A two-parameter optical fiber sensor based on heart-shaped air holes and exposed core

Abstract

The application relates to a double-parameter optical fiber sensor based on a heart-shaped air hole and an exposed core, belonging to the technical field of optical sensing, which comprises a photonic crystal fiber base with an exposed core structure, a temperature-sensitive material layer, a first metal film layer and a second metal film layer; the fiber base is provided with a double-layer asymmetric heart-shaped air hole array, forming a lower refractive index detection channel and a left temperature detection channel; the refractive index detection channel surface is plated with a gold film, and the temperature detection channel inner wall is plated with a silver film and filled with the temperature-sensitive material layer. The application enhances light and material interaction through the collaborative design of the exposed core and the heart-shaped air hole, reduces the difficulty of filling the temperature-sensitive material by using a large-size air hole, realizes high-sensitivity synchronous measurement of the refractive index and the temperature, and eliminates cross interference.

Description

Technical Field

[0001] This invention belongs to the field of optical devices and sensing technology, and particularly relates to a dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core. Background Technology

[0002] Fiber optic sensing technology, with its advantages of electromagnetic interference resistance, corrosion resistance, small size, and light weight, is playing an increasingly important role in environmental monitoring, biochemical detection, and industrial production. In recent years, photonic crystal fiber (PCF) combined with surface plasmon resonance (SPR) technology has significantly enhanced the interaction between light and matter, and has become an important research direction for high-sensitivity sensors.

[0003] To improve sensor detection sensitivity and achieve multi-parameter measurement, researchers have made numerous improvements and optimizations to the geometry of optical fibers. Early asymmetric structures, such as C-shaped and D-shaped (side-projected) microstructured fibers, improved detection performance and achieved multi-parameter demodulation to some extent by changing the fiber cross-section. In 2017, Wu et al. first integrated C-shaped fibers with a PCF-Sagnac interferometric structure for microfluidic refractive index sensing, verifying the applicability of C-shaped structures in liquid sensing. In 2021, Liu et al. proposed an SPR sensor based on parallel side-projected plastic fiber (POF). Through symmetrical double-sided polishing and coating different regions with gold films and PDMS thermosensitive materials, they achieved simultaneous measurement of refractive index and temperature, experimentally measuring a refractive index sensitivity of 1174 nm / RIU and a temperature sensitivity of -0.7 nm / °C. These asymmetric structures improve detection sensitivity to some extent by guiding the interaction between the evanescent field and the external environment. In the same year, Zhao Yong et al. conducted a systematic theoretical analysis of C-shaped microstructured fibers, verifying their potential in seawater temperature and salinity measurement.

[0004] Building upon the aforementioned research, the "exposed-core" structure has garnered significant attention in recent years due to its ability to further enhance detection sensitivity. By removing part of the fiber cladding, the exposed-core structure allows evanescent waves generated in the fiber core to penetrate directly and unimpeded into the measured medium, thus achieving extremely high environmental response capabilities. In 2020, Li et al. proposed an exposed-core sensing scheme combining a fiber Bragg grating (FBG) with a multimode Mach-Zehnder interferometer (MZI), achieving dual-parameter measurement of temperature and refractive index. In 2021, Yang et al. designed an SPR sensor based on an exposed-core microstructure fiber, achieving a wavelength sensitivity of up to 3000 nm / RIU in the refractive index range of 1.33–1.39 by sequentially depositing indium tin oxide (ITO) and gold layers on the bare fiber core surface. These studies confirm the enormous potential of exposed-core structures in improving sensor sensitivity.

[0005] However, existing exposed-core fiber optic sensors still have significant limitations when it comes to two-parameter measurements. In temperature detection, current sensors typically achieve temperature response by filling the cladding pores of the fiber with a temperature-sensitive material (such as PDMS). However, the cladding pores of traditional PCF microstructured fibers are only on the order of micrometers, making it extremely difficult to uniformly fill these tiny pores with a temperature-sensitive material. This often requires expensive pressurization equipment, and the filling process is complex, inefficient, and difficult to guarantee uniformity. This manufacturing bottleneck severely restricts the practical application of exposed-core fiber optic sensors in the field of two-parameter temperature measurement. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core. While maintaining the ultra-high refractive index sensitivity of the exposed core structure, it achieves easy manufacturing, convenient filling, and cross-sensitivity measurement of temperature and refractive index as dual parameters.

[0007] A dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core, characterized in that it comprises an optical fiber substrate (1), a temperature-sensitive material layer (2), a first metal film layer (3), and a second metal film layer (4). The optical fiber substrate (1) is a photonic crystal fiber with an exposed core structure. A multi-layer asymmetric pore array is arranged around the core. The multi-layer asymmetric pore array forms a lower refractive index detection channel and a left temperature detection channel. The lower refractive index detection channel is located in the Y-axis exposed channel of the optical fiber, and the surface is coated with the second metal film layer (4), which is used to contact the medium to be tested and excite surface plasma resonance to sense the refractive index change; the left temperature detection channel is located in the X-axis pore of the optical fiber, the inner wall of the pore is coated with the first metal film layer (3) and filled with the temperature-sensitive material layer (2).

[0008] The multi-layer asymmetric pore array includes an inner layer of air pores and an outer layer of air pores; The inner air pores include multiple large heart-shaped air pores (8), small circular air pores (10), extra-large circular air pores (11), and small heart-shaped air pores (12). The outer air vents include multiple large circular air vents (6), large elliptical air vents (7), and small elliptical air vents (9).

[0009] The axial length of the large heart-shaped air hole (8), that is, the straight-line distance from the concave point of the heart-shaped contour to the opposite convex vertex along the axis of symmetry, is 2.7μm~3.1μm, the center distance from the fiber core is 0.9μm~1.3μm, and the air hole angle is 50°~56°; the axial length of the small heart-shaped air hole (12), that is, the straight-line distance from the concave point of the heart-shaped contour to the opposite convex vertex along the axis of symmetry, is 2.8μm~3.1μm, the center distance from the fiber core is 1μm~1.2μm, and the air hole angle is 28~32°.

[0010] The large elliptical air hole (7) has a major semi-axis of 1μm~1.6μm, a minor semi-axis of 0.5μm~0.9μm, and a distance from the fiber core of 4.1μm~4.2μm; the small elliptical air hole (9) has a major semi-axis of 0.9μm~1.1μm, a minor semi-axis of 0.5μm~0.7μm, and a distance from the fiber core of 3.85μm~3.95μm; the small circular air hole (10) has a diameter of 0.7μm~0.9μm and a distance from the fiber core of 0.4μm~0.8μm.

[0011] The diameter of the large circular air hole (6) is 1.5μm~1.9μm and the distance from the fiber core is 4.1μm~4.2μm; the diameter of the ultra-large circular air hole (11) is 1.5μm~2.1μm and the distance from the fiber core is 1.5μm~2μm.

[0012] The second metal film layer (4) is a gold film with a thickness of 30nm~50nm and a height of 0.8μm~1.2μm; the first metal film layer (3) is a silver film with a thickness of 20nm~36nm and a height of 1.4μm~2μm.

[0013] The optical fiber substrate (1) is made of fused silica, and the temperature-sensitive material layer (2) is made of polydimethylsiloxane.

[0014] The lower refractive index detection channel generates a first resonance peak under Y-polarized light excitation. The wavelength of the first resonance peak shifts with the refractive index of the medium under test. The refractive index detection range is 1.33–1.38, and the maximum wavelength sensitivity is 24390 nm / RIU. The left-layer temperature detection channel generates a second resonance peak under X-polarized light excitation. The wavelength of the second resonance peak drifts with changes in ambient temperature. The temperature detection range is 100°C to 150°C, and the maximum wavelength sensitivity is 13.2 nm / °C.

[0015] The temperature-sensitive material layer in the left-side temperature detection channel senses temperature changes while acting as a protective layer to isolate air and prevent the first metal film layer (3) from oxidizing in a high-temperature environment.

[0016] The lower refractive index detection channel and the left temperature detection channel achieve synchronous decoupled detection of refractive index and temperature based on spatially separated resonance peaks under different polarization states, thus eliminating the cross-interference of temperature on refractive index measurement.

[0017] By employing the above technical solution, the present invention has at least the following beneficial effects: (1) The present invention exposes the core structure so that the medium under test can directly contact the light field, and the optimized heart-shaped air hole design significantly enhances the interaction between light and matter; at the same time, the use of large-sized circular air holes greatly reduces the difficulty of penetrating and filling colloidal temperature-sensitive materials, and solves the technical bottleneck of filling existing PCF microstructure optical fibers.

[0018] (2) The refractive index detection range of the present invention covers 1.33–1.38, and the temperature detection range is 100–150°C. By filling the internal air holes with PDMS, on the one hand, the high thermo-optic coefficient is used to achieve sensitive temperature sensing, and on the other hand, PDMS is cleverly used as a protective layer to effectively isolate the air and prevent the internal silver film from oxidizing in a high-temperature environment, thereby ensuring the long-term stability and reliability of the sensor under harsh working conditions.

[0019] (3) This invention, through its asymmetric double-layer pore structure, can precisely control the mode field distribution of the optical fiber, thereby enhancing the penetration depth of the evanescent field in the Y direction while ensuring effective confinement of the fiber core energy. The maximum wavelength sensitivity of the sensor's refractive index reaches 24390 nm / RIU, and the maximum wavelength sensitivity of the temperature is 13.2 nm / °C. Furthermore, the goodness of fit R² for both is close to 1, demonstrating excellent linear response characteristics.

[0020] (4) By rationally arranging the double-layer pores, the present invention effectively confines the light field energy to the region near the metal film, reduces energy leakage to the cladding, enhances the coupling efficiency between the fiber core mode and the plasma mode on the surface of the metal film, thereby improving the response sensitivity of the SPR sensor. Attached Figure Description

[0021] Figure 1 A cross-sectional schematic diagram of a dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the X-polarization phase matching and confined loss spectrum of the sensor in an embodiment of the present invention; Figure 3 This is a schematic diagram of the Y-polarization phase matching and confined loss spectrum of the sensor in an embodiment of the present invention; Figure 4 This is a graph showing the X-polarization confined loss spectrum of the sensor's refractive index sensing characteristics as a function of the refractive index of the medium under test, according to an embodiment of the present invention. Figure 5 This is a linear fitting graph of the resonant wavelength as a function of temperature for the refractive index sensing characteristics of the sensor in an embodiment of the present invention. Figure 6 This is a graph showing the Y-polarization confined loss spectrum of the sensor temperature sensing characteristics as a function of ambient temperature, according to an embodiment of the present invention. Figure 7 This is a linear fitting graph of the resonant wavelength as a function of refractive index for the temperature sensing characteristics of the sensor in an embodiment of the present invention. In the picture: 1-Fiber optic substrate; 2-Temperature-sensitive material layer; 3-First metal film layer; 4-Second metal film layer; 5-Medium channel under test; 6-Large circular air hole; 7-Large elliptical air hole; 8-Large heart-shaped air hole; 9-Small elliptical air hole; 10-Small circular air hole; 11-Extra-large circular air hole; 12-Small heart-shaped air hole. Detailed Implementation

[0022] To better explain and facilitate understanding of the present invention, the technical solution and effects of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] like Figure 1 As shown, this embodiment provides a dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core, including a photonic crystal fiber substrate 1, a temperature-sensitive material layer 2, a first metal film layer 3, and a second metal film layer 4. The photonic crystal fiber substrate 1 is made of fused silica and has an exposed core structure. A double-layer asymmetric vent array is arranged around the fiber core, forming a refractive index detection channel and a temperature detection channel, and also exhibiting an exposed core structure. Specifically, the cladding below the fiber core is removed to form an exposed channel, allowing one side of the core to be directly exposed, forming the lower refractive index detection channel, i.e., the channel for the measured medium 5. Simultaneously, a large circular air hole 11 is provided on the left side of the fiber core as the left-layer temperature detection channel.

[0024] The double-layer air hole array of the photonic crystal fiber substrate 1 includes an inner layer of air holes and an outer layer of air holes. The inner layer of air holes consists of two large heart-shaped air holes 8, two small circular air holes 10, two extra-large circular air holes 11, and two small small air holes 12 arranged symmetrically in the fiber core. The outer layer of air holes includes six small elliptical air holes 9 located between the inner layer air holes, five large circular air holes 6 located above the inner layer air holes, and two large elliptical air holes 7. Specifically, the axial length of the large heart-shaped air hole 8, i.e., the straight-line distance from the concave point of the heart-shaped profile to the opposite convex vertex along the axis of symmetry, is r1=3μm, its center is 1μm away from the fiber core, and the air hole angle θ1=55°; the axial length of the small heart-shaped air hole 12, i.e., the straight-line distance from the concave point of the heart-shaped profile to the opposite convex vertex along the axis of symmetry, is r2=3μm, its distance from the fiber core is 1μm, and the air hole angle θ2=32°; the semi-major axis R of the large elliptical air hole 7 is... a =1.5μm, short half-axis R b =0.9μm, distance from the fiber core is 4.2μm; the major semi-axis r of the small elliptical air hole 9 a =1μm, short half-axis r b =0.6μm, distance from the fiber core is 3.9μm; the diameter d1 of the small circular air hole 10 is 0.9μm, distance from the fiber core is 0.5μm; the diameter d3 of the large circular air hole 6 is 1.8μm, distance from the fiber core is 4.15μm; the diameter d2 of the ultra-large circular air hole 11 is 2μm, distance from the fiber core is 1.9μm. Through this asymmetric double-layer heart-shaped air hole design, the optical field energy is effectively confined to the region near the metal film layer, reducing energy leakage to the cladding, thereby enhancing the coupling efficiency between the fiber core fundamental mode and the plasmonic mode on the surface of the metal film layer, laying the foundation for subsequent high-sensitivity SPR sensing.

[0025] Furthermore, the lower refractive index detection channel, i.e., the test medium channel 5, is an exposed channel located on the Y-axis of the optical fiber. A second metal film layer 4, which is a gold film, is deposited on the exposed surface of this channel, with a thickness of t. Au =50nm, and the height of the gold film is the excision depth h1 of the exposed core = 1μm. When the test medium enters this channel, the surface plasmon resonance effect will be excited on the surface of the gold film. The change in the refractive index of the test medium will directly cause the resonant wavelength to shift, thereby realizing the quantitative detection of the refractive index. The left layer temperature detection channel is located in a super-large circular air hole 11 on the X-axis. The inner wall of the super-large circular air hole 11 is deposited with a first metal film layer 3, which is a silver film with a thickness t. Ag=30nm, the distance h2 between the silver film and the fiber core is 1.9μm. A temperature-sensitive material layer 2 is filled after the silver film. This layer 2 uses polydimethylsiloxane (PDMS) with a substrate-to-curing agent mass ratio of 15:1. Because the diameter of the ultra-large circular air hole 11 reaches 2μm, which is much larger than the micron-sized pores in traditional photonic crystal fibers, the PDMS material can be easily and uniformly filled using capillary effects, eliminating the need for expensive pressurization equipment. This greatly reduces the difficulty of filling the temperature-sensitive material while ensuring the uniformity and reliability of the filling.

[0026] The working principle and process of the above sensor are as follows: Light emitted from a broadband light source is modulated into X-polarized and Y-polarized light by a polarization controller before entering the sensor. Comparative analysis of the transmission characteristics of the two polarization fundamental modes reveals that, for example… Figure 2 As shown, the resonance peak in the X-polarization mode originates from the coupling between the core mode and the interface of the left-side silver film. Since the temperature detection channel in the left layer is filled with a thermosensitive material, which exhibits a significant thermo-optic effect and whose refractive index decreases with increasing temperature, changes in the refractive index of the thermosensitive material modulate the SPR resonance conditions on the silver film surface when the ambient temperature changes, causing the resonance peak to shift. Therefore, this resonance peak is mainly affected by the ambient temperature and is used for synchronous temperature detection. Figure 3 As shown, the resonance peak in the Y-polarization mode originates from the coupling between the core mode and the interface of the underlying gold film. Since the fifth channel of the analyte is located on the exposed core surface and is in direct contact with the analyte, changes in the refractive index of the analyte directly affect the SPR resonance wavelength on the gold film surface. Therefore, this resonance peak is highly sensitive to external refractive index changes and is used for quantitative refractive index detection. This design, based on the spatial separation of the resonance peaks in two channels under different polarization states, achieves independent responses to refractive index and temperature, effectively eliminating the cross-interference of temperature on refractive index measurement in traditional SPR sensors.

[0027] To verify the performance of the sensor in this embodiment, the refractive index sensing characteristics and temperature sensing characteristics were tested. In the refractive index sensing test, a test medium with a refractive index range of 1.33–1.38 was sequentially introduced into the refractive index detection channel, and the changes in the limiting loss spectrum were recorded. Figure 4 As shown, as the refractive index of the tested medium gradually increases from 1.33 to 1.38, a significant blue shift, i.e., a shift towards shorter wavelengths, is observed in the resonance peak. Linear fitting was performed on the resonance wavelength data corresponding to each refractive index point, and the results are as follows: Figure 5 As shown, the maximum wavelength sensitivity for refractive index detection reaches 24390 nm / RIU, and the linear fit goodness R² is close to 1, indicating that the sensor has excellent linear response characteristics. This ultra-high sensitivity is attributed to the exposed core structure that allows the analyte to directly contact the light field, and the optimized control of the mode field by the heart-shaped air hole.

[0028] In temperature sensing tests, a temperature detection channel filled with a temperature-sensitive material layer is placed in a controlled heating environment with a temperature range set from 100°C to 150°C. For example... Figure 6 As shown, as the ambient temperature gradually increases from 100°C to 150°C, the resonance peak exhibits a significant redshift, that is, it shifts towards longer wavelengths. Figure 7 As shown, by linearly fitting the resonant wavelength data corresponding to each temperature point, the temperature sensitivity was measured to be 13.2 nm / °C, and R² was also close to 1. The temperature-sensitive material layer filled within the ultra-large circular air hole 11 not only achieves the temperature-sensitive function but also isolates the sensor from air, effectively preventing oxidation of the internal silver film at high temperatures. This ensures the long-term stability and reliability of the sensor within the temperature range of 100–150°C.

[0029] In summary, the sensor provided in this embodiment achieves high-sensitivity and high-linearity synchronous measurement of two parameters, namely refractive index (1.33–1.38) and temperature (100–150°C), through the synergistic design of exposed core and asymmetric double-layer heart-shaped pores, combined with a dual-channel spatially separated SPR resonance peak scheme. At the same time, it solves the process bottleneck of difficult filling of temperature-sensitive materials in traditional microstructure optical fibers.

Claims

1. A dual-parameter fiber optic sensor based on a cardioid vent and an exposed core, characterized in that: It includes an optical fiber substrate (1), a temperature-sensitive material layer (2), a first metal film layer (3), and a second metal film layer (4); The optical fiber substrate (1) is a photonic crystal fiber with an exposed core structure. A multi-layer asymmetric pore array is arranged around the core. The multi-layer asymmetric pore array forms a lower refractive index detection channel and a left temperature detection channel. The lower refractive index detection channel is located in the Y-axis exposed channel of the optical fiber. The cladding below the fiber core is removed, so that one side of the fiber core is directly exposed to the outside. The exposed core surface is coated with the second metal film layer (4), which is used to contact the medium to be tested and excite surface plasma resonance to sense the refractive index change. The left temperature detection channel is located in the X-axis pore of the optical fiber. The inner wall of the pore is coated with the first metal film layer (3) and filled with the temperature-sensitive material layer (2). The multilayer asymmetric pore array includes an inner layer air pore and an outer layer air pore. The inner layer air pore includes multiple large heart-shaped air pores (8), small circular air pores (10), super-large circular air pores (11) and small heart-shaped air pores (12). The tips of the large heart-shaped air pores (8) and the small heart-shaped air pores (12) are all set towards the fiber core. The axial length of the large heart-shaped air hole (8), that is, the straight-line distance from the concave point of the heart-shaped contour to the opposite convex vertex along the axis of symmetry, is 2.7μm~3.1μm, the center distance from the fiber core is 0.9μm~1.3μm, and the air hole angle is 50°~56°; the axial length of the small heart-shaped air hole (12), that is, the straight-line distance from the concave point of the heart-shaped contour to the opposite convex vertex along the axis of symmetry, is 2.8μm~3.1μm, the center distance from the fiber core is 1μm~1.2μm, and the air hole angle is 28~32°; The diameter of the super-large circular air hole (11) is 1.5μm~2.1μm, and the distance from the fiber core is 1.5μm~2μm; The left layer temperature detection channel is an ultra-large circular air hole (11), and the temperature-sensitive material layer (2) is filled in the ultra-large circular air hole (11). The temperature-sensitive material layer (2) acts as a protective layer to isolate air while sensing temperature changes, and is used to prevent the first metal film layer (3) from oxidizing in a high-temperature environment. The lower refractive index detection channel generates a refractive index resonance peak under Y-polarized light excitation, and the left temperature detection channel generates a temperature resonance peak under X-polarized light excitation. The lower refractive index detection channel and the left temperature detection channel achieve synchronous decoupled detection of refractive index and temperature based on spatially separated resonance peaks under different polarization states, thereby eliminating the cross-interference of temperature on refractive index measurement.

2. The dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core according to claim 1, characterized in that: The outer air vents include multiple large circular air vents (6), large elliptical air vents (7), and small elliptical air vents (9).

3. The dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core according to claim 2, characterized in that: The large elliptical air hole (7) has a major semi-axis of 1μm~1.6μm, a minor semi-axis of 0.5μm~0.9μm, and a distance from the fiber core of 4.1μm~4.2μm; the small elliptical air hole (9) has a major semi-axis of 0.9μm~1.1μm, a minor semi-axis of 0.5μm~0.7μm, and a distance from the fiber core of 3.85μm~3.95μm; the small circular air hole (10) has a diameter of 0.7μm~0.9μm and a distance from the fiber core of 0.4μm~0.8μm.

4. The dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core according to claim 2, characterized in that: The diameter of the large circular air hole (6) is 1.5μm~1.9μm, and the distance from the fiber core is 4.1μm~4.2μm.

5. The dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core according to claim 1, characterized in that: The second metal film layer (4) is a gold film with a thickness of 30nm~50nm and a height of 0.8μm~1.2μm; the first metal film layer (3) is a silver film with a thickness of 20nm~36nm and a height of 1.4μm~2μm.

6. The dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core according to claim 1, characterized in that: The optical fiber substrate (1) is made of fused silica, and the temperature-sensitive material layer (2) is made of polydimethylsiloxane.

7. The dual-parameter fiber optic sensor based on a heart-shaped vent and an exposed core according to claim 1, characterized in that: The lower refractive index detection channel generates a first resonance peak under Y-polarized light excitation. The wavelength of the first resonance peak shifts with the refractive index of the medium under test. The refractive index detection range is 1.33–1.38, and the maximum wavelength sensitivity is 24390 nm / RIU. The left-layer temperature detection channel generates a second resonance peak under X-polarized light excitation. The wavelength of the second resonance peak drifts with changes in ambient temperature. The temperature detection range is 100°C to 150°C, and the maximum wavelength sensitivity is 13.2 nm / °C.

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