CN109632075B - Vibration monitoring system and method based on double fiber bragg grating arrays - Google Patents

Vibration monitoring system and method based on double fiber bragg grating arrays Download PDF

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CN109632075B
CN109632075B CN201910080918.4A CN201910080918A CN109632075B CN 109632075 B CN109632075 B CN 109632075B CN 201910080918 A CN201910080918 A CN 201910080918A CN 109632075 B CN109632075 B CN 109632075B
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coupler
grating array
light
fiber
optical
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CN109632075A (en
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李政颖
王加琪
王洪海
刘佳佩
王立新
姜德生
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Wuhan University of Technology WUT
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Wuhan University of Technology WUT
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01HMEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
    • G01H9/00Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
    • G01H9/004Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors

Abstract

The invention relates to a vibration monitoring system based on a double fiber grating array, which is characterized in that a narrow-linewidth laser, a pulse light modulator, a pulse light amplifier and an optical filter are sequentially connected, the output end of the optical filter is connected with a first communication end of a circulator, a second communication end of the circulator is connected with a first communication end on the front side of a coupler, the first communication end on the rear side of the coupler is knotted and stopped, a third communication end of the circulator is connected with a first signal input end of an embedded signal processor, a second communication end on the front side of the coupler is connected with a second signal input end of the embedded signal processor, a third communication end on the front side of the coupler is connected with a third signal input end of the embedded signal processor, a second communication end on the rear side of the coupler is connected with a first optical fiber of the double fiber grating array, and a third communication end on the rear side of the coupler is connected with a second optical fiber of the double fiber grating array. The invention can more truly restore the vibration signal on the premise of not increasing the hardware cost.

Description

Vibration monitoring system and method based on double fiber bragg grating arrays
Technical Field
The invention relates to the technical field of fiber bragg grating sensing, in particular to a vibration monitoring system and method based on a double fiber bragg grating array.
Background
With the development of vibration sensing monitoring technology, high-precision and high-sensitivity test requirements are continuously provided for monitoring vibration signals in engineering, long-term monitoring of the vibration signals becomes one of the targets of the engineering world, and research and development of a high-performance vibration monitoring system is imperative. Compared with the traditional mechanical, piezoelectric, magnetoelectric sensors and the like, the fiber grating sensor has a series of advantages of small volume, light weight, good flexibility, strong anti-electromagnetic interference capability, high precision, wide dynamic range and the like, can realize the real-time monitoring function of high resolution, high signal-to-noise ratio, high reliability and long-term stability, and has wide application prospect in the fields of petroleum, civil engineering, ocean, aviation, aerospace, machinery and the like.
The traditional distributed optical fiber vibration sensing method is based on Rayleigh scattering and optical fiber grating reflection, wherein distributed vibration signals reflected by the optical fiber grating are widely applied due to the advantages of high signal-to-noise ratio, high sensitivity, strong temperature tolerance and the like. The distributed vibration monitoring method based on the fiber bragg grating converts the change of the length of all optical fibers between two adjacent gratings caused by vibration into the phase change of an interference optical signal, so that the phase of the interference signal is demodulated to obtain a vibration signal.
Theoretically, the longer the optical fiber between the gratings, the stronger the sensing ability to the outside, and the higher the sensitivity of the system. However, in practical applications, since the change in length of each small segment on the optical fiber will cause the phase change of the interference signal, the resulting phase change is the summation of the length changes of the whole segment of the optical fiber between gratings, and when the vibration wave signal is transmitted to each point on the optical fiber, the response at the point will be delayed with distance, so that the possibility of vibration cancellation of the detected signal will occur. As the length of the optical fiber increases, the influence of external noise on the length of the optical fiber is far greater than that of the vibration source, thereby causing the signal-to-noise ratio of the system to be poor. The phase noise introduced by the frequency instability of the light source also increases with the length of the fiber. To avoid this, the spacing between the gratings may be reduced, however this approach is again limited by hardware circuitry such as the pulse width of the pulse signal, the bandwidth of the hardware circuitry, the sampling rate of the signal acquisition, the rate of data transmission, etc.
Disclosure of Invention
The invention aims to provide a vibration monitoring system and method based on a double fiber bragg grating array, which can more truly restore vibration signals and improve the signal-to-noise ratio of a system on the premise of not increasing the hardware cost, and meanwhile, an embedded system is utilized to preprocess data in the signal acquisition process so as to improve the dynamic range of the system.
In order to solve the technical problem, the vibration monitoring system based on the double fiber bragg grating array is characterized in that: the device comprises a narrow-linewidth laser, a pulse light modulator, a pulse light amplifier, an optical filter, a circulator, a coupler, a double-fiber grating array and an embedded signal processor, wherein a continuous light signal output end of the narrow-linewidth laser is connected with an input end of the pulse light modulator, a pulse light signal output end of the pulse light modulator is connected with an input end of the pulse light amplifier, an output end of the pulse light amplifier is connected with an input end of the optical filter, an output end of the optical filter is connected with a first communication end of the circulator, a second communication end of the circulator is connected with a first communication end on the front side of the coupler, the first communication end on the rear side of the coupler is tied for stopping use, a third communication end of the circulator is connected with a first signal input end of the embedded signal processor, a second communication end on the front side of the coupler is connected with a second signal input end of the embedded signal processor, and a third communication end on the front side of the coupler is connected with, the second communication end at the back side of the coupler is connected with the second fiber bragg grating array of the double fiber bragg grating array, and the third communication end at the back side of the coupler is connected with the second fiber bragg grating array of the double fiber bragg grating array.
In the above technical solution, the coupler is configured to divide an input filtered pulse signal into three optical signals with equal power, where the two optical signals respectively enter a first fiber grating array and a second fiber grating array, the two optical signals respectively encounter each fiber bragg grating in the first fiber grating array and the second fiber grating array to be reflected, the two reflected optical signals are interfered in the coupler to form a reflected light interference signal, the coupler divides the reflected light interference signal into three reflected light interference signals, the first reflected light interference signal is transmitted through the circulator 5 and subjected to photoelectric conversion (photoelectric conversion is realized through a photoelectric converter, the photoelectric converter is composed of a photodiode and a low-noise precision operational amplifier, and has a bandwidth of 80MHz) and then transmitted to a first signal input end of the embedded signal processor, and the second reflected light interference signal is transmitted through the photoelectric conversion and then transmitted to a second signal input end of the embedded signal processor, the third reflected light interference signal is photoelectrically converted and transmitted to the third signal input terminal of the embedded signal processor 7.
The double fiber bragg grating array is composed of two chirp fiber bragg grating sensing networks which are tightly attached, m equally spaced fiber bragg gratings are respectively engraved on the two optical fibers, the distance between every two adjacent fiber bragg gratings on each optical fiber is L, the fiber bragg gratings on the two optical fibers are respectively numbered according to the same numbering sequence, the distance between the two fiber bragg gratings with the same number on the two optical fibers is L, the part between the two fiber bragg gratings with the same number on the two optical fibers is regarded as a vibration detection point, and in addition, the vibration detection point is formed by considering the part between the two fiber bragg gratings with the same number on the two optical fibers as a vibration detection point, and the vibration detection point is formed by arrangingThe double fiber grating array has m detection points.
A double fiber grating array vibration monitoring method of the system is characterized by comprising the following steps:
step 1: the narrow linewidth laser emits continuous light, and a pulse light signal with the pulse width T and the period T is formed after modulation of the pulse light modulator, wherein the pulse width T and the period T meet the following formula:
in the formula, m is the number of detection points in a double fiber bragg grating array, n is the refractive index of a fiber core, c is the light speed in vacuum, L is the distance between two fiber bragg gratings with the same number on two optical fibers, and L is the distance between two adjacent fiber bragg gratings;
step 2: the pulse light signal modulated by the pulse light modulator is subjected to power amplification by a pulse light amplifier, and light noise in the amplified pulse light signal is filtered after the pulse light signal is input into a light filter;
and step 3: the filtered pulse light signals enter a coupler through a circulator, the coupler divides the filtered pulse light signals into three paths of light signals with equal power, the tail end of an output end corresponding to one path of light signals is knotted and stops using, and the other two paths of light signals respectively enter a first fiber bragg grating array and a second fiber bragg grating array in a double fiber bragg grating array;
and 4, step 4: two paths of optical signals enter a double fiber bragg grating array and are reflected when encountering each fiber bragg grating, the two paths of reflected optical signals form reflected light interference signals after being interfered in a coupler, the coupler divides the reflected light interference signals into three paths of reflected light interference signals with phase differences, the first path of reflected light interference signals are transmitted through a circulator and are transmitted to a first signal input end of an embedded signal processor after being subjected to photoelectric conversion, the second path of reflected light interference signals are transmitted to a second signal input end of the embedded signal processor after being subjected to photoelectric conversion, and the third path of reflected light interference signals are transmitted to a third signal input end of the embedded signal processor after being subjected to photoelectric conversion;
and 5: the embedded signal processor is used for judging the arrival time of the two reflected light signals through the collected three reflected light interference signals, obtaining the spatial position of the vibration event in the double fiber bragg grating array by utilizing an optical time domain reflection technology, and obtaining the phase change of each detection point through a phase demodulation algorithm so as to realize the restoration of the vibration signal.
Compared with the traditional distributed optical fiber vibration monitoring technology, the invention provides the method for converting the traditional line type sensing into the point type sensing by using the double optical fiber grating array on the premise of not changing a hardware circuit, effectively avoids the interference of external environment disturbance on the transmission optical fiber and the phase noise caused by the instability of the light source frequency, can more truly reduce the signal, improves the spatial positioning resolution, improves the signal-to-noise ratio of the system and reduces the signal distortion caused by phase jump.
Drawings
FIG. 1 is a block diagram showing the overall structure of the method of the present invention;
FIG. 2 is a partial detail view of a dual fiber grating array according to the present invention;
FIG. 3 shows the principle of optical signal interference in the method of the present invention.
The optical fiber grating array comprises a 1-narrow linewidth laser, a 2-pulse optical modulator, a 3-pulse optical amplifier, a 4-optical filter, a 5-circulator, a 6-coupler, a 7-embedded signal processor, an 8-double optical fiber grating array and a 9-photoelectric converter.
Detailed Description
The present invention is further illustrated in detail by the following specific examples:
the invention relates to a vibration monitoring system based on a double fiber bragg grating array, which comprises a narrow-linewidth laser 1, a pulse light modulator 2, a pulse light amplifier 3, an optical filter 4, a circulator 5, a coupler 6, an embedded signal processor 7 and a double fiber bragg grating array 8, wherein a continuous light signal output end of the narrow-linewidth laser 1 is connected with an input end of the pulse light modulator 2, a pulse light signal output end of the pulse light modulator 2 is connected with an input end of the pulse light amplifier 3, an output end of the pulse light amplifier 3 is connected with an input end of the optical filter 4, an output end of the optical filter 4 is connected with a first communication end of the circulator 5, a second communication end of the circulator 5 is connected with a first communication end on the front side of the coupler 6, a first communication end on the rear side of the coupler 6 is not used when a knot is made, a third communication end of the circulator 5 is connected with a first signal input end of the embedded signal processor 7, the front second communication end of the coupler 6 is connected with the second signal input end of the embedded signal processor 7, the front third communication end of the coupler 6 is connected with the third signal input end of the embedded signal processor 7, the rear second communication end of the coupler 6 is connected with the second fiber bragg grating array of the dual fiber bragg grating array 8, and the rear third communication end of the coupler 6 is connected with the second fiber bragg grating array of the dual fiber bragg grating array 8.
In the above technical solution, the coupler 6 is configured to divide an input filtered pulse signal into three optical signals with equal power, where two optical signals respectively enter the first fiber grating array and the second fiber grating array, the two optical signals respectively encounter each fiber bragg grating in the first fiber grating array and the second fiber grating array to be reflected, the two reflected optical signals interfere in the coupler 6 to form a reflected light interference signal, the coupler 6 divides the reflected light interference signal into three reflected light interference signals, the first reflected light interference signal is transmitted through the circulator 5 (the reflected light interference signal passes through the second communication end of the circulator 5 and is output from the third communication end of the circulator 5) and is subjected to photoelectric conversion (photoelectric conversion is realized through the photoelectric converter 9, the photoelectric converter 9 is composed of a photodiode, a low-noise precision operational amplifier, and the like, and has a bandwidth of 80MHz) and then is transmitted to the first signal input end of the embedded signal processor 7, the second path of reflected light interference signal is subjected to photoelectric conversion and then transmitted to the second signal input end of the embedded signal processor 7, and the third path of reflected light interference signal is subjected to photoelectric conversion and then transmitted to the third signal input end of the embedded signal processor 7.
In the technical scheme, the embedded signal processor 7 is composed of an FPGA, and controls 2 analog-to-digital conversion chips of 250Msps and 14bit to acquire signals, preprocesses the signals to obtain phase information of vibration signals, and controls a USB3.0 chip transmission interface to interact with an upper computer.
In the above technical solution, the embedded signal processor 7 is configured to determine arrival time of the three-way signal for the collected three-way reflected light interference signal, obtain a spatial position of a vibration event in the dual fiber grating array 8 by using an optical time domain reflection technique, and obtain a phase change of each detection point by using a phase demodulation algorithmAnd further, the restoration of the vibration signal is realized.
The positioning method comprises the following steps: relationship between time of acquisition of reflected signal and position of fiber grating sensor:
Dx=ctx/2n
wherein DxIs the distance from the demodulation system to the x-th grating, c is the transmission speed of light in vacuum, txIn order to acquire the time of the reflection signal of the xth fiber grating sensor, n is the refractive index of the fiber core. And the position of the fiber grating sensor is judged through tx, so that the positioning of a disturbance point is realized.
The phase demodulation method comprises the following steps:
wherein, I1、I2And I3Respectively representing three paths of reflected light interference signals with phase difference.
In the technical scheme, the pulse light modulator 2 consists of a Semiconductor Optical Amplifier (SOA) and a driving circuit, wherein the SOA is an Inphenix switch type product, the working current is 200mA, the switching speed is less than 5ns, the extinction ratio is more than 40dB, and the driving circuit is designed by the driving circuit and is designed based on the principle of a high-frequency triode switch circuit;
in the above technical solution, the coupler 6 is a 3 × 3 fused tapered coupler, and the coupling coefficients of the coupler 6 have equal | k |12|=|k23|=|κ31The phase factors of the three optical fibers in the coupler 6 are the same, and in this embodiment, the phase factor is 120 °, where the coupling coefficient between the first communication end on the front side and the second communication end on the back side of the coupler 6 is | k12The coupling coefficient between the front second communication end and the rear third communication end of the coupler 6 is | k23The coupling coefficient between the front third communication end and the rear first communication end of the coupler 6 is | k |31|。
In the above technical solution, the optical filter 4 is a narrow band optical filter, and has a 100GHz optical wavelength division multiplexing module with a center wavelength of 1550.12 nm. The pulse light Amplifier 3 is a pulse Erbium-doped Fiber Amplifier (P-EDFA), and the P-EDFA can reduce the nonlinear effect of the Optical Fiber to the minimum when outputting pulse light and has the advantages of high gain and low noise;
the narrow linewidth laser 1 is a common continuous narrow linewidth laser, the output power is 10dBm, the linewidth is 100kHz, and the central wavelength is 1550.12 nm;
in the technical scheme, the output light intensity ratio of the coupler 6 is 1:1:1, and each two paths have the same phase difference of 2 pi/3.
In the above technical solution, as shown in fig. 2, the dual fiber grating array 8 is composed of two chirped fiber grating sensing networks closely attached to each other, wherein m (m is 100) equally spaced fiber bragg gratings are engraved on two optical fibers, a distance between two adjacent fiber bragg gratings on each optical fiber is L (L is 5m), the fiber bragg gratings on the two optical fibers are respectively numbered according to the same numbering sequence, a distance between two fiber bragg gratings with the same numbering on the two optical fibers is L (denoted by numbers 1,2,3, … m and 1 ', 2', 3 ', … m' in order of space, wherein a distance between a number 1 and 1 ', a number 2 and a number … … m and a distance between a number m' are L), a portion between two fiber bragg gratings with the same numbering on the two optical fibers is regarded as a vibration detection point, and isThe other part is transmission optical fiber, and m detection points are arranged in the double fiber bragg grating array. The design changes the traditional line type sensing into point type sensing, effectively avoids the interference of external environment disturbance on transmission optical fibers and phase noise caused by light source frequency instability, and simultaneously can more truly reduce signals compared with the line type sensing. The spatial localization resolution of the disturbance point can be improved.
In the above technical solution, as shown in fig. 2, since the two optical paths forming the dual fiber grating array are closely attached, the corresponding positions on the two optical paths are affected by the vibration wave equally. If the detection point 1 (point I) is influenced by vibration, the vibration signal causes the phase change of the reflected signal of all the gratings behind the fiber grating 1 and simultaneously causes the phase change of the reflected signal of all the gratings behind the fiber grating 1 and the fiber grating 1, when the reflected signals of all the gratings are finally interfered, the change of the optical path difference between the fiber grating 1 and the fiber grating 1 will cause the change, and other detection points cannot be influenced by crosstalk because of the same phase change. If the transmission fiber (point:) is disturbed by the external environment at a position other than the detection point, the phases of the reflected signals of the fiber grating No. 1 and the fiber grating No. 1' are not affected, and the reflected signals of the gratings behind the reflected signals have the same phase change, so that the interference signal is not affected. Therefore, the change of the optical path difference between the gratings can be only caused by the vibration generated on the detection point, such as the vibration at the point I, and is not influenced by other interference, the signal recovery authenticity is strong, and the spatial positioning resolution is high
A double fiber grating array vibration monitoring method of the system comprises the following steps:
step 1: the narrow linewidth laser 1 emits continuous light, and forms a pulse light signal with a pulse width of T and a period of T after being modulated by the pulse light modulator 2, wherein the pulse width is 30ns, and the period is 1kHz, and the pulse width T and the period T satisfy the following formula:
in the formula, m is the number of detection points in a double fiber bragg grating array, n is the refractive index of a fiber core, c is the light speed in vacuum, L is the distance between two fiber bragg gratings with the same number on two optical fibers, and L is the distance between two adjacent fiber bragg gratings;
the size of the pulse width t influences the design of a hardware driving circuit of a system pulse light modulator and subsequent circuits for photoelectric conversion, data acquisition, data transmission and the like, the spatial resolution of the traditional distributed optical fiber vibration monitoring method depends on the size of t, and the hardware circuit is required to reach the bandwidth of 10ns (100MHz) to reach the spatial resolution of 1 m. The invention utilizes the double fiber grating array to adjust the relative distance of two light paths, the pulse width is determined by L, namely the distance between adjacent detection points;
step 2: the pulse light signal modulated by the pulse light modulator 2 is subjected to power amplification by the pulse light amplifier 3, and the optical noise in the amplified pulse light signal is filtered after the light is input into the optical filter 4, so that the signal-to-noise ratio is improved;
and step 3: the filtered pulse light signals enter a coupler 6 through a circulator 5, the coupler 6 divides the filtered pulse light signals into three paths of light signals with equal power, the tail end of an output end corresponding to one path of light signals is knotted and stops using, and the other two paths of light signals respectively enter a first fiber bragg grating array and a second fiber bragg grating array in a double fiber bragg grating array 8;
and 4, step 4: after entering the dual fiber bragg grating array 8, the two paths of optical signals are reflected by each fiber bragg grating, the two paths of reflected optical signals are interfered in the coupler 6 to form reflected light interference signals, the coupler 6 divides the reflected light interference signals into three paths of reflected light interference signals with phase differences, the first path of reflected light interference signals are transmitted through the circulator 5 and are transmitted to the first signal input end of the embedded signal processor 7 after photoelectric conversion, the second path of reflected light interference signals are transmitted to the second signal input end of the embedded signal processor 7 after photoelectric conversion, and the third path of reflected light interference signals are transmitted to the third signal input end of the embedded signal processor 7 after photoelectric conversion;
the embedded signal processor 7 is configured to determine arrival times of the two reflected light signals according to the collected three reflected light interference signals, obtain a spatial position of a vibration event in the dual fiber grating array 8 by using an optical time domain reflection technique, and obtain a phase change of each detection point by using a phase demodulation algorithm, thereby implementing restoration of the vibration signal.
In the step 4, the coupler 6 divides the reflected light interference signal into three paths of reflected light interference signals with phase differences, which are expressed as:
wherein D is a direct current component generated after the interference of the two beams of light,in order to interfere with the phase change of the optical signal,is a phase change caused by a change in the length of the optical fiber excited by external vibration,representing the phase noise caused by the wavelength change a lambda of the light in the dual fiber grating array 8,showing the initial phase difference when the two beams interfere,
because the two light paths forming the double fiber grating array are tightly attached, the corresponding positions on the two light paths are influenced by the environment the same, and because l is shorter,the signal can be only caused by the vibration generated on the detection point, is slightly influenced by other interference, the signal recovery reality is strong, and the change can be expressed as:
wherein, Δ l is the fiber length increment caused by vibration on the detection point, n is the fiber core refractive index of the dual fiber grating array 8, λ is the wavelength of light in the dual fiber grating array 8, it can be seen that Δ l has um-level increment change (10u) to cause 12rad phase change, the bottom noise of the system can be 0.05rad, and the sensitivity is high;
the frequency instability of the light source can introduce noise to the system, and the phase noise caused by the wavelength change delta lambdaCan be expressed as:
wherein, Δ λ is the wavelength variation of light in the dual fiber grating array 8 caused by the instability of the light source of the narrow linewidth laser 1, the same wavelength drift, the larger the optical path difference is, the larger the phase noise is, the phase variation caused by the wavelength variation of the light source of 0.1pm is 1.91rad at the wavelength of 1.55um, if l is 5m, the phase noise caused by the wavelength drift of the visible light source is very serious. Therefore, reducing/can reduce the noise introduced by the light source.
In step 4, as shown in fig. 3, the signal returned by the grating No. 1 and the signal returned by the grating No. 1' arrive at the PD at the same time within a period of time, and interference occurs, and a process of interference of the two reflected optical signals in the coupler 6 can be represented as:
wherein, t1Indicates the starting time of the occurrence of interference, t2The end time of the interference is shown, Δ t shows the time of the interference, n is the refractive index of the fiber core of the dual fiber grating array 8, c is the speed of light in vacuum, l is the distance between two fiber bragg gratings with the same number on two fibers, and t is the pulse width of a pulse light signal. Optical signals arrive at the PD at the same time within 2 ns-30 ns time, and interference is generated. Since the interference signals interfere in 29ns, the bandwidth requirements on the hardware circuit parts are low. The invention has low bandwidth requirement on hardware circuit part.
Compared with the traditional distributed optical fiber vibration monitoring technology, the vibration monitoring method based on the double optical fiber grating array has the advantages that the traditional linear sensing is converted into the point sensing on the premise of not changing a hardware circuit, the interference of external environment disturbance on the transmission optical fiber and the phase noise caused by the instability of the light source frequency are effectively avoided, the signal can be more truly recovered, the spatial positioning resolution is improved, the signal to noise ratio of the system is improved, and the signal distortion caused by phase jump is reduced.
Details not described in this specification are within the skill of the art that are well known to those skilled in the art.

Claims (9)

1. A vibration monitoring system based on double fiber bragg grating arrays is characterized in that: the device comprises a narrow-linewidth laser (1), a pulse light modulator (2), a pulse light amplifier (3), an optical filter (4), a circulator (5), a coupler (6), an embedded signal processor (7) and a double-fiber grating array (8), wherein a continuous light signal output end of the narrow-linewidth laser (1) is connected with an input end of the pulse light modulator (2), a pulse light signal output end of the pulse light modulator (2) is connected with an input end of the pulse light amplifier (3), an output end of the pulse light amplifier (3) is connected with an input end of the optical filter (4), an output end of the optical filter (4) is connected with a first communication end of the circulator (5), a second communication end of the circulator (5) is connected with a first communication end on the front side of the coupler (6), a first communication end on the rear side of the coupler (6) is tied off and is disabled, a third communication end of the circulator (5) is connected with a first signal input end of the embedded signal processor (7), the front second communication end of the coupler (6) is connected with the second signal input end of the embedded signal processor (7), the front third communication end of the coupler (6) is connected with the third signal input end of the embedded signal processor (7), the rear second communication end of the coupler (6) is connected with the first fiber bragg grating array of the dual-fiber bragg grating array (8), and the rear third communication end of the coupler (6) is connected with the second fiber bragg grating array of the dual-fiber bragg grating array (8);
the coupler (6) is used for dividing an input filtered pulse signal into three paths of optical signals with equal power, wherein the two paths of light respectively enter the first fiber bragg grating array and the second fiber bragg grating array, the two paths of light respectively encounter each fiber bragg grating in the first fiber bragg grating array and the second fiber bragg grating array to be reflected, the two paths of reflected optical signals are interfered in the coupler (6) to form reflected light interference signals, the coupler (6) divides the reflected light interference signals into three paths of reflected light interference signals, the first path of reflected light interference signals are transmitted through the circulator (5) and are transmitted to the first signal input end of the embedded signal processor (7) after being subjected to photoelectric conversion, the second path of reflected light interference signals are transmitted to the second signal input end of the embedded signal processor (7) after being subjected to photoelectric conversion, and the third path of reflected light interference signals are transmitted to the third signal input end of the embedded signal processor (7) after being subjected to photoelectric conversion.
2. A dual fiber grating array based vibration monitoring system according to claim 1 wherein: the embedded signal processor (7) is used for judging the arrival time of the two reflected light signals through the collected three reflected light interference signals, obtaining the space position of the vibration event in the double fiber grating array (8) by using an optical time domain reflection technology, and obtaining the phase change of each detection point through a phase demodulation algorithm so as to realize the restoration of the vibration signal.
3. A dual fiber grating array based vibration monitoring system according to claim 1 wherein: the coupler (6) is a 3 x 3 fused taper coupler, and the coupling coefficients of the coupler (6) are equal in size | k |12|=|k23|=|κ31The phase factors of the three optical fibers in the coupler (6) are the same, wherein the coupling coefficient between the first communication end on the front side and the second communication end on the back side of the coupler (6) is | kappa12The coupling coefficient between the front second communication end and the rear third communication end of the coupler (6) is | kappa |23The coupling coefficient between the front third communication end and the rear first communication end of the coupler (6) is | k31|。
4. A dual fiber grating array based vibration monitoring system according to claim 1 wherein: the pulse light amplifier (3) is a pulse type erbium-doped fiber amplifier, and the optical filter (4) is a narrow-band optical filter.
5. A dual fiber grating array based vibration monitoring system according to claim 1 wherein: the output light intensity ratio of the coupler (6) is 1:1:1, and each two paths have the same phase difference of 2 pi/3.
6. A dual fiber grating array based vibration monitoring system according to claim 1 wherein: the double fiber bragg grating array (8) consists of two chirp fiber bragg grating sensing networks which are tightly attached, m equally-spaced fiber bragg gratings are respectively engraved on two optical fibers, the distance between every two adjacent fiber bragg gratings on each optical fiber is L, the fiber bragg gratings on the two optical fibers are respectively numbered according to the same numbering sequence, the distance between every two fiber bragg gratings with the same number on the two optical fibers is L, the part between every two fiber bragg gratings with the same number on the two optical fibers is regarded as a vibration detection point, and the vibration detection point is a part between every two fiber bragg gratings with the same number on the two optical fibers, and the vibration detection point is a vibration detection pointThe double fiber grating array has m detection points.
7. A method for dual fiber grating array vibration monitoring of the system of claim 6, comprising the steps of:
step 1: the narrow linewidth laser (1) emits continuous light, and a pulse light signal with the pulse width T and the period T is formed after the continuous light is modulated by the pulse light modulator (2);
step 2: the pulse light signal modulated by the pulse light modulator (2) is subjected to power amplification by a pulse light amplifier (3), and light noise in the amplified pulse light signal is filtered after the pulse light signal is input into a light filter (4);
and step 3: the filtered pulse light signals enter a coupler (6) through a circulator (5), the coupler (6) divides the filtered pulse light signals into three paths of light signals with equal power, the tail end of an output end corresponding to one path of light signals is knotted and stops using, and the other two paths of light signals respectively enter a first fiber bragg grating array and a second fiber bragg grating array in a double fiber bragg grating array (8);
and 4, step 4: after entering a double fiber bragg grating array (8), two paths of optical signals are reflected when encountering each fiber bragg grating, the two paths of reflected optical signals are interfered in a coupler (6) to form reflected light interference signals, the coupler (6) divides the reflected light interference signals into three paths of reflected light interference signals with equal power, the first path of reflected light interference signals are transmitted through a circulator (5) and are transmitted to a first signal input end of an embedded signal processor (7) after photoelectric conversion, the second path of reflected light interference signals are transmitted to a second signal input end of the embedded signal processor (7) after photoelectric conversion, and the third path of reflected light interference signals are transmitted to a third signal input end of the embedded signal processor (7) after photoelectric conversion;
and 5: the embedded signal processor (7) is used for judging the arrival time of the two reflected light signals through the collected three reflected light interference signals, obtaining the space position of the vibration event in the double fiber grating array (8) by using an optical time domain reflection technology, and obtaining the phase change of each detection point through a phase demodulation algorithm so as to realize the restoration of the vibration signal.
8. The dual fiber grating array vibration monitoring method of claim 7, wherein: in the step 4, the coupler (6) divides the reflected light interference signal into three paths of reflected light interference signals, which are expressed as:
wherein D is a direct current component generated after the interference of the two beams of light,in order to interfere with the phase change of the optical signal,is a phase change caused by a change in the length of the optical fiber excited by external vibration,representing the phase noise caused by the wavelength variation Delta lambda of the light in the dual fiber grating array (8),representing the initial phase difference when two beams interfere;
wherein, delta l is the length increment of the optical fiber caused by the vibration on the detection point, n is the refractive index of the optical fiber core of the double optical fiber grating array (8), and lambda is the wavelength of the light in the double optical fiber grating array (8);
wherein, Delta lambda is the wavelength change of the light in the double fiber grating array (8) caused by the unstable light source of the narrow linewidth laser (1).
9. The dual fiber grating array vibration monitoring method of claim 8, wherein: in the step 4, the process of interference of the two reflected optical signals in the coupler (6) can be expressed as
Wherein, t1Indicates the starting time of the occurrence of interference, t2The method is characterized in that the end time of interference is shown, delta t is the time of interference, n is the refractive index of the fiber core of the double fiber grating array (8), c is the speed of light in vacuum, l is the distance between two fiber Bragg gratings with the same number on two fibers, and t is the pulse width of a pulse light signal.
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