Weak reflection fiber grating array weak vibration intensity and position demodulation system and method

Abstract

This invention discloses a weak reflection fiber Bragg grating array demodulation system for measuring the intensity and position of weak vibrations, comprising: a pulse signal generation module, two identical demodulation modules, and a weak reflection fiber Bragg grating array; the pulse signal generation module is connected to both demodulation modules; both ends of the weak reflection fiber Bragg grating array are connected to the two demodulation modules. This invention can double the measurement accuracy compared to a single Michelson interferometric demodulation system and can measure weak signals with even lower intensities.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing system technology. Specifically, it relates to a system and method for simultaneously demodulating the intensity and position of weak vibrations in a weakly reflective fiber Bragg grating array. Background Technology

[0002] Fiber optic vibration sensing technology can detect seismic sound waves, foreign object intrusion, oil and gas pipeline leaks, and vibrations of buildings and bridges, providing important technical support for human safety early warning activities. Weakly reflective fiber grating arrays, fabricated by online fiber drawing and laser single-pulse exposure, have been widely used in human production and daily life [Huang Junbin, Ding Peng, Tang Jinsong. Progress in the fabrication, demodulation and application of weakly reflective fiber grating arrays [J]. Progress in Lasers and Optoelectronics, 2021, 58(17):1700005.].

[0003] The time-domain demodulation technique for weak reflection fiber grating arrays generally uses a Mach-Zenhder interferometer, a Michelson interferometer, or a fiber delay line to match the weak reflection fiber grating array to form an interference effect, thereby demodulating the interference phase signal of the weak reflection fiber grating array. This type of method can obtain the phase change of the sensing fiber between adjacent weak reflection fiber gratings and obtain information on the intensity of external disturbances, but it cannot obtain the vibration position of the sensing fiber. To address this, the input light source is divided into two paths. One path uses a traditional distributed optical fiber method to detect vibration position, solving the problem that weakly reflective fiber grating arrays cannot obtain vibration position [Zhang XP, Sun ZH, Shan YY, et al. A high performance distributed optical fiber sensor based on Φ-OTDR for dynamic strain measurement[J]. IEEE Photocics Journal, 2017, 9(3): 6802412.]. The other path uses a spatial multiplexing method to detect the phase change of the weakly reflective fiber grating array, and then switches the optical pulse to detect the vibration position of the weakly reflective fiber grating array [Tong YH, Li ZY, Wang J Q, et al. High-speed Mach-Zehnder-OTDR distributed optical fiber vibration sensor using medium-coherence laser[J]. Photonic Sensors, 2018, 8(3): 203-212.]. Essentially, both methods measure the two physical quantities of vibration intensity and position separately, and mechanically superimpose the vibration intensity demodulation optical path and the vibration position optical path. However, the two methods mentioned above will result in weakened signal light intensity, low signal-to-noise ratio, and discontinuity in measurement time, making it impossible to obtain the intensity and location of high-frequency vibration signals. Summary of the Invention

[0004] This invention provides a weak reflection fiber Bragg grating array weak vibration intensity and position demodulation system and method to solve the problem that vibration intensity and position need to be detected separately, cannot be detected simultaneously, and cannot detect even weaker vibration signals.

[0005] This invention discloses a weak reflection fiber grating array weak vibration intensity and position demodulation system, comprising: a pulse signal generation module, two identical demodulation modules, and a weak reflection fiber grating array;

[0006] The pulse signal generation module is connected to two demodulation modules respectively; the two ends of the weak reflection fiber grating array are connected to two demodulation modules respectively.

[0007] Furthermore, the pulse signal generation module includes: a first arbitrary signal generator, a continuous laser, a first acousto-optic modulator, an optical fiber amplifier, and a 1×2 coupler;

[0008] The output of the continuous laser is connected to the signal input of the first acousto-optic modulator; the first arbitrary signal generator is connected to the modulation signal input of the first acousto-optic modulator; the output of the first acousto-optic modulator is connected to the input of the fiber optic amplifier; the output of the fiber optic amplifier is connected to the input of the 1×2 coupler; and the two outputs of the 1×2 coupler are respectively connected to the two modulation modules.

[0009] Furthermore, the pulse width of the first acousto-optic modulator needs to be less than 2nL / c, where n is the effective refractive index of the optical fiber, c is the speed of light in vacuum, and L is the grating pitch of the weakly reflective fiber grating array.

[0010] The pulse period of the first acousto-optic modulator needs to be greater than (N-1)L, where N is the number of gratings in the weakly reflective fiber grating array and L is the grating pitch of the weakly reflective fiber grating array.

[0011] Furthermore, the demodulation module includes: a first circulator, a second circulator, a second acousto-optic modulator, a second arbitrary signal generator, an optical fiber delay line, three photodetectors, a Michelson interferometer, and a zero-difference symmetric demodulator;

[0012] The pulse signal generation module is connected to pin 1 of the first circulator; pin 2 of the first circulator is connected to the signal input of the second acousto-optic modulator; the second arbitrary signal generator is connected to the modulation signal input of the second acousto-optic modulator; the output of the second acousto-optic modulator is connected to one end of the fiber delay line; the other end of the fiber delay line is connected to the weakly reflective fiber optic grating array; pin 3 of the first circulator is connected to pin 1 of the second circulator; pin 2 of the second circulator is connected to the input of the Michelson interferometer; pin 3 of the second circulator is connected to a photodetector; the two outputs of the Michelson interferometer are connected to two photodetectors respectively; and the three photodetectors are connected to the zero-difference symmetric demodulator.

[0013] Furthermore, the arm difference of the Michelson interferometer is the same as the grating pitch of the weakly reflective fiber grating array.

[0014] Furthermore, the second acousto-optic modulator is de-lighted during the period from [L1+2L0+(N-1)L]n / c to [L1+2L0+(N-1)L]n / c+τ within one cycle, and is on during the other times of one cycle, where n is the effective refractive index of the optical fiber, c is the speed of light in vacuum, L is the grating pitch of the weakly reflective fiber grating array, N is the number of gratings in the weakly reflective fiber grating array, L0 is the length of the optical fiber delay line, and L1 is the connection length between the coupler and the second acousto-optic modulator.

[0015] This invention also provides a demodulation method for a weak reflection fiber grating array weak vibration intensity and position demodulation system, comprising:

[0016] When the detected interference range shifts, the interference phase signals of the light intensity signals received by the photodetectors in the two demodulation modules are obtained by the zero-difference symmetric demodulation algorithm. The average value of the two interference phase signals is calculated to obtain the vibration intensity information. Then, any one photodetector is selected from the two demodulation modules, and the cross-correlation calculation of the interference light intensity obtained by the two photodetectors is performed to obtain the position information of the vibration point.

[0017] The present invention has the following beneficial effects:

[0018] This invention uses an acousto-optic modulator in the demodulation module to control the on / off state of optical pulses, allowing useful weak-reflection fiber Bragg grating array reflected optical pulses to pass through, while optical pulses transmitted through the weak-reflection fiber Bragg grating array by useless light sources cannot pass through, thus protecting the photodetector.

[0019] This invention uses two Michelson interferometers matched with a weakly reflective fiber grating array to form an interference effect. The average value of the interference phase signals of the two interferometers is used as the vibration intensity signal. The measurement accuracy can be doubled compared to a single Michelson interferometric demodulation system, and it can measure weak signals with lower intensity.

[0020] This invention obtains the vibration signal location information by selecting any one of the interference light intensity signals from the two demodulation modules and performing cross-correlation calculations while acquiring the vibration intensity signal. This method is simple to calculate and can obtain location information in real time. Attached Figure Description

[0021] The features and advantages of the invention will be more clearly understood by referring to the accompanying drawings, which are schematic and should not be construed as limiting the invention in any way. In the drawings:

[0022] Figure 1 This is a system diagram of a specific embodiment of the present invention;

[0023] Figure 2 This is a time-domain diagram of the light intensity received by detectors 1-3 in a specific embodiment of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention; all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0025] This invention discloses a weak reflection fiber grating array weak vibration intensity and position demodulation system, such as... Figure 1 As shown: The pulse signal generation module includes a 1550nm wavelength continuous laser 1 connected to an acousto-optic modulator 2 (driven by a pulse signal from an arbitrary signal generator 3), which is then connected in sequence to an erbium-doped fiber amplifier 4 and a 1×2 coupler 5 with a splitting ratio of 50:50. Two identical demodulation modules are also present. The demodulation module on the left includes: a circulator 6, a circulator 7, an acousto-optic modulator 8, an arbitrary signal generator 9, a fiber delay line 10, three photodetectors 11-13, a Michelson interferometer 15, and a zero-difference symmetric demodulator 14. The demodulation module on the right includes: a circulator 16, a circulator 17, an acousto-optic modulator 18, an arbitrary signal generator 19, a fiber delay line 20, three photodetectors 21-23, a Michelson interferometer 25, and a zero-difference symmetric demodulator 24.

[0026] The A output port of the 1×2 coupler 5 is connected to port 1 of the circulator 6. Port 2 of the circulator 6 is connected to the acousto-optic modulator 8 (driven by the signal from the arbitrary signal generator 9). The acousto-optic modulator 8 is connected to an optical fiber delay line 10 with a length of L0, and then to one end of a weakly reflective fiber optic grating array 26 with a grating pitch of L. Port 3 of the circulator 6 is connected to port 1 of the circulator 7. Port 2 of the circulator 7 is connected to a Michelson interferometer 25 with an arm difference of L. Port 3 of the circulator 7 and each of the two output ports of the Michelson interferometer 25 are connected to a photodetector, referred to as photodetectors 11-13. Photodetectors 11-13 are connected to a 3×3 homodyne symmetric demodulator 14. The subsequent optical path of the B output port of the 1×2 coupler is the same as that of the A port. In the system, the pulse width of acousto-optic modulator 1 needs to be less than 2nL / c, where n is the effective refractive index of the optical fiber and c is the speed of light in vacuum. This ensures that the light pulses reflected by adjacent weakly reflective fiber Bragg gratings will not overlap. The pulse period of acousto-optic modulator 1 needs to be greater than (N-1)L, ensuring that the light pulses reflected by adjacent periodic pulses in the weakly reflective fiber Bragg grating array will not overlap. Acousto-optic modulators 8 and 18 are switched off during the time interval [L1+2L0+(N-1)L]n / c to [L1+2L0+(N-1)L]n / c+τ of one cycle, and are switched on during the rest of the cycle. This ensures that light pulses reflected from the front end of the weakly reflective fiber Bragg grating array can pass through, while light pulses transmitted into the array from the rear end cannot pass through, thus protecting photodetectors 11-13 and 21-23.

[0027] Assuming there is a vibration point between WFBG1 and WFBG2 in the weak reflection fiber grating array 26, and the distance of this vibration point from WFBG1 is R, then the distance from WFBG2 is LR. The time-domain plot of the light intensity received by photodetectors 11-13 is shown below. Figure 2As shown, the pulse width T1T0 of the acousto-optic modulator 2 is less than 2nL / c. The light pulse reflected from WFBG1 passes through the long and short arms of the Michelson interferometer 15 with an arm difference of L, resulting in two light pulses in the time domain diagram. These two pulses are separated by a distance T2T0 = 2nL / c. Similarly, two identical light pulses appear in the time domain diagram of WFBG2. Since the grating pitch of the weakly reflective fiber grating array 26 is L, the duration of the preceding light pulse in the time domain diagram of WFBG2 from the preceding light pulse in the time domain diagram of WFBG1 is T2T0 = 2nL / c. Therefore, the following light pulse of WFBG1 overlaps with the preceding light pulse of WFBG2, resulting in interference. Therefore, the interference phase signal of the vibration can be calculated from the light intensity signal received by photodetectors 11-13 using the zero-difference symmetric demodulation algorithm. The specific calculation method can be obtained from [Ding Peng, Wu Jing, Kang De, et al. Acoustic wave direction detection using weak reflection fiber Bragg grating [J]. Chinese Journal of Lasers, 2020, 47(5): 0506002]. The time for photodetectors 11-13 to receive the interference light intensity of the vibration point is t1 = [L1 + 2(R + L0)]n / c. Similarly, the interference phase signal of the vibration can be calculated from the light intensity signal received by photodetectors 21-23 using the zero-difference symmetric demodulation algorithm. The time for photodetectors 21-23 to receive the interference light intensity of the vibration point is t2 = [L1 + 2(L - R + L0) + 2(N - 2)L]n / c. Finally, the vibration intensity information is obtained by calculating the average value of the two interference phase signals. The position information of the vibration point can be obtained by cross-correlation between any interference light intensity of photodetectors 11-13 and any interference light intensity of photodetectors 21-23. The specific calculation method can be found in [Liu Bo, Yang Yifei, Zhang Jian, et al. Experimental study on fiber optic fence system based on MZ interference [J]. Acta Photonica Sinica, 2007, 36(6):1013-1017].

[0028] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A weak reflection fiber grating array weak vibration intensity and position demodulation system, characterized in that, include: Pulse signal generation module, two identical demodulation modules, and a weakly reflective fiber Bragg grating array; The pulse signal generation module includes: a first arbitrary signal generator, a continuous laser, a first acousto-optic modulator, an optical fiber amplifier, and a 1×2 coupler; The output of the continuous laser is connected to the signal input of the first acousto-optic modulator; the first arbitrary signal generator is connected to the modulation signal input of the first acousto-optic modulator; the output of the first acousto-optic modulator is connected to the input of the fiber optic amplifier; the output of the fiber optic amplifier is connected to the input of a 1×2 coupler; and the two outputs of the 1×2 coupler are respectively connected to two modulation modules. The two ends of the weak reflection fiber grating array are connected to two demodulation modules respectively; The demodulation module includes: a first circulator, a second circulator, a second acousto-optic modulator, a second arbitrary signal generator, an optical fiber delay line, three photodetectors, a Michelson interferometer, and a zero-difference symmetric demodulator; One output of the 1×2 coupler is connected to pin 1 of the first circulator; pin 2 of the first circulator is connected to the signal input of the second acousto-optic modulator; the second arbitrary signal generator is connected to the modulation signal input of the second acousto-optic modulator; the output of the second acousto-optic modulator is connected to one end of an optical fiber delay line; the other end of the optical fiber delay line is connected to a weakly reflective fiber optic grating array; pin 3 of the first circulator is connected to pin 1 of the second circulator; pin 2 of the second circulator is connected to the input of a Michelson interferometer; pin 3 of the second circulator is connected to a photodetector; the two outputs of the Michelson interferometer are connected to two photodetectors respectively; the three photodetectors are connected to a zero-difference symmetric demodulator. The pulse width of the first acousto-optic modulator needs to be less than 2. nL / c ,in, n The effective refractive index of the optical fiber, c The speed of light in a vacuum. L The grating pitch of a weakly reflective fiber Bragg grating array; The pulse period of the first acoustic-optic modulator needs to be greater than ( N -1) L ,in, N This represents the number of gratings in a weakly reflective fiber Bragg grating array. L The grating pitch is the grating pitch of a weakly reflective fiber Bragg grating array.

2. The weak reflection fiber optic grating array weak vibration intensity and position demodulation system as described in claim 1, characterized in that, The arm difference of the Michelson interferometer is the same as the grating pitch of the weakly reflective fiber grating array.

3. The weak reflection fiber optic grating array weak vibration intensity and position demodulation system as described in claim 2, characterized in that, The second acousto-optic modulator in one cycle [ L 1+2 L 0+( N -1) L ] n / c to[ L 1+2 L 0+( N -1) L ] n / c + τ The light is cut off during a specific time period, and on during the rest of the cycle. n The effective refractive index of the optical fiber, c The speed of light in a vacuum. L The grating pitch of a weakly reflective fiber Bragg grating array, N This represents the number of gratings in a weakly reflective fiber Bragg grating array. L 0 represents the length of the fiber optic delay line. L 1 represents the length of the connection between the coupler and the second acousto-optic modulator.

4. A demodulation method for a weak reflection fiber grating array weak vibration intensity and position demodulation system, applicable to the weak reflection fiber grating array weak vibration intensity and position demodulation system as described in any one of claims 1-3, characterized in that, The demodulation method of the weak reflection fiber grating array weak vibration intensity and position demodulation system includes: When the detected interference range shifts, the interference phase signals of the light intensity signals received by the photodetectors in the two demodulation modules are obtained by the zero-difference symmetric demodulation algorithm. The average value of the two interference phase signals is calculated to obtain the vibration intensity information. Then, any one photodetector is selected from the two demodulation modules, and the cross-correlation calculation of the interference light intensity obtained by the two photodetectors is performed to obtain the position information of the vibration point.

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