Inverse conical special optical fiber and preparation method thereof

Abstract

The invention discloses an inverted-cone-shaped special optical fiber and a preparation method thereof, and belongs to the technical field of special optical fiber preparation, and the method comprises the following steps: carrying out impurity removal and rare earth particle doping treatment on a quartz tube to prepare a hollow quartz preform; penetrating a standard traction rod into the quartz preform, wherein the head of the standard traction rod is an etalon head; fixing the standard traction rod and the quartz preform after rod penetration; heating the quartz preform to draw the quartz preform into a hollow optical fiber; heating the hollow optical fiber and the head of the standard traction rod, and enabling the head of the etalon to drive the softened hollow optical fiber to be uniformly enlarged due to thermal expansion so as to form a gradually-changed optical fiber inner core; coating and curing the optical fiber for the first time; and carrying out secondary coating on the optical fiber after the primary coating. The thermal expansion characteristic of the molybdenum or tungsten standard traction rod is utilized, uniform gradual change of the inner diameter is achieved in optical fiber drawing to form a reverse cone, accurate control of the inner diameter is achieved, and the mechanical strength and transmission stability of the optical fiber are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of preparation technology of special optical fiber, especially to a kind of reverse conical special optical fiber and its preparation method. BACKGROUND

[0002] In the application of high-power laser, traditional optical fiber is prone to cause laser system instability due to stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) and other nonlinear effects, which limits the improvement of output power. Reverse conical optical fiber, with its special structure design, can effectively reduce nonlinear effects and become a key element for high-power laser transmission.

[0003] In related technologies, the preparation of reverse conical optical fiber is mostly by mold forming or chemical etching method, which has the following defects: (1) Mold forming method is difficult to accurately control the inner diameter of the gradual change, resulting in increased fiber transmission loss; (2) Chemical etching method is complex, easy to introduce impurities, and cannot realize continuous preparation of large length optical fiber; (3) Preform rod shrinking process will lead to uneven doping, affecting the optical performance of the optical fiber.

[0004] Therefore, how to design a reverse conical special optical fiber preparation method that can accurately control the inner diameter of the gradual change is a technical problem to be solved. SUMMARY

[0005] The present application aims to overcome the defects of low inner diameter control accuracy, complex process and uneven doping in the preparation of existing reverse conical optical fiber, and provides a preparation method based on thermal expansion traction technology, which realizes the accurate forming of reverse conical structure and ensures the optical performance and mechanical strength of the optical fiber.

[0006] In a first aspect, the present application provides a preparation method of a reverse conical special optical fiber, comprising the following steps: S1: Impurities are removed from a quartz tube and rare earth particles are doped to obtain a hollow quartz preform rod; S2: A standard traction rod is inserted into the inside of the quartz preform rod, and the head of the standard traction rod is a standard etalon head; S3: The standard traction rod and the quartz preform rod after being inserted are fixed respectively; S4: The quartz preform rod is heated to draw a hollow optical fiber that meets the outer diameter requirements; S5: The hollow optical fiber and the head of the standard traction rod are heated to make the hollow optical fiber enter a softened state, and the standard etalon head drives the inner diameter of the hollow optical fiber to uniformly increase due to thermal expansion, forming a fiber inner core with a gradually changing inner diameter; S6: The hollow optical fiber is coated once and solidified; S7: The hollow optical fiber after the first coating is coated twice to form a reverse conical special optical fiber.

[0007] Preferably, the rare earth particle doping is doping of erbium, ytterbium or neodymium element, and the doping concentration is 0.1-1 mol%.

[0008] Preferably, the material of the standard traction rod is molybdenum or tungsten, the linear expansion coefficient of the standard traction rod is 4.1*10 -6 ~5.1*10 -6 / K, and the tensile strength at a temperature above 2000℃ is ≥379 MPa.

[0009] Preferably, in the step S3, the standard traction rod is fixed at the top clamp of the drawing tower, and the preform rod is fixed at the preform rod fixing clamp.

[0010] Preferably, in the step S4, the heating temperature is 1700-2000℃, and the outer diameter of the hollow optical fiber formed by drawing is less than 100 μm.

[0011] Preferably, in the step S5, the heating temperature is 1300-1650℃, and the outer diameter parameter of the optical fiber is monitored in real time by the outer diameter control etalon during drawing to feedback and adjust the heating temperature.

[0012] Between the step S5 and the step S6, the preparation method further comprises: optically detecting the optical fiber core of the hollow optical fiber, and repeating the steps S4-S5 if the detection is unqualified.

[0013] Preferably, in the step S6, the material of the primary coating is polyolefin or PBT, and the curing is ultraviolet curing or thermal curing.

[0014] Preferably, the special optical fiber is a reverse conical structure with symmetrical and gradual changes in the side surface, and the inner diameter changes from the input end to the output end as d0→c0→b0→a0→b0→c0→d0, wherein a0

[0015] In a second aspect, the embodiments of the present application provide a reverse conical special optical fiber made based on the preparation method of the first aspect, comprising: a central transmission layer, an active layer, an outer cladding layer and a coating layer.

[0016] Preferably, the reverse conical special optical fiber is a reverse conical structure with symmetrical and gradual changes in the side surface.

[0017] Preferably, the reverse conical special optical fiber is a reverse conical structure with symmetrical and gradual changes in the side surface.

[0018] Compared with the prior art, the present application has the following beneficial effects: (1) Precise control of reverse conical structure: by heating the head of the standard pulling rod and the quartz preform rod, which utilizes the synergistic effect of metal thermal expansion and quartz softening, the precise control of the inner diameter gradient is realized, so that the prepared reverse conical special optical fiber can effectively suppress the nonlinear effect, can improve the SBS threshold when high-power laser transmission, and the reverse conical special optical fiber has high damage resistance and high conversion rate; (2) Process simplification: the method flow is simple, only the standard pulling rod needs to be inserted into the quartz preform rod, and the method omits the rod shrinking process; through the continuous process of once drawing and inner core drawing of hollow optical fiber, the preparation of long length optical fiber is realized; (3) Superior material matching: when making, molybdenum or tungsten is selected to make the standard pulling rod, and the thermal expansion coefficient of the standard pulling rod is significantly different from the thermal expansion coefficient of quartz (5.3~5.8×10 -7 / K), which can ensure that the expansion force during processing is sufficient to drive the gradual change of the inner diameter of the optical fiber, thereby forming an inner diameter gradient optical fiber core; at the same time, the standard pulling rod can avoid deformation due to the high temperature resistance of the material, thereby ensuring the stability of the manufacturing process; (4) Excellent mechanical properties: the hollow optical fiber after the inner core is made is double-coated, so that the tensile strength of the optical fiber is increased to more than 1000MPa, which can meet the use requirements in harsh environments. BRIEF DESCRIPTION OF DRAWINGS

[0019] The exemplary embodiments of the present application can be more completely understood in reference to the following drawings. The accompanying drawings are used to provide further understanding of the embodiments of the present application and constitute a part of the specification, which together with the embodiments of the present application serve to explain the present application and do not constitute a limitation thereof. In the drawings, the same reference numerals generally represent the same components or steps.

[0020] Figure 1 A flow chart of a preparation method of a reverse conical special optical fiber is provided for an exemplary embodiment of the present application; Figure 2 A structural schematic diagram of a quartz tube is provided for an exemplary embodiment of the present application; Figure 3 A structural schematic diagram of a quartz preform rod is provided for an exemplary embodiment of the present application; Figure 4 A structural schematic diagram of a standard pulling rod is provided for an exemplary embodiment of the present application; Figure 5 A structural schematic diagram of the standard pulling rod inserted into the quartz preform rod is provided for an exemplary embodiment of the present application; Figure 6 A drawing process schematic diagram of step S4 is provided for an exemplary embodiment of the present application; Figure 7 A cross-sectional view of an anti-tapered special optical fiber is provided for an exemplary embodiment of the present application.

[0021] Reference signs 1-quartz tube, 2-hollow interior, 3-doped layer, 4-standard drawing rod, 41-drawing wire, 42-standard head, 5-outer diameter control standard, 6-heating flame, a-central transmission layer, b-active layer, c-cladding layer, d-coating layer. DETAILED DESCRIPTION

[0022] Exemplary embodiments of the present disclosure will be described in greater detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure can be more thoroughly understood and the scope of the present disclosure can be accurately conveyed to those skilled in the art.

[0023] In the description of the present application, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application. In addition, the terms "first", "second", "third" are only for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0024] In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as there is no conflict between them.

[0025] Reference Figure 1 The present application discloses a preparation method of an anti-tapered special optical fiber. Specifically, the anti-tapered special optical fiber is a kind of non-uniform cross-section special optical fiber with a structure of "narrow in the middle and wide at both ends". Its core feature is that there is one or more "tapered regions" in the axial direction of the optical fiber, i.e. the diameter of the optical fiber gradually decreases to a minimum value from the input end (or output end) to the middle region, and then gradually increases from the middle region to the other end, forming a cross-sectional change trend completely opposite to that of the conventional tapered optical fiber with "one end wide and one end narrow". Through material customization, structure optimization or functional doping, it is specially adapted to the optical transmission, optical coupling or optical sensing needs in specific scenarios, and belongs to the sub-category of special optical fibers. The core of the present embodiment is to utilize the thermal expansion characteristics of high-melting-point metal materials to achieve uniform and gradual change of the inner diameter during optical fiber drawing, thereby producing an anti-tapered special optical fiber. The specific steps of preparation are as follows: S1: Impurity rejection and rare earth particle doping treatment are performed on the quartz tube to obtain a hollow quartz preform rod; Specifically, referring to Figures 2-3 , a high-purity (e.g., purity ≥ 99.99%) quartz tube 1 is selected. Impurities in the tube can be removed (e.g., to a content ≤ 1 ppm) by plasma etching technology, and rare earth particles (e.g., erbium, ytterbium, or neodymium) are doped into the inner wall of the quartz tube by vapor deposition method. The doping concentration can be 0.1-1 mol%. In this way, a uniform doping layer 3 is formed. Compared with the traditional process, the steps of the present embodiment omit the rod shrinking process, and the hollow interior 2 of the quartz tube is preserved, leaving room for subsequent inner diameter control.

[0026] It can be understood that impurity rejection can also be performed by chemical cleaning method, and rare earth particle doping treatment can also be performed by sol-gel method. Specifically, during impurity rejection, a high-purity (purity ≥ 99.99%) quartz tube 1 is selected, and it is immersed in a high-purity mixed acid solution (e.g., a mixture of 40% hydrofluoric acid and 68% nitric acid at a volume ratio of 1:3, with an impurity content ≤ 0.1 ppm) at a constant temperature of 25-40°C for 1-2 hours. Through chemical reaction between the acid solution and the impurities (e.g., Na, K, Fe, and other metal ions) on the inner wall of the quartz tube, soluble salts are generated. Subsequently, the inner wall of the quartz tube is washed with 18.2 MΩ·cm high-purity water for 3-5 times, with each washing time ≥ 15 minutes. Finally, the quartz tube is dried in a clean oven at 120-150°C for 2 hours, so that the impurity content on the inner wall of the quartz tube is reduced to ≤ 1 ppm. During rare earth particle doping, tetraethyl orthosilicate is used as the silicon source, ethanol is used as the solvent, and rare earth nitrate (e.g., erbium nitrate, ytterbium nitrate, or neodymium nitrate, with a purity ≥ 99.99%) is added. The pH of the system is adjusted to 3-4 with 0.1 mol / L hydrochloric acid, and the rare earth doped sol is prepared by stirring at 60-80°C for 3-4 hours. The sol is uniformly coated on the inner wall of the quartz tube by dip-coating method (pulling speed 5-10 mm / s). Subsequently, the sol is dried at 80-120°C for 2-4 hours, and then sintered in a muffle furnace at 500-700°C for 1-2 hours to form a uniform doping layer 3 with a doping concentration of 0.1-1 mol%. This embodiment also does not require a rod shrinking process, and the hollow interior 2 of the quartz tube is stably preserved. The chemical cleaning method causes less damage to the inner wall of the quartz tube, and the sol-gel method can achieve more uniform dispersion of rare earth particles, which is suitable for scenarios with high requirements for doping uniformity.

[0027] S2: A standard pulling rod is inserted into the interior of the quartz preform rod, and the head of the standard pulling rod is a standard etalon head; Specifically, a standard pulling rod 4 made of modified molybdenum or tungsten (diameter 2-5 mm) is selected, such as Figures 4-5As shown, the standard pulling rod 4 includes a pulling wire 41 and a standard gauge head 42, which in a preferred embodiment can be machined into a conical standard gauge head 42 (the taper angle can be 30°~60°).

[0028] In this embodiment, molybdenum or tungsten material can still maintain a relatively high tensile strength (tensile strength of tungsten ≥379 MPa at 2200℃) and linear expansion coefficient (4.1×10 -6 ~5.1×10 -6 / K) at high temperature above 2000℃. The characteristics of molybdenum or tungsten material can produce uniform expansion when heated. Specifically, high-purity molybdenum or tungsten can avoid impurities volatilizing to form pollution at high temperature, preventing the quartz preform from introducing impurity defects (such as bubbles, stones) during the pulling process, thereby ensuring the stability of the optical performance of the final product; at the same time, the linear expansion coefficient in the range of this embodiment is close to that of quartz glass (about 5.5×10 -6 / K), which can significantly reduce the thermal stress difference between the standard pulling rod and the quartz preform during high-temperature pulling (1800~2200℃) and cooling process, and reduce the risk of preform cracking or deformation caused by thermal expansion mismatch; finally, the tensile strength of the standard pulling rod is ≥379 MPa at a temperature above 2000℃, and the high-temperature tensile strength and low creep characteristics can ensure that the standard pulling rod does not occur plastic deformation or fracture under the action of high temperature above 2000℃ and continuous pulling load (50~200N), ensuring the dimensional accuracy of the preform during the pulling process and improving the process stability.

[0029] S3: Fix the standard pulling rod and the quartz preform after the rod is inserted; Specifically, the top end of the standard pulling rod 4 is fixed on the precision displacement platform at the top of the drawing tower, and the axial deviation during the drawing process can be corrected in real time through the fine adjustment function of the platform, avoiding uneven stress caused by the shaking of the pulling rod, thereby ensuring that its axial direction is consistent with the drawing direction; at the same time, the hollow quartz preform rod is fixed on the movable fixing clamp below, so that the elastic clamping structure (such as a silicon nitride material buffer) of the clamp can avoid local stress cracking of the preform rod due to rigid fixation, and the position can be adjusted flexibly according to the subsequent drawing requirements (such as adjusting the preform rod feeding speed). The fixing methods of the above two in this step are adapted to the high-temperature tensile strength (≥379 MPa at 2000°C or above) of the standard pulling rod, so that the stable support of the displacement platform can fully exert the anti-fracture ability of the pulling rod, avoid the pulling rod bending or the preform rod falling off due to loose fixation at high temperature, and increase the continuous running time of the entire drawing process to ≥8 hours (compared with the traditional fixing method, the interruption rate is reduced by 60%). Finally, based on the fixing requirement that the axial direction of the pulling rod is consistent with the drawing direction, combined with the control of the coaxiality of the standard pulling rod and the quartz preform rod (controlled to ≤0.1 mm), the hollow interior of the quartz preform rod can be kept axial centering during the drawing process, avoiding the hollow channel deviation or deformation, and in the subsequent drawing, only the drawing speed needs to be adjusted by the displacement platform, so as to accurately control the inner diameter and outer diameter of the hollow optical fiber, without additional correction process, so that the size qualified rate of the hollow optical fiber can be greatly improved, especially for the product preparation of the reverse tapered special optical fiber which has very high requirements on cross-sectional size precision.

[0030] S4: heating the quartz preform rod to draw a hollow optical fiber meeting the outer diameter requirement; Exemplarily, the lower end of the quartz preform rod can be heated by a hydrogen-oxygen flame, and the flame temperature is controlled at 1700-2000°C (above the softening point of quartz). In this way, since the hydrogen-oxygen flame temperature is accurately controlled to be above the softening point of quartz (about 1713°C) and below the excessive melting temperature (above 2000°C), the lower end of the quartz preform rod can be fully softened, avoiding the breakage of the preform rod during stretching due to insufficient softening. In addition, the temperature is prevented from being too high to cause excessive flow of the quartz material, thereby reserving the key structural basis for subsequent preparation of hollow reverse-taper optical fiber and other products. At the same time, the optical fiber is drawn from an outer diameter of 10 mm or more to an outer diameter of 100 μm or less at a speed of 5-20 m / min. This pulling speed range is accurately matched with the size change requirement of the preform rod of 10 mm or more drawn to the optical fiber of 100 μm or less. When a relatively thick hollow optical fiber (such as an outer diameter of 80-100 μm) is needed to be drawn, a low-speed traction of 5-10 m / min is used to reduce the diameter fluctuation in the stretching process. When a relatively thin hollow optical fiber (such as an outer diameter of 50-80 μm) is needed to be drawn, a high-speed traction of 10-20 m / min is used to improve the production efficiency while ensuring the accuracy. Compared with the fixed traction speed, the variable speed range in this embodiment can cover more hollow optical fiber requirements of different outer diameters, without the need to change the equipment parameters, thereby improving the process compatibility. At the same time, the stable traction speed can avoid the necking phenomenon (local diameter sudden decrease) of the preform rod due to sudden stress, and ensure the uniformity of the outer diameter of the optical fiber in the axial direction, thereby providing a uniform base material for the subsequent processing of the tapering region of the reverse-taper optical fiber.

[0031] With reference to Figure 6In the drawing process of the embodiment, the outer diameter of the hollow optical fiber is monitored in real time by the outer diameter control standard gauge 5 (laser diameter measuring instrument) to feedback and adjust the pulling speed, so as to ensure that the deviation is ≤±2 μm; the closed-loop control mechanism can effectively avoid the problem of batch waste caused by no real-time monitoring - the traditional process relies on manual sampling inspection, if the fluctuation of the pulling speed is not found in time, the length of the unqualified optical fiber is easy to produce, and real-time monitoring can reduce the waste rate. In addition, the high-precision outer diameter control of ±2 μm can directly meet the high requirement scene (such as the matching accuracy requirement of the hollow optical fiber and the sensing probe in the optical fiber sensing system), without subsequent grinding processing, which can save process cost. Finally, in order to avoid structural damage and ensure the adaptability of subsequent processing, it is necessary to control the entire drawing process to be completed before reaching the standard gauge head 42, because if the drawing process exceeds the range of the standard gauge head 42, the hollow optical fiber is easy to rub with the standard gauge parts, which will cause surface scratches, affect the uniformity of light transmission, or cause the optical fiber to cool down too fast due to the unstable temperature field outside the standard gauge, resulting in internal stress. The completion of the drawing before the standard gauge head 42 can ensure that the optical fiber is always in the stable process interval of “heating-monitoring-pulling”, which not only can maintain the surface smoothness, but also can make the axial straightness of the hollow channel meet the positioning requirement of the subsequent reverse conical fiber tapering processing, avoid the problem of uneven wall thickness in the tapering area caused by channel deviation, and finally ensure the overall process performance.

[0032] S5: heating the head of the drawn hollow optical fiber and the standard gauge head 42, so that the hollow optical fiber enters a softened state, and at the same time the standard gauge head 42 uniformly expands the inner diameter of the hollow optical fiber due to thermal expansion, to form a fiber core with a gradually changing inner diameter; Specifically, when the hollow optical fiber enters the standard gauge head 42 area, the heating flame temperature is switched to 1300-1650°C, which is precisely below the quartz softening point (1713°C) and above the critical temperature of plastic deformation (1300°C), so that the hollow optical fiber is in an ideal state of being plastic but not melting: on the one hand, it avoids the quartz melting and flowing caused by too high temperature (≥1713°C), prevents the hollow channel from shrinking, closing or having uneven wall thickness due to surface tension; on the other hand, it ensures that the temperature is sufficient to make the inner wall of the optical fiber have plastic deformation ability, which can uniformly expand under the expansion force of the standard gauge head, avoiding cracking during stretching due to excessive rigidity. At the same time, the viscosity of quartz in this temperature range is moderate, which can not only respond to the expansion force of the standard gauge head to achieve slow deformation, but also maintain the structure shaping ability to ensure the size stability after the inner diameter gradually changes.

[0033] At this temperature interval, the etalon head 42 axially and radially expands due to high temperature; the softened hollow optical fiber inner wall plastically deforms under the traction rod expansion force, and the inner diameter uniformly and slowly increases with the expansion amount; during preparation, by controlling the drawing speed (for example, controlled at 1-5 m / min) and the heating temperature gradient, the inner diameter gradually changes from a0 to d0, forming a reverse tapered optical fiber structure, as shown in Figure 7 The inner diameter of the reverse tapered special optical fiber changes from d0 to c0 to b0 to a0 to b0 to c0 to d0, where a0 < b0 < c0 < d0. Specifically, at this temperature interval, the thermal expansion amount of the etalon head 42 is positively linearly related to the temperature, and the expansion amount can be quantitatively controlled by precise temperature control (for example, the radial expansion amount is stably controlled at 5-8 μm per 100 ℃ temperature rise), thereby driving the uniform increase of the inner diameter of the hollow optical fiber. Compared with mechanical external force expansion (such as metal rod pushing), the deformation force driven by thermal expansion is softer and uniformly distributed in the circumferential direction, which can avoid sudden changes in the inner diameter and reduce micro-cracks caused by stress concentration. This uniform gradual change characteristic is the core guarantee for forming the "d0→c0→b0→a0→b0→c0→d0" symmetrical reverse tapered structure, and lays a foundation for subsequent low-loss transition of optical transmission.

[0034] S6: once-coating the hollow optical fiber and curing; Specifically, before coating, that is, between steps S5 and S6, an optical detection step is further included: optically detecting the optical fiber core, and if the detection is unqualified, repeating steps S4-S5; for example, the core diameter parameters of the optical fiber, such as core diameter consistency, refractive index distribution, and transmission loss, can be detected by an optical time domain reflectometer (OTDR), and the qualified standard can be set as: core diameter deviation ≤±1 μm, and loss ≤0.2 dB / km. When the optical detection is qualified, the subsequent preparation process is continued, and when the optical detection is unqualified, the step S4 is started again.

[0035] After the optical detection is completed, a polyolefin or PBT resin is coated on the surface of the drawn optical fiber as a protective layer, and in a preferred embodiment, the coating thickness can be 50-100 μm. After coating, the coating is cured by ultraviolet curing (the power can be 300-500 mW / cm 2 ) or thermal curing (for example, the temperature is 120-150 ℃) to enhance the wear resistance of the optical fiber.

[0036] S7: twice-coating the once-coated hollow optical fiber to form a reverse tapered special optical fiber.

[0037] Specifically, a layer of high-strength resin (such as polyimide) is coated outside the once-coating layer, and in a preferred embodiment, the thickness can be 30-80 μm, which is to further improve the tensile strength and temperature resistance of the optical fiber.

[0038] Compared with the prior art, the present application has the following beneficial effects: (1) Precise control of reverse conical structure: by heating the head of the standard pulling rod and the quartz preform rod, the synergistic effect of metal thermal expansion and quartz softening is utilized to achieve precise control of the gradual change of the inner diameter, so that the prepared reverse conical special optical fiber can effectively suppress nonlinear effects, can improve the SBS threshold when high-power laser is transmitted, and the reverse conical special optical fiber has high damage resistance and high conversion rate; (2) Process simplification: the method flow is simple, only the standard pulling rod needs to be inserted into the quartz preform rod, and the method omits the rod shrinking process; through the continuous process of once drawing and inner core drawing of hollow optical fiber, the preparation of long-length optical fiber is realized; (3) Superior material matching: the standard pulling rod is made of molybdenum or tungsten, and the thermal expansion coefficient of the standard pulling rod is significantly different from the thermal expansion coefficient of quartz (5.3~5.8×10 -7 / K), which can ensure that the expansion force during processing is sufficient to drive the gradual change of the inner diameter of the optical fiber, thereby forming an optical fiber inner core with a gradually changing inner diameter; at the same time, the standard pulling rod can avoid deformation due to the high-temperature resistance of the material, thereby ensuring the stability of the manufacturing process; (4) Excellent mechanical properties: the hollow optical fiber after the inner core is made is double-coated, so that the tensile strength of the optical fiber is increased to more than 1000MPa, which can meet the use requirements in harsh environments.

[0039] Based on the same inventive concept, the present application also provides a reverse conical special optical fiber prepared by the preparation method of the reverse conical special optical fiber. The implementation scheme for solving the problem provided by the special optical fiber is similar to the implementation scheme described in the above method, so the specific limitations in the following reverse conical special optical fiber embodiment can be referred to the limitations of the preparation method of the reverse conical special optical fiber in the above text, which will not be repeated here.

[0040] The present application discloses a reverse conical special optical fiber, comprising: a center transmission layer a, an active layer b, an outer cladding layer c and a coating layer d.

[0041] Specifically, the reverse conical special optical fiber is a reverse conical structure with symmetrical gradual change of the side surface, and the inner diameter changes from the input end to the output end as d0→c0→b0→a0→b0→c0→d0, wherein a0

[0042] Specifically, the anti-taper special optical fiber is a non-uniform cross-section special optical fiber with a structure of "narrow in the middle and wide at both ends". Its core feature is that there is one or more "tapered sections" in the axial direction of the optical fiber, that is, the diameter of the optical fiber gradually decreases to a minimum value from the input end (or the output end) to the middle region, and then gradually increases from the middle region to the other end, forming a cross-section change trend completely opposite to that of the conventional tapered optical fiber with "one end wide and one end narrow". Through material customization, structure optimization or functional doping, it is specially adapted to the optical transmission, optical coupling or optical sensing needs in specific scenarios, and belongs to the sub-category of special optical fibers. The inner diameter structure of the anti-taper special optical fiber forms a structure of narrow in the middle and wide at both ends, which has multiple optimization effects on optical transmission, as follows: (1) Low loss coupling: the gradual change from the input end d0 to the middle a0 can gradually compress the incident light field (such as Gaussian beam), reducing the reflection loss caused by mode mismatch; the gradual change from a0 to d0 at the output end can smoothly release the compressed light field, adapting to the interface requirements of subsequent coarse-aperture devices (such as lasers and detectors); (2) Evanescent field enhancement: the core diameter of the middle a0 section (the smallest inner diameter) is the smallest, and the distance that the evanescent field of the optical signal extends to the cladding is the largest (increased by 3-5 times compared with the d0 section), which can significantly improve the sensing sensitivity to the surrounding environment, especially suitable for biological detection, environmental monitoring and other scenarios; (3) High power tolerance: the symmetrical gradual change structure can disperse the optical power density, avoiding the material damage (such as photo-darkening of quartz) caused by power concentration in a certain section of the straight-tube optical fiber, so that the high-power laser transmission threshold of the anti-taper optical fiber is increased to 500W or more, which is at least 1.5 times that of the same specification straight-tube optical fiber.

[0043] It should be noted that the flowcharts and block diagrams in the drawings show the possible implementation architecture, functions and operations of the systems, methods and computer program products according to the embodiments of the present application. In this regard, each block in the flowchart or block diagram can represent a module, a program segment or a part of code containing one or more executable instructions for implementing the specified logical function. It should also be noted that in some alternative implementations, the functions noted in the blocks can occur in different order than that shown in the drawings. For example, two consecutive blocks can actually be executed in parallel, and sometimes in reverse order, depending on the functions involved. It should also be noted that each block in the block diagram and / or flowchart, and the combination of blocks in the block diagram and / or flowchart, can be implemented by a dedicated hardware-based system that performs the specified functions or actions, or can be implemented by a combination of dedicated hardware and computer instructions.

[0044] Those skilled in the art can clearly understand that, for the convenience and brevity of description, the specific working process of the system, device and unit described above can refer to the corresponding process in the foregoing method embodiments, and will not be repeated here.

[0045] In several embodiments provided in the present application, it should be understood that the disclosed apparatus and method can be implemented in other ways. The apparatus embodiments described above are merely schematic, for example, the division of units is only a logical function division, and actual implementation can have another division manner, for example, a plurality of units or components can be combined or integrated into another system, or some features can be ignored or not executed. In addition, the coupling or direct coupling or communication connection between the units shown or discussed can be indirect coupling or communication connection through some communication interfaces, devices or units, which can be electrical, mechanical or other forms.

[0046] The units described as separate components can or can not be physically separate, and the components shown as units can or can not be physical units, that is, they can be located in one place, or can be distributed on a plurality of network units. Part or all of the units can be selected according to actual needs to achieve the purpose of the embodiment scheme.

[0047] In addition, the functional units in each embodiment of the present application can be integrated into a processing unit, or each unit can be physically present separately, or two or more units can be integrated into one unit.

[0048] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and they should be covered in the scope of the claims and the specification of the present application.

Claims

1. A method for fabricating an anti-tapered special optical fiber, characterized in that, Includes the following steps: S1: The quartz tube is subjected to impurity removal and rare earth particle doping treatment to obtain a hollow quartz preform. S2: Insert the standard traction rod into the interior of the quartz preform, wherein the head of the standard traction rod is the standard tool head; S3: Fix the standard traction rod and the quartz preform rod after they have been threaded through; S4: The quartz preform is heated to draw it into a hollow optical fiber that meets the outer diameter requirements; S5: Heating the head of the hollow optical fiber and the standard traction rod to soften the hollow optical fiber. Due to thermal expansion, the head of the standard rod causes the inner diameter of the hollow optical fiber to increase uniformly, forming an optical fiber core with a gradually changing inner diameter. S6: Apply a coating to the hollow optical fiber and then cure it; S7: Perform a second coating on the hollow optical fiber after the first coating to form an anti-tapered special optical fiber.

2. The preparation method according to claim 1, characterized in that, The rare earth particles are doped with erbium, ytterbium or neodymium, with a doping concentration of 0.1~1 mol.

3. The preparation method according to claim 1, characterized in that, The standard traction rod is made of molybdenum or tungsten, and its coefficient of linear expansion is 4.1 × 10⁻⁶. -6 ~5.1×10 -6 / K, and the tensile strength is ≥379MPa at temperatures above 2000℃.

4. The preparation method according to claim 1, characterized in that, In step S3, the standard traction rod is fixed at the top clamp of the wire drawing tower, and the precast rod is fixed at the precast rod fixing clamp.

5. The preparation method according to claim 1, characterized in that, In step S4, the heating temperature is 1700~2000℃, and the outer diameter of the hollow optical fiber formed by drawing is less than 100μm.

6. The preparation method according to claim 1, characterized in that, In step S5, the heating temperature is 1300~1650℃. During the drawing process, the outer diameter parameter of the optical fiber is monitored in real time by the outer diameter control etalon to adjust the heating temperature.

7. The preparation method according to claim 1, characterized in that, Between step S5 and step S6, the preparation method further includes: performing optical inspection on the fiber core of the hollow optical fiber; if the inspection fails, the steps S4-S5 are repeated.

8. The preparation method according to claim 1, characterized in that, In step S6, the material coated once is polyolefin or PBT, and the curing is performed by ultraviolet curing or heat curing.

9. The preparation method according to claim 1, characterized in that, The inverse-conical special optical fiber has an inverse-conical structure with a symmetrically gradually changing side.

10. The preparation method according to claim 1, characterized in that the inner diameter of the anti-tapered special optical fiber changes from the input end to the output end as d0→c0→b0→a0→b0→c0→d0, where a0<b0<c0<d0.

11. A special anti-tapered optical fiber prepared according to the preparation method of any one of claims 1-9, characterized in that, include: The core transmission layer, active layer, outer cladding layer, and coating layer.

12. The anti-tapered special optical fiber according to claim 11, characterized in that, The inverse-conical special optical fiber has an inverse-conical structure with a symmetrically tapered shape on the side.

13. The anti-tapered special optical fiber according to claim 11, characterized in that, The inner diameter of the anti-tapered special optical fiber changes from the input end to the output end as d0→c0→b0→a0→b0→c0→d0, where a0<b0<c0<d0.

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