阅读说明：本技术 一种宽带宽、长工作距离的光子晶体光纤微透镜及其制备方法 (Photonic crystal fiber micro lens with wide bandwidth and long working distance and preparation method thereof ) 是由 肖力敏 陈雨星 于 2021-01-10 设计创作，主要内容包括：本发明属于光纤通信技术领域,具体为一种宽带宽、长工作距离的光子晶体光纤微透镜及其制备方法。本发明方法包括：精确对准两根光子晶体光纤并调整间距；通过石墨加热的方式对光纤进行加热处理,制得不同结构的光纤微透镜；测量各组光纤微透镜的端面光斑质量,确定结构参数阈值；测量光纤微透镜的插入损耗随着两根光纤透镜纵向以及横向偏移距离的变化,确定工作距离与光纤微透镜结构参数的关系；根据测量的结构参数阈值调节加热参数,制得工作距离最长的光纤微透镜；测量光纤微透镜的插入损耗随着波长的变化趋势。本发明可以降低光纤微透镜的材料成本以及制备精度要求,有效提高光子晶体光纤微透镜的工作距离并扩充光纤微透镜的使用波长范围。(The invention belongs to the technical field of optical fiber communication, and particularly relates to a photonic crystal optical fiber micro-lens with wide bandwidth and long working distance and a preparation method thereof. The method comprises the following steps: accurately aligning two photonic crystal fibers and adjusting the distance; heating the optical fiber in a graphite heating mode to prepare optical fiber micro lenses with different structures; measuring the end surface light spot quality of each group of optical fiber micro lenses, and determining a structural parameter threshold; measuring the insertion loss of the optical fiber micro lens along with the change of the longitudinal and transverse offset distances of the two optical fiber lenses, and determining the relation between the working distance and the structural parameters of the optical fiber micro lens; adjusting heating parameters according to the measured structural parameter threshold value to obtain the optical fiber micro lens with the longest working distance; the variation trend of the insertion loss of the fiber microlens with the wavelength was measured. The invention can reduce the material cost and the preparation precision requirement of the optical fiber micro lens, effectively improve the working distance of the photonic crystal optical fiber micro lens and expand the use wavelength range of the optical fiber micro lens.)

1. A preparation method of a photonic crystal fiber micro-lens with wide bandwidth and long working distance is characterized by comprising the following specific steps:

step 1: selecting two photonic crystal fibers to be processed, stripping a coating layer of the photonic crystal fibers, cleaning the optical fibers, accurately aligning the two optical fibers, and measuring the variation trend of the insertion loss of the optical fibers along with the alignment distance under a single wavelength;

step 2: heating the tail end of the optical fiber by the graphite electrode, changing different heating time lengths and heating powers, preparing a plurality of groups of optical fiber micro lenses by adopting a single heating mode or a multi-step heating mode, and determining the influence of the adjustment of the heating parameters of the graphite electrode on the structural parameters of the optical fiber micro lenses;

and step 3: preparing a plurality of groups of optical fiber micro lenses with different structural parameters, measuring the quality of light spots on the emergent end surface, and determining the structural parameter threshold of the optical fiber micro lenses;

and 4, step 4: accurately aligning the prepared photonic crystal fiber micro-lens again, measuring the variation trend of the insertion loss of the fiber micro-lens along with the longitudinal and transverse offset distances of the fiber micro-lens, and determining the variation relation between the working distance and the structural parameters;

and 5: adjusting graphite heating parameters according to the measured structural parameter threshold of the optical fiber micro lens, and preparing the optical fiber micro lens with the longest working distance within the structural parameter threshold range in a graphite step-by-step heating mode;

step 6: and introducing a continuous white light source, and measuring the variation trend of the insertion loss of the optical fiber micro lens along with the wavelength in a wide wavelength range.

2. The method according to claim 1, wherein the step 2 of preparing the plurality of groups of fiber microlenses comprises:

selecting a plurality of photonic crystal fibers, stripping coating layers of the optical fibers, and cleaning the optical fibers;

adjusting different heating parameters to heat the graphite electrode at the tail end of the photonic crystal fiber, comparing the influence of different heating parameters on the arc-shaped structural parameters at the tail end of the photonic crystal, and corresponding the structural parameters of the optical fiber micro lens with the graphite heating parameters.

3. The method of claim 1, wherein the plurality of step heats comprises:

the air hole at the tail end of the photonic crystal fiber is collapsed to the target length in a low-power and long-time heating mode, and the curvature radius of the arc at the tail end is basically unchanged in the collapsing process;

and adjusting the position of the optical fiber, only heating the tail end of the optical fiber, and melting the tail end of the photonic crystal optical fiber to a target curvature radius in a high-power short-time heating mode.

4. The method for preparing the optical waveguide fiber laser device according to claim 1, wherein the method for measuring the spot quality of the emergent end face in the step 3 comprises the following steps:

observing the cross section of the optical fiber through the infrared CCD without accessing a light source, and focusing the CCD on the end surface of the optical fiber micro lens;

the positions of the optical fiber micro lens and the CCD are unchanged, a light source is accessed, the light spot intensity obtained by CCD observation is observed and measured, the power of the light source is adjusted until the maximum light spot intensity just reaches the vicinity of the saturation intensity of the CCD measurement, and the intensity distribution of the whole light spot is measured;

and measuring the quality of the light spots on the emergent end surfaces of the multiple groups of optical fiber micro lenses by using the method for measuring the quality of the light spots on the emergent end surfaces and comparing the light spots.

5. The method of claim 4, wherein the step of focusing the CCD onto the end face of the fiber microlens comprises:

connecting the infrared CCD with a microscope, and vertically placing the fiber micro lens under an objective lens of the microscope;

illuminating the end position of the optical fiber by using a continuous light source, observing a focused image on a computer, and adjusting the positions of the objective lens and the end surface of the optical fiber to fix the focused image on the end surface of the optical fiber micro lens.

6. The method of claim 1, wherein the step 6 of measuring the trend of the insertion loss of the fiber microlens along with the wavelength in the wide bandwidth wavelength range comprises:

welding a fiber microlens with one end of a common single-mode fiber, and connecting a spectrometer to the other end of the single-mode fiber; welding the other fiber microlens with one end of a common single-mode fiber, and connecting the white light continuous light source to the other end of the single-mode fiber; and accurately aligning and fixing the two optical fiber micro lenses at the position with the minimum insertion loss, and observing the curve of the spectrometer after the white light wide light source is input.

7. The method of claim 1, wherein the process of precisely aligning two optical fibers comprises:

welding a photonic crystal fiber with one end of a single-mode fiber, and connecting a power meter to the other end of the single-mode fiber;

welding the other photonic crystal fiber with one end of a common single-mode fiber, and connecting a laser to the other end of the single-mode fiber;

and the positions of the two photonic crystal fibers are transversely and two-dimensionally adjusted, the readings of the power meter in the moving process are observed, and the two fibers are fixed at the position with the minimum loss.

8. The method according to claim 1 or 2, wherein the structural parameters of the fiber microlens include:

the length of collapse of the air hole at the tail end of the photonic crystal fiber, the size of the curvature radius of an arc formed by thermally melting the tail end of the fiber and the bending degree of the fiber.

9. The method according to claim 6 or 7, wherein the average fusion loss between the photonic crystal fiber and a common single mode fiber is less than 0.5 dB.

10. A wide bandwidth, long working distance photonic crystal fiber microlens prepared by the method of any one of claims 1 to 9 wherein the air holes at one end of the photonic crystal fiber are completely collapsed to form a coreless region of uniform refractive index, a graded-index region exists between the uncollapsed region and the completely collapsed region of the air holes, and the end facet of the coreless region of the fiber has an arcuate lens structure.

Technical Field

The invention belongs to the technical field of optical fiber communication, and particularly relates to an all-fiber micro lens and a preparation method thereof.

Background

With the continuous development of optical communication and optical fiber sensing technologies, the demand for advanced optical coupling and focusing technologies is increasing. The fiber lens, also called a fiber microlens or a lensed fiber, is fabricated in the shape of a lens by processing the end face of the fiber. The optical fiber laser can expand, collimate or focus light beams from the optical fiber, thereby playing a role in optical path change or mode conversion. The optical fiber microlens is not only widely used in various optical experiments to realize various functions such as free space optical interconnection, beam collimation, optical sensing and optical microfluidics, but also is a very large number of components required in an optical communication system in actual production and use. Meanwhile, the optical fiber microlens is also an important component of various optical elements such as an optical switch, an optical isolator, an optical attenuator, and a wavelength division multiplexer. Therefore, the research on the working performance of the optical fiber microlens is urgent.

The traditional optical fiber micro lens technology mainly comprises two aspects, namely selection of the lens and assembly mode. The choice of fiber optic microlenses is currently largely divided by the shape of their lens end faces, including beveled, wedge, spherical, conical, curved, and the like. The shape of the lens is selected depending on the application of the optical fiber micro lens, the inclined plane-shaped optical fiber lens can enable the light path to generate wide-angle total emission and can also increase the light receiving area of light, and the optical fiber lens is mainly applied to the fields of optical fiber laser, optical fiber communication and the like; the wedge-shaped fiber lens is mostly used for realizing the coupling of light beams and increasing the quality of the light beams which are coupled into the optical fiber by the LD; the conical fiber lens can enlarge the numerical aperture of the optical fiber, and is therefore often used for coupling of DFB, SLD laser, etc., whose output beam cross section is circular. Although these kinds of lasers have some advantages in some fields, the processing of the optical fiber microlenses with these structures often involves complicated and precise manufacturing processes including fusion splicing, precision micro-cutting, mechanical polishing, fusion and thermal forming, and the material cost is expensive, so that the development of optical devices toward low cost and high performance cannot be satisfied. In comparison, in the fiber lenses of various structures at present, the arc-shaped fiber microlens is a fiber lens with the widest application range, can be applied to various fields such as optical coupling, biomedicine, optical sensing and the like, and can replace fiber lenses with other shapes and structures to a certain extent for use. Meanwhile, compared with other optical fiber micro lenses, the arc micro lens has the advantages of low cost, low loss, long working distance and the like, the focal length can be adjusted conveniently by adjusting the arc structure of the lens to meet different working requirements of the optical fiber micro lens, and the arc micro lens has tunability, so that the arc micro lens is widely used in the field of optical communication at present. In the assembly of the optical fiber micro lens, at present, there are two main assembly methods, one is to have a tiny air gap between the lens and the optical fiber, and two adjacent surfaces need to be coated with antireflection films to increase the transmission of light beams, and the other is to directly weld the lens and the optical fiber, which is relatively simpler in manufacturing method and relatively smaller in insertion loss.

The all-fiber arc microlens is one of arc fiber microlenses because of its small size and the superiority of beam quality, which has attracted much attention in recent years. The traditional method for manufacturing the all-fiber micro lens is to weld an optical fiber with a section of coreless quartz optical fiber with a precise length, and then form an arc lens at the tail end of the coreless quartz optical fiber in forms of arc discharge and the like. The light output from the fiber will be expanded in the region of the coreless silica fiber and then collimated or focused by the end curved lens. The manufacturing method has advantages in that various kinds of coreless silica fibers of different diameters can be fusion-spliced, different working performances can be achieved, and a relatively long working distance can be achieved. Kim et al achieve an all-fiber microlens with a working distance of approximately 6 mm by fusion splicing a single mode fiber to a large aperture coreless silica fiber. Although the all-fiber microlens has many superior performances compared with the conventional fiber microlens, the manufacturing process thereof has certain defects, a precise cutting technology is required to obtain a section of coreless silica fiber with a precise length, and precise melting parameters are also required to ensure that the coreless silica fiber can be accurately spliced on a target fiber, so the precision requirement on the process is high.

The defects of the coreless quartz fiber for manufacturing the fiber micro-lens can be completely overcome by using a Photonic Crystal Fiber (PCF). Compared with single-mode optical fiber, the photonic crystal fiber is a novel optical microstructure material with the dielectric constant periodically changing along with space. Due to the structural particularity, the photonic crystal fiber has the optical characteristics that the single-mode transmission of light can be realized in a large frequency range, the dispersion and the dispersion slope can be flexibly adjusted, and broadband dispersion compensation and the like are provided. Meanwhile, due to the special structural characteristics of the periodic arrangement of the air holes, the air holes at the tail end of the photonic crystal fiber can be collapsed and form a drop-shaped arc through arc discharge or other heating modes to form a lens structure, and parallel Gaussian beams are output, so that an all-fiber system is formed to realize the effect of beam focusing, and the integration of an optical system is facilitated. There is also considerable progress in the current research on photonic crystal all-fiber microlenses, and g.j. Kong collapsed a large mode field photonic crystal fiber (LMA-10) by means of arc discharge heating to obtain a fiber microlens with a working distance of about 1 mm. In addition, h.y. Choi et al reported a lensed PCF probe with side focusing achieved by side polishing with a femtosecond laser; G. mudhana et al propose a fiber optic probe for liquid refractive index measurement based on a dual function lens PCF. However, for the photonic crystal fiber with a small diameter, due to the difficulty in optimizing the parameters of the collapse region of the air holes of the fiber and the limitation of the preparation precision of the fiber micro-lens, the working characteristic of a long working distance is not realized, and the wide bandwidth characteristic of the photonic crystal fiber is not fully utilized.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a photonic crystal fiber micro-lens with wide bandwidth and long working distance and a preparation method thereof.

The preparation method of the wide-bandwidth long-working-distance photonic crystal fiber micro-lens provided by the invention is realized by heating the graphite electrode, namely, the graphite electrode is used for heating the tail end of the photonic crystal fiber, so that the periodically arranged air holes at the tail end are collapsed and a water-drop-shaped lens arc structure is formed on the end surface; the beneficial effects can be evaluated from at least the following three aspects: firstly, the graphite electrode has a larger heating area, so that the optical fiber is heated more uniformly, and the excessive bending deformation of the optical fiber is avoided; secondly, the graphite electrode heating process can be monitored in real time through the CCD and parameters can be adjusted in time, so that the accuracy of the collapse area and length of the air hole can be adjusted, and the more accurate collapse range can be realized; and thirdly, the heating time and power can be conveniently adjusted, and the manufacturing of a plurality of groups of optical fiber micro lenses with different structural parameters is facilitated and compared.

The invention provides a preparation method of a photonic crystal fiber micro-lens with wide bandwidth and long working distance, which comprises the following steps:

step 1: selecting two photonic crystal fibers to be processed, stripping a coating layer of the photonic crystal fibers, cleaning the optical fibers, accurately aligning the two optical fibers, and measuring the variation trend of the insertion loss of the optical fibers along with the alignment distance under a single wavelength;

step 2: heating the tail end of the optical fiber by the graphite electrode, changing different heating time lengths and heating powers, preparing a plurality of groups of optical fiber micro lenses by adopting a single heating mode or a multi-step heating mode, and determining the influence of the adjustment of the heating parameters of the graphite electrode on the structural parameters of the optical fiber micro lenses;

and step 3: preparing a plurality of groups of optical fiber micro lenses with different structural parameters, measuring the quality of light spots on the emergent end surface, and determining the structural parameter threshold of the optical fiber micro lenses;

and 4, step 4: accurately aligning the prepared photonic crystal fiber micro-lens again, measuring the variation trend of the insertion loss of the fiber micro-lens along with the longitudinal and transverse offset distances of the fiber micro-lens, and determining the variation relation between the working distance and the structural parameters;

and 5: adjusting graphite heating parameters according to the measured structural parameter threshold of the optical fiber micro lens, and preparing the optical fiber micro lens with the longest working distance within the structural parameter threshold range in a graphite step-by-step heating mode;

step 6: and introducing a continuous white light source, and measuring the variation trend of the insertion loss of the optical fiber micro lens along with the wavelength in a wide wavelength range.

Wherein, the step 2 of preparing the plurality of groups of optical fiber micro lenses comprises the following steps:

selecting a plurality of photonic crystal fibers, stripping coating layers of the optical fibers, and cleaning the optical fibers;

adjusting different heating parameters to heat the graphite electrode at the tail end of the photonic crystal fiber, comparing the influence of different heating parameters on the arc-shaped structural parameters at the tail end of the photonic crystal, and corresponding the structural parameters of the optical fiber micro lens with the graphite heating parameters.

Wherein the multiple step heating in step 2 or 5 comprises:

the air hole at the tail end of the photonic crystal fiber is collapsed to the target length in a low-power and long-time heating mode, and the curvature radius of the arc at the tail end is basically unchanged in the collapsing process;

and adjusting the position of the optical fiber, only heating the tail end of the optical fiber, and melting the tail end of the photonic crystal optical fiber to a target curvature radius in a high-power short-time heating mode.

The method for measuring the quality of the light spot on the emergent end face in the step 3 comprises the following steps:

observing the cross section of the optical fiber through the infrared CCD without accessing a light source, and focusing the CCD on the end surface of the optical fiber micro lens;

the positions of the optical fiber micro lens and the CCD are unchanged, a light source is accessed, the light spot intensity obtained by CCD observation is observed and measured, the power of the light source is adjusted until the maximum light spot intensity just reaches the vicinity of the saturation intensity of the CCD measurement, and the intensity distribution of the whole light spot is measured;

and measuring the quality of the light spots on the emergent end surfaces of the multiple groups of optical fiber micro lenses by using the method for measuring the quality of the light spots on the emergent end surfaces and comparing the light spots.

The method for focusing the CCD on the end face of the fiber micro lens in the step 4 comprises the following steps:

connecting the infrared CCD with a microscope, and vertically placing the fiber micro lens under an objective lens of the microscope;

illuminating the end position of the optical fiber by using a continuous light source, observing a focused image on a computer, and adjusting the positions of the objective lens and the end surface of the optical fiber to fix the focused image on the end surface of the optical fiber micro lens.

Wherein, the method for measuring the variation trend of the insertion loss of the optical fiber micro lens along with the wavelength in the wide bandwidth wavelength range in the step 6 comprises the following steps:

welding a fiber microlens with one end of a common single-mode fiber, and connecting a spectrometer to the other end of the single-mode fiber;

welding the other fiber microlens with one end of a common single-mode fiber, and connecting the white light continuous light source to the other end of the single-mode fiber;

and accurately aligning and fixing the two optical fiber micro lenses at the position with the minimum insertion loss, and observing the curve of the spectrometer after the white light wide light source is input.

Wherein, the process of precisely aligning the two optical fibers in step 1 comprises:

welding a photonic crystal fiber with one end of a common single-mode fiber, and connecting a power meter to the other end of the single-mode fiber;

welding the other photonic crystal fiber with one end of a common single-mode fiber, and connecting a laser to the other end of the single-mode fiber;

and the positions of the two photonic crystal fibers are transversely and two-dimensionally adjusted, the readings of the power meter in the moving process are observed, and the two fibers are fixed at the position with the minimum loss.

The structural parameters of the fiber microlens in the step 1 or 2 include:

the length of collapse of the air hole at the tail end of the photonic crystal fiber, the size of the curvature radius of an arc formed by thermally melting the tail end of the fiber and the bending degree of the fiber.

The air holes at one end of the photonic crystal fiber micro-lens prepared by the invention are completely collapsed to form a section of coreless area with uniform refractive index, a section of refractive index gradual change area exists between the uncollapsed area and the completely collapsed area of the air holes, and the end face of the coreless area of the fiber is provided with an arc-shaped lens structure.

The wide-bandwidth long-working-distance photonic crystal all-fiber micro-lens provided by the invention has the advantages that the range of transverse deviation within the tolerance range of 1dB loss reaches +/-20 mu m, the alignment precision requirement in actual use can be reduced, and the optical coupling efficiency of a free space is effectively improved; the maximum alignment distance which can be realized by longitudinally offsetting 1dB loss within the tolerance range of 1dB loss is as long as 4.84mm, so that the working distance and the working range of the photonic crystal fiber all-fiber micro-lens are greatly improved; the fiber microlens achieves an ultra-wide operating bandwidth in excess of 770 nm, within a tolerance of 1dB loss, which is nearly impossible to achieve with conventional fiber lenses.

In addition, the invention is an all-fiber fusion system, which is simple and high in integration level, and can accelerate more practical applications of the fiber micro lens in the fiber communication system.

Drawings

FIG. 1 is a schematic diagram of a photonic crystal fiber microlens with a long working distance and a wide bandwidth according to an embodiment of the present invention.

Fig. 2 is a flowchart of a method for manufacturing a long-working-distance and wide-bandwidth photonic crystal fiber microlens according to an embodiment of the present invention.

Fig. 3 is a schematic diagram of step-by-step and multiple graphite electrode heating in the method for manufacturing a long-working-distance and wide-bandwidth photonic crystal fiber microlens according to an embodiment of the present invention.

FIG. 4 is a side view of a long working distance, wide bandwidth photonic crystal fiber microlens made using different graphite electrode heating parameters according to an embodiment of the present invention.

Fig. 5 is a light spot diagram of an exit end face with different air hole collapse lengths manufactured in a long-working-distance and wide-bandwidth photonic crystal fiber microlens according to an embodiment of the present invention.

FIG. 6 is a side view of a long working distance, wide bandwidth photonic crystal fiber microlens fabricated using step and multiple graphite electrode heating in accordance with an embodiment of the present invention.

Reference numbers in the figures: 1-lens arc area, 2-photonic crystal fiber air hole collapse complete area, 3-photonic crystal fiber air hole collapse complete section, 4-photonic crystal fiber air hole collapse incomplete area, 5-photonic crystal fiber air hole collapse starting section, 6-photonic crystal fiber uncollapsed area, 7-photonic crystal fiber input end face, and 8-graphite heating electrode.

Detailed Description

To more clearly illustrate the objects and advantages of the present invention, the present invention is further described in detail below with reference to the accompanying drawings. It is to be understood that the following examples are illustrative of the present invention and are not intended to limit the scope of the present invention. The technical features of the embodiments of the present invention may be combined with each other as long as they do not conflict with each other.

Fig. 1 is a schematic diagram of a photonic crystal fiber microlens provided by an embodiment of the present invention, including: the photonic crystal fiber air hole collapse detection method comprises a lens arc area 1, a photonic crystal fiber air hole collapse area 2, a photonic crystal fiber air hole collapse completion cross section 3, a photonic crystal fiber air hole collapse incomplete area 4, a photonic crystal fiber air hole collapse starting cross section 5, a photonic crystal fiber uncollapsed area 6 and a photonic crystal fiber input end face 7. The specific preparation method can be carried out according to the following steps in the flow chart shown in fig. 2:

step 1: selecting two photonic crystal fibers to be processed, stripping a coating layer of the photonic crystal fibers, cleaning the optical fibers, accurately aligning the two optical fibers, and measuring the variation trend of the insertion loss of the optical fibers along with the alignment distance under a single wavelength;

step 2: heating the tail end of the optical fiber by the graphite electrode, changing different heating time lengths and heating powers, preparing a plurality of groups of optical fiber micro lenses by adopting a single heating mode or a multi-step heating mode, and determining the influence of the adjustment of the heating parameters of the graphite electrode on the structural parameters of the optical fiber micro lenses;

and step 3: preparing a plurality of groups of optical fiber micro lenses with different structural parameters, measuring the quality of light spots on the emergent end surface, and determining the structural parameter threshold of the optical fiber micro lenses;

and 4, step 4: accurately aligning the prepared photonic crystal fiber micro-lens again, measuring the variation trend of the insertion loss of the fiber micro-lens along with the longitudinal and transverse offset distances of the fiber micro-lens, and determining the variation relation between the working distance and the structural parameters;

and 5: adjusting graphite heating parameters according to the measured structural parameter threshold of the optical fiber micro lens, and preparing the optical fiber micro lens with the longest working distance within the structural parameter threshold range in a graphite step-by-step heating mode;

step 6: and introducing a continuous white light source, and measuring the variation trend of the insertion loss of the optical fiber micro lens along with the wavelength in a wide wavelength range.

In step 1, the coating layer of the photonic crystal fiber is stripped and cleaned. It should be noted that in order to avoid as much as possible micro-cracks on the surface of the photonic crystal fiber caused by the stripping and cleaning process that weaken the fiber strength during later use, commercially available fiber stripping equipment may be used. And secondly, the precise alignment of the two optical fibers is realized through loss monitoring.

In step 2, adjusting heating parameters of the graphite electrode, wherein the heating parameters to be adjusted include heating power of the graphite electrode, heating time, spacing distance of optical fiber alignment, and position of optical fiber heating. The parameters for heating the graphite electrode are as follows: the heating power is set to 70-85W, the heating time is set to 0.5-5 s, and the distance between two optical fibers is greater than 20 μm.

Fig. 3 is a schematic diagram of step-by-step and multiple heating of graphite in preparation of a photonic crystal fiber all-fiber microlens based on graphite heating according to an embodiment of the present invention. The heating area of the graphite heating electrode 8 is large, the electrode heating power is uniform step by step, and the two optical fibers are aligned and placed in the graphite heating electrode at a certain interval distance. The region 6 with one end of the photonic crystal fiber placed in the graphite heating electrode collapses because of heating in the process of heating the electrode, so that a region 2 with uniform refractive index is formed, and simultaneously, the tail end of the fiber forms an arc-shaped lens structure 1 because of heating and melting. If the photonic crystal fiber micro-lens with the required structural parameters is not obtained by single heating, the fiber micro-lens can be accurately aligned and placed in the graphite electrode again, and the graphite heating treatment is carried out on the fiber micro-lens again by adjusting the heating position of the fiber micro-lens and the heating parameters of the graphite electrode.

Fig. 4 is a side view of a fiber microlens sample obtained by adjusting different graphite electrode heating parameters in the preparation of a photonic crystal fiber microlens based on graphite heating according to an embodiment of the present invention. FIG. 4 (a) shows a plurality of groups of optical fiber microlenses prepared by fixing the heating time of the graphite electrode to 2.5s and adjusting the heating power of the graphite electrode. With the increase of the graphite heating power, the collapse area of the air hole gradually increases, but the curvature radius of the terminal lens is hardly changed; FIG. 4 (b) is a view showing a plurality of sets of optical fiber microlenses obtained by fixing the graphite electrode at a heating power of 85W and adjusting the graphite heating time. The length of the air hole collapse region was almost constant as the graphite heating time was increased, and was maintained at about 610 μm, but the radius of curvature of the end lens arc was gradually increased as the heating time of the graphite electrode was increased. By adjusting the heating power and the heating time of the graphite electrode, the photonic crystal fiber micro-lens with target structural parameters can be accurately manufactured.

FIG. 5 shows the variation of the measured spot on the emergent end surface of the fiber microlens with the increase of the collapse length of the air hole of the fiber after a plurality of different sets of photonic crystal fiber microlenses are manufactured. It can be seen that when the collapse distance of the air hole is too long, the quality of the light spot on the emergent end face of the optical fiber micro lens is reduced, so that the collapse length of the air hole of the optical fiber micro lens needs to be kept within a certain threshold range.

FIG. 6 is a schematic diagram of a photonic crystal fiber microlens finally manufactured by using the multi-step graphite electrode heating method proposed by the present invention.

Based on the above descriptions of fig. 1, 2, 3, 4, 5, and 6, in general, the method for manufacturing a long-working-distance and wide-bandwidth photonic crystal fiber microlens according to an embodiment of the present invention can achieve the following advantages. The range of the transverse deviation within the tolerance range of 1dB loss reaches +/-20 mu m, which shows that the sensitivity of the loss to the transverse displacement is reduced, the requirement on alignment precision in actual use can be effectively reduced, and the optical coupling efficiency of a free space is effectively improved; the maximum alignment distance which can be realized by longitudinally offsetting 1dB loss within the tolerance range of 1dB loss is as long as 4.84mm, so that the working distance and the working range of the photonic crystal fiber micro-lens are greatly improved; the fiber microlens achieves an ultra-wide operating bandwidth in excess of 770 nm, within a tolerance of 1dB loss, which is nearly impossible to achieve with conventional fiber lenses.

On the basis of the above embodiment, the photonic crystal fiber is a commercial large mode field photonic crystal fiber.

The core of the photonic crystal fiber is solid silica, and the cladding comprises regular hexagonal periodically arranged air holes.

It should be noted that the optical fiber microlens obtained in the embodiment of the present invention can be used after being fusion-spliced with other various optical fibers. Preferably, however, embodiments of the present invention self-provide for use after fusion splicing with conventional single mode optical fibers.

Finally, the method of the present application is only a preferred embodiment and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.