Optical fiber seawater salinity sensor with temperature self-compensation function, preparation method and test system

Abstract

The invention belongs to the technical field of optical fiber sensing, and discloses an optical fiber seawater salinity sensor with a temperature self-compensation function, a preparation method and a test system. The sensor is based on a single-mode fiber-hollow fiber-single-mode fiber (SHS) structure partially filled with a temperature-sensitive material, and simultaneously excites Fabry-Perot interference and an anti-resonance effect. The femtosecond laser micromachining technology is adopted for optical fiber microdefect etching, temperature-sensitive material filling and seawater flowing into the SHS structure through the microdefects are facilitated, the problem that the anti-resonance phenomenon cannot be excited under the liquid immersion condition is solved, and high-sensitivity salinity measurement is achieved. The temperature crosstalk in the salinity measurement process is compensated based on the temperature-sensitive material filled Fabry-Perot interferometer, and the measurement error is reduced. The optical fiber sensor is compact in structure, has good stability, repeatability and reproducibility, has wide application potential in the field of microfluidics, and provides technical support for application of optical sensing in the ocean.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber sensing, and in particular to an optical fiber seawater salinity sensor with a temperature self-compensation function, a preparation method and a testing system. Background Art

[0002] Seawater salinity is an important parameter in the marine environment, which has a profound impact on marine life, ocean circulation and climate change. Measuring seawater salinity can help humans understand the density, temperature and salinity distribution of seawater, and thus infer changes and trends in the marine environment. The British Antarctic Survey found that the formation rate of Antarctic sea ice has slowed down since 1992, causing the volume of the bottom water of the ocean to shrink by about 30%, changing the distribution of seawater salinity, and having a profound impact on deep ocean currents and global climate [Document: 10.1038 / s41558-023-01695-4]. At present, the measurement of seawater salinity mainly uses electrical equipment based on conductivity, which has the characteristics of large linear working range, high sensitivity and short response time. However, there are problems of electrode polarization and temperature crosstalk in the use of conductivity meters, which affect the accuracy of the measurement results. In addition, conductivity meters need regular maintenance and calibration, which will increase the detection cost and are not convenient for long-term monitoring. In addition to using conductivity to infer seawater salinity, salinity can also be calculated by measuring the refractive index of seawater. Fiber optic sensing technology is widely used in the field of liquid refractive index detection. It has the advantages of anti-electromagnetic interference, corrosion resistance, and multi-parameter sensitivity. It has great potential in the field of in-situ measurement of seawater salinity.

[0003] In recent years, many researchers at home and abroad have explored the technology of optical fiber seawater parameter measurement, and various types of optical fiber salinity sensors based on different principles have been designed and developed. As early as 1999, O. Esteban et al. deposited multiple layers of media on the side-polished single-mode optical fiber and measured the liquid salinity by exciting the surface plasmon resonance (SPR) effect. The experimental results showed that the sensor can be used as a salinity measurement device for environmental applications [Reference: 10.1364 / AO.38.005267]. However, the problem of easy detachment of the metal film in the marine environment limits the application of optical fiber SPR sensors in long-term in-situ measurement of seawater salinity. Unlike the SPR effect, the evanescent field can be enhanced by tapering the optical fiber, so that the optical path can directly interact with the external medium for salinity measurement [Reference: 10.1016 / j.sna.2011.09.024]. However, the preparation process of micro-nano optical fiber sensors is random, and it is difficult to ensure good processing consistency. Fiber Bragg grating sensors are known for their good stability. They can be used to detect seawater salinity by coating polymer or polyelectrolyte multilayer films on the grating surface [Reference: 10.1021 / acsomega.8b02823]. However, the refractive index of the sensitive film changes slowly, resulting in a long response time and being greatly affected by temperature crosstalk. Fiber optic sensors based on the interference principle can quickly and directly reflect salinity changes through changes in output light intensity or characteristic wavelengths, and can design matching sensing structures according to measurement requirements [Reference: 10.1016 / j.rinp.2020.103022]. Cascade interferometers can stimulate the vernier effect and thus improve measurement sensitivity [Reference: 10.1016 / j.sna.2024.115603]. However, the peak aliasing problem of fiber optic interferometers limits their measurement range. The antiresonance effect of optical fiber has attracted more and more attention and investment due to its theoretical breakthroughs, technological development and sensing applications. J. Zhao et al. proposed an antiresonance sensor based on double-sided perforation, which has outstanding potential in high-resolution measurement of ocean hydrological parameters [Reference: 10.1109 / TIM.2024.3398078]. In addition, antiresonant hollow-core optical fiber has unique structural advantages in microfluidic channel preparation and chip integration, and has the conditions for constructing multifunctional sensors.

[0004] Therefore, inspired by the principle of optical fiber antiresonance, a new hollow-core optical fiber microfluidic structure based on micro-filling of sensitive materials is proposed for in-situ measurement of seawater salinity with temperature compensation, which has broad application prospects in complex marine environments. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides an optical fiber seawater salinity sensor with temperature self-compensation function, a preparation method and a testing system.

[0006] The invention discloses an optical fiber seawater salinity sensor with a temperature self-compensation function, which is composed of a single-mode fiber-hollow-core fiber-single-mode fiber (SHS) structure; microgrooves are etched on the adjacent ends of two single-mode fibers, which are located in the single-mode fiber cladding and do not contact the single-mode fiber core, and the depth of the microgrooves is greater than the thickness of the hollow-core fiber cladding; microchannels are etched on the side walls of the hollow-core fibers, which vertically penetrate the hollow-core fiber cladding and are 100 μm-300 μm away from the single-mode fiber-hollow-core fiber fusion point; and a temperature-sensitive material is filled into the hollow-core fiber through the microgrooves on one of the single-mode fibers, and the filling length is less than the length of the hollow-core fiber.

[0007] Furthermore, the hollow core optical fiber has a length of 1 mm-3.5 mm, an outer diameter of 120 μm-140 μm, and an inner diameter of 30 μm-100 μm.

[0008] Furthermore, the microgrooves are cubic defects with a depth of 40 μm-50 μm; and the microchannels are 40 μm-60 μm wide.

[0009] Furthermore, the filling length of the temperature sensitive material is 10 μm-50 μm.

[0010] Furthermore, seawater flows into the hollow-core optical fiber through the microgrooves and microchannels on another single-mode optical fiber, so that the hollow-core optical fiber meets the refractive index matching conditions and thus stimulates the anti-resonance effect, which is used for seawater salinity measurement; the single-mode optical fiber-temperature-sensitive material interface and the temperature-sensitive material-seawater interface in the hollow-core optical fiber region serve as two reflection surfaces, forming a Fabry-Perot interferometer with the temperature-sensitive material as the resonant cavity.

[0011] The temperature sensitive material is polydimethylsiloxane (PDMS) or ultraviolet glue.

[0012] Furthermore, the microgrooves and microchannels are both etched by femtosecond laser micromachining technology.

[0013] The method for preparing the optical fiber seawater salinity sensor with temperature self-compensation function comprises the following steps:

[0014] Step 1: Use a femtosecond laser to etch microgrooves at the proximal ends of two single-mode optical fibers and etch microchannels on the hollow-core optical fiber, blowing away debris during the etching process;

[0015] Step 2: Align and fuse the single-mode fiber and the hollow-core fiber through a fusion splicer, and cut the hollow-core fiber to a fixed length using a femtosecond laser;

[0016] Step 3: Place the fused single-mode fiber-hollow-core fiber structure on a six-dimensional adjustment frame, dip the temperature-sensitive material through the optical fiber microprobe, and fill the temperature-sensitive material into the hollow-core fiber through the microgroove and solidify it;

[0017] Step 4: Fusion-splice the single-mode optical fiber-hollow-core optical fiber structure after curing the temperature-sensitive material with another single-mode optical fiber etched with microgrooves.

[0018] Furthermore, when the temperature-sensitive material is PDMS, it is cured by heating; specifically, the single-mode optical fiber-hollow-core optical fiber structure is placed in a constant temperature box for heating and curing, the heating temperature is 80° C., and the curing time is 1 hour.

[0019] The test system composed of the optical fiber seawater salinity sensor with temperature self-compensation function is connected in the following order: light source - circulator - optical fiber seawater salinity sensor with temperature self-compensation function - 2×1 converter - spectrometer; the light source is connected to the first port of the circulator, the second port of the circulator is connected to the optical fiber seawater salinity sensor with temperature self-compensation function, the third port of the circulator and the output single-mode optical fiber of the optical fiber seawater salinity sensor with temperature self-compensation function are connected to the 2×1 converter and connected to the spectrometer.

[0020] The sensing principle of the optical fiber seawater salinity sensor with temperature self-compensation function is as follows: the change of seawater salinity will change its refractive index, thereby causing the characteristic wavelength of the hollow-core optical fiber anti-resonance structure to shift, thereby realizing seawater salinity detection. At the same time, temperature will also change the refractive index of seawater, and the Fabry-Perot interferometer based on temperature-sensitive materials only responds to temperature changes, so it can be used for temperature compensation.

[0021] The SHS structure is used to simultaneously excite interference and antiresonance effects to obtain transmission and reflection spectral information, and its response characteristics are obtained by wavelength demodulation. Then, a decoupling function is constructed to achieve temperature compensation based on the salinity measurement.

[0022] The fiber-optic seawater salinity sensor with temperature self-compensation function stimulates Fabry-Perot interference and anti-resonance effects by micro-filling temperature-sensitive materials in the SHS structure, thereby achieving compact integration of fiber-optic sensing elements and expansion of detection functions.

[0023] Beneficial effects of the present invention: Compared with existing sensors, the optical fiber sensor based on the SHS structure in the present invention realizes the simultaneous excitation of Fabry-Perot interference and antiresonance effect, and is used for high-sensitivity seawater salinity measurement with temperature self-compensation. Based on femtosecond laser micromachining and sensitive material filling technology, a sensor element with compact integration of the sensing area is prepared. The sensor in the present invention has high sensitivity, good linearity, strong stability, and good repeatability and reproducibility, providing effective data support for seawater salinity monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a structural schematic diagram of the optical fiber seawater salinity sensor with temperature self-compensation function in the present invention.

[0025] Figure 2 The schematic diagram is a method for preparing the optical fiber seawater salinity sensor with temperature self-compensation function in the present invention. (a) is step one, (b) is step two, (c) is step three, and (d) is step four.

[0026] Figure 3 It is the numerical simulation transmission spectrum and reflection spectrum of the optical fiber seawater salinity sensor with temperature self-compensation function in the present invention.

[0027] Figure 4 It is a schematic diagram of a test system of the optical fiber seawater salinity sensor with temperature self-compensation function in the present invention.

[0028] Figure 5 The transmission spectrum salinity response of the optical fiber seawater salinity sensor with temperature self-compensation function in the present invention. (a) is the wavelength-transmittance relationship diagram, and (b) is the salinity-wavelength relationship diagram.

[0029] Figure 6 The salinity response of the reflection spectrum of the optical fiber seawater salinity sensor with temperature self-compensation function in the present invention. (a) is the wavelength-reflectivity relationship diagram, and (b) is the salinity-wavelength relationship diagram.

[0030] Figure 7 The temperature response of the transmission spectrum of the optical fiber seawater salinity sensor with temperature self-compensation function in the present invention. (a) is the wavelength-transmittance relationship diagram, and (b) is the temperature-wavelength relationship diagram.

[0031] Figure 8 The temperature response of the reflection spectrum of the optical fiber seawater salinity sensor with temperature self-compensation function in the present invention. (a) is the wavelength-reflectivity relationship diagram, and (b) is the temperature-wavelength relationship diagram.

[0032] In the figure, 1. input single-mode optical fiber; 2. core; 3. cladding; 4. hollow-core optical fiber; 5. quartz cladding; 6. air core; 7. output single-mode optical fiber; 8. microgroove; 9. temperature-sensitive material; 10. microchannel; 11. femtosecond laser beam; 12. welding machine electrode; 13. optical fiber microprobe; 14. light source; 15. circulator; 16. first port; 17. second port; 18. third port; 19. optical fiber seawater salinity sensor with temperature self-compensation function; 20. 2×1 converter; 21. spectrometer. DETAILED DESCRIPTION

[0033] In order to make the objectives, technical solutions and advantages of the present invention more clear, the specific structure, principle and function of the present invention are further described in detail below in conjunction with the accompanying drawings and embodiments.

[0034] Figure 1The present invention is a structural schematic diagram of an optical fiber seawater salinity sensor 19 with a temperature self-compensation function. The optical fiber seawater salinity sensor 19 with a temperature self-compensation function is composed of an input single-mode optical fiber 1, a hollow-core optical fiber 4, and an output single-mode optical fiber 7 to form a SHS structure; the two single-mode optical fibers include a core 2 and a cladding 3; the hollow-core optical fiber 4 includes a cladding and an air core 6, and the cladding is a quartz cladding 5; wherein the hollow-core optical fiber 4 has a length of 1mm-3.5mm, an outer diameter of 120μm-140μm, and an inner diameter of 30μm-100μm; in addition, microgrooves 8 are engraved on the input single-mode optical fiber 1 and the output single-mode optical fiber 7, which are located at the end where the single-mode optical fiber and the hollow-core optical fiber are fused; one end of the hollow-core optical fiber 4 is filled with a temperature-sensitive material 9, such as PDMS, ultraviolet glue, etc., and a microchannel 10 is provided on the side wall of the hollow-core optical fiber 4. The microgroove is a cubic defect with a side length of 40-50 μm, which is located in the cladding 3 without contacting the core 2; a microchannel 10 is etched on the side wall of the hollow-core optical fiber 4, the microchannel 10 has a width of 40-60 μm, vertically penetrates the hollow-core optical fiber quartz cladding 5, and is located 100-300 μm away from the single-mode optical fiber-hollow-core optical fiber fusion point.

[0035] The temperature-sensitive material 9 is filled into the hollow-core optical fiber through the microgroove 8 on the input single-mode optical fiber 1, and the filling length is 10-50 μm.

[0036] Seawater flows into the hollow-core optical fiber 4 through the microgrooves 8 and microchannels 10 of the output single-mode optical fiber 7, so that the hollow-core optical fiber 4 meets the refractive index matching condition and thus stimulates the anti-resonance effect; in addition, the hollow-core optical fiber 4 is filled with a temperature-sensitive material 9 to form a Fabry-Perot interferometer with the temperature-sensitive material as the resonant cavity.

[0037] The incident light is reflected at the single-mode fiber-temperature-sensitive material interface and the temperature-sensitive material-air interface, forming Fabry-Perot interference, and the output interference curve is observed in the reflection spectrum.

[0038] Because the refractive index of the hollow-core fiber quartz cladding 5 is higher than that of the seawater on both sides, the cladding can be regarded as another form of a resonant cavity. When the incident light is transmitted to the hollow-core fiber 4 through the temperature-sensitive material, the light that meets the resonance condition is reflected multiple times in the hollow-core fiber quartz cladding, while the light that does not meet the resonance condition is concentrated inside the hollow-core fiber 4 as an anti-resonance guided mode, and is finally coupled to the lead-out output single-mode fiber 7, and the output anti-resonance curve is observed in the transmission spectrum.

[0039] Preferably, in a specific embodiment, the inner diameter and outer diameter of the hollow core optical fiber 4 are 75 μm and 125 μm respectively.

[0040] Preferably, in a specific embodiment, the temperature-sensitive material is PDMS, and a Fabry-Perot interferometer with PDMS as the resonant cavity is formed. The incident light is reflected at the single-mode fiber-PDMS interface and the PDMS-air interface to form Fabry-Perot interference, and the output interference curve is observed in the reflection spectrum.

[0041] The manufacturing method and steps of the optical fiber seawater salinity sensor 19 with temperature self-compensation function are as follows:

[0042] Step 1: Fix two single-mode optical fibers on a femtosecond laser micromachining platform, set parameters such as femtosecond laser power, wavelength, and frequency, set parameters such as the processing platform movement path and processing speed, and etch microgrooves 8 and microchannels 10 on the ends of the two single-mode optical fibers and the side walls of the hollow-core optical fiber 4 based on the femtosecond laser beam 11. During the etching process, use air to blow away debris to prevent clogging of the processing area.

[0043] Specifically, the microgroove 8 is a cubic defect with a depth of 50 μm, a microchannel width of 50 μm, vertically penetrates the hollow-core optical fiber quartz cladding, and is located 150 μm away from the single-mode optical fiber-hollow-core optical fiber fusion point.

[0044] Step 2: Align and fuse the input single-mode optical fiber 1 etched with microgrooves and the hollow-core optical fiber 4 etched with microchannels through a fusion splicer. Pay attention to the discharge intensity and discharge time during the fusion splicing process of the fusion splicer electrode 12, and avoid collapse at the fusion point while ensuring the fusion strength. Then, place the fusion-finished structure on a micro-machining platform, and use a femtosecond laser to achieve fixed-length cutting of the hollow-core optical fiber.

[0045] Specifically, the cutting position is 1.5 mm away from the welding point.

[0046] Step 3: Place the fused single-mode fiber-hollow-core fiber structure on a six-dimensional adjustment frame, use the optical fiber microprobe 13 to dip PDMS, and fill the hollow-core fiber with PDMS through the microgroove, with a filling length of 30 μm; then put the optical fiber structure into a constant temperature box for heating and curing, the heating temperature is 80°C, and the curing time is 1 hour;

[0047] Step 4: Fusion-splice the single-mode fiber-hollow-core fiber structure with another output single-mode fiber 7 etched with microgrooves.

[0048] In the present invention, Comsol software is used to perform numerical simulation analysis on the optical fiber seawater salinity sensor with temperature self-compensation function based on the finite element method, and the transmission spectrum curve and reflection spectrum curve are obtained as shown in FIG. Figure 3 As shown, it can be seen that the sensor in the present invention realizes multifunctional integration and simultaneously stimulates Fabry-Perot interference and anti-resonance effects.

[0049] Figure 4The schematic diagram of the test system composed of the optical fiber seawater salinity sensor with temperature self-compensation function of the present invention includes a light source 14, a single-mode optical fiber, a circulator 15, an optical fiber seawater salinity sensor with temperature self-compensation function 19, a 2×1 converter 20 and a spectrometer 21, wherein the circulator includes a first port 16, a second port 17 and a third port 18.

[0050] Combined test system diagram Figure 4 Fiber optic seawater salinity sensor structure with temperature self-compensation function Figure 1 , introduce the specific working principle:

[0051] The incident light from the light source 14 reaches the first port 16 of the circulator through the single-mode optical fiber, and reaches the optical fiber seawater salinity sensor 19 with temperature self-compensation function through the second port 17 of the circulator. The light beam is reflected on the single-mode optical fiber-PDMS interface and the PDMS-air interface. The reflected light is transmitted to the converter 20 through the third port 18 of the circulator, and finally output to the spectrometer 21. The Fabry-Perot interference curve is observed in the reflection spectrum. The transmitted light through PDMS is transmitted to the hollow-core optical fiber. The inside and outside of the hollow-core optical fiber are seawater, which excites the anti-resonance effect. The light in the anti-resonance state is transmitted in the seawater core in the form of a guided mode and coupled to the output single-mode optical fiber, transmitted to the 2×1 converter 20 and finally output to the spectrometer 21. The resonant wavelength and anti-resonance region are observed in the transmission spectrum curve.

[0052] The refractive index of seawater will change with salinity, which will cause the characteristic wavelength of the anti-resonance structure to shift, thereby realizing the detection of seawater salinity. At the same time, temperature will also change the refractive index of seawater, and the PDMS-based Fabry-Perot interferometer only responds to temperature changes, so it can be used for temperature compensation.

[0053] Sensing performance test:

[0054] In an environment of 25°C, the salinity of the seawater around the sensor is changed by injecting standard seawater of different salinities (salinities are: 0‰, 5‰, 20‰, 30‰, 35‰, 40‰). The changes in the transmission spectrum curve collected by the spectrometer are shown in the figure below. Figure 5 As shown in the figure, since the increase of seawater salinity will increase the refractive index of seawater, it can be seen that the resonant wavelength shifts to the shorter wavelength direction with the increase of salinity. The corresponding fitting curve shows that the sensitivity of salinity measurement is -0.961nm / ‰; while the optical path difference of Fabry-Perot interferometer is not affected by the change of salinity, Figure 6 The interference wavelength positions shown do not shift.

[0055] Afterwards, under the condition of salinity of 0‰, the temperature around the sensor was changed in the range of 10℃ to 40℃. As the temperature increased, the refractive index of seawater decreased, so the resonant wavelength of the hollow-core fiber anti-resonance structure moved toward the long wavelength direction. The change of the transmission spectrum curve and the fitting of the characteristic wavelength are shown in the figure. Figure 7 As shown in the figure, it can be seen that the shift of the characteristic wavelength presents a quadratic fitting function with the change of temperature; at the same time, due to the thermo-optical effect and thermal expansion effect of PDMS, the optical path difference of the Fabry-Perot interferometer changes significantly due to the influence of temperature, causing its interference wavelength to shift toward the long wavelength direction. The change of its reflection spectrum curve and the fitting of the characteristic wavelength are shown in Figure 8 As shown, its temperature measurement sensitivity is 2.775nm / ℃.

[0056] According to the above test results, the following decoupling function is established:

[0057]

[0058] Among them, T0 and S0 are the initial seawater temperature (25℃) and salinity (0‰), λ FPI0 and λ Resonance0 are the characteristic wavelengths of the reflection spectrum and the transmission spectrum under the initial conditions. k1, k2, k3 and k4 are sensitivity coefficients, which are 0.005nm / ℃, 0.405nm / ℃, -0.961nm / ‰ and 2.775nm / ℃ respectively.

[0059] According to the wavelength position of the characteristic valley in the collected spectral data, the salinity of the measured seawater can be obtained through the data decoupling equation in formula (1), and temperature self-compensation can be achieved at the same time.

[0060] In the present invention, hollow-core optical fiber is taken as the research object, an SHS structure is constructed, and femtosecond laser micromachining technology is used as the main means to etch microgrooves and microchannel structures on the optical fiber. Combined with sensitive material filling, the co-excitation of Fabry-Perot interference and antiresonance effect is achieved, and a multifunctional integrated sensing unit is established to realize seawater salinity measurement with temperature self-compensation function, providing a new idea for seawater salinity monitoring.

Claims

1. An optical fiber seawater salinity sensor with temperature self-compensation function, characterized in that: The optical fiber seawater salinity sensor with temperature self-compensation function is composed of a single-mode optical fiber-hollow-core optical fiber-single-mode optical fiber structure; microgrooves are etched on the adjacent ends of the two single-mode optical fibers, which are located in the single-mode optical fiber cladding and do not contact the single-mode optical fiber core, and the depth of the microgrooves is greater than the thickness of the hollow-core optical fiber cladding; a microchannel is etched on the side wall of the hollow-core optical fiber, and the microchannel vertically penetrates the hollow-core optical fiber cladding and is 100μm-300μm away from the single-mode optical fiber-hollow-core optical fiber fusion point; a temperature-sensitive material is filled into the hollow-core optical fiber through the microgroove on one of the single-mode optical fibers, and the filling length is less than the length of the hollow-core optical fiber.

2. The optical fiber seawater salinity sensor with temperature self-compensation function according to claim 1, characterized in that: The hollow core optical fiber has a length of 1 mm to 3.5 mm, an outer diameter of 120 μm to 140 μm, and an inner diameter of 30 μm to 100 μm.

3. The optical fiber seawater salinity sensor with temperature self-compensation function according to claim 1, characterized in that: The microgrooves are cubic defects with a depth of 40 μm-50 μm; the width of the microchannels is 40 μm-60 μm.

4. The optical fiber seawater salinity sensor with temperature self-compensation function according to claim 1, characterized in that: The filling length of the temperature sensitive material is 10 μm-50 μm.

5. The optical fiber seawater salinity sensor with temperature self-compensation function according to any one of claims 1 to 4, characterized in that: Seawater flows into the hollow-core optical fiber through the microgrooves and microchannels on another single-mode optical fiber, so that the hollow-core optical fiber meets the refractive index matching conditions and thus stimulates the anti-resonance effect, which is used for seawater salinity measurement; the single-mode optical fiber-temperature-sensitive material interface and the temperature-sensitive material-seawater interface in the hollow-core optical fiber area serve as two reflection surfaces to form a Fabry-Perot interferometer with the temperature-sensitive material as the resonant cavity.

6. The optical fiber seawater salinity sensor with temperature self-compensation function according to claim 5, characterized in that: The temperature-sensitive material is polydimethylsiloxane or ultraviolet glue.

7. The optical fiber seawater salinity sensor with temperature self-compensation function according to claim 6, characterized in that: The microgrooves and microchannels are both etched by femtosecond laser micromachining technology.

8. A method for preparing the optical fiber seawater salinity sensor with temperature self-compensation function according to any one of claims 1 to 7, characterized in that: The following steps are involved: Step 1: Use a femtosecond laser to etch microgrooves at the proximal ends of two single-mode optical fibers and etch microchannels on the hollow-core optical fiber, blowing away debris during the etching process; Step 2: Align and fuse the single-mode fiber and the hollow-core fiber through a fusion splicer, and cut the hollow-core fiber to a fixed length using a femtosecond laser; Step 3: Place the fused single-mode fiber-hollow-core fiber structure on a six-dimensional adjustment frame, dip the temperature-sensitive material through the optical fiber microprobe, and fill the temperature-sensitive material into the hollow-core fiber through the microgroove and solidify it; Step 4: Fusion-splice the single-mode optical fiber-hollow-core optical fiber structure after curing the temperature-sensitive material with another single-mode optical fiber etched with microgrooves.

9. A test system composed of the optical fiber seawater salinity sensor with temperature self-compensation function as claimed in any one of claims 1 to 7, characterized in that: The test system is connected in the following order: light source—circulator—optical fiber seawater salinity sensor with temperature self-compensation function—2×1 converter—spectrometer; the light source is connected to the first port of the circulator, the second port of the circulator is connected to the optical fiber seawater salinity sensor with temperature self-compensation function, the third port of the circulator and the output single-mode optical fiber of the optical fiber seawater salinity sensor with temperature self-compensation function are connected to the 2×1 converter and connected to the spectrometer.

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