A fade-free DAS system based on multi-frequency delay loop and its working method

Abstract

The present invention discloses a fade-free DAS system based on a multi-frequency delay loop and its operating method, relating to the field of distributed acoustic wave sensing technology. The DAS system includes a narrow-linewidth laser, a fiber coupler, a polarization-maintaining optical isolator, an acousto-optic modulator, a polarization-maintaining delay fiber, an erbium-doped fiber amplifier, a polarization-maintaining dense wavelength division multiplexer, a fiber circulator, a fiber Bragg grating, a piezoelectric transducer, a polarization controller, a photoelectric balanced detector, a data acquisition card, an arbitrary waveform generator, and the like. The present invention utilizes a multi-frequency delay loop for multi-frequency modulation. The number of frequencies of the light wave increases with the number of times the light wave circulates in the delay loop. Without increasing the use of a large-bandwidth electro-optic modulator, a low-bandwidth acousto-optic modulator is used to achieve flexible frequency modulation, which is not affected by the carrier frequency and various order sidebands. The use of a low-bandwidth acousto-optic modulator for frequency modulation in the multi-frequency delay loop effectively saves modulation costs compared to electro-optic modulation or multi-acoustic-optic / multi-light source modulation schemes.

Description

Technical Field

[0001] The present invention relates to the field of distributed acoustic wave sensing technology, and in particular to a fading-free DAS system based on a multi-frequency delay loop and a working method thereof. Background Art

[0002] Distributed acoustic sensing (DAS) technology quantitatively analyzes vibration signals along the sensing fiber by demodulating the phase changes of the Rayleigh scattered light signal. Compared with other types of distributed optical fiber sensors (DOFS), it has unique advantages such as fast measurement speed and high detection sensitivity. It has broad application prospects in oil and gas pipeline monitoring, seismic wave monitoring, and perimeter intrusion detection.

[0003] However, due to the high coherence of optical signals in DAS systems, the optical signals within the half-width of the detection pulse light interfere with each other, and the position where the coherent signals cancel each other is called the coherent fading point. The signal-to-noise ratio of the optical signal is extremely low, resulting in demodulation distortion of the phase information at the corresponding position. The main means to solve coherent fading in DAS systems include using special optical fibers such as multi-mode, few-mode and multi-core optical fibers, scattering-enhanced point optical fibers as sensing units, or using multi-frequency, multi-wavelength, and multi-phase pulse modulation multiplexing. Traditional acousto-optic modulation devices have a good extinction ratio and are single-frequency modulation, but the modulation bandwidth is low and the number of modulated frequencies is small; while traditional electro-optic modulation devices have a high modulation bandwidth, but multi-frequency modulation is difficult and is affected by the carrier and various order sidebands. Therefore, it is necessary to achieve flexible multi-frequency modulation. Summary of the Invention

[0004] In view of the above problems, the present invention proposes a fade-free DAS system based on a multi-frequency delay loop and a working method thereof.

[0005] According to one aspect of the present invention, a fade-free DAS system based on a multi-frequency delay ring is proposed. The system includes a narrow linewidth laser 1, a first fiber coupler 2, a first polarization-maintaining optical isolator 3, a second fiber coupler 4, a first acousto-optic modulator 5, a polarization-maintaining delay fiber 6, a first erbium-doped fiber amplifier 7, a polarization-maintaining dense wavelength division multiplexer 8, a second polarization-maintaining optical isolator 9, a second acousto-optic modulator 10, a second erbium-doped fiber amplifier 11, a first fiber circulator 12, a first fiber Bragg grating 13, a second fiber circulator 14, a piezoelectric transducer 15, a third erbium-doped fiber amplifier 16, a third fiber circulator 17, a second fiber Bragg grating 18, a polarization controller 19, a third fiber coupler 20, a photoelectric balanced detector 21, a data acquisition card 22, and an arbitrary waveform generator 23.

[0006] The optical signal output end of the narrow linewidth laser 1 is connected to the optical signal input end of the first optical fiber coupler 2, and the optical signal output end of the first optical fiber coupler 2 is connected to the optical signal input end of the first polarization-maintaining optical isolator 3 and the optical signal input end of the polarization controller 19 respectively;

[0007] The optical signal output end of the first polarization-maintaining optical isolator 3 is connected to the first optical signal input end 4-1 of the second optical fiber coupler 4, and the first optical signal output end 4-2 of the second optical fiber coupler 4 is connected to the optical signal input end of the second acousto-optic modulator 10;

[0008] The optical signal output end of the second acousto-optic modulator 10 is connected to the optical signal input end of the second erbium-doped fiber amplifier 11, the optical signal output end of the second erbium-doped fiber amplifier 11 is connected to the first port 12-1 of the first fiber circulator 12, and the second port 12-2 of the first fiber circulator 12 is connected to the first fiber Bragg grating 13;

[0009] The second optical signal output terminal 4-3 of the second optical fiber coupler 4 is connected to the optical signal input terminal of the first acousto-optic modulator 5, the optical signal output terminal of the first acousto-optic modulator 5 is connected to one end of a polarization-maintaining delay optical fiber 6, the other end of the polarization-maintaining delay optical fiber 6 is connected to the optical signal input terminal of the first erbium-doped fiber amplifier 7, the optical signal output terminal of the first erbium-doped fiber amplifier 7 is connected to the optical signal input terminal of a polarization-maintaining dense wavelength division multiplexer 8, the optical signal output terminal of the polarization-maintaining dense wavelength division multiplexer 8 is connected to the optical signal input terminal of a second polarization-maintaining optical isolator 9, and the optical signal output terminal of the second polarization-maintaining optical isolator 9 is connected to the second optical signal input terminal 4-4 of the second optical fiber coupler 4;

[0010] Port No. 3 12-3 of the first fiber circulator 12 is in communication with port No. 14-1 of the second fiber circulator 14, port No. 2 14-2 of the second fiber circulator 14 is in communication with one end of a sensing fiber, the other end of the sensing fiber is in communication with a piezoelectric transducer 15, port No. 3 14-3 of the second fiber circulator 14 is in communication with an optical signal input end of a third erbium-doped fiber amplifier 16, an optical signal output end of the third erbium-doped fiber amplifier 16 is in communication with port No. 17-1 of a third fiber circulator 17, and port No. 2 17-2 of the third fiber circulator 17 is in communication with a second fiber Bragg grating 18;

[0011] The optical signal output end of the third port 17-3 of the third optical fiber circulator 17 and the polarization controller 19 are respectively connected to the optical signal input end of the third optical fiber coupler 20, the optical signal output end of the third optical fiber coupler 20 is connected to the optical signal input end of the photoelectric balance detector 21, and the electrical signal output end of the photoelectric balance detector 21 is connected to the data acquisition card 22;

[0012] The RF output terminal of the arbitrary waveform generator 23 is connected to the RF signal input terminal of the first acousto-optic modulator 5, the RF signal input terminal of the second acousto-optic modulator 10, the RF signal input terminal of the piezoelectric transducer 15 and the trigger signal input terminal of the data acquisition card 22 respectively.

[0013] In one possible implementation, the narrow linewidth laser 1 is a polarization-maintaining laser with an output power of 20 mW, a wavelength of 1550.12 nm, and a linewidth of 1 kHz.

[0014] In one possible implementation, the first fiber coupler 2 is a 1×2 polarization-maintaining coupler with a splitting ratio of 90:10; the second fiber coupler 4 is a 2×2 polarization-maintaining coupler with a splitting ratio of 50:50; and the third fiber coupler 20 is a 2×2 coupler with a splitting ratio of 50:50.

[0015] In one possible implementation, the first AOM 5 is a broadband polarization-maintaining modulator with a 3dB bandwidth of 100 MHz, an operating range of 30 MHz to 130 MHz, and an extinction ratio of 50 dB; the second AOM 10 is a polarization-maintaining modulator with a carrier frequency of 200 MHz and an extinction ratio of 50 dB.

[0016] In one possible implementation, the input and output ends of the first erbium-doped fiber amplifier 7 are polarization-maintaining fibers; the central transmission wavelength of the polarization-maintaining dense wavelength division multiplexer 8 is 1550.12 nm, and the 3dB transmission bandwidth is 0.20 nm.

[0017] In one possible implementation, the central reflection wavelength of the first fiber Bragg grating 13 and the second fiber Bragg grating 18 is 1550.12 nm, and the 3dB reflection bandwidth is 0.08 nm; the 3dB operating bandwidth of the photoelectric balanced detector 21 is 2 GHz; the sampling rate of the data acquisition card 22 is 5 GS / s, and the sampling resolution is 14 bits.

[0018] According to another aspect of the present invention, a method for operating a fade-free DAS system based on a multi-frequency delay loop is provided. The method is implemented based on the aforementioned fade-free DAS system based on a multi-frequency delay loop; the method comprises:

[0019] The single-frequency continuous light output by the narrow linewidth laser 1 is divided into two paths, upper and lower, by the first fiber coupler 2; the upper path is used as the detection pulse light modulation of the DAS system; the lower path continuous light is used as the reference light of the DAS system after the polarization state is adjusted by the polarization controller 20;

[0020] The uplink continuous light passes through the first polarization-maintaining optical isolator 3 and is divided into two upper and lower branches by the second optical fiber coupler 4: a first upper branch and a first lower branch. The first lower branch continuous light sequentially enters the multi-frequency delay loop composed of the first acousto-optic modulator 5, the polarization-maintaining delay fiber 6, the first erbium-doped fiber amplifier 7, the polarization-maintaining dense wavelength division multiplexer 8, and the second polarization-maintaining optical isolator 9 for multi-frequency modulation. The modulated optical signal is further divided into two upper and lower branches by the second optical fiber coupler 4: a second upper branch and a second lower branch. The first upper branch continuous light and the second upper branch continuous light are periodically modulated into pulsed light by the second acousto-optic modulator 10 and then sent to the second erbium-doped fiber amplifier 11 for multi-frequency modulation. The power is pre-amplified and then injected into the first fiber Bragg grating 13 through the first fiber circulator 12 to filter out ASE noise. The pulsed light after ASE noise filtering is injected into the sensing fiber and the piezoelectric transducer 15 in sequence through the second fiber circulator 14. The second lower branch continuous light continues to be multi-frequency modulated again by the multi-frequency delay loop. Among them, in the multi-frequency delay loop, the first acousto-optic modulator 5 is used for frequency shift modulation, the polarization-maintaining delay fiber 6 is used to extend the delay of the delay loop, the first erbium-doped fiber amplifier 7 is used to compensate for the loss of the delay loop, the polarization-maintaining dense wavelength division multiplexer 8 is used to filter out ASE noise, and the second polarization-maintaining optical isolator 9 is used to prevent reverse transmission of the optical signal.

[0021] The backscattered Rayleigh light signal returned by the sensing fiber is injected into the third erbium-doped fiber amplifier 16 for power pre-amplification via the second fiber circulator 14. The signal is then injected into the second fiber Bragg grating 18 by the third fiber circulator 17 to filter out ASE noise. After that, the signal is injected into the third fiber coupler 20 together with the reference light of the drop-down DAS system for coherent frequency beat. The signal is then converted to photoelectricity by the photoelectric balanced detector 21 and recorded by the data acquisition card 22.

[0022] The arbitrary waveform generator 23 is used to control the frequency shift of the first AOM 5 and the second AOM 10 and the on-off switching of the optical signal, and to control the data acquisition cycle of the data acquisition card 22 .

[0023] In one possible implementation, the multi-frequency beat signal collected by the data acquisition card 22 is transmitted to a computer, and the computer performs the following processing: filtering the multi-frequency beat signal into multiple single-frequency beat signals; demodulating the amplitude and phase information of each single-frequency beat signal through Hilbert transform; and performing weighted averaging on the phase information of each single-frequency beat signal using the amplitude information as the weight.

[0024] In one possible implementation, the modulation process of the multi-frequency delay loop on the continuous light includes:

[0025] Define ω0 and are the angular frequency and initial phase of the continuous light output by the narrow linewidth laser 1, Δω1, T AOM1 and TP1 are the modulation frequency, period and pulse width of the first acousto-optic modulator 5 respectively; L loop is the length of the multi-frequency delay loop; the length of the sensing fiber is L; the modulation period T of the first acousto-optic modulator 5 AOM1 Satisfies the following relationship:

[0026]

[0027] Where n is the effective refractive index of the optical fiber; c is the propagation speed of light waves in vacuum;

[0028] Multi-frequency continuous light E modulated by multi-frequency delay ring loop (t) is expressed as follows:

[0029]

[0030] Among them, A loop For E loop (t) is the amplitude; m is the number of multi-frequency delay ring modulation, t represents time.

[0031] In one possible implementation, the process of the second acousto-optic modulator 10 modulating the multi-frequency continuous light into the multi-frequency detection pulse light includes:

[0032] Define Δω2 as the carrier frequency of the second acousto-optic modulator 10, and the multi-frequency detection pulse light E modulated by it probe The expression of (t) is as follows:

[0033]

[0034] Among them, A probe For E probe (t) amplitude; T P2 is the pulse width of the multi-frequency detection pulse light; j is the frequency number of the multi-frequency detection pulse light.

[0035] In one possible implementation, the 3dB operating bandwidth f of the photoelectric balanced detector 22 is BPD Satisfies the following relationship:

[0036] f BPD ≥2π(Δω2+jΔω1);

[0037] In one possible implementation, the sampling rate S of the data acquisition card 23 is a Satisfies the following relationship:

[0038] S a ≥2max(Δω2+jΔω1).

[0039] The beneficial technical effects of the present invention are:

[0040] The present invention proposes a fade-free DAS system based on a multi-frequency delay loop and its operating method, which have the following advantages: 1) Flexible frequency modulation: Multi-frequency modulation is performed using a multi-frequency delay loop. The number of lightwave frequencies increases with the number of times the lightwave circulates through the delay loop. Without increasing the use of a large-bandwidth electro-optical modulator, flexible frequency modulation is achieved using a low-bandwidth acousto-optic modulator, which is not affected by the carrier frequency and various order sidebands. 2) Low modulation cost: The use of a low-bandwidth acousto-optic modulator for frequency modulation in the multi-frequency delay loop effectively reduces modulation costs compared to electro-optical modulation or multi-acoustic-optic / multi-light source modulation schemes. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily apparent by reading the following detailed description with reference to the accompanying drawings, in which several embodiments of the present invention are shown by way of example and not limitation, in which:

[0042] Figure 1 This is a structural diagram of a fade-free DAS system based on a multi-frequency delay loop according to an embodiment of the present invention;

[0043] Figure 2 1 is an example diagram of an original beat frequency signal measured in an embodiment of the present invention;

[0044] Figure 3 This is an example diagram of frequency information of an original beat signal in an embodiment of the present invention;

[0045] Figure 4 Schematic diagram of the relationship between the number of multiplexing frequencies and fading probability in an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The principles and spirit of the present invention will be described below with reference to several exemplary embodiments. It should be understood that these embodiments are provided solely to enable those skilled in the art to better understand and implement the present invention, and are not intended to limit the scope of the present invention in any way. Rather, these embodiments are provided to make this disclosure more thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art.

[0047] The embodiment of the present invention proposes a non-fading DAS system based on a multi-frequency delay loop, such as Figure 1As shown, the system includes: a narrow linewidth laser 1, a first fiber coupler 2, a first polarization-maintaining optical isolator 3, a second fiber coupler 4, a first acousto-optic modulator 5, a polarization-maintaining delay fiber 6, a first erbium-doped fiber amplifier 7, a polarization-maintaining dense wavelength division multiplexer 8, a second polarization-maintaining optical isolator 9, a second acousto-optic modulator 10, a second erbium-doped fiber amplifier 11, a first fiber circulator 12, a first fiber Bragg grating 13, a second fiber circulator 14, a piezoelectric transducer 15, a third erbium-doped fiber amplifier 16, a third fiber circulator 17, a second fiber Bragg grating 18, a polarization controller 19, a third fiber coupler 20, a photoelectric balanced detector 21, a data acquisition card 22, and an arbitrary waveform generator 23;

[0048] The optical signal output end of the narrow linewidth laser 1 is connected to the optical signal input end of the first optical fiber coupler 2, and the optical signal output end of the first optical fiber coupler 2 is connected to the optical signal input end of the first polarization-maintaining optical isolator 3 and the optical signal input end of the polarization controller 19 respectively;

[0049] The optical signal output end of the first polarization-maintaining optical isolator 3 is connected to the first optical signal input end 4-1 of the second optical fiber coupler 4, and the first optical signal output end 4-2 of the second optical fiber coupler 4 is connected to the optical signal input end of the second acousto-optic modulator 10;

[0050] The optical signal output end of the second acousto-optic modulator 10 is connected to the optical signal input end of the second erbium-doped fiber amplifier 11, the optical signal output end of the second erbium-doped fiber amplifier 11 is connected to the first port 12-1 of the first fiber circulator 12, and the second port 12-2 of the first fiber circulator 12 is connected to the first fiber Bragg grating 13;

[0051] The second optical signal output terminal 4-3 of the second optical fiber coupler 4 is connected to the optical signal input terminal of the first acousto-optic modulator 5, the optical signal output terminal of the first acousto-optic modulator 5 is connected to one end of a polarization-maintaining delay optical fiber 6, the other end of the polarization-maintaining delay optical fiber 6 is connected to the optical signal input terminal of the first erbium-doped fiber amplifier 7, the optical signal output terminal of the first erbium-doped fiber amplifier 7 is connected to the optical signal input terminal of a polarization-maintaining dense wavelength division multiplexer 8, the optical signal output terminal of the polarization-maintaining dense wavelength division multiplexer 8 is connected to the optical signal input terminal of a second polarization-maintaining optical isolator 9, and the optical signal output terminal of the second polarization-maintaining optical isolator 9 is connected to the second optical signal input terminal 4-4 of the second optical fiber coupler 4;

[0052] Port No. 3 12-3 of the first fiber circulator 12 is in communication with port No. 14-1 of the second fiber circulator 14, port No. 2 14-2 of the second fiber circulator 14 is in communication with one end of a sensing fiber, the other end of the sensing fiber is in communication with a piezoelectric transducer 15, port No. 3 14-3 of the second fiber circulator 14 is in communication with an optical signal input end of a third erbium-doped fiber amplifier 16, an optical signal output end of the third erbium-doped fiber amplifier 16 is in communication with port No. 17-1 of a third fiber circulator 17, and port No. 2 17-2 of the third fiber circulator 17 is in communication with a second fiber Bragg grating 18;

[0053] The optical signal output end of the third port 17-3 of the third optical fiber circulator 17 and the polarization controller 19 are respectively connected to the optical signal input end of the third optical fiber coupler 20, the optical signal output end of the third optical fiber coupler 20 is connected to the optical signal input end of the photoelectric balance detector 21, and the electrical signal output end of the photoelectric balance detector 21 is connected to the data acquisition card 22;

[0054] The RF output terminal of the arbitrary waveform generator 23 is connected to the RF signal input terminal of the first acousto-optic modulator 5, the RF signal input terminal of the second acousto-optic modulator 10, the RF signal input terminal of the piezoelectric transducer 15 and the trigger signal input terminal of the data acquisition card 22 respectively.

[0055] In this embodiment, preferably, the narrow linewidth laser 1 is a polarization-maintaining laser with an output power of 20 mW, a wavelength of 1550.12 nm, and a linewidth of 1 kHz.

[0056] In this embodiment, preferably, the first optical fiber coupler 2 is a 1×2 polarization-maintaining coupler with a splitting ratio of 90:10; the second optical fiber coupler 4 is a 2×2 polarization-maintaining coupler with a splitting ratio of 50:50; and the third optical fiber coupler 20 is a 2×2 coupler with a splitting ratio of 50:50.

[0057] In this embodiment, preferably, the first acousto-optic modulator 5 is a broadband polarization-maintaining modulator with a 3dB bandwidth of 100 MHz, an operating range of 30 MHz-130 MHz, and an extinction ratio of 50 dB; the second acousto-optic modulator 10 is a polarization-maintaining modulator with a carrier frequency of 200 MHz and an extinction ratio of 50 dB.

[0058] In this embodiment, preferably, the central transmission wavelength of the polarization-maintaining dense wavelength division multiplexer 8 is 1550.12 nm, and the 3dB transmission bandwidth is 0.20 nm; the central reflection wavelength of the first fiber Bragg grating 13 and the second fiber Bragg grating 18 is 1550.12 nm, and the 3dB reflection bandwidth is 0.08 nm.

[0059] In this embodiment, preferably, the input end and the output end of the first erbium-doped fiber amplifier 7 are polarization-maintaining fibers.

[0060] In this embodiment, preferably, the 3dB operating bandwidth of the photoelectric balance detector 21 is 2 GHz.

[0061] In this embodiment, preferably, the sampling rate of the data acquisition card 22 is 5 GS / s and the sampling resolution is 14 bits.

[0062] Another embodiment of the present invention provides a method for operating a fade-free DAS system based on a multi-frequency delay loop. The method is implemented based on the fade-free DAS system based on a multi-frequency delay loop described in the above embodiment. The method includes:

[0063] The single-frequency continuous light output by the narrow linewidth laser 1 is divided into two paths, upper and lower, by the first fiber coupler 2; the upper path is used as the detection pulse light modulation of the DAS system; the lower path continuous light is used as the reference light of the DAS system after the polarization state is adjusted by the polarization controller 19;

[0064] The uplink continuous light passes through the first polarization-maintaining optical isolator 3 and is divided into two upper and lower branches by the second optical fiber coupler 4: a first upper branch and a first lower branch. The first lower branch continuous light sequentially enters the multi-frequency delay loop composed of the first acousto-optic modulator 5, the polarization-maintaining delay optical fiber 6, the first erbium-doped fiber amplifier 7, the polarization-maintaining dense wavelength division multiplexer 8, and the second polarization-maintaining optical isolator 9 for multi-frequency modulation. The modulated optical signal is further divided into two upper and lower branches by the second optical fiber coupler 4: a second upper branch and a second lower branch. The first upper branch continuous light and the second upper branch continuous light are periodically modulated into pulsed light by the second acousto-optic modulator 10 and then transmitted to the second erbium-doped fiber amplifier 11. The power is pre-amplified and then injected into the first fiber Bragg grating 13 through the first fiber circulator 12 to filter out ASE noise. The pulsed light after ASE noise filtering is injected into the sensing fiber and the piezoelectric transducer 15 in sequence through the second fiber circulator 14. The second lower branch continuous light is further modulated by the multi-frequency delay loop. Among them, in the multi-frequency delay loop, the first acousto-optic modulator 5 is used for frequency shift modulation, the polarization-maintaining delay fiber 6 is used to extend the delay of the delay loop, the first erbium-doped fiber amplifier 7 is used to compensate for the loss of the delay loop, the polarization-maintaining dense wavelength division multiplexer 8 is used to filter out ASE noise, and the second polarization-maintaining optical isolator 9 is used to prevent reverse transmission of the optical signal.

[0065] The backscattered Rayleigh light signal returned by the sensing fiber is injected into the third erbium-doped fiber amplifier 16 for power pre-amplification through the second fiber circulator 14. It is then injected into the second fiber Bragg grating 18 by the third fiber circulator 17 to filter out ASE noise. After that, it is injected into the third fiber coupler 20 together with the reference light of the downlink DAS system for coherent frequency beat. It is then converted into electrical and electronic signals by the photoelectric balanced detector 21 and recorded by the data acquisition card 22.

[0066] The arbitrary waveform generator 23 is used to control the frequency shift of the first AOM 5 and the second AOM 10 and the on-off switching of the optical signal, and to control the data acquisition cycle of the data acquisition card 22 .

[0067] The collected multi-frequency beat signals are filtered to produce multiple single-frequency beat signals, and the amplitude and phase information of each single-frequency beat signal are demodulated separately through Hilbert transform. The phase information of each single-frequency beat signal is weighted and averaged using the amplitude information as the weight, so as to suppress the phase demodulation distortion caused by the fading point due to the coherent fading effect in the DAS system.

[0068] Figure 2 The time domain distribution of the beat frequency signal measured for the system; Figure 3 The frequency domain distribution of the beat signal measured by the system shows the 20 frequency information modulated by the multi-frequency delay loop; Figure 4 This is the relationship between the number of frequencies reused and the fading probability of the multi-frequency delay loop modulation. The more frequencies there are, the lower the fading probability.

[0069] In this embodiment, preferably, the modulation process of the continuous light in the multi-frequency delay loop is as follows:

[0070] First, define ω0 and are the angular frequency and initial phase of the continuous light output by the narrow linewidth laser 1, Δω1, T AOM1 and T P1 are the modulation frequency, period and pulse width of the first acousto-optic modulator 5 respectively; L loop is the length of the multi-frequency delay loop; the length of the sensing fiber is L; the modulation period T of the first acousto-optic modulator 5 AOM1 The following relationship should be satisfied:

[0071]

[0072] Where n is the effective refractive index of the optical fiber; c is the propagation speed of light waves in vacuum. loop (t) is expressed as follows:

[0073]

[0074] Among them, A loop For E loop (t) amplitude; m is the modulation times of the multi-frequency delay loop, the modulation times m and T P1 The relationship is as follows:

[0075]

[0076] It can be seen that the number of lightwave frequencies increases with the number of times the lightwave circulates in the delay loop. Without increasing the use of large-bandwidth electro-optical modulators, a low-bandwidth acousto-optic modulator can be used to achieve flexible frequency modulation without being affected by the carrier frequency and various order sidebands, effectively controlling the cost of multi-frequency modulation.

[0077] In this embodiment, preferably, the process of the second acousto-optic modulator 10 modulating the multi-frequency continuous light into the multi-frequency detection pulse light is as follows:

[0078] Define Δω2 as the carrier frequency of the second acousto-optic modulator 10; the multi-frequency detection pulse light E modulated by it probe The expression of (t) is as follows:

[0079]

[0080] Among them, A probe For E probe (t) amplitude; T P2 is the pulse width of the multi-frequency detection pulse light; j is the frequency number of the multi-frequency detection pulse light.

[0081] In this embodiment, preferably, the 3dB working bandwidth f of the photoelectric balance detector 21 BPD The following relationship should be satisfied:

[0082] f BPD ≥2π(Δω2+jΔω1) (5)

[0083] In this embodiment, preferably, the sampling rate S of the data acquisition card 22 is a The following relationship should be satisfied:

[0084] S a ≥2max(Δω2+jΔω1) (6)

[0085] The present invention proposes a fade-free DAS system based on a multi-frequency delay loop and its operating method, which have the following advantages: 1) Flexible frequency modulation: Multi-frequency modulation is performed using a multi-frequency delay loop. The number of lightwave frequencies increases with the number of times the lightwave circulates through the delay loop. Without increasing the use of a large-bandwidth electro-optical modulator, flexible frequency modulation is achieved using a low-bandwidth acousto-optic modulator, which is not affected by the carrier frequency and various order sidebands. 2) Low modulation cost: The use of a low-bandwidth acousto-optic modulator for frequency modulation in the multi-frequency delay loop effectively reduces modulation costs compared to electro-optical modulation or multi-acoustic-optic / multi-light source modulation schemes.

[0086] Although the spirit and principles of the present invention have been described with reference to several specific embodiments, it should be understood that the present invention is not limited to the specific embodiments disclosed, and the division into various aspects does not mean that the features of these aspects cannot be combined to benefit. Such division is only for the convenience of expression. The present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A fade-free DAS system based on a multi-frequency delay loop, characterized in that: The invention comprises a narrow linewidth laser (1), a first optical fiber coupler (2), a first polarization-maintaining optical isolator (3), a second optical fiber coupler (4), a first acousto-optic modulator (5), a polarization-maintaining delay optical fiber (6), a first erbium-doped optical fiber amplifier (7), a polarization-maintaining dense wavelength division multiplexer (8), a second polarization-maintaining optical isolator (9), a second acousto-optic modulator (10), a second erbium-doped optical fiber amplifier (11), a first optical fiber circulator (12), a first optical fiber Bragg grating (13), a second optical fiber circulator (14), a piezoelectric transducer (15), a third erbium-doped optical fiber amplifier (16), a third optical fiber circulator (17), a second optical fiber Bragg grating (18), a polarization controller (19), a third optical fiber coupler (20), a photoelectric balance detector (21), a data acquisition card (22), and an arbitrary waveform generator (23); The optical signal output end of the narrow linewidth laser (1) is connected to the optical signal input end of the first optical fiber coupler (2), and the optical signal output end of the first optical fiber coupler (2) is respectively connected to the optical signal input end of the first polarization-maintaining optical isolator (3) and the optical signal input end of the polarization controller (19); The optical signal output end of the first polarization-maintaining optical isolator (3) is connected to the first optical signal input end (4-1) of the second optical fiber coupler (4), and the first optical signal output end (4-2) of the second optical fiber coupler (4) is connected to the optical signal input end of the second acousto-optic modulator (10); The optical signal output end of the second acousto-optic modulator (10) is connected to the optical signal input end of the second erbium-doped fiber amplifier (11), the optical signal output end of the second erbium-doped fiber amplifier (11) is connected to the first port (12-1) of the first optical fiber circulator (12), and the second port (12-2) of the first optical fiber circulator (12) is connected to the first fiber Bragg grating (13); The second optical signal output end (4-3) of the second optical fiber coupler (4) is connected to the optical signal input end of the first acousto-optic modulator (5), the optical signal output end of the first acousto-optic modulator (5) is connected to one end of the polarization-maintaining delay optical fiber (6), the other end of the polarization-maintaining delay optical fiber (6) is connected to the optical signal input end of the first erbium-doped optical fiber amplifier (7), the optical signal output end of the first erbium-doped optical fiber amplifier (7) is connected to the optical signal input end of the polarization-maintaining dense wavelength division multiplexer (8), the optical signal output end of the polarization-maintaining dense wavelength division multiplexer (8) is connected to the optical signal input end of the second polarization-maintaining optical isolator (9), and the optical signal output end of the second polarization-maintaining optical isolator (9) is connected to the second optical signal input end (4-4) of the second optical fiber coupler (4); The third port (12-3) of the first optical fiber circulator (12) is connected to the first port (14-1) of the second optical fiber circulator (14), the second port (14-2) of the second optical fiber circulator (14) is connected to one end of the sensing optical fiber, the other end of the sensing optical fiber is connected to the piezoelectric transducer (15), the third port (14-3) of the second optical fiber circulator (14) is connected to the optical signal input end of the third erbium-doped optical fiber amplifier (16), the optical signal output end of the third erbium-doped optical fiber amplifier (16) is connected to the first port (17-1) of the third optical fiber circulator (17), and the second port (17-2) of the third optical fiber circulator (17) is connected to the second optical fiber Bragg grating (18); The third port (17-3) of the third optical fiber circulator (17) and the optical signal output end of the polarization controller (19) are respectively connected to the optical signal input end of the third optical fiber coupler (20), the optical signal output end of the third optical fiber coupler (20) is connected to the optical signal input end of the photoelectric balance detector (21), and the electrical signal output end of the photoelectric balance detector (21) is connected to the data acquisition card (22); The radio frequency output end of the arbitrary waveform generator (23) is respectively connected to the radio frequency signal input end of the first acousto-optic modulator (5), the radio frequency signal input end of the second acousto-optic modulator (10), the radio frequency signal input end of the piezoelectric transducer (15), and the trigger signal input end of the data acquisition card (22); The input and output ends of the first erbium-doped fiber amplifier (7) are polarization-maintaining fibers; the central transmission wavelength of the polarization-maintaining dense wavelength division multiplexer (8) is 1550.12 nm, and the 3dB transmission bandwidth is 0.20 nm; the central reflection wavelengths of the first fiber Bragg grating (13) and the second fiber Bragg grating (18) are 1550.12 nm, and the 3dB reflection bandwidth is 0.08 nm; the 3dB operating bandwidth of the photoelectric balanced detector (21) is 2 GHz; the sampling rate of the data acquisition card (22) is 5 GS / s, and the sampling resolution is 14 bits.

2. The non-fading DAS system based on a multi-frequency delay loop according to claim 1, characterized in that: The narrow linewidth laser (1) is a polarization-maintaining laser with an output power of 20 mW, a wavelength of 1550.12 nm, and a linewidth of 1 kHz.

3. The non-fading DAS system based on a multi-frequency delay loop according to claim 1, characterized in that: The first optical fiber coupler (2) is a 1×2 polarization-maintaining coupler with a splitting ratio of 90:10; the second optical fiber coupler (4) is a 2×2 polarization-maintaining coupler with a splitting ratio of 50:50; and the third optical fiber coupler (20) is a 2×2 coupler with a splitting ratio of 50:

50.

4. The non-fading DAS system based on a multi-frequency delay loop according to claim 1, characterized in that: The first acousto-optic modulator (5) is a broadband polarization-maintaining modulator with a 3dB bandwidth of 100MHz, an operating range of 30MHz-130MHz, and an extinction ratio of 50dB; the second acousto-optic modulator (10) is a polarization-maintaining modulator with a carrier frequency of 200MHz and an extinction ratio of 50dB.

5. A method for operating a fade-free DAS system based on a multi-frequency delay loop, characterized in that: The working method is implemented based on a fade-free DAS system based on a multi-frequency delay loop according to any one of claims 1 to 4; the working method comprises: The single-frequency continuous light output by the narrow linewidth laser (1) is divided into two paths, an upper path and an lower path, by a first optical fiber coupler (2); the upper path is used as a detection pulse light modulation of the DAS system; the lower path continuous light is used as a reference light of the DAS system after the polarization state is adjusted by a polarization controller (20); The upper continuous light is divided into two upper and lower branches: a first upper branch and a first lower branch by the second optical fiber coupler (4) through the first polarization-maintaining optical isolator (3). The first lower branch continuous light sequentially enters a multi-frequency delay loop composed of a first acousto-optic modulator (5), a polarization-maintaining delay optical fiber (6), a first erbium-doped optical fiber amplifier (7), a polarization-maintaining dense wavelength division multiplexer (8), and a second polarization-maintaining optical isolator (9) for multi-frequency modulation. The modulated optical signal is further divided into two upper and lower branches: a second upper branch and a second lower branch by the second optical fiber coupler (4). The first upper branch continuous light and the second upper branch continuous light are periodically modulated into pulse light by the second acousto-optic modulator (10) and then enter the second erbium-doped optical fiber amplifier (11). The power of the optical fiber is pre-amplified and then injected into the first fiber Bragg grating (13) through the first fiber circulator (12) to filter out ASE noise; the pulse light after filtering out ASE noise is injected into the sensing fiber and the piezoelectric transducer (15) in sequence through the second fiber circulator (14); the second lower branch continuous light is further multi-frequency modulated by the multi-frequency delay loop; wherein, in the multi-frequency delay loop, the first acousto-optic modulator (5) is used for frequency shift modulation, the polarization-maintaining delay optical fiber (6) is used to expand the delay of the delay loop, the first erbium-doped fiber amplifier (7) is used to compensate for the loss of the delay loop, the polarization-maintaining dense wavelength division multiplexer (8) is used to filter out ASE noise, and the second polarization-maintaining optical isolator (9) is used to prevent the reverse transmission of the optical signal; The backscattered Rayleigh light signal returned by the sensing optical fiber is injected into the third erbium-doped fiber amplifier (16) through the second optical fiber circulator (14) for power pre-amplification, and is injected into the second fiber Bragg grating (18) through the third optical fiber circulator (17) to filter out ASE noise, and then injected into the third optical fiber coupler (20) together with the reference light of the downlink DAS system for coherent frequency beat, and then is photoelectrically converted by the photoelectric balanced detector (21) and recorded by the data acquisition card (22); The arbitrary waveform generator (23) is used to control the frequency shift and optical signal switching of the first acousto-optic modulator (5) and the second acousto-optic modulator (10), and to control the data acquisition cycle of the data acquisition card (22).

6. The operating method of a fade-free DAS system based on a multi-frequency delay loop according to claim 5, characterized in that: The multi-frequency beat signal collected by the data acquisition card (22) is transmitted to the computer, and the computer performs the following processing: filtering the multi-frequency beat signal into multiple single-frequency beat signals; demodulating the amplitude and phase information of each single-frequency beat signal by Hilbert transform; and performing weighted averaging operation on the phase information of each single-frequency beat signal with the amplitude information as the weight.

7. The operating method of a fade-free DAS system based on a multi-frequency delay loop according to claim 6, characterized in that: The modulation process of the multi-frequency delay loop on continuous light includes: definition and are the angular frequency and initial phase of the continuous light output by the narrow linewidth laser (1), 、 and are respectively the modulation frequency, period and pulse width of the first acousto-optic modulator (5); is the length of the multi-frequency delay loop; the length of the sensing fiber is L; then the modulation period of the first acousto-optic modulator (5) is Satisfies the following relationship: in, is the effective refractive index of the optical fiber; c is the propagation speed of light waves in vacuum; Multi-frequency continuous light modulated by multi-frequency delay ring The expression is as follows: ； in, for Amplitude; is the number of multi-frequency delay ring modulation, ; t represents time.

8. The operating method of a non-fading DAS system based on a multi-frequency delay loop according to claim 7, characterized in that: The process of the second acousto-optic modulator (10) modulating the multi-frequency continuous light into the multi-frequency detection pulse light includes: definition is the carrier frequency of the second acousto-optic modulator (10), and the multi-frequency detection pulse light modulated by it The expression is as follows: ； in, for Amplitude; is the pulse width of the multi-frequency detection pulse light; is the frequency number of the multi-frequency detection pulse light.

9. The operating method of a non-fading DAS system based on a multi-frequency delay loop according to claim 8, characterized in that: 3dB operating bandwidth of the photoelectric balanced detector (22) Satisfies the following relationship: ； Sampling rate of the data acquisition card (23) Satisfies the following relationship: 。

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