Large range quasi-distributed fiber optic sensing method and system

Abstract

According to the application, a wide-range quasi-distributed optical fiber sensing method and system are provided, which comprises: linear sweep light outputting local light and signal light through a linear sweep light source module; the signal light enters a weak reflection point array sensing module to generate reflected light after being reflected by the reflection points; the reflected light and the local light enter a coherent detection module to interfere to obtain interference light; the coherent detection module converts the interference light signal into an electric signal; the electric signal enters a signal processing module to perform Fourier transform to obtain a corresponding frequency spectrum signal; the frequency spectrum signal selects beat frequency signals corresponding to weak reflection points A and B at two ends of a target optical fiber by using a band-pass filter, obtains a current time domain signal based on the beat frequency signals, and thus obtains a frequency value of a difference frequency term; the optical path between the A and B points is calculated according to the frequency value of the difference frequency term; and the temperature change and strain of the optical fiber are obtained by detecting the optical path change.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, specifically to a large-range quasi-distributed fiber optic sensing method and system, and more specifically to a large-range quasi-distributed fiber optic sensor method and system capable of accurately measuring the optical path between weak reflection points, thereby enabling the measurement of physical quantities such as temperature and strain. Background Technology

[0002] Optical reflectometer technology, based on the phenomena of light scattering and reflection in optical fibers, can measure information such as backscattering and reflection along fiber optic links, thereby obtaining physical quantities such as environmental temperature and strain. By fabricating an array of weak reflection points with a certain reflectivity on the optical fiber, a stronger reflected light signal than backscattered light can be obtained, improving the signal-to-noise ratio and sensitivity of the sensor and realizing a quasi-distributed optical fiber sensing system with a high signal-to-noise ratio. Most quasi-distributed optical fiber sensing systems detect the phase of the beat frequency signal when the light signals of two adjacent weak reflection points interfere. Due to the periodicity of phase change, this approach has a very small dynamic range and is mainly used for the detection of dynamic signals. When using low-reflectivity fiber gratings to create weak reflection points, a wide-range tunable narrow-linewidth laser can be used as the light source to acquire the spectrum of each grating. The detection of dynamic or static physical quantities can be achieved based on the change in the center wavelength of the spectrum. However, the measurement range of this method is limited by the sweep frequency range of the laser. In addition, the phase noise introduced by the nonlinearity of the sweep frequency will seriously affect the measurement accuracy of the system.

[0003] Patent document CN207036249U (application number: 201621320115.X) discloses a high-sensitivity distributed fiber optic vibration sensing system. The system obtained in this invention uses a highly coherent, narrow-linewidth laser as the light source, which is split into two paths by a coupler. One path is modulated into a sequence of optical pulses by a pulse modulator, amplified by an optical amplifier, and then injected into the vibration sensing fiber via a circulator. The backscattered Rayleigh light in the sensing fiber is transmitted back through the circulator and undergoes heterodyne beat frequency interaction with the local oscillator light, which has been frequency-shifted by Δf by a frequency shifter. The beat frequency optical signal is converted into an electrical signal by a detector, and then the data is acquired and processed. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the purpose of this invention is to provide a large-range quasi-distributed optical fiber sensing method and system.

[0005] A large-range quasi-distributed optical fiber sensing system provided by the present invention includes: a linear sweep frequency light source module, a weak reflection point sensing array module, a coherent detection module, and a signal processing module;

[0006] The linearly swept frequency light passes through the linearly swept frequency light source module to output local light and signal light; the signal light enters the weak reflection point array sensing module and is reflected by the reflection points to generate reflected light; the reflected light and the local light enter the coherent detection module together to interfere and obtain interference light, and the coherent detection module converts the interference light signal into an electrical signal; the electrical signal enters the signal processing module to perform Fourier transform to obtain the corresponding spectrum signal; the spectrum signal uses a bandpass filter to select the beat frequency signal corresponding to the weak reflection points A and B at both ends of the target optical fiber, and the current time domain signal is obtained based on the beat frequency signal, thereby obtaining the frequency value of the difference frequency term; the optical path between points A and B is calculated based on the frequency value of the difference frequency term; the temperature change and strain of the optical fiber are obtained by detecting the change in optical path.

[0007] Preferably, the electrical signal adopts:

[0008]

[0009] Where E0 represents the electric field amplitude; M represents the total number of reflection points; R m The reflectivity of the reflection point is represented by τ; k represents the laser sweep rate; τ represents the laser frequency sweep rate. m θ(t) represents the transmission time of the swept light from transmission to reflection at the m-th reflection point on the optical fiber and back to the receiver; f0 represents the initial frequency of the laser; θ(t) represents the phase noise of the laser; and I(t) represents the received electrical signal.

[0010] Preferably, the spectral signal is filtered using a bandpass filter to select the beat frequency signals corresponding to the weak reflection points A and B at both ends of the target optical fiber. The current time-domain signal is obtained based on the beat frequency signals by: filtering the spectral signal using a bandpass filter to extract the spectrum corresponding to the two reflection points, and then performing an inverse Fourier transform to obtain the current time-domain signal I. AB (t); or, after filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, the inverse Fourier transforms are performed on each to obtain the current time-domain signal I. A (t) and I B (t).

[0011] Preferably, after filtering out the spectrum containing the two reflection points using a bandpass filter, an inverse Fourier transform is performed to obtain the current time-domain signal I. AB After (t), the current time-domain signal is squared to obtain Calculated using the Chirp-Z transform The frequency value f of the difference frequency term AB The The mean difference frequency term is:

[0012]

[0013] f AB =κ(τ) B-τ A (3)

[0014] Among them, R d R represents the response coefficient of a photoelectric balanced detector. A With R B Let A and B be the reflectivities of reflection points A and B, respectively; E0 represents the electric field amplitude; k represents the laser sweep rate; τ B τ represents the transmission time of the swept light from its transmission to its reflection at the B-th reflection point on the optical fiber and back to the receiver; A f0 represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; f0 is the initial frequency of the laser. Indicates time (τ) B -τ A Phase noise of the internal laser, The variance is 2πδf(τ) B -τ A ), where δf is the linewidth of the laser.

[0015] Preferably, after filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, inverse Fourier transforms are performed on each to obtain the current time-domain signal I. A (t) and I B After (t), the time-domain signal I A (t) and I B (t) multiplied together yields Calculated using the Chirp-Z transform The frequency value f of the difference frequency term AB The The mean difference frequency term is:

[0016]

[0017] f AB =κ(τ) B -τ A (5);

[0018] Among them, R d R represents the response coefficient of a photoelectric balanced detector. A With R B Let A and B be the reflectivities of reflection points A and B, respectively; E0 represents the electric field amplitude; k represents the laser sweep rate; τ B τ represents the transmission time of the swept light from its transmission to its reflection at the B-th reflection point on the optical fiber and back to the receiving end; A f0 represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; f0 is the initial frequency of the laser. Indicates time (τ) B -τ APhase noise of the internal laser, The variance is 2πδf(τ) B -τ A ), where δf is the linewidth of the laser.

[0019] Preferably, the calculation of the optical path between points A and B based on the frequency value of the difference frequency term uses the following method:

[0020]

[0021] Among them, L AB Indicates optical path length; f AB κ represents the frequency value of the difference frequency term; κ is the sweep rate of the light source; and c is the speed of light in a vacuum.

[0022] Preferably, the method of obtaining the temperature change and strain of the optical fiber by detecting the optical path change is achieved by: obtaining L through multiple measurements. AB Furthermore, using the first set as a reference value, differential calculations are performed with subsequent sets, and the changes in external temperature and stress during the current process are demodulated through optical path change; the frequency changes between the remaining reflection points are calculated in turn, and the corresponding temperature changes and strain magnitudes are measured.

[0023] ΔL AB / L AB =(α+ξ)ΔT+(1-p e )ε (7)

[0024] Where, ΔL AB α represents the change in optical path length; ξ represents the thermal expansion coefficient of the optical fiber; p represents the thermo-optic coefficient. e ε represents the photoelastic coefficient; ε represents stress; ΔT represents the temperature change of the optical fiber.

[0025] Preferably, the linear sweep frequency light source module includes a linear sweep frequency light source and a first fiber coupler;

[0026] The weak reflection point sensing array module includes a weak reflection point array and a third fiber optic circulator.

[0027] The coherent detection module includes a second fiber coupler and a photodetector.

[0028] A large-range quasi-distributed optical fiber sensing method provided by the present invention includes:

[0029] Step S1: The linearly swept frequency light passes through the linearly swept frequency light source module to output local light and signal light;

[0030] Step S2: The signal light enters the weak reflection point array sensing module and is reflected by the reflection points to generate reflected light;

[0031] Step S3: The reflected light and the local light enter the coherent detection module together and interfere to obtain interference light. The coherent detection module converts the interference light signal into an electrical signal.

[0032] Step S4: The electrical signal enters the signal processing module for Fourier transform to obtain the corresponding spectrum signal;

[0033] Step S5: The spectrum signal is used to select the beat frequency signal corresponding to the weak reflection points A and B at both ends of the target optical fiber using a bandpass filter. The current time domain signal is obtained based on the beat frequency signal, thereby obtaining the frequency value of the difference frequency term.

[0034] Step S6: Calculate the optical path between points A and B based on the frequency value of the difference frequency term;

[0035] Step S7: Obtain the temperature change and strain of the optical fiber by detecting the change in optical path length.

[0036] Preferably, the electrical signal in step S3 is:

[0037]

[0038] Where E0 represents the electric field amplitude; M represents the total number of reflection points; R m The reflectivity of the reflection point is represented by τ; k represents the laser sweep rate; τ represents the laser frequency sweep rate. m The transmission time of the swept light from transmission to reflection at the m-th reflection point on the optical fiber and back to the receiver is represented by f0; the initial frequency of the laser is represented by θ(t); the phase noise of the laser is represented by θ(t); and the received electrical signal is represented by I(t).

[0039] Step S5 involves: filtering out the spectrum corresponding to the two reflection points using a bandpass filter from the spectral signal, and then performing an inverse Fourier transform to obtain the current time-domain signal I. AB (t); or, after filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, the inverse Fourier transforms are performed on each to obtain the current time-domain signal I. A (t) and I B (t);

[0040] After filtering out the spectrum containing the two reflection points using a bandpass filter, an inverse Fourier transform is performed to obtain the current time-domain signal I. AB After (t), the current time-domain signal is squared to obtain Calculated using the Chirp-Z transform The frequency value f of the difference frequency term AB The The mean difference frequency term is:

[0041]

[0042] fAB =κ(τ) B -τ A (3)

[0043] Among them, R d R represents the response coefficient of a photoelectric balanced detector. A With R B Let A and B be the reflectivities of reflection points A and B, respectively; E0 represents the electric field amplitude; k represents the laser sweep rate; τ B τ represents the transmission time of the swept light from its transmission to its reflection at the B-th reflection point on the optical fiber and back to the receiving end; A f0 represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; f0 is the initial frequency of the laser. Indicates time (τ) B -τ A Phase noise of the internal laser, The variance is 2πδf(τ) B -τ A ), where δf is the linewidth of the laser;

[0044] After filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, and then performing inverse Fourier transforms on each, the current time-domain signal I is obtained. A (t) and I B After (t), the time-domain signal I A (t) and I B (t) multiplied together yields Calculated using the Chirp-Z transform The frequency value f of the difference frequency term AB The The mid-difference frequency term is:

[0045]

[0046] f AB =κ(τ) B -τ A (5);

[0047] Among them, R d R represents the response coefficient of a photoelectric balanced detector. A With R B Let A and B be the reflectivities of reflection points A and B, respectively; E0 represents the electric field amplitude; k represents the laser sweep rate; τ B τ represents the transmission time of the swept light from its transmission to its reflection at the B-th reflection point on the optical fiber and back to the receiving end; A f0 represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; f0 is the initial frequency of the laser. Indicates time (τ)B -τ A Phase noise of the internal laser, The variance is 2πδf(τ) B -τ A ), where δf is the linewidth of the laser;

[0048] Step S6 employs the following:

[0049]

[0050] Among them, f AB κ represents the frequency value of the difference frequency term; κ is the sweep rate of the light source; and c is the speed of light in a vacuum.

[0051] Step S7 involves obtaining L through multiple measurements. AB Furthermore, using the first set as a reference value, differential calculations are performed with subsequent sets, and the changes in external temperature and stress during the current process are demodulated through optical path change; the frequency changes between the remaining reflection points are calculated in turn, and the corresponding temperature changes and strain magnitudes are measured.

[0052] ΔL AB / L AB =(α+ξ)ΔT+(1-p e )ε (7)

[0053] Where, ΔL AB α represents the change in optical path length; ξ represents the thermal expansion coefficient of the optical fiber; p represents the thermo-optic coefficient. e ε represents the photoelastic coefficient; ε represents stress; ΔT represents the temperature change of the optical fiber.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] 1. This invention combines optical frequency domain reflectometer technology with a weak reflection point array. By monitoring the frequency difference between reflection points on the array, the absolute optical path between reflection points is obtained, thereby realizing the monitoring of changes in the parameter to be measured. A large range and high precision sensing can be achieved using a small-range swept frequency light source.

[0056] 2. In terms of measurement range, the measurement range of this invention is extended to the limits of temperature and strain range that optical fibers can withstand. In contrast, the traditional method based on detecting the phase difference between weak reflection points cannot detect a phase difference exceeding π, resulting in a very small measurement range, which is only applicable to the monitoring of dynamic vibration signals.

[0057] 3. The detection method based on weak fiber grating reflection spectrum has a measurement range that is related to the sweep frequency range of the sweep frequency light source, and a large sweep frequency light source is very expensive. In terms of measurement accuracy, the signal processing method used in this invention can significantly reduce the influence of light source sweep frequency nonlinearity and phase noise, thereby achieving high-precision measurement.

[0058] 4. Significantly improves the measurement range. For example, in the example, the measurement range according to the conventional method is only 5°C, while our method can measure more than 200°C, greatly expanding the measurement range. Attached Figure Description

[0059] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0060] Figure 1 This is a schematic diagram of the measurement system in this embodiment.

[0061] Figure 2 This is a flowchart of the signal demodulation method in this embodiment.

[0062] Figure 3 This is the result of a temperature measurement experiment.

[0063] Figure 4 The temperature measurement accuracy is shown in the example.

[0064] Among them, 1-sweep laser; 9-first fiber coupler; 10-third fiber circulator; 11-weak reflection point array; 12-second fiber coupler; 13-photodetector; 14-data acquisition card; 15-data processing module. Detailed Implementation

[0065] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0066] The purpose of this invention is to overcome the shortcomings of existing solutions and reduce the cost of sensing systems. It proposes a quasi-distributed fiber optic sensing system based on absolute optical path measurement. By measuring the change in absolute optical path between reflection points, it obtains physical quantities such as ambient temperature or strain on the fiber optic segments between these points, offering advantages such as high precision and a large measurement range. This invention employs the principle of an optical frequency domain reflectometer to identify the reflected light signals generated by each weak reflection point. Through signal processing technology, it accurately extracts the optical path length between adjacent reflection points, thereby enabling the detection of strain and temperature.

[0067] Example 1

[0068] A large-range quasi-distributed optical fiber sensing system provided by the present invention includes:

[0069] Linear sweep frequency light source module, weak reflection point sensor array module, coherent detection module, and signal processing module;

[0070] The linearly swept frequency light passes through the linearly swept frequency light source module to output local light and signal light; the signal light enters the weak reflection point array sensing module and is reflected by the reflection points to generate reflected light; the reflected light and the local light enter the coherent detection module together to interfere and obtain interference light, and the coherent detection module converts the interference light signal into an electrical signal; the electrical signal enters the signal processing module to perform Fourier transform to obtain the corresponding spectrum signal; the spectrum signal uses a bandpass filter to select the beat frequency signal corresponding to the weak reflection points A and B at both ends of the target optical fiber, and the current time domain signal is obtained based on the beat frequency signal, thereby obtaining the frequency value of the difference frequency term; the optical path between points A and B is calculated based on the frequency value of the difference frequency term; the temperature change and strain of the optical fiber are obtained by detecting the change in optical path.

[0071] Specifically, the electrical signal adopts:

[0072]

[0073] Where E0 represents the electric field amplitude; M represents the total number of reflection points; R m The reflectivity of the reflection point is represented by τ; k represents the laser sweep rate; τ represents the laser frequency sweep rate. m θ(t) represents the transmission time of the swept light from transmission to reflection at the m-th reflection point on the optical fiber and back to the receiver; f0 represents the initial frequency of the laser; θ(t) represents the phase noise of the laser; and I(t) represents the received electrical signal.

[0074] Specifically, the spectral signal is filtered using a bandpass filter to select the beat frequency signals corresponding to weak reflection points A and B at both ends of the target optical fiber. The current time-domain signal is obtained based on the beat frequency signals by: filtering the spectral signal using a bandpass filter to extract the spectrum corresponding to the two reflection points, and then performing an inverse Fourier transform to obtain the current time-domain signal I. AB (t); or, after filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, the inverse Fourier transforms are performed on each to obtain the current time-domain signal I. A (t) and I B (t).

[0075] Specifically, after filtering out the spectrum containing the two reflection points using a bandpass filter, an inverse Fourier transform is performed to obtain the current time-domain signal I. AB After (t), the current time-domain signal is squared to obtain Calculated using the Chirp-Z transform The frequency value f of the difference frequency term AB The The mean difference frequency term is:

[0076]

[0077] f AB =κ(τ) B -τ A (3)

[0078] Among them, R d R represents the response coefficient of a photoelectric balanced detector. A With R B Let A and B be the reflectivities of reflection points A and B, respectively; E0 represents the electric field amplitude; k represents the laser sweep rate; τ B τ represents the transmission time of the swept light from its transmission to its reflection at the B-th reflection point on the optical fiber and back to the receiving end; A f0 represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; f0 is the initial frequency of the laser. Indicates time (τ) B -τ A Phase noise of the internal laser, The variance is 2πδf(τ) B -τ A ), where δf is the linewidth of the laser.

[0079] Specifically, after filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, and then performing inverse Fourier transforms on each, the current time-domain signal I is obtained. A (t) and I B After (t), the time-domain signal I A (t) and I B (t) multiplied together yields Calculated using the Chirp-Z transform The frequency value f of the difference frequency term AB The The mid-difference frequency term is:

[0080]

[0081] f AB =κ(τ) B -τ A (5);

[0082] Among them, R d R represents the response coefficient of a photoelectric balanced detector. A With R B Let A and B be the reflectivities of reflection points A and B, respectively; E0 represents the electric field amplitude; k represents the laser sweep rate; τ B τ represents the transmission time of the swept light from its transmission to its reflection at the B-th reflection point on the optical fiber and back to the receiving end; Af0 represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; f0 is the initial frequency of the laser. Indicates time (τ) B -τ A Phase noise of the internal laser, The variance is 2πδf(τ) B -τ A ), where δf is the linewidth of the laser.

[0083] Specifically, the calculation of the optical path between points A and B based on the frequency value of the difference frequency term uses the following method:

[0084]

[0085] Among them, f AB κ represents the frequency value of the difference frequency term; κ is the sweep rate of the light source; and c is the speed of light in a vacuum.

[0086] Specifically, the method of obtaining the temperature change and strain of the optical fiber by detecting the optical path change is achieved by: obtaining L through multiple measurements. AB Furthermore, using the first set as a reference value, differential calculations are performed with subsequent sets, and the changes in external temperature and stress during the current process are demodulated through optical path change; the frequency changes between the remaining reflection points are calculated in turn, and the corresponding temperature changes and strain magnitudes are measured.

[0087] ΔL AB / L AB =(α+ξ)ΔT+(1-p e )ε (7)

[0088] Where, ΔL AB α represents the change in optical path length; ξ represents the thermal expansion coefficient of the optical fiber; p represents the thermo-optic coefficient. e ε represents the photoelastic coefficient; ε represents stress; ΔT represents the temperature change of the optical fiber.

[0089] Specifically, the linear sweep frequency light source module includes a linear sweep frequency light source and a first fiber coupler, which output signal light and local light respectively; the linear sweep frequency light source can be an internally modulated sweep frequency light source or an externally modulated sweep frequency laser light source generated based on a frequency-stabilized laser and a modulator.

[0090] The weak reflection point sensing array module includes a weak reflection point array and a third fiber optic circulator, and has two ports: input and output.

[0091] The coherent detection module includes a second fiber optic coupler and a photodetector, and includes two optical input ports and an electrical signal output port; wherein the two optical input ports are respectively connected to the local light output by the linear sweep frequency light source module and the reflected light output by the weak reflection point sensor array module.

[0092] The signal processing module includes a data acquisition card and a data processing module.

[0093] A large-range quasi-distributed optical fiber sensing method provided by the present invention includes:

[0094] Step S1: The linearly swept frequency light passes through the linearly swept frequency light source module to output local light and signal light;

[0095] Step S2: The signal light enters the weak reflection point array sensing module and is reflected by the reflection points to generate reflected light;

[0096] Step S3: The reflected light and the local light enter the coherent detection module together and interfere to obtain interference light. The coherent detection module converts the interference light signal into an electrical signal.

[0097] Step S4: The electrical signal enters the signal processing module for Fourier transform to obtain the corresponding spectrum signal;

[0098] Step S5: The spectrum signal is used to select the beat frequency signal corresponding to the weak reflection points A and B at both ends of the target optical fiber using a bandpass filter. The current time domain signal is obtained based on the beat frequency signal, thereby obtaining the frequency value of the difference frequency term.

[0099] Step S6: Calculate the optical path between points A and B based on the frequency value of the difference frequency term;

[0100] Step S7: Obtain the temperature change and strain of the optical fiber by detecting the change in optical path length.

[0101] Specifically, the electrical signal used in step S3 is:

[0102]

[0103] Where E0 represents the electric field amplitude; M represents the total number of reflection points; R m The reflectivity of the reflection point is represented by τ; k represents the laser sweep rate; τ represents the laser frequency sweep rate. m θ(t) represents the transmission time of the swept light from transmission to reflection at the m-th reflection point on the optical fiber and back to the receiver; f0 represents the initial frequency of the laser; θ(t) represents the phase noise of the laser; and I(t) represents the received electrical signal.

[0104] Specifically, step S5 involves: filtering out the spectrum corresponding to the two reflection points using a bandpass filter from the spectral signal, and then performing an inverse Fourier transform to obtain the current time-domain signal I. AB (t); or, after filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, the inverse Fourier transforms are performed on each to obtain the current time-domain signal I. A (t) and I B (t).

[0105] Specifically, after filtering out the spectrum containing the two reflection points using a bandpass filter, an inverse Fourier transform is performed to obtain the current time-domain signal I. AB After (t), the current time-domain signal is squared to obtain Calculated using the Chirp-Z transform The frequency value f of the difference frequency term AB The The mean difference frequency term is:

[0106]

[0107] f AB =κ(τ) B -τ A (3)

[0108] Among them, R d R represents the response coefficient of a photoelectric balanced detector. A With R B Let A and B be the reflectivities of reflection points A and B, respectively; E0 represents the electric field amplitude; k represents the laser sweep rate; τ B τ represents the transmission time of the swept light from its transmission to its reflection at the B-th reflection point on the optical fiber and back to the receiving end; A f0 represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; f0 is the initial frequency of the laser. Indicates time (τ) B -τ A Phase noise of the internal laser, The variance is 2πδf(τ) B -τ A ), where δf is the linewidth of the laser.

[0109] Specifically, after filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, and then performing inverse Fourier transforms on each, the current time-domain signal I is obtained. A (t) and I B After (t), the time-domain signal I A (t) and I B (t) multiplied together yields Calculated using the Chirp-Z transform The frequency value f of the difference frequency term AB The The mid-difference frequency term is:

[0110]

[0111] f AB =κ(τ) B -τ A (5);

[0112] Among them, R d R represents the response coefficient of a photoelectric balanced detector. A With R B Let A and B be the reflectivities of reflection points A and B, respectively; E0 represents the electric field amplitude; k represents the laser sweep rate; τ B τ represents the transmission time of the swept light from its transmission to its reflection at the B-th reflection point on the optical fiber and back to the receiving end; A f0 represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; f0 is the initial frequency of the laser. Indicates time (τ) B -τ A Phase noise of the internal laser, The variance is 2πδf(τ) B -τ A ), where δf is the linewidth of the laser.

[0113] Specifically, step S6 employs the following:

[0114]

[0115] Among them, f AB κ represents the frequency value of the difference frequency term; κ is the sweep rate of the light source; and c is the speed of light in a vacuum.

[0116] Specifically, step S7 involves obtaining L through multiple measurements. AB Furthermore, using the first set as a reference value, differential calculations are performed with subsequent sets, and the changes in external temperature and stress during the current process are demodulated through optical path change; the frequency changes between the remaining reflection points are calculated in turn, and the corresponding temperature changes and strain magnitudes are measured.

[0117] ΔL AB / L AB =(α+ξ)ΔT+(1-p e )ε (7)

[0118] Where, ΔL AB α represents the change in optical path length; ξ represents the thermal expansion coefficient of the optical fiber; p represents the thermo-optic coefficient. e ε represents the photoelastic coefficient; ε represents stress; ΔT represents the temperature change of the optical fiber.

[0119] Specifically, the linear sweep frequency light source module includes a linear sweep frequency light source and a first fiber coupler, which output signal light and local light respectively; the linear sweep frequency light source can be an internally modulated sweep frequency light source or an externally modulated sweep frequency laser light source generated based on a frequency-stabilized laser and a modulator.

[0120] The weak reflection point sensing array module includes a weak reflection point array and a third fiber optic circulator, and has two ports: input and output.

[0121] The coherent detection module includes a second fiber optic coupler and a photodetector, and includes two optical input ports and an electrical signal output port; wherein the two optical input ports are respectively connected to the local light output by the linear sweep frequency light source module and the reflected light output by the weak reflection point sensor array module.

[0122] The signal processing module includes a data acquisition card and a data processing module.

[0123] Example 2

[0124] Example 2 is a preferred example of Example 1.

[0125] According to the present invention, a large-range quasi-distributed sensing method includes: identifying the reflected light signals generated by each weak reflection point using the principle of an optical frequency domain reflectometer, and accurately extracting the optical path length between adjacent reflection points through signal processing technology, thereby realizing the detection of strain and temperature. Specifically, the linearly swept laser light is split into two paths by a back-end fiber coupler: one is local light, and the other is signal light, which enter the weak reflection point array. The current signal obtained after the interference of the reflected light generated by the local light and signal light through the weak reflection point array with the local light is collected, and a Fourier transform is performed to obtain its frequency domain signal. According to the principle of the optical frequency domain reflectometer, each reflection point corresponds to a different beat frequency signal. Using the optical fiber between any two adjacent weak reflection points A and B on the weak reflection point array as the sensing unit, a bandpass filter is used to simultaneously select the beat frequency signals corresponding to weak reflection points A and B at both ends of the target optical fiber. An inverse Fourier transform is performed to obtain the current time domain signal. The current signal is squared in the time domain, and the frequency value of the squared signal at the difference frequency is detected, which corresponds to the temperature or strain information on the sensing unit.

[0126] The specific method is as follows: Step 1: The linearly swept frequency light is split into two paths after passing through an optical fiber coupler. One path serves as the local light, and the other path serves as the signal light, entering the weak reflection point array. The optical fiber between two weak reflection points A and B in the weak reflection point array is used as the sensing unit. Using the local light as the reference, the optical path lengths corresponding to the two weak reflection points are denoted as Z. A With Z B The current signal obtained by the interference of reflected light generated by the weak reflection point array between the local light and the signal light is collected, and its frequency domain signal is obtained by Fourier transform. According to the principle of the optical frequency domain reflectometer, each reflection point corresponds to a different beat frequency signal.

[0127] Step 2: Using a bandpass filter, select the beat frequency signal corresponding only to the weak reflection points A and B at both ends of the target fiber. Obtain the current time-domain signal through inverse Fourier transform. Square the current signal in the time domain, and then detect the frequency value of the squared signal at the difference frequency, denoted as f. AB Another way to implement this step is to use a bandpass filter to select the beat frequency signals corresponding to the weak reflection points A and B at both ends of the target optical fiber, respectively, and then perform inverse Fourier transform to obtain the current time-domain signals. The two current signals are then multiplied in the time domain, and the frequency value of the multiplied signal at the difference frequency, i.e., f... AB Based on the frequency difference f AB The optical path Z between points A and B B -Z A Relationship That is, the optical path L between points A and B can be obtained. AB , where κ is the sweep rate of the light source and c is the speed of light in a vacuum.

[0128] Step 3: When the external temperature changes by ΔT or there is a strain ε between points A and B, the optical path difference L increases due to the thermo-optical effect and the photoelastic effect. AB The change will occur, and the amount of change is denoted as ΔL. AB ΔL AB Satisfying ΔL AB / L AB =(α+ξ)ΔT+(1-p e )ε, where α is the thermal expansion coefficient of the optical fiber, ξ is the thermo-optical coefficient, and p e Let be the photoelastic coefficient. Therefore, by detecting the change in optical path ΔL... AB The temperature change ΔT and strain ε of the optical fiber can then be obtained.

[0129] Example 3

[0130] Example 3 is a preferred example of Example 1.

[0131] like Figure 1 As shown, the present invention provides a large-range quasi-distributed optical fiber sensing system, including: a swept-frequency laser 1, a first optical fiber coupler 9, a third optical fiber circulator 10, a weak reflection point array 11, a second optical fiber coupler 12, a photodetector 13, a data acquisition card 14, and a data processing module 15.

[0132] Wherein: the swept-frequency laser 1 is connected to the first port a of the first fiber coupler 9, the second port b of the first fiber coupler 9 is connected to the second port b of the second fiber coupler 12, the third port c of the first fiber coupler 9 is connected to the first port a of the third fiber circulator 10, the third port c of the third fiber circulator 10 is connected to the first port a of the second fiber coupler 12, the second port b of the third fiber circulator 10 is connected to the weak reflection point array 11, the third port c and the fourth port d of the second fiber coupler 12 are both connected to the balanced photodetector 13, and the balanced photodetector 13 is connected to the data acquisition card 14 and the data processing module 15 in sequence.

[0133] The first fiber optic coupler is a 90:10 fiber optic coupler, and the second fiber optic coupler is a 50:50 fiber optic coupler.

[0134] The weak reflection point array is a weak reflection point array with low reflectivity, which is made by point-by-point writing using femtosecond laser processing technology. The reflectivity of each reflection point is almost the same for incident light of different wavelengths.

[0135] The weak reflection point array can be replaced with a weak fiber grating array with a wider reflection spectrum.

[0136] The data acquisition card 14 has a sampling rate of 250 MSPS and a quantization resolution of 8 bits.

[0137] The bandwidth of the balanced photodetector 13 is AC-400MHz.

[0138] This embodiment discloses a large-range quasi-distributed optical fiber sensing method and system, one specific implementation is as follows: Figure 2 As shown.

[0139] In each measurement, the linear sweep frequency light source module outputs local light and signal light. The signal light enters the weak reflection point array sensing module, and after reflection by the reflection points, it generates reflected light. The reflected light and local light enter the coherent detection module together and interfere. The coherent detection module converts the interference light signal into an electrical signal I(t). This electrical signal I(t) is acquired by the data acquisition card 14 and converted into a digital signal. The signal processing module 15 performs a Fourier transform on the electrical signal I(t) to obtain its spectrum signal S(f).

[0140] From the spectral signal S(f), use a bandpass filter to filter out the spectral components S(f) that simultaneously contain the corresponding reflection points A and B. A ) and S(f B The filtered signal is subjected to an inverse Fourier transform to obtain the time-domain signal I. l (t). Squaring this time-domain signal yields... The signal Includes S(f)A ) and S(f B The frequency components such as harmonics, combination frequencies, difference frequencies, and DC components introduced by this process have an impact on... The signal is subjected to chirp-z transform, and the result is calculated. The frequency value f of the difference frequency term AB According to the frequency difference f AB The optical path Z between points A and B B -Z A Relationship That is, the optical path L between points A and B can be obtained. AB Where κ is the sweep rate of the light source, and c is the speed of light in vacuum. L was obtained through multiple measurements. AB Furthermore, using the first set as a reference value, differential calculations are performed with subsequent sets. By measuring the change in optical path length, the changes in external temperature and stress during this process can be demodulated. The changes in optical path length between the remaining reflection points are calculated sequentially, and the corresponding temperature changes and strain magnitudes are measured.

[0141] The output photoelectric field of the linear sweep frequency optical module is: E(t)=E0exp[j(2πf0t+πκt)] 2 +θ(t))];where E0 is the electric field amplitude; f0 is the initial frequency of the laser; θ(t) is the phase noise of the laser. k represents the sweep rate of the linear sweep frequency source;

[0142] The reflected light signal I(t) is:

[0143]

[0144] Where, τ m R is the transmission time of the swept-frequency light from its transmission point to its reflection at the m-th reflection point on the optical fiber and back to the receiver; m Let be the reflectivity of the reflecting point. M represents the total number of reflecting points.

[0145] The The mid-difference frequency term is:

[0146]

[0147] f AB =κ(τ) B -τ A );

[0148] Through specific practical experiments, a weak reflection point array with a total length of 1km, a spacing of 10m, and an average reflectivity of -45dB was used as the measurement medium, containing 100 reflection points. Approximately 100 meters of the weak reflection point array (reflection points 7 to 17) was placed in a heating chamber, while the remaining weak reflection point array was placed in a temperature- and vibration-isolated chamber. The modulated optical signal had a frequency sweep range of 5GHz and a duration of 1ms. Running the above method, the following results were obtained: Figure 3 The measurement results, from top to bottom, represent curves measured at 120℃, 100℃, 80℃, 60℃, 44℃, and room temperature (20℃). Placing all optical fibers in a temperature- and vibration-isolated chamber allows for the acquisition of... Figure 4 The measurement results show that the temperature measurement accuracy is 0.3℃.

[0149] Compared with existing technologies, this example achieves array-based large-scale measurement of temperature and strain based on a small-range swept frequency light source. The highest measurable temperature and strain are the highest temperatures and strains that optical fibers can withstand. The temperature measurement accuracy is better than 0.3℃. If this scheme is used for strain measurement, based on the conversion factor between the optical path difference of standard single-mode optical fiber and strain, the strain measurement accuracy can reach 2.6με, and the scale of the measurement array can reach hundreds.

[0150] Those skilled in the art will understand that, in addition to implementing the system, apparatus, and their modules provided by this invention in purely computer-readable program code, the same program can be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system, apparatus, and their modules provided by this invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; alternatively, modules for implementing various functions can be considered both software programs implementing the method and structures within the hardware component.

[0151] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A large range quasi-distributed optical fiber sensing system, characterized in that, include: Linear sweep frequency light source module, weak reflection point sensor array module, coherent detection module, and signal processing module; The linearly swept frequency light passes through the linearly swept frequency light source module, outputting local light and signal light. The signal light enters the weak reflection point array sensing module and is reflected by the reflection points to generate reflected light. The reflected light and local light enter the coherent detection module together and interfere to obtain interference light. The coherent detection module converts the interference light signal into an electrical signal. The electrical signal enters the signal processing module and undergoes Fourier transform to obtain the corresponding spectrum signal. The spectrum signal is used with a bandpass filter to select the beat frequency signal corresponding to the weak reflection points A and B at both ends of the target optical fiber. Based on the beat frequency signal, the current time domain signal is obtained, thereby obtaining the frequency value of the difference frequency term. The optical path between points A and B is calculated based on the frequency value of the difference frequency term. The temperature change and strain of the optical fiber are obtained by detecting the change in optical path. The electrical signal uses: （1） in, M represents the electric field amplitude; M represents the total number of reflection points. Indicates the reflectivity of the reflection point; Indicates the laser sweep rate; This represents the transmission time of the swept light from transmission to reflection at the m-th reflection point on the optical fiber and back to the receiving end; Indicates the initial frequency of the laser; Indicates laser phase noise; This indicates the received electrical signal; The spectral signal is filtered using a bandpass filter to select the beat frequency signals corresponding to weak reflection points A and B at both ends of the target optical fiber. The current time-domain signal is then obtained based on the beat frequency signals by performing an inverse Fourier transform on the spectral signal to extract the spectra corresponding to the two reflection points. Alternatively, the current time-domain signal can be obtained by filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, and then performing inverse Fourier transforms on each of them. and ; After filtering out the spectrum corresponding to the two reflection points using a bandpass filter, an inverse Fourier transform is performed to obtain the current time-domain signal. Then, the current time-domain signal is squared to obtain... Calculated by chirp-z transform Frequency value of the middle difference frequency term The The mean difference frequency term is: （2） （3） in, The response coefficient of the photoelectric balance detector is represented. and The reflectances of reflection points A and B are respectively. Indicates the amplitude of the electric field; Indicates the laser sweep rate; This represents the transmission time of the swept light from transmission to reflection at the Bth reflection point on the optical fiber and back to the receiving end; This represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; The initial frequency of the laser; Indicates time ( Phase noise of the internal laser, The variance is ,in The linewidth of the laser; After filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, inverse Fourier transforms are performed on each to obtain the current time-domain signal. and Then, the time-domain signal and Multiplying together Calculated by chirp-z transform Frequency value of the middle difference frequency term The The mean difference frequency term is: （4） （5）； in, The response coefficient of the photoelectric balance detector is represented. and The reflectances of reflection points A and B are respectively. Indicates the amplitude of the electric field; Indicates the laser sweep rate; This represents the transmission time of the swept light from transmission to reflection at the Bth reflection point on the optical fiber and back to the receiving end; This represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; The initial frequency of the laser; Indicates time ( Phase noise of the internal laser, The variance is ,in The linewidth of the laser; The method for calculating the optical path between points A and B based on the frequency value of the difference frequency term is as follows: （6） in, Indicates optical path length; This represents the frequency value of the difference frequency term; The sweep rate of the light source. The speed of light in a vacuum; The temperature change and strain of the optical fiber are obtained by detecting the change in optical path length using multiple measurements. Furthermore, using the first set as a reference value, differential calculations are performed with subsequent sets, and the changes in external temperature and stress during the current process are demodulated through optical path change; the frequency changes between the remaining reflection points are calculated in turn, and the corresponding temperature changes and strain magnitudes are measured. （7） in, Indicates the change in optical path length; This represents the coefficient of thermal expansion of the optical fiber; Indicates the thermo-optic coefficient; Indicates the photoelastic coefficient; Indicates stress; This indicates the amount of temperature change in the optical fiber.

2. The large-range quasi-distributed optical fiber sensing system according to claim 1, characterized in that, The linear sweep frequency light source module includes a linear sweep frequency light source and a first fiber optic coupler; The weak reflection point sensing array module includes a weak reflection point array and a third fiber optic circulator. The coherent detection module includes a second fiber coupler and a photodetector.

3. A large-range quasi-distributed optical fiber sensing method, characterized in that, include: Step S1: The linearly swept frequency light passes through the linearly swept frequency light source module to output local light and signal light; Step S2: The signal light enters the weak reflection point array sensing module and is reflected by the reflection points to generate reflected light; Step S3: The reflected light and the local light enter the coherent detection module together and interfere to obtain interference light. The coherent detection module converts the interference light signal into an electrical signal. Step S4: The electrical signal enters the signal processing module for Fourier transform to obtain the corresponding spectrum signal; Step S5: The spectrum signal is used to select the beat frequency signal corresponding to the weak reflection points A and B at both ends of the target optical fiber using a bandpass filter. The current time domain signal is obtained based on the beat frequency signal, thereby obtaining the frequency value of the difference frequency term. Step S6: Calculate the optical path between points A and B based on the frequency value of the difference frequency term; Step S7: Obtain the temperature change and strain of the optical fiber by detecting the change in optical path length; The electrical signal used in step S3 is: （1） in, M represents the electric field amplitude; M represents the total number of reflection points. Indicates the reflectivity of the reflection point; Indicates the laser sweep rate; This represents the transmission time of the swept light from transmission to reflection at the m-th reflection point on the optical fiber and back to the receiving end; Indicates the initial frequency of the laser; Indicates laser phase noise; This indicates the received electrical signal; Step S5 involves: filtering out the spectrum corresponding to the two reflection points using a bandpass filter from the spectral signal, and then performing an inverse Fourier transform to obtain the current time-domain signal. Alternatively, the current time-domain signal can be obtained by filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, and then performing inverse Fourier transforms on each of them. and ; After filtering out the spectrum corresponding to the two reflection points using a bandpass filter, an inverse Fourier transform is performed to obtain the current time-domain signal. Then, the current time-domain signal is squared to obtain... Calculated by chirp-z transform Frequency value of the middle difference frequency term The The mean difference frequency term is: （2） （3） in, The response coefficient of the photoelectric balance detector is represented. and The reflectances of reflection points A and B are respectively. Indicates the amplitude of the electric field; Indicates the laser sweep rate; This represents the transmission time of the swept light from transmission to reflection at the Bth reflection point on the optical fiber and back to the receiving end; This represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; The initial frequency of the laser; Indicates time ( Phase noise of the internal laser, The variance is ,in The linewidth of the laser; After filtering out the spectra corresponding to the two reflection points A and B using bandpass filters, inverse Fourier transforms are performed on each to obtain the current time-domain signal. and Then, the time-domain signal and Multiplying together Calculated by chirp-z transform Frequency value of the middle difference frequency term The The mid-difference frequency term is: （4） （5）； in, The response coefficient of the photoelectric balance detector is represented. and The reflectances of reflection points A and B are respectively. Indicates the amplitude of the electric field; Indicates the laser sweep rate; This represents the transmission time of the swept light from transmission to reflection at the Bth reflection point on the optical fiber and back to the receiving end; This represents the transmission time of the swept light from transmission to reflection at the A-th reflection point on the optical fiber and back to the receiving end; The initial frequency of the laser; Indicates time ( Phase noise of the internal laser, The variance is ,in The linewidth of the laser; Step S6 employs the following: （6） in, This represents the frequency value of the difference frequency term; The sweep rate of the light source. The speed of light in a vacuum; Step S7 involves obtaining data through multiple measurements. Furthermore, using the first set as a reference value, differential calculations are performed with subsequent sets, and the changes in external temperature and stress during the current process are demodulated through optical path change; the frequency changes between the remaining reflection points are calculated in turn, and the corresponding temperature changes and strain magnitudes are measured. （7） in, Indicates the change in optical path length; This represents the coefficient of thermal expansion of the optical fiber; Indicates the thermo-optic coefficient; Indicates the photoelastic coefficient; Indicates stress; This indicates the amount of temperature change in the optical fiber.

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