Preparation method and application of optical fiber direct writing processing head based on photon nano jet flow

Abstract

The invention discloses a preparation method and application of an optical fiber direct-writing processing head based on photon nano jet, and the method comprises the following steps: firstly, removing a coating layer of an optical fiber by using a Miller clamp, retaining a fiber core and a cladding, then wiping the surface of the cladding by using degreasing cotton dipped in ethanol, and cutting the end face of the optical fiber to be flat by using an optical fiber cutter, observing the end face of the optical fiber through a confocal microscope; a group of optical fibers are fixed on a Z-axis displacement table, the optical fiber part with the length of 1-2 cm is put into a hydrofluoric acid solution to be corroded, and the integrity of a fiber core is not damaged; and taking out the optical fiber from the hydrofluoric acid, carrying out a CO2 laser hot melting processing experiment on the end surface of the optical fiber, and firing the corroded end surface of the optical fiber to obtain the optical fiber microsphere. According to the technical scheme, the curvature radius of the manufactured optical fiber microsphere is close to the radius of a fiber core, light beam focusing can be achieved, the electric field intensity is enhanced to be multiple times of an incident light field, and photon nano jet micro-nano machining is achieved; and meanwhile, the material cost is low, the structure and the manufacturing process are simple, and mass production can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser near-field processing, and particularly relates to a preparation method and application of a fiber direct writing processing head based on photonic nanobubble. BACKGROUND

[0002] Laser processing technology is an advanced, little-wearing, and capable of meeting various complex material requirements processing method. It has been widely applied in many fields such as aviation, biology, medicine, and national defense due to its excellent precision, little pollution, and automatic control characteristics. According to the action distance, laser processing can be divided into far-field and near-field modes. The far-field processing precision is difficult to break through the micron-level processing bottleneck due to the limitation of the optical diffraction limit. Relatively, the near-field processing technology can push the processing resolution to the nanometer scale through the local light field enhancement effect, which not only deepens the understanding of the laser and material interaction mechanism, but also provides a new solution for micro-nano lithography technology.

[0003] In recent years, microsphere-assisted near-field processing technology has attracted much attention due to its ability to achieve super-diffraction-limited nanofabrication. This technology utilizes the backscattering near-field enhancement effect of micro / nanospheres under laser irradiation to form a high-intensity evanescent wave focusing light field, thereby breaking through the traditional optical resolution limit. This phenomenon is called photonic nanobubble, and since it was introduced into the field of laser processing, it has shown significant advantages in processing precision and efficiency. However, current super-diffraction processing based on photonic nanobubble mainly focuses on the preparation of regular nanohole arrays, and the direct writing of large-area arbitrary patterns still heavily relies on precise scanning systems such as atomic force microscopes. Such systems have inherent defects such as slow scanning speed, complex process flow, high equipment cost, and lack of sufficient degrees of freedom, which greatly limits the industrial application prospects of this technology.

[0004] Therefore, there is a need for a method to prepare a fiber direct writing processing head based on photonic nanobubble with simple structure, low cost, and the ability to realize arbitrary patterning processing, and to minimize the full width at half maximum of the photonic nanobubble and increase the working distance. SUMMARY

[0005] In view of the deficiencies in the prior art, the present application aims to provide a preparation method and application of a fiber direct writing processing head based on photonic nanobubble with simple preparation process, low manufacturing cost, and long working distance.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical solution: a preparation method of a fiber direct writing processing head based on photonic nanobubble, comprising the following steps:

[0007] Step 1: First, use Miller pliers to remove the coating of the optical fiber, leaving the core and cladding. Then, use absorbent cotton dipped in ethanol to wipe the cladding surface. Use a fiber cleaver to cut the fiber end face flat. Then, observe the fiber end face under a confocal microscope to ensure that there are no obvious defects on the end face after cutting and the roughness Ra is less than 0.1um.

[0008] Step 2: Fix a group of optical fibers on a Z-axis translation stage and place a 1-2 cm long section of the fiber in a 20% hydrofluoric acid solution for etching. Remove one fiber every 5-10 minutes during etching and measure its diameter using a confocal microscope to determine whether the core layer has been etched. This ensures that the fiber cladding is substantially removed while the core integrity is not compromised.

[0009] Step 3: Remove the optical fiber from the hydrofluoric acid and place the corroded part in an ultrasonic cleaner. Use deionized water and anhydrous ethanol to ultrasonically clean it for 10 minutes to remove residual hydrofluoric acid.

[0010] Step 4: Use a rotating fiber clamp to fix the optical fiber on the six-axis micro-motion platform, with the CO2 laser's laser vertically facing downward. Keep the CCD industrial camera aligned with the fiber end face position to monitor the status of the fiber end face during processing in real time.

[0011] Step 5: Conduct a CO2 laser hot-melt processing experiment on the fiber end face, burn out a fiber microsphere on the etched fiber end face, and fix the laser spot size, which is required to be larger than the fiber core diameter;

[0012] Step 6: Changing the CO2 laser heat-melting output power and irradiation time processing parameters; by fixing one laser heat-melting processing parameter and changing another parameter, a curve showing the effect of each processing parameter on the fiber microsphere structural parameters is obtained; the fiber microsphere structural parameters are measured using a laser scanning confocal microscope at a magnification of 50 to 100 times;

[0013] Step 7: Conduct simulation experiments to establish a geometric model of the fiber microsphere, define the material as silica, set the light source to a wavelength of 1030 nm, and the diameter to the core diameter, and obtain the influence curve of the fiber microsphere structural parameters on the focal length and full width at half maximum of the photonic nanojet;

[0014] Step 8: Preset a target value Y0 of the full width at half maximum of the photonic nanojet;

[0015] Step 9: Using the change curve obtained in step 7, find the preset target value combination of the optical fiber microsphere structural parameters that achieves the target value Y0;

[0016] Step 10: Using the change curve obtained in step 6, find a combination of laser hot-melt processing parameters that can achieve the preset target combination of optical fiber microsphere structural parameters in step 9;

[0017] Step 11: According to the numerical combination of laser hot-melt processing parameters determined in step 10, prepare optical fiber microspheres and test the full width at half maximum (FWHM) of all optical fiber microsphere photonic nanojets by knife-edge scanning method. i , obtaining samples that meet the target value of the full width at half maximum of the photonic nanojet;

[0018] Step 12: Find the sample with the smallest full width at half maximum of the fiber microsphere photonic nanojet in step 11, obtain the corresponding numerical combination of laser hot-melt processing parameters, and prepare it in batches;

[0019] Step 13: The optical fiber with a microsphere at the front end prepared in step 12 is used as a direct writing processing head in subsequent processing experiments.

[0020] As a preferred embodiment: in step 1, the single-mode optical fiber adopts SMF-28 optical fiber.

[0021] Preferably, in step 5, the spot size is 5 to 10 times larger than the fiber core diameter.

[0022] As a preferred embodiment: in step 6, the CO2 laser output power is fixed at 9W, 10 different irradiation times are selected, the irradiation time range is 0.04s to 0.4s, and the interval is 0.04s. The CO2 laser performs laser hot-melt processing on 10 optical fiber end faces in sequence at the selected irradiation time in single-pulse working mode to obtain a curve of the change of irradiation time to the structural parameters of the optical fiber microsphere; then, the irradiation time is fixed at 0.4s, 6 different laser output powers are selected, the output power range is 1.5W to 9W, and the interval is 1.5W. The above steps are repeated to obtain a curve of the change of output power to the structural parameters of the optical fiber microsphere.

[0023] In addition, the present invention also provides an application of a fiber direct writing processing head based on photonic nanojet, comprising the following steps:

[0024] Step (1): Fix the fiber direct writing processing head on a six-axis micro-motion platform, and couple the femtosecond laser into the optical fiber through the objective lens; use two CCD industrial cameras to monitor the surface state of the processing substrate and the distance between the processing head and the substrate surface in real time;

[0025] Step (2): simulate the distribution of electric field enhancement generated by the optical fiber microsphere under laser irradiation to provide a theoretical basis and verify the accuracy of the subsequent experimental plan;

[0026] Step (3): changing the laser power and the distance between the processing head and the processing surface, performing a near-field groove processing test on the direct writing processing head, and obtaining the variation pattern of the substrate groove depth and width under different processing parameters;

[0027] Step (4): The incident Gaussian beam is transformed into a Bessel beam through an aconic lens, and steps (1) to (3) are repeated to obtain the influence of different light source types on the minimum processing resolution and working distance;

[0028] In step (1), the femtosecond laser uses a wavelength of 1030 nm.

[0029] The beneficial effects of the present invention are:

[0030] 1. Compared to traditional fiber microsphere preparation methods, chemical etching can minimize the effects of the cladding. By controlling the parameters of the CO2 laser, the size of the prepared fiber microspheres can be freely adjusted. When the microsphere diameter approaches the fiber core diameter, the photonic nanojet effect can be greatly enhanced.

[0031] 2. The method of the present invention is not limited to single-mode optical fibers, and multi-mode optical fibers can also be used to prepare optical fiber microspheres with similar structures.

[0032] 3. The fiber direct-write processing head fabricated by the method described in this invention utilizes the photonic nanojet effect, achieving a processing resolution approaching the optical diffraction limit. Compared to existing near-field processing technologies, this processing head is simpler and more reliable to implement and can be directly applied to near-field processing. By combining optical simulation (COMSOL) with a fiber microsphere model and simulating the electric field distribution of the fiber microsphere, the mechanism by which the microsphere shape and size influence the photonic nanojet can be studied, providing theoretical guidance for adjusting and optimizing process parameters.

[0033] 4. The present invention also studies the influence of different incident beam types on the photonic nanojet effect, proving that the photonic nanojet formed by the Bessel beam can achieve smaller resolution processing and longer working distance.

[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of preparing optical fiber microspheres by chemical etching and laser melting according to an embodiment of the present invention;

[0036] Figure 2 This is a physical picture of the optical fiber microsphere according to an embodiment of the present invention;

[0037] Figure 3 This is a schematic diagram of a Bessel beam coupling optical fiber according to an embodiment of the present invention;

[0038] Figure 4 This is the electric field distribution diagram of the Gaussian beam fiber microsphere established by COMSOL simulation in an embodiment of the present invention;

[0039] Figure 5This is the electric field distribution diagram of the Bessel beam fiber microsphere established by COMSOL simulation in an embodiment of the present invention. DETAILED DESCRIPTION

[0040] In the description of this embodiment, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," "outer," "front," and "rear" and the like are used to indicate positions or locations based on those shown in the accompanying drawings. These terms are intended solely to facilitate the description of the present invention and simplify the description, and are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation. Therefore, these terms should not be construed as limitations on the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not intended to indicate or imply relative importance.

[0041] See also Figures 1 to 5 The present invention discloses a method for preparing a fiber direct writing processing head based on photonic nanojet, comprising the following steps:

[0042] Step 1: First, use Miller pliers to remove the coating of the optical fiber, leaving the core and cladding. Then, use absorbent cotton dipped in ethanol to wipe the cladding surface. Use a fiber cleaver to cut the fiber end face flat. Then, observe the fiber end face under a confocal microscope to ensure that there are no obvious defects on the end face after cutting and the roughness Ra is less than 0.1um.

[0043] Step 2: Fix a group of optical fibers on a Z-axis translation stage and place a 1-2 cm long section of the fiber in a 20% hydrofluoric acid solution for etching. Remove one fiber every 5-10 minutes during etching and measure its diameter using a confocal microscope to determine whether the core layer has been etched. This ensures that the fiber cladding is substantially removed while the core integrity is not compromised.

[0044] Step 3: Remove the optical fiber from the hydrofluoric acid and place the corroded part in an ultrasonic cleaner. Use deionized water and anhydrous ethanol to ultrasonically clean it for 10 minutes to remove residual hydrofluoric acid.

[0045] Step 4: Use a rotating fiber clamp to fix the optical fiber on the six-axis micro-motion platform, with the CO2 laser's laser vertically facing downward. Keep the CCD industrial camera aligned with the fiber end face position to monitor the status of the fiber end face during processing in real time.

[0046] Step 5: Conduct a CO2 laser hot-melt processing experiment on the fiber end face, burn out a fiber microsphere on the etched fiber end face, and fix the laser spot size, which is required to be larger than the fiber core diameter;

[0047] Step 6: change the hot melting output power and irradiation time processing parameters of the CO2 laser; by fixing one laser hot melting processing parameter, changing the other parameter, the change curve of each processing parameter on the structure parameters of the fiber microsphere is obtained; the structure parameters of the fiber microsphere are measured by a laser scanning confocal microscope at 50-100 times magnification;

[0048] Step 7: carry out simulation experiment, establish a geometric model of the fiber microsphere, define the material as silica, set the light source as 1030nm wavelength, and the diameter as the core diameter, and obtain the influence curve of the structure parameters of the fiber microsphere on the focal length and full width at half maximum of the photonic nanobubble jet;

[0049] Step 8: preset a target value Y0 of the full width at half maximum of the photonic nanobubble jet;

[0050] Step 9: find the preset target value combination of the structure parameters of the fiber microsphere to achieve the target value Y0 through the change curve obtained in step 7;

[0051] Step 10: find the laser hot melting processing parameter value combination that can achieve the preset target value combination of the structure parameters of the fiber microsphere in step 9 through the change curve obtained in step 6;

[0052] Step 11: according to the laser hot melting processing parameter value combination determined in step 10, prepare the fiber microsphere, and test the full width at half maximum Y of the photonic nanobubble jet of all the fiber microspheres by knife-edge scanning method, and obtain the sample meeting the target value of the full width at half maximum of the photonic nanobubble jet; i

[0053] Step 12: find the sample with the smallest full width at half maximum of the photonic nanobubble jet in step 11, obtain the corresponding laser hot melting processing parameter value combination, and prepare in batches;

[0054] Step 13: use the fiber with microsphere prepared in step 12 as a direct writing processing head in subsequent processing experiments.

[0055] As preferred, in step 1, the single-mode optical fiber is SMF-28 optical fiber.

[0056] As preferred, in step 5, the spot size is greater than 5-10 times of the core diameter.

[0057] ​Preferably, in step 6, the CO2 laser output power is fixed at 9W, 10 different irradiation times are selected, the irradiation time range is 0.04s to 0.4s, and the interval is 0.04s. The CO2 laser performs laser hot-melt processing on 10 optical fiber end faces in sequence at the selected irradiation time in single-pulse working mode to obtain a curve of the change of irradiation time to the structural parameters of the optical fiber microsphere; then, the irradiation time is fixed at 0.4s, 6 different laser output powers are selected, the output power range is 1.5W to 9W, and the interval is 1.5W, and the above steps are repeated to obtain a curve of the change of output power to the structural parameters of the optical fiber microsphere.

[0058] In practical application, a method for preparing a fiber direct writing processing head based on photonic nanojet includes the following steps:

[0059] 1. Use Miller pliers to remove the protective layer and coating of the single-mode optical fiber, leaving only the core and cladding. Then, use absorbent cotton dipped in ethanol to wipe the cladding surface and use a fiber cleaver to cut the fiber end face flat. Observe the fiber end face under a confocal microscope to ensure that there are no obvious defects on the end face after cutting and that the roughness Ra is less than 0.1μm.

[0060] 2. A group of optical fibers was mounted on a Z-axis stage. A 1-2 cm long section of fiber was placed in a 20% hydrofluoric acid solution for etching. Considering that temperature fluctuations and the consumption of the etching solution affect the etching rate, one of the fibers was used as a monitoring device. During the etching process, the fiber was removed and its diameter measured using a confocal microscope to determine if the core layer had been etched. After an average etching time of approximately 190 minutes at room temperature, the fiber diameter was reduced to 9 μm. The fiber cladding was substantially removed while the core integrity was intact.

[0061] 3. Remove the optical fiber from the hydrofluoric acid, place the corroded part in an ultrasonic cleaning machine, and use deionized water and anhydrous ethanol to ultrasonically clean it at 20°C for 10 minutes to remove residual hydrofluoric acid.

[0062] 4. Use a rotating fiber optic clamp to fix the optical fiber on the six-axis micro-motion platform. The optical fiber is perpendicular to the horizontal plane and upward, and the CO2 laser is perpendicular to the horizontal plane and downward. The CCD industrial camera is always aligned with the position of the optical fiber end face to monitor the status of the optical fiber end face during processing in real time.

[0063] 5. Carry out CO2 laser hot-melt processing experiments on the end face of optical fiber, burn out optical fiber microspheres on the corroded end face of optical fiber, and fix the laser spot size, which is about 10 times the diameter of the fiber core.

[0064] 6. By changing the CO2 laser heat-melting processing parameters (output power and irradiation time), fiber microspheres were burned onto the etched fiber end face. The CO2 laser output power was fixed at 9W, and 10 different irradiation times (irradiation time ranged from 0.04s to 0.4s, with an interval of 0.04s) were selected. The CO2 laser was used in single-pulse mode to perform laser heat-melting on 10 fiber end faces in sequence with the selected irradiation time. The curve of the change of irradiation time on the structural parameters of the fiber microsphere (R, eccentricity e) was obtained. Then, the irradiation time was fixed at 0.4s, and 6 different laser output powers (output power ranged from 1.5W to 9W, with an interval of 1.5W) were selected. The above steps were repeated to obtain the curve of the change of output power on the structural parameters of the fiber microsphere (R, eccentricity e). The structural parameters of the fiber microsphere were measured by laser scanning confocal microscopy (LSCM, OSL4100) at 50-100 times magnification.

[0065] 7. Use COMSOL to establish a geometric model of fiber optic microspheres, define the material as silica, set the light source to a wavelength of 1030nm, a diameter of 9um, and a microsphere curvature radius of 5um to 20um (with an interval of 1um). Run the software and obtain the control law of the fiber optic microsphere structural parameters (R, eccentricity e) on the focal length and full width at half maximum (FWHM) of the photonic nanojet, that is, the change curve of any fiber optic microsphere structural parameter on the full width at half maximum (FWHM) of its photonic nanojet.

[0066] 8. Preset a target value Y0 of the half-width at half maximum of a photonic nanojet to 2 μm.

[0067] 9. Through the change curve obtained in step 7, find the preset target value combination of the optical fiber microsphere structural parameters that can achieve the target value Y0.

[0068] 10. Through the change curve obtained in step 6, find the laser hot melt processing parameter value combination that can achieve the preset target value combination of the fiber microsphere structure parameters in step 9.

[0069] 11. According to the numerical combination of laser hot melt processing parameters determined in step 10, prepare fiber microspheres and test the full width at half maximum Y of all fiber microsphere photonic nanojets by knife-edge scanning method. i , and obtain the target value of the full width at half maximum of the photon nanojet (Y i ≤2um) samples.

[0070] 12. See Figure 2 , find the sample with the smallest full width at half maximum of the fiber microsphere photonic nanojet in step 11, obtain the corresponding numerical combination of laser hot-melt processing parameters, and prepare the ideal fiber microsphere;

[0071] In addition, this embodiment also provides a fiber direct writing processing head based on photonic nanojets, which is prepared using the preparation method of the fiber direct writing processing head based on photonic nanojets described in the above technical solution.

[0072] An application of a fiber direct writing processing head based on photonic nanojet includes the following steps:

[0073] Step (1): Fix the fiber direct writing processing head on a six-axis micro-motion platform, and couple the femtosecond laser into the optical fiber through the objective lens; use two CCD industrial cameras to monitor the surface state of the processing substrate and the distance between the processing head and the substrate surface in real time;

[0074] Step (2): simulate the distribution of electric field enhancement generated by the optical fiber microsphere under laser irradiation to provide a theoretical basis and verify the accuracy of the subsequent experimental plan;

[0075] Step (3): changing the laser power and the distance between the processing head and the processing surface, performing a near-field groove processing test on the direct writing processing head, and obtaining the variation pattern of the substrate groove depth and width under different processing parameters;

[0076] Step (4): The incident Gaussian beam is transformed into a Bessel beam through an aconic lens, and steps (1) to (3) are repeated to obtain the influence of different light source types on the minimum processing resolution and working distance;

[0077] See also Figure 3 , a fiber direct writing processing experimental device was built. The fiber direct writing processing experimental device consists of a femtosecond laser, a lens group, a CCD industrial camera, an illumination light source, a six-axis micro-motion platform, a stage, and a cover glass / sample holder. The fiber direct writing processing head is fixed on a rotating fiber clamp, and the fiber clamp is installed on the six-axis micro-motion platform. The 1030nm femtosecond laser is coupled into the single-mode optical fiber through a conical lens and adjusted in real time using an optical power meter. Two CCD industrial cameras are respectively installed on the top and side of the six-axis micro-motion platform to monitor the surface status of the processed substrate and the distance between the processing head and the substrate surface in real time. The Gaussian beam is converted into a Bessel beam through a conical lens, and a photonic nanojet is realized through the fiber microsphere.

[0078] See also Figure 4 Using COMSOL simulation software, a fiber microsphere model was established. The microsphere had a diameter of 10 μm and was made of silica. The light source was set to a 1030 nm Gaussian beam with a beam waist diameter equal to the single-mode fiber mode field diameter. The software was then run to obtain the electric field distribution of the fiber microsphere, the focal length of the photonic nanojet, and the full width at half maximum (FWHM) at the focal point. The focal length of the photonic nanojet was 6.35 μm, and the FWHM at the focal point was 1.5 μm. This provides theoretical guidance for process parameters in actual processing.

[0079] When studying the processing characteristics of the optical fiber direct writing processing head, the distance between the processing head and the substrate surface can be changed through the six-axis micro-motion platform, and the laser power can be changed to perform near-field groove processing tests on the direct writing processing head to obtain the changing patterns of substrate groove depth and width under different processing parameters.

[0080] See also Figure 5 A fiber microsphere model was constructed using COMSOL simulation software. All other conditions were the same, with a 1030nm wavelength, zero-order Bessel beam as the light source, and a main lobe larger than the fiber core diameter. The resulting photonic nanojet had a focal length of 7.52µm and a full width at half maximum (FWHM) of 1.44µm at the focal point. Compared to Gaussian beams, Bessel beams have a longer focal length and offer advantages in achieving higher processing resolution.

[0081] This invention uses hydrofluoric acid etching combined with CO2 laser melting to produce optical fiber microspheres. The curvature radius of the optical fiber microspheres produced using the technical solution provided by this invention is close to the core radius, enabling beam focusing and enhancing the electric field intensity to several times that of the incident light field, thus enabling photonic nanofluidic micro-nanofabrication. Furthermore, the invention features low material cost, a simple structure, and a simple manufacturing process, enabling mass production.

[0082] The specific description of the present invention in the above embodiments is only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Technical engineers in this field may make some non-essential improvements and adjustments to the present invention based on the contents of the above invention, which fall within the scope of protection of the present invention.

Claims

1. A method for preparing a fiber direct writing processing head based on photonic nanojet, characterized by: The steps include: Step 1: First, use Miller pliers to remove the coating of the optical fiber, leaving the core and cladding. Then, use absorbent cotton dipped in ethanol to wipe the cladding surface. Use a fiber cleaver to cut the fiber end face flat. Then, observe the fiber end face under a confocal microscope to ensure that there are no obvious defects on the end face after cutting and the roughness Ra is less than 0.1um. Step 2: Fix a group of optical fibers on a Z-axis translation stage and immerse a 1-2 cm long section of the fiber in a 20% hydrofluoric acid solution for etching. Remove one fiber every 5-10 minutes during etching and measure its diameter using a confocal microscope to determine whether the core layer has been etched. This ensures that the fiber cladding is substantially removed while the core integrity is intact. Step 3: Remove the optical fiber from the hydrofluoric acid and place the corroded part in an ultrasonic cleaner. Use deionized water and anhydrous ethanol to ultrasonically clean it for 10 minutes to remove residual hydrofluoric acid. Step 4: Use the rotating fiber clamp to fix the optical fiber on the six-axis micro-motion platform. CO2 The laser's vertical horizontal plane is downward, so that the CCD industrial camera is always aligned with the position of the optical fiber end face, which is used to monitor the status of the optical fiber end face during processing in real time; Step 5: Conduct fiber end-face CO2 In the laser hot-melt processing experiment, optical fiber microspheres are burned on the end face of the etched optical fiber, and the laser spot size is fixed, which is required to be larger than the fiber core diameter; Step 6: Change CO2 The laser heat-melting output power and irradiation time processing parameters were used. By fixing one laser heat-melting processing parameter and changing another, a curve showing the effect of each processing parameter on the fiber microsphere structural parameters was obtained. The fiber microsphere structural parameters were measured using a laser scanning confocal microscope at 50-100 times magnification. Step 7: Conduct simulation experiments to establish a geometric model of the fiber microsphere, define the material as silica, set the light source to a wavelength of 1030 nm, and the diameter to the core diameter, and obtain the influence curve of the fiber microsphere structural parameters on the focal length and full width at half maximum of the photonic nanojet; Step 8: Preset a target value for the full width at half maximum of the photonic nanojet Y0 ; Step 9: Find the target value by the change curve obtained in step 7 Y0 Preset target value combination of optical fiber microsphere structural parameters; Step 10: Using the change curve obtained in step 6, find a combination of laser hot-melt processing parameters that can achieve the preset target combination of optical fiber microsphere structural parameters in step 9; Step 11: Prepare fiber microspheres according to the laser hot melt processing parameter combination determined in step 10, and test the full width at half maximum of all fiber microsphere photonic nanojets by knife-edge scanning method. Yi , obtaining samples that meet the target value of the full width at half maximum of the photonic nanojet; Step 12: Find the sample with the smallest full width at half maximum of the fiber microsphere photonic nanojet in step 11, obtain the corresponding numerical combination of laser hot-melt processing parameters, and prepare it in batches; Step 13: The optical fiber with a microsphere at the front end prepared in step 12 is used as a direct writing processing head in subsequent processing experiments.

2. The method for preparing a fiber direct writing processing head based on photonic nanojet according to claim 1, characterized in that: In step 1, the single-mode optical fiber uses SMF-28 optical fiber.

3. The method for preparing a fiber direct writing processing head based on photonic nanojet according to claim 1, characterized in that: In step 5, the spot size is 5 to 10 times larger than the fiber core diameter.

4. The method for preparing a fiber direct writing processing head based on photonic nanojet according to claim 1, characterized in that: In step 6, fix CO2 The laser output power is 9W, and 10 different irradiation times are selected, with the irradiation time range from 0.04s to 0.4s and the interval is 0.04s. CO2 The laser was used in single-pulse operating mode to perform laser heat-melting on 10 optical fiber end faces in sequence with a selected irradiation time, obtaining a curve showing how the structural parameters of the optical fiber microspheres change with irradiation time. Then, the irradiation time was fixed at 0.4s, and six different laser output powers were selected, ranging from 1.5W to 9W with an interval of 1.5W. These steps were repeated to obtain a curve showing how the output power changes with the structural parameters of the optical fiber microspheres.

5. An application of a fiber direct writing processing head based on photonic nanojet, characterized by: The steps include: Step (1): Fix the fiber direct writing processing head on the six-axis micro-motion platform, and couple the femtosecond laser into the optical fiber through the objective lens; use two CCD industrial cameras to monitor the surface state of the processing substrate and the distance between the processing head and the substrate surface in real time; Step (2): simulate the distribution of electric field enhancement generated by the optical fiber microsphere under laser irradiation to provide a theoretical basis and verify the accuracy of the subsequent experimental plan; Step (3): changing the laser power and the distance between the processing head and the processing surface, performing a near-field groove processing test on the direct writing processing head, and obtaining the variation pattern of the substrate groove depth and width under different processing parameters; Step (4): Convert the incident Gaussian beam into a Bessel beam through a conical lens, repeat steps (1) to (3) to obtain the influence of different light source types on the minimum processing resolution and working distance.

6. The application of the optical fiber direct writing processing head based on photonic nanojet according to claim 5, characterized in that: In step (1), the femtosecond laser uses a wavelength of 1030 nm.

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