CN116026447B - Polarization fading inhibition device and method for distributed optical fiber acoustic wave sensing system - Google Patents
Polarization fading inhibition device and method for distributed optical fiber acoustic wave sensing system Download PDFInfo
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Abstract
A polarization fading suppression device and method for a distributed optical fiber acoustic wave sensing system is composed of a narrow linewidth laser, a 1x2 optical fiber coupler, an acousto-optic modulator, an electro-optic modulator, an optical fiber amplifier, an optical circulator, a polarization beam splitter, a photoelectric detector, an AD conversion module and a data processing module. The method comprises the steps of modulating an optical pulse signal containing a plurality of different frequencies by adjusting bias voltage of an electro-optical modulator and amplitude of an input driving signal, inputting the optical pulse signal into a tested optical fiber after amplification of the optical pulse signal by an amplifier, dividing a Rayleigh scattering signal returned along the optical fiber into lights with different polarization states by a polarization beam splitter, interfering each path of optical signal with local oscillation light waves which also pass through the polarization beam splitter, carrying out analog-digital conversion, frequency separation, selection, phase separation, single-frequency weighted average and multi-frequency average algorithm processing on each path of obtained interference signal, and obtaining a phase signal with effectively suppressed fading.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a polarization fading inhibition device and method of a distributed optical fiber acoustic wave sensing system.
Background
For the past decades, fiber optic sensing technology has been continuously studied and, due to its great potential and unique advantages, has been widely used in the fields of perimeter security and structural monitoring. The distributed optical fiber acoustic wave sensing technology is an interference type optical fiber sensing technology and has the advantages of high sensitivity, long sensing distance, real-time monitoring and the like. At present, two fading mechanisms for influencing signal quality in a distributed optical fiber acoustic wave sensing system are mainly adopted, namely polarization fading and coherent fading. The former is mainly caused by the mismatch of the polarization states of the local oscillation light and the Rayleigh scattering light after frequency shift returned from the sensing optical fiber. The birefringence effect in the fiber, as well as the environmental changes, can affect the polarization state of the return signal light, making the polarization state of the return rayleigh signal spatially and temporally unstable. When the polarization direction of the return signal light is inconsistent with the polarization direction of the local oscillation light, the interference efficiency is reduced, the polarization fading occurs, and the time-varying Rayleigh signal polarization state can lead to the random fluctuation of the amplitude of the finally obtained interference signal, so that the noise floor of the system is changed after demodulation. Particularly, when the polarization states of the two interference signals are orthogonal, the interference signals disappear, and the signals cannot be demodulated. At present, in a distributed optical fiber acoustic wave sensing system, methods of adding a polarization controller, using a full polarization maintaining optical fiber structure, adding a Faraday rotator at the tail end of an optical fiber, using a sensing optical fiber added with a weak optical fiber grating, using polarization diversity and the like are mainly used for inhibiting polarization fading. In a long distance sensing optical fiber system of several tens kilometers, the effect of the method for adding the polarization controller is remarkably reduced due to the fact that the birefringence caused by the optical fiber and the external environment becomes more complex. Both the polarization maintaining fiber and the grating fiber are more expensive than the common single mode fiber, and the system cost is increased. Because the interference type optical fiber sensing system has influence on the polarization state, the addition of the Faraday rotator mirror can not completely eliminate the polarization fading phenomenon, and the method has special requirements on the system structure and the measured optical fiber, thereby reducing the system adaptability. The principle of the polarization diversity receiving method is that a one-to-two polarization beam splitter or a one-to-three polarization beam splitter is used at a receiving end to split an optical signal into different polarization states, when a signal in one polarization state is weakened, signal light in other polarization states can be strengthened, and then selection or average operation is carried out to restrain polarization fading. Coherent fading results from a non-uniform distribution of the refractive index of the fiber. The current method for inhibiting coherent fading mainly comprises the technologies of frequency diversity, phase diversity and the like. Since interference fading varies with the frequency of the pulsed light, the basic idea of the frequency diversity technique is to generate multiple frequency pulsed light and then process the different frequency signals. Polarization fading and coherent fading can have serious influence on signal quality, and currently, methods capable of simultaneously inhibiting the polarization fading and the coherent fading are rarely reported, so that a system and a processing method capable of simultaneously inhibiting the polarization fading and the coherent fading in one measurement are needed.
Disclosure of Invention
The invention aims to solve the technical problem of providing a polarization fading inhibition device and method for a distributed optical fiber acoustic wave sensing system, which have the characteristic of being capable of simultaneously inhibiting polarization fading and coherent fading.
In order to achieve the above purpose, the technical scheme provided by the invention is as follows:
the polarization fading suppression device of the distributed optical fiber acoustic wave sensing system comprises a narrow linewidth laser, a first optical fiber coupler, an acousto-optic modulator, an electro-optic modulator, an optical fiber amplifier, an optical circulator, a first polarization beam splitter, a second optical fiber coupler, a third optical fiber coupler, a first photoelectric detector, a second photoelectric detector, an AD analog-to-digital conversion module and a data processing module which are sequentially arranged along the direction of an optical path; light emitted by the narrow linewidth laser is divided into a light beam a and a light beam b by a first optical fiber coupler, wherein the light beam a enters an optical circulator after passing through an acousto-optic modulator, an electro-optic modulator and an optical fiber amplifier, is output to a sensing optical fiber by the optical circulator, and Rayleigh scattered light which is returned by the sensing optical fiber and carries vibration information is divided into Pa light and Sa light by a second polarization beam splitter; the light beam b passes through the first polarization beam splitter and is divided into Pb light and Sb light with mutually perpendicular vibration directions; the Pa light and the Pb light are coupled through a second optical fiber coupler and then received by a first photoelectric detector, and then converted into digital information through an AD analog-to-digital conversion module and sent to a data processing module for processing; the Sa light and the Sb light are coupled through a third optical fiber coupler and then received by a second photoelectric detector, and then converted into digital information through an AD analog-to-digital conversion module and sent to a data processing module for processing.
Preferably, the output signal end of the optical circulator is connected with the tested optical fiber, and the return signal end is connected with the second polarization beam splitter.
Preferably, the application wavelength of the acousto-optic modulator is consistent with the central wavelength of the narrow linewidth laser, the frequency shift frequency fa is greater than 100M, and the extinction ratio is greater than 40dB.
Preferably, at least one frequency multiplication modulation phenomenon occurs in the frequency of the optical signal modulated by the electro-optical modulator.
Preferably, the data processing module comprises a combination of different bandpass filters whose passband frequencies filter out respectively a plurality of frequencies of the acousto-optic modulator and the electro-optic modulator combined with each other.
Preferably, the first optical fiber coupler is a 1×2 optical fiber coupler, the second optical fiber coupler and the third optical fiber coupler are 2×2 optical fiber couplers, the splitting ratio of the first optical fiber coupler is 90:10, 90% of the first optical fiber coupler is the light beam a, and 10% of the first optical fiber coupler is the light beam b.
Preferably, the first polarizing beam splitter and the second polarizing beam splitter are one-to-two state polarizing beam splitters or one-to-three state polarizing beam splitters.
The invention also provides a polarization fading inhibition method of the distributed optical fiber acoustic wave sensing system, which comprises the following steps:
s1: the continuous light with the frequency f0 is output by the narrow linewidth laser, the continuous light is divided into a light beam a and a light beam b by the first optical fiber coupler, the light beam b is divided into Pb light and Sb light with mutually perpendicular vibration directions after being input into the first polarization beam splitter as local oscillation light, the light beam a is input into the electro-optical modulator, and the signal frequency of the light beam a modulated by the electro-optical modulator comprises five types of f0-2fe, f0-fe, f0, f0+fe and f0+2fe assuming that the driving frequency of the electro-optical modulator is fe and the maximum available high harmonic frequency is 2 fe.
S2: the beam a signal output from the electro-optic modulator is transmitted to the acousto-optic modulator through the optical fiber to carry out secondary modulation, and the modulated beam a signal is pulse light with a certain width on the assumption that the frequency shift frequency of the acousto-optic modulator is fa, and the frequency of the beam a signal comprises five types of f0-2E+fa, f0-E+fa, f0+fa, f0+E+fa and f0+2E+fa.
S3: the light beams a with different frequencies are amplified by the optical fiber amplifier and then output to the sensing optical fiber through the optical circulator, and Rayleigh scattering light which is returned by the sensing optical fiber and carries vibration information is separated into Pa light and Sa light after passing through the second polarization beam splitter.
S4: the Pa light of the Rayleigh scattered light and the Pb light of the local oscillation light interfere, the interference is outputted to the first photoelectric detector through the second optical fiber coupler, the optical signal is converted into an electric signal through the first photoelectric detector, and then the electric signal is converted into a digital signal through the AD analog-to-digital conversion module and is sent to the data processing module; the Sa light of the Rayleigh scattered light and the Sb light of the local oscillation light interfere, the interference is outputted to a second photoelectric detector through a third optical fiber coupler, an optical signal is converted into an electric signal through the second photoelectric detector, and then the electric signal is converted into a digital signal through an AD analog-to-digital conversion module and is sent to a data processing module;
s5: the data processing module carries out filtering processing on the received digital signals to respectively obtain signals of five frequencies A, A-E, A+E, A-2*E and A+2*E corresponding to P light and S light; processing the signals of the five frequencies to obtain a phase signal representing that the fading of vibration information is effectively suppressed
Preferably, the processing the signals of five frequencies in the step S5 includes the following substeps:
s5.1: respectively inputting the P optical signal and the S optical signal received by the data processing module into a parallel array of multi-center wavelength band-pass filters based on FPGA to filter out five frequency signals A-2E, A-E, A, A +E and A+2E;
s5.2: the signal intensities of the P optical signal and the S optical signal of each frequency are respectively compared, and the path with the highest intensity in the P optical signal and the S optical signal of the same frequency is selected for storage;
s5.3: performing Hilbert transformation plus arctan solution phase operation on the selected optical signals with different frequencies to obtain phase signals corresponding to the signals with different frequencies respectively;
s5.4: inputting phase signals of all frequencies into a subtracter in sequence, subtracting the phase signals into current time signals, subtracting the phase signals into next time signals, and subtracting the phase signals from each other in sequence;
s5.5: grouping the data after the difference (whether the difference refers to the operation of S5.4) by taking M point data as a group, wherein M is the pulse width divided by the period corresponding to the sampling frequency of the ADC, and selecting the minimum point in the M points;
s5.6: m points are respectively subtracted from the minimum value of the M points to obtain M weights, and the M point data are respectively multiplied by the weights and then are subjected to average operation;
s5.7: five frequency signals after weighted average are obtained for M points, and then direct average is carried out to obtain a final phase value;
s5.8: and repeating the operations S5.1-S5.7, wherein each operation can obtain a phase signal which effectively inhibits the fading of the characterization vibration information in one measurement process, and the polarization fading and the coherent fading can be effectively inhibited.
The invention relates to a polarization fading inhibition device and a method for a distributed optical fiber acoustic wave sensing system, which are characterized in that an acousto-optic modulator and an electro-optic modulator can be combined to modulate optical pulse signals with a plurality of different frequencies by adjusting bias voltage of the electro-optic modulator and amplitude of an input driving signal, the optical pulse signals are amplified by an amplifier and then input into a tested optical fiber, a Rayleigh scattering signal returned along the optical fiber is divided into lights with different polarization states by a polarization beam splitter, each path of optical signal is interfered by local oscillation light waves of the polarization beam splitter, and the obtained interference signals are subjected to analog-digital conversion, frequency separation, selection, phase separation, single-frequency weighted average and multi-frequency average algorithm processing to obtain phase signals representing effective inhibition of fading of vibration. The invention can effectively and simultaneously inhibit the problems of polarization fading and coherent fading in the distributed acoustic wave sensing system.
Description of the drawings:
FIG. 1 is a diagram of an implementation system employing a two-state polarizing beam splitter approach;
FIG. 2 is a signal processing flow diagram of a method of using a two-state polarizing beam splitter;
FIG. 3 is a flow chart of another signal processing using a two-state polarizing beam splitter approach;
fig. 4 is a diagram of an implementation system using a tri-state polarizing beamsplitter approach.
Detailed Description
The present invention will be further described with reference to examples and drawings for the purpose of clearly illustrating the structure and measuring method of the present measuring apparatus, but should not be construed as limiting the scope of the present invention.
Example 1
Combining digital filtering with two-state polarization beam splitter mode:
referring to fig. 1, a polarization fading suppression device of a distributed optical fiber acoustic wave sensing system is composed of a narrow linewidth laser 1, a first optical fiber coupler 2, an acousto-optic modulator 3, an electro-optic modulator 4, an optical fiber amplifier 5, an optical circulator 6, a first polarization beam splitter 7, a second polarization beam splitter 7', a second optical fiber coupler 8, a third optical fiber coupler 8', a first photoelectric detector 9, a second photoelectric detector 9', an AD analog-to-digital conversion module 10 and a data processing module 11 which are sequentially arranged along an optical path direction; the first optical fiber coupler 2 is a 1x2 optical fiber coupler, the first polarization beam splitter 7 and the second polarization beam splitter 7 'are both two-state polarization beam splitters, and the second optical fiber coupler 8 and the third optical fiber coupler 8' are both 2x2 optical fiber couplers. The narrow linewidth laser 1 is divided into 90% and 10% parts by a first optical fiber coupler 2, the 90% part is assumed to be light a, the 10% part is assumed to be light b, the light a shifts the frequency of an optical signal by an acousto-optic modulator 3 and modulates the optical signal into pulse light, the pulse light is subjected to secondary modulation and superposition of the optical signal by an electro-optic modulator 4 to multiple frequencies, the amplified optical signal is amplified by an optical fiber amplifier 5, the amplified optical signal is input into a 1 port of an optical circulator 6, a 2 port of the optical circulator 6 is connected with a tested optical fiber, a 3 port of the optical circulator 6 is connected with a second polarization beam splitter 7', the optical signal is output from the 1 port of the optical circulator 6 into the tested optical fiber, the tested optical fiber returns Rayleigh scattered light carrying vibration information, and the returned optical signal is decomposed into Pa light and Sa light with different polarization states by the second polarization beam splitter 7'. The optical b signal is used as local oscillation light to be connected with the first polarization beam splitter 7, and the first polarization beam splitter 7 splits the optical b signal into Pb light and Sb light with different polarization states. The Pa light and the Pb light are respectively connected to the two input ends of the second optical fiber coupler 8, and the Sa light and the Sb light are respectively connected to the two input ends of the third optical fiber coupler 8'. The output ends of the second optical fiber coupler 8 and the third optical fiber coupler 8 'are respectively connected with the first photoelectric detector 9 and the second photoelectric detector 9', pa light and Pb light are coupled into P light through the second optical fiber coupler 8 and then are converted into analog electric signals after being captured by the first photoelectric detector 9 and then are output to the AD analog-digital conversion module 10, sa light and Sb light are coupled into S light through the third optical fiber coupler 8 'and then are converted into analog electric signals after being captured by the second photoelectric detector 9' and are output to the AD analog-digital conversion module 10, the AD analog-digital conversion module 10 converts the received two paths of analog electric signals into digital signals and sends the digital signals to the data processing module 11, and the data processing module 11 carries out algorithm processing on the received digital signals.
The specific algorithm process flow is shown in fig. 2, and firstly, the digital signals from the P light and the S light are respectively subjected to band-pass filtering, the number of the band-pass filters is equal to the number of frequencies combined by the required acousto-optic modulator 3 and the electro-optic modulator 4, and the corresponding frequencies can be filtered out. And selecting the filtered P optical signals and S optical signals with the same frequency according to the intensity, leaving one signal with high intensity, performing Hilbert transformation and arctangent phase solving operation on the signals to obtain processed P optical signals and S optical signals, performing phase extraction operation on the processed P optical signals and S optical signals, and performing weighted average on each M points to obtain phase signals, wherein the weight is the percentage of the sum of the total difference value of each point in the M points and the minimum value. (M is the period corresponding to the pulse width divided by the ADC sampling frequency); and performing direct average operation on the data obtained after the weighted average of the multiple frequencies to obtain a final phase signal value.
The algorithm processing of the signal may also adopt another processing method as shown in fig. 3, which is different in that, during the processing of the signal, the hilbert transform and the arctan phase resolving operation are performed first without selecting the intensity of the signal, so as to obtain a processed P optical signal and an S optical signal, and then the processed P optical signal and the S optical signal are added to obtain a phase signal.
Example 2
Using digital filtering in combination with tri-state polarization beam splitting:
as shown in fig. 4, embodiment 2 differs from embodiment 1 mainly in that the first polarizing beam splitter 7 and the second polarizing beam splitter 7 'in fig. 2 are replaced with a first tri-state polarizing beam splitter 13 and a second tri-state polarizing beam splitter 13'. Corresponding to the addition of a path of detection signals, the photoelectric detector 9' is correspondingly added. The data processing manner is basically the same as that in fig. 2, except that a path of data processing flow is to be newly added, and the selection, the phase separation, the weighted average and the average operations are performed on the three paths of multi-frequency signals at the end, which are not described herein again.
The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. The polarization fading inhibition method of the distributed optical fiber acoustic wave sensing system is characterized by comprising the following steps of:
s1: the method comprises the steps that continuous light with the output frequency of f0 is output by a narrow linewidth laser, the continuous light is divided into a light beam a and a light beam b by a first optical fiber coupler, the light beam b is input into a first polarization beam splitter as local oscillation light and then is divided into Pb light and Sb light with mutually perpendicular vibration directions, the light beam a is input into an electro-optical modulator, when the driving frequency of the electro-optical modulator is fe, the maximum available high-order harmonic frequency generated by the electro-optical modulator is 2fe, and the signal frequency of the light beam a modulated by the electro-optical modulator comprises five types of f0-2fe, f0-fe, f0, f0+fe and f0+2fe;
s2: the light beam a signal output from the electro-optic modulator is transmitted to the acousto-optic modulator through an optical fiber to carry out secondary modulation, when the frequency of the acousto-optic modulator is fa, the modulated light beam a signal is pulse light with a certain width, and the frequency of the light beam a signal comprises five types of f0-2fe+fa, f0-fe+fa, f0+fa, f0+fe+fa and f0+2fe+fa;
s3: the light beams a with different frequencies are amplified by an optical fiber amplifier and then output to a sensing optical fiber through an optical circulator, and Rayleigh scattered light which is returned by the sensing optical fiber and carries vibration information is divided into Pa light and Sa light after passing through a second polarization beam splitter;
s4: the Pa light of the Rayleigh scattered light and the Pb light of the local oscillation light interfere, the interference is outputted to the first photoelectric detector through the second optical fiber coupler, the optical signal is converted into an electric signal through the first photoelectric detector, and then the electric signal is converted into a digital signal through the AD analog-to-digital conversion module and is sent to the data processing module; the Sa light of the Rayleigh scattered light and the Sb light of the local oscillation light interfere, the interference is outputted to a second photoelectric detector through a third optical fiber coupler, an optical signal is converted into an electric signal through the second photoelectric detector, and then the electric signal is converted into a digital signal through an AD analog-to-digital conversion module and is sent to a data processing module;
s5: the data processing module carries out filtering processing on the received digital signals to respectively obtain signals of five frequencies, namely fa, fa-fe, fa+fe, fa-2 xfe and fa+2 xfe, corresponding to P light and S light; processing the signals of the five frequencies to obtain a phase signal representing that the fading of vibration information is effectively inhibited;
wherein: the processing of the signals of five frequencies in S5 includes the following substeps:
s5.1: respectively inputting the P optical signal and the S optical signal received by the data processing module into a multi-center wavelength band-pass filter parallel array based on FPGA to filter out five frequency signals fa-2fe, fa-fe, fa+fe and fa+2fe;
s5.2: the signal intensities of the P optical signal and the S optical signal of each frequency are respectively compared, and the path with the highest intensity in the P optical signal and the S optical signal of the same frequency is selected for storage;
s5.3: performing Hilbert transformation plus arctan solution phase operation on the selected optical signals with different frequencies to obtain phase signals corresponding to the signals with different frequencies respectively;
s5.4: inputting phase signals of all frequencies into a subtracter in sequence, subtracting the phase signals into current time signals, subtracting the phase signals into next time signals, and subtracting the phase signals from each other in sequence;
s5.5: grouping the data subjected to the operation of S5.4 by taking M point data as a group, wherein M is the period corresponding to the pulse width divided by the ADC sampling frequency, and selecting the minimum point in the M points;
s5.6: m points are respectively subtracted from the minimum value of the M points to obtain M weights, and the M point data are respectively multiplied by the weights and then are subjected to average operation;
s5.7: five frequency signals after weighted average are obtained for M points, and then direct average is carried out to obtain a final phase value;
s5.8: repeating the operations of S5.1-S5.7, and inhibiting polarization fading and coherent fading in the phase signal.
2. A polarization fading suppression device of a distributed optical fiber acoustic wave sensing system is characterized in that: the polarization fading suppression method for realizing the distributed optical fiber acoustic wave sensing system according to claim 1, comprising a narrow linewidth laser, a first optical fiber coupler, an acousto-optic modulator, an electro-optic modulator, an optical fiber amplifier, an optical circulator, a first polarization beam splitter, a second optical fiber coupler, a third optical fiber coupler, a first photoelectric detector, a second photoelectric detector, an AD analog-to-digital conversion module and a data processing module which are sequentially arranged along the optical path direction; light emitted by the narrow linewidth laser is divided into a light beam a and a light beam b by a first optical fiber coupler, wherein the light beam a enters an optical circulator after being modulated by an acousto-optic modulator, an electro-optic modulator and an optical fiber amplifier, the light enters a sensing optical fiber after being output by the optical circulator, and Rayleigh scattered light which is returned by the sensing optical fiber and carries vibration information is divided into Pa light and Sa light by a second polarization beam splitter; the light beam b passes through the first polarization beam splitter and is divided into Pb light and Sb light with mutually perpendicular vibration directions; the Pa light and the Pb light are coupled through a second optical fiber coupler and then received by a first photoelectric detector, and then converted into digital information through an AD analog-to-digital conversion module and sent to a data processing module for processing; the Sa light and the Sb light are coupled through a third optical fiber coupler and then received by a second photoelectric detector, and then converted into digital information through an AD analog-to-digital conversion module and sent to a data processing module for processing.
3. The polarization fading suppression device of a distributed optical fiber acoustic wave sensing system according to claim 2, wherein: and an output signal end of the optical circulator is connected with the tested optical fiber, and a return signal end of the optical circulator is connected with the second polarization beam splitter.
4. The polarization fading suppression device of a distributed optical fiber acoustic wave sensing system according to claim 2, wherein: the application wavelength of the acousto-optic modulator is consistent with the central wavelength of the narrow linewidth laser, the frequency shift frequency fa is greater than 100M, and the extinction ratio is greater than 40dB.
5. The polarization fading suppression device of a distributed optical fiber acoustic wave sensing system according to claim 2, wherein: at least one frequency multiplication modulation phenomenon occurs to the frequency of the optical signal modulated by the electro-optical modulator.
6. The polarization fading suppression device of a distributed optical fiber acoustic wave sensing system according to claim 2, wherein: the data processing module comprises a combination of different bandpass filters whose passband frequencies filter out respectively a plurality of frequencies of the acousto-optic modulator and the electro-optic modulator combined with each other.
7. The polarization fading suppression device of a distributed optical fiber acoustic wave sensing system according to claim 2, wherein: the first optical fiber coupler is a 1 multiplied by 2 optical fiber coupler, the second optical fiber coupler and the third optical fiber coupler are 2 multiplied by 2 optical fiber couplers, the splitting ratio of the first optical fiber coupler is 90:10, 90% of the first optical fiber coupler is light beam a, and 10% of the first optical fiber coupler is light beam b.
8. The polarization fading suppression device of a distributed optical fiber acoustic wave sensing system according to claim 2, wherein: the first polarizing beam splitter and the second polarizing beam splitter are one-to-two state polarizing beam splitters or one-to-three state polarizing beam splitters.
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| Theoretical and Experimental Analysis of -OTDR Based on Polarization Diversity Detection;Meiqi Ren 等;IEEE PHOTONICS TECHNOLOGY LETTERS;第28卷(第6期);第697-700页 * |
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