A refractive index fiber optic sensor, method of manufacture and use

Abstract

The application discloses a refractive index fiber sensor, a preparation method and application, relates to the field of fiber sensors, and aims to solve the problems of complex structure and short service life in seawater in the prior art.The technical scheme is that a first single-mode optical fiber, a coreless optical fiber, a corrosion dispersion optical fiber and a second single-mode optical fiber are arranged in a cascade structure, and the diameter of the corrosion dispersion optical fiber is 45-55 mu m.The application realizes the detection of the refractive index of a liquid medium by detecting the resonance wavelength of the optical signal in the optical fiber.The miniaturization of the sensor structure is realized, the sensitivity of the detection is directly improved from 21.8 nm / RIU to 278.8 nm / RIU due to the direct detection of the optical signal, the detection sensitivity of the equipment is greatly improved, the refractive index fiber sensor does not need complex coating or packaging, the preparation cost is low, the stability is high, and the refractive index fiber sensor is easy to integrate with other equipment.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensor technology, specifically to a refractive index fiber optic sensor, its fabrication method, and its application. Background Technology

[0002] Currently, commonly used methods for measuring the refractive index of seawater include the traditional refractometer method and fiber optic sensing methods.

[0003] Traditional refractometer methods are simple to operate, but they are greatly affected by environmental interference, have limited measurement accuracy, and are difficult to monitor online in real time. In contrast, fiber optic sensors have significant advantages such as small size, light weight, resistance to electromagnetic interference, corrosion resistance, and remote transmission capability, showing broad application prospects in the field of seawater refractive index measurement.

[0004] Existing fiber-optic refractive index sensors mainly include fiber Bragg grating sensors and fiber interferometer sensors. Fiber Bragg grating sensors have high sensitivity, but are significantly affected by temperature cross-sensitivity, and their selectivity in narrow-range refractive index measurements needs improvement. Fiber interferometer sensors have diverse structures and good measurement accuracy, but their manufacturing process is complex, costly, and requires stringent installation environments. Furthermore, the seawater environment is characterized by high humidity, high salinity, and strong corrosiveness. The packaging structure and sensing materials of existing fiber optic sensors are difficult to adapt to long-term seawater immersion, easily leading to performance degradation and structural damage, thus limiting their widespread application in the marine field.

[0005] Chinese patent CN113340849A discloses a polyvinyl alcohol-sensitized Mach-Zehnder interferometer sensor for simultaneous measurement of humidity and temperature. The technical solution involves a staggered fusion splicing of a single-mode fiber and a dispersion-compensating fiber. An interferometer arm, which is essentially a clad dispersion-compensating fiber, is connected to the rear end of the dispersion-compensating fiber. The interferometer arm is then connected to a combiner composed of coreless optical fibers for output. The interferometer arm is obtained by chemical etching with a 40% hydrofluoric acid solution for 10 minutes. The interferometer arm is coated with a polyvinyl alcohol film.

[0006] This existing patent relies on the strong hydrophilicity of the polyvinyl alcohol (PVA) film to enhance the sensor's response to humidity. Therefore, upon contact with seawater, the high concentration of salt ions in the seawater generates enormous osmotic pressure, causing the film to absorb water violently, leading to excessive swelling, a loose structure, and a sharp drop in strength. This, combined with the seawater scouring during the detection process, directly accelerates the peeling and wear of the already swollen and softened film, resulting in rapid failure of the PVA film. Furthermore, this existing patent requires ensuring equal lengths of the two interference arms and symmetrical offset of the beam splitter core (8-10 μm offset on one side), demanding high precision in structural assembly.

[0007] Therefore, there is an urgent need to develop a sensor with optimized structure, high sensitivity, strong stability, and suitable for refractive index detection in liquids with high salinity (especially seawater with a refractive index of 1.336 to 1.343). Summary of the Invention

[0008] The technical problem to be solved by the present invention is to overcome the existing defects and provide a refractive index fiber optic sensor, its preparation method and application, which can effectively solve the problems in the background art.

[0009] To achieve the above objectives, this invention discloses a refractive index fiber optic sensor, its fabrication method, and its application. The technical solution includes a first single-mode fiber, which is coaxially connected to a coreless fiber, an etched dispersive fiber, and a second single-mode fiber in sequence. The diameter of the thinnest part of the etched dispersive fiber is 45-55 μm. By precisely controlling the diameter of the thinnest part of the etched dispersive fiber within a fixed range of 45-55 μm, the reduced diameter enhances the evanescent field and strengthens the interaction with the surrounding liquid medium. This allows the refractive index change of the liquid medium to correlate with the resonant wavelength of the transmitted light, causing it to drift and achieving the detection effect. If the diameter is too small (less than 45 μm), it will affect the structural integrity and reduce performance.

[0010] As a preferred embodiment of the present invention, the diameter of the thinnest part of the etched dispersive optical fiber is 51 μm.

[0011] This invention also discloses a method for fabricating the aforementioned refractive index fiber optic sensor. The technical solution involves fusing a first single-mode fiber, a coreless fiber, an etched dispersion fiber, and a second single-mode fiber. The etched dispersion fiber is obtained by etching the dispersion compensation fiber using a 30-50% concentration hydrofluoric acid solution for at least 60 minutes. Due to the high requirements on the diameter of the etched dispersion fiber, if the hydrofluoric acid solution concentration is too high, the etching will be too fast, hindering precise control; if the concentration is too low, the etching will be too slow and time-consuming.

[0012] As a preferred embodiment of the present invention, the etched dispersion fiber is obtained by etching dispersion compensation fiber with a 40% hydrofluoric acid solution at 25°C for 95 minutes. Since the evaporation rate of hydrofluoric acid solution varies at different ambient temperatures, the degree of corrosion of the fiber cladding by the same concentration of hydrofluoric acid solution for the same duration will differ depending on the ambient temperature.

[0013] The present invention also discloses the application of the above-mentioned refractive index fiber optic sensor, which can be used for the detection of liquid refractive index, especially the detection of seawater refractive index that varies in the range of 1.336 to 1.343.

[0014] As a preferred embodiment of the present invention, the above-mentioned refractive index fiber optic sensor is placed in the liquid environment to be tested, and the two ends of the refractive index fiber optic sensor are respectively connected to a light source and a spectrometer; the resonant wavelength of the light signal is detected by the spectrometer; the correspondence between the resonant wavelength and the refractive index of the liquid to be tested is as follows: in, The resonant wavelength, To improve the effective refractive index of the core layer of the etched dispersive fiber, To improve the effective refractive index of the cladding of the etched dispersive fiber, It is the equivalent grating period of the etched dispersive fiber.

[0015] As a preferred technical solution of the present invention, when the refractive index of the liquid to be tested changes At that time, the amount of drift of the resonant wavelength satisfy: .

[0016] Compared with existing technologies, the beneficial effects of this invention are as follows: By reducing the diameter of the dispersion-compensating fiber to 45-55 μm, this invention forms an etched dispersion fiber, enhancing the evanescent field and the interaction with the surrounding liquid medium. This allows the refractive index of the liquid medium to be directly correlated with the resonant wavelength of the optical signal in the fiber, thereby enabling the detection of the refractive index of the liquid medium by detecting the resonant wavelength of the optical signal within the fiber. This achieves miniaturization of the sensor structure. Due to the direct detection of the optical signal, the detection sensitivity is directly increased from 21.8 nm / RIU (Refractive Index Unit) to 278.8 nm / RIU, significantly improving the detection sensitivity of the device. Furthermore, the refractive index fiber sensor of this invention does not require complex coating or packaging, resulting in low manufacturing costs, high stability, and easy integration with other devices. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the refractive index fiber optic sensor structure of the present invention; Figure 2 This is a schematic diagram of the optical fiber structure before the etching process of this invention; Figure 3 This is a graph showing the relationship between the spectrum of the fiber optic sensor of the present invention and the refractive index. Figure 4 This is a graph showing the relationship between the resonant wavelength shift and refractive index variation of the fiber optic sensor of the present invention in solution; Figure 5 This is a graph showing the relationship between the spectrum of Comparative Example 1 and the refractive index. Figure 6 The graph shows the relationship between the resonant wavelength shift and the refractive index change in Comparative Example 1 in solution. Figure 7This is a graph showing the relationship between the spectrum of Example 2 and the refractive index. Figure 8 This is a graph showing the relationship between the resonant wavelength shift and the refractive index change in Comparative Example 2 in solution. Figure 9 This is a graph showing the relationship between the spectrum of Example 3 and the refractive index. Figure 10 The graph shows the relationship between the resonant wavelength shift and the refractive index change in Comparative Example 3 in solution. Figure 11 This is a graph showing the relationship between the spectrum and refractive index in Comparative Example 4. Figure 12 This is a graph showing the relationship between the resonant wavelength shift and the refractive index in solution for Comparative Example 4. Figure 13 This is a graph showing the relationship between the spectrum and refractive index in Comparative Example 5. Figure 14 This is a graph showing the relationship between the resonant wavelength shift and the refractive index change in Comparative Example 5 in solution. Figure 15 This is a graph showing the relationship between the spectrum and refractive index in Comparative Example 6. Figure 16 This is a graph showing the relationship between the resonant wavelength shift and the refractive index change in Comparative Example 6 in solution; Figure 17 This is a graph showing the relationship between the spectrum of Example 7 and the refractive index. Figure 18 The graph shows the relationship between the resonant wavelength shift and the refractive index change in Comparative Example 7 in solution.

[0018] In the diagram: 1. First single-mode fiber; 2. Coreless fiber; 3. Etched dispersion fiber; 4. Second single-mode fiber; 5. Dispersion-compensating fiber. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1 like Figure 1As shown, this invention discloses a refractive index fiber optic sensor. The technical solution includes a first single-mode fiber 1, coaxially fused with a coreless fiber 2, an etched dispersion fiber 3, and a second single-mode fiber 4. The etched dispersion fiber 3 is a dispersion compensation fiber 5 with a diameter of 51 μm (at its thinnest point). Due to the small diameter of the etched dispersion fiber 3, to improve safety and stability, the refractive index fiber optic sensor is encapsulated in a capillary tube using UV adhesive. Connection ends are provided at both ends of the refractive index fiber optic sensor, located outside the capillary tube. The etched dispersion fiber 3 is positioned between two adjacent UV adhesive fixing points within the capillary tube.

[0021] This embodiment also discloses the fabrication method of the above-mentioned refractive index fiber optic sensor. The technical solution adopted is that the first single-mode fiber 1 and the second single-mode fiber 4 both adopt standard single-mode fibers with a core diameter of 9μm and a cladding diameter of 125μm to ensure efficient transmission of optical signals; the coreless fiber 2 is a pure quartz fiber with a diameter of 125μm and a length of 5mm, which is used to excite higher-order modes and enhance the interaction between the subsequent sensing segment and the external medium; the dispersion compensation fiber 5 has a core diameter of 8.5μm, a cladding diameter (i.e., the fiber diameter including the core) of 125μm, and a length of 10mm.

[0022] Using a fiber optic fusion splicer, the first single-mode fiber 1, the coreless fiber 2, the dispersion-compensating fiber 5, and the second single-mode fiber 4 are sequentially fused together to produce the following: Figure 2 The cascaded structure is shown. The first single-mode fiber 1 is connected to a broadband light source, and the second single-mode fiber 4 is connected to a spectrometer. The dispersion-compensating fiber 5 is immersed in a 40% hydrofluoric acid solution for etching. The etching process is performed in two steps: first, etching is carried out at an ambient temperature of 25°C for 30 minutes to remove impurities and defects from the cladding surface; second, the ambient temperature is maintained at 25°C, and etching continues for 65 minutes to uniformly reduce the diameter to 51 μm. The spectrum is then observed to ensure optimal sensitivity response of the evanescent wave within the refractive index range of 1.336–1.343. After etching, the fiber surface is repeatedly rinsed with deionized water to remove residual etchant. The dispersion-compensating fiber 5 is then fabricated into an etched dispersion fiber 3. The broadband light source and spectrometer are then separated to obtain a refractive index fiber sensor.

[0023] Furthermore, the refractive index fiber sensor is installed in a capillary tube, and UV adhesive is used to fix both ends of the capillary tube to the first single-mode fiber 1 and the second single-mode fiber 4, as follows. Figure 1 As shown.

[0024] During testing, a fine needle is used to draw the liquid to be tested. The needle tip is then inserted into the capillary through the UV adhesive, and the liquid is injected into the capillary. It is ensured that the etched dispersive fiber 3 is completely immersed in the solution without any air bubbles. The first single-mode fiber 1 is connected to a broadband light source, with the operating wavelength set to 1500-1600 nm. The second single-mode fiber 4 is connected to a spectrometer. The light signal emitted by the broadband light source is transmitted via Example 1, and the spectrometer collects spectral data, refractive index changes, and spectral drift data. When the refractive index of the liquid to be tested changes, it causes a change in the effective refractive index of the cladding of the etched dispersive fiber 3, which in turn changes the mode coupling conditions, causing a drift in the resonant wavelength of the transmitted light. Finally, the light signal is transmitted to the spectrometer via the second single-mode fiber 4. By detecting the amount of resonant wavelength drift, the refractive index value of the seawater can be deduced.

[0025] The relationship between resonant wavelength and refractive index satisfies the following formula: In the formula: The resonant wavelength, To determine the effective refractive index of the three core layers of the etched dispersive fiber, To determine the effective refractive index of the 3rd cladding of the etched dispersive fiber, It is the equivalent grating period of the etched dispersive fiber 3, where, and To determine the values, in this embodiment, values ​​of 1.474 and 119 nm were used respectively. When the refractive index of seawater changes... At that time, the resonant wavelength shift satisfy: Within the refractive index range of 1.336 to 1.343, The refractive index of seawater exhibits a good linear relationship with the change in refractive index, thus accurate measurement can be achieved by calibrating the linear relationship between the resonant wavelength shift and the change in refractive index.

[0026] In this embodiment, seawater with refractive indices of 1.336, 1.337, 1.338, 1.339, 1.340, 1.341, 1.342, and 1.343 was used sequentially for spectral detection at different broadband wavelengths. Data processing was performed using Origin software, and the detection results are as follows: Figure 3 As shown, the relationship between the resonant wavelength shift and the change in seawater refractive index in Example 1 was then calibrated, and the results are as follows. Figure 4 As shown, the sensitivity of Example 1 is approximately 278.8 nm / RIU, and the linear relationship between the resonant wavelength shift and the refractive index change has a goodness of fit of 0.97208.

[0027] To verify the impact of different etching locations and etching times on the detection results of optical fiber structures, the following comparative example was set up: Comparative Example 1: The difference between this comparative example and Example 1 is that the etched part is a coreless optical fiber 2.

[0028] Comparative Example 2: The difference between this comparative example and Example 1 is that the etched parts are coreless optical fiber 2 and dispersion-compensating optical fiber 5.

[0029] Comparative Example 3: The difference between this comparative example and Example 1 is that no corrosion operation was performed.

[0030] Comparative Example 4: The difference between this comparative example and Example 1 is that the dispersion compensation fiber 5 was etched for 35 minutes.

[0031] Comparative Example 5: The difference between this comparative example and Example 1 is that the dispersion compensation fiber 5 was etched for 55 minutes.

[0032] Comparative Example 6: The difference between this comparative example and Example 1 is that the dispersion compensation fiber 5 was etched for 119 min.

[0033] Comparative Example 7: The difference between this comparative example and Example 1 is that the dispersion compensation fiber 5 was etched for 75 minutes.

[0034] Interference spectra and linear fitting plots for each comparison example, as follows: Figures 5 to 18 As shown, the following is a comparison of the linear fit between the sensitivity, resonant wavelength shift, and refractive index change of Example 1 and Comparative Examples 1-7: The above comparison shows that Example 1 exhibits the highest sensitivity and a very high linear fit. This is a result of the reduced fiber diameter, enhanced evanescent field, and increased interaction with the surrounding seawater medium. Different corrosion sites alter the optical transmission characteristics, evanescent field distribution, and mode coupling effect of the fiber, ultimately leading to significant differences in detection sensitivity and resonant wavelength drift. The core reason is the different functional roles of different fiber segments, resulting in completely different effects of corrosion on their optical performance. The core function of coreless fiber is to excite higher-order modes; it is not a sensing segment itself. Corroding only the coreless portion enhances mode excitation but provides no effective sensing, resulting in a slight increase in sensitivity. Simultaneous corrosion of both coreless and dispersive fibers disrupts the modes, causing coupling imbalance, moderate sensitivity, but a sharp drop in linearity. Corroding only the dispersive portion precisely enhances the coupling of the sensing segment, matching mode excitation with sensing, achieving peak sensitivity.

[0035] Note: Since the cutting and splicing processes cannot be guaranteed to be 100% identical in actual operation, there will be some errors in the structure, which will lead to different refractive index test results for dispersion compensation fiber 5 before corrosion, coreless fiber 2 before corrosion, and coreless fiber 2 and dispersion compensation fiber 5 before corrosion, but this does not affect the final conclusion.

[0036] The relationship between corrosion time and the changes in cladding diameter and sensitivity is as follows: The above comparison shows that as the corrosion time increases, the cladding of the dispersion compensation fiber 5 gradually thins, the coupling between the fiber evanescent field and the outside world gradually increases, and the sensitivity gradually improves. However, when the corrosion time is too long (such as reaching 119 min), the integrity of the fiber core structure is destroyed, and the interference spectrum almost disappears, so the sensitivity is extremely low.

[0037] Example 1 can be used not only for laboratory refractive index measurements, but also for a variety of other applications: Marine environmental monitoring: It can be integrated into marine observation equipment such as buoys and underwater moorings for long-term indirect measurement of seawater refractive index and salinity.

[0038] Aquaculture: Used to monitor changes in the refractive index of aquaculture water, indirectly reflecting water quality and providing data support for aquaculture management.

[0039] Marine engineering: Embedded in structures such as marine platforms, monitoring the impact of changes in seawater refractive index on equipment, and assisting in engineering safety assessments.

[0040] Components not described in detail in this article are existing technologies.

[0041] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A refractive index fiber optic sensor, characterized in that: It includes a first single-mode fiber (1), which is coaxially connected to a coreless fiber (2), an etched dispersion fiber (3), and a second single-mode fiber (4); the diameter of the thinnest part of the etched dispersion fiber (3) is 45-55 μm.

2. The refractive index fiber optic sensor according to claim 1, characterized in that: The diameter of the thinnest part of the etched dispersive fiber (3) is 51 μm.

3. A method for preparing the refractive index fiber optic sensor as described in claim 1, characterized in that: The first single-mode fiber (1), the coreless fiber (2), the etched dispersion fiber (3), and the second single-mode fiber (4) are fused together; wherein the etched dispersion fiber (3) is obtained by etching the dispersion compensation fiber (5) with a 30-50% concentration hydrofluoric acid solution for a time of not less than 60 minutes.

4. The method according to claim 3, characterized in that: The etched dispersion fiber (3) was obtained by etching the dispersion compensation fiber (5) with a 40% hydrofluoric acid solution at 25°C for 95 minutes.

5. An application of a refractive index fiber optic sensor, characterized in that: The refractive index fiber optic sensor as described in claim 1 is used for the detection of the refractive index of liquids.

6. The application according to claim 5, characterized in that: The refractive index fiber optic sensor as described in claim 1 is used to detect the refractive index of seawater, where the refractive index of seawater varies in the range of 1.336 to 1.

343.

7. The application according to claim 5, characterized in that: The refractive index fiber optic sensor as described in claim 1 is placed in the liquid environment to be tested, with a light source and a spectrometer connected to its two ends respectively; the resonant wavelength of the light signal is detected by the spectrometer; the correspondence between the resonant wavelength and the refractive index of the liquid to be tested is as follows: in, The resonant wavelength, To improve the effective refractive index of the core layer of the etched dispersive fiber (3), To improve the effective refractive index of the cladding of the dispersive fiber (3), It is the equivalent grating period of the etched dispersive fiber (3).

8. The application according to claim 7, characterized in that: When the refractive index of the liquid being tested changes At that time, the amount of drift of the resonant wavelength satisfy: 。

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