CN116182917A - Dual-wavelength distributed weak grating array sensing system and method based on heterodyne detection - Google Patents
Dual-wavelength distributed weak grating array sensing system and method based on heterodyne detection Download PDFInfo
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Abstract
The invention discloses a heterodyne detection-based dual-wavelength distributed weak grating array sensing system which comprises a first narrow linewidth laser, a second narrow linewidth laser, a 2 multiplied by 2 coupler, a first acousto-optic modulator, a second acousto-optic modulator, a pulse signal generator, a 2 multiplied by 1 coupler, an erbium-doped optical fiber amplifier, a circulator, a sensing optical fiber fused with a dual-wavelength weak grating array, a wavelength division multiplexer, a first photoelectric detector, a second photoelectric detector, a data acquisition card and a data processor. The invention can prolong the sensing distance by nearly two times under the condition of unchanged spatial resolution, or improve the spatial resolution by two times under the condition of unchanged sensing distance.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a dual-wavelength distributed weak grating array sensing system and method based on heterodyne detection.
Background
With the practical use of optical fibers and the development of optical fiber communication technologies, optical fiber sensing technologies have started. The optical fiber sensing technology uses optical waves as a carrier, the optical fiber is used as a transmission medium, when the environment near the optical fiber along the line changes, parameters such as the phase, the frequency, the polarization state, the intensity and the like of an optical signal can be changed, and by detecting the parameters, the information such as the temperature, the strain, the vibration, the defect and the like of any point on the optical fiber along the line can be extracted, so that the sensing in a large range is realized.
The sensing principle of the distributed weak grating array is as follows: the weak gratings embedded in the optical fiber are equivalent to weak mirrors, the detection pulses are reflected at the weak mirrors, stable and intensity-controllable reflected light signals can be provided at the designated positions of the optical fiber by uniformly writing the weak gratings into the optical fiber, the reflected light signals are overlapped with reference light in an interference manner, and disturbance signals applied to the optical fiber can be recovered by demodulating information of phase, power and the like of the interference light.
Compared with spontaneous Rayleigh scattering signals in a traditional optical fiber sensing system, the reflected light signals obtained by the distributed weak grating array sensing system are more stable, the signal to noise ratio is higher, but at the same time, the energy loss of the reflected signals for detection pulses is higher than that of scattered signals, when the detection pulses are transmitted in the weak grating array, a part of energy is lost for each reflected signal generated until the reflected signals generated by the reflected signals are too weak to perform disturbance detection, the sensing distance of the distributed weak grating array sensing system is limited, the spatial resolution of the distributed weak grating array sensing system is related to the grating interval, the longer sensing distance is needed, and the density of gratings is reduced, so that the spatial resolution of the sensing system is reduced. It is an urgent problem to increase the sensing distance while maintaining the spatial resolution or to increase the spatial resolution while maintaining the sensing distance.
The invention with publication number of CN110806259A discloses a device for positioning and detecting optical fiber sensing high-frequency disturbance, a traditional optical time domain reflection system greatly improves the sensing distance and positioning precision, and a frequency measuring range can not identify a disturbance signal with higher frequency. Aiming at the problem that the prior art is difficult to realize the frequency detection of a high-frequency disturbance event, the invention provides a method for combining a coherent optical time domain reflection technology and an interferometer technologyAnd meanwhile, the positioning of the disturbance point and the detection of the high-frequency signal are realized. The first laser and the second laser respectively emit light with wavelength lambda 1 and λ2 Is injected into the sensing optical fiber by wavelength division multiplexing technique, lambda 1 and λ2 The interference of the two wavelengths carries disturbance information and frequency information. For lambda 1 Quadrature demodulating the interference signal with wavelength to obtain amplitude information of the signal, and then performing lambda demodulation 1 Performing variance processing on the interference signals of the wavelengths to obtain the position information of external disturbance; for lambda 2 And carrying out phase demodulation on the interference signals with the wavelength to obtain frequency information of external disturbance. The sensing optical fiber used in the invention comprises a Bragg fiber grating with single center wavelength, the frequency detection of disturbance signals is realized by utilizing reflected light generated by the Bragg fiber grating through a Mach-Zehnder interferometer structure, and the positioning and amplitude measurement of the disturbance signals are realized by utilizing back Rayleigh scattered light in the sensing optical fiber through an optical time domain reflection technology.
Disclosure of Invention
The technical problems to be solved are as follows: in the traditional sensing system based on the distributed weak grating array, the spatial resolution is related to the grating interval, the grating density needs to be increased to improve the spatial resolution, the sensing distance is shortened, and in the same way, the grating density needs to be reduced to prolong the sensing distance, so that the spatial resolution is reduced.
The technical scheme is as follows:
the dual-wavelength distributed weak grating array sensing system based on heterodyne detection comprises a first narrow linewidth laser, a second narrow linewidth laser, a 2X 2 coupler, a first acousto-optic modulator, a second acousto-optic modulator, a pulse signal generator, a 2X 1 coupler, an erbium-doped fiber amplifier, a circulator, a sensing fiber fused with a dual-wavelength weak grating array, a wavelength division multiplexer, a first photoelectric detector, a second photoelectric detector, a data acquisition card and a data processor;
the first narrow linewidth laser emits light with a center wavelength lambda 1 And the second narrow linewidth laser emits light having a center wavelength lambda 2 Continuous light of (2); the 2 x 2 coupler sends two paths of continuous light with different central wavelengths into two branches and exists in the two branches at the same time; the pulse signal generator sends out a first pulse electric signal and a second pulse electric signal to respectively control the first acoustic optical modulator and the second acoustic optical modulator to modulate continuous light, and simultaneously transmits a synchronous trigger signal to the data acquisition card, wherein the first acoustic optical modulator outputs first pulse light and second pulse light which are overlapped together, and the second acoustic optical modulator outputs third pulse light and fourth pulse light which are overlapped together; the pulse width of the first pulse light, the second pulse light, the third pulse light and the fourth pulse light is the same; the frequency shift frequencies of the first acoustic optical modulator and the second acoustic optical modulator are different; the 2X 1 coupler sends the first pulse light, the second pulse light, the third pulse light and the fourth pulse light into the same branch and then amplifies the same branch by the erbium-doped fiber amplifier, and the amplified first pulse light, second pulse light, third pulse light and fourth pulse light enter a sensing fiber of the fused dual-wavelength weak grating array through a circulator; the sensing optical fiber fused with the dual-wavelength weak grating array comprises a central wavelength lambda 1 Is a first weak grating array with a center wavelength lambda 2 A second weak grating array of (a); the first pulse light and the third pulse light are reflected only on adjacent gratings of the first weak grating array and perform superposition interference; the second pulse light and the fourth pulse light are reflected only on adjacent gratings of the second weak grating array and perform superposition interference; the wavelength division multiplexer has a center wavelength lambda 1 and λ2 Is divided into two paths; the first photoelectric detector has a collection center wavelength lambda 1 And converts the optical signal into an electrical signal, wherein the second photodetector acquires the optical signal with the center wavelength lambda 2 And converts the optical signal of (a) into an electrical signal; the data acquisition card acquires and digitizes the electric signals of the first photoelectric detector and the second photoelectric detector; the data processor logarithmDisturbance positioning and quantitative measurement are carried out according to the two paths of digital electric signals acquired by the acquisition card;
the data processor demodulates beat signals between adjacent gratings in each path of electric signals, the demodulated vibration represents vibration existing between the grating pairs, a disturbed grating area in each path of data is observed, when the position of the disturbed grating pair demodulated by the first weak grating array overlaps with the position of the disturbed grating pair demodulated by the second weak grating array, the two gratings which are overlapped with each other between the grating pair in the first weak grating array and the grating pair in the second weak grating array are taken as disturbance generating areas, and the disturbance position is reduced to be in adjacent gratings with different center wavelengths.
Further, the 2×2 coupler is replaced by an optical switch or modulator, and the optical switch or modulator is used for controlling the time sequence of the output light wave of the first narrow linewidth laser and the output light wave of the second narrow linewidth laser so that the central wavelength is lambda 1 and λ2 Respectively, are incident on the system.
Further, the grating intervals in the first weak grating array are equal, the grating intervals in the second weak grating array are equal, and the grating intervals in the first weak grating array and the second weak grating array are L; the grating interval between the grating in the first weak grating array and the grating in the adjacent second weak grating array is L respectively 1 and L2 。
Further, the grating interval between the first weak grating array and the adjacent second weak grating array is
Further, the pulse signal generator generates a first pulse electric signal and a second pulse electric signal with pulse width t 1 Pulse spacing of
t 1 ＜t 2 The method comprises the steps of carrying out a first treatment on the surface of the Wherein c is the propagation speed of light in vacuum, n is the equivalent refractive index of the optical fiber, and L is the gratingThe spacing of adjacent gratings in the array having the same center wavelength.
Further, assume that the electric field intensities of the first pulse light, the second pulse light, the third pulse light and the fourth pulse light are E 0 The angular frequencies are ω 1 、ω 2 、ω 3 and ω4, wherein ω3 -ω 1 Equal to omega 4 -ω 2 The initial phase of the first pulse light and the third pulse light is determined by the frequency shift difference of the first acousto-optic modulator and the second acousto-optic modulator
The initial phase of the second pulse light and the fourth pulse light is +.>
The interval of the gratings in the first weak grating array and the second weak grating array is L, the reflectivity is r, and delta L is the length change of the optical fiber caused by external disturbance;
the reflected optical electric fields of the four paths of pulse light are respectively expressed as:
the reflected light of the first pulse light and the reflected light of the third pulse light are overlapped and then interfere, and the generated optical signal is expressed as:
wherein ,
is the phase change caused by external disturbance;
the reflected light of the second pulse light and the reflected light of the fourth pulse light are overlapped and then interfere, and the generated optical signal is expressed as:
wherein ,
is the phase change caused by external disturbances.
The invention discloses a heterodyne detection-based dual-wavelength distributed weak grating array sensing method, which is executed based on the heterodyne detection-based dual-wavelength distributed weak grating array sensing system;
the dual-wavelength distributed weak grating array sensing method comprises the following steps:
step one: the 2 multiplied by 2 coupler injects the continuous light waves generated by the first narrow linewidth laser and the continuous light waves generated by the second narrow linewidth laser into the same two branches;
step two: the pulse signal generator sends a first pulse electric signal to control the first acousto-optic modulator to modulate the continuous light generated by the first narrow linewidth laser and the second narrow linewidth laser into first pulse light and second pulse light, the pulse signal generator sends a second pulse electric signal to control the second acousto-optic modulator to modulate the continuous light generated by the first narrow linewidth laser and the second narrow linewidth laser into third pulse light and fourth pulse light, the first pulse light and the second pulse light are overlapped together, the third pulse light and the fourth pulse light are overlapped together, pulse widths of the first pulse light, the second pulse light, the third pulse light and the fourth pulse light are the same, and pulse intervals of the first pulse light, the second pulse light, the third pulse light and the fourth pulse light are the same;
step three: the 2X 1 coupler sends the first pulse light, the second pulse light, the third pulse light and the fourth pulse light into one branch and then amplifies the first pulse light, the second pulse light, the third pulse light and the fourth pulse light by an erbium-doped fiber amplifier, and the amplified first pulse light, the second pulse light, the third pulse light and the fourth pulse light enter a sensing fiber fused with the double-wavelength weak grating array through a circulator;
step four: the first pulse light and the third pulse light are reflected only on adjacent gratings of the first weak grating array and perform superposition interference, and the second pulse light and the fourth pulse light are reflected only on adjacent gratings of the second weak grating array and perform superposition interference;
step five: the wavelength division multiplexer divides the superimposed light with different center wavelengths into two paths, the two paths are respectively collected by the first photoelectric detector and the second photoelectric detector and converted into electric signals, and the data collection card digitizes the collected electric signals and transmits the electric signals to the data processor;
step six: the data processor performs signal processing on two paths of digital electric signals acquired by the data acquisition card, demodulates beat signals between adjacent gratings in each path of electric signals, demodulates vibration to represent vibration existing between the grating pairs, observes a grating area generating disturbance in each path of data, and takes two gratings which are overlapped between the grating pair in the first weak grating array and the grating pair in the second weak grating array at the moment as areas generated by disturbance when the positions of the grating pair generating disturbance in the first weak grating array overlap with the positions of the grating pair generating disturbance in the second weak grating array, so that the disturbance positions are reduced to adjacent gratings with different center wavelengths.
The beneficial effects are that:
the invention discloses a heterodyne detection-based dual-wavelength distributed weak grating array sensing system and a heterodyne detection-based dual-wavelength distributed weak grating array sensing method, wherein the sensing system is integrated with a dual-wavelength weak grating array, and a sensing optical fiber comprises two sets of weak grating arrays with different center wavelengths, wherein each weak grating array only reflects pulse light with the corresponding center wavelength and transmits pulse light with the other center wavelength; the invention can prolong the sensing distance by nearly two times under the condition of unchanged spatial resolution, or improve the spatial resolution by two times under the condition of unchanged sensing distance.
Drawings
FIG. 1 is a schematic diagram of the disturbance localization principle using the scheme of the present invention;
FIG. 2 is a schematic diagram of a conventional heterodyne detection-based distributed weak grating array sensing system;
FIG. 3 is a schematic diagram of a dual-wavelength distributed weak grating array sensing system based on heterodyne detection according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of two paths of time domain signals collected by a data collection card according to an embodiment of the present invention;
fig. 5 is a schematic diagram of vibration results of adjacent beat demodulation recovery of corresponding disturbance positions in two paths of time domain signals acquired by a data acquisition card according to an embodiment of the present invention.
Fig. 6 is a schematic diagram of demodulation results according to an embodiment of the present invention.
Detailed Description
The following examples will provide those skilled in the art with a more complete understanding of the invention, but are not intended to limit the invention in any way.
Referring to fig. 3, in the dual-wavelength distributed weak grating array sensing system based on heterodyne detection provided in this embodiment, two sets of weak grating arrays with different center wavelengths are embedded in a sensing optical fiber used in this embodiment, reflected light generated by the weak grating arrays is used to replace backward rayleigh scattered light in the optical fiber to perform sensing, the signal-to-noise ratio of the generated interference signal is higher, and the positioning and quantitative measurement of the disturbance signal can be realized by simultaneously utilizing the reflected light generated by the two sets of weak grating arrays through an optical time domain reflection technology.
Specifically, the dual-wavelength distributed weak grating array sensing system of the embodiment includes the following components:
narrow linewidth laser 1: continuous light with a center wavelength of 1535nm is produced.
Narrow linewidth laser 2: producing continuous light with a center wavelength of 1550 nm.
2 x 2 coupler: the continuous light generated by the narrow linewidth laser 1 and the narrow linewidth laser 2 is output to the acousto-optic modulator 1 and the acousto-optic modulator 2.
Acousto-optic modulator 1: by modulating the continuous light emitted from the narrow linewidth lasers 1 and 2, pulse light having a specific period and a specific width is generated.
An acousto-optic modulator 2: by modulating the continuous light emitted from the narrow linewidth lasers 1 and 2, pulse light having a specific period and a specific width is generated.
Pulse signal generator: two pulse electric signals having a specific period, a specific width and a specific pulse interval are generated to drive the acousto-optic modulator 1 and the acousto-optic modulator 2, respectively.
2 x 1 coupler: the pulsed light generated by the acousto-optic modulator is fed into a branch.
Erbium-doped fiber amplifier: amplifying the pulsed optical power at the output end of the 2 x 1 coupler.
The circulator is as follows: the optical fiber is used for guiding the pulse light subjected to power amplification into the sensing optical fiber fused with the dual-wavelength weak grating array, and guiding the returned superimposed light into the wavelength division multiplexer.
Sensing optical fiber fused with dual-wavelength weak grating array: two sets of weak grating arrays 1 and 2 which respectively correspond to the central wavelengths of the narrow linewidth laser 1 and the narrow linewidth laser 2 are embedded in the sensing optical fiber, each grating of the weak grating array 1 can only reflect the pulse light corresponding to the central wavelength of the narrow linewidth laser 1, and each grating of the weak grating array 2 can only reflect the pulse light corresponding to the central wavelength of the narrow linewidth laser 2.
A wavelength division multiplexer: for separating the interference light corresponding to the narrow linewidth laser 1 and the narrow linewidth laser 2.
Photodetector 1: the optical signal conversion module is used for converting one path of optical signals output by the wavelength division multiplexer into electric signals and outputting the electric signals to the acquisition card.
Photodetector 2: the optical signal conversion module is used for converting one path of optical signals output by the wavelength division multiplexer into electric signals and outputting the electric signals to the acquisition card.
And (3) an acquisition card: the analog signal acquisition module is used for digitizing the acquired analog signal and outputting the digitized analog signal to the processor.
And (3) a computer: and analyzing and processing the acquired data, so as to realize disturbance measurement and further positioning of the sensing optical fibers along the line fusing the dual-wavelength weak grating array.
Generating 1535nm continuous light by using a narrow linewidth laser 1, modulating the continuous light into first pulse light and second pulse light by using an acousto-optic modulator 1 and an acousto-optic modulator 2 after passing through a 2X 2 coupler, generating 1550nm continuous light by using the narrow linewidth laser 2, modulating the continuous light into third pulse light and fourth pulse light by using the acousto-optic modulator 1 and the acousto-optic modulator 2 after passing through the 2X 2 coupler, superposing the first pulse light and the second pulse light together, superposing the third pulse light and the fourth pulse light together, amplifying the four paths of pulse light by using an erbium-doped fiber amplifier after passing through the 2X 1 coupler, then entering a sensing fiber fused with a dual-wavelength grating array by using a circulator, reflecting and superposing the first pulse light and the third pulse light at the weak grating array 1, reflecting and superposing the second pulse light at the weak grating array 2, superposing the second pulse light and the fourth pulse light, transmitting and acquiring the four paths of reflected light by using a wave division detector, multiplexing the optical signal and the photoelectric detector to acquire the data by using a digital interference card, and calculating the data by using the photoelectric detector and the digital interference card; disturbance measurement is realized by demodulating two paths of digital electric signals, a disturbance-generating grating area in each path of data is observed, and when the grating positions with disturbance and different center wavelengths overlap, the disturbance positions can be reduced to be between the overlapped gratings.
The period and pulse width of two paths of pulse electric signals sent by the pulse signal generator are equal, and the pulse interval is equal to
c is the propagation speed of light in vacuum, n is the equivalent refractive index of the optical fiber, and L is the sameThe spacing between adjacent gratings in the wavelength grating array; .
The pulse signal generator gives out a synchronous signal as a trigger signal of the data acquisition card.
Let the center wavelength of the narrow linewidth laser 1 be λ 1 The center wavelength of the narrow linewidth laser 2 is lambda 2 The electric field intensity of the first pulse light, the second pulse light, the third pulse light and the fourth pulse light is E 0 The angular frequencies are ω 1 、ω 2 、ω 3 and ω4, wherein ω3 -ω 1 Equal to omega 4 -ω 2 The initial phases of the first pulse light and the third pulse light are determined by the frequency shift difference of the acousto-optic modulator 1 and the acousto-optic modulator 2
The initial phase of the second pulse light and the fourth pulse light is +.>
The interval between the weak grating array 1 and the grating in the weak grating array 2 is L, the reflectivity is r, and DeltaL is the length change of the optical fiber caused by external disturbance.
The reflected optical electric fields of the four paths of pulse light can be expressed as:
the reflected light of the first pulse light and the reflected light of the third pulse light are overlapped to generate interference, and the optical signal can be expressed as:
wherein ,
is the phase change caused by external disturbances.
The reflected light of the second pulse light and the reflected light of the fourth pulse light are overlapped to generate interference, and the optical signal can be expressed as:
wherein ,
is the phase change caused by external disturbances.
For demodulation of beat signals of two paths of interference light, referring to fig. 1, when a disturbance is applied between two gratings with different center wavelengths, the disturbance also exists between gratings with the same center wavelength, and demodulation of beat signals generated by two gratings with the same center wavelength containing the disturbance can restore the vibration, and the positions of the gratings at the moment are positioned to obtain two groups of gratings with different center wavelengths, and overlapped parts of the gratings are areas of the disturbance. Compared with a single-wavelength weak grating array, the spatial resolution is improved by 1 time.
Fig. 2 is a schematic diagram of a conventional distributed weak grating array sensing system, in which continuous light emitted by an ultra-narrow linewidth laser enters two acousto-optic modulators through a coupler and is modulated into two paths of narrow pulse light, power is amplified by an erbium-doped fiber amplifier after passing through the coupler, then the power enters a sensing fiber through a circulator, and returned interference light is received by a detector. By adopting the scheme, the above situation is solved, and the following detailed description is carried out according to experimental description:
adopting an experimental device shown in fig. 3, emitting continuous light by a narrow linewidth laser with central wavelength of 1535nm and 1550nm, and simultaneously entering an acousto-optic modulator with modulation frequency of 200MHz and-60 MHz to obtain four pulse lights with pulse width of 60ns by modulation, wherein two pulse lights modulated by the same acousto-optic modulator are overlapped together, and the pulse interval between the pulse lights modulated by the two acousto-optic modulators is 100ns; four pulse lights enter an erbium-doped fiber amplifier, are amplified and then are sent into a dual-wavelength grating array by a circulator to be sensed, the grating spacing of the same-wavelength weak grating array is 10m, and the grating spacing of the different-wavelength weak grating arrays is 5m; the returned superimposed light is divided into two paths of interference light by the wavelength division multiplexer after passing through the circulator, and the interference light is received by the two detectors, collected by the data collecting card and sent into the computer for disturbance positioning and measurement.
Fig. 4 shows two paths of beat signals collected by the data collection card after being received by the two detectors, and a certain time delay exists between the two paths of beat signals, wherein the time delay is caused by the distribution mode of the dual-wavelength weak grating array, and the time delay is related to grating intervals of different center wavelengths.
Sinusoidal signals with the frequency of 200Hz and the voltage of 10V are applied near 2000 meters of the grating, the demodulation results of two adjacent beats in two paths of beat signals are shown in fig. 5, the beat signals just correspond to two pairs of adjacent gratings with different center wavelengths, such as two beats indicated by black arrows in fig. 6, the demodulation results of each path of beat signals only can reduce disturbance to a range of 10m, the disturbance can be reduced to a range of 5m after the two paths of beat signals are combined, and finally the positioned vibration position is consistent with the actual position.
Compared with the traditional scheme, the heterodyne detection-based dual-wavelength distributed weak grating array sensing system and method have the advantages that a special dual-wavelength weak grating array is used, two paths of interference light can be formed for sensing through a unique staggered arrangement mode of different center wavelength gratings, and the same disturbance is measured and positioned twice, so that the spatial resolution is improved by two times under the condition that the sensing distance is unchanged, and the sensing distance is prolonged by nearly two times under the condition that the spatial resolution is unchanged.
The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above examples, and all technical solutions belonging to the concept of the present invention belong to the protection scope of the present invention. It should be noted that modifications and adaptations to the invention without departing from the principles thereof are intended to be within the scope of the invention as set forth in the following claims.
Claims (7)
1. The dual-wavelength distributed weak grating array sensing system based on heterodyne detection is characterized by comprising a first narrow linewidth laser, a second narrow linewidth laser, a 2X 2 coupler, a first acousto-optic modulator, a second acousto-optic modulator, a pulse signal generator, a 2X 1 coupler, an erbium-doped optical fiber amplifier, a circulator, a sensing optical fiber fused with a dual-wavelength weak grating array, a wavelength division multiplexer, a first photoelectric detector, a second photoelectric detector, a data acquisition card and a data processor;
the first narrow linewidth laser emits light with a center wavelength lambda 1 And the second narrow linewidth laser emits light having a center wavelength lambda 2 Continuous light of (2); the 2 x 2 coupler sends two paths of continuous light with different central wavelengths into two branches and exists in the two branches at the same time; the pulse signal generator sends out a first pulse electric signal and a second pulse electric signal to respectively control the first acoustic optical modulator and the second acoustic optical modulator to modulate continuous light, and simultaneously transmits a synchronous trigger signal to the data acquisition card, wherein the first acoustic optical modulator outputs first pulse light and second pulse light which are overlapped together, and the second acoustic optical modulator outputs third pulse light and fourth pulse light which are overlapped together; the pulse width of the first pulse light, the second pulse light, the third pulse light and the fourth pulse light is the same; the frequency shift frequencies of the first acoustic optical modulator and the second acoustic optical modulator are different; the 2X 1 coupler is used for transmitting the first pulse light, the second pulse light,The third pulse light and the fourth pulse light are sent into the same branch and then amplified by the erbium-doped fiber amplifier, and the amplified first pulse light, second pulse light, third pulse light and fourth pulse light enter a sensing fiber fused with a dual-wavelength weak grating array through a circulator; the sensing optical fiber fused with the dual-wavelength weak grating array comprises a central wavelength lambda 1 Is a first weak grating array with a center wavelength lambda 2 A second weak grating array of (a); the first pulse light and the third pulse light are reflected only on adjacent gratings of the first weak grating array and perform superposition interference; the second pulse light and the fourth pulse light are reflected only on adjacent gratings of the second weak grating array and perform superposition interference; the wavelength division multiplexer has a center wavelength lambda 1 and λ2 Is divided into two paths; the first photoelectric detector has a collection center wavelength lambda 1 And converts the optical signal into an electrical signal, wherein the second photodetector acquires the optical signal with the center wavelength lambda 2 And converts the optical signal of (a) into an electrical signal; the data acquisition card acquires and digitizes the electric signals of the first photoelectric detector and the second photoelectric detector; the data processor performs disturbance positioning and quantitative measurement on two paths of digital electric signals acquired by the data acquisition card;
the data processor demodulates beat signals between adjacent gratings in each path of electric signals, the demodulated vibration represents vibration existing between the grating pairs, a disturbed grating area in each path of data is observed, when the position of the disturbed grating pair demodulated by the first weak grating array overlaps with the position of the disturbed grating pair demodulated by the second weak grating array, the two gratings which are overlapped with each other between the grating pair in the first weak grating array and the grating pair in the second weak grating array are taken as disturbance generating areas, and the disturbance position is reduced to be in adjacent gratings with different center wavelengths.
2. The heterodyne detection based dual wavelength distributed weak grating array sensing system according to claim 1, wherein the 2 x 2 coupler is replaced with an optical switch or modulator by which the first is controlledThe timing of the output light wave of one narrow linewidth laser and the output light wave of the second narrow linewidth laser is such that the center wavelength is lambda 1 and λ2 Respectively, are incident on the system.
3. The heterodyne detection-based dual-wavelength distributed weak grating array sensing system according to claim 1, wherein grating intervals in the first weak grating array are equal, grating intervals in the second weak grating array are equal, and grating intervals in the first weak grating array and the second weak grating array are all L; the grating interval between the grating in the first weak grating array and the grating in the adjacent second weak grating array is L respectively 1 and L2 。
4. The heterodyne detection based dual wavelength distributed weak grating array sensing system according to claim 3, wherein the grating spacing in the first weak grating array and the grating spacing in the adjacent second weak grating array are each
5. The heterodyne detection based dual wavelength distributed weak grating array sensing system as recited in claim 1, wherein the pulse signal generator generates a first pulse electrical signal and a second pulse electrical signal each having a pulse width t 1 Pulse spacing of
t 1 ＜t 2 The method comprises the steps of carrying out a first treatment on the surface of the Where c is the propagation speed of light in vacuum, n is the equivalent refractive index of the fiber, and L is the spacing between adjacent gratings in the grating array having the same center wavelength.
6. The heterodyne detection-based dual wavelength distributed weak grating array sensing system according to claim 1, wherein the first pulse light, the second pulse light, the third pulse light are assumed to beThe electric field intensity of the four pulse lights is E 0 The angular frequencies are ω 1 、ω 2 、ω 3 and ω4, wherein ω3 -ω 1 Equal to omega 4 -ω 2 The initial phase of the first pulse light and the third pulse light is determined by the frequency shift difference of the first acousto-optic modulator and the second acousto-optic modulator
The initial phase of the second pulse light and the fourth pulse light is +.>
The interval of the gratings in the first weak grating array and the second weak grating array is L, the reflectivity is r, and delta L is the length change of the optical fiber caused by external disturbance;
the reflected optical electric fields of the four paths of pulse light are respectively expressed as:
the reflected light of the first pulse light and the reflected light of the third pulse light are overlapped and then interfere, and the generated optical signal is expressed as:
wherein ,
is the phase change caused by external disturbance;
the reflected light of the second pulse light and the reflected light of the fourth pulse light are overlapped and then interfere, and the generated optical signal is expressed as:
wherein ,
is the phase change caused by external disturbances.
7. A heterodyne detection-based dual-wavelength distributed weak grating array sensing method, characterized in that the dual-wavelength distributed weak grating array sensing method is performed based on the heterodyne detection-based dual-wavelength distributed weak grating array sensing system as claimed in any one of claims 1 to 6;
the dual-wavelength distributed weak grating array sensing method comprises the following steps:
step one: the 2 multiplied by 2 coupler injects the continuous light waves generated by the first narrow linewidth laser and the continuous light waves generated by the second narrow linewidth laser into the same two branches;
step two: the pulse signal generator sends a first pulse electric signal to control the first acousto-optic modulator to modulate the continuous light generated by the first narrow linewidth laser and the second narrow linewidth laser into first pulse light and second pulse light, the pulse signal generator sends a second pulse electric signal to control the second acousto-optic modulator to modulate the continuous light generated by the first narrow linewidth laser and the second narrow linewidth laser into third pulse light and fourth pulse light, the first pulse light and the second pulse light are overlapped together, the third pulse light and the fourth pulse light are overlapped together, pulse widths of the first pulse light, the second pulse light, the third pulse light and the fourth pulse light are the same, and pulse intervals of the first pulse light, the second pulse light, the third pulse light and the fourth pulse light are the same;
step three: the 2X 1 coupler sends the first pulse light, the second pulse light, the third pulse light and the fourth pulse light into one branch and then amplifies the first pulse light, the second pulse light, the third pulse light and the fourth pulse light by an erbium-doped fiber amplifier, and the amplified first pulse light, the second pulse light, the third pulse light and the fourth pulse light enter a sensing fiber fused with the double-wavelength weak grating array through a circulator;
step four: the first pulse light and the third pulse light are reflected only on adjacent gratings of the first weak grating array and perform superposition interference, and the second pulse light and the fourth pulse light are reflected only on adjacent gratings of the second weak grating array and perform superposition interference;
step five: the wavelength division multiplexer divides the superimposed light with different center wavelengths into two paths, the two paths are respectively collected by the first photoelectric detector and the second photoelectric detector and converted into electric signals, and the data collection card digitizes the collected electric signals and transmits the electric signals to the data processor;
step six: the data processor performs signal processing on two paths of digital electric signals acquired by the data acquisition card, demodulates beat signals between adjacent gratings in each path of electric signals, demodulates vibration to represent vibration existing between the grating pairs, observes a grating area generating disturbance in each path of data, and takes two gratings which are overlapped between the grating pair in the first weak grating array and the grating pair in the second weak grating array at the moment as areas generated by disturbance when the positions of the grating pair generating disturbance in the first weak grating array overlap with the positions of the grating pair generating disturbance in the second weak grating array, so that the disturbance positions are reduced to adjacent gratings with different center wavelengths.
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