Phase demodulation method and system

Abstract

The invention relates to the technical field of optical fiber sensing, in particular to a phase demodulation method and system. The phase demodulation method comprises the following steps: acquiring a discrete signal corresponding to a beat frequency signal; determining a first target signal corresponding to the discrete signal based on an intermediate frequency filtering technology; based on the first target signal and the orthogonal frequency shift signal, a second target signal and a third target signal are determined, and the second target signal and the third target signal are obtained after the first target signal and the orthogonal frequency shift signal are subjected to frequency mixing and low-pass filtering; converting the amplitudes of the second target signal and the third target signal into 1; constructing a first function of the second target signal about time and position and a second function of the third target signal about time and position based on the second target signal and the third target signal; and demodulating the differential phase of the phase-sensitive optical time domain reflection fiber sensing system based on the first function and the second function. Demodulation efficiency is improved, noise and errors are reduced, and system complexity and cost are reduced.

Description

Technical Field

[0001] The present application relates to the field of optical fiber sensing technology, and in particular to a phase demodulation method and system. Background Art

[0002] The phase-sensitive optical time-domain reflectometry fiber optic sensing system is capable of distributed multi-point real-time monitoring of tiny signals over long distances. It has the characteristics of high sensitivity, large spatial dynamic range and high positioning accuracy. Therefore, it has broad market application prospects in large-scale facility health monitoring, industrial production process detection, perimeter security, pipeline protection, marine engineering, etc.

[0003] At present, the phase demodulation of phase-sensitive optical time-domain reflectometry fiber optic sensor systems is mainly based on hardware phase demodulation schemes including phase generation carrier, phase demodulation based on 3×3 coupler structure, I / Q demodulation, etc. The phase generation carrier method requires the introduction of phase modulators and demodulators, which increases the complexity and cost of the system, and relies on accurate phase modulation. Any modulation error will affect the final demodulation accuracy. The phase demodulation method based on 3×3 couplers is limited by the manufacturing error of the coupler, which may lead to inaccurate phase demodulation. Due to the use of multiple couplers and other optical components, the system loss is large, and the light intensity signal may be weakened, affecting the demodulation effect. I / Q demodulation requires complex circuit design, including orthogonal modulators, balanced detectors, and high-performance signal processing circuits, which increases the complexity and cost of the system. It usually requires real-time processing of a large amount of data, and has high requirements on the system's computing power and processing speed. It is very sensitive to the phase noise in the system, especially the light source phase noise and phase jitter in the system, which will cause demodulation errors.

[0004] Therefore, the existing demodulation methods have problems such as high cost, complex system, high noise, and large amount of calculation, which seriously affect the practical application and industrialization of the sensing system. Summary of the invention

[0005] The phase demodulation method and system provided in the embodiment of the present application demodulate and recover the phase information of the acoustic signal and the vibration signal in the phase-sensitive optical time-domain reflectometry fiber optic sensing system, realize the phase digital demodulation analysis of the system, improve the demodulation efficiency, reduce the noise and error introduced by the demodulation technology method, simplify the system and reduce the system cost.

[0006] In some embodiments, a phase demodulation method is provided, and the phase demodulation method is applied to a phase demodulation system, and the phase demodulation system includes a phase-sensitive optical time-domain reflection fiber sensing system, and the phase demodulation method includes: obtaining a discrete signal corresponding to a beat frequency signal; the beat frequency signal is generated by interference between a signal of backward Rayleigh scattered light generated by an optical pulse and a signal of local oscillator light in the phase-sensitive optical time-domain reflection fiber sensing system; based on an intermediate frequency filtering technique, determining a first target signal corresponding to the discrete signal; based on the first target signal and the orthogonal frequency shift signal, determining a second target signal and a third target signal, the second target signal and the third target signal being obtained by mixing and low-pass filtering the first target signal and the orthogonal frequency shift signal; converting the amplitudes of the second target signal and the third target signal to 1; based on the second target signal and the third target signal, constructing a first function of the second target signal with respect to time and position, and a second function of the third target signal with respect to time and position; and based on the first function and the second function, demodulating the differential phase of the phase-sensitive optical time-domain reflection fiber sensing system.

[0007] The phase demodulation method provided in the embodiment of the present application can be used to demodulate and recover the phase information of the acoustic signal and the vibration signal in the phase-sensitive optical time-domain reflection fiber sensing system. It avoids the huge computational pressure caused by the phase unwrapping and detrending algorithms in the inverse tangent demodulation algorithm, improves the algorithm calculation efficiency, reduces the calculation time, and reduces the system demodulation cost. It can realize the phase digital demodulation analysis of the system, improve the demodulation efficiency, eliminate the noise and errors introduced by the demodulation technology method, simplify the system and reduce the system cost.

[0008] Optionally, based on the intermediate frequency filtering technology, determining the first target signal corresponding to the discrete signal includes: filtering out the discrete signal except for the central angular frequency The other harmonic components of the first target signal are obtained.

[0009] Optionally, based on the first target signal and the orthogonal frequency shift signal, determining the second target signal and the third target signal includes: determining the first mixing signal and the second mixing signal corresponding to the first target signal and the orthogonal frequency shift signal; based on the low-pass filtering technology, filtering out the high-frequency items in the first mixing signal and the second mixing signal to determine the second target signal and the third target signal.

[0010] Optionally, the first target signal is ; Orthogonal frequency shift signals include and ; Determine the first mixing signal and the second mixing signal corresponding to the first target signal and the orthogonal frequency shift signal, including: based on the following first formula, calculate the first mixing signal ; Based on the following second formula, the second mixing signal is calculated ; The first formula is: ; The second formula is: ; Wherein, is the angular frequency difference between the local oscillator light and the backward Rayleigh scattered light, T is the pulse interval, N is the order of the optical pulse, and t is the preset time.

[0011] Optionally, the high-frequency term is a correlation term with an angular frequency coefficient of 2Ω.

[0012] Optionally, the phase demodulation system further includes a photoelectric balanced detector that converts the amplitudes of the second target signal and the third target signal to 1, including: the second target signal is ; the third target signal is ; wherein, R is the responsivity of the photoelectric balanced detector, is the amplitude of the local oscillator light, is the amplitude of the backward Rayleigh scattered light at time; Calculate the normalization parameter according to the following third formula ; The third formula is: ; Convert the amplitude of the second target signal to 1 according to the following fourth formula; The fourth formula is: ; Convert the amplitude of the third target signal to 1 according to the following fifth formula; The fifth formula is: .

[0013] Optionally, construct a first function of the second target signal with respect to time NT and position z, and a second function of the third target signal with respect to time NT and position z, including: according to the relationship between time t and detection position z, z = ct / 2n f , represent the second target signal as the first function , represent the third target signal as the second function ; wherein, 0 ≤ t < T, T is the pulse interval, c is the speed of light, and n f is the refractive index of light with a frequency of f in the sensing optical fiber; ; .

[0014] Optionally, the phase-sensitive optical time domain reflectometry fiber optic sensing system includes a plurality of measurement positions, and based on the first function and the second function, the differential phase of the phase-sensitive optical time domain reflectometry fiber optic sensing system is demodulated, including: taking each measurement position as a target position in turn; determining the target position The corresponding first objective function And the second objective function ; Calculate the first differential of the first objective function and the second differential of the second objective function ; Based on the first objective function, the first differential, the second objective function and the second differential, calculate the third differential of the phase ; Determine the phase corresponding to the target position by integrating and restoring the third differential ; According to the sixth formula below, the differential phase corresponding to the target position is calculated ; The sixth formula is: ; Among them, s is the preset spatial difference step size.

[0015] Optionally, based on the first objective function , first differential , the second objective function and the second differential , calculate the third differential of the phase ,include: The third differential of the phase is calculated according to the seventh formula as follows; The seventh formula is: .

[0016] In some embodiments, a phase demodulation system is provided, comprising: a phase-sensitive optical time-domain reflection fiber sensing system, a photoelectric balanced detector, and a data acquisition and processing module; in the phase-sensitive optical time-domain reflection fiber sensing system, a beat frequency signal is generated by interference between a backward Rayleigh scattered light signal generated by an optical pulse and a signal of a local oscillator light; the photoelectric balanced detector is configured to receive the beat frequency signal generated by the phase-sensitive optical time-domain reflection fiber sensing system; the data acquisition and processing module is communicatively connected to the photoelectric balanced detector; the data acquisition and processing module is configured to obtain a discrete signal corresponding to the beat frequency signal; the beat frequency signal is generated by the backward Rayleigh scattered light generated by the optical pulse in the phase-sensitive optical time-domain reflection fiber sensing system. interference between the signal of the local oscillator light and the signal of the local oscillator light; based on the intermediate frequency filtering technology, the first target signal corresponding to the discrete signal is determined; based on the first target signal and the orthogonal frequency shift signal, the second target signal and the third target signal are determined, and the second target signal and the third target signal are obtained by mixing the first target signal and the orthogonal frequency shift signal and low-pass filtering; the amplitudes of the second target signal and the third target signal are converted to 1; based on the second target signal and the third target signal, a first function of the second target signal with respect to time and position, and a second function of the third target signal with respect to time and position are constructed; based on the first function and the second function, the differential phase of the phase-sensitive optical time domain reflection fiber optic sensing system is demodulated.

[0017] It can be understood that the beneficial effects that can be achieved by the technical solution provided by the above-mentioned phase demodulation system can be referred to the beneficial effects in the phase demodulation method and any of its optional implementations, and will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solution of the present application, the drawings required for use in the embodiments are briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0019] Figure 1 A structural block diagram of a phase demodulation system provided in an embodiment of the present application; Figure 2 A structural diagram of a phase demodulation system provided in an embodiment of the present application; Figure 3 A flow chart of a phase demodulation method provided in an embodiment of the present application; Figure 4 A comparison chart of demodulation results provided in an embodiment of the present application.

[0020] Reference numerals: 10. Phase-sensitive optical time-domain reflectometry fiber-optic sensing system; 101. Narrow linewidth laser; 102. First fiber coupler; 103. Acousto-optic modulator; 104. First erbium-doped fiber amplifier; 105. Fiber circulator; 1051. First port; 1052. Second port; 1053. Third port; 106. Sensing fiber; 107. Second erbium-doped fiber amplifier; 108. Second fiber coupler; 20. Photoelectric balanced detector; 30. Data acquisition and processing module; 301. Computer; 302. Mobile phone. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present application will be described clearly below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments of the present application, other embodiments obtained by ordinary technicians in this field without making creative work all belong to the protection scope of the present application.

[0022] In the following, the terms "first", "second", etc. are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0023] In addition, in the present application, directional terms such as "upper", "lower", "inner" and "outer" are defined relative to the orientation of the components schematically placed in the drawings. It should be understood that these directional terms are relative concepts. They are used for relative description and clarification, and they can change accordingly according to the changes in the orientation of the components placed in the drawings.

[0024] In order to facilitate the understanding of the technical solution of the application, the relevant technologies involved in this application are first explained below.

[0025] The phase-sensitive optical time-domain reflectometry fiber optic sensing system is capable of distributed multi-point real-time monitoring of tiny signals over long distances. It has the characteristics of high sensitivity, large spatial dynamic range and high positioning accuracy. Therefore, it has broad market application prospects in health monitoring of large facilities, industrial production process detection, perimeter security, pipeline protection, marine engineering, etc.

[0026] At present, phase demodulation of interferometric fiber optic sensing systems (including phase-sensitive optical time-domain reflectometry fiber optic sensing systems) is mainly based on hardware phase demodulation solutions including phase-generated carrier, phase demodulation based on 3×3 coupler structure, I / Q demodulation, etc.

[0027] Phase-Generated Carrier (PGC) is a common phase demodulation method used in interferometric fiber optic sensing systems. The phase-generated carrier method modulates the phase of light by applying a high-frequency carrier to the interference signal, so that the phase information that is originally difficult to demodulate directly can be extracted through spectrum transformation. Since the phase-generated carrier method requires the introduction of a phase modulator and demodulator, it increases the complexity and cost of the system, and relies on precise phase modulation, any modulation error will affect the final demodulation accuracy.

[0028] The phase demodulation method based on the 3×3 coupler structure is a phase demodulation technology widely used in fiber interferometers. It can divide the fiber interference signal into three paths by utilizing the special structure of the 3×3 fiber coupler, and realize phase demodulation through mathematical calculation. This method is usually used in systems such as fiber optic gyroscopes (FOGs) and has high demodulation accuracy and stability. However, the demodulation accuracy of the phase demodulation method based on the 3×3 coupler structure is limited by the manufacturing error of the coupler, which may lead to inaccurate phase demodulation. Due to the use of multiple couplers and other optical components, the system loss is large, and the light intensity signal may be weakened, affecting the demodulation effect.

[0029] I / Q demodulation (In-phase and Quadrature Demodulation) is a common method for extracting useful information from modulated signals. It is widely used in communication systems, radar, software-defined radio, fiber optic sensors and other fields. However, I / Q demodulation requires complex circuit design, including orthogonal modulators, balanced detectors and high-performance signal processing circuits, which increases the complexity and cost of the system. It usually requires real-time processing of large amounts of data, and has high requirements for the system's computing power and processing speed. It is very sensitive to phase noise in the system, especially light source phase noise and phase jitter within the system, which can lead to demodulation errors.

[0030] Therefore, the existing phase demodulation methods have problems such as high cost, complex system, high noise, and large amount of calculation, which seriously affect the practical application and industrialization of phase-sensitive optical time-domain reflectometry fiber optic sensing systems.

[0031] Figure 1 This is a structural block diagram of the phase demodulation system provided in an embodiment of the present application.

[0032] In order to solve the problems existing in the above-mentioned related technologies, Figure 1As shown, the present application provides a phase demodulation system including: a phase-sensitive optical time-domain reflection fiber sensing system 10, a photoelectric balance detector 20 and a data acquisition processing module 30. In the phase-sensitive optical time-domain reflection fiber sensing system 10, the signal of the backward Rayleigh scattered light generated by the optical pulse and the signal of the local oscillator light interfere to generate a beat frequency signal. The photoelectric balance detector 20 is configured to receive the beat frequency signal generated by the phase-sensitive optical time-domain reflection fiber sensing system 10. The data acquisition processing module 30 is connected to the photoelectric balance detector 20 for communication ( Figure 1 The dashed arrow in the middle is schematically connected, and the arrow indicates the direction of data transmission). The data acquisition and processing module 30 is configured to acquire a discrete signal corresponding to the beat frequency signal, and demodulate the discrete signal to obtain a differential phase.

[0033] The phase demodulation system provided in the embodiment of the present application can demodulate and recover the phase information of the acoustic signal and the vibration signal based on the backward Rayleigh scattered light signal and the local oscillator light signal in the phase-sensitive optical time-domain reflectometry fiber sensing system. This can realize the phase digital demodulation analysis of the system, reduce the amount of calculation, improve the demodulation efficiency, reduce the noise and error introduced by the demodulation technology, simplify the system and reduce the system cost.

[0034] Specifically, after the photoelectric balance detector 20 acquires the beat frequency signal, it also converts the beat frequency signal into an electrical signal. The data acquisition and processing module 30 acquires the electrical signal to obtain a discrete signal.

[0035] Alternatively, if Figure 1 As shown, the data acquisition and processing module 30 includes but is not limited to one or more of a computer 301, a mobile phone 302, a tablet computer (not shown in the figure) and a mobile wearable device (not shown in the figure). It is sufficient that the data acquisition and processing module 30 can realize the above functions.

[0036] Figure 2 A structural diagram of a phase demodulation system provided in an embodiment of the present application.

[0037] Combination Figure 2 As shown, the phase-sensitive optical time-domain reflectometry fiber optic sensing system includes: a narrow linewidth laser 101, a first fiber coupler 102, an acousto-optic modulator 103, a first erbium-doped fiber amplifier 104, a fiber circulator 105, a sensing fiber 106, a second erbium-doped fiber amplifier 107, and a second fiber coupler 108.

[0038] Specifically, the laser output by the narrow linewidth laser 101 is divided into two paths by the first fiber coupler 102 according to the first coupling ratio, and the optical signal in the optical fiber is output as a first optical signal and a second optical signal. The second optical signal is the local oscillator optical signal. The first optical signal enters the acousto-optic modulator 103 through the optical fiber, and after being modulated by the acousto-optic modulator 103, it passes through the first erbium-doped fiber amplifier 104 to amplify the first optical signal. The first optical signal enters the optical fiber circulator 105 through the first port 1051 of the optical fiber circulator 105, and then outputs through the second port 1052 of the optical fiber circulator 105 and is transmitted to the sensing optical fiber 106. After generating a backward Rayleigh scattered optical signal, it enters the optical fiber circulator 105 through the second port 1052 of the optical fiber circulator 105, and then is transmitted to the second erbium-doped fiber amplifier 107 through the third port 1053 of the optical fiber circulator 105 for further amplification to form a third optical signal. The third optical signal is coupled with the second optical signal through the second fiber coupler 108 to form a fourth optical signal. The third optical signal is the signal of the backscattered light generated by the optical pulse. The fourth optical signal is the beat frequency signal generated by the interference between the backscattered light signal generated by the optical pulse and the signal of the local oscillator light. The fourth optical signal is received by the photoelectric balance detector 20 and converted into an electrical signal, which is then collected by the data acquisition processing module 30 to obtain a discrete signal.

[0039] It should be understood that Figure 2 The arrow in the figure indicates the transmission direction of the optical signal.

[0040] Specifically, the splitting ratio of the first optical fiber coupler 102 is 90:10, and the coupling ratio of the second optical fiber coupler 108 is 50:50.

[0041] For example, a single light pulse generates a Rayleigh scattered light signal through the sensing fiber. Formula (1): Formula (1); Among them, ω=ω0+Ω, ω0 represents the central angular frequency of the laser light source, and Ω is the frequency shift center of the AOM. To detect the light amplitude, is the time after the detection pulse is emitted, is the time delay corresponding to the i-th Rayleigh scattering center, To detect the optical pulse width, is the backscattering coefficient of the ith Rayleigh scattering center, τ i =2z i n f / c,z i is the distance from the i-th Rayleigh scattering center to the input end of the sensing optical fiber, is the optical fiber attenuation coefficient. Where c is the speed of light, n fis the refractive index of light with frequency f.

[0042] Specifically, the time corresponding to t=0 is the time corresponding to when the light pulse enters the sensing optical fiber.

[0043] .

[0044] From the above formula (1), it can be seen that the Rayleigh scattered light obtained at a certain time t is the result of the interference of the scattered light of all Rayleigh scattering centers in the pulse, and the signal at time t corresponds to the detection fiber position information.

[0045] Assuming the pulse interval is T, the Nth light pulse generates a Rayleigh scattered light signal through the sensing fiber, which is expressed as (2): Formula (2); The Rayleigh scattering signal is expressed as formula (3) using complex amplitude: Formula (3); in, is the additional phase in the process of generating the backward Rayleigh scattered light signal, j represents the imaginary part, is the amplitude of the Rayleigh scattered light at time NT+t.

[0046] For example, the local oscillator optical signal Formula (4): Formula (4); Among them, E0 is the amplitude of the local oscillation light, is the initial phase of the local oscillator light.

[0047] Specifically, the backward Rayleigh scattered light signal and the local oscillator light signal undergo beat frequency interference through the 3dB fiber coupler, and the output light intensities of the two ports of the 3dB fiber coupler are the first light intensity I1 and the second light intensity I2, respectively. The 3dB fiber coupler is also called a 50:50 fiber coupler, that is, the second fiber coupler 108 given in the above embodiment.

[0048] Exemplarily, the first light intensity I1 and the second light intensity I2 are as follows: Formula (5); Formula (6); in, for The complex conjugate of for The complex conjugate of for The complex conjugate of .

[0049] From equations (5) and (6), we can see that both I1 and I2 have three terms, of which the first two terms represent the DC part of the light intensity, and the third term represents the AC part of the light intensity, i.e., the beat frequency signal.

[0050] Specifically, the beat frequency signals in the first light intensity I1 and the second light intensity I2 are extracted.

[0051] In this step, the third term in the above equations (5) and (6) is extracted as the beat signal. For example, the first beat signal corresponding to the first light intensity is , the second beat frequency signal corresponding to the second light intensity is .

[0052] Specifically, the photoelectric balance detector 20 collects a discrete signal corresponding to the beat frequency signal, including the following steps S01 and S02, which are specifically as follows: Step S01 : using a balanced photodetector (BPD) to convert a beat frequency signal into an electrical signal.

[0053] Specifically, due to the suppression of common-mode signals by BPD, what BPD detects is the AC part, and the output voltage u BPD It can be expressed as formula (7): Formula (7).

[0054] Step S02: The electrical signal output by the photoelectric balance detector is collected by a data acquisition card (DAQ) and converted into a discrete signal.

[0055] Figure 3 A flow chart of a phase demodulation method provided in an embodiment of the present application.

[0056] Combination Figure 1 and Figure 2 The phase demodulation system shown in the embodiment of the present application also provides a phase demodulation method, the execution body of the method can be a data acquisition processing module, and the phase demodulation method is applied to a phase-sensitive optical time-domain reflection fiber sensing system, such as Figure 3 As shown, the phase demodulation method includes steps S100 to S600, which are specifically as follows: Step S100, obtaining a discrete signal corresponding to a beat signal; the beat signal is generated by interference between a backward Rayleigh scattered light signal generated by an optical pulse and a local oscillator light signal in a phase-sensitive optical time-domain reflectometry optical fiber sensing system.

[0057] Specifically, the interference between the backward Rayleigh scattered light signal generated by the optical pulse and the local oscillator light signal refers to the fact that a single optical pulse generates a backward Rayleigh scattered light signal through the sensing optical fiber, which merges with the local oscillator light signal and causes coherent interference.

[0058] Step S200: determining a first target signal corresponding to the discrete signal based on intermediate frequency filtering technology.

[0059] Optionally, step S200 includes step S201: Step S201, filtering out harmonic components other than the one with a central angular frequency of Ω in the discrete signal to obtain a first target signal.

[0060] In this step, theoretically, the central angular frequency of the voltage signal generated by the BPD is Ω, but there are also DC and high-order harmonic components with higher angular frequencies, such as harmonic components with angular frequencies of 2Ω, 3Ω, 4Ω, etc. In order to reduce system noise and reduce the impact of DC and high-frequency signals on the demodulation results, the voltage signal obtained by the BPD is band-pass filtered with a central angular frequency of Ω, that is, other harmonic components in the discrete signal except the central angular frequency of Ω are filtered out.

[0061] Step S300: determining a second target signal and a third target signal based on the first target signal and the orthogonal frequency shift signal, wherein the second target signal and the third target signal are obtained by mixing and low-pass filtering the first target signal and the orthogonal frequency shift signal.

[0062] Optionally, step S300 includes step S301 and step S302: Step S301: determine a first mixing signal and a second mixing signal corresponding to a first target signal and an orthogonal frequency shift signal.

[0063] Optionally, the first target signal is ; Orthogonal frequency shift signals include and ; Determine the first mixing signal and the second mixing signal corresponding to the first target signal and the orthogonal frequency shift signal (step S401), comprising: Based on the following first formula, the first mixing signal is calculated: ; Based on the following second formula, the second mixing signal is calculated: ; The first formula is: ; The second formula is: ; in, is the angular frequency difference between the local oscillator light and the backscattered Rayleigh light, T is the pulse interval, N is the order of the light pulses, and t is the preset time.

[0064] Specifically, For the time discrete signal.

[0065] Specifically, the filtered signal and and Digital mixing is performed respectively to obtain a first mixing signal and a second mixing signal. The first mixing signal is shown in formula (8), and the second mixing signal is shown in formula (9).

[0066] Formula (8).

[0067] Formula (9).

[0068] Step S302: Based on a low-pass filtering technique, high-frequency terms in the first mixed signal and the second mixed signal are filtered out to determine a second target signal and a third target signal.

[0069] In this embodiment, the influence of the high frequency term in subsequent demodulation is reduced by filtering out the high frequency term.

[0070] Optionally, the high frequency term is a related term having an angular frequency coefficient of 2Ω.

[0071] Specifically, the second target signal is (as shown in the following formula (10)), the third target signal is (As shown in the following formula (11)), specifically: Formula (10).

[0072] Formula (11).

[0073] Combining the above equations (8) and (10), it can be seen that by filtering out the high-frequency terms in equation (8), the second target signal shown in equation (10) is obtained, where the transformation rule based on the trigonometric formula is simplified to obtain .

[0074] Combining the above equations (9) and (11), it can be seen that by filtering out the high-frequency terms in equation (9), the third target signal shown in equation (11) is obtained, where the transformation rule based on the trigonometric formula is simplified to obtain .

[0075] Step S400: converting the amplitudes of the second target signal and the third target signal to 1.

[0076] In this step, converting the amplitude to 1 can facilitate subsequent signal processing and reduce the complexity of the calculation caused by the amplitude processing.

[0077] Optionally, the phase-sensitive optical time-domain reflectometry fiber optic sensing system includes a photoelectric balance detector, and converts the amplitudes of the second target signal and the third target signal into 1 (step S400), including steps S401 to S403, which are specifically as follows: The second target signal is ; The third target signal is ; Where R is the responsivity of the photoelectric balance detector, is the amplitude of the local oscillation light, for The amplitude of the backscattered Rayleigh light at time ; Step S401, calculate the normalization parameter according to the following third formula: ; The third formula is: .

[0078] Step S402, converting the amplitude of the second target signal to 1 according to the following fourth formula; The fourth formula is: .

[0079] Step S403, converting the amplitude of the third target signal to 1 according to the following fifth formula; The fifth formula is: .

[0080] In this embodiment, the normalization parameter is calculated, and the amplitudes of the second target signal and the third target signal are converted to 1, so as to reduce the amount of calculation caused by the amplitude in the subsequent noise reduction process and improve the efficiency of phase demodulation.

[0081] It should be understood that in order to make the description of this application more complete, the third formula is set after the above formula (11) as formula (12); the fourth formula is set after the above formula (12) as formula (13); and the fifth formula is set after the above formula (13) as formula (14).

[0082] Step S500: constructing a first function of the second target signal with respect to time and position, and a second function of the third target signal with respect to time and position based on the second target signal and the third target signal.

[0083] In this step, a first function and a second function are constructed to facilitate demodulation to obtain a differential phase.

[0084] Optionally, constructing a first function of the second target signal with respect to time NT and position z, and a second function of the third target signal with respect to time NT and position z (step S500) includes step S501, which is specifically as follows: Step S501, according to the relationship between time t and detection position z z=ct / 2n f, represent the second target signal as a first function , represent the third target signal as a second function .

[0085] where 0 ≤ t < T, T is the pulse interval, c is the speed of light, and n f is the refractive index of light with optical frequency f in the sensing optical fiber; ; .

[0086] In this embodiment, by constructing the first function and the second function, it is convenient to demodulate the differential phase according to the first function and the second function.

[0087] It should be understood that for the sake of more complete description of this application, the above first function is taken as Equation (15); the above second function is taken as Equation (16).

[0088] Step S600, based on the first function and the second function, demodulate the differential phase of the phase-sensitive optical time domain reflectometry fiber optic sensing system.

[0089] Optionally, the phase-sensitive optical time domain reflectometry fiber optic sensing system includes multiple measurement positions. Based on the first function and the second function, demodulating the differential phase of the phase-sensitive optical time domain reflectometry fiber optic sensing system (Step S600) includes Steps S601 to S606, specifically as follows: Step S601, sequentially take each measurement position as the target position.

[0090] Specifically, when performing phase demodulation, it is necessary to select the target position z1 among the multiple measurement positions of the phase-sensitive optical time domain reflectometry fiber optic sensing system, and perform demodulation operations on the data x(NT, z1) and y(NT, z1) for N = 0, 1, 2,....

[0091] Step S602, determine the first target function corresponding to the target position and the second target function .

[0092] In this step, substitute the relevant parameters of the target position into the first function and the second function respectively to obtain the first target function and the second target function.

[0093] Step S603, calculate the first differential of the first target function and the second differential of the second target function.

[0094] Specifically, Step S603 is detailed in the following Equations (17) and (18).

[0095] Step S604, based on the first objective function, the first differential, the second objective function and the second differential, calculate the third differential of the phase .

[0096] Optionally, based on the first objective function , first differential , the second objective function and the second differential , calculate the third differential of the phase ,include: The third differential of the phase is calculated according to the seventh formula as follows; The seventh formula is: .

[0097] It should be understood that in order to make the description of the embodiments of the present application more complete, the seventh formula is taken as the following formula (19). For a complete description, see the following formula (19).

[0098] Step S605, restore the third differential by integration to determine the phase corresponding to the target position .

[0099] Specifically, step S605 is described in detail in the following formula (20).

[0100] Specifically, when performing phase demodulation, it is necessary to select the target position z1 from multiple measurement positions of the phase-sensitive optical time domain reflection fiber optic sensor system. Demodulate the data x(NT, z1) and y(NT, z1) of N=0, 1, 2, ..., to obtain the differential phase. First, calculate the first differential of the first objective function (as shown in the following formula (17)) and the second differential of the second objective function (as shown in the following formula (18)), and then calculate the third differential (as shown in the following formula (19)) based on the first objective function, the first differential, the second objective function and the second differential. The third differential is the differential of the phase. The phase can be obtained by integrating and restoring the third differential (as shown in the following formula (20)).

[0101] Formula (17).

[0102] Formula (18).

[0103] Formula (19).

[0104] Formula (20).

[0105] Step S606: Calculate the differential phase corresponding to the target position according to the following sixth formula: ; The sixth formula is: ; Among them, s is the preset spatial difference step size.

[0106] It should be understood that in order to make the description of this application more complete, the above sixth formula is taken as formula (21).

[0107] Specifically, to obtain the differential phase of equation (21), we can also first obtain (as shown in equation (22) below) and (As shown in formula (23) below).

[0108] Formula (22).

[0109] Formula (23).

[0110] In separately seeking The corresponding fourth differential and The corresponding fifth differential Based on the following formula (24), the sixth differential is obtained, which is the differential of the phase. The phase can be obtained by integrating and restoring the sixth differential (see the above formula (20)).

[0111] Formula (24).

[0112] Optionally, step S706 further includes step S707, which is as follows: Step S707: Use a high-pass filter or a band-pass filter to filter the differential phase Filter, remove The low frequency signal in .

[0113] In this step, select an appropriate high-pass filter or band-pass filter to filter the differential phase Filtering is performed to reduce the impact of low-frequency signals introduced during the integration process.

[0114] The phase demodulation method provided in the embodiment of the present application can be used to demodulate and recover the phase information of the acoustic signal and the vibration signal in the phase-sensitive optical time-domain reflection fiber sensing system. It avoids the huge computational pressure caused by the phase unwrapping and detrending algorithms in the inverse tangent demodulation algorithm, improves the algorithm calculation efficiency, reduces the calculation time, and reduces the system demodulation cost. It can realize the phase digital demodulation analysis of the system, improve the demodulation efficiency, eliminate the noise and errors introduced by the demodulation technology method, simplify the system and reduce the system cost.

[0115] The phase demodulation method provided in the embodiment of the present application realizes the effective extraction and reconstruction of the phase signal contained in the COTDR system signal, and then realizes the phase digital demodulation of the phase-sensitive optical time domain reflectometry fiber optic sensing system. The huge computational pressure brought by the unwinding of the traditional algorithm is avoided by the product and difference method, the algorithm calculation efficiency is improved, the calculation time is reduced, and the noise and error introduced by the demodulation technical method are effectively eliminated, so that the system is simplified and the system cost is reduced.

[0116] For example, if the number of pulses N = 1000, the detrending algorithm in the inverse tangent algorithm needs to save an additional 20,000 data points to determine the adjacent moments. and The detrending operation requires polynomial fitting of the 1000 phase values ​​at each position z, and then subtracting them from the original phase value to obtain the final phase value. This results in a large amount of calculation in the system and low calculation efficiency.

[0117] Figure 4 A comparison chart of demodulation results provided in an embodiment of the present application.

[0118] Combination Figure 4 As shown, for the same set of data, the differential phase result obtained by the phase demodulation method provided in the embodiment of the present application is as follows Figure 4 As shown in the middle curve L1 (solid line), the traditional inverse tangent algorithm obtains the differential phase result as follows Figure 4 As shown in the middle curve L2 (dashed line), it can be seen that the minimum noise level of the result of the phase demodulation method provided in the embodiment of the present application is less than 0.001 rad, while the minimum noise level of the traditional inverse tangent algorithm is about 0.005 rad.

[0119] Corresponding to the embodiments of the aforementioned phase demodulation method, the present application also provides an embodiment of a phase demodulation system. The phase demodulation system includes: a phase-sensitive optical time-domain reflection fiber sensing system, a photoelectric balance detector, and a data acquisition and processing module; in the phase-sensitive optical time-domain reflection fiber sensing system, a beat frequency signal is generated by interference between the signal of the backward Rayleigh scattered light generated by the optical pulse and the signal of the local oscillator light; the photoelectric balance detector is configured to receive the beat frequency signal generated by the phase-sensitive optical time-domain reflection fiber sensing system; the data acquisition and processing module is communicatively connected to the photoelectric balance detector; the data acquisition and processing module is configured to obtain a discrete signal corresponding to the beat frequency signal; the beat frequency signal is generated by the interference between the signal of the backward Rayleigh scattered light generated by the optical pulse and the signal of the local oscillator light in the phase-sensitive optical time-domain reflection fiber sensing system. Interference is generated between optical signals; based on the intermediate frequency filtering technology, the first target signal corresponding to the discrete signal is determined; based on the first target signal and the orthogonal frequency shift signal, the second target signal and the third target signal are determined, and the second target signal and the third target signal are obtained by mixing the first target signal and the orthogonal frequency shift signal and low-pass filtering; the amplitudes of the second target signal and the third target signal are converted to 1; based on the second target signal and the third target signal, a first function of the second target signal with respect to time and position, and a second function of the third target signal with respect to time and position are constructed; based on the first function and the second function, the differential phase of the phase-sensitive optical time domain reflection fiber optic sensing system is demodulated.

[0120] The phase demodulation system provided in the embodiment of the present application can be used to demodulate and recover the phase information of the acoustic signal and the vibration signal in the phase-sensitive optical time-domain reflection fiber sensing system based on the data acquisition and processing module. It avoids the huge computing pressure caused by the phase unwrapping and detrending algorithms in the inverse tangent demodulation algorithm, improves the algorithm computing efficiency, reduces the computing time, and reduces the system demodulation cost. It can realize the phase digital demodulation analysis of the system, improve the demodulation efficiency, eliminate the noise and errors introduced by the demodulation technology method, simplify the system and reduce the system cost.

[0121] It should be noted that those skilled in the art will easily think of other embodiments of the present application after considering the specification and practicing the application disclosed herein. The present application is intended to cover any modification, use or adaptation of the present application, which follows the general principles of the present application and includes common knowledge or customary technical means in the art that are not disclosed in the present application. The specification and examples are only regarded as exemplary, and the true scope of the present application is indicated by the claims.

[0122] It should be understood that the present application is not limited to the precise structures that have been described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof. The scope of the present application is limited only by the appended claims.

Claims

1. A phase demodulation method, characterized in that: The phase demodulation method is applied to a phase demodulation system, the phase demodulation system includes a phase-sensitive optical time-domain reflectometry fiber sensing system, and the phase demodulation method includes: Acquire a discrete signal corresponding to a beat frequency signal; the beat frequency signal is generated by interference between a signal of backward Rayleigh scattered light generated by an optical pulse and a signal of local oscillator light in the phase-sensitive optical time-domain reflectometry optical fiber sensing system; Based on the intermediate frequency filtering technology, determining the first target signal corresponding to the discrete signal; Based on the first target signal and the orthogonal frequency-shifted signal, determining a second target signal and a third target signal, wherein the second target signal and the third target signal are obtained by mixing and low-pass filtering the first target signal and the orthogonal frequency-shifted signal; Converting the amplitudes of the second target signal and the third target signal to 1; Based on the second target signal and the third target signal, construct a first function of the second target signal with respect to time and position, and a second function of the third target signal with respect to time and position; Based on the first function and the second function, the differential phase of the phase-sensitive optical time-domain reflectometry fiber optic sensing system is demodulated.

2. The phase demodulation method according to claim 1, characterized in that: The determining the first target signal corresponding to the discrete signal based on the intermediate frequency filtering technology includes: Filter out the discrete signal except the central angular frequency The other harmonic components of the first target signal are obtained.

3. The phase demodulation method according to claim 1, characterized in that: The determining of the second target signal and the third target signal based on the first target signal and the orthogonal frequency shift signal comprises: Determine a first mixing signal and a second mixing signal corresponding to the first target signal and the orthogonal frequency shift signal; Based on the low-pass filtering technology, high-frequency items in the first mixed signal and the second mixed signal are filtered out to determine the second target signal and the third target signal.

4. The phase demodulation method according to claim 3, characterized in that: The first target signal is ; The orthogonal frequency shift signal includes and ; The determining of the first mixing signal and the second mixing signal corresponding to the first target signal and the orthogonal frequency shift signal includes: Based on the following first formula, the first mixing signal is calculated: ; Based on the following second formula, the second mixing signal is calculated: ; The first formula is: ； The second formula is: ； in, is the angular frequency difference between the local oscillator light and the backscattered Rayleigh light, T is the pulse interval, N is the order of the light pulses, and t is the preset time.

5. The phase demodulation method according to claim 3, characterized in that: The high frequency term is a related term with an angular frequency coefficient of 2Ω.

6. The phase demodulation method according to claim 1, characterized in that: The phase demodulation system further includes a photoelectric balance detector, and the step of converting the amplitudes of the second target signal and the third target signal into 1 includes: The second target signal is ; The third target signal is ; Wherein, R is the responsivity of the photoelectric balance detector, is the amplitude of the local oscillation light, for The amplitude of the backscattered Rayleigh light at time t; The normalization parameter is calculated according to the third formula as follows: ; The third formula is: ； The amplitude of the second target signal is converted into 1 according to the following fourth formula; The fourth formula is: ； The amplitude of the third target signal is converted into 1 according to the following fifth formula; The fifth formula is: 。 7. The phase demodulation method according to claim 1, characterized in that: The constructing of a first function of the second target signal with respect to time NT and position z, and a second function of the third target signal with respect to time NT and position z, comprises: According to the relationship between time t and detection position z, z=ct / 2n f , the second target signal is expressed as a first function , the third target signal is expressed as the second function ; where \(0\leq t < T\), \(T\) is the pulse interval, \(c\) is the speed of light, and \(n\) f is the refractive index of light with optical frequency \(f\) in the sensing optical fiber; ； 。 8. The phase demodulation method according to claim 1, characterized in that: The phase-sensitive optical time-domain reflectometry fiber-optic sensing system includes a plurality of measurement positions, and the demodulating the differential phase of the phase-sensitive optical time-domain reflectometry fiber-optic sensing system based on the first function and the second function includes: taking each of the measurement positions as a target position in turn; Determine the target location The corresponding first objective function And the second objective function ; Calculate the first differential of the first objective function and the second differential of the second objective function ; Based on the first objective function, the first differential, the second objective function and the second differential, a third differential of the phase is calculated. ; The phase corresponding to the target position is determined by integrating and restoring the third differential. ; According to the sixth formula below, the differential phase corresponding to the target position is calculated: ; The sixth formula is: ； Among them, s is the preset spatial difference step size.

9. The phase demodulation method according to claim 8, characterized in that: based on the first objective function 、The first differential The second objective function and the second differential , calculate the third differential of the phase ,include: The third differential of the phase is calculated according to the following seventh formula; The seventh formula is: 。 10. A phase demodulation system, characterized in that: include: Phase-sensitive optical time-domain reflectometry fiber optic sensing system, photoelectric balance detector and data acquisition and processing module; In the phase-sensitive optical time-domain reflectometry optical fiber sensing system, a beat frequency signal is generated by interference between a backward Rayleigh scattered light signal generated by an optical pulse and a local oscillator light signal; The photoelectric balance detector is configured to receive the beat frequency signal generated by the phase-sensitive optical time-domain reflectometry fiber optic sensing system; The data acquisition and processing module is communicatively connected to the photoelectric balance detector; the data acquisition and processing module is configured to obtain a discrete signal corresponding to a beat frequency signal; the beat frequency signal is generated by interference between a signal of backward Rayleigh scattered light generated by an optical pulse and a signal of local oscillator light in the phase-sensitive optical time-domain reflectometry optical fiber sensing system; Based on the intermediate frequency filtering technology, a first target signal corresponding to the discrete signal is determined; based on the first target signal and the orthogonal frequency shift signal, a second target signal and a third target signal are determined, wherein the second target signal and the third target signal are obtained by mixing and low-pass filtering the first target signal and the orthogonal frequency shift signal; Convert the amplitudes of the second target signal and the third target signal to 1; construct a first function of the second target signal with respect to time and position, and a second function of the third target signal with respect to time and position based on the second target signal and the third target signal; and demodulate the differential phase of the phase-sensitive optical time domain reflectometry fiber optic sensing system based on the first function and the second function.

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