A dispersion fourier transform based optical frequency reflectometer system

Abstract

The present application relates to the field of distributed optical fiber measurement, and particularly relates to an optical frequency reflectometer (OFDR) system based on dispersion Fourier transform. The present application introduces dispersion Fourier transform technology in the traditional OFDR system, through the dispersion characteristics of the dispersion element, the ultra-short pulse of the optical frequency comb is expanded into continuous chirp pulse in time domain, and then the optical frequency comb is applied to the OFDR system, so that the dependence of the traditional OFDR system on the swept light source is got rid of; the present application breaks through the limitation of the swept frequency speed of the internal modulation swept light source and the external modulation swept light source, simultaneously realizes the nm-level swept frequency range and the MHz-level scanning frequency in the OFDR system without relying on the swept light source, and realizes the high-speed dynamic signal measurement of the OFDR system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of distributed optical fiber measurement, and particularly relates to an optical frequency reflectometer (OFDR) system based on dispersion Fourier transform. BACKGROUND

[0002] Distributed acoustic sensor can locate vibration events on the measured optical fiber link and quantify specific dynamic signal variables, and the sensor system has the advantages of distributed measurement, high sensitivity and high temperature corrosion resistance, and is widely used in geological exploration, building structure monitoring, railway safety monitoring and other fields. However, in the field of high-precision aerospace measurement, such as health monitoring of aircraft wings in working condition, there are usually requirements for millimeter-level spatial resolution and high-speed dynamic strain signal detection. The distributed acoustic sensor based on OFDR can meet the measurement requirements of high spatial resolution within a short distance by scanning the light source in a large frequency range, but the measurement bandwidth of the dynamic signal is limited by the scanning period of the light source.

[0003] At present, the dynamic signal detection based on the OFDR system mainly includes phase demodulation method and spectrum demodulation method. The spectrum demodulation method extracts Rayleigh backscattering light information at the target position by setting a distance domain window, and then transforms it to the spectral domain to obtain the spectral shift through cross-correlation, so as to realize the demodulation of the dynamic signal. In the spectrum demodulation method, the detection bandwidth of the dynamic signal is limited by the scanning period of the scanning light source. A faster scanning period is required for detecting high-speed dynamic signals, but it will bring about a sharp increase in the phase noise of the detection light source, resulting in the degradation of the spatial resolution. Therefore, the spatial resolution and the detection bandwidth in the traditional spectrum demodulation method are limited by the hardware devices and have a mutual restraining relationship. Compared with the spectrum demodulation method, the phase demodulation method can obtain higher strain spatial resolution under the same scanning range, but it is also limited by the scanning nonlinearity and phase noise of the scanning light source, and the dynamic range is low, so the detection bandwidth needs to be sacrificed to improve the dynamic range. Therefore, the OFDR system still has challenges in realizing high-speed dynamic signal detection. SUMMARY

[0004] In view of the above problems or deficiencies, in order to solve the problem that the traditional OFDR system is difficult to realize high spatial resolution and respond to high-speed dynamic signals at the same time, the present application provides an optical frequency reflectometer system based on dispersion Fourier transform. The ultra-short pulse in the time domain of the optical frequency comb is expanded into a chirped pulse in the time domain, so as to serve as the scanning light source of the OFDR system, which can realize the detection of high-speed dynamic signals while ensuring high spatial resolution.

[0005] To achieve the above purpose, the technical scheme adopted by the present application is:

[0006] A kind of optical frequency reflectometer system based on dispersion fourier transform, comprising: signal femtosecond pulse mode-locked laser, dispersion element, fiber coupler 1, fiber coupler 2, delay fiber, fiber coupler 3, fiber coupler 4, polarization controller, circulator, probe fiber, piezoelectric ceramic, fiber coupler 5, photodetector 1, photodetector 2, A / D acquisition device and computer.

[0007] The signal femtosecond pulse mode-locked laser emits signal femtosecond pulse sequence, and the signal femtosecond pulse mode-locked laser repetition frequency f rep >50kHz, and output to the dispersion element.

[0008] The dispersion element carries out dispersion fourier transform to the signal femtosecond pulse sequence, and expands into nanometer width linear chirp pulse on time window, and output to fiber coupler 1.

[0009] The fiber coupler 1 connects the output of the dispersion element, and the linear chirp pulse output by the dispersion element is split into two ways, one way is connected to fiber coupler 4 as probe light and the other way is connected to fiber coupler 2 as local oscillator light.

[0010] The fiber coupler 2 connects two optical fibers, one of which is a delay fiber, and then the two optical fibers connect the fiber coupler 3 to form a double-beam interference structure as an auxiliary interferometer, and the output end of the fiber coupler 3 is connected to the photodetector 1.

[0011] The fiber coupler 4 connects two optical fibers, one of which is connected to the fiber coupler 5 after the polarization controller; the other is connected to the 1 port of the circulator, the 2 port of the circulator is connected to the probe fiber, and the 3 port of the circulator is connected to the fiber coupler 5, forming a double-beam interference structure as a main interferometer, and the output end of the fiber coupler 5 is connected to the photodetector 2.

[0012] The input of the A / D acquisition device is connected to the output of the photodetector 1 and the photodetector 2, and the output is connected to the computer.

[0013] The computer processes the digital signal input by the A / D acquisition device: Fourier transforms the digital signal to obtain the distance domain signal of the probe fiber in the vibration state and the non-vibration state, and the distance domain window signal of the probe fiber at the corresponding window position z n In the distance domain is intercepted by N continuous sliding windows, the inverse fourier transform is carried out on all the obtained distance domain window signals, and the Rayleigh scattering spectrum at each spatial position on the probe fiber in the vibration state and the non-vibration state is obtained, and the Rayleigh scattering spectrum at the same spatial position z n In the vibration state and the non-vibration state is correlated, and the spectrum shift T is the total number of sampling periods, v0 is the instantaneous frequency of the swept light, n = 1, 2, …, N, t = 1, 2, …, T, according to the formula The strain amount of the spatial position is calculated Through multiple measurements, high-speed dynamic signal detection at each spatial position on the entire optical fiber link is obtained.

[0014] Further, the signal femtosecond pulse mode-locked laser adopts a passive mode-locked laser, specifically a mode-locked laser based on nonlinear polarization rotation effect, the repetition frequency of the signal femtosecond pulse sequence is 50k-20MHz, and the pulse width is in the femtosecond order.

[0015] Further, the dispersion element is an optical fiber element with a group velocity dispersion parameter, and the total dispersion amount of the dispersion element causes the signal femtosecond pulse sequence to be time-domain widened, and the time τ of the single pulse after the widening pulse is less than the repetition period of the femtosecond pulse sequence

[0016] Further, the optical fiber element with a group velocity dispersion parameter is a standard single-mode optical fiber, a dispersion compensation optical fiber or a chirped Bragg grating.

[0017] Further, the bandwidth f aux of the photoelectric detector 1 is greater than the frequency f aux of the interference signal output by the auxiliary interferometer, and f main of the photoelectric detector 2 is greater than the beat signal frequency corresponding to the longest detection optical fiber L is the longest detection length, γ is the actual pulse sweeping speed, v c is the speed of light in the medium, f main >12G.

[0018] Further, the sampling rate f sample of the A / D acquisition device is 2-4 times the detection bandwidth of the photoelectric detector 2, f sample >24G.

[0019] Further, the detection optical fiber is a single-mode optical fiber, such as a G.652 type single-mode optical fiber, a G.655 type single-mode optical fiber and a G.657 type single-mode optical fiber.

[0020] Further, the delay optical fiber length is 0.01m-1m, different lengths are set to change the arm length difference of the auxiliary interferometer, and each period of the interference signal output by the optical fiber coupler 3 corresponds to a frequency change of the scanning light source of 200M-20G. When the optical fiber length is less than 0.01m, the compensation effect on the swept nonlinear noise is weak, and when the optical fiber length is greater than 1m, the required detector bandwidth is further increased, the system calculation complexity is increased, and the compensation effect on the swept nonlinear noise is slightly improved.

[0021] The working process of the above-mentioned optical frequency reflectometer system based on dispersion Fourier transform is as follows:

[0022] The signal femtosecond pulse mode-locked laser emits a signal femtosecond probe pulse sequence, and the signal femtosecond pulse sequence is subjected to dispersion Fourier transform by a dispersion element to expand the femtosecond pulse into a nanosecond-width linearly chirped pulse in a time window.

[0023] The linearly chirped pulse is split into a probe light and a local oscillator light after entering the fiber coupler 1:

[0024] The local oscillator light is split into two lights after passing through the fiber coupler 2 and entering two optical fibers, one of which passes through a delay optical fiber, and then the two lights are coupled into the fiber coupler 3 to form a double-beam interference structure as an auxiliary interferometer, and the double-beam interference signal of the auxiliary interferometer enters the photodetector 1 through the fiber coupler 3 to be converted into an electrical signal.

[0025] The probe light is split into two lights by the fiber coupler 4: one of which enters the fiber coupler 5 after adjusting the polarization state by a polarization controller; the other of which enters a probe optical fiber from the 2 port of the circulator, and the Rayleigh backscattering light generated returns to the 2 port of the circulator and enters the fiber coupler 5 from the 3 port of the circulator; the two lights are combined into the fiber coupler 5 to form a double-beam interference structure as a main interferometer, and the double-beam interference signal of the main interferometer enters the photodetector 2 through the fiber coupler 5 to be converted into an electrical signal.

[0026] The A / D acquisition device converts the electrical signals output by the photodetector 1 and the photodetector 2 into digital signals, and finally the digital signals are demodulated in a computer to obtain the Rayleigh scattering spectra at each spatial position on the probe optical fiber in the vibration state and the non-vibration state, and the Rayleigh scattering spectrum at the same spatial position z n in the vibration state and the Rayleigh scattering spectrum at the same spatial position z Through multiple measurements, the high-speed dynamic signal detection at each spatial position on the entire optical fiber link is obtained.

[0027] Optical frequency comb refers to a spectrum composed of a series of discrete frequency components with equal interval and locked phase relationship, which is also a series of pulse sequences with equal time interval in time domain according to Fourier transform, and is a natural time-frequency reference and highly stable pulsed laser source. The traditional optical frequency comb cannot be used as a probe light source of the OFDR system because different frequency components thereof are concentrated in the same pulse, and the OFDR system needs continuous chirped pulses. The present application solves the problem by using dispersion Fourier transform technology, and expands the ultra-short pulse of the optical frequency comb into continuous chirped pulses in time domain through the dispersion characteristics of the dispersion element, thereby overcoming the limitation of the scanning speed of the traditional light source and realizing the nm-level scanning range and MHz-level scanning frequency.

[0028] In summary, the present application introduces dispersion Fourier transform technology into the traditional OFDR system, expands the ultra-short pulse of the optical frequency comb into continuous chirped pulses in time domain through the dispersion characteristics of the dispersion element, and then applies the optical frequency comb to the OFDR system, thereby getting rid of the dependence of the traditional OFDR system on the scanning light source. The present application breaks through the limitation of the scanning speed of the internal modulation scanning light source and the external modulation scanning light source, realizes the nm-level scanning range and the MHz-level scanning frequency in the OFDR system without relying on the scanning light source, and realizes the high-speed dynamic signal measurement of the OFDR system. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 Fig. 1 is a structural schematic block diagram of the OFDR system of the present application.

[0030] Figure 2 Fig. 2 is a distance domain signal of the main interferometer part in the embodiment.

[0031] Figure 3 Fig. 3 is a time domain interference waveform of the auxiliary interferometer in the embodiment.

[0032] Figure 4 Fig. 4 is a power density spectrum of the high-speed dynamic signal at a certain position on the measured optical fiber obtained by demodulation in the embodiment.

[0033] Fig. 1 is a structural schematic block diagram of the OFDR system of the present application. DETAILED DESCRIPTION

[0034] The present application will be further described in detail below in combination with the drawings and embodiments.

[0035] The embodiment provides a light frequency reflectometer system based on dispersion Fourier transform, which has a structure as shown in the figure Figure 1 and comprises: 1, a signal femtosecond pulse mode-locked fiber laser; 2, a dispersion element; 3, a fiber coupler 1; 4, a fiber coupler 2; 5, a delay fiber; 6, a fiber coupler 3; 7, a fiber coupler 4; 8, a polarization controller; 9, a circulator; 10, a probe fiber; 11, a piezoelectric ceramic; 12, a fiber coupler 5; 13, a photodetector 1; 14, a photodetector 2; 15, an A / D acquisition device; and 16, a computer.

[0036] The signal femtosecond pulse mode-locked fiber laser 1 (optical frequency comb) generates a signal femtosecond pulse sequence with a repetition frequency of f r1 , the femtosecond pulse is stretched into a chirped pulse light in the time domain after passing through the dispersion element with a specific dispersion amount, the chirped pulse light is injected into the fiber coupler 1, the fiber coupler 1 injects the input chirped pulse light into the fiber coupler 2 (auxiliary interferometer incident end) and the fiber coupler 4 (main interferometer incident end) respectively, the chirped pulse light entering the auxiliary interferometer part forms an interferometer structure after passing through two different optical paths of single-mode fibers, the two light signals enter the photodetector 1 through the fiber coupler 3 (auxiliary interferometer output end) and are detected, the chirped pulse light entering the main interferometer part is divided into two light signals through the fiber coupler 4, one of the two light signals is used as an intrinsic light signal and passes through the polarization controller to realize polarization state regulation, the other light signal is used as a probe light signal and enters the circulator 1 port, the Rayleigh scattering signal generated in the measured fiber link returns to the circulator 2 port and is output from the circulator 3 port, the two light signals enter the photodetector 2 through the fiber coupler 5 (main interferometer output end) and are detected, the A / D acquisition device is used for analog-to-digital conversion, and finally the computer is used for data processing.

[0037] The embodiment specifically comprises the following steps:

[0038] Step A: the repetition frequency f r1 of the signal femtosecond pulse mode-locked fiber laser is adjusted, the dispersion amount of the dispersion element is controlled, and a periodic sweep pulse with a pulse width of τ pulse in each period is generated.

[0039] Step B: when the measured fiber is in a non-vibration state, the digital signal input by the A / D acquisition device is subjected to Fourier transform to obtain a distance domain signal of the probe fiber in the non-vibration state, N continuous sliding windows are used to intercept distance domain window signals of the probe fiber at corresponding window positions z n , and inverse Fourier transform is performed on all the obtained distance domain window signals to obtain Rayleigh reflection spectra of reference signals at the window positions of the measured fiber link

[0040] Step C: The piezoelectric ceramic is placed at an arbitrary position on the probe fiber, and the piezoelectric ceramic is set to generate a vibration signal of 100 kHz. The distance domain signal of the probe fiber under vibration is obtained by performing Fourier transform on the digital signal input by the A / D acquisition device. N continuous sliding windows are used to intercept the distance domain window signal of the probe fiber at the corresponding window position z n The Rayleigh reflection spectrum of the measurement signal at each window position on the fiber link to be measured changes with the sampling time, which is obtained by performing inverse Fourier transform on all the obtained distance domain window signals T is the total number of sampling periods;

[0041] Step D: The Rayleigh reflection spectrum of the N measurement signals at z1 is respectively correlated with the Rayleigh reflection spectrum of the reference signal at the position, and the strain at the corresponding fiber position z1 is calculated to obtain the strain at the corresponding fiber position z1

[0042] Step D is repeated for each spatial position on the fiber link to be measured, and the strain information at all fiber link positions is obtained n

[0043] In this embodiment, the signal femtosecond pulse mode fiber laser is a mode-locked laser based on nonlinear polarization rotation effect. Compared with other optical frequency combs, it has the advantages of simple structure, low noise, high power and self-starting. The center wavelength of the optical frequency comb is 1550 nm, the 3dB spectral width is 20 nm, and the actual use optical signal is filtered to have a spectral width of 1 nm. The repetition frequency of the signal femtosecond mode-locked laser frequency comb is f r1 = 5 MHz, and the corresponding pulse period T r1 = 200 ns. According to the dispersion amount, the length Q used should satisfy 20 km ≤ Q < 100 km. Here, 80 km is selected, and the pulse can be broadened to τ pulse = 100 ns, and the corresponding theoretical light source scanning speed is 1.25EHz / s.

[0044] The length of the delay fiber introduced by the auxiliary interferometer part is 0.1 m. Each cycle of the interference signal corresponds to a frequency change of 2 GHz of the frequency-swept light source, and the interference signal frequency is 0.625 GHz, as shown in Figure 3 The measured light source scanning speed is the interference signal frequency multiplied by the frequency change of the frequency-swept light source corresponding to each cycle, and the result is consistent with the theoretical light source scanning speed. In addition, photoelectric detector 1 and photoelectric detector 2 with a bandwidth of 12 GHz are selected, and the A / D acquisition device uses a high-speed oscilloscope with a bandwidth of 15 GHz and a maximum sampling rate of 60 Gs / s.

[0045] ​Finally, the OFDR system used in this embodiment realizes the measurement of 100 kHz high-speed dynamic signals, as shown in Figure 4 Compared with the traditional OFDR system (quasi-static measurement or kHz dynamic signal measurement), there is a great improvement; it is worth mentioning that the OFDR system provided in this embodiment uses a dispersion element to realize frequency sweeping, and the frequency sweeping stability is not affected by external environmental noise, and the frequency sweeping stability is only affected by the pulse jitter and broadening caused by the frequency drift of the light source over time, and if the light source is further controlled by feedback stability, the signal-to-noise ratio of the measured dynamic signal can be further improved; at the same time, by using a wider-band optical frequency comb spectrum as the signal femtosecond pulse, the spatial resolution of the system can be further improved.

Claims

1. A system for optical frequency reflectometry based on dispersion Fourier transform, characterized in that, It comprises: a signal femtosecond pulse mode-locked laser, a dispersion element, a fiber coupler 1, a fiber coupler 2, a delay fiber, a fiber coupler 3, a fiber coupler 4, a polarization controller, a circulator, a probe fiber, a piezoelectric ceramic, a fiber coupler 5, a photodetector 1, a photodetector 2, an A / D acquisition device and a computer; The signal femtosecond pulse mode-locked laser emits a signal femtosecond pulse sequence, and the signal femtosecond pulse mode-locked laser has a repetition frequency f rep >50 kHz, and is output to the dispersive element; the dispersion element dispersively Fourier transforms the signal femtosecond pulse sequence, expands the femtosecond pulse into a nanosecond linear chirp pulse in a time window, and outputs to the fiber coupler 1; the fiber coupler 1 receives the output of the dispersion element, splits the linear chirp pulse output by the dispersion element into two paths, one path is connected to the fiber coupler 4 as a probe light, and the other path is connected to the fiber coupler 2 as a local oscillator light; the fiber coupler 2 receives two optical fibers, one of which is a delay fiber, and then the two optical fibers are connected to the fiber coupler 3 to form a double-beam interference structure as an auxiliary interferometer, and the output end of the fiber coupler 3 is connected to the photodetector 1; the fiber coupler 4 receives two optical fibers, one of which is connected to the fiber coupler 5 through the polarization controller; the other is connected to the 1 port of the circulator, the 2 port of the circulator is connected to the probe fiber, and the 3 port of the circulator is connected to the fiber coupler 5, forming a double-beam interference structure as a main interferometer, and the output end of the fiber coupler 5 is connected to the photodetector 2; the input of the A / D acquisition device is connected to the output of the photodetector 1 and the photodetector 2, and the output is connected to the computer; The computer processes the digital signal inputted by the A / D acquisition device: Fourier transform is performed on the digital signal to obtain the distance domain signal of the probe optical fiber in the vibration state and the non-vibration state, N continuous sliding windows are used to intercept the distance domain window signal of the probe optical fiber at the corresponding window position z n of the probe optical fiber, inverse Fourier transform is performed on all the obtained distance domain window signals to obtain the Rayleigh scattering spectrum at each spatial position of the probe optical fiber in the vibration state and the non-vibration state, cross-correlation calculation is performed on the Rayleigh scattering spectrum at the same spatial position z n of the probe optical fiber in the vibration state and the non-vibration state to obtain the spectrum shift of the spatial position T is the total sampling period number, v0 is the instantaneous frequency of the sweep frequency light, n=1, 2, …, N, t=1, 2, …, T, according to the formula , the strain of the spatial position is calculated Through multiple measurements, the high-speed dynamic signal detection at each spatial position on the entire optical fiber link is obtained.

2. The optical frequency reflectron system based on dispersion Fourier transform of claim 1, wherein: the signal femtosecond pulse mode-locked laser adopts a passive mode-locked laser, specifically a mode-locked laser based on nonlinear polarization rotation effect, the repetition frequency of the signal femtosecond pulse sequence is 50k-20MHz, and the pulse width is in femtosecond level.

3. The optical frequency reflectron system based on dispersion Fourier transform of claim 1, wherein: The dispersion element is an optical fiber element having a group velocity dispersion parameter, the total dispersion of the dispersion element broadening the sequence of femtosecond pulses in the time domain, and the individual pulses of the broadened sequence having a time τ pulse less than the repetition period of the sequence of femtosecond pulses 4. The optical frequency reflectron system based on dispersion Fourier transform of claim 3, wherein: The optical fiber element with group velocity dispersion parameter is a standard single-mode fiber, a dispersion compensation fiber or a chirped Bragg grating.

5. The optical frequency reflectometer system based on dispersion Fourier transform according to claim 1, characterized in that: The bandwidth f of the photodetector 1 aux greater than the frequency of the auxiliary interferometer output interference signal, f aux > 200 M; Bandwidth f of the photodetector 2 main greater than the beat signal frequency corresponding to the farthest probe fiber distance L is the farthest probe length, γ is the actual pulse sweep speed, v c is the speed of light in the medium, f main >12G.

6. The optical frequency reflectron system based on dispersion Fourier transform of claim 1, wherein: The sampling rate f of the A / D acquisition device sample 2-4 times the detection bandwidth of the photodetector 2, f sample >24G.

7. The optical frequency reflectron system based on dispersion Fourier transform of claim 1, wherein: the probe fiber is a single-mode fiber.

8. The optical frequency reflectron system based on dispersion Fourier transform of claim 1, wherein: The length of the delay fiber is 0.01m-1m, by setting different lengths to change the arm length difference of the auxiliary interferometer, the interference signal output by the fiber coupler 3 corresponds to the scanning light source frequency change of 200M-20G per cycle.

9. The optical frequency reflectron system based on dispersion Fourier transform of claim 1, wherein, Workflow: The signal femtosecond pulse mode-locked laser emits a signal femtosecond probe pulse sequence, the signal femtosecond pulse sequence is dispersedly Fourier transformed by the dispersion element, and the femtosecond pulse is expanded into a nanosecond linear chirp pulse in a time window; the linear chirp pulse enters the fiber coupler 1 and is split into a probe light and a local oscillator light; the local oscillator light is split into two paths after passing through the fiber coupler 2, one of which passes through the delay fiber, and then the two paths are coupled into the fiber coupler 3 to form a double-beam interference structure as an auxiliary interferometer, and the double-beam interference signal of the auxiliary interferometer enters the photodetector 1 through the fiber coupler 3 to be converted into an electrical signal; The probe light is split into two paths by the fiber coupler 4: one path enters the fiber coupler 5 after the polarization state is adjusted by the polarization controller; the other path enters the fiber coupler 5 from the 3 port of the circulator after the Rayleigh backscattering light generated by the probe light entering the probe fiber from the 1 port of the circulator and returning to the 2 port of the circulator; the two paths of light are combined into the double-beam interference structure as the main interferometer by the fiber coupler 5, and the double-beam interference signal of the main interferometer enters the photodetector 2 through the fiber coupler 5 to be converted into an electric signal; The A / D acquisition device converts the electrical signals output by the photodetector 1 and the photodetector 2 into digital signals, and finally the digital signals are demodulated in a computer to obtain Rayleigh scattering spectra at each spatial position on the detection optical fiber in the vibration state and the non-vibration state. The Rayleigh scattering spectrum in the vibration state and the Rayleigh scattering spectrum at the same spatial position z n in the non-vibration state are cross-correlated to calculate the strain amount of the spatial position Through multiple measurements, high-speed dynamic signal detection at each spatial position on the entire optical fiber link is obtained.

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