JPS6346012B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an apparatus for manufacturing an optical fiber preform. [Prior art] Methods for producing a preform with a core-clad waveguide structure for optical fibers include the VAD method (vapor phase axis method) and the vapor phase axis method for silica-based optical fibers.
There is an MCVD method, and a theoretical limit of 0.2 dB/Km has been achieved (T. Miya, et al., Electron. Lett.,
15, 106). Further, when using a silicate oxide multicomponent glass, a double crucible method is used. On the other hand, since fluoride glass can transmit light with longer wavelengths than silica-based glass, it is thought that Rayleigh scattering will be smaller in the 3 μm band, and it is expected to have even lower loss values than quartz fiber. However, unlike oxide glass, fluoride glass has an extremely steep temperature-viscosity curve and is likely to crystallize in a high temperature range. Therefore, the VAD method and CVD method used to form silica-based glass preforms cannot be applied, and it has also been difficult to apply the double crucible drawing method. For this purpose, build-in casting methods and rotational casting methods have been developed. However, even if these methods are used, it is not possible to completely eliminate the occurrence of scattering centers, and it has not yet been possible to reduce the loss in a long fiber of 100 m or more. Therefore, in order to increase the length and reduce the loss, there is a need to develop a method for manufacturing a preform with fewer scattering centers. In addition, the build-in casting method
In the Rotational Casting method, there is room for water vapor in the atmosphere to get mixed in when forming the core-clad interface, and the absorption of OH groups is 2000~2000 at a wavelength of 3 μm.
Reaching 5000dB/Km. Therefore, there is also a need for a method for producing a preform that does not allow OH group contamination. [Purpose] The present invention was made in view of the above situation, and its purpose is to achieve low scattering and low OH when manufacturing an optical fiber using low viscosity glass.
An object of the present invention is to provide an apparatus for manufacturing a preform for forming a long, low-loss optical fiber. [Structure of the Invention] In order to achieve the above object, the present invention has a double-structured container consisting of a core melting section and a clad melting section surrounding the core melting section, and the container is placed in a predetermined atmosphere. A heating member for heating the core and cladding fusion zone is arranged around the core and cladding fusion zone, and a heating member is arranged downstream of the core and cladding fusion zone and connected to the core and cladding fusion zone, respectively. A nozzle is provided, a member for controlling opening and closing of the double nozzle is provided, a mold for forming an optical fiber preform is disposed downstream of the double nozzle, and a core flows out from the double nozzle. and a temperature control member is disposed around the mold to cool and solidify the melt in the mold, and a core-clad waveguide structure is provided in the mold. The present invention is characterized in that a preform for an optical fiber is formed. [Example] Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 shows an embodiment of an optical fiber preform manufacturing apparatus according to the present invention. In FIG. 1, 1 is a crucible stand, and on this crucible stand 1 is a double metal crucible which has a double structure of a core glass melting part 2 and a clad glass melting part 3 surrounding this core glass melting part 2. 4 will be placed. 5 is the gold lid of this crucible 4. Core and cladding glass melting sections 2 and 3
Nozzles 6 and 7 are formed at the bottom of each of the core and cladding outlets, respectively. Reference numeral 8 denotes a gold stopper that controls the opening and closing of these nozzles 6 and 7 to control the outflow of the melt. The above-described gold double crucible 4 is housed in a quartz container 9 and hermetically sealed, and dry Ar gas is introduced into the container 9. A heater 10 is arranged around the container 9 to heat and melt the core and clad glass raw materials filled in the core and clad glass melting sections 2 and 3, respectively. Inside the crucible stand 1, that is, on the downstream side of the double crucible 4, a hollow cylindrical brass mold 11 that can be divided into three parts in the vertical direction, surrounding the stopper 8, is fitted with a ring 1.
2 and a bottom ring 13, and are fixedly supported and arranged. Furthermore, a heater 14 for heating the mold as a temperature regulating member surrounds the mold 11.
are arranged to cool and solidify the melt of the core and cladding that flows out from the nozzles 6 and 7 and falls and is received in the mold 11. 15 is a base that supports the crucible stand 1, the bottom ring 13, and the heater 14, and the tip of the above-mentioned stopper 8 is made to protrude from an opening 16 formed in the center, so that the stopper 8 can be moved up and down. This opens and closes the plug 8 for the nozzles 6 and 7. An example in which a preform was manufactured using the manufacturing apparatus of the present invention described above will be shown below. First, 32.01BaF ₂ −3.88GdF ₃ −61.11ZrF ₄ −
41.42 g of a mixture with the composition of 3AlF ₃ (mol%)
A mixture of 24 g of NH ₄ F and HF was charged into the core glass melting section 2 shown in FIG. Next, 30.69BaF ₂ −
A mixture of 58.72 g of a mixture having a composition of 3.72GdF ₃ -58.59ZrF ₄ -7AlF ₃ (mol %) and 30 g of NH ₄ F.HF was charged into the clad glass melting section 3 . Next, the lid 5 of the double crucible 4 was closed. At this time, the tips of the nozzles 6 and 7 are closed in advance by inserting a gold stopper 8 as shown in Figure 1, so that the glass melt flows out from the double crucible 4 and the core glass melt and clad glass Make sure to prevent mixing with the melt. Next, heater 1
400~ for 1 hour in double crucible 4 by 0
Heated to 500°C. This heating decomposes the NH ₄ F・HF mixed in the raw materials in advance, resulting in the
HF fluorinated the oxides contained in the raw materials.
After this, the double crucible 4 is further heated by the heater 10.
was heated to 900° C. for 2 hours to dissolve the core and cladding materials. At this time, the quartz container 9
The atmosphere inside was maintained with dry Ar. During this time, the mold 11 is kept warm at 260°C by the heater 14, and the stopper 8 is pulled out to guide the melt maintaining the core-clad structure into the hollow part of the mold 11 through the nozzles 6 and 7. The mixture was cooled and solidified to obtain a preform for fluoride optical fiber. The obtained preform had an outer diameter of 8φ, a core diameter of 3φ, and a length of 270 mm. This is Teflon
The FEP tube was coated and drawn. The optical fiber thus obtained had a relative refractive index difference of 0.25%, a core diameter of 54 μm, and a cladding diameter of 145 μm. The minimum loss is 6 dB/Km at a wavelength of 2.1 μm, and the scattering loss due to structural imperfections such as crystallization of the entire optical fiber is extremely low.
It was possible to measure loss over the entire length of an optical fiber of 500 m. In addition, in the device of the present invention, the core does not come into contact with the atmosphere, and there is no room for OH to enter the core-clad interface, so the absorption of OH groups is about 3 μm, which is less than 1/100 of the conventional method. It was found that the reduction was 20dB/Km. Furthermore, in the present invention, since the temperature conditions at the tips of the nozzles 6 and 7 are constant due to the arrangement of the heater 14, the entire preform is placed under uniform cooling conditions, making it possible to manufacture a fluoride optical fiber with extremely low scattering loss. I understood. In addition, since the core melt is always cooled and solidified in the mold 11 while being surrounded by the cladding melt, no OH groups are introduced into the core from the atmosphere, making it extremely effective for producing low-OH fluoride optical fibers. I found out something. [Effects] As explained above, according to the present invention, a fluoride glass preform with low scattering loss and low OH can be manufactured, and therefore, the possibility of ultra-low loss values is suggested. In manufacturing fluoride optical fibers, it has the advantage of contributing to longer lengths and lower losses. Another advantage is that by making the double crucible and stopper from platinum instead of gold, the present invention can be used to manufacture optical fiber preforms using oxide glasses and chalcogenide glasses, which require melting at even higher temperatures. There is.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an optical fiber preform manufacturing apparatus according to the present invention. 1... Crucible stand, 2... Core glass melting section, 3
... Clad glass melting section, 4 ... Gold double crucible, 5 ... Gold lid for double crucible, 6 ... Nozzle as core outflow part, 7 ... Nozzle as clad outflow part, 8 ... Gold plug , 9...Quartz container, 1
0... Core and cladding raw material heater, 11
... Brass mold, 12 ... Ring, 13 ... Bottom ring, 14 ... Heater for heating the mold, 15 ... Base, 16 ... Opening.

Claims (1)

[Claims] 1. It has a double-structured container consisting of a core melting section and a cladding melting section surrounding the core melting section, the container is placed in a predetermined atmosphere, and the core and cladding melting section are surrounded by the core and cladding melting section. A heating member for heating the cladding melting section is disposed, a double nozzle is provided on the downstream side of the core and the cladding melting section, each of which is connected to the core and the cladding melting section, and a member is disposed for controlling opening and closing of the double nozzle. A mold for forming an optical fiber preform is arranged downstream of the double nozzle to receive the melt of the core and cladding flowing out from the double nozzle, and a mold for forming an optical fiber preform is arranged downstream of the double nozzle, and A temperature control member for cooling and solidifying the melt in the mold is arranged to form an optical fiber preform having a core-clad waveguide structure in the mold. Fiber preform manufacturing equipment.