Kilometer-level OFDR distributed sensing system and demodulation method and device thereof

Abstract

The invention discloses a demodulation method for a kilometer-level OFDR distributed sensing system. The demodulation method comprises the following steps: acquiring a beat frequency signal of a main interferometer and a beat frequency signal of an auxiliary interferometer; processing the beat frequency signal of the auxiliary interferometer to obtain phase information of the auxiliary interferometer; according to the phase information of the auxiliary interferometer, performing equal-phase interval resampling on the beat frequency signal of the auxiliary interferometer to obtain an equal-phase signal; estimating the time delay of the auxiliary interferometer according to the equiphase signal; and performing nonlinear compensation on the beat frequency signal of the main interferometer by adopting a de-skew filter according to the time delay of the auxiliary interferometer. The invention discloses a kilometer-level OFDR distributed sensing system and a demodulation device thereof based on the demodulation method.

Description

Technical Field

[0001] The present invention relates to optical frequency domain sensing technology, and in particular to a kilometer-level OFDR distributed sensing system and a demodulation method and device thereof. Background Art

[0002] Optical frequency domain reflectometry (OFDR), a high-performance distributed fiber-optic sensing technology, has attracted widespread attention and in-depth research from both domestic and international academic and engineering communities in recent years due to its ultra-high spatial resolution and excellent sensing accuracy. The 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 verified in a variety of fields, including aerospace, civil engineering, and energy and power. However, OFDR's application in long-distance sensing is significantly limited by the phase noise introduced by the nonlinear frequency sweep of the tunable light source, which 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 the spatial resolution. However, the sensing distance is still affected by the length of the delay fiber in the auxiliary interferometer, and the sensing distance is limited to tens of meters.

[0004] The de-skew filter compresses the sweep signal from the main interferometer from a wide pulse to a narrow pulse by compressing the time width of the sweep signal, thereby improving spatial resolution and sensing accuracy, and achieving kilometer-scale sensing distances. However, the use of the de-skew filter requires estimating the delay of the auxiliary interferometer. Inaccurate delay estimation can significantly degrade the spatial resolution of kilometer-scale sensing, significantly affecting the effectiveness of nonlinear compensation.

[0005] Currently, when estimating the delay of the auxiliary interferometer, the presence of nonlinear swept-frequency interference in the beat signal of the auxiliary interferometer is not taken into account, which leads to large errors in the delay estimation. In addition, the traditional de-skewing filter needs to be further optimized. Summary of the Invention

[0006] In order to address the deficiencies of the prior art described above, the present invention provides a kilometer-level OFDR distributed sensing system and a demodulation method and device thereof, 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 solutions:

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

[0009] Step 100: Obtaining a beat signal of a main interferometer and a beat signal of an auxiliary interferometer;

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

[0011] Step 300: resampling the beat frequency signal of the auxiliary interferometer at equal phase intervals according to the phase information of the auxiliary interferometer to obtain an equal phase signal;

[0012] Step 400: estimating the time delay of the auxiliary interferometer according to the equal phase signal;

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

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

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

[0016] Step 220: performing forward and reverse tangent and unwrapping processing on the complex signal of the auxiliary interferometer in sequence to obtain phase information of the auxiliary interferometer.

[0017] Furthermore, in step 400, when estimating the delay of the auxiliary interferometer according to the equal phase signal, the following steps are included:

[0018] Step 410: performing a fast Fourier transform on the equal-phase signal to obtain its range domain spectrum;

[0019] Step 420: Determine the end reflection peak in the distance domain spectrum of the equal phase signal using a peak-finding algorithm, and determine the fiber length of the delay fiber according to the fiber position of the end reflection peak;

[0020] Step 430: Calculate the delay of the auxiliary interferometer according to the fiber length of the delay fiber.

[0021] Furthermore, in step 500, when a de-skew filter is used to perform nonlinear compensation on the beat frequency signal of the main interferometer according to the time delay of the auxiliary interferometer, the following steps are included:

[0022] Step 510: estimating the nonlinear phase of the local oscillator of the main interferometer according to the time delay of the auxiliary interferometer;

[0023] Step 520: constructing a conjugate phase factor according to the estimated local oscillator nonlinear phase;

[0024] Step 530: Eliminate the local oscillator nonlinear phase in the beat signal of the main interferometer according to the conjugate phase factor to obtain the beat signal of the main interferometer after primary compensation.

[0025] Step 540: constructing the de-skew filter, filtering the beat frequency signal of the main interferometer after the primary compensation through the de-skew filter to obtain the beat frequency signal of the main interferometer after the secondary compensation.

[0026] Furthermore, in step 510, estimating the nonlinear phase of the local oscillator of the main interferometer according to the time delay of the auxiliary interferometer includes the following steps:

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

[0028] Step 512: Subtracting 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 signal of the main interferometer according to the conjugate phase factor to obtain the beat signal of the main interferometer after primary compensation, the following steps are included:

[0031] Step 531: performing Hilbert transform on the beat frequency signal of the main interferometer, and rewriting the complex signal into an exponential form;

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

[0033] Furthermore, in step 500, when a de-skew filter is used to perform nonlinear compensation on the beat frequency signal of the main interferometer according to the time delay of the auxiliary interferometer, the following steps are further included:

[0034] Step 550: multiplying the beat frequency signal of the main interferometer after quadratic compensation by the conjugate phase factor to obtain the beat frequency signal of the main interferometer after cubic compensation.

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

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

[0037] Furthermore, the beat signal of the main interferometer includes a reference beat signal and a measurement beat signal. In step 600, the beat signal of the main interferometer after compensation is demodulated to obtain a measurement distribution curve of the measurement optical fiber, including the following steps:

[0038] Step 610: performing fast Fourier transform on the compensated reference beat frequency signal and the measured beat frequency signal to obtain a reference range domain spectrum and a measured range domain spectrum respectively;

[0039] Step 620: setting a sliding window size and a sliding window step distance, and performing sliding window sampling on the reference distance domain spectrum and the measured distance domain spectrum to obtain a plurality of reference local spectra and a plurality of measured distance domain spectra;

[0040] Step 630: performing a cross-correlation operation on the reference local spectrum and the measured local spectrum at the same optical fiber position to obtain a plurality of cross-correlation peaks;

[0041] Step 640: splicing the cross-correlation peaks at different optical fiber positions to obtain a cross-correlation image;

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

[0043] Step 660: According to the relationship curve between the measurement parameters and the cross-correlation peaks, the filtered cross-correlation image is converted into a corresponding measurement distribution curve.

[0044] A demodulation device for a kilometer-level OFDR distributed sensing system comprises a processor and a memory connected to each other, wherein the memory stores a computer program for execution by the processor; when the processor executes the computer program, the processor performs the above-mentioned demodulation method.

[0045] A kilometer-level OFDR distributed sensing system includes a data acquisition device, an optical frequency domain reflectometer, a measuring optical fiber, and the above-mentioned demodulation device, wherein the sensing end of the optical frequency domain reflectometer is connected to the measuring 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 optical 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 respectively connected to the auxiliary interferometer and the main interferometer through the first optical fiber coupler, the auxiliary interferometer is connected to the first photoelectric balanced detector, the main interferometer is respectively connected to the second photoelectric balanced detector and the third photoelectric balanced detector, and the first photoelectric balanced detector, the second photoelectric balanced detector, and the third photoelectric balanced detector 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, which extracts the phase information of the auxiliary interferometer and performs equal-phase interval resampling on the beat frequency signal of the auxiliary interferometer according to the phase information of the auxiliary interferometer, so as to estimate the time delay of the auxiliary interferometer using the equal-phase main interferometer obtained by the equal-phase interval resampling, thereby eliminating the influence of nonlinear frequency sweep interference on the 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 nonlinear phase of the local oscillator of the main interferometer, and then constructs the conjugate phase factor to achieve cubic 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is a schematic diagram of the structural principle of the kilometer-level OFDR distributed sensing system provided by the present invention.

[0048] Figure 2 This is a schematic diagram of the steps of the demodulation method provided by the present invention.

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

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

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

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

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

[0054] Figure 8 This is a schematic diagram of step 600 in the demodulation method provided by the present invention. DETAILED DESCRIPTION

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

[0056] In the description of the present invention, it should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present 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 the technical features being referred to. Thus, a feature identified as "first," "second," or "third" may explicitly or implicitly include one or more of such features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0058] In the present invention, unless otherwise expressly specified or limited, terms such as "installed," "connected," "connect," "fixed," and "disposed" should be understood in a broad sense. For example, they may refer to fixed or detachable connections, or integration; mechanical or electrical connections; direct or indirect connections through an intermediate medium; or internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0059] Example 1

[0060] like Figure 1 As shown, a kilometer-level OFDR distributed sensing system includes a demodulation device 1 and a data acquisition device 2, an optical frequency domain reflectometer 3 and a measuring optical fiber 4. The sensing end of the optical frequency domain reflectometer 3 is connected to the measuring 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 collects the beat frequency signal and outputs the collected beat frequency signal to the demodulation device 1. The demodulation device 1 demodulates the beat frequency signal to finally obtain 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 coupler 32, an auxiliary interferometer, a main interferometer, a first photoelectric balanced detector 313, a second photoelectric balanced detector 314 and a third photoelectric balanced detector 315. The tunable light source 31 is connected to the auxiliary interferometer and the main interferometer respectively through the first fiber coupler 32, the auxiliary interferometer is connected to the first photoelectric balanced detector 313, and the main interferometer is connected to the second photoelectric balanced detector 314 and the third photoelectric balanced detector 315 respectively. The first photoelectric balanced detector 313, the second photoelectric balanced detector 314 and the third photoelectric balanced 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 reflector 35, and a second Faraday rotator reflector 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 balanced detector 313, the third port of the second fiber coupler 33 is connected to the first Faraday rotator reflector 35 via the time-delay fiber 34, and the fourth port of the second fiber coupler 33 is connected to the second Faraday rotator reflector 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, 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, and 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 measurement 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 balanced detector 314 and the third photoelectric balanced detector 315 respectively via the first polarization beam splitter 311, and the fourth port of the fourth fiber coupler 310 is connected to the second photoelectric balanced detector 314 and the third photoelectric balanced detector 315 respectively via the second polarization beam splitter 312.

[0065] During use, the swept-frequency optical signal emitted by the tunable light source 31 passes through the first fiber coupler 32 and is split at a ratio of 1:99, wherein 1% of the swept-frequency optical signal enters the auxiliary interferometer and the other 99% of the swept-frequency optical signal enters the main interferometer; the swept-frequency optical signal in the auxiliary interferometer is divided into two beams after passing through the second fiber coupler 33, one of which passes through the time-delay fiber 34 and reaches the first Faraday rotator 35, and the other directly reaches the second Faraday rotator 36. After being reflected by the first Faraday rotator 35 and the second Faraday rotator 36 respectively, the two swept-frequency optical signals return to the second fiber coupler 33 along the original path to generate a first beat-frequency interference light, which is finally input into the first photoelectric balance detector 313 for photoelectric signal conversion to form a A group of beat frequency signals; the swept frequency light signal in the main interferometer is split by the third optical fiber coupler 37, and one swept frequency light signal passes through the polarization controller 38 to enter the fourth optical fiber coupler 310, and the other swept frequency light signal passes through the optical fiber circulator 39 to enter the measuring optical fiber 4, and after forming Rayleigh scattered light in the measuring optical fiber 4, passes through the optical fiber circulator 39 to enter the fourth optical fiber coupler 310, the Rayleigh scattered light and the swept frequency light signal generate a second beat frequency interference light in the fourth optical fiber coupler 310, and the second beat frequency interference light is split by the fourth optical fiber coupler 310, and then polarization split by the first polarization beam splitter 311 and the second polarization beam splitter 312 respectively, and finally input into the second photoelectric balance detector 314 and the third photoelectric balance detector 315 for photoelectric signal conversion to form another group of beat frequency signals.

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

[0067] Theoretically, the swept-frequency optical signal emitted by the tunable light source 31 should be linear swept-frequency light. However, no matter how the manufacturing process of the tunable light source 31 is improved, the swept-frequency optical signal emitted by the tunable light source 31 inevitably contains a small amount of nonlinear swept-frequency light. The nonlinear swept-frequency interference caused by the nonlinear swept-frequency 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-level OFDR distributed sensing system described in Example 1.

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

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

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

[0073] The auxiliary interferometer has two groups of beat frequency signals. One group of beat frequency signals is collected by the data acquisition device on the auxiliary interferometer when the measurement parameters such as temperature and strain in the environment where the measuring optical fiber is located have not changed. This group of beat frequency signals is used to perform nonlinear compensation on the reference beat frequency signal collected by the data acquisition device on the main interferometer in the same unchanged environment; the other group of beat frequency signals is collected by the data acquisition device on the auxiliary interferometer when the measurement parameters such as temperature and strain in the environment where the measuring optical fiber is located have changed. This group of beat frequency signals is used to perform nonlinear compensation on the measurement beat frequency signal collected by the data acquisition device on the main interferometer in the same changed environment.

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

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

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

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

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

[0079] Specifically, the beat signal of the auxiliary interferometer is phase-shifted by 90° using Hilbert transform to generate an imaginary part orthogonal to the beat signal of the auxiliary interferometer. The beat signal of the auxiliary interferometer is then 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 signal of the auxiliary interferometer, and H[s(t)] is the result of Hilbert transform of the beat signal of the auxiliary interferometer.

[0082] Step 220: performing forward and reverse tangent and unwrapping processing on the complex signal of the auxiliary interferometer in sequence to obtain phase information of the auxiliary interferometer.

[0083] In step 220, the instantaneous phase is extracted from the complex signal of the auxiliary interferometer by means of tangent and antitangent operations to map the complex signal of the auxiliary interferometer into the angle domain, and phase unwrapping processing is performed to eliminate the phase jump problem during instantaneous phase extraction, thereby restoring a truly continuous phase curve.

[0084] The phase curve after the tangent operation is as follows:

[0085]

[0086] Wherein, atan2 is a four-quadrant inverse tangent function with an output range of (-π, π].

[0087] The phase curve after phase unwrapping operation is as follows:

[0088]

[0089] Wherein, k(t) is an integer compensation value calculated according to the phase jump situation.

[0090] Step 300: resampling the beat signal of the auxiliary interferometer at equal phase intervals according to the phase information of the auxiliary interferometer to obtain an equal phase signal.

[0091] In step 300, the beat 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 signals are evenly distributed in the phase dimension, thereby eliminating the interference caused by nonlinear frequency scanning.

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

[0093] Assuming the original sampling point is {ti,s(ti)}, the corresponding phase is Define the target phase sequence as in For the preset phase interval (such as M is the number of sampling points in each sweep cycle); through the inverse function Calculate the time point corresponding to the target phase, and then obtain the equal phase signal by interpolation

[0094] Step 400: Estimate the time delay of the auxiliary interferometer according to 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 eliminates the interference caused by nonlinear frequency sweeping and is evenly distributed in the phase dimension, so the delay of the auxiliary interferometer can be estimated more accurately.

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

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

[0098] In step 410, the distance domain spectrum of the equal-phase signal has the fiber position of the time-delay fiber as the horizontal axis and the signal intensity as the vertical axis, and has multiple reflection peaks. One 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 starting point of the fiber.

[0099] Step 420: In the distance domain spectrum of the equal phase signal, a peak-finding algorithm is used to determine the end reflection peak, and the fiber length of the 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). The distance between the corresponding fiber position and the starting point of the fiber is the fiber length of the time-delay fiber. The peak-finding algorithm is a commonly used reflection peak location algorithm in the art, so its principle and data processing are not described here.

[0101] Step 430: Calculate the delay of the auxiliary interferometer according to the fiber length of the delay fiber.

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

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

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

[0105] Example 3

[0106] As an optimization solution of the second embodiment, in this embodiment, if Figure 5 As shown, in step 500, when a de-skew filter is used to perform nonlinear compensation on the beat frequency signal of the main interferometer according to the time delay of the auxiliary interferometer, the following steps are included:

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

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

[0109] Specifically, such as Figure 6 As shown, in step 510, estimating the nonlinear phase of the local oscillator of the main interferometer according to the time delay of the auxiliary interferometer includes the following steps:

[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 subjected to linear regression processing by using, but not limited to, the least square method, the robust linear regression method, the polynomial linear regression method, or the FFT frequency domain regression method.

[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 a linear phase and a 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 the 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] After the third-order Taylor expansion of e(t) ref Integrate to obtain the local oscillator nonlinear phase of the main interferometer

[0117] Step 520: Construct a conjugate phase factor according to 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 ], and then conjugate the local oscillator phase factor to obtain the conjugate phase factor where j represents the imaginary unit.

[0119] Step 530: Eliminate the local oscillator nonlinear phase in the beat signal of the main interferometer according to the conjugate phase factor to obtain the beat signal of the main interferometer after primary compensation.

[0120] In step 530, the beat signal of the master interferometer can be expressed as Where A is the signal amplitude, f b is the beat frequency, f0 is the initial frequency of the tunable light source, τ test is the time delay of the measurement fiber, t is time, γ is the sweep acceleration, e(t) main The estimated value of the local oscillator nonlinear phase can be used to eliminate the actual value of the local oscillator nonlinear phase contained in the beat signal of the main interferometer 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 signal of the main interferometer according to the conjugate phase factor to obtain the beat signal of the main interferometer after primary compensation, the following steps are included:

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

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

[0124] The complex signal of the main interferometer is expressed as an exponential form

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

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

[0127] Step 540: constructing the de-skew filter, filtering the beat frequency signal of the main interferometer after the primary compensation through the de-skew filter to obtain the beat frequency signal of the main interferometer after the secondary compensation.

[0128] In step 540, the de-skewing filter is The beat frequency signal of the main interferometer after secondary compensation FFT is fast Fourier transform, and IFFT is inverse fast Fourier transform.

[0129] Step 550: multiplying the beat frequency signal of the main interferometer after quadratic compensation by the conjugate phase factor to obtain the beat frequency signal of the main interferometer after cubic compensation.

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

[0131] Example 4

[0132] As an optimization solution of the second or third embodiment, in this embodiment, if 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 a measurement distribution curve of the measurement optical fiber.

[0134] The beat signal of the main interferometer includes a reference beat signal and a measurement beat signal. Specifically, 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 optical 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 a reference range domain spectrum and a measured range domain spectrum.

[0136] Step 620: Setting a sliding window size and a sliding window step distance, and performing sliding window sampling on the reference distance domain spectrum and the measured distance domain spectrum to obtain a plurality of reference local spectra and a plurality of measured distance domain spectra.

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

[0138] Step 640: Splice the cross-correlation peaks at different optical fiber positions to obtain a cross-correlation image.

[0139] Step 650: Use a two-dimensional Gaussian filter to filter the cross-correlation image.

[0140] Step 660: According to the relationship curve between the measurement parameters and the cross-correlation peaks, the filtered cross-correlation image is converted into a corresponding measurement distribution curve.

[0141] Example 5

[0142] A demodulation device is used in the kilometer-level OFDR distributed sensing system described in Example 1. The demodulation device includes a processor and a memory connected to each other, wherein the memory stores a computer program for execution by the processor; when the processor executes the computer program, it performs the demodulation method described in any one of Examples 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 rather than to limit them. Although the embodiments of the present invention are described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the embodiments of the present invention can still be modified or replaced by equivalents, and these modifications or equivalent replacements 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 kilometer-level OFDR distributed sensing system, characterized in that: The steps include: Step 100: Obtaining a beat signal of a main interferometer and a beat signal of an auxiliary interferometer; Step 200: Processing the beat signal of the auxiliary interferometer to obtain phase information of the auxiliary interferometer; Step 300: resampling the beat frequency signal of the auxiliary interferometer at equal phase intervals according to the phase information of the auxiliary interferometer to obtain an equal phase signal; Step 400: estimating the time delay of the auxiliary interferometer according to the equal phase signal; Step 500: According to the time delay of the auxiliary interferometer, a de-skew filter is used to perform nonlinear compensation on the beat frequency signal of the main interferometer.

2. The demodulation method according to claim 1, wherein: In step 200, processing the beat signal of the auxiliary interferometer to obtain the phase information of the auxiliary interferometer includes the following steps: Step 210: performing Hilbert transform on the beat frequency signal of the auxiliary interferometer to obtain a complex signal of the auxiliary interferometer; Step 220: performing forward and reverse tangent and unwrapping processing on the complex signal of the auxiliary interferometer in sequence to obtain phase information of the auxiliary interferometer.

3. The demodulation method according to claim 1, wherein: In step 400, estimating the delay of the auxiliary interferometer according to the equal phase signal includes the following steps: Step 410: performing a fast Fourier transform on the equal-phase signal to obtain its range domain spectrum; Step 420: Determine the end reflection peak in the distance domain spectrum of the equal phase signal using a peak-finding algorithm, and determine the fiber length of the delay fiber according to the fiber position of the end reflection peak; Step 430: Calculate the delay of the auxiliary interferometer according to the fiber length of the delay fiber.

4. The demodulation method according to claim 1, wherein: In step 500, when a de-skew filter is used to perform nonlinear compensation on the beat frequency signal of the main interferometer according to the time delay of the auxiliary interferometer, the following steps are included: Step 510: estimating the nonlinear phase of the local oscillator of the main interferometer according to the time delay of the auxiliary interferometer; Step 520: constructing a conjugate phase factor according to the estimated local oscillator nonlinear phase; Step 530: Eliminate the local oscillator nonlinear phase in the beat signal of the main interferometer according to the conjugate phase factor to obtain the beat signal of the main interferometer after primary compensation. Step 540: constructing the de-skew filter, filtering the beat frequency signal of the main interferometer after the primary compensation through the de-skew filter to obtain the beat frequency signal of the main interferometer after the secondary compensation.

5. The demodulation method according to claim 4, wherein: In step 510, estimating the nonlinear phase of the local oscillator of the main interferometer according to the time delay of the auxiliary interferometer includes the following steps: Step 511: performing linear regression on the phase information of the auxiliary interferometer to obtain the linear phase of the auxiliary interferometer; Step 512: Subtracting the linear phase from the phase information of the auxiliary interferometer to obtain the nonlinear phase of the auxiliary interferometer; 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.

6. The demodulation method according to claim 4, wherein: In step 530, when eliminating the local oscillator nonlinear phase in the beat signal of the main interferometer according to the conjugate phase factor to obtain the beat signal of the main interferometer after primary compensation, the following steps are included: Step 531: performing Hilbert transform on the beat frequency signal of the main interferometer, and rewriting the complex signal into an exponential form; Step 532: multiply the complex signal of the main interferometer by the conjugate phase factor to eliminate the nonlinear phase of the local oscillator, and obtain the beat frequency signal of the main interferometer after primary compensation.

7. The demodulation method according to claim 6, wherein: In step 500, the following steps are also included: Step 550: multiplying the beat frequency signal of the main interferometer after quadratic compensation by the conjugate phase factor to obtain the beat frequency signal of the main interferometer after cubic compensation.

8. The demodulation method according to claim 1, wherein: The demodulation method further comprises the following steps: Step 600: Demodulate the compensated beat frequency signal of the main interferometer to obtain a measurement distribution curve of the measurement optical fiber.

9. The demodulation method according to claim 8, wherein: The beat signal of the main interferometer includes a reference beat signal and a measurement beat signal. In step 600, the beat signal of the main interferometer after compensation is demodulated to obtain a measurement distribution curve of the measurement optical fiber, including the following steps: Step 610: performing fast Fourier transform on the compensated reference beat frequency signal and the measured beat frequency signal to obtain a reference range domain spectrum and a measured range domain spectrum respectively; Step 620: setting a sliding window size and a sliding window step distance, and performing sliding window sampling on the reference distance domain spectrum and the measured distance domain spectrum to obtain a plurality of reference local spectra and a plurality of measured distance domain spectra; Step 630: performing a cross-correlation operation on the reference local spectrum and the measured 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: Using a two-dimensional Gaussian filter to filter the cross-correlation image; Step 660: According to the relationship curve between the measurement parameters and the cross-correlation peaks, the filtered cross-correlation image is converted into a corresponding measurement distribution curve.

10. A demodulation device for a kilometer-level OFDR distributed sensing system, characterized in that: The device comprises a processor and a memory connected to each other, wherein the memory stores a computer program for execution by the processor; when the processor executes the computer program, the processor performs the demodulation method according to claim 1.

11. A kilometer-level OFDR distributed sensing system, characterized in that: The optical frequency domain reflectometer comprises a data acquisition device, an optical frequency domain reflectometer, a measuring optical fiber, and the demodulation device according to claim 10, wherein the sensing end of the optical frequency domain reflectometer is connected to the measuring 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 comprises a tunable light source, a first optical 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 respectively connected to the auxiliary interferometer and the main interferometer through the first optical fiber coupler, the auxiliary interferometer is connected to the first photoelectric balanced detector, the main interferometer is respectively connected to the second photoelectric balanced detector and the third photoelectric balanced detector, and the first photoelectric balanced detector, the second photoelectric balanced detector, and the third photoelectric balanced detector are all connected to the data acquisition device; the auxiliary interferometer comprises a time-delay optical fiber.

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