A kilometer-scale OFDR distributed sensing system and its demodulation method and apparatus

Abstract

The application discloses a demodulation method for a kilometer-level OFDR distributed sensing system, comprising the following steps: obtaining beat frequency signals of a main interferometer and beat frequency signals of an auxiliary interferometer; processing the beat frequency signals of the auxiliary interferometer to obtain phase information of the auxiliary interferometer; performing equal-phase interval resampling on the beat frequency signals of the auxiliary interferometer according to the phase information of the auxiliary interferometer to obtain an equal-phase signal; estimating a time delay size of the auxiliary interferometer according to the equal-phase signal; and performing nonlinear compensation on the beat frequency signals of the main interferometer by using a desmearing filter according to the time delay size of the auxiliary interferometer. The application discloses a kilometer-level OFDR distributed sensing system and a demodulation device thereof, which are based on the above-mentioned demodulation method.

Description

Technical Field

[0001] This invention relates to optical frequency domain sensing technology, and more particularly to a kilometer-scale OFDR distributed sensing system and its demodulation method and apparatus. Background Technology

[0002] Optical Frequency Domain Reflectometry (OFDR), a high-performance distributed fiber optic sensing technology, has attracted widespread attention and in-depth research from academic and engineering communities both domestically and internationally in recent years due to its ultra-high spatial resolution and excellent sensing accuracy. This technology has shown great potential in large-scale structural health monitoring, large-scale strain measurement, and real-time monitoring in complex environments, and has been practically applied and validated in various fields such as aerospace, civil engineering, and energy. However, the application of OFDR in long-distance sensing is greatly limited by the phase noise introduced by the nonlinear frequency sweep of the tunable light source, and this phase noise gradually accumulates with increasing sensing distance.

[0003] To address the nonlinear phase compensation problem of OFDR, existing technologies add an auxiliary interferometer to the optical frequency domain reflectometer. The phase information of the auxiliary interferometer is used to perform nonlinear compensation on the beat frequency signal of the main interferometer to improve spatial resolution. However, the sensing distance is still affected by the length of the time-delay fiber in the auxiliary interferometer, limiting the sensing distance to the order of tens of meters.

[0004] The deskewing filter compresses the sweep frequency signal, reducing it from a wide pulse signal to a narrow pulse signal, thereby improving spatial resolution and sensing accuracy, enabling sensing distances on the order of kilometers. However, when using the deskewing filter, the time delay of the auxiliary interferometer needs to be estimated. Inaccurate estimation of the time delay will drastically degrade the spatial resolution of the kilometer-level sensing, significantly impacting the nonlinear compensation effect.

[0005] Currently, the estimation of the time delay of the auxiliary interferometer does not take into account the nonlinear sweep frequency interference present in the beat frequency signal of the auxiliary interferometer, leading to a large estimation error in the time delay. Furthermore, the traditional descrambling filter needs further optimization. Summary of the Invention

[0006] To address the shortcomings of the prior art, this invention provides a kilometer-scale OFDR distributed sensing system and its demodulation method and apparatus, which can improve the estimation accuracy of the time delay of the auxiliary interferometer.

[0007] The technical problem to be solved by the present invention is achieved through the following technical solution:

[0008] A demodulation method for a kilometer-scale OFDR distributed sensing system includes the following steps:

[0009] Step 100: Acquire the beat frequency signal of the main interferometer and the beat frequency signal of the auxiliary interferometer;

[0010] Step 200: Process the beat frequency signal of the auxiliary interferometer to obtain the phase information of the auxiliary interferometer;

[0011] Step 300: Based on the phase information of the auxiliary interferometer, the beat frequency signal of the auxiliary interferometer is resampled at equal phase intervals to obtain an equal phase signal;

[0012] Step 400: Estimate the time delay of the auxiliary interferometer based on the equal-phase signal;

[0013] Step 500: Based on the time delay of the auxiliary interferometer, a deskewing filter is used to perform nonlinear compensation on the beat frequency signal of the main interferometer.

[0014] Furthermore, in step 200, when processing the beat frequency signal of the auxiliary interferometer to obtain the phase information of the auxiliary interferometer, the following steps are included:

[0015] Step 210: Perform Hilbert transform on the beat frequency signal of the auxiliary interferometer to obtain the complex signal of the auxiliary interferometer;

[0016] Step 220: Perform forward and reverse tangent and unwinding processes on the complex signals of the auxiliary interferometer in sequence to obtain the phase information of the auxiliary interferometer.

[0017] Furthermore, in step 400, when estimating the time delay of the auxiliary interferometer based on the equal-phase signal, the following steps are included:

[0018] Step 410: Perform a fast Fourier transform on the isophase signal to obtain its range domain spectrum;

[0019] Step 420: In the distance domain spectrum of the equal-phase signal, the peak-finding algorithm is used to determine the end reflection peak, and the fiber length of the time-delay fiber is determined according to the fiber position of the end reflection peak.

[0020] Step 430: Calculate the time delay of the auxiliary interferometer based on the fiber length of the time delay fiber.

[0021] Furthermore, in step 500, when performing nonlinear compensation on the beat frequency signal of the main interferometer using a descrambling filter based on the time delay of the auxiliary interferometer, the following steps are included:

[0022] Step 510: Estimate the local oscillator nonlinear phase of the main interferometer based on the time delay of the auxiliary interferometer;

[0023] Step 520: Construct the conjugate phase factor based on the estimated local oscillator nonlinear phase;

[0024] Step 530: Based on the conjugate phase factor, eliminate the local oscillator nonlinear phase in the beat frequency signal of the main interferometer to obtain the beat frequency signal of the main interferometer after one compensation.

[0025] Step 540: Construct the deskewing filter, and filter the beat frequency signal of the main interferometer after the first compensation through the deskewing filter to obtain the beat frequency signal of the main interferometer after the second compensation.

[0026] Furthermore, in step 510, when estimating the local oscillator nonlinear phase of the main interferometer based on the time delay of the auxiliary interferometer, the following steps are included:

[0027] Step 511: Perform linear regression on the phase information of the auxiliary interferometer to obtain the linear phase of the auxiliary interferometer;

[0028] Step 512: Subtract the linear phase from the phase information of the auxiliary interferometer to obtain the nonlinear phase of the auxiliary interferometer;

[0029] Step 513: Perform a third-order Taylor expansion and integration on the nonlinear phase of the auxiliary interferometer to obtain the local oscillator nonlinear phase of the main interferometer.

[0030] Furthermore, in step 530, when eliminating the local oscillator nonlinear phase in the beat frequency signal of the main interferometer according to the conjugate phase factor to obtain the beat frequency signal of the main interferometer after one compensation, the following steps are included:

[0031] Step 531: Perform Hilbert transform on the beat frequency signal of the main interferometer and rewrite its complex signal into exponential form;

[0032] Step 532: Multiply the complex signal of the main interferometer by the conjugate phase factor to eliminate the local oscillator nonlinear phase, and obtain the beat frequency signal of the main interferometer after one compensation.

[0033] Furthermore, in step 500, when performing nonlinear compensation on the beat frequency signal of the main interferometer using a descrambling filter based on the time delay of the auxiliary interferometer, the following steps are also included:

[0034] Step 550: Multiply the beat frequency signal of the main interferometer after secondary compensation by the conjugate phase factor to obtain the beat frequency signal of the main interferometer after tertiary compensation.

[0035] Furthermore, the demodulation method also includes the following steps:

[0036] Step 600: Demodulate the compensated beat frequency signal of the main interferometer to obtain the measurement distribution curve of the measurement fiber.

[0037] Furthermore, the beat frequency signal of the main interferometer includes a reference beat frequency signal and a measured beat frequency signal. In step 600, when demodulating the compensated beat frequency signal of the main interferometer to obtain the measurement distribution curve of the measurement fiber, the following steps are included:

[0038] Step 610: Perform Fast Fourier Transform on the compensated reference beat frequency signal and the measured beat frequency signal respectively to obtain the reference range domain spectrum and the measured range domain spectrum;

[0039] Step 620: Set the sliding window size and sliding window step distance, and perform sliding window values ​​on the reference range domain spectrum and the measurement range domain spectrum to obtain multiple reference local spectra and multiple measurement range domain spectra;

[0040] Step 630: Perform cross-correlation calculations on the reference local spectrum and the measured local spectrum at the same fiber location to obtain multiple cross-correlation peaks;

[0041] Step 640: Segment the cross-correlation peaks at different fiber locations to obtain a cross-correlation image;

[0042] Step 650: Apply a two-dimensional Gaussian filter to the cross-correlation image;

[0043] Step 660: Based on the relationship curve between the measured parameters and the cross-correlation peaks, convert the filtered cross-correlation image into the corresponding measurement distribution curve.

[0044] A demodulation device for a kilometer-scale OFDR distributed sensing system includes a processor and a memory connected together. The memory stores a computer program that the processor can execute. When the processor executes the computer program, it performs the aforementioned demodulation method.

[0045] A kilometer-scale OFDR distributed sensing system includes a data acquisition device, an optical frequency domain reflectometer, a measurement optical fiber, and the aforementioned demodulation device. The sensing end of the optical frequency domain reflectometer is connected to the measurement optical fiber, and the output end of the optical frequency domain reflectometer is connected to the data acquisition device. The data acquisition device is connected to the demodulation device. The optical frequency domain reflectometer includes a tunable light source, a first fiber coupler, an auxiliary interferometer, a main interferometer, a first photoelectric balanced detector, a second photoelectric balanced detector, and a third photoelectric balanced detector. The tunable light source is connected to the auxiliary interferometer and the main interferometer respectively through the first fiber coupler. The auxiliary interferometer is connected to the first photoelectric balanced detector, and the main interferometer is connected to the second photoelectric balanced detector and the third photoelectric balanced detector. The first, second, and third photoelectric balanced detectors are all connected to the data acquisition device. The auxiliary interferometer includes a time-delay optical fiber.

[0046] The present invention has the following beneficial effects: The demodulation method of the present invention is used in a kilometer-level OFDR distributed sensing system. It extracts the phase information of the auxiliary interferometer and performs equal-phase interval resampling of the beat frequency signal of the auxiliary interferometer based on the phase information of the auxiliary interferometer. The equal-phase main interferometer obtained by equal-phase interval resampling is used to estimate the time delay of the auxiliary interferometer, thereby eliminating the influence of nonlinear sweep frequency interference on the time delay estimation accuracy, improving the estimation accuracy of the time delay of the auxiliary interferometer and the nonlinear compensation effect of the de-skewing filter on the beat frequency signal of the main interferometer. In addition, the demodulation method of the present invention also uses the time delay of the auxiliary interferometer to estimate the local oscillator nonlinear phase of the main interferometer, and then constructs the conjugate phase factor to achieve triple compensation of the beat frequency signal of the main interferometer, optimize the nonlinear compensation effect of the de-skewing filter, and combine the optimized de-skewing filter with a two-dimensional Gaussian filter to further improve the sensing spatial resolution and sensing sensitivity of OFDR. Attached Figure Description

[0047] Figure 1 The schematic diagram of the kilometer-level OFDR distributed sensing system provided by the present invention.

[0048] Figure 2 A schematic diagram illustrating the steps of the demodulation method provided by this invention.

[0049] Figure 3 This is a schematic diagram illustrating the principle of step 200 in the demodulation method provided by the present invention.

[0050] Figure 4 This is a schematic diagram illustrating the principle of step 400 in the demodulation method provided by the present invention.

[0051] Figure 5 This is a schematic diagram illustrating the principle of step 500 in the demodulation method provided by the present invention.

[0052] Figure 6 This is a schematic diagram illustrating the principle of step 510 in the demodulation method provided by the present invention.

[0053] Figure 7 This is a schematic diagram illustrating the principle of step 530 in the demodulation method provided by the present invention.

[0054] Figure 8 This is a schematic diagram illustrating the principle of step 600 in the demodulation method provided by the present invention. Detailed Implementation

[0055] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, examples of which are shown in the drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0056] In the description of this invention, it should be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0057] Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first," "second," or "third" may explicitly or implicitly include one or more of that feature. In the description of this invention, "multiple" means two or more, unless otherwise explicitly specified.

[0058] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," "fixing," and "setting," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0059] Example 1

[0060] like Figure 1 As shown, a kilometer-scale OFDR distributed sensing system includes a demodulation device 1, a data acquisition device 2, an optical frequency domain reflectometer 3, and a measurement optical fiber 4. The sensing end of the optical frequency domain reflectometer 3 is connected to the measurement optical fiber 4, and the output end of the optical frequency domain reflectometer 3 is connected to the data acquisition device 2. The data acquisition device 2 is connected to the demodulation device 1.

[0061] During operation, the swept-frequency optical signal emitted by the optical frequency domain reflectometer 3 enters the measuring optical fiber 4, and the Rayleigh scattered light generated by the measuring optical fiber 4 re-enters the optical frequency domain reflectometer 3. The optical frequency domain reflectometer 3 converts the Rayleigh scattered light into a beat frequency signal and outputs it to the data acquisition device 2. The data acquisition device 2 acquires the beat frequency signal and outputs the acquired beat frequency signal to the demodulation device 1. The demodulation device 1 demodulates the beat frequency signal and finally obtains the measurement distribution curve of the measuring optical fiber 4.

[0062] The optical frequency domain reflectometer 3 includes a tunable light source 31, a first fiber optic coupler 32, an auxiliary interferometer, a main interferometer, a first photoelectric balance detector 313, a second photoelectric balance detector 314, and a third photoelectric balance detector 315. The tunable light source 31 is connected to the auxiliary interferometer and the main interferometer respectively through the first fiber optic coupler 32. The auxiliary interferometer is connected to the first photoelectric balance detector 313. The main interferometer is connected to the second photoelectric balance detector 314 and the third photoelectric balance detector 315 respectively. The first photoelectric balance detector 313, the second photoelectric balance detector 314, and the third photoelectric balance detector 315 are all connected to the data acquisition device 2.

[0063] The auxiliary interferometer includes a second fiber coupler 33, a time-delay fiber 34, a first Faraday rotator mirror 35, and a second Faraday rotator mirror 36. The first port of the second fiber coupler 33 is connected to the first fiber coupler 32, the second port of the second fiber coupler 33 is connected to the first photoelectric balance detector 313, the third port of the second fiber coupler 33 is connected to the first Faraday rotator mirror 35 via the time-delay fiber 34, and the fourth port of the second fiber coupler 33 is connected to the second Faraday rotator mirror 36.

[0064] The main interferometer includes a third fiber coupler 37, a polarization controller 38, a fiber circulator 39, a fourth fiber coupler 310, a first polarization beam splitter 311, and a second polarization beam splitter 312. The first port of the third fiber coupler 37 is connected to the first fiber coupler 32, and the second port of the third fiber coupler 37 is connected to the first port of the fourth fiber coupler 310 via the polarization controller 38. The third port of the third fiber coupler 37 is connected to the first port of the fiber circulator 39. The second port of the fiber circulator 39 is connected to the measuring fiber 4, and the third port of the fiber circulator 39 is connected to the second port of the fourth fiber coupler 310. The third port of the fourth fiber coupler 310 is connected to the second photoelectric balance detector 314 and the third photoelectric balance detector 315 via the first polarization beam splitter 311, and the fourth port of the fourth fiber coupler 310 is connected to the second photoelectric balance detector 314 and the third photoelectric balance detector 315 via the second polarization beam splitter 312.

[0065] In use, the swept-frequency light signal emitted by the tunable light source 31 is split at a ratio of 1:99 after passing through the first fiber coupler 32. 1% of the swept-frequency light signal enters the auxiliary interferometer, and the remaining 99% enters the main interferometer. The swept-frequency light signal in the auxiliary interferometer is split into two beams after passing through the second fiber coupler 33. One beam travels through the time-delay fiber 34 to the first Faraday rotator mirror 35, while the other beam directly reaches the second Faraday rotator mirror 36. After being reflected by the first and second Faraday rotator mirrors 35 and 36 respectively, the two beams return to the second fiber coupler 33 to generate a first beat-frequency interference beam. This first beat-frequency interference beam is ultimately input to the first photoelectric balance detector 313 for photoelectric signal conversion to form a... The sweeping light signal in the main interferometer is split by the third fiber coupler 37. One sweeping light signal passes through the polarization controller 38 and enters the fourth fiber coupler 310, while the other sweeping light signal passes through the fiber circulator 39 and enters the measurement fiber 4. Rayleigh scattered light is formed in the measurement fiber 4 and then enters the fourth fiber coupler 310 through the fiber circulator 39. The Rayleigh scattered light and the sweeping light signal generate a second beat frequency interference light in the fourth fiber coupler 310. The second beat frequency interference light is split by the fourth fiber coupler 310 and then polarized by the first polarization beam splitter 311 and the second polarization beam splitter 312, respectively. Finally, it is input to the second photoelectric balance detector 314 and the third photoelectric balance detector 315 for photoelectric signal conversion to form another set of beat frequency signals.

[0066] The data acquisition device 2 simultaneously acquires the beat frequency signals of the main interferometer and the auxiliary interferometer, and provides the two sets of acquired beat frequency signals to the demodulation device 1 for nonlinear compensation and measurement demodulation. When the environment in which the measuring optical fiber 4 is located does not experience changes in measurement parameters such as temperature or strain, the beat frequency signal output by the main interferometer is the reference beat frequency signal. When the environment in which the measuring optical fiber 4 is located has experienced changes in measurement parameters such as temperature or strain, the beat frequency signal output by the main interferometer is the measurement beat frequency signal.

[0067] In theory, the frequency sweep light signal emitted by the tunable light source 31 should be linear frequency sweep light. However, no matter how the manufacturing process of the tunable light source 31 is improved, the frequency sweep light signal emitted by the tunable light source 31 inevitably contains a small amount of nonlinear frequency sweep light. The nonlinear frequency sweep interference caused by the nonlinear frequency sweep light reduces the spatial resolution and sensing accuracy of OFDR, which greatly limits the application of OFDR in the field of long-distance sensing.

[0068] Example 2

[0069] A demodulation method is used in the kilometer-scale OFDR distributed sensing system described in Embodiment 1.

[0070] like Figure 2 As shown, the demodulation method includes the following steps:

[0071] Step 100: Obtain the beat frequency signal of the main interferometer and the beat frequency signal of the auxiliary interferometer.

[0072] In step 100, the demodulation device simultaneously acquires the beat frequency signals output by the main interferometer and the auxiliary interferometer through the data acquisition device, so as to simultaneously obtain the beat frequency signal of the main interferometer and the beat frequency signal of the auxiliary interferometer.

[0073] The auxiliary interferometer has two sets of beat frequency signals. One set of beat frequency signals is acquired by the data acquisition device when the environment in which the measuring fiber is located remains unchanged, such as when the measuring parameters (e.g., temperature, strain) do not change. This set of beat frequency signals is used to perform nonlinear compensation on the reference beat frequency signal acquired by the data acquisition device from the main interferometer in the same unchanged environment. The other set of beat frequency signals is acquired by the data acquisition device when the environment in which the measuring fiber is located has changed, such as when the measuring parameters (e.g., temperature, strain) have changed. This set of beat frequency signals is used to perform nonlinear compensation on the measuring beat frequency signal acquired by the data acquisition device from the main interferometer in the same changed environment.

[0074] Step 200: Process the beat frequency signal of the auxiliary interferometer to obtain the phase information of the auxiliary interferometer.

[0075] In step 200, the phase information of the auxiliary interferometer reflects the change of the noise phase in the beat frequency signal of the auxiliary interferometer over time.

[0076] Specifically, such as Figure 3 As shown, in step 200, when processing the beat frequency signal of the auxiliary interferometer to obtain the phase information of the auxiliary interferometer, the following steps are included:

[0077] Step 210: Perform Hilbert transform on the beat frequency signal of the auxiliary interferometer to obtain the complex signal of the auxiliary interferometer.

[0078] In step 210, the beat frequency signal of the auxiliary interferometer is converted from a one-dimensional time-domain signal to a rotating vector on a two-dimensional complex plane by Hilbert transform, so as to facilitate subsequent phase extraction.

[0079] Specifically, a Hilbert transform is used to perform a 90° phase shift on the beat frequency signal of the auxiliary interferometer to generate an imaginary part orthogonal to the beat frequency signal of the auxiliary interferometer. Then, the beat frequency signal of the auxiliary interferometer is used as the real part to construct the following complex signal:

[0080] s a (t)=s(t)+H[s(t)]

[0081] Wherein, s(t) is the beat frequency signal of the auxiliary interferometer, and H[s(t)] is the result of the beat frequency signal of the auxiliary interferometer after Hilbert transform.

[0082] Step 220: Perform forward and reverse tangent and unwinding processes on the complex signals of the auxiliary interferometer in sequence to obtain the phase information of the auxiliary interferometer.

[0083] In step 220, the instantaneous phase is extracted from the complex signal of the auxiliary interferometer through tangent-inverse operation to map the complex signal of the auxiliary interferometer into the angular domain. The phase unwinding process is then used to eliminate the phase jump problem during instantaneous phase extraction, thereby restoring the true and continuous phase curve.

[0084] The phase curves after the forward and reverse tangent operations are as follows:

[0085]

[0086] Here, atan2 is the arctangent function in the fourth quadrant, with an output range of (-π, π).

[0087] The phase curve after phase unwinding is as follows:

[0088]

[0089] Where k(t) is an integer compensation value calculated based on the phase transition.

[0090] Step 300: Based on the phase information of the auxiliary interferometer, the beat frequency signal of the auxiliary interferometer is resampled at equal phase intervals to obtain an equal phase signal.

[0091] In step 300, the beat frequency signal of the auxiliary interferometer is resampled at equal phase intervals based on its own phase information to rearrange the original sampling points of the auxiliary interferometer, so that the resampled equal phase signal is uniformly distributed in the phase dimension, thereby eliminating the interference caused by nonlinear frequency sweep.

[0092] Specifically, a set of equally spaced target phase points are defined on the phase axis. (like (For a fixed phase interval), the phase information from the auxiliary interferometer The inverse mapping is used to find the actual time point corresponding to each target phase point. Then, interpolation algorithms (such as linear interpolation and spline interpolation) are used to extract the beat frequency signal s(t) from the auxiliary interferometer. Signal value at time Generate equal-phase signals with equal phase intervals.

[0093] Assume the original sampling points are {ti, s(ti)}, and the corresponding phase is Define the target phase sequence as in For the preset phase interval (e.g.) M is the number of sampling points per sweep cycle; through the inverse function Calculate the time point corresponding to the target phase, and then obtain the isophase signal through interpolation.

[0094] Step 400: Estimate the time delay of the auxiliary interferometer based on the equal-phase signal.

[0095] In step 400, after equal-phase interval resampling, compared with the beat frequency signal of the auxiliary interferometer, the equal-phase signal is uniformly distributed in the phase dimension because the interference caused by nonlinear frequency sweep is eliminated, thus making the estimation of the time delay of the auxiliary interferometer more accurate.

[0096] Specifically, such as Figure 4As shown, in step 400, estimating the time delay of the auxiliary interferometer based on the equal-phase signal includes the following steps:

[0097] Step 410: Perform a fast Fourier transform on the isophase signal to obtain its range domain spectrum.

[0098] In step 410, the distance domain spectrum of the equal-phase signal has multiple reflection peaks with the fiber position of the time-delay fiber as the horizontal axis and the signal intensity as the vertical axis. Each reflection peak corresponds to an equivalent reflection point on the time-delay fiber, and the fiber position of each reflection peak is the distance between the corresponding reflection point and the fiber starting point.

[0099] Step 420: In the distance domain spectrum of the equal-phase signal, the peak-finding algorithm is used to determine the end reflection peak, and the fiber length of the time-delay fiber is determined according to the fiber position of the end reflection peak.

[0100] In step 420, the end reflection peak refers to the reflection peak corresponding to the end reflection point of the time-delay fiber (i.e., the first Faraday rotator mirror), and the distance between the corresponding fiber position and the fiber starting point is the fiber length of the time-delay fiber. The peak-finding algorithm is a commonly used reflection peak localization algorithm in this field, so its principle and data processing procedure will not be described.

[0101] Step 430: Calculate the time delay of the auxiliary interferometer based on the fiber length of the time delay fiber.

[0102] In step 430, the time delay of the auxiliary interferometer is... Z ref Let n be the fiber length of the time-delay fiber, n be the refractive index of the time-delay fiber, and c be the speed of light.

[0103] Step 500: Based on the time delay of the auxiliary interferometer, a deskewing filter is used to perform nonlinear compensation on the beat frequency signal of the main interferometer.

[0104] In step 500, the beat frequency signal of the main interferometer includes a reference beat frequency signal and a measured beat frequency signal. The reference beat frequency signal is the time delay estimated by the data acquisition device from the beat frequency signal acquired by the auxiliary interferometer in the same unchanged environment, and is used for nonlinear compensation in conjunction with the de-skew filter. The measured beat frequency signal is the time delay estimated by the data acquisition device from the beat frequency signal acquired by the auxiliary interferometer in the same changed environment, and is used for nonlinear compensation in conjunction with the de-skew filter.

[0105] Example 3

[0106] As an optimization of Embodiment 2, in this embodiment, such as Figure 5 As shown, in step 500, when using a descrambling filter to perform nonlinear compensation on the beat frequency signal of the main interferometer based on the time delay of the auxiliary interferometer, the following steps are included:

[0107] Step 510: Estimate the local oscillator nonlinear phase of the main interferometer based on the time delay of the auxiliary interferometer.

[0108] In step 510, the local oscillator nonlinear phase is caused by the nonlinear phase of the tunable light source.

[0109] Specifically, such as Figure 6 As shown, in step 510, when estimating the local oscillator nonlinear phase of the main interferometer based on the time delay of the auxiliary interferometer, the following steps are included:

[0110] Step 511: Perform linear regression on the phase information of the auxiliary interferometer to obtain the linear phase of the auxiliary interferometer.

[0111] In step 511, the phase information of the auxiliary interferometer may be linearly regressed using methods such as least squares, robust linear regression, polynomial linear regression, or FFT frequency domain regression, but not limited to these methods.

[0112] Step 512: Subtract the linear phase from the phase information of the auxiliary interferometer to obtain the nonlinear phase of the auxiliary interferometer.

[0113] In step 512, the phase information of the auxiliary interferometer includes linear phase and nonlinear phase. The linear phase is removed, and what remains is the nonlinear phase.

[0114] Step 513: Perform a third-order Taylor expansion and integration on the nonlinear phase of the auxiliary interferometer to obtain the local oscillator nonlinear phase of the main interferometer.

[0115] In step 513, it is assumed that the nonlinear phase of the auxiliary interferometer is e(t). ref Then, after performing a third-order Taylor expansion, e(t) ref =a0+a1t+a2t 2 +a3t 3 , a0 is e(t) ref The function value at time t, a1 is e(t). ref The first derivative at time t, a2 is e(t). ref The second derivative at time t, a3 is e(t). ref The third derivative at time t.

[0116] The third-order Taylor expansion of e(t) ref By integrating, the local oscillator nonlinear phase of the main interferometer is obtained.

[0117] Step 520: Construct the conjugate phase factor based on the estimated local oscillator nonlinear phase.

[0118] In step 520, the local oscillator phase factor S is first constructed based on the estimated local oscillator nonlinear phase. e (t)=exp[j2πe(t) main Then, the conjugate phase factor is obtained by conjugating the local oscillator phase factor. Where j represents the imaginary unit.

[0119] Step 530: Based on the conjugate phase factor, eliminate the local oscillator nonlinear phase in the beat frequency signal of the main interferometer to obtain the beat frequency signal of the main interferometer after one compensation.

[0120] In step 530, the beat frequency signal of the main interferometer can be expressed as: Where A is the signal amplitude, f b f0 is the beat frequency, f0 is the initial frequency of the tunable light source, and τ is the beat frequency. test Let be the time delay of the measuring fiber, t be time, γ be the sweep acceleration, and e(t) be the value of the time delay. main The nonlinear phase of the local oscillator can be eliminated from the beat frequency signal of the main interferometer by using the estimated value of the nonlinear phase of the local oscillator through the constructed conjugate phase factor.

[0121] Specifically, such as Figure 7 As shown, in step 530, when eliminating the local oscillator nonlinear phase in the beat frequency signal of the main interferometer according to the conjugate phase factor to obtain the beat frequency signal of the main interferometer after one compensation, the following steps are included:

[0122] Step 531: Perform Hilbert transform on the beat frequency signal of the main interferometer and rewrite its complex signal into exponential form.

[0123] In step 531, the beat frequency signal of the main interferometer is first subjected to Hilbert transform to obtain the complex signal of the main interferometer. This is the same as the method of performing Hilbert transform on the beat frequency signal of the auxiliary interferometer in step 210, and will not be described again. Then, Euler's formula is used to rewrite the complex signal into exponential form.

[0124] The complex signal of the master interferometer is represented by an exponential formation as follows:

[0125] Step 532: Multiply the complex signal of the main interferometer by the conjugate phase factor to eliminate the local oscillator nonlinear phase, and obtain the beat frequency signal of the main interferometer after one compensation.

[0126] In step 532, the beat frequency signal of the main interferometer after one compensation

[0127] Step 540: Construct the deskewing filter, and filter the beat frequency signal of the main interferometer after the first compensation through the deskewing filter to obtain the beat frequency signal of the main interferometer after the second compensation.

[0128] In step 540, the descrambling filter is The beat frequency signal of the main interferometer after secondary compensation FFT stands for Fast Fourier Transform, and IFFT stands for Inverse Fast Fourier Transform.

[0129] Step 550: Multiply the beat frequency signal of the main interferometer after secondary compensation by the conjugate phase factor to obtain the beat frequency signal of the main interferometer after tertiary compensation.

[0130] In step 550, the beat frequency signal of the main interferometer after three compensations The nonlinear sweep frequency signal has been completely eliminated.

[0131] Example 4

[0132] As an optimization of Embodiment 2 or Embodiment 3, in this embodiment, such as Figure 2 As shown, the demodulation method further includes the following steps:

[0133] Step 600: Demodulate the compensated beat frequency signal of the main interferometer to obtain the measurement distribution curve of the measurement fiber.

[0134] The beat frequency signal of the master interferometer includes a reference beat frequency signal and a measured beat frequency signal, specifically, as follows: Figure 8 As shown, in step 600, when demodulating the compensated beat frequency signal of the main interferometer to obtain the measurement distribution curve of the measurement fiber, the following steps are included:

[0135] Step 610: Perform Fast Fourier Transform on the compensated reference beat frequency signal and the measured beat frequency signal respectively to obtain the reference range domain spectrum and the measured range domain spectrum.

[0136] Step 620: Set the sliding window size and sliding window step distance, and perform sliding window values ​​on the reference range domain spectrum and the measurement range domain spectrum to obtain multiple reference local spectra and multiple measurement range domain spectra.

[0137] Step 630: Perform cross-correlation calculations on the reference local spectrum and the measured local spectrum at the same fiber location to obtain multiple cross-correlation peaks.

[0138] Step 640: Segment the cross-correlation peaks at different fiber locations to obtain a cross-correlation image.

[0139] Step 650: The cross-correlation image is filtered using a two-dimensional Gaussian filter.

[0140] Step 660: Based on the relationship curve between the measured parameters and the cross-correlation peaks, convert the filtered cross-correlation image into the corresponding measurement distribution curve.

[0141] Example 5

[0142] A demodulation device is provided for the kilometer-scale OFDR distributed sensing system described in Embodiment 1. The demodulation device includes a processor and a memory connected together, the memory storing a computer program for execution by the processor; when the processor executes the computer program, it performs the demodulation method described in any of Embodiments 2 to 4.

[0143] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention and not to limit them. Although the embodiments of the present invention have been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the embodiments of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A demodulation method for a kilometric OFDR distributed sensing system, characterized in that, The method comprises the following steps: Step 100: obtaining beat frequency signals of a main interferometer and beat frequency signals of an auxiliary interferometer; Step 200: processing the beat frequency signals of the auxiliary interferometer to obtain phase information of the auxiliary interferometer; Step 300: performing equal-phase interval resampling on the beat frequency signals of the auxiliary interferometer according to the phase information of the auxiliary interferometer to obtain an equal-phase signal; Step 400: estimating a time delay size of the auxiliary interferometer according to the equal-phase signal; Step 500: performing nonlinear compensation on the beat frequency signals of the main interferometer by using a desquaring filter according to the time delay size of the auxiliary interferometer; Step 600: demodulating the compensated beat frequency signals of the main interferometer to obtain a measurement distribution curve of a measurement optical fiber; In step 200, when the beat frequency signals of the auxiliary interferometer are processed to obtain the phase information of the auxiliary interferometer, the following steps are included: Step 210: performing Hilbert transform on the beat frequency signals of the auxiliary interferometer to obtain complex signals of the auxiliary interferometer; Step 220: sequentially performing arctangent and unwrapping processing on the complex signals of the auxiliary interferometer to obtain the phase information of the auxiliary interferometer; In step 400, when the time delay size of the auxiliary interferometer is estimated according to the equal-phase signal, the following steps are included: Step 410: performing fast Fourier transform on the equal-phase signal to obtain a distance domain spectrum thereof; Step 420: determining an end reflection peak in the distance domain spectrum of the equal-phase signal by using a peak searching algorithm, and determining a fiber length of a time delay fiber according to a fiber position of the end reflection peak; Step 430: calculating the time delay size of the auxiliary interferometer according to the fiber length of the time delay fiber; In step 500, when the beat frequency signals of the main interferometer are compensated by using the desquaring filter according to the time delay size of the auxiliary interferometer, the following steps are included: Step 510: estimating a local oscillator nonlinear phase of the main interferometer according to the time delay size of the auxiliary interferometer; Step 520: constructing a conjugate phase factor according to the estimated local oscillator nonlinear phase; Step 530: eliminating the local oscillator nonlinear phase in the beat frequency signals of the main interferometer according to the conjugate phase factor to obtain beat frequency signals of the main interferometer after first compensation; Step 540: constructing the desquaring filter, and filtering the beat frequency signals of the main interferometer after first compensation by using the desquaring filter to obtain beat frequency signals of the main interferometer after second compensation; Step 550: multiplying the beat frequency signals of the main interferometer after second compensation by the conjugate phase factor to obtain beat frequency signals of the main interferometer after third compensation.

2. The demodulation method according to claim 1, characterized in that, In step 510, when the local oscillator nonlinear phase of the main interferometer is estimated according to the time delay size of the auxiliary interferometer, the following steps are included: Step 511: performing linear regression on the phase information of the auxiliary interferometer to obtain a linear phase of the auxiliary interferometer; Step 512: subtracting the linear phase of the auxiliary interferometer from the phase information of the auxiliary interferometer to obtain the nonlinear phase of the auxiliary interferometer; Step 513: performing third-order Taylor expansion and integration on the nonlinear phase of the auxiliary interferometer to obtain the local oscillator nonlinear phase of the main interferometer.

3. The demodulation method according to claim 1, characterized by, In step 530, according to the conjugate phase factor, the local oscillator nonlinear phase in the beat signal of the main interferometer is eliminated to obtain the beat signal of the main interferometer after first compensation, including the following steps: Step 531: performing Hilbert transform on the beat signal of the main interferometer, and rewriting the complex signal into exponential form; Step 532: multiplying the complex signal of the main interferometer by the conjugate phase factor to eliminate the local oscillator nonlinear phase, and obtaining the beat signal of the main interferometer after first compensation.

4. The demodulation method according to claim 1, characterized by, The beat signal of the main interferometer includes a reference beat signal and a measurement beat signal, and in step 600, the compensated beat signal of the main interferometer is demodulated to obtain the measurement distribution curve of the measurement optical fiber, including the following steps: Step 610: performing fast Fourier transform on the compensated reference beat signal and the measurement beat signal respectively to obtain the reference distance domain spectrum and the measurement distance domain spectrum; Step 620: setting the size of the sliding window and the sliding window step distance, and performing sliding window value on the reference distance domain spectrum and the measurement distance domain spectrum to obtain a plurality of reference local spectra and a plurality of measurement distance domain spectra; Step 630: performing cross-correlation operation on the reference local spectrum and the measurement local spectrum at the same optical fiber position to obtain a plurality of cross-correlation peaks; Step 640: splicing the cross-correlation peaks at different optical fiber positions to obtain a cross-correlation image; Step 650: filtering the cross-correlation image by using a two-dimensional Gaussian filter; Step 660: converting the filtered cross-correlation image into the corresponding measurement distribution curve according to the relationship curve between the measurement parameter and the cross-correlation peak.

5. A demodulation device for a kilometre-scale OFDR distributed sensing system, characterised in that, The device comprises a processor and a memory connected to each other, and the memory stores a computer program for the processor to execute; when the processor executes the computer program, the demodulation method of claim 1 is performed.

6. A kilometre scale OFDR distributed sensing system characterised in that, The device comprises a data acquisition device, an optical frequency domain reflectometer, a measurement optical fiber, and a demodulation device as claimed in claim 5, a sensing end of the optical frequency domain reflectometer being connected to the measurement optical fiber, an output end of the optical frequency domain reflectometer being connected to the data acquisition device; the data acquisition device being connected to the demodulation device; the optical frequency domain reflectometer comprising a tunable light source, a first optical fiber coupler, an auxiliary interferometer, a main interferometer, a first photoelectric balance detector, a second photoelectric balance detector, and a third photoelectric balance detector, the tunable light source being connected to the auxiliary interferometer and the main interferometer through the first optical fiber coupler, the auxiliary interferometer being connected to the first photoelectric balance detector, the main interferometer being connected to the second photoelectric balance detector and the third photoelectric balance detector, respectively, and the first photoelectric balance detector, the second photoelectric balance detector, and the third photoelectric balance detector all being connected to the data acquisition device; the auxiliary interferometer comprising a time-delay optical fiber.

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