JPH0255385B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Fields The present invention relates to communication optical fibers with excellent radiation resistance, multiple optical fibers for image guides,
Or it relates to a silica glass-based optical transmission path such as a light guide for illumination. BACKGROUND OF THE INVENTION In recent years, optical transmission lines have been frequently used for communication, measurement, and control in places that may be exposed to radiation, such as nuclear reactors, nuclear ships, and artificial satellites. In general, an optical transmission path consists of a core, which is a path for optical energy (optical signals), and a cladding layer, which is an optical energy reflecting layer provided around the core.In the case of a multimode optical fiber, the cladding layer is a single layer. Since there is little light energy seeping into the cladding layer, which occurs in the case of mode optical fibers, the cladding layer thickness can be reduced to several μm.
It is extremely thin. Such an optical transmission path has γ
When exposed to radiation such as beams or electron beams, optical transmission loss increases rapidly. For this reason, problems have arisen in the good transmission of optical signals through optical transmission lines in radiation fields that may be affected by radiation and in the long-term use of optical transmission lines. In general, silica glass-based optical transmission paths are evaluated to have superior radiation resistance compared to multi-component glass-based ones, but according to detailed research by the present inventors, Even though it is an optical transmission line, its radiation resistance varies greatly depending on the materials constituting the core and cladding layers. An object of the present invention is to provide various optical transmission paths made of silica glass for the visible light region and having excellent radiation resistance under high dose rates. Another object of the present invention is to provide a silica glass-based communication optical fiber with excellent radiation resistance. Still another object of the present invention is to provide multiple optical fibers with excellent radiation resistance for use as image transmission paths in industrial image scopes. Still another object of the present invention is to provide a light guide for illumination with excellent radiation resistance. Means for Solving the Problems In the present invention, the core serving as a light path is made of pure silica glass with a chlorine content of 10 ppm or less, and the cladding layer provided outside the core contains B and F as dopants. It is an object of the present invention to provide a radiation-resistant optical transmission path that is made of quartz glass and can be used under high doses and in the visible light region. Effects The above configuration satisfies the requirements that the pure silica glass that makes up the core has a chlorine content of 10 ppm or less and the type of dopant in the silica glass that makes up the cladding layer. For example if the total dose is at least
Especially at high doses of 10 ⁴ R, at least 1×10 ⁴ R/
An optical transmission line having excellent radiation resistance under high H dose rates and in the visible light region can be obtained. In FIGS. 1 to 4, each optical transmission line 1 consists of a core 2 made of high-purity quartz glass and a cladding layer 3 made of doped quartz glass. The optical transmission line shown in FIG.
Further, there is a support layer 4 made of quartz glass. The optical transmission line (multiple optical fiber) 1 shown in FIG. 3 has a large number of unit optical transmission lines 1' each consisting of a core 2, a cladding layer 3, and a support layer 4, whose support layers 4 are fused together. Has a structure. The optical transmission line 1 having the structure shown in FIG. 1 can be constructed, for example, by externally attaching doped quartz glass to form the cladding layer 3 on a pure silica glass rod to form the core 2, or by attaching doped quartz glass to form the cladding layer 3 externally, or by using a rod-in-tube as described later. It can be manufactured by removing the support layer 4 (Fig. 2) of the three-layer structure preform obtained by the method, for example, by flame polishing to obtain a two-layer structure consisting of the core 2 and the cladding layer 3, and then drawing it. be. The optical transmission line 1 having the structure shown in FIG. 2 can be obtained by adding a support layer externally on the above-mentioned doped silica glass layer and drawing the line, or by adding a cladding layer inside the quartz glass tube which is to become the support layer 4. It can be manufactured by inserting a pure silica glass rod to become the core 2 into a glass tube with a core 3 attached therein, manufacturing a base material by the rod-in-tube method, and then drawing the base material. The optical transmission line 1 having the structure shown in FIG. 3 can be manufactured by drawing the optical transmission line ^itself having the structure shown in ^FIG . can. Apart from the embodiment shown in FIG. 3, a multiple optical fiber formed by bundling and drawing a large number of two-layered optical transmission lines or their base material shown in FIG. 1 in the same manner as described above is also an important aspect of the present invention. This is an embodiment. Under the high doses mentioned above, especially at least 1x10 ⁵ R/
In order to achieve excellent radiation resistance under high dose rates of H, it is essential that the core 2 used in the present invention has a chlorine content of 10 ppm or less.
It is desirable that the content be 1 ppm or less, more preferably 0.1 ppm or less. In addition, the OH group content of core 2 is 200~
There is no problem if the content is in the range of 1500 ppm, but in order to obtain even better radiation resistance, it is desirable that the OH group content be in the range of 300 to 1200 ppm, particularly 500 to 1000 ppm. Silica glass that satisfies the above two requirements can be obtained, for example, by the following method. A mixed gas consisting of O ₂ and at least one chlorine-free silicon compound such as Si(OC ₂ H ₅ ) ₄ , Si(OCH ₃ ) ₄ , SiH ₄ is combusted. In this way, the silicon compound is oxidized by O ₂ in the mixed gas to produce chlorine-free SiO ₂ . The clad layer employed in the present invention must be made of quartz glass containing B and F as dopants. Even if either dopant B or dopant F is missing, the object of the present invention cannot be achieved. However, in the present invention, the state of existence of both of the above-mentioned dopants in the quartz glass does not matter. For example, B and F may be present separately and physically or chemically bonded to silica glass constituent molecules, or B and F may be physically or chemically bonded to silica glass constituent molecules. It may also exist in combination with molecules. Both B and F may exist in the form of oxides or compounds with other elements. Such doped quartz glass is made of, for example, BCl ₃ ,
Mixed gas with BF ₃ , SiCl ₄ and oxygen, BCl ₃ ,
Mixed gas with SiF ₄ and oxygen, or BF ₃ or
It can be formed by a well-known chemical vapor deposition method (CVD method) using a mixed gas of BCl ₃ , SiF ₄ and oxygen as a raw material. The amounts of B and F in the raw material gas are 5 to 200 parts by weight, preferably 10 to 100 parts by weight, and 10 to 100 parts by weight of B per 100 parts by weight of Si (weight ratio converted to atomic state).
-50 parts by weight, preferably 50-400 parts by weight. Among the above-mentioned raw material mixed gases, a mixed gas with BCl ₃ , SiF ₄ and O ₂ is particularly preferred for producing an optical transmission line with even better radiation resistance. In the present invention, the refractive index difference (Δn) between the core 2 and the cladding layer 3 is at least 0.008, especially from 0.10 to
Approximately 0.015 is preferable. In general, the larger the diameter of the core 2 is relative to the thickness of the cladding layer 3, the more advantageous it is in terms of radiation resistance.In particular, the ratio of the outer diameter _d1 of the core 2 to the outer diameter d2 of the cladding layer ₃ ( _d1 / _d2 ) is 0.45 to 0.9,
Among these, it is preferably 0.55 to 0.8, particularly 0.6 to 0.75. When the optical transmission line of the present invention is a multiple optical fiber, as described above, a bundle obtained by bundling a large number of optical fibers or their base material, for example, 10 ² to 10 ⁵ fibers, is heated at 1800 to 2200°C. Heat it to a certain temperature to soften it, then draw a line to a predetermined thickness, e.g. 0.4
It can be manufactured by making it 3 mm, especially 0.7 to 2 mm. In the present invention, each unit optical transmission line 1' in the multiple optical fiber has a core of 5 to 30 μm, and a cladding layer 3 of 1.5 μm.
A thickness of ~10 μm is preferable from the viewpoint of radiation resistance. In the embodiment shown in FIGS. 2 and 3, the support layer 4
However, if the constituent material of the support layer 4 is quartz glass containing a large amount of impurities, radiation resistance may be adversely affected. Therefore, the constituent material of the support layer 4 is quartz glass whose wire drawing temperature is at least 1800°C, such as natural quartz glass or synthetic quartz glass, especially high-purity synthetic glass with a purity of 99.99% by weight or more, particularly 99.9999% by weight or more. Quartz glass is preferred. In addition, when manufacturing multiple optical fibers, for example, a quartz glass pipe is filled with optical fibers and then drawn, and a skin corresponding to the pipe is added to the outer periphery of the group of optical fibers that are fused together. It is preferable to have the layers fused together in terms of flexibility and resistance to bending of the resulting multiple optical fiber. In the present invention, each quartz glass constituting the clad layer, support layer, or skin layer may contain chlorine, but in order to further improve the radiation resistance of the optical transmission line of the present invention, any of the silica glasses may contain chlorine. It is desirable that the chlorine content be 50 ppm or less, especially 10 ppm or less. Examples Examples 1 Si(OC ₂ H ₅ ) ₄ and oxygen are mixed and combusted, and the flame is blown onto a quartz rod target to produce quartz glass according to the so-called gas-phase Bernoulli method, with an outer diameter of approx. A quartz glass rod of 35 mm and 200 mm in length was obtained.
The chlorine content of the glass rod is 1 ppm or less, the OH group content is 850 ppm, and the refractive index at 20°C is
It was 1.4585. The chlorine content in quartz glass is determined by ESCA (Electron
Spectroscopic Chemical Analysis) method, and the OH group content was measured by the following method. Measurement of OH group content: When the OH group content is 1 ppm or more, formula (1)
, and when it is less than 1ppm, the formula ()
It was determined by OH=1.2x (L ₁ − L ₀ ) ……(1) OH=1.85x (L ₃ − L ₂ ) ……() Here, L ₁ is the loss value (dB/ Km) and L ₀ are loss values assuming that the OH group content of the optical transmission line is zero at the same wavelength. L ₃ is a loss value (dB/Km) of the optical transmission line at a wavelength of 1.38 μm, and L ₂ is a loss value assuming that the OH group content of the optical transmission line at the same wavelength is zero. A core rod with an outer diameter of 11 mm made of the above pure silica glass, as well as SiCl ₄ , BF ₃ , O ₂ , and a natural quartz glass tube (outer diameter 26 mm, wall thickness 1.5 mm, n ²⁰ : 1.459)
B and F were formed using the MCVD method using
A preform with a ^three -layer structure (outer diameter
18.9 mm), this was drawn under heating (2000°C) to obtain an optical fiber with an outer diameter of 300 μm. 6,000 of the above optical fibers (length 20 cm) were bundled so that the cross section was in a close-packed shape, one end of which was inserted into a quartz glass tube, and then heated and fused.
This was ultrasonically cleaned in a hydrofluoric acid aqueous solution (5% by volume) and then in distilled water, and then dried. The obtained bundle of optical fibers is heated to 2000℃ and drawn.
Multiple optical fibers with an outer diameter of 1.0 mm were obtained in which adjacent optical fibers were fused to each other. The obtained multiple optical fiber had a core diameter of 7.5 μm, a clad layer thickness of 2.0 μm, an optical fiber diameter of 12 μm, and a refractive index difference (Δn) between the core and the clad layer of 0.012. The multiple optical fibers also had accurate image alignment over their entire length. Next, the radiation resistance of the obtained multiple optical fibers was investigated. In the evaluation test, as shown in Figure 4, a 10 m length of multiple optical fibers of 20 m length was bundled into a coil at a position with a predetermined dose rate from a Co ⁶⁰ γ-ray irradiation source, and both ends of the multiple optical fibers were tied together. The light from the multi-halogen mercury light source (300W) is introduced from one end of the shielding wall.
The light emitted from the other end was measured with a monochromator/photometer and recorded on a recorder. Irradiation was performed in air, and the multiple optical fibers were removed from the light source except during measurements to prevent the effects of photobleaching. Cut Back the loss value of multiple optical fibers
It was measured by the method. Example 2 The three-layered preform obtained in Example 1 was flame-polished to remove the natural quartz glass tube, resulting in a two-layered preform (optical fiber diameter 11.8 μm).
A multiple optical fiber was obtained in the same manner as in Example 1 except that the radiation resistance thereof was examined. Examples 3 and 4 Using 3000 optical fibers, the core diameter of the optical fiber in the multiple optical fiber is 11 μm, the thickness of the cladding layer is 2.5 μm, and the optical fiber diameter is 17 μm (Example 3), or 2000 fibers are used. A multiple optical fiber was obtained in the same manner as in Example 1, except that the core diameter of the optical fiber in the multiple optical fiber was 10 μm, the thickness of the cladding layer was 2.0 μm, and the optical fiber diameter was 15 μm (Example 4). I looked into gender. Examples 5, 6, 7 Using 6000 optical fibers, the core diameter of the optical fiber in a multiple optical fiber is 7 μm, the cladding layer thickness is 1.9 μm, and the optical fiber diameter is 12 μm (Example 5)
Or, using 3000 optical fibers, the core diameter of the optical fiber in multiple optical fibers is
12μm, cladding layer thickness 2.2μm, optical fiber diameter
17 μm (Example 6) and 1000 optical fibers, the core diameter of the optical fiber in the multiple optical fiber is 14 μm,
A multiple optical fiber was obtained in the same manner as in Example 1, except that the cladding layer thickness was 2.5 μm and the optical fiber diameter was 20 μm (Example 7), and its radiation resistance was examined. Comparative Example 1 A multiple optical fiber was obtained in the same manner as in Example 1, except that a core material with a chlorine content of approximately 450 ppm was used, which was prepared by a plasma method using silicon tetrachloride and oxygen, and its radiation resistance was investigated. . Comparative Example 2 A multiple optical fiber was obtained in the same manner as in Example 1, except that a core material with a chlorine content of approximately 600 ppm, which was produced by flame hydrolysis using an oxyhydrogen burner, was used, and its radiation resistance was investigated. . Comparative Example 3 Same as Example 1 except that SiF ₄ was used and a natural quartz glass tube having an F-doped quartz glass layer on the inner periphery was used.
Multiple optical fibers were obtained in the same manner as described above, and their radiation resistance was investigated. The radiation resistance of each of the above examples and comparative examples is shown in the table below.

【table】

[Table] Effects As explained above, the optical transmission line of the present invention has the following effects.
Since it has excellent radiation resistance even under high dose rates, it can be used in places where there is irradiation or where there is a possibility of being irradiated. As a result, if the fiber is used, optical communication using visible light becomes possible in such locations, and observation, control, or measurement using various types of visible light becomes possible using an image guide.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views of an embodiment of the optical transmission line of the present invention used as an optical fiber for communication or a light guide for illumination. FIG. 3 is a sectional view of an embodiment of a multiple optical fiber, which is one type of optical transmission line of the present invention. FIG. 4 is an explanation of a method for testing the radiation resistance of an optical transmission line in the atmosphere using Co ⁶⁰ γ-rays as a radiation source. 1: Optical transmission line, 1': Unit optical transmission line, 2: Core,
3: Clad layer, 4: Support layer.

Claims (1)

[Claims] 1. The core that serves as a path for light has a chlorine content of 10 ppm.
A radiation-resistant optical transmission line which is made of the following pure silica glass and whose cladding layer provided on the outside of the core is made of quartz glass containing B and F as dopants and which is used under high doses and in the visible light region. 2. The optical transmission line according to claim 1, wherein the pure silica glass constituting the core has an OH group content of 200 to 1500 ppm. 3. The optical transmission line according to claims 1 and 2, which has a support layer outside the cladding layer. 4. Claim ₁ , wherein the ratio (d ₁ /d ₂ ) of the outer diameter d ₁ of the core to the outer diameter d 2 of the cladding layer is 0.55 to 0.8.
The optical transmission line according to items 1 to 3.