阅读说明：本技术 一种纤维集成的光纤凹透镜及其制备方法 (Fiber-integrated optical fiber concave lens and preparation method thereof ) 是由 苑立波 孟令知 陈宫傣 于 2020-05-10 设计创作，主要内容包括：本发明提供的是一种纤维集成的光纤凹透镜及其制备方法。其特征是：它由环形芯光纤经热扩散制备而成。光纤凹透镜是在恒温场中,经热扩散制备而成,精细设计的环形芯光纤的纤芯掺杂剂扩散后,折射率分布变为圆周对称的反向准高斯分布,可以等效为凹透镜。本发明主要解决纤维集成的光纤凹透镜的问题,同时提供一种成本低,操作简单的制备方法。本发明具有制作简单、成本低的优点。本发明可用于纤维集成凹透镜的制备,可广泛应用于基于纤维集成凹透镜的微型内窥镜、细胞生物光纤成像系统、光纤光镊系统、微型无人机等领域。(The invention provides a fiber-integrated optical fiber concave lens and a preparation method thereof. The method is characterized in that: it is prepared by ring core optical fiber through thermal diffusion. The fiber concave lens is prepared by thermal diffusion in a constant temperature field, and after the fiber core dopant of the finely designed annular core fiber is diffused, the refractive index distribution is changed into reverse quasi-Gaussian distribution with circumferential symmetry, which can be equivalent to a concave lens. The invention mainly solves the problem of fiber-integrated optical fiber concave lens, and provides a preparation method with low cost and simple operation. The invention has the advantages of simple manufacture and low cost. The invention can be used for preparing the fiber integration concave lens, and can be widely applied to the fields of miniature endoscopes, cell biological optical fiber imaging systems, optical fiber optical tweezers systems, miniature unmanned aerial vehicles and the like based on the fiber integration concave lens.)

1. A fiber-integrated optical fiber concave lens and a preparation method thereof. The method is characterized in that: it is prepared by ring core optical fiber through thermal diffusion. The fiber concave lens is prepared by thermal diffusion in a constant temperature field, and after the fiber core dopant of the finely designed annular core fiber is diffused, the refractive index distribution is changed into reverse quasi-Gaussian distribution with circumferential symmetry, which can be equivalent to a concave lens.

2. The fiber-integrated optical fiber concave lens according to claim 1, which is prepared by thermal diffusion in a constant temperature field. The temperature of the constant temperature field is above 1000 ℃.

3. The fiber-integrated optical fiber concave lens according to claim 1, wherein the optical fiber concave lens with different focal lengths can be prepared by cutting the ring-core optical fiber after heat diffusion for a certain length after heating and diffusion in a constant temperature field.

4. The fiber-integrated optical fiber concave lens according to claim 1, wherein the ring-core optical fiber can be finely designed, including the design of the geometric dimension of the fiber core, the dopant species, the numerical aperture and the like.

5. The method of manufacturing a fiber-integrated optical fiber concave lens according to claim 1, comprising the steps of:

1) fine design of ring core optical fiber

The geometric dimension, dopant species and numerical aperture of the fiber core are designed.

2) Performing thermal diffusion treatment on the ring-shaped core optical fiber

The annular core optical fiber is placed in a constant temperature field for thermal diffusion treatment, and after heating for a certain time, the refractive index distribution of the annular core optical fiber is gradually changed into stable reverse quasi-Gaussian distribution with circumferential symmetry.

3) Cutting the ring core optical fiber

The annular core optical fiber after thermal diffusion is cut in a fixed length, and optical fiber concave lenses with different focal lengths can be prepared.

(I) technical field

The invention relates to a fiber-integrated optical fiber concave lens and a preparation method thereof, which can be used for preparing the fiber-integrated concave lens, can be widely applied to the fields of miniature endoscopes, cell biological optical fiber imaging systems, optical fiber tweezers systems, miniature unmanned aerial vehicles and the like based on the fiber-integrated concave lens, and belongs to the technical field of fiber integration.

(II) background of the invention

With the development of modern industry and scientific technology, people have gradually entered the information-based era. The rapid development of information technology requires that a complete information system can realize as many functions as possible in as small a space as possible, which requires that devices for realizing various functions be as small as possible, and the development is directed toward miniaturization and miniaturization.

The fiber-integrated micro-optical element has the advantages of small volume, light weight, flexible design and manufacture, low manufacturing cost, easy realization of arraying and batch production and the like, can realize the function which is difficult to realize by a common optical element, and has important application value in the fields of optical fiber communication, information processing, aerospace, biomedicine, laser technology, optical calculation and the like.

With the continuous and deep research, many methods for manufacturing micro-optical elements are proposed, mainly including semiconductor lithography, single-point diamond turning, electron beam etching, femtosecond laser direct writing, and the like. The semiconductor photoetching process needs to use a mask plate, and the microstructure is transferred onto the photoresist through development by utilizing ultraviolet light exposure. The method has mature process, is suitable for mass production and has low average cost. The defects that the processed structure only can be planar, multiple times of alignment are needed when a multi-stage structure is processed, the requirement on alignment precision is high, and the cost is increased sharply. The surface roughness of single-point diamond turning is small, the surface roughness is generally below 10nm, and the method is suitable for processing structures with any rotary appearance. The machining precision depends on the tool bit and the machine tool, the precision requirement on the machine tool is high, the machined material is limited, and the size of a machined structure cannot be too small. The electron beam etching is divided into a scanning type and a projection type, a mask plate is not needed in the scanning type, the alignment and the splicing are automatically controlled by a computer, and the processing precision is extremely high. The disadvantages are complex equipment, high cost, small single exposure area and too long time for manufacturing large-size structures. The projection type processing speed is fast, but the mask preparation is difficult. Both methods need to be carried out in vacuum, which greatly limits the application range. The femtosecond laser processing is a processing method of a non-contact high-precision micro-nano photoelectric device, and has strong universality on applicable materials. The defects are high equipment cost, complex processing technology and low processing efficiency.

Current lens systems are limited in shape and size due to manufacturing process concerns. Fabrication techniques for fiber integration of optical fibers with micro-optical elements have recently been proposed for fabrication of micro-optical elements directly on the end face of an optical fiber using different methods of fabrication techniques such as focused ion beam milling, interference lithography, nanoimprint techniques, lithography, polishing techniques, etc. However, they have the disadvantages of difficult processing, complicated manufacturing device, etc.

The thermal diffusion processing technology has the advantages of easiness in implementation, low cost, simplicity in operation and the like, and has great application potential in micro-electro-mechanical systems, optical integrated devices, optical communication and optical fiber sensing. The optical fiber is subjected to thermal diffusion treatment, so that smooth gradual change of the refractive index can be formed in a thermal diffusion processing area, and the smooth gradual change refractive index area has the effect of a micro lens. The finely designed annular core optical fiber is subjected to thermal diffusion processing, and a fiber-integrated optical fiber concave lens can be prepared.

Patent CN01144937.3 discloses an optical fiber having a lens function and a method for manufacturing the same, which is effective for an optical fiber having an abrupt refractive index by using a graded-index optical fiber having a period length indicating lens function. This method can collimate a single-mode optical fiber, but does not have the function of a concave lens.

Patent CN201210011571.6 discloses a single mode fiber connector with large mode area and a manufacturing method thereof, which is to perform thermal diffusion of core doping elements on a step multimode fiber to form a graded index lens with a refractive index decreasing outward in the radial direction, and is mainly used for connecting the single mode fiber with large mode area without the function of a concave lens.

Patent US4269648A discloses a method of mounting a microsphere coupling lens onto an optical fiber, where the microsphere coupling lens can be mounted onto the end of the optical fiber using an adhesive. A method of manufacturing a microlens at an optical fiber end is disclosed, but the method is complicated in manufacturing process and does not have the function of a concave lens.

Patent US7013678B2 discloses a method for manufacturing a graded index fiber lens, which is an important component in a fiber optic communication system and can be used as a lens, but the graded index fiber lens does not have the function of a concave lens, and the method has complicated process and high production cost.

Patent US7228033B2 discloses an optical waveguide lens and method of making the same by fusion splicing a uniform glass lens blank to the distal end of an optical fiber, heating and stretching the lens blank to separate it into two segments, and attaching the segments to the optical fiber defining a tapered end, and then heating the lens blank above its softening point to form a spherical lens. The optical waveguide lens can be used for collimation or focusing of light beams, but the lens manufactured by the method does not have the function of a concave lens.

The invention discloses a fiber-integrated optical fiber concave lens and a preparation method thereof, which can be used for preparing the fiber-integrated concave lens and can be widely applied to the fields of miniature endoscopes, cell biological optical fiber imaging systems, optical fiber tweezers systems, miniature unmanned aerial vehicles and the like based on the fiber-integrated concave lens. The method adopts a thermal diffusion technology to carry out thermal diffusion treatment on a finely designed annular core optical fiber in a constant temperature field, a circumferentially symmetrical reverse quasi-Gaussian-distribution refractive index gradual change area is formed in the thermal diffusion area, and the annular core optical fiber after thermal diffusion is cut at a fixed length, so that the optical fiber concave lenses with different focal lengths can be prepared. Compared with the prior art, the fiber-integrated optical fiber concave lens can be prepared in batch and high efficiency at low cost by adopting the thermal diffusion technology and the ring-core optical fiber with fine design, integrating the concave lens on the optical fiber and realizing the function of the concave lens on the optical fiber.

Disclosure of the invention

The invention aims to provide a fiber-integrated optical fiber concave lens which is simple to manufacture, low in cost and capable of being produced in batch and a preparation method thereof.

The purpose of the invention is realized as follows:

the fiber-integrated optical fiber concave lens is prepared by ring-core optical fibers through thermal diffusion. The fiber concave lens is prepared by thermal diffusion in a constant temperature field, and after the fiber core dopant of the finely designed annular core fiber is diffused, the refractive index distribution is changed into reverse quasi-Gaussian distribution with circumferential symmetry, which can be equivalent to a concave lens.

Thermal diffusion techniques are commonly used for expansion of the fundamental mode field, which enables the dopant profile in the fiber to be graded into a stable quasi-gaussian profile. The ring-shaped core optical fiber with the fine design is placed in a constant temperature field for heating, the dopant distribution in the ring-shaped core gradually changes into stable reverse quasi-Gaussian distribution with circumferential symmetry, and the normalization frequency of the optical fiber is not changed in the heating process. The reverse quasi-Gaussian distribution of the dopant gradually changes the refractive index distribution of the annular core fiber into the reverse quasi-Gaussian distribution, and the annular core fiber after thermal diffusion has the function of a concave lens because the annular core fiber is bent towards a region with higher refractive index in the process of light beam propagation.

During thermal diffusion, the local doping concentration C can be expressed as:

d in formula (1) is the dopant diffusion coefficient; t is the heating time. D depends mainly on the type of different dopants, the host material and the local heating temperature. In most cases, considering the diffusion of germanium in the core of an optical fiber, the heating temperature of the fiber is almost uniformly constant with respect to the radial position r on its axisymmetric geometry, and the diffusion coefficient D is assumed to be constant with respect to the radial position r. In practice, neglecting the diffusion of dopants in the axial direction, the simplified diffusion equation (1) in cylindrical coordinates is:

the doping concentration C of the dopant is a function of the radial distance r and the heating time t. The diffusion coefficient D is also affected by the heating temperature and is expressed as:

t (z) in the formula (3) represents the heating temperature in K, which is related to the longitudinal position of the optical fiber in the furnace; r-8.3145 (J/K/mol) is an ideal gas constant; parameter D₀And Q can be obtained from experimental data. Consider the initial boundary conditions:

where a is a constant and represents the diameter of the optical fiber.

The dopant local doping concentration profile C can be expressed as:

in the formula (5), f (r) is an initial concentration distribution, and the concentration at the fiber boundary surface r ═ a is 0. J. the design is a square₀Is a first class zero order Bessel function with characteristic value α_nIs the root of it

J₀(aα_n)＝0 (6)

Assuming that the refractive index profile of the optical fiber over the thermal diffusion region is proportional to the dopant profile, the refractive index profile of the optical fiber after thermal diffusion can be expressed as:

n in formula (7)_clAnd n_coThe refractive indices of the fiber cladding and the core, respectively. The refractive index profile of the ring-core optical fiber changes with the heating time t when the heating temperature field is 1600 ℃ (see fig. 2 a). Curves 21, 22, 23, and 24 are refractive index distributions along the radial direction of the optical fiber after the annular core optical fiber is heated for 0h, 1h, 2h, and 3h, respectively. After 3h of thermal diffusion treatment, the refractive index profile of the ring core fiber tended to be a more stable inverted quasi-gaussian profile (see fig. 2 b).

Graded index lenses have been widely used in optical components and devices for collimation, focusing and coupling. A graded index lens refers to a lens in which the refractive index varies continuously in the axial, radial, or spherical directions. For a radially graded-index annular-core fiber-concave lens, the central index of the fiber is lowest and increases with increasing radial distance from the central axis. The refractive index profile follows a square ratio profile:

n in formula (8)₀Is an optical fiberThe index of refraction at the center, r is the radial distance from the center axis, and g is the gradient constant. The focal length of the annular core fiber concave lens with the axial length L is

The cross-sectional refractive index of the fiber-integrated optical fiber concave lens prepared after the annular core optical fiber is subjected to thermal diffusion for 3h is shown in fig. 3 a. FIG. 3b is a three-dimensional representation of the cross-sectional refractive index of the fiber-integrated optical fiber concave lens. As can be seen from the figure, the central refractive index of the annular core fiber concave lens is the lowest and increases as the distance from the central axis in the radial direction increases.

When the fiber-integrated optical fiber concave lens is prepared, the ring-core optical fiber can be finely designed, including the design of the geometric dimension of the fiber core, the type of the dopant, the numerical aperture and the like.

The fiber-integrated optical fiber concave lens is prepared by thermal diffusion in a constant temperature field. The temperature of the constant temperature field is above 1000 ℃. The thermal diffusivity of ring core fibers with different core dopants is different.

When the fiber-integrated optical fiber concave lens is prepared, after heating and diffusing for a certain time in a constant temperature field, the annular core optical fiber after thermal diffusion is cut at a fixed length, and the optical fiber concave lens with different focal lengths can be prepared according to the formula (9).

The invention discloses a preparation method of a fiber integrated optical fiber concave lens, which is characterized by comprising the following steps:

in the first step, the ring core fiber is finely designed, including the design of the geometric dimension of the fiber core, the dopant species, the numerical aperture and the like.

And secondly, carrying out thermal diffusion treatment on the annular core optical fiber, placing the annular core optical fiber in a constant temperature field for thermal diffusion treatment, and after heating for a certain time, gradually changing the refractive index distribution of the annular core optical fiber into stable reverse quasi-Gaussian distribution with circumferential symmetry.

And thirdly, cutting the annular core optical fiber, and cutting the annular core optical fiber subjected to thermal diffusion at a fixed length to prepare the optical fiber concave lenses with different focal lengths.

When the fiber-integrated optical fiber concave lens is prepared, after a certain time of thermal diffusion treatment, the refractive index distribution of the annular core optical fiber tends to be more stable reverse quasi-Gaussian distribution with circumferential symmetry, the refractive index at the center is the lowest, and the refractive index is increased along with the increase of the distance from the radial direction to the central axis. After the annular core optical fiber is subjected to thermal diffusion treatment, the dopant forms smooth reverse quasi-Gaussian distribution in a thermal diffusion processing area. The distribution of the dopant is reverse quasi-Gaussian distribution, the refractive index distribution of the annular core optical fiber is also reverse quasi-Gaussian distribution, and the annular core optical fiber is bent towards a region with higher refractive index in the light beam propagation process, so that the annular core optical fiber after heat diffusion has the function of a concave lens.

As shown in fig. 4, an incident parallel light beam 41 becomes a diverging exit light beam 42 as it passes through the concave lens. The fiber-integrated optical fiber concave lens prepared by the invention has the function of a concave lens, and can realize the diverging effect on light beams. And the fiber-integrated optical fiber concave lens is cut in a fixed length, so that the optical fiber concave lenses with different focal lengths can be prepared.

When the ring core optical fiber is finely designed, the dopant of the fiber core can be one or more different doped dopants according to the requirement. When the annular core optical fiber is used for preparing the optical fiber concave lens, the larger diameter of the annular core is designed, the heating time is prolonged, the heating temperature is increased, and the optical fiber concave lens with the larger diameter of the mode field can be prepared. One or more different doping agents are used, so that the function of the fiber concave lens is not affected.

The fiber-integrated optical fiber concave lens provided by the invention is prepared by ring-core optical fibers through thermal diffusion. Compared with the prior art, the fiber-integrated optical fiber concave lens can be prepared in batch and high efficiency at low cost by adopting the thermal diffusion technology and the ring-core optical fiber with fine design, integrating the concave lens on the optical fiber and realizing the function of the concave lens on the optical fiber.

(IV) description of the drawings

FIG. 1 is a schematic diagram of refractive index profile changes before and after a fiber-integrated optical fiber concave lens is prepared by thermal diffusion.

Fig. 2a is a graph showing the change of the refractive index profile of the ring-core optical fiber with the change of the heating time t in a temperature field of 1600 c, and fig. 2b is a graph showing the refractive index profile of the ring-core optical fiber after heating for 3 h.

FIG. 3a is a cross-sectional refractive index profile of the ring-core optical fiber after heating for 3h, and FIG. 3b is a three-dimensional representation of the cross-sectional refractive index profile of the ring-core optical fiber after heating for 3 h.

Fig. 4 is a schematic beam propagation diagram of a concave lens. An incident beam is denoted by 41 and an outgoing beam is denoted by 42.

FIG. 5 is a schematic cross-sectional view of a ring core optical fiber in the embodiment. Reference numeral 51 denotes a cladding of the ring core fiber, and 52 denotes a core of the ring core fiber.

Fig. 6 is a schematic structural diagram of a single-mode fiber + fiber integrated fiber concave lens in an embodiment. 61 is a single mode fiber and 62 is a fiber-integrated fiber concave lens made from a ring-core fiber.

Fig. 7a is a refractive index profile of a single mode fiber + fiber integrated fiber optic concave lens in an example embodiment, and fig. 7b is a three-dimensional representation of the refractive index profile of the single mode fiber + fiber integrated fiber optic concave lens in an example embodiment.

Fig. 8a is a light field distribution of a fiber end outgoing light of a single-mode fiber in the embodiment, fig. 8b is a light field distribution of a fiber end outgoing light of a fiber end concave lens of a single-mode fiber + fiber integration in the embodiment, fig. 8c is a light intensity distribution of a fiber end outgoing light field of a single-mode fiber in the embodiment, and fig. 8d is a light intensity distribution of a fiber end outgoing light field of a fiber end concave lens of a single-mode fiber + fiber integration in the embodiment.

(V) detailed description of the preferred embodiments

The invention is further illustrated below with reference to specific examples.