CN113091782B - PGC-based phase-sensitive optical time domain reflection system and phase demodulation method - Google Patents
PGC-based phase-sensitive optical time domain reflection system and phase demodulation method Download PDFInfo
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Abstract
The invention belongs to the technical field of phase-sensitive optical time domain reflection distributed optical fiber sensing, and particularly relates to a PGC (phase generated coherent) based phase-sensitive optical time domain reflection system and a phase demodulation method. The method comprises the following steps: s1, two paths of signals output by the interferometer are subjected to 3 x 3 coupler to generate signals with fixed phase difference as input signals, wherein one path of signals is subjected to synchronous carrier modulation, and a fundamental frequency signal and a double frequency signal are output; s2, mixing the fundamental frequency signal and the double frequency mixing signal output in the step S1 with the other path of signal respectively to obtain a fundamental frequency mixing signal and a double frequency mixing signal; s3, performing low-pass filtering on the fundamental frequency mixing signal and the double frequency mixing signal respectively, and performing normalization processing according to the optical fiber attenuation coefficient; and S4, performing phase extraction on the signal after the low-pass filtering and normalization processing. The invention eliminates the influence of optical fiber loss and carrier phase delay on the system, and can ensure the stability and accuracy of phase demodulation of the sensing system.
Description
Technical Field
The invention belongs to the technical field of phase-sensitive optical time domain reflection distributed optical fiber sensing, and particularly relates to a PGC (phase generated coherent) based phase-sensitive optical time domain reflection system and a phase demodulation method.
Background
In recent years, the frequent occurrence of road traffic accidents seriously threatens the life and property safety of people, how to eliminate traffic hidden dangers as early as possible and timely positioning the occurrence point of the accidents are key links for realizing quick rescue. The distributed phase-sensitive optical time domain reflection system is an effective solution for realizing the intelligent guardrail, when the system is applied to pre-buried optical fibers in a road, the refractive index of the optical fibers can be changed by sound/vibration signals, the variable quantity of phase information carried by coherent backward Rayleigh scattering light at the corresponding position and the optical fiber strain are in a linear correlation relationship at the moment, real-time and all-weather acquisition and analysis of traffic information in the road can be realized through phase recovery processing, and the system has the advantages of electromagnetic interference resistance, corrosion resistance, electric insulation, high sensitivity, good reliability and low cost. Therefore, the selection of a proper phase demodulation method is the key to realize the quantitative and accurate demodulation of the sound/vibration signals.
The phase demodulation method based on the distributed phase-sensitive optical time domain reflection system mainly comprises a digital I/Q demodulation method based on a heterodyne coherent structure and a passive homodyne demodulation method based on an interferometer structure. Among them, the digital I/Q demodulation method based on the heterodyne coherent demodulation structure (liu limin, wang xu, vast, etc. the photonics newspaper 2018, 47 (08): 219-. Therefore, the practical engineering application mostly adopts a demodulation structure based on an interferometer, including a 3 × 3 coupler demodulation method and a phase generation carrier algorithm. A demodulation algorithm (He Q, Zhu T, Xiao X, et al. IEEE Photonics Technology Letters,2013,25(20):1955 and 1957.) based on a 3X 3 coupler introduces three paths of output optical signals into fixed phase differences of 120 degrees, the demodulation algorithm is simple, but three identical photodetectors are required in the signal acquisition process, which inevitably brings measurement errors and increases the system cost. The patent application with publication number CN103759750A discloses a distributed optical fiber sensing system based on a Phase Generation Carrier (PGC) technology, which introduces a carrier signal outside a measured bandwidth band, effectively improves the signal-to-noise ratio of the system, and therefore, the system has high sensitivity and good linear responsivity. However, the PGC demodulation algorithm implemented by the arctangent method has a problem of phase unwrapping, and the PGC demodulation algorithm implemented by differential cross phase multiplication (DCM) has a problem of being greatly affected by light source fluctuation and easily generating harmonic distortion.
In patent cn201911072212.x, zhangchunyu et al adopts michelson interferometer and improved PGC demodulation scheme, and realizes elimination of polarization fading and carrier modulation depth influence by combining DCM and arctan algorithm, but the problem of carrier phase delay caused by unequal interferometer arm differences still exists. In summary, the existing PGC phase demodulation method based on the phase-sensitive optical time domain reflectometry system still lacks improvement work. Aiming at the technical problems that the PGC demodulation technology is easily influenced by light intensity fluctuation and carrier phase delay, the PGC method in the existing phase-sensitive optical time domain reflection system needs to be improved.
Disclosure of Invention
The invention overcomes the defects of the prior art, and provides a phase sensitive optical time domain reflection system and a phase demodulation method based on PGC (phase generated carrier) in order to solve the problems of light intensity fluctuation influence and carrier phase delay of the conventional PGC technology based on a distributed phase sensitive optical time domain reflection technology.
In order to solve the technical problems, the invention adopts the technical scheme that: a phase demodulation method of a phase sensitive optical time domain reflection system based on PGC comprises the following steps:
s1, two paths of signals output by the interferometer are subjected to 3 x 3 coupler to generate signals with fixed phase difference as input signals, wherein one path of signals is subjected to synchronous carrier modulation, and a fundamental frequency signal and a double frequency signal are output;
s2, mixing the fundamental frequency signal and the double frequency mixing signal output in the step S1 with the other path of signal respectively to obtain a fundamental frequency mixing signal and a double frequency mixing signal;
s3, performing low-pass filtering on the fundamental frequency mixing signal and the double frequency mixing signal respectively, and performing normalization processing according to the optical fiber attenuation coefficient;
and S4, performing phase extraction on the low-pass filtered and normalized fundamental frequency mixing signal and double frequency mixing signal.
The step S1 specifically includes the following steps:
one path of signals is divided into two paths after high-pass filtering, normalization processing, inverse cosine operation and high-pass filtering are sequentially carried out on the signals, one path of signals is used as base frequency signals to be output, and the other path of signals passes through a double frequency modulation module to be output as double frequency signals.
In step S4, the specific method for extracting the phase includes:
and respectively differentiating the fundamental frequency mixing signals, mixing the differentiated fundamental frequency mixing signals with the double-frequency mixing signals, outputting the mixed fundamental frequency mixing signals, differentiating the double-frequency mixing signals, mixing the differentiated double-frequency mixing signals with the original fundamental frequency mixing signals, outputting the mixed fundamental frequency mixing signals, and performing subtraction processing, integration processing and high-pass filtering processing on the two paths of output signals to obtain phase information.
In step S3, the coefficients of the normalization process are:
wherein, w1LPAnd w2LPRespectively representing the low-pass filtered fundamental and double frequency mixed signals, C1And C2Normalized coefficients for the fundamental and double frequency mixing signals, c represents the propagation velocity of the light wave in vacuum, a represents the attenuation coefficient in the optical fiber, nfThe refractive index of the laser in the optical fiber is shown as optical frequency f, and t represents the sampling time of the high-speed data acquisition card.
In addition, the invention also provides a phase sensitive optical time domain reflection system based on PGC, which is used for implementing the phase demodulation method and comprises a signal modulation module, an interference output module, a signal acquisition module and a data processing system;
the interference output module comprises a 3 multiplied by 3 coupler, a piezoelectric transducer, a first Faraday rotator mirror and a second Faraday rotator mirror;
coherent Rayleigh scattered light output by the signal modulation module enters a port a of the 3 multiplied by 3 coupler and outputs two paths of signals through a port d and a port e, wherein one path of signals enters the first Faraday rotating mirror; the other path of the signal is subjected to carrier modulation by a piezoelectric transducer and then is incident to a second Faraday rotating mirror; signals reflected by the first Faraday rotator mirror and the second Faraday rotator mirror return to the 3 multiplied by 3 coupler through the port d and the port e to form an interferometer, two paths of signals respectively output from the port b and the port c of the 3 multiplied by 3 coupler are transmitted to the data processing system after being acquired by the signal acquisition module, and the data processing system is used for carrying out phase demodulation on the acquired data and extracting phase information.
The signal modulation module comprises a narrow-linewidth laser, a signal generator, an acousto-optic modulator, a pulse optical amplifier, an optical circulator, a sensing optical fiber and an erbium-doped optical fiber amplifier;
the narrow linewidth laser emits high-coherence laser, the high-coherence laser is modulated into pulse light with pulse width T and light frequency f by the acousto-optic modulator, the pulse light is amplified by the pulse light amplifier and then injected into the sensing optical fiber through the optical circulator to interact with non-uniform scattering points in the optical fiber to generate coherent backward Rayleigh scattering light, and the coherent backward Rayleigh scattering light is output by the optical circulator and then amplified and output by the erbium-doped optical fiber amplifier.
The signal acquisition module comprises a first avalanche photodetector, a second avalanche photodetector and a dual-channel high-speed data acquisition card, wherein the input ends of the first avalanche photodetector and the second avalanche photodetector are respectively connected with a port b and a port c of the 3 x 3 coupler, and the output ends of the first avalanche photodetector and the second avalanche photodetector are connected with the dual-channel high-speed data acquisition card.
The data processing system includes:
a synchronous carrier modulation module: the signal processing circuit is used for respectively outputting a base frequency signal and a double frequency signal according to one path of signal;
synchronous carrier frequency mixing module: the frequency mixer is used for respectively mixing the other path of signals with the fundamental frequency signal and the double frequency signal output by the synchronous carrier modulation module to obtain two paths of mixing signals;
a normalization processing module: the device is used for respectively carrying out low-pass filtering on the two paths of frequency mixing signals output by the synchronous carrier frequency mixing module and then respectively carrying out normalization processing on the two paths of frequency mixing signals according to the optical fiber attenuation coefficient;
a phase information extraction module: and the device is used for calculating the two paths of frequency mixing signals after normalization processing and extracting phase information.
The synchronous carrier modulation module comprises: the system comprises a first high-pass filter, a first normalization module, an inverse cosine operation module, a second high-pass filter and a double-frequency modulation module, wherein one path of signal is divided into two paths after passing through the first high-pass filter, the first normalization module, the inverse cosine operation module and the second high-pass filter, one path of signal directly outputs a base frequency signal, and the other path of signal outputs a double-frequency signal after passing through the double-frequency modulation module;
the synchronous carrier frequency mixing module comprises a first frequency mixer and a second frequency mixer, wherein the first frequency mixer is used for mixing the other signal with a fundamental frequency signal, and the second frequency mixer is used for mixing the other signal with a double frequency signal;
the normalization processing module comprises a first low-pass filter, a second normalization module and a third normalization module;
the phase information extraction module comprises a first differentiator, a second differentiator, a first multiplier, a second multiplier, a subtracter and an integrator; in the two paths of mixed signals normalized by the normalization processing module, the first path of mixed signals is divided into two parts, one part is input into a first multiplier after passing through a first differentiator, the other part is directly input into a second multiplier, the second path of mixed signals is divided into two parts, one part is input into the second multiplier after passing through the second differentiator, the other part is directly input into the first multiplier, and output signals of the first multiplier and the second multiplier pass through a subtracter, an integrator and a third high-pass filter and then phase information is output.
Compared with the prior art, the invention has the following beneficial effects:
the invention discloses a phase sensitive optical time domain reflection system based on PGC and a phase demodulation method, on one hand, after a 3 x 3 coupler is used for generating a fixed phase difference, only two paths of output light are needed to be used as input signals of a PGC demodulation algorithm, wherein one path is used as a synchronous carrier mixing signal, and the other path is used as a signal to be demodulated, compared with a 3 x 3 coupler demodulation method, the invention reduces the hardware overhead cost of one path of photoelectric detector, reduces the system complexity and realizes the synchronous carrier recovery on a hardware structure. On the other hand, the invention provides a normalized PGC phase demodulation algorithm based on a distributed phase-sensitive optical time domain reflection system, the optical fiber attenuation coefficient is considered in the calculation process, normalization processing is carried out through normalization factors related to the optical fiber attenuation coefficient, the influence of optical fiber loss on the system is eliminated, the influence of light intensity disturbance on the demodulation algorithm is also eliminated, the operation process is simplified, and the function of accurately demodulating vibration information in a long-distance and high-speed linear mode can be realized.
Drawings
Fig. 1 is a schematic structural diagram of a PGC-based phase-sensitive optical time domain reflectometry system according to an embodiment of the present invention;
in the figure: 10-coherent Rayleigh scattering sensing signal modulation module, 101-narrow linewidth laser, 102-signal generator, 103-acousto-optic modulator, 104-pulsed light amplifier, 105-optical circulator, 106-sensing optical fiber, 107-erbium-doped optical fiber amplifier, 11-3 x 3 interference output module, 111-3 x 3 coupler, 112-piezoelectric transducer, 113-first Faraday rotator mirror, 114-second Faraday rotator mirror, 12-signal acquisition module, 121-first avalanche photodetector, 122-second avalanche photodetector, 123-double-channel high-speed data acquisition card, 13-data processing system.
Fig. 2 is a schematic diagram of an algorithm flow for implementing light intensity disturbance elimination and carrier synchronization recovery PGC according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments; all other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example one
As shown in fig. 1, a distributed optical fiber sensing system based on phase-based carrier generation according to an embodiment of the present invention includes a signal modulation module 10, an interference output module 11, a signal acquisition module 12, and a data processing system 13.
The interference output module 11 includes a 3 × 3 coupler 111, a piezoelectric transducer 112, a first faraday rotator mirror 113 and a second faraday rotator mirror 114.
Coherent rayleigh scattered light output by the signal modulation module 10 is incident to a port a of the 3 × 3 coupler 111, and two paths of signals are output through a port d and a port e, wherein one path of signals is incident to the first faraday rotator mirror 113; the other path of the signal is subjected to carrier modulation by the piezoelectric transducer 112 and then enters the second Faraday rotator mirror 114; signals reflected by the first faraday rotator 113 and the second faraday rotator 114 return to the 3 × 3 coupler 111 through the port d and the port e, and two paths of signals respectively output from the port b and the port c of the 3 × 3 coupler 111 are transmitted to the data processing system after data are acquired by the signal acquisition module 12, and the data processing system is used for performing phase demodulation on the acquired data and extracting phase information.
Further, as shown in fig. 1, in the present embodiment, the signal modulation module 10 includes a narrow linewidth laser 101, a signal generator 102, an acousto-optic modulator 103, a pulsed optical amplifier 104, an optical circulator 105, a sensing fiber 106, and an erbium-doped fiber amplifier 107; the narrow linewidth laser 101 emits high-coherence laser, which is modulated into pulse light with a pulse width T and an optical frequency f by the acousto-optic modulator 103, amplified by the pulse optical amplifier 104, injected into the sensing fiber 106 through the optical circulator 105, interacted with non-uniform scattering points in the fiber to generate coherent backward rayleigh scattering light, returned to the optical circulator 105, output and amplified by the erbium-doped fiber amplifier 107.
Specifically, in this embodiment, the output signal of the optical circulator 105 is amplified by the erbium-doped fiber amplifier 107, input through the port a of the 3 × 3 coupler 111, and output from the ports d and f respectively and enter the two arms of the unbalanced michelson interferometer, where the half-arm difference of the interferometer is d. Wherein the piezoelectric transducer 112 is disposed between the output port d of the 3 × 3 coupler 111 and the faraday rotary mirror 113 for carrier modulation at a modulation frequency ω0The phase modulation depth C is 2.63. The first faraday rotating mirror 113 and the second faraday rotating mirror 114 eliminate the influence of the birefringence of the fiber on the input and output light polarization states. Two signals respectively reflected by the first faraday rotator 113 and the second faraday rotator 114 interfere with each other inside the 3 × 3 coupler 111, and are output from ports b and c, which perform phase modulation on the reflected light interference signals so that the modulated phases thereof are 120 ° with respect to each other. The ports a, b, c are located on one side of the 3 × 3 coupler 111, and the ports d, e, f are located on the other side of the coupler.
Further, as shown in fig. 1, in the present embodiment, the signal acquisition module 12 includes a first avalanche photodetector 121, a second avalanche photodetector 122, and a dual-channel high-speed data acquisition card 123, wherein input ends of the first avalanche photodetector 121 and the second avalanche photodetector 122 are respectively connected to the port b and the port c of the 3 × 3 coupler 111, and an output end is connected to the dual-channel high-speed data acquisition card 123.
Further, as shown in fig. 2, in this embodiment, the data processing system 13 includes:
the synchronous carrier modulation module 131: the signal processing circuit is used for respectively outputting a base frequency signal and a double frequency signal according to one path of signal;
the synchronous carrier mixing module 132: the frequency mixer is used for respectively mixing the other path of signals with the fundamental frequency signal and the double frequency signal output by the synchronous carrier modulation module 131 to obtain two paths of mixing signals;
the normalization processing module 133: the two channels of frequency mixing signals output by the synchronous carrier frequency mixing module 132 are respectively subjected to low-pass filtering, and then are respectively subjected to normalization processing through optical fiber attenuation coefficients;
a phase information extraction module: and the device is used for calculating the two paths of frequency mixing signals after normalization processing and extracting phase information.
Further, as shown in fig. 2, the synchronous carrier modulation module 131 includes: the first high-pass filter 1311, the first normalization module 1312, the inverse cosine operation module 1313, the second high-pass filter 1314 and the frequency doubling modulation module 1315, wherein one path of signal passes through the first high-pass filter 1311, the first normalization module 1312, the inverse cosine operation module 1313 and the second high-pass filter 1314 and then is divided into two paths, one path of signal directly outputs a baseband signal, and the other path of signal passes through the frequency doubling modulation module 1315 and then outputs a frequency doubling signal.
The synchronous carrier mixing module 132 includes a first mixer 1321 and a second mixer 1322, the first mixer 1321 is configured to mix the other signal with the baseband signal, and the second mixer 1322 is configured to mix the other signal with the frequency-doubled signal; the normalization processing module 133 includes a first low pass filter 1331, a second low pass filter 1332, a second normalization module 1333, and a third normalization module 1334.
The phase information extraction module includes a first differentiator 1341, a second differentiator 1342, a first multiplier 1343, a second multiplier 1344, a subtractor 1345, and an integrator 1351; in the two paths of mixed signals normalized by the normalization processing module 133, the first path of mixed signal is divided into two parts, one part is input to the first multiplier 1343 after passing through the first differentiator 1341, the other part is directly input to the second multiplier 1344, the second path of mixed signal is divided into two parts, one part is input to the second multiplier 1344 after passing through the second differentiator 1342, the other part is directly input to the first multiplier 1343, and output signals of the first multiplier 1343 and the second multiplier 1344 pass through the subtractor 1345, the integrator 1351 and the third high-pass filter 1352, and then phase information is output.
The demodulation principle of the embodiment of the present invention is described below.
The photoelectric detector 121/122 receives the reflected light interference signals of the two ports b/c of the 3X 3 coupler 111 for detection gain amplification, and two paths of output I are obtained by the two-channel high-speed data acquisition card 1231And I2The expression is as follows:
in the formula (I), the compound is shown in the specification,
Irorepresents the photocurrent reflected back at input port f of 3 x 3 coupler 111 at time t, when the interference arm length is s. I isrdRepresents the photocurrent reflected back at input port d of 3 × 3 coupler 111 at time t, when the interference arm is s + d long. a isi，τi，aj，τjRespectively representing the scattering amplitude and the round trip time of the probe pulse at the ith and the jth scattering points, N representing the total number of the scattering points, alpha representing the attenuation coefficient in the optical fiber, c representing the propagation speed of the light wave in vacuum, NfRefractive index, tau, in an optical fiber of laser light representing optical frequency fs＝2snfC represents the round-trip time of the incident light traveling through the fiber over a distance s, τd＝2dnfC represents the round-trip time of the incident light traveling through the fiber at a distance d, T represents the pulse width, C represents the piezoelectric transducer112 phase modulation depth, ω0Representing carrier modulation angular frequency, phis(t) represents the phase information to be measured including the ambient phase noise. D represents the average optical power of the dc component of the signal collected by the high-speed data acquisition card 123,
which is indicative of the magnitude of the alternating current component,
and
the phase angle parameters introduced for the 3 x 3 coupler 111, which are at 120 ° angles to each other, rect () represent square-wave pulse shape functions.
Two paths of output signals with fixed phase difference generated by a 3 x 3 coupler at the rear end of the interferometer are used as input signals of a PGC demodulation algorithm, wherein one path of output signals respectively outputs fundamental frequency and double-frequency mixed frequency signals through a synchronous carrier modulation module 131, and the influence of carrier phase delay is eliminated; the other path of output signal is used as a signal to be demodulated, and the elimination of the light intensity disturbance can be realized by improving the PGC demodulation algorithm, including the synchronous carrier mixing module 132, the normalization processing module 133, the differential cross multiplication module 134 and the phase recovery module 135.
The synchronous carrier modulation module 131 outputs one of the two output signals of the signal acquisition module 12 to the high-pass filter 1311 to filter out the dc component, and the cut-off frequency is determined according to the modulation frequency ω of the piezoelectric transducer 1110Setting, then eliminating the problem of uneven light intensity ratio introduced by the 3 × 3 interference output module 11 through the first normalization module 1312, performing solution operation processing through the inverse cosine operation module 1313, wherein the signal contains phase information to be measured and environmental phase noise introduced by the coherent rayleigh scattering sensing signal modulation module 10, and outputting u-Ccos ω as a fundamental frequency signal after filtering through the high-pass filter 13140t. The baseband output signal may further be passed through a double frequency modulation module 1315, squared, subtracted, and divided to remove C2/2, the frequency doubling signal v ═ Cco can be outputs2ω0t。
The synchronous carrier frequency mixing module 132 eliminates the problem of carrier phase delay caused by an external modulation method and a michelson interference depolarization structure by using the output signals u and v of the synchronous carrier modulation module 131 as frequency mixing signals, and can ensure that the frequency and the phase of the fundamental frequency signal and the double frequency signal of the frequency mixing signals are completely aligned with those of interference signals during multiplication frequency mixing. I is2After mixing with u, v respectively, the obtained fundamental frequency mixing signal w1And a frequency-doubled mixed signal w2The Bessel function expansions of (a) can be expressed as:
in the formula, Jn(C) For the nth order Bessel function,
the introduction of (2) can avoid demodulation failure caused by too small disturbance phase. By the demodulation algorithm, the synchronous carrier recovery based on the phase-sensitive optical time domain reflection system can be realized so as to eliminate the problem of carrier phase delay.
In this embodiment, the normalization processing module 133 is disposed behind the synchronous carrier mixing module 132, so as to solve the problem that when the phase of the phase-sensitive optical time domain reflection system is demodulated by the conventional DCM-PGC algorithm, the stability and accuracy of the demodulation result are affected by the light intensity fluctuation. At the time t, the signal collected by the system is the superposition of all scattering amplitudes within the half pulse width length transmitted by the light in the optical fiber, is related to the position of the optical fiber, has relative stability, and the amplitude is modulated by the attenuation coefficient alpha of the optical fiber, namely A ^ alpha-alpha ct/nf. Therefore, two paths of output signals of the synchronous carrier frequency mixing module (132) are filtered by low-pass filters (1331, 1332) and input into normalization modules (1333, 1334), and an optical fiber attenuation coefficient alpha is considered during normalization, and a specific demodulation algorithm is as follows:
fundamental frequency mixing signal w1And a frequency-doubled mixed signal w2Respectively pass through a low-pass filter (for mixed-frequency multiplication filtering)Signals of first frequency and above, and signals of second frequency and above are filtered when mixing second frequency) to obtain signal w1LPAnd w2LPThe classification can be expressed as:
setting a normalized parameter expression C1And C2Comprises the following steps:
wherein, C1，C2Respectively, the signal w after passing through a low-pass filter1LPAnd w2LPA represents the amplitude of the AC component of the output signal of the interference output module 11, C represents the phase modulation depth of the piezoelectric transducer, J1(C) And J2(C) The Bessel functions of order 1 and 2, respectively.
After normalization, the influence of light intensity fluctuation can be eliminated, and then the normalization processing module 133 processes the two signals w1NormalAnd w2NormalAre respectively:
finally, the extraction of the phase information can be realized by a phase information extraction module, the phase information extraction module includes a differential cross multiplication module 134 and a phase recovery module 135, the differential cross multiplication module 134 includes first and second differentiators 1341, 1342, first and second multipliers 1343, 1344, and a subtractor 1345, and the extraction of the phase information can be realized after the processed data passes through an integrator 1351 and a high pass filter 1352 in the phase recovery module 135, where the expression is:
φsignal(t)＝B cos ωt； (6)
where B denotes the amplitude to be measured and ω denotes the phase angular frequency to be measured.
Example two
An embodiment of the present invention provides a phase demodulation method for a distributed optical fiber sensing system based on a phase generated carrier shown in fig. 1, referring to fig. 2, which mainly includes the following steps:
s1, taking the two signals collected by the signal collection module 12 as input signals of phase demodulation, where one signal is subjected to synchronous carrier modulation, and outputs a fundamental frequency signal and a double frequency signal.
The step S1 specifically includes the following steps:
one path of signals is divided into two paths after high-pass filtering, normalization processing, inverse cosine operation and high-pass filtering are sequentially carried out on the signals, one path of signals is used as base frequency signals to be output, and the other path of signals passes through a double-frequency modulation module 1315 to be output as double-frequency signals.
And S2, mixing the baseband signal and the double frequency mixing signal output in the step S1 with the other path of signal respectively to obtain a baseband mixing signal and a double frequency mixing signal.
And S3, low-pass filtering and normalizing the fundamental frequency mixing signal and the frequency doubling mixing signal respectively.
Wherein, the normalization processing coefficients are respectively:
wherein, C1，C2Respectively a signal fundamental frequency mixing signal w after passing through a low-pass filter1LPAnd a frequency-doubled mixed signal w2LPC represents the propagation velocity of the light wave in vacuum, a represents the attenuation coefficient in the optical fiber, nfThe refractive index of the laser in the fiber, which represents the optical frequency f, and t represents the sampling time of the high speed data acquisition card 123.
And S4, performing phase extraction on the low-pass filtered and normalized fundamental frequency mixing signal and double frequency mixing signal.
In step S4, the specific method for extracting the phase includes:
and respectively differentiating the fundamental frequency mixing signals, mixing the differentiated fundamental frequency mixing signals with the double-frequency mixing signals, outputting the mixed fundamental frequency mixing signals, differentiating the double-frequency mixing signals, mixing the differentiated double-frequency mixing signals with the original fundamental frequency mixing signals, outputting the mixed fundamental frequency mixing signals, and performing subtraction processing, integration processing and high-pass filtering processing on the two paths of output signals to obtain phase information.
In summary, the present invention discloses a PGC-based distributed optical fiber sensing system and a phase demodulation method. The rear end of an interferometer is provided with a 3 x 3 coupler, two paths of output light with fixed phase difference generated by the 3 x 3 coupler are used as input signals of a PGC demodulation algorithm, wherein one path of output light passes through a synchronous carrier modulation module and outputs fundamental frequency and double frequency mixed frequency signals, and the influence of carrier phase delay is eliminated; and the other path of output signal is used as a signal to be demodulated, and elimination of light intensity disturbance is realized by improving a PGC demodulation algorithm comprising a synchronous carrier mixing module, a normalization processing module and a phase information extraction module. In addition, the invention considers that the signal collected by the system is the superposition of all scattering amplitudes within the half pulse width length transmitted by the light in the optical fiber and is related to the position of the optical fiber, the amplitude is modulated by the optical fiber attenuation coefficient alpha, and the signal is normalized by the normalization coefficient related to the optical fiber attenuation coefficient alpha, thereby eliminating the influence of the optical fiber loss on the system.
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.
Claims (4)
1. A phase demodulation method of a phase sensitive optical time domain reflection system based on PGC is characterized by comprising the following steps:
s1, two paths of signals output by the interferometer are subjected to 3 x 3 coupler to generate signals with fixed phase difference as input signals, wherein one path of signals is subjected to synchronous carrier modulation, and a fundamental frequency signal and a double frequency signal are output;
s2, mixing the fundamental frequency signal and the double frequency mixing signal output in the step S1 with the other path of signal respectively to obtain a fundamental frequency mixing signal and a double frequency mixing signal;
s3, performing low-pass filtering on the fundamental frequency mixing signal and the double frequency mixing signal respectively, and performing normalization processing according to the optical fiber attenuation coefficient;
s4, phase extraction is carried out on the fundamental frequency mixing signal and the double frequency mixing signal after the low-pass filtering and normalization processing;
the step S1 specifically includes the following steps:
one path of signals is divided into two paths after high-pass filtering, normalization processing, inverse cosine operation and high-pass filtering are sequentially carried out on the signals, one path of signals is used as fundamental frequency signals to be output, and the other path of signals passes through a double frequency modulation module (1315) and then double frequency signals are output;
in step S4, the specific method for extracting the phase includes:
differentiating the base frequency mixing signal, mixing the differentiated base frequency mixing signal with the original base frequency mixing signal, outputting the mixed signal, differentiating the double frequency mixing signal, mixing the differentiated double frequency mixing signal with the original double frequency mixing signal, outputting the mixed signal, and performing subtraction processing, integration processing and high-pass filtering processing on two paths of output signals to obtain phase information;
in step S3, the coefficients of the normalization process are:
wherein, w1LPAnd w2LPRespectively representing the low-pass filtered fundamental and double frequency mixed signals, C1And C2Respectively, the normalization coefficients of the fundamental frequency mixing signal and the double frequency mixing signal,crepresenting light waves in vacuumThe speed of propagation of the beam of light,αwhich represents the attenuation coefficient of the optical fiber,n f representing optical frequenciesfThe refractive index of the laser in the optical fiber, and t represents the sampling time of the high-speed data acquisition card.
2. A PGC-based phase-sensitive optical time domain reflectometry system for implementing a phase demodulation method according to claim 1, comprising a signal modulation module (10), an interference output module (11), a signal acquisition module (12) and a data processing system (13);
the interference output module (11) comprises a 3 x 3 coupler (111), a piezoelectric transducer (112), a first Faraday rotator mirror (113) and a second Faraday rotator mirror (114);
coherent Rayleigh scattered light output by the signal modulation module (10) enters a port a of the 3 multiplied by 3 coupler (111), and two paths of signals are output through a port d and a port e, wherein one path of signals enters the first Faraday rotating mirror (113); the other path of the signal is subjected to carrier modulation by a piezoelectric transducer (112) and then enters a second Faraday rotating mirror (114); signals reflected by the first Faraday rotator mirror (113) and the second Faraday rotator mirror (114) return to the 3 × 3 coupler (111) through the port d and the port e to form an interferometer, two paths of signals respectively output from the port b and the port c of the 3 × 3 coupler (111) are transmitted to a data processing system after data are acquired by the signal acquisition module (12), and the data processing system is used for performing phase demodulation on the acquired data and extracting phase information;
the data processing system (13) comprises:
synchronous carrier modulation module (131): the signal processing circuit is used for respectively outputting a base frequency signal and a double frequency signal according to one path of signal;
synchronous carrier mixing module (132): the frequency mixer is used for respectively mixing the other path of signals with a fundamental frequency signal and a double frequency signal output by the synchronous carrier modulation module (131) to obtain two paths of mixing signals;
normalization processing module (133): the low-pass filtering module is used for performing low-pass filtering on the two paths of frequency mixing signals output by the synchronous carrier frequency mixing module (132) respectively and performing normalization processing on the two paths of frequency mixing signals according to the optical fiber attenuation coefficient;
a phase information extraction module: the device is used for calculating the two paths of frequency mixing signals after normalization processing and extracting phase information;
the synchronous carrier modulation module (131) comprises: the device comprises a first high-pass filter (1311), a first normalization module (1312), an arcsine operation module (1313), a second high-pass filter (1314) and a double-frequency modulation module (1315), wherein one path of signals are divided into two paths after passing through the first high-pass filter (1311), the first normalization module (1312), the arccosine operation module (1313) and the second high-pass filter (1314), one path of signals directly outputs fundamental frequency signals, and the other path of signals outputs double-frequency signals after passing through the double-frequency modulation module (1315);
the synchronous carrier mixing module (132) comprises a first mixer (1321) and a second mixer (1322), wherein the first mixer (1321) is used for mixing the other signal with a baseband signal, and the second mixer (1322) is used for mixing the other signal with a frequency doubling signal;
-the normalization processing module (133) comprises a first low-pass filter (1331), a second low-pass filter (1332), a second normalization module (1333) and a third normalization module (1334);
the phase information extraction module comprises a first differentiator (1341), a second differentiator (1342), a first multiplier (1343), a second multiplier (1344), a subtracter (1345) and an integrator (1351); in the two paths of mixed signals normalized by the normalization processing module (133), the first path of mixed signal is divided into two parts, one part is input into a first multiplier (1343) after passing through a first differentiator (1341), the other part is directly input into a second multiplier (1344), the second path of mixed signal is divided into two parts, one part is input into the second multiplier (1344) after passing through a second differentiator (1342), the other part is directly input into the first multiplier (1343), and output signals of the first multiplier (1343) and the second multiplier (1344) pass through a subtracter (1345), an integrator (1351) and a third high-pass filter (1352) and then phase information is output.
3. The PGC-based phase sensitive optical time domain reflectometry system according to claim 2, wherein the signal modulation module (10) comprises a narrow linewidth laser (101), a signal generator (102), an acousto-optic modulator (103), a pulsed optical amplifier (104), an optical circulator (105), a sensing fiber (106) and an erbium-doped fiber amplifier (107);
the narrow linewidth laser (101) emits high-coherence laser, and the high-coherence laser is modulated into pulse width by the acousto-optic modulator (103)TOptical frequency offThe pulse light is amplified by a pulse light amplifier (104), injected into a sensing optical fiber (106) through an optical circulator (105), interacted with a non-uniform scattering point in the optical fiber to generate coherent backward Rayleigh scattering light, returned to the optical circulator (105), output and amplified and output by an erbium-doped optical fiber amplifier (107).
4. The PGC-based phase sensitive optical time domain reflectometry system according to claim 2, wherein the signal acquisition module (12) comprises a first avalanche photodetector (121), a second avalanche photodetector (122) and a dual channel high speed data acquisition card (123), the input terminals of the first avalanche photodetector (121) and the second avalanche photodetector (122) are respectively connected to the port b and the port c of the 3 x 3 coupler (111), and the output terminal is connected to the dual channel high speed data acquisition card (123).
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