CN110108346B - Optical fiber vibration sensor based on delay phase modulation chirp pulse pair - Google Patents
Optical fiber vibration sensor based on delay phase modulation chirp pulse pair Download PDFInfo
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- CN110108346B CN110108346B CN201910323418.9A CN201910323418A CN110108346B CN 110108346 B CN110108346 B CN 110108346B CN 201910323418 A CN201910323418 A CN 201910323418A CN 110108346 B CN110108346 B CN 110108346B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
- G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
- G01H9/004—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/09—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on magneto-optical elements, e.g. exhibiting Faraday effect
- G02F1/095—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on magneto-optical elements, e.g. exhibiting Faraday effect in an optical waveguide structure
- G02F1/0955—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on magneto-optical elements, e.g. exhibiting Faraday effect in an optical waveguide structure used as non-reciprocal devices, e.g. optical isolators, circulators
Abstract
A high-performance optical fiber vibration sensor based on a delay phase modulation chirp pulse pair comprises a narrow-linewidth laser, a first optical fiber coupler, an electro-optic modulator, an acousto-optic modulator, an optical amplifier, a second optical fiber coupler, a delay optical fiber, an optical fiber expansion device, a first Faraday rotating mirror, a second Faraday rotating mirror, a circulator, an optical fiber to be detected, a third optical fiber coupler, a first polarization controller, a fourth optical fiber coupler, a second polarization controller, a fifth optical fiber coupler, a sixth optical fiber coupler, a first double-balance detector, a second double-balance detector, a first analog-to-digital converter, a second analog-to-digital converter, a first digital signal processing unit, a second digital signal processing unit and an arbitrary waveform generator. The invention can solve the contradiction relation between the spatial resolution and the sensing distance in the traditional sensing system based on the time domain reflectometer, can solve the interference fading problem in the interference sensing system, and can simultaneously realize the indexes of long sensing distance, high spatial resolution, high signal-to-noise ratio and the like.
Description
Technical Field
The invention relates to the technical field of distributed optical fiber sensing, in particular to a high-performance optical fiber vibration sensor based on a delay phase modulation chirp pulse pair.
Background
The optical fiber sensor has the advantages of strong anti-electromagnetic interference capability, non-invasiveness, easy realization of remote monitoring of a detected signal, corrosion resistance, explosion resistance, flexibility of an optical path, convenience in connection with an optical fiber system and the like. In recent years, the method is widely applied to the fields of natural gas and petroleum pipeline safety monitoring, bridge crack monitoring, gas concentration detection, boundary security and the like.
Phase sensitive optical time domain reflectometer (Φ -OTDR) is a novel distributed optical fiber acoustic sensing technology (DAS), and distributed dynamic detection can be realized by using backward rayleigh scattering in the optical fiber. The main performance indicators include sensing distance, spatial resolution, detection bandwidth, sensitivity, signal-to-noise ratio (SNR), and the like. phi-OTDR has attracted extensive attention from foreign and domestic researchers and made many advances since 1993 [ F.Tarlor H., E.Lee C.apparatus and method for fiber optic intervention sending: US,5194847[ P/OL ].1993-3-16 ]. For example, the signal-to-noise ratio [ Lu Yuelan, Zhu Tao, Chen Liang, et al distributed sensing Sensor Based on Coherent Detection of Phase-OTDR [ J ]. J Lightwave technique, 2010,28(22):3243-3249 ] is improved by time-series multi-frequency sources, bandwidth [ Wang Z.Y. ], Pan Z.Q., et al.ultra-branched Phase-sensitive optical time-domain reflection with a temporal sequence of multi-frequency sources, 40(22): 5192-5195) is realized by quantitative Detection of Ivyuelan, Zhu Tao, Chen Liang, et al.S. Detection of Phase-OTDR [ J.Op, Pal.S.P.: 15. distribution of Phase-sensing [ J.S. P.S.: fifth, N.S. P.S. N.S. P.S. N. P.S. sub.S. N. P.S. M. sub.S. M. S. M. sub.S. S. M. A. M. sub.S. M. Ser. No. 3, M. 3, M. sub.S. M., liang k.z., Ye q., et al.phase-sensitive OTDR system based on digital coherent detection [ M ].2011Asia Communications and Photonics Conference and inhibition.new York; ieee.2012 ], solves the interference fading problem by Phase modulating pulses [ Xiao Wang, Bin Lu, et al, interference-fading-free Φ -OTDR Based on Differential Phase Shift Pulsing Technology [ J ]. IEEE Photonics Technology Letters,2019,31(1), 1041-channel 1135 ], etc.
The above documents respectively optimize certain indexes of the phi-OTDR and obtain certain results. However, many indexes in the Φ -OTDR are mutually restricted and mutually influenced, the improvement of a single index often leads to the reduction of other indexes, the performance of the system cannot be comprehensively improved, and the indexes such as spatial resolution, sensitivity and the like in the current technology need to be further improved. For example, sensitivity and spatial resolution have a mutually restrictive, conflicting relationship with sensing distance. When the sensing distance is more than ten kilometers, the sensing spatial resolution can only reach several meters to dozens of meters, the sensitivity is about the magnitude of nano strain, the spatial resolution cannot meet the application requirement of smaller-scale events, and the sensitivity needs to be further improved so as to meet the detection of weak signals.
In the document [ Bin Lu, Zhengqing Pan, et al, high spatial resolution phase-sensitive optical time domain reflectometer with a frequency-swept pulse [ J ]. Opt Lett,2017,42(3):391 and 394 ], it is proposed that the contradictory relationship between sensing distance and spatial resolution can be solved by using the frequency-swept pulse technology, and sub-meter-scale spatial resolution can be realized, but the interference fading problem existing in the system is not solved at the same time.
Disclosure of Invention
The invention aims to overcome the relationship of mutual restriction and mutual contradiction between the sensitivity and the spatial resolution of the conventional optical fiber sensing system and the sensing distance, solve the problem of interference fading in the system, improve multiple key indexes of phi-OTDR and construct a high-performance phi-OTDR sensing system.
The core idea of the invention is as follows: the optical signal injected into the optical fiber is a chirp pulse pair with a certain delay, and matched filtering processing is carried out in the digital signal processing module, so that two chirp pulses can be compressed into two narrow pulses with a certain delay. The pulse width of each narrow pulse is inversely proportional to the swept frequency range of the chirped pulse, regardless of the pulse width of the transmitted chirped pulse. At this point, the spatial resolution of the sensing system is determined by the two narrow pulse widths and the delay between the two pulses after compression. By performing phase modulation of 0 or pi on the rear pulse, the interference pattern obtained by odd and even times of measurement is changed when the phase modulation amount of the odd and even pulse pairs is different. In the odd and even times of measurement results, interference fading cannot occur at the same position, the measurement results are screened, and the influence of interference fading can be removed by selecting the measurement result with high signal intensity. Therefore, the technology can break through the relation between the spatial resolution and the pulse width in the traditional phi-OTDR, increase the pulse width while improving the spatial resolution, improve the sensing distance, solve the problem of interference fading simultaneously and comprehensively improve a plurality of key indexes of the system.
The technical solution of the invention is as follows:
a high-performance optical fiber vibration sensor based on a delay phase modulation chirp pulse pair comprises a narrow line width laser, a first optical fiber coupler, an electro-optic modulator, an acousto-optic modulator, an optical amplifier, a second optical fiber coupler, a delay optical fiber, an optical fiber expansion device, a first Faraday rotating mirror, a second Faraday rotating mirror, a circulator, an optical fiber to be detected, a third optical fiber coupler, a first polarization controller, a fourth optical fiber coupler, a second polarization controller, a fifth optical fiber coupler, a sixth optical fiber coupler, a first double-balance detector, a second double-balance detector, a first analog-to-digital converter, a second analog-to-digital converter, a first digital signal processing unit, a second digital signal processing unit and an arbitrary waveform generator, wherein an output port of the narrow line width laser is connected with an input port of the first optical fiber coupler, a first output port of the first optical fiber coupler is connected with an optical signal input port of the electro-optic modulator, the second output port of the first optical fiber coupler is connected with the input port of the fourth optical fiber coupler, the optical signal output port of the electro-optical modulator is connected with the optical signal input port of the acousto-optic modulator, the optical signal output port of the acousto-optic modulator is connected with the input port of the optical amplifier, the output port of the optical amplifier is connected with the first optical port of the second optical fiber coupler, the third port of the second optical fiber coupler is connected with the first port of the delay optical fiber, the second port of the delay optical fiber is connected with the optical input port of the optical fiber expander, the optical output port of the optical fiber expander is connected with the first Faraday rotator mirror, the fourth port of the second optical fiber coupler is connected with the second Faraday rotator mirror, and the second port of the second optical fiber coupler is connected with the first optical port of the circulator, the second optical port of the circulator is connected with an optical fiber to be tested, the third optical port of the circulator is connected with the optical input port of the third optical fiber coupler, the first output port of the third optical fiber coupler is connected with the input port of the first polarization controller, the output port of the first polarization controller is connected with the first input port of the sixth optical fiber coupler, the second output port of the third optical fiber coupler is connected with the first input port of the fifth optical fiber coupler, the first output port of the fourth optical fiber coupler is connected with the second input port of the sixth optical fiber coupler, the second output port of the fourth optical fiber coupler is connected with the input port of the second polarization controller, and the output port of the second polarization controller is connected with the second input port of the fifth optical fiber coupler, two output ports of the fifth optical fiber coupler are connected with two input ports of the first double balanced detector, two output ports of the sixth optical fiber coupler are connected with two input ports of the second double balanced detector, an output port of the second double balanced detector is connected with an input port of the second analog-to-digital converter, an output port of the second analog-to-digital converter is connected with a data input port of the second digital signal processing unit, an output port of the first double balanced detector is connected with an input port of the first analog-to-digital converter, an output port of the first analog-to-digital converter is connected with a data input port of the first digital signal processing unit, a sweep frequency signal a generated by the arbitrary waveform generator is input to an electrical input port of the electro-optic modulator, and a pulse signal c generated by the arbitrary waveform generator is input to a mode control port of the acousto-optic modulator And the arbitrary waveform generator generates a step voltage signal b and is connected with the electrical control signal input port of the optical fiber expansion piece.
The detection signal injected into the optical fiber to be detected through the circulator is two frequency chirp pulses, a certain delay exists between the two pulses, the rear pulse is subjected to phase modulation, the phase modulation amount of the rear pulse in the frequency chirp pulse pair transmitted by odd-even times is different, and the odd-even phase modulation amount is 0 and pi respectively. The frequency-chirped pulse signal injected into the fiber can be represented by the following equation:
Ep＝E0rect(t/T)exp(j2πfct+jπKt2)+E0rect((t-τ)/T)exp(j2πfc(t-τ)+jπK(t-τ)2+jφm) Where T is the pulse width, fcIs the carrier frequency, K is the chirp rate of the LFM pulse, rect (T/T) is a rectangular function, τ is the delay between two pulses, φmFor the phase modulation amount, the transmission odd pulse pair time is 0, and the transmission even pulse pair time is pi.
The first digital signal processing unit and the second digital signal processing unit are similar in structure and mainly comprise a matched filter, a quadrature phase demodulation module and a low-pass filter. Wherein the matched filter is designed according to the transmitted frequency chirped pulse pair. Can be expressed as:
the digital signal processing unit output signals are amplitude and phase information at each demodulated position.
After the first digital signal processing unit and the second digital signal processing unit demodulate the amplitude and phase information of each position: a. amplitude signals obtained by odd-number measurement and amplitude information obtained by even-number measurement at the same position can be compared, the amplitude minimum value is achieved because the odd-number measurement and the even-number measurement cannot simultaneously generate interference fading, phases demodulated by the odd-number (or even-number) measurement with larger amplitude are selected as effective data, and the effective data are spliced with phase signals demodulated at the last moment; b. besides the scheme of optimally selecting and splicing the signals obtained by demodulation, the signals obtained by odd-even times measurement can be combined and superposed.
The narrow linewidth laser is a narrow linewidth optical fiber laser or a semiconductor laser, the central wavelength is 1550nm, the linewidth is about 2.5kHz, and other kinds of narrow linewidth lasers can also be adopted.
The first optical fiber coupler is a polarization maintaining coupler with 1550nm waveband, 1 multiplied by 2 of port and 9:1 of splitting ratio, and optical fiber couplers with slightly different splitting ratios can also be adopted.
The second optical fiber coupler, the third optical fiber coupler, the fourth optical fiber coupler, the fifth optical fiber coupler and the sixth optical fiber coupler are common single-mode optical fiber couplers, and the splitting ratio is 1: 1.
the electro-optical modulator is an intensity modulator with a high bandwidth, suppresses carrier waves by adjusting direct-current voltage, and performs frequency modulation by adjusting radio-frequency signals loaded on the electro-optical modulator.
The acousto-optic modulator makes the local oscillation light and the detection light generate frequency difference of dozens of MHz, and chops the continuous light to generate light pulse.
The optical fiber expansion device is used for generating 0 or pi phase shift quantity by winding an optical fiber on a piezoelectric ceramic tube or a phase modulator.
The optical fiber circulator is a three-port optical fiber circulator, and a method of accessing an optical fiber coupler and an isolator can be adopted to achieve the effect equivalent to that of the optical fiber circulator.
The double-balance detector is a high-speed detector which converts optical signals into electric signals, can filter out direct current components and reserves alternating current components.
In a high-performance optical fiber vibration sensing system based on a delay phase modulation chirp pulse pair, a receiving end adopts polarization division receiving, the polarization state is controlled by a first polarization controller and a second polarization controller, and two optical signals in different polarization states are respectively received by a first double-balanced detector and a second double-balanced detector, so that the influence of polarization fading on the signal-to-noise ratio of the system can be eliminated.
The invention has the advantages that:
1. the delay phase modulation chirped pulse is used as a detection pulse, and the spatial resolution is inversely proportional to the sweep frequency range and is independent of the pulse width. By increasing the sweep range, sub-meter spatial resolution can be obtained without sacrificing sensing distance.
2. The problem of interference fading can be solved by adjusting the phase modulation amount of the pulse after the odd-even number measurement, changing the scattering pattern obtained by the odd-even number measurement, and optimally selecting the signal with higher signal intensity or carrying out vector combination at different positions.
3. The receiving end adopts polarization division receiving, so that the influence caused by polarization fading can be greatly reduced.
Drawings
FIG. 1 is a block diagram of an optical fiber vibration sensing structure based on a delayed phase modulation chirped pulse pair according to the present invention
FIG. 2 is a block diagram of a digital signal processing unit
FIG. 3 shows an embodiment of generating delayed phase-modulated chirped pulse pairs
FIG. 4 shows a second embodiment of generating delayed phase-modulated chirped pulse pairs
Detailed Description
The present invention will be further described with reference to the following examples and drawings, but the scope of the present invention should not be limited thereto.
Referring to fig. 1, fig. 1 is a block diagram of an overall structure of an optical fiber vibration sensor based on a delay phase modulation chirped pulse pair according to the present invention. As shown in fig. 1, the high-performance optical fiber vibration sensing device based on the delayed phase modulation chirped pulse pair of the present invention includes a narrow-linewidth laser 1, a first optical fiber coupler 2, an electro-optic modulator 3, an acousto-optic modulator 4, an optical amplifier 5, a second optical fiber coupler 6, a delay optical fiber 7, an optical fiber expander 8, a first faraday rotator mirror 9, a second faraday rotator mirror 10, a circulator 11, a third optical fiber coupler 13, a first polarization controller 14, a fourth optical fiber coupler 15, a second polarization controller 16, a fifth optical fiber coupler 17, a sixth optical fiber coupler 18, a first double-balanced detector 19, a second double-balanced detector 20, a first analog-to-digital converter 21, a second analog-to-digital converter 22, a first digital signal processing unit 23, a second digital signal processing unit 24, and an arbitrary waveform generator 25, wherein an output port of the narrow-linewidth laser 1 is connected to an input port of the first optical fiber coupler 2, the first output port of the first optical fiber coupler 2 is connected to the optical signal input port of the electro-optical modulator 3, the second output port of the first optical fiber coupler 2 is connected to the input port of the fourth optical fiber coupler 15, the optical signal output port of the electro-optical modulator 3 is connected to the optical signal input port of the acousto-optic modulator 4, the optical signal output port of the acousto-optic modulator 4 is connected to the input port of the optical amplifier 5, the output port of the optical amplifier 5 is connected to the first optical port of the second optical fiber coupler 6, the third port of the second optical fiber coupler 6 is connected to the first port of the delay optical fiber 7, the second port of the delay optical fiber 7 is connected to the optical input port of the optical fiber expander 8, the optical output port of the optical fiber expander 8 is connected to the first faraday rotator 9, the fourth port of the second optical fiber coupler 6 is connected to the second faraday rotator mirror 10, the second port of the second optical fiber coupler 6 is connected to the first optical port of the circulator 11, the second optical port of the circulator 11 is connected to the optical fiber 12 to be measured, the third optical port of the circulator 11 is connected to the optical input port of the third optical fiber coupler 13, the first output port of the third optical fiber coupler 13 is connected to the input port of the first polarization controller 14, the output port of the first polarization controller 14 is connected to the first input port of the sixth optical fiber coupler 18, the second output port of the third optical fiber coupler 13 is connected to the first input port of the fifth optical fiber coupler 17, the first output port of the fourth optical fiber coupler 15 is connected to the second input port of the sixth optical fiber coupler 18, a second output port of the fourth optical fiber coupler 15 is connected to an input port of the second polarization controller 16, an output port of the second polarization controller 16 is connected to a second input port of the fifth optical fiber coupler 17, two output ports of the fifth optical fiber coupler 17 are connected to two input ports of the first double balanced detector 19, two output ports of the sixth optical fiber coupler 18 are connected to two input ports of the second double balanced detector 20, an output port of the second double balanced detector 20 is connected to an input port of the second analog-to-digital converter 22, an output port of the second analog-to-digital converter 22 is connected to a data input port of the second digital signal processing unit 24, an output port of the first double balanced detector 19 is connected to an input port of the first analog-to-digital converter 21, an output port of the first analog-to-digital converter 21 is connected to a data input port of the first digital signal processing unit 23, a sweep frequency signal a generated by the arbitrary waveform generator 25 is input to an electrical input port of the electro-optical modulator 3, a pulse signal c generated by the arbitrary waveform generator 25 is input to a mode control input port of the acousto-optical modulator 4, and the arbitrary waveform generator 25 generates a step voltage signal b and is connected to an electrical control signal input port of the optical fiber expander 8.
The detection signal injected into the optical fiber to be detected through the circulator is two frequency chirp pulses, a certain delay exists between the two pulses, the rear pulse is subjected to phase modulation, the phase modulation amount of the rear pulse in the frequency chirp pulse pair transmitted by odd-even times is different, and the odd-even phase modulation amount is 0 and pi respectively.
The frequency-chirped pulse signal injected into the fiber can be represented by the following equation:
Ep＝E0rect(t/T)exp(j2πfct+jπKt2)+E0rect((t-τ)/T)exp(j2πfc(t-τ)+jπK(t-τ)2+jφm) Where T is the pulse width, fcIs the carrier frequency, K is the chirp rate of the LFM pulse, rect (T/T) is a rectangular function, τ is the delay between two pulses, φmFor the phase modulation amount, the transmission odd pulse pair time is 0, and the transmission even pulse pair time is pi.
The first digital signal processing unit 23 and the second digital signal processing unit 24 are similar in structure, and as shown in fig. 2, mainly include a matched filter D1, a quadrature phase demodulation module D2, and a low pass filter D3. Wherein the matched filter is designed according to the transmitted frequency chirped pulse pair. Can be expressed as:
the digital signal processing unit output signals are amplitude and phase information at each demodulated position.
After the first digital signal processing unit 23 and the second digital signal processing unit 24 demodulate the amplitude and phase information at each position: a. amplitude signals obtained by odd-number measurement and amplitude information obtained by even-number measurement at the same position can be compared, the amplitude minimum value is achieved because the odd-number measurement and the even-number measurement cannot simultaneously generate interference fading, phases demodulated by the odd-number (or even-number) measurement with larger amplitude are selected as effective data, and the effective data are spliced with phase signals demodulated at the last moment; b. besides the scheme of optimally selecting and splicing the signals obtained by demodulation, the signals obtained by odd-even times measurement can be combined and superposed.
The basic principle of the device of the invention is as follows:
the optical signal output by the first narrow linewidth laser is divided into detection light and local oscillator light through the first optical fiber coupler, the detection light passes through the electro-optic modulator, and the chirp radio frequency signal drives the electro-optic modulator to modulate the input continuous laser.
Secondly, the modulated optical signal generates frequency shift through an acousto-optic modulator, and is chopped into optical pulses to generate frequency chirp pulses: ep0＝E0rect(t/T)exp(j2πfct+jπKt2)。
Thirdly, after the frequency chirp pulse is optically amplified, the frequency chirp pulse passes through a Michelson interferometer with a certain arm length difference to generate a delayed frequency chirp optical pulse pair, the post chirp optical pulse is subjected to phase modulation of 0 or pi through an optical fiber expander or a phase modulator, the phase modulation amounts of two adjacent measurements are different, and then, a signal of the delayed phase modulation frequency chirp optical pulse pair is injected into an optical fiber to be measured through a circulator:
Ep＝E0rect(t/T)exp(j2πfct+jπKt2)+E0rect((t-τ)/T)exp(j2πfc(t-τ)+jπK(t-τ)2+jφm). Where T is the pulse width, fcIs the carrier frequency, K is the chirp rate of the LFM pulse, rect (T/T) is a rectangular function, τ is the delay between two pulses, φmIs the amount of phase modulation.
Fourthly, the optical signal scattered back from the optical fiber and the light after frequency shift respectively pass through the first polarization controller and the second polarization controller, and heterodyne polarization reception is ensured by using a double-balanced detector. Due to polarization fading, the amplitude of the signal reflected at certain time and the amplitude of the local oscillator optical beat signal are close to zero, so that phase demodulation errors occur, and the influence of polarization fading can be greatly reduced by polarization division reception.
And fifthly, after the electric signals output by the double-balanced detector are converted into digital signals through an analog-to-digital converter, the digital signals are accessed to a digital signal processing unit for performing matched filtering processing and quadrature phase demodulation. Wherein, the matched filter is:
after the first digital signal processing unit 23 and the second digital signal processing unit 24 demodulate the amplitude and phase information at each position: a. amplitude signals obtained by odd-number measurement and amplitude information obtained by even-number measurement at the same position can be compared, the amplitude minimum value is achieved because the odd-number measurement and the even-number measurement cannot simultaneously generate interference fading, phases demodulated by the odd-number (or even-number) measurement with larger amplitude are selected as effective data, and the effective data are spliced with phase signals demodulated at the last moment; b. besides the scheme of optimally selecting and splicing the signals obtained by demodulation, the signals obtained by odd-even times measurement can be combined and superposed. The influence of interference fading can be eliminated by optimal selection or vector time combination.
The core of the present invention is to transmit a delay phase modulation chirped pulse pair as sensing light, and perform matched filtering and other processing on the received signal, and the manner of generating the delay phase modulation chirped pulse pair in the above embodiment, i.e., the first embodiment, as shown in fig. 3, in practical applications, there are many similar embodiments that can generate the delay phase modulation chirped pulse pair.
The second embodiment of the present invention for generating a delay phase modulation chirped pulse pair is as shown in fig. 4, and directly adopts electro-optic modulation to generate a delay phase modulation chirped signal, and generates a pulse pair through chopping by an acousto-optic modulator, and then amplifies the pulse pair by an optical amplifier, and the structure of the present invention includes an arbitrary waveform generator 2_1, an electro-optic modulator 2_2, an acousto-optic modulator 2_3, an optical amplifier 2_4, and the like, wherein an optical input port of the electro-optic modulator 2_2 is connected to a first output port of a first optical fiber coupler 2 in fig. 1, an optical output port of the electro-optic modulator 2_2 is connected to an optical input port of the acousto-optic modulator 2_3, an optical output port of the acousto-optic modulator 2_3 is connected to an optical input port of the optical amplifier 2_4, and an optical output port of the optical amplifier 2_4 outputs the delay phase modulation chirped, and is connected to the first optical port of the circulator 11 shown in fig. 1, the radio frequency chirp signal 2_ a generated by the arbitrary waveform generator 2_1 is connected to the radio frequency input port of the electro-optic modulator 2_2, the radio frequency chirp signal 2_ a includes two radio frequency chirp signals with a certain delay, and performs periodic phase modulation of 0 or pi on the following chirp signals, and the pulse signal 2_ b generated by the arbitrary waveform generator 2_1 is connected to the radio frequency signal input port of the acousto-optic modulator 2_3, and chops the continuous delay phase modulation chirp optical signal to generate a delay phase modulation chirp pair signal.
When the two measurements are carried out in the invention, the modulation quantity of the rear pulse phase of the chirp pulse pair is respectively 0 or pi in the odd-even number measurement, and then the best selection or vector combination can be carried out on the odd-even number measurement result. In practical implementation, n times of chirped pulse pair transmission can be performed before and after, the phase modulation amount of the pulse pair after pulse pair transmission is respectively 0, pi/n … … and pi (n-1)/n during each measurement, and then the best selection or vector synthesis is performed on the n times of measurement results, and when n is larger, the vector synthesis effect is better.
The technical scheme can realize the high-performance optical fiber vibration sensor based on the delay phase modulation chirped pulse pair. While the invention has been described in detail and with reference to specific embodiments thereof, it will be understood that the invention is not limited to the disclosed embodiments and examples, but is capable of various modifications in form and detail as would be apparent to those skilled in the art. For example, the operating band of the laser may be replaced with other bands; the circulator may be replaced by a coupler and the interferometer producing the delayed phase modulated chirped pulse pair of figure 3 may have the michelson interferometer replaced by a mach-zehnder interferometer; the front pulse of the chirp pulse pair is phase-modulated, and the phase of the rear pulse is not changed. It should be understood that the above-mentioned embodiments are merely exemplary of the present invention, and are not intended to limit the present invention, and any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (7)
1. The optical fiber vibration sensor based on the delay phase modulation chirped pulse pair is characterized by comprising a narrow-linewidth laser (1), a first optical fiber coupler (2), an electro-optic modulator (3), an acousto-optic modulator (4), an optical amplifier (5), a second optical fiber coupler (6), a delay optical fiber (7), an optical fiber expansion piece (8), a first Faraday rotating mirror (9), a second Faraday rotating mirror (10), a circulator (11), a third optical fiber coupler (13), a first polarization controller (14), a fourth optical fiber coupler (15), a second polarization controller (16), a fifth optical fiber coupler (17), a sixth optical fiber coupler (18), a first double-balanced detector (19), a second double-balanced detector (20), a first analog-to-digital converter (21), a second analog-to-digital converter (22), a first digital signal processing unit (23), A second digital signal processing unit (24) and an arbitrary waveform generator (25);
the Michelson interferometer consists of the second optical fiber coupler (6), the delay optical fiber (7), the optical fiber expansion piece (8), the first Faraday rotator mirror (9) and the second Faraday rotator mirror (10);
the output port of the narrow linewidth laser (1) is connected with the input port of the first optical fiber coupler (2), the first output port of the first optical fiber coupler (2) is connected with the optical signal input port of the electro-optical modulator (3), the second output port of the first optical fiber coupler (2) is connected with the input port of the fourth optical fiber coupler (15), the optical signal output port of the electro-optical modulator (3) is connected with the optical signal input port of the acousto-optic modulator (4), the optical signal output port of the acousto-optic modulator (4) is connected with the input port of the optical amplifier (5), the output port of the optical amplifier (5) is connected with the first port of the second optical fiber coupler (6), the third port of the second optical fiber coupler (6) is connected with the first port of the delay optical fiber (7), the second port of the delay optical fiber (7) is connected with the optical input port of an optical fiber expander (8), the optical output port of the optical fiber expander (8) is connected with the first Faraday rotator mirror (9), the fourth port of the second optical fiber coupler (6) is connected with the second Faraday rotator mirror (10), the second port of the second optical fiber coupler (6) is connected with the first optical port of the circulator (11), the second optical port of the circulator (11) is connected with the optical fiber (12) to be tested, the third optical port of the circulator (11) is connected with the optical input port of the third optical fiber coupler (13), the first output port of the third optical fiber coupler (13) is connected with the input port of the first polarization controller (14), the output port of the first polarization controller (14) is connected with the first input port of the sixth optical fiber coupler (18), a second output port of the third optical fiber coupler (13) is connected to a first input port of the fifth optical fiber coupler (17), a first output port of the fourth optical fiber coupler (15) is connected to a second input port of the sixth optical fiber coupler (18), a second output port of the fourth optical fiber coupler (15) is connected to an input port of the second polarization controller (16), an output port of the second polarization controller (16) is connected to a second input port of the fifth optical fiber coupler (17), two output ports of the fifth optical fiber coupler (17) are respectively connected to two input ports of the first double balanced detector (19), two output ports of the sixth optical fiber coupler (18) are respectively connected to two input ports of the second double balanced detector (20), the output port of the second double balanced detector (20) is connected with the input port of the second analog-to-digital converter (22), the output port of the second analog-to-digital converter (22) is connected with the data input port of the second digital signal processing unit (24), the output port of the first double balanced detector (19) is connected with the input port of the first analog-to-digital converter (21), the output port of the first analog-to-digital converter (21) is connected with the data input port of the first digital signal processing unit (23), the sweep frequency signal (a) generated by the arbitrary waveform generator (25) is input to the electrical input port of the electro-optic modulator (3), the pulse signal (c) generated by the arbitrary waveform generator (25) is input to the mode control input port of the acousto-optic modulator (4), the arbitrary waveform generator (25) generates a step voltage signal (b) which is connected with an electrical control signal input port of the optical fiber expansion piece (8);
chopping a chirped continuous optical signal output by the electro-optic modulator (3) into a chirped pulse optical signal, amplifying the chirped pulse optical signal by the optical amplifier (5), and injecting the chirped pulse optical signal into the Michelson interferometer to generate a delayed-frequency chirped optical pulse pair;
the detection signal injected into the optical fiber (12) to be detected is two frequency chirp pulses, a certain delay exists between the two frequency chirp pulses, the latter frequency chirp pulse is subjected to phase modulation, the phase modulation amount of the frequency chirp pulse pair emitted by odd-even times to the latter frequency chirp pulse is different, and the odd-even phase modulation amount is 0 and pi respectively;
the frequency chirp pulse signal is expressed as follows:
Ep＝E0rect(t/T)exp(j2πfct+jπKt2)+E0rect((t-τ)/T)exp(j2πfc(t-τ)+jπK(t-τ)2+jφm)，
where T is the pulse width, fcIs the carrier frequency, K is the chirp rate, rect (T/T) is a rectangular function, τ is the delay between two pulses, φmAs phase modulation amount, the time-to-phase modulation amount of the transmitted odd pulse is 0, and the time-to-phase modulation amount of the transmitted even pulse is pi, EpFor chirped pulses to the optical field, E0Amplitude of the light field of the chirped pulse, tIs the time;
the first digital signal processing unit (23) and the second digital signal processing unit (24) both comprise a matched filter (D1), a quadrature phase demodulation module (D2) and a low-pass filter (D3);
the matched filter (D1) is designed based on the transmitted frequency chirped pulse pair and is expressed as:
2. the fiber vibration sensor according to claim 1, wherein the output signals of the first digital signal processing unit (23) and the second digital signal processing unit (24) are amplitude and phase information at each demodulated position.
3. The fiber vibration sensor according to claim 2, wherein the first digital signal processing unit (23) and the second digital signal processing unit (24) demodulate the amplitude and phase information at each position: a. amplitude signals obtained by odd-number measurement and amplitude information obtained by even-number measurement at the same position can be compared, the amplitude minimum value is reached because the odd-number measurement and the even-number measurement cannot simultaneously generate interference fading, phases demodulated by the odd-number measurement or the even-number measurement with larger amplitude are selected as effective data, and are spliced with phase signals demodulated at the last moment; b. besides the scheme of optimally selecting and splicing the signals obtained by demodulation, the signals obtained by odd-even times measurement can be combined and superposed.
4. The fiber optic vibration sensor according to claim 1, wherein said narrow linewidth laser is a narrow linewidth fiber laser or a semiconductor laser having a center wavelength of 1550nm and a linewidth of 2.5 kHz.
5. The fiber optic vibration sensor according to claim 1, wherein the first fiber coupler is a polarization maintaining coupler having a splitting ratio of 9:1, and the second, third, fourth, fifth and sixth fiber couplers have a splitting ratio of 1: 1.
6. the fiber optic vibration sensor according to claim 1, wherein said electro-optic modulator is an intensity modulator with high bandwidth, suppressing carrier by adjusting dc voltage and performing frequency modulation by adjusting rf signal loaded thereon; the acousto-optic modulator makes the local oscillation light and the detection light generate frequency difference of dozens of MHz, and chops the continuous light to generate light pulse.
7. The fiber optic vibration sensor according to claim 1, wherein the fiber optic expanders are formed by winding optical fibers around a piezo-ceramic tube, or a phase modulator, for producing a phase shift of 0 or pi.
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