Fast quantitative distributed fiber vibration detection method and system based on frequency comb

Abstract

The application discloses a kind of fast quantitative distributed optical fiber vibration detection method and system based on frequency comb, belong to optical fiber sensing field technical field, method includes: S1, design target frequency comb, including the comb tooth interval Δf of the target frequency comb is designed C And the initial phase of the target frequency comb;S2, the target frequency comb is converted into time domain signal, and the time domain signal is modulated on probe light, obtains modulated time domain optical carrier signal;S3, the modulated time domain optical carrier signal is carried out optical domain frequency shift and probe pulse modulation, obtains modulated frequency comb pulse sequence;S4, the frequency comb pulse sequence is input to the optical fiber FUT to be measured, and quantitative distributed optical fiber vibration detection is carried out.The application realizes the modulation of target frequency comb on probe pulse, compresses single detection time, to improve the frequency response capability to external disturbance, can realize the high signal-to-noise ratio detection of dynamic signal.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of optical fiber sensing, and more particularly, to a fast quantitative distributed optical fiber vibration detection method and system based on a frequency comb. BACKGROUND

[0002] Optical fiber sensors have the advantages of small size, wide frequency band, high sensitivity, immunity to electromagnetic interference, corrosion resistance, high temperature resistance, high pressure resistance, and adaptability to harsh environments. Distributed sensing is a technology in which every point on the optical fiber link serves as a sensing element. The optical fiber serves as both an information transmission medium and a sensing element. It can continuously measure environmental parameters such as temperature and strain along the length of the optical fiber, with a sensing length of up to tens of kilometers. It has applications in many fields such as oil pipelines, bridges, dams, tunnels, power lines, building construction, aircraft, earthquake warning, and border defense. It is an ideal distributed measurement tool that combines intelligence and environmental protection.

[0003] The main information that can be applied to signal sensing in optical fibers is Rayleigh scattering, Brillouin scattering, and Raman scattering. The most common distributed sensing scheme based on Rayleigh scattering is the phase-sensitive optical time domain reflectometer Phase-sensitive optical time domain reflectometer which is achieved by using a highly coherent light source. Due to the high coherence of the light source, the backscattered Rayleigh scattering signals collected by the φ-OTDR technology form an interference pattern, reflecting the inherent stress conditions of the optical fiber and other information. When the environment of the optical fiber changes in temperature or the optical fiber itself is subjected to external stress, the Rayleigh interference pattern will change, allowing real-time positioning of the external disturbance location.

[0004] Phase-sensitive optical time domain reflectometer including single-frequency detection and frequency shift detection Single-frequency detection schemes use a single frequency for each detection, acquiring external disturbance information by intensity / phase demodulating the backscattered Rayleigh signal. This allows for signal strength detection, but its sensitivity is low. Frequency-shift detection schemes use a set of probes at different frequencies for each detection. The Rayleigh scattering signals obtained from these pulses reconstruct the Rayleigh frequency shift spectrum along the fiber, allowing for quantitative analysis of the magnitude of external disturbances. This enables highly sensitive quantitative detection. However, a single detection takes a long time and has a slow response, making real-time monitoring of dynamic signals impossible. Specifically, in a frequency-shift detection system, each detection group consists of several independent probes. Assuming N independent probes are performed in each group, a single detection takes at least N times longer than a single-frequency detection system. Adding the frequency switching time of the signal generator further increases the time, meaning that even with expensive frequency-agile devices, existing frequency-shift detection systems can only achieve a response speed on the order of Hz. Therefore, they are unsuitable for detecting dynamic signals and can only monitor slowly changing quasi-static signals.

[0005] Existing frequency comb detection technology generally generates a frequency comb by hardware modulation of hardware devices (such as lasers). Its performance characteristics are a large bandwidth, but the frequency comb spacing is also large, making the frequency comb very sparse. Due to the time-frequency commutation characteristics of Fourier transform, the time repetition period corresponding to this frequency comb is very long, resulting in a large detection pulse time width and extremely low spatial resolution, making it unsuitable for use in frequency shift detection φ-OTDR. Summary of the Invention

[0006] In response to the shortcomings and improvement needs of existing technologies, this invention provides a rapid quantitative distributed optical fiber vibration detection method and system based on frequency combs, with the aim of achieving ultra-high sensitivity quantitative detection of dynamic signals.

[0007] To achieve the above objectives, according to one aspect of the present invention, a rapid quantitative distributed optical fiber vibration detection method based on a frequency comb is provided, comprising:

[0008] S1. Design the target frequency comb, including designing the comb tooth spacing Δf of the target frequency comb. C and the initial phase of the target frequency comb; wherein, the comb tooth spacing Δf of the target frequency comb C The initial phase of the target frequency comb is not less than twice the reciprocal of the probe pulse width, and the initial phase of the target frequency comb is the phase corresponding to the minimum peak-to-average power ratio of the time-domain waveform of the target frequency comb.

[0009] S2. Convert the target frequency comb into a time-domain signal, and modulate the time-domain signal onto the probe light to obtain a modulated time-domain optical carrier signal;

[0010] S3. Perform optical domain frequency shifting and probe pulse modulation on the modulated time-domain optical carrier signal to obtain a modulated frequency comb pulse sequence;

[0011] S4. Input the frequency comb pulse sequence into the fiber under test (FUT) to perform quantitative distributed fiber vibration detection.

[0012] Furthermore, before S2, the following steps are also included:

[0013] The target frequency comb is sequentially frequency-shifted m times, with each frequency shift step being the comb tooth spacing Δf. C of The tooth spacing of the m+1 frequency combs obtained after frequency shifting remains unchanged. The m+1 frequency combs are used as the new target frequency combs, where m≥1.

[0014] Furthermore, in S3, the optical frequency shift and the target frequency comb satisfy the following:

[0015] The total bandwidth f of the target frequency comb BW The difference between the starting frequency of the target frequency comb and the optical frequency shift is not greater than twice the optical frequency shift, and the difference between the optical frequency shift and the comb tooth spacing Δf is not less than the optical frequency shift and the comb tooth spacing Δf. C The sum of.

[0016] Further, in S1, calculating the phase corresponding to the minimum peak-to-average power ratio of the target frequency comb time-domain waveform includes:

[0017] The initial phase of each tooth of the target frequency comb is randomly set, and the peak-to-average power ratio corresponding to the time-domain waveform of the target frequency comb is calculated.

[0018] Through iterative optimization, the phase corresponding to the minimum peak-to-average power ratio of the target frequency comb is obtained.

[0019] Furthermore, in S4, quantitative distributed fiber optic vibration detection includes:

[0020] S41. After interfering the continuous backscattered Rayleigh signal generated by the fiber under test (FUT) with the local light, two continuous coupled signals are output.

[0021] S42. Extract the intensity information of the two continuous coupled signals after beat frequency;

[0022] S43. The intensity information is periodically trimmed and rearranged according to the detection pulse interval; the rearranged intensity information is then filtered to obtain the frequency information for each detection; the frequency information is arranged in frequency order to obtain the Rayleigh scattering trajectory matrix for each detection.

[0023] S44, taking the Rayleigh scattering trajectory matrix at a moment as a reference spectrum, taking the Rayleigh scattering trajectory matrix in a period of the moment as a measurement spectrum, calculating the correlation degree of each measurement spectrum and the reference spectrum, obtaining the frequency shift size corresponding to each position of the fiber to be measured, and using the frequency shift size to represent the external disturbance size.

[0024] Further, in S4, pulse amplification is further included.

[0025] Or / and, after denoising the amplified pulse sequence, the pulse sequence is input to the fiber to be measured FUT.

[0026] According to a second aspect of the present application, a frequency comb-based fast quantitative distributed fiber vibration detection system is provided, comprising:

[0027] A target frequency comb design module is configured to design a target frequency comb, including designing the comb tooth interval Δf C of the target frequency comb and the initial phase of the target frequency comb; wherein the comb tooth interval Δf C of the target frequency comb is not less than twice the reciprocal of the pulse width, and the initial phase of the target frequency comb is the phase corresponding to the minimum peak average power ratio of the time-domain waveform of the target frequency comb;

[0028] An arbitrary waveform generator is configured to generate a time-domain signal corresponding to the target frequency comb;

[0029] An electro-optical modulator is configured to modulate the time-domain signal on a probe light to obtain a modulated time-domain optical carrier signal;

[0030] An acousto-optic modulator is configured to perform optical domain frequency shift and probe pulse modulation on the modulated time-domain optical carrier signal to obtain a modulated frequency comb pulse sequence;

[0031] A second circulator is configured to input the frequency comb pulse sequence to a fiber to be measured FUT and output a back Rayleigh scattering signal generated by the fiber to be measured FUT;

[0032] A 3dB coupler is configured to output two continuous coupling signals after interference between the back Rayleigh scattering signal and a local light;

[0033] A balanced photodetector is configured to extract intensity information after beat frequency of the two continuous coupling signals;

[0034] A data processing module is configured to perform quantitative distributed fiber vibration detection according to the intensity information.

[0035] Further, a target frequency comb frequency shift module is further included, configured to sequentially perform m times of frequency shift on the target frequency comb, and the frequency shift step each time is the comb tooth interval ΔfC The The comb tooth interval of the m+1 groups of frequency combs obtained after frequency shifting is unchanged, and the m+1 groups of frequency combs are taken as new target frequency combs, m≥1.

[0036] Further, the bandwidth f AOM The frequency difference Δf between the target frequency comb and the frequency comb satisfies:

[0037] The total frequency bandwidth f BW of the target frequency comb is not greater than twice the bandwidth f AOM of the acousto-optic modulator, and the frequency difference Δf between the start frequency of the target frequency comb and the bandwidth f AOM of the acousto-optic modulator is not less than the bandwidth f AOM of the acousto-optic modulator and the comb tooth interval Δf C of the target frequency comb.

[0038] Further, the data processing module comprises a Rayleigh scattering trajectory matrix generating unit and an external disturbance quantitative calculation unit.

[0039] The Rayleigh scattering trajectory matrix generating unit is configured to periodically clip and rearrange the intensity information according to the detection pulse interval; filter the rearranged intensity information to obtain frequency information of each detection; and arrange the frequency information in frequency order to obtain a Rayleigh scattering trajectory matrix of each detection.

[0040] The external disturbance quantitative calculation unit is configured to take a Rayleigh scattering trajectory matrix at a moment as a reference spectrum, take Rayleigh scattering trajectory matrices in a period of time at the moment as measurement spectra, calculate a correlation degree of each measurement spectrum and the reference spectrum to obtain a frequency shift amount corresponding to each position of the optical fiber, and use the frequency shift amount to represent a size of an external disturbance.

[0041] Overall, the above technical solutions conceived by the present application can achieve the following beneficial effects:

[0042] (1) The present application designs a target frequency comb based on the width of a detection pulse in a digital domain and designs an initial phase of the target frequency comb, modulates a corresponding time domain waveform on an optical carrier based on frequency shift detection, realizes loading of multiple different detection frequencies on a single pulse, and only occupies one detection time for one frequency comb, which greatly compresses the detection time compared with the traditional method of occupying one detection time for each frequency, thereby improving the frequency response capability and realizing ultra-high sensitivity quantitative detection of dynamic signals, while avoiding the problem of extremely low signal-to-noise ratio caused by modulation of a frequency comb in a single pulse.

[0043] (2) In the new target frequency comb, m+1 detection pulses constitute a set of detection frequency combs, and a set of frequency combs only occupies m+1 detection time, compared with the traditional frequency occupying one detection time, the detection time is greatly compressed, thereby improving the frequency response capability, and the super-high sensitivity quantitative detection of dynamic signals can be realized, at the same time, the frequency resolution of the new target frequency comb is increased by m+1 times, which is suitable for scenes with higher requirements for frequency resolution and spatial resolution.

[0044] (3) Based on the relationship between the designed optical domain frequency shift and the target frequency comb, the target frequency comb designed by the application is organically combined with the coherent detection scheme, which greatly improves the signal-to-noise ratio of the detection signal, realizes the high signal-to-noise ratio single pulse frequency comb detection effect not lower than the traditional single frequency detection scheme, and solves the problem of low signal-to-noise ratio of the detection signal caused by the dispersion of pulse energy by multiple different frequencies and the limitation of total pulse energy based on the nonlinear effect of optical fiber in the modulation of frequency comb in single pulse, and the maximum energy that each frequency component in single pulse can load is lower than that in single frequency detection.

[0045] (4) The application also provides a specific method for quantitative distributed optical fiber vibration detection, which combines the coherent detection technology with the designed frequency comb organically, the designed target frequency comb is beat into a double sideband signal, the Rayleigh scattering trajectory matrix of each detection is calculated based on the double sideband signal, and then the position information of the external disturbance is obtained, and the disturbance size is dynamically monitored.

[0046] (5) As a preferred, the pulse sequence of the frequency comb is amplified, and the noise in the amplified pulse sequence is filtered out, further improving the signal-to-noise ratio of the detection.

[0047] In summary, the application realizes the modulation of the target frequency comb on the detection pulse, compresses the single detection time, thereby improving the frequency response capability, enabling the detection of dynamic signals, and improving the signal-to-noise ratio of the detection. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 The flow chart of the application is a fast quantitative distributed optical fiber vibration detection method based on frequency comb.

[0049] Figure 2 The target frequency comb designed by the application.

[0050] Figure 3 The time domain waveform diagram of the modulated frequency comb of the application.

[0051] Figure 4 The pulse time domain waveform diagram of the frequency comb modulation of the application.

[0052] Figure 5 The new target frequency comb designed by the application.

[0053] Figure 6 This is a schematic diagram of the upper sideband and lower sideband of the modulation frequency comb of the present invention.

[0054] Figure 7 This is a schematic diagram of the rapid quantitative distributed optical fiber vibration detection system based on frequency comb provided by the present invention.

[0055] Figure 8 This is a schematic diagram of the Rayleigh scattering trajectory matrix and cross-correlation calculation obtained after filtering and reconstructing the sampling results.

[0056] Figure 9 This is the cross-correlation spectrum obtained in an embodiment of the present invention.

[0057] Figure 10 The figure shows the peak fitting vibration measurement results obtained in an embodiment of the present invention. Detailed Implementation

[0058] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0059] In this invention, the terms "first," "second," etc., used in the invention and accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0060] like Figure 1 As shown, the rapid quantitative distributed optical fiber vibration detection method based on frequency comb of the present invention mainly includes the following steps:

[0061] S1. Design target frequency comb: including the comb tooth spacing Δf of the design target frequency comb. C and the initial phase of the target frequency comb; the spacing Δf between the comb teeth of this frequency comb. C Not less than twice the reciprocal of the probe pulse width; the initial phase design of the frequency comb includes: randomly setting the initial phase of each tooth of the target frequency comb, calculating the peak-to-average power ratio (PAPR) corresponding to the time-domain waveform of the frequency comb, and obtaining the phase of the frequency comb corresponding to the time-domain waveform with the smallest PAPR through iterative optimization, and using this phase as the initial phase of the frequency comb.

[0062] S2. Convert the designed target frequency comb into a time-domain signal, and modulate the time-domain signal onto the probe light to obtain the modulated time-domain optical carrier signal;

[0063] S3, performing optical domain frequency shift and probe pulse modulation on the modulated time domain optical carrier signal to obtain a modulated frequency comb pulse sequence, and realizing loading of n equal-interval probe frequencies on a single pulse;

[0064] S4, inputting the modulated frequency comb pulse sequence into the optical fiber FUT to perform back Rayleigh scattering type distributed optical fiber sensing, that is, to perform quantitative distributed optical fiber vibration detection.

[0065] In S1, the designed target frequency comb is as shown in the figure Figure 2 . BW f represents the total frequency bandwidth of the designed target frequency comb, including n equal-interval probe frequencies, and n represents the number of teeth of the designed target frequency comb, which is determined by the total frequency bandwidth of the target frequency comb and the interval between each frequency component.

[0066] After converting the designed target frequency comb into a time domain signal, the time domain waveform diagram thereof is as shown in the figure Figure 3 . It can be seen that the time domain waveform is a periodically repeated signal, and the length of the repetition period is equal to the reciprocal of the interval Δf C of the frequency comb.

[0067] As shown in the figure Figure 4 , when the interval Δf C of the frequency comb is not less than twice the reciprocal of the probe pulse width, it can make each probe pulse after the probe pulse modulation in the subsequent S3 contain at least two complete periods of the time domain waveform in the pulse width, and then enable the designed frequency comb spectrum to be restored according to the modulated pulse signal.

[0068] By taking the phase of the frequency comb corresponding to the time domain waveform with the minimum peak average power ratio PAPR as the initial phase of the frequency comb, the total optical power in a single pulse can be improved under the threshold limit of the optical fiber nonlinearity, and the signal-to-noise ratio of the detection can be improved.

[0069] As a further preferred design of the present application, before S2, it further includes the step of:

[0070] The target frequency comb designed in S1 is sequentially shifted m times (i.e. the next frequency shift is performed on the basis of the last frequency shift), and the frequency shift step each time is The comb tooth interval of the m+1 groups of frequency combs obtained after frequency shifting is unchanged, that is, the comb tooth interval of the m+1 groups of frequency combs after frequency shifting is the same as the comb tooth interval of the frequency combs before frequency shifting, and the m+1 groups of frequency combs obtained after frequency shifting are taken as new target frequency combs, m≥1. The frequency resolution of the new target frequency combs is increased by m+1 times, and is suitable for scenarios with higher requirements for frequency resolution and spatial resolution. At this time, the new target frequency combs designed can be applied to a situation where the Fourier time-frequency reciprocity relationship is not satisfied between the detection pulse width corresponding to the required spatial resolution and the frequency size corresponding to the required monitoring sensitivity.

[0071] In the embodiment of the present application, taking the case of sequentially performing four times of frequency shifting on the target frequency combs as an example, as shown in the figure, Figure 5 the target frequency combs designed in S1 are sequentially frequency shifted four times, the next frequency comb is frequency shifted based on the previous frequency comb, and the frequency shift step is 1 / 5 of the frequency comb tooth interval. The five groups of frequency combs obtained after frequency shifting are taken as a whole, and are taken as new target frequency combs. Figure 5 t1-t5 in the figure represent that the first group to the fifth group of frequency combs are respectively modulated on five continuous single pulses at t1-t5.

[0072] Based on the new target frequency combs, S3 further includes: each detection pulse sequentially loads one frequency comb in the m+1 groups of frequency combs, and m+1 detection pulses collectively constitute a group of detections, that is, n equally spaced detection frequencies are loaded on each pulse, and m+1 groups of frequency combs are sequentially loaded on m+1 pulses, and each group of frequency combs includes n equally spaced detection frequencies.

[0073] In S2, in the embodiment of the present application, after the designed target frequency combs are converted into time domain signals, the time domain signals are generated by an arbitrary waveform generator, and the time domain signals are modulated on the detection light by an electro-optical modulator controlled by the arbitrary waveform generator. The arbitrary waveform generator can be an arbitrary waveform generator AWG or an arbitrary function generator AFG. Since the time domain signals converted from the target frequency combs have the characteristics of large bandwidth and high sampling rate, the arbitrary waveform generator AWG is preferably used to generate the time domain signals in this embodiment. At the same time, the bandwidth of the modulation speed is high, and the electro-optical modulator EOM is preferably used to modulate the time domain signals on the detection light in this embodiment. In other embodiments, a Mach-Zehnder modulator MZM can also be selected.

[0074] In S3, in the embodiment of the present application, in order to realize coherent detection in the subsequent quantitative distributed optical fiber vibration detection process, the following relationship needs to be satisfied between the optical domain frequency shift amount and the designed target frequency combs:

[0075] The total width f of the target frequency combs BW is not greater than twice the optical domain frequency shift amount, and the difference between the starting frequency of the frequency combs and the optical domain frequency shift amount is not less than the optical domain frequency shift amount and the comb tooth interval Δf.C and.

[0076] As preferred, the modulated time domain signal can be frequency shifted in optical domain by an acousto-optic modulator AOM and the probe pulse is modulated simultaneously, and the acousto-optic modulator bandwidth f AOM And the designed target frequency comb also needs to satisfy:

[0077] The total width f BW of the frequency comb is not greater than twice of the acousto-optic modulator bandwidth f AOM The frequency difference Δf between the frequency comb start frequency and the acousto-optic modulator bandwidth f AOM is not less than the acousto-optic modulator bandwidth f AOM The sum of the comb tooth interval Δf C and the frequency shift amount in optical domain is to realize the coherent detection of double sideband coexistence; in the specific implementation of the method of the present application, the difference between the frequency comb start frequency and the frequency shift amount in optical domain is equal to Δf.

[0078] As preferred, S4 further includes the step of: pulse amplifying the modulated frequency comb pulse sequence to improve the detection signal-to-noise ratio.

[0079] Specifically, in S4, the quantitative distributed fiber vibration detection includes:

[0080] S41, filtering the modulated pulse sequence after pulse amplification and entering the to-be-detected optical fiber FUT, and the to-be-detected optical fiber FUT generates continuous backscattering signals;

[0081] The backscattering signals are interfered with local light to output two continuous coupling signals; wherein the detection light and the local light can be generated by a laser, and the continuous light generated by the laser is divided into two paths after passing through a coupler, one path is used as detection light, and the other path is used as coherent local light;

[0082] S42, extracting the intensity information of the beat frequency of the two continuous coupling signals (i.e. heterodyne Rayleigh scattering signals);

[0083] S43, periodically clipping the intensity information after the beat frequency according to the detection pulse interval, and rearranging according to the correspondence between each detection pulse and the modulated frequency comb loaded thereon, and filtering the rearranged intensity information to obtain the frequency information of each detection, i.e. if a single detection uses one frequency comb, each detection obtains 2n frequency information, and if a single detection uses the above designed m+1 frequency combs, each detection obtains 2*(m+1)*n frequency information; then arrange the frequency information obtained by each detection in frequency order to obtain the Rayleigh scattering trajectory matrix of each detection, such as Figure 8The obtained filtered signal contains three frequency bands: the reference carrier frequency, the upper sideband of the modulation frequency comb and the lower sideband of the modulation frequency comb; when filtering the rearranged intensity information, the upper sideband of the modulation frequency comb and the lower sideband of the modulation frequency comb are calculated according to the position of each frequency of the target frequency comb and the size of the optical domain frequency shift, and then the filter is designed.

[0084] That is, the original designed target frequency comb is shot as a double sideband signal by combining the coherent detection technology and the designed frequency comb. In the frequency domain, the upper sideband of the modulation frequency comb is the designed target frequency comb frequency which is shifted upward by an optical domain frequency shift, and the lower sideband of the modulation frequency comb is the designed target frequency comb frequency which is shifted downward by an optical domain frequency shift. In this embodiment, the optical domain frequency shift is equal to the bandwidth f AOM of the acousto-optic modulator. Based on the designed target frequency comb, the obtained upper sideband of the modulation frequency comb and the lower sideband of the modulation frequency comb together form a continuous long frequency comb or form two frequency comb signals with a certain frequency interval. The reference carrier frequency will exist separately at a lower frequency band, and there is an easily distinguishable frequency interval between the two frequency combs, as shown in Figure 6 .

[0085] S44, the measured Rayleigh scattering trajectory matrix at a certain moment is taken as a reference spectrum, and the measured Rayleigh scattering trajectory matrix within a period of time before or / and after the moment is taken as a measurement spectrum. Each measurement spectrum is calculated with the reference spectrum by a cross-correlation algorithm or other vector correlation degree estimation algorithm to obtain the correlation degree of each measurement spectrum with the reference spectrum, and the size of the frequency shift corresponding to each position of the measured optical fiber is obtained, that is, the position information of the external disturbance can be obtained from the size of the frequency shift, and the disturbance size is dynamically monitored. The size of the frequency shift corresponds to the size of the external disturbance, and in common communication optical fibers, the corresponding relationship is generally about 150MHz / με. The upper sideband of the modulation frequency comb and the lower sideband of the modulation frequency comb can be processed respectively, and the cross-correlation operation is performed respectively, and then the obtained results are averaged. The two frequency combs detect the same external disturbance information in different dimensions, which can improve the signal-to-noise ratio of the detection.

[0086] By the method of the present application, a large number of measurement results can be obtained within a period of time, and a frequency comb only occupies one detection time, or a group of frequency combs only occupies m+1 detection times. Compared with the traditional method in which each frequency occupies one detection time, the detection time of the present application is greatly shortened, that is, the measurement speed can be greatly improved to realize dynamic detection of signals. As shown in Figure 7 , based on the above detection method, the present application provides a fast quantitative distributed optical fiber vibration detection system based on frequency comb, mainly comprising:

[0087] The target frequency comb design module, the laser, the coupler 1, the polarization controller PC, the electro-optical modulator, the arbitrary waveform generator, the acousto-optic modulator AOM, the amplifier, the first circulator, the Bragg fiber grating FBG, the second circulator, the 3dB coupler 2, the balanced photodetector BPD, the high-speed oscilloscope Oscilloscope and the data processing module;

[0088] The target frequency comb design module is used for designing a target frequency comb: the interval Δf of the frequency comb is C The interval Δf of the frequency comb is not less than twice the reciprocal of the pulse width of the probe pulse, and the initial phase of the frequency comb is determined, including calculating the peak-to-average power ratio PAPR of the time-domain waveform corresponding to the frequency comb, obtaining the phase of the frequency comb corresponding to the time-domain waveform with the minimum peak-to-average power ratio PAPR through iterative optimization, and taking the phase as the initial phase of the frequency comb.

[0089] As a preferred design of the application, the application further comprises a target frequency comb frequency shift module, which is used for performing m times of frequency shift on the target frequency comb designed by the target frequency comb design module in sequence, and the frequency shift step is The intervals of the m+1 groups of frequency combs obtained after the frequency shift are unchanged, and the m+1 groups of frequency combs after the frequency shift are taken as new target frequency combs. In the embodiment of the application, the new target frequency combs are used for frequency shift detection.

[0090] The laser is used for outputting laser, and the output laser is divided into two paths of laser through the coupler 1, one path of laser is used as probe light, and the other path is used as coherent local light. In the embodiment, a narrow-line-width laser is selected as the laser; the probe light is input into the electro-optical modulator after being adjusted in polarization state through the polarization controller PC.

[0091] The arbitrary waveform generator is used for generating a time-domain signal converted from the target frequency comb, wherein the arbitrary waveform generator can be an arbitrary waveform generator AWG or an arbitrary function generator AFG.

[0092] The electro-optical modulator is used for modulating the time-domain signal on the probe light to obtain a modulated time-domain optical carrier signal, wherein the electro-optical modulator can be an electro-optical modulator EOM or a Mach-Zehnder modulator MZM.

[0093] The acousto-optic modulator AOM is used for optical domain frequency shift and probe pulse modulation of the modulated time domain optical carrier signal, to obtain a modulated frequency comb pulse sequence, and to realize loading of n equally spaced probe frequencies on a single pulse; in this embodiment, the acousto-optic modulator AOM is provided with a periodic rectangular pulse radio frequency signal by an arbitrary function generator AFG; the time interval between two optical pulses is greater than twice the optical pulse transit time. A larger bandwidth acousto-optic modulator AOM (such as 200 MHz or 300 MHz) is preferably used to realize a larger signal strength measurement range.

[0094] In order to avoid aliasing when subsequent correlation demodulation is performed, the target frequency comb also needs to satisfy:

[0095] The total width f of the frequency comb BW is not greater than twice the bandwidth f of the acousto-optic modulator used AOM The frequency difference Δf between the starting frequency of the frequency comb and the bandwidth f of the acousto-optic modulator AOM is not less than the bandwidth f of the acousto-optic modulator AOM The sum of the target frequency comb tooth interval Δf C is used to realize double sideband coexistence coherent detection.

[0096] Based on the target frequency comb designed in the application, two waveform change periods can be distinguished in each optical pulse after probe pulse modulation; at the same time, the acousto-optic modulator AOM introduces a fixed frequency shift, so that subsequent signals can be heterodyne coherent demodulated.

[0097] As a preferred, the modulated frequency comb pulse sequence is amplified by a pulse amplifier, in this embodiment, the amplifier is an erbium-doped fiber amplifier EDFA. After pulse amplification, the pulse sequence passes through a first circulator and a Bragg fiber grating FBG to filter out the spontaneous emission noise generated by the erbium-doped fiber amplifier EDFA, and then enters the optical fiber to be measured FUT through a second circulator; wherein the probe optical fiber includes but is not limited to ordinary single-mode optical fiber, multi-core optical fiber, few-mode optical fiber, photonic crystal optical fiber, plastic optical fiber, etc. In this embodiment, an ordinary single-mode optical fiber is selected.

[0098] The backscattered Rayleigh scattering signal generated by the optical fiber to be measured FUT is output through the other port of the second circulator, and the output backscattered Rayleigh scattering signal and the other coherent local light are respectively connected to a 3dB coupler coupler2 to interfere, and two continuous coupling signals are output; the light signal is sampled by a balanced photodetector BPD and converted into an electrical signal, and the intensity information of the beat frequency of the two continuous coupling signals, i.e. the heterodyne Rayleigh scattering signal, is extracted; the heterodyne Rayleigh scattering signal is collected by a high-speed oscilloscope Oscilloscope.

[0099] The data processing module comprises a Rayleigh scattering trajectory matrix generating unit and an external disturbance quantitative calculation unit, the Rayleigh scattering trajectory matrix generating unit is used for executing step S43 in the above-mentioned fast quantitative distributed optical fiber vibration detection method based on a frequency comb, and the external disturbance quantitative calculation unit is used for executing S44 in the above-mentioned fast quantitative distributed optical fiber vibration detection method based on a frequency comb.

[0100] In the embodiment, the Rayleigh scattering trajectory matrix at t0 is taken as a reference spectrum, the Rayleigh scattering trajectory matrix in t1-t n is taken as a measurement spectrum, cross-correlation calculation is performed, and the cross-correlation spectrum is obtained. Figure 8

[0101] In the embodiment, the upper sideband of the modulation frequency comb and the lower sideband of the modulation frequency comb are processed respectively, and the obtained cross-correlation spectrum is as shown in Figure 9 When there is an external disturbance on the optical fiber, the correlation peak at the corresponding position of the cross-correlation spectrum deviates from zero value, and the size of the deviation is linearly related to the size of the external disturbance, and if a non-standard single-mode optical fiber is used, the linear relationship needs to be calibrated in advance. The cross-correlation spectrum obtained by the lower sideband signal of the modulation frequency comb is flipped along the frequency component direction, and then averaged with the cross-correlation spectrum obtained by the upper sideband signal of the modulation frequency comb, so that the fiber along the line frequency shift correlation spectrum pattern is obtained, and the external disturbance signal of the fiber along the line is obtained by calculating the correlation spectrum at each time and fitting the peak value of the correlation spectrum at each time. As shown in Figure 10 , the peak fitting results of several 500Hz sine signals with different amplitudes at the vibration signal position on Figure 9 are shown, which verifies the dynamic quantitative monitoring capability of the application; the system response speed is equal to the total time consumption of each group of measurements, and in the embodiment, the total time consumption is the sum of the required time of 5 single pulse measurements. The resolution of the system to the disturbance signal intensity is determined by the frequency comb interval set by the experiment and the specific peak fitting algorithm used, and in general case, the strain sensitivity is in the order of nε or even pε, which is more than three orders of magnitude higher than the sensitivity (vibration signal amplitude measurement accuracy) of the traditional single frequency coherent detection scheme in the order of με, wherein the vibration signal amplitude measurement accuracy is related to the size of the spatial resolution, and the vibration signal amplitude measurement accuracy is determined by the total bandwidth f BW and the interval Δf C of the frequency comb teeth, and the size of the spatial resolution is determined by the pulse width of the detection pulse. Moreover, the frequency comb designed based on the application directly obtains the frequency shift information corresponding to the external disturbance signal according to the intensity information of the detection result, so that the application does not need to perform phase demodulation and is not affected by interference fading, and therefore the monitoring and sensing of the vibration are more reliable.

[0102] Compared with the traditional frequency-by-frequency scanning frequency shift detection ​The scheme realizes loading of n equally-spaced detection frequencies on a single pulse through a designed frequency comb. One frequency comb occupies one detection time, or a group of frequency combs occupies m detection times. Compared with the traditional scheme in which each frequency occupies one detection time, the long frequency scanning process is compressed into single or several detection times, the detection time is greatly compressed, and the frequency response capability is improved. The frequency response speed is limited by the sensing fiber length and the measurement precision, and can easily reach the kHz level. Since coherent detection is used, averaging is not required, and the signal-to-noise ratio is higher than that of the conventional agile frequency scheme.

[0103] In a specific application, the system can be installed according to a conventional coherent detection phi-OTDR single-end sensing system. After installation, the frequency comb signal to be used is designed according to the target sensing scene, and the signal processing is performed according to the above steps to realize high-sensitivity quantitative monitoring of dynamic signals.

[0104] In the present application, dynamic signal response equivalent to that of a conventional single-frequency detection system can be realized, while the advantages of the ultra-high sensitivity of the frequency-shift detection system are maintained, thereby realizing ultra-high sensitivity and rapid quantitative vibration signal monitoring.

[0105] Those skilled in the art will readily understand that the above description is only a preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.​​

Claims

1. A rapid quantitative distributed optical fiber vibration detection method based on frequency comb, characterized in that, include: S1. Design the target frequency comb, including designing the comb tooth spacing Δf of the target frequency comb. C and the initial phase of the target frequency comb; wherein, the comb tooth spacing Δf of the target frequency comb C The initial phase of the target frequency comb is not less than twice the reciprocal of the probe pulse width, and the initial phase of the target frequency comb is the phase corresponding to the minimum peak-to-average power ratio of the time-domain waveform of the target frequency comb. S2. Convert the target frequency comb into a time-domain signal, and modulate the time-domain signal onto the probe light to obtain a modulated time-domain optical carrier signal; S3. Perform optical domain frequency shifting and probe pulse modulation on the modulated time-domain optical carrier signal to obtain a modulated frequency comb pulse sequence; S4. Input the frequency comb pulse sequence into the fiber under test (FUT) to perform quantitative distributed fiber vibration detection.

2. The method according to claim 1, characterized in that, Before S2, the following steps are also included: The target frequency comb is subjected to m frequency shifts in sequence, and the step size of each frequency shift is the comb tooth spacing Δf. C of The tooth spacing of the m+1 frequency combs obtained after frequency shifting remains unchanged. The m+1 frequency combs are used as the new target frequency combs, where m≥1.

3. The method according to claim 1 or 2, characterized in that, In S3, the optical frequency shift and the target frequency comb satisfy the following: The total bandwidth f of the target frequency comb BW The difference between the starting frequency of the target frequency comb and the optical frequency shift is not greater than twice the optical frequency shift, and the difference between the optical frequency shift and the comb tooth spacing Δf is not less than the optical frequency shift and the comb tooth spacing Δf. C The sum of.

4. The method according to claim 1 or 2, characterized in that, In S1, the phase corresponding to the minimum peak-to-average power ratio of the target frequency comb time-domain waveform is calculated, including: The initial phase of each tooth of the target frequency comb is randomly set, and the peak-to-average power ratio corresponding to the time-domain waveform of the target frequency comb is calculated. Through iterative optimization, the phase corresponding to the minimum peak-to-average power ratio of the target frequency comb is obtained.

5. The method according to claim 1 or 2, characterized in that, In S4, quantitative distributed fiber optic vibration detection includes: S41. After interfering the continuous backscattered Rayleigh signal generated by the fiber under test (FUT) with the local light, two continuous coupled signals are output. S42. Extract the intensity information of the two continuous coupled signals after beat frequency; S43. The intensity information is periodically trimmed and rearranged according to the detection pulse interval; the rearranged intensity information is then filtered to obtain the frequency information for each detection; the frequency information is arranged in frequency order to obtain the Rayleigh scattering trajectory matrix for each detection. S44. Using the Rayleigh scattering trajectory matrix at a certain moment as the reference spectrum, and the Rayleigh scattering trajectory matrix over a period of time at that moment as the measurement spectrum, calculate the correlation between each measurement spectrum and the reference spectrum to obtain the magnitude of the frequency shift corresponding to each position of the fiber under test, and use the magnitude of the frequency shift to characterize the magnitude of the external disturbance.

6. The method according to claim 5, characterized in that, S4 also includes: pulse amplification of the frequency comb pulse sequence; Or / and also includes: denoising the amplified pulse sequence before inputting it to the fiber under test (FUT).

7. A rapid quantitative distributed optical fiber vibration detection system based on a frequency comb, characterized in that, include: The target frequency comb design module is used to design a target frequency comb, including designing the comb tooth spacing Δf of the target frequency comb. C and the initial phase of the target frequency comb; wherein, the comb tooth spacing Δf of the target frequency comb C The initial phase of the target frequency comb is not less than twice the reciprocal of the probe pulse width, and the initial phase of the target frequency comb is the phase corresponding to the minimum peak-to-average power ratio of the time-domain waveform of the target frequency comb. An arbitrary waveform generator is used to generate the time-domain signal corresponding to the target frequency comb; An electro-optic modulator is used to modulate the time-domain signal onto a probe light to obtain a modulated time-domain optical carrier signal. An acousto-optic modulator is used to perform optical domain frequency shifting and probe pulse modulation on the modulated time-domain optical carrier signal to obtain a modulated frequency comb pulse sequence. The second circulator is used to input the frequency comb pulse sequence into the fiber under test (FUT) and output the backscattered Rayleigh signal generated by the FUT. A 3dB coupler is used to interfere the backscattered Rayleigh signal with the local light and output two continuous coupled signals. A balanced photodetector is used to extract the intensity information of the two continuous coupled signals after beat frequency. The data processing module is used to perform quantitative distributed optical fiber vibration detection based on the intensity information.

8. The system according to claim 7, characterized in that, It also includes a target frequency comb frequency shift module, used to sequentially perform m frequency shifts on the target frequency comb, wherein the step size of each frequency shift is the comb tooth spacing Δf of the target frequency comb. C of The tooth spacing of the m+1 frequency combs obtained after frequency shifting remains unchanged. The m+1 frequency combs are used as the new target frequency combs, where m≥1.

9. The system according to claim 7 or 8, characterized in that, The bandwidth f of the acousto-optic modulator AOM The following conditions must be met with the frequency comb: The total bandwidth f of the target frequency comb BW Not greater than the bandwidth f of the acousto-optic modulator AOM twice the target frequency comb starting frequency and the bandwidth f of the acousto-optic modulator. AOM The frequency difference Δf is not less than the bandwidth f of the acousto-optic modulator. AOM The comb tooth spacing Δf with respect to the target frequency C The sum of.

10. The system according to claim 9, characterized in that, The data processing module includes a Rayleigh scattering trajectory matrix generation unit and an external disturbance quantitative calculation unit; The Rayleigh scattering trajectory matrix generation unit is used to periodically trim and rearrange the intensity information according to the detection pulse interval; then filter the rearranged intensity information to obtain the frequency information of each detection; and arrange the frequency information in frequency order to obtain the Rayleigh scattering trajectory matrix of each detection. The external disturbance quantitative calculation unit is used to take the Rayleigh scattering trajectory matrix at a certain moment as the reference spectrum, the Rayleigh scattering trajectory matrix over a period of time at that moment as the measurement spectrum, calculate the correlation between each measurement spectrum and the reference spectrum, obtain the magnitude of the frequency shift corresponding to each position of the fiber under test, and use the magnitude of the frequency shift to characterize the magnitude of the external disturbance.

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