A Sensing Method and Device Based on LFM Signals in the FrFT Domain

Abstract

The present invention relates to the field of optical fiber sensing, and particularly to a sensing method and device based on an LFM signal in the FrFT domain. It mainly includes: generating a probe optical signal and a local oscillator optical signal, converting the probe optical signal into a FrFT-DC optical signal, and injecting the converted FrFT-DC optical signal into a sensing optical fiber; receiving the backward Rayleigh scattering optical signal of the FrFT-DC optical signal returned by the sensing optical fiber, collecting the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal, and demodulating the sensing data from the beat signal. The present invention can achieve high LSPR and narrow main lobes. At the same time, by using orthogonal frequency division multiplexing of FrFT-DC signals with high spectral efficiency, the interference fading is significantly reduced.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing, and particularly to a sensing method and device based on LFM signals in the FrFT domain. Background Art

[0002] Distributed acoustic sensing (DAS) based on phase-sensitive optical time-domain reflectometer (Phase-Sensitive Optical Time-Domain Reflectometer, abbreviated as φ-OTDR) has attracted wide attention in the fields of perimeter security monitoring, seismic wave detection, traffic tracking, and pipeline monitoring, due to its ability to perform long-distance and high-sensitivity distributed vibration monitoring. For intensity-demodulated φ-OTDR, the vibration amplitude or phase cannot be recovered. To quantitatively reconstruct the vibration amplitude, phase demodulation techniques including heterodyne detection, IQ demodulation, Kramers-Kronig receiver, self-interference detection, and phase-generated carrier have been proposed. Then, in-depth research has been carried out to improve the performance of DAS, including sensing distance, spatial resolution, and frequency response.

[0003] Specifically, the sensing range of DAS is limited by the received average signal-to-noise ratio (SNR), which is directly related to the energy of the injected pump pulse. Increasing the pulse power is an effective method to improve the SNR, but it is limited by non-linear effects such as modulation instability. Another method is to increase the pulse duration, but this will lead to a decrease in spatial resolution. Distributed optical amplification compensates for fiber losses by introducing Raman amplification, Brillouin amplification, and the combination of both, and has successfully extended the sensing distance to more than 100 kilometers, thus alleviating these contradictions. However, these methods require high-power pump sources and introduce additional noise.

[0004] To improve the contradiction between the sensing distance and the spatial resolution, an optical pulse compression method by deploying linear frequency modulation (LFM) pulses and a matched filter has been proposed. In this way, if a long LFM optical pulse with a duration up to the microsecond level is launched into the sensing optical fiber, a spatial resolution of one meter or even sub-meter can be achieved. However, a key problem with pulse compression using a matched filter is the crosstalk resulting from a relatively low peak side lobe ratio (PSLR). Generally, applying a window function to the matched filter in demodulation can enhance the PSLR. However, it will lead to SNR loss due to the mismatch of the filter. Replacing the rectangular window of the transmitted optical pulse with a Gaussian window and a Hanning window can effectively increase the PSLR to 26 dB and 46 dB respectively, but at the cost of lower pulse power or poorer spatial resolution. Nonlinear frequency modulation (NLFM) pulses inherently have a high PSLR of approximately 42 dB, but have a wider main lobe compared to LFM.

[0005] Another problem affecting the φ-OTDR sensing performance is signal fading, including polarization fading and interference fading. The former is usually eliminated using orthogonal polarization detection. Based on its frequency dependence, a multi-frequency source can mitigate interference fading. For φ-OTDR using LFM pulses, internal pulse frequency division and the method of rotating vector sum are widely used. The multi-carriers generated by the EOM have also been verified in the NLFM pulse technology. However, these segmentation or multiplexing operations of the LFM signal reduce the spectral efficiency and occupy more bandwidth because only a small part of the entire bandwidth is considered in pulse compression. Therefore, the full width at half maximum (FWHM) of the compressed pulse after matched filtering becomes wider and the spatial resolution becomes poorer.

[0006] In view of this, how to overcome the defects of the existing technology and reduce the interference phenomenon in the current LFM signal sensing is a problem to be solved in this technical field. Summary of the Invention

[0007] Aiming at the above defects or improvement requirements of the existing technology, the present invention solves the problems of signal crosstalk and interference fading in the current LFM signal sensing.

[0008] The embodiments of the present invention adopt the following technical solutions:

[0009] In the first aspect, the present invention provides a sensing method based on LFM signals in the FrFT domain, specifically as follows: generating a probe optical signal and a local oscillator optical signal, converting the probe optical signal into a FrFT-DC optical signal, and injecting the converted FrFT-DC optical signal into a sensing optical fiber; receiving the backward Rayleigh scattering optical signal of the FrFT-DC optical signal returned by the sensing optical fiber, collecting the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal, and demodulating the sensing data from the beat signal.

[0010] Preferably, converting the probe optical signal into a FrFT-DC optical signal specifically includes: generating a FrFT-DC electrical signal using FrFT-DC signal data; converting the probe optical signal into a FrFT-DC optical signal through the FrFT-DC electrical signal.

[0011] Preferably, before generating the FrFT-DC electrical signal using FrFT-DC signal data, it further includes: generating LFM signal data through the p-order FrFT operation of DC signal data.

[0012] Preferably, generating LFM signal data through the p-order FrFT operation of DC signal data specifically includes: the FrFT operation is specifically a discrete fractional Fourier transform.

[0013] Preferably, collecting the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal specifically includes: performing polarization diversity and 90° mixing reception on the backward Rayleigh scattering optical signal and the local oscillator optical signal; converting the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal into an electrical signal of multiple polarization channels; collecting data on the electrical signals of all polarization channels.

[0014] Preferably, when the number of polarization channels is 4 channels, converting the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal into an electrical signal of multiple polarization channels specifically includes: converting the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal into a 4-channel current signal; recombining the 4-channel signals into two-channel complex signals, and the two-channel complex signals respectively correspond to the polarization signals of the X and Y polarization channels.

[0015] Preferably, demodulating the sensing data from the beat signal specifically includes: performing sliding processing on the original electrical signal of each polarization channel along the time axis segment by segment using the (1-p)-order FrFT operation to generate the corresponding fractional Fourier domain data for each segment; taking the peak value of the corresponding fractional Fourier domain data for each segment to form a complex vector, and calculating the combined vector by the method of rotating vector sum, and using the combined vector as the sensing data.

[0016] Preferably, after converting the probe optical signal into a FrFT-DC optical signal, it further includes:

[0017] Modulate the FrFT-DC optical signal to chop the FrFT-DC optical signal, where the modulation signal source is synchronized with the FrFT-DC signal data source.

[0018] On the other hand, the present invention provides a sensing device based on the FrFT-domain LFM signal, including: a laser, a coupler, an I / Q modulator, an arbitrary waveform generator, an erbium-doped amplifier, a filter, a circulator, a sensing optical fiber, an integrated coherent receiver, a data acquisition card, and a processor, to implement the sensing method based on the FrFT-domain LFM signal provided in the first aspect. Specifically: the laser outputs a highly coherent continuous optical signal, which is split into two paths by the coupler. One path of the optical signal enters the integrated coherent receiver as the local oscillator optical signal, and the other path of the optical signal enters the I / Q modulator as the probe optical signal; the arbitrary waveform generator generates a FrFT-DC electrical signal, and the FrFT-DC electrical signal drives the I / Q modulator to modulate the probe optical signal into a FrFT-DC optical signal. The FrFT-DC optical signal passes through the erbium-doped fiber amplifier and the filter in sequence, and is injected into the sensing optical fiber through the circulator; the backward Rayleigh scattering optical signal of the FrFT-DC optical signal returned by the sensing optical fiber enters the signal port of the integrated coherent receiver through the circulator and is converted into electrical signals of multiple polarization channels. The data acquisition card performs data acquisition on the electrical signals of all polarization channels, and the collected multi-channel digital signal data is demodulated by the processor to obtain the sensing signal.

[0019] Preferably, between the I / Q modulator and the erbium-doped amplifier, a semiconductor optical amplifier is further included. Specifically: the arbitrary waveform generator is connected to the semiconductor optical amplifier; the semiconductor optical amplifier is connected in series between the I / Q modulator and the erbium-doped fiber amplifier and acts as an optical switch to chop the FrFT-DC optical signal.

[0020] Compared with the prior art, the beneficial effects of the embodiments of the present invention are as follows: the probe optical signal is converted into a FrFT-DC optical signal, and the FrFT is introduced into the LFM signal generation and pulse compression, which can achieve high LSPR and narrow main lobes; at the same time, by using orthogonal frequency division multiplexing of FrFT-DC signals with high spectral efficiency, these signals are highly overlapped in the Fourier domain but orthogonal in the fractional Fourier domain, significantly reducing the interference fading and improving the spectral efficiency. Description of the Drawings

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required to be used in the embodiments of the present invention will be briefly introduced below. Obviously, the following described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0022] Figure 1Flowchart of a sensing method based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0023] Figure 2 Schematic diagram of the coordinate transformation of the rotation of the fractional Fourier transform in the time-frequency plane;

[0024] Figure 3 Schematic diagram of the coordinate transformation of the multiplexing of the FrFT-DC signal in the time-frequency plane;

[0025] Figure 4 Flowchart of another sensing method based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0026] Figure 5 Flowchart of another sensing method based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0027] Figure 6 Flowchart of another sensing method based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0028] Figure 7 Flowchart of another sensing method based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0029] Figure 8 Schematic diagram of the structure of a sensing device based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0030] Figure 9 Schematic diagram of the structure of another sensing device based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0031] Figure 10 Schematic diagram of the measured data of a sensing device based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0032] Figure 11 Schematic diagram of the measured data of another sensing device based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0033] Figure 12 Schematic diagram of the measured data of another sensing device based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0034] Figure 13 Schematic diagram of the measured data of another sensing device based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0035] Figure 14 Schematic diagram of the measured data of another sensing device based on LFM signals in the FrFT domain provided by an embodiment of the present invention;

[0036] Figure 15 Another schematic diagram of the measured data of the sensing device based on the LFM signal in the FrFT domain provided by the embodiment of the present invention. Detailed implementation manners

[0037] In order to make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0038] The present invention is an architecture of a specific functional system. Therefore, in specific embodiments, the functional logic relationships of each structural module are mainly described, and no specific software and hardware implementation manners are limited.

[0039] In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other. The present invention will be described in detail below with reference to the drawings and embodiments.

[0040] Embodiment 1:

[0041] In the method provided in this embodiment, the fractional Fourier transform (FrFT) is introduced into the generation and pulse compression of LFM signals. The FrFT is a generalization of the traditional Fourier transform (FFT) and introduces rotation in the time-frequency domain. The decomposition basis function of the FrFT is extended from a single-frequency sine signal to a linear frequency modulation signal. Therefore, the FrFT can be regarded as a matching algorithm for LFM signals and has been widely used in the parameter estimation of LFM signals in the radar and communication fields. Using the additivity of the FrFT transform order, a p-order FrFT of a direct current (DC) signal (hereinafter referred to as a FrFT-DC signal) is used to generate an LFM pulse, and then it is compressed by a (1 - p)-order FrFT to complete demodulation. Theoretically, these two-order FrFTs are equivalent to the direct FFT of the DC signal. Therefore, it can be expected that it has a fully compressed LFM pulse with high PSLR and narrow FWHM. In order to eliminate the interference fading of the system, orthogonal frequency division multiplexing (OFDM) of LFM is proposed in the fractional Fourier domain. The generated FrFT-DC signal is multiplexed by introducing an integer multiple of the base frequency shift in the fractional Fourier domain. The frequency shift can be made much smaller than the bandwidth of the FrFT-DC signal in the Fourier domain, thereby generating a spectrally overlapping multi-carrier LFM signal with higher spectral efficiency.

[0042] As Figure 1 shown, the specific steps of the sensing method based on the LFM signal in the FrFT domain provided by the embodiment of the present invention are as follows:

[0043] Step 101: Generate a probe optical signal and a local oscillator optical signal, convert the probe optical signal into a FrFT-DC optical signal, and inject the converted FrFT-DC optical signal into a sensing optical fiber.

[0044] The fractional Fourier transform can be defined from different perspectives. To explain it more intuitively, the FrFT with order p can be regarded as a rotation of coordinates in the time-frequency plane with an angle of α = pπ / 2. Among them, the value of p can be set according to the actual experimental results. Then, the traditional FFT can be interpreted as a counterclockwise rotation with an angle of π / 2 from the time axis to the frequency axis. Figure 2 The time-domain DC signal shown by the thick solid line in (a) is transformed into an LFM through a rotation Figure 2 with an angle of pπ / 2 marked as α in (b). After increasing the rotation angle by (1 - p)π / 2, the LFM signal is compressed to the maximum extent as shown in Figure 2 (c). In this embodiment, in order to perform a fractional Fourier transform on the sampled digital sequence, the specific fractional Fourier transform used is a discrete fractional Fourier transform.

[0045] Mathematically, a typical LFM signal can be expressed in the form of Equation (1):

[0046]

[0047] In Equation (1), A is the amplitude, φ0, f0, and k represent the phase, center frequency, and modulation rate respectively, t is the time point within each period of the data sequence of a single LFM signal period, and exp is the exponential function with the natural constant e as the base.

[0048] In practice, the LFM signal has a finite duration, and a rectangular function can be introduced in the form of Equation (2):

[0049]

[0050] Among them, T is a clock period, t is the time point, and g(t) is the rectangular function.

[0051] The FrFT of c(t)g(t) is in the form of Equation (3) when the rotation angle α = -arccot(k):

[0052]

[0053] Among them, α is the rotation angle, A is the amplitude, k is the modulation rate, c(t) represents the LFM signal, g(t) is the rectangular function, F a is the fractional Fourier transform with a rotation angle of α, u represents the fractional-domain data points after transformation, and j is the imaginary symbol.

[0054] When \(k + \cot\theta=\alpha\), the integral in equation (3) can be regarded as the FFT of \(g(t)\). Then the FrFT of the \(2\alpha / \pi\) order LFM signal can be simplified to the form of equation (4):

[0055]

[0056] where \(A\) is the amplitude, \(\alpha\) is the rotation angle, \(T\) is a clock period, \(j\) is the imaginary symbol, \(u\) represents the fractional domain data points after transformation, and \(f_0\) is the center frequency.

[0057] It can be seen from equation (4) that the amplitude envelope of the LFM signal with a finite duration in the \(\alpha = -\arccot(k)\) order FrFT domain is a Sinc signal. The full width at half maximum (FWHM) of the main lobe is in the form of equation (5):

[0058] \(\Delta u=\sin\alpha / T = \cos\alpha / Tk\ (5)\)

[0059] where \(\alpha\) is the rotation angle, \(k\) is the modulation rate, \(\Delta u\) is the full width at half maximum of the main lobe, and \(T\) is a clock period.

[0060] In the method provided in this embodiment, the LFM signal is compressed by FrFT in a manner similar to the matched filtering method. The difference is that the main lobe is smaller than that of the matched filter because there is a product factor \(\cos\alpha\) compared with the matched filtering result. More importantly, by using the coordinate rotation of FrFT, the LFM signal is generated by the \(p\) order FrFT of the direct current (DC) signal, and then it is compressed by the \((1 - p)\) order FrFT. In theory, these two order FrFTs are equal to the direct FFT of the DC signal. Therefore, a fully compressed LFM pulse with high peak sidelobe ratio (PSLR) and narrow full width at half maximum (FWHM) can be expected.

[0061] The interference fading in DAS is usually eliminated through multiple uncorrelated sensing channels, including the frequency channels in the optical fiber and different optical field modes. In DAS using LFM as the probing light, the multi-frequency channel mostly adopts the internal pulse division method. Orthogonal frequency division multiplexing (OFDM) is a typical multi-carrier technology in the communication field, which has the characteristic of high spectral efficiency. The frequency offset between adjacent subcarriers is \(1 / T\). This concept is introduced into the FrFT-DC signal by shifting the signal with the frequency offset \(f\) N as Figure 3 (a) and Figure 3 (b) show the schematic diagram of the coordinate transformation of the multiplexing of the FrFT-DC signal in the time-frequency plane. In the figure, the FrFT-DC signal moves up and down in the time-frequency plane, generating LFM signals of three frequencies. According to equation (4), \(f\) NIt should satisfy \(f_N = N\Delta f = N\csc\alpha / T\), where \(N\) is an integer. When the frequency shift is small, the LFM signals overlap with each other and cannot be distinguished by a matched filter. However, through the 1 - p order FrFT, the LFM signals are compressed into three orthogonal components in the fractional Fourier domain, as shown in Figure 3 (c). In practical use, orthogonal frequency division maintains a high PSLR because there is no crosstalk between different subcarriers in the fractional Fourier domain, thus solving the problem of signal crosstalk in LFM signal sensing.

[0062] Furthermore, in order to obtain an optical pulse with a higher extinction ratio, the FrFT - DC optical signal can also be modulated to chop the FrFT - DC optical signal. Among them, the signal source for modulation is synchronized with the data source of the FrFT - DC signal. The extinction ratio represents the ratio of the optical power \(P_1\) when the laser emits all "1" codes to the optical power \(P_0\) when it emits all "0" codes. By chopping, the value of \(P_0\) can be made to become 0 to generate a darker background.

[0063] Step 102: Receive the backward Rayleigh scattering optical signal of the FrFT - DC optical signal returned by the sensing optical fiber, collect the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal (Rayleigh backscattered signal, abbreviated as RBS), and demodulate the sensing data from the beat signal. The frequencies of the two waves generating the beat signal are close but different. The beat signal is the result of using the interference technique in fiber sensing, and this result can reduce the frequency of the sensing signal light and lower the equipment cost required for detection.

[0064] When the FrFT - DC signal using orthogonal frequency division multiplexing passes through the optical fiber, the backward - scattered multi - carrier signal carries the same fiber scattering center information. However, the intensity of each sub - carrier is different, and some sub - carriers may deteriorate due to interference fading. The independent phase and amplitude information of each sub - carrier can be extracted by the 1 - p order FrFT, and the phase is denoted as \(\varphi\) n \(=\text{angle}(r\) t，τ，n ). Among them, \(\varphi_0\) is the initial phase, and \(r\) t，τ，n is the complex vector of each sub - carrier, where \(t\) represents the slow time axis with respect to the detection period, and \(\tau\) is the fast time axis along the sensing optical fiber. In order to more effectively combine the responses of all sub - carriers, the rotated - vector - sum (abbreviated as RVS) method is applied to the complex vector. The combined vector is in the form of equation (6):

[0065]

[0066] where \(R\) t,τ,n is the rotated - vector sum, and \(r\) t,τ,n is the complex vector of each sub - carrier.

[0067] In actual implementation, when using a polarization diversity receiver, the dual-polarization signal can also be combined by RVS to improve the SNR, and then the final phase information can be obtained through the beat phase between different optical fiber segments. In a φ-OTDR system, the change in physical quantity can be represented by the change in phase..

[0068] After steps 101-step 102 provided in this embodiment, an LFM pulse can be generated by the p-order FrFT of a direct current (DC) signal (hereinafter referred to as the FrFT-DC signal), and after receiving the scattered signal, it is demodulated by the 1-p order FrFT, thereby solving the problem of signal crosstalk in LFM signal sensing.

[0069] In specific implementation, as Figure 4 shown, the process of converting the probe optical signal into a FrFT-DC optical signal in step 101 can be completed through the following steps.

[0070] Step 201: Generate a FrFT-DC electrical signal using FrFT-DC signal data.

[0071] In order to convert the probe optical signal, it is first necessary to obtain FrFT-DC signal data, and then generate a FrFT-DC electrical signal for modulation according to the coding of the FrFT-DC signal data. In actual implementation, a p-order FrFT-DC electrical signal can be generated by an arbitrary waveform generator.

[0072] Step 202: Convert the probe optical signal into a FrFT-DC optical signal through the FrFT-DC electrical signal.

[0073] After generating the FrFT-DC electrical signal, the probe optical signal can be modulated by the FrFT-DC electrical signal to convert the probe optical signal into a FrFT-DC optical signal. In actual implementation, the modulation of the probe optical signal can be completed by an I / Q modulator.

[0074] After steps 201-step 202 provided in this embodiment, the probe optical signal can be converted into a FrFT-DC optical signal.

[0075] Furthermore, in order to obtain FrFT-DC signal data, an LFM signal data can be generated by the p-order FrFT operation of the DC signal data. In this embodiment, the LFM pulse is generated by the p-order FrFT of the DC signal, and then compressed by the 1-p order FrFT in the demodulation stage to achieve a high sidelobe suppression ratio.

[0076] After converting the probe optical signal into a FrFT-DC optical signal, it is also necessary to perform steps such as Figure 5 to collect the beat signal between the local oscillator optical signal and the backward Rayleigh scattered optical signal.

[0077] Step 301: Perform polarization diversity and 90° mixing reception on the backward Rayleigh scattering optical signal and the local oscillator light.

[0078] Step 302: Convert the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal into an electrical signal of multiple polarization channels.

[0079] Step 303: Perform data acquisition on the electrical signals of all polarization channels.

[0080] After passing through steps 301 - 303 provided in this embodiment, the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal can be acquired.

[0081] In a specific implementation scenario, when the number of polarization channels is 4 channels, as Figure 6 shown, the beat signal between the local oscillator optical signal and the processed backward Rayleigh scattering optical signal can be specifically converted into an electrical signal of multiple polarization channels through the following steps.

[0082] Step 401: Convert the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal into a 4-channel current signal.

[0083] Step 402: Recombine the 4-channel signals into two-channel complex signals, and the two-channel complex signals respectively correspond to the polarization signals of the X and Y polarization channels.

[0084] After passing through steps 401 - 402 provided in this embodiment, the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal in the 4-channel scenario can be completed.

[0085] After acquiring the beat signal between the local oscillator optical signal and the backward Rayleigh scattering optical signal, as Figure 7 shown, the sensing data can be demodulated from the beat signal according to the following steps.

[0086] Step 501: Use the (1 - p)-order FrFT operation to process each segment of the original electrical signal of each polarization channel by sliding along the time axis, generating the corresponding fractional Fourier domain data for each segment.

[0087] Step 502: Take the peak value of the corresponding fractional Fourier domain data of each segment to form a complex vector, and calculate the combined vector by the method of rotating vector sum, and use the combined vector as the sensing data.

[0088] After passing through steps 501 - 502 provided in this embodiment, the final sensing data can be obtained. This sensing data is not the actual physical parameter of sensing. In actual implementation, the sensing data also needs to be correspondingly operated according to specific requirements to obtain the required physical value.

[0089] The sensing method based on the LFM signal in the FrFT domain provided by this embodiment has the following beneficial effects.

[0090] 1. The method provided by this embodiment introduces the FrFT into LFM signal generation and pulse compression, which can achieve a high peak sidelobe ratio and a narrow main lobe, reducing the crosstalk to the sensing system caused by a relatively low peak sidelobe ratio (PSLR).

[0091] 2. The method provided by this embodiment uses FrFT-DC signals with high spectral efficiency through orthogonal frequency division multiplexing. These signals are highly overlapped in the Fourier domain but orthogonal in the fractional Fourier domain, significantly reducing interference fading and improving spectral efficiency.

[0092] Embodiment 2:

[0093] Based on the sensing method based on the LFM signal in the FrFT domain provided in the above Embodiment 1, the present invention further provides a sensing device based on the LFM signal in the FrFT domain that can be used to implement the above method, as Figure 8 shown, which is a schematic diagram of the device architecture of an embodiment of the present invention.

[0094] The device provided by this embodiment specifically includes: a laser, a coupler, an I / Q modulator (I: in-phase, Q: quadrature), an arbitrary waveform generator (abbreviated as AWG), an erbium-doped fiber amplifier (abbreviated as EDFA), a filter, a circulator, a sensing optical fiber, an integrated coherent receiver (abbreviated as ICR), a data acquisition card, and a processor.

[0095] The laser outputs a highly coherent continuous optical signal. The continuous optical signal is split into two paths by the coupler. One path of the optical signal enters the integrated coherent receiver as the local oscillator optical signal, and the other path of the optical signal enters the I / Q modulator as the probe optical signal. In a specific implementation, the coupler is preferably a polarization-maintaining coupler, and the splitting ratio is specifically 50:50; the I / Q modulator is preferably a single-polarization I / Q modulator.

[0096] The arbitrary waveform generator generates a FrFT-DC electrical signal. The FrFT-DC electrical signal drives the I / Q modulator to modulate the probe optical signal into a FrFT-DC optical signal. The FrFT-DC optical signal sequentially passes through the erbium-doped fiber amplifier and the filter, and is injected into the sensing optical fiber through the circulator. In a specific implementation, the filter is preferably a fiber Bragg grating.

[0097] The backward Rayleigh scattering optical signal of the FrFT-DC optical signal transmitted back by the sensing optical fiber enters the signal port of the integrated coherent receiver through the circulator and is converted into electrical signals of multiple polarization channels. The data acquisition card collects the electrical signals of all polarization channels, and the collected multi-channel digital signal data is demodulated by the processor to obtain the sensing signal.

[0098] Further, as Figure 9 shown, between the I / Q modulator and the erbium-doped amplifier, there is also a semiconductor optical amplifier (abbreviated as SOA). The arbitrary waveform generator is connected to the semiconductor optical amplifier; the semiconductor optical amplifier is connected in series between the I / Q modulator and the erbium-doped fiber amplifier and serves as an optical switch to chop the FrFT-DC optical signal. When the arbitrary waveform generator is single-channel, an arbitrary function generator can also be added between the arbitrary waveform generator and the semiconductor optical amplifier as the chopping signal source of the semiconductor optical amplifier. The arbitrary function generator is synchronized with the arbitrary waveform generator through an external clock source signal from the arbitrary waveform generator to generate a chopping signal and outputs the chopping signal to the semiconductor optical amplifier.

[0099] In actual implementation, the following process can be referred to. Use the device provided in this embodiment to complete the sensing method of the LFM signal in the FrFT domain provided in Embodiment 1. The following process is only an actual usage example in a specific implementation scenario. In actual implementation, the specific models, configurations, parameters, etc. of each component in the device provided in this embodiment can be adjusted according to actual situations to obtain the best usage effect.

[0100] In the example provided in this embodiment, the specific usage plan of the sensing device is as Figure 10 shown. The sensing optical fiber is a 10-km single-mode optical fiber. The FrFT-DC generates an LFM pulse with a duration of about 1 μs and a transformation order of 0.4. A fiber laser (model: NKT E15) with an ultra-narrow linewidth (less than 1 kHz) operating at 1550.12 nm is the laser source.

[0101] According to step 101, the laser outputs highly coherent continuous light, which is split into two parts by a polarization-maintaining coupler with a splitting ratio of 50:50.

[0102] A part of the continuous light is guided to the integrated coherent receiver (model: Fujitsu FIM24706 / 301) as the local oscillator (LO) signal.

[0103] Another part of the continuous light is sent into a single-polarization I / Q modulator to generate a probe optical signal. The arbitrary waveform generator pre-stores the LFM signal data generated by the p-order FrFT operation of the DC signal data, and outputs the generated FrFT-DC electrical signal to drive the I / Q modulator to complete the modulation of the FrFT-DC optical signal.

[0104] Furthermore, in order to obtain optical pulses with a higher extinction ratio, a semiconductor optical amplifier serving as an optical switch is cascaded to the I / Q modulator to chop the FrFT-DC optical signal, and the SOA is controlled by the AWG.

[0105] The chopped FrFT-DC optical signal passes through an erbium-doped fiber amplifier to increase the optical power of the FrFT-DC optical signal. The amplified spontaneous emission of the EDFA is filtered by a filter with a passband of about 0.8 nm (optical model: fiber Bragg grating). Then, the amplified FrFT-DC optical signal is used as a detection pulse and injected into the sensing fiber through an optical circulator.

[0106] According to step 102, the backward Rayleigh scattering optical signal from the sensing fiber enters the signal port of the ICR. After polarization diversity and 90° mixing, the beat signal between the LO and the RBS is converted into electrical signals of 4 polarization channels by the integrated coherent receiver in the ICR (model: 4-channel ICR). At the integrated coherent receiver, a 4-channel oscilloscope is used as an acquisition card for data acquisition. Digital signal processing is performed offline on a processor (model: personal computer). In the scenario of the 4-channel integrated correlation receiver and 4-channel acquisition card in this embodiment, the received electrical signals of the 4 polarization channels are recombined into two-channel complex signals, corresponding to the X and Y polarizations respectively.

[0107] After the above process, the beat signal between the local oscillator optical signal LO and the backward Rayleigh scattering optical signal RBS can be acquired. According to steps 501 - step 502, referring to the following process, the sensing signal can be demodulated from the beat signal. The following is only a specific example in the scenario of this embodiment. In specific implementations, appropriate demodulation algorithms and parameters can be selected according to actual needs.

[0108] For each polarization channel, the original signal is processed by sliding FrFT segment by segment along the optical fiber. The size of the moving window is the same as the length of the FrFT-DC signal, and the sliding step is one sampling point. The peak value of each segment in the fractional Fourier domain is taken to form a complex vector r t,τ,n . Finally, the complex vectors are combined by the RVS method to suppress signal fading.

[0109] To generate and process digital FrFT-DC electrical signals, discrete algorithms are required. There are mainly two discrete algorithms for numerical calculation: (1) The eigen-decomposition of the Discrete Fourier Transform (DFT) matrix is accurate but has a high complexity; (2) The discrete algorithm proposed by Ozaktas has a comparable computational complexity to the FFT, so this algorithm is preferably adopted. In this scenario, the sampling rate of the AWG is 1 Gsa / s and the number of sampling points is 1024. Then the duration of the generated FrFT-DC signal is approximately 1 μs. The FrFT order of the DC signal is 0.4. The FrFT-DC signal is as Figure 10 shown, and its spectral width is approximately 280 MHz. As Figure 11 shown, the signal is compressed by further performing a 1-p order, that is, a 0.6-order FrFT. To better illustrate the signal compression performance of the two-step FrFT, the intensity of the compression result is represented logarithmically, as Figure 12 shown. The compressed FrFT-DC signal has an ultra-narrow compression peak and a high PSLR greater than 80 dB. In addition, the maximum background noise is still lower than -60 dB. As a comparison, the PSLR of the result compressed by the matched filter is less than 20 dB, and the main lobe is also wider, as Figure 13 shown.

[0110] The orthogonal frequency division multiplexed FrFT-DC signal is generated by shifting the signal frequency by an offset f N . According to the system settings, f N = N csc(0.6*π / 2) / T, where T = 1024 ns, to generate an orthogonal frequency division multiplexed FrFT-DC signal with three frequencies. As Figure 14 shown, the multiplexed FrFT-DC signal is compressed into three frequencies in the fractional Fourier domain. Figure 15 shows the intensity of the compression result, which still maintains a high PSLR close to 80 dB but has a higher background noise near -50 dB. Therefore, the impact of orthogonal frequency division multiplexing on the compression performance of the FrFT-DC signal can be ignored.

[0111] According to the implementation results in the above actual scenario, when using the device provided in this embodiment to execute the method provided in Embodiment 1, using FrFT-DC pulses with a PSLR higher than 60 dB can significantly suppress crosstalk. In addition, by deploying orthogonal frequency division multiplexed FrFT-DC signals in the fractional Fourier domain, interference fading is greatly reduced.

[0112] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A sensing method based on FrFT domain LFM signal, characterized in that: Specifically include: Generate a detection light signal and a local oscillator light signal, convert the detection light signal into a FrFT-DC light signal, and inject the converted FrFT-DC light signal into the sensing optical fiber; Receive the backscattered Rayleigh light signal of the FrFT-DC optical signal transmitted back by the sensing fiber, collect the beat signal between the local oscillator optical signal and the backscattered Rayleigh light signal, and demodulate the sensing data from the beat signal; The demodulation of the sensing data from the beat signal specifically includes: using a (1-p)-order FrFT operation to slide the original electrical signal of each polarization channel along the time axis segment by segment to generate fractional Fourier domain data corresponding to each segment, taking the peak value of the fractional Fourier domain data corresponding to each segment to form a complex vector, and calculating a combined vector of the complex vector by the rotation vector sum method, and using the combined vector as the sensing data.

2. The sensing method based on FrFT domain LFM signal according to claim 1, characterized in that: The converting of the detection optical signal into the FrFT-DC optical signal specifically includes: generating a FrFT-DC electrical signal using the FrFT-DC signal data; The detection optical signal is converted into an FrFT-DC optical signal through the FrFT-DC electrical signal.

3. The sensing method based on FrFT domain LFM signal according to claim 2, characterized in that: Before generating the FrFT-DC electrical signal using the FrFT-DC signal data, the method further includes: The LFM signal data is generated by performing a p-th order FrFT operation on the DC signal data.

4. The sensing method based on FrFT domain LFM signal according to claim 3, characterized in that: The generating of LFM signal data by performing a p-order FrFT operation on the DC signal data specifically includes: The FrFT operation is specifically a discrete fractional Fourier transform.

5. The sensing method based on FrFT domain LFM signal according to claim 1, characterized in that: The collecting of the beat signal between the local oscillator light signal and the backscattered Rayleigh light signal specifically includes: The backscattered Rayleigh light signal and the local oscillator light are received by polarization diversity and 90° mixing; Converting the beat signal between the local oscillator light signal and the backscattered Rayleigh light signal into an electrical signal in multiple polarization channels; Data acquisition is performed on the electrical signals of all polarization channels.

6. The sensing method based on FrFT domain LFM signal according to claim 5, characterized in that: When the number of polarization channels is 4, converting the beat signal between the local oscillator light signal and the backscattered Rayleigh light signal into an electrical signal of multiple polarization channels specifically includes: Convert the beat signal between the local oscillator light signal and the backscattered Rayleigh light signal into a 4-channel current signal; The four-channel signals are recombined into two-channel complex signals, and the two-channel complex signals correspond to the polarization signals of the X and Y polarization channels respectively.

7. The sensing method based on FrFT domain LFM signal according to claim 1, characterized in that: After converting the detection optical signal into the FrFT-DC optical signal, the method further includes: The FrFT-DC optical signal is modulated to chop the FrFT-DC optical signal, wherein a modulated signal source is synchronized with a FrFT-DC signal data source.

8. A sensing device based on FrFT domain LFM signal, characterized in that: include: A laser, a coupler, an I / Q modulator, an arbitrary waveform generator, an erbium-doped amplifier, a filter, a circulator, a sensing fiber, an integrated coherent receiver, a data acquisition card, and a processor to implement the sensing method based on FrFT domain LFM signals according to any one of claims 1 to 7, specifically: The laser outputs a highly coherent continuous wave optical signal, which is split into two paths by a coupler. One path enters the integrated coherent receiver as the local oscillator signal, and the other path enters the I / Q modulator as the detection signal. An arbitrary waveform generator generates a FrFT-DC electrical signal, which drives an I / Q modulator to modulate the detection optical signal into a FrFT-DC optical signal. The FrFT-DC optical signal then passes through an erbium-doped fiber amplifier and a filter, and is injected into the sensing fiber through a circulator. The backscattered Rayleigh scattered light signal of the FrFT-DC optical signal transmitted back by the sensing fiber enters the signal port of the integrated coherent receiver through the circulator and is converted into electrical signals of multiple polarization channels. The data acquisition card collects data on the electrical signals of all polarization channels. The collected multi-channel digital signal data is demodulated by the processor to obtain the sensing signal.

9. The sensing device based on FrFT domain LFM signal according to claim 8, characterized in that: A semiconductor optical amplifier is also included between the I / Q modulator and the erbium-doped amplifier. Specifically: One output channel of the arbitrary waveform generator is connected to a semiconductor optical amplifier; The semiconductor optical amplifier is connected in series between the I / Q modulator and the erbium-doped fiber amplifier and acts as an optical switch to chop the FrFT-DC optical signal.

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