A multi-channel grating array optical fiber vibration sensing system and method

Abstract

The application discloses a kind of multi-channel grating array optical fiber vibration sensing system, its pulse light source module output delay is ts pulse light signal p1 and p2;Pulse light signal p1 and p2 form pulse light sequence signal in phase modulation unit;Multi-channel optical fiber splitter divides pulse light sequence signal into multiple power equal sub-pulse light sequence signals;Each pulse signal in each sub-pulse light sequence signal encounters each grating in corresponding grating array sensing optical fiber and is reflected;Each grating array sensing optical fiber reflected pulse signal is output to corresponding multi-channel photoelectric conversion module by multi-channel optical fiber splitter and is converted to corresponding sensing electric signal by photoelectric signal conversion;Signal processing module carries out phase demodulation processing to sensing electric signal.The application improves the signal-to-noise ratio of system while not amplifying reflected signal, and only needs to collect single-channel signal for demodulation of each grating array optical fiber, which reduces system cost and complexity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fiber grating sensing, in particular to a multi-channel grating array fiber vibration sensing system and method. BACKGROUND

[0002] The grating array phase demodulation technology has the advantages of long detection distance, high sensitivity and simple signal processing, and can well adapt to the demand of deep sea and underground long distance distributed vibration monitoring, so it has been widely concerned by domestic and foreign research units and enterprises. The current grating array phase demodulation method mainly includes active phase compensation method, heterodyne method, coupler multi-term demodulation method, etc. The coupler multi-term demodulation method based on 3*3 coupler uses the phase difference of the output end of the coupler to construct the demodulation interference signal, has the advantages of large dynamic range and no need of active device, and is a simple and effective demodulation method. The existing demodulation technology generally places the Michelson interferometer based on 3*3 coupler at the reflection signal end of the grating array fiber sensing network, uses the arm length difference of the interferometer to realize the interference of the front and rear two grating reflection signals, and then uses the 120° phase difference between the two output ends of the 3*3 coupler to realize the phase demodulation of the three interference signals.

[0003] Since the reflectivity of the grating in the large-capacity grating array sensing network is less than -45dB, the reflection signal of the sensing network is weak, and the loss of the Michelson interferometer based on 3*3 coupler is about -10dB. Such a large loss seriously affects the signal-to-noise ratio of the reflection signal, thereby causing the demodulation accuracy of the system to deteriorate. In order to solve this problem, the existing solution often uses a fiber amplifier to improve the power of the reflection signal, and the reflection signal after amplification often introduces strong direct current pollution, which limits the input dynamic range of the photodetector.

[0004] On the other hand, in order to share expensive optical devices and consider the complexity of the system, the multi-channel grating array fiber vibration sensing system generally divides the detection light signal into multiple paths, and each path is equipped with a signal processing system based on the 3*3 coupler multi-phase detection method to detect one grating array fiber. With the increase of the number of sensing channels, the signal processing system based on the 3*3 coupler multi-phase detection method also increases, and the three output signals of the 3*3 coupler need to be collected and processed synchronously inside the signal processing system. The system cost and overall complexity are high, which is not conducive to the mass industrial application of the grating array phase demodulation technology, and it is urgent to invent a simple and practical compact multi-channel grating array fiber vibration sensing system. SUMMARY

[0005] The application aims to provide a multi-channel grating array optical fiber vibration sensing system and method.

[0006] To achieve the above-mentioned purpose, the multi-channel grating array optical fiber vibration sensing system comprises a pulse light source module, a phase modulation unit, a multi-channel optical fiber splitter, multi-channel grating array sensing optical fibers, a signal processing module and a multi-channel photoelectric conversion module.

[0007] The pulse light source module is configured to output pulse light signals p1 and p2 with a time delay ts in a cycle.

[0008] The pulse light signal p1 forms reflected light signals p11, p12, p13 and p14 in the phase modulation unit, and the pulse light signal p2 forms reflected light signals p21, p22, p23 and p24 in the phase modulation unit, which form a pulse light sequence signal.

[0009] The multi-channel optical fiber splitter is configured to divide the pulse light sequence signal into multiple sub-pulse light sequence signals with equal power, and each sub-pulse light sequence signal enters a corresponding grating array sensing optical fiber.

[0010] Each pulse signal in each sub-pulse light sequence signal is reflected when encountering each grating in the corresponding grating array sensing optical fiber, and the time delay of the first pulse signal reflected by adjacent gratings in the grating array sensing optical fiber is tg.

[0011] The pulse signal reflected by each grating array sensing optical fiber is output to the corresponding multi-channel photoelectric conversion module through the multi-channel optical fiber splitter to convert the optical signal into a corresponding sensing electric signal; wherein, the reflected light signals p22, p23 and p24 reflected by the front grating of the adjacent two gratings in each grating array sensing optical fiber interfere with the reflected light signals p11, p12 and p13 reflected by the rear grating of the adjacent two gratings to generate a corresponding sensing electric signal.

[0012] The signal processing module is used to perform phase demodulation processing on the sensing electrical signals.

[0013] Compared to traditional multi-channel grating array fiber optic vibration sensing systems that utilize multiple interferometers, this invention retains the advantages of grating array sensing technology, such as long detection distance, large multiplexing capacity, and high sensitivity, while significantly simplifying the optical path and structural design of traditional phase demodulation systems. It avoids the loss of reflected signals through Michelson interferometers based on 3×3 couplers, thereby improving the system's signal-to-noise ratio without amplifying the reflected signals. Furthermore, demodulation of each grating array fiber only requires the acquisition of a single signal, reducing system cost and complexity, and facilitating the large-scale engineering application of grating array fiber optic vibration sensing technology. Attached Figure Description

[0014] Figure 1 This is a block diagram of the overall structure of the vibration sensing system described in this invention;

[0015] Figure 2 This is a schematic diagram illustrating the principle of the phase demodulation method described in this invention;

[0016] Figure 3 The three original signals described in this invention example are Data1, Data2, and Data3, which represent I1, I2, and I3 of the first sensor signal acquired, respectively.

[0017] Figure 4 The ellipse fitting result described in the example of this invention;

[0018] Figure 5 This refers to the phase demodulation result described in the example of the present invention. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0020] like Figure 1 The multi-channel grating array fiber optic vibration sensing system shown is characterized by comprising a pulse source module (narrow linewidth pulse source module), a circulator, a pulse signal amplifier, a phase modulation unit, a multi-channel fiber optic splitter, a multi-channel grating array sensing fiber, a signal processing module, and a multi-channel photoelectric conversion module.

[0021] The output end of the narrow line width pulse light source module is connected with the first communication end of the circulator, the second communication end of the circulator is connected with the input / output end of the phase modulation unit, the third communication end of the circulator is connected with the input end of the pulse light signal amplifier, the output end of the pulse light signal amplifier is connected with the input end of the multi-path optical fiber splitter, the multi-path optical fiber splitter divides the output optical signal of the pulse light signal amplifier into multiple optical signals and outputs the multiple optical signals to the multi-path grating array sensing optical fiber, the reflected signals of the multi-path grating array sensing optical fiber are output to the multi-path photoelectric conversion module through the multi-path optical fiber splitter, and the photoelectric signal conversion is carried out through the multi-path photoelectric conversion module, the signal processing unit synchronizes the synchronization signals output by the narrow line width pulse light source module, collects and processes the electrical signals converted by the multi-path photoelectric conversion module to realize the demodulation of the multi-path grating array sensing optical fiber.

[0022] The narrow line width pulse light source module is used to output pulse light signals p1 and p2 with a delay of ts (80 ns) in a period, the pulse width of the pulse light signals p1 and p2 is 10 ns, and the modulation frequency is 10 kHz, as shown in the following figure: Figure 2 The 10 kHz synchronization TTL electric pulse signal is also output.

[0023] The pulse light signal p1 forms reflected light signals p11, p12, p13 and p14 in the phase modulation unit, the pulse light signal p2 forms reflected light signals p21, p22, p23 and p24 in the phase modulation unit, and the reflected light signals p11, p12, p13 and p14, the reflected light signals p21, p22, p23 and p24 form a pulse light sequence signal.

[0024] The multi-path optical fiber splitter is used to divide the pulse light sequence signal into multiple sub-pulse light sequence signals with equal power (each sub-pulse light sequence signal is a pulse light sequence signal of p11, p12, p13, p14, p21, p22, p23 and p24, the power is 1 / 4 of the original, and the time delay and phase are the same as those of the original multi-phase pulse light sequence signal), and each sub-pulse light sequence signal detects a corresponding grating array sensing optical fiber.

[0025] Each pulse signal in each sub-pulse light sequence signal encounters each grating in the corresponding grating array sensing optical fiber and is reflected, that is, each grating returns eight pulse light signals, and the time delay of the first pulse signal reflected by adjacent gratings in the grating array sensing optical fiber is tg(100 ns). Since the distance between adjacent gratings in the grating array sensing optical fiber is 10 m, the time delay of the first pulse signal reflected by adjacent gratings is 100 ns. By adjusting the length of each delay optical fiber in the phase modulation unit to control the time delay of each reflected light signal in the output pulse light sequence signal, the interference of the two groups of pulse light signals reflected by adjacent gratings is realized, thereby avoiding the problem that in the traditional vibration sensing system, the demodulation of each channel grating array optical fiber needs to pass through a Michelson interferometer based on a 3*3 coupler to interfere, and thereby avoiding the problem that the reflection signal becomes weak due to the loss of the interferometer;

[0026] The pulse signals reflected by each grating array sensing optical fiber are output to the corresponding multi-channel photoelectric conversion module through a multi-channel optical fiber splitter to convert the optical signals into corresponding sensing electrical signals. The reflected light signals p22, p23 and p24 reflected by the former one of the two adjacent gratings of each grating array sensing optical fiber interfere with the reflected light signals p11, p12 and p13 reflected by the latter one of the two adjacent gratings (that is, the reflected light signal p22 interferes with the reflected light signal p11, the reflected light signal p23 interferes with the reflected light signal p12, and the reflected light signal p24 interferes with the reflected light signal p13), to generate corresponding sensing electrical signals. The three pulse signals that interfere are regarded as sensing signals. Since the paths of the reflected light signals are different, the phase differences caused by the reflected light signals are also different. Therefore, the three pulse signals that interfere contain the phase change caused by the to-be-measured signal and the different phase differences caused by the reflected light signals. Therefore, the to-be-measured signal can be demodulated by performing mathematical operations on the three pulse signals. The reflected light signals p21 and p14 reflected by all the gratings reach the photoelectric conversion module separately and do not interfere, and are regarded as invalid signals, which are discarded by the subsequent signal processing module without processing.

[0027] The signal processing module is used for phase demodulation processing of the sensing electrical signals, to realize the demodulation of the grating array optical fiber, so as to obtain the vibration information corresponding to the grating array sensing optical fiber.

[0028] Phase demodulation principle (demodulation method):

[0029] Since the phase difference between two of the three output signals of the 3x3 coupler is 120°, different phase delays will be caused by different devices and delay optical fibers. Therefore, assuming that the initial phase values of the two pulse light signals output by the pulse modulation unit are phi1 and phi2, and the phase delays of the light passing through the paths containing the first, second, third, and fourth Faraday rotator mirrors are g1, g2, g3, and g4 respectively, the phase difference between the p22 reflected by the former grating and the p11 reflected by the latter grating after interference is phi1+phi2-phi1+g3-g1+120°, the phase difference between the p23 reflected by the former grating and the p12 reflected by the latter grating after interference is phi1+phi2-phi1+g4-g3, and the phase difference between the p24 reflected by the former grating and the p13 reflected by the latter grating after interference is phi1+phi2-phi1+g2-g4-120°. Therefore, the above three pulse signal intensities can be represented as:

[0030]

[0031] wherein A1, A2, and A3 are the direct current amounts of the three interference signals respectively, B1, B2, and B3 are the alternating current amounts of the three interference signals respectively, phi1=2n eff piV / lambda, phi1 represents the phase change amount of the transmission light caused by the length change of the fiber length between two adjacent gratings of the grating array sensing fiber due to the contraction caused by the vibration of the piezoelectric ceramic ring, lambda represents the wavelength of the transmission light, and theta1=phi2-phi1+g3-g1+120°, theta2=phi2-phi1+g4-g3, and theta3=phi2-phi1+g2-g4-120° are the fixed phase errors introduced by different paths in the phase modulation unit.

[0032] According to the Lissajous figure principle, the ellipse curve can be drawn by simultaneously associating I1, I2, and I3, and the coefficient values of the ellipse equation can be obtained by using the ellipse fitting algorithm, so that A1, A2, A3, B1, B2, B3, theta1, theta2, and theta3 can be obtained. Further, the value of phi1 can be obtained by simultaneously associating I1, I2, and I3, and further, the demodulation of the external vibration signal can be realized. In order to ensure that the double-beam interference occurs between the adjacent gratings and that there are at least three groups of signals I1, I2, and I3 to realize the demodulation of the vibration signal, the signals reflected by the adjacent gratings need to interfere, and the signals reflected by the non-adjacent gratings cannot interfere. Therefore, the number of signals reflected by a single grating is at least six, and each of the signals interferes with the signals reflected by the front and rear gratings. At the same time, in order to ensure the difference of theta1, theta2, and theta3 in the three groups of signals I1, I2, and I3, the three groups of signals need to have different constant phase differences. In order to achieve the above purpose, the present application adopts the scheme of double-pulse modulation and a phase modulation unit.

[0033] In the above technical solution, the phase modulation unit is composed of a 3*3 coupler, two 1*2 couplers, four Faraday rotators and six delay optical fibers (L11, L12, L13, L21, L22, L23). The first coupler (3*3 coupler) in the phase modulation unit is used to divide the pulse light signal p1 into first-level sub-pulse light signals p100 and p101 with a phase difference of 120°. The first-level sub-pulse light signal p100 is transmitted to the second coupler (1*2 coupler) in the phase modulation unit through the delay optical fiber L11, and the first-level sub-pulse light signal p101 is transmitted to the third coupler in the phase modulation unit through the delay optical fiber L21. The second coupler in the phase modulation unit is used to divide the first-level sub-pulse light signal p100 into second-level sub-pulse light signals p1001 and p1002 with a phase difference of 90°. The third coupler (1*2 coupler) in the phase modulation unit is used to divide the first-level sub-pulse light signal p101 into second-level sub-pulse light signals p1011 and p1012 with a phase difference of 90°.

[0034] The second-level sub-pulse light signal p1001 is input to the first Faraday rotator through the delay optical fiber L12 and reflected to form a reflected light signal p11. The second-level sub-pulse light signal p1002 is input to the second Faraday rotator through the delay optical fiber L13 and reflected to form a reflected light signal p12. The second-level sub-pulse light signal p1011 is input to the third Faraday rotator through the delay optical fiber L22 and reflected to form a reflected light signal p13. The second-level sub-pulse light signal p1012 is input to the fourth Faraday rotator through the delay optical fiber L23 and reflected to form a reflected light signal p14. All the signals reflected by the Faraday rotators have a phase difference of 180° (the reflected signals have a phase change of 180° when passing through the Faraday rotators).

[0035] The first coupler in the phase modulation unit is used to divide the pulse light signal p2 into first-level sub-pulse light signals p200 and p201 with a phase difference of 120°. The first-level sub-pulse light signal p200 is transmitted to the second coupler in the phase modulation unit through the delay optical fiber L11, and the first-level sub-pulse light signal p201 is transmitted to the third coupler in the phase modulation unit through the delay optical fiber L21. The second coupler in the phase modulation unit is used to divide the first-level sub-pulse light signal p200 into second-level sub-pulse light signals p2001 and p2002 with a phase difference of 90°. The third coupler in the phase modulation unit is used to divide the first-level sub-pulse light signal p201 into second-level sub-pulse light signals p2011 and p2012 with a phase difference of 90°.

[0036] The second-stage sub-pulse light signal p2001 is input to the first Faraday rotating mirror through the delay optical fiber L12 and reflected to form a reflected light signal p21, the second-stage sub-pulse light signal p2002 is input to the second Faraday rotating mirror through the delay optical fiber L13 and reflected to form a reflected light signal p22, the second-stage sub-pulse light signal p2011 is input to the third Faraday rotating mirror through the delay optical fiber L22 and reflected to form a reflected light signal p23, and the second-stage sub-pulse light signal p2012 is input to the fourth Faraday rotating mirror through the delay optical fiber L23 and reflected to form a reflected light signal p24. All the signals reflected by the Faraday rotating mirrors have a phase difference of 180°.

[0037] Since the phase difference between the three output signals of the 3x3 coupler is 120°, different phase delays will be caused by different devices and delay optical fibers. After the implementation of the above scheme, assuming that the initial phase values of the two pulse light signals p1 and p2 output by the pulse modulation unit are phi1 and phi2, the phase delays of the light passing through the paths containing the first, second, third, and fourth Faraday rotating mirrors are g1, g2, g3, and g4, respectively, and the phase delays are different due to the different lengths of the delay optical fibers, the phase difference between p22 reflected by the previous grating and p11 reflected by the subsequent grating is phis+phi2-phi1+g3-g1+120°, the phase difference between p23 reflected by the previous grating and p12 reflected by the subsequent grating is phis+phi2-phi1+g4-g3, and the phase difference between p24 reflected by the previous grating and p13 reflected by the subsequent grating is phis+phi2-phi1+g2-g4-120°, i.e. θ1=phi2-phi1+g3-g1+120°, θ2=phi2-phi1+g4-g3, θ3=phi2-phi1+g2-g4-120°, each of which is different.

[0038] In the above technical solution, in order to ensure that the signals reflected by adjacent gratings interfere with each other and the signals reflected by non-adjacent gratings do not interfere with each other, the time delay between the reflected light signal p11 and the reflected light signal p12 is t1 (20 ns), the time delay between the reflected light signal p21 and the reflected light signal p22 is t1, the time delay between the reflected light signal p11 and the reflected light signal p13 is t2 (40 ns), the time delay between the reflected light signal p21 and the reflected light signal p23 is t2, the time delay between the reflected light signal p11 and the reflected light signal p14 is t3 (60 ns), and the time delay between the reflected light signals p21 and p24 is t3.

[0039] In the above technical solution, the delay between the reflected light signal p11 and the reflected light signal p21 is equal to the delay ts (80ns) between the pulse light signal p1 and the pulse light signal p2, and the delay between the reflected light signal p11 and the reflected light signal p22 is equal to the delay corresponding to the grating spacing (100ns).

[0040] In the above technical solution, the pulse light source module is used to output a synchronous trigger electrical signal to the signal processing module during each output cycle of pulse light signal p1 and pulse light signal p2, so that the signal processing module can start to collect the electrical signal converted by the photoelectric conversion module.

[0041] In the above technical solution, pulsed optical signal p1 and pulsed optical signal p2 are transmitted to the phase modulation unit through a circulator;

[0042] The phase modulation unit transmits the pulsed optical sequence signal through a circulator to a pulse signal amplifier, which amplifies it before sending it to a multi-channel fiber optic splitter. The signal amplification is used to compensate for the power loss caused by the subsequent splitting into four channels, while also increasing the optical signal energy and improving the system signal-to-noise ratio.

[0043] In the above technical solution, the delay ts, delay tg, delay t1, delay t2, and delay t3 respectively satisfy the following formulas:

[0044] ts = tg - t1

[0045] tg = 2n eff d / c

[0046] t1=2n eff (L21+L22-L11-L12) / c

[0047] t2=2n eff (L21+L23-L11-L12) / c

[0048] t3=2n eff (L13-L12) / c

[0049] Where, n eff θ1 is the core refractive index of the grating array fiber, c is the speed of light in vacuum, d is the spacing between adjacent gratings, and L11, L12, L21 and L22 are the lengths of delay fibers L11, L12, L21 and L22, respectively. The above formula is designed to cause optical signal interference and ensure the difference between θ1, θ2 and θ3.

[0050] The interference signal amplitude of the reflected light signal p22 reflected by the former grating and the reflected light signal p11 reflected by the latter grating is I1, the interference signal amplitude of the reflected light signal p23 reflected by the former grating and the reflected light signal p12 reflected by the latter grating is I2, and the interference signal amplitude of the reflected light signal p24 reflected by the former grating and the reflected light signal p13 reflected by the latter grating is I3.

[0051]

[0052] Wherein, A1, A2 and A3 are the direct current amounts of the three interference signals respectively, B1, B2 and B3 are the cross current amounts of the three interference signals respectively, phis=2n eff pi, phi2 represent the initial phase values of the pulse light signal p1 and the pulse light signal p2 respectively, phi1 and phi2 are not fixed after each modulation but the phase difference (phi2-phi1) is constant; the phase delays of the transmission light passing through the paths containing the first Faraday rotating mirror, the second Faraday rotating mirror, the third Faraday rotating mirror and the fourth Faraday rotating mirror in the phase modulation unit are g1, g2, g3 and g4 respectively, for the same phase modulation unit, g1, g2, g3 and g4 are constant values, then the initial phase value of the reflected light signal p11 is phi1+g1, the initial phase value of the reflected light signal p12 is phi1+g2, the initial phase value of the reflected light signal p13 is phi1+g3, the initial phase value of the reflected light signal p14 is phi1+g4, the initial phase value of the reflected light signal p21 is phi2+g1, the initial phase value of the reflected light signal p22 is phi2+g2, the initial phase value of the reflected light signal p23 is phi2+g3, and the initial phase value of the reflected light signal p24 is phi2+g4.

[0053] Because the devices and delay optical fibers through which the light is transmitted are different, different phase delays will be caused. Therefore, it is assumed that the initial phase values of the pulse light signal p1 and the pulse light signal p2 are phi1 and phi2 respectively, phi1 and phi2 are not fixed after each modulation but the phase difference (phi2-phi1) is constant; the phase delays of the transmission light passing through the paths containing the first Faraday rotating mirror, the second Faraday rotating mirror, the third Faraday rotating mirror and the fourth Faraday rotating mirror in the phase modulation unit are g1, g2, g3 and g4 respectively, for the same phase modulation unit, g1, g2, g3 and g4 are constant values, then the initial phase value of the reflected light signal p11 is phi1+g1, the initial phase value of the reflected light signal p12 is phi1+g2, the initial phase value of the reflected light signal p13 is phi1+g3, the initial phase value of the reflected light signal p14 is phi1+g4, the initial phase value of the reflected light signal p21 is phi2+g1, the initial phase value of the reflected light signal p22 is phi2+g2, the initial phase value of the reflected light signal p23 is phi2+g3, and the initial phase value of the reflected light signal p24 is phi2+g4.

[0054] The interference signal amplitude I1, the interference signal amplitude I2 and the interference signal amplitude I3 are taken as the parametric equations of an ellipse, and the solutions of A1, A2, A3, B1, B2, B3, theta1, theta2 and theta3 are obtained through an elliptical fitting algorithm. Figure 4The ellipse fitting results of I2, I3, and I1 are shown. Substituting the results of each variable into the equations I1, I2, and I3 yields the value of phis. Based on the ellipse fitting results, the values ​​of A1, A2, A3, B1, B2, B3, θ1, θ2, and θ3 are obtained. Furthermore, the simultaneous equations I1, I2, and I3 are used to demodulate the external vibration signal, verifying the effectiveness of the invention. Figure 5 The demodulated PIS signal is shown in the figure. The demodulation result is consistent with the excitation signal, which illustrates the effectiveness of the present invention.

[0055] In the above technical solution, the narrow linewidth pulse light source module is a 1550nm narrow linewidth pulse light source module, which has a dual-pulse light signal output function, a dual-pulse delay time adjustable range of 10ns to 1us, a pulse width adjustable range of 5ns to 255ns, and a pulse synchronization output signal for triggering the signal processing module to perform signal acquisition.

[0056] The circulator is a conventional fiber optic circulator;

[0057] The pulsed optical signal amplifier is a conventional pulsed optical signal amplifier used to amplify the power of the pulsed optical signal;

[0058] The multi-channel fiber optic splitter adopts a conventional 4-channel fiber optic splitter, which divides the input optical signal into 4 channels. Each channel is connected to a grating array sensing fiber, and the return signal of each grating array sensing fiber is transmitted to the corresponding return signal output port.

[0059] The multi-channel optical signal amplifier uses a conventional 4-channel optical signal amplifier to amplify the power of the return signal from each grating array sensing fiber.

[0060] All the optical fibers used in the grating array are broadband grating array fibers with a grating spectral width of 3nm, a center wavelength of 1550nm, a sensing network length of 5km, a grating spacing of 10m, and 500 gratings.

[0061] The multi-channel photoelectric conversion module adopts a conventional 4-channel photoelectric conversion module to convert the return light signal of each grating array sensing fiber into an electrical signal.

[0062] In the phase modulation unit, the 3x3 coupler 13 adopts a triangular fused tapered fiber coupler, the splitting ratio of the three output signals is 1:1:1, and the phase difference of each output signal is 120°. The 1x2 couplers all adopt conventional 1:1 fiber couplers. The delay fibers L11, L12, L13, L21, L22, and L23 are ordinary single-mode fibers with lengths of 1m, 1m, 7m, 3m, 1m, and 3m, respectively. The Faraday rotator is a conventional Faraday rotator.

[0063] The signal processing module controls the 4-channel analog-to-digital conversion chip to collect each channel of the electrical signals converted by the multi-channel photoelectric conversion module and to perform phase demodulation processing on the signals to realize demodulation of the sensing network.

[0064] The grating array fibers are all wide-spectrum grating array fibers, the grating spectral width is 3 nm, the central wavelength is 1550 nm, the sensing network length is 5 km, the grating pitch is 10 m, and the number of gratings is 500; the sensing signals are 499, and each sensing signal contains three interfering pulse signals.

[0065] A multi-channel grating array fiber vibration sensing method based on the above system, which comprises the following steps:

[0066] Step 1: the pulse light source module outputs a pulse light signal p1 and a pulse light signal p2 with a delay of ts in a period;

[0067] Step 2: the pulse light signal p1 forms reflected light signals p11, p12, p13 and p14 in the phase modulation unit, the pulse light signal p2 forms reflected light signals p21, p22, p23 and p24 in the phase modulation unit, and the reflected light signals p11, p12, p13 and p14, the reflected light signals p21, p22, p23 and p24 form a pulse light sequence signal;

[0068] Step 3: the multi-channel fiber splitter is used to divide the pulse light sequence signal into multiple sub-pulse light sequence signals with equal power, and each sub-pulse light sequence signal enters a corresponding grating array sensing fiber;

[0069] Step 4: each pulse signal in each sub-pulse light sequence signal is reflected by each grating in the corresponding grating array sensing fiber, and the time delay of the first pulse signal reflected by adjacent gratings in the grating array sensing fiber is tg;

[0070] Step 5: the pulse signals reflected by each grating array sensing fiber are output to the corresponding multi-channel photoelectric conversion module through the multi-channel fiber splitter for conversion of photoelectric signals to obtain corresponding sensing electrical signals; wherein the reflected light signals p22, p23 and p24 reflected by the former grating of the adjacent two gratings of each grating array sensing fiber interfere with the reflected light signals p11, p12 and p13 reflected by the latter grating of the adjacent two gratings to produce corresponding sensing electrical signals;

[0071] Step 6: The signal processing module performs phase demodulation processing on the sensing electrical signal, realizes demodulation of the grating array fiber, and thus obtains vibration information corresponding to the grating array sensing fiber.

[0072] In order to verify the effectiveness of the present application, a 2m long optical fiber between the first grating and the second grating is wound on a piezoelectric ceramic ring, and a 10Vpp, 100Hz sinusoidal signal is applied to the piezoelectric ceramic ring. The piezoelectric ceramic expands with the voltage, the length of the optical fiber between the two gratings increases with the expansion of the piezoelectric ceramic ring, and thus affects the phase phis of the interference signal. According to the calculation of the diameter of the used pzt being 3cm and the expansion coefficient being 5.4nm / Vpp, the peak-to-peak value of phis is about 7rad. Figure 5 The demodulation results show that the peak-to-peak value and the frequency are consistent with the excited signal.

[0073] Figure 3 The amplitude I1, I2, I3 of the three pulse signals of the first sensing signal is shown to change with time;

[0074] The contents not described in detail in the specification belong to the prior art known to those skilled in the art.

Claims

1. A multi-channel grating array fiber optic vibration sensing system, characterized in that: It includes a pulsed light source module, a phase modulation unit, a multi-channel fiber optic splitter, a multi-channel grating array sensing fiber, a signal processing module, and a multi-channel photoelectric conversion module; The pulse light source module is used to output pulse light signals p1 and p2 with a delay of ts within one cycle; In the phase modulation unit, pulsed light signal p1 forms reflected light signals p11, p12, p13, and p14. Pulsed light signal p2 forms reflected light signals p21, p22, p23, and p24 in the phase modulation unit. The reflected light signals p11, p12, p13, p14, p21, p22, p23, and p24 form a pulsed light sequence signal. The multi-path fiber optic splitter is used to divide the pulsed light sequence signal into multiple sub-pulse light sequence signals with equal power, and each sub-pulse light sequence signal is used to detect the corresponding grating array sensing fiber. Each pulse signal in each sub-pulse optical sequence signal is reflected when it encounters each grating in the corresponding grating array sensing fiber. The time delay of the first pulse signal reflected by the adjacent grating in the grating array sensing fiber is tg. The pulse signal reflected back from each grating array sensing fiber is output to the corresponding multi-channel photoelectric conversion module through a multi-channel fiber splitter to convert the photoelectric signal into the corresponding sensing electrical signal. Among them, the reflected light signals p22, p23 and p24 reflected by the first grating of each pair of adjacent gratings interfere with the reflected light signals p11, p12 and p13 reflected by the second grating of the two adjacent gratings, respectively, to generate the corresponding sensing electrical signal. The signal processing module is used to perform phase demodulation processing on the sensing electrical signals.

2. The multi-channel grating array fiber optic vibration sensing system according to claim 1, characterized in that: The first coupler in the phase modulation unit is used to split the pulsed optical signal p1 into a first-level sub-pulse optical signal p100 and a first-level sub-pulse optical signal p101 with a phase difference of A. The first-level sub-pulse optical signal p100 is transmitted to the second coupler in the phase modulation unit through the delay fiber L11. The first-level sub-pulse optical signal p101 is transmitted to the third coupler in the phase modulation unit through the delay fiber L21. The second coupler in the phase modulation unit is used to split the first-level sub-pulse optical signal p100 into a second-level sub-pulse optical signal p1001 and a second-level sub-pulse optical signal p1002 with a phase difference of B. The third coupler in the phase modulation unit is used to split the first-level sub-pulse optical signal p101 into a second-level sub-pulse optical signal p1011 and a second-level sub-pulse optical signal p1012 with a phase difference of B. The second-stage sub-pulse optical signal p1001 is input to the first Faraday rotator through the delay fiber L12 and is reflected to form the reflected optical signal p11. The second-stage sub-pulse optical signal p1002 is input to the second Faraday rotator through the delay fiber L13 and is reflected to form the reflected optical signal p12. The second-stage sub-pulse optical signal p1011 is input to the third Faraday rotator through the delay fiber L22 and is reflected to form the reflected optical signal p13. The second-stage sub-pulse optical signal p1012 is input to the fourth Faraday rotator through the delay fiber L23 and is reflected to form the reflected optical signal p14. The first coupler in the phase modulation unit is used to split the pulsed optical signal p2 into a first-level sub-pulse optical signal p200 and a first-level sub-pulse optical signal p201 with a phase difference of A. The first-level sub-pulse optical signal p200 is transmitted to the second coupler in the phase modulation unit through the delay fiber L11, and the first-level sub-pulse optical signal p201 is transmitted to the third coupler in the phase modulation unit through the delay fiber L21. The second coupler in the phase modulation unit is used to split the first-level sub-pulse optical signal p200 into a second-level sub-pulse optical signal p2001 and a second-level sub-pulse optical signal p2002 with a phase difference of B. The third coupler in the phase modulation unit is used to split the first-level sub-pulse optical signal p201 into a second-level sub-pulse optical signal p2011 and a second-level sub-pulse optical signal p2012 with a phase difference of B. The second-stage sub-pulse optical signal p2001 is input to the first Faraday rotator through the delay fiber L12 and is reflected to form the reflected optical signal p21. The second-stage sub-pulse optical signal p2002 is input to the second Faraday rotator through the delay fiber L13 and is reflected to form the reflected optical signal p22. The second-stage sub-pulse optical signal p2011 is input to the third Faraday rotator through the delay fiber L22 and is reflected to form the reflected optical signal p23. The second-stage sub-pulse optical signal p2012 is input to the fourth Faraday rotator through the delay fiber L23 and is reflected to form the reflected optical signal p24.

3. The multi-channel grating array fiber optic vibration sensing system according to claim 2, characterized in that: The time delay between reflected light signals p11 and p12 is t1, the time delay between reflected light signals p21 and p22 is t1, the time delay between reflected light signals p11 and p13 is t2, the time delay between reflected light signals p21 and p23 is t2, the time delay between reflected light signals p11 and p14 is t3, and the time delay between reflected light signals p21 and p24 is t3.

4. The multi-channel grating array fiber optic vibration sensing system according to claim 2 or 3, characterized in that: The delay between the reflected light signals p11 and p21 is equal to the delay ts between the pulsed light signals p1 and p2, and the delay between the reflected light signals p11 and p22 is equal to the delay corresponding to the grating spacing.

5. The multi-channel grating array fiber optic vibration sensing system according to claim 2 or 3, characterized in that: The pulse light source module is used to output a synchronous trigger electrical signal to the signal processing module during each output cycle of pulse light signal p1 and pulse light signal p2, so that the signal processing module can start to collect the electrical signal converted by the photoelectric conversion module.

6. The multi-channel grating array fiber optic vibration sensing system according to claim 1, characterized in that: It also includes a circulator and a pulse signal amplifier, and the pulse optical signal p1 and pulse optical signal p2 are transmitted to the phase modulation unit through the circulator; The phase modulation unit transmits the pulsed optical sequence signal through a circulator to a pulse signal amplifier, which amplifies it before sending it to a multi-channel fiber optic splitter.

7. The multi-channel grating array fiber optic vibration sensing system according to claim 3, characterized in that: The delays ts, tg, t1, t2, and t3 respectively satisfy the following formulas: ts = tg - t1 tg=2n eff d / c t1=2n eff (L21+L22-L11-L12) / c t2=2n eff (L21+L23-L11-L12) / c t3=2n eff (L13-L12) / c Where n eff denoted as the core refractive index of the grating array fiber, c as the speed of light in a vacuum, d as the spacing between adjacent gratings, and L11, L12, L21, and L22 as the lengths of delay fibers L11, L12, L21, and L22, respectively.

8. The multi-channel grating array fiber optic vibration sensing system according to claim 1, characterized in that: The interference signal amplitude of the interference between the reflected light signal p22 from the previous grating and the reflected light signal p11 from the next grating is I1; the interference signal amplitude of the interference between the reflected light signal p23 from the previous grating and the reflected light signal p12 from the next grating is I2; and the interference signal amplitude of the interference between the reflected light signal p24 from the previous grating and the reflected light signal p13 from the next grating is I3. Where A1, A2, and A3 are the DC quantities of the three interference signals, and B1, B2, and B3 are the AC quantities of the three interference signals, phis = 2n eff πV / λ, phis represents the phase change of the transmitted light caused by the change in fiber length between two adjacent gratings in the grating array sensing fiber due to the vibration and contraction of the piezoelectric ceramic ring. λ represents the wavelength of the transmitted light. θ1=phi2-phi1+g3-g1+120°, θ2=phi2-phi1+g4-g3, θ3=phi2-phi1+g2-g4-120° are fixed phase errors introduced by different paths in the phase modulation unit. Let the initial phase values ​​of pulsed optical signal p1 and pulsed optical signal p2 be phi1 and phi2, respectively. phi1 and phi2 are not fixed after each modulation, but the phase difference is constant. The path phase delays of the transmitted light through the phase modulation unit containing the first Faraday rotator mirror, the second Faraday rotator mirror, the third Faraday rotator mirror, and the fourth Faraday rotator mirror are g1, g2, g3, and g4, respectively.

9. The multi-channel grating array fiber optic vibration sensing system according to claim 8, characterized in that: The amplitudes of the interference signals I1, I2, and I3 are combined in pairs as the parametric equations of the ellipse. The equations A1, A2, A3, B1, B2, B3, θ1, θ2, and θ3 are solved using an ellipse fitting algorithm.

10. A method for fiber optic vibration sensing based on a multi-channel grating array according to claim 1, characterized in that, It includes the following steps: Step 1: The pulse light source module outputs pulse light signals p1 and p2 with a delay of ts within one cycle; Step 2: Pulsed light signal p1 forms reflected light signals p11, p12, p13, and p14 in the phase modulation unit; pulsed light signal p2 forms reflected light signals p21, p22, p23, and p24 in the phase modulation unit; reflected light signals p11, p12, p13, p14, p21, p22, p23, and p24 form a pulsed light sequence signal. Step 3: The multi-path fiber optic splitter is used to divide the pulsed light sequence signal into multiple sub-pulse light sequence signals with equal power, and each sub-pulse light sequence signal enters the corresponding grating array sensing fiber. Step 4: Each pulse signal in each sub-pulse optical sequence signal will be reflected when it encounters each grating in the corresponding grating array sensing fiber. The time delay of the first pulse signal reflected by the adjacent grating in the grating array sensing fiber is tg. Step 5: The pulse signal reflected back from each grating array sensing fiber is output to the corresponding multi-channel photoelectric conversion module through a multi-channel fiber splitter to convert the photoelectric signal into the corresponding sensing electrical signal; wherein, the reflected light signals p22, p23 and p24 reflected by the first grating in each pair of adjacent gratings of the grating array sensing fiber interfere with the reflected light signals p11, p12 and p13 reflected by the second grating in the two adjacent gratings, respectively, to generate the corresponding sensing electrical signal; Step 6: The signal processing module performs phase demodulation processing on the sensing electrical signal to demodulate the grating array fiber, thereby obtaining the vibration information corresponding to the grating array sensing fiber.

Images