JPH07325237A - Production of metallic tube coated optical fiber cable - Google Patents

Abstract

PURPOSE:To provide a production method of an optical fiber cable covered with a metallic tube which prevents increase in the transmission loss of the optical fiber. CONSTITUTION:This optical fiber cable covered with a metallic tube is produced by covering an optical fiber with a metallic tube. In this method, the metallic tube is prepared by annealing while the gaseous atmosphere in the metallic tube is purged by inert gas through the one end of the tube under pressure to obtain inert gas atmosphere, and then the optical fiber is inserted into this metallic tube.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a metal tube-coated optical fiber cable. The metal tube coated optical fiber cable is used for temperature measurement by an optical fiber and optical communication under an environment exposed to high temperature or high tension.

[0002]

2. Description of the Related Art In recent years, a metal tube-coated optical fiber cable has been used as an optical fiber cable which requires high tensile strength or environment resistance. In this metal tube-coated optical fiber cable, one or a plurality of optical fibers are passed through a metal tube such as a stainless steel tube, a common steel tube, a copper tube, an incoloy tube, etc., and the optical fiber is protected by the metal tube.

The manufacturing method thereof is disclosed in JP-A-63-19561.
As disclosed in Japanese Unexamined Patent Publication No. 0-202, a raw metal tube is drawn and annealed, and an optical fiber is inserted into the metal tube that has been cleaned.
An example of the manufacturing process of a stainless steel pipe is shown in FIG. A stainless steel hoop is molded into a round shape and welded to form a mother tube. Anneal to improve work hardening and weld metal structure by cold rolling of hoop. Then, the softened mother tube is expanded. The number of cycles of tube-annealing-tube depends on the size of the mother tube and the size of the final product. After that, the inner and outer surfaces of the tube are cleaned to remove the surface scale and the lubricant for the tube expansion, and the metal tube is covered with a metal tube for a fiber optic cable. If the metal tube needs to be soft, wash it without annealing after annealing. The annealing atmosphere is a non-oxidizing atmosphere such as hydrogen gas, decomposition gas of ammonia, argon gas or vacuum in order to prevent oxidation of stainless steel. Generally, in consideration of cost, hydrogen gas or decomposition gas of ammonia is used. In order to obtain a desired inner diameter and circularity, a so-called floating die method is generally used for a drawn tube, and a lubricant for drawing is used on the inner and outer surfaces of the tube.
In particular, as the lubricant used for the inner surface of the pipe, a high boiling point Teflon-based oil containing an extreme pressure reducing agent is used in consideration of the fact that it can endure high processing heat and high processing degree. The lubricant remaining on the inner surface of the pipe is washed by injecting a cleaning liquid such as trichlorethylene at a pressure of several tens of atmosphere from the end of the inlet pipe and discharging it from the end of the outlet pipe. For thin and long tubes,
It takes 10 hours or more. The cleaning liquid adhering to the inner wall surface of the pipe is purged by injecting N ₂ gas of several tens atm. Other metal tubes are manufactured by substantially the same manufacturing process.

[0004]

FIG. 2 shows an example of changes over time in transmission loss of a metal tube-coated optical fiber in which an optical fiber is inserted into a metal tube manufactured by a conventional method by a desired method.
The cleaning time is the time from when the cleaning liquid starts flowing out from the outlet pipe end. The transmission loss shows a tendency to saturate from the initial value of 0.78 dB / km to about 2.4 dB / km. Even if the cleaning time is increased, the time required to start increasing the loss is increased, but the saturation level is not so changed. Further, when the absorption spectrum of the optical fiber was examined after 12 days, an increase in the absorption spectrum due to hydrogen was recognized. Furthermore, 2.1 vol% (100-hour washed product) in the gas in the pipe and 3.2
Vol% (10-hour washed product) hydrogen gas was detected. Inferring from these phenomena, (1) hydrogen is present in the tube and is absorbed by the optical fiber, increasing the transmission loss. (2) Even a small amount of hydrogen generation product causes an increase in transmission loss.

The change in transmission loss with time when the cleaning time is 100 hours or more has not been confirmed, but it is not industrially feasible considering the cost. On the other hand, the following are possible causes of hydrogen generation. (1) Hydrogen contained in the pyrolysis gas of the lubricant for pipe expansion during annealing. (2) Hydrogen in an annealing atmosphere. (3) Hydrogen diffused in the metal. It is considered that the hydrogen of (1) and (2) is partially diffused in the metal.

The present invention is intended to provide a method of manufacturing a metal tube-coated optical fiber cable capable of preventing an increase in transmission loss of an optical fiber caused by these causes.

[0007]

According to the method for manufacturing a metal tube-coated optical fiber cable of the present invention, a metal is annealed as an inert gas atmosphere while injecting an inert gas from one end of the tube to purge the atmosphere gas in the tube. It is characterized by inserting an optical fiber into the tube. Needless to say, it is more effective to make the atmosphere outside the tube an inert gas atmosphere.

The metal tube is made of stainless steel, copper, incoloy, ordinary steel, etc., and has an outer diameter of about 0.5 to 8 mm and an inner diameter of about 0.4 to 3 mm. The optical fiber inserted in the metal tube is a bare optical fiber, an optical fiber elemental wire, an optical fiber core wire, or an optical fiber cord. The number of optical fibers inserted into the metal tube may be one or plural.

Annealing of a metal tube is disclosed in JP-A-63-195610.
Strand furnaces and batch furnaces are used as disclosed in the publication. The annealing heating conditions should be appropriately selected depending on the type of metal. As an example of stainless steel, in the case of a batch furnace, 1050 ° C. × 1 to 4 hours,
In the case of a strand furnace, 1050 ° C x 1 minute (soaking zone residence time) is applied. The inert gas for purging should be selected in consideration of cost depending on the type of metal.

The inert gas referred to in the present invention may be any gas that does not react with the metal under the annealing and heating conditions applied with a gas other than hydrogen. It is appropriately selected from nitrogen gas, argon gas, helium gas, carbon dioxide gas, carbon monoxide gas and the like. As an example, in the case of stainless steel, argon gas is applied. The gas press-in conditions differ depending on the pipe inner diameter and length. That is, if the diameter is small and the length is long, the gas pressure for press-fitting must be high. Inner diameter 1.
In the case of a tube with a diameter of 8 mm and a length of 2000 m, the gas pressure is 10 kgf /
cm ² is required. In the case of a strand furnace, if gas is supplied from the heat treatment tip side, the pyrolysis gas of the tube expansion lubricant can be effectively purged. The gas flow rate is 5-10 of the internal volume
About 0 times the amount is flown during the heating time.

The hydrogen concentration in the gas inside the metal tube which had been treated for 12 days at room temperature was 1.0 vol% or less.

To insert an optical fiber element wire into a metal tube, a known method such as a method of directly pushing the optical fiber into the metal tube, a method of applying a vibrating force to the metal tube, or a method of using an air flow flowing in the metal tube is used. Is used.

[0013]

[Function] An optical fiber is inserted into a metal tube which has been annealed in an inert gas atmosphere while injecting an inert gas from one end of the tube to purge the atmosphere gas on the inner surface of the tube. It is possible to effectively and easily remove hydrogen contained in gas, hydrogen in an annealing atmosphere, and diffusible hydrogen in metal, including hydrogen diffused in the metal.
As a result, the hydrogen concentration in the metal tube becomes 1.0 vol% or less, and the amount of hydrogen absorbed by the optical fiber during use of the optical fiber cable is extremely small. Therefore, the transmission loss due to hydrogen absorption of the optical fiber is reduced.

[0014]

EXAMPLES Table 1 shows the atmospheric conditions and the treatment results in annealing of the examples in which the same metal tubes and optical fibers as shown in FIG. 2 were used and comparative experiments were conducted.

[Table 1]

The results shown in the conventional example show that even if the atmosphere in the tube is simply annealed and annealed, the hydrogen concentration after leaving for 12 days hardly changes, and the transmission loss greatly increases. . On the other hand, in the embodiment, the hydrogen concentration is 1 vol% or less and the transmission loss is almost the same as the initial value due to the effect of the positive pipe purging.

[0016]

According to the method of manufacturing a metal tube-coated optical fiber cable of the present invention, an optical fiber is inserted into a metal tube annealed as an inert gas atmosphere while purging the atmosphere gas in the tube with an inert gas. The generated hydrogen gas can be suppressed, and the increase in transmission loss due to the absorption of hydrogen in the optical fiber can be effectively and easily suppressed.

[Brief description of drawings]

FIG. 1 is a manufacturing process diagram of a stainless steel tube into which an optical fiber is inserted.

FIG. 2 is a diagram showing changes over time in transmission loss of a conventional metal tube-coated optical fiber cable.

Claims (2)

[Claims] 1. In manufacturing a metal tube-coated optical fiber cable in which an optical fiber is covered with a metal tube, a metal annealed as an inert gas atmosphere while injecting an inert gas from one end of the tube to purge the atmosphere gas in the tube. A method of manufacturing an optical fiber cable coated with a metal tube, which comprises inserting an optical fiber into the tube. 2. The method for producing an optical fiber cable coated with a metal tube according to claim 1, wherein the atmosphere outside the tube is an inert gas atmosphere.