Phase Demodulation Method and System

Abstract

This application relates to the field of optical fiber sensing technology, and particularly to a phase demodulation method and system. Among them, the phase demodulation method includes: obtaining a discrete signal corresponding to a beat frequency signal; determining a first target signal corresponding to the discrete signal based on intermediate frequency filtering technology; determining a second target signal and a third target signal based on the first target signal and an orthogonal frequency shift signal, where the second target signal and the third target signal are obtained after 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; 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; demodulating the differential phase of a phase-sensitive optical time domain reflectometry fiber sensing system based on the first function and the second function. The demodulation efficiency is improved, the noise and error are reduced, and the system complexity and cost are reduced.

Description

Technical Field

[0001] The present application relates to the technical field of fiber optic sensing, and specifically relates to a phase demodulation method and system. Background Art

[0002] Based on the phase-sensitive optical time domain reflectometry (OTDR) fiber optic sensing system, it can perform distributed multi-point real-time monitoring of tiny signals over a long distance. It has the characteristics of high sensitivity, large spatial dynamic range, and high positioning accuracy. Therefore, it has broad market application prospects in aspects such as health monitoring of large facilities, industrial production process detection, perimeter security, pipeline protection, and ocean engineering.

[0003] Currently, the phase demodulation of the phase-sensitive OTDR fiber optic sensing system is mainly based on hardware phase demodulation schemes including phase-generated carrier, phase demodulation based on a 3×3 coupler, I / Q demodulation, etc. The phase-generated carrier method requires the introduction of a phase modulator and a demodulator, which increases the complexity and cost of the system, and depends on precise phase modulation. Any modulation error will affect the final demodulation accuracy. For the phase demodulation method based on a 3×3 coupler, the demodulation accuracy 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 optical intensity signal may be weakened, affecting the demodulation effect. I / Q demodulation requires complex circuit design, including quadrature modulators, balanced detectors, and high-performance signal processing circuits, which increases the complexity and cost of the system. Usually, a large amount of data needs to be processed in real time, requiring high computing power and processing speed of the system. It is very sensitive to the phase noise in the system, especially the phase noise of the light source and the phase jitter in the system, which will cause demodulation errors.

[0004] Therefore, the existing demodulation methods have problems such as high cost, complex system, large 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 by the embodiments of the present application demodulate and recover the phase information of acoustic signals and vibration signals in the phase-sensitive OTDR fiber optic sensing system. It realizes digital phase demodulation analysis of the system, improves the demodulation efficiency, reduces the noise and error introduced by the demodulation technical method, simplifies the system, and reduces the system cost.

[0006] In some embodiments, a phase demodulation method is provided. The phase demodulation method is applied to a phase demodulation system, which includes a phase-sensitive optical time domain reflectometry (OTDR) fiber sensing system. The phase demodulation method includes: obtaining a discrete signal corresponding to a beat frequency signal, where the beat frequency signal is generated by the interference between the signal of the backward Rayleigh scattered light generated by an optical pulse and the signal of a local oscillator light in the phase-sensitive OTDR fiber sensing system; determining a first target signal corresponding to the discrete signal based on intermediate frequency filtering technology; determining a second target signal and a third target signal based on the first target signal and an orthogonal frequency shift signal, where the second target signal and the third target signal are obtained after 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; 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; and demodulating the differential phase of the phase-sensitive OTDR fiber sensing system based on the first function and the second function.

[0007] By using the phase demodulation method provided in the embodiments of the present application, it is possible to demodulate and recover the phase information of acoustic signals and vibration signals in a phase-sensitive OTDR fiber sensing system. It avoids the huge computational pressure brought by the phase unwrapping and detrending algorithms in the arctangent demodulation algorithm, improves the computational efficiency of the algorithm, reduces the computational time, and reduces the system demodulation cost. It can realize digital phase 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, determining a first target signal corresponding to the discrete signal based on intermediate frequency filtering technology includes: filtering out other harmonic components in the discrete signal except those with a center angular frequency of to obtain the first target signal.

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

[0010] Optionally, the first target signal is ; the orthogonal frequency shift signal includes and ; determining a first mixed signal and a second mixed signal corresponding to the first target signal and the orthogonal frequency shift signal includes: calculating the first mixed signal based on the following first formula; and calculating the second mixed signal based on the following second formula;

[0011] The first formula is:

[0012] ;

[0013] The second formula is:

[0014] ;

[0015] 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.

[0016] Optionally, the high-frequency term is a relevant term with an angular frequency coefficient of 2Ω.

[0017] 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;

[0018] Calculate the normalization parameter according to the following third formula ;

[0019] The third formula is:

[0020] ;

[0021] Convert the amplitude of the second target signal to 1 according to the following fourth formula;

[0022] The fourth formula is:

[0023] ;

[0024] Convert the amplitude of the third target signal to 1 according to the following fifth formula;

[0025] The fifth formula is:

[0026] .

[0027] 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 ; 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 a frequency of \(f\) in the sensing optical fiber;

[0028] ;

[0029] .

[0030] Optionally, the phase-sensitive optical time-domain reflectometry (OTDR) fiber sensing system includes multiple measurement positions. Based on the first function and the second function, the differential phase of the phase-sensitive OTDR fiber sensing system is demodulated, including: sequentially taking each measurement position as the target position; determining the first target function corresponding to the target position and the second target function ; calculating the first differential of the first target function and the second differential of the second target function ; calculating the third differential of the phase based on the first target function, the first differential, the second target function, and the second differential ; determining the phase corresponding to the target position by integrating and restoring the third differential ;

[0031] Calculating the differential phase corresponding to the target position according to the following sixth formula ;

[0032] The sixth formula is:

[0033] ;

[0034] where \(s\) is a preset spatial differential step size.

[0035] Optionally, based on the first target function , the first differential , the second target function and the second differential , calculating the third differential of the phase , including:

[0036] Calculating the third differential of the phase according to the following seventh formula;

[0037] The seventh formula is:

[0038] .

[0039] In some embodiments, a phase demodulation system is provided, including: a phase-sensitive optical time domain reflectometry (OTDR) fiber sensing system, a photoelectric balanced detector, and a data acquisition and processing module; in the phase-sensitive OTDR fiber sensing system, an interference between the signal of the backward Rayleigh scattered light generated by the optical pulse and the signal of the local oscillator light generates a beat signal; the photoelectric balanced detector is configured to receive the beat signal generated by the phase-sensitive OTDR 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 the discrete signal corresponding to the beat signal; the beat 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 OTDR 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 quadrature 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 mixing and low-pass filtering the first target signal and the quadrature frequency shift signal; 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 OTDR fiber sensing system is demodulated.

[0040] It can be understood that for the beneficial effects that can be achieved by the technical solution provided by the above-mentioned phase demodulation system, reference can be made to the beneficial effects in the phase demodulation method and any of its optional embodiments, which will not be elaborated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solutions of the present application, the drawings required for use in the embodiments will be briefly introduced below. Obviously, for those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative efforts.

[0042] Figure 1 It is a structural block diagram of the phase demodulation system provided by the embodiment of the present application;

[0043] Figure 2 It is a structural diagram of the phase demodulation system provided by the embodiment of the present application;

[0044] Figure 3 It is a flowchart of the phase demodulation method provided by the embodiment of the present application;

[0045] Figure 4 It is a comparison diagram of the demodulation results provided by the embodiment of the present application.

[0046] REFERENCE SIGNS:

[0047] 10. Phase-sensitive optical time domain reflectometry (OTDR) fiber sensing system; 101. Narrow linewidth laser; 102. First fiber coupler; 103. Acousto-optic modulator; 104. First erbium-doped fiber amplifier; 105. Fiber optical 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 implementation manners

[0048] Next, the technical solutions in the embodiments of the present application will be clearly described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0049] Hereinafter, terms such as "first" and "second" are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of the present application, unless otherwise specified, the meaning of "a plurality" is two or more.

[0050] In addition, in the present application, orientation terms such as "upper", "lower", "inner", "outer", etc. are defined relative to the orientation of the components shown in the drawings. It should be understood that these directional terms are relative concepts, which are used for relative description and clarification, and they may change correspondingly according to the change of the orientation of the components placed in the drawings.

[0051] For the convenience of understanding the technical solutions of the application, the relevant technologies involved in the present application will be described first below.

[0052] The phase-sensitive optical time domain reflectometry (OTDR) fiber sensing system can perform distributed multi-point real-time monitoring on tiny signals over a long distance. It has the characteristics of high sensitivity, large spatial dynamic range, and high positioning accuracy. Therefore, it has broad market application prospects in the health monitoring of large facilities, industrial production process detection, perimeter security, pipeline protection, ocean engineering, etc.

[0053] Currently, the phase demodulation of interferometric fiber sensing systems (including phase-sensitive optical time domain reflectometry (OTDR) fiber sensing systems) is mainly based on hardware phase demodulation schemes including phase-generated carrier, phase demodulation based on a 3×3 coupler structure, I / Q demodulation, etc.

[0054] The Phase-Generated Carrier (PGC) method is a common phase demodulation method used in interferometric fiber optic sensing systems. The Phase-Generated Carrier method modulates the optical phase by applying a high-frequency carrier to the interference signal, enabling the extraction of the originally difficult-to-demodulate phase information through spectral transformation. Since the Phase-Generated Carrier method requires the introduction of a phase modulator and a demodulator, it increases the complexity and cost of the system and depends on precise phase modulation. Any modulation error will affect the final demodulation accuracy.

[0055] The phase demodulation method based on a 3×3 coupler is a phase demodulation technique widely used in fiber optic interferometers. By utilizing the special structure of the 3×3 fiber coupler, it can split the fiber optic interference signal into three paths and achieve phase demodulation through mathematical calculations. This method is commonly used in systems such as fiber optic gyroscopes (FOG) and has high demodulation accuracy and stability. However, the demodulation accuracy of the phase demodulation method based on a 3×3 coupler 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 optical intensity signal may weaken, affecting the demodulation effect.

[0056] I / Q demodulation (In-phase and Quadrature Demodulation) is a common method for extracting useful information from modulated signals. It is widely used in fields such as communication systems, radar, software-defined radio, and fiber optic sensors. However, I / Q demodulation requires complex circuit design, including quadrature modulators, balanced detectors, and high-performance signal processing circuits, increasing the complexity and cost of the system. It usually requires real-time processing of a large amount of data, has high requirements for the computing power and processing speed of the system, and is very sensitive to the phase noise in the system. In particular, the phase noise of the light source and the phase jitter within the system will both cause demodulation errors.

[0057] Therefore, the existing phase demodulation methods have problems such as high cost, complex systems, large noise, and large computational load, seriously affecting the practical application and industrialization of phase-sensitive optical time domain reflectometry fiber optic sensing systems.

[0058] Figure 1 This is the structural block diagram of the phase demodulation system provided by the embodiments of this application.

[0059] To solve the problems existing in the above related technologies, in combination with Figure 1As shown in the figure, the present application provides a phase demodulation system, which includes a phase-sensitive optical time domain reflectometry (OTDR) fiber sensing system 10, a photoelectric balanced detector 20, and a data acquisition and processing module 30. In the phase-sensitive OTDR fiber sensing system 10, an interference occurs between the signal of the backward Rayleigh scattered light generated by the optical pulse and the signal of the local oscillator light to generate a beat signal. The photoelectric balanced detector 20 is configured to receive the beat signal generated by the phase-sensitive OTDR fiber sensing system 10. The data acquisition and processing module 30 is communicatively connected to the photoelectric balanced detector 20 ( Figure 1 The dotted arrow in the figure indicates a schematic connection, and the arrow represents the direction of data transmission). The data acquisition and processing module 30 is configured to acquire the discrete signal corresponding to the beat signal and demodulate the differential phase based on the discrete signal.

[0060] By using the phase demodulation system provided in the embodiments of the present application, it is possible to demodulate and recover the phase information of acoustic signals and vibration signals based on the backward Rayleigh scattered light signal and the local oscillator light signal in the phase-sensitive OTDR fiber sensing system. It realizes the digital phase demodulation analysis of the system, reduces the calculation amount, improves the demodulation efficiency, reduces the noise and error introduced by the demodulation technical method, simplifies the system, and reduces the system cost.

[0061] Specifically, after the photoelectric balanced detector 20 obtains the beat signal, it also converts the beat signal into an electrical signal. The data acquisition and processing module 30 acquires the electrical signal to obtain a discrete signal.

[0062] Optionally, as Figure 1 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), as long as the data acquisition and processing module 30 can implement the above functions.

[0063] Figure 2 It is a structural diagram of the phase demodulation system provided in the embodiments of the present application.

[0064] Combined with Figure 2 shown, the phase-sensitive OTDR fiber 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, an optical fiber circulator 105, a sensing optical fiber 106, a second erbium-doped fiber amplifier 107, and a second fiber coupler 108.

[0065] Specifically, the laser output by the narrow linewidth laser 101 is split into two paths of optical signals in the optical fiber by the first optical fiber coupler 102 according to a first coupling ratio, namely the first path of optical signal and the second path of optical signal. The second path of optical signal is the local oscillator optical signal. The first path of optical signal enters the acousto-optic modulator 103 through an optical fiber. After being modulated by the acousto-optic modulator 103, it is amplified by the first erbium-doped optical fiber amplifier 104 to amplify the first path of optical signal. The first path of optical signal enters the optical fiber circulator 105 through the first port 1051 of the optical fiber circulator 105, and then is output through the second port 1052 of the optical fiber circulator 105 and transmitted to the sensing optical fiber 106. After generating a backward Rayleigh scattering optical signal, it enters the optical fiber circulator 105 through the second port 1052 of the optical fiber circulator 105 again, and then is transmitted to the second erbium-doped optical fiber amplifier 107 through the third port 1053 of the optical fiber circulator 105 for amplification again to form a third path of optical signal. The third path of optical signal and the second path of optical signal are coupled by the second optical fiber coupler 108 to form a fourth path of optical signal. The third path of optical signal is the signal of the backward Rayleigh scattering light generated by the optical pulse. The fourth path of optical signal is the beat frequency signal generated by the interference between the signal of the backward Rayleigh scattering light generated by the optical pulse and the local oscillator optical signal. The fourth path of optical signal is received by the photoelectric balanced detector 20 and converted into an electrical signal, so as to be collected by the data acquisition and processing module 30 to obtain a discrete signal.

[0066] It should be understood that Figure 2 the arrows in

[0067] are the transmission directions of the optical signals.

[0068] Exemplarily, a single optical pulse generates a backward Rayleigh scattering optical signal through the sensing optical fiber as shown in Equation (1):

[0069] Equation (1);

[0070] where ω = ω0 + Ω, ω0 represents the central angular frequency of the laser light source, and Ω is the frequency shift center of the acousto-optic modulator. is the amplitude of the probe light, is the time after the emission of the probe pulse, is the time delay corresponding to the i-th Rayleigh scattering center, is the width of the probe optical pulse, is the backward Rayleigh scattering coefficient of the i-th 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. Wherein, c is the speed of light, and n f is the refractive index of light with a frequency of f.

[0071] Specifically, the moment corresponding to t = 0 is the moment when the optical pulse enters the sensing optical fiber.

[0072] .

[0073] As can be seen from the above formula (1), the Rayleigh scattered light obtained at a certain moment t is the result of the interference of the scattered light of all Rayleigh scattering centers within the pulse, and the signal at moment t corresponds to the information of the position of the detection optical fiber .

[0074] Assume that the pulse interval is T, and the backward Rayleigh scattered light signal generated by the Nth optical pulse passing through the sensing optical fiber is given by formula (2):

[0075] Formula (2);

[0076] The Rayleigh scattered signal is represented by the complex amplitude as formula (3):

[0077] Formula (3);

[0078] Wherein, 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 the moment of NT + t.

[0079] Exemplarily, the local oscillator optical signal is given by formula (4):

[0080] Formula (4);

[0081] Wherein, E0 is the amplitude of the local oscillator optical signal, is the initial phase of the local oscillator optical signal.

[0082] Specifically, the backward Rayleigh scattered light signal and the local oscillator optical signal undergo beat frequency interference through a 3dB fiber coupler. The optical intensities output from the two ports of the 3dB fiber coupler are the first optical intensity I1 and the second optical 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.

[0083] Exemplarily, the first optical intensity I1 and the second optical intensity I2 are specifically as follows:

[0084] Formula (5);

[0085] Formula (6);

[0086] Among them, is the complex conjugate of is the complex conjugate of is the complex conjugate of

[0087] As can be seen from the above equations (5) and (6), both I1 and I2 have three terms, where the first two terms represent the DC part of the light intensity, and the third term is the AC part of the light intensity, that is, the beat signal.

[0088] Specifically, extract the beat signals in the first light intensity I1 and the second light intensity I2.

[0089] In this step, it is to extract the third term in the above equations (5) and (6) as the beat signal. Exemplarily, the first beat signal corresponding to the first light intensity is , and the second beat signal corresponding to the second light intensity is .

[0090] Specifically, the photoelectric balanced detector 20 collects the discrete signal corresponding to the beat signal, including the following steps S01 and S02, specifically as follows:

[0091] Step S01, use a balanced photodetector (BPD) to convert the beat signal into an electrical signal.

[0092] Specifically, due to the suppression of the common-mode signal by the BPD, what the BPD detects is the AC part, and the output voltage u BPD can be expressed as Equation (7):

[0093] Equation (7).

[0094] Step S02, collect and convert the electrical signal output by the photoelectric balanced detector into a discrete signal by a data acquisition card (DAQ).

[0095] Figure 3 This is the flowchart of the phase demodulation method provided by the embodiment of the present application.

[0096] Combined with Figure 1 and Figure 2 the phase demodulation system shown, the embodiment of the present application also provides a phase demodulation method. The execution subject of this method can be a data acquisition and processing module. The phase demodulation method is applied to a phase-sensitive optical time domain reflectometry fiber sensing system, as Figure 3 shown, the phase demodulation method includes steps S100 to S600, specifically as follows:

[0097] Step S100, obtaining the discrete signal corresponding to the beat signal; the beat 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 reflectometry fiber sensing system.

[0098] Specifically, the interference between the signal of the backward Rayleigh scattered light generated by the optical pulse and the signal of the local oscillator light means that the backward Rayleigh scattered light signal generated by a single optical pulse passes through the sensing fiber, converges with the local oscillator light signal, and undergoes coherent interference.

[0099] Step S200, determining the first target signal corresponding to the discrete signal based on the intermediate frequency filtering technology.

[0100] Optionally, step S200 includes step S201:

[0101] Step S201, filtering out other harmonic components in the discrete signal except those with the central angular frequency of Ω to obtain the first target signal.

[0102] In this step, theoretically, the central angular frequency of the voltage signal generated by the BPD is Ω, but there are also DC components and high-order harmonic components with relatively high angular frequencies, such as harmonic components with angular frequencies of 2Ω, 3Ω, 4Ω, etc. To reduce the system noise and the influence of the DC component and the high-frequency signal on the demodulation result, the voltage signal obtained by the BPD is subjected to band-pass filtering with the central angular frequency of Ω, that is, filtering out other harmonic components in the discrete signal except those with the central angular frequency of Ω.

[0103] Step S300, determining the second target signal and the third target signal based on the first target signal and the quadrature frequency shift signal. The second target signal and the third target signal are obtained after mixing and low-pass filtering the first target signal and the quadrature frequency shift signal.

[0104] Optionally, step S300 includes step S301 and step S302:

[0105] Step S301, determining the first mixing signal and the second mixing signal corresponding to the first target signal and the quadrature frequency shift signal.

[0106] Optionally, the first target signal is ; the quadrature frequency shift signal includes and ; determining the first mixing signal and the second mixing signal corresponding to the first target signal and the quadrature frequency shift signal (step S401) includes:

[0107] Based on the following first formula, calculating to obtain the first mixing signal ;

[0108] Based on the following second formula, calculating to obtain the second mixing signal ;

[0109] The first formula is:

[0110] ;

[0111] The second formula is:

[0112] ;

[0113] 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.

[0114] Specifically, is the discrete signal at time .

[0115] Specifically, the filtered signal is digitally mixed with and respectively to obtain a first mixed signal and a second mixed signal. The first mixed signal is shown in Equation (8), and the second mixed signal is shown in Equation (9).

[0116] Equation (8).

[0117] Equation (9).

[0118] Step S302: Based on the low-pass filtering technique, filter out the high-frequency terms in the first mixed signal and the second mixed signal to determine a second target signal and a third target signal.

[0119] In this embodiment, by filtering out the high-frequency terms, the influence of the high-frequency terms in the subsequent demodulation is reduced.

[0120] Optionally, the high-frequency term is a related term with an angular frequency coefficient of 2Ω.

[0121] Specifically, the second target signal is (as shown in the following Equation (10)), and the third target signal is (as shown in the following Equation (11)), specifically as follows:

[0122] Equation (10).

[0123] Equation (11).

[0124] Combining the above Equation (8) and Equation (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, wherein, based on the transformation rules of the trigonometric formula, it is simplified to obtain .

[0125] 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 simplification is performed based on the transformation rules of trigonometric formulas to obtain .

[0126] Step S400: Convert the amplitudes of the second target signal and the third target signal to 1.

[0127] In this step, converting the amplitude to 1 can facilitate subsequent signal processing and reduce the problem of complex computational complexity caused by processing the amplitude.

[0128] Optionally, the phase-sensitive optical time-domain reflectometry fiber optic sensing system includes a photoelectric balanced detector. Converting the amplitudes of the second target signal and the third target signal to 1 (step S400) includes steps S401 to S403, which are specifically as follows:

[0129] The second target signal is ;

[0130] The third target signal is ;

[0131] where 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

[0132] Step S401: Calculate the normalization parameter according to the following third formula ;

[0133] The third formula is:

[0134] .

[0135] Step S402: Convert the amplitude of the second target signal to 1 according to the following fourth formula;

[0136] The fourth formula is:

[0137] .

[0138] Step S403: Convert the amplitude of the third target signal to 1 according to the following fifth formula;

[0139] The fifth formula is:

[0140] .

[0141] In this embodiment, by calculating the normalization parameter and converting the amplitudes of the second target signal and the third target signal to 1, it is convenient to reduce the computational amount brought by the amplitude in the subsequent noise reduction process and improve the efficiency of phase demodulation.

[0142] It should be understood that for the sake of more complete description of this application, the above third formula is set after the above formula (11) as formula (12); the above fourth formula is set after the above formula (12) as formula (13); the above fifth formula is set after the above formula (13) as formula (14).

[0143] Step S500: 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.

[0144] In this step, by constructing the first function and the second function, it is convenient to demodulate the differential phase.

[0145] 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:

[0146] Step S501: 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 .

[0147] 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; ; .

[0148] 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.

[0149] It should be understood that for the sake of more complete description of this application, the above first function is used as formula (15); the above second function is used as formula (16).

[0150] Step S600: Based on the first function and the second function, demodulate the differential phase of the phase-sensitive optical time domain reflectometry fiber sensing system.

[0151] Optionally, the phase-sensitive optical time domain reflectometry (OTDR) fiber sensing system includes a plurality of measurement positions. Based on a first function and a second function, the differential phase of the phase-sensitive OTDR fiber sensing system is demodulated (step S600), which includes steps S601 to S606, specifically as follows:

[0152] Step S601: Successively take each measurement position as the target position.

[0153] Specifically, when performing phase demodulation, it is necessary to select a target position z1 among the multiple measurement positions of the phase-sensitive OTDR fiber sensing system, and perform demodulation operations on the data x(NT, z1) and y(NT, z1) for N = 0, 1, 2,....

[0154] Step S602: Determine the corresponding first target function and the second target function

[0155] In this step, the relevant parameters of the target position are substituted into the first function and the second function to obtain the first target function and the second target function respectively.

[0156] Step S603: Calculate the first differential of the first target function and the second differential of the second target function.

[0157] Specifically, the details of step S603 are shown in the following formulas (17) and (18).

[0158] Step S604: Based on the first target function, the first differential, the second target function, and the second differential, calculate the third differential of the phase.

[0159] Optionally, based on the first target function , the first differential , the second target function , and the second differential , calculate the third differential of the phase, including:

[0160] Calculate the third differential of the phase according to the following seventh formula;

[0161] The seventh formula is:

[0162]

[0163] It should be understood that for the sake of more complete description of the embodiments of the present application, the above seventh formula is used as the following formula (19), and the complete description can be seen in the following formula (19). ​​​

[0164] Step S605, determine the phase corresponding to the target position by integral reduction of the third differential .

[0165] Specifically, Step S605 is detailed in the following formula (20).

[0166] Specifically, during phase demodulation, it is necessary to select the target position z1 among multiple measurement positions of the phase-sensitive optical time domain reflectometry fiber sensing system. Demodulation operations are performed on the data x(NT, z1) and y(NT, z1) for 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)) respectively, 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 integral reduction of the third differential (as shown in the following formula (20)).

[0167] Formula (17).

[0168] Formula (18).

[0169] Formula (19).

[0170] Formula (20).

[0171] Step S606, calculate the differential phase corresponding to the target position according to the following sixth formula ;

[0172] The sixth formula is:

[0173] ;

[0174] where s is the preset spatial differential step size.

[0175] It should be understood that for the sake of the completeness of the description of this application, the above sixth formula is taken as formula (21).

[0176] Specifically, to obtain the differential phase of formula (21), it is also possible to first obtain (as shown in the following formula (22)) and (as shown in the following formula (23)).

[0177] Formula (22).

[0178] Formula (23).

[0179] After separately obtaining the corresponding fourth differential and the corresponding fifth differential . And based on the following formula (24), the sixth differential is obtained. The sixth differential is the differential of the phase. The phase can be obtained by integrating and restoring the sixth differential (refer to that shown in the above formula (20)).

[0180] Formula (24).

[0181] Optionally, after step S706, step S707 is further included, which is specifically as follows:

[0182] Step S707, filtering the differential phase using a high-pass filter or a band-pass filter to filter out the low-frequency signal in

[0183] In this step, a suitable high-pass filter or band-pass filter is selected to filter the differential phase to reduce the influence of the low-frequency signal introduced during the integration process.

[0184] Adopting the phase demodulation method provided by the embodiments of the present application can demodulate and recover the phase information of the acoustic signal and the vibration signal in the phase-sensitive optical time domain reflectometry fiber sensing system. It avoids the huge computational pressure brought by the phase unwrapping and detrending algorithms in the arctangent demodulation algorithm, improves the computational efficiency of the algorithm, reduces the computational time, and reduces the system demodulation cost. It can realize the digital demodulation analysis of the phase of the system, improve the demodulation efficiency, eliminate the noise and errors introduced by the demodulation technical method, simplify the system and reduce the system cost.

[0185] The phase demodulation method provided by the embodiments of the present application realizes the effective extraction and reconstruction of the phase signal contained in the COTDR system signal, and further realizes the digital demodulation of the phase of the phase-sensitive optical time domain reflectometry fiber sensing system. By means of the product-to-sum formula, it avoids the huge computational pressure brought by the traditional algorithm for unwrapping, improves the computational efficiency of the algorithm, reduces the computational time, and effectively eliminates the noise and errors introduced by the demodulation technical method, simplifies the system and reduces the system cost.

[0186] For example, when the number of pulses N = 1000, the detrending algorithm in the arctangent algorithm needs to additionally store 20,000 data, and judge the relationship between adjacent moments and to confirm the unwrapping value. The detrending operation needs to perform polynomial fitting on 1000 phase values at each position z, and subtract from the original phase value to obtain the final phase value. In this way, the computational amount of the system is large and the computational efficiency is low.

[0187] Figure 4 This is a comparison chart of demodulation results provided by the embodiments of the present application.

[0188] Combined with Figure 4 As shown, for the same set of data, the differential phase results obtained by using the phase demodulation method provided by the embodiments of the present application are as shown by the curve L1 (solid line) in Figure 4 The differential phase results obtained by the traditional arctangent algorithm are as shown by the curve L2 (dashed line) in Figure 4 It can be seen that the lowest noise level of the results of the phase demodulation method provided by the embodiments of the present application is less than 0.001 rad, while the lowest noise level of the traditional arctangent algorithm is about 0.005 rad.

[0189] Corresponding to the foregoing embodiments of the 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 reflectometry (OTDR) fiber sensing system, a photoelectric balanced detector, and a data acquisition and processing module; in the phase-sensitive OTDR fiber sensing system, an interference occurs between the signal of the backward Rayleigh scattered light generated by the optical pulse and the signal of the local oscillator light to generate a beat signal; the photoelectric balanced detector is configured to receive the beat signal generated by the phase-sensitive OTDR 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 the discrete signal corresponding to the beat signal; the beat 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 OTDR fiber sensing system; based on the intermediate frequency filtering technology, determine the first target signal corresponding to the discrete signal; based on the first target signal and the quadrature frequency shift signal, determine the second target signal and the third target signal, where the second target signal and the third target signal are obtained after mixing and low-pass filtering the first target signal and the quadrature frequency shift signal; convert 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, demodulate the differential phase of the phase-sensitive OTDR fiber sensing system.

[0190] By using the phase demodulation system provided by the embodiments of the present application, it is possible to demodulate and recover the phase information of the acoustic signal and the vibration signal in the phase-sensitive OTDR fiber sensing system based on the data acquisition and processing module. It avoids the huge computational pressure brought by the phase unwrapping and detrending algorithms in the arctangent demodulation algorithm, improves the computational efficiency of the algorithm, reduces the computational time, and reduces the system demodulation cost. It can realize the digital demodulation analysis of the phase of the system, improve the demodulation efficiency, eliminate the noise and errors introduced by the demodulation technical method, simplify the system and reduce the system cost.

[0191] It should be noted that those skilled in the art will readily conceive 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 variations, uses, or adaptations of the present application, which follow the general principles of the present application and include the common general knowledge or conventional technical means in the technical field not disclosed in the present application. The specification and examples are only regarded as exemplary, and the true scope of the present application is pointed out by the claims.

[0192] It should be understood that the present application is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present application is only limited by the appended claims.

Claims

1. A phase demodulation method, characterized in that, The phase demodulation method is applied to a phase demodulation system, and the phase demodulation system includes a phase-sensitive optical time domain reflectometry (OTDR) fiber sensing system. The phase demodulation method includes: Obtaining a discrete signal corresponding to a beat signal; the beat signal is generated by the interference between the signal of the backward Rayleigh scattered light generated by an optical pulse and the signal of a local oscillator light in the phase-sensitive optical time domain reflectometry fiber sensing system; Based on intermediate frequency filtering technology, determining a first target signal corresponding to the discrete signal; Based on the first target signal and a quadrature frequency shift signal, determining a second target signal and a third target signal, where the second target signal and the third target signal are obtained after mixing and low-pass filtering the first target signal and the quadrature 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; Based on the first function and the second function, demodulating the differential phase of the phase-sensitive optical time domain reflectometry fiber sensing system; The phase demodulation system further includes a photoelectric balanced detector. The converting the amplitudes of the second target signal and the third target signal to 1 includes: The second target signal is ; The third target signal is ; wherein, R is the responsivity of the optoelectronic balanced detector, and is the amplitude of the local oscillator light, and is the amplitude of the backward Rayleigh scattered light at a certain moment. Calculate the normalization parameter according to the following third formula ; The third formula is ; Converting the amplitude of the second target signal to 1 according to the following fourth formula; The fourth formula is: ； Converting the amplitude of the third target signal to 1 according to the following fifth formula; The fifth formula is: ； The phase-sensitive optical time domain reflectometry fiber sensing system includes multiple measurement positions. The demodulating the differential phase of the phase-sensitive optical time domain reflectometry fiber sensing system based on the first function and the second function includes: Sequentially taking each of the measurement positions as a target position; Determine 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 ; Calculate a third derivative of the phase based on the first objective function, the first derivative, the second objective function, and the second derivative ; Determine the phase corresponding to the target position by integrating and restoring the third differential ; Calculate the differential phase corresponding to the target position according to the following sixth formula ; The sixth formula is: ； where s is a preset spatial differential step size; Based on the first objective function , the first differential , the second objective function and the second differential , calculate the third differential of the phase , including: Calculating a third derivative of the phase according to the following seventh formula; The seventh formula is: ； The determining the first target signal corresponding to the discrete signal based on intermediate frequency filtering technology includes: Filtering out other harmonic components in the discrete signal except those with a center angular frequency of Ω to obtain a first target signal.

2. The phase demodulation method according to claim 1, wherein The determining the second target signal and the third target signal based on the first target signal and a quadrature frequency shift signal includes: Determining a first mixed signal and a second mixed signal corresponding to the first target signal and the quadrature frequency shift signal; Based on low-pass filtering technology, filtering out high-frequency terms in the first mixed signal and the second mixed signal to determine the second target signal and the third target signal.

3. The phase demodulation method according to claim 2, wherein The first target signal is ; The orthogonal frequency shift signal includes and ; The determining the first mixed signal and the second mixed signal corresponding to the first target signal and the quadrature frequency shift signal includes: The first mixing signal is calculated based on the following first formula ; The second mixing signal is calculated based on the following second formula ; 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.

4. The phase demodulation method according to claim 2, wherein The high-frequency term is a related term with an angular frequency coefficient of 2Ω.

5. The phase demodulation method according to claim 1, wherein 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 includes: 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 ; 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; ； 。 6. A phase demodulation system, characterized in that, Including: A phase-sensitive optical time domain reflectometry (OTDR) fiber sensing system, an optoelectronic balanced detector, and a data acquisition and processing module; In the phase-sensitive OTDR fiber sensing system, an interference between the signal of the backward Rayleigh scattering light generated by an optical pulse and the signal of a local oscillator light generates a beat signal; The optoelectronic balanced detector is configured to receive the beat signal generated by the phase-sensitive OTDR fiber sensing system; The data acquisition and processing module is communicatively connected to the optoelectronic balanced detector; the data acquisition and processing module is configured to obtain a discrete signal corresponding to the beat signal; the beat signal is generated by an interference between the signal of the backward Rayleigh scattering light generated by an optical pulse and the signal of a local oscillator light in the phase-sensitive OTDR 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 a quadrature frequency shift signal, determining a second target signal and a third target signal, where the second target signal and the third target signal are obtained by mixing and low-pass filtering the first target signal and the quadrature 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; based on the first function and the second function, demodulating the differential phase of the phase-sensitive OTDR fiber sensing system; The phase demodulation system further includes a photoelectric balanced detector. Converting the amplitudes of the second target signal and the third target signal to 1 includes: The second target signal is ; The third target signal is ; where 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: ; 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 includes: sequentially taking each measurement position as the target position; determining the first target function corresponding to the target position and the second target function ; calculating the first differential of the first target function and the second differential of the second target function; calculating the third differential of the phase based on the first target function, the first differential, the second target function and the second differential; restoring the third differential by integration to determine the phase corresponding to the target position; calculating the differential phase corresponding to the target position according to the following sixth formula; The sixth formula is: ; where s is a preset spatial differential step size; Based on the first target function , the first differential , the second target function and the second differential , calculating the third differential of the phase includes: calculating the third differential of the phase according to the following seventh formula; The seventh formula is: ; Based on the intermediate frequency filtering technology, determining the first target signal corresponding to the discrete signal includes: filtering out other harmonic components in the discrete signal except the center angular frequency of Ω to obtain the first target signal.

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