High-sensitivity fiber FP temperature sensor based on PDMS end-face filling and preparation method

Abstract

The present application belongs to the technical field of optical fiber sensing, and aims to solve the problems of large size, high cost and complex preparation of existing optical fiber sensors. The temperature sensor comprises a single-mode optical fiber, a hollow-core optical fiber and a temperature-sensitive filling medium; the single-mode optical fiber and the hollow-core optical fiber are coaxially butted and fixed, the temperature-sensitive filling medium is filled in the internal cavity of the hollow-core optical fiber, and the temperature-sensitive filling medium is in contact with the end face of the single-mode optical fiber; the end face of the single-mode optical fiber forms a first reflecting surface of a Fabry-Perot interference cavity, the end face of the temperature-sensitive filling medium away from the single-mode optical fiber forms a second reflecting surface of the Fabry-Perot interference cavity, and the temperature-sensitive filling medium segment between the first reflecting surface and the second reflecting surface forms a cavity length of the Fabry-Perot interference cavity; the temperature-sensitive filling medium is polydimethylsiloxane, which is used for causing wavelength shift of an interference spectrum through synchronous change of refractive index and cavity length when the ambient temperature changes, and high-sensitivity sensitization effect is realized through the PDMS filling type interference cavity structure.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, specifically relating to a high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling and its fabrication method. Background Technology

[0002] Fiber optic sensors have been widely studied and applied in temperature and pressure measurement due to their advantages such as high sensitivity, simple and compact structure, and resistance to electromagnetic interference. They have shown great application potential in fields such as power and electrical engineering, industrial production, and energy and transportation.

[0003] In recent years, various temperature sensors based on the principle of fiber optic interferometry have been proposed to improve sensitivity, including Mach-Zehnder interferometers, FP interferometers, Michelson interferometers, and Sagnac interferometers. Among them, the FP interferometer has attracted widespread attention due to its advantages such as compact structure, simple fabrication, and good fringe contrast.

[0004] To further improve temperature sensitivity, researchers have attempted to introduce temperature-sensitive materials with high thermo-optical coefficients and coefficients of thermal expansion into fiber optic sensing structures, such as polymethyl methacrylate (PMMA), UV adhesive, polydimethylsiloxane (PDMS), silicone oil, and photopolymer materials. Among these, PDMS stands out due to its excellent light transmittance and high thermo-optical coefficient (-4.66 × 10⁻⁶). -4 (°C) and high coefficient of thermal expansion (3×10) -4 / ℃), and PMMA (thermophotometric coefficient 193×10 -6 / ℃, coefficient of thermal expansion 61×10 -6 It has advantages over other materials such as ( / ℃) and is considered an ideal temperature-sensitizing material.

[0005] Previous studies have shown that coating PDMS onto the end face of an optical fiber to form a PDMS cap has yielded a temperature sensitivity of 0.698 nm / ℃, or using two-photon polymerization lithography to fabricate a spring microstructure on the end face of an optical fiber and then filling it with PDMS has yielded a temperature sensitivity of 3.449 nm / ℃. However, these methods have limitations such as large size, high cost, and complex fabrication. Summary of the Invention

[0006] (a) Purpose of the invention The purpose of this invention is to provide a high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling and its fabrication method, aiming to solve the problems of large size, high cost and complex fabrication of existing fiber optic sensors.

[0007] (II) Technical Solution To address the above problems, a first aspect of the present invention provides a high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling, the temperature sensor comprising a single-mode fiber, a hollow fiber, and a temperature-sensitive filling medium; The single-mode optical fiber is coaxially connected and fixed to the hollow-core optical fiber, and the temperature-sensitive filling medium is filled in the internal cavity of the hollow-core optical fiber, and the temperature-sensitive filling medium is in contact with the end face of the single-mode optical fiber. The end face of the single-mode fiber forms the first reflecting surface of the Fabry-Perot interferometer cavity, and the end face of the temperature-sensitive filling medium on the side away from the single-mode fiber forms the second reflecting surface of the Fabry-Perot interferometer cavity. The section of temperature-sensitive filling medium between the first and second reflecting surfaces forms the cavity length of the Fabry-Perot interferometer cavity. The temperature-sensitive filling medium is polydimethylsiloxane, which is used to achieve temperature sensing by causing a wavelength shift in the interference spectrum through synchronous changes in refractive index and cavity length when the ambient temperature changes.

[0008] Preferably, the hollow optical fiber and the single-mode optical fiber are coaxially fixedly connected by fusion splicing, and the quartz tube wall of the hollow optical fiber and the cladding material of the single-mode optical fiber are fused together at the splice point to form a continuous quartz matrix structure.

[0009] Preferably, the internal cavity of the hollow optical fiber is a cylindrical structure, the axis of the internal cavity coincides with the optical fiber axis of the single-mode optical fiber, and the diameter of the internal cavity is greater than or equal to the diameter of the optical fiber.

[0010] Preferably, the outer diameter of the single-mode optical fiber is equal to the outer diameter of the hollow-core optical fiber.

[0011] Preferably, the outer diameter of the hollow optical fiber is in the range of 100μm-150μm, and the diameter of the internal cavity is in the range of 60μm-90μm.

[0012] Preferably, the length of the internal cavity is 50μm-80μm.

[0013] Another aspect of the present invention provides a method for fabricating a high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling, the method comprising: Align one end of the hollow fiber with the end face of the single-mode fiber, and perform coaxial fusion splicing using a fiber optic fusion splicer to form a fusion fiber optic assembly. The hollow fiber in the fusion spliced ​​fiber assembly is cut to a predetermined axial length to form an initial cavity structure inside the hollow fiber that is closed at one end and open at the other. The main agent of polydimethylsiloxane and the curing agent are mixed in a predetermined ratio, and then degassed to obtain liquid polydimethylsiloxane prepolymer. The liquid polydimethylsiloxane prepolymer is attached to the end face of the auxiliary optical fiber, and the liquid polydimethylsiloxane prepolymer is introduced into the initial cavity of the hollow optical fiber by capillary force until the initial cavity is completely filled. The filled fusion-bonded optical fiber assembly is placed in a heating environment to cause the liquid polydimethylsiloxane prepolymer to undergo a cross-linking and curing reaction, forming a solid temperature-sensitive filling medium. The cured sensor end face is subjected to arc discharge treatment to remove surface impurities and smooth the port edge, thus obtaining the temperature sensor.

[0014] Preferably, the step of mixing the polydimethylsiloxane main agent and curing agent in a predetermined ratio and obtaining a liquid polydimethylsiloxane prepolymer through degassing treatment includes: The main agent and the curing agent are weighed at a mass ratio of 10:1 and placed in a clean container. The mixture is then centrifuged and stirred to ensure that the main agent and the curing agent are fully and evenly mixed. Vacuum treatment removes air bubbles from the mixture, yielding a bubble-free liquid polydimethylsiloxane prepolymer.

[0015] Preferably, placing the filled fusion-bonded optical fiber assembly in a heating environment to cause the liquid polydimethylsiloxane prepolymer to undergo a crosslinking and curing reaction to form a solid temperature-sensitive filling medium includes: Place the filled fusion spliced ​​fiber assembly vertically with the open end of the hollow fiber facing upwards. The curing temperature is set to 50℃-70℃, and the curing time is 4-6 hours. At the curing temperature, the polydimethylsiloxane prepolymer gradually transforms from a liquid state to a solid cross-linked structure.

[0016] Preferably, the cured sensor end face is subjected to arc discharge treatment to remove surface impurities and smooth the port edges, including: Set the discharge parameters for the fiber optic fusion splicer, where the discharge intensity is set to 50 bits and the discharge time is set to 100 ms. The high temperature generated by the electric arc discharge causes the quartz material at the edge of the hollow fiber port to melt slightly and form a smooth rounded corner, and removes the organic impurities remaining on the free end face of the temperature-sensitive filling medium. After discharge, the free end face remains optically flat and is flush with the outer end face of the hollow fiber.

[0017] (III) Beneficial Effects The above-described technical solution of the present invention has the following beneficial technical effects: 1. The sensor head is constructed using a pure optical fiber structure, avoiding the influence of electromagnetic interference on the sensing signal. The all-fiber sensor can maintain stable and reliable measurement performance. At the same time, the optical fiber material itself has insulating properties, so the sensor head will not generate electric sparks or leakage risks. It can be directly applied to dangerous places such as flammable and explosive electrical, petrochemical, and coal mines without the need for additional explosion-proof isolation measures. In addition, both optical fiber and PDMS have good chemical stability and corrosion resistance, and can work for a long time in harsh environments such as humidity, salinity, and corrosive gases. The placement inside the hollow optical fiber further reduces the overall size of the sensor and simplifies the overall structure.

[0018] 2. By filling the hollow fiber with PDMS to form a Fabry-Perot interferometer cavity, the dual thermo-sensitive properties of PDMS are fully utilized. On the one hand, PDMS has a significant thermo-optic effect, and its refractive index changes with temperature much more than that of quartz. Temperature changes directly cause changes in the refractive index of the interferometer cavity medium. On the other hand, PDMS has a large coefficient of thermal expansion. Temperature changes cause changes in the axial length of the cavity. The thermo-optic effect and the thermal expansion effect are superimposed in the same direction and work together to affect the optical path difference of the interferometer cavity, causing a significant wavelength shift in the interference spectrum. This improves the temperature sensitivity, enabling the resolution of smaller temperature changes and meeting the requirements of high-precision temperature detection. At the same time, the hollow structure of the hollow fiber provides a complete physical support for PDMS, allowing the thermo-sensitive material to fully interact with the optical field. The entire optical path is within the thermo-sensitive medium, resulting in a higher sensitivity enhancement efficiency than surface-coated or partially contact thermo-sensitive structures.

[0019] 3. The internal cavity of hollow optical fiber is used as the filling container for PDMS. The shape and size of PDMS are precisely determined by the internal cavity size of the hollow optical fiber, and are not affected by factors such as surface tension and drop volume. The cavity size and shape of different sensors are highly consistent, thus ensuring batch stability of sensing performance. The cured PDMS columnar body is tightly attached to the inner wall of the hollow optical fiber. The friction of the inner wall and the interface adhesion force form a mechanical anchoring effect on the filler, which can effectively prevent the filler from axial displacement or falling off during temperature cycling, thus improving the long-term reliability of the sensor. In addition, the closed end of the cavity is composed of the end face of a single-mode optical fiber, which provides a definite axial positioning reference for PDMS, ensuring that the position of the first reflecting surface of the interference cavity is accurate and stable, and is not affected by the fluctuation of the filling process. This is conducive to ensuring precise control of cavity length and repeated preparation. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of the high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling provided by the present invention; Figure 2 This is the optical path schematic diagram of the high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling provided by the present invention; Figure 3 This is a schematic flowchart of the fabrication method of the high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling provided by the present invention. Figure 4 This is an optical microscope image of a high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling provided in an embodiment of the present invention; Figure 5 This is a reflection interference spectrum of a temperature sensor provided in one embodiment of the present invention at different temperatures; Figure 6 This is a linear fitting graph of wavelength shift at an interference valley and temperature provided by a temperature sensor according to an embodiment of the present invention; Figure 7 This is a linear fitting graph of the wavelength shift at another interference valley and temperature provided by a temperature sensor according to one embodiment of the present invention; Figure 8 This is a wavelength stability test diagram of a temperature sensor provided in one embodiment of the present invention at 30°C.

[0021] Figure label: 1. Single-mode optical fiber; 11. First reflecting surface; 2. Hollow-core optical fiber; 2a. Internal cavity; 3. Temperature-sensitive filling medium; 31. Second reflective surface; 100. Auxiliary optical fiber; 200. Optical fiber clamp. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. It should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0023] The accompanying drawings illustrate a layer structure according to an embodiment of the present invention. These drawings are not to scale, and some details have been enlarged for clarity, and some details may have been omitted. The shapes of the various regions and layers shown in the drawings, as well as their relative sizes and positional relationships, are merely exemplary and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0024] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0025] Combination Figures 1 to 8 The first aspect of the present invention provides a high-sensitivity fiber optic FP temperature sensor based on PDMS end face filling, the temperature sensor comprising a single-mode fiber 1, a hollow fiber 2 and a temperature-sensitive filling medium 3. Single-mode fiber 1 and hollow fiber 2 are coaxially connected and fixed. Temperature-sensitive filling medium 3 is filled in the internal cavity 2a of hollow fiber 2 and is in contact with the end face of single-mode fiber 1. The end face of the single-mode fiber 1 forms the first reflecting surface 11 of the Fabry-Perot interferometer cavity, and the end face of the temperature-sensitive filling medium 3 away from the single-mode fiber 1 forms the second reflecting surface 31 of the Fabry-Perot interferometer cavity. The section of the temperature-sensitive filling medium 3 between the first reflecting surface 11 and the second reflecting surface 31 forms the cavity length of the Fabry-Perot interferometer cavity. The temperature-sensitive filling medium 3 is polydimethylsiloxane, or PDMS, which is used to achieve temperature sensing by causing a wavelength shift in the interference spectrum through synchronous changes in refractive index and cavity length when the ambient temperature changes.

[0026] Specifically, single-mode fiber 1 serves as the transmission carrier and incident port for optical signals, guiding the incident light from the external detection system into the sensing structure while simultaneously exporting the reflected interference signal to the detection end. The core-layer confinement light of single-mode fiber 1 is transmitted in a single mode, ensuring the mode purity of the incident light and providing a high-quality coherent light source for the Fabry-Perot interferometer. The end face of the single-mode fiber 1 also serves as the carrier of the first reflecting surface 11 of the interference cavity, providing a flat optical reflection interface. The hollow fiber 2 serves as the physical carrier structure of the temperature-sensitive filling medium 3. Its internal cavity 2a provides filling space and radial constraint for PDMS. The quartz tube wall of the hollow fiber 2 provides mechanical support and structural protection for the sensing head. At the same time, after being fused with the single-mode fiber 1, it forms a cavity structure with one end closed, which confines the temperature-sensitive medium within a defined spatial range, ensuring the stability and consistency of the interference cavity length. The temperature-sensitive filling medium 3 uses PDMS as a temperature-sensitive element and is the core functional material for realizing temperature sensing. Its refractive index and volume change with the ambient temperature, converting the temperature quantity into the optical path difference change of the interference cavity. It is the medium for converting the temperature signal into an optical signal. At the same time, the free end face of the PDMS constitutes the second reflecting surface 31 of the interference cavity, which, together with the first reflecting surface 11, forms a Fabry-Perot resonant structure.

[0027] When light is incident on the sensing structure from single-mode fiber 1, at the end face of single-mode fiber 1, i.e., the first reflecting surface 11, due to the refractive index difference between the quartz material of single-mode fiber 1 and PDMS, part of the light is directly reflected to form the first reflected beam; the other part of the light is transmitted into the cavity filled with PDMS, and propagates along the cavity axis to the free end face of PDMS, i.e., the second reflecting surface 31. Due to the refractive index difference between PDMS and air, this part of the light is reflected at the second reflecting surface 31 and returns along the original path, passing through the PDMS cavity again and being transmitted back to single-mode fiber 1 through the first reflecting surface 11, forming the second reflected beam. There is a phase difference between the two reflected beams, determined by the round-trip optical path of the cavity. Multi-beam interference occurs in single-mode fiber 1, forming an interference spectrum with periodic troughs and peaks. When the ambient temperature changes, the refractive index of PDMS changes due to the thermo-optic effect, and at the same time, the axial length of the cavity changes due to the thermal expansion effect. The combined effect of these two factors causes a change in the optical path difference of the interference cavity, which in turn causes a wavelength shift in the positions of the troughs and peaks in the interference spectrum. By detecting the shift in the characteristic wavelength of the interference spectrum, the change in ambient temperature can be deduced, thus achieving temperature sensing.

[0028] By using this setup, the sensor head is constructed with a pure optical fiber structure, avoiding the influence of electromagnetic interference on the sensing signal. The all-fiber sensor can maintain stable and reliable measurement performance. At the same time, the optical fiber material itself has insulating properties, so the sensor head will not generate electric sparks or leakage risks. It can be directly applied to dangerous places such as flammable and explosive petrochemical and coal mines without the need for additional explosion-proof isolation measures. In addition, both optical fiber and PDMS have good chemical stability and corrosion resistance, and can work for a long time in harsh environments such as humidity, salinity, and corrosive gases. Setting it inside the hollow optical fiber 2 further reduces the overall size of the sensor and simplifies the overall structure. PDMS is filled inside hollow fiber 2 to form a Fabry-Perot interferometer cavity, which fully utilizes the dual thermo-sensitive properties of PDMS material. On the one hand, PDMS has a significant thermo-optic effect, and its refractive index changes with temperature much more than that of quartz material. Temperature changes directly cause changes in the refractive index of the interferometer cavity medium. On the other hand, PDMS has a large coefficient of thermal expansion. Temperature changes will cause changes in the axial length of the cavity. The thermo-optic effect and the thermal expansion effect are superimposed in the same direction and work together to affect the optical path difference of the interferometer cavity, causing a significant wavelength shift in the interference spectrum. This improves the temperature sensitivity, enabling the resolution of smaller temperature changes and meeting the requirements of high-precision temperature detection. At the same time, the cavity structure of hollow fiber 2 provides a complete physical support for PDMS, allowing the thermosensitive material to fully interact with the optical field. The entire optical path is within the thermosensitive medium, resulting in a higher sensitivity enhancement efficiency than surface-coated or partially contact thermosensitive structures. The internal cavity 2a of the hollow fiber 2 is used as the filling container for PDMS. The shape and size of the PDMS are precisely determined by the internal cavity size of the hollow fiber 2, and are not affected by factors such as surface tension and drop volume. The cavity size and shape of different sensors are highly consistent, thus ensuring batch stability of sensing performance. The cured PDMS columnar body is tightly attached to the inner wall of the hollow fiber 2. The friction of the inner wall and the interface adhesion force form a mechanical anchoring effect on the filler, which can effectively prevent the filler from axial displacement or falling off during temperature cycling, thus improving the long-term reliability of the sensor. In addition, the closed end of the cavity is formed by the end face of the single-mode fiber 1, which provides a definite axial positioning reference for the PDMS, ensuring that the position of the first reflecting surface 11 of the interference cavity is accurate and stable, and is not affected by the fluctuation of the filling process. This is conducive to ensuring the precise control of the cavity length and repeated preparation.

[0029] In a preferred embodiment, the hollow fiber 2 and the single-mode fiber 1 are coaxially fixedly connected by fusion splicing. The quartz tube wall of the hollow fiber 2 and the cladding material of the single-mode fiber 1 are fused together at the splice point to form a continuous quartz matrix structure.

[0030] Specifically, the fusion splice structure serves to achieve a robust mechanical connection and efficient optical coupling between the hollow-core fiber 2 and the single-mode fiber 1. The fusion splice fixes the two fibers coaxially, and at the splice point, the quartz tube wall and cladding material fuse together to form a continuous quartz matrix. This ensures the mechanical strength between the two fiber segments while eliminating the additional reflection loss caused by air gaps. This allows light to couple from the single-mode fiber 1 into the hollow-core fiber 2 region with low loss, while reflected light can also be efficiently coupled back to the single-mode fiber 1. The continuous quartz matrix structure avoids the introduction of intermediate media such as adhesives, improving the structure's temperature stability and long-term reliability. Fiber optic fusion splicing utilizes the high temperature generated by electric arc discharge to heat the end faces of two fiber segments to a molten state. Under the combined action of surface tension and axial pressure, the molten end faces of the two fiber segments fuse together, forming a continuous, integrated quartz structure upon cooling. During the splicing process, a precision alignment mechanism ensures that the central axes of the two fiber segments coincide, achieving coaxial connection. The quartz tube wall of the hollow fiber 2 and the cladding of the single-mode fiber 1 are both made of quartz, possessing the same thermophysical properties and refractive index. In the molten state, they can fully fuse to form a continuous matrix with uniform composition and no obvious interface. This continuous quartz structure optically eliminates the air interface at the splice point of the two fiber segments, reducing Fresnel reflection loss; mechanically, it forms a complete stress-bearing structure, ensuring the mechanical strength of the splice point.

[0031] With this setup, hollow fiber 2 and single-mode fiber 1 are connected by fusion splicing, forming a continuous quartz matrix structure at the splice point. There is no air gap or adhesive medium layer between the two fiber segments, eliminating Fresnel reflection loss and coupling loss at the interface. Low-loss optical coupling means that more light energy can enter the interference cavity to participate in interference. At the same time, the reflected interference signal can also return to single-mode fiber 1 more efficiently and be received by the detection system, thereby improving the signal-to-noise ratio and fringe contrast of the interference spectrum. The high-quality interference signal is beneficial for subsequent wavelength demodulation algorithms to more accurately identify the location of interference valleys, improve the resolution and accuracy of temperature measurement, and ensure that the signal quality remains stable during long-term use. Furthermore, at the fusion point, the quartz tube wall and the cladding material are fused together to form a continuous, integrated quartz matrix structure. This metallurgical bonding provides mechanical strength far exceeding that of bonding or mechanical clamping. The continuous quartz matrix exhibits uniform mechanical properties, with a smooth and continuous stress distribution and no weak interfaces with stress concentration. It can withstand mechanical loads such as bending, tension, and vibration that may be encountered during the use of optical fibers, improving the mechanical reliability and impact resistance of the sensor head. At the same time, the entire fusion point is made of homogeneous quartz material with a completely consistent coefficient of thermal expansion. During temperature changes, there will be no additional stress or interface separation due to differences in thermal expansion and contraction between different materials. It has excellent temperature cycling stability. In environments with high humidity and chemical corrosion, the quartz material itself remains stable and does not contain easily aged or degraded organic materials such as adhesives, ensuring the structural integrity and performance reliability of the sensor for long-term use in harsh environments.

[0032] In a preferred embodiment, the internal cavity 2a of the hollow optical fiber 2 is a cylindrical structure, the axis of the internal cavity 2a coincides with the optical fiber axis of the single-mode optical fiber 1, and the diameter of the internal cavity 2a is greater than or equal to the diameter of the optical fiber.

[0033] Specifically, the cylindrical internal cavity 2a provides a regular filling space for the PDMS. The cylindrical shape ensures that the filling medium has uniform constraint and symmetrical thermal expansion space in all radial directions. The cavity axis coincides with the axis of the single-mode fiber 1, ensuring that the incident light is transmitted along the center of the cavity, so that the light field and the temperature-sensitive medium can fully interact. The cavity diameter is greater than or equal to the diameter of the single-mode fiber 1, ensuring that the diverging beam emitted from the single-mode fiber 1 can be completely contained by the cavity, avoiding light energy leakage to the outside of the cavity and causing loss. The cylindrical cavity possesses rotational symmetry, and PDMS fills it to form a cylindrical dielectric column. When light enters the cavity from the end face of a single-mode fiber, it propagates axially within the cylindrical medium. Since the cavity diameter is larger than the fiber mode field diameter, the beam has sufficient radial expansion space and is not restricted by the sidewalls, thus avoiding additional mode coupling or loss. The cylindrical symmetry structure ensures that the radial and circumferential expansion of the PDMS is uniformly constrained by the inner wall of the cavity when the temperature changes, while allowing free axial expansion and contraction. This constraint mode is beneficial for the cavity length to exhibit a good linear relationship with temperature changes. The coincidence of the axes ensures that the center of the incident light is aligned with the center of the cavity, maintaining a symmetrical optical path when the light propagates back and forth between the two reflecting surfaces. The two reflected beams can overlap to the maximum extent, achieving the best interference effect.

[0034] With this design, the internal cavity 2a adopts a cylindrical structure with a smooth and continuous inner wall and a uniform cross-section, providing an ideal channel for the capillary filling of liquid PDMS prepolymer. The uniform circumferential curvature of the cylindrical cavity ensures that the capillary force is distributed consistently in all directions, allowing the liquid prepolymer to advance uniformly along the axial direction during the filling process. The filling front maintains a flat liquid surface, preventing local rapid advances or depressions, which is conducive to forming a complete, bubble-free filler. The uniform filling process ensures that the density and cross-linking degree of the cured PDMS column are uniformly distributed radially, resulting in consistent overall optical performance. This avoids refractive index gradients or internal defects caused by uneven filling, thereby improving the quality of the interference spectrum and the uniformity of the temperature response. At the same time, the cylindrical structure facilitates the observation and judgment of the filling degree. The liquid column advancement position can be clearly determined through a microscope, which is beneficial for process control and yield improvement. The structural design, in which the cavity axis coincides with the axis of single-mode fiber 1, ensures that the light beam emitted from single-mode fiber 1 is incident along the central axis of the cavity, concentrating the light energy in the central region of the PDMS filler. The entire optical path is completely within the temperature-sensitive medium, maximizing the interaction between the light field and the temperature-sensitive material. Temperature-induced refractive index changes are fully reflected in the phase changes of the light, maximizing the sensitizing effect of PDMS. The symmetrical structure with coincident axes also ensures that the mode fields of the two interfering beams are perfectly matched, resulting in the highest interference efficiency and the strongest interference signal, which is beneficial for improving the sensitivity and signal-to-noise ratio of temperature detection.

[0035] In a preferred embodiment, the outer diameter of the single-mode fiber 1 is equal to the outer diameter of the hollow fiber 2.

[0036] Specifically, the structural design of having equal outer diameters for single-mode fiber 1 and hollow fiber 2 ensures a perfectly flush transition of the outer cylindrical surfaces after splicing, forming a unified fiber structure with consistent outer diameters. This equal outer diameter guarantees matching end-face areas during splicing, resulting in symmetrical and uniform heat distribution in the molten region, which is beneficial for forming a high-quality splice. The flush outer surface also facilitates subsequent coating, encapsulation, or clamping operations, avoiding stress concentration caused by steps. When two fibers with equal outer diameters are spliced, the volume and heat capacity of the quartz material on both sides are comparable. The degree of melting on both sides is symmetrical during arc heating, and the material flows uniformly during splicing, reducing the likelihood of defects such as eccentricity, collapse, or burrs. The consistent outer diameter ensures a smooth transition of the outer surface at the splice point, without steps or protrusions, and the stress is evenly distributed radially in the splice area, eliminating stress concentration points. In optics, equal outer diameters mean that the quartz tube wall thickness of hollow fiber 2 matches the cladding thickness of single-mode fiber 1. After splicing, the transmission of cladding light is continuous and will not cause excitation or loss of cladding modes due to abrupt changes in outer diameter.

[0037] This design, with single-mode fiber 1 and hollow fiber 2 having equal outer diameters, ensures high process compatibility during splicing. Conventional splicing procedures and standard fixtures can be used directly, eliminating the need for specialized equipment or dedicated processes. This reduces manufacturing difficulty and cost. The equal outer diameter of the two fiber segments results in symmetrical heating during splicing, a uniform melting area, and balanced material flow during advancement. This facilitates the production of high-quality splices free from collapse, bubbles, and eccentricity. Stable splicing quality directly guarantees the consistency of sensor optical performance, minimizing performance differences between different batches, which is beneficial for large-scale production and product standardization. The equal outer diameters of the two fiber segments result in a smooth transition on the outer cylindrical surface after splicing, forming a slender fiber structure with a consistent diameter throughout the entire sensor head, without radial steps or protrusions. During coating protection, the coating thickness is uniform, preventing stress concentration or coating defects at steps. During sleeve encapsulation, the sensor can smoothly insert into a standard inner diameter protective sleeve, ensuring a tight fit and preventing movement. For installation and fixation, standard-sized fiber optic clamps (200mm) or adhesive mounts can be used, offering convenient clamping and uniform force distribution. This improves the sensor head's bending and fatigue resistance, reducing the likelihood of breakage near the fusion splice under repeated bending or vibration, thus extending the sensor's lifespan.

[0038] In a preferred embodiment, the outer diameter of the hollow fiber 2 is in the range of 100μm-150μm, and the diameter of the internal cavity 2a is in the range of 60μm-90μm.

[0039] Specifically, the outer diameter of the hollow fiber 2 determines the wall thickness and overall mechanical strength. The larger the outer diameter, the thicker the wall and the higher the mechanical strength, but the flexibility of the fiber decreases accordingly and the bending loss increases. The outer diameter range of 100μm to 150μm balances strength and flexibility, is close to the size system of conventional communication optical fibers, and is easy to use. The cavity diameter determines the cross-sectional area of ​​the filling medium and the space for the light field. The diameter range of 60μm to 90μm can fully cover the divergence range of the beam emitted from the single-mode fiber 1, while ensuring that the wall has sufficient thickness to maintain structural integrity. The moderate wall thickness is conducive to forming a stable fusion point during fusion splicing and can also provide sufficient mechanical protection for the internal filling material.

[0040] With this configuration, the outer diameter of the hollow fiber 2 is set within the range of 100μm to 150μm, and the cavity diameter is set within the range of 60μm to 90μm. This size range has been reasonably optimized to achieve a good balance between optical and mechanical performance. From an optical perspective, the cavity diameter is not less than 60μm, which is much larger than the mode field diameter of the single-mode fiber 1. This ensures that the diverging beam emitted from the single-mode fiber 1 can be completely contained within the cavity along the entire cavity length, without light energy leakage or sidewall scattering, thus guaranteeing the intensity and purity of the interference light. From a mechanical perspective, the outer diameter does not exceed 150μm and the tube wall has an appropriate thickness, which allows the hollow fiber 2 to maintain sufficient structural rigidity and bending resistance, making it less prone to breakage or deformation. At the same time, the outer diameter is not too large, maintaining the basic flexibility of the fiber, which is convenient for bending cabling and installation in confined spaces. A moderate tube wall thickness is also beneficial for the conduction and distribution of heat during the fusion splicing process, resulting in stable fusion quality. For example, if the cavity is too small, the optical field will be limited, which will introduce additional loss and mode noise. If the outer diameter is too large, the optical fiber will be too rigid, inconvenient to use, and the fusion splicing difficulty will increase.

[0041] In a preferred embodiment, the length of the internal cavity 2a is 50 μm-80 μm.

[0042] Specifically, the free spectral range of a Fabry-Perot interferometer is inversely proportional to the cavity length; the longer the cavity length, the smaller the free spectral range and the denser the interference fringes. A cavity length of 50 μm to 80 μm corresponds to a free spectral range in the tens of nanometers to hundreds of nanometers range, and several clear interference valleys can be observed in the near-infrared band, facilitating wavelength tracking and multi-point verification. From the perspective of temperature sensitivity, the cavity length is positively correlated with sensitivity within a certain range, and appropriately increasing the cavity length can improve temperature sensitivity; however, an excessively long cavity length will lead to an excessively small free spectral range, which is prone to valley overlap or aliasing, increasing the difficulty of demodulation. This length range comprehensively considers sensitivity and demodulation convenience, and is a relatively optimized design range.

[0043] By setting the length of the internal cavity 2a to within the range of 50μm to 80μm, the free spectral range of the Fabry-Perot interferometer is placed within a reasonable range, which can well adapt to the operating band of conventional spectroscopic detection equipment. In the near-infrared communication band, commonly used spectrometers have a wide measurement range, but their resolution and sampling point density are limited. If the free spectral range is too small and the interference fringes are too dense, the spectrometer may not be able to clearly distinguish adjacent interference valleys, leading to demodulation difficulties or even wavelength aliasing errors. The cavity length range corresponds to a moderate free spectral range, which can present several clearly distinguishable interference valleys within the typical operating band. This facilitates accurate identification of the location of individual valleys and allows for simultaneous demodulation of multiple valleys to improve measurement reliability. At the same time, the free spectral range falls within the effective bandwidth of conventional light sources, which can fully utilize the spectral energy of the light source to obtain interference signals with good signal-to-noise ratio. This allows the sensor to be directly demodulated using commercially available general-purpose spectroscopic detection equipment without the need for customized instruments, reducing operating costs and system complexity.

[0044] Another aspect of the present invention provides a method for fabricating a high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling, the method comprising: Align one end of the hollow fiber 2 with the end face of the single-mode fiber 1, and perform coaxial fusion splicing using a fiber optic fusion splicer to form a fusion fiber optic assembly. The hollow fiber 2 in the fusion spliced ​​fiber assembly is cut to a predetermined axial length to form an initial cavity structure inside the hollow fiber 2 that is closed at one end and open at the other. The main component of PDMS and the curing agent are mixed in a predetermined ratio, and then degassed to obtain liquid PDMS prepolymer. Liquid PDMS prepolymer is attached to the end face of auxiliary optical fiber 100, and capillary force is used to introduce the liquid PDMS prepolymer into the initial cavity of hollow optical fiber 2 until the initial cavity is completely filled. The filled fusion spliced ​​fiber assembly is placed in a heating environment to cause the liquid PDMS prepolymer to undergo a cross-linking and curing reaction, forming a solid temperature-sensitive filling medium 3; The cured sensor end face is subjected to arc discharge treatment to remove surface impurities and smooth the port edge, thus obtaining a temperature sensor.

[0045] Specifically, in the fiber fusion splicing step, single-mode fiber 1 and hollow fiber 2 are coaxially fixedly connected to form a basic structure with one end closed, providing a sealed cavity substrate for subsequent filling, while ensuring optical coupling between the two fiber segments; in the cavity cutting step, hollow fiber 2 is precisely cut to a predetermined length to form an initial cavity with a defined axial dimension, providing a precise length reference for the Fabry-Perot interferometer cavity; in the prepolymer preparation step, the PDMS main agent and curing agent are mixed and degassed to prepare a uniform, bubble-free liquid prepolymer for filling. The process provides qualified raw materials; in the capillary filling step, the liquid prepolymer is introduced into the cavity of the hollow fiber 2 by the capillary force to achieve precise filling of the temperature-sensitive material, which is a key process for forming the interference cavity medium; in the heating and curing step, the liquid prepolymer undergoes a cross-linking reaction by heating, transforming into a solid elastomer, fixing the shape and position of the filler, and forming a stable interference cavity structure; in the end face post-processing step, the sensor end face is trimmed by arc discharge to remove impurities and smooth the port edge, improving the optical quality and structural integrity of the end face.

[0046] This method, employing capillary force for PDMS filling, offers significant advantages in terms of ease of operation and stable filling quality. Capillary force, a natural effect of liquid surface tension within a narrow tube, drives the liquid to flow spontaneously into the cavity. The entire filling process is smooth and uniform, requiring no external pressure or complex micro-injection equipment. Process control is simple, and the liquid prepolymer propagates steadily under capillary force, resulting in a flat filling front that minimizes the risk of air bubbles or voids, leading to a uniform and complete filled column. The filling progress can be visually observed under a microscope, facilitating precise control of the filling degree and ensuring consistent filling quality for each sensor. The capillary filling method requires relatively low operational skills, requiring only simple training for operators, thus improving production efficiency and product yield. Furthermore, this method exhibits good adaptability to prepolymer viscosity, allowing for adjustments to the number of filling cycles to accommodate materials of varying viscosities. The process is robust, and the simple equipment, convenient operation, and high yield of the capillary filling method make it particularly suitable for filling micro-sized cavities such as hollow optical fibers. Furthermore, all the processes involved in the preparation method are completed using conventional equipment in the field of optical fiber communication. Fiber fusion splicers, fiber cleavers, ovens, microscopes, etc. are standard equipment in optical fiber processing laboratories and production lines. There is no need to purchase expensive two-photon lithography systems, femtosecond laser processing systems, or other special micro-nano manufacturing equipment. The equipment investment cost is low. Ordinary optical fiber device manufacturers or research institutions have the corresponding production conditions. The technical threshold is low and it is easy to promote and apply. The consumable and maintenance costs of conventional equipment are also relatively low, which further reduces the overall manufacturing cost of the sensor.

[0047] In a preferred embodiment, the PDMS main agent and curing agent are mixed in a predetermined ratio, and then subjected to degassing treatment to obtain a liquid PDMS prepolymer, comprising: Weigh the main agent and curing agent at a mass ratio of 10:1 and place them in a clean container. Centrifuge and stir to ensure that the main agent and curing agent are fully mixed. Vacuum treatment removes air bubbles from the mixture, resulting in a bubble-free liquid PDMS prepolymer.

[0048] Specifically, PDMS curing is a hydrosilylation polymerization reaction. The vinyl groups in the main agent and the hydrogen-containing groups in the curing agent undergo an addition reaction under the action of a platinum catalyst, forming a three-dimensional cross-linked network. The ratio of the main agent to the curing agent directly affects the cross-linking density and the final material properties. Deviations in the ratio can lead to insufficient or excessive cross-linking, affecting the material's optical, thermal, and mechanical properties. Centrifugal stirring utilizes a centrifugal force field to induce convection and shear motion in the liquid, accelerating molecular diffusion and mixing of the two components, achieving uniform mixing at both the macroscopic and microscopic scales. Vacuum degassing utilizes a low-pressure environment to cause the bubbles inside the liquid to expand in volume and increase buoyancy, thus causing them to float to the surface and escape, achieving the purpose of removing bubbles. Weigh the PDMS main agent and curing agent separately at a mass ratio of 10:1 using a precision electronic balance, and place them in a clean plastic or glass container. Put the container into a centrifuge and set an appropriate speed and time, such as about 2,000 revolutions per minute, and stir for several minutes to ensure that the two components are thoroughly mixed. After stirring, remove the mixture and observe the initial mixing effect. Place the container containing the mixture into a vacuum desiccator, connect a vacuum pump, and maintain a vacuum level above 0.09 MPa for at least ten minutes. Observe the surface of the mixture during this period. When no more bubbles escape and the liquid becomes clear and transparent, stop the vacuuming, slowly release the gas, and remove the prepolymer for later use. The prepared prepolymer should be used as soon as possible to avoid premature curing due to prolonged storage.

[0049] By strictly adhering to a 10:1 mass ratio of the main agent and curing agent, this method ensures that the crosslinking reaction of PDMS proceeds fully according to stoichiometry. After curing, a three-dimensional network structure with a defined crosslinking density is formed. Various physical parameters of the material, including refractive index, thermo-optic coefficient, coefficient of thermal expansion, elastic modulus, and hardness, are closely related to the crosslinking density. Only precise proportioning can guarantee the stability and consistency of these parameters. Precise proportioning also ensures complete curing of PDMS, preventing localized softening or brittleness. This guarantees the material's mechanical properties and long-term stability, extending the sensor's lifespan. Vacuuming thoroughly removes air bubbles from the mixture, ensuring that the PDMS filled into the cavity is a uniform, transparent, and defect-free medium. This bubble-free, uniform medium ensures a flat wavefront during light propagation within the cavity, resulting in good wavefront matching between the two reflected beams. This leads to the formation of high-contrast, smooth, high-quality interference fringes, improving the accuracy and sensitivity of wavelength detection. Furthermore, the absence of air bubbles avoids the additional stress or interfacial debonding introduced by bubble expansion during temperature changes, improving the sensor's temperature stability and long-term reliability.

[0050] In a preferred embodiment, the filled fusion-bonded fiber assembly is placed in a heating environment to cause the liquid PDMS prepolymer to undergo a cross-linking and curing reaction, forming a solid temperature-sensitive filling medium 3, comprising: Place the filled fusion spliced ​​fiber assembly vertically with the open end of the hollow fiber 2 facing upwards. The curing temperature is set to 50℃-70℃, and the curing time is 4-6 hours. At the curing temperature, the PDMS prepolymer gradually changes from a liquid state to a solid cross-linked structure.

[0051] Specifically, vertical placement utilizes gravity to maintain the filling state, preventing the liquid prepolymer from flowing out of the opening during curing, ensuring the integrity of the filler's length and shape. The curing temperature and time range provide suitable heat and reaction time for the crosslinking reaction, ensuring the prepolymer fully cures to form a stable solid elastomer, while avoiding excessive temperature that could lead to material degradation or excessive internal stress. The addition curing reaction rate of PDMS increases with temperature; an appropriate heating temperature ensures the curing reaction is completed within a reasonable time. The temperature range of 50℃ to 70℃ is considered a medium-temperature curing range, with a moderate reaction rate, allowing curing to be completed within a few hours without causing excessively vigorous reaction, rapid volume shrinkage, or internal stress or defects due to excessively high temperatures. During curing, the prepolymer gradually transforms from a liquid to a solid state, and its viscosity continuously increases. When placed vertically with the open end facing upwards, the direction of gravity is consistent with the direction of filling, which helps the liquid prepolymer stay at the bottom of the cavity and prevents it from flowing out due to its own weight; at the same time, the bubbles are more likely to rise and escape, and the curing time of several hours ensures that the cross-linking reaction is complete, so that the material reaches a stable performance state.

[0052] In one specific embodiment, after filling is complete, the fusion splice fiber assembly is removed from the fusion splicer and vertically suspended or placed upright using a dedicated bracket, ensuring that the open end of the hollow fiber 2 faces upwards and the end of the single-mode fiber 1 faces downwards. This allows the liquid prepolymer within the cavity to remain filled under gravity. The bracket is then placed entirely into a constant-temperature forced-air oven, the oven door is closed, and the curing temperature is set, for example, to 60°C. Heating is started, and timing begins after the oven temperature stabilizes. Curing takes 4 to 6 hours, for example, 5 hours. The oven is kept closed during curing to avoid temperature fluctuations. After curing is complete, heating is turned off, and the sensor is allowed to cool naturally to room temperature with the oven before being removed to prevent thermal stress caused by sudden temperature changes. After removal, the filling status is checked to confirm that the PDMS has been completely cured and its morphology is intact.

[0053] This method employs a medium-low temperature (50℃ to 70℃) combined with a relatively long curing time of 4 to 6 hours, a mild curing process that effectively reduces the internal stress generated during PDMS curing. The curing reaction is accompanied by volume shrinkage. If the curing temperature is too high or the reaction rate is too fast, the surface layer cures first, followed by the interior. This limited shrinkage leads to significant internal stress within the material, potentially even causing microcracks. Low-temperature, long-duration curing allows the cross-linking reaction to proceed slowly and uniformly, with volume shrinkage occurring gradually. This provides ample time for stress relaxation and release, resulting in very low residual stress within the cured material. Low internal stress means a more uniform refractive index distribution in the PDMS filler, preventing the introduction of additional optical anisotropy due to stress birefringence, and improving interference spectral quality. More importantly, the low internal stress results in less interfacial bonding stress between the filler and the cavity wall, reducing the likelihood of interfacial debonding or filler displacement during temperature cycling. This leads to better long-term sensor stability, less zero-point drift, and higher measurement reliability. During the curing process, the sensor is placed vertically with the open end of the hollow fiber 2 facing upwards. Gravity keeps the liquid PDMS prepolymer at the bottom of the cavity, preventing it from flowing out of the open end or sagging and deforming. This ensures the integrity of the axial length of the filler after curing and guarantees the accuracy of the cavity length. Vertical placement also helps residual micro-bubbles to float upwards and escape from the free end face during curing, further reducing bubble defects in the filler and improving optical quality. The free end face naturally forms a flat liquid surface under the action of surface tension, which becomes a flat optical reflection surface after curing. Good end face quality can be obtained without additional processing, while ensuring the morphological integrity of the filler and the accuracy of the cavity length.

[0054] In a preferred embodiment, the cured sensor end face is subjected to arc discharge treatment to remove surface impurities and smooth the port edges, including: Set the discharge parameters for the fiber optic fusion splicer, where the discharge intensity is set to 50 bits and the discharge time is set to 100 ms. The high temperature generated by the electric arc discharge causes the quartz material at the edge of the hollow fiber 2 port to melt slightly and form a smooth rounded corner, and removes the organic impurities remaining on the free end face of the temperature-sensitive filling medium 3. After discharge, the free end face remains optically flat and is flush with the outer end face of the hollow fiber 2.

[0055] Specifically, the post-processing of the arc discharge utilizes high-temperature vaporization to remove residual organic impurities and excess PDMS from the free end face, resulting in a clean and tidy end face. Simultaneously, it slightly melts the quartz at the edge of the hollow fiber 2 port, forming a smooth, rounded transition, eliminating burrs and microcracks caused by cutting. Furthermore, it smooths the flatness of the free end face, improving the optical quality of the second reflective surface 31 and ensuring uniform and stable reflectivity. The discharge function of the fiber optic fusion splicer generates an arc between high-voltage electrodes, instantly releasing a large amount of heat energy, causing a rapid increase in temperature in the arc region. When the sensor end face is placed in the arc region, the high temperature first causes the organic impurities and residual prepolymers attached to the free end face to burn or vaporize and be removed. Simultaneously, the quartz edge at the hollow fiber 2 port slightly melts at high temperature, and under the action of surface tension, sharp edges and burrs become rounded, forming a smooth port edge. Microcracks caused by cutting are also melted and repaired. Appropriate discharge parameter control ensures that only the surface of the quartz undergoes micro-melting, preventing excessive melting that could deform or block the cavity, and avoiding thermal damage to the internal PDMS.

[0056] This method effectively improves the optical quality of the free end face by removing organic impurities, excess PDMS burrs, and surface contaminants that may remain during the filling and curing process. This results in a clean and smooth reflective end face. The clean and flat end face forms uniform and stable Fresnel reflection with minimal wavefront distortion, exhibiting good coherence with the reflected light from the first reflecting surface 11. This leads to high-contrast, smooth interference fringes. The high-contrast interference spectrum makes the characteristics of interference valleys more pronounced, making it easier for the wavelength demodulation algorithm to accurately locate the valley center, thus improving the resolution and accuracy of temperature measurement. The high temperature generated by the arc discharge causes slight melting of the quartz surface layer at the port edge. Under surface tension, sharp edges naturally round off, burrs are fused and eliminated, and microcracks are repaired and healed, forming a smooth and rounded port edge. This smooth edge eliminates stress concentration points, improving the mechanical strength and fatigue resistance of the sensor head, enabling it to withstand harsher mechanical environments and extending the sensor's lifespan. Simultaneously, edge smoothing reduces light scattering at the end face, helping to reduce optical loss.

[0057] In one specific implementation, the temperature sensor is composed of a single-mode fiber 1 and a hollow fiber 2. One end of the hollow fiber 2 is fused to the single-mode fiber 1. The hollow fiber 2 is filled with PDMS. The end face of the single-mode fiber 1 and the free end face of the PDMS form two reflecting surfaces of the FP interference cavity.

[0058] like Figure 2 As shown, when light enters the FP cavity from single-mode fiber 1, part of the light is reflected at the end face of single-mode fiber 1, i.e., the first reflecting surface 11, to form beam I1. The other part of the light is transmitted into the PDMS filling cavity, reflected at the free end face of the PDMS, i.e., the second reflecting surface 31, and returns to the fiber core, forming beam I2. The two reflected beams have a phase difference due to the optical path difference, and thus interfere. The intensity of the reflected light can be expressed as: in The relative phase difference This is the initial phase difference. The relative phase difference... =2πδ / λ, where δ=2n PDMS ·L PDMS For optical path difference, n PDMS L is the refractive index of PDMS. PDMS This refers to the length of the PDMS cavity.

[0059] The FSR between adjacent troughs or peaks in the interference spectrum is: As shown in the above equation, the cavity length is inversely proportional to the free spectral range. When the incident light wavelength is constant, L PDMS The longer the length, the smaller the FSR and the denser the interference fringes.

[0060] The relationship between wavelength shift and temperature change is as follows: Where α and β are the thermal expansion coefficient and thermo-optic coefficient of PDMS, respectively. As can be seen from the above formula, longer wavelengths and larger thermo-optic coefficients and thermal expansion coefficients can achieve higher temperature sensitivity.

[0061] In the specific fabrication process of a temperature sensor, the fabrication procedure is as follows: Figure 3 As shown, firstly, hollow fiber 2 and single-mode fiber 1 are fused together using a fiber optic fusion splicer. The fusion parameters are set to the standard single-mode fusion mode to ensure that there is no collapse at the fusion point.

[0062] Next, the hollow fiber 2 is cut to a predetermined length using a fiber optic cleaver, which is approximately 60 μm in this embodiment. Figure 3 The cutting length is 53μm, forming the initial FP cavity.

[0063] Next, the PDMS prepolymer, main agent A and curing agent B are mixed at a mass ratio of 10:1, centrifuged and stirred and vacuum degassed, and placed in a clean container. The end face of another optical fiber with the same structure as single-mode optical fiber 1, namely auxiliary optical fiber 100, is immersed in the PDMS prepolymer so that an appropriate amount of liquid PDMS is attached to the end face.

[0064] Then, the single-mode fiber 1 with PDMS attached is fixed to the fiber clamp 200 of the fiber fusion splicer, and moved towards the initial FP cavity by the fusion splicer motor. Under capillary action, the PDMS on the end face of the single-mode fiber 1 is introduced into the hollow fiber 2. The above filling operation is repeated several times until the hollow fiber 2 is completely filled with PDMS.

[0065] Finally, the filled sensor is placed vertically in an oven and heated at 60°C for 5 hours to cure the PDMS completely. After curing, the sensor end face is subjected to arc discharge treatment with a discharge intensity of approximately 50 bits and a discharge time of approximately 100 ms to remove surface impurities.

[0066] Figure 4 The optical microscope image of the fabricated sensor is shown, which shows that there is no collapse at the weld point, the PDMS is well filled, and the FP cavity length is 60.92 μm.

[0067] In a specific temperature response test, the sensor is placed on a high-precision temperature platform. The light emitted by the supercontinuum broadband light source is transmitted to the sensor through an optical fiber circulator. The reflected interference signal generated by the sensor is received and recorded by a spectrometer through the optical fiber circulator.

[0068] The temperature was started at 20°C and increased in increments of 4°C to 40°C, with each temperature point held for 5 minutes to allow for thermal equilibrium. Subsequently, the temperature was decreased from 40°C to 20°C in equal increments to perform a heating-cooling cycle test.

[0069] Figure 5 The reflection interference spectra of the sensor at different temperatures are shown. At 20°C, interference valleys Dip A and Dip B are observed at 1485.640 nm and 1546.342 nm, respectively. As the temperature increases, the interference fringes shift towards longer wavelengths.

[0070] Figure 6 and Figure 7 The linear fitting results of the interference valley wavelength shift versus temperature are presented. For Dip A, the temperature sensitivity during the heating process is 2.062 nm / ℃ (R0). 2 =0.9995), and during the cooling process, it is 1.977 nm / ℃ (R 2 =0.998), with a sensitivity deviation of 4.1%. For Dip B, the temperature sensitivity during heating is 2.155 nm / ℃ (R0). 2 =0.9997), and during the cooling process, it is 2.037 nm / ℃ (R 2=0.9985), with a sensitivity deviation of 5%. The results show that the sensor has good linearity and repeatability, and the maximum temperature sensitivity can reach 2.155 nm / ℃.

[0071] In a specific stability test of a temperature sensor, the reflectance spectrum was recorded at 5-minute intervals under a constant temperature of 30°C to test the sensor's wavelength stability. For example... Figure 8 As shown, the wavelength fluctuations of Dip A and Dip B are less than 1.09 nm and 1.192 nm, respectively. The fluctuations mainly stem from minute changes in ambient temperature and the control precision of the heating platform.

[0072] It should be understood that the specific embodiments described above are merely illustrative or explanatory of the principles of the invention and do not constitute a limitation thereof. Therefore, any modifications, equivalent substitutions, improvements, etc., made without departing from the spirit and scope of the invention should be included within the protection scope of the invention. Furthermore, the appended claims are intended to cover all variations and modifications falling within the scope and boundaries of the appended claims, or equivalent forms of such scope and boundaries.

Claims

1. A high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling, characterized in that, The temperature sensor includes a single-mode optical fiber (1), a hollow optical fiber (2), and a temperature-sensitive filling medium (3). The single-mode optical fiber (1) is coaxially connected and fixed with the hollow optical fiber (2), and the temperature-sensitive filling medium (3) is filled in the internal cavity (2a) of the hollow optical fiber (2). The temperature-sensitive filling medium (3) is in contact with the end face of the single-mode optical fiber (1). The end face of the single-mode fiber (1) forms the first reflecting surface (11) of the Fabry-Perot interferometer cavity, and the end face of the temperature-sensitive filling medium (3) on the side away from the single-mode fiber (1) forms the second reflecting surface (31) of the Fabry-Perot interferometer cavity. The section of the temperature-sensitive filling medium (3) between the first reflecting surface (11) and the second reflecting surface (31) forms the cavity length of the Fabry-Perot interferometer cavity. The temperature-sensitive filling medium (3) is polydimethylsiloxane, which is used to achieve temperature sensing by causing a wavelength shift in the interference spectrum through synchronous changes in refractive index and cavity length when the ambient temperature changes.

2. The temperature sensor according to claim 1, characterized in that, The hollow fiber (2) and the single-mode fiber (1) are coaxially fixedly connected by fusion splicing. The quartz tube wall of the hollow fiber (2) and the cladding material of the single-mode fiber (1) are fused together at the splice point to form a continuous quartz matrix structure.

3. The temperature sensor according to claim 1, characterized in that, The internal cavity (2a) of the hollow optical fiber (2) is a cylindrical structure. The axis of the internal cavity (2a) coincides with the optical fiber axis of the single-mode optical fiber (1). The diameter of the internal cavity (2a) is greater than or equal to the diameter of the optical fiber.

4. The temperature sensor according to claim 3, characterized in that, The outer diameter of the single-mode optical fiber (1) is equal to the outer diameter of the hollow optical fiber (2).

5. The temperature sensor according to claim 4, characterized in that, The outer diameter of the hollow optical fiber (2) ranges from 100μm to 150μm, and the diameter of the internal cavity (2a) ranges from 60μm to 90μm.

6. The temperature sensor according to claim 5, characterized in that, The length of the internal cavity (2a) is 50μm-80μm.

7. A method for fabricating a high-sensitivity fiber optic FP temperature sensor based on PDMS end-face filling, characterized in that, The preparation method includes: Align one end of the hollow fiber (2) with the end face of the single-mode fiber (1), and perform coaxial fusion splicing using a fiber optic fusion splicer to form a fusion fiber optic assembly. The hollow fiber (2) in the fusion spliced ​​fiber assembly is cut to a predetermined axial length to form an initial cavity structure with one end closed and the other end open inside the hollow fiber (2). The main agent of polydimethylsiloxane and the curing agent are mixed in a predetermined ratio and then subjected to degassing treatment to obtain liquid polydimethylsiloxane prepolymer; The liquid polydimethylsiloxane prepolymer is attached to the end face of the auxiliary optical fiber (100), and the liquid polydimethylsiloxane prepolymer is introduced into the initial cavity of the hollow optical fiber (2) by capillary force until the initial cavity is completely filled. The filled fusion fiber assembly is placed in a heating environment to cause the liquid polydimethylsiloxane prepolymer to undergo a cross-linking and curing reaction to form a solid temperature-sensitive filling medium (3). The cured sensor end face is subjected to arc discharge treatment to remove surface impurities and smooth the port edge, thus obtaining the temperature sensor.

8. The preparation method according to claim 7, characterized in that, The process of mixing the polydimethylsiloxane main agent and curing agent in a predetermined ratio, followed by degassing treatment to obtain a liquid polydimethylsiloxane prepolymer, includes: The main agent and the curing agent are weighed at a mass ratio of 10:1 and placed in a clean container. The mixture is then centrifuged and stirred to ensure that the main agent and the curing agent are fully and evenly mixed. Vacuum treatment removes air bubbles from the mixture, yielding a bubble-free liquid polydimethylsiloxane prepolymer.

9. The preparation method according to claim 7, characterized in that, The process of placing the filled fusion-bonded optical fiber assembly in a heating environment to cause the liquid polydimethylsiloxane prepolymer to undergo a crosslinking and curing reaction to form a solid temperature-sensitive filling medium (3) includes: Place the filled fusion fiber assembly vertically with the open end of the hollow fiber (2) facing upwards. The curing temperature is set to 50℃-70℃, and the curing time is 4-6 hours. At the curing temperature, the polydimethylsiloxane prepolymer gradually transforms from a liquid state to a solid cross-linked structure.

10. The preparation method according to claim 7, characterized in that, The cured sensor end face is subjected to arc discharge treatment to remove surface impurities and smooth the port edges, including: Set the discharge parameters for the fiber optic fusion splicer, where the discharge intensity is set to 50 bits and the discharge time is set to 100 ms. The high temperature generated by the electric arc discharge causes the quartz material at the port edge of the hollow optical fiber (2) to melt slightly and form a smooth rounded corner, and removes the organic impurities remaining on the free end face of the temperature-sensitive filling medium (3); After discharge, the free end face remains optically flat and is flush with the outer end face of the hollow fiber (2).

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