A dual-microchannel NHCPCF fiber optic seawater salinity sensor, its fabrication method, and its application.

Abstract

This invention relates to the field of fiber optic sensing technology, specifically to an NHCPCF fiber optic seawater salinity sensor with dual microchannels, its fabrication method, and its application. The sensor comprises: an introductory single-mode fiber, a nodeless hollow-core photonic crystal fiber, and an outtroductory single-mode fiber, sequentially fused together. The nodeless hollow-core photonic crystal fiber includes a first cladding and a main vent. Uniformly distributed silica tubes are disposed inside the main vent, with the tube wall tangent to the inner wall of the main vent and the axis of the main vent parallel to the axis of the silica tubes. The sensor utilizes the anti-resonance reflection waveguide principle of NHCPCF and specific microchannels to measure seawater salinity. It avoids the traditional method of large-area cavitation in the cladding, employing microchannels to improve integration, effectively prevent fiber breakage, and enhance mechanical strength during testing. This sensor exhibits high sensitivity (salinity sensitivity -0.88 nm / ‰), good repeatability, and stability.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, specifically to an NHCPCF fiber optic seawater salinity sensor comprising dual microchannels, its fabrication method, and its application. Background Technology

[0002] Seawater salinity is a core parameter in marine environmental monitoring, directly affecting seawater density, osmotic pressure, and acoustic and optical propagation characteristics. Its measurement accuracy is directly related to the research results of major topics such as ocean current simulation, ocean circulation prediction, marine ecosystem health assessment, and nearshore pollution source tracing. At the same time, it plays an irreplaceable role in ship navigation safety, mariculture monitoring, oil and gas development environmental assessment, and military oceanography applications.

[0003] For a long time, the development of seawater salinity measurement technology has been constrained by the complexity of the marine environment and the upgrading of measurement requirements. Early salinity measurements relied on traditional methods such as chemical titration and specific gravity methods. Although these methods had a certain degree of accuracy, they were cumbersome and time-consuming, unable to achieve real-time on-site monitoring, and easily affected by factors such as seawater impurities and temperature fluctuations, making it difficult to meet the measurement needs of complex scenarios such as the open ocean and deep sea. With the development of electronic technology, electronic salinity sensors such as electrode-type and capacitive-type sensors have been gradually promoted. However, they have exposed obvious shortcomings in the extreme marine environment: the high salinity, high pressure, and high humidity of seawater easily lead to corrosion of sensor electrodes and sensitivity decay. Electromagnetic interference and biofouling in the marine environment further affect the measurement stability. Moreover, these sensors are usually large in size and consume a lot of energy, making it difficult to adapt to the deployment requirements of long-term fixed-point observation or distributed monitoring networks.

[0004] Against this backdrop, new measurement technologies based on non-contact sensing mechanisms such as optics and acoustics are gradually emerging. Among them, fiber optic sensing technology, with its unique advantages such as resistance to electromagnetic interference, tolerance to harsh environments, small size, remote transmission capability, and distributed measurement, effectively avoids the inherent defects of traditional electronic sensors and has become a research hotspot in the field of accurate seawater salinity measurement, providing a new technical path for realizing real-time, efficient, and long-term monitoring of the marine environment.

[0005] Existing technology discloses a salinity sensor based on fiber Bragg gratings (FBGs) with core etching. By thinning the fiber cladding using chemical wet etching, this sensor exhibits significant sensitivity to changes in the refractive index of the surrounding medium and combines wavelength drift for salinity monitoring, utilizing a high-sensitivity fiber optic sensor with coreless fiber. The design leverages the unique optical properties of coreless fiber to detect salinity by sensing changes in refractive index. When changes in salinity cause changes in the refractive index of the surrounding medium, the distribution of higher-order modes within the NCF adjusts accordingly, thereby achieving accurate signal sensing. Existing technology also discloses a fiber optic reflective probe that combines surface plasmon resonance (SPR) effects to measure salinity, temperature, and pressure. This probe uses photonic crystal fiber (PCF) as the mode excitation field and utilizes a gold film coating to sense changes in the environmental refractive index. This sensor can simultaneously record salinity and temperature changes through multi-parameter measurements, demonstrating its potential for monitoring complex marine environments.

[0006] Chinese invention patent CN118258768A discloses an in-situ high-sensitivity fiber optic seawater salinity sensing device and method, but the overall structure is not compact enough and the measurement does not cover the entire ocean salinity range. Chinese invention patent CN116519634A discloses a salinity measurement method, but the measurement method is complicated.

[0007] In summary, there is an urgent need to provide a highly sensitive and compact sensor for measuring seawater salinity. Summary of the Invention

[0008] This invention addresses the technical problem of providing a highly sensitive and compact sensor for measuring seawater salinity.

[0009] In this invention, the NHCPCF (Nodeless Hollow-Core Photonic Crystal Fiber) is a nodeless hollow-core photonic crystal fiber;

[0010] The SMF (Single-Mode Optical Fiber) is a single-mode optical fiber;

[0011] The Lead-inSMF is an introduced single-mode optical fiber;

[0012] The Lead-out SMF is a single-mode fiber with an outgoing lead.

[0013] To achieve the above objectives, a first aspect of the present invention provides an NHCPCF fiber optic seawater salinity sensor comprising dual microchannels, wherein the dual microchannel NHCPCF fiber optic seawater salinity sensor includes:

[0014] The sequentially fused single-mode fiber, nodeless hollow-core photonic crystal fiber, and pull-out single-mode fiber;

[0015] The nodeless hollow photonic crystal fiber includes a first cladding and a main vent. Uniformly distributed silica tubes are arranged inside the main vent. The wall of the silica tube is tangent to the inner wall of the main vent, and the axis of the main vent is parallel to the axis of the silica tube.

[0016] The introduced single-mode fiber includes a second cladding, a first core, and a first microchannel. One end of the first microchannel has an opening on the outer surface of the second cladding, and the opening at the other end of the first microchannel is connected to the main air hole and the silica tube. The first microchannel is not in contact with the first core of the introduced single-mode fiber.

[0017] The first microchannel and the second microchannel are stretchable bodies;

[0018] The lead-out single-mode fiber includes a third cladding, a second core, and a second microchannel. One end of the second microchannel has an opening on the outer surface of the third cladding, and the opening at the other end of the second microchannel is connected to the main air hole and the silica tube. The second microchannel is not in contact with the second core of the lead-out single-mode fiber.

[0019] The introduced single-mode fiber, nodeless hollow-core photonic crystal fiber, and extracted single-mode fiber are coaxial.

[0020] A second aspect of the present invention provides a method for fabricating the above-mentioned NHCPCF fiber optic seawater salinity sensor comprising dual microchannels, wherein the method includes the following steps:

[0021] Step 1: Fusion splice one end of the nodeless hollow core photonic crystal fiber NHCPCF to the introduced single-mode fiber;

[0022] Step 2: Fusion splice the other end of the nodeless hollow core photonic crystal fiber NHCPCF to the lead-out single-mode fiber to form a Lead-inSMF-NHCPCF-Lead-outSMF structure;

[0023] Step 3: The structure obtained in Step 2 is placed in a femtosecond laser processing three-dimensional displacement platform. The core center of the single-mode fiber is used as a reference for leveling and calibration. Microchannels are processed on the SMF using a femtosecond laser to form a Lead-inSMF-L-NHCPCF-Lead-outSMF structure.

[0024] A third aspect of the present invention provides the application of the above-mentioned NHCPCF fiber optic seawater salinity sensor with dual microchannels in seawater salinity testing, wherein the testing method of the application includes the following steps:

[0025] S1: Set up a salinity testing system;

[0026] S2: Record the salinity response test results;

[0027] S3: Salinity repeatability test: Under constant room temperature conditions, the structure was subjected to three salinity rise and fall experiments, and the spectrum at the salinity stabilization point was recorded each time. The spectrum of each salinity was then linearly fitted with the corresponding trough wavelength.

[0028] S4: Salinity stability test: The NHCPCF fiber optic seawater salinity sensor containing dual microchannels was placed in a standard seawater environment with constant concentration. The transmission spectrum was recorded every 10 minutes for a total of two hours, and the data of the trough wavelength of each spectrum was tracked.

[0029] The beneficial effects of this invention are as follows:

[0030] (1) The NHCPCF fiber optic seawater salinity sensor with dual microchannels provided by the present invention utilizes nodeless hollow core photonic crystal fiber (NHCPCF) to actively break the "isolation" state of the communication fiber and introduce the seawater to be measured directly into the main pore of the NHCPCF through dual microchannels.

[0031] (2) The L-shaped channel allows seawater to freely enter and exit the nodeless hollow core photonic crystal fiber (NHCPCF). One end of the microchannel is precisely aligned with the main air hole of the nodeless hollow core photonic crystal fiber (NHCPCF), and the other end opens into the fiber cladding and connects to the outside, ensuring that seawater flows directly into the main air hole and the silica tube area and interacts directly with the light. The structure has a high degree of integration.

[0032] (3) The sensing principle of the sensor is not the conventional Fabry-Perot interferometer (FPI) or Mach-Zehnder interferometer (MZI), but is based on the principle of anti-resonant reflecting optical waveguide (ARROW). It allows light of a specific wavelength (anti-resonant wavelength) to be transmitted efficiently in a low-loss air core and prevents light of other wavelengths from leaking into the cladding. It has the advantages of strong anti-interference and adaptability to liquid flow. The change in the refractive index of seawater (which varies with salinity) will directly modulate the anti-resonant condition of NHCPF, causing the wavelength of its characteristic transmission / reflection spectrum to drift. By monitoring this wavelength shift (Δλ), the seawater salinity (ΔS) can be inferred.

[0033] (4) Experimental results show that the sensor has a salinity sensitivity of -0.88 nm / ‰ in the salinity range of 5‰-40‰. It has higher sensitivity (wavelength shift caused by unit salinity change), good linearity, repeatability, stability and fast response, making the signal "extremely sensitive" to environmental changes and producing measurable and stable changes. Attached Figure Description

[0034] Figure 1This is a schematic diagram of the NHCPCF fiber optic seawater salinity sensor containing dual microchannels according to the present invention.

[0035] Figure 2 This is a microscopic cross-sectional view of the nodeless hollow core photonic crystal fiber (NHCPCF) of the present invention. In the figure, 1 is the main pore; 2 is the silica tube; and 3 is the cladding.

[0036] Figure 3 This is a schematic diagram illustrating the fabrication process of the NHCPCF fiber optic seawater salinity sensor containing dual microchannels according to an embodiment of the present invention. Figure 3 (a) is a schematic diagram of fusion splicing of single-mode fiber and nodeless hollow core photonic crystal fiber (NHCPCF). Figure 3 (b) is a schematic diagram of femtosecond laser processing of an L-shaped microchannel. Figure 3 (c) is a photograph of the sensor under a 10x microscope.

[0037] Figure 4 This is a schematic diagram of the salinity test system of the present invention.

[0038] Figure 5 This is a salinity response transmission spectrum of an NHCPCF fiber optic seawater salinity sensor containing dual microchannels according to an embodiment of the present invention.

[0039] Figure 6 This is a salinity response fitting diagram of the NHCPCF fiber optic seawater salinity sensor containing dual microchannels according to an embodiment of the present invention.

[0040] Figure 7 This is a salinity repeatability fitting diagram of the NHCPCF fiber optic seawater salinity sensor containing dual microchannels according to an embodiment of the present invention.

[0041] Figure 8 This is a stability test diagram of the NHCPCF fiber optic seawater salinity sensor containing dual microchannels according to an embodiment of the present invention at a salinity of 20‰. Detailed Implementation

[0042] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0043] A first aspect of the present invention provides an NHCPCF fiber optic seawater salinity sensor comprising dual microchannels, wherein the dual microchannel NHCPCF fiber optic seawater salinity sensor includes:

[0044] The sequentially fused single-mode fiber, nodeless hollow-core photonic crystal fiber, and pull-out single-mode fiber;

[0045] The nodeless hollow photonic crystal fiber includes a first cladding and a main vent. Uniformly distributed silica tubes are arranged inside the main vent. The wall of the silica tube is tangent to the inner wall of the main vent, and the axis of the main vent is parallel to the axis of the silica tube.

[0046] The introduced single-mode fiber includes a second cladding, a first core, and a first microchannel. One end of the first microchannel has an opening on the outer surface of the second cladding, and the opening at the other end of the first microchannel is connected to the main air hole and the silica tube. The first microchannel is not in contact with the first core of the introduced single-mode fiber.

[0047] The first microchannel and the second microchannel are stretchable bodies;

[0048] The lead-out single-mode fiber includes a third cladding, a second core, and a second microchannel. One end of the second microchannel has an opening on the outer surface of the third cladding, and the opening at the other end of the second microchannel is connected to the main air hole and the silica tube. The second microchannel is not in contact with the second core of the lead-out single-mode fiber.

[0049] The introduced single-mode fiber, nodeless hollow-core photonic crystal fiber, and extracted single-mode fiber are coaxial.

[0050] In this invention, the stretching body refers to the fact that any cross-section of the first microchannel and the second microchannel has the same shape and area.

[0051] In this invention, the NHCPCF is transformed from a closed information channel into an open miniature optical sensing unit. Its working mechanism changes from maintaining stable transmission of optical signals to actively using changes in external parameters to modulate the characteristics of optical signals. The seawater to be measured is actively introduced into the main pore of the NHCPCF to maintain the airtightness and integrity of its structure, so as to avoid interference from the external environment and achieve low loss and stable transmission of optical signals, thereby realizing high sensitivity and measurement of seawater salinity.

[0052] In this invention, the principle of the NHCPCF fiber optic seawater salinity sensor containing dual microchannels is as follows:

[0053] When seawater flows through the interior of a nodeless hollow-core photonic crystal fiber (NHCPCF), the refractive index inside the fiber core is lower than the refractive index of the fiber itself. When light passes through the NHCPCF, the seawater and the fiber structure together form an anti-resonant reflection waveguide. The formula for calculating seawater salinity using the anti-resonant reflection waveguide principle (ARROW) is as follows:

[0054]

[0055] In the formula, λ m The resonant wavelength of the anti-resonant reflective waveguide is given by n, where t is the wall thickness of the silica tube in the nodeless hollow core photonic crystal fiber (NHCPCF), and n is the wavelength of the reflective waveguide. G n is the refractive index of nodeless hollow-core photonic crystal fiber (NHCPCF). S Let m be the refractive index of seawater, and m be the order of the interference trough.

[0056] When the refractive index of seawater changes, the m-th characteristic wavelength in the transmission spectrum will change with the refractive index of seawater, λ. m With n S It changes with the environment. Therefore, the formula for calculating salinity sensitivity K is:

[0057]

[0058] In the formula, K is the sensitivity of the sensor to salinity, S represents the seawater salinity, and dn s / dS is the coefficient for the variation of seawater refractive index with salinity.

[0059] The above formula is based on the silica tube wall thickness t of the nodeless hollow photonic crystal fiber (NHCPCF), the refractive index of the nodeless hollow photonic crystal fiber (NHCPCF), and the refractive index n of seawater. S The correlation is used to calculate salinity through the resonant wavelength shift.

[0060] In this invention, the shape of the microchannel can be arc-shaped or a common pipe shape in the art, provided that the liquid to be tested can enter the main vent and / or silica tube of the nodeless hollow photonic crystal fiber from the outer surface of the single-mode fiber.

[0061] According to the present invention, the first microchannel is L-shaped or arc-shaped, and the second microchannel is L-shaped or arc-shaped.

[0062] According to the present invention, the first microchannel can be either L-shaped or arc-shaped, and the second microchannel can be either L-shaped or arc-shaped.

[0063] According to the present invention, the first microchannel is a first L-shaped microchannel, and the second microchannel is a second L-shaped microchannel;

[0064] The ratio of the radial cross-sectional area of ​​the first L-shaped microchannel to the outer surface area of ​​the introduced single-mode optical fiber is 1:9-766.

[0065] The ratio of the radial cross-sectional area of ​​the second L-shaped microchannel to the outer surface area of ​​the lead-out single-mode fiber is 1:9-766.

[0066] According to the present invention, the ratio of the radial cross-sectional area of ​​the first L-shaped microchannel to the cross-sectional area of ​​the main vent is 1:3-240;

[0067] The ratio of the radial cross-sectional area of ​​the second L-shaped microchannel to the cross-sectional area of ​​the main vent is 1:3-240.

[0068] In this invention, when the ratio of the radial cross-sectional area of ​​the microchannel to the cross-sectional area of ​​the main air hole is within a limited range, seawater can be directly introduced into the nodeless hollow photonic crystal fiber, thereby improving the response speed.

[0069] According to the present invention, the length of the first L-shaped microchannel is 5-45 μm, and the length of the first L-shaped microchannel is the distance from the opening of the first L-shaped microchannel on the outer surface of the introduced single-mode fiber to the point where the first L-shaped microchannel is closest to the core of the introduced single-mode fiber.

[0070] The length of the second L-shaped microchannel is 5-45μm. The length of the second L-shaped microchannel is the distance from the opening of the second L-shaped microchannel on the outer surface of the lead-out single-mode fiber to the point where the second L-shaped microchannel is closest to the core of the lead-out single-mode fiber.

[0071] Preferably, the length-to-width ratio of the first L-shaped microchannel is 1:0.5-2;

[0072] Preferably, the length-to-width ratio of the second L-shaped microchannel is 1:0.5-2;

[0073] In this invention, the width of the first L-shaped microchannel is the distance from the fusion splice of the first L-shaped microchannel and the nodeless hollow photonic crystal fiber to the end of the first L-shaped microchannel away from the nodeless hollow photonic crystal fiber.

[0074] The width of the second L-shaped microchannel is the distance from the splice between the second L-shaped microchannel and the nodeless hollow photonic crystal fiber to the end of the second L-shaped microchannel away from the nodeless hollow photonic crystal fiber.

[0075] According to the present invention, the ratio of the inner diameter of the main vent to the inner diameter of the silica tube is 1:0.1-0.2;

[0076] The ratio of the distance between the axis of the main vent and the axis of the silica tube to the inner diameter of the main vent is 1:0.6-0.8.

[0077] According to the present invention, the first L-shaped microchannel is divided into two parts: one parallel to the core axis of the introduced single-mode fiber and the other perpendicular to the core axis of the introduced single-mode fiber. The length of the first L-shaped microchannel is less than the distance from the core surface of the introduced single-mode fiber to the outer surface of the cladding of the introduced single-mode fiber.

[0078] The width of the first L-shaped microchannel is the distance of the portion parallel to the core axis of the introduced single-mode fiber.

[0079] The second L-shaped microchannel is divided into two parts: one parallel to the core axis of the lead-out single-mode fiber and the other perpendicular to the core axis of the lead-out single-mode fiber. The length of the second L-shaped microchannel is less than the distance from the core surface of the lead-out single-mode fiber to the outer surface of the cladding of the lead-out single-mode fiber.

[0080] The width of the second L-shaped microchannel is the distance of the portion parallel to the core axis of the lead-out single-mode fiber.

[0081] The first L-shaped microchannel and the second L-shaped microchannel are symmetrically arranged on both sides of the nodeless hollow photonic crystal fiber.

[0082] According to the present invention, the number of silicon dioxide tubes is 2n, where n is a natural number from 2 to 6.

[0083] In this invention, the silica tube provides a periodic negative curvature boundary, forming a photonic bandgap. It supports the main vent wall, preventing collapse. It also regulates the light field distribution, enabling single-mode transmission.

[0084] According to a preferred embodiment of the present invention, the number of silica tubes is 8, n is 4, and the 8 tubes are evenly distributed along the inner wall of the main vent (at 45° intervals), which can form a perfect eight-fold symmetric structure, ensuring uniform radial refractive index distribution of the optical fiber and reducing mode distortion during optical transmission.

[0085] A second aspect of the present invention provides a method for fabricating the above-mentioned NHCPCF fiber optic seawater salinity sensor comprising dual microchannels, wherein the method includes the following steps:

[0086] Step 1: Fusion splice one end of the nodeless hollow core photonic crystal fiber NHCPCF to the introduced single-mode fiber;

[0087] Step 2: Fusion splice the other end of the nodeless hollow core photonic crystal fiber NHCPCF to the lead-out single-mode fiber to form a Lead-in SMF-NHCPCF-Lead-out SMF structure;

[0088] Step 3: The structure obtained in Step 2 is placed in a femtosecond laser processing three-dimensional displacement platform. Leveling and calibration are performed with the core center of the single-mode fiber being introduced and the single-mode fiber being led out as the reference. Microchannels are processed on the SMF using a femtosecond laser to form a Lead-in SMF-L-NHCPCF-Lead-out SMF structure.

[0089] A third aspect of the present invention provides the application of the above-mentioned NHCPCF fiber optic seawater salinity sensor with dual microchannels in seawater salinity testing, wherein the testing method of the application includes the following steps:

[0090] S1: Set up a salinity testing system;

[0091] S2: Record the salinity response test results;

[0092] S3: Salinity repeatability test: Under constant room temperature conditions, the structure was subjected to three salinity rise and fall experiments, and the spectrum at the salinity stabilization point was recorded each time. The spectrum of each salinity was then linearly fitted with the corresponding trough wavelength.

[0093] S4: Salinity stability test: The NHCPCF fiber optic seawater salinity sensor containing dual microchannels was placed in a standard seawater environment with constant concentration. The transmission spectrum was recorded every 10 minutes for a total of two hours, and the data of the trough wavelength of each spectrum was tracked.

[0094] According to the present invention, in S1, the salinity testing system includes a supercontinuum light source, a spectrometer, an isolator, and a water bath.

[0095] The specific steps of S2 include: placing the NHCPCF fiber optic seawater salinity sensor containing dual microchannels in a water bath to simulate a constant temperature environment of 20-30°C and a pressure of standard atmospheric pressure, specifically, a temperature of 22.8°C. By changing the concentration of seawater in the external environment, the salinity response characteristics of the sensor are tested, with the seawater salinity increasing from 5‰ to 40‰.

[0096] The technical solution of the present invention will be further described in detail below with reference to the embodiments. Obviously, the embodiments described herein are only some embodiments of the present invention and are not intended to limit the present invention. All other embodiments implemented by those skilled in the art based on the embodiments of the present invention without creative improvements are within the protection scope of the present invention.

[0097] Example 1

[0098] The NHCPCF fiber optic seawater salinity sensor containing dual L-shaped microchannels includes: a lead-in single-mode fiber (Lead-in SMF), a nodeless hollow-core photonic crystal fiber (NHCPCF), and a lead-out single-mode fiber (Lead-out SMF). The first L-shaped microchannel and the second L-shaped microchannel are formed by femtosecond laser etching on the lead-in SMF and the lead-out SMF, respectively. The first L-shaped microchannel and the second L-shaped microchannel are symmetrically arranged on both sides of the nodeless hollow-core photonic crystal fiber.

[0099] L-shaped microchannels were fabricated in lead-in and lead-out SMFs using femtosecond laser processing technology. These channels serve as inflow and outflow pathways for liquids, connecting the core of the nodeless hollow-core photonic crystal fiber (NHCPCF) to the external environment. The core and cladding diameters of the lead-in and lead-out SMFs are the same, at 8.2 μm and 125 μm, respectively. A microscopic cross-sectional image of the nodeless hollow-core photonic crystal fiber (NHCPCF) is shown below. Figure 2 As shown, its inner and outer diameters are 70 μm and 125 μm, respectively. Eight silica tubes are uniformly distributed within the main pore. These tubes have an inner diameter of 11 μm and a wall thickness of 1.2 μm.

[0100] The length ratio of the introduced single-mode fiber, the nodeless hollow-core photonic crystal fiber, and the extracted single-mode fiber is 0.1:1:0.1;

[0101] The ratio of the inner diameter of NHCPCF to the inner diameter of the lead-in single-mode fiber and the lead-out single-mode fiber is 1:0.12:0.12;

[0102] The ratio of the radial cross-sectional area of ​​the first L-shaped microchannel to the outer surface area of ​​the introduced single-mode fiber is 1:30.66.

[0103] The ratio of the radial cross-sectional area of ​​the second L-shaped microchannel to the outer surface area of ​​the lead-out single-mode fiber is 1:30.66.

[0104] The ratio of the radial cross-sectional area of ​​the first L-shaped microchannel to the cross-sectional area of ​​the main vent is 1:9.62;

[0105] The ratio of the radial cross-sectional area of ​​the second L-shaped microchannel to the cross-sectional area of ​​the main vent is 1:9.62;

[0106] According to the instruction manual Figure 3 (c) The length of the first L-shaped microchannel is 40 μm, and the width and thickness of the length portion are both 20 μm;

[0107] The second L-shaped microchannel has a length of 40 μm, and the width and thickness of the length portion are both 20 μm.

[0108] The ratio of the inner diameter of the main vent to that of the silica tube is 1:0.16;

[0109] The ratio of the distance between the axis of the main vent and the axis of the silica tube to the inner diameter of the main vent is 1:0.6.

[0110] A method for manufacturing an NHCPCF fiber optic seawater salinity sensor comprising dual L-shaped microchannels includes the following steps:

[0111] In this embodiment, the materials used to fabricate the fiber optic sensor are single-mode fiber (SMF-28) and nodeless hollow-core photonic crystal fiber (NHCPCF). The fabrication equipment includes a fiber optic fusion splicer (S179), fiber optic stripping pliers, a microscope, a femtosecond laser micromachining platform, and a fixed-length cleaving system. The fixed-length cleaving system includes a fiber optic cleaver, a fiber optic clamp, a fixed pulley, and a three-dimensional displacement platform.

[0112] Step 1: Process diagram as shown Figure 3 As shown in (a), the cladding of the introduced single-mode fiber is removed using fiber strippers and wiped with alcohol. The cladding-removed end of the introduced single-mode fiber is placed in a cleaver, the end face is cut flat, and it is placed on one side of the motor of the fiber fusion splicer (S179). Similarly, one end of the nodeless hollow-core photonic crystal fiber (NHCPCF) is treated in the same way, placed on the other side of the motor of the fiber fusion splicer, and the introduced single-mode fiber and the nodeless hollow-core photonic crystal fiber (NHCPCF) are fused together. The length is then cut to 1.5 cm using a fixed-length cleaver.

[0113] Step 2: Use fiber strippers to remove the cladding of the single-mode fiber and wipe it with alcohol. Place the decladding end of the single-mode fiber in a cleaver, cut the end face flat, and place it on one side of the motor of the fiber fusion splicer (S179). Similarly, place one end of the nodeless hollow-core photonic crystal fiber (NHCPCF) after length cutting on the other side of the motor of the fiber fusion splicer, and fuse the single-mode fiber and the nodeless hollow-core photonic crystal fiber to form an SMF-NHCPCF-SMF structure.

[0114] Step 3: Place the SMF-NHCPCF-SMF structure on a three-dimensional displacement platform and perform precise leveling and calibration using the core centers of the introduced and extracted single-mode fibers as a reference. Using femtosecond laser processing technology, fabricate L-shaped microchannels with identical parameters on both the introduced and extracted single-mode fibers. The fabrication parameters for the L-shaped microchannels are: length 40 μm, width and thickness both 20 μm. The center of the L-shaped microchannel is located 30 μm from the splice point between the single-mode fiber and the nodeless hollow-core photonic crystal fiber (NHCPCF).

[0115] After processing, the integrity of the microchannel inscription is inspected under a microscope at 10x magnification to ensure there is no blockage or damage. See the actual product image below. Figure 3 As shown in (c).

[0116] Use the instruction manual included Figure 4The salinity testing system shown tests the salinity performance of the sensor. A supercontinuum light source (SCS) is used for output light, with an emission range of 400-2100 nm. An OSA (AQ6370D) is used to acquire spectral data, with an operating range of 600-1700 nm. An isolator (OI) is used to protect the supercontinuum light source. A fiber optic probe is placed in a thermostat to simulate a constant temperature environment. Furthermore, standard seawater is used to simulate salinity variations. The concentrations of the standard seawater are 5‰, 12.5‰, 20‰, 30‰, 35‰, and 40‰, where the 12.5‰ concentration standard seawater is obtained by mixing equal masses of 5‰ and 20‰ standard seawater. By changing the concentration of the standard seawater, the salinity response characteristics of the sensor are tested.

[0117] With the ambient temperature stabilized at room temperature, the salinity of the seawater was increased sequentially from 5‰ to 40‰, and the spectral data at each salinity was recorded by OSA. It is important to note that the sensor should be rinsed with alcohol each time the standard seawater is replaced. The temperature should be kept stable at room temperature throughout this process. The transmission spectra of the sensor at different salinities in the 1100-1200 nm range are shown in the attached instruction manual. Figure 5 As shown, a significant blue shift occurs in the transmission spectrum as salinity increases from 5‰ to 40‰. The wavelengths of the interference troughs in the spectra at each salinity are fitted with the corresponding salinity values, yielding the wavelength-salinity fitting relationship as shown in the appendix to the instruction manual. Figure 6 As shown, the salinity sensitivity is -0.88 nm / ‰, and the linearity is 0.9945. Combined with the minimum resolution of the spectrometer (0.02 nm), the salinity resolution of the sensor is calculated to be 0.023‰.

[0118] In practical applications, the repeatability of salinity sensors is crucial. To test the repeatability of the sensor, three salinity rise and fall experiments were conducted on the structure under constant room temperature conditions, and the spectrum at the salinity stabilization point was recorded for each experiment. The spectra of each salinity level were linearly fitted to the corresponding trough wavelengths, yielding the results shown in the appendix to the specification. Figure 7 The salinity sensitivities for the three tests were -0.88 nm / ‰, -0.875 nm / ‰, and -0.845 nm / ‰, respectively, with corresponding linearities of 0.9945, 0.9958, and 0.9972, indicating that the sensor provided by this invention has good repeatability.

[0119] As per the instruction manual Figure 8 As shown, the stability of the sensor was tested over a period of time. The sensor was placed in a standard seawater environment with a salinity of 20‰, and the transmission spectrum was recorded every 10 minutes for a total of two hours. Data on the trough wavelengths of each spectrum were tracked, and the maximum fluctuation was 0.5 nm. This indicates that the sensor provided by this invention has good spectral stability.

[0120] This invention combines microchannels with nodeless hollow photonic crystal fiber (NHCPCF) to successfully fabricate a highly integrated NHCPCF fiber seawater salinity sensor containing dual microchannels. This sensor has high sensitivity (salinity sensitivity -0.88nm / ‰), good repeatability, and stability.

[0121] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An NHCPCF fiber optic seawater salinity sensor comprising dual microchannels, characterized in that, The dual-microchannel NHCPCF fiber optic seawater salinity sensor includes: The sequentially fused single-mode fiber, nodeless hollow-core photonic crystal fiber, and pull-out single-mode fiber; The nodeless hollow photonic crystal fiber includes a first cladding and a main vent. Uniformly distributed silica tubes are arranged inside the main vent. The wall of the silica tube is tangent to the inner wall of the main vent, and the axis of the main vent is parallel to the axis of the silica tube. The introduced single-mode fiber includes a second cladding, a first core, and a first microchannel. One end of the first microchannel has an opening on the outer surface of the second cladding, and the opening at the other end of the first microchannel is connected to the main air hole and the silica tube. The first microchannel is not in contact with the first core of the introduced single-mode fiber. The first microchannel and the second microchannel are stretchable bodies; The lead-out single-mode fiber includes a third cladding, a second core, and a second microchannel. One end of the second microchannel has an opening on the outer surface of the third cladding, and the opening at the other end of the second microchannel is connected to the main air hole and the silica tube. The second microchannel is not in contact with the second core of the lead-out single-mode fiber. The introduced single-mode fiber, nodeless hollow-core photonic crystal fiber, and extracted single-mode fiber are coaxial.

2. The NHCPCF fiber optic seawater salinity sensor comprising dual microchannels according to claim 1, characterized in that, The first microchannel is L-shaped or arc-shaped, and the second microchannel is L-shaped or arc-shaped.

3. The NHCPCF fiber optic seawater salinity sensor comprising dual microchannels according to claim 1, characterized in that, The first microchannel is a first L-shaped microchannel, and the second microchannel is a second L-shaped microchannel; The ratio of the radial cross-sectional area of ​​the first L-shaped microchannel to the outer surface area of ​​the introduced single-mode optical fiber is 1:9-766. The ratio of the radial cross-sectional area of ​​the second L-shaped microchannel to the outer surface area of ​​the lead-out single-mode fiber is 1:9-766; The ratio of the radial cross-sectional area of ​​the first L-shaped microchannel to the cross-sectional area of ​​the main vent is 1:3-240; The ratio of the radial cross-sectional area of ​​the second L-shaped microchannel to the cross-sectional area of ​​the main vent is 1:3-240.

4. The NHCPCF fiber optic seawater salinity sensor comprising dual microchannels according to claim 3, characterized in that, The length of the first L-shaped microchannel is 5-45μm. The length of the first L-shaped microchannel is the distance from the opening of the first L-shaped microchannel on the outer surface of the introduced single-mode fiber to the point where the first L-shaped microchannel is closest to the core of the introduced single-mode fiber. The length of the second L-shaped microchannel is 5-45μm. The length of the second L-shaped microchannel is the distance from the opening of the second L-shaped microchannel on the outer surface of the lead-out single-mode fiber to the point where the second L-shaped microchannel is closest to the core of the lead-out single-mode fiber.

5. The NHCPCF fiber optic seawater salinity sensor comprising dual microchannels according to claim 1, characterized in that, The ratio of the inner diameter of the main vent to the inner diameter of the silica tube is 1:0.1-0.2; The ratio of the distance between the axis of the main vent and the axis of the silica tube to the inner diameter of the main vent is 1:0.6-0.

8.

6. The NHCPCF fiber optic seawater salinity sensor comprising dual microchannels according to claim 3, characterized in that, The first L-shaped microchannel is divided into two parts: one parallel to the core axis of the introduced single-mode fiber and the other perpendicular to the core axis of the introduced single-mode fiber. The length of the first L-shaped microchannel is less than the distance from the core surface of the introduced single-mode fiber to the outer surface of the cladding of the introduced single-mode fiber. The second L-shaped microchannel is divided into two parts: one parallel to the core axis of the lead-out single-mode fiber and the other perpendicular to the core axis of the lead-out single-mode fiber. The length of the second L-shaped microchannel is less than the distance from the core surface of the lead-out single-mode fiber to the outer surface of the cladding of the lead-out single-mode fiber. The first L-shaped microchannel and the second L-shaped microchannel are symmetrically arranged on both sides of the nodeless hollow photonic crystal fiber.

7. The NHCPCF fiber optic seawater salinity sensor comprising dual microchannels according to claim 1, characterized in that, The number of silicon dioxide tubes is 2n, where n is a natural number from 2 to 6.

8. The method for fabricating the NHCPCF fiber optic seawater salinity sensor comprising dual microchannels as described in any one of claims 1-7, characterized in that, Includes the following steps: Step 1: Fusion splice one end of the nodeless hollow core photonic crystal fiber NHCPCF to the introduced single-mode fiber; Step 2: Fusion splice the other end of the nodeless hollow core photonic crystal fiber NHCPCF to the lead-out single-mode fiber to form a Lead-in SMF-NHCPCF-Lead-out SMF structure; Step 3: The structure obtained in Step 2 is placed in a femtosecond laser processing three-dimensional displacement platform. Leveling and calibration are performed with the core center of the single-mode fiber being introduced and the single-mode fiber being led out as the reference. Microchannels are processed on the SMF using a femtosecond laser to form a Lead-in SMF-L-NHCPCF-Lead-out SMF structure.

9. The application of the NHCPCF fiber optic seawater salinity sensor comprising dual microchannels as described in any one of claims 1-7 in seawater salinity testing, characterized in that, The testing method for the application includes the following steps: S1: Set up a salinity testing system; S2: Record the salinity response test results; S3: Salinity repeatability test: Under constant room temperature conditions, the structure was subjected to three salinity rise and fall experiments, and the spectrum at the salinity stabilization point was recorded each time. The spectrum of each salinity was then linearly fitted with the corresponding trough wavelength. S4: Salinity stability test: The NHCPCF fiber optic seawater salinity sensor containing dual microchannels was placed in a standard seawater environment with constant concentration. The transmission spectrum was recorded every 10 minutes for a total of two hours, and the data of the trough wavelength of each spectrum was tracked.

10. The application according to claim 9, characterized in that, In S1, the salinity testing system includes a supercontinuum light source, a spectrometer, an isolator, and a water bath. The specific steps of S2 include: placing the NHCPCF fiber optic seawater salinity sensor containing dual microchannels in the water bath to simulate a constant temperature environment of 20-30°C and a pressure of standard atmospheric pressure; and testing the salinity response characteristics of the sensor by changing the concentration of seawater in the external environment, with the seawater salinity increasing from 5‰ to 40‰.

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