Whispering-gallery mode refractive index sensor and its fabrication method

Abstract

This invention relates to a whispering-gallery mode refractive index sensor and its fabrication method. The fabrication method of the whispering-gallery mode refractive index sensor includes the following steps: writing a microstructure on the end face of an optical fiber using photolithography; the microstructure includes a grating base, a suspended grating, a microsphere base, and a microfunnel, sequentially arranged along the length of the fiber core from the side closest to the end face to the side furthest from the end face; developing and curing the microstructure; assembling the microspheres into the microsphere base through the microfunnel until the microspheres are coupled to the suspended grating. The whispering-gallery mode refractive index sensor fabricated using the above method has the advantages of small space requirements, high measurement accuracy, and suitability for long-distance measurement, and can adapt to refractive index detection in confined spaces.

Description

Technical Field

[0001] This invention relates to the field of optical technology, and in particular to a whispering-gallery mode refractive index sensor and its fabrication method. Background Technology

[0002] As research into optical devices deepens and expands, their application areas are also broadening. In detecting the concentration of substances in liquid environments, existing technologies typically use refractive index measuring instruments. Some existing refractive index measuring instruments utilize whispering-gallery mode microcavity (SCVM) technology. SCVM technology leverages the high sensitivity of a microcavity's high-quality factor (high sensitivity factor) to external stimuli. Specifically, when light propagates within a microspherical resonant cavity and the angle of incidence is greater than the critical angle for total internal reflection, the light is confined within the cavity, forming a closed whispering-gallery mode. Devices employing whispering-gallery mode sensing usually require a large coupling device to connect the spherical resonant cavity to the detection area. However, this large coupling device is unsuitable for detection in confined spaces due to its size. Summary of the Invention

[0003] Therefore, it is necessary to provide a whispering-gallery mode refractive index sensor and its fabrication method to address the issue of the large size of existing whispering-gallery mode sensing devices.

[0004] A method for fabricating a whispering-gallery mode refractive index sensor includes the following steps:

[0005] A microstructure is created on the end face of an optical fiber using photolithography. The microstructure includes a grating base, a suspended grating, a microsphere base, and a microfunnel, which are sequentially arranged along the length of the fiber core from the side closest to the end face to the side furthest from the end face.

[0006] The microstructure is developed and cured;

[0007] The microspheres are assembled onto the microsphere base through the micro-funnel until the microspheres are coupled to the suspended grating.

[0008] The aforementioned method for fabricating a whispering-gallery mode refractive index sensor effectively reduces the space required by the sensor by using photolithography to create a microstructure on the end face of the optical fiber. This allows the sensor to be used in confined spaces and enables efficient coupling between the microstructure and the optical fiber, improving the utilization efficiency of the signal light in the fiber and the measurement accuracy of the whispering-gallery mode refractive index sensor. Assembling the microspheres onto the microsphere base using a microfunnel improves the assembly positioning accuracy and efficiency, thus enhancing the sensor's stability and reducing measurement errors. Through the coupling between the microspheres and the suspended grating, light emitted from the optical fiber passes through the grating and diffracts. The diffracted light then couples into the microsphere cavity, creating resonance and exciting the whispering-gallery mode within the cavity, thereby enabling refractive index detection.

[0009] In one embodiment, after the step "developing and curing the microstructure", the following step is also included:

[0010] The microstructure was placed in a vacuum environment;

[0011] A thin metal film is deposited on the side of the suspended grating away from the grating base.

[0012] In one embodiment, prior to the step "writing the microstructure on the end face of the multi-core optical fiber using photolithography", the following steps are included:

[0013] Remove the cladding from the optical fiber;

[0014] The end face of the optical fiber is cut.

[0015] A whispering-gallery mode refractive index sensor, comprising:

[0016] Optical fiber, wherein the optical fiber has a core inside for transmitting signal light;

[0017] A microstructure is disposed on one end face of the optical fiber. The microstructure includes a grating base, a suspended grating, and a microsphere base arranged sequentially along the length extension direction of the fiber core from the side near the end face to the side away from the end face.

[0018] The microsphere is disposed on the microsphere base, and the interior of the microsphere has a microsphere cavity. The microsphere is coupled to the suspended grating.

[0019] The aforementioned whispering-gallery mode refractive index sensor, by placing a microstructure on one end face of the optical fiber, effectively reduces the space required for the sensor, thus enabling it to adapt to detection in confined spaces. Furthermore, it achieves efficient coupling between the microstructure and the optical fiber, improving the utilization efficiency of signal light within the fiber and the measurement accuracy of the whispering-gallery mode refractive index sensor. By placing the microstructure along the length extension of the fiber core and the microsphere on a microsphere base, a certain coupling distance exists between the suspended grating and the microsphere, allowing them to couple. Light emitted from the fiber core passes through the suspended grating and diffracts. The diffracted light then couples into the microsphere cavity, creating resonance and exciting the whispering-gallery mode within the cavity, thereby enabling refractive index detection.

[0020] In one embodiment, a metal film is provided on the side of the suspended grating away from the grating base.

[0021] In one embodiment, the thickness of the metal thin film ranges from 100 nm to 300 nm; and / or,

[0022] The material of the metal film is gold.

[0023] In one embodiment, the suspended grating is a hybrid two-dimensional suspended grating; and / or,

[0024] The period of the suspended grating ranges from 1.0 μm to 1.2 μm; and / or,

[0025] The duty cycle of the suspended grating ranges from 40% to 60%; and / or,

[0026] The thickness of the suspended grating ranges from 0.9 μm to 1.1 μm; and / or,

[0027] The shortest distance between the microsphere and the suspended grating ranges from 0.8 μm to 1.2 μm.

[0028] In one embodiment, the microstructure further includes a microfunnel disposed on the side of the microsphere base away from the end face; the microsphere is assembled to the microsphere base through the microfunnel.

[0029] In one embodiment, the micro funnel includes a funnel body and a support wall, one end of the support wall is connected to the small end of the funnel body, and a connecting groove is formed on the side of the support wall.

[0030] In one embodiment, the optical fiber is characterized in that the number of microstructures matches the number of cores in the multi-core optical fiber; and / or,

[0031] The microstructure is a photoresist material. Attached Figure Description

[0032] Figure 1 This is a flowchart illustrating a method for fabricating a whispering-gallery mode refractive index sensor according to one embodiment;

[0033] Figure 2 This is a schematic diagram of the structure of a whispering-gallery mode refractive index sensor according to one embodiment;

[0034] Figure 3 This is a schematic diagram of the stability test spectrum of a whispering-gallery mode refractive index sensor according to one embodiment;

[0035] Figure 4 This is a schematic diagram of the sensitivity test spectrum of a whispering-gallery mode refractive index sensor according to one embodiment;

[0036] Figure 5 This is a graph showing the linear relationship between refractive index and wavelength offset for a whispering-gallery mode refractive index sensor according to one embodiment.

[0037] In the picture:

[0038] 100. Whispering-gallery mode refractive index sensor; 10. Optical fiber; 20. Microstructure; 21. Grating base; 22. Suspended grating; 23. Microsphere base; 24. Micro funnel; 241. Funnel body; 242. Support wall; 30. Microsphere. Detailed Implementation

[0039] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0040] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0041] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0042] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0043] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0044] It should be noted that when an element is referred to as being "fixed to" or "set on" another element, it can be directly on the other element or there may be an intervening element. When an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0045] On one hand, this application provides a method for fabricating a whispering-gallery mode refractive index sensor. Please refer to... Figure 1 , Figure 1 A method for fabricating a whispering-gallery mode refractive index sensor, as one embodiment, includes the following steps:

[0046] S1. A microstructure is created on the end face of an optical fiber using photolithography. The microstructure includes a grating base, a suspended grating, a microsphere base, and a microfunnel, which are sequentially arranged along the length of the fiber core from the side closest to the end face to the side furthest from the end face.

[0047] Specifically, by inscribing microstructures onto the end face of optical fibers, the space required for whispering-gallery mode refractive index sensors can be effectively reduced, enabling them to adapt to detection in confined spaces. Furthermore, efficient coupling between the microstructures and the optical fiber can be achieved, improving the utilization efficiency of signal light within the fiber and the measurement accuracy of the whispering-gallery mode refractive index sensor. Moreover, since the length of the optical fiber can be extended or shortened according to actual needs, the fabricated whispering-gallery mode refractive index sensor can perform remote monitoring, is suitable for long-distance measurements, and can be applied to a wider range of scenarios. Specifically, the suspended grating can cover the fiber core, thereby improving the propagation efficiency and utilization rate of optical information within the fiber core.

[0048] In this microstructure, the grating base controls the distance between the suspended grating and the end face, ensuring the grating is suspended relative to the end face. This allows light emitted from the fiber core to travel a distance through the environment, resulting in sufficient diffraction of the light as it passes through the grating. The suspended grating diffracts the light to excite whispering-gallery modes within the microsphere cavity, enabling refractive index measurement. The microsphere base fixes the coupling distance between the suspended grating and the microsphere, ensuring the stability of the diffracted light propagation. The micro-funnel facilitates easier assembly of the microsphere onto the base in step S3.

[0049] In some embodiments, the photolithography technology is 3D two-photon femtosecond laser lithography using a 3D lithography machine. By performing three-dimensional photolithography on the end face of a multi-core optical fiber, the shape of the microstructure can be determined, allowing for development and curing in step S2. Specifically, 3D two-photon femtosecond laser lithography includes two-photon polymerization, two-photon photoreduction, and multi-photon processing. Because femtosecond lasers have extremely short pulse widths and extremely high peak power, they primarily rely on photon absorption mechanisms to process transparent materials (such as photoresist materials) that long-pulse lasers cannot reach. Due to the short duration and negligible thermal effect of femtosecond lasers, the processing accuracy of the fabrication method in this embodiment can be improved, and it can penetrate deep into the interior of transparent materials, thus enabling the fabrication method of the whispering-gallery mode refractive index sensor to achieve true three-dimensional micromachining.

[0050] In some embodiments, the optical fiber is a multi-core optical fiber. Further, the multi-core optical fiber is a seven-core optical fiber, thereby enabling high-speed, high-capacity transmission of optical information. In other embodiments, the multi-core optical fiber may also be a two-core, four-core, or six-core optical fiber, with at least two cores. Specifically, a seven-core optical fiber is a seven-core single-mode optical fiber, which can provide a larger bandwidth for optical information transmission and improve the transmission speed. In some embodiments, the optical fiber may have only one core, with each core corresponding to a set of microstructures, thereby minimizing material costs while ensuring the measurement function of the whispering-gallery mode refractive index sensor.

[0051] S2. Develop and cure the microstructure.

[0052] Specifically, the microstructure is a photoresist material, enabling development and curing. The photoresist material can be positive or negative. When the photoresist material is positive, the non-exposed areas form the desired structure shape, while the exposed areas dissolve in the developer during development. When the photoresist material is negative, the exposed areas form the desired structure shape, while the non-exposed areas dissolve in the developer during development. In some embodiments, the microstructure is a negative photoresist, which facilitates the formation of a three-dimensional structure and helps reduce photoresist material loss and manufacturing costs. Specifically, the microstructure can be cured by placing it in an organic solution or by baking. In some embodiments, the organic solution can be isopropanol, etc.

[0053] S3. Assemble the microspheres onto the microsphere base through the micro-funnel until the microspheres are coupled to the suspended grating.

[0054] Specifically, the microspheres are assembled onto the microsphere base via a microfunnel using a template-assisted self-assembly method. This method utilizes the surface tension of a liquid to facilitate the self-assembly of the microspheres onto the base, allowing them to transition from a disordered to an ordered state more quickly. This improves assembly efficiency and reduces the preparation time. The microspheres are made of polymer materials such as polybutadiene, polyisoprene, or polystyrene. In some embodiments, the microspheres are polystyrene microspheres. Polystyrene microspheres possess excellent physical properties such as small particle size, good relative stability, strong hydrophobicity, and low adhesion, enabling the fabricated whispering-gallery mode temperature sensor to exhibit fast response, small size, high stability, and ease of assembly.

[0055] Specifically, the coupling between the microsphere and the suspended grating refers to the following: after the light information in the fiber core is emitted from the end face, it is diffracted by the suspended grating. The first diffraction sequence of the diffracted light information matches the basic WGM (Whispering Gallery Mode) in the microsphere, thereby exciting the Whispering Gallery Mode within the microsphere cavity. Furthermore, the principle behind the refractive index measurement through the coupling between the microsphere and the suspended grating is as follows: when the Whispering Gallery Mode refractive index measuring device is placed in a liquid or gaseous environment, changes in the liquid or gas concentration cause changes in the refractive index of the environment. Therefore, when the microsphere cavity is excited by the diffracted light information to create the Whispering Gallery Mode, the refractive index of the different environments causes a shift in the position of the resonant wavelength within the microsphere cavity. By measuring the value of the resonant wavelength, the refractive index value and the change in refractive index can be reflected.

[0056] In some embodiments, the shortest distance between the microsphere and the suspended grating ranges from 0.8 μm to 1.2 μm, thereby enabling the diffracted light information to be more effectively coupled into the microsphere cavity. This ensures critical coupling between the suspended grating and the microsphere, preventing over-coupling or under-coupling that could affect the transmission of light information. Simultaneously, it reduces light information loss and improves the quality factor of the fabricated whispering-gallery mode refractive index sensor. Specifically, the shortest distance between the optical waveguide 123 and the microsphere 133 can be 0.916, 0.982, 1.000, 1.035, or 1.072 μm, etc.

[0057] Furthermore, in some embodiments, the suspended grating in the fabricated whispering-gallery mode refractive index sensor is a hybrid two-dimensional suspended grating with a period ranging from 1.0 μm to 1.2 μm, a duty cycle ranging from 40% to 60%, and a thickness ranging from 0.9 μm to 1.1 μm. When these ranges are met, the first diffraction sequence of the suspended grating can achieve a better match with the fundamental WGM phase in the microsphere, making it easier to excite the whispering-gallery mode within the microsphere cavity. Furthermore, the hybrid two-dimensional suspended grating refers to a grating with multiple diffraction apertures arranged in an array on the same plane, which can improve the propagation stability of the diffracted light information. The shape of the diffraction apertures can be circular, rectangular, etc.; when using circular diffraction apertures, it is more suitable for coupling with the microsphere.

[0058] The aforementioned method for fabricating a whispering-gallery mode refractive index sensor effectively reduces the space required by the sensor by using photolithography to create a microstructure on the end face of the optical fiber. This allows the sensor to be used in confined spaces and enables efficient coupling between the microstructure and the optical fiber, improving the utilization efficiency of the signal light in the fiber and the measurement accuracy of the whispering-gallery mode refractive index sensor. Assembling the microspheres onto the microsphere base using a microfunnel improves the assembly positioning accuracy and efficiency, thus enhancing the sensor's stability and reducing measurement errors. Through the coupling between the microspheres and the suspended grating, light emitted from the optical fiber passes through the grating and diffracts. The diffracted light then couples into the microsphere cavity, creating resonance and exciting the whispering-gallery mode within the cavity, thereby enabling refractive index detection.

[0059] In one embodiment, after the step "developing and curing the microstructure", the following step is also included:

[0060] S2-1. Place the microstructure in a vacuum environment.

[0061] Specifically, placing the microstructure in a vacuum environment can improve coating efficiency, prevent metal oxidation, and make the metal film denser during subsequent metal thin film deposition.

[0062] S2-2, Deposit a thin metal film on the side of the suspended grating away from the grating base.

[0063] Specifically, when the refractive index of the environment to be measured by the whispering-gallery mode refractive index sensor differs little from the refractive index of the suspended grating, it affects the diffraction effect of the suspended grating and causes a certain degree of energy and optical information loss. By depositing a thin metal film on the side of the suspended grating away from the grating base, the diffraction efficiency of the optical information after passing through the suspended grating can be fully guaranteed, thereby ensuring the integrity and accuracy of the optical information during transmission. In some embodiments, the material used for the thin metal film is gold, which has the characteristics of being easy to deposit, having high stability, and not easily oxidized. In some embodiments, the thickness of the thin metal film ranges from 100nm to 300nm, thereby ensuring both the density of the deposited thin metal film and saving the cost of metal raw materials.

[0064] In one embodiment, prior to the step "writing the microstructure on the end face of the multi-core optical fiber using photolithography", the following steps are included:

[0065] S0-1, Remove the cladding of the optical fiber.

[0066] Specifically, the cladding of the optical fiber can be removed by using fiber strippers, which facilitates the cutting of the end face of the multi-core optical fiber in subsequent steps.

[0067] S0-2, Cut the end face of the optical fiber.

[0068] Specifically, optical fibers can be cut using a fiber optic cleaver. Cutting the end face of the fiber ensures a flat surface, reduces roughness, facilitates subsequent photolithography processes on the end face, improves the coupling efficiency between the microstructure and the end face, and reduces optical information loss and fluctuation at the end face, thereby improving the measurement efficiency and accuracy of the whispering-gallery mode refractive index sensor.

[0069] On the other hand, this application provides a whispering-gallery mode refractive index sensor 100. Please refer to... Figure 2 One embodiment of a whispering-gallery mode refractive index sensor 100 includes: an optical fiber 10, a microstructure 20, and a microsphere 30. The optical fiber 10 has a core for transmitting signal light.

[0070] In some embodiments, the optical fiber 10 is a multi-core optical fiber. Further, the multi-core optical fiber is a seven-core optical fiber, thereby enabling high-speed, high-capacity transmission of optical information. In other embodiments, the multi-core optical fiber may also be a two-core, four-core, or six-core optical fiber, etc., with at least two cores. Specifically, the seven-core optical fiber is a seven-core single-mode optical fiber, thereby providing a larger bandwidth for optical information transmission and improving the transmission speed of optical information. In some embodiments, the optical fiber may have only one core, with one core corresponding to a set of microstructures 20, thereby minimizing material costs while ensuring the measurement function of the whispering-gallery mode refractive index sensor 100.

[0071] Specifically, the microstructure 20 is disposed on one end face of the optical fiber 10, including a grating base 21, a suspended grating 22, and a microsphere base 23 arranged sequentially along the length of the fiber core from the side closest to the end face to the side furthest from the end face. The grating base 21 is correspondingly disposed to the fiber core of the optical fiber 10, thereby enabling the optical information in the fiber core to be fully diffracted by the suspended grating 22, improving the utilization efficiency of the optical information.

[0072] The function of the grating base 21 is to control the distance between the suspended grating 22 and the end face, so that the suspended grating 22 is suspended relative to the end face. This allows the light information in the fiber core to travel a certain distance in the environment after exiting from the end face, thus ensuring sufficient diffraction of the light information as it passes through the suspended grating 22. The function of the suspended grating 22 is to diffract the light information to excite the whispering gallery mode in the microsphere cavity of the microsphere 30, thereby enabling the measurement of the refractive index. The function of the microsphere base 23 is to fix the coupling distance between the suspended grating 22 and the microsphere 30, ensuring the stability of the propagation of the diffracted light information.

[0073] Furthermore, in some embodiments, the number of microstructures 20 matches the number of cores in a multi-core optical fiber. For example, when the optical fiber 10 used is a seven-core optical fiber, the number of microstructures 20 is also seven, with each of the seven microstructures 20 corresponding to a core of the seven-core optical fiber.

[0074] Specifically, the microsphere 30 is disposed on the microsphere base 23, and the interior of the microsphere 30 has a microsphere cavity. The microsphere 30 is coupled to the suspended grating 22. In some embodiments, the microsphere 30 is a polystyrene microsphere 30. Polystyrene microspheres 30 have excellent physical properties such as small particle size, good relative stability, strong hydrophobicity, and low adhesion, thereby enabling the fabricated whispering-gallery mode temperature sensor to have the characteristics of fast response speed, small size, high stability in use, and convenient assembly.

[0075] Specifically, the coupling between the microsphere 30 and the suspended grating 22 means that after the light information in the fiber core is emitted from the end face, it is diffracted by the suspended grating 22. The first diffraction sequence of the diffracted light information matches the basic WGM in the microsphere 30, thereby exciting the whispering gallery mode in the microsphere cavity within the microsphere 30. Furthermore, the principle by which the coupling between the microsphere 30 and the suspended grating 22 enables refractive index measurement is as follows: when the whispering gallery mode refractive index measuring device is placed in a liquid or gaseous environment, changes in the liquid or gas concentration will cause changes in the refractive index of the environment. Therefore, when the microsphere cavity within the microsphere 30 is excited by the diffracted light information to create a whispering gallery mode, the different refractive indices of the environment will cause a shift in the position of the resonant wavelength within the microsphere cavity. Thus, by measuring the value of the resonant wavelength, the value of the refractive index and the change in refractive index can be reflected.

[0076] The aforementioned whispering-gallery refractive index mode sensor, by placing the microstructure 20 on one end face of the optical fiber 10, effectively reduces the space required for the whispering-gallery mode refractive index sensor 100, thus enabling it to adapt to detection in confined spaces. Furthermore, it achieves efficient coupling between the microstructure 20 and the optical fiber 10, which improves the utilization efficiency of the signal light in the optical fiber 10 and the measurement accuracy of the whispering-gallery mode refractive index sensor 100. By placing the microstructure 20 along the length extension direction of the fiber core and the microsphere 30 on the microsphere base 23, a certain coupling distance exists between the suspended grating 22 and the microsphere 30. This allows the microsphere 30 and the suspended grating 22 to couple with each other. The light information emitted from the fiber core can pass through the suspended grating 22 and diffract, and the diffracted light information can couple into the microsphere cavity of the microsphere 30 to form resonance, thereby exciting the whispering-gallery mode in the microsphere cavity. Therefore, it is possible to detect the refractive index.

[0077] In some embodiments, a metal thin film is provided on the side of the suspended grating 22 away from the grating base 21. When the refractive index of the environment to be measured by the whispering-gallery mode refractive index sensor 100 is small compared with the refractive index of the suspended grating 22, it will affect the diffraction effect of the suspended grating 22 and cause a certain degree of energy and optical information loss. By providing a metal thin film on the side of the suspended grating 22 away from the grating base 21, the diffraction efficiency of the optical information after passing through the suspended grating 22 can be fully guaranteed, thereby ensuring the integrity and accuracy of the optical information during transmission. In some embodiments, the material used for the metal thin film is gold, which has the characteristics of easy vapor deposition, high stability, and resistance to oxidation. In some embodiments, the thickness of the metal thin film is in the range of 100nm-300nm, thereby ensuring the compactness of the metal thin film and saving the cost of metal raw materials. Specifically, the thickness of the metal thin film can be: 160.03, 180.75, 200.00, 220.60, or 240.00, etc. (all units are μm).

[0078] In some embodiments, the shortest distance between the microsphere 30 and the suspended grating 22 ranges from 0.8 μm to 1.2 μm, thereby enabling the diffracted light information to be more effectively coupled into the microsphere cavity. This ensures the critical coupling between the suspended grating 22 and the microsphere 30, preventing over-coupling or under-coupling that could affect the transmission of light information. Simultaneously, it reduces light information loss and improves the quality factor of the fabricated whispering-gallery mode refractive index sensor 100. Specifically, the shortest distance between the optical waveguide 123 and the microsphere 30133 can be 0.916, 0.982, 1.000, 1.035, or 1.072 μm, etc.

[0079] Furthermore, in some embodiments, the suspended grating 22 in the fabricated whispering-gallery mode refractive index sensor 100 is a hybrid two-dimensional suspended grating 22. The period of the suspended grating 22 ranges from 1.0 μm to 1.2 μm, the duty cycle ranges from 40% to 60%, and the thickness ranges from 0.9 μm to 1.1 μm. It should be noted that the thickness of the suspended grating 22 already includes the thickness of the metal thin film. When the above ranges are met, the first diffraction sequence of the suspended grating 22 can achieve a better match with the fundamental WGM phase in the microsphere 30, making it easier to excite the whispering-gallery mode in the microsphere cavity within the microsphere 30. Furthermore, the hybrid two-dimensional suspended grating 22 refers to a grating with multiple diffraction apertures disposed on the same plane, arranged in an array, which can improve the propagation stability of the diffracted light information. The shape of the diffraction apertures can be circular, rectangular, etc. When using circular diffraction apertures, it is more suitable for coupling with the microsphere 30.

[0080] In some more specific embodiments, the suspended grating 22 has a period of 1.1 μm, a duty cycle of 50%, and a thickness of 1 μm. The shortest distance between the suspended grating 22 and the microsphere 30 is 1 μm, which enables the coupling effect between the suspended grating 22 and the microsphere 30 to be better, and helps to improve the measurement accuracy and precision of the whispering-gallery mode refractive index sensor 100.

[0081] In one embodiment, the microstructure 20 further includes a microfunnel 24, which is mounted on the side of the microsphere base 23 away from the end face. Thus, the microsphere 30 can be assembled onto the microsphere base 23 via the microfunnel 24. The application of a template self-assembly method allows the microsphere 30 to transition from a disordered state to an ordered state more quickly, meaning the microsphere 30 can be combined with the microsphere base 23 more rapidly, thereby improving the assembly efficiency of the whispering-gallery mode temperature sensor.

[0082] Furthermore, the micro-funnel 24 includes a funnel body 241 and a support wall 242. One end of the support wall 242 is connected to the small end of the funnel body 241. Specifically, the support wall 242 surrounds the outer periphery of the microsphere 30, serving to define the position of the microsphere 30 and support the micro-funnel 24. Furthermore, the side of the support wall 242 has a communicating groove, which allows the microsphere 30 to be more fully placed in the test environment, helping to improve detection efficiency and accuracy.

[0083] Specifically, the microstructure 20 is a photoresist material, which can be formed using photolithography. The photoresist material can be positive or negative. When the photoresist material is positive, the non-exposed areas of the positive photoresist form the shape of the desired structure, while the exposed areas dissolve in the developer during the development process. When the photoresist material is negative, the exposed areas of the negative photoresist form the shape of the desired structure, while the non-exposed areas dissolve in the developer during the development process. In some embodiments, the microstructure 20 is a negative photoresist, which facilitates the formation of a three-dimensional structure and helps reduce photoresist material loss and manufacturing costs.

[0084] In some more specific embodiments, the optical fiber 10 is a seven-core single-mode optical fiber with a diameter of 124.5 μm and a maximum distance of 70 μm between each pair of cores; the grating base 21 has a height of 2 μm, a bottom outer diameter of 17.5 μm, a bottom inner diameter of 15.5 μm, a top outer diameter of 14 μm, a top inner diameter of 12 μm, and a wall thickness of 2 μm; the microsphere base 23 has a height of 10 μm, a bottom outer diameter of 16 μm, a bottom inner diameter of 14 μm, a top outer diameter of 14 μm, a top inner diameter of 12 μm, and a wall thickness of 2 μm; the microfunnel 24 has a height of 30 μm, a top radius of 18 μm, and a wall thickness of 1 μm.

[0085] In some embodiments, when using the whispering-gallery mode refractive index sensor 100 for detection, the wavelength of the optical signal used is 1520nm-1570nm. Figure 3 A schematic diagram of the stability test spectrum of a whispering-gallery mode refractive index sensor 100 according to one embodiment is shown. From... Figure 3 As can be seen, the sharp resonance peak in the 1520nm-1570nm wavelength band is the whispering-gallery mode. The applicant verified this through experiments, placing the whispering-gallery mode refractive index sensor 100 in 50 ml of aqueous solution and recording the spectrum at 20-minute intervals for a total experimental time of 120 minutes. No shift in the resonance wavelength was observed, demonstrating that the whispering-gallery mode refractive index sensor 100 of this application exhibits good stability.

[0086] Please refer to Figure 4 , Figure 4 A schematic diagram of the sensitivity test spectrum of a whispering-gallery mode refractive index sensor 100 according to an embodiment is shown. From... Figure 4 As can be seen, the sharp resonance peak (e.g., at 1542.16418 nm) in the 1520 nm-1570 nm wavelength band represents the whispering-gallery mode. At 1542.16418 nm, the calculated quality factor of the whispering-gallery mode refractive index sensor 100 is 3000. The applicant experimentally verified this by placing the whispering-gallery mode refractive index sensor 100 in salt solutions of different concentrations and recording the spectral changes. The figure shows that the resonance peak redshifts with increasing solution concentration. After further analysis... Figure 4 After processing the data, the refractive index detection sensitivity of the multi-core fiber 10 end-face grating coupled whispering-gallery mode microsphere cavity refractive index sensor can reach 146.0517. Therefore, the whispering-gallery mode refractive index sensor 100 of this application has excellent refractive index sensing capabilities.

[0087] Please refer to Figure 5 , Figure 5 A linear relationship between refractive index and wavelength shift is shown for a whispering-gallery mode refractive index sensor 100 according to one embodiment. The horizontal axis represents different refractive indices, and the vertical axis represents wavelength shift. Experimental and fitting results show a linear relationship between refractive index and wavelength shift: Δλ = 146.0517R - 194.5783. Calculations show that the detection sensitivity of the whispering-gallery mode refractive index sensor 100 can reach 146.0517 nm. Therefore, the whispering-gallery mode refractive index sensor 100 of this application has excellent refractive index sensing capabilities.

[0088] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0089] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for fabricating a whispering-gallery mode refractive index sensor, characterized in that, Includes the following steps: A microstructure is created on the end face of an optical fiber using photolithography. The microstructure includes a grating base, a suspended grating, a microsphere base, and a microfunnel, which are sequentially arranged along the length of the fiber core from the side closest to the end face to the side furthest from the end face. The microstructure is developed and cured; The microspheres are assembled onto the microsphere base through the micro-funnel until the microspheres are coupled to the suspended grating. After the light information in the fiber core is emitted from the end face, it is diffracted by the suspended grating. The first diffraction sequence of the diffracted light information matches the basic whispering gallery mode in the microsphere, thereby exciting the whispering gallery mode in the microsphere cavity. The microspheres are assembled onto the microsphere base via the micro-funnel using a template-assisted self-assembly method. The minimum distance between the microsphere and the suspended grating is set between 0.8 μm and 1.2 μm. In the fabricated whispering-gallery mode refractive index sensor, the suspended grating is a hybrid two-dimensional suspended grating; and / or, the period of the suspended grating ranges from 1.0 μm to 1.2 μm; and / or, the duty cycle of the suspended grating ranges from 40% to 60%; and / or, the thickness of the suspended grating ranges from 0.9 μm to 1.1 μm. The hybrid two-dimensional suspended grating refers to a grating having multiple diffraction apertures disposed on the same plane, with the multiple diffraction apertures arranged in an array.

2. The method for fabricating a whispering-gallery mode refractive index sensor according to claim 1, characterized in that, Following the step "developing and curing the microstructure", the following steps are also included: The microstructure was placed in a vacuum environment; A thin metal film is deposited on the side of the suspended grating away from the grating base.

3. The method for fabricating a whispering-gallery mode refractive index sensor according to claim 1, characterized in that, Before the step "writing the microstructure on the end face of the optical fiber using photolithography", the following steps are included: Remove the cladding from the optical fiber; The end face of the optical fiber is cut.

4. A whispering-gallery mode refractive index sensor, characterized in that, include: Optical fiber, wherein the optical fiber has a core inside for transmitting signal light; A microstructure is disposed on one end face of the optical fiber. The microstructure includes a grating base, a suspended grating, and a microsphere base arranged sequentially along the length extension direction of the fiber core from the side near the end face to the side away from the end face. The microstructure also includes a microfunnel, which is mounted on the side of the microsphere base away from the end face; The microsphere is disposed on the microsphere base and has a microsphere cavity inside. The microsphere is coupled to the suspended grating. After the light information in the fiber core is emitted from the end face, it is diffracted by the suspended grating. The first diffraction sequence of the diffracted light information matches the basic whispering gallblade mode in the microsphere, thereby exciting the whispering gallblade mode in the microsphere cavity. The microsphere is assembled to the microsphere base through the micro-funnel using a template-assisted self-assembly method. The shortest distance between the microsphere and the suspended grating ranges from 0.8 μm to 1.2 μm; The suspended grating is a hybrid two-dimensional suspended grating; and / or, the period of the suspended grating is in the range of 1.0 μm-1.2 μm; and / or, the duty cycle of the suspended grating is in the range of 40%-60%; and / or, the thickness of the suspended grating is in the range of 0.9 μm-1.1 μm. The hybrid two-dimensional suspended grating refers to a grating having multiple diffraction apertures disposed on the same plane, with the multiple diffraction apertures arranged in an array.

5. The whispering-gallery mode refractive index sensor according to claim 4, characterized in that, A thin metal film is provided on the side of the suspended grating away from the grating base.

6. The whispering-gallery mode refractive index sensor according to claim 5, characterized in that, The thickness of the metal film ranges from 100nm to 300nm.

7. The whispering-gallery mode refractive index sensor according to claim 5, characterized in that, The material of the metal film is gold.

8. The whispering-gallery mode refractive index sensor according to claim 4, characterized in that, The micro funnel includes a funnel body and a support wall. One end of the support wall is connected to the small end of the funnel body, and a connecting groove is opened on the side of the support wall.

9. The whispering-gallery mode refractive index sensor according to any one of claims 4-8, characterized in that, The optical fiber is a multi-core optical fiber, and the number of the microstructures matches the number of the cores of the multi-core optical fiber.

10. The whispering-gallery mode refractive index sensor according to claim 9, characterized in that, The microstructure is a photoresist material.

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