JPS62176941A - Optical fiber - Google Patents

Abstract

PURPOSE:To obtain the titled highly reliable optical fiber with less defects in the glass and having low initial loss, etc., in a radioactive environment by composing the fiber of a core consisting of quartz glass contg. specified amts. of an OH group and Cl and a clad consisting of quartz glass contg. fluorine. CONSTITUTION:The core consisting of quartz glass contg. only SiO2 as the metallic oxide, 0-1ppm OH group, and 50-500ppm Cl as the network terminator, and the clad wherein the refractive index is decreased by adding fluorine to a value lower than that of the core and the Cl concn. is balanced with the Cl concn. in the core are bonded to couple the dangling bonds. Consequently, a network is formed, and the optical fiber is obtained.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to the structure of an optical fiber, and more specifically to a highly reliable optical fiber with a low hydroxyl group concentration and few defects in the glass, particularly in a radiation environment. The present invention relates to the closing of multimode or single mode optical fibers with extremely small initial loss and transmission loss increase.

[Prior Art] The presence of defects in glass is considered to be one of the causes of increased loss in optical fibers exposed to radiation or in a hydrogen atmosphere. In order to reduce this increase in loss, optical fibers with fewer defects are being studied, and currently SiO□
Optical fibers with a core of Furthermore, conventionally, such 5102 cores that do not contain chlorine and contain a large amount of hydroxyl groups have been considered to have the fewest defects and excellent radiation resistance.

[Problem that the invention seeks to solve]

However, since the conventional radiation-resistant optical fiber with sio□ core contains a large amount of hydroxyl groups, its initial transmission loss is as large as 10 dB/km or more at the wavelength of 1.5 μm, where the increase in transmission loss is small. However, it had the disadvantage that it was practically unusable.

The purpose of the present invention is to eliminate this drawback, and to provide an optical fiber with a new configuration that has a low hydroxyl group concentration, few defects in the glass, is highly reliable, and has significantly less initial loss and increase in transmission loss even in a radiation environment. It is something to do.

[Means for solving problems]

The present invention provides an optical fiber having a core and a cladding, wherein the core contains hydroxyl groups at 1 ppm or less (including 0),
The optical fiber is made of quartz glass containing 50 to 500 ppm of chlorine, and the cladding is made of quartz glass containing fluorine, thereby achieving the above object.

It is well known that the radiation-resistant properties of optical fibers are better when the glass has fewer defects, and silica glass has the lowest concentration of defects within the glass, but impurities may be present in this quartz glass. It was already explained in the previous section that chlorine, in particular, was thought to significantly increase the amount of defects, while hydroxyl groups were thought to reduce the amount of defects. Therefore, 100p was applied to the quartz glass core.
Optical fibers doped with hydroxyl groups of pm or higher were used as radiation-resistant fibers.

However, according to the results of our research, 90
It has become clear that even in multimode fibers in which more than 100% power propagates through the core, the glass material of the cladding is strongly affected by radiation. This fact indicates that the defect concentration not only inside the core glass but also at the core-clad interface changes depending on the combination of core-clad glasses. As a result of more detailed experiments, the inventors obtained the surprising finding that transmission loss due to radiation is lower in fibers containing chlorine than in fibers with no chlorine in the core. The invention was achieved.

Most of the previous studies have focused on plastic cladding, and the effects of chlorine on fluorine-doped quartz cladding have not been investigated. Although the mechanism is unknown, at the core/cladding interface, the cladding is stabilized by fluorine bonded to the edges of the network, whereas the core has atoms (which stabilize it) bonded to the edges of such a network. called a network terminator,
For example, if hydrogen, halogen, etc. are not present, the defect concentration will be high and the radiation resistance will be poor. In contrast,
If a network terminator such as chlorine is also added to the core, chlorine will bond to the network ends of the core and stabilize it, reducing the defect concentration at the interface and improving radiation resistance.

The core of the optical fiber of the present invention contains substantially only 8i02 as a metal oxide, and the hydroxyl group contained is l.
ppm or less, and the chlorine concentration is preferably 50 to 5.
00 ppm, particularly preferably 50-40 u ppm
It is. Further, the cladding of the optical fiber of the present invention is preferably 5 in 2 doped with fluorine, and has a lower refractive index than the core due to the fluorine doping.

In the present invention, the concentration of the halogen element in the cladding,
It is preferable that the chlorine concentration in the core is balanced.

In the mechanism described above, if there is no network terminator at the interface and the concentrations of broken parts of the network (dangling bonds) are equal (balanced) in the core and cladding, dangling bonds between the core and cladding occur. Ring bonds combine to form a network and stabilize it, but if it is not balanced, dangling bonds remain at the core-clad interface, degrading radiation characteristics.

In addition, in the present invention, the hydroxyl group concentration is at wavelengths of 1 and 39
μ? 1 for absorption peak near FL of 60 dB/km
ppm, and the chlorine concentration is EPMA (XM
A).

〔Example〕

Example 1 A single mode fiber whose cladding is silica containing 0.9 times fluorine (relative refractive index difference -03%) and whose core is pure silica, with a core diameter of 8 μm and a quad diameter (outer diameter) of 125 μm. In products, the chlorine content of the core is reduced to 10
It was varied in the range of ~1200 ppm. The chlorine content was adjusted by preparing cysuto by the VAD method and changing the chlorine concentration in the dehydration step.

Further, the hydroxyl group content of the core was 10 ppb or less.

These fibers are connected to 10'R/H gamma rays (ray source 2
o) After leaving it in the environment for 1 hour, we measured the increase in transmission loss (dB/km) divided into wavelengths of 1.3 μm, and found that the chlorine concentration in the core (ppm) and the increase in loss (dB/km) were as shown in Figure 1. /km) was obtained. Note that the initial transmission loss is Q, 55 d at a wavelength of 1.3 μm.
It was an extremely low value of B/km.

Example 2 The cladding is silica containing 3% by weight of fluorine (relative refractive index difference -1%), the core is silica, and the cladding diameter is 8
A stepped multimode fiber with a core diameter of 0 μm and a core diameter of 50 μm and 125 μm was fabricated.

The core is made by VAD method using only 8iC4 as raw material.
A soot of the same size as a book was prepared and then dehydrated with chlorine in an electric furnace, but the amount of chlorine at this time was 500 c.
c was varied in the range of 1 min to 1 t/min. Next, it was made transparent in an electric furnace to obtain five core materials.

For each of the five core materials, 5iCt4, 02 and CC2 were blown out from a plasma torch on the outside.
, F, and fluorine-doped 5i02 was deposited while being directly vitrified to form a cladding with a relative refractive index difference of -1 inch, which was used as a preform.

The preform was drawn into fibers with an outer diameter of 125 μm, and each of the five fibers obtained was 10'
When irradiated with γ-rays of R/H and Ii yield for 1 hour,
The amount of increase in transmission loss at a wavelength of 1.3 μm was as shown in FIG.

Note that the initial transmission loss for both fibers is at wavelength 1.3.
It was a low value of 1.5 dB/km or less in μm. In addition, the hydroxyl group in all fibers has a peak of 6 dB/km or less around 1.39 μm, which is 0.1
This corresponds to less than ppm.

〔Effect of the invention〕

The present invention has a core containing OH of 1 ppm or less and chlorine of 50 ppm.
The fiber is made of silica glass with a fluorine content from pm to 500 ppm, and the cladding is made of silica glass doped with fluorine.Because the defect concentration in the core, cladding, and interface is low, the increase in loss due to radiation is extremely small, and the OH
Due to the low concentration, the initial loss at 1.3 μm is also 1 dB.
/km, making it possible to obtain a fiber that can be used in a radiation environment.

[Brief explanation of drawings]

Figure 1 shows the chlorine concentration (ppm) in the core and the γ-ray 105
It is a graph showing the relationship of increase in transmission loss after R irradiation. Figure 2 shows the amount of hydroxyl groups in the core is 0. The chlorine concentration (1) pm in the core of a stepped multimode fiber with a relative refractive index difference of the cladding of less than j ppm (1) pm) and the γ-ray 10
It is a graph showing the relationship of increase in transmission loss after 5R irradiation.

Claims (1)

[Claims] In an optical fiber having a core and a cladding,
The core contains 1 ppm or less of hydroxyl groups (including 0) and 50% of chlorine.
An optical fiber characterized in that it is made of quartz glass containing up to 500 ppm of fluorine, and its cladding is made of quartz glass that contains fluorine.