Digital dechirped fiber optic sensing system and method

Abstract

This invention provides a fiber optic sensing system and method based on digital dechirping. A coupler splits a narrow-linewidth laser into two paths, which are transmitted to a sweep frequency modulation module and an optical mixer, respectively. The sweep frequency modulation module modulates the narrow-linewidth laser into sweep frequency long pulses, which are then transmitted to the sensing fiber. The sensing fiber returns scattered light to the optical mixer. The optical mixer coherently mixes the narrow-linewidth laser and the scattered light to generate four mixed-frequency beams. A data acquisition and processing module acquires the first and second coherent signals obtained after detecting the mixed-frequency beams by first and second balanced detectors to obtain complex scattered signals. The complex scattered signals are dechirped to obtain dechirped scattered signals. The dechirped scattered signals are converted into frequency-domain scattered signals and demodulated to obtain the perturbation magnitude at various locations on the sensing fiber. This invention has the advantages of high spatial resolution, low sampling frequency, and demodulation unaffected by fading, while also enabling long-distance sensing.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing, specifically relating to a fiber optic sensing system and method based on digital dechirping. Background Technology

[0002] Commonly used devices in fiber optic sensing include phase-sensitive optical time-domain reflectometers (Φ-OTDRs) and optical frequency-domain reflectometers (OFDRs). Currently, fiber optic sensing systems based on Φ-OTDRs typically improve spatial resolution by compressing pulses. However, this increases modulation and detection bandwidth, and higher spatial resolution requires higher analog-to-digital conversion sampling frequencies, leading to higher system costs. Furthermore, Φ-OTDRs generally employ phase demodulation, which is susceptible to polarization and coherent fading, resulting in poor local signal-to-noise ratios and even frequent demodulation errors. Suppressing fading requires additional hardware or computational costs, such as polarization diversity and rotation vector summation, further increasing system cost. On the other hand, for OFDR-based fiber optic sensing systems, the sampling frequency is generally much smaller than the system modulation bandwidth and is unaffected by fading, resulting in higher spatial resolution. However, the sensing distance is typically much shorter than that of Φ-OTDRs. It is evident that current fiber optic sensing systems face challenges in achieving long-distance sensing. To achieve high spatial resolution, the sampling frequency needs to be increased, and the demodulation phase is affected by fading. Summary of the Invention

[0003] This invention provides a fiber optic sensing system and method based on digital dechirping to solve the problems of current fiber optic sensing systems in achieving long-distance sensing, where increasing the sampling frequency is required to achieve high spatial resolution, and the demodulation phase is affected by fading.

[0004] According to a first aspect of the present invention, a digitally chirped fiber optic sensing system is provided, comprising a narrow-linewidth laser, a coupler, a sweep frequency modulation module, an optical circulator, a sensing fiber, an optical mixer, a first balanced detector, a second balanced detector, and a data acquisition and processing module. The output of the narrow-linewidth laser is connected to the input of the coupler. The first output of the coupler is connected to the first end of the optical circulator via the sweep frequency modulation module. The second output is connected to the first input of the optical mixer. The second end of the optical circulator is connected to the sensing fiber. The third end of the optical circulator is connected to the second input of the optical mixer. The first and second outputs of the optical mixer are connected to the input of the first balanced detector. The third and fourth outputs are connected to the input of the second balanced detector. The outputs of the first and second balanced detectors are connected to the data acquisition and processing module.

[0005] The coupler splits the narrow-linewidth laser provided by the narrow-linewidth laser into two paths, one of which is transmitted to the sweep frequency modulation module and the other to the optical mixer; the sweep frequency modulation module modulates the narrow-linewidth laser into a sweep frequency long pulse light, and transmits the sweep frequency long pulse light to the sensing fiber through the optical circulator; the sensing fiber transmits the returned scattered light to the optical mixer through the optical circulator.

[0006] The optical mixer coherently mixes the narrow-linewidth laser with the scattered light to generate four mixed light signals; the first balanced detector detects two of the mixed light signals to obtain a first coherent signal; the second balanced detector detects the other two mixed light signals to obtain a second coherent signal.

[0007] The data acquisition and processing module acquires the first coherent signal and the second coherent signal to obtain the complex scattering signal I. OTDR (n; for the complex scattering signal I) OTDR (n dechirp, obtain the dechirped scattering signal I) OFDR (n; the dechirped scattering signal I) OFDR (n is converted into a frequency domain scattering signal, and the frequency domain scattering signal is demodulated to obtain the disturbance magnitude at each position on the sensing fiber.)

[0008] In an optional solid line configuration, an optical amplification module is also included. The optical amplification module is located between the sweep frequency modulation module and the first end of the optical circulator. The optical amplification module amplifies the sweep frequency long pulse light output by the sweep frequency modulation module and transmits the amplified sweep frequency long pulse light to the sensing optical fiber through the optical circulator.

[0009] According to a second aspect of the present invention, a sensing method for the above-described digitally chirped fiber optic sensing system is provided, comprising:

[0010] Step S110: Obtain the complex scattering signal I OTDR (n;

[0011] Step S120: The complex scattering signal I... OTDR (n dechirp, obtain the dechirped scattering signal I) OFDR (n;

[0012] Step S130: The dechirped scattering signal I OFDR (n is converted into a frequency domain scattering signal, and the frequency domain scattering signal is demodulated to obtain the disturbance magnitude at each position on the sensing fiber.)

[0013] In an optional solid line method, step S110 specifically includes: step S111, splitting the narrow linewidth laser into two paths, one path being modulated into a frequency-sweeping long pulse light and transmitted to the sensing fiber, and the other path being directly transmitted to the optical mixer.

[0014] Step S112: After receiving the swept long pulse light, the sensing fiber transmits the returned scattered light to the optical mixer.

[0015] Step S123: The optical mixer coherently mixes the narrow linewidth laser with the scattered light to generate four mixed beams.

[0016] Step S124: The first balanced detector detects two of the mixed optical signals to obtain a first coherent signal; the second balanced detector detects the other two mixed optical signals to obtain a second coherent signal.

[0017] Step S125: Use the first coherent signal as the complex scattering signal I. OTDR (The real part of n, taking the second coherent signal as the complex scattering signal I) OTDR (The imaginary part of n).

[0018] In another alternative solid-line approach, step S120 specifically includes: using the generated local oscillator signal R(n) to scatter the complex scattering signal I. OTDR (n performs digital dechirping to obtain the dechirped scattering signal I) OFDR (n; the local oscillator signal R(n) is a discrete signal of the reference signal R(t) in the digital domain. The reference signal R(t) is related to the characteristics of the sweep frequency modulation module and is continuous in time. The complex scattering signal I OTDR (n) and its local oscillator signal R(n) have the same frequency variation pattern on the time axis.

[0019] In another alternative solid-line representation, the reference signal R(t) is: R(t=exp[j*(2πf0tπkt)] 2 ], where f0 represents the starting frequency of modulation by the sweep frequency modulation module, k represents the modulation rate, and t represents time;

[0020] In step S120, the complex scattering signal I is processed according to the following formula. OTDR (n performs digital dechirping to obtain the dechirped scattering signal I) OFDR (n:I OFDR (n=conj OTDR (n. / (n, where “. / ” represents dot division and “conj” represents complex conjugate operation.)

[0021] In another alternative solid-line approach, let the sampling frequency range be [-f D / 2,+f D / 2], the complex scattering signal I at each position on the sensing fiber. OTDR (The frequency change start time of n is different for each, and the frequency change start time of the local oscillator signal R(n) is 0;

[0022] The complex scattering signal I at each position on the sensing optical fiber OTDR The frequency variation law of 9n and the local oscillator signal R(n) on the time axis is the same, wherein the frequency variation law on the time axis is as follows: as time increases, the frequency starts from 0 and monotonically increases at a set slope, and after time T, the frequency reaches +f. D When / 2, the frequency drops to -f D / 2, when the frequency drops to -f D After / 2, the slope is monotonically increased according to the set value until, after a time of 2T, the frequencies of both reach +f. D / 2, after which the frequency drops to -f D / 2, and so on, where T is the frequency sweep modulation time of the frequency sweep modulation module, f D This is the maximum sampling frequency;

[0023] For each position on the sensing fiber, at any time, the local oscillator signal R(n) and the corresponding complex scattered signal I at that position... OTDR (The frequency difference of n is the same, and the frequency difference is the difference between the local oscillator signal R(n) and the complex scattered signal I corresponding to that position.) OTDR (n's beat frequency f) i And for that location, the corresponding dechirped scattering signal I OFDR (frequency f of n) i The different positions on the sensing fiber correspond to the dechirped scattering signal I. OFDR (frequency f of n) i They are all different, the frequency f i Less than f D / 2.

[0024] In another optional solid line method, the sweep modulation time T is the duration of the sweep long pulse light transmission to the end of the sensing fiber, which is used to characterize the sensing length of the sensing fiber.

[0025] When the frequency sweep modulation duration T is constant, the maximum sampling frequency f is adjusted by changing the set slope. D Adjustments are made to meet the sampling frequency requirements, wherein the smaller the set slope, the higher the maximum sampling frequency f. D The smaller the value; at the maximum sampling frequency f DAt a certain time, the sweep frequency modulation duration T is adjusted by adjusting the set slope, thereby adjusting the sensing length. The smaller the set slope, the larger the sensing length. The adjustment of the set slope is achieved by adjusting the sweep frequency modulation parameters of the sweep frequency modulation module.

[0026] In another alternative solid line method, the sweep frequency modulation parameters are adjusted according to the following steps: Step S201: Determine the duration required for the sweep frequency long pulse light to be transmitted to the end of the sensing fiber of the required sensing length, i.e., the sweep frequency modulation time T, according to the required sensing length.

[0027] Step S202: Based on the determined sweep frequency modulation time T and the required maximum sampling frequency f D According to the formula Determine the set slope k;

[0028] Step S203: Adjust the sweep frequency modulation parameters of the sweep frequency modulation module according to the determined set slope k.

[0029] In another alternative solid line approach, step S130 specifically includes: step S131, for each position on the sensing fiber, using different sampling frequencies to obtain the corresponding dechirped scattering signal I. OFDR (n is sampled;

[0030] Step S132: For each location on the sensing fiber, the dechirped scattering signal I... OFDR (n, for the dechirped scattering signal I) OFDR (n is subjected to Fourier Transform (FFT) and converted into a frequency domain scattering signal;

[0031] Step S133: After windowing in the frequency domain, perform an inverse Fourier transform (IFFT) on the frequency domain scattering signal to convert it into a time domain scattering signal;

[0032] Step S134: The time-domain scattering signal is compared with the dechirped scattering signal I. OFDR (The time-domain signal corresponding to the first pulse in n is used as the reference time-domain signal, and this reference time-domain signal is compared with the dechirped scattering signal I) OFDR (The time-domain signals of other pulses in n are cross-correlated to obtain the shift of the time-domain scattered signal, thereby demodulating the magnitude of the disturbance at that position on the sensing fiber.)

[0033] The beneficial effects of this invention are:

[0034] 1. This invention modulates a narrow-linewidth laser into a swept-frequency long pulse, which is then transmitted to a sensing fiber. The narrow-linewidth laser is then coherently mixed with the scattered light returned from the sensing fiber. The four mixed-frequency beams generated by the coherent mixing are detected to obtain a first coherent signal and a second coherent signal. These two coherent signals are then acquired to obtain a complex scattered signal. After dechirping the complex scattered signal to obtain a dechirped scattered signal, it can be converted into a frequency-domain scattered signal. At this point, OFDR demodulation can be used to demodulate the frequency-domain scattered signal. Since OFDR has the characteristics of high spatial resolution, low sampling frequency, and demodulation unaffected by fading, ... This invention obtains complex scattered signals based on Φ-OTDR, thus combining the advantages of both OFDR and Φ-OTDR. It features high spatial resolution, low sampling frequency, and demodulation unaffected by fading, while also enabling long-distance sensing. Furthermore, this invention improves measurement sensitivity. The frequency response of this invention can be enhanced through time-frequency multiplexing and other methods, enabling it to detect high-frequency dynamic strain. Since the local oscillator signal can be arbitrarily extended in the digital domain, this invention overcomes the end-effect in traditional OFDR, where the duration of the beat frequency signal at the fiber end is shorter than that at the fiber front end, resulting in a more balanced signal-to-noise ratio and facilitating the implementation of time-frequency multiplexing and other methods.

[0035] 2. This invention amplifies the frequency-sweeping long pulse light transmitted to the sensing fiber by setting an optical amplifier, which can further improve the sensing distance.

[0036] 3. This invention utilizes the local oscillator signal R(n) to scatter the complex scattering signal I. OTDR (n performs digital dechirping and calculates I) OTDR (n. / (n), that is, by performing a dot division operation, the complex scattered signal I is... OTDR (The amplitude of n is divided by the amplitude of the local oscillator signal R(n), and the phase is subtracted. Since the local oscillator signal R(n) corresponds to the complex scattered signal I at each position on the sensing fiber...) OTDR (The frequency variation of n on the time axis follows the same pattern, therefore, for a certain position on the sensing fiber, at any time, the local oscillator signal R(n) and the corresponding complex scattered signal I at that position are...) OTDR (The frequency difference of n is the same, that is, the dechirped scattering signal I corresponding to this position is calculated by dot division operation) OFDR (n has the same frequency on the time axis; and since each position on the sensing fiber corresponds to a complex scattered signal I) OTDR (The frequency changes of n start at different times, therefore the dechirped scattering signal I corresponds to different positions on the sensing fiber.) OFDR (frequency f of n) i Each one is different, thus ensuring that the acquired dechirped scattering signal I OFDR(Signal-to-noise ratio of n;

[0037] 4. This invention utilizes the local oscillator signal R(n) to scatter the complex scattering signal I. OTDR (n performs digital dechirping, so that the dechirped scattered signal I) OFDR (n has the same frequency on the time axis, so even if the complex scattering signal I) OTDR (n exhibits spectral aliasing, generating a dechirped scattering signal I) OFDR (n) spectral aliasing will not occur, therefore, this invention addresses the issue of dechirped scattering signal I. OFDR (When sampling n, it is not necessary to adjust the maximum sampling frequency f according to the spectral aliasing situation) D The system can be adjusted accordingly. Traditionally, spectral aliasing only occurs when the modulation bandwidth is greater than the sampling frequency. However, this invention acquires dechirped scattering signals generated based on complex scattering signals, which do not suffer from spectral aliasing. Therefore, the sampling frequency of this invention is not limited by the pulse width of the swept long pulse light, thus ensuring that the signal-to-noise ratio of the dechirped scattering signal acquired by this invention is not limited by the pulse width. Furthermore, this invention can undersample the dechirped scattering signal, which reduces the detection cost of the system.

[0038] 5. This invention is for complex scattering signals I OTDR The frequency variation of n on the time axis can be adjusted by adjusting the sweep frequency modulation parameters of the sweep frequency modulation module to adjust the set slope when the frequency changes. By adjusting the set slope, the sensing length of the sensing fiber can be adjusted while meeting the sampling frequency requirements, thereby improving the sensing flexibility of the system. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of an embodiment of the fiber optic sensing system based on digital dechirping of the present invention;

[0040] Figure 2 This is a schematic diagram of another embodiment of the fiber optic sensing system based on digital dechirping of the present invention;

[0041] Figure 3 This is a flowchart of an embodiment of the sensing method of the present invention;

[0042] Figure 4 This is a schematic diagram showing the frequency variation of a real-valued scattered signal and its local oscillator signal;

[0043] Figure 5 This is a schematic diagram illustrating the frequency variation of the complex scattering signal and its local oscillator signal according to the present invention. Detailed Implementation

[0044] To enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention, and to make the above-mentioned objectives, features and advantages of the embodiments of the present invention more apparent and understandable, the technical solutions in the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0045] In the description of this invention, unless otherwise specified and limited, it should be noted that the term "connection" should be interpreted broadly. For example, it can be a mechanical connection or an electrical connection, or it can be a connection between two internal components. It can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above term according to the specific circumstances.

[0046] See Figure 1 This is a schematic diagram of an embodiment of the fiber optic sensing system based on digital dechirped fiber optic sensing according to the present invention. The fiber optic sensing system may include a narrow-linewidth laser, a coupler, a swept-frequency modulation module, an optical circulator, a sensing fiber, an optical mixer, a first balanced detector, a second balanced detector, and a data acquisition and processing module. The output of the narrow-linewidth laser is connected to the input of the coupler. The first output of the coupler is connected to the first end of the optical circulator via the swept-frequency modulation module. The second output is connected to the first input of the optical mixer. The second end of the optical circulator is connected to the sensing fiber, and the third end is connected to the second input of the optical mixer. The first and second outputs of the optical mixer are connected to the input of the first balanced detector, and the third and fourth outputs are connected to the input of the second balanced detector. The outputs of the first and second balanced detectors are connected to the data acquisition and processing module. The optical mixer may be a 90-degree optical mixer.

[0047] The coupler splits the narrow-linewidth laser provided by the narrow-linewidth laser into two paths: one path is transmitted to the sweep modulation module, and the other path is transmitted to the optical mixer. The sweep modulation module modulates the narrow-linewidth laser into sweep long pulses, and transmits these pulses to the sensing fiber via the optical circulator. The sensing fiber transmits the returned scattered light to the optical mixer via the optical circulator. The optical mixer coherently mixes the narrow-linewidth laser with the scattered light to generate four mixed-frequency signals. The first balanced detector detects two of the mixed-frequency signals to obtain a first coherent signal. The second balanced detector detects the other two mixed-frequency signals to obtain a second coherent signal. Specifically, the first balanced detector converts two of the mixed-frequency signals into first mixed-frequency electrical signals, and subtracts the two first mixed-frequency electrical signals to obtain a first coherent signal. The second balanced detector converts the other two mixed-frequency signals into second mixed-frequency electrical signals, and subtracts the two second mixed-frequency electrical signals to obtain a second coherent signal.

[0048] The data acquisition and processing module acquires the first coherent signal and the second coherent signal to obtain the complex scattering signal I. OTDR (n; for the complex scattering signal I) OTDR (n dechirp, obtain the dechirped scattering signal I) OFDR (n; the dechirped scattering signal I) OFDR (n is converted into a frequency domain scattering signal, and the frequency domain scattering signal is demodulated to obtain the disturbance magnitude at each position on the sensing fiber.)

[0049] As can be seen from the above embodiments, the present invention modulates a narrow-linewidth laser into a swept-frequency long pulse light, which is then transmitted to a sensing fiber. Subsequently, the narrow-linewidth laser is coherently mixed with the scattered light returned from the sensing fiber. The four mixed-frequency beams formed by the coherent mixing are detected to obtain a first coherent signal and a second coherent signal. The first and second coherent signals are acquired to obtain a complex scattering signal. After dechirping the complex scattering signal to obtain a dechirped scattering signal, it can be converted into a frequency-domain scattering signal. At this point, the OFDR demodulation method can be used to demodulate the frequency-domain scattering signal. Since OFDR has the characteristics of high spatial resolution, low sampling frequency, and demodulation unaffected by fading, and the present invention obtains the complex scattering signal based on Φ-OTDR, the present invention combines the advantages of both OFDR and Φ-OTDR, possessing high spatial resolution, low sampling frequency, and demodulation unaffected by fading. It can also achieve long-distance sensing. Furthermore, the present invention can improve measurement sensitivity.

[0050] See Figure 2 This is a schematic diagram of another embodiment of the fiber optic sensing system based on digital dechirping according to the present invention. Figure 2 and Figure 1 The difference in the illustrated fiber optic sensing system lies in the inclusion of an optical amplification module. This module is located between the sweep frequency modulation module and the first end of the optical circulator. The optical amplification module amplifies the sweep frequency long pulse light output from the sweep frequency modulation module and transmits the amplified sweep frequency long pulse light to the sensing fiber through the optical circulator. By incorporating an optical amplifier to amplify the sweep frequency long pulse light transmitted to the sensing fiber, this invention can further increase the sensing distance.

[0051] As can be seen from the above embodiments, the present invention modulates a narrow-linewidth laser into a swept-frequency long pulse light, which is then transmitted to a sensing fiber. Subsequently, the narrow-linewidth laser is coherently mixed with the scattered light returned from the sensing fiber. The four mixed-frequency beams formed by the coherent mixing are detected to obtain a first coherent signal and a second coherent signal. The first and second coherent signals are acquired to obtain a complex scattering signal. After dechirping the complex scattering signal to obtain a dechirped scattering signal, it can be converted into a frequency-domain scattering signal. At this point, the OFDR demodulation method can be used to demodulate the frequency-domain scattering signal. Since OFDR has the characteristics of high spatial resolution, low sampling frequency, and demodulation unaffected by fading, and the present invention obtains the complex scattering signal based on Φ-OTDR, the present invention combines the advantages of both OFDR and Φ-OTDR, possessing high spatial resolution, low sampling frequency, and demodulation unaffected by fading. It can also achieve long-distance sensing. Furthermore, the present invention can improve measurement sensitivity. The frequency response of this invention can be improved by time-frequency multiplexing and other methods, so that this invention has the ability to detect high-frequency dynamic strain. Since the local oscillator signal can be arbitrarily extended in the digital domain, this invention overcomes the end effect in traditional OFDR, that is, the duration of the beat frequency signal at the end of the optical fiber is shorter than that at the front end of the optical fiber, so that the signal-to-noise ratio is more balanced, which is conducive to the implementation of time-frequency multiplexing and other methods.

[0052] In addition, the present invention also provides a sensing method for the above-mentioned digitally chirped fiber optic sensing system, such as... Figure 3 As shown, it may include the following steps:

[0053] Step S110: Obtain the complex scattering signal I OTDR (n.)

[0054] The step S110 may specifically include: step S111, splitting the narrow linewidth laser into two paths, one path being modulated into a swept frequency long pulse light and transmitted to the sensing fiber, and the other path being directly transmitted to the optical mixer.

[0055] Step S112: After receiving the swept long pulse light, the sensing fiber transmits the returned scattered light to the optical mixer.

[0056] Step S123: The optical mixer coherently mixes the narrow-linewidth laser with the scattered light to generate four mixed beams.

[0057] Step S124: The first balanced detector detects two of the mixed optical signals to obtain a first coherent signal; the second balanced detector detects the other two mixed optical signals to obtain a second coherent signal. Specifically, the first balanced detector converts two of the mixed optical signals into first mixed electrical signals, and subtracts the two first mixed electrical signals to obtain the first coherent signal; the second balanced detector converts the other two mixed optical signals into second mixed electrical signals, and subtracts the two second mixed electrical signals to obtain the second coherent signal.

[0058] Step S125: Use the first coherent signal as the complex scattering signal I. OTDR (The real part of n, taking the second coherent signal as the complex scattering signal I) OTDR (The imaginary part of n. The complex scattering signal of this invention can distinguish between positive and negative frequencies.)

[0059] Step S120: The complex scattering signal I... OTDR (n dechirp, obtain the dechirped scattering signal I) OFDR (n. Step S120 may specifically include: using the generated local oscillator signal R(n) to scatter the complex scattering signal I. OTDR (n performs digital dechirping to obtain the dechirped scattering signal I) OFDR (n; the local oscillator signal R(n) is a discrete signal of the reference signal R(t) in the digital domain. The reference signal R(t) is related to the characteristics of the sweep frequency modulation module and is continuous in time. The complex scattering signal I OTDR (n) and its local oscillator signal R(n) exhibit the same frequency variation pattern on the time axis. The local oscillator signal R(n) can be generated by the data acquisition and processing module or a computer connected to the data acquisition and processing module.

[0060] Wherein, the reference signal R9t) is: R9t=exp[j*(2πf0tπkt) 2 [], f0 represents the starting frequency of modulation by the sweep frequency modulation module, k represents the modulation rate, and t represents time; in step S120, the complex scattering signal I is processed according to the following formula. OTDR (n performs digital dechirping to obtain the dechirped scattering signal I) OFDR (n:I OFDR (n=conj OTDR (n. / (n, where ". / " represents dot division and "conj" represents complex conjugate. This invention uses dot division to transform the complex scattered signal I...) OTDR Divide the amplitude of (n) and the phase of the local oscillator signal R(n) by the phase difference; perform complex conjugate operations on the complex scattered signal I. OTDRThe phase of the point division result of (n) and the local oscillator signal R(n) is inverted to restore the position of the scattered signal on the frequency axis in OFDR.

[0061] For complex scattering signals I that can distinguish between positive and negative frequencies OTDR (n, when spectral aliasing occurs, its frequency variation on the time axis is as follows) Figure 4 As shown in the figure, the sampling frequency range is [-f D / 2,+f D / 2], the complex scattering signal I at each position on the sensing fiber. OTDR (The frequency changes of n start at different times, for example, t in the figure) i This represents the complex scattered signal I corresponding to the i-th position on the sensing fiber. OTDR The frequency change start time of the local oscillator signal R(n) is 0. The complex scattered signal I at each position on the sensing fiber... OTDR (n) and the local oscillator signal R(n) exhibit the same frequency variation pattern on the time axis. Specifically, the frequency variation pattern on the time axis is as follows: as time increases, the frequency monotonically increases from 0 according to a set slope, reaching +f after time T. D When / 2, the frequency drops to -f D / 2, when the frequency drops to -f D After / 2, the slope is monotonically increased according to the set value until, after a time of 2T, the frequencies of both reach +f. D / 2, after which the frequency drops to -f D / 2, and so on, where T is the frequency sweep modulation time of the frequency sweep modulation module, f D This is the maximum sampling frequency.

[0062] Due to the complex scattered signals I at various locations on the sensing fiber OTDR (n and its local oscillator signal r(n) have the same frequency variation law on the time axis. Therefore, for each position on the sensing fiber, at any time, the local oscillator signal R(n) and the corresponding complex scattered signal I OTDR (The frequency difference of n is the same, and the frequency difference is the difference between the local oscillator signal R(n) and the corresponding complex scattered signal I.) oTDR (n's beat frequency f) i And for the dechirped scattering signal I oFDR (n) frequency; the local oscillator signal R(n) and the complex scattered signals I corresponding to each position on the sensing fiber. OTDR (The frequency difference of n is different, that is, the dechirped scattering signal I corresponds to different positions on the sensing fiber.) OFDR (frequency f of n) i They are all different, the frequency f iLess than f D / 2.

[0063] For real-number scattered signals that cannot distinguish between positive and negative frequencies, when spectral aliasing occurs, their frequency variation on the time axis follows the pattern shown below. Figure 5 As shown in the figure, the frequency of the real-number scattered signal oscillates back and forth on the positive frequency axis. With increasing time, its frequency exhibits a cyclical pattern of monotonically increasing, monotonically decreasing, and then monotonically increasing again. However, when spectral aliasing occurs in the real-number scattered signal, the aliasing frequency f... j They are not the same; the signal-to-noise ratio of the real-valued scattering signal or the dechirped scattering signal acquired at this time is low.

[0064] This invention utilizes the local oscillator signal R(n) to scatter the complex scattering signal I. OTDR (n performs digital dechirping and calculates I) OTDR (n. / (n), that is, by performing a dot division operation, the complex scattered signal I is... OTDR (The amplitude of n is divided by the amplitude of the local oscillator signal R(n), and the phase is subtracted. Since the local oscillator signal R(n) corresponds to the complex scattered signal I at each position on the sensing fiber...) OTDR (The frequency variation of n on the time axis follows the same pattern, therefore, for a certain position on the sensing fiber, at any time, the local oscillator signal R(n) and the corresponding complex scattered signal I at that position are...) OTDR (The frequency difference of n is the same, that is, the dechirped scattering signal I corresponding to this position is calculated by dot division operation) OFDR (n has the same frequency on the time axis; and since each position on the sensing fiber corresponds to a complex scattered signal I) DTDR (The frequency changes of n start at different times, therefore the dechirped scattering signal I corresponds to different positions on the sensing fiber.) OFDR (frequency f of n) i Each one is different, thus ensuring that the acquired dechirped scattering signal I OFDR (the signal-to-noise ratio of n).

[0065] in addition, Figure 4 In the aliasing region shown in the figure, the complex scattered signal will experience spectral aliasing. If the local oscillator signal R(n) is not used to alias the complex scattered signal I... OTDR (n performs digital dechirping, directly processing the complex scattered signal I) OTDR (If n is used for sampling, then in order to collect the complex scattering signal in the aliasing region, the maximum sampling frequency f needs to be increased.) D This invention utilizes the local oscillator signal R(n) to scatter the complex scattered signal I. OTDR (n performs digital dechirping, so that the dechirped scattered signal I) OFDR (n has the same frequency on the time axis, so even if the complex scattering signal I) OTDR(n exhibits spectral aliasing, generating a dechirped scattering signal I) OFDR (n) spectral aliasing will not occur, therefore, this invention addresses the issue of dechirped scattering signal I. OFDR (When sampling n, it is not necessary to adjust the maximum sampling frequency f according to the spectral aliasing situation) D The method involves adjusting the sampling frequency. Traditionally, spectral aliasing only occurs when the modulation bandwidth exceeds the sampling frequency. However, this invention acquires dechirped scattering signals generated from complex scattering signals, eliminating spectral aliasing. Therefore, the sampling frequency of this invention is not limited by the pulse width of the swept long pulse light, resulting in an unrestricted signal-to-noise ratio (SNR) of the acquired dechirped scattering signal. Furthermore, this invention allows for undersampling of the dechirped scattering signal, reducing the system's detection cost.

[0066] Wherein, the sweep modulation time T is the duration of the sweep long pulse light transmission to the end of the sensing fiber, used to characterize the sensing length of the sensing fiber; when the sweep modulation time T is constant, the complex scattering signal I can be adjusted. OTDR (n) is the set slope for the maximum sampling frequency f D Adjustments are made to meet the sampling frequency requirements, wherein the smaller the set slope, the higher the maximum sampling frequency f. D The smaller the value; at the maximum sampling frequency f D At a certain time, the sweep frequency modulation duration T can be adjusted by adjusting the set slope, thereby adjusting the sensing length. The smaller the set slope, the larger the sensing length. The adjustment of the set slope is achieved by adjusting the sweep frequency modulation parameters of the sweep frequency modulation module.

[0067] Specifically, the sweep frequency modulation parameters can be adjusted according to the following steps:

[0068] Step S201: Based on the required sensing length, determine the duration required for the swept frequency long pulse light to be transmitted to the end of the sensing fiber of the required sensing length, i.e., the swept frequency modulation time T.

[0069] Step S202: Based on the determined sweep frequency modulation time T and the required maximum sampling frequency f D According to the formula Determine the set slope k;

[0070] Step S203: Adjust the sweep frequency modulation parameters of the sweep frequency modulation module according to the determined set slope k.

[0071] This invention targets complex scattering signal I OTDRThe frequency variation of n on the time axis can be adjusted by adjusting the sweep frequency modulation parameters of the sweep frequency modulation module to adjust the set slope when the frequency changes. By adjusting the set slope, the sensing length of the sensing fiber can be adjusted while meeting the sampling frequency requirements, thereby improving the sensing flexibility of the system.

[0072] Step S130: The dechirped scattering signal I OFDR (n is converted into a frequency domain scattering signal, and the frequency domain scattering signal is demodulated to obtain the disturbance magnitude at each position on the sensing fiber.)

[0073] Step S130 may specifically include: Step S131, for each position on the sensing fiber, using different sampling frequencies to obtain the corresponding dechirped scattering signal I. OFDR (n is sampled;

[0074] Step S132: For each location on the sensing fiber, the dechirped scattering signal I... OFDR (n, for the dechirped scattering signal I) OFDR (n is subjected to Fourier Transform (FFT) and converted into a frequency domain scattering signal;

[0075] Step S133: After windowing in the frequency domain, perform an inverse Fourier transform (IFFT) on the frequency domain scattering signal to convert it into a time domain scattering signal;

[0076] Step S134: The time-domain scattering signal is compared with the dechirped scattering signal I. OFDR (The time-domain signal corresponding to the first pulse in n is used as the reference time-domain signal, and this reference time-domain signal is compared with the dechirped scattering signal I) OFDR (The time-domain signals of other pulses in n are cross-correlated to obtain the shift of the time-domain scattered signal, thereby demodulating the magnitude of the disturbance at that location on the sensing fiber. This invention uses the dechirped scattered signal I) OFDR (n is converted into a frequency domain scattering signal, and the frequency domain scattering signal is demodulated using the OFDR demodulation method. In this way, the present invention can avoid the influence of signal fading during phase adjustment.)

[0077] As can be seen from the above embodiments, the present invention modulates a narrow-linewidth laser into a swept-frequency long pulse light, which is then transmitted to a sensing fiber. Subsequently, the narrow-linewidth laser is coherently mixed with the scattered light returned from the sensing fiber. The four mixed-frequency beams formed by the coherent mixing are detected to obtain a first coherent signal and a second coherent signal. The first and second coherent signals are acquired to obtain a complex scattering signal. After dechirping the complex scattering signal to obtain a dechirped scattering signal, it can be converted into a frequency-domain scattering signal. At this point, the OFDR demodulation method can be used to demodulate the frequency-domain scattering signal. Since OFDR has the characteristics of high spatial resolution, low sampling frequency, and demodulation unaffected by fading, and the present invention obtains the complex scattering signal based on Φ-OTDR, the present invention combines the advantages of both OFDR and Φ-OTDR, possessing high spatial resolution, low sampling frequency, and demodulation unaffected by fading. It can also achieve long-distance sensing. Furthermore, the present invention can improve measurement sensitivity. The frequency response of this invention can be improved by time-frequency multiplexing and other methods, so that this invention has the ability to detect high-frequency dynamic strain. Since the local oscillator signal can be arbitrarily extended in the digital domain, this invention overcomes the end effect in traditional OFDR, that is, the duration of the beat frequency signal at the end of the optical fiber is shorter than that at the front end of the optical fiber, so that the signal-to-noise ratio is more balanced, which is conducive to the implementation of time-frequency multiplexing and other methods.

[0078] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

[0079] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is defined solely by the appended claims.

Claims

1. A fiber optic sensing system based on digital dechirping, characterized in that, The system includes a narrow-linewidth laser, a coupler, a sweep frequency modulation module, an optical circulator, a sensing fiber, an optical mixer, a first balanced detector, a second balanced detector, and a data acquisition and processing module. The output of the narrow-linewidth laser is connected to the input of the coupler. The first output of the coupler is connected to the first end of the optical circulator via the sweep frequency modulation module. The second output is connected to the first input of the optical mixer. The second end of the optical circulator is connected to the sensing fiber, and the third end is connected to the second input of the optical mixer. The first and second outputs of the optical mixer are connected to the input of the first balanced detector, and the third and fourth outputs are connected to the input of the second balanced detector. The outputs of the first and second balanced detectors are connected to the data acquisition and processing module. The coupler splits the narrow-linewidth laser provided by the narrow-linewidth laser into two paths, one of which is transmitted to the sweep frequency modulation module and the other to the optical mixer; the sweep frequency modulation module modulates the narrow-linewidth laser into a sweep frequency long pulse light, and transmits the sweep frequency long pulse light to the sensing fiber through the optical circulator; the sensing fiber transmits the returned scattered light to the optical mixer through the optical circulator. The optical mixer coherently mixes the narrow-linewidth laser with the scattered light to generate four mixed light signals; the first balanced detector detects two of the mixed light signals to obtain a first coherent signal; the second balanced detector detects the other two mixed light signals to obtain a second coherent signal. The data acquisition and processing module acquires the first coherent signal and the second coherent signal to obtain a complex scattering signal. ; for the complex scattering signal Dechirp to obtain the dechirped scattering signal. ; The dechirped scattering signal The signal is converted into a frequency domain scattering signal, and the frequency domain scattering signal is demodulated to obtain the disturbance magnitude at each location on the sensing fiber.

2. The fiber optic sensing system based on digital dechirping according to claim 1, characterized in that, It also includes an optical amplification module, which is located between the sweep frequency modulation module and the first end of the optical circulator. The optical amplification module amplifies the sweep frequency long pulse light output by the sweep frequency modulation module and transmits the amplified sweep frequency long pulse light to the sensing optical fiber through the optical circulator.

3. A sensing method for a fiber optic sensing system based on digital dechirping as described in claim 1 or 2, characterized in that, include: Step S110: Obtain the complex scattering signal ; Step S120: The complex scattering signal Dechirp to obtain the dechirped scattering signal. ; Step S130: The dechirped scattering signal The signal is converted into a frequency domain scattering signal, and the frequency domain scattering signal is demodulated to obtain the disturbance magnitude at each location on the sensing fiber.

4. The sensing method according to claim 3, characterized in that, The specific steps of step S110 include: step S111, splitting the narrow linewidth laser into two paths, one path being modulated into a frequency-sweeping long pulse light and transmitted to the sensing fiber, and the other path being directly transmitted to the optical mixer. Step S112: After receiving the swept long pulse light, the sensing fiber transmits the returned scattered light to the optical mixer. Step S123: The optical mixer coherently mixes the narrow linewidth laser with the scattered light to generate four mixed beams. Step S124: The first balanced detector detects two of the mixed optical signals to obtain a first coherent signal; the second balanced detector detects the other two mixed optical signals to obtain a second coherent signal. Step S125: Use the first coherent signal as the complex scattering signal. The real part of the second coherent signal is used as the complex scattering signal. The imaginary part.

5. The sensing method according to claim 3, characterized in that, Step S120 specifically includes: utilizing the generated local oscillator signal For the complex scattering signal Digital dechirping is performed to obtain the dechirped scattering signal. The local oscillator signal Reference signal In the digital domain, the discrete signal, the reference signal The complex scattering signal is related to the characteristics of the sweep frequency modulation module and is continuous in time. Its local oscillator signal The frequency changes follow the same pattern on the time axis.

6. The sensing method according to claim 5, characterized in that, The reference signal for: ,in This indicates the starting frequency at which the sweep frequency modulation module initiates modulation, k represents the modulation rate, and t represents time. In step S120, the complex scattering signal is processed according to the following formula. Digital dechirping is performed to obtain the dechirped scattering signal. : , where ". / " represents dot division and "conj" represents complex conjugate operation.

7. The sensing method according to claim 5 or 6, characterized in that, Let the sampling frequency range be [-f D / 2,+f D / 2], the complex scattered signals at each position on the sensing fiber. The frequency changes of the local oscillator signal have different start times. The frequency change begins at time 0; The complex scattered signal corresponding to each position on the sensing optical fiber and the local oscillator signal The frequency changes along the time axis follow the same pattern: as time increases, the frequency starts from 0 and monotonically increases at a set slope, reaching +f after time T. D When / 2, the frequency drops to -f D / 2, when the frequency drops to -f D After / 2, the slope is monotonically increased according to the set value until, after a time of 2T, the frequencies of both reach +f. D / 2, after which the frequency drops to -f D / 2, and so on, where T is the frequency sweep modulation time of the frequency sweep modulation module, f D This is the maximum sampling frequency; For each position on the sensing fiber, at any time, the local oscillator signal This location corresponds to a complex scattering signal. The frequency differences are all the same, and the frequency difference is the local oscillator signal. This location corresponds to a complex scattering signal. beat frequency f i And for the dechirped scattering signal corresponding to that location. frequency f i The different positions on the sensing fiber correspond to the dechirped scattering signals. frequency f i They are all different, the frequency f i Less than f D / 2.

8. The sensing method according to claim 7, characterized in that, The sweep modulation time T is the duration of the sweep long pulse light transmission to the end of the sensing fiber, and is used to characterize the sensing length of the sensing fiber. by adjusting the set slope, when the sweep modulation time T is constant, to adjust the maximum sampling frequency f D to meet the sampling frequency requirement, wherein the smaller the set slope, the smaller the maximum sampling frequency f D ; and by adjusting the set slope, when the maximum sampling frequency f D is constant, to adjust the sweep modulation time T, thereby adjusting the sensing length, wherein the smaller the set slope, the larger the sensing length; the adjustment of the set slope is realized by adjusting the sweep modulation parameters of the sweep modulation module.

9. The sensing method according to claim 8, characterized in that, Adjust the sweep frequency modulation parameters according to the following steps: Step S201: Based on the required sensing length, determine the duration required for the swept frequency long pulse light to be transmitted to the end of the sensing fiber of the required sensing length, i.e., the swept frequency modulation time T. Step S202: Based on the determined sweep frequency modulation time T and the required maximum sampling frequency f D According to the formula Determine the set slope k; Step S203: Adjust the sweep frequency modulation parameters of the sweep frequency modulation module according to the determined set slope k.

10. The sensing method according to claim 3, characterized in that, Step S130 specifically includes: Step S131, for each position on the sensing optical fiber, using different sampling frequencies to obtain the corresponding dechirped scattering signal. Perform sampling; Step S132: For the chirped scattering signal corresponding to each position on the sensing fiber... Regarding the dechirped scattering signal Perform a Fourier transform (FFT) to convert it into a frequency domain scattering signal; Step S133: After windowing in the frequency domain, perform an inverse Fourier transform (IFFT) on the frequency domain scattering signal to convert it into a time domain scattering signal; Step S134: The time-domain scattering signal is compared with the dechirped scattering signal. The time-domain signal corresponding to the first pulse is used as the reference time-domain signal, and this reference time-domain signal is compared with the dechirped scattering signal. Cross-correlation is performed on the time-domain signals of other pulses to obtain the shift of the time-domain scattered signal, thereby demodulating the magnitude of the disturbance at that location on the sensing fiber.

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