Experimental device, experimental method and demodulation method based on tilted fiber grating refractive index sensing

Abstract

The invention belongs to the technical field of optics, and discloses an experimental device, experimental method and demodulation method based on tilted fiber bragg grating refractive index sensing, the experimental device comprises a broadband light source, a polarization controller, a tilted fiber bragg grating, a liquid sample pool, a data acquisition module, a temperature automatic compensation module and a data processing module, the light output end of the broadband light source is connected with the light input end of the polarization controller, the light output end of the polarization controller is connected with the light input end of the tilted fiber bragg grating, the light output end of the tilted fiber bragg grating is connected with the data acquisition module, and the data acquisition module is connected with the automatic temperature compensation module. The temperature automatic compensation module is connected with the data processing module, and the tilted fiber bragg grating is arranged in the liquid sample pool. The experimental device is relatively simple in structure, good in experimental real-time performance and capable of meeting the requirements for high sensitivity, microscale and dynamic monitoring.

Description

Technical Field

[0001] The invention belongs to the field of optical technology, and in particular relates to an experimental device, an experimental method and a demodulation method based on tilted optical fiber grating refractive index sensing. Background Art

[0002] Tilted Fiber Bragg Grating (TFBG) is a new type of fiber optic sensor device that achieves coupling between the core mode and the cladding mode of an optical fiber. It has attracted widespread attention due to its high sensitivity to the refractive index. The measurement of refractive index has important applications in optical sensing, biomedicine, chemical analysis, and environmental monitoring. TFBG can accurately detect small changes in the refractive index of the external environment through the coupling of the core mode and the cladding mode, and is an important tool for measuring the refractive index.

[0003] In the relevant technical field, traditional refractive index measurement methods mainly rely on refractometers and interferometers. Although these methods have high measurement accuracy, they have problems such as large size, poor real-time performance, and high sample requirements, and cannot meet the needs of modern high-sensitivity, micro-quantification, and dynamic monitoring. Summary of the invention

[0004] In view of the deficiencies in the prior art, the purpose of the present invention is to provide an experimental device, an experimental method and a demodulation method based on tilted fiber grating refractive index sensing. The experimental device of the present invention has a relatively simple structure and good real-time performance of the experiment, and can meet the needs of high sensitivity, micro-quantification and dynamic monitoring.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows: The invention discloses an experimental device based on tilted fiber grating refractive index sensing, comprising a broadband light source, a polarization controller, a tilted fiber grating, a liquid sample pool, a data acquisition module, an automatic temperature compensation module, and a data processing module, wherein the light output end of the broadband light source is connected to the light input end of the polarization controller, the light output end of the polarization controller is connected to the light input end of the tilted fiber grating, the light output end of the tilted fiber grating is connected to the data acquisition module, the data acquisition module is connected to the automatic temperature compensation module, the automatic temperature compensation module is connected to the data processing module, the tilted fiber grating is arranged in the liquid sample pool, the liquid sample pool is used for holding the liquid sample, the polarization controller is used for modulating the received measurement light into a single polarized light component; the data acquisition module is used for forming a spectrum according to the single polarized light component of the reflected or transmitted tilted fiber grating; the automatic temperature compensation module is used for calibrating the offset of the core and cladding modes of the tilted fiber grating caused by external temperature perturbation; the data processing module is used for grouping the cladding modes in the spectrum calibrated by the automatic temperature compensation module, and calculating the refractive index sensitivity.

[0006] Preferably, the polarization controller adopts a three-ring polarizer, which includes a fiber jumper and three circular rings. The fiber jumper is wound around the three circular rings twice, three times, and two times in sequence to form a quarter wave plate, a half wave plate, and a quarter wave plate in sequence.

[0007] Preferably, the liquid sample pool in the experimental device includes a transparent rectangular box, the inner wall of the rectangular box is coated with a right hydrophilic film, the top of the rectangular box is provided with an inclined liquid injection inlet and an exhaust hole, the water outlet end of the inclined liquid injection inlet faces the side wall of the rectangular box, and the bottom of the rectangular box is provided with a groove for fixing the inclined fiber grating, and the inclined fiber grating is arranged in the groove.

[0008] Preferably, the groove extends from one end to the other end of the bottom of the rectangular box, and TFBG inlets and outlets for the tilted fiber grating to pass through are correspondingly opened on the side walls of the rectangular box at both ends of the groove. Both ends of the tilted fiber grating pass through the rectangular box from the TFBG inlets and outlets, and the tilted fiber grating is sealed and connected to the rectangular box at the TFBG inlets and outlets.

[0009] Preferably, a capacity mark is provided on the rectangular box and a drain valve is provided at the bottom.

[0010] Preferably, the wavelength of the measuring light output by the broadband light source is greater than or equal to 1500 nm and less than or equal to 1590 nm.

[0011] The present invention also provides an experimental method based on tilted fiber Bragg grating refractive index sensing, which is carried out using the experimental device based on tilted fiber Bragg grating refractive index sensing as described above. The experimental method comprises: A liquid sample is placed in a liquid sample pool, and a measuring light is emitted by a broadband light source. A polarization controller modulates the measuring light emitted by the broadband light source into a single polarized light component. The single polarized light component after modulation by the polarization controller enters a tilted fiber grating, and the tilted fiber grating measures the refractive index of the liquid sample in the liquid sample pool. During the measurement process, changes in ambient temperature will cause changes in the effective refractive index of the cladding mode in the tilted fiber grating. A data acquisition module receives the single polarized light component reflected or transmitted through the tilted fiber grating to form a spectrum. An automatic temperature compensation module calibrates the offset of the core and cladding modes of the tilted fiber grating caused by external temperature perturbations in the spectrum to obtain a calibrated spectrum. A data processing module groups the cladding modes in the calibrated spectrum and calculates the refractive index sensitivity.

[0012] The present invention also provides a demodulation method based on tilted fiber Bragg grating refractive index sensing as described above. The demodulation method is performed using the experimental device based on tilted fiber Bragg grating refractive index sensing as described above. The demodulation method comprises: Acquire a plurality of spectra, wherein the plurality of spectra are spectra of transmission or reflection of the tilted fiber Bragg grating under different ambient refractive indices; Obtaining a spectrum of the tilted fiber Bragg grating in air at room temperature, and using the spectrum as a reference spectrum; The ambient temperature perturbation during the measurement of each spectrum is determined by comparing the change of the core mode position of the spectrum under different ambient refractive indices with the core mode position of the reference spectrum. Keep the core mode wavelength position of each spectrum graph consistent with the core mode wavelength position of the reference spectrum graph to achieve temperature calibration; Convert the ambient temperature perturbation of the spectrum at different refractive indices into the refractive index perturbation; Eliminate the wavelength offset of each spectral cladding mode caused by refractive index perturbation, and draw the relationship between the wavelength position of each spectral cladding mode and the ambient refractive index under different refractive indices; Determine the ratio of the cladding mode offset value to the ambient refractive index change value as the cladding mode refractive index sensitivity; The cladding mode sensitivity is grouped into odd and even groups according to the wavelength; Determine the refractive index sensitivity model of odd-group cladding modes and the refractive index sensitivity model of even-group cladding modes; A measurement is performed, and based on the measurement value, the change in the ambient refractive index of the tilted fiber grating is determined through two groups of different refractive index sensitivity models, odd and even.

[0013] Preferably, determining the ambient temperature perturbation when measuring each spectrum graph by comparing the change of the core mode position in the spectrum graph under different ambient refractive indices with the core mode position in the reference spectrum graph comprises the following process: Obtain the wavelength position of the core mode under different refractive indices; Subtract the core mode wavelength position under different refractive indices from the core mode wavelength position of the reference spectrum to obtain a difference value; Substituting the difference into the relationship between the core mode position and the temperature, the ambient temperature perturbation under different refractive indices is obtained.

[0014] Preferably, the wavelength offset of each spectral cladding mode caused by the refractive index perturbation is eliminated, and a relationship diagram between the wavelength position of each spectral cladding mode and the ambient refractive index under different refractive indices is drawn, which includes the following process: Obtain the wavelength position of each cladding mode under different refractive indices; Substituting the values ​​of refractive index perturbation caused by ambient temperature perturbation under different refractive indices into the relationship between the wavelength position of each cladding mode and the refractive index, the wavelength drift of each cladding mode under different refractive index perturbations is obtained; The temperature calibration of the cladding mode is achieved by subtracting the wavelength drift of each cladding mode caused by the refractive index perturbation from the original wavelength position of each cladding mode under different refractive indices; The relationship between the calibrated wavelength position of each cladding mode and the ambient refractive index in different refractive index spectra is plotted.

[0015] Compared with the prior art, the present invention has the following effects: In the experimental device based on tilted fiber grating refractive index sensing of the present invention, a broadband light source can emit measurement light, a polarization controller can modulate the measurement light emitted by the broadband light source into a single polarized light component, and the tilted fiber grating can be used to measure the refractive index of the liquid sample in the liquid sample pool. During the measurement process, the temperature change of the environment will cause the effective refractive index of the cladding mode in the tilted fiber grating to change. At the same time, liquid samples with different refractive indices can be loaded into the liquid sample pool for measurement; the single polarized light component of the reflected or transmitted tilted fiber grating can be received by the data acquisition module to form a spectrum; the offset of the core and cladding modes of the tilted fiber grating caused by the external temperature perturbation in the spectrum is calibrated by the temperature automatic compensation module to obtain the calibrated spectrum; finally, the data processing module can be used to group the cladding modes in the calibrated spectrum and calculate the refractive index sensitivity. It can be seen from the experimental device based on tilted fiber grating refractive index sensing of the present invention that, compared with the traditional refractive index measurement method mainly relying on the refractometer and the interferometer, the experimental device of the present invention has a relatively simple structure, and through actual use, it has good real-time performance and can meet the requirements of high sensitivity, micro-quantification and dynamic monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0017] Figure 1 It is a schematic diagram of the structure of the experimental device based on tilted fiber Bragg grating refractive index sensing according to the present invention; Figure 2 A schematic diagram of the structure of a tilted fiber Bragg grating in an embodiment of the present invention; Figure 3 This is a schematic diagram of the structure of a liquid sample pool in an embodiment of the present invention; Figure 4 The Bragg wavelength spectra at 25°C, 27°C, 34°C, 47°C and 56°C in the embodiment of the present invention; Figure 5 Spectra of cladding modes in the range of 1528nm to 1532nm at 25°C, 27°C, 34°C, 47°C and 56°C in an embodiment of the present invention; Figure 6 A graph showing the relationship between the temperature perturbation change and the resulting wavelength position drift slope of each cladding mode in an embodiment of the present invention; Figure 7 This is a simulation result diagram of the refractive index sensitivity of odd and even groups of cladding modes in an embodiment of the present invention; Figure 8 This is a graph showing the refractive index sensitivity test results for odd and even arrays of cladding modes in an embodiment of the present invention; Fig. 9 Spectra of tilted fiber Bragg gratings at different refractive indices in an embodiment of the present invention; Fig.10 Schematic diagram of the process of the tilted fiber grating refractive index sensing experimental method according to the embodiment of the present invention; Fig.11 A schematic diagram of a process for determining an ambient temperature perturbation in an embodiment of the present invention; Fig.12 A schematic diagram of a flow chart for obtaining the relationship between wavelength position changes and ambient refractive index under multiple cladding modes in an embodiment of the present invention; Fig.13 A linear fitting diagram showing the relationship between the change in wavelength position and the ambient refractive index in different cladding modes in an embodiment of the present invention; Fig.14 Schematic diagram of the distribution of odd-group and even-group cladding modes in an embodiment of the present invention; Fig.15 This is a refractive index sensitivity model diagram of even and odd arrays within the wavelength range of 1520nm-1568nm in an embodiment of the present invention.

[0018] In the figure, 1- broadband light source, 2- polarization controller, 3- tilted fiber grating, 4- liquid sample pool, 41- tilted liquid injection inlet, 42- exhaust hole, 43- TFBG inlet and outlet, 44- drain valve, 45- groove, 5- data acquisition module, 6- automatic temperature compensation module, 7- data processing module. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0020] Please refer to Figure 1As shown, this embodiment provides an experimental device based on tilted fiber Bragg grating refractive index sensing, including a broadband light source 1, a polarization controller 2, a tilted fiber Bragg grating 3, a liquid sample pool 4, a data acquisition module 5, an automatic temperature compensation module 6 and a data processing module 7, wherein the light output end of the broadband light source 1 is connected to the light input end of the polarization controller 2, the light output end of the polarization controller 2 is connected to the light input end of the tilted fiber Bragg grating 3, the light output end of the tilted fiber Bragg grating 3 is connected to the data acquisition module 5, the data acquisition module 5 is connected to the automatic temperature compensation module 6, the automatic temperature compensation module 6 is connected to the data processing module 7, and the tilted fiber Bragg grating 3 is arranged in the liquid sample pool 4.

[0021] Specifically, the experimental method of the experimental device provided by the above embodiment of the present invention includes the following process: The liquid sample is placed in the liquid sample pool 4, and the measurement light is emitted by the broadband light source 1. The polarization controller 2 modulates the measurement light emitted by the broadband light source 1 into a single polarized light component. The single polarized light component after modulation by the polarization controller 2 enters the tilted fiber grating 3, and the tilted fiber grating 3 measures the refractive index of the liquid sample in the liquid sample pool 4. During the measurement process, the temperature change of the environment will cause the effective refractive index of the cladding mode in the tilted fiber grating 3 to change; the data acquisition module 5 receives the single polarized light component reflected or transmitted by the tilted fiber grating 3 to form a spectrum; the temperature automatic compensation module 6 calibrates the offset of the core and cladding modes of the tilted fiber grating 3 caused by the external temperature perturbation in the spectrum to obtain the calibrated spectrum. The temperature automatic compensation module 6 is set after the data acquisition module 5 to compensate for the influence of the environmental temperature perturbation on the accurate measurement of the refractive index; the data processing module 7 groups the cladding mode in the calibrated spectrum and calculates the refractive index sensitivity. Specifically, in the above scheme of the present invention, the polarization controller 2 is used to modulate the measurement light emitted by the broadband light source 1 into a single polarized light component, that is, modulate it into P polarized light or S polarized light. In this embodiment, the polarization controller 2 may use a three-ring polarizer. Specifically, the three-ring polarizer includes a fiber jumper and three circular rings. The fiber jumper is wound around the three circular rings for two, three, and two turns respectively, to form a quarter wave plate, a half wave plate, and a quarter wave plate. In order to maximize the modulation of P polarized light or S polarized light, the angles of the three circular rings are further adjusted to achieve the maximum modulation of P polarized light or S polarized light.

[0022] In the above solution of the present invention, the tilted fiber grating 3 is a new type of passive optical device in which the grating plane forms a certain angle with the axial direction of the light, and its structure is as follows: Figure 2As shown. Further, in this embodiment, the tilted fiber grating 3 is made of a single-mode fiber through a phase mask method. When a beam of light reaches the tilted fiber grating 3 through a path, the light that meets the Bragg condition will undergo Bragg reflection, and the light that meets the conditions of a certain order of cladding radiation mode will be coupled into the cladding and exchange energy with the external environment. Therefore, when the external environment is different, the spectral characteristics of the tilted fiber grating 3 will also change accordingly. Therefore, when the tilted fiber grating 3 is used to detect changes in various environmental factors, the purpose of measurement can be achieved by detecting the spectral changes of the tilted fiber grating and then analyzing it through relevant demodulation methods.

[0023] In the above solution of the present invention, the liquid sample pool 4 is used to hold the liquid sample, so as to change the working environment of the tilted fiber grating 3. Figure 3 In this embodiment, the liquid sample pool 4 includes a transparent rectangular box, which is made of highly transparent material. An inclined liquid injection inlet 41 and a small exhaust hole 42 are provided on the top of the rectangular box. The water outlet end of the inclined liquid injection inlet 41 faces the side wall of the rectangular box. The inner wall of the rectangular box is coated with a hydrophilic film, and the hydrophilic film can reduce the influence of bubbles on the measurement. A drain valve 44 is designed at the bottom of the rectangular box. The drain valve 44 is seamlessly connected to the bottom of the rectangular box. The drain valve 44 is opened and closed by rotation, and the rectangular box is also provided with a precise liquid capacity mark. The bottom of the rectangular box is flat and has a groove 45. The groove 45 is used to place the inclined fiber grating 3. The groove 45 extends from the left end to the right end of the bottom of the rectangular box. The side wall of the rectangular box is provided with TFBG inlets and outlets 43 for the inclined fiber grating 3 to pass through at both ends of the groove 45. The two ends of the inclined fiber grating 3 pass through the rectangular box from the TFBG inlets and outlets 43. The inclined fiber grating 3 is sealed and connected to the rectangular box at the TFBG inlets and outlets 43.

[0024] In the above scheme of the present invention, the data acquisition module 5 and the data processing module 7 are used to receive the transmission / reflection spectrum and perform post-processing and calculation on the spectrum data, which is helpful to realize real-time dynamic monitoring. In this embodiment, the data acquisition module 5 controls the spectrometer based on LabVIEW to obtain the transmission / reflection light of the fiber grating 3 with different refractive index tilt under the P polarization component light or S polarization component light; the transmission / reflection light data under the P polarization component light or S polarization component light are automatically stored in the computer. Specifically, in this embodiment, the data acquisition module 5 includes an image processing component and a software system, for example, the image processing component is a spectrometer, the software system is LabVIEW software, the spectrometer realizes the acquisition of the transmission / reflection light of the fiber grating with different refractive index tilt under the P polarization component light or S polarization component light; LabVIEW software is used to realize automatic data acquisition. Generate a spectrum diagram of the transmission / reflection light under the P polarization component light or S polarization component light. The temperature automatic compensation module 6 is used to calibrate the offset of the core and cladding modes of the tilted fiber grating 3 caused by external temperature perturbations.

[0025] As a preferred embodiment of the present invention, in this embodiment, the broadband light source 1 can adopt an ASE broadband light source, and the wavelength of the light measured by the ASE broadband light source is greater than or equal to 1500nm and less than or equal to 1590nm.

[0026] In this embodiment, the outgoing light (i.e., the measuring light) of the broadband light source 1 passes through the polarization controller 2 on the path, and the polarization controller 2 modulates the outgoing light into a single polarized light component. The modulated single polarized light component enters the tilted fiber grating 3. Liquids with different refractive indices can be placed in the liquid sample pool 4. During the measurement process, the temperature change of the environment will cause the effective refractive index of the cladding mode in the tilted fiber grating 3 to change, thereby affecting the accuracy of the refractive index measurement.

[0027] To this end, the present invention sets an automatic temperature compensation module 6 after the data acquisition module 5, and uses the automatic temperature compensation module 6 to compensate for the influence of the ambient temperature perturbation on the accurate measurement of the refractive index. Specifically, the automatic temperature compensation module 6 takes the position of the transmission / reflection spectrum of the tilted fiber grating in the air at room temperature as a reference, and automatically aligns the core wavelength position of the spectrum under different refractive indices with it. By detecting the change of the core wavelength position under different refractive index conditions, the specific temperature change can be obtained. The temperature change is brought into the "fitting function of temperature-induced refractive index change and wavelength drift of each cladding mode" to obtain the wavelength drift of each cladding mode caused by the temperature-induced refractive index change, and the drift is subtracted from the corresponding cladding mode. By performing temperature calibration on the cladding mode wavelength drift of the tilted fiber grating, the relationship between the temperature-induced refractive index change and the cladding mode wavelength drift is established. Taking the temperature of the tilted fiber grating at room temperature as a reference, the slight change in refractive index caused by temperature change is calculated by formula (1.1), as shown in formula (1.1): (1.1) in is room temperature, To increase the temperature, is the temperature-induced refractive index change; by fitting the relationship between the cladding mode wavelength drift and the temperature-induced refractive index change, the fitting function of the temperature-induced refractive index change and the wavelength drift of each cladding mode is obtained.

[0028] The data processing module 7 receives the single polarized light components transmitted / reflected by the tilted fiber grating under different refractive indices to form a spectrum, and establishes a database between the refractive index and the odd / even array cladding mode.

[0029] In this embodiment, the temperature automatic compensation module 6 can automatically align the core wavelength positions of multiple spectra under different refractive indices, and calculate the specific temperature change by detecting the change of the core wavelength position under different refractive index conditions. Through formula (1.2), we get: (1.2) According to mode coupling theory, the Bragg wavelength and cladding mode wavelength for: (1.3) in, and The core mode and the i The effective refractive index of the order cladding mode and the core mode are independent of the ambient refractive index. is the inclination angle of the tilted fiber Bragg grating, is the angle of tilting the fiber Bragg grating; the change of the ambient refractive index only affects the wavelength of the cladding mode, causing the wavelength of each cladding mode to shift, as shown in formula (1.4): (1.4) Furthermore, the automatic temperature compensation module 6 can eliminate the deviation of each cladding mode caused by temperature change based on the position of each cladding mode at different refractive indices, thereby achieving temperature calibration in a true sense.

[0030] Based on the temperature automatic compensation module 6 of the embodiment of the present invention, the tilted fiber Bragg grating 3 is placed in a water bath heating pot, and the temperature is set from the initial temperature 23 Gradually increase the temperature to the target temperature. After each temperature increase, keep at each temperature point for 2-3 minutes to ensure that the liquid sample reaches thermal equilibrium. At each temperature point, record the transmission / reflection spectrum of the tilted fiber Bragg grating 3 at the current temperature, repeat the measurement 3 times, and take the average value as the final result. Figure 4 and Figure 5 is the tilted FBG 3 at 25 , 27 , 34 , 47 and 56 The Bragg peak and multiple cladding mode spectra. It is understandable that the temperature range is not limited to the above examples, and the temperature can be any value that the experimental device can withstand. In the embodiment of the present application, the tilted fiber Bragg grating transmission / reflection spectrum at the initial temperature is used as a reference, and the position of the Bragg peak at different temperatures is first calibrated, that is, the position of the Bragg peak at other temperatures is consistent with the Bragg peak in the reference spectrum, and then the relationship between the position change of each cladding mode and the temperature change is obtained. Figure 6 It is the relationship between the cladding mode position at 1530.94nm and temperature.

[0031] The demodulation method based on tilted fiber Bragg grating refractive index sensing in this embodiment uses a matrix mode solver to simulate and numerically calculate the mode characteristics of the tilted fiber Bragg grating 3 under different ambient refractive indices, and analyzes the refractive index sensitivity of odd and even modes. For example, the core diameter of the tilted fiber Bragg grating is selected to be 8 , refractive index 1.45; cladding diameter is 125 , refractive index is 1.44; the refractive index of the external environment The refractive index of the fiber is 1.32, which is used to simulate the refractive index liquid. The fiber cross section is discretized by the finite difference method to generate the refractive index distribution matrix of the fiber structure. The input light is P polarized light, and its polarization direction matches the coupling characteristics of the tilted fiber grating. By solving the matrix equation after finite difference discretization, the effective refractive index of different modes in the fiber is obtained. , calculate the effective refractive index of mode numbers L = 0, 1, 2, 3, 4, 5, 6, 7 respectively. Change the ambient refractive index from =1.32 to 1.33, keeping the core and cladding parameters unchanged, repeat the mode solution process to obtain the effective refractive index changes of each mode under different ambient refractive indices. Figure 7 The odd and even modes are grouped as shown, such as l = 1, 3, 5 ... for the odd group, l = 2, 4, 6 ... for the even group, and the refractive index sensitivity is calculated (the simulation results are shown in Figure 7 ). This demodulation method improves the accuracy, resolution and single-shot utilization rate of tilted fiber Bragg grating refractive index measurement.

[0032] In order to verify the simulation results of the tilted fiber Bragg grating refractive index sensing demodulation method in the embodiment of the present application, under the same environment, the tilted fiber Bragg grating was used to measure the refractive index liquids of 1.31, 1.315, 1.32, 1.325 and 1.33, and the refractive index sensitivity of each cladding mode was calculated. The results are shown in Figure 2. Figure 8 As shown in the figure, the refractive index sensitivities of the odd and even cladding modes are clearly divided into two groups, Figure 7 The simulation results shown are consistent.

[0033] Please refer to Fig.10 As shown, this embodiment provides an experimental method, which is applied to a control end and includes the following steps: S100, obtaining multiple spectra, which are spectra of transmission / reflection of tilted fiber Bragg gratings under different environmental refractive indices; please refer to Fig. 9 , which is the spectrum formed by the tilted fiber Bragg grating in the refractive index liquids with different refractive indices. The refractive indices of the refractive liquids are 1.31, 1.33, 1.35, 1.37, 1.39 and 141 respectively.

[0034] S200, obtaining a tilted fiber Bragg grating spectrum in air at room temperature as a reference spectrum, wherein the reference spectrum is used to implement subsequent temperature perturbation calibration.

[0035] S300, determine the ambient temperature perturbation during the measurement of each spectrum graph by comparing the change of the core mode position of the spectrum graph under different ambient refractive indices with the core mode position of the reference spectrum graph; for specific steps, please refer to Fig.11 First, obtain the wavelength position of the core mode under different refractive indices. When the refractive index and the refractive index are 1.31, 1.33, 1.35, 1.37, 1.39 and 1.41 respectively (such as Fig. 9 ), find the wavelength position of the core mode. Secondly, subtract these values ​​from the core mode wavelength position of the reference spectrum. Finally, according to the core mode temperature sensitivity (10 ), and obtain the temperature perturbation under different refractive indices. Substitute the difference obtained in the above difference process into the relationship between the core mode position and temperature to obtain the temperature perturbation under different refractive indices.

[0036] S400, making the core mode wavelength position in each spectrum diagram consistent with the core mode wavelength position in the reference spectrum diagram to achieve temperature calibration: specifically, the spectrum under different refractive indices is overall shifted by the "difference" calculated by S300, so that the core mode wavelength position in the spectrum under different refractive indices is consistent with the reference value.

[0037] S500, converting the ambient temperature perturbation of the optical spectrum at different refractive indices into a refractive index perturbation: specifically, substituting the temperature perturbation calculated by S300 at different refractive indices into the relationship between the core mode position and temperature (i.e., equation (1.2)) to obtain the refractive index perturbation caused by the temperature perturbation.

[0038] S600, remove the wavelength offset of each spectral cladding mode caused by the refractive index perturbation, and draw the relationship between the wavelength position of each spectral cladding mode and the ambient refractive index under different refractive indices; please refer to Fig.12 First, obtain the wavelength position of each cladding mode under different refractive indices. Then, substitute the values ​​of refractive index perturbation caused by temperature perturbation under different refractive indices into the relationship between the wavelength position of each cladding mode and the refractive index (i.e., formula (1.4)) to obtain the wavelength drift of each cladding mode under different refractive index perturbations. Then, subtract the calculated wavelength drift of each cladding mode caused by the refractive index perturbation from the original wavelength position of each cladding mode under different refractive indices to achieve temperature calibration of the cladding mode. Finally, draw a graph showing the relationship between the wavelength position of each cladding mode and the ambient refractive index in different refractive index spectra. Please refer to Fig.13 , the relationship between the wavelength position change and the ambient refractive index in multiple cladding modes such as 1519.8298nm, 1521.09116nm, 1522.38735nm, 1523.64571nm, and 1524.91685nm.

[0039] S700, determine the ratio of the cladding mode offset value to the ambient refractive index change value as the cladding mode refractive index sensitivity: For details, please refer to Fig.13 The slope of the linear fitting relationship between the wavelength position change and the ambient refractive index in all cladding modes is the refractive index sensitivity of the cladding mode.

[0040] S800, group the cladding mode sensitivity by wavelength size and odd / even: For details, please refer to Fig.14In the tilted grating spectrum, whether it is the P polarization state or the S polarization state, the cladding modes are always distributed in odd and even states. For a pair of cladding modes, the even group mode has a larger refractive index sensitivity and is located at the short wavelength. Therefore, when performing odd-even grouping, all cladding modes are first labeled, starting from the short wavelength, and labeled from 2 to the long wavelength in sequence. Among them, the cladding modes with serial numbers 2, 4, 6, etc. are recorded as the even group, and the cladding modes with serial numbers 1, 3, 5, etc. are recorded as the odd group.

[0041] S900, determine the odd-group cladding mode refractive index sensitivity model and the even-group cladding mode refractive index sensitivity model: For details, please refer to Fig.15 , all the calculated cladding mode refractive index sensitivities are divided into two groups, odd and even, and plotted out to form odd-group and even-group cladding mode refractive index sensitivity models.

[0042] S1000. According to a single measurement value, the change in the ambient refractive index of the tilted fiber Bragg grating can be determined by using two sets of different refractive index sensitivity models, odd and even. In actual refractive index measurement, using this method to perform a single measurement can obtain results under two sets of different refractive index sensitivity models, which can meet the sensitivity requirements in different application scenarios and improve the accuracy of the measurement.

[0043] In the above scheme, the spectrum formed by the single polarized light component divides the two adjacent cladding modes into two groups, the short wavelength part is recorded as an even group, and the long wavelength part is recorded as an odd group, and the refractive index sensitivities of the odd group and the even group at the full wavelength are obtained respectively by the cladding refractive index sensitivity formula, and 3-5 repeated measurements are performed. After the sensitivity of each cladding mode is averaged, two refractive index sensitivity models of the odd group and the even group are obtained. The two refractive index sensitivity models of the odd group and the even group can improve the resolution, accuracy and single measurement utilization rate of tilted fiber grating refractive index sensing. The automatic temperature feedback system realizes the precise control of the refractive index liquid and the real-time compensation of the wavelength deviation caused by temperature; the demodulation method based on the odd and even grouping of the cladding mode effectively improves the measurement resolution, accuracy, data processing efficiency and single measurement utilization rate.

[0044] Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. Experimental device based on tilted fiber Bragg grating refractive index sensing, characterized in that: The invention comprises a broadband light source (1), a polarization controller (2), a tilted optical fiber Bragg grating (3), a liquid sample pool (4), a data acquisition module (5), an automatic temperature compensation module (6), and a data processing module (7), wherein the light output end of the broadband light source (1) is connected to the light input end of the polarization controller (2), the light output end of the polarization controller (2) is connected to the light input end of the tilted optical fiber Bragg grating (3), the light output end of the tilted optical fiber Bragg grating (3) is connected to the data acquisition module (5), the data acquisition module (5) is connected to the automatic temperature compensation module (6), and the automatic temperature compensation module (6) is connected to the data processing module (7). The tilted fiber Bragg grating (3) is arranged in a liquid sample pool (4), the liquid sample pool (4) is used to hold a liquid sample, the polarization controller (2) is used to modulate the received measurement light into a single polarized light component; the data acquisition module (5) is used to form a spectrum according to the single polarized light component reflected or transmitted through the tilted fiber Bragg grating (3); the temperature automatic compensation module (6) is used to calibrate the offset of the core and cladding modes of the tilted fiber Bragg grating (3) caused by external temperature perturbations; the data processing module (7) is used to group the cladding modes in the spectrum calibrated by the temperature automatic compensation module (6) and calculate the refractive index sensitivity.

2. The experimental device based on tilted fiber Bragg grating refractive index sensing according to claim 1 is characterized in that: The polarization controller (2) adopts a three-ring polarizer, which comprises an optical fiber jumper and three circular rings. The optical fiber jumper is wound around the three circular rings twice, three times, and two times in sequence to form a quarter wave plate, a half wave plate, and a quarter wave plate in sequence.

3. The experimental device based on tilted fiber Bragg grating refractive index sensing according to claim 1, characterized in that: The liquid sample pool (4) in the experimental device comprises a transparent rectangular box, the inner wall of the rectangular box is coated with a right hydrophilic film, the top of the rectangular box is provided with an inclined liquid injection inlet (41) and an exhaust hole (42), the water outlet end of the inclined liquid injection inlet (41) faces the side wall of the rectangular box, and the bottom of the rectangular box is provided with a groove (45) for fixing the inclined optical fiber Bragg grating (3), and the inclined optical fiber Bragg grating (3) is arranged in the groove (45).

4. The experimental device based on tilted fiber Bragg grating refractive index sensing according to claim 3 is characterized in that: The groove (45) extends from one end of the bottom of the rectangular box to the other end. The side wall of the rectangular box is provided with TFBG inlets and outlets (43) for the tilted optical fiber grating (3) to pass through at both ends of the groove (45). Both ends of the tilted optical fiber grating (3) pass through the rectangular box from the TFBG inlets and outlets (43). The tilted optical fiber grating (3) and the rectangular box are sealed and connected at the TFBG inlets and outlets (43).

5. The experimental device based on tilted fiber Bragg grating refractive index sensing according to claim 3 is characterized in that: A capacity mark is provided on the rectangular box and a drain valve (44) is provided at the bottom.

6. The experimental device based on tilted fiber Bragg grating refractive index sensing according to claim 1, characterized in that: The wavelength of the measuring light output by the broadband light source (1) is greater than or equal to 1500 nm and less than or equal to 1590 nm.

7. An experimental method based on tilted fiber Bragg grating refractive index sensing, characterized in that: The experimental method is carried out using the experimental device based on tilted fiber grating refractive index sensing according to any one of claims 1 to 6, and the experimental method comprises: A liquid sample is placed in a liquid sample pool (4), and a broadband light source (1) emits a measuring light. A polarization controller (2) modulates the measuring light emitted by the broadband light source (1) into a single polarized light component. The single polarized light component after modulation by the polarization controller (2) enters a tilted fiber grating (3). The tilted fiber grating (3) measures the refractive index of the liquid sample in the liquid sample pool (4). During the measurement process, changes in the ambient temperature will cause the effective refractive index of the cladding mode in the tilted fiber grating (3) to change. A data acquisition module (5) receives the single polarized light component reflected or transmitted through the tilted fiber grating (3) to form a spectrum. An automatic temperature compensation module (6) calibrates the offset of the core and cladding modes of the tilted fiber grating (3) caused by external temperature perturbations in the spectrum to obtain a calibrated spectrum. A data processing module (7) groups the cladding modes in the calibrated spectrum and calculates the refractive index sensitivity.

8. The demodulation method based on tilted fiber Bragg grating refractive index sensing according to any one of claims 1 to 6, characterized in that: The demodulation method is performed using the experimental device based on tilted fiber grating refractive index sensing according to any one of claims 1 to 6, and the demodulation method comprises: Acquiring a plurality of spectra, wherein the plurality of spectra are spectra of transmission or reflection of the tilted fiber Bragg grating (3) under different ambient refractive indices; Obtaining a spectrum of the tilted fiber grating (3) in air at room temperature, and using the spectrum as a reference spectrum; The ambient temperature perturbation during the measurement of each spectrum is determined by comparing the change of the core mode position of the spectrum under different ambient refractive indices with the core mode position of the reference spectrum. Keep the core mode wavelength position of each spectrum graph consistent with the core mode wavelength position of the reference spectrum graph to achieve temperature calibration; Convert the ambient temperature perturbation of the spectrum at different refractive indices into the refractive index perturbation; Eliminate the wavelength offset of each spectral cladding mode caused by refractive index perturbation, and draw the relationship between the wavelength position of each spectral cladding mode and the ambient refractive index under different refractive indices; Determine the ratio of the cladding mode offset value to the ambient refractive index change value as the cladding mode refractive index sensitivity; The cladding mode sensitivity is grouped into odd and even groups according to the wavelength; Determine the refractive index sensitivity model of odd-group cladding modes and the refractive index sensitivity model of even-group cladding modes; A measurement is performed, and based on the measurement value, the change in the ambient refractive index of the tilted fiber grating is determined through two groups of different refractive index sensitivity models, odd and even.

9. The demodulation method based on tilted fiber Bragg grating refractive index sensing according to claim 8, characterized in that: By comparing the changes of the core mode positions in the spectrum graphs under different ambient refractive indices with the core mode positions in the reference spectrum graph, the ambient temperature perturbation during the measurement of each spectrum graph is determined, including the following process: Obtain the wavelength position of the core mode under different refractive indices; Subtract the core mode wavelength position under different refractive indices from the core mode wavelength position of the reference spectrum to obtain a difference value; Substituting the difference into the relationship between the core mode position and the temperature, the ambient temperature perturbation under different refractive indices is obtained.

10. The demodulation method based on tilted fiber Bragg grating refractive index sensing according to claim 8, characterized in that: Eliminate the wavelength offset of each spectral cladding mode caused by the refractive index perturbation, and draw the relationship between the wavelength position of each spectral cladding mode and the ambient refractive index under different refractive indices, including the following process: Obtain the wavelength position of each cladding mode under different refractive indices; Substituting the values ​​of refractive index perturbation caused by ambient temperature perturbation under different refractive indices into the relationship between the wavelength position of each cladding mode and the refractive index, the wavelength drift of each cladding mode under different refractive index perturbations is obtained; The temperature calibration of the cladding mode is achieved by subtracting the wavelength drift of each cladding mode caused by the refractive index perturbation from the original wavelength position of each cladding mode under different refractive indices; The relationship between the calibrated wavelength position of each cladding mode and the ambient refractive index in different refractive index spectra is plotted.

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