Optical fiber sensor based on polymer whispering gallery mode resonant cavity and preparation method thereof

Abstract

The present invention discloses an optical fiber sensor based on a polymer whispering gallery mode resonant cavity, wherein the optical fiber includes a core and a cladding; the optical fiber has a groove; the bottom height of the groove is lower than the core, and the groove cuts the core into a first core and a second core; the groove has a waveguide with a polymer structure and a whispering gallery resonant cavity with a polymer structure; the two ends of the waveguide are respectively connected to the first core and the second core. The present invention also discloses a method for preparing the above-mentioned optical fiber sensor based on a polymer whispering gallery mode resonant cavity. The present invention proposes for the first time to integrate a whispering gallery resonant cavity inside an optical fiber using laser polymerization technology. Compared with the existing technology, the most prominent advantage of the present invention is that it has a higher degree of integration, can ensure the integration of the resonator, and has a compact structure; the processing time is reduced, the structural size is reduced, the stability of the device is improved, and the structural design is more flexible, which provides great guarantee for meeting the needs of different environments.

Description

Technical Field

[0001] The present invention relates to the field of sensor technology, and in particular to an optical fiber sensor based on a polymer whispering gallery mode resonant cavity and a preparation method thereof. Background Art

[0002] When light propagates in a direction greater than the critical angle within the boundary of a closed cavity, a total reflection effect will occur on the surface of the cavity. When the light wave path meets certain phase matching conditions, they can be superimposed and enhanced to form a standing wave field and maintain a stable traveling wave propagation mode. This mode is called the whispering gallery mode, and the cavity formed is called the whispering gallery mode resonant cavity. There are several different structures of typical whispering gallery mode resonant cavities, such as waveguide micro-ring cavities, micro-ring core cavities, micro-sphere cavities, etc. Whispering gallery resonant cavities are widely used in the development of high-sensitivity sensors due to their high sensitivity to changes in the external environment. They have broad application prospects in many fields such as nonlinear optics, narrow-band optical filtering, and ultra-high sensitivity micro-sensors. At present, the production of optical fiber sensors with whispering gallery resonators is still difficult to achieve.

[0003] The known micro-nano fiber coupling method uses the strong evanescent field of the micro-nano fiber to couple the light in the micro-nano fiber to the micro-disk or micro-ring resonant cavity to form a resonator. However, the resonator formed by this method is not integrated, the preparation steps are relatively cumbersome, the system is unstable, and the micro-nano fiber needs to be precisely operated.

[0004] A known optical fiber end face coupling method is to use ultraviolet glue to bond microspheres on the optical fiber end face and connect a circulator to form a resonator. However, this method has poor flexibility and large optical energy loss. Summary of the invention

[0005] In order to overcome the problems existing in the prior art, the present invention proposes an optical fiber sensor based on a polymer whispering gallery mode resonant cavity and a preparation method thereof.

[0006] The present application provides an optical fiber sensor based on a polymer whispering gallery mode resonant cavity, comprising an optical fiber, wherein the optical fiber comprises a core and a cladding; the optical fiber has a groove; the bottom height of the groove is lower than the core, and the groove cuts the core into a first core and a second core; the groove has a waveguide with a polymer structure and a whispering gallery resonant cavity with a polymer structure; the two ends of the waveguide are respectively connected to the first core and the second core.

[0007] As an improvement of the optical fiber sensor based on the polymer whispering gallery mode resonant cavity provided by the present invention, the whispering gallery resonant cavity is a micro-disk cavity, a micro-ring cavity, a micro-sphere cavity, a micro-dish cavity, or a micro-column cavity.

[0008] As an improvement of the optical fiber sensor based on the polymer whispering gallery mode resonant cavity provided by the present invention, the polymer structure is formed by laser polymerization of polymer monomers.

[0009] As an improvement of the optical fiber sensor based on the polymer whispering gallery mode resonant cavity provided by the present invention, the polymer structure is a photoresist cured structure.

[0010] The present application also provides a method for preparing an optical fiber sensor based on a polymer whispering gallery mode resonant cavity, comprising the following steps:

[0011] Step S1: removing material from the optical fiber to obtain a groove, and the fiber core at the groove is removed together;

[0012] Step S2: cleaning the optical fiber;

[0013] Step S3: polymerization processing, fixing the optical fiber on the laser micromachining system so that the optical fiber groove is immersed in the monomer solution; using laser to polymerize the monomer solution to form a solidified polymer structure waveguide and a polymer structure whispering gallery resonant cavity in the monomer solution, and obtaining an optical fiber sensor sample based on the polymer whispering gallery mode resonant cavity;

[0014] Step S4: developing.

[0015] As an improvement to the method for preparing the optical fiber sensor provided by the present invention, in step S3, the optical fiber is fixed on a glass slide, the optical fiber groove is immersed in the monomer solution, and a cover glass is placed on it; the glass slide with the optical fiber is fixed on a three-dimensional displacement platform of a femtosecond laser micromachining system, and a polymer structure waveguide and a polymer structure whispering gallery resonant cavity are obtained through femtosecond laser polymerization processing.

[0016] As an improvement to the method for preparing the optical fiber sensor provided by the present invention, in step S3, the monomer solution is polymerized using femtosecond laser two-photon polymerization technology.

[0017] As an improvement to the method for preparing the optical fiber sensor provided by the present invention, in step S3, support parts are provided on the glass slides on both sides of the optical fiber to prevent the cover glass from squeezing the optical fiber; after the polymerization process is completed and the cover glass is removed, the support parts are removed.

[0018] As an improvement to the method for preparing the optical fiber sensor provided by the present invention, in step S1, after the coating layer of the optical fiber is stripped off, a groove is obtained by femtosecond laser etching.

[0019] As an improvement to the method for preparing the optical fiber sensor provided by the present invention, in step S2, the sample with the processed groove is placed in anhydrous ethanol for ultrasonic cleaning, and then taken out and dried.

[0020] This application has the following beneficial effects:

[0021] The present invention proposes for the first time to integrate a whispering gallery resonator inside an optical fiber using laser polymerization technology. Compared with existing technologies, the most prominent advantage of the present invention is that it has a higher degree of integration, can ensure the integration of the resonator, and has a compact structure. The polymer structure whispering gallery resonator directly integrated inside the optical fiber through polymerization technology reduces processing time, reduces structural size, improves device stability, and makes structural design more flexible, providing great guarantees for meeting the needs of different environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the structure of an optical fiber sensor based on a polymer whispering gallery mode resonant cavity according to an embodiment of the present invention;

[0023] Figure 2 A schematic diagram of an optical path system of a processing device for an optical fiber sensor of a polymer whispering gallery mode resonant cavity according to an embodiment of the present invention;

[0024] Figure 3 This is a scanning electron microscope image of an optical fiber sensor based on a polymer whispering gallery mode resonant cavity prepared by the method of an embodiment of the present invention;

[0025] Figure 4 for Figure 3 Shown is the transmission spectrum of an optical fiber sensor based on a polymer whispering gallery mode resonator.

[0026] Reference numerals:

[0027] A fiber core (1), a cladding (2), a groove (3), a waveguide (4), and a whispering gallery resonant cavity (5). DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0029] In the description of the present invention, it is to be understood that the terms “center”, “longitudinal”, “lateral”, “length”, “width”, “thickness”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside”, “clockwise”, “counterclockwise”, “axial”, “radial”, “circumferential”, etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0030] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0031] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral one; it can be a mechanical connection, an electrical connection, or communication with each other; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0032] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments.

[0033] Figure 1 It is a schematic structural diagram of an optical fiber sensor based on a polymer whispering gallery mode resonant cavity according to an embodiment of the present invention.

[0034] like Figure 1 As shown, an optical fiber sensor based on a polymer whispering gallery mode resonant cavity comprises an optical fiber. The optical fiber comprises a core (1) and a cladding (2). The optical fiber of the embodiment of the present application can be a single-mode optical fiber or a multi-mode optical fiber, preferably a single-mode optical fiber.

[0035] A groove (3) is provided in the middle of the optical fiber, and the groove (3) is obtained by removing material. A waveguide (4) and a whispering gallery resonant cavity (5) of a polymer structure are provided in the groove (3).

[0036] The polymer structure of the embodiment of the present application is a solidified structure formed by laser polymerization of polymer monomers. Specifically, the monomer solution contains polymer monomers, photoinitiators and photosensitizers. After irradiating the monomer solution, the polymer monomers undergo polymerization reaction to form a polymer structure, and the light-induced polymerization reaction occurs. Preferably, the polymer structure is a photoresist solidified structure, which is formed after the photoresist is exposed and solidified.

[0037] Figure 1 FIG. 2 schematically shows the structure of the optical fiber sensor when the whispering gallery resonant cavity (5) is a microdisk cavity.

[0038] However, it is understandable that the specific structure of the whispering gallery resonant cavity (5) is not limited to a micro-disk cavity, but may also be a micro-ring cavity, a micro-sphere cavity, a micro-disk cavity, or a micro-column cavity.

[0039] In a specific embodiment, the waveguide (4) may be a core (1) of an optical fiber; the core (1) is exposed when the groove (3) is processed.

[0040] Alternatively, in another specific embodiment, the waveguide (4) is a polymer structure, which is a solidified structure formed by laser polymerization of polymer monomers.

[0041] If the fiber core (1) is exposed when the groove (3) is processed, and the exposed fiber core (1) is used as the waveguide (4), due to the limitation of the processing technology, the surface of the waveguide (4) is easily roughened, which affects the light guiding. Therefore, as a preferred solution, the waveguide (4) is a polymer structure, and the fiber core (1) in the groove (3) is first removed, and then the waveguide (4) with a polymer structure is formed.

[0042] Specifically, the bottom height of the groove (3) is lower than the fiber core (1), and the fiber core (1) in the groove (3) is also removed at the same time, so that the groove (3) cuts the fiber core (1) into a first fiber core located on the left side of the groove (3) and a second fiber core located on the right side of the groove (3). A waveguide (4) with a polymer structure is formed in the groove (3), and the two ends of the waveguide (4) are respectively connected to the first fiber core and the second fiber core.

[0043] When the whispering gallery resonant cavity (5) is a micro-disk cavity, a micro-ring cavity, a micro-dish cavity, or a micro-column cavity, the bottom surface of the groove (3) may be a platform, such as Figure 2 As shown. However, it can be understood that the bottom surface of the groove (3) is used to carry the waveguide (4) and the whispering gallery resonant cavity (5), and the specific structure of the bottom surface of the groove (3) is not limited thereto. When the whispering gallery resonant cavity (5) is specifically a microsphere cavity, the bottom surface of the groove (3) can be in a stepped shape, and the bottom surface of the groove (3) carrying the waveguide (4) is higher than the bottom surface of the groove (3) carrying the microsphere cavity.

[0044] The whispering gallery resonant cavity (5) and the waveguide (4) are fixed on the bottom surface of the groove (3). The waveguide (4) and the whispering gallery resonant cavity (5) are tangent to each other, and the gap between the waveguide (4) and the whispering gallery resonant cavity (5) is 0-2 μm, and the size of the gap is appropriately set so as to generate resonance between the waveguide (4) and the whispering gallery resonant cavity (5).

[0045] In a preferred embodiment, the diameter of the waveguide (4) is 1 μm-2 μm, and the diameter of the whispering gallery resonant cavity (5) is 20 μm-60 μm.

[0046] The optical fiber also includes a coating layer (not shown in the figure), which covers the cladding (2).

[0047] The method for preparing the optical fiber sensor based on the polymer whispering gallery mode resonant cavity comprises the following steps:

[0048] Step S1: sample preparation and processing of groove (3): removing material from the optical fiber to obtain the groove (3), and the fiber core (1) at the groove (3) is removed together;

[0049] Step S2: cleaning the optical fiber;

[0050] Step S3: polymerization processing, fixing the optical fiber on a laser micromachining system so that the optical fiber groove (3) is immersed in the monomer solution; using laser to polymerize the monomer solution to form a solidified polymer structure waveguide (4) and a polymer structure whispering gallery resonant cavity (5) in the monomer solution, thereby obtaining an optical fiber sensor sample based on a polymer whispering gallery mode resonant cavity;

[0051] Step S4: developing, immersing the sample obtained in step S3 in a developing solution, dissolving the unpolymerized and solidified monomers, and obtaining an optical fiber sensor based on a polymer whispering gallery mode resonant cavity.

[0052] The monomer solution contains polymer monomers, photoinitiator and photosensitizer. The monomer solution is preferably photoresist.

[0053] In step S1, the groove (3) can be processed by various methods. Preferably, in a specific embodiment of the present application, the groove (3) is processed by laser etching.

[0054] After stripping the coating of the optical fiber to a suitable length, the coating debris is wiped off. The optical fiber is then fixed on a glass slide; the glass slide with the optical fiber is fixed on the three-dimensional displacement platform of the femtosecond laser micromachining system, and the material is removed from the position where the coating of the optical fiber is removed by femtosecond laser etching to obtain a groove (3).

[0055] In a more detailed embodiment, sample preparation and groove machining respectively include the following steps:

[0056] Sample preparation: Take a section of single-mode optical fiber, strip off the coating layer in the middle of the optical fiber to an appropriate length, such as 5mm-20mm, and use lens cleaning paper or dust-free paper dipped in anhydrous ethanol to wipe off the coating debris.

[0057] Processing grooves: Fix this section of optical fiber on a glass slide by UV glue or other means to keep the optical fiber straight and loose. Adsorb the glass slide with the optical fiber fixed on it onto the three-dimensional displacement platform of the femtosecond laser micromachining system by vacuum adsorption. Adjust the energy and repetition frequency of the femtosecond laser pulse. Select a low-magnification (low numerical aperture) microscope objective (NA<0.3) as the focusing element of the laser beam. Design a suitable laser etching scanning path by computer, set the length, height, width of the laser etching area, as well as the scanning layer spacing, scanning line spacing, and displacement speed. The start and end of the processing are controlled by a shutter. After femtosecond laser etching processing, a platform lower than the height of the core (1) can be processed inside the optical fiber. The height of the platform is appropriately set so that it can support the waveguide (4) of the polymer structure.

[0058] In step S2, the detailed steps of cleaning include: placing the sample with the processed groove (3) in anhydrous ethanol for ultrasonic cleaning for 5-10 minutes, and then taking it out to dry.

[0059] In step S3, the optical fiber is fixed on a glass slide, photoresist is dripped into the optical fiber groove (3), the optical fiber groove (3) is immersed in the photoresist, and then a cover glass is covered. The detailed steps of adding the photoresist include: providing support parts on both sides of the optical fiber etching position to prevent the cover glass from squeezing the optical fiber, dripping photoresist into the position of the groove (3) after the optical fiber is etched, the optical fiber is immersed in the photoresist without generating bubbles, and then covering the cover glass to seal the photoresist. Specifically, adhesive tape can be attached to both sides of the optical fiber etching position as a support part.

[0060] In step S3, the laser micromachining system is preferably a femtosecond laser micromachining system. Compared with other laser processing methods, femtosecond laser is an ultrafast laser. The power density of a single pulse of a femtosecond laser is very high. When it acts on a material, the thermal effect is small. It is a cold process. The surface of the prepared waveguide (4) and the whispering gallery resonant cavity (5) is smoother and more precise. Furthermore, it is preferred to use femtosecond laser two-photon polymerization technology to polymerize the photoresist to prepare the waveguide (4) and the whispering gallery resonant cavity (5). Using femtosecond laser two-photon polymerization technology, the polymerization spatial resolution is higher, and a more delicate structure can be made.

[0061] After fixing the glass slide with the optical fiber on the three-dimensional displacement platform of the femtosecond laser micromachining system, a polymer structure waveguide (4) and a polymer structure whispering gallery resonant cavity (5) are obtained through femtosecond laser polymerization processing.

[0062] The detailed steps of the polymerization process include: fixing the sample with photoresist dripped into it on a three-dimensional displacement platform by vacuum adsorption, adjusting the energy and repetition frequency of the femtosecond laser pulse. Selecting a high-magnification (high numerical aperture) microscope objective (NA>0.4) as the focusing element of the laser beam. Setting the appropriate scanning layer spacing, scanning line spacing, and displacement speed, and starting and stopping the processing process by shutter control. After femtosecond laser polymerization processing, the polymer waveguide (4) and whispering gallery resonant cavity (5) structure can be processed inside the optical fiber.

[0063] The optical path system of the processing equipment used to prepare the polymer whispering gallery mode resonant cavity inside the polymer optical fiber is as follows Figure 2 shown.

[0064] First, the femtosecond laser beam is expanded by a beam expander to expand the laser beam diameter by 2-3 times, and then passes through a laser attenuator and an optical power meter. The laser attenuator is used to adjust the laser power value, the optical power meter is used to detect the laser power value, and the computer-driven switch is used to control the laser exposure time. The transmitted illumination light passes through the dichroic mirror and then passes through a filter to filter out excess laser light before entering the CCD imaging for real-time observation of the processed image. The femtosecond laser beam is reflected by the dichroic mirror and enters the microscope objective lens to focus on the inside of the glue for processing. The sample is fixed on a three-dimensional precision displacement platform, and the displacement platform is controlled by a computer to move in the X, Y, and Z directions.

[0065] As an improvement to the method for preparing the optical fiber sensor provided by the present invention, in step S4, the developing solution is a mixed solution of acetone and isopropanol.

[0066] The detailed steps of the development process include: after processing is completed, the cover glass on the sample that has been polymerized is removed, the support parts on both sides for protecting the optical fiber are removed, and the sample and the slide are immersed in a mixed solution of acetone:isopropanol (volume ratio of 1:4) and allowed to stand for 1-5 minutes, so that the unpolymerized photoresist is dissolved and the polymerized waveguide (4) and whispering gallery resonant cavity (5) structures are retained.

[0067] When the whispering gallery resonant cavity (5) is a micro-ring whispering gallery resonant cavity (5), Figure 3 The figure in the figure is a scanning electron microscope image of an optical fiber sensor based on a polymer whispering gallery mode resonant cavity prepared by the above method.

[0068] Figure 4 for Figure 3 The transmission spectrum of a fiber optic sensor based on a polymer whispering gallery mode resonator is shown in Figure 1. At a wavelength of 1560 nm, the free spectrum range is about 13 nm. Since the microdisk diameter is 40 μm, the microdisk radius r is 20 μm, and the photoresist refractive index is 1.5, the free spectrum FSR is consistent with the relationship between the radius r FSR = (λ2 ) / (2πrn).

[0069] The application proposes an optical fiber sensor based on a polymer whispering gallery mode resonant cavity, wherein the fiber core (1) at the groove (3) is first removed together, and then a waveguide (4) is prepared by polymerization. The surface of the prepared waveguide (4) is relatively smooth.

[0070] If the fiber core (1) located at the groove (3) is retained when the groove (3) is processed, and this part of the fiber core (1) is used as a waveguide (4), and the groove (3) is processed by removing material while retaining part of the fiber core (1), the surface of the fiber core (1) will become rough, thereby affecting the light guiding effect.

[0071] The polymer whispering gallery resonant cavity structure integrated in the optical fiber solidified by femtosecond laser two-photon polymerization technology proposed in the present invention has the characteristics of compact size and high integration. Polymer has a high thermo-optical coefficient, and the optical fiber sensor of the polymer micro-ring whispering gallery resonant cavity can be used as a highly sensitive temperature sensor; because polymer easily absorbs water and expands, the optical fiber sensor of the polymer micro-ring whispering gallery resonant cavity can also be used as a humidity sensor. In addition, it can also be used as a filter in communication optical fiber.

[0072] Obviously, the embodiments described above are only some embodiments of the present application, rather than all embodiments. The preferred embodiments of the present application are given in the accompanying drawings, but they do not limit the patent scope of the present application. The present application can be implemented in many different forms. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive. Although the present application is described in detail with reference to the aforementioned embodiments, for those skilled in the art, it is still possible to modify the technical solutions recorded in the aforementioned specific implementation methods, or to perform equivalent replacement of some of the technical features therein. Any equivalent structure made using the contents of the specification and drawings of this application, directly or indirectly used in other related technical fields, is similarly within the scope of patent protection of this application.

Claims

1. An optical fiber sensor based on a polymer whispering gallery mode resonator, comprising an optical fiber, wherein the optical fiber comprises a core and a cladding; characterized in that: The optical fiber has a groove; the bottom height of the groove is lower than the core, and the groove cuts the core into a first core and a second core; the groove has a waveguide with a polymer structure and a whispering gallery resonator with a polymer structure, and the polymer structure is formed by laser polymerization of polymer monomers; the two ends of the waveguide are respectively connected to the first core and the second core; the waveguide and the whispering gallery resonator are both supported and fixed on the bottom surface of the groove.

2. The optical fiber sensor according to claim 1, characterized in that: The whispering gallery resonant cavity is a micro-disk cavity, a micro-ring cavity, a micro-sphere cavity, a micro-dish cavity, or a micro-column cavity.

3. The optical fiber sensor according to claim 1, characterized in that: The polymer structure is a photoresist-cured structure.

4. A method for preparing an optical fiber sensor based on a polymer whispering gallery mode resonator, characterized in that: The steps include: Step S1: removing material from the optical fiber to obtain a groove, wherein the bottom height of the groove is lower than the fiber core, and the fiber core at the groove is removed together, and the groove cuts the fiber core into a first fiber core and a second fiber core; Step S2: cleaning the optical fiber; Step S3: polymerization processing, fixing the optical fiber on the laser micromachining system so that the optical fiber groove is immersed in the monomer solution; using laser to polymerize the monomer solution to form a solidified polymer structure waveguide and a polymer structure whispering gallery resonant cavity in the monomer solution, wherein the two ends of the waveguide are respectively connected to the first fiber core and the second fiber core, and the waveguide and the whispering gallery resonant cavity are both supported and fixed on the bottom surface of the groove, so as to obtain an optical fiber sensor sample based on the polymer whispering gallery mode resonant cavity; Step S4: developing.

5. The method for preparing an optical fiber sensor according to claim 4, characterized in that: In step S3, the optical fiber is fixed on a glass slide, the optical fiber groove is immersed in the monomer solution, and a coverslip is placed on it; the glass slide with the optical fiber is fixed on a three-dimensional displacement platform of a femtosecond laser micromachining system, and a polymer structure waveguide and a polymer structure whispering gallery resonant cavity are obtained through femtosecond laser polymerization processing.

6. The method for preparing an optical fiber sensor according to claim 5, characterized in that: In step S3, the monomer solution is polymerized using femtosecond laser two-photon polymerization technology.

7. The method for preparing an optical fiber sensor according to claim 5, characterized in that: In step S3, support parts are provided on the glass slides on both sides of the optical fiber to prevent the cover glass from squeezing the optical fiber; after the polymerization process is completed, the cover glass is removed and the support parts are removed.

8. The method for preparing an optical fiber sensor according to claim 4, characterized in that: In step S1, after the coating layer of the optical fiber is stripped off, a groove is obtained by femtosecond laser etching.

9. The method for preparing an optical fiber sensor according to claim 4, characterized in that: In step S2, the sample with the processed grooves is placed in anhydrous ethanol for ultrasonic cleaning, and then taken out and dried.

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