CN117968738A - Forward Brillouin scattering double-parameter sensing device and method based on optical fiber single-end reflection structure - Google Patents
Forward Brillouin scattering double-parameter sensing device and method based on optical fiber single-end reflection structure Download PDFInfo
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Abstract
The invention provides a forward Brillouin scattering double-parameter sensing device and method based on an optical fiber single-end reflection structure, which obtain a forward Brillouin scattering spectrum excited by a torsion-radial acoustic mode in a sensing optical fiber, and have the advantages of narrow line width and high signal to noise ratio. And the sensitivity of the frequency shift and the linewidth to the temperature and the salinity is obtained by adopting a sensing optical fiber with a linear relation between a measured parameter and a parameter to be measured and utilizing the frequency shift and linewidth response characteristics of the forward Brillouin scattering excited by the torsional-radial acoustic mode in the optical fiber with different temperatures and salinity. The temperature and the salinity can be solved by utilizing a binary one-time linear equation system through measuring the frequency shift change and the linewidth change corresponding to the forward Brillouin scattering peak with higher sensitivity, and the high-precision simultaneous measurement of the temperature and the salinity can be realized. In addition, the retention of the optical fiber coating layer in the experiment enables the sensing optical fiber to have higher mechanical strength, can reduce the packaging requirement of the sensing unit, and is applicable to more use environments.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a forward Brillouin scattering double-parameter sensing device and method based on an optical fiber single-end reflection structure.
Background
With rapid developments over the past decades, fiber optic sensors based on forward brillouin scattering remain challenging in monitoring the surrounding environment. The simultaneous measurement of temperature and salinity has been attracting attention in many application fields such as marine research, marine environment, marine structure health monitoring, etc. Forward Brillouin scattering, also known as acoustic waveguide Brillouin scattering, has received attention from researchers because of its multiple spectral peaks and multiparameter sensing capability. The monitoring of the forward brillouin spectrum can be used to quantitatively analyze the external environment of the cladding or coating of the optical fiber. The forward brillouin scattering process involves two modes, namely a radial acoustic mode and a torsional-radial acoustic mode. The torsional-radial acoustic mode can cause not only radial resonance but also torsional resonance. These acoustic modes in the fibre interact with the incident optical field, producing forward brillouin scattered light. The radial acoustic mode may be excited by circularly polarized light or linearly polarized light. For the torsional-radial acoustic mode, when the vibration direction of the radial component is along the fiber axis, the induced birefringence direction is also along the fiber axis, and the inherent birefringence axis of the fiber is unchanged, referred to as the 90 °/0 ° vibration mode. When the vibration direction of the radial component is + -45 DEG to the fiber axis, the birefringence axis changes due to the modulation of the additional birefringence, and this deviation from the inherent birefringence axis of the fiber is called 45 DEG/-45 DEG vibration pattern. The intensity of forward brillouin scattered light excited by the torsional-radial acoustic mode is related to the ellipticity of the incident light, which is most scattered when linearly polarized and least scattered when circularly polarized.
In practical application, because forward Brillouin scattering is simultaneously sensitive to two or more parameters such as temperature and salinity, only a single parameter is measured in an environment where a plurality of parameters are changed, sensitivity crosstalk can be caused, and therefore measurement accuracy is reduced. Therefore, in an environment with only two parameters changed, the two parameters are measured simultaneously by utilizing forward Brillouin scattering, so that the problem of cross sensitivity can be avoided, and the simultaneous high-precision measurement of the two parameters can be obtained.
In prior art 1(W.J.Zhang,Y.G.Lu,C.J.He,"Simultaneous measurement of temperature,refractive index,and axial strain based on forward Brillouin scattering in polyimide-coated SMF,"IEEE Sensors J.,23(22),27361-27368(2023).) published by w.j.zhang et al, forward brillouin light excited by radial acoustic modes and its sensing applications are mainly discussed. The forward brillouin scattering excited by the radial acoustic mode has been used for dual parameter measurement, including strain and temperature, temperature and acoustic impedance, temperature and pressure, relative humidity and temperature, to be measured. In fact, the forward brillouin scattering of torsional-radial acoustic mode excitation is also sensitive to changes in the parameters of the surrounding environment of the optical fibre. In addition, compared with the forward Brillouin scattering excited by the radial acoustic mode, the line width of the forward Brillouin scattering excited by the torsion-radial acoustic mode is much smaller, and only a few megahertz can be used for carrying out accurate Lorentz fitting, so that the sensing precision is improved. However, most of the current optical fiber sensing systems based on forward brillouin scattering are based on a Sagnac loop structure, which is designed to obtain forward brillouin scattered light excited by a radial acoustic mode, and the forward brillouin scattered light excited by a torsion-radial acoustic mode is obtained by using the structure, so that the defects of low signal intensity and low signal to noise ratio exist. In the prior art 2(Z.L.Zhang,Y.G.Lu,J.Q.Peng,and Z.Y.Ji,"Simultaneous measurement of temperature and acoustic impedance based on forward Brillouin scattering in LEAF,"Opt.Lett.,Vol.46,no.7,pp.1776-1779,2021) published by z.l.zhang et al, a certain linear relationship between the optical fiber forward brillouin scattering spectrum and the temperature and acoustic impedance is utilized to realize the double-parameter measurement of the temperature and the acoustic impedance, but the used optical fiber needs to be stripped to protect the outer coating of the fiber core and the cladding, so that the optical fiber sensing unit has low mechanical strength and is easy to break.
How to use forward Brillouin scattering light excited by torsion-radial acoustic mode for double-parameter sensing, and overcome the inherent defects of the existing Sagnac ring structure and the coating sensing fiber, is an important challenge for high-precision measurement of double parameters of temperature and salinity.
Disclosure of Invention
The invention aims to: the forward Brillouin scattering double-parameter sensing device and method based on the optical fiber single-end reflection structure have the advantages of being simple and convenient to manufacture, simple in structure, high in measurement accuracy, capable of eliminating temperature crosstalk and the like, and the problems in the prior art are solved.
In a first aspect, a forward brillouin scattering dual-parameter sensing device based on an optical fiber single-end reflection structure is provided, and the device comprises a laser light source, an optical fiber single-end reflection structure, an optical polarizer, an optical polarization controller, a photoelectric detector and a data processing module.
The optical fiber single-end reflecting structure consists of an optical circulator, a sensing optical fiber and an optical reflecting mirror.
The optical polarizer is connected with the optical polarization controller and is used for detecting beat frequency signals of incident light and forward light excited by the torsional-radial acoustic mode, converting the beat frequency signals into corresponding electric signals and transmitting the corresponding electric signals to the data processing module.
In the single-ended reflective structure, the transverse acoustic field phase modulates the incident light due to thermal vibration of the optical fiber, thereby generating spontaneous forward brillouin scattering, and both the radial acoustic mode and the torsional-radial acoustic mode are excited. The beat signals of the forward light and the incident light are output through the 3 ports of the optical circulator. The optical polarizer is used for filtering out components perpendicular to the polarization plane of the optical polarizer, so that the influence of signals in other polarization directions on beat signals is effectively eliminated. The optical polarization controller phase delays the light (incident light, forward brillouin scattering excited by the radial acoustic mode, and forward brillouin scattering excited by the twist-radial acoustic mode). The relative phase of the transmitted light is changed by adjusting the optical polarization controller, so that the forward Brillouin scattered light of the torsion-radial acoustic mode with high signal-to-noise ratio and the forward Brillouin scattered light of the radial acoustic mode are excited. The function of the optical polariser and the optical polarisation controller is to obtain high signal to noise ratio forward brillouin scattered light for torsional-radial acoustic mode excitation while suppressing as much as possible the forward brillouin scattered light for radial acoustic mode excitation. The optical wave signals after phase delay are converted into electric signals through the photoelectric detector and are sent to the data processing module, and analysis of the signal spectrum is completed.
In a further embodiment of the first aspect, an optical isolator is further disposed between the laser light source and the input end of the optical circulator, and after the optical wave signal emitted by the laser light source passes through the optical isolator, the optical wave signal is incident to the input end of the optical circulator.
In a further embodiment of the first aspect, the sensing fiber employs a G657 fiber with a polyimide coating.
In a further embodiment of the first aspect, the coated optical fiber has a length of 38m, a cladding diameter and a coating diameter of 125 μm and 145 μm, respectively, and a transmission loss in the 1550nm band of 0.50dB/km.
In a further embodiment of the first aspect, the laser light source used is output by a narrow linewidth single frequency laser with a center wavelength of 1550.12nm, the output laser linewidth being 5kHz.
In a second aspect of the present invention, a forward brillouin scattering double-parameter sensing method based on an optical fiber single-end reflection structure is provided, and the temperature and salinity response characteristics of a sensing optical fiber are obtained by using the sensing device based on the optical fiber single-end reflection structure, including:
S1, controlling the temperature and the salinity of a medium around a sensing optical fiber to change for a plurality of times according to a set change amount, obtaining forward Brillouin scattering spectra of the sensing optical fiber in different states in the change process, performing Lorentz fitting on formants of forward Brillouin scattering excited by one torsion-radial acoustic mode in different states to obtain change amounts of frequency shift and linewidth in different states, and performing linear fitting on the change amounts of the frequency shift and the linewidth and the temperature and the salinity change amounts in different states to obtain response coefficients of the frequency shift and the linewidth;
S2, acquiring a forward Brillouin scattering spectrum of the sensing optical fiber at a certain temperature and a certain salinity, taking the state as a reference state, acquiring the forward Brillouin scattering spectrum of the sensing optical fiber at a state to be measured, comparing the forward Brillouin scattering spectrum of the sensing optical fiber at the reference state with the forward Brillouin scattering spectrum of the sensing optical fiber at the state to be measured, acquiring a frequency shift variation and a line width variation of forward Brillouin scattering excited by a torsion-radial acoustic mode at the state to be measured, and calculating a measurement result according to the frequency shift variation and the line width variation at the state to be measured and the response coefficient acquired in the step S1, wherein the measurement result is the variation of the temperature and the salinity.
In a further embodiment of the second aspect, in step S2, a calculation formula for calculating a measurement result according to the frequency shift variation and the line width variation in the to-be-measured state and the response coefficient is as follows:
wherein: delta T is the temperature variation, delta S is the salinity variation; deltav i and Deltaf i are the frequency shift variation and linewidth variation of a torsional-radial acoustic mode, respectively; for frequency shift-temperature sensitivity,/> For the frequency shift-salinity sensitivity,For line width-temperature sensitivity,/>Line width-salinity sensitivity; the i in the parameter subscript is used for identifying the torsion-radial sound mode corresponding to the relevant parameter, and the value of the i is any mode order in different torsion-radial sound modes.
In a further embodiment of the second aspect, the selected one of the torsional-radial acoustic modes (TR 2,m, m is an integer) is TR 2,15.
In a further embodiment of the second aspect, in step S1, the method for controlling the temperature and the salinity of the medium around the sensing optical fiber to change multiple times according to the set change amount includes:
The method for controlling seven equally-spaced changes of one parameter (salinity) is carried out by keeping the temperature/salinity constant and controlling the temperature and the salinity of the other parameter (salinity/temperature) to be respectively controlled.
In a further embodiment of the second aspect, in step S1, the method for controlling the temperature and the salinity of the medium around the sensing optical fiber to change multiple times according to the set change amount includes:
the temperature of the medium around the sensing optical fiber is changed by adjusting the water bath temperature of the immersed sensing optical fiber;
the salinity of the medium surrounding the sensing optical fiber is changed by adjusting the concentration of NaCl solution immersed in the sensing optical fiber.
In a further embodiment of the second aspect, the dual parameter sensing method of the present invention further comprises:
and S3, calculating and evaluating the uncertainty of the measurement result, and if the obtained uncertainty exceeds a preset threshold, returning to the step S1 and reselecting a torsion-radial sound mode.
In a further embodiment of the second aspect, in step S3, the formula for calculating the uncertainty of the measurement is:
Wherein: δν i and δf i are respectively the frequency shift and the line width measurement error of the forward brillouin scattering of one torsion-radial acoustic mode, and the values of δν i and δf i are obtained by calculating the standard deviation of multiple groups of data of the corresponding torsion-radial acoustic mode in the same state; δT is uncertainty in temperature measurement and δS is uncertainty in salinity measurement.
The beneficial effects are that: the invention utilizes the frequency and linewidth response characteristics of forward Brillouin scattering excited by different torsion-radial acoustic modes in an optical fiber single-end reflecting structure and having different temperatures and salinity to obtain the sensitivity of frequency shift and linewidth to the temperatures and the salinity, and realizes the temperature and salinity double-parameter measurement by measuring the frequency shift change and the linewidth change corresponding to the forward Brillouin scattering peak excited by the torsion-radial acoustic mode with higher sensitivity. Compared with the existing single-parameter optical fiber sensor based on the forward Brillouin scattered light excited by the radial acoustic mode, the scheme of the invention can realize high-precision and effective measurement of double parameters; compared with the traditional Sagnac ring structure, the scheme of the invention can effectively inhibit the radial acoustic mode, excite forward Brillouin scattered light excited by the torsion-radial acoustic mode with high signal-to-noise ratio and narrow line width, and obtain higher measurement precision; compared with a biological optical fiber sensor based on the simultaneous measurement of double parameters of surface plasma resonance, the scheme of the invention has the advantages of simple manufacture and low cost; the retention of the optical fiber coating layer in the experiment enables the optical fiber sensor to have higher mechanical strength, be applicable to more application scenes, effectively avoid the temperature cross sensitivity problem, and realize high-precision double-parameter simultaneous measurement.
Drawings
Fig. 1 is a schematic structural diagram of a forward brillouin scattering dual-parameter sensing system based on an optical fiber single-end reflection structure.
Fig. 2 is a diagram showing a forward brillouin scattering spectrum of a sensing optical fiber excited by a torsional-radial acoustic mode at normal temperature in an embodiment.
Fig. 3 (a) is a graph showing the frequency shift-temperature sensitivity (C ν-T), line width-temperature sensitivity (C Г-T), and experimental measurement results corresponding to a certain torsional-radial acoustic mode (TR 2,15).
Fig. 3 (b) is a graph of experimental measurements of frequency shift-salinity sensitivity (C ν-S) and linewidth-salinity sensitivity (C Г-S) for this mode.
Fig. 4 (a) is a forward brillouin scattering spectrum corresponding to the torsion-radial acoustic mode (TR 2,15) under the condition of state 1.
Fig. 4 (b) is a forward brillouin scattering spectrum of the torsional-radial acoustic mode excitation under the condition of state 2.
Detailed Description
In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the invention may be practiced without one or more of these details. In other instances, well-known features have not been described in detail in order to avoid obscuring the invention.
The invention will now be described in further detail with reference to the accompanying drawings.
The forward Brillouin scattering double-parameter sensing device based on the optical fiber single-end reflection structure shown in fig. 1 comprises a laser light source, an optical isolator, an optical circulator, an optical polarizer, an optical polarization controller, a photoelectric detector, a data processing module and the like.
The laser source adopts a narrow linewidth single-frequency semiconductor laser, can generate continuous light waves with stable wavelength, and in the embodiment, the laser source is output by the narrow linewidth single-frequency laser with the center wavelength of 1550.12nm, and the linewidth of the output laser is 5kHz.
The optical isolator is used for preventing light reflected from the optical circulator from damaging the laser light source.
The 2 port of the optical circulator is connected with the input end of the sensing optical fiber with the coating layer, and the sensing optical fiber with the coating layer is connected with the high-reflectivity optical reflector. In this embodiment, the sensing fiber uses a G657 fiber with a polyimide coating. The coated optical fiber had a length of 38m, a cladding diameter and a coating diameter of 125 μm and 145 μm, respectively, and a transmission loss at 1550nm band of 0.50dB/km.
The 3 port of the optical circulator is connected with the optical polarizer, and the light passing through the optical polarizer becomes linearly polarized light in a specific direction.
The optical polarizer is connected with the optical polarization controller and is used for detecting beat frequency signals of incident light and forward light excited by the torsional-radial acoustic mode, converting the beat frequency signals into corresponding electric signals and sending the corresponding electric signals to the data processing module, and finally, analyzing the electric signals in the data processing module to acquire detailed information of forward Brillouin scattering spectral characteristic change.
In the single-ended reflection structure, due to thermal vibration of the optical fiber, the transverse acoustic field generates phase modulation on the incident light, so that spontaneous forward brillouin scattering is generated, and both the forward brillouin scattering corresponding to the radial acoustic mode and the forward brillouin scattering corresponding to the torsion-radial acoustic mode are excited. The beat signals of the forward light and the incident light are output through the 3 ports of the optical circulator. The optical polarizer is used for filtering out components perpendicular to the polarization plane of the optical polarizer, so that the influence of signals in other polarization directions on beat signals is effectively eliminated. The optical polarization controller phase delays the light (incident light, forward brillouin scattering caused by radial acoustic mode excitation, and forward brillouin scattering caused by torsional-radial acoustic mode excitation). The relative phase of the transmitted light is changed by adjusting the optical polarization controller, and the forward Brillouin scattering corresponding to the torsion-radial acoustic mode with high signal-to-noise ratio and the forward Brillouin scattering corresponding to the radial acoustic mode are excited. The function of the optical polariser and the optical polarisation controller is to obtain high signal to noise ratio forward brillouin scattered light for torsional-radial acoustic mode excitation while suppressing as much as possible the forward brillouin scattered light for radial acoustic mode excitation. The optical wave signals after phase delay are converted into electric signals through the photoelectric detector and are sent to the data processing module, and analysis of the signal spectrum is completed.
Based on the forward Brillouin scattering sensing device of the optical fiber single-end reflection structure, the temperature and salinity double-parameter sensing is carried out, and the method specifically comprises the following steps:
S1, controlling the temperature and the salinity of a medium around the sensing optical fiber to change for a plurality of times according to set variables, obtaining forward Brillouin scattering frequency spectrums of the sensing optical fiber in different states in the changing process, carrying out Lorentz fitting on formants of forward Brillouin scattering excited by one torsion-radial acoustic mode in different states to obtain frequency shift and linewidth variation of the formants in different states, and then carrying out linear fitting on the frequency shift and linewidth variation and the temperature and salinity variation in different states to obtain response coefficients of the frequency shift and the linewidth, wherein the response coefficients are coefficient matrixes formed by the sensitivity of the frequency shift and the linewidth to the temperature and the salinity.
In the process, the method for controlling the temperature and the salinity of the medium around the sensing optical fiber to change for a plurality of times according to the set change amount comprises the following steps:
immersing the sensing optical fiber in a water bath, and changing the temperature of a medium around the sensing optical fiber by adjusting the temperature of the water bath;
Immersing the sensing optical fiber in NaCl solution, and changing the salinity of the medium around the sensing optical fiber by adjusting the concentration of the NaCl solution.
In the process of changing two parameters, each change is to keep one parameter unchanged, control the other parameter to change, and each parameter is changed at least 7 times at equal intervals, so as to complete the change control of the two parameters of temperature and salinity.
S2, acquiring a forward Brillouin scattering spectrum of the sensing optical fiber under a certain temperature and salinity, taking the forward Brillouin scattering spectrum of the sensing optical fiber under a certain temperature and salinity as a reference state, comparing the forward Brillouin scattering spectrum of the sensing optical fiber under the reference state with the forward Brillouin scattering spectrum under the reference state, acquiring a frequency shift variation and a linewidth variation of a torsion-radial acoustic mode under the state to be measured, bringing the frequency shift variation and the linewidth variation under the state to be measured and a response coefficient acquired in the step S1 into a formula (1), and calculating to obtain a measurement result, wherein the measurement result is the variation of the temperature and the salinity of the state to be measured compared with the reference state.
Wherein: delta T is the temperature variation, delta S is the salinity variation; deltav i and Deltaf i are the frequency shift variation and linewidth variation of a torsional-radial acoustic mode, respectively; for frequency shift-temperature sensitivity,/> For the frequency shift-salinity sensitivity,For line width-temperature sensitivity,/>Line width-salinity sensitivity; and i in each parameter subscript is used for identifying the torsion-radial sound mode corresponding to the relevant parameter. For example, in the condition of i=15, the selected torsion-radial acoustic mode is TR 2,15.
In order to verify the accuracy of the dual-parameter sensing method of the present invention, in the test phase, the measurement result uncertainty can be calculated and evaluated, specifically as follows:
And obtaining a forward Brillouin scattering spectrum of the sensing optical fiber under a certain temperature and salinity, taking the state as a reference state, controlling the temperature and salinity of a medium around the sensing optical fiber to change simultaneously, taking the changed state as a state to be measured, obtaining the forward Brillouin scattering spectrum of the sensing optical fiber under the state to be measured, comparing the forward Brillouin scattering spectrum of the sensing optical fiber under the reference state with the forward Brillouin scattering spectrum under the state to be measured, obtaining the frequency shift change amount and the linewidth change amount of forward Brillouin scattering excited by a torsion-radial acoustic mode under the state to be measured, and obtaining a frequency shift measurement error and a linewidth measurement error through multiple measurements.
And (2) calculating and evaluating the uncertainty of the measurement result through a formula, and if the obtained uncertainty exceeds a preset threshold value, returning to the step (1) to reselect a torsion-radial sound mode and executing the subsequent steps until the uncertainty reaches the standard.
Wherein: δν i is a frequency shift measurement error of forward brillouin scattering excited by a torsional-radial acoustic mode, δΓ i is a linewidth measurement error of the torsional-radial acoustic mode, and values of δν i and δΓ i are obtained by calculating standard deviations of multiple sets of data of the corresponding torsional-radial acoustic mode in the same state.
ΔT is uncertainty in temperature measurement and δS is uncertainty in salinity measurement.
In this embodiment, considering the signal-to-noise ratio, the spectral shape, and the high-order mode sensitivity of the torsional-radial acoustic mode TR 2,m in combination, the torsional-radial acoustic mode selected in this embodiment is TR 2,15. The sensing optical fiber has the temperature ranging from 32.90+/-0.05 ℃ to 56.90 +/-0.05 ℃, the salinity ranging from 0 to 10.76+/-0.01%, the temperature measurement error and the salinity measurement error are respectively 0.09 ℃ and 0.06%, the error is small, the measurement precision is high, and the high-precision double-parameter sensing requirement can be met.
As described above, although the present invention has been shown and described with reference to certain preferred embodiments, it is not to be construed as limiting the invention itself. Various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (10)
1. The forward Brillouin scattering double-parameter sensing device based on the optical fiber single-end reflection structure is characterized by comprising a laser light source, an optical fiber single-end reflection structure, an optical polarizer, an optical polarization controller, a photoelectric detector and a data processing module;
The optical fiber single-end reflecting structure comprises an optical circulator, a sensing optical fiber and an optical reflector;
The optical polarizer is connected with the optical polarization controller and is used for detecting beat frequency signals of incident light and forward light excited by a torsion-radial acoustic mode, converting the beat frequency signals into corresponding electric signals and transmitting the corresponding electric signals to the data processing module;
In the optical fiber single-end reflection structure, due to the thermal vibration of an optical fiber, a transverse sound field generates phase modulation on incident light so as to generate spontaneous forward Brillouin scattering, and a forward Brillouin scattering spectrum corresponding to a radial acoustic mode and a forward Brillouin scattering spectrum corresponding to a torsion-radial acoustic mode are excited;
The beat frequency signals of the forward light and the incident light are output through the optical circulator;
the light polarizer is used for filtering out components perpendicular to the polarization plane of the light polarizer;
The optical polarization controller performs phase delay on incident light, forward Brillouin scattering excited by a radial acoustic mode and forward Brillouin scattering excited by a torsion-radial acoustic mode;
The optical polarizer and the optical polarization controller are used for obtaining forward Brillouin scattered light with high signal to noise ratio excited by the torsional-radial acoustic mode and simultaneously inhibiting the forward Brillouin scattered light excited by the radial acoustic mode;
the optical wave signals after phase delay are converted into electric signals through the photoelectric detector and are sent to the data processing module, and analysis of the signal spectrum is completed.
2. The forward brillouin scattering two-parameter sensing device according to claim 1, wherein: an optical isolator is further arranged between the laser light source and the input end of the optical circulator, and after the light wave signals emitted by the laser light source pass through the optical isolator, the light wave signals are incident to the input end of the optical circulator.
3. The forward brillouin scattering two-parameter sensing device according to claim 1, wherein: the sensing optical fiber adopts a G657 optical fiber with a polyimide coating layer.
4. The forward brillouin scattering two-parameter sensing device according to claim 1, wherein: the laser source is output by a narrow linewidth single-frequency laser with a center wavelength of 1550.12nm, and the linewidth of the output laser is 5kHz.
5. A forward brillouin scattering double-parameter sensing method, using the forward brillouin scattering double-parameter sensing device based on the optical fiber single-end reflection structure according to any one of claims 1 to 4, characterized in that the temperature and salinity response characteristics of the sensing optical fiber are obtained by:
s1, controlling the temperature and the salinity of a medium around a sensing optical fiber to change for a plurality of times according to a set change amount, obtaining forward Brillouin scattering spectra of the sensing optical fiber in different states in the change process, performing Lorentz fitting on formants of the forward Brillouin scattering spectra excited by one torsion-radial acoustic mode in different states to obtain change amounts of frequency shift and linewidth in different states, and performing linear fitting on the change amounts of the frequency shift and the linewidth and the temperature and the salinity change amounts in different states to obtain response coefficients of the frequency shift and the linewidth;
S2, acquiring a forward Brillouin scattering spectrum of the sensing optical fiber at a certain temperature and a certain salinity, taking the state as a reference state, acquiring the forward Brillouin scattering spectrum of the sensing optical fiber at a state to be measured, comparing the forward Brillouin scattering spectrum of the sensing optical fiber at the reference state with the forward Brillouin scattering spectrum of the sensing optical fiber at the state to be measured, acquiring a frequency shift variation and a line width variation of the forward Brillouin scattering spectrum excited by a torsion-radial acoustic mode at the state to be measured, and calculating a measurement result according to the frequency shift variation and the line width variation at the state to be measured and the response coefficient acquired in the step S1, wherein the measurement result is the variation of the temperature and the salinity.
6. The method of dual-parameter sensing for forward brillouin scattering according to claim 5, wherein in step S1, the temperature and salinity of the medium around the control sensing fiber change multiple times according to a set change amount, including:
The method for controlling the other parameter to change at equal intervals for several times is used for controlling the change of the temperature and the salinity respectively.
7. The method of claim 6, wherein the controlling the temperature and salinity of the medium around the sensing fiber to change a plurality of times according to the set amount of change comprises:
the temperature of the medium around the sensing optical fiber is changed by adjusting the water bath temperature of the immersed sensing optical fiber;
the salinity of the medium surrounding the sensing optical fiber is changed by adjusting the concentration of NaCl solution immersed in the sensing optical fiber.
8. The method of claim 5, wherein in step S2, the measurement result is calculated according to the frequency shift variation and the line width variation under the to-be-measured state and the response coefficient obtained in step S1, and the calculation formula is as follows:
Wherein DeltaT is the temperature variation and DeltaS is the salinity variation; deltav i and Deltaf i are the frequency shift variation and linewidth variation of a torsional-radial acoustic mode, respectively; for frequency shift-temperature sensitivity,/> For frequency shift-salinity sensitivity,/>For line width-temperature sensitivity,/>Line width-salinity sensitivity; the i in the parameter subscript is used for identifying the torsion-radial sound mode corresponding to the relevant parameter, and the value of the i is any mode order in different torsion-radial sound modes.
9. The forward brillouin scattering two-parameter sensing method according to claim 5, further comprising:
and S3, calculating and evaluating the uncertainty of the measurement result, and if the obtained uncertainty exceeds a preset threshold, returning to the step S1 and reselecting a torsion-radial sound mode.
10. The forward brillouin scattering two-parameter sensing method according to claim 9, wherein the uncertainty of the measurement result in step S3 is calculated as follows:
Wherein δν i and δf i are respectively the frequency shift of forward brillouin scattering and the line width measurement error of a torsion-radial acoustic mode, and the values of δν i and δf i are obtained by calculating the standard deviation of a plurality of groups of data of the corresponding torsion-radial acoustic mode in the same state; δT is uncertainty in temperature measurement and δS is uncertainty in salinity measurement.
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