JP2547068B2 - Radiation resistant multiple fibers - Google Patents

Description

TECHNICAL FIELD The present invention has excellent radiation resistance in the visible light region,
Therefore, the present invention relates to a multiple fiber suitable as an image transmitter for an image scope.

2. Description of the Related Art In recent years, image scopes have been widely used in places such as nuclear reactors, nuclear ships, and artificial satellites that may be exposed to radiation.

Two types of image transmission materials for image scopes are known, a silica glass-based multiple fiber and a multi-component glass-based multiple fiber. Among them, the silica glass-based multiple fiber is generally a multi-component glass-based multiple fiber. Since it is superior in radiation resistance as compared with a fiber, it is exclusively used for observation in the above-mentioned radiation field.

Problems to be solved However, according to in-depth research by the present inventors, even in the case of a silica glass-based multiple fiber, its radiation resistance is different, and the materials, dimensions, and materials constituting the core and clad layers,
Or it varies greatly depending on the structure. Therefore, an object of the present invention is to provide a silica glass-based multiple fiber that is more excellent in radiation resistance in the visible light region.
Another object of the present invention is to provide a silica glass-based multiple fiber suitable as an image transmitter for an industrial imagescope used for observation in a radiation field.

Means for Solving the Problems That is, according to the present invention, a large number of optical fibers having a fused silica glass clad layer on a pure silica glass core and further having a silica glass support layer thereon are fused and assembled together. A multiple fiber having a structure, wherein the pure silica glass core has a chlorine content of 1 ppm or less and an OH group content of 10 ppm.
And the core space factor of each optical fiber is 20-60%
And that the thickness of the clad layer is 1.0 μm or more, and the optical fibers present in a portion within at least a radius of 80% from the center of the cross section of the multiple fibers are fused to each other in a regular alignment structure. A feature of the present invention is to provide a radiation-resistant multiple fiber.

Action of the Invention Conventionally, regarding the radiation resistance of an optical fiber having a doped silica glass clad layer on a pure silica glass core, and optionally a silica glass support layer thereon, the chlorine content of the pure silica glass core is It was recognized that the smaller the amount, the larger the OH group content, the better. Under the general recognition, the present inventor has unexpectedly (1) to (3) below.
It has been found that a multiple fiber satisfying the above conditions simultaneously exhibits excellent radiation resistance. Furthermore (1)-(3)
It has been found that the multiple fiber satisfying the conditions (4) to (5) at the same time as the condition (1) above exhibits even more excellent radiation resistance.

(1) The chlorine content of the pure silica glass core is 1 ppm or less and the OH group content is 10 ppm or less, (2) The core space factor of each optical fiber is within the range of 20 to 60%. (3) The thickness of the cladding layer is at least 1.0 μm, (4) The conditions (1) to (3) are satisfied, and a quartz glass support layer is further provided on the doped quartz glass cladding layer. (5) The optical fibers existing at least within a radius of 80% from the center of the cross section of the multiple fibers are fused to each other in a regular alignment structure.

By satisfying all the above configurations, a multiple fiber having excellent radiation resistance in the visible light region can be obtained.

DETAILED DESCRIPTION OF THE INVENTION FIGS. 1 and 2 are cross-sectional views of a multiple fiber according to an embodiment of the present invention. In the embodiment shown in FIG. 1, the multiple fiber 1 has a structure in which a large number of unit optical fibers 1'are fused together, and each unit optical fiber 1'is
It is composed of a core 2 made of high-purity quartz glass and a cladding layer 3 made of doped quartz glass, and the cladding layers 3 of adjacent unit optical fibers 1'are fused together. In the embodiment shown in FIG. 2, the multiple fiber 1 is composed of a large number of unit optical fibers 1 ′ each having a support layer 4 made of quartz glass on the cladding layer 3, and the support layers 4 are connected to each other. It has a fused structure. The refractive index difference (Δn) between the core 2 and the cladding layer 3 is preferably at least 0.008, and particularly preferably 0.01 to 0.015.

The multiple fiber 1 having the structure shown in FIG. 1 has, for example, a pure silica glass rod to be the core 2 and a doped silica glass to be the cladding layer 3 externally attached thereto, or a rod-in-tube method described later. The support layer 4 (FIG. 2) of the three-layer structure base material obtained in step 1 is removed by, for example, a flame polishing method to form a two-layer structure of the core 2 and the clad layer 3, and then drawn to draw the mother fiber of the unit optical fiber 1 '. A material is obtained, and then a large number of the base materials, for example 10 ² to 10 ^5, are bundled and heated to a temperature of, for example, 1,800 to 2,200 ° C. to be softened, and this is drawn to have a predetermined thickness, for example, 0.1 to 5 mm. Medium, 0.5
It can be manufactured by setting the thickness to be ~ 3 mm.

The multiple fiber 1 having the structure shown in FIG. 2 can be manufactured by the same method as described above using the three-layer structure base material obtained by the rod-in-tube method described later.

As described above, in the present invention, (1) the chlorine content of the pure silica glass core is 1 ppm or less and the OH group content is 10 ppm or less, (2) the core space factor of each optical fiber is It is in the range of 20 to 60%, (3) the thickness of the cladding layer is at least 1.0 μm, and preferably (4) further having a silica glass support layer on the cladding layer, and (5) ) It is essential that all the optical fibers existing within a portion of at least 80% radius from the center of the cross section of the multiple fibers are regularly fused to the aligned structure, and that all the conditions are satisfied.
In order to obtain multiple fibers with even better radiation resistance, the pure silica glass with a core content of 0.
It is preferably 5 ppm or less, particularly 0.2 ppm or less, and the core 2 has an OH group content of 7 ppm or less, more preferably 5 ppm or less. The clad layer 3 preferably has a thickness of at least 1.5 μm, particularly at least 1.8 μm, and the core space factor of each optical fiber is within the range of 25 to 55%, and particularly within the range of 30 to 50%. preferable.

Next, the condition (5) will be described. As described above, the multiple fiber of the present invention is manufactured by drawing a large number of bundles of optical fiber preforms. However, there is a large variation in outer diameter between the optical fiber preforms to be used, or when drawing. If the heating temperature or drawing speed is asymmetrical, the random force generated during drawing makes the arrangement of each optical fiber irregular, and the clad layer is partially thinned and the parts where the cores are abnormally close to each other are generated. Many bubbles may be generated, or many bubbles may remain between the fused optical fibers. The occurrence of a large number of such irregularly arranged portions, abnormally close portions of cores, or bubbles
Even if the above conditions (1) to (3) are satisfied, the radiation resistance of the multiple fiber may be reduced. Therefore, in the present invention, it is preferable that the optical fibers existing in a portion having a radius of at least 80% from the center of the cross section of the multiple fibers are regularly arranged and fused, for example, in a bales stacking shape. However, this radius
As long as it is local to 80% or less and is extremely mild, irregular arrangement, abnormally close parts between cores, or bubbles may be used. In the present invention, the optical fiber existing in a portion within a radius of at least 80% from the center of the multiple fiber cross section has a core cross section of a circular shape or a shape close to that, and each optical fiber has a hexagonal cross section or It is particularly preferable that the particles have a close shape and are fused to form a regular honeycomb structure. The multiple fiber having such a regular honeycomb structure is heated and drawn by the above-described method by using the optical fiber preform having the support layer 4 made of silica glass having a higher drawing temperature than the constituent glass of the cladding layer 3 as an example. It can be manufactured by

The cladding layer 3 is made of quartz glass containing, for example, B and / or F as a dopant. Such doped quartz glass has, for example, BCl ₃ , BF ₃ , S
Well-known chemical vapor deposition using a mixed gas of iCl ₄ and oxygen, a mixed gas of BCl ₃ , SiF ₄ and oxygen, or a mixed gas of BF ₃ or BCl ₃ and SiF ₄ and oxygen as a raw material. Can be formed by the CVD method.

Among the above-mentioned raw material mixed gas, a particularly preferable one in order to produce an optical transmission line having more excellent radiation resistance is B
It is a mixed gas of F ₃ , SiCl ₄ , and O ₂ .

The embodiment shown in FIG. 2 has the support layer 4, but if the constituent material of the support layer 4 is silica glass containing a large amount of impurities, the radiation resistance may be adversely affected.
Therefore, as a constituent material of the support layer 4, a quartz glass having a drawing temperature of at least 1800 ° C., for example, natural quartz glass or synthetic quartz glass, particularly 99% by weight in purity is used.
Above all, high-purity synthetic quartz glass of 99.9% by weight or more is preferable.

When manufacturing a multiple optical fiber, for example, a quartz glass pipe is used to draw the optical fiber in a filled state, and thus a skin layer corresponding to the pipe is further formed on the outer circumference of the optical fiber group fused to each other. The fused optical fibers are preferable in terms of flexibility and difficulty in breaking of the obtained multiple optical fibers.

In the present invention, each of the silica glass constituting the clad layer, the support layer, or the skin layer may contain chlorine, but in order to further improve the radiation resistance of the multiple fiber of the present invention, all of them are chlorine. It is desirable that the content is 500 ppm or less, particularly 100 ppm or less.

As described above, the multiple fiber of the present invention is more excellent in radiation resistance in the visible light region,
It is very suitable as an image transmitter for an industrial image scope used for observation in a radiation field.

Examples Example 1 Si (OC ₂ H ₅ ) ₄ and oxygen are mixed and burned, the flame is blown onto a quartz rod target, quartz glass is generated according to the so-called vapor phase Bernoulli method, and the outer diameter is about 35mm, length
A 200 mm quartz glass rod was obtained. The chlorine content of the glass rod was 0.02 ppm, the OH group content was 1.5 ppm, and the refractive index at 20 ° C. was 1.4585.

The chlorine content in quartz glass is determined by ESCA (Electron Spectroscopy).
The OH group content was measured by the following method.

Measurement of OH group content: When the OH group content was 1 ppm or more, it was determined by the formula (1), and when it was less than 1 ppm, it was determined by the formula (II).

OH = 1.2x (L ₁ −L ₀ ) ・ ・ ・ (1) OH = 1.85x (L ₃ −L ₂ ) ・ ・ ・ (II) where L ₁ is the loss value of the optical transmission line at a wavelength of 0.94 μm. (DB / km), L ₀ is a loss value when the OH group content of the optical transmission line at the same wavelength is assumed to be zero. L ₃ is the wavelength
The loss value (dB / km) of the optical transmission line at 1.38 μm, L ₂ is the loss value when it is assumed 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-mentioned pure quartz glass,
In addition, B, F-based doped quartz glass formed under the application of the MCVD method using SiCl ₄ , BF ₃ , O ₂ and a synthetic quartz glass tube (outer diameter 26 mm, wall thickness 1.5 mm, n ²⁰ : 1.459) Layer (n ²⁰ : 1.
4465) having the inner circumference of the glass tube, and applying the rod-in-tube method, a preform with a three-layer structure (outer diameter)
After obtaining 18.9 mm), this was drawn under heating (2,000 ° C.) to obtain an optical fiber having an outer diameter of 300 μm.

The above 6,000 optical fibers (20 cm in length) were bundled so that the cross-section would be a close-packed and aligned shape, one end of which was inserted into a quartz glass tube, and then heat fusion was performed. %), Ultrasonic cleaning in distilled water, and drying. The obtained bundle of optical fibers was heated to 2,000 ° C. and drawn to obtain a multiple fiber having an outer diameter of 1.0 mm in which adjacent optical fibers were fused to each other.

The core diameter of the obtained multiple fiber is 7.3μ.
m, clad layer thickness 2.1 μm, optical fiber diameter 12 μm
m, the refractive index difference (Δn) between the core and the clad layer is 0.012,
The core space factor was 33%. Further, in the multiple optical fiber, each optical fiber having a radius of 95% in the cross section along the entire length had a regular honeycomb arrangement.

Examples 2-5, Comparative Examples 1-5 Examples 2-5, Comparative Examples 1- 5
Five multiple fibers (the number of contained optical fibers is 6000 in each case) were obtained. Their structural characteristics are shown in Table 1 together with that of the multiple fiber of Example 1.

Next, the radiation resistance of each obtained multiple fiber was investigated.

As shown in Fig. 3, the evaluation test consisted of bundling 10m length of a 30m long multiple fiber test sample into a coil at the position of the prescribed dose rate (2 × 10 ⁴ R / H) by the Co ⁶⁰ γ-ray irradiation source. Irradiation was performed at a total dose of 3 × 10 ⁵ R. Both ends of the multiple fiber test sample are taken out through the shielding wall. The light from the white light source (500W) is made incident from one end, and the light emitted from the other end is measured by the monochromator / photometer and recorded in the recorder. did. The above irradiation was performed in air, and the multiple fibers were removed from the light source except during measurement to prevent the influence of photobleaching.

The loss value (at 480 nm) of multiple fiber is CUT B
It was measured by the ACK method. Further, using the irradiated multiple fibers, image observation was performed on a standard test diagram of Kodak color standard color (9 colors).

 The results are shown in Table 1.

[Brief description of drawings]

1 and 2 are sectional views of a multiple fiber according to an embodiment of the present invention. FIG. 3 is an explanation of the radiation resistance test method for multiple fibers in the atmosphere using Co ⁶⁰ γ rays as a radiation source. 1: Multiple fiber 1 ': Unit optical fiber 2: Core 3: Clad layer 4: Support layer

Claims (1)

(57) [Claims] 1. A multiple fiber having a structure in which a fused silica glass clad layer is provided on a pure silica glass core, and a silica glass support layer is further provided thereon, and a plurality of optical fibers are fused and assembled together. The chlorine content of the pure silica glass core is 1 ppm or less, the OH group content is 10 ppm or less, the core space factor of each optical fiber is 20 to 60%, and the thickness of the clad layer is 1.0 μm or more. In addition, each of the optical fibers existing within a portion of at least a radius of 80% from the center of the cross section of the multiple fiber is fused to each other in a regular alignment structure.