Alkali metal doping device and method for silicon dioxide tube

Abstract

The invention belongs to the field of optical fiber processing, and particularly discloses an alkali metal doping device and method for a silicon dioxide tube, the device comprises a shell, and the shell is hollow and is provided with a containing cavity and an opening communicated with the containing cavity; the cover body covers the opening; the induction heat conduction rod is arranged in the containing cavity and rotationally connected to the cover body; the induction heat conduction connecting piece is connected to the end, away from the cover body, of the induction heat conduction rod, a material groove is formed in the induction heat conduction connecting piece, and the tail end of the induction heat conduction rod is inserted into the material groove; the first induction coil is wound on the outer wall of the shell and is opposite to the induction heat conduction rod, and the induction heat conduction rod emits heat when the first induction coil is electrified; and the second induction coil is wound on the outer wall of the shell and is opposite to the induction heat conduction connecting piece, and the induction heat conduction connecting piece emits heat when the second induction coil is electrified. Through the structural design, the doping effect of the silicon dioxide tube doped with the alkali metal can be effectively improved, and the doping process is more efficient and energy-saving.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical fiber processing, and more particularly relates to an alkali metal doping device and method for a silica tube. BACKGROUND

[0002] Optical fiber has become the core medium of long-distance trunk, metropolitan and access network due to its characteristics of large transmission capacity, long transmission distance and fast transmission speed. By reducing the attenuation coefficient of the optical fiber, the transmission loss can be effectively reduced, thereby prolonging the non-repeater transmission distance, reducing the number of repeater stations and operation and maintenance costs, optimizing the network structure and reducing the whole life cycle operation expenditure.

[0003] Alkali metal doping in the core layer and inner cladding layer of the optical fiber can optimize the viscosity matching between the core and cladding, thereby effectively reducing the attenuation of the optical fiber. At present, the diffusion method is generally used to dope alkali metal elements into the silica tube. This method uses high-purity alkali metal elements or alkali metal salt (purity above 99.9%) vapor as raw material to introduce alkali metal elements into the tube, and then heats the glass tube by using an external local heat source to diffuse the alkali metal elements to the inner surface of the glass tube.

[0004] In the related art, the common silica tube alkali metal doping device usually adopts short hot zone moving doping. The specific implementation is to move back and forth by using a short hot zone heating device, and the alkali metal vapor inside the silica tube is diffused and doped into the inner wall of the silica tube by the high-temperature heat source. However, this method can cause the silica tube to be suddenly heated and suddenly cooled, forming a stress concentration area. When the tensile stress exceeds the tensile strength of the silica tube, micron-level cracks will be generated on the surface of the silica tube. The diffusion path of the alkali metal ions is blocked, the alkali metal doping concentration fluctuation is increased, and the alkali metal doping efficiency is decreased. SUMMARY

[0005] In view of the defects of the prior art, the present application provides an alkali metal doping device and method for a silica tube, which aims to solve the problems of low doping efficiency and poor effect of the existing silica tube alkali metal doping device.

[0006] The present application provides an alkali metal doping device for a silica tube, comprising: a housing, the housing is hollow and provided with a containing cavity and an opening communicating with the containing cavity; a cover body, the cover body is arranged on the opening; an induction heat conduction rod, the induction heat conduction rod is arranged in the containing cavity and rotationally connected to the cover body; An induction heat conduction connecting piece is connected to one end of the induction heat conduction rod away from the cover, and a material groove is arranged in the induction heat conduction connecting piece, and the end of the induction heat conduction rod is inserted into the material groove and is arranged spaced apart from the inner side wall of the material groove. A first induction coil is arranged on the outer wall of the shell in a position corresponding to the induction heat conduction rod, and the induction heat conduction rod generates heat when the first induction coil is energized. A second induction coil is arranged on the outer wall of the shell in a position corresponding to the induction heat conduction connecting piece, and the induction heat conduction connecting piece generates heat when the second induction coil is energized.

[0007] Compared with the prior art, the above technical scheme conceived by the present application can avoid the formation of stress concentration areas due to sudden heating and cooling by heating the entire interior of the silica tube through the induction heat conduction rod, and can avoid the re-formation of crystals and adhesion under cold conditions by doping the interior of the silica tube with alkali metal vapor, while the heating temperature control is more accurate, and the doping of alkali metal can be followed by slow cooling and annealing to optimize the relaxation of the silica tube structure, thereby achieving the beneficial effects of improved doping effect and more energy-saving doping process.

[0008] As a further preferred, the induction heat conduction rod comprises a center induction heat conduction rod and upper and lower heat insulation rods connected to both ends of the center induction heat conduction rod, the upper heat insulation rod is detachably connected to the cover, and the lower heat insulation rod is detachably connected to the induction heat conduction connecting piece.

[0009] As a further preferred, the middle part of the cover is rotatably connected to a rotatable hook, the rotation axis of the rotatable hook is coaxially arranged with the central axis of the shell, and the upper heat insulation rod is detachably mounted on the rotatable hook.

[0010] As a further preferred, the alkali metal doping device further comprises a rotary drive member, the rotary drive member is arranged on the cover, and the output shaft of the rotary drive member is connected to the rotatable hook.

[0011] As a further preferred, the outer peripheral wall of the rotatable hook is provided with a positioning protrusion, a plurality of raised protrusions are arranged on the cover around the peripheral side of the rotatable hook, and the positioning protrusion intermittently contacts the raised protrusions when the rotatable hook rotates.

[0012] As a further preferred, the alkali metal doping device comprises a magnetic fluid seal arranged on the rotary drive member for sealing the gap between the cover and the rotary drive member.

[0013] As a further preferred, the silica tube comprises a main tube and upper and lower extension tubes coaxially connected to opposite ends of the main tube, the alkali metal doping device comprises a positioning cover, the positioning cover is sleeved on the outer periphery of the upper heat insulation rod and connected to the end face of the upper extension tube, the induction heat conduction connecting piece is provided with an assembly groove around the trough, and the lower extension tube is inserted into the assembly groove.

[0014] As a further preferred, the shell is cylindrically arranged and comprises a bottom wall and a side wall connected to the outer periphery of the bottom wall, and the central axis of the induction heat conduction rod coincides with the central axis of the shell.

[0015] As a further preferred, the shell is provided with an air inlet pipeline and a vacuum pipeline, the inner wall of the shell is coated with a protective layer, and the shell is connected with an infrared temperature measuring tube.

[0016] The application provides a method for doping alkali metal into a silica tube, which is performed by using the above device and comprises the following steps. S1: connecting the induction heat conduction rod with the induction heat conduction connecting piece and placing alkali metal raw materials in the trough of the induction heat conduction connecting piece; S2: sleeving the silica tube on the outer periphery of the induction heat conduction rod, connecting the silica tube with the induction heat conduction connecting piece, and then integrally installing the silica tube and the induction heat conduction connecting piece into the containing cavity and covering the cover body; S3: after the containing cavity is vacuumized, starting the first induction coil, the first induction coil inducing the induction heat conduction rod to heat and preheating the silica tube; S4: after the preheating of the silica tube is completed, starting the second induction coil, the second induction coil inducing the induction heat conduction connecting piece to heat, heating the alkali metal raw materials to make the alkali metal raw materials gasify, alkali metal vapor diffusing into the preheated silica tube and completing the doping, and obtaining a silica doping tube; S5: stopping the heating, introducing inert gas into the containing cavity to replace residual alkali metal vapor, and then cooling the silica doping tube to room temperature.

[0017] Overall, compared with the prior art, the above technical solutions conceived by the application mainly have the following technical advantages: 1. The application effectively improves the doping effect of the silica tube through the synergistic effect of the vacuum environment and internal overall heating. The vacuum environment effectively reduces the boiling point of alkali metals, making them more easily vaporized, while reducing impurity interference and ensuring the purity of the vapor. The internal overall heating method avoids the temperature gradient problem caused by traditional external heating, ensuring that the silica tube is evenly heated and avoiding uneven doping and thermal stress cracking caused by local overheating or uneven heating. In addition, the vibration generated by the rotatable hook in combination with the raised protrusion further promotes the uniform distribution of alkali metal vapor in the silica tube, reducing the attachment of bubbles and impurities, significantly improving the uniformity and purity of doping, and thus greatly improving the doping quality.

[0018] 2. The application directly introduces alkali metal vapor into the sealed space of the silica tube, which effectively avoids the ineffective diffusion of the vapor in the receiving cavity, significantly improves the utilization rate of the alkali metal raw material, and reduces the waste of raw materials. The heating assembly of the tank precisely heats the induction heat-conducting tank with the second induction coil, which can accurately control the vaporization rate and vapor pressure of the alkali metal, further optimize the doping process, ensure the uniformity and stability of the doping, and thus greatly improve the doping efficiency and achieve high-efficiency doping.

[0019] 3. The tube heating assembly of the application uses an induction heat-conducting rod and a first induction coil to achieve internal overall heating of the silica tube, which can accurately control the temperature inside the tube and ensure the stability and uniformity of the doping process. After doping is completed, the device can slowly cool and anneal, optimizing the structure relaxation of the silica tube and further improving the doping effect. At the same time, the vacuum environment and internal overall heating method improve the thermal efficiency and reduce heat loss; the vibration mechanism and precise heating control optimize the doping process, shorten the heating time and reduce energy consumption, achieve energy saving and consumption reduction, and make the entire doping process more efficient and energy-saving. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is the overall structure schematic diagram of the alkali metal doping device for the silica tube provided by the embodiment of the application; Figure 2 is Figure 1 the enlarged structure schematic diagram of part A in Figure 3 is Figure 1 the enlarged structure schematic diagram of part B in Figure 4 is the top view structure schematic diagram of the shell provided by the embodiment of the application; Figure 5 is the stress birefringence value comparison diagram of the silica doping tube prepared by the embodiment of the application and the short hot zone mobile doping tube; Figure 6is a core rod doping concentration distribution diagram prepared when the potassium doping amount of the silica tube is 148 ppm, provided by an embodiment of the present application; Figure 7 is a core rod doping concentration distribution diagram prepared when the potassium doping amount of the silica tube is 199 ppm, provided by an embodiment of the present application; Figure 8 is a core rod doping concentration distribution diagram prepared when the potassium doping amount of the silica tube is 465 ppm, provided by an embodiment of the present application.

[0021] In all the drawings, the same reference signs are used to represent the same elements or structures, wherein: 1, shell; 11, air inlet pipe; 12, vacuum pipe; 13, infrared temperature measurement pipe; 14, containing cavity; 2, cover; 21, rotatable hook; 22, positioning protrusion; 23, elevated protrusion; 3, induction heat conduction rod; 31, central induction heat conduction rod; 32, upper heat insulation rod; 33, lower heat insulation rod; 4, induction heat conduction connecting piece; 41, trough; 42, assembly groove; 5, first induction coil; 6, second induction coil; 7, silica tube; 71, main tube; 72, upper extension tube; 73, lower extension tube; 8, positioning cover; 9, magnetic fluid seal. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical scheme and advantages of the present application more clear and obvious, the present application is further described in detail below in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0023] The alkali metal doping device for the silica tube disclosed in the present application constructs a controllable vacuum heating environment, heats the alkali metal source in the vacuum environment, effectively reduces the boiling point of the alkali metal in the vacuum environment, and makes it more easily vaporized. Meanwhile, the combination of internal heating of the silica tube and vapor introduction is used to realize efficient and uniform bulk doping of the silica tube. In the doping process, the interior of the silica tube is constructed as a closed space.

[0024] Reference Figures 1-4In the embodiment, the device comprises a shell 1, a cover 2, an induction heat-conducting rod 3, an induction heat-conducting connecting piece 4, a first induction coil 5 and a second induction coil 6. The shell 1 is the core reaction chamber of the whole device, which is in a cylindrical structure and comprises a bottom wall and a side wall connected to the outer periphery of the bottom wall. The shell 1 is hollow inside and is provided with a cylindrical accommodating cavity 14. The top of the shell 1 is provided with an opening communicating with the accommodating cavity 14. The cover 2 is arranged on the opening. The silica tube 7 and the induction heat-conducting rod 3 are both installed in the accommodating cavity 14, and the central axes of the two coincide with the central axis of the shell 1. The accommodating cavity 14 can be constructed as a vacuum space. After the cover 2 is arranged on the opening, the accommodating cavity 14 can be connected with a vacuum system (not shown in the figure) to be pumped to and maintain a required vacuum degree, so as to provide a suitable environment for the alkali metal doping process of the silica tube 7.

[0025] The induction heat-conducting rod 3 is located inside the silica tube 7 and is rotationally connected to the top end of the cover 2. The induction heat-conducting connecting piece 4 is connected to one end of the induction heat-conducting rod 3 away from the cover 2. A groove 41 for placing alkali metal is formed in the induction heat-conducting connecting piece 4. The end of the induction heat-conducting rod 3 is inserted into the groove 41 and is spaced apart from the inner side wall of the groove 41. When the induction heat-conducting connecting piece 4 is installed in place, the groove 41 communicates with the internal sealed space of the silica tube 7, so that the alkali metal vapor can be smoothly introduced into the sealed space of the silica tube 7, thereby realizing effective doping of the silica tube 7 by the alkali metal.

[0026] The first induction coil 5 is wound on the outer wall of the shell 1 and is arranged in position with the induction heat-conducting rod 3. The induction heat-conducting rod 3 generates heat when the first induction coil 5 is powered on. This heating method can perform omnidirectional and uniform overall radiation heating on the wall of the silica tube 7 from the inside of the silica tube 7, and can accurately control the temperature in the silica tube 7, so as to ensure that the silica tube 7 is evenly heated in the whole doping process, and effectively avoid problems such as uneven doping or thermal stress cracking caused by temperature gradient, thereby improving the quality and reliability of the doping. The second induction coil 6 is wound on the outer wall of the shell 1 and is arranged in position with the induction heat-conducting connecting piece 4. The induction heat-conducting connecting piece 4 generates heat when the second induction coil 6 is powered on. This heating method can accurately and locally heat the induction heat-conducting connecting piece 4, so as to make the alkali metal gasify in the groove 41, and the gasification rate and vapor pressure are easy to control. The design of the groove 41 of the induction heat-conducting connecting piece 4 should ensure that the alkali metal raw material can be uniformly heated, and local overheating or uneven heating should be avoided. The induction heat-conducting connecting piece 4 and the induction heat-conducting rod 31 are both made of one of aluminum nitride, high-thermal-conductivity silicon carbide and hexagonal boron nitride-based composite materials.

[0027] The silica tube 7 in the embodiment is processed by internal overall and continuous heating in a vacuum environment. Compared with the traditional heating method, the heating method has higher thermal efficiency and can more efficiently transfer heat to each part of the silica tube 7. At the same time, since the heating is from the inside, the heat distribution is more uniform, forming a uniform temperature field. This uniform temperature field ensures that the diffusion dynamics process of the alkali metal in the wall of the silica tube 7 remains consistent, thereby significantly improving the uniformity and effect of doping. In addition, in the embodiment, the alkali metal vapor is directly introduced into the sealed space to be doped. This design avoids the invalid diffusion of the alkali metal vapor in the vacuum containing cavity 14 of the shell 1, so that the alkali metal vapor can directly act on the silica tube 7 to be doped. This not only improves the utilization rate of raw materials and reduces waste, but also improves the doping efficiency. At the same time, since the alkali metal vapor does not diffuse in the vacuum containing cavity 14, the pollution to the vacuum containing cavity 14 is reduced, the cleaning and maintenance cost is reduced, and the operation efficiency and reliability of the entire device are improved.

[0028] Specifically, to form a vacuum environment inside the containing cavity 14, the cover 2 is arranged on the opening to seal the top opening of the shell 1, ensuring that a vacuum environment can be formed and maintained in the tube. The shell 1 is also provided with a gas inlet pipe 11 and a vacuum pumping pipe 12. The gas inlet pipe 11 is located at the bottom left side of the shell 1, and high-purity helium, chlorine, nitrogen and other process gases can be introduced through the gas inlet pipe 11 during the doping process. The vacuum pumping pipe 12 is located on the side wall of the shell 1 and is connected with a vacuum pump for pumping out the gas in the containing cavity 14 to form and maintain the required vacuum degree. Specifically, the shell 1 is a high-purity glass cylinder with an inner diameter of 400 mm, an outer diameter of 415 mm and a height of 4000 mm, and the content of impurities therein is ≤100 ppm. In addition, the inner wall of the shell 1 is also coated with a protective layer, which is zirconium oxide with a thickness of 30-60 μm. This can effectively prevent the diffusion and penetration of alkali metal vapor into the containing cavity 14, thereby protecting the tube body from corrosion by alkali metal vapor and prolonging the service life of the equipment, while ensuring the stability and reliability of the doping process. In order to realize accurate temperature control of the doping process, a plurality of infrared temperature measuring tubes 13 are fixedly installed on the shell 1. These infrared temperature measuring tubes 13 are connected with an optical fiber infrared temperature measuring instrument and can monitor the temperature change in the containing cavity 14 in real time. Through the temperature feedback system, the required doping and subsequent annealing process temperature can be accurately adjusted according to the actual measured temperature data. This temperature control method ensures the temperature uniformity and stability of the entire doping process, further improving the uniformity and effect of doping.

[0029] More specifically, the induction heat-conducting rod 3 comprises a central induction heat-conducting rod 31 and an upper heat-insulating rod 32 and a lower heat-insulating rod 33 connected to both ends of the central induction heat-conducting rod 31. The central induction heat-conducting rod 31 has good thermal conductivity, can quickly transfer heat to the inside of the silica tube 7, is resistant to high temperature and oxidation, can work stably in an operating environment of 800-1400℃, has extremely low impurity content, OH content <1ppm, total impurity content <10ppm, ensures the purity of the doping process, has high tensile strength (1200-1400 MPa), does not react with chlorine, oxygen and alkali metals in a high temperature environment, and ensures the stability and safety of the heating process; the diameter of the central induction heat-conducting rod 31 is 30mm, and the length is greater than that of the silica tube 7, so that it can penetrate the silica tube 7 coaxially after installation. The central induction heat-conducting rod 31 generates heat through electromagnetic induction, heats the inside of the silica tube 7, and provides a stable and uniform heat source. This heating method ensures that the silica tube 7 is evenly heated during the entire doping process, and avoids uneven doping or thermal stress cracking caused by temperature gradient. The first induction coil 5 is coaxially sleeved outside the outer shell 1 and corresponds to the position of the central induction heat-conducting rod 31. Through the principle of electromagnetic induction, the first induction coil 5 can generate heat for the central induction heat-conducting rod 31, thereby heating the inside of the silica tube 7.

[0030] In the embodiment, the upper heat-insulating rod 32 and the lower heat-insulating rod 33 are coaxially fixedly connected to the upper end and the lower end of the central induction heat-conducting rod 31 respectively, the upper heat-insulating rod 32 is detachably connected with the cover 2, and the lower heat-insulating rod 33 is detachably connected with the induction heat-conducting connector 4. The upper heat-insulating rod 32 and the lower heat-insulating rod 33 are both silicon nitride rods, have good heat-insulating performance, and will not be heated by electromagnetic eddy current, are resistant to oxidation (1400℃) and corrosion, have a tensile strength of 800-1000 MPa (1400℃), and the main function is to lock the high temperature zone, prevent the central induction heat-conducting rod 31 from being heated too quickly, and thus ensure that heat is mainly concentrated in the inside of the silica tube 7, thereby improving the thermal efficiency and doping effect.

[0031] In this embodiment, the silica tube 7 includes a main tube 71 and upper and lower extension tubes 72 and 73 coaxially welded to opposite ends of the main tube 71, and the alkali metal doping device includes a positioning cover 8 sleeved on the outer periphery of the upper heat insulation rod 32 and connected to the end face of the upper extension tube 72. The induction heat conduction connector 4 is provided with an assembly groove 42 around the material groove 41, and the lower extension tube 73 is inserted into the assembly groove 42. The positioning cover 8 and the induction heat conduction connector 4 are respectively used for plugging the two ends of the silica tube 7 and positioning the central induction heat conduction rod 31, so that the inside of the main tube 71 forms a sealed space. Specifically, the positioning cover 8 is coaxially sleeved on the outside of the upper heat insulation rod 32, used for plugging the upper end of the silica tube 7 and positioning the upper end of the central induction heat conduction rod 31, ensuring that the central induction heat conduction rod 31 remains stable during heating and avoiding displacement due to thermal expansion or other factors. The induction heat conduction connector 4 is detachably connected to the lower heat insulation rod 33 and used for placing alkali metal raw materials. The material groove 41 of the induction heat conduction connector 4 is added with an appropriate amount of alkali metal raw materials, such as potassium bromide, potassium chloride, potassium iodide and other alkali metal halides. In this embodiment, potassium bromide is selected as the alkali metal dopant. The material groove 41 of the induction heat conduction connector 4 is in communication with the lower end of the silica tube 7, ensuring that the alkali metal vapor can smoothly enter the sealed space of the silica tube 7. The inner and outer diameters of the upper extension tube 72 are consistent with those of the silica tube 7, and the inner and outer diameters of the lower extension tube 73 are adapted to those of the induction heat conduction connector 4. The fusion part of the lower extension tube 73 and the main tube 71 adopts smooth transition, which is conducive to the flow of alkali metal vapor.

[0032] Further, to realize the installation and dismounting of the silica tube 7, a rotatable hook 21 is rotationally connected to the middle part of the cover body 2. The rotatable hook 21 is located in the accommodating cavity 14 and the rotation axis is coaxially arranged with the middle axis of the outer shell 1. The upper heat insulation rod 32 is detachably mounted on the rotatable hook 21. The silica tube 7 is driven to rotate by the rotatable hook 21, so that the silica tube 7 can be heated uniformly in all directions during heating. A rotary drive is mounted on the cover body 2. The rotatable hook 21 is connected with the output shaft of the rotary drive. The rotary drive provides rotary power for the rotatable hook 21. By accurately controlling the rotation speed and direction, it is ensured that the silica tube 7 can rotate uniformly during heating. This automatic driving mode not only improves the accuracy of operation, but also reduces manual intervention and reduces the risk of operation. In addition, a magnetic fluid seal 9 is arranged on the cover body 2. The magnetic fluid seal 9 is arranged on the rotary drive and used for sealing the gap between the cover body 2 and the rotary drive. The magnetic fluid seal 9 is a device that uses the magnetism and fluidity of magnetic fluid to realize sealing. It can effectively prevent gas leakage in the rotating state of the rotatable hook 21 and ensure that the vacuum environment in the accommodating cavity 14 remains stable. It is a commonly used device in the field and will not be described in detail here.

[0033] In this embodiment, in order to further optimize the heating and doping process of the silicon dioxide tube 7, a positioning protrusion 22 is arranged on the outer peripheral wall of the rotatable hook 21, the protruding direction of the positioning protrusion 22 is perpendicular to the axis direction of the rotatable hook 21, a plurality of raised protrusions 23 are arranged on the cover body 2 around the peripheral side of the rotatable hook 21, the raised protrusions 23 protrude from the upper surface of the cover body 2, when the rotatable hook 21 rotates, the positioning protrusion 22 intermittently contacts the raised protrusions 23, and when the two contact, the positioning protrusion 22 drives the rotatable hook 21 to rise, and then the positioning protrusion 22 and the rotatable hook 21 descend together, through this movement mode, the silicon dioxide tube 7 generates regular and slight vibration, specifically, the contact surfaces of the positioning protrusion 22 and the raised protrusions 23 are arc surfaces, so as to avoid the interference of the raised protrusions 23 on the rotation of the positioning protrusion 22, and facilitate the synchronous rotation of the positioning protrusion 22 with the rotatable hook 21. The vibration helps the alkali metal vapor in the silicon dioxide tube 7 to be more uniformly distributed, avoids the problem of uneven doping caused by uneven distribution of the vapor, and also makes the alkali metal loose, can be uniformly heated, improves the diffusion of the alkali metal vapor, and improves the doping efficiency.

[0034] The application also discloses a method for doping alkali metal in a silicon dioxide tube, which is performed by using the device and comprises the following steps. S1: first, connect the induction heat conductor 3 with the induction heat connecting piece 4, place the alkali metal raw material in the trough 41 of the induction heat connecting piece 4, then connect the silicon dioxide tube 7 with the induction heat connecting piece 4 and install the whole into the containing cavity 14, specifically, the silicon dioxide tube 7 is sleeved on the periphery of the induction heat conductor 3, the two ends of the silicon dioxide tube 7 are respectively sealed by the positioning cover 8 and the induction heat connecting piece 4, the trough 41 of the induction heat connecting piece 4 is filled with 5g of the alkali metal potassium bromide raw material 5, then the silicon dioxide tube 7 is hung on the rotatable hook 21 on the cover body 2 through the upper heat insulation rod 32, and the installation is completed. When installing, the silicon dioxide tube 7 is slowly lowered to avoid the contact between the central induction heat conductor 31 and the inner wall of the silicon dioxide tube 7 caused by shaking.

[0035] S2: after the containing cavity 14 is vacuumized, the first induction coil 5 is started to preheat the silicon dioxide tube 7, the inside of the shell 1 is vacuumized, and the process needs to be repeated for multiple times. The specific operation is as follows: a vacuum pump and a pressure gauge are used to control the pressure in the containing cavity 14, when the pressure in the containing cavity 14 reaches 10Kpa, the vacuumization is stopped and high-purity helium is introduced, until the pressure approaches normal pressure. After the above process is repeated for about 6 times, the containing cavity 14 is filled with helium.

[0036] The first induction coil 5 is powered to heat the center induction heat rod 31, and the temperature rising rate is 20°C / min. When the temperature is heated to 900°C, high-purity chlorine gas, helium gas, and the mixed gas of the above two processes (the flow rate of chlorine is 1 L / min, and the flow rate of helium is 15 L / min) are introduced. The action is to purify the atmosphere in the containing cavity 14 and the impurity atmosphere after the reaction, and the exhaust is discharged into the waste gas recovery pipeline through another pipeline connected to the vacuum pipeline. The exhaust pipeline and the vacuum pipeline are separated by a corrosion-resistant electromagnetic valve to prevent chlorine-containing waste gas from entering the vacuum pump and corroding the rotor. The entire purification process takes 30 min, after which the chlorine gas and helium gas are turned off, and 30 min of high-purity argon gas (30 L / min) is introduced to completely replace the chlorine gas in the containing cavity 14.

[0037] S3: After the preheating of the silicon dioxide tube 7 is completed, the second induction coil 6 is started to heat the induction heat connecting piece 4 to gasify the alkali metal, and the alkali metal vapor diffuses into the preheated silicon dioxide tube 7 and completes the doping to obtain a silicon dioxide doped tube. The first induction coil 5 is used to continue heating the center induction heat rod 31 to 1200°C at a temperature rising rate of 15°C / min. At the same time, the second induction coil 6 is powered to heat the induction heat connecting piece 4 to 900°C at a temperature rising rate of 20°C / min. The pressure in the containing cavity 14 is drawn to 5 Kpa by the vacuum pump and maintained. In this vacuum environment of 5 Kpa, the boiling point of potassium bromide will be reduced to about 1250°C, and the potassium bromide vapor will rapidly and uniformly diffuse in the vacuum state, without causing uneven distribution of potassium bromide vapor in the silicon dioxide tube 7 and the containing cavity 14.

[0038] At 20 min, the center induction heat rod 31 is heated to 1200°C, and at 50 min, the induction heat connecting piece 4 is heated to 1000°C. At this time, the temperature of the induction heat connecting piece 4 exceeds the melting point of potassium bromide (734°C), and the stable release of potassium bromide vapor begins. Because the temperature of the center induction heat rod 31 reaches 1200°C in advance, it continuously heats the silicon dioxide tube 7, making the crystal lattice of the silicon dioxide tube 7 relatively loose at high temperature, and the high temperature allows the potassium ion to enter by decomposing the potassium bromide into 2KBr→2K+Br2. At the same time, the pressure change in the containing cavity 14 is observed. With the stable release and diffusion of potassium bromide vapor, the tube pressure of the outer shell 1 begins to rise, and the pressure in the containing cavity 14 finally changes from the initial 5 KPa to about 5.3 KPa and no longer increases, indicating that the potassium bromide has been completely dissolved, and thus the relative vapor pressure of potassium bromide in the silicon dioxide tube 7 is 0.3 KPa.

[0039] The center induction heat rod 31 is used to continuously heat the silica tube 7 for 2 hours, and the potassium doping amount of the silica doping tube reaches 148 ppm; the center induction heat rod 31 is used to continuously heat the silica tube 7 for 3 hours, and the potassium doping amount of the silica doping tube reaches 199 ppm; the potassium bromide raw material in the induction heat connecting piece 4 is increased from 5 g to 8 g, the center induction heat rod 31 is used to continuously heat the silica tube 7 for 3 hours, and the potassium doping amount of the silica doping tube reaches 465 ppm.

[0040] S4: stop heating, and inert gas is introduced into the containing cavity 14 to replace residual alkali metal vapor; then the silica doping tube is cooled to room temperature, and the doping of the silica tube 7 with alkali metal is completed.

[0041] The unreacted potassium bromide vapor in the containing cavity 14 is replaced, specifically, high-purity nitrogen gas (10 L / min) is introduced into the containing cavity 14, and when the pressure in the containing cavity 14 reaches close to normal pressure (50 Kpa~80 Kpa), the nitrogen gas is closed, and then the containing cavity 14 is pumped by a vacuum pump until the pressure is 10 Kpa, and then nitrogen gas is introduced again, and the above inflation and pumping process is repeated. At the same time, the center induction heat rod 31 is slowly cooled at a rate of 1 ℃ / min. This slow cooling can anneal and optimize the structure relaxation of the silica tube 7, eliminate the Si-O bond defects of the silica tube 7, greatly improve the axial and radial uniformity of alkali metal doping, and ensure that there is no crystallization and bubble in the tube during the subsequent high-temperature collapse of the tube. After 5 hours, the temperature of the center induction heat rod 31 and the silica tube 7 is reduced to 900 ℃, and the above inflation and pumping actions (10~20 times) need to be repeated multiple times to ensure that there is no potassium bromide vapor in the silica tube 7 to be recondensed and crystallized at low temperature. At this time, the containing cavity 14 is full of nitrogen gas. By using inert gas at high temperature, the unreacted alkali metal vapor is replaced to prevent the vapor from re-crystallizing on the wall of the silica tube 7 in a low-temperature environment.

[0042] When the temperature of the center induction heat rod 31 is reduced to 900 ℃, the cooling rate is increased to 2~3 ℃ / min, and the process is carried out at normal pressure, and the nitrogen gas is always on, and the process gas is extracted by the exhaust pipeline; when the temperature of the center induction heat rod 31 is reduced to 500 ℃, the heating can be stopped, and the natural cooling to room temperature is started.

[0043] By the above preparation steps, a silica tube doped with alkali metal (hereinafter referred to as silica doped tube) is obtained, which has a lower stress distribution, which is crucial to improve the performance and reliability of optical fiber. To accurately evaluate the stress level of the silica doped tube, the present embodiment uses a birefringence analyzer to test the stress of the silica tube. The test method measures the refractive index value to determine the stress size. The smaller the amplitude of the stress birefringence value, the smaller the tube stress. Specifically, during the stress test, first select the point from the outer wall of the tube to the inner wall of the tube for testing, and then select the point from the inner wall to the outer wall of the tube for testing. As shown in Figure 5 , by this bidirectional testing method, a wave-like stress birefringence value distribution can be obtained, so as to comprehensively evaluate the distribution of the stress in the tube. According to the test results, the stress birefringence value amplitude of the conventional short hot zone moving doped tube (comparative example) is usually about 20 nm; in contrast, the stress birefringence value amplitude of the doped tube prepared by the present application (embodiment) is significantly reduced, only about 10 nm. According to the results, the preparation process of the present application can effectively reduce the stress in the silica tube, thereby improving the overall performance and stability of the optical fiber.

[0044] The silica doped tube collapses into a solid rod at high temperature, which will become the core rod part of the low-loss optical fiber preform rod. According to the silica doped tube prepared above, when the silica doped tube with a potassium doping amount of 148 ppm is made into a core rod, the doping concentration distribution in the core rod is as shown in Figure 6 ; when the silica doped tube with a potassium doping amount of 199 ppm is made into a core rod, the doping concentration distribution in the core rod is as shown in Figure 7 ; when the silica doped tube with a potassium doping amount of 465 ppm is made into a core rod, the doping concentration distribution in the core rod is as shown in Figure 8 .

[0045] To realize the collapse of the silica tube doped with alkali metal, the cooled silica doped tube is first installed on a longitudinal collapsing device. Before the collapsing operation, the silica doped tube needs to be reduced in diameter.

[0046] During the diameter reduction process, a mixed gas composed of chlorine and helium is introduced into the interior of the silica doped tube, wherein the flow rate of chlorine is controlled to be more than 1.0 standard liter per minute (SLM) and not more than 2.0 SLM, and the flow rate of helium is controlled to be more than 2.0 SLM and not more than 5.0 SLM. The mixed gas is used to dehydrate and remove impurities in the interior of the silica doped tube, so as to ensure the purity of the tube and create a good internal environment for the subsequent collapsing operation.

[0047] Meanwhile, the silica-doped tube is heated by an external heat source. The temperature of the outer surface of the silica-doped tube is raised to 1700-2000°C by precisely controlling the heating temperature. During the heating process, the external heat source needs to move longitudinally to ensure that the silica-doped tube is uniformly heated. In addition, the silica-doped tube is controlled to rotate at a speed of 10 revolutions per minute (r / min), which helps to further improve the uniformity of heating and avoid local overheating or insufficient heating.

[0048] During the diameter reduction operation, the pressure inside the silica-doped tube also needs to be precisely controlled. Specifically, the pressure inside the tube needs to be maintained at a micro-negative pressure state, i.e., the pressure difference between the inside and outside of the tube is controlled to be between 300-500 Pa. This micro-negative pressure state helps to better control the gas flow inside the tube during the diameter reduction process, further promoting the removal of impurities, and also helps the silica-doped tube to maintain a stable shape and structure during the diameter reduction process. Through the above operation, the inner diameter of the silica-doped tube is reduced to more than 3 mm and not more than 8 mm.

[0049] During the etching process, in order to remove the impurities on the inner surface of the silica-doped tube, it needs to be treated by gas phase etching. The specific operation is as follows: the mixed gas of SF6 (sulfur hexafluoride) and oxygen is introduced into the inside of the silica-doped tube, wherein the flow rate of SF6 is controlled to be more than 0.5 SLM and not more than 2.0 SLM, and the flow rate of oxygen is also controlled to be more than 0.5 SLM and not more than 2.0 SLM; at the same time of introducing the mixed gas, the silica-doped tube is continuously heated by the external heat source. Through the synergistic effect, the impurities on the inner surface of the silica-doped tube can be effectively removed, and the etching thickness is controlled to be between 0.1-0.2 mm, thereby ensuring the purity and flatness of the inner surface and providing a high-quality material basis for subsequent processing and application.

[0050] During the solidification process, the silica-doped tube is converted into a solid structure. First, a single or mixed gas of oxygen and helium is introduced into the inside of the glass tube, wherein the flow rate of oxygen is controlled to be more than 0.1 SLM and not more than 0.5 SLM, and the flow rate of helium is controlled to be more than 0.5 SLM and not more than 1.0 SLM. At the same time of gas introduction, the absolute pressure inside the glass tube is gradually reduced to below 97 kPa, and the surface temperature of the glass tube is set to be above 1700°C and below 2100°C. In addition, the moving speed of the hot zone is controlled to be 10-20 mm / min to ensure that the glass tube is uniformly heated and gradually solidified. Through the above operation, a solid silica-doped core rod with a diameter of 41-43 mm is finally obtained.

[0051] Subsequently, the solid silica doped core rod containing the alkali metal dopant is placed in a heat treatment furnace for heat treatment. During the heat treatment, the core rod stays in the furnace for 15 hours, the furnace temperature is slowly decreased from 1200 °C to 500 °C at a rate of 1 °C / min, and then naturally cooled to room temperature. The heat treatment helps to stabilize the structure and properties of the core rod, ensuring its reliability and consistency in subsequent applications.

[0052] The prepared solid silica doped core rod is subjected to a stretching treatment to reduce its diameter. Subsequently, the core rod with reduced diameter is placed in an OVD (Outside Vapor Deposition) device for an outside spray matching cladding operation, thereby generating a porous powder rod with an alkali metal core rod at the center. The porous powder rod is subjected to fluorine doping treatment at a high temperature of 1000 °C to 1200 °C to optimize the optical performance of the optical fiber. After that, the fluorine-doped porous powder rod is sintered at a temperature range of 1300 °C to 1350 °C. At the end of the sintering stage, heat treatment needs to be performed again, i.e., the temperature is slowly decreased from 1200 °C to 500 °C, to ensure the structural stability and performance consistency of the preform rod. Through this series of process steps, an optical fiber preform rod with a diameter of 180 mm is finally prepared. Finally, the optical fiber preform rod is placed in a drawing tower, and a drawing operation is performed at a linear speed of 2000 m / min with an applied tension of 60 g, thereby preparing an optical fiber with low attenuation rate.

[0053] It is to be understood that the terms such as "include" and "may include" used in the present application indicate the presence of the disclosed functions, operations, or constituent elements, and do not limit one or more additional functions, operations, and constituent elements. In the present application, terms such as "include" and / or "have" can be interpreted to mean that a specific characteristic, number, operation, constituent element, component, or a combination thereof is present, but cannot be interpreted to mean that one or more other characteristics, numbers, operations, constituent elements, components, or combinations thereof are excluded.

[0054] It should be understood that the terms "center", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", 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 convenience of describing the present application and simplifying the description, and do not indicate or imply that the indicated device or element must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0055] In addition, the terms "first", "second", etc. are used only for the purpose of description and do not imply or imply relative importance or imply the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more features. In the description of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly specified and limited.

[0056] In the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connection", "fixing" and the like should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0057] Those skilled in the art will readily understand that the above description is only a preferred embodiment of the present application, and is not intended to limit the present application, and any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. An alkali metal doping apparatus for a silicon dioxide tube, characterized by, The alkali metal doping device comprises a shell (1) with a receiving cavity (14) and an opening communicating with the receiving cavity (14); a cover (2) covering the opening; an induction heat conduction rod (3) arranged in the receiving cavity (14) and rotationally connected to the cover (2); an induction heat conduction connecting piece (4) connected to one end of the induction heat conduction rod (3) away from the cover (2), the induction heat conduction connecting piece (4) being provided with a trough (41) in which the end of the induction heat conduction rod (3) is arranged; a first induction coil (5) wound on the outer wall of the shell (1) and arranged in position with the induction heat conduction rod (3), the induction heat conduction rod (3) being heated when the first induction coil (5) is powered; and a second induction coil (6) wound on the outer wall of the shell (1) and arranged in position with the induction heat conduction connecting piece (4), the induction heat conduction connecting piece (4) being heated when the second induction coil (6) is powered. The induction heat conduction rod (3) comprises a central induction heat conduction rod (31) and upper and lower heat insulation rods (32, 33) connected to the two ends of the central induction heat conduction rod (31), the upper heat insulation rod (32) being detachably connected to the cover (2), and the lower heat insulation rod (33) being detachably connected to the induction heat conduction connecting piece (4). The middle part of the cover (2) is rotationally connected to a rotatable hook (21), the rotation axis of the rotatable hook (21) is coaxially arranged with the central axis of the shell (1), and the upper heat insulation rod (32) is detachably mounted on the rotatable hook (21). The alkali metal doping device further comprises a rotary driving member arranged on the cover (2), and the output shaft of the rotary driving member is connected to the rotatable hook (21). The outer peripheral wall of the rotatable hook (21) is provided with a positioning protrusion (22), and a plurality of raised protrusions (23) are arranged on the cover (2) around the peripheral side of the rotatable hook (21), the positioning protrusion (22) intermittently contacts the raised protrusions (23) when the rotatable hook (21) rotates. The alkali metal doping device comprises a magnetic fluid seal (9) arranged on the rotary driving member for sealing the gap between the cover (2) and the rotary driving member. The silica tube (7) comprises a main tube (71) and upper and lower extension tubes (72, 73) coaxially connected to the opposite ends of the main tube (71), the alkali metal doping device comprises a positioning cover (8) sleeved on the outer periphery of the upper heat insulation rod (32) and connected to the end face of the upper extension tube (72), and the induction heat conduction connecting piece (4) is provided with a fitting groove (42) around the trough (41), and the lower extension tube (73) is arranged in the fitting groove (42). ​ 2. The alkali metal doping apparatus of claim 1, wherein ​ 3. The alkali metal doping apparatus of claim 2, wherein ​ 4. The alkali metal doping apparatus of claim 3, wherein ​ 5. The alkali metal doping apparatus of claim 3, wherein ​ 6. The alkali metal doping apparatus of claim 3, wherein ​ 7. The alkali metal doping apparatus of claim 2, wherein ​ 8. The alkali metal doping apparatus of claim 1, wherein The shell (1) is cylindrically arranged and comprises a bottom wall and a side wall connected to the outer periphery of the bottom wall, and the central axis of the induction heat conduction rod (3) coincides with the central axis of the shell (1).

9. The alkali metal doping apparatus of claim 1, wherein The shell (1) is provided with an air inlet pipeline (11) and a vacuum pipeline (12), the inner wall of the shell (1) is coated with a protective layer, and an infrared temperature measuring tube (13) is connected to the shell (1).

10. A method of doping a silicon dioxide tube with alkali metal using the alkali metal doping apparatus according to any one of claims 1 to 9, characterized by, The method comprises the following steps: S1: connecting the induction heat conduction rod (3) with the induction heat conduction connecting piece (4), and placing the alkali metal raw material in the trough (41) of the induction heat conduction connecting piece (4); S2: sleeving the silica tube (7) on the periphery of the induction heat conduction rod (3), connecting the silica tube (7) with the induction heat conduction connecting piece (4), and then integrally installing them into the containing cavity (14) and covering the cover (2); S3: after the containing cavity (14) is vacuumized, the first induction coil (5) is started, the first induction coil (5) induces the induction heat conduction rod (3) to heat, and the silica tube (7) is preheated; S4: after the preheating of the silica tube (7) is completed, the second induction coil (6) is started, the second induction coil (6) induces the induction heat conduction connecting piece (4) to heat, the alkali metal raw material is heated to be gasified, the alkali metal vapor diffuses into the preheated silica tube (7) and completes the doping, and a silica doped tube is obtained; S5: stop heating, introduce inert gas into the containing cavity (14) to replace the residual alkali metal vapor, and then cool the silica doped tube to room temperature.

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