Long-distance frequency-expanded distributed optical fiber sensing system and sensing method

Abstract

A long-distance frequency-expanded distributed optical fiber sensing system and sensing method, comprising: a narrow-linewidth laser, an optical fiber isolator, a dense wavelength division multiplexer, an optical fiber coupler, a continuous optical amplifier, a narrowband filter, an acousto-optic modulator, an optical fiber circulator, a balanced detector, an acquisition card, and a host computer. By selecting the narrow-linewidth laser and controlling an arbitrary waveform generator, a mode-division multiplexed pulse sequence is obtained. After the pulse sequence enters the optical fiber to be measured, a Rayleigh backscattered light will be returned. After beating with the local light, the original data of the system is obtained. Subsequently, a compensation method is used to demodulate the signal, and the unwanted signals are suppressed successively, solving the problem of limited spurious-free dynamic range caused by phase mismatch of different modes.

Description

Technical Field

[0001] The present invention mainly relates to the field of distributed optical fiber sensing technology, and particularly to a long-distance frequency-expanded distributed optical fiber sensing system and a sensing method. Background Art

[0002] Distributed optical fiber sensing (DAS) technology has been widely applied in practical scenarios due to its advantages such as continuous distribution, real-time monitoring, and high sensitivity, providing new solutions for high-performance sensing and monitoring in fields such as oil and gas, power, chemical industry, construction, and transportation.

[0003] In order to extract rich information from distributed optical fiber sensing data to distinguish various environmental states, the development of high-performance distributed optical fiber sensing systems has become increasingly crucial. This not only includes the pursuit of sensitivity, measurement distance, and spatial resolution, but also aspects unique to distributed optical fiber sensing systems, such as the expansion of the response frequency band range.

[0004] In 2014, Zhengqing Pan et al. achieved a 20 kHz vibration detection bandwidth within a 10 km sensing range by multiplexing four different frequencies. This method of transmitting pulse sequences of different modes and splicing their demodulated signals can increase the response bandwidth. Research shows that this method is universal and applicable to coherent and incoherent detection, as well as uniform and non-uniform sampling. However, it is not easy to achieve compatibility between wide dynamic range and wide response bandwidth measurement. The limitation of the dynamic range is usually caused by sensor non-linearity, such as the random distribution of Rayleigh scattering centers and phase unwrapping errors. Directly splicing the demodulated signals of different modes will generate unexpected vibration peaks (showing waveform distortion in the time domain), and these peaks do not exist on the optical fiber to be measured. This not only hinders the long-distance detection ability of the distributed optical fiber sensing system, but also severely reduces the spurious-free dynamic range of measurable vibrations, which is contrary to the predictions of early research.

[0005] Therefore, it is a technical problem urgently to be solved in this field to design a distributed optical fiber sensing system that can solve the problem of time-domain waveform distortion caused by sensor non-linearity during different mode fusion and improve the dynamic range of measurable vibration amplitude. Summary of the Invention

[0006] Aiming at the problem of time-domain waveform distortion caused by sensor non-linearity during different mode fusion in the existing distributed optical fiber sensing system, the present invention proposes a long-distance frequency-expanded distributed optical fiber sensing system and a sensing method, integrating wavelength division multiplexing and frequency division multiplexing, enriching the method of broadening the response frequency band. At the same time, it solves the problem of time-domain waveform distortion caused by sensor non-linear response during different mode fusion, and further improves the dynamic range of measurable vibration amplitude.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] On the one hand, the present invention provides a long-distance frequency-expanded distributed optical fiber sensing system, including: a narrow-linewidth laser, an optical fiber isolator, a dense wavelength division multiplexer, an optical fiber coupler, a continuous optical amplifier, a narrow-band filter, an acousto-optic modulator, an optical fiber circulator, a balanced detector, a data acquisition card, and a host computer;

[0009] There are n narrow-linewidth lasers, where n is greater than or equal to 2, and the output wavelengths of the n narrow-linewidth lasers are different, and the optical frequencies are f1, f2,..., f n ;

[0010] The output ends of the narrow-linewidth lasers are respectively connected to a first dense wavelength division multiplexer through corresponding optical fiber isolators. The first dense wavelength division multiplexer is connected to a first optical fiber coupler. The first optical fiber coupler has two output ports. The first output port of the first optical fiber coupler is used to output the laser with most of the power. The first output port of the first optical fiber coupler is connected to a second dense wavelength division multiplexer. The second dense wavelength division multiplexer has n output ports. A continuous optical amplifier, a narrow-band filter, and an acousto-optic modulator are respectively connected to the optical paths corresponding to the n output ports of the second dense wavelength division multiplexer;

[0011] The third dense wavelength division multiplexer has n input ports. The output ends of the n acousto-optic modulators are respectively connected to an input port of the third dense wavelength division multiplexer. The output end of the third dense wavelength division multiplexer is connected to the first port of the optical fiber circulator. The interrogation light input through the first port of the optical fiber circulator is output from the second port of the optical fiber circulator. The second port of the optical fiber circulator is connected to the optical fiber to be measured. The Rayleigh backscattered light generated in the optical fiber to be measured returns to the third port of the optical fiber circulator through the second port of the optical fiber circulator and is output;

[0012] The second output port of the first optical fiber coupler outputs the laser with the remaining power. The second output port of the first optical fiber coupler is connected to the input end of the fourth dense wavelength division multiplexer. The output end of the fourth dense wavelength division multiplexer has n output ports. The i-th output port of the fourth dense wavelength division multiplexer is correspondingly connected to an input port on the input side of the (i + 1)-th optical fiber coupler for beating between the continuous local light and the subsequent generated Rayleigh backscattered light, where i = 1, 2,..., n;

[0013] The third port of the optical fiber circulator is connected to the input end of the fifth dense wavelength division multiplexer. The output end of the fifth dense wavelength division multiplexer has n output ports. The fifth dense wavelength division multiplexer is used to distinguish Rayleigh backscattered light of different wavelengths. The i-th output port of the fifth dense wavelength division multiplexer is correspondingly connected to another input port on the input side of the (i + 1)-th optical fiber coupler to beat the Rayleigh backscattered light with the local light;

[0014] The output port on the output side of the (i + 1)-th optical fiber coupler is correspondingly connected to a balanced detector, which converts the optical signal into an electrical signal. The acquisition card acquires the electrical signal and sends it to the upper computer for signal demodulation.

[0015] On the other hand, the present invention provides a sensing method for the above-mentioned long-distance frequency-expanded distributed optical fiber sensing system, including:

[0016] (S1) Start the long-distance frequency-expanded distributed optical fiber sensing system, and receive the original data of the vibration signal acquired by the acquisition card;

[0017] (S2) Perform frequency division processing on the received original data, and extract the corresponding signals by using different frequencies of frequency division multiplexing;

[0018] (S3) Perform IQ demodulation on the extracted signals to obtain 2n signals after demodulation in different modes;

[0019] (S4) Respectively set the DC terms of the 2n signals after demodulation in different modes to zero to achieve harmonic suppression;

[0020] (S5) Respectively perform Fourier transform on the 2n signals after demodulation in different modes, and extract the amplitudes of the signal aliasing frequencies in different modes , and calculate the average value of the amplitudes of the signal aliasing frequencies in 2n different modes ;

[0021] (S6) For the 2n signals after demodulation in different modes after harmonic suppression, in the time domain, compensate the amplitudes of the signal aliasing frequencies in different modes to , so as to perform average compensation on the signals after demodulation in each mode, achieve sum-frequency and difference-frequency suppression, and obtain a set of effective samples of the vibration signal;

[0022] (S7) Stitch the effective samples in chronological order to achieve the fusion and reconstruction of signals in different modes;

[0023] (S8) Perform Fourier transform on the signal after fusion and reconstruction to obtain the frequency information of the vibration event.

[0024] Compared with the prior art, the technical effects of the present invention:

[0025] By selecting a narrow linewidth laser and controlling an arbitrary waveform generator, the present invention obtains a pulse sequence of mode division (wavelength division, frequency division) multiplexing. After this sequence enters the optical fiber to be measured, a Rayleigh backscattered light will be returned, and the system original data is obtained after beating with the local light. Subsequently, a compensation method is used to demodulate the signal, and the non-expected signals (harmonics, sum-frequency, difference-frequency) are suppressed successively, solving the problem of limited spur-free dynamic range caused by phase mismatch in different modes. Brief Description of the Drawings

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0027] Figure 1 Schematic structural diagram of a long-distance frequency-expanded distributed optical fiber sensing system provided for an embodiment;

[0028] Figure 2 Schematic working flow diagram of a sensing method of a long-distance frequency-expanded distributed optical fiber sensing system provided for an embodiment;

[0029] Figure 3 Schematic structural diagram of a long-distance frequency-expanded distributed optical fiber sensing system provided for another embodiment. Where n = 2;

[0030] Figure 4 For Figure 3 Frequency domain diagrams of mode fusion reconstruction before and after processing at the vibration point position of the shown embodiment, where Figure 4 (a) is the frequency domain diagram of mode fusion reconstruction before processing at the vibration point position, Figure 4 (b) is the frequency domain diagram of mode fusion reconstruction after processing at the vibration point position;

[0031] Figure 5 For Figure 3 Time-distance power spectral density diagrams of the signals before and after processing of the shown embodiment, where Figure 5 (a) is the time-distance power spectral density diagram of the signal before processing, Figure 5 (b) is the time-distance power spectral density diagram of the signal after processing;

[0032] Reference numerals in the figures:

[0033] 101. First narrow linewidth laser; 102. Second narrow linewidth laser; 10(n). n-th narrow linewidth laser; 201. First optical fiber isolator; 202. Second optical fiber isolator; 20(n). n-th optical fiber isolator; 301. First dense wavelength division multiplexer; 302. Second dense wavelength division multiplexer; 303. Third dense wavelength division multiplexer; 304. Fourth dense wavelength division multiplexer; 305. Fifth dense wavelength division multiplexer; 401. First optical fiber coupler; 402. Second optical fiber coupler; 403. Third optical fiber coupler; 40(n + 1). (n + 1)-th optical fiber coupler; 501. First continuous optical amplifier; 502. Second continuous optical amplifier; 50(n). n-th continuous optical amplifier; 601. First narrowband filter; 602. Second narrowband filter; 60(n). n-th narrowband filter; 701. First acousto-optic modulator; 702. Second acousto-optic modulator; 70(n). n-th acousto-optic modulator; 801. Optical fiber circulator; 901. First balanced detector; 902. Second balanced detector; 90(n). n-th balanced detector; 1001. Acquisition card; 1101. Host computer; 1201. Arbitrary waveform generator; 1301. Driver; 1401. Optical fiber under test; 1402. Piezoelectric ceramic. Detailed implementation mode

[0034] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0035] Refer to Figure 1 An embodiment provides a long-distance frequency-expanded distributed optical fiber sensing system, including: narrow linewidth lasers, optical fiber isolators, first dense wavelength division multiplexer 301, second dense wavelength division multiplexer 302, third dense wavelength division multiplexer 303, fourth dense wavelength division multiplexer 304, fifth dense wavelength division multiplexer 305, optical fiber couplers, continuous optical amplifiers, narrowband filters, first acousto-optic modulator 701, second acousto-optic modulator 702, optical fiber circulator 801, first balanced detector 901, second balanced detector 902, acquisition card 1001, host computer 1101, arbitrary waveform generator 1201, driver 1301.

[0036] There are n narrow linewidth lasers, namely the first narrow linewidth laser 101, the second narrow linewidth laser 102,..., the n-th narrow linewidth laser 10(n), where n is greater than or equal to 2. The output wavelengths of the n narrow linewidth lasers are different, and the optical frequencies are f1, f2,..., f n。

[0037] There are n optical fiber isolators, namely the first optical fiber isolator 201, the second optical fiber isolator 202,......, the nth optical fiber isolator 20(n).

[0038] There are n continuous optical amplifiers, namely the first continuous optical amplifier 501, the second continuous optical amplifier 502,......, the nth continuous optical amplifier 50(n).

[0039] There are n narrowband filters, namely the first narrowband filter 601, the second narrowband filter 602,......, the nth narrowband filter 60(n).

[0040] There are n acousto-optic modulators, namely the first acousto-optic modulator 701, the second acousto-optic modulator 702,......, the nth acousto-optic modulator 70(n).

[0041] There are n + 1 optical fiber couplers, namely the first optical fiber coupler 401, the second optical fiber coupler 402, the third optical fiber coupler 403,...... the (n + 1)th optical fiber coupler 40(n + 1).

[0042] There are n balanced detectors, namely the first balanced detector 901, the second balanced detector 902,......, the nth balanced detector 90(n).

[0043] The output ends of the first narrow linewidth laser 101, the second narrow linewidth laser 102,......, the nth narrow linewidth laser 10(n) are respectively connected to the input ends of the first optical fiber isolator 201, the second optical fiber isolator 202,......, the nth optical fiber isolator 20(n). The output ends of the first optical fiber isolator 201, the second optical fiber isolator 202,......, the nth optical fiber isolator 20(n) are connected to the first dense wavelength division multiplexer 301. The first dense wavelength division multiplexer 301 is connected to the first optical fiber coupler 401. The first optical fiber coupler 401 is a 1×2 optical fiber coupler, which has two output ports with a splitting ratio of 90 / 10. The first output port of the first optical fiber coupler 401 outputs laser with 90% power. The first output port of the first optical fiber coupler 401 is connected to the second dense wavelength division multiplexer 302, and the laser with 90% power is further input to the second dense wavelength division multiplexer 302 and further modulated as interrogation light.

[0044] The second dense wavelength division multiplexer 302 has n output ports. The n output ports of the second dense wavelength division multiplexer 302 are respectively connected to the input ends of the first continuous optical amplifier 501, the second continuous optical amplifier 502, ……, the nth continuous optical amplifier 50(n). The output ends of the first continuous optical amplifier 501, the second continuous optical amplifier 502, ……, the nth continuous optical amplifier 50(n) are respectively connected to the input ends of the first narrowband filter 601, the second narrowband filter 602, ……, the nth narrowband filter 60(n). The output ends of the first narrowband filter 601, the second narrowband filter 602, ……, the nth narrowband filter 60(n) are respectively connected to the input ends of the first acousto-optic modulator 701, the second acousto-optic modulator 702, ……, the nth acousto-optic modulator 70(n). The third dense wavelength division multiplexer 303 has n input ports. The output ends of the first acousto-optic modulator 701, the second acousto-optic modulator 702, ……, the nth acousto-optic modulator 70(n) are respectively correspondingly connected to an input port of the third dense wavelength division multiplexer 303. The output end of the third dense wavelength division multiplexer 303 is connected to the first port of the optical fiber circulator 801. The interrogation light input through the first port of the optical fiber circulator 801 is output from the second port of the optical fiber circulator 801. The second port of the optical fiber circulator 801 is connected to the optical fiber under test 1401. The Rayleigh backscattered light generated in the optical fiber under test 1401 returns to the optical fiber circulator 801 through the second port and is output from the third port of the optical fiber circulator 801.

[0045] The second output port of the first optical fiber coupler 401 outputs the laser with the remaining 10% power. The second output port of the first optical fiber coupler 401 is connected to the input end of the fourth dense wavelength division multiplexer 304. The laser with 10% power enters the fourth dense wavelength division multiplexer 304. The output end of the fourth dense wavelength division multiplexer 304 has n output ports. The ith output port of the fourth dense wavelength division multiplexer 304 is correspondingly connected to an input port on the input side of the (i + 1)th optical fiber coupler to perform beat frequency with the subsequent generated Rayleigh backscattered light as continuous local light, where i = 1, 2, ……, n. The third port of the optical fiber circulator 801 is connected to the input end of the fifth dense wavelength division multiplexer 305. The output end of the fifth dense wavelength division multiplexer 305 has n output ports. The fifth dense wavelength division multiplexer 305 is used to distinguish Rayleigh backscattered light of different wavelengths. The ith output port of the fifth dense wavelength division multiplexer 305 is correspondingly connected to another input port on the input side of the (i + 1)th optical fiber coupler to perform beat frequency between the Rayleigh backscattered light and the local light.

[0046] The output ports on the output side of the (i + 1)-th optical fiber coupler are correspondingly connected to a balanced detector, namely the first balanced detector 901, the second balanced detector 902,......, the n-th balanced detector 90(n). The first balanced detector 901, the second balanced detector 902,......, the n-th balanced detector 90(n) convert the optical signal into an electrical signal. The first balanced detector 901, the second balanced detector 902,......, the n-th balanced detector 90(n) are connected to the acquisition card 1001, and the acquisition card 1001 acquires the electrical signal and sends it to the host computer 1101 for signal demodulation.

[0047] Among them, the first continuous optical amplifier 501, the second continuous optical amplifier 502,......, the n-th continuous optical amplifier 50(n) are used to amplify the continuous optical signal. The first narrowband filter 601, the second narrowband filter 602,......, the n-th narrowband filter 60(n) are used to filter out the amplifier spontaneous noise. The first acousto-optic modulator 701, the second acousto-optic modulator 702,......, the n-th acousto-optic modulator 70(n) are used to modulate the continuous light and output a pulse sequence containing multiple modes.

[0048] In this embodiment, an arbitrary waveform generator 1201 generates a pulse signal and is connected to the drivers 1301 of the first acousto-optic modulator 701, the second acousto-optic modulator 702,......, the n-th acousto-optic modulator 70(n) to modulate the first acousto-optic modulator 701, the second acousto-optic modulator 702,......, the n-th acousto-optic modulator 70(n). Specifically, the first acousto-optic modulator 701 modulates the continuous light with an optical frequency of f1 and outputs a single-frequency pulse sequence, and this single-frequency pulse sequence cycles with f1, as the period; at the same time, the second acousto-optic modulator 702 modulates the continuous light with an optical frequency of f2 and outputs a single-frequency pulse sequence, and this single-frequency pulse sequence cycles with f2, as the period...... The n-th acousto-optic modulator 70(n) modulates the continuous light with an optical frequency of f n and outputs a single-frequency pulse sequence, and this single-frequency pulse sequence cycles with f n 、 as the period,

[0049] The periods of each group of single-frequency pulse sequences are all T, but the starting positions differ by T / 2n.

[0050] The first acousto-optic modulator 701, the second acousto-optic modulator 702, ..., the nth acousto-optic modulator 70(n) are connected to the third dense wavelength division multiplexer 303, and are used to multiplex n pulse sequences with different wavelengths to obtain a single-frequency pulse sequence with a pulse interval of T / 2n, and the mode polling number is increased to 2n, so as to expand the response frequency band range. The clock of the arbitrary waveform generator 1201 is used as the reference clock, and at the same time, any output thereof is used as the trigger signal of the acquisition card 1001.

[0051] Refer to Figure 2 , Figure 2 FIG. is a schematic working flow diagram of a sensing method of a long-distance frequency-expanded distributed optical fiber sensing system provided for an embodiment, and includes the following steps:

[0052] (S1) Start the long-distance frequency-expanded distributed optical fiber sensing system, and receive the original data of the vibration signal collected by the acquisition card;

[0053] (S2) Perform frequency division processing on the received original data, and extract the corresponding signals by using different frequencies of frequency division multiplexing;

[0054] (S3) Perform IQ demodulation on the extracted signals to obtain the signals after demodulation in 2n different modes;

[0055] (S4) Respectively perform DC term zeroing on the signals after demodulation in 2n different modes to achieve harmonic suppression;

[0056] (S5) Respectively perform Fourier transform on the signals after demodulation in 2n different modes, and extract the amplitudes of the signal aliasing frequencies in different modes , and calculate the average value of the amplitudes of the signal aliasing frequencies in 2n different modes ;

[0057] (S6) For the signals after demodulation in 2n different modes after harmonic suppression, in the time domain, compensate the amplitudes of the signal aliasing frequencies in different modes to , so as to perform average compensation on the signals after demodulation in each mode, achieve sum-frequency and difference-frequency suppression, and obtain a set of effective samples of the vibration signal;

[0058] (S7) Stitch the effective samples in chronological order to achieve the fusion and reconstruction of signals in different modes;

[0059] (S8) Perform Fourier transform on the signal after fusion and reconstruction to obtain the frequency information of the vibration event.

[0060] In step (S4), the present invention proposes two available methods, and any one of the methods can effectively suppress the harmonic signal:

[0061] The first method: The signals demodulated from 2n different patterns are respectively processed by detrend to remove the drift trend, so as to achieve harmonic suppression.

[0062] The second method: The signals demodulated from 2n different patterns are respectively subtracted by the corresponding zero-frequency components, which can also effectively suppress the harmonic signals.

[0063] Referring to Figure 3 , an embodiment provides a long-distance frequency-expanded distributed optical fiber sensing system, including: a first narrow-linewidth laser 101, a second narrow-linewidth laser 102, a first optical fiber isolator 201, a second optical fiber isolator 202, a first dense wavelength division multiplexer 301, a second dense wavelength division multiplexer 302, a third dense wavelength division multiplexer 303, a fourth dense wavelength division multiplexer 304, a fifth dense wavelength division multiplexer 305, a first optical fiber coupler 401, a second optical fiber coupler 402, a third optical fiber coupler 403, a first continuous optical amplifier 501, a second continuous optical amplifier 502, a first narrowband filter 601, a second narrowband filter 602, a first acousto-optic modulator 701, a second acousto-optic modulator 702, an optical fiber circulator 801, a first balanced detector 901, a second balanced detector 902, a data acquisition card 1001, a host computer 1101, an arbitrary waveform generator 1201, and a driver 1301.

[0064] The output wavelengths of the first narrow-linewidth laser 101 and the second narrow-linewidth laser 102 are different, and they respectively generate continuous light with wavelengths of CH-30 and CH-34, and the optical frequencies are f1 and f2 respectively. The output ends of the first narrow-linewidth laser 101 and the second narrow-linewidth laser 102 are respectively connected to the input ends of the first optical fiber isolator 201 and the second optical fiber isolator 202. The output ends of the first optical fiber isolator 201 and the second optical fiber isolator 202 are connected to the first dense wavelength division multiplexer 301, and the first dense wavelength division multiplexer 301 is connected to the first optical fiber coupler 401. The first optical fiber coupler 401 is a 1×2 optical fiber coupler, which has two output ports, and the splitting ratio is 90 / 10. The first output port of the first optical fiber coupler 401 outputs laser with 90% power. The first output port of the first optical fiber coupler 401 is connected to the second dense wavelength division multiplexer 302, and the laser with 90% power is further input to the second dense wavelength division multiplexer 302 and further modulated as interrogation light.

[0065] The two output ports of the second dense wavelength division multiplexer 302 are respectively connected to the input ends of the first continuous optical amplifier 501 and the second continuous optical amplifier 502. The output ends of the first continuous optical amplifier 501 and the second continuous optical amplifier 502 are respectively connected to the input ends of the first narrowband filter 601 and the second narrowband filter 602. The output ends of the first narrowband filter 601 and the second narrowband filter 602 are respectively connected to the input ends of the first acousto-optic modulator 701 and the second acousto-optic modulator 702. The output ends of the first acousto-optic modulator 701 and the second acousto-optic modulator 702 are connected to the two input ends of the third dense wavelength division multiplexer 303. The output end of the third dense wavelength division multiplexer 303 is connected to the first port of the optical fiber circulator 801. The interrogation light input through the first port of the optical fiber circulator 801 is output from the second port of the optical fiber circulator 801. The second port of the optical fiber circulator 801 is connected to the optical fiber under test 1401. The Rayleigh backscattered light generated in the optical fiber under test 1401 returns to the optical fiber circulator 801 through the second port of the optical fiber circulator 801 and is output from the third port of the optical fiber circulator 801.

[0066] The second output port of the first optical fiber coupler 401 outputs the laser with the remaining 10% power. The second output port of the first optical fiber coupler 401 is connected to the input end of the fourth dense wavelength division multiplexer 304. The laser with 10% power enters the fourth dense wavelength division multiplexer 304. The two output ports at the other end of the fourth dense wavelength division multiplexer 304 are respectively connected to one input port on the input side of the second optical fiber coupler 402 and the third optical fiber coupler 403, and are used as continuous local light to beat with the subsequent generated Rayleigh backscattered light. The third port of the optical fiber circulator 801 is connected to one end of the fifth dense wavelength division multiplexer 305, which is used to distinguish Rayleigh backscattered light of different wavelengths. The two ports at the other end of the fifth dense wavelength division multiplexer 305 are respectively connected to the other input port on the input side of the second optical fiber coupler 402 and the third optical fiber coupler 403, and beat the Rayleigh backscattered light with the local light.

[0067] The output ports on the output sides of the second optical fiber coupler 402 and the third optical fiber coupler 403 are respectively connected to the first balanced detector 901 and the second balanced detector 902. The first balanced detector 901 and the second balanced detector 902 convert the optical signal into an electrical signal. The first balanced detector 901 and the second balanced detector 902 are connected to the acquisition card 1001, and the acquisition card 1001 collects the electrical signal and sends it to the upper computer 1101 for signal demodulation.

[0068] The first narrow linewidth laser 101 and the second narrow linewidth laser 102 are respectively connected to the first optical fiber isolator 201 and the second optical fiber isolator 202, which are used to prevent the returned laser from damaging the laser source. The first optical fiber isolator 201 and the second optical fiber isolator 202 are connected to the first dense wavelength division multiplexer 301, which is used to multiplex two continuous lights with different wavelengths. The first dense wavelength division multiplexer 301 is connected to the first optical fiber coupler 401, which is used to output two continuous lights with the same mode components. One of them enters the second dense wavelength division multiplexer 302 and is further modulated as interrogation light, and the other enters the fourth dense wavelength division multiplexer 304 as local light to beat with the subsequently generated Rayleigh backscattered light.

[0069] The first continuous optical amplifier 501 and the second continuous optical amplifier 502 are used to amplify the continuous optical signal. The first narrowband filter 601 and the second narrowband filter 602 are used to filter out the amplifier spontaneous noise. The first acousto-optic modulator 701 and the second acousto-optic modulator 702 are used to modulate the continuous light and output a pulse sequence containing multiple modes. In this embodiment, a pulse signal is generated by an arbitrary waveform generator 1201 and connected to the drivers 1301 of the first acousto-optic modulator 701 and the second acousto-optic modulator 702 to modulate the first acousto-optic modulator 701 and the second acousto-optic modulator 702. Specifically, the first acousto-optic modulator 701 modulates the continuous light with an optical frequency of f1 and outputs a single-frequency pulse sequence, and the single frequency cycles with f1, as the period; at the same time, the second acousto-optic modulator 702 modulates the continuous light with an optical frequency of f2 and outputs a single-frequency pulse sequence, and the single frequency cycles with f2, as the period; the periods of the above two single-frequency pulse sequences are both T, but the starting positions differ by T / 4, Figure 3 A pulse sequence of a complete period is plotted in.

[0070] The first acousto-optic modulator 701 and the second acousto-optic modulator 702 are connected to the third dense wavelength division multiplexer 303, which is used to multiplex two pulse sequences with different wavelengths to obtain a single-frequency pulse sequence with a pulse interval of T / 4, and the mode polling number is increased to 4 to realize the expansion of the response frequency band range. The clock of the arbitrary waveform generator 1201 is used as the reference clock, and its arbitrary output is used as the trigger signal of the acquisition card 1001.

[0071] The third dense wavelength division multiplexer 303 is connected to the first port of the optical fiber circulator 801. A single-frequency pulse sequence with a pulse interval of T / 4 obtained after multiplexing by the third dense wavelength division multiplexer 303 is input into the first port of the optical fiber circulator 801, and enters the fiber under test 1401 through the second port of the optical fiber circulator 801, realizing the transmission of the pulse sequence. The Rayleigh backscattered light generated in the fiber under test 1401 returns to the third port of the optical fiber circulator 801 through the second port of the optical fiber circulator 801 and is output. The third port of the optical fiber circulator 801 is connected to the fifth dense wavelength division multiplexer 305, which is used to distinguish Rayleigh backscattered light of different wavelengths.

[0072] The second output port of the first optical fiber coupler 401 outputs the laser with the remaining 10% power. The second output port of the first optical fiber coupler 401 is connected to the input end of the fourth dense wavelength division multiplexer 304. The laser with 10% power enters the fourth dense wavelength division multiplexer 304 as the local light. The local light part is demultiplexed by the fourth dense wavelength division multiplexer 304, and the Rayleigh backscattered light is demultiplexed by the fifth dense wavelength division multiplexer 305. After demultiplexing, they are respectively input into the second optical fiber coupler 402 and the third optical fiber coupler 403. Both the second optical fiber coupler 402 and the third optical fiber coupler 403 are 2×2 couplers. After beat frequency, they are respectively received by the first balanced detector 901 and the second balanced detector 902 and converted into electrical signals. The acquisition card 1001 acquires the electrical signals and sends them to the host computer 1101 for signal demodulation.

[0073] In this embodiment, the clock of an arbitrary waveform generator is used as the reference clock, and its arbitrary output is used as the trigger signal of the acquisition card. The acquisition card sends the acquired raw data to the host computer for signal demodulation, and at the same time completes harmonic, sum frequency, and difference frequency suppression, finally improving the dynamic range of the measurable vibration amplitude. The sensing method of the long-distance frequency-expanded distributed optical fiber sensing system provided in this embodiment includes the following steps:

[0074] (S1) Start the long-distance frequency-expanded distributed optical fiber sensing system and receive the raw data of the vibration signal acquired by the acquisition card;

[0075] (S2) Perform frequency division processing on the received raw data and extract the corresponding signals using different frequencies of frequency division multiplexing;

[0076] (S3) Perform IQ demodulation on the extracted signals to obtain a set of phase information of different modes; in this embodiment, the number of mode polling is 4, and the optical frequencies are f1, f2, 、 . According to the Nyquist sampling theorem, under the condition of undersampling, the frequency of the vibration event detected will be aliased, and there is a The mathematical relationship. Therefore, the phase information of the four modes does not present the vibration frequency of the event, but the aliased frequency of 500 Hz. The detection pulse of each mode provides a valid sample of a vibration signal, and the valid samples are staggered in time. After signal fusion and reconstruction, the response frequency band can be extended. The response frequency band range of the conventional system is half of the detection pulse frequency, that is, 1 / 2T. The detection pulses of the four different modes increase the response frequency band range of the system to 4 / 2T. Due to the non-linear response of the sensor, there are differences in the amplitudes of the zero frequency and the signal aliasing frequency. Direct fusion will produce large distortion and non-expected frequency components, such as Figure 4 (a) shows Figure 4 For Figure 3 The frequency domain diagrams of the mode fusion and reconstruction before and after the vibration point position processing in the shown embodiment, where Figure 4 (a) is the frequency domain diagram of the mode fusion and reconstruction before the vibration point position processing. The above frequency components can be divided into two categories according to the generation reasons: harmonics and sum frequencies, difference frequency quantities. The present invention proposes a dynamic range improvement method to eliminate the above non-expected signals in sequence, so as to restore the vibration event signal.

[0077] (S4) Set the DC term of the signals demodulated from the 4 different modes in (S3) to zero to achieve harmonic suppression.

[0078] (S5) Perform Fourier transform on the signals demodulated from the 4 different modes respectively, extract the amplitudes of the signal aliasing frequencies of different modes , and calculate the average value of the amplitudes of the signal aliasing frequencies of the 4 different modes ;

[0079] (S5) Perform Fourier transform on the signals demodulated from the 4 different modes in (S3) respectively, extract the amplitudes of the signal aliasing frequencies of different modes , and calculate the average value of the amplitudes of the signal aliasing frequencies of the 4 different modes ;

[0080] (S6) For the signals demodulated from the 4 different modes after harmonic suppression, compensate the amplitudes of the signal aliasing frequencies of different modes to in the time domain to perform average compensation on the signals demodulated from each mode and achieve sum frequency and difference frequency suppression, and obtain a set of valid samples of the vibration signal.

[0081] (S7) Stitch the valid samples in chronological order to achieve the fusion and reconstruction of the signals of different modes. The fusion and reconstruction result is as shown in Figure 4 (b), Figure 4 (b) is the frequency domain diagram of the mode fusion and reconstruction after the vibration point position processing.

[0082] (S8) Perform a Fourier transform on the signal after fusion reconstruction in (S7) to obtain the frequency information of the vibration event. Refer to Figure 5 , Figure 5 is Figure 3 the time-distance power spectral density diagram of the signals before and after processing in the shown embodiment, where Figure 5 (a) is the time-distance power spectral density diagram of the signal before processing, Figure 5 (b) is the time-distance power spectral density diagram of the signal after processing. Figure 5 (b) Compared with Figure 5 (a), the unwanted signals are effectively eliminated, while the vibration event signals are restored, and the dynamic range of the measurable vibration amplitude is increased. To more clearly show the improvement effect of the method proposed by the present invention, the field of view is enlarged to the vicinity of the vibration event, and the lighter-colored part in the figure is the position of the signal.

[0083] This embodiment only analyzes the case where the number of mode polling is 4, and other situations (such as Figure 1 shown) can be processed by the same process. That is, the extended long-distance frequency-expanded distributed optical fiber sensing system shown in Figure 1 can be used, and narrow linewidth lasers with multiple different wavelengths are selected to increase the number of wavelength division multiplexing. The interrogation optical path part needs to be adjusted accordingly, and a fiber coupler and a balanced detector are added during beat frequency.

[0084] In summary, the present invention first proposes a long-distance frequency-expanded distributed optical fiber sensing system based on mode polling, integrating wavelength division multiplexing and frequency division multiplexing, enriching the method of broadening the response frequency band, and then demodulating the signal using a compensation method, suppressing the unwanted signals (harmonics, sum frequency, difference frequency) one after another, and solving the problem of limited spurious-free dynamic range caused by phase mismatch of different modes.

[0085] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0086] The above-described embodiments only represent several implementation manners of the present application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

[0087] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. Sensing method of a long-distance frequency-expanded distributed optical fiber sensing system, characterized in that, Including the following steps: (S1) Start the long-distance frequency-expanded distributed optical fiber sensing system, and receive the original data of the vibration signal collected by the acquisition card. The long-distance frequency-expanded distributed optical fiber sensing system includes: a narrow linewidth laser, an optical fiber isolator, a dense wavelength division multiplexer, an optical fiber coupler, a continuous optical amplifier, a narrowband filter, an acousto-optic modulator, an optical fiber circulator, a balanced detector, an acquisition card, and a host computer; There are n narrow-linewidth lasers, where n is greater than or equal to 2, and the output wavelengths of the n narrow-linewidth lasers are different, and the optical frequencies are f1, f2,......, f n ; Lasers with different wavelengths output by each narrow linewidth laser are input into the first dense wavelength division multiplexer. The first dense wavelength division multiplexer is connected to the first optical fiber coupler. The first optical fiber coupler has two output ports. The first output port of the first optical fiber coupler is used to output the laser with most of the power. The first output port of the first optical fiber coupler is connected to the second dense wavelength division multiplexer; The second dense wavelength division multiplexer has n output ports, and a continuous optical amplifier, a narrowband filter, and an acousto-optic modulator are respectively connected to the optical paths corresponding to the n output ports of the second dense wavelength division multiplexer; The third dense wavelength division multiplexer has n input ports. The output ends of the n acousto-optic modulators are respectively connected to an input port of the third dense wavelength division multiplexer. The output end of the third dense wavelength division multiplexer is connected to the first port of the optical fiber circulator. The interrogation light input through the first port of the optical fiber circulator is output from the second port of the optical fiber circulator. The second port of the optical fiber circulator is connected to the optical fiber to be measured. The Rayleigh backscattered light generated in the optical fiber to be measured returns to the third port of the optical fiber circulator through the second port of the optical fiber circulator and is output; The second output port of the first optical fiber coupler outputs the laser with the remaining power. The second output port of the first optical fiber coupler is connected to the input end of the fourth dense wavelength division multiplexer. The output end of the fourth dense wavelength division multiplexer has n output ports. The i-th output port of the fourth dense wavelength division multiplexer is correspondingly connected to an input port on the input side of the (i + 1)-th optical fiber coupler for beating the continuous local light with the subsequent generated Rayleigh backscattered light, where i = 1, 2,... n; The third port of the optical fiber circulator is connected to the input end of the fifth dense wavelength division multiplexer. The output end of the fifth dense wavelength division multiplexer has n output ports. The fifth dense wavelength division multiplexer is used to distinguish Rayleigh backscattered light with different wavelengths; The i-th output port of the fifth dense wavelength division multiplexer is correspondingly connected to another input port on the input side of the (i + 1)-th optical fiber coupler to beat the Rayleigh backscattered light with the local light; The output port on the output side of the (i + 1)-th optical fiber coupler is correspondingly connected to a balanced detector. The balanced detector converts the optical signal into an electrical signal, and the acquisition card collects the electrical signal and sends it to the host computer for signal demodulation; (S2) Perform frequency division processing on the received original data, and extract the corresponding signals using different frequencies of frequency division multiplexing; (S3) Perform IQ demodulation on the extracted signals to obtain signals after demodulation in 2n different modes; (S4) Respectively set the DC terms of the signals after demodulation in 2n different modes to zero to achieve harmonic suppression; (S5) Perform Fourier transforms on the signals demodulated from 2n different modes respectively, extract the amplitudes of the signal aliasing frequencies of different modes , and calculate the average value of the amplitudes of the signal aliasing frequencies of 2n different modes ; (S6) For the signals after demodulation of 2n different modes after harmonic suppression, in the time domain, the amplitudes of the aliasing frequencies of the signals of different modes are compensated to , so as to perform average compensation on the signals after demodulation of each mode, realize the suppression of sum frequency and difference frequency, and obtain a set of effective samples of vibration signals; (S7) Stitch the effective samples in chronological order to achieve the fusion and reconstruction of signals in different modes; (S8) Perform a Fourier transform on the fused and reconstructed signal to obtain the frequency information of the vibration event.

2. The sensing method of the long-distance frequency-expanded distributed optical fiber sensing system according to claim 1, characterized in that: In step (S4), the method for suppressing the harmonic signal is as follows: Remove the drift trend from the signals demodulated in 2n different modes respectively to achieve harmonic suppression.

3. The sensing method of the long-distance frequency-expanded distributed optical fiber sensing system according to claim 1, characterized in that: In step (S4), the method for suppressing the harmonic signal is as follows: Subtract the corresponding zero-frequency quantity from the signals demodulated in 2n different modes respectively to effectively suppress the harmonic signal.

4. The sensing method of the long-distance frequency-expanded distributed optical fiber sensing system according to claim 1, characterized in that: An optical fiber isolator is connected to the output end of each narrow-linewidth laser, and is connected to the first dense wavelength division multiplexer through the corresponding optical fiber isolator.

5. The sensing method of the long-distance frequency-expanded distributed optical fiber sensing system according to any one of claims 1 to 4, characterized in that, The first optical fiber coupler is a 1×2 optical fiber coupler, and the splitting ratio of its two output ports is 90 / 10. The first output port of the first optical fiber coupler outputs laser with 90% power. The laser with 90% power is input into the second dense wavelength division multiplexer and further modulated as the interrogation light. The second output port of the first optical fiber coupler outputs the remaining laser with 10% power.

6. The sensing method of the long-distance frequency-expanded distributed optical fiber sensing system according to claim 5, characterized in that There are n acousto-optic modulators, namely the first acousto-optic modulator, the second acousto-optic modulator,..., the nth acousto-optic modulator. The acousto-optic modulator is used to modulate the continuous light and output a pulse sequence containing multiple modes.

7. The sensing method of the long-distance frequency-expanded distributed optical fiber sensing system according to claim 5, characterized in that, It also includes an arbitrary waveform generator and a driver. The arbitrary waveform generator generates a pulse signal and is connected to the drivers of the first acousto-optic modulator, the second acousto-optic modulator,..., the nth acousto-optic modulator to modulate the first acousto-optic modulator, the second acousto-optic modulator,..., the nth acousto-optic modulator.

8. The sensing method of the long-distance frequency-expanded distributed optical fiber sensing system according to claim 5, characterized in that, The first acousto-optic modulator modulates the continuous light with an optical frequency of f1, and outputs a single-frequency pulse sequence, and this single-frequency pulse sequence cycles with f1, as the period; the second acousto-optic modulator modulates the continuous light with an optical frequency of f2, and outputs a single-frequency pulse sequence, and this single-frequency pulse sequence cycles with f2, as the period...... The nth acousto-optic modulator modulates the continuous light with an optical frequency of f n , and outputs a single-frequency pulse sequence, and this single-frequency pulse sequence cycles with f n , as the period. The periods of each group of single-frequency pulse sequences are all T, and the starting position differences are T / 2n.

9. The sensing method of the long-distance frequency-expanded distributed optical fiber sensing system according to claim 6, characterized in that, The first acousto-optic modulator, the second acousto-optic modulator,..., the nth acousto-optic modulator are connected to the third dense wavelength division multiplexer, and are used to multiplex n pulse sequences with different wavelengths to obtain a single-frequency pulse sequence with a pulse interval of T / 2n, and the mode polling number is increased to 2n to achieve an expansion of the response frequency band range; the clock of the arbitrary waveform generator is used as the reference clock, and at the same time, its arbitrary output is used as the trigger signal of the acquisition card.

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