A long-distance distributed optical fiber broadband acoustic vibration sensing system

Abstract

The application belongs to the technical field of distributed optical fiber sensing, and particularly relates to a long-distance distributed optical fiber broadband acoustic vibration sensing system. The application utilizes the same laser to transmit and generate back Rayleigh scattering light and interference light in the same sensing optical fiber, and further constructs a distributed optical fiber acoustic vibration sensing system combined with Φ-OTDR and MZI, which comprehensively utilizes the performance advantages of both, utilizes a very simplified optical system scheme, realizes the function fusion of the two sensing structures on one sensing optical fiber, reduces the resource waste caused by the transmission of sensing signals by different sensing optical cables respectively, maximizes the optical fiber resource utilization rate, and reduces the system cost; utilizes a frequency division multiplexing demodulation scheme in the digital domain, expands the frequency response range on the basis of ensuring the high positioning accuracy of the system, realizes the simultaneous detection of the two systems by a single core, simultaneously realizes the high positioning accuracy and the continuous non-blind area detection in the broadband frequency range, and improves the reliability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of distributed optical fiber sensing, and specifically relates to a long-distance distributed optical fiber broadband acoustic vibration sensing system. BACKGROUND

[0002] Due to the unique advantages of simple structure and fast response speed, the distributed optical fiber vibration sensing system has become one of the research focuses of domestic and foreign optical fiber sensing research teams in recent decades, and is widely applied to oil and gas pipeline detection, perimeter security, structure health monitoring and other fields.

[0003] The phase sensitive-optical time domain reflectometry (Ф-OTDR) based on backscattered light has been widely applied due to its excellent multi-point detection capability and high sensitivity. The principle of Ф-OTDR is that the light signal emitted by the laser is modulated into a pulse signal and injected into the sensing optical fiber, and the light signal will produce backscattered light in the sensing optical fiber; when a disturbance occurs at a certain place of the sensing optical fiber, the phase of the backscattered light will change, and through demodulation of the backscattered light, the accurate position and frequency information of the vibration signal can be obtained. However, the length of the sensing optical fiber limits the detectable frequency range of the Ф-OTDR.

[0004] On the other hand, the maximum frequency response of the optical fiber interferometer structure sensor can be as high as hundreds of kHz or even tens of MHz, and it is only limited by the original sampling rate of the data acquisition device, but it cannot determine the position of the vibration because the continuous probe light does not carry position information. Through the fusion of multiple interferometers such as Mach-Zehnder interferometer (MZI), Sagnac interferometer (SI) and Michelson interferometer (MI), a spatial resolution of only tens of meters can be achieved, which is not enough to accurately locate the position of the acoustic vibration event.

[0005] To enhance the frequency response of the Ф-OTDR system, Q.He et al., in “All fiber distributed vibration sensing using modulated time-difference pulse,” IEEE Photonics Technol. Lett. 25(20), 1955-1957 (2013), used a time-division multiplexing pulse modulation scheme to integrate the Ф-OTDR system and the MZI interferometer. The disadvantage is that there is a blind zone in the detectable frequency range. H.He et al., in “Multiple vibration measurement using phase-sensitive OTDR merged with Mach-Zehnder interferometer based on frequency division multiplexing,” Opt. Exp., vol. 24, no. 5, pp. 4842, Mar. 2016, integrated the Ф-OTDR system and the MZI interferometer using a frequency division multiplexing scheme. However, the ring structure of this system halves the detectable distance and cannot achieve single-end detection, requiring a large amount of fiber optic resources. In addition, Chinese patent CN 113607261 B discloses "a fiber optic sensing system that integrates phase-sensitive optical time-domain reflectometry and fiber optic interferometry", which uses a polarization diversity structure to integrate a Ф-OTDR and a fiber optic interferometer. This scheme suffers from problems such as interference fading and polarization fading, which limit the sensing performance of the integrated system. Chinese patent CN 110657878 A discloses "a distributed fiber optic sensing system based on a Mach-Zehnder interferometer and a Ф-OTDR", which uses multi-core optical fibers to integrate a Ф-OTDR and a fiber optic interferometer. This scheme also requires a large amount of optical fiber resources.

[0006] In practical engineering applications, distributed fiber optic acoustic vibration sensing systems, in addition to high-precision disturbance localization, also need to address the issues of wider frequency response and lower fiber optic resource consumption. Summary of the Invention

[0007] To address the problems or shortcomings in the aforementioned background technology, this invention aims to provide a long-distance distributed fiber optic broadband acoustic vibration sensing system. It utilizes the same laser transmitted in the same sensing fiber to generate backscattered Rayleigh light and interference light, creating a simplified optical structure that integrates Ф-OTDR and MZI distributed fiber optic acoustic vibration sensing. While ensuring high positioning accuracy, it expands the frequency response range and enables simultaneous detection of two systems per core, improving fiber optic resource utilization. Furthermore, by employing a frequency division multiplexing demodulation scheme in the digital domain, it simultaneously achieves high positioning accuracy and continuous, blind-zone-free detection over a wide frequency range.

[0008] To achieve the above objectives, the present invention adopts the following solution:

[0009] A long-distance distributed fiber optic broadband acoustic and vibration sensing system includes a laser, a first coupler, a modulator, a fiber amplifier, a second coupler, a third coupler, a circulator, a sensing fiber, an FBG, a fourth coupler, a photodetector, and a data acquisition and processing module.

[0010] The laser is connected to the first coupler, which splits the generated continuous optical signal into two paths:

[0011] The continuous optical signal in the first optical path is modulated into a pulsed optical signal by a modulator, then power-compensated by an optical fiber amplifier, and connected to port two of the third coupler. Port one of the third coupler is connected to port one of the circulator, and the optical pulse is guided into the sensing optical fiber through port two of the circulator. The backscattered Rayleigh light signal carrying sensing information returns to port two of the circulator and is input to port two of the fourth coupler through port three of the circulator. The continuous optical signal in the second optical path is connected to port one of the second coupler, and port three of the second coupler is connected to port one of the fourth coupler. The fourth coupler couples the two signals input from its ports one and two, and the resulting coherent light is processed by the data acquisition and processing module through a photodetector to obtain the position and frequency information of the sensing signal. This optical path is a Ф-OTDR sensing system.

[0012] The continuous optical signal (which can be frequency-shifted) in the second optical path is connected to port one of the second coupler. Port two of the second coupler is connected to port three of the third coupler, and port three of the second coupler is connected to port one of the fourth coupler. Port one of the third coupler is connected to port one of the circulator, and the optical pulse is guided into the sensing fiber through port two of the circulator. The tail of the sensing fiber is connected to the FBG (Fiber Optic Gateway), which propagates the continuous light wave backward to port two of the circulator, and then inputs it to port two of the fourth coupler through port three of the circulator. The fourth coupler couples the two signals input from port one and port two, and the resulting coherent light is transmitted to the data acquisition and processing module via a photodetector. After processing, the frequency information of the sensing signal is obtained. This optical path is a fiber optic MZI sensing system.

[0013] The laser, first coupler, second coupler, third coupler, circulator, sensing fiber, fourth coupler, photodetector, and data acquisition and processing module are shared by the two sensing systems.

[0014] Preferably, the modulator of the first optical path is a combination structure in which a continuous optical signal is first modulated into a chirped pulse optical signal by an electro-optic modulator and then the extinction ratio is enhanced by an acousto-optic modulator, so as to make the pulse signal better.

[0015] Preferably, the continuous optical signal of the second optical path is frequency-shifted by a modulator and then connected to port one of the second coupler, which is different from the first optical path to form a difference frequency Ф-OTDR sensing system, which is used to reduce the amount of data and reduce the performance requirements of the data acquisition and processing module.

[0016] Preferably, the first coupler is a 1×2 single-mode fiber coupler with a splitting ratio of 99:1.

[0017] Preferably, the second and third couplers are both 1×2 single-mode fiber couplers with a splitting ratio of 50:50; the fourth coupler is a 2×2 single-mode fiber coupler with a splitting ratio of 50:50.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] (1) This invention utilizes the same laser to transmit and generate backscattered Rayleigh light and interference light in the same sensing fiber, thereby constructing a system that integrates Ф-OTDR and MZI, reducing the waste of resources caused by transmitting sensing signals separately using different sensing optical cables, maximizing the utilization rate of optical fiber resources, reducing system costs, and improving reliability.

[0020] (2) The present invention utilizes a frequency division multiplexing demodulation scheme in the digital domain to achieve both high positioning accuracy and continuous blind-zone-free detection in a wide frequency range. Attached Figure Description

[0021] Figure 1 This is a schematic block diagram of the long-distance distributed fiber optic broadband acoustic and vibration sensing system in Example 1;

[0022] Figure 2 This is a schematic block diagram of the long-distance distributed fiber optic broadband acoustic and vibration sensing system in Example 2;

[0023] Figure 3 The backscattered Rayleigh signal obtained by the Ф-OTDR sensing system in Example 2;

[0024] Figure 4 This is the positioning result of the Ф-OTDR sensing system in Example 2 with PZT perturbation applied at 500m of the sensing fiber;

[0025] Figure 5 This is the positioning result of the Ф-OTDR sensing system in Example 2 with PZT perturbation applied at 5600m of the sensing fiber;

[0026] Figure 6 This is the positioning result of the Ф-OTDR sensing system in Example 2 with PZT perturbation applied at 26000m of the sensing fiber;

[0027] Figure 7The time and frequency domain results of the Ф-OTDR sensing system in Example 2 with a PZT perturbation signal applied at 500m of the sensing fiber;

[0028] Figure 8 The time and frequency domain results are shown for the MZI sensing system in Example 2 when a PZT perturbation signal is applied at 500m of the sensing fiber. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0030] Example 1

[0031] The long-distance distributed fiber optic broadband acoustic and vibration sensing system provided in this embodiment, such as Figure 1 As shown:

[0032] In the Ф-OTDR sensing system: the laser is connected to the first coupler, splitting the continuous optical signal into two paths. The two signals enter the electro-optic modulator and the second coupler respectively. The electro-optic modulator modulates the continuous light into a chirped pulse light signal, which is then enhanced by the acousto-optic modulator and connected to the fiber amplifier. The fiber amplifier performs power compensation on the input optical pulse signal and connects to port two of the third coupler. Port one of the third coupler is connected to port one of the circulator, and the optical pulse is guided into the sensing fiber through port two of the circulator. The backscattered Rayleigh light signal carrying the sensing information returns to port two of the circulator and is input to port two of the fourth coupler through port three of the circulator. The fourth coupler couples the two signals input from port one and port two. The resulting coherent light is processed by the data acquisition and processing module through the photodetector to obtain the sensing position and frequency information.

[0033] In the fiber optic MZI sensing system: the laser is connected to port 1 of the second coupler, port 2 of the second coupler is connected to port 3 of the third coupler, and port 3 of the second coupler is connected to port 1 of the fourth coupler; port 1 of the third coupler is connected to port 1 of the circulator, and optical pulses are introduced into the sensing fiber through port 2 of the circulator. The tail of the sensing fiber is connected to the FBG, which propagates the continuous light wave in the reverse direction to port 2 of the circulator, and then inputs it to port 2 of the fourth coupler through port 3 of the circulator. The fourth coupler couples the two signals input from port 1 and port 2, and the resulting coherent light is processed by the data acquisition and processing module through the photodetector to obtain the sensing frequency information.

[0034] Example 2

[0035] The long-distance distributed fiber optic broadband acoustic and vibration sensing system provided in this embodiment, such as Figure 2 As shown:

[0036] In the Ф-OTDR sensing system: the laser is connected to the first coupler, splitting the continuous optical signal into two paths. The two signals enter the first and second acousto-optic modulators, respectively. The first acousto-optic modulator modulates the continuous light into a specific pulsed optical signal and then connects it to the fiber amplifier. The fiber amplifier performs power compensation on the input optical pulse signal and then connects it to port two of the third coupler. Port one of the third coupler is connected to port one of the circulator, and the optical pulse is guided into the sensing fiber through port two of the circulator. The backscattered Rayleigh light signal carrying the sensing information returns to port two of the circulator and is input to port two of the fourth coupler through port three. The fourth coupler couples the two signals input from port one and port two. The resulting coherent light is processed by the data acquisition and processing module through the photodetector to obtain the sensing position and frequency information.

[0037] In the fiber optic MZI sensing system: the second acousto-optic modulator frequency-shifts the input continuous light and connects it to port one of the second coupler; port two of the second coupler is connected to port three of the third coupler; and port three of the second coupler is connected to port one of the fourth coupler. Port one of the third coupler is connected to port one of the circulator and guides the optical pulse into the sensing fiber through port two of the circulator. The tail of the sensing fiber is connected to the FBG, which propagates the continuous light wave in the reverse direction to port two of the circulator and inputs it to port two of the fourth coupler through port three of the circulator. The fourth coupler couples the two signals input from port one and port two, and the resulting coherent light is processed by the data acquisition and processing module through the photodetector to obtain the sensing frequency information.

[0038] In Example 2: the first coupler is a 1×2 single-mode fiber coupler with a splitting ratio of 99:1; the second and third couplers are both 1×2 single-mode fiber couplers with a splitting ratio of 50:50; the fourth coupler is a 2×2 single-mode fiber coupler with a splitting ratio of 50:50; in the fiber MZI sensing system, the center wavelength of the FBG is 1550.120nm. By applying pressure to the FBG to change its center wavelength, the interference light intensity of the fiber MZI sensing system is controlled.

[0039] Taking the system in Example 2 as an example, the backscattered Rayleigh light obtained is as follows: Figure 3 As shown, the positioning results obtained after processing the backscattered Rayleigh light collected when PZT perturbation was applied at 500m, 5600m and 26000m of the sensing fiber are as follows: Figure 4 , Figure 5 and Figure 6 As shown, Figure 3 The time and frequency domain plots of the corresponding PZT signal are as follows: Figure 7 As shown. The time and frequency domain diagrams of the interference signal acquired during the disturbance are as follows. Figure 8 As shown.

[0040] As can be seen from the above embodiments, this invention utilizes the same laser to transmit and generate backscattered Rayleigh light and interference light in the same sensing fiber, thereby constructing a distributed fiber optic acoustic vibration sensing system that integrates Ф-OTDR and MZI. It combines the performance advantages of both, and uses an extremely simplified optical system scheme to achieve the functional integration of two sensing structures on a single sensing fiber. This reduces the resource waste caused by transmitting sensing signals separately using different sensing optical cables, maximizes the utilization rate of fiber resources, and reduces system costs. By using a frequency division multiplexing demodulation scheme in the digital domain, the frequency response range is expanded while ensuring high positioning accuracy of the system, enabling simultaneous detection of dual systems on a single core. This achieves both high positioning accuracy and continuous, blind-zone-free detection in a wide frequency range, improving reliability.

Claims

1. A long-distance distributed fiber optic broadband acoustic vibration sensing system, characterized in that: It includes a laser, a first coupler, a modulator, an optical fiber amplifier, a second coupler, a third coupler, a circulator, a sensing optical fiber, an FBG, a fourth coupler, a photodetector, and a data acquisition and processing module; The laser is connected to a first coupler, which splits the generated continuous optical signal into two paths: The continuous optical signal in the first optical path is modulated into a pulsed optical signal by a modulator, then power-compensated by an optical fiber amplifier, and connected to port two of the third coupler. Port one of the third coupler is connected to port one of the circulator, and the optical pulse is guided into the sensing optical fiber through port two of the circulator. The backscattered Rayleigh light signal carrying sensing information returns to port two of the circulator and is input to port two of the fourth coupler through port three of the circulator. The continuous optical signal in the second optical path is connected to port one of the second coupler, and port three of the second coupler is connected to port one of the fourth coupler. The fourth coupler couples the two signals input from its ports one and two, and the resulting coherent light is processed by the data acquisition and processing module through a photodetector to obtain the position and frequency information of the sensing signal. This optical path is a Ф-OTDR sensing system; The continuous optical signal of the second optical path is connected to port one of the second coupler. Port two of the second coupler is connected to port three of the third coupler. Port three of the second coupler is connected to port one of the fourth coupler. Port one of the third coupler is connected to port one of the circulator. The optical pulse is guided into the sensing fiber through port two of the circulator. The tail of the sensing fiber is connected to the FBG, which propagates the continuous light wave in the reverse direction to port two of the circulator. The light is then input to port two of the fourth coupler through port three of the circulator. The fourth coupler couples the two signals input from port one and port two. The resulting coherent light is transmitted to the data acquisition and processing module through the photodetector. After processing, the frequency information of the sensing signal is obtained. This optical path is a fiber optic MZI sensing system; The laser, first coupler, second coupler, third coupler, circulator, sensing fiber, fourth coupler, photodetector, and data acquisition and processing module are shared by the two sensing systems.

2. The long-distance distributed fiber optic broadband acoustic and vibration sensing system as described in claim 1, characterized in that: The modulator of the first optical path is a combination structure that first uses an electro-optic modulator to modulate the continuous optical signal into a chirped pulse optical signal, and then uses an acousto-optic modulator to enhance the extinction ratio.

3. The long-distance distributed fiber optic broadband acoustic and vibration sensing system as described in claim 1, characterized in that: The continuous optical signal of the second optical path is frequency-shifted by a modulator and then connected to port one of the second coupler, which is different from the first optical path to form a difference frequency Ф-OTDR sensing system.

4. The long-distance distributed fiber optic broadband acoustic and vibration sensing system as described in claim 1, characterized in that: The first coupler is a 1×2 single-mode fiber coupler with a splitting ratio of 99:

1.

5. The long-distance distributed fiber optic broadband acoustic and vibration sensing system as described in claim 1, characterized in that: The second and third couplers are both 1×2 single-mode fiber couplers with a splitting ratio of 50:50; the fourth coupler is a 2×2 single-mode fiber coupler with a splitting ratio of 50:

50.

6. The long-distance distributed fiber optic broadband acoustic and vibration sensing system as described in claim 1, characterized in that: In the fiber optic MZI sensing system, the center wavelength of the FBG is 1550.120nm. By applying pressure to the FBG to change its center wavelength, the interference light intensity of the fiber optic MZI sensing system can be controlled.

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