Dual-parameter sensing device and method based on forward brillouin scattering temperature and axial strain of a direct fiber probing structure

Abstract

The application discloses a forward Brillouin scattering temperature and axial strain dual-parameter sensing device and method based on a fiber direct detection structure, and a forward Brillouin scattering spectrum with a narrow linewidth and a high signal-to-noise ratio excited by a torsional-radial acoustic mode is obtained by using a light polarization controller and a light polarizer for the first time. The light polarization controller enhances the optical beat of the forward Brillouin scattering corresponding to the pump light and the torsional-radial acoustic mode, and weakens the optical beat of the forward Brillouin scattering corresponding to the pump light and the radial acoustic mode. The light polarizer is used for filtering out the component perpendicular to the polarization plane, and effectively eliminates the influence of signals of other polarization directions on the optical beat signal. By simultaneously measuring the frequency shift-temperature and the frequency shift-axial strain two response coefficients, the 2*2 coefficient matrix is used to effectively avoid the temperature cross-sensitivity problem, and high-precision temperature and axial strain dual-parameter measurement can be realized. The sensing device has a simple manufacturing method and realizes high-precision simultaneous measurement.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical fiber sensing, and particularly to a forward Brillouin scattering temperature and axial strain dual-parameter sensing device and method based on a direct detection structure of an optical fiber. BACKGROUND

[0002] Due to the multiple resonance peaks of the forward Brillouin scattering spectrum, simultaneous detection of multiple parameters in the surrounding environment is one of the most basic and challenging problems in the research of forward Brillouin scattering optical fiber sensors. Among them, temperature and axial strain sensors are extremely attractive due to their unique ability to monitor infrastructure, structural health monitoring, and various industrial manufacturing processes.

[0003] The phenomenon of forward Brillouin scattering in optical fibers has been studied since the mid-1980s. Due to the thermal motion characteristics of the sensing optical fiber itself, the phase / polarization state of the incident light is modulated by the transverse acoustic field, resulting in the excitation of spontaneous forward Brillouin scattering spectrum of radial acoustic mode and torsional-radial acoustic mode. So far, a large number of forward Brillouin scattering spectrum measurement methods based on Sagnac loop structure have been studied, and these methods have the disadvantages of low signal-to-noise ratio and wide line width, which leads to low accuracy of Lorentz fitting. Forward Brillouin scattering excited by radial acoustic mode based on Sagnac loop structure has been used for dual-parameter measurement, and the measured dual parameters include strain and temperature, temperature and acoustic impedance, relative humidity and temperature. In the prior art 1 (Lu Yuangang, Zhang Zelin, Peng Keqin, Ji Zhengyuan, Temperature and acoustic impedance dual-parameter sensing method and device based on forward Brillouin scattering, application number CN 202110222185.0, invention patent granted) published by Lu Yuangang et al., the two optical polarization controllers in the Sagnac loop structure sensing device increase the complexity of adjusting the polarization state of the forward transmission light wave in the experimental process, and half of the output light energy is lost because half of the light energy enters the laser light source. In the prior art 2 (Lu Yuangang, Zhang Wujun, He Chongjun, Zhu Meng, Liu Peng, Kai Li Mengli, Sun Rui, Forward Brillouin scattering dual-parameter sensing device and method based on optical fiber single-end reflection structure, application number CN 202410151319.8, invention patent published) published by Lu Yuangang et al., there is a circulator, an optical polarization controller and a plane mirror in the optical fiber single-end reflection structure, and the light wave passing through the plane mirror cannot be fully reflected, resulting in energy loss. How to further reduce the loss of signal light energy and reduce the complexity of the detection system needs further research. At the same time, in the past experimental research and practical application, the bending resistance of some conventional single-mode optical fibers is poor, which makes the optical fiber sensing unit unable to bend, affecting the normal monitoring in engineering.

[0004] How to build a simple sensing device to produce forward Brillouin scattering spectrum excited by torsional-radial acoustic mode for dual-parameter sensing measurement, while overcoming the disadvantages of low signal-to-noise ratio, wide linewidth and poor bending resistance, is an important challenge for high-precision temperature and axial strain dual-parameter simultaneous measurement. SUMMARY

[0005] The present application provides a forward Brillouin scattering temperature and axial strain dual-parameter sensing device and method based on a fiber direct detection structure, which has the advantages of high measurement accuracy, high sensitivity, strong bending resistance, simple manufacturing method, simple structure, and can eliminate temperature crosstalk.

[0006] In order to achieve the above object, the technical scheme of the present application is:

[0007] A forward Brillouin scattering temperature and axial strain dual-parameter sensing device based on a fiber direct detection structure, comprising a laser light source, an optical isolator, a sensing optical fiber, an optical polarization controller, an optical polarizer, a photodetector and a data processing module, the laser light source, the optical isolator, the sensing optical fiber, the optical polarization controller, the optical polarizer and the photodetector are arranged in sequence along the propagation direction of light, and the whole sensing system structure is in the form of a single line. Compared with the Sagnac ring structure and the single-end reflection structure, the single line structure is simpler and the energy loss of the signal light is smaller. The polarization state of the optical polarizer is set to the x direction, and the vibration directions of the pump light, the forward Brillouin scattering light corresponding to the radial acoustic mode and the forward Brillouin scattering light corresponding to the torsional-radial acoustic mode are all along the x axis. At this time, the initial state of the polarization controller is the state that has the least effect on the light intensity. Slowly adjust the first and second rotating wave plates of the optical polarization controller, and at the same time observe the optical beat effect between the pump light and the forward Brillouin scattering light corresponding to the torsional-radial acoustic mode, to determine a position that makes the polarization states of the pump light and the forward Brillouin scattering light corresponding to the torsional-radial acoustic mode as same as possible (i.e. their electric field vectors in the polarization space are as consistent as possible, usually along the x axis). This will obtain the enhancement of the optical beat amplitude between the pump light and the forward Brillouin scattering light corresponding to the torsional-radial acoustic mode. While maintaining the optical beat enhancement between the pump light and the forward Brillouin scattering light corresponding to the torsional-radial acoustic mode, adjust the position of the third rotating wave plate of the polarization controller to change the polarization state of the forward Brillouin scattering light corresponding to the radial acoustic mode, so that it is as different as possible from the polarization state of the pump light to weaken the optical beat between the pump light and the forward Brillouin scattering light corresponding to the radial acoustic mode. The polarizer is used to filter out the components perpendicular to its polarization plane, effectively eliminating the influence of signals with other polarization directions on the beat frequency signal.

[0008] Optionally, the laser light source comprises a narrow linewidth single frequency laser.

[0009] Optionally, the sensing optical fiber is a high-bending-resistant BI 1011-A single-mode optical fiber with a coating layer.

[0010] The application further provides a dual-parameter sensing method for forward Brillouin scattering temperature and axial strain based on a fiber direct detection structure, which is applied to the dual-parameter sensing device for forward Brillouin scattering temperature and axial strain based on a fiber direct detection structure and comprises the following steps:

[0011] In step S1, the acoustic characteristics of the sensing optical fiber are changed by different temperatures and axial strains so that the frequency shift of the spectral resonance peak has different response characteristics, and the frequency shift response coefficients of the frequency shift to the temperature and the axial strain are obtained.

[0012] In step S2, the measurement results including the temperature change and the axial strain change are solved by simultaneously measuring the frequency shift change amounts of the two resonance peaks.

[0013] In step S3, the uncertainty of the measurement results is calculated for verification.

[0014] In step S4, if the obtained measurement uncertainty excessively exceeds the theoretical calculation value, the two torsional-radial acoustic modes are reselected in step S1, and the subsequent steps are executed until the measurement uncertainty reaches the ideal value.

[0015] Further, in step S1, the temperature around the sensing optical fiber and the axial strain applied to the sensing optical fiber are changed multiple times, and the forward Brillouin scattering spectrum of the sensing optical fiber in multiple states during the change is obtained. The resonance peaks of the two torsional-radial acoustic modes in the multiple states are Lorenz fitted to obtain the corresponding frequency shift change amounts. Then, the frequency shift change amounts and the temperature and axial strain change amounts in the multiple states are linearly fitted to obtain the frequency shift response coefficients of the frequency shift to the temperature and the axial strain.

[0016] Further, the method for changing the temperature around the sensing optical fiber and the axial strain applied to the sensing optical fiber multiple times comprises the following steps.

[0017] The sensing optical fiber is fixed on a heating platform, and the sensing optical fiber is in a relaxed and strain-free state. The temperature around the sensing optical fiber is changed.

[0018] Part of the sensing optical fiber is fixed on a heating platform, and another part of the sensing optical fiber is fixed on a micro-displacement platform. The axial strain applied to the other part of the sensing optical fiber is changed by adjusting the micro-displacement platform in a normal temperature state.

[0019] According to the set values, the two parameters of temperature and axial strain are changed, each time the change is keeping one parameter unchanged and controlling the other parameter to change, and each parameter changes at least 8 times at equal intervals, and the control of the two parameters of temperature and axial strain is completed respectively.

[0020] Further, in the step S2, the forward Brillouin scattering spectrum of the sensing fiber under a certain state of temperature and axial strain is obtained, and the forward Brillouin scattering spectrum under the state is taken as a reference state; the forward Brillouin scattering spectrum of the sensing fiber under another state of temperature and axial strain is obtained, and the forward Brillouin scattering spectrum under the state is taken as a to-be-measured state; the forward Brillouin scattering spectra of the sensing fiber under the two states are compared, the frequency shift change amounts of the two torsional-radial acoustic modes under the two states are obtained, and the measurement result is calculated according to the frequency shift change amounts under the two states and the frequency shift response coefficients obtained in the step S1, the measurement result includes the temperature change amount and the axial strain change amount, and the expression is as follows:

[0021]

[0022] Wherein, ΔT is the temperature change amount, Δε is the axial strain change amount, Δv i and Δv j are the frequency shift change amounts of the two different orders i-th and j-th torsional-radial acoustic modes respectively; is the i-th order frequency shift-temperature coefficient, is the i-th order frequency shift-axial strain coefficient, is the j-th order frequency shift-temperature coefficient, is the j-th order frequency shift-axial strain coefficient; i and j take values of any two mode orders in the different torsional-radial acoustic modes.

[0023] Further, in the step S3, the forward Brillouin scattering spectrum of the sensing fiber under a certain state of temperature and axial strain is obtained, and the state is taken as a reference state; the temperature around the sensing fiber and the axial strain applied to the sensing fiber are controlled to change simultaneously, and the state is taken as a to-be-measured state, and the forward Brillouin scattering spectrum of the sensing fiber under the to-be-measured state is collected; the forward Brillouin scattering spectra of the sensing fiber under the reference state and the to-be-measured state are compared, the frequency shift change amounts of the torsional-radial acoustic modes under the two states are obtained, and the frequency shift measurement error of the forward Brillouin scattering excited by the two torsional-radial acoustic modes is obtained by measuring multiple times under a fixed state.

[0024] Further, in the step S3, the measurement uncertainty of the measurement result is calculated by the following formula:

[0025]

[0026] where, δv i and δv j are the measurement errors of the frequency shift of the forward Brillouin scattering excited by the two torsional-radial acoustic modes, respectively, obtained by the standard deviation of a plurality of groups of data collected by the corresponding two torsional-radial acoustic modes in a certain state; δT is the measurement uncertainty of the temperature, and δε is the measurement uncertainty of the axial strain.

[0027] Beneficial effects, the optical fiber direct detection structure built by the application, including laser light source, optical isolator, optical polarization controller, optical polarizer, photodetector and data processing module, the use of an optical polarization controller makes the sensing device simple. Different forward Brillouin scattering spectrum resonance peaks have different frequency response characteristics of temperature and axial strain, realizing the simultaneous measurement of temperature and axial strain dual parameters. Compared with the existing single-parameter optical fiber sensor based on forward Brillouin scattering spectrum, the application scheme can realize effective measurement of dual parameters; compared with the existing traditional Sagnac ring structure, the application scheme can effectively suppress the radial acoustic mode and reduce energy loss, excite the forward Brillouin scattering spectrum of the torsional-radial acoustic mode with high signal-to-noise ratio, narrow linewidth and more resonance peaks in the same frequency range, and obtain higher measurement accuracy; compared with the optical fiber single-end reflection structure, the sensing device of the application scheme is simple and has high energy transmittance; compared with the biological or chemical optical fiber sensor based on microstructure for dual-parameter simultaneous measurement, the application scheme has the advantages of simple manufacturing method, low cost and high success rate; the selection of special single-mode optical fiber in the specific embodiment makes the optical fiber sensor have higher bending resistance and can be applied to more engineering applications, effectively avoiding the temperature cross-sensitivity problem while realizing high-precision dual-parameter simultaneous measurement.

[0028] In order to make the above features and advantages of the application more obvious and easy to understand, the following specific examples are described in detail below, and the accompanying drawings are described as follows. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 It is a structural schematic diagram of the forward Brillouin scattering temperature and axial strain dual-parameter measurement sensing device based on the optical fiber direct detection structure of the application.

[0030] Figure 2 It is a flow chart of the forward Brillouin scattering temperature and axial strain dual-parameter measurement sensing method based on the optical fiber direct detection structure of the application.

[0031] Figure 3 It is the forward Brillouin scattering spectrum excited by the torsional-radial acoustic mode of the sensing optical fiber in a specific embodiment under normal temperature conditions.

[0032] Fig. 4(a) is the forward Brillouin scattering spectrum excited by the two torsional-radial acoustic modes (TR 2,23 and TR2,29 ) corresponding frequency shift-temperature coefficient (C ν-T ) experimental measurement linear fitting results figure.

[0033] Figure 4 (b) is a two torsional-radial acoustic mode (TR 2,23 and TR 2,29 ) corresponding frequency shift-axial strain coefficient (C ν-ε ) experimental measurement linear fitting results figure

[0034] Figure 5 (a) is a forward Brillouin scattering spectrum diagram of the torsional-radial acoustic mode (TR 2,23 and TR 2,29 ) excited in the reference state.

[0035] Figure 5 (b) is a forward Brillouin scattering spectrum diagram of the torsional-radial acoustic mode (TR 2,23 and TR 2,29 ) excited in the to-be-measured state. DETAILED DESCRIPTION

[0036] In order to make the purpose and technical scheme of the embodiments of the present application clearer, the technical scheme of the embodiments of the present application will be described clearly and completely below in combination with the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the described embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without any creative effort belong to the scope of protection of the present application.

[0037] Figure 1 A structure schematic diagram of a forward Brillouin scattering temperature and axial strain dual-parameter sensing device based on a fiber direct detection structure is shown, and the forward Brillouin scattering temperature and axial strain dual-parameter sensing device based on the fiber direct detection structure comprises a laser light source 1, an optical isolator 2, a sensing optical fiber 3, an optical polarization controller 4, an optical polarizer 5, a photodetector 6 and a data processing module 7. The laser light source 1, the optical isolator 2, the sensing optical fiber 3, the optical polarization controller 4, the optical polarizer 5 and the photodetector 6 are sequentially arranged along the propagation direction of light, and the entire sensing system structure is in the form of a “single line”.

[0038] More specifically, the laser emitted by the laser light source 1 enters the sensing optical fiber 3 after passing through the optical isolator 2, the sensing optical fiber 3 is used to sense the changes of temperature and axial strain in the external environment, the changes of the temperature and the axial strain will cause the changes of the laser spectrum passing through the sensing optical fiber 3, the optical polarization controller 4 and the optical polarizer 5 modulate the phase / polarization state of the laser passing through the sensing optical fiber 3, to generate the beat frequency signal of the pump light and the forward Brillouin scattering light excited by the torsional-radial acoustic mode, the photoelectric detector 6 converts the beat frequency signal into the corresponding electrical signal and sends it to the data processing module 7, the data processing module 7 collects and analyzes the frequency spectrum of the electrical signal to obtain the forward Brillouin scattering spectrum characteristic information, and calculates the values of the temperature and the axial strain according to the forward Brillouin scattering spectrum characteristic information.

[0039] Optionally, the laser light source 1 comprises a narrow linewidth single-frequency laser; the output laser linewidth of the narrow linewidth single-frequency laser is 5 kHz, the maximum output power is 14.5 dBm, and the center wavelength is 1550.12 nm.

[0040] Further, the optical isolator 2 is used to prevent the reflection of the pump light signal emitted by the laser light source 1 from damaging the laser light source 1.

[0041] Further, all the light passing through the optical polarizer 5 is converted into linearly polarized light with a specific direction.

[0042] Optionally, the sensing optical fiber 3 comprises a high-bending-resistant BI 1011-A single-mode optical fiber with a coating layer.

[0043] Optionally, the sensing optical fiber 3 has a length of 30 m, a cladding diameter and a coating layer diameter of 125 μm and 250 μm respectively, and a transmission loss of 0.21 dB / km at a wavelength of 1550 nm.

[0044] Further, in the optical fiber direct detection structure of the present application, the thermal vibration in the sensing fiber induces phase / polarization state modulation of the incident light through the transverse acoustic field. Therefore, this phase / polarization state modulation triggers the forward Brillouin scattering spectrum of the radial acoustic mode and the torsional-radial acoustic mode excitation. Due to the birefringence in the sensing fiber, the forward propagating light waves, including the pump light and the forward Brillouin scattering light of the radial acoustic mode and the torsional-radial acoustic mode excitation, are highly likely to be elliptically polarized light. The spatial polarization mode interaction caused by the random birefringence of the non-polarization maintaining fiber changes the electric field distribution of the fiber along its two principal axes, resulting in the beat note phenomenon between the light beam projections along the x and y axes of the fiber coordinate system. Without loss of generality, it is assumed that the intensities of the forward Brillouin scattering light of the radial acoustic mode and the torsional-radial acoustic mode excitation are equal, and the electric field vector in the x direction is greater than that in the y direction. For the sake of clarity, the linear polarization direction of the forward propagating light wave through the optical polarizer 5 is defined as the x axis.

[0045] By carefully adjusting the rotatable wave plate position of the optical polarization controller 4, the beat note between the pump light and the forward Brillouin scattering light of the torsional-radial acoustic mode excitation is enhanced, and the beat note between the pump light and the forward Brillouin scattering light of the radial acoustic mode excitation is weakened. Slowly adjust the first and second rotating wave plates of the optical polarization controller while observing the optical beat effect between the pump light and the forward Brillouin scattering light of the torsional-radial acoustic mode, to determine a position that makes the polarization states of the pump light and the forward Brillouin scattering light of the torsional-radial acoustic mode as identical as possible (i.e., their electric field vectors in the polarization space are as consistent as possible, usually the x axis). This will obtain the enhancement of the optical beat amplitude between the pump light and the forward Brillouin scattering light of the torsional-radial acoustic mode. While maintaining the optical beat enhancement between the pump light and the forward Brillouin scattering light of the torsional-radial acoustic mode, adjust the position of the third rotating wave plate of the polarization controller to change the polarization state of the forward Brillouin scattering light of the radial acoustic mode, so that it is as different as possible from the polarization state of the pump light to weaken the optical beat between the pump light and the forward Brillouin scattering light of the radial acoustic mode. After the light passing through the optical polarization controller 4 passes through the optical polarizer 5, only the projection component of the pump light on the x axis and the projection component of the radial acoustic mode on the x axis are left, indicating that the optical polarizer 5 filters out all the y axis components. Assuming that the insertion loss in the optical polarizer 5 is negligible, the light wave propagating forward through the optical polarization controller 4 and the optical polarizer 5 maintains the same projection component along the x axis. The optical wave signal after the acousto-optic interaction is converted into an electrical signal by the photodetector 6 and sent to the data processing module 7 for analysis and collection of the frequency spectrum of the electrical signal.

[0046] As Figure 2As shown, the forward Brillouin scattering temperature and axial strain dual-parameter sensing method based on the optical fiber direct detection structure specifically comprises the following steps:

[0047] In step S1, the acoustic characteristics of the sensing optical fiber are changed by different temperatures and axial strains so that the frequency shift of the spectral resonance peak has different response characteristics, and the frequency shift response coefficients of the frequency shift to the temperature and the axial strain are obtained.

[0048] More specifically, the temperature around the sensing optical fiber and the axial strain applied to the sensing optical fiber are changed multiple times, and the forward Brillouin scattering spectrum of the sensing optical fiber in multiple states during the change is obtained; and the resonance peaks of the two torsional-radial acoustic modes in multiple states are Lorenz fitted to obtain the corresponding frequency shift change; then the frequency shift change and the temperature and axial strain change in multiple states are linearly fitted to obtain the frequency shift response coefficients of the frequency shift to the temperature and the axial strain.

[0049] Further, the method for controlling the temperature around the sensing optical fiber 3 and the axial strain applied to the sensing optical fiber 3 to change multiple times comprises:

[0050] The sensing optical fiber 3 is fixed on the heating platform, and the sensing optical fiber 3 is in a relaxed and strain-free state, and the temperature around the sensing optical fiber 3 is changed.

[0051] A part of the sensing optical fiber 3 is fixed on the heating platform, and another part of the sensing optical fiber 3 is fixed on the micro-displacement platform, and the axial strain applied to the other part of the sensing optical fiber is changed by adjusting the micro-displacement platform in a normal temperature state.

[0052] According to the set value, the two parameters of temperature and axial strain are changed, each change keeps one parameter unchanged and controls the other parameter to change, and each parameter changes at least 8 times at equal intervals, and the control of the two parameters of temperature and axial strain is completed respectively.

[0053] In step S2, the frequency shift change corresponding to the two resonance peaks is measured simultaneously to solve the measurement result, and the measurement result includes the temperature change and the axial strain change.

[0054] More specifically, the forward Brillouin scattering spectrum of the sensing fiber in a certain state is obtained, and the forward Brillouin scattering spectrum in the certain state is taken as a reference state; the forward Brillouin scattering spectrum of the sensing fiber in another state is obtained, and the forward Brillouin scattering spectrum in the another state is taken as a to-be-measured state; the forward Brillouin scattering spectrum of the sensing fiber in the two states is compared, the frequency shift variation of the two torsional-radial acoustic modes in the two states is obtained, and the measurement result is calculated according to the frequency shift variation in the two states and the frequency shift response coefficient obtained in step S1, the measurement result includes the temperature variation and the axial strain variation, and the expression is as follows:

[0055]

[0056] Wherein, ΔT is the temperature variation, Δε is the axial strain variation; Δv i and Δv j are the frequency shift variation of the two different orders i-th and j-th torsional-radial acoustic modes, respectively. is the i-th order frequency shift-temperature coefficient, is the i-th order frequency shift-axial strain coefficient, is the j-th order frequency shift-temperature coefficient, is the j-th order frequency shift-axial strain coefficient; i and j are values of any two mode orders in different torsional-radial acoustic modes.

[0057] Step S3, the uncertainty of the measurement result is calculated for verification.

[0058] More specifically, the forward Brillouin scattering spectrum of the sensing fiber in a certain state is obtained, and the certain state is taken as a reference state; the temperature around the sensing fiber and the axial strain applied to the sensing fiber are controlled to change simultaneously, and the certain state is taken as a to-be-measured state, and the forward Brillouin scattering spectrum of the sensing fiber in the to-be-measured state is collected; the forward Brillouin scattering spectrum of the sensing fiber in the reference state is compared with the forward Brillouin scattering spectrum of the sensing fiber in the to-be-measured state, the frequency shift variation of the torsional-radial acoustic mode in the two states is obtained, and the frequency shift measurement error of the forward Brillouin scattering excited by the two torsional-radial acoustic modes is obtained by measuring multiple times in a fixed state.

[0059] Further, the measurement uncertainty of the measurement result is calculated by the following formula:

[0060]

[0061] Wherein, δv i and δv jrespectively, are obtained from the standard deviation of the multiple sets of data collected in a certain state by the corresponding two torsional-radial acoustic modes; δT is the measurement uncertainty of temperature, and δε is the measurement uncertainty of axial strain.

[0062] Step S4: If the obtained measurement uncertainty exceeds the theoretical calculation value, return to step S1 to reselect the two torsional-radial acoustic modes and perform the subsequent steps until the measurement uncertainty reaches the ideal value.

[0063] In summary, the present application discloses a forward Brillouin scattering temperature and axial strain dual-parameter sensing device and method based on a fiber direct detection structure. The sensing device (fiber direct detection structure) includes a laser light source, an optical isolator, an optical polarization controller, an optical polarizer, a photodetector, and a data processing module. The optical polarization controller and the optical polarizer modulate the polarization state of the forward transmission light wave. The torsional-radial acoustic mode excited by the acousto-optic interaction of the sensing fiber excites the forward Brillouin scattering spectrum with a narrow linewidth and a high signal-to-noise ratio. The different temperature and axial strain changes the acoustic characteristics of the sensing fiber, making the frequency shift of the spectral resonance peak have different response characteristics. The frequency shift response coefficients of the frequency shift to the temperature and axial strain are obtained. By simultaneously measuring the frequency shift changes of the two resonance peaks, the temperature change and the axial strain change are solved by using a 2x2 coefficient matrix. The temperature cross-sensitivity problem is effectively avoided, and high-precision temperature and axial strain dual-parameter measurement can be realized. The sensing device of the present application has a simple manufacturing method and is expected to become a high-precision temperature and axial strain sensing method with high sensitivity, light weight, strong anti-electromagnetic interference ability, and strong bending resistance. It has potential application prospects in the fields of infrastructure, structural health monitoring, and various industrial manufacturing processes.

[0064] Reference Figures 3 to 5(b) In a specific embodiment, according to the mode selection criteria, the selected torsional-radial acoustic mode is TR 2,23 and TR 2,29 The temperature measurement error and the axial strain measurement error are 0.17℃ and 21με, respectively, when the sensing fiber is in the temperature range of 28.8-63.1℃ and the axial strain range of 0-1400με. As can be seen, the forward Brillouin scattering temperature and axial strain dual-parameter sensing device based on the fiber direct detection structure has high measurement accuracy and can meet the requirements of high-precision dual-parameter sensing measurement.

[0065] Although the present application has been disclosed as above with examples, it is not intended to limit the present application, and anyone with ordinary knowledge in the art can make some changes and modifications without departing from the spirit and scope of the present application. Therefore, the protection scope of the present application shall be subject to the appended patent claim scope.

Claims

1. A dual-parameter sensing device for forward Brillouin scattering temperature and axial strain based on an optical fiber direct detection structure, characterized in that, The system includes a laser source, an optical isolator, a sensing fiber, an optical polarization controller, an optical polarizer, a photodetector, and a data processing module. The laser source, the optical isolator, the sensing fiber, the optical polarization controller, the optical polarizer, and the photodetector are arranged sequentially along the direction of light propagation, and the entire sensing system has a "single-line" structure. The sensing fiber is used to sense changes in temperature and axial strain in the external environment. By altering the acoustic properties of the sensing fiber with different temperatures and axial strains, the frequency shift of the spectral resonance peak of the sensing fiber is made to have different response characteristics, and the frequency shift response coefficients to temperature and axial strain are obtained. The optical polarization controller includes a rotatable waveplate, which, by adjusting the position of the rotatable waveplate, enhances the optical beat between the forward Brillouin scattered light excited by the torsional-radial acoustic mode and the pump light, and weakens the optical beat between the forward Brillouin scattered light excited by the radial acoustic mode and the pump light.

2. The dual-parameter sensing device for forward Brillouin scattering temperature and axial strain based on a fiber optic direct detection structure as described in claim 1, characterized in that, The laser source includes a narrow-linewidth single-frequency laser.

3. The dual-parameter sensing device for forward Brillouin scattering temperature and axial strain based on a fiber optic direct detection structure as described in claim 1, characterized in that, The sensing fiber includes a BI 1011-A single-mode fiber with a coating for high bending resistance.

4. A dual-parameter sensing method for forward Brillouin scattering temperature and axial strain based on an optical fiber direct sensing structure, applied to the dual-parameter sensing device for forward Brillouin scattering temperature and axial strain based on an optical fiber direct sensing structure as described in any one of claims 1-3, characterized in that, include: Step S1: By changing the acoustic properties of the sensing fiber with different temperatures and axial strains, the frequency shift of its spectral resonance peak has different response characteristics, and the frequency shift response coefficients of the frequency shift to temperature and axial strain are obtained. Step S2: By simultaneously measuring the frequency shift changes corresponding to the two resonance peaks, the measurement results are obtained, including the temperature change and the axial strain change. Step S3: Calculate and verify the uncertainty of the measurement results; In step S4, if the obtained measurement uncertainty exceeds the theoretical calculation value excessively, return to step S1 to reselect two torsional-radial acoustic modes and execute subsequent steps until the measurement uncertainty reaches the ideal value.

5. The dual-parameter sensing method for forward Brillouin scattering temperature and axial strain based on a fiber optic direct detection structure as described in claim 4, characterized in that... In step S1, the temperature around the sensing fiber and the axial strain applied to the sensing fiber are controlled to change multiple times to obtain the forward Brillouin scattering spectra of the sensing fiber under various states of temperature and axial strain during the change process; and Lorentz fitting is performed on the resonance peaks of the two torsional-radial acoustic modes under various states to obtain the corresponding frequency shift changes; then the frequency shift changes are linearly fitted with the temperature and axial strain changes under various states to obtain the frequency shift response coefficients of the frequency shift to temperature and axial strain.

6. The dual-parameter sensing method for forward Brillouin scattering temperature and axial strain based on an optical fiber direct detection structure as described in claim 5, characterized in that, Methods for controlling multiple variations in the temperature surrounding the sensing fiber and the axial strain applied to the sensing fiber include: The sensing fiber is fixed to the heating platform and is in a relaxed, strain-free state. The temperature around the sensing fiber is changed. One part of the sensing fiber is fixed on the heating platform, and the other part of the sensing fiber is fixed on the micro-displacement platform. At room temperature, the axial strain applied to the other part of the sensing fiber is changed by adjusting the micro-displacement platform. The temperature and axial strain parameters are changed according to the set values. Each change keeps one parameter constant while controlling the other parameter to change. Each parameter changes at least 8 times at equal intervals to control the temperature and axial strain parameters respectively.

7. The dual-parameter sensing method for forward Brillouin scattering temperature and axial strain based on a fiber optic direct detection structure as described in claim 6, characterized in that... In step S2, the forward Brillouin scattering spectrum of the temperature and axial strain of the sensing fiber under a certain state is obtained, and the forward Brillouin scattering spectrum under that state is used as a reference state. Next, obtain the forward Brillouin scattering spectrum of the sensing fiber under another state, considering its temperature and axial strain. Use this forward Brillouin scattering spectrum as the state to be measured. Compare the forward Brillouin scattering spectra of the sensing fiber under the two states to obtain the frequency shift changes of the two torsional-radial acoustic modes under these two states. Calculate the measurement results based on the frequency shift changes under the two states and the frequency shift response coefficients obtained in step S1. The measurement results include the temperature change and the axial strain change, expressed as follows: in: The change in temperature This represents the change in axial strain. and They are two different orders. i-th and j-th The frequency shift of the torsional-radial acoustic mode; for i-th Frequency shift-temperature coefficient of the order, for i-th Frequency shift-axial strain coefficient of the order, for j-th Frequency shift-temperature coefficient of the order, for j-th Frequency shift of the order - axial strain coefficient; and The value is taken as the order of any two modes in different torsional-radial acoustic modes.

8. The dual-parameter sensing method for forward Brillouin scattering temperature and axial strain based on a fiber optic direct detection structure as described in claim 7, characterized in that, In step S3, the forward Brillouin scattering spectrum of the temperature and axial strain of the sensing fiber under a certain state is obtained, and this state is used as a reference state. The temperature around the sensing fiber and the axial strain applied to the sensing fiber are changed simultaneously. This state is taken as the test state, and the forward Brillouin scattering spectrum of the sensing fiber under the test state is collected. By comparing the forward Brillouin scattering spectrum of the sensing fiber in the reference state and the forward Brillouin scattering spectrum in the test state, the frequency shift change of the torsional-radial acoustic mode in the two states is obtained. The frequency shift measurement error of the forward Brillouin scattering excited by the two torsional-radial acoustic modes is obtained by multiple measurements in a fixed state.

9. The dual-parameter sensing method for forward Brillouin scattering temperature and axial strain based on a fiber optic direct detection structure as described in claim 8, characterized in that, In step S3, the measurement uncertainty of the measurement result is calculated using the following formula: in, δ and δ The frequency shift measurement error of the forward Brillouin scattering excited by the two torsional-radial acoustic modes is obtained from the standard deviation of multiple sets of data collected by the two torsional-radial acoustic modes under a certain state. δT For the measurement uncertainty of temperature, δε This represents the measurement uncertainty of the axial strain.

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