Ф-OTDR system based on phase-shifted dual-pulse detection technology and its demodulation method

Abstract

The present invention relates to a Ф-OTDR system based on phase-shifted dual-pulse detection technology and a demodulation method thereof. The system includes a front-end component, the front-end component generates a dual-pulse laser signal input to a sensing optical fiber to be tested through an optical circulator, the dual-pulse laser signal is two pulse laser signals with an initial phase difference generated after driving an acousto-optic modulator via a driving signal source; the dual-pulse laser signal generates a backward Rayleigh scattered light signal through Rayleigh scattering of the sensing optical fiber to be tested, and the backward Rayleigh scattered light signal is output to a detection processing component through an optical circulator; the detection processing component is used to perform photoelectric conversion on the interference signal of the backward Rayleigh scattered light signal output through the optical circulator and analyze the electrical signal to obtain the position information, vibration waveform and spectrum information of the vibration event. The system solves the technical problems of severe noise interference, complex optical path and high hardware cost.

Description

Technical Field

[0001] The invention relates to the technical field of distributed optical fiber vibration sensing, and in particular to a Ф-OTDR system based on phase-shifted double-pulse detection technology and a demodulation method thereof. Background Art

[0002] In the infrastructure construction of hydropower and water conservancy, such as oil and gas pipelines, rail transit, bridges and tunnels, there are a large number of physical fields that need to be monitored.

[0003] In practical applications, the widely used existing technology is distributed optical fiber sensing technology. Among them, Ф-OTDR (phase-sensitive optical time domain detector) is a rapidly developing distributed vibration sensing technology, which is currently commonly used as a system optical path based on intensity demodulation and a system optical path based on phase demodulation.

[0004] The system optical path based on intensity demodulation has a simple structure and the defect that the intensity information can only be used to qualitatively analyze the vibration event, but cannot analyze the waveform and spectrum information of the vibration event. It has few uses and poor application prospects.

[0005] The system optical path based on phase demodulation uses heterodyne detection technology to obtain a beat signal with a center frequency of tens to hundreds of MHz, and requires the use of a large-bandwidth optoelectronic converter and data acquisition equipment that matches the beat signal frequency. This results in a complex optical path, serious signal impact from laser source noise, high system hardware cost, and difficulty in data processing. Summary of the invention

[0006] 1. Technical issues to be resolved

[0007] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a Ф-OTDR system based on phase-shifted dual-pulse detection technology and a demodulation method thereof, which solves the technical problems of complex optical path, severe noise interference, high hardware cost and great difficulty in data processing.

[0008] (II) Technical solution

[0009] In order to achieve the above objectives, in a first aspect, an embodiment of the present invention provides a Ф-OTDR system based on phase-shifted double-pulse detection technology. The system comprises:

[0010] A front-end component, wherein the front-end component generates a double-pulse laser signal which is output to the sensing optical fiber to be tested through an optical circulator, wherein the double-pulse laser signal is two pulse laser signals with an initial phase difference which are generated after the acousto-optic modulator is driven by a driving signal source;

[0011] The double-pulse laser signal generates a backscattered Rayleigh light signal through Rayleigh scattering of the sensing optical fiber to be measured, and the backscattered Rayleigh light signal is output to the detection processing component through the optical circulator;

[0012] The detection and processing component is used to perform photoelectric conversion on the interference signal of the backward Rayleigh scattered light signal output by the optical circulator and analyze the electrical signal to obtain the position information, vibration waveform and / or spectrum information of the vibration event.

[0013] Optionally, the front-end component includes a narrow linewidth laser, an optical isolator, an acousto-optic modulator, a driving signal source and a first Er-doped fiber amplifier;

[0014] The narrow line width laser, the optical isolator, the acousto-optic modulator and the first Er-doped fiber amplifier are connected in sequence, and the driving signal source is connected to the acousto-optic modulator through a radio frequency line.

[0015] Optionally, the narrow linewidth laser has a linewidth less than 10 kHz; the narrow linewidth laser outputs incident laser light, which is output to the acousto-optic modulator through the optical isolator;

[0016] The acousto-optic modulator is driven by a driving signal source to modulate the incident laser and output it to the first Er-doped fiber amplifier;

[0017] The first Er-doped fiber amplifier, as an output end of the front-end component, is connected to the first port of the optical circulator;

[0018] The second port of the optical circulator is connected to the sensing optical fiber to be tested;

[0019] The third port of the optical circulator is output to the detection processing component.

[0020] Optionally, the dual-pulse laser signal has the same repetition frequency, pulse width, time delay and phase difference between the two pulse signals as the radio frequency dual-pulse signal generated by the driving signal source for driving the acousto-optic modulator;

[0021] The pulse width value range is: 50ns≤W≤200ns;

[0022] The time delay between the two pulse signals is 50ns≤τ≤300ns;

[0023] The phase difference is satisfy:

[0024]

[0025] i is the number of times the double-pulse laser signal is generated, θ is a phase shift constant, and the value of θ is π / 2 or π / 3.

[0026] Optionally, the detection processing component includes:

[0027] A second erbium-doped fiber amplifier, an optical filter, a photodetector, a data acquisition card and a host computer connected in sequence;

[0028] The second erbium-doped fiber amplifier is connected to the third port of the optical circulator as the input end of the detection processing component;

[0029] The photoelectric detector receives the interference signal of the backscattered Rayleigh light signal, performs photoelectric conversion, and generates an electrical signal to output to the data acquisition card;

[0030] The data acquisition card samples the input electrical signal, outputs the sampled electrical signal data to the host computer for demodulation, and obtains the position information, vibration waveform and / or spectrum information of the vibration event.

[0031] Optionally, the host computer calculates and determines the differential phase result of the electrical signal data The differential phase result A two-dimensional data matrix The two-dimensional data matrix is ​​stored in the form of The standard deviation in the direction of variable i is compared with a preset threshold to obtain the position information, vibration waveform and / or spectrum information of the vibration event.

[0032] In a second aspect, the present application provides a demodulation method based on the Ф-OTDR system described in the first aspect, comprising:

[0033] The host computer combines the interference signals I of multiple groups of backscattered Rayleigh light signals with different phase differences through a pre-built calculation formula. i (t), and obtain the differential phase results of multiple interference signals

[0034] Combine variable i with the differential phase result Create a two-dimensional data matrix

[0035] For the two-dimensional data matrix The standard deviation D(t) is calculated, and the vibration event is determined based on the standard deviation D(t), and the position information, time domain waveform information and / or spectrum information of the vibration event are determined.

[0036] Optionally, the pre-built calculation formula is:

[0037]

[0038] The interference signal I of the backscattered Rayleigh light signal i(t) is:

[0039]

[0040] The host computer uses the i-1th and i+1th backward Rayleigh scattered light interference signals to calculate the i-th backward Rayleigh scattered interference signal I i The differential phase of (t)

[0041] ω is the center frequency of the narrow linewidth laser output, t is the time when the detection and processing component detects the interference signal, is the phase of the backward Rayleigh scattered light interference signal at time t, I' and I" are the DC and AC component amplitudes of the backward Rayleigh scattered light interference signal, respectively. is the phase difference between the two backward Rayleigh scattered light signals at time t.

[0042] Optionally, the vibration event is determined as follows:

[0043] Compare the standard deviation D(t) with a preset threshold value D0;

[0044] If at time t=t0, D(t0)>D0, it is determined that a vibration event exists;

[0045] The distance from the position where the vibration event is detected by the sensing optical fiber to the second port of the optical circulator is:

[0046] v·t0 / 2;

[0047] v is the speed of the probe light propagating in the optical fiber.

[0048] Optionally, the vibration waveform of the vibration event is

[0049] Performing a fast Fourier transform on the vibration waveform to determine frequency spectrum information of the vibration event.

[0050] (III) Beneficial effects

[0051] The Ф-OTDR system based on phase-shifted dual-pulse detection technology in one embodiment of the present application uses a dual-pulse laser signal with a certain time delay and phase difference as detection light, obtains the interference signal of the backward Rayleigh scattered light signal returned by the dual-pulse laser signal in the sensing optical fiber to be measured after Rayleigh scattering, and obtains the position information, time domain waveform information and spectrum information of the vibration event by demodulating the differential phase of the backward Rayleigh scattered light. The fixed phase shift between adjacent interference signals is used to directly calculate the optical phase information from the light intensity information collected by direct detection. Compared with the prior art, the complex optical path of heterodyne detection is not required, the laser source phase noise introduced by the reference light in heterodyne detection is avoided, the bandwidth requirements for the photoelectric detector and the data acquisition card are reduced, and the hardware cost is saved.

[0052] The Ф-OTDR system based on phase-shifted dual-pulse detection technology in one embodiment of the present application uses an acousto-optic modulator to simultaneously modulate the intensity and phase of the input laser. Compared with the prior art, it eliminates the need for an additional optical phase modulator, saves hardware costs and the complexity of hardware connections, simplifies the system optical path, and reduces the difficulty of structural connection. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 A structural diagram of a Ф-OTDR system based on a phase-shifted dual-pulse detection technology provided by an embodiment of the present invention;

[0054] Figure 2 A schematic diagram of the structure of a Ф-OTDR system based on a phase-shifted double-pulse detection technology provided by another embodiment of the present invention;

[0055] Figure 3 The present invention is a flowchart of a demodulation method of a Ф-OTDR system based on a phase-shifted double-pulse detection technology according to yet another embodiment of the present invention.

[0056] [Description of Reference Numerals]

[0057] 1: Narrow linewidth laser;

[0058] 2: Optical isolator;

[0059] 3: Acousto-optic modulator;

[0060] 4: driving signal source;

[0061] 5: The first erbium-doped fiber amplifier;

[0062] 6: Optical circulator;

[0063] 7: Sensing optical fiber to be tested;

[0064] 8: Second erbium-doped fiber amplifier;

[0065] 9: optical filter;

[0066] 10: Photodetector;

[0067] 11: Data acquisition card;

[0068] 12: Host computer. DETAILED DESCRIPTION

[0069] In order to better explain the present invention and facilitate understanding, the present invention is described in detail below in conjunction with the accompanying drawings through specific implementation methods. It is understood that the specific embodiments described below are only used to explain the relevant inventions, rather than to limit the invention. It should also be noted that the embodiments and features in the embodiments of this application can be combined with each other in the absence of conflict; for ease of description, only the parts related to the invention are shown in the accompanying drawings.

[0070] like Figure 1 As shown, Figure 1 This is a structural diagram of a Ф-OTDR system provided in one embodiment of the present invention. In this embodiment, the Ф-OTDR system includes: a front-end component, an optical circulator, a sensing optical fiber to be tested, and a detection processing component. The optical circulator includes a first port, a second port, and a third port.

[0071] The front-end component is input to the first port of the optical circulator to generate a double-pulse laser signal which is output to the sensing optical fiber to be tested through the optical circulator, wherein the double-pulse laser signal is two pulse laser signals with an initial phase difference which are generated after the acousto-optic modulator is driven by a driving signal source;

[0072] The sensing optical fiber to be tested is connected to the second port of the optical circulator, and the double-pulse laser signal is Rayleigh scattered by the sensing optical fiber to generate a backward Rayleigh scattered light signal, and the backward Rayleigh scattered light signal is output to the detection processing component through the optical circulator.

[0073] The third port of the optical circulator is output to the detection processing component, and the detection processing component is used to perform photoelectric conversion on the interference signal of the back Rayleigh scattered light signal output by the optical circulator and analyze the electrical signal to obtain the position information, time domain waveform information and / or spectrum information of the vibration event.

[0074] An Ф-OTDR system based on phase-shifted dual-pulse detection technology provided in an embodiment of the present application uses a dual-pulse laser signal with a certain time delay and phase difference as detection light, obtains an interference signal of a backward Rayleigh scattered light signal returned by Rayleigh scattering of the dual-pulse laser signal in a sensing optical fiber to be measured, and obtains position information, time domain waveform information and spectrum information of a vibration event by demodulating the differential phase of the backward Rayleigh scattered light. The fixed phase shift between adjacent interference signals is used to directly calculate the optical phase information from the light intensity information collected by direct detection. Compared with the prior art, the complex optical path of heterodyne detection is not required, the phase noise of the laser source introduced by the reference light in heterodyne detection is avoided, the bandwidth requirements for photoelectric detectors and data acquisition cards are reduced, and the hardware cost is saved.

[0075] Figure 2 A schematic diagram of the structure of a Ф-OTDR system provided for another embodiment of the present invention is shown in FIG2 , wherein the Ф-OTDR system structure mainly includes a narrow linewidth laser 1 having a linewidth less than 10 kHz, an optical isolator 2, an acousto-optic modulator 3, a driving signal source 4 and a first erbium-doped fiber amplifier 5, an optical circulator 6, a sensing fiber to be tested 7, a second erbium-doped fiber amplifier 8, an optical filter 9, a photodetector 10, a data acquisition card 11 and a host computer 12.

[0076] The narrow linewidth laser 1, the optical isolator 2, the acousto-optic modulator 3, the driving signal source 4 and the first erbium-doped fiber amplifier 5 are front-end components, and the first erbium-doped fiber amplifier is connected to the first port of the optical circulator as the output end of the front-end component; the second erbium-doped fiber amplifier 8, the optical filter 9, the photodetector 10, the data acquisition card 11 and the host computer 12 are detection and processing components, and the second erbium-doped fiber amplifier 8 is connected to the third port of the optical circulator 6 as the input end of the detection and processing component.

[0077] In this embodiment, the connection method of each device is as follows:

[0078] The laser output port of the narrow linewidth laser 1 is connected to the input port of the optical isolator 2, and the output port of the optical isolator 2 is connected to the optical input port of the acousto-optic modulator 3. The modulation signal input port of the acousto-optic modulator 3 is connected to the output port of the driving signal source 4 via a radio frequency line, and the optical output port of the acousto-optic modulator 3 is connected to the input port of the first erbium-doped fiber amplifier 5 via an optical fiber. The output port of the first erbium-doped fiber amplifier 5 is connected to the first port of the optical circulator 6. The second port of the optical circulator 6 is connected to the input port of the sensing optical fiber 7 to be measured, and the third port of the optical circulator 6 is connected to the input port of the second erbium-doped fiber amplifier 8. The output port of the second erbium-doped fiber amplifier 8 is connected to the input port of the optical filter 9, and the output port of the optical filter 9 is connected to the optical signal input port of the photodetector 10, and the electrical signal output port of the photodetector 10 is connected to the input port of the data acquisition card 11 via a radio frequency line. The output port of the data acquisition card 11 is connected to the host computer 12.

[0079] Specifically, in this embodiment, the narrow linewidth laser 1 outputs an incident laser, which is input into the acousto-optic modulator 3 through the optical isolator 2; the acousto-optic modulator 3 is driven by a driving signal source 4 to modulate the incident laser, and outputs a double-pulse laser signal which enters the sensing fiber 7 to be tested through the first Er-doped fiber amplifier 5 and the first port 601 and the second port 602 of the optical circulator 6; the double-pulse laser signal is Rayleigh scattered in the sensing fiber 7 to be tested, generating two backward Rayleigh scattered light signals which are transmitted through the optical circulator The second port 602 and the third port 603 of the device 6 enter the second Er-doped fiber amplifier 8, and the two backward Rayleigh scattered light signals interfere with each other during the transmission process, and the interference signal of the backward Rayleigh scattered light signal is directly detected by the photodetector 10 and photoelectrically converted to generate an electrical signal which is input to the data acquisition card 11; the data acquisition card 11 samples the input electrical signal, and inputs the sampled electrical signal data to the host computer 12 for demodulation, and the host computer 12 calculates and determines the differential phase result of the electrical signal data The differential phase result A two-dimensional data matrix The two-dimensional data matrix is ​​stored in the form of The standard deviation in the direction of variable i is compared with a preset threshold to obtain the position information, time domain waveform information and / or spectrum information of the vibration event.

[0080] The acousto-optic modulator 3 is driven by a driving signal source 4 to modulate the incident laser and output a double-pulse laser signal. The double-pulse laser signal has the same repetition frequency, pulse width, time delay and phase difference between the two pulse signals as the radio frequency double-pulse signal generated by the driving signal source for driving the acousto-optic modulator, specifically:

[0081] The driving signal source 4 generates a center frequency f, a repetition frequency R, a pulse width W, a time delay τ and a phase difference The RF double pulse signal is used to drive the acousto-optic modulator 3; the phase difference satisfy:

[0082]

[0083] Wherein, i is the number of times the RF double pulse signal is generated, θ is the phase shift constant, and the center frequency f is consistent with the center frequency of the acousto-optic modulator 3 .

[0084] After being driven by the driving signal source 4, the acousto-optic modulator 3 modulates the incident laser to generate a repetition frequency of R, a pulse width of W, a time delay of τ and a phase difference of In the present embodiment, the number of times the RF dual-pulse signal is generated is equal to the number of times the dual-pulse laser signal is generated, so i is also the number of times the dual-pulse laser signal is generated.

[0085] For example, when detecting a vibration event for the first time, the number of times the RF double pulse signal is generated is i=1, and the phase difference of the generated RF double pulse signal is θ, that is, the initial phase modulation of the first RF pulse signal is 0, and the initial phase modulation of the second RF pulse signal is θ. After the acousto-optic modulator 3 is driven by the RF double pulse signal, the initial phase of the first pulse laser signal of the generated double pulse laser signal is 0, and the initial phase of the second pulse laser signal is θ; when detecting a vibration event for the second time, the number of times the RF double pulse signal is generated is i=2, and the phase difference of the generated RF double pulse signal is 2θ, that is, the initial phase modulation of the first RF pulse signal is 0, and the initial phase modulation of the second RF pulse signal is 2θ. After the acousto-optic modulator 3 is driven by the RF double pulse signal, the initial phase of the first pulse laser signal of the generated double pulse laser signal is 0, and the initial phase of the second pulse laser signal is 2θ. By analogy, when i=3, 4, 5…, the phase differences are 3θ, 4θ, 5θ… respectively.

[0086] The size of i is determined by the actual detection process and is not limited here. The θ needs to satisfy cosθ not equal to 1, sinθ not equal to 0, and the preferred range is 0<θ<π. In practical applications, the θ is preferably π / 2 or π / 3; the pulse width W is preferably in the range of 50ns≤W≤200ns; the time delay between the two pulse signals is preferably in the range of 50ns≤τ≤300ns, and the preferred value is 100ns. The value of the repetition frequency is mutually constrained by the value of the sensing optical fiber to be measured. In practical applications, the repetition frequency is determined based on the specific required length of the optical fiber to be measured.

[0087] Figure 3 A schematic flow chart of a demodulation method of a Ф-OTDR system based on a phase-shifted dual-pulse detection technology provided by another embodiment of the present invention is as follows: Figure 3 As shown, the host computer, through a pre-built calculation formula, combines the interference signal I of multiple groups of backward Rayleigh scattered light signals with different phase differences i (t), and obtain the differential phase results of multiple interference signals Combine variable i with the differential phase result Create a two-dimensional data matrix For the two-dimensional data matrix The standard deviation D(t) is calculated, and the vibration event is determined based on the standard deviation D(t), and the position information, time domain waveform information and / or spectrum information of the vibration event are determined.

[0088] According to the number of times i of the RF double pulse signal, the phase difference of the double pulse laser signal input to the sensing optical fiber to be tested is determined to be Then the two paths of backward Rayleigh scattered light signals can be expressed as:

[0089]

[0090]

[0091] Based on the above expression, the interference signal I of the backscattered Rayleigh light signal is determined as i (t) is:

[0092]

[0093] ω is the center frequency of the narrow linewidth laser output, t is the moment when the detection and processing component detects the interference signal of the backscattered Rayleigh light signal, with the moment when the double-pulse laser signal is input into the sensing optical fiber as time 0; is the phase of the interference signal at time t, I' and I" are the DC and AC component amplitudes of the interference signal, respectively. It is the phase difference between the two backward Rayleigh scattered light signals at time t. The differential phase contains the position information, waveform and spectrum information of the vibration events along the sensing optical fiber.

[0094] The host computer uses the pre-built calculation formula to calculate the interference signal I of the i-1th and i+1th backscattered Rayleigh light. i-1 (t), I i+1 (t), calculate and determine the differential phase result of the i-th backward Rayleigh scattered light interference signal

[0095] The pre-built calculation formula is:

[0096]

[0097] Furthermore, based on the differential phase result Combine variable i to create a two-dimensional data matrix That is, the differential phase signal is obtained, and the standard deviation D(t) along the direction of the variable i is calculated; the standard deviation D(t) is compared with a preset threshold value D0, and if at a time t=t0, the standard deviation D(t) is greater than the threshold value D0, it is determined that a vibration event exists;

[0098] The distance from the position where the vibration event is detected by the sensing optical fiber to the second port of the optical circulator is:

[0099] v·t0 / 2;

[0100] v is the speed of the detection light propagating in the optical fiber;

[0101] The time domain waveform information of the vibration event is the same as the waveform change shape of the differential phase signal at this position, that is,

[0102] By performing a fast Fourier transform on the time domain waveform information, the frequency spectrum information of the vibration event can be determined.

[0103] In order to better demonstrate the Ф-OTDR system and demodulation method thereof provided by the present application, a more detailed explanation and description will be given below in conjunction with a specific embodiment.

[0104] This embodiment adopts Figure 2 The Ф-OTDR system structure shown in the figure includes the following components:

[0105] The narrow linewidth laser 1 generates a continuous incident laser with a central wavelength of 1550 nanometers, a linewidth of 5 kHz, and a power of 20 dBm, which enters the acousto-optic modulator 3 through the optical isolator 2.

[0106] The optical isolator 2 is used to protect the narrow line width laser 1 from being disturbed by the laser reflected back in the optical path.

[0107] Acousto-optic modulator 3, the central modulation frequency is 80 MHz.

[0108] The signal source 4 is driven to generate a radio frequency double pulse signal with a center frequency of 80 MHz, a repetition frequency of 16 kHz, a pulse width of 100 nanoseconds, a preferred time delay of 100 nanoseconds, and a phase difference of i·π / 2, which is input through the modulation input port of the acousto-optic modulator 3, and the acousto-optic modulator 3 is driven to modulate the intensity and phase of the incident laser to generate a double pulse laser signal with the same parameters as the radio frequency double pulse signal, such as a center frequency of 80 MHz, a repetition frequency of 16 kHz, a pulse width of 100 nanoseconds, a time delay of 100 nanoseconds between two pulse signals, and a phase difference of i·π / 2, which enters the input port of the first erbium-doped fiber amplifier 5 through the output port of the acousto-optic modulator 3. i is the number of transmissions of the radio frequency double pulse signal.

[0109] The double-pulse laser signal is amplified by the first erbium-doped fiber amplifier 5, input through the first port 601 of the fiber circulator 6, output through the second port 602, and injected into the sensing fiber 7 to be tested as detection light.

[0110] During the forward transmission of the detection light in the sensing optical fiber 7 to be tested, the two pulsed light signals are Rayleigh scattered respectively, each generating a backward Rayleigh scattered light signal, and the two backward Rayleigh scattered light signals are input from the second port 602 of the optical fiber circulator 6 and output from the third port 603 to the detection processing component.

[0111] The detection and processing component receives two backward Rayleigh scattered light signals and the interference signal generated by their interference, which is amplified by the second erbium-doped fiber amplifier 8. The optical filter 9 filters out the amplifier spontaneous radiation noise introduced by the first erbium-doped fiber amplifier 5 and the second erbium-doped fiber amplifier 8.

[0112] The photoelectric detector 10 directly detects the interference signal and performs photoelectric conversion to convert it into an analog electrical signal, which is then collected by the data acquisition card 11 as a digital electrical signal and input into the host computer 12 for demodulation processing.

[0113] In this embodiment, the total length of the sensing optical fiber 7 to be tested is 5.1 kilometers, the bandwidth of the photodetector 10 is 10 MHz, the bandwidth of the data acquisition card 11 is 200 MHz, the sampling rate is 10 MHz, and the host computer 12 is a computer.

[0114] In this embodiment, a vibration event is simulated on a cylindrical piezoelectric ceramic with a 1-meter-long optical fiber wrapped around the surface of the sensing optical fiber 7 to be tested, which is 5 kilometers away from the second port 602 of the optical circulator. The piezoelectric ceramic is driven by a sinusoidal electrical signal with a frequency of 10 Hz and a peak-to-peak voltage of 5 volts to simulate the sensing optical fiber 7 at this location being affected by the vibration event. In this embodiment, the moment when the double-pulse laser signal is input into the sensing optical fiber is taken as time 0, and the moment when the detection and processing component detects the interference signal of the backward Rayleigh scattered light signal generated at a certain point in the sensing optical fiber is taken as time t, and the time t used is regarded as the sum of the time when the double-pulse laser signal reaches the point and the time when the corresponding backward Rayleigh scattered light signal is transmitted in the sensing optical fiber.

[0115] In this embodiment, a total of 16000 groups of double-pulse laser signals are emitted and input into the sensing optical fiber 7 to be tested, i.e., i=[1,16000], and the phase shift constant θ is selected as π / 2. Then, the demodulation formula of the differential phase information of the interference signal of the backward Rayleigh scattered light generated by the i-th group of emitted double-pulse laser signals is:

[0116]

[0117] After the computer demodulates the differential phase information of the interference signal of the backscattered Rayleigh light generated by each set of double-pulse laser signals, it is converted into a two-dimensional data matrix. And store, calculate the standard deviation D(t) of each column data of the two-dimensional data matrix along the direction of variable i, and compare it with the threshold value D0=100. There is a time period from t=t0 to t=t1, that is, in the time period t0-t1, D(t)>100, and it is judged that there is a vibration event at the position of the sensing optical fiber vt0 / 2 to vt1 / 2, and locate that there is a vibration event at the sensing optical fiber 7 4995 meters to 5005 meters away from the second port of the optical circulator. The corresponding differential phase information at this place is affected by the vibration event and changes in a sinusoidal function. The peak-to-peak value of the phase change is 15 radians. Through fast Fourier transform, the frequency of the sine function is obtained to be 10 Hz, that is, the spectrum information of the vibration event.

[0118] In this embodiment, based on the actual needs of this embodiment, the threshold D0 is set to 100, and a total of 16,000 groups of dual-pulse laser signal emissions are set. In other embodiments, the threshold D0 and the number of dual-pulse laser signal emission groups are set according to the actual needs of other embodiments and are not limited here.

[0119] The Ф-OTDR system based on phase-shifted dual-pulse detection technology provided in the embodiment of the present application has the advantages of long monitoring distance, high positioning accuracy, and immunity to electromagnetic interference, and can perform real-time dynamic detection over a large range and with high density.

[0120] The Ф-OTDR system based on phase-shifted dual-pulse detection technology in one embodiment of the present application uses an acousto-optic modulator to simultaneously modulate the intensity and phase of the input laser. Compared with the prior art, it eliminates the need for an additional optical phase modulator, saves hardware costs and the complexity of hardware connections, simplifies the system optical path, and reduces the difficulty of structural connection.

[0121] The Ф-OTDR system based on phase-shifted dual-pulse detection technology provided in the embodiment of the present application can realize real-time distributed measurement of vibration events along the sensing optical fiber, and accurately restore the vibration waveform and spectrum information. Without adding additional modulation devices, the phase-shifted dual-pulse detection technology is adopted, and the generated backward Rayleigh scattered light interference signal is also subjected to phase shift modulation. By using the fixed phase shift between adjacent interference signals and a simple direct detection method, the differential phase information of the backward Rayleigh scattered light can be demodulated, and the vibration waveform and spectrum information can be further obtained, without the need to introduce local reference light to assist phase demodulation. Compared with the heterodyne detection method that requires local reference light, it significantly suppresses the influence of laser source phase noise and improves the phase information demodulation signal-to-noise ratio of the system; the heterodyne detection beat frequency signal with a center frequency of tens to hundreds of megahertz is converted into a zero-frequency backward Rayleigh scattered light interference signal, which significantly reduces the system's requirements for the frequency response bandwidth of the photodetector, the maximum sampling rate and bandwidth of the data acquisition card, and saves hardware costs.

[0122] In the description of the present invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0123] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0124] In the description of this specification, the description of the terms "one embodiment", "some embodiments", "embodiment", "example", "specific example" or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, unless they are contradictory.

[0125] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may alter, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A Ф-OTDR system based on phase-shifted dual-pulse detection technology, characterized in that: include: A front-end component, wherein the front-end component generates a double-pulse laser signal which is output to the sensing optical fiber to be tested through an optical circulator, wherein the double-pulse laser signal is two pulse laser signals with an initial phase difference which are generated after the acousto-optic modulator is driven by a driving signal source; The double-pulse laser signal generates a backscattered Rayleigh light signal through Rayleigh scattering of the sensing optical fiber to be measured, and the backscattered Rayleigh light signal is output to the detection processing component through the optical circulator; The detection and processing component is used to perform photoelectric conversion on the interference signal of the backward Rayleigh scattered light signal output by the optical circulator and analyze the electrical signal to obtain the position information, vibration waveform and / or spectrum information of the vibration event; The front-end component includes a narrow linewidth laser, an optical isolator, an acousto-optic modulator, a driving signal source and a first Er-doped fiber amplifier; The narrow linewidth laser, the optical isolator, the acousto-optic modulator and the first Er-doped fiber amplifier are connected in sequence, and the driving signal source is connected to the acousto-optic modulator via a radio frequency line; The narrow linewidth laser has a linewidth of less than 10 kHz; the narrow linewidth laser outputs incident laser light, which is output to the acousto-optic modulator through the optical isolator; The acousto-optic modulator is driven by a driving signal source to modulate the incident laser and output it to the first Er-doped fiber amplifier; The first Er-doped fiber amplifier, as an output end of the front-end component, is connected to the first port of the optical circulator; The second port of the optical circulator is connected to the sensing optical fiber to be tested; The third port of the optical circulator is output to the detection processing component; The detection processing component includes: A second erbium-doped fiber amplifier, an optical filter, a photodetector, a data acquisition card and a host computer connected in sequence; The second erbium-doped fiber amplifier is connected to the third port of the optical circulator as the input end of the detection processing component; The photoelectric detector receives the interference signal of the backscattered Rayleigh light signal, performs photoelectric conversion, and generates an electrical signal to output to the data acquisition card; The data acquisition card samples the input electrical signal, outputs the sampled electrical signal data to the host computer for demodulation, and obtains the position information, vibration waveform and / or spectrum information of the vibration event.

2. The Φ-OTDR system according to claim 1, characterized in that: The double-pulse laser signal has the same repetition frequency, pulse width, time delay and phase difference between the two pulse signals as the radio frequency double-pulse signal generated by the driving signal source for driving the acousto-optic modulator; The pulse width value range is: 50ns≤W≤200ns; The time delay between the two pulse signals is 50ns≤τ≤300ns; The phase difference is satisfy: i is the number of times the double-pulse laser signal is generated, θ is a phase shift constant, and the value of θ is π / 2 or π / 3.

3. The Φ-OTDR system according to claim 1, characterized in that: The host computer calculates and determines the differential phase result of the electrical signal data The differential phase result A two-dimensional data matrix The two-dimensional data matrix is ​​stored in the form of The standard deviation in the direction of variable i is compared with a preset threshold to obtain the position information, vibration waveform and / or spectrum information of the vibration event.

4. A demodulation method of the Ф-OTDR system according to any one of claims 1 to 3, characterized in that: include: The host computer combines the interference signals I of multiple groups of backscattered Rayleigh light signals with different phase differences through a pre-built calculation formula. i (t), and obtain the differential phase results of multiple interference signals Combine variable i with the differential phase result Create a two-dimensional data matrix For the two-dimensional data matrix The standard deviation D(t) is calculated, and the vibration event is determined based on the standard deviation D(t), and the position information, time domain waveform information and / or spectrum information of the vibration event are determined.

5. The demodulation method according to claim 4, characterized in that: The pre-built calculation formula is: The interference signal I of the backscattered Rayleigh light signal i (t) is: The host computer uses the i-1th and i+1th backward Rayleigh scattered light interference signals to calculate the i-th backward Rayleigh scattered interference signal I i The differential phase of (t) ω is the center frequency of the narrow linewidth laser output, t is the time when the detection and processing component detects the interference signal, is the phase of the backward Rayleigh scattered light interference signal at time t, I' and I" are the DC and AC component amplitudes of the backward Rayleigh scattered light interference signal, respectively. is the phase difference between the two backward Rayleigh scattered light signals at time t.

6. The demodulation method according to claim 4, characterized in that: Determine the vibration event, specifically: Compare the standard deviation D(t) with a preset threshold value D0; If at time t=t0, D(t0)>D0, it is determined that a vibration event exists; The distance from the position where the vibration event is detected by the sensing optical fiber to the second port of the optical circulator is: v·t0 / 2; v is the speed of the probe light in the optical fiber.

7. The demodulation method according to claim 6, characterized in that: The vibration waveform of the vibration event is: Performing a fast Fourier transform on the vibration waveform to determine frequency spectrum information of the vibration event.

Images