A few-mode optical fiber distributed acoustic sensing system and signal processing method thereof

Abstract

The invention discloses a few-mode fiber distributed acoustic sensing system and a signal processing method thereof, which relates to the field of distributed optical fiber sensing, and includes: a light source module, a modulation module, a multi-channel injection module, a mode multiplexer / demultiplexer, a few-mode optical fiber, a reference light beam splitting module, a signal detection module, a data acquisition card and a signal processing module. The invention combines the few-mode optical fiber with the MIMO technology. In the modulation module, different frequency labels are added to each mode injected into the few-mode optical fiber. In the signal processing module, the output signal of each detector is separated by bandpass filtering to separate the Rayleigh light scattering signals of each transmission mode with different frequencies, and the MIMO equalization algorithm is used to eliminate the influence of mode coupling during signal transmission. Finally, multiple diversity signals are effectively merged, and by injecting M modes and receiving N modes, not only can signal fading be suppressed, but also 10log(M*N)dB of background noise suppression can be achieved, which provides the possibility for developing a distributed optical fiber sensing system with a longer distance and a higher signal-to-noise ratio.

Description

Technical Field

[0001] The present invention relates to the field of distributed optical fiber sensing with high signal-to-noise ratio, and in particular to a few-mode optical fiber distributed acoustic sensing system and a signal processing method thereof. Background Art

[0002] With the development of low-loss optical fiber manufacturing technology and optical fiber communication technology, optical fiber sensing technology has gradually formed and has made great progress. In recent years, distributed optical fiber sensing technology has been widely used in many fields such as perimeter security and oil and gas exploration due to its unique advantages such as strong anti-electromagnetic interference ability, strong environmental adaptability, long distance, and high sensitivity. Among them, phase-sensitive optical time domain reflectometer (Φ-OTDR) is the most representative type of distributed optical fiber sensing technology. It uses the phase change of backward Rayleigh scattered light caused by the external disturbance signal acting on the sensing optical fiber to realize the distributed detection of acoustic wave signals. Since the optical phase signal is linearly related to the dynamic signal acting on the optical fiber, Φ-OTDR technology can realize real-time quantitative detection of vibration and sound waves on the optical fiber link, which is a strategic high technology that is focused on development at home and abroad.

[0003] Φ-OTDR technology uses the interference effect of Rayleigh backscattering (RBS) for sensing, but the interference effect will cause the problem of coherent fading. Due to the random distribution of the spatial refractive index of the optical fiber, the signal amplitude is close to zero at some positions, resulting in the loss of phase information, which greatly affects the reliability of the system. In addition, compared with the power of the laser emission pulse, the Rayleigh scattered light is several orders of magnitude weaker, resulting in a low backscattered light level and a corresponding high noise level. The system noise floor is much higher than that based on reflection points or interferometer-type systems, which limits the detection capability of weak signals. However, the existing Φ-OTDR technology generally uses ordinary single-mode fiber (SMF) as the sensing fiber. The nonlinear effect threshold of ordinary SMF is low, resulting in the limited power of the pulsed light injected into the sensing fiber, which has a great limitation on improving the sensing distance and signal-to-noise ratio of the system.

[0004] Due to the advantages of higher nonlinear effect threshold and greater scattered light capture efficiency, few-mode fiber or multimode fiber has the potential to solve the two limitations of single-mode fiber system. Matthew J. Murray et al. proposed a phase measurement Φ-OTDR system based on multimode fiber. By using a high-speed camera to detect the entire backscattered field, the system bandwidth reaches 20kHz and the minimum phase noise is 80[dBrad 2 / Hz] [MJMurray, A.Davis, and B.Redding,"Multimode fiber Phi-OTDR with holographic demodulation,"Optics Express, vol.26, no.18, pp.23019-23030, Sep 3 2018.] However, the sensor position is fixed by the reference arm delay, and the distributed multimode fiber acoustic sensor has not been realized. Ali Masoudi et al. proposed a distributed fiber dynamic strain sensor based on dual-mode fiber, which achieved a 2.52dB noise improvement by combining two different modes [MMChen, A.Masoudi, F.Parmigiani, and G.Brambilla,"Distributed acoustic sensor based ona two-mode fiber,"Optics Express, vol.26, no.19, pp.25399-25407, Sep 17 2018.]. However, N-mode few-mode optical fibers can only achieve 10logN (dB) noise floor suppression, which is still a long way from the Φ-OTDR system with lower noise floor. Therefore, it is very meaningful to invent a new low-noise few-mode optical fiber acoustic sensing system and method based on MIMO technology, which can provide technical support for Φ-OTDR systems with higher signal-to-noise ratio, so as to be suitable for monitoring longer distances and weaker signals. Summary of the invention

[0005] The purpose of the present invention is to propose a few-mode fiber distributed acoustic sensing system and a signal processing method thereof, in order to break through the current Φ-OTDR system based on single-mode fiber, which is faced with the problem of limited system signal-to-noise ratio caused by the low amplitude of the Rayleigh scattering signal, and cannot realize a low-noise sensing system to meet the application requirements of weak acoustic signal detection.

[0006] The present invention uses a new type of optical fiber, a few-mode optical fiber, and changes the sensing optical fiber in the Φ-OTDR system from a traditional single-mode optical fiber to a few-mode optical fiber, and proposes a few-mode optical fiber distributed acoustic sensing system. Since each mode in the few-mode optical fiber has a relatively large mode field area, the few-mode optical fiber has a higher nonlinear effect threshold, which can increase the optical power injected into the sensing optical fiber and avoid the interference of the nonlinear effect on the system. In addition, since the core radius of the few-mode optical fiber is much larger than the core diameter of the single-mode optical fiber, scattered light at multiple angles (corresponding to each mode) can be allowed to be transmitted in the optical fiber, so that more backward transmitted light energy can be captured to improve the back Rayleigh scattering signal. In addition, several orthogonal modes in the few-mode optical fiber are used as independent transmission channels to transmit signals, and the natural diversity of the modes can be used to solve the problem of signal fading. Furthermore, the MIMO technology of multi-channel (multiple modes) injection and multi-channel reception is adopted, and several modes in the few-mode optical fiber are used to perceive external sound waves / vibration information, and then demultiplexed at the detection end or the signal end, and the signals injected by M modes and received by N modes are merged and superimposed, thereby greatly reducing the background noise of the system.

[0007] The specific core idea of ​​the present invention is to propose a few-mode fiber distributed acoustic sensing system and its signal processing method. Compared with the traditional Φ-OTDR system, the sensing fiber in the present invention adopts few-mode fiber, combines the few-mode fiber with MIMO technology, makes full use of the energy of each mode in the few-mode fiber, and realizes larger-scale diversity. Since multiple modes are injected into the optical fiber and transmitted simultaneously, the Rayleigh scattering signal of a certain mode received at the detection end is actually the aliasing of the scattering signals from multiple transmission modes. Therefore, it is not easy to separate the signals of each mode (injection) in the optical domain. Therefore, the present invention proposes a frequency labeling method. At the signal injection end, different frequency labels are added to the pulses of different modes injected into the few-mode fiber. The multi-channel injection module controls the delay time of each channel through the optical fiber delay line, and strictly controls the light pulses of each mode to be injected into the sensing fiber at the same time. The multiple modes in the few-mode fiber are used to sense the external sound wave / vibration information respectively, and then the signal light of each scattering mode is demultiplexed at the detection end through the mode demultiplexer. After each mode is independently received, it is coherently amplified by beating with the local reference light. Then, during signal demodulation, bandpass filtering is used to separate the Rayleigh light scattering signals of each transmission mode with different frequencies in the frequency domain. In addition, due to the "imperfect" production of optical fiber or micro-bending or twisting of optical fiber, coupling between transmission modes will occur, affecting the transmission quality of the system. During signal processing, the MIMO equalization algorithm is required to compensate for the crosstalk between modes, so as to effectively separate the signals of each mode and reduce the impact of coupling between transmission modes on system performance. Finally, the coherent merging method is used to merge and superimpose the signals injected by M modes and received by N modes, and finally demodulate the phase information, thereby suppressing coherent fading and obtaining more reliable external disturbance information. Finally, the M*N different mode signals are coherently superimposed, which can greatly increase the intensity of the Rayleigh scattering signal. Without significantly increasing the hardware cost, MIMO technology has achieved a 10log(M*N)dB noise floor suppression, which has great potential to significantly improve the performance of the sensing system.

[0008] The technical solution of the present invention is as follows:

[0009] A few-mode fiber distributed acoustic sensing system includes: a light source module, a modulation module, a multi-channel injection module, a mode multiplexer / demultiplexer, a few-mode fiber, a reference light beam splitter module, a signal detection module, a data acquisition card and a signal processing module. Among them:

[0010] The light source module includes a laser and a first optical fiber coupler;

[0011] The modulation module includes a pulse chopping part, a pulse amplification part, a frequency modulation part and a connection part;

[0012] The multi-channel injection module includes N optical fiber delay lines and N connecting devices;

[0013] The reference light beam splitting module includes a continuous erbium-doped fiber amplifier and a third fiber coupler;

[0014] The signal detection part includes N fourth optical fiber couplers and N double-balanced detectors;

[0015] The output light of the laser is divided into detection light and reference light through a first optical fiber coupler. The detection light is sequentially injected into each mode in the few-mode optical fiber through a modulation module, a multi-channel injection module, and a mode multiplexer / demultiplexer for transmission. The backward Rayleigh scattered signal light scattered back by the few-mode optical fiber returns to the mode multiplexer / demultiplexer to separate the signal lights of N scattered modes, and respectively enters the signal detection part through N connecting devices and N reference lights for beat frequency and detection, and is collected by a data acquisition card and then enters the signal processing module for analysis and processing.

[0016] Preferably, the laser is a narrow linewidth laser.

[0017] Furthermore, the central wavelength of the narrow linewidth laser is 1550 nm.

[0018] Furthermore, the linewidth of the narrow linewidth laser is about 2.5 kHz.

[0019] Preferably, the output of the laser is continuous light.

[0020] Preferably, each port of the first optical fiber coupler is a polarization-maintaining optical fiber.

[0021] Furthermore, the wavelength band of the first optical fiber coupler is 1550 nm.

[0022] Furthermore, the first optical fiber coupler has a 1×2 port.

[0023] Furthermore, the splitting ratio of the first optical fiber coupler is 9:1.

[0024] The pulse chopping part is a semiconductor optical amplifier. The pulse amplification part is a pulsed erbium-doped fiber amplifier. The frequency modulation part is N acousto-optic modulators. The connection part is a second fiber coupler. The detection light output by the light source module is connected to the input port of the semiconductor optical amplifier, the output port of the semiconductor optical amplifier is connected to the input port of the pulsed erbium-doped fiber amplifier, the output port of the pulsed erbium-doped fiber amplifier is connected to the input port of the second fiber coupler, and the N pulse lights separated from the N output ports of the second fiber coupler are frequency modulated by N acousto-optic modulators respectively.

[0025] Preferably, the semiconductor optical amplifier chops the light output by the light source portion into pulse light.

[0026] Furthermore, the semiconductor optical amplifier has a frequency shift Δω0=0.

[0027] Preferably, the number of ports of the second optical fiber coupler is 1×N.

[0028] Furthermore, each port of the second optical fiber coupler is a common single-mode optical fiber.

[0029] Furthermore, the splitting ratio of the second optical fiber coupler is 1:1.

[0030] Furthermore, the wavelength band of the second optical fiber coupler is 1550 nm.

[0031] Preferably, the frequency modulation part is N frequency shifts Δω1, Δω2, Δω3, ..., Δω N The acousto-optic modulator. The detection pulses injected into different modes are modulated with different frequencies Δω0+Δω1, Δω0+Δω1, Δω0+Δω2, Δω0+Δω2, Δω0+Δω3, …, Δω0+Δω N .

[0032] Preferably, the length of the optical fiber delay line can be adjusted according to the delay when each frequency modulated pulse (corresponding to each mode) is injected, thereby strictly ensuring that the pulses of each mode can be injected into the mode multiplexer / demultiplexer at the same time.

[0033] Preferably, the connection device is a circulator, or a 2×2 fiber coupler with a unidirectional isolator connected to one of its ports.

[0034] Preferably, the mode multiplexer / demultiplexer is a mode selective photonic lantern with compact design, high integration and low insertion loss.

[0035] Preferably, the few-mode optical fiber is a gradient-index few-mode optical fiber having a lower differential group delay.

[0036] Preferably, the ports of the third optical fiber coupler are 1×N.

[0037] Furthermore, each port of the third optical fiber coupler is a common single-mode optical fiber.

[0038] Furthermore, the splitting ratio of the third optical fiber coupler is 1:1.

[0039] Furthermore, the wavelength band of the third optical fiber coupler is 1550 nm.

[0040] Preferably, the ports of the fourth optical fiber coupler are 2×2.

[0041] Furthermore, each port of the fourth optical fiber coupler is a common single-mode optical fiber.

[0042] Furthermore, the splitting ratio of the fourth optical fiber coupler is 1:1.

[0043] Furthermore, the wavelength band of the fourth optical fiber coupler is 1550 nm.

[0044] Preferably, the dual-balanced detector is a high-speed detector that converts optical signals into electrical signals, which can effectively eliminate the DC component and common-mode noise of the photodetector and effectively amplify the differential-mode signal, greatly improving the signal-to-noise ratio of the system.

[0045] The pulse chopping part is an acousto-optic modulator. The pulse amplification part is N pulse erbium-doped fiber amplifiers. The frequency modulation part is an electro-optic modulator and N fiber gratings. The connection part is a second fiber coupler. The detection light output by the light source module is connected to the input port of the electro-optic modulator, and the output port of the electro-optic modulator is connected to the input port of the acousto-optic modulator. The output port of the acousto-optic modulator is connected to the input port of the second fiber coupler, and the N pulse lights separated from the N output ports of the second fiber coupler pass through N fiber gratings respectively, so that the N pulse lights are modulated by a different frequency respectively. The N fiber grating output ports are connected to the input ports of the N pulse erbium-doped fiber amplifiers respectively.

[0046] Preferably, the acousto-optic modulator chops the light output from the light source into pulse light and simultaneously causes the frequency of the pulse light to produce a frequency shift of Δω0, Δω0≠0.

[0047] Preferably, the electro-optic modulator can generate several equally spaced frequency sidebands around the center frequency of the optical carrier.

[0048] Preferably, the cutoff frequencies of the N fiber gratings are different, so that the detection pulses injected into different modes are modulated with different frequencies Δω0+Δω1, Δω0+Δω2, Δω0+Δω3, ..., Δω0+Δω N .

[0049] The pulse chopping part is a semiconductor optical amplifier. The pulse amplification part is a pulsed erbium-doped fiber amplifier. The frequency modulation part is an acousto-optic modulator and an adjustable filter. The connecting part is a fifth optical fiber coupler and an optical switch. The detection light output by the light source module is connected to the input port of the semiconductor optical amplifier, the output port of the semiconductor optical amplifier is connected to the first input port of the fifth optical fiber coupler, the first output port of the fifth optical fiber coupler is connected to the input port of the acousto-optic modulator, the output port of the acousto-optic modulator is connected to the input port of the pulsed erbium-doped fiber amplifier, the output port of the pulsed erbium-doped fiber amplifier is connected to the input port of the adjustable filter, and the output port of the adjustable filter is connected to the second input port of the fifth optical fiber coupler to form a closed loop. The fifth output port of the fifth optical fiber coupler is connected to the input port of the optical switch.

[0050] Preferably, the ports of the fifth optical fiber coupler are 2×2.

[0051] Furthermore, each port of the fifth optical fiber coupler is a common single-mode optical fiber.

[0052] Furthermore, the splitting ratio of the fifth optical fiber coupler is 1:1.

[0053] Furthermore, the wavelength band of the fifth optical fiber coupler is 1550 nm.

[0054] Furthermore, the first output port of the fifth optical fiber coupler is connected to the second input port to form a closed loop.

[0055] Preferably, the optical switch is a high-speed optical switch with a response time of ˜10 ns.

[0056] Furthermore, the ports of the optical switch are 1×N.

[0057] Preferably, the acousto-optic modulator in the frequency modulation part causes the pulse light frequency to produce a frequency shift of Δω1, and each time the detection pulse goes around the optical fiber ring, it will be frequency modulated by a frequency of Δω1. Until the N+1th time, after the frequency shift is Δω0+(N+1)·Δω1, it is filtered out by the adjustable filter. The detection pulses injected into different modes are modulated with different frequencies Δω0+Δω1, Δω0+Δω2, Δω0+Δω3, ..., Δω0+Δω N , where Δω2=2Δω1…, Δω N =NΔω1.

[0058] The detection signals of each mode injected from each port of the mode multiplexer / demultiplexer are x1, x2, ..., x N , corresponding to different frequencies Δω0+Δω1, Δω0+Δω2, Δω0+Δω3,…, Δω0+ΔωN In the actual transmission process, there will be mutual coupling between modes. At this time, the transmission signals corresponding to each frequency are y1, y2, ...y N When the detection light is transmitted in the optical fiber, Rayleigh scattering occurs. When the external disturbance signal to be measured is applied to the few-mode optical fiber, the backward Rayleigh scattered signal light scattered back by each mode in the few-mode optical fiber returns to the output port of the mode multiplexer / demultiplexer and is divided into multiple signal lights. Together with their respective reference lights, they enter the fourth optical fiber coupler for beat frequency and enter the double balanced detector for detection. The output signals of each detector are z1, z2, ..., z N ,After the data acquisition card collects the data, it enters the signal processing module for analysis and processing.

[0059] A method for diversity combining Rayleigh scattered signals of a few-mode fiber distributed acoustic sensing system, characterized in that the signal processing module comprises the following steps: the output signal of each double balanced detector of the signal detection module is z i (i=1,2...,N); separate the Rayleigh light scattering signals of different transmission modes in the frequency domain through bandpass filtering to obtain z i ′(y1,y2,...,y N );The coupling between transmission modes is eliminated by MIMO equalization algorithm, so that a certain transmission mode y i (i=1,2...,N) contains only x i , excluding the signal x coupled from other modes i′ (i≠i′). Compensation mode coupling x i =W(n)*y i , where W(n) is the equalizer weight for eliminating interference from the ith mode to the ith' mode, and n is the number of equalizer taps. Therefore, the final output signal is z i ″(x1,x2,...,x N ); then the separated signals are effectively aggregated through the coherent combining method; finally, the phase of the resultant vector is demodulated to recover the disturbance information imposed on the optical fiber by the outside world.

[0060] The acoustic sensor of the present invention can be applied to fields including phase-sensitive optical time domain reflectometers for coherent detection, phase-sensitive optical time domain reflectometers for direct detection, optical frequency domain reflectometers, and distributed optical fiber sensors based on the Brillouin scattering principle.

[0061] The advantages of the present invention are:

[0062] 1. A new type of optical fiber, few-mode optical fiber, is used to change the sensing optical fiber in the Φ-OTDR system from the traditional single-mode optical fiber to few-mode optical fiber. Few-mode optical fiber has a higher threshold of nonlinear effect, which can increase the optical power injected into the sensing optical fiber and avoid the interference of nonlinear effect on the system. It can effectively increase the sensing distance of the system without sacrificing the spatial resolution of the system.

[0063] 2. Using several orthogonal modes in the few-mode fiber as independent transmission channels to transmit signals, a few-mode fiber acoustic sensing system based on mode diversity technology is proposed. Multiple modes in the few-mode fiber are used to sense external sound waves / vibration information respectively, and then the signal light of each different scattering mode is demultiplexed at the detection end through the mode demultiplexer. By realizing diversity detection of each mode in the few-mode fiber and then merging and receiving, the strength of the signal to be measured is greatly increased, thereby reducing the background noise of the system. In addition, the natural diversity of modes can be used to solve the problem of signal fading.

[0064] 3. Drawing on the multi-antenna systems in wireless communications and phased array radars, few-mode fiber is combined with MIMO technology to fully utilize the energy of each mode in the few-mode fiber, thereby achieving greater diversity. At the signal transmitter, each injected mode is given a different frequency label, so that the Rayleigh light scattering signals of different frequency transmission modes can be distinguished in the frequency domain. In addition, in the post-signal processing process, the MIMO equalization algorithm is used to eliminate the coupling problem between transmission modes. Combining few-mode fiber with MIMO technology, by injecting M modes and receiving N modes, it is possible to achieve 10log(M*N)dB of noise floor suppression without significantly increasing hardware costs, which has great potential to meet the needs of advanced acoustic applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 This is a schematic diagram of the structure of Example 1 of the present invention.

[0066] Figure 2 This is a schematic diagram of a connection device used in Example 1 of the present invention - a 2×2 optical fiber coupler in which one port is connected to a unidirectional isolator.

[0067] Figure 3 This is a schematic structural diagram of the mode multiplexer / demultiplexer 110 used in Example 1 of the present invention, which is an all-fiber photonic lantern.

[0068] Figure 4 This is a schematic diagram of the structure of Example 2 of the present invention.

[0069] Figure 5 This is a schematic diagram of the structure of Example 3 of the present invention.

[0070] Figure 6 Schematic diagram of the physical transmission process of each mode signal in the system.

[0071] Figure 7 Schematic diagram of the superimposed signal received by the detection end when two signals of different modes are transmitted simultaneously in the optical fiber.

[0072] Figure 8 The figure is a flow chart of a signal processing method for a few-mode fiber MIMO system.

[0073] Fig. 9 Schematic diagram of the signal with two modes coherently superimposed after frequency modulation and demultiplexing.

[0074] Fig.10 FIG. 5 is a timing diagram of multi-channel digital signal control of the optical switch 504 in Embodiment 3 of the present invention.

[0075] In the figure: 101-laser, 102-first fiber coupler, 103-semiconductor optical amplifier, 104-continuous erbium-doped fiber amplifier, 105-pulse erbium-doped fiber amplifier, 106-second fiber coupler, 107-N acousto-optic modulators, 108-N fiber delay lines, 109-N connecting devices, 110-mode multiplexer / demultiplexer, 111-few-mode fiber, 112-third fiber coupler, 113-N fourth fiber couplers, 114-N double-balanced detectors, 115-data acquisition card, 116-data processing module, a, b, c are the first port, second port and third port of the connecting device 109-circulator respectively.

[0076] 201-2×2 fiber coupler, 202-unidirectional isolator.

[0077] 301-single-mode optical fiber, 302-fluorine-doped glass sleeve, 303-few-mode optical fiber.

[0078] 401 - electro-optic modulator, 402 - acousto-optic modulator, 403 - N fiber gratings, 404 - N pulsed erbium-doped fiber amplifiers.

[0079] 501 - fifth optical fiber coupler, 502 - acousto-optic modulator, 503 - tunable filter, 504 - optical switch. DETAILED DESCRIPTION

[0080] The principles and technical solutions of the present invention will be clearly and completely described below in conjunction with the specific embodiments and drawings of the present invention. It should be understood that the described embodiments are only part of the embodiments of the present invention and should not be regarded as limiting the scope of protection of the present invention. The components of the embodiments of the present invention generally described and shown in the drawings herein can be arranged and designed in various different configurations. All other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0081] The few-mode optical fiber distributed acoustic sensing system and method described in the present invention are based on Figure 1 For example, it includes: a light source module, a modulation module, a multi-channel injection module, a mode multiplexer / demultiplexer, a few-mode optical fiber, a reference light splitting module, a signal detection module, a data acquisition card and a signal processing module.

[0082] The light source part includes a laser 101 and a first fiber coupler 102. The modulation module includes a pulse chopping part, a pulse amplification part, a frequency modulation part and a connection part. The pulse chopping part is a semiconductor optical amplifier 103, the pulse amplification part includes a pulse erbium-doped fiber amplifier 105, and the frequency modulation part includes N acousto-optic modulators 107. The connection part is a second fiber coupler 106. The multi-channel injection module includes N fiber delay lines 108 and N connection devices 109. A mode multiplexer / demultiplexer 110 and a few-mode fiber 111. The reference light splitting module includes a continuous erbium-doped fiber amplifier 104 and a third fiber coupler 112. The signal detection part includes N fourth fiber couplers 113 and N double-balanced detectors 114. A data acquisition card 115 and a signal processing module 116.

[0083] The output light of the laser is divided into detection light and reference light through the first optical fiber coupler. The detection light is injected into each mode in the few-mode optical fiber through the modulation module, the multi-channel injection module and the mode multiplexer / demultiplexer in sequence for transmission. The backward Rayleigh scattered signal light scattered back by the few-mode optical fiber returns to the mode multiplexer / demultiplexer, so that the signal lights of N scattered modes are separated from each other, and enter the signal detection part through N connecting devices and N reference lights for beat frequency and detection respectively, and enter the signal processing module for analysis and processing after being collected by the data acquisition card.

[0084] The connecting device 109 may be a circulator or a Figure 2 A 2x2 fiber coupler is shown with a unidirectional isolator connected to one of the ports.

[0085] The mode multiplexer / demultiplexer 110 may be Figure 3The basic structure of the mode-selective photon lantern shown in the figure is to embed a single-mode optical fiber 301 into a low-refractive-index fluorine-doped glass sleeve 302 for melt taper, and form a waveguide structure similar to the few-mode optical fiber 303 at the tip of the taper. The fundamental mode input from the single-mode optical fiber 301 can be converted into various high-order modes in the few-mode optical fiber 303. The external disturbance information to be measured is applied to the few-mode sensing optical fiber, and the backward Rayleigh scattered signal light of each mode scattered back from the few-mode optical fiber enters the few-mode output port of the photon lantern and enters the fundamental mode of each corresponding single-mode optical fiber, thereby effectively separating the signal light of each scattering mode.

[0086] In one case, the pulse chopping part is a semiconductor optical amplifier 103. The frequency modulation part includes N acousto-optic modulators 107. The detection light separated by the first fiber coupler 102 is connected to the input port of the semiconductor optical amplifier 103, the output port of the semiconductor optical amplifier 103 is connected to the input port of the pulse erbium-doped fiber amplifier 105, the output port of the pulse erbium-doped fiber amplifier 105 is connected to the input port of the second fiber coupler 106, and the N pulse lights separated from the N output ports of the second fiber coupler 106 are frequency modulated by N acousto-optic modulators 107 respectively. The frequency shifts of the N acousto-optic modulators are different.

[0087] In another case, if Figure 4 As shown, the pulse chopping part is an acousto-optic modulator 402. The frequency modulation part is an electro-optic modulator 401 and N fiber gratings 403. The detection light separated by the first fiber coupler 102 is connected to the input port of the electro-optic modulator 401, and the output port of the electro-optic modulator 401 is connected to the input port of the acousto-optic modulator 402. The electro-optic modulator can generate several equally spaced frequency sidebands near the center frequency of the optical carrier. The output port of the acousto-optic modulator 402 is connected to the input port of the second fiber coupler 106, and the N pulse lights separated from the N output ports of the second fiber coupler 106 pass through N fiber gratings 403 respectively, so that the N pulse lights are modulated by a different frequency. The output ports of the N fiber gratings 403 are connected to the input ports of N pulsed erbium-doped fiber amplifiers 404 respectively.

[0088] In other cases, such as Figure 5As shown, the pulse chopping part is a semiconductor optical amplifier 103. The frequency modulation part is an acousto-optic modulator 502 and an adjustable filter 503. The connecting part includes a fifth fiber coupler 501 and an optical switch 504. The detection light separated by the first fiber coupler 102 is connected to the input port of the semiconductor optical amplifier 103, the output port of the semiconductor optical amplifier 103 is connected to the first input port of the fifth fiber coupler 501, the first output port of the fifth fiber coupler 501 is connected to the input port of the acousto-optic modulator 502, the output port of the acousto-optic modulator 502 is connected to the input port of the pulse erbium-doped fiber amplifier 105, the output port of the pulse erbium-doped fiber amplifier 105 is connected to the input port of the adjustable filter 503, and the output port of the adjustable filter 503 is connected to the second input port of the fifth fiber coupler 501, forming a closed loop. The second output port of the fifth fiber coupler 501 is connected to the input port of the optical switch 504. The N output ports of the optical switch 504 are connected to N optical fiber delay lines 108 respectively.

[0089] The principle of improving the signal-to-noise ratio of the system by using few-mode fiber MIMO technology:

[0090] The variance of the phase noise term of the Φ-OTDR system can be expressed as:

[0091]

[0092] In the formula, is the variance of the system noise, A(z1) and A(z2) are the amplitudes of the signals at the two differential positions. As can be seen from the above formula, the smaller the amplitude of the scattered signal, the larger the phase noise term. The weak intensity of the Rayleigh scattering signal will limit the noise floor level of the Φ-OTDR system. In addition, the extinction ratio, shot noise, etc. It is also an important factor that limits the background noise of the acoustic sensor system. Since the above noise is system additive noise and conforms to Gaussian distribution, the influence of system additive noise on the background noise level of the system can be reduced through mode diversity technology.

[0093] Referring to the definition of signal-to-noise ratio (SNR) by HANIEL GABAI et al., see [Gabai, H. and A. Eyal (2016). "On the sensitivity of distributed acoustic sensing." Optics Letters 41(24): 5648-5651.] for details, the signal-to-noise ratio (SNR) of a distributed acoustic sensing system can be expressed as:

[0094]

[0095] It is assumed that z1 and z2 are close enough to ignore the fiber loss, that is, A2 (z1) = A 2 (z2) = A 2 , is the variance of the signal to be measured.

[0096] After adopting the mode diversity technology based on single-channel injection (single mode) and multi-channel output (SIMO), one measurement is equivalent to the superposition of N measurements (the number of diversity N is the number of selected modes). The signal-to-noise ratio after diversity combining can be expressed as:

[0097]

[0098] It can be seen that without considering laser phase noise, SNR increases linearly with N. If we take full advantage of the high scattering energy of multiple modes in few-mode fiber to achieve larger-scale diversity, we can use MIMO technology based on M mode injection and N mode reception. Since each mode is orthogonal, it can be used as an independent transmission channel to transmit signals. At this time, the signal-to-noise ratio after diversity combining can be expressed as:

[0099] SNR″=SNR+10log(M*N) (4)

[0100] The use of MIMO technology based on multi-channel injection (multiple modes) and multi-channel output can not only significantly increase the amplitude of the signal light, but also reduce the additive noise in the system and the system's base noise. In addition, without significantly increasing the hardware cost, MIMO technology has achieved a 10log(M*N)dB base noise suppression, which has great potential to significantly improve the performance of the sensor system.

[0101] In the Φ-OTDR system based on few-mode fiber MIMO technology, the physical process of each mode signal includes three parts: signal emission, transmission and reception. Figure 6 As shown. The detection signals injected into each mode are x1, x2, ..., x N , corresponding to different frequencies Δω0+Δω1, Δω0+Δω2, Δω0+Δω3,…, Δω0+Δω N In the actual transmission process, there will be mutual coupling between modes. At this time, the transmission signals corresponding to each frequency are y1, y2, ...y N When the detection light is transmitted in the optical fiber, Rayleigh scattering occurs, and the output signals of each detector are z1, z2, ..., z N Among them, due to the inter-mode coupling, each transmission signal y i It contains not only x i , and also includes the signal x coupled from other modes i′(i≠i′). Secondly, any mode transmitted in the few-mode fiber will excite the scattering signals of other modes (all modes allowed by the few-mode fiber). Therefore, the output signal of each detector is z i All of them are the superposition of scattered light from multiple transmission modes, that is, z i ′(y1,y2,...,y N ).

[0102] MIMO technology principle of multi-channel injection (multiple modes) and multi-channel output:

[0103] The present invention adopts MIMO technology of multi-channel (multi-mode) injection and multi-channel reception. Since each mode is orthogonal, it can be used as an independent transmission channel to transmit signals. However, it is not easy to separate the signals of each transmission mode in the optical domain at the detection end or the signal end. The scattered signal detected at the signal receiving end is actually the result of the superposition of the scattered signals of each transmission mode in the optical domain, that is, z i ′(y1,y2,...,y N ).

[0104] like Figure 7 As shown in the figure, two signals of different modes are transmitted simultaneously in the optical fiber, and the amplitude and phase of the signals (and vectors) received at the receiving end are:

[0105]

[0106] Where A1 and A2 are the signal amplitudes when modes 1 and 2 are injected. The transmission phase difference between the two modes is The light source frequency ω=2πc / λ is about 10 15 Hz. Since the transmission constants of each mode are different and the phase difference is large, the amplitude of the sum vector is not the sum of the amplitudes of the signals in each mode. At this time, the signal amplitude cannot be increased to improve the signal-to-noise ratio.

[0107] Therefore, the present invention proposes a frequency labeling method. At the signal injection end, the pulses of different modes injected into the few-mode optical fiber are labeled with different frequencies Δω0+Δω1, Δω0+Δω2, Δω0+Δω3, …, Δω0+Δω N Modulation. Finally, the signal light of each scattering mode received by each detector is actually the superposition result of the scattering signals of multiple transmission modes.

[0108] The flowchart of the signal processing method of the few-mode fiber MIMO system is as follows Figure 8 As shown: The output signal of each double-balanced detector collected by the signal detection module is z i(i=1,2...,N); separate the Rayleigh light scattering signals of different transmission modes (corresponding to different frequency labels) in the frequency domain through bandpass filtering to obtain z i ′(y1,y2,...,y N );The coupling between transmission modes is eliminated by MIMO equalization algorithm, so that a certain transmission mode y i (i=1,2...,N) contains only x i , excluding the signal x coupled from other modes i′ (i≠i′), compensation mode coupling x i =W(n)*y i , so the final output signal is z i ″(x1,x2,...,x N ); then the separated signals are effectively aggregated by coherent merging method, such as Figure 8 As shown; Finally, the phase of the resultant vector is demodulated to restore the disturbance information imposed on the optical fiber by the outside world. At this time, the signal amplitude is greatly increased and the system noise is greatly suppressed.

[0109] The basic principles of the present invention are as follows:

[0110] First, the narrow linewidth laser outputs continuous light, which is divided into two paths through the first fiber coupler: one path is used as reference light; the other path is chopped into pulse light by an acousto-optic modulator (AOM) or a semiconductor optical amplifier (SOA), and a frequency shift of Δω0 (MHz) is generated. The pulse light is amplified by a pulsed erbium-doped fiber amplifier (EDFA). The amplified pulse light is equally divided into N paths through a 1×N fiber coupler as optical signals of each mode for injection.

[0111] Second, at the signal injection end, the pulses of different modes injected into the few-mode fiber are modulated at different frequencies through N AOMs with different frequency shifts. At this time, each signal injected in each mode is given a frequency "mark". The pulse light of each mode modulated with different frequencies is adjusted through the fiber delay line to adjust the delay time of each channel, and the light pulses of each mode are strictly controlled to be injected into the photon lantern and enter the sensing fiber for transmission at the same time.

[0112] Third, each mode in the few-mode fiber is used to sense the external sound wave / vibration information, and then the signal light of each different scattering mode is demultiplexed at the detection end through the mode demultiplexer. After each mode is independently received, it is coherently amplified by beating with the local reference light. The beat frequency output is converted into an electrical signal by a double balanced detector (BPD), and the data is synchronously collected using a multi-channel data acquisition card (DAQ).

[0113] Fourth, in the signal processing module, the Rayleigh light scattering signals of different transmission modes are separated through bandpass filtering in the frequency domain. In addition, the crosstalk between modes is compensated through the MIMO equalization algorithm, thereby effectively separating the signals of each mode and reducing the impact of coupling between transmission modes on system performance. Finally, the signals injected into N modes and received by N modes are effectively combined and superimposed through the coherent combining method, and the phase information is finally demodulated, thereby suppressing coherent fading, greatly suppressing system noise, and obtaining more reliable external disturbance information.

[0114] Embodiment 1:

[0115] Figure 1The present invention is a schematic diagram of the overall structure of a few-mode fiber distributed acoustic sensing system. The output port of the laser 101 is connected to the input port of the first fiber coupler 102, the first output port of the first fiber coupler 102 is connected to the input port of the semiconductor optical amplifier 103, the second output port of the first fiber coupler 102 is connected to the input port of the continuous erbium-doped fiber amplifier 104, and the output port of the continuous erbium-doped fiber amplifier 104 is connected to the input port of the third fiber coupler 112. The output port of the semiconductor optical amplifier 103 is connected to the input port of the pulsed erbium-doped fiber amplifier 105, the output port of the pulsed erbium-doped fiber amplifier 105 is connected to the input port of the second fiber coupler 106, the N output ports of the second fiber coupler 106 are respectively connected to the input ports of N acousto-optic modulators 107, and the frequency shift amount of each acousto-optic modulator is different. The output ports of the N acousto-optic modulators 107 are connected to N optical fiber delay lines 108. The length of the optical fiber delay line can be adjusted according to the delay when each frequency modulated pulse (different mode) is injected, so as to strictly ensure that the pulses of each mode can be simultaneously injected into the mode multiplexer / demultiplexer 110. The N optical fiber delay lines 108 are respectively connected to the first ports of the N connecting devices 109, the second ports of the N connecting devices 109 are respectively connected to the N single-mode input ports of the mode multiplexer / demultiplexer 110, and the few-mode output port of the mode multiplexer / demultiplexer 110 is connected to the few-mode optical fiber 111. The backward Rayleigh scattered signal light scattered back from each mode in the few-mode fiber 111 returns to the few-mode output port of the mode multiplexer / demultiplexer 110 and is divided into signal lights of each mode (received), and returns from each single-mode input port of the mode multiplexer / demultiplexer 110, and is output from the third port through the second ports of the N connecting devices 109 respectively, and enters the two input ports of the N fourth fiber couplers 113 respectively with the multi-path reference light output by the third fiber coupler 112 for frequency beat. The two output ports of the fourth fiber coupler 113 are connected to the two input ports of the dual-balanced detector 114, and the output of the dual-balanced detector 114 is connected to the input port of the data acquisition card 115. The data acquisition card 115 collects the data, and then enters the signal processing module 116 for analysis and processing. The signals injected into the N modes and received by the N modes are demultiplexed and effectively coherently combined after compensating for mode coupling through the MIMO equalization algorithm, and finally the amplitude and phase information of the signal are demodulated.

[0116] The connection device 109 of this embodiment can be a circulator, or a 2×2 optical fiber coupler with a unidirectional isolator connected to one port, such as Figure 2As shown. The pulse light is input from port a, enters the 2×2 fiber coupler, and outputs from port b to the detection fiber. The signal light scattered back from the detection fiber enters from port b, passes through the 2×2 fiber coupler, and is output from ports a and c. A one-way isolator is connected to port a to isolate the scattered light, and the signal detection part is connected to port c to detect the signal.

[0117] The laser 101 of this embodiment can be a narrow linewidth laser. The central wavelength of the narrow linewidth laser is 1550nm, the linewidth is about 2.5kHz, and the output is continuous light. Each port of the first fiber coupler 102 is a polarization-maintaining fiber with a wavelength of 1550nm, a 1×2 port, and a splitting ratio of 9:1. A fiber coupler with a slightly different splitting ratio can also be used. The device used for the pulse chopping part of this embodiment is a semiconductor optical amplifier 103, which can chop the output light of the light source part into pulse light, or an acousto-optic modulator, which chop the output light of the light source part into pulse light while causing the pulse light frequency to produce a frequency shift of Δω0. Each port of the second fiber coupler 106 is an ordinary single-mode optical fiber with a wavelength of 1550nm and 1×N ports. The frequency shifts of the N acousto-optic modulators 107 are Δω1, Δω2, Δω3,…, Δω respectively. N The mode multiplexer / demultiplexer 110 is a mode selective photon lantern. The few-mode fiber 111 is a gradient few-mode fiber with a low differential group delay. Each port of the third fiber coupler 112 is an ordinary single-mode fiber with a wavelength of 1550nm and a port of 1×N. Each port of the fourth fiber coupler 113 is an ordinary single-mode fiber with a wavelength of 1550nm and a port of 2×2, and the splitting ratio is 1:1.

[0118] Embodiment 2:

[0119] The chopping part of the sensor of the present invention may also be an acousto-optic modulator, and the frequency modulation part may also include an electro-optic modulator and a fiber grating. The system structure diagram of this embodiment is shown in FIG. Figure 4As shown. The output port of the laser 101 is connected to the input port of the first fiber coupler 102, and the first output port of the first fiber coupler 102 is connected to the input port of the electro-optic modulator 401. The electro-optic modulator can generate several equally spaced frequency sidebands near the center frequency of the optical carrier. The second output port of the first fiber coupler 102 is connected to the input port of the continuous erbium-doped fiber amplifier 104, and the output port of the continuous erbium-doped fiber amplifier 104 is connected to the input port of the third fiber coupler 112. The output port of the electro-optic modulator 401 is connected to the input port of the acousto-optic modulator 402, and the output port of the acousto-optic modulator 402 is connected to the input port of the second fiber coupler 106. The N output ports of the second fiber coupler 106 are respectively connected to the input ports of the N fiber gratings 403, and the cutoff frequency of each fiber grating 403 is different. The N fiber grating 403 output ports are respectively connected to the input ports of the N pulsed erbium-doped fiber amplifiers 404. Since the amplitudes of the various frequency sidebands generated by the electro-optical modulator are not the same, each frequency modulation needs to be amplified by its own pulsed erbium-doped fiber amplifier to adjust the signal light amplitudes of each mode (different frequency sidebands) to be roughly the same. The output ports of the N pulsed erbium-doped fiber amplifiers 404 are connected to the N optical fiber delay lines 108. The length of the optical fiber delay line can be adjusted according to the delay when each frequency modulated pulse (different mode) is injected, so as to strictly ensure that the pulses of each mode can be injected into the mode multiplexer / demultiplexer 110 at the same time. The N optical fiber delay lines 108 are respectively connected to the first ports of the N connecting devices 109, and the second ports of the N connecting devices 109 are respectively connected to the N single-mode input ports of the mode multiplexer / demultiplexer 110, and the few-mode output port of the mode multiplexer / demultiplexer 110 is connected to the few-mode optical fiber 111. The backward Rayleigh scattered signal light scattered back from each mode in the few-mode fiber 111 returns to the few-mode output port of the mode multiplexer / demultiplexer 110 and is divided into signal lights of each scattered mode, and returns from each single-mode input port of the mode multiplexer / demultiplexer 110, and is output from the third port through the second ports of the N connecting devices 109 respectively, and enters the two input ports of the N fourth fiber couplers 113 respectively with the multi-path reference light output by the third fiber coupler 112 for beat frequency. The two output ports of the fourth fiber coupler 113 are connected to the two input ports of the dual-balanced detector 114, and the output of the dual-balanced detector 114 is connected to the input port of the data acquisition card 115. The data acquisition card 115 collects the data, and then enters the signal processing module 116 for analysis and processing. The signals injected into the N modes and received by the N modes are demultiplexed and effectively coherently combined after compensating for mode coupling through the MIMO equalization algorithm, and finally the amplitude and phase information of the signal are demodulated.The advantage of this method is that it does not require multiple acousto-optic modulators. An electro-optic modulator is used to generate several equally spaced frequency sidebands near the center frequency of the optical carrier, and then through fiber gratings with different cutoff frequencies, the frequency of each pulse light transmitted through the fiber grating is different. In this way, different frequency modulations are added to the pulses of different modes injected into the few-mode optical fiber.

[0120] The device used for the pulse chopping part in this embodiment is an acousto-optic modulator 402, which chops the light output from the light source part into pulse light and makes the pulse light frequency produce a frequency shift of Δω0. The electro-optic modulator 401 generates several equally spaced frequency sidebands near the center frequency of the optical carrier, and then passes through N fiber gratings 403 with different cutoff frequencies, so that the pulses injected into different modes are modulated with different frequencies Δω0+Δω1, Δω0+Δω2, Δω0+Δω3, ..., Δω0+Δω N .

[0121] Embodiment 3:

[0122] The sensor of the present invention can also frequency modulate the pulse signals injected into each mode by adding an electro-optic modulator and an adjustable filter to the optical fiber ring, and then control the light pulses of each mode to be injected into the few-mode optical fiber at the same time by using an optical switch and an optical fiber delay line. Figure 5As shown. The output port of the laser 101 is connected to the input port of the first fiber coupler 102, the first output port of the first fiber coupler 102 is connected to the input port of the semiconductor optical amplifier 103, the second output port of the first fiber coupler 102 is connected to the input port of the continuous erbium-doped fiber amplifier 104, and the output port of the continuous erbium-doped fiber amplifier 104 is connected to the input port of the third fiber coupler 112. The output port of the semiconductor optical amplifier 103 is connected to the first input port of the fifth fiber coupler 501, the first output port of the fifth fiber coupler 501 is connected to the input port of the acousto-optic modulator 502, the output port of the acousto-optic modulator 502 is connected to the input port of the pulsed erbium-doped fiber amplifier 105, the output port of the pulsed erbium-doped fiber amplifier 105 is connected to the input port of the tunable filter 503, and the output port of the tunable filter 503 is connected to the second input port of the fifth fiber coupler 501, forming a closed loop. The second output port of the fifth optical fiber coupler 501 is connected to the input port of the optical switch 504. The N output ports of the optical switch 504 are respectively connected to N optical fiber delay lines 108. The length of the optical fiber delay line can be adjusted according to the delay when each frequency modulated pulse (different mode) is injected, so as to strictly ensure that the pulses of each mode can be simultaneously injected into the mode multiplexer / demultiplexer 110. The N optical fiber delay lines 108 are respectively connected to the first ports of the N connecting devices 109. The second ports of the N connecting devices 109 are respectively connected to the N single-mode input ports of the mode multiplexer / demultiplexer 110. The few-mode output port of the mode multiplexer / demultiplexer 110 is connected to the few-mode optical fiber 111. The backward Rayleigh scattered signal light scattered back from each mode in the few-mode fiber 111 returns to the few-mode output port of the mode multiplexer / demultiplexer 110 and is divided into signal lights of each scattered mode, and returns from each single-mode input port of the mode multiplexer / demultiplexer 110, and is output from the third port through the second ports of the N connecting devices 109 respectively, and enters the two input ports of the N fourth fiber couplers 113 respectively with the multi-path reference light output by the third fiber coupler 112 for beat frequency. The two output ports of the fourth fiber coupler 113 are connected to the two input ports of the dual-balanced detector 114, and the output of the dual-balanced detector 114 is connected to the input port of the data acquisition card 115. The data acquisition card 115 collects the data, and then enters the signal processing module 116 for analysis and processing. The signals injected into the N modes and received by the N modes are demultiplexed and effectively coherently combined after compensating for mode coupling through the MIMO equalization algorithm, and finally the amplitude and phase information of the signal are demodulated.

[0123] The advantage of this method is that it does not require multiple acousto-optic modulators, and the pulse signals injected into each mode are frequency modulated by adding an electro-optic modulator and an adjustable filter to the optical fiber ring. The first optical pulse output from the semiconductor optical amplifier 103 is divided into two after passing through the fifth optical fiber coupler 501, and half of the optical pulse passes through the second output port of the fifth optical fiber coupler 501 and enters the optical switch 504, and the frequency modulation Δω0=0 enters the optical switch 504. At this time, the state of the optical switch is off. The other half of the optical pulse passes through the first output port of the fifth optical fiber coupler 501 and enters the acousto-optic modulator 502. The frequency shift of the acousto-optic modulator 502 is Δω1, and the frequency modulation of the optical pulse is Δω0+Δω1. The output port of the acousto-optic modulator 502 is connected to the input port of the pulse erbium-doped fiber amplifier 105, and the amplification factor of the pulse erbium-doped fiber amplifier 105 is twice, so as to adjust the signal light amplitudes of different frequency modulations (representing different mode injections) output after passing through the optical fiber ring each time to be roughly the same. The optical pulse (frequency modulated Δω0+Δω1) passes through the output port of the adjustable filter 503 and enters the fifth optical fiber coupler 501, where it is split into two again. Half of the optical pulse passes through the second output port of the fifth optical fiber coupler 501 and enters the first channel of the optical switch 504, where it is frequency modulated Δω0+Δω1. The other half of the optical pulse passes through the first output port of the fifth optical fiber coupler 501 and enters the acousto-optic modulator 502 again. At this time, the frequency of the optical pulse is modulated Δω0+Δω2, where Δω2=2Δω1. And so on. The detection pulses injected into each channel of the optical switch are modulated with different frequencies Δω0+Δω1, Δω0+Δω2, Δω0+Δω3,…, Δω0+Δω N , where Δω2=2Δω1,…Δω N =NΔω1. The cutoff frequency of the adjustable filter 503 is set to Δω0+(N+1)Δω1 to prevent the optical pulse in the optical fiber ring from transmitting endlessly until the semiconductor optical amplifier 103 outputs the second optical pulse. The switching duration and repetition frequency of each channel of the optical switch 504 need to be strictly controlled according to the length L of the optical fiber ring and the pulse repetition frequency fs, such as Fig.10 As shown. The time nL / c for one circle of the fiber ring needs to be greater than the pulse width T to avoid signals of different frequencies from overlapping each other. The pulse repetition frequency fs is set to 1 / f s>>N*nL / c, to avoid the mutual overlap of optical pulses of different time sequences. The repetition frequency of the optical switch 504 can be set to be the same as the pulse repetition frequency. Each channel of the high-speed optical switch is controlled by a digital signal, and its response time is ~10ns. When controlled by a digital signal, Δt>10ns can be set. The switching duration of each output port can be set slightly larger than the pulse width T. The length of the optical fiber delay line is adjusted according to the delay of the pulses (different modes) modulated by different frequencies output by each channel of the optical switch, thereby strictly ensuring that the pulses of each mode can be injected into the mode multiplexer / demultiplexer at the same time.

[0124] Each port of the fifth fiber coupler 501 of this embodiment is a common single-mode fiber, the wavelength is 1550nm, the ports are 2×2, and the splitting ratio is 1:1. The frequency shift of the acousto-optic modulator 502 is Δω1. The optical switch 504 is a high-speed optical switch, the response time is 10ns, and the ports are 1×N.

[0125] It should be understood that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, various changes can be made to its form and details. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A few-mode optical fiber distributed acoustic sensing system, comprising: A light source module and a signal detection module for outputting reference light and detection light, characterized in that: a modulation module, a multi-channel injection module, a mode multiplexer / demultiplexer and a few-mode optical fiber are sequentially arranged on the optical path of the detection light; a beam splitting module is arranged on the optical path of the reference light, for splitting one reference light into N reference lights; The multi-channel injection module includes N optical fiber delay lines and N connecting devices; The detection light is sequentially injected into each mode of the few-mode optical fiber through the modulation module, the multi-channel injection module, and the mode multiplexer / demultiplexer for transmission. The backward Rayleigh scattered signal light scattered back by the few-mode optical fiber returns to the mode multiplexer / demultiplexer, so that the signal lights of N scattering modes are separated from each other, and enter the signal detection module through N connecting devices and N reference lights for beat frequency and detection, and are collected by the data acquisition card and then enter the signal processing module for analysis and processing; The modulation module adds different frequency labels to the detection pulses of different modes injected into the few-mode optical fiber, and the optical fiber delay line controls the delay time of each channel so that the optical pulses of each mode are injected into the few-mode optical fiber at the same time; The modulation module includes a pulse chopping part, a pulse amplification part, a frequency modulation part and a connection part; The pulse chopping part is an acousto-optic modulator, the pulse amplification part is N pulse erbium-doped fiber amplifiers, the frequency modulation part is an electro-optic modulator and N fiber gratings, the connection part is a second fiber coupler, the detection light output by the light source module is injected into the input port of the electro-optic modulator, the output port of the electro-optic modulator is connected to the input port of the acousto-optic modulator, the electro-optic modulator generates several equally spaced frequency sidebands near the center frequency of the optical carrier, the output port of the acousto-optic modulator is connected to the input port of the second fiber coupler, the N pulse lights separated from the N output ports of the second fiber coupler pass through N fiber gratings with different cutoff frequencies respectively, so that the N pulse lights are modulated by a different frequency respectively, and the N fiber grating output ports are connected to the input ports of N pulse erbium-doped fiber amplifiers respectively. Since the amplitudes of the frequency sidebands generated by the electro-optic modulator are different, each frequency modulation needs to be amplified by its own pulse erbium-doped fiber amplifier to adjust the signal light amplitudes corresponding to different frequency sidebands of each mode to be roughly the same.

2. The few-mode fiber distributed acoustic sensing system according to claim 1, characterized in that: The pulse chopping part can also be a semiconductor optical amplifier, the pulse amplification part is a pulse erbium-doped fiber amplifier, the frequency modulation part is an acousto-optic modulator and a tunable filter, the connecting part is a fifth fiber coupler and an optical switch, the detection light output by the light source module is connected to the input port of the semiconductor optical amplifier, the output port of the semiconductor optical amplifier is connected to the first input port of the fifth fiber coupler, the first output port of the fifth fiber coupler is connected to the input port of the acousto-optic modulator, the output port of the acousto-optic modulator is connected to the input port of the pulse erbium-doped fiber amplifier, the output port of the pulse erbium-doped fiber amplifier is connected to the input port of the tunable filter, the output port of the tunable filter is connected to the second input port of the fifth fiber coupler to form a closed loop, and the fifth output port of the fifth fiber coupler is connected to the input port of the optical switch.

3. The few-mode fiber distributed acoustic sensing system according to claim 1, characterized in that: The signal detection module includes N fourth optical fiber couplers and N double-balanced detectors.

4. The few-mode fiber distributed acoustic sensing system according to claim 1, characterized in that: The connecting device is a circulator, or a 2×2 optical fiber coupler with a unidirectional isolator connected to one port.

5. The few-mode fiber distributed acoustic sensing system according to claim 1, characterized in that: The data acquisition card should be a multi-channel data acquisition card with synchronization function, or a plurality of dual-channel acquisition cards.

6. A method for performing diversity combining on the Rayleigh scattered signals of the few-mode fiber distributed acoustic sensing system according to claim 3, characterized in that: The signal processing module includes the following steps: the output signal of each double-balanced detector collected by the signal detection module is z i , i = 1, 2..., N; separate the Rayleigh light scattering signals corresponding to different frequency labels in different transmission modes in the frequency domain through bandpass filtering to obtain z i ′(y1,y2,...,y N );The coupling between transmission modes is eliminated by MIMO equalization algorithm, so that a certain transmission mode y i Contains only x i , i = 1, 2..., N and does not include the signal x coupled from other modes i′ ,i≠i′, compensation mode coupling x i =W(n)*y i , so the final output signal is z i ″(x1,x2,...,x N ); where W(n) is the equalizer weight for eliminating interference of the ith mode with the ith' mode, and n is the number of equalizer taps. The separated signals are then effectively aggregated by the coherent combining method; finally, the resultant vector is phase demodulated to recover the disturbance information imposed on the optical fiber by the outside world.

Images