JPS6245531B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an optical fiber cable that prevents an increase in transmission loss in a high temperature range of an optical fiber cable made by twisting fiber cores around a tension member.

Silica fiber, which can keep transmission loss extremely low compared to other fibers, holds great promise especially for communication applications, and many attempts are being made to create cables using it. be. Generally, the transmission loss of silica glass used as the core of a silica-based fiber does not change unless the temperature is raised to several hundred degrees Celsius. By the way, when manufacturing an optical fiber cable using this quartz fiber, the quartz fiber that becomes the fiber core is coated with synthetic resin and is integrated with other cable constituent materials such as tension members and intervening wires. It is bundled into a cable. in this case,
It is known that there is a serious drawback of increased transmission loss in the low temperature range due to the difference in temperature characteristics, especially linear expansion coefficient, between the synthetic resin coating material or other cable constituent materials and the quartz fiber. . for example,
As shown in Figure 1, which shows the cross section of the fiber core, when the coating material 2 covering the silica fiber 1 is made of nylon, its coefficient of linear expansion is 1 x 10 ^-4 cm/deg. The coefficient of linear expansion is 6×10 ^-7 cm/
deg, and the coefficient of linear expansion of the covering material 2 is greater than that of the quartz fiber 1 by two orders of magnitude or more. Therefore, as the fiber core 3 is cooled, the coating material also undergoes a large contraction, but when this contraction exceeds a certain level, the quartz fiber 1 occupying the center of the fiber core 3 buckles and causes microbending. A meandering phenomenon called deing occurs, which causes the light propagating within the silica fiber 1 to leak to the outside.
This is the main cause of increased transmission loss.

Therefore, this fiber core wire 3 in the low temperature range
In order to prevent the occurrence of microbending,
As shown in FIG. 2, which shows the cross-sectional structure of the fiber core wire 3 made into a cable, a buffer layer 6 consisting of an intervening wire is formed between the jacket 4 and the tension member 5.
A plurality of fiber core wires 3 are twisted spirally around the inner tension member 5, and the linear expansion coefficient of the metal tension member 5 is higher than the linear expansion coefficient of the optical fiber core wire 3 as a composite material. An optical fiber cable 7 has been developed that takes advantage of its small size to maintain the fiber core 3 in a taut state in a low temperature range. The ratio of linear expansion strain to temperature of such an optical fiber cable 7 is shown in Figure 3, with the strain state at 20°C, which is the factory room temperature in Japan, being zero. The broken line corresponds to the tension member 5. From this graph, it can be seen that in the temperature range lower than 20°C, the amount of contraction of the tension member 5 is smaller than the amount of contraction of the fiber core 3, and this fiber core 3 is in a state under tension by the tension member 5. can be easily determined, thereby making it possible to prevent microbending of the fiber core 3 in the low temperature range.

However, in a temperature range higher than 20°C, the amount of expansion of the fiber core 3 is greater than the expansion amount of the tension member 5, so microbending occurs in the fiber core 3, resulting in an increase in transmission loss. There was found.

Based on such knowledge, the present invention aims to provide an optical fiber cable in which microbending does not occur in the fiber core even in a high temperature range, and the present invention aims to manufacture the optical fiber cable of the present invention to achieve this purpose. The configuration according to the method includes an optical fiber cable in which a plurality of fiber core wires having a tension member at the center and a plurality of fiber core wires having a coefficient of linear expansion larger than the coefficient of linear expansion of the tension member are twisted around the tension member, The present invention is characterized in that the fiber core wires are twisted together while being heated to a temperature higher than that at which they are used as the optical fiber cable.

In other words, the present invention is designed to heat the fiber to a high temperature when winding it around the tension member so that the fiber is always kept under tension with respect to the tension member. It is. For example, assuming that the heating temperature of the fiber core is T, the ratio of linear expansion strain to the temperature of the optical fiber cable is shown in Figure 4, which is based on this temperature. , the amount of contraction of the tension member shown by the broken line is always smaller than the amount of contraction of the fiber core shown by the solid line, and the fiber core is always under tension by the tension member. No microbending occurs in the wire and therefore no increase in transmission loss. There is no practical problem if the temperature of T is set to the maximum operating temperature of the optical fiber cable, and generally around 60°C is considered appropriate.

On the other hand, regarding heating, there is no need to keep the entire device at a high temperature; it is sufficient to heat only the fiber cores just before they are twisted together.
For example, by heating the twisting die, by blowing hot air onto the fiber core just before twisting, or by surrounding the fiber core just before twisting in a heating furnace, the fiber core can be heated to 60°C as described above. It is best to heat it to a certain degree.

As described above, according to the method of manufacturing an optical fiber cable of the present invention, the shrinkage strain of the tension member is reduced by making the temperature when the fiber core wires are twisted higher than the temperature when used as an optical fiber cable. Since the shrinkage strain of the fiber core was always larger than that of the fiber core, the fiber core was in a state of tension against the tension member.
Microbending does not occur in the fiber core, and therefore, there is no possibility that the transmission loss of the optical fiber cable will increase in a high temperature range.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of a fiber core that is a component of an optical fiber cable, Figure 2 is an example of a cross-sectional view of a six-core optical fiber cable, and Figure 3 is a linear expansion strain and temperature of a conventional optical fiber cable. This is a graph showing the relationship between
Dashed lines represent tension members. FIG. 4 is a graph showing the relationship between linear expansion strain and temperature of the optical fiber cable according to the present invention, in which the solid line represents the fiber core and the broken line represents the tension member. In the drawing, 3 is a fiber core, 5 is a tension member, and 7 is an optical fiber cable.

Claims (1)

[Claims] 1. An optical fiber cable having a tension member at the center and a plurality of fiber cores twisted around the tension member having a linear expansion coefficient larger than that of the tension member, A method for manufacturing an optical fiber cable, characterized in that the fiber core wires are twisted together while being heated to a temperature higher than that at which they are used as the optical fiber cable. 2. The optical fiber cable according to claim 1, wherein the fiber core is heated by heating a twisting die that twists the fiber core around a tension member. manufacturing method. 3. The method for manufacturing an optical fiber cable according to claim 1, wherein the fiber core wires before twisting located immediately before the twisting die are heated with hot air. 4. Install a heating furnace around the untwisted fiber core located just before the twisting die,
2. The method for manufacturing an optical fiber cable according to claim 1, wherein the fiber core is heated in this heating furnace.