JPH0225846B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of manufacturing an optical fiber that aims to improve the phenomenon of increased transmission loss due to aging. Background and Purpose It has recently been found that transmission loss increases with time in optical fiber cables.
The reasons for this are currently thought to be as follows. (1) Hydrogen gas is generated by hydrolysis or thermal oxidation of the plastic that is the constituent material of the optical fiber, and this hydrogen gas diffuses into the optical fiber, causing an increase in loss. (2) Some of the diffused hydrogen gas is

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[Formula], and some of it is retained as hydrogen molecules in the network structure of the glass. (3) Hydrogen gas can be generated not only from the above-mentioned plastics, but also when water gets into optical fiber cables made of dissimilar metals, hydrogen gas is generated by forming a battery. (4) Even when the same amount of hydrogen gas is generated, the loss increase characteristics differ depending on the composition and composition ratio of the optical fiber. In other words, the increase in OH groups will not bond with hydrogen unless defective oxygen with dangling bonds is already present in the glass. (5) The number and type of defects strongly depend not only on the composition and composition ratio of the optical fiber, but also on the manufacturing method and manufacturing conditions. This invention aims to provide a method for manufacturing an optical fiber that can reduce oxygen defects in the optical fiber as much as possible, particularly in consideration of the above points (4) and (5). This prevents the formation of OH groups even if hydrogen gas is generated within the cable and diffuses into the optical fiber, thereby improving loss increase characteristics. It is. Composition of the Invention The present invention includes the following steps in the optical fiber manufacturing process:
It is particularly characterized by the dehydration process of the soot preform, which includes: (1) performing the dehydration treatment of the soot preform by heating it in a heating furnace equipped with a furnace pressure adjustment device; ) Oxygen gas is added to the gas sent into the heating furnace, but the flow rate of the chlorine gas that has been sent conventionally is not changed; (3) The inside of the furnace is pressurized by adjusting the exhaust flow rate with the adjustment device. It is characterized by keeping it. More detailed explanation of the structure Generally, when manufacturing an optical fiber by the VAD method, the created soot preform is dehydrated and sintered. "FIG. 1" is a schematic explanatory diagram of a conventional dewatering device. 10 is a soot preform, 12 is a heating furnace, 14 is a heater, and 16 is a gas inlet, through which chlorine-based gas such as chlorine and He gas are fed.
18 is a gas outlet, and 20 is a device for adjusting the pressure inside the furnace, which adjusts the pressure inside the furnace by adjusting the flow rate of exhaust gas. An example of conventional dehydration conditions is shown in the following "Table 1".

【table】

[Table] Following this dehydration treatment, sintering treatment is performed. In that case, change the pulling speed and heating temperature to appropriate values. Note that dehydration and sintering may not be performed in two stages as described above, but may be performed in one process. By the way, in the present invention, during the above-mentioned dehydration treatment, oxygen gas is added to chlorine gas and He gas, and the mixture is fed through the gas inlet 16. However, at that time, the flow rates of chlorine gas and He gas remain the same as before. Further, by adjusting the exhaust flow rate using the regulating device 20, the inside of the furnace is kept in a pressurized state, and as a result, the oxygen gas is kept in a pressurized state. By performing dehydration treatment in such a gas atmosphere, the number of oxygen defects within the glass is to be greatly reduced. The reason why the number of oxygen defects in the optical fiber is extremely reduced when oxygen gas is added to chlorine gas or He gas and dehydration treatment is performed in an atmosphere where oxygen gas is pressurized is as follows. by. In other words, when glass is brought to a high temperature, oxygen defects occur with a certain probability. If there is excess oxygen in the surroundings at this time, oxygen defects will be less likely to occur than in the case where the reducing atmosphere and oxygen partial pressure are low. Example 1 A soot preform was produced by the VAD method,
It was dehydrated in a heating furnace under the conditions shown in Table 2 while varying the gas pressure in the furnace.

[Table] A transparent base material was then created by sintering under conventional conditions without oxygen gas. It was covered with a quartz jacket, spun in a conventional spinning furnace, and provided with a primary coating (two layers of a modified silicone resin and a conventional silicone resin) and a secondary coating (nylon) to obtain an optical fiber core. Its specifications are shown in "Table 3" below.

[Table] The transmission loss increase characteristics were determined by an accelerated test method. In other words, the fabricated optical fiber core wire is
The sample was kept in an oven at 200°C for 2 hours, and the loss wavelength characteristics before and after the oven were measured using a monochromator. The value of increase in loss was determined from the increase value at a wavelength of 1.39 μm, where the increase in loss due to OH groups is significant. The value of the loss increase is shown in the following "Table 4" together with the value of the conventional optical fiber produced by dehydration treatment without adding oxygen gas.

[Table] Example 2 The dehydration treatment was carried out under the same conditions as in "Table 2" above, and the sintering treatment was subsequently carried out under the conditions in the following "Table 5" with the addition of oxygen gas.

[Table] Then, spin and coat as above,
Optical fiber core wires having the same specifications as in Table 4 above were prepared, and loss increase characteristics were determined by accelerated tests in the same manner as above. The values are shown in the following "Table 6".

[Table] From the above experimental results, it is possible to add oxygen gas and pressurize the furnace, resulting in dehydration treatment or dehydration and sintering treatment in an atmosphere with pressurized oxygen gas. It can be seen that when this is done, the transmission loss increase characteristics are improved. Effects of the Invention The present invention was made based on the clarification of the cause of increase in transmission loss due to aging of optical fibers, which is caused by hydrogen gas generated within the optical fibers. In the present invention, in the dehydration and sintering process of the soot preform produced by the VAD method, at least the dehydration treatment is carried out in the following atmosphere,
That is, the heating furnace is provided with an internal pressure adjustment device, and oxygen gas is added to the gas fed into the heating furnace without changing the flow rate of the chlorine gas,
In addition, by adjusting the exhaust flow rate using the adjustment device, the inside of the furnace is pressurized, and as a result, the process is carried out in an atmosphere in which oxygen gas is pressurized, so that oxygen defects in the optical fiber are extremely reduced. Therefore, even if the generated hydrogen gas diffuses into the optical fiber, the amount of OH groups increases is very small. As a result, transmission loss will not increase significantly even after many years have passed since installation.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the dehydration sintering process common to the prior art and the present invention. 10: Soot preform, 12: Heating furnace, 1
6: Gas inlet, 20: Furnace gas pressure adjustment device.

Claims (1)

[Claims] 1. When performing the dehydration treatment and sintering treatment of the produced soot preform by heating it in an atmosphere containing chlorine gas in a heating furnace, the heating furnace is provided with an in-furnace pressure regulating device, and the heating Oxygen gas is added to the gas sent into the furnace without changing the flow rate of the chlorine gas, and the exhaust flow rate is adjusted by the adjustment device to make the inside of the furnace pressurized, and in such an atmosphere, A method for manufacturing an optical fiber, comprising performing at least the dehydration treatment.