Long-distance distributed optical fiber broadband acoustic vibration sensing system

Abstract

The invention 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. According to the invention, backward Rayleigh scattering light and interference light are transmitted and generated in the same sensing optical fiber by using the same laser, so that a phi-OTDR and MZI fused distributed optical fiber acoustic vibration sensing system is constructed, the performance advantages of the phi-OTDR and MZI fused distributed optical fiber acoustic vibration sensing system are integrated, and the function fusion of two sensing structures is realized on one sensing optical fiber by using an extremely simplified optical system scheme; the resource waste caused by respectively transmitting sensing signals by using different sensing optical cables is reduced, the utilization rate of optical fiber resources is maximized, and the system cost is reduced; by utilizing a frequency division multiplexing demodulation scheme of a digital domain, on the basis of ensuring high positioning precision of the system, the frequency response range is expanded, simultaneous detection of a single core of the double systems is realized, meanwhile, high positioning precision and continuous non-blind area detection in a broadband frequency range are realized, and the reliability is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of distributed optical fiber sensing, and in particular relates to a long-distance distributed optical fiber broadband acoustic vibration sensing system. Background Art

[0002] Due to the unique advantages of distributed fiber optic vibration sensing systems such as simple structure and fast response speed, it has become one of the research focuses of fiber optic sensing research teams at home and abroad in recent decades, and is widely used in many fields such as oil and gas pipeline inspection, perimeter security, and structural health monitoring.

[0003] Phase sensitive-optical time domain reflectometry (Ф-OTDR) based on backscattered Rayleigh light has been widely used due to its excellent multi-point detection capability and high sensitivity. The principle of Ф-OTDR is as follows: the optical signal emitted by the laser is modulated into a pulse signal and injected into the sensing fiber, and the optical signal will generate backscattered Rayleigh light in the sensing fiber; when a disturbance occurs somewhere in the sensing fiber, the phase of the backscattered Rayleigh light will change, and the precise position and frequency information of the vibration signal can be obtained by demodulating the backscattered Rayleigh light. However, the length of the sensing fiber limits the detectable frequency range of Ф-OTDR.

[0004] On the other hand, the maximum frequency response of fiber interferometer structure sensors can reach hundreds of kHz or even tens of MHz, which is only limited by the original sampling rate of the data acquisition device, but they cannot determine the position of the vibration because the continuous detection 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] In order to enhance the frequency response of the Ф-OTDR system, Q. He et al. adopted a time-division multiplexing pulse modulation scheme to realize the integration of the Ф-OTDR system and the MZI interferometer in “All fiber distributed vibration sensing using modulated time-difference pulse,” IEEE Photonics Technol. Lett. 25 (20), 1955-1957 (2013). However, the disadvantage is that there is a blind spot in the detectable frequency range. H. He et al. used a frequency-division multiplexing scheme to integrate the Ф-OTDR system and the MZI interferometer in “Multiple vibrations 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. However, the ring structure of the system reduces the monitoring distance by half, and single-ended detection cannot be achieved, which requires a large amount of optical fiber resources. In addition, Chinese patent CN 113607261 B discloses "a fiber optic sensing system integrating phase-sensitive optical time-domain reflection and fiber optic interference", which realizes the integration of Ф-OTDR and fiber optic interferometer by using polarization diversity structure. This solution has problems such as interference fading and polarization fading, which limits the sensing performance of the fusion system; Chinese patent CN 110657878 A discloses "a sound-collecting distributed fiber optic sensing system based on Mach-Zehnder interferometer and Ф-OTDR", which realizes the integration of Ф-OTDR and fiber optic interferometer by using multi-core optical fiber. This solution also requires a large amount of optical fiber resources.

[0006] In actual engineering applications, in addition to high-precision disturbance positioning, distributed fiber optic acoustic vibration sensing systems also need to solve the problems of wider frequency band response and lower fiber resource occupancy. Summary of the invention

[0007] In view of the problems or shortcomings in the above-mentioned background technology, the present invention aims to provide a long-distance distributed optical fiber broadband acoustic vibration sensing system, which uses the same laser to transmit and generate backward Rayleigh scattered light and interference light in the same sensing optical fiber. It is a Ф-OTDR and MZI fusion distributed optical fiber acoustic vibration sensing system with a simplified optical structure. On the basis of ensuring the high positioning accuracy of the system, the frequency response range is expanded, and dual-system single-core simultaneous detection is realized, thereby improving the utilization rate of optical fiber resources; by using the frequency division multiplexing demodulation scheme in the digital domain, high positioning accuracy and continuous blind-spot-free detection within a broadband frequency range are simultaneously achieved.

[0008] To achieve the above object, the present invention adopts the following scheme:

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

[0010] The laser is connected to a first coupler, and the generated continuous optical signal is divided into two paths through the first coupler:

[0011] The continuous optical signal of the first optical path is modulated into a pulse optical signal by a modulator, and then power is compensated by an optical fiber amplifier, and then connected to the second port of the third coupler. The first port of the third coupler is connected to the first port of the circulator, and the optical pulse is introduced into the sensing optical fiber through the second port of the circulator. The backward Rayleigh scattered optical signal carrying the sensing information returns to the second port of the circulator, and is input to the second port of the fourth coupler through the third port of the circulator; the continuous optical signal of the second optical path is connected to the first port of the second coupler, and the third port of the second coupler is connected to the first port of the fourth coupler; the fourth coupler couples the two signals input from its first port and the second port, and the generated coherent light is processed by the data acquisition and processing module through the photoelectric detector 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 of the second optical path (which can be frequency-shifted) is connected to the port 1 of the second coupler, the port 2 of the second coupler is connected to the port 3 of the third coupler, the port 3 of the second coupler is connected to the port 1 of the fourth coupler, the port 1 of the third coupler is connected to the port 1 of the circulator, and the optical pulse is introduced into the sensing optical fiber through the port 2 of the circulator. The tail of the sensing optical fiber is connected to the FBG, and the continuous light wave is propagated back to the port 2 of the circulator, and is input to the port 2 of the fourth coupler through the port 3 of the circulator. The fourth coupler couples the two signals input from its port 1 and port 2, and the generated coherent light is transmitted to the data acquisition and processing module through the photodetector, and the frequency information of the sensing signal is obtained after processing. This optical path is a fiber MZI sensing system.

[0013] Among them, the laser, the first coupler, the second coupler, the third coupler, the circulator, the sensing optical fiber, the fourth coupler, the photoelectric detector, and the data acquisition and processing module are common to the two sensing systems.

[0014] Preferably, the modulator of the first optical path is a combined structure that first modulates the continuous optical signal into a chirped pulse optical signal using an electro-optic modulator and then enhances the extinction ratio using 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 1 of the second coupler, which is different from the first optical path to form a difference frequency Ф-OTDR sensing system for reducing the amount of data and lowering 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 coupler and the third coupler are both 1×2 single-mode optical fiber couplers with a splitting ratio of 50:50; and the fourth coupler is a 2×2 single-mode optical fiber coupler with a splitting ratio of 50:50.

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

[0019] (1) The present invention utilizes the same laser to transmit and generate backward Rayleigh scattered light and interference light in the same sensing optical fiber, thereby constructing a system integrating Ф-OTDR and MZI, thereby reducing the resource waste caused by using different sensing optical cables to transmit sensing signals separately, maximizing the utilization 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 simultaneously achieve high positioning accuracy and continuous detection without blind spots within a wideband frequency range. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

[0024] Figure 4 The positioning result of the Ф-OTDR sensing system in Example 2 when the PZT disturbance is applied at 500 m of the sensing optical fiber;

[0025] Figure 5 The positioning result of the Ф-OTDR sensing system in Example 2 when applying PZT disturbance at 5600m of the sensing optical fiber;

[0026] Figure 6 The positioning result of the Ф-OTDR sensing system in Example 2 when applying PZT disturbance at 26000m of the sensing optical fiber;

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

[0028] Figure 8 The time and frequency domain results of the MZI sensing system in Example 2 applying a PZT disturbance signal at 500 m of the sensing optical fiber. DETAILED DESCRIPTION

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

[0030] Example 1

[0031] The long-distance distributed optical fiber broadband acoustic vibration sensing system provided in this embodiment is as follows: Figure 1 As shown:

[0032] In the Ф-OTDR sensing system: the laser is connected to the first coupler to divide the continuous optical signal into two paths, and 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 optical signal, and then enhances the extinction ratio through the acousto-optic modulator and connects to the optical fiber amplifier; the optical fiber amplifier performs power compensation on the input optical pulse signal and connects to the second port of the third coupler; the first port of the third coupler is connected to the first port of the circulator, and the optical pulse is introduced into the sensing optical fiber through the second port of the circulator; the backward Rayleigh scattered optical signal carrying the sensing information returns to the second port of the circulator, and is input to the second port of the fourth coupler through the third port of the circulator; the fourth coupler couples the two signals input from its first port and the second port, and the generated coherent light is processed by the data acquisition processing module through the photoelectric detector to obtain the sensing position and frequency information.

[0033] In the optical fiber MZI sensing system: the laser 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 introduced into the sensing optical fiber through port two of the circulator, the tail of the sensing optical fiber is connected to the FBG, the continuous light wave is propagated back 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 its port one and port two, and the generated coherent light is processed by the data acquisition processing module through the photoelectric detector to obtain the sensing frequency information.

[0034] Example 2

[0035] The long-distance distributed optical fiber broadband acoustic vibration sensing system provided in this embodiment is as follows: Figure 2 As shown:

[0036] In the Ф-OTDR sensing system: the laser is connected to the first coupler to divide the continuous optical signal into two paths, and the two signals enter the first acousto-optic modulator and the second acousto-optic modulator respectively; the first acousto-optic modulator modulates the continuous light into a specific pulse optical signal and then connects to the optical fiber amplifier, the optical fiber amplifier performs power compensation on the input optical pulse signal and then connects to the second port of the third coupler, the first port of the third coupler is connected to the first port of the circulator, and the optical pulse is introduced into the sensing optical fiber through the second port of the circulator, the backward Rayleigh scattered optical signal carrying the sensing information returns to the second port of the circulator, and is input to the second port of the fourth coupler through the third port of the circulator, the fourth coupler couples the two signals input from its first port and the second port, and the generated coherent light is processed by the data acquisition processing module through the photoelectric detector to obtain the sensing position and frequency information.

[0037] In the optical fiber MZI sensing system: the second acousto-optic modulator performs frequency shift on the input continuous light and is connected to the first port of the second coupler, the second port of the second coupler is connected to the third port of the third coupler, and the third port of the second coupler is connected to the first port of the fourth coupler; the first port of the third coupler is connected to the first port of the circulator, and the optical pulse is introduced into the sensing optical fiber through the second port of the circulator, the tail of the sensing optical fiber is connected to the FBG, the continuous light wave is propagated back to the second port of the circulator, and is input to the second port of the fourth coupler through the third port of the circulator, the fourth coupler couples the two signals input from the first port and the second port, and the generated coherent light is processed by the data acquisition processing module through the photoelectric detector 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 coupler and the third coupler 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 central wavelength of FBG is 1550.120nm, and the central wavelength is changed by applying pressure to FBG, which is used for interference light intensity control of the fiber MZI sensing system.

[0039] Taking the system in Example 2 as an example, the backscattered Rayleigh light obtained is as follows: Figure 3 As shown in Figure 2, the positioning results obtained after processing the backscattered Rayleigh light collected when PZT disturbance is 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 corresponding time and frequency domain diagrams of the pzt signal are as follows: Figure 7 The time and frequency domain diagrams of the interference signal collected during disturbance are shown in Figure 8 shown.

[0040] It can be seen from the above embodiments that the present invention utilizes the same laser to transmit and generate backward Rayleigh scattered light and interference light in the same sensing optical fiber, thereby constructing a distributed optical fiber acoustic vibration sensing system integrating Ф-OTDR and MZI, combining the performance advantages of both, utilizing an extremely simplified optical system solution, realizing the functional fusion of two sensing structures on one sensing optical fiber, reducing the resource waste caused by utilizing different sensing optical cables to transmit sensing signals separately, maximizing the utilization rate of optical fiber resources, and reducing the system cost; utilizing the frequency division multiplexing demodulation solution in the digital domain, expanding the frequency response range on the basis of ensuring the high positioning accuracy of the system, realizing dual-system single-core simultaneous detection, realizing high positioning accuracy and continuous detection without blind spots within a broadband frequency range, and improving reliability.

Claims

1. A long-distance distributed optical fiber 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, and the generated continuous optical signal is divided into two paths through the first coupler: The continuous optical signal of the first optical path is modulated into a pulse optical signal by the modulator, and then power-compensated by the optical fiber amplifier, and then connected to the second port of the third coupler, the first port of the third coupler is connected to the first port of the circulator, and the optical pulse is introduced into the sensing optical fiber through the second port of the circulator, and the backward Rayleigh scattered optical signal carrying the sensing information returns to the second port of the circulator, and is input to the second port of the fourth coupler through the third port of the circulator; the continuous optical signal of the second optical path is connected to the first port of the second coupler, and the third port of the second coupler is connected to the first port of the fourth coupler; the fourth coupler couples the two signals input from its first port and the second port, and the generated coherent light is processed by the data acquisition processing module through the photoelectric detector to obtain the position and frequency information of the sensing signal; This optical path is the Ф-OTDR sensing system; The continuous optical signal of the second optical path is connected to the port 1 of the second coupler, the port 2 of the second coupler is connected to the port 3 of the third coupler, the port 3 of the second coupler is connected to the port 1 of the fourth coupler, the port 1 of the third coupler is connected to the port 1 of the circulator, and the optical pulse is introduced into the sensing optical fiber through the port 2 of the circulator. The tail of the sensing optical fiber is connected to the FBG, and the continuous optical wave is propagated back to the port 2 of the circulator, and is input to the port 2 of the fourth coupler through the port 3 of the circulator. The fourth coupler couples the two-way signals input from its port 1 and port 2, and the generated coherent light is transmitted to the data acquisition and processing module through the photoelectric detector, and the frequency information of the sensing signal is obtained after being processed by the data acquisition and processing module. This optical path is a fiber optic MZI sensing system; Among them, the laser, the first coupler, the second coupler, the third coupler, the circulator, the sensing optical fiber, the fourth coupler, the photoelectric detector, and the data acquisition and processing module are common to the two sensing systems.

2. The long-distance distributed optical fiber broadband acoustic vibration sensing system according to claim 1, characterized in that: The modulator of the first optical path is a combined structure that first modulates a continuous optical signal into a chirped pulse optical signal using an electro-optic modulator and then enhances the extinction ratio using an acousto-optic modulator.

3. The long-distance distributed optical fiber broadband acoustic vibration sensing system according to 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 1 of the second coupler, which is different from the first optical path to form a difference frequency Φ-OTDR sensing system.

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

1.

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

50.

6. The long-distance distributed optical fiber broadband acoustic vibration sensing system according to claim 1, characterized in that: In the optical fiber MZI sensing system, the central wavelength of the FBG is 1550.120 nm, and the central wavelength is changed by applying pressure to the FBG, which is used for interfering light intensity control of the optical fiber MZI sensing system.

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