin optical time domain reflection method can be used for measurement. The strain in the optical fiber is sensed by using the frequency information of Brillouin scattered light. 4. The fully distributed optical fiber strain and vibration sensing device based on orthogonal polarization state switching according to claim 3, characterized in that when the polarization state switching device (7) is in any working state, due to the pulsed light in The polarization states at different positions in the optical fiber are different, and the Brillouin signal generated by the pulsed light obtained after coherent heterodyne detection by the balanced photodetector (9) along the optical fiber has corresponding fluctuations; multiple measurements at different times After the Brillouin signal in the optical fiber, the polarization state of the pulsed light in the optical fiber at a certain position can be obtained by comparing the fluctuations between them or analyzing the frequency spectrum of the fluctuations at various positions in the optical fiber by using the time-frequency transformation method. Changes. 5. The fully distributed optical fiber strain and vibration sensing device based on orthogonal polarization switching according to claim 3, characterized in that when the pulsed light is transmitted in the sensing optical fiber, Brillouin scattered light will be continuously generated along the optical fiber , its center frequency has a certain offset relative to the center frequency of the probe light, this offset is called the Brillouin frequency shift ν _B ; the frequency of the microwave source (10) is set to ν _M , and ν _M is close to ν _B , the center frequency of the high-frequency signal will be converted to ∣ ν _B -ν _M ∣ after passing through the mixer (11); the output signal of the mixer (11) is adjusted by scanning the frequency of the microwave source (10) When the signal power of the bandpass filter (12) reaches the maximum, the Brillouin frequency shift of the Brillouin scattered light can be obtained, and its magnitude is ν _B = ν _M -f _BPF , where f _BPF is Center frequency of the bandpass filter (12).