JPH0476442B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a quartz glass-based multiple optical transmission body suitable as an optical transmission line for an industrial image scope. Conventional technology A multi-wire optical transmitter is manufactured by drawing a bundle of multiple optical fiber base materials having a cladding layer of doped silica glass on a core of pure silica glass, and thus has a structure in which multiple optical fibers are fused together. The optical transmission base material has advantages such as excellent radiation resistance. On the other hand, since the optical transmission bodies are fused together, the multiple optical transmission bodies obtained by drawing are no different in mechanical properties from a single quartz glass fiber or rod. Therefore, in order to obtain a multiple optical transmission body having the above structure with excellent flexibility,
It is necessary to make the finished outer diameter as small as possible. By the way, in an optical transmission body having a core made of pure silica glass, compared to an optical transmission body having a doped quartz glass core and a doped silica glass cladding layer, it is impossible to increase the difference in refractive index between the core cladding layers for reasons described later. Therefore, it is necessary to increase the thickness of the cladding layer, and there is a limit to reducing the diameter of the multiple optical transmission body due to the thickness of the cladding layer. Although it is possible to reduce the refractive index of pure silica glass by adding a large amount of dopants such as boron or fluorine, the thermal expansion coefficient and fluidity of the resulting doped silica glass during glass melting also sharply increase. Due to the difference in thermal expansion coefficient from pure silica glass, it is not possible to use both materials in direct contact with each other (cracks occur in doped silica glass), and there is also the problem of uneven thickness in the cladding layer due to excessive fluidity. be. Problems to be solved and means for solving them For the reasons mentioned above, it is important to develop a multiple optical transmission body in which each core is made of pure silica glass, and which can be made as small in diameter and as sharp in image quality as possible. In order to solve this important problem, the present invention has a core made of pure silica glass, and a first cladding layer made of doped silica glass, a second cladding layer made of doped silica glass, and a cladding layer made of pure silica glass. It has a structure in which a large number of silica glass-based optical transmitter base materials having a third cladding layer are fused together, the first cladding layer has a lower refractive index than the second cladding layer, has a lower refractive index than the third cladding layer, and the first cladding layer has a refractive index at least 0.014 less than the refractive index of the core;
Moreover, it is an object of the present invention to provide a silica glass-based multiple optical transmission body characterized by a space factor of 5 to 25% in the quartz glass-based optical transmission body matrix. Actions and Effects If the layer is so thin that the space factor of the first cladding layer in the base material of the silica glass optical transmission body is 25% or less, even if it is used as a constituent material of the first cladding layer, a large amount of refractive index reduction is required. Even if a doped quartz glass with a high expansion coefficient obtained by blending a dopant with the above-mentioned dopant is used, the above-mentioned problems of cracking and overflow do not occur. Moreover, by blending a large amount of a dopant for reducing the refractive index to lower the refractive index of the first cladding layer, a large difference in refractive index between the first cladding layer and the second cladding layer can be obtained. An optical transmission body that can transmit clear images even if the total thickness is made thinner than the cladding layer of conventional products can be obtained. Note that if the refractive index of the first cladding layer is lowered as described above,
There is a problem that the first cladding layer cracks when the present invention is produced by drawing a bundle of silica glass optical transmitter base material or after drawing due to the excessive difference in coefficient of thermal expansion with the pure quartz core. be. However, this problem requires that the space factor of the first cladding layer in the base material be 25% or less, and that the third cladding layer is made of pure silica glass.
This can be prevented by placing a cladding layer on top of the second cladding layer. Furthermore, the third cladding layer has the effect of preventing excessive flow during drawing of the cladding layer. Therefore, the multiple optical transmission body of the present invention takes advantage of its excellent image quality, radiation resistance, and flexibility, and can be used in nuclear power plants, high-temperature reactors, etc. by attaching a member that bends the tip end vertically and horizontally. It is useful as a means of observing the inside of a EXAMPLES The present invention will be described below based on the drawings. 1st
The figure shows a cross section of a reference example, and FIG. 3 shows a cross section of an example of the present invention. FIG. 2a is a partially enlarged sectional view of FIG. 1, and FIG. FIG. 4a is a partial enlarged sectional view of FIG. 3, and FIG. 4b is a diagram showing the refractive index distribution of each part in FIG. 4a. In FIGS. 1 to 4, 1 is a multiple optical transmission body, and 2 is an optical fiber constituting the multiple optical transmission body 1. In FIG. Each of the multiple optical fibers 2 has a core 21 and a first cladding layer 22 and a second cladding layer provided thereon, and the embodiment shown in FIG. It has three cladding layers 24. In the reference example, adjacent second cladding layers are fused together, while in the embodiment, adjacent third cladding layers are fused together to form a multiple optical transmission body. 3 is a skin layer provided on the outermost side of the multiple optical transmission body 1. As shown in FIG. D _c is the core 2 of each optical fiber 2 constituting the multiple optical transmission body 1
1 is the diameter, D _f is the diameter of the same optical fiber 2, and T ₁ is the diameter of the same optical fiber 2.
The thickness of the cladding layer, T ₂ indicates the thickness of the second cladding layer, and T ₃ indicates the thickness of the third cladding layer. In the present invention, each of the above optical fibers 2 is
Preferably, the space factor of the core 21 in the cross section of the optical fiber is at least 20%. If the core space factor is less than 20%, the amount of light transmitted by the core 21 is insufficient, making it difficult to transmit bright images, and on the other hand, if the core space factor is excessive, flexibility may be sacrificed. However, there is a problem in that the cladding layer becomes too thin and a blurring phenomenon occurs in the transmitted image, making it difficult to obtain a clear image. Therefore, it is particularly preferable that the core space factor is 70% or less, particularly 25 to 60%. The core 21 is made of pure silica glass, and the purity of pure silica glass is 99.99% by weight or more.
In particular, synthetic products with a purity of 99.9999% by weight or more are preferred. In the present invention, the first cladding layer 22 has a lower refractive index than the second cladding layer 23 as well as the core 21. The multiple optical transmission body 1 of the present invention is usually made of natural silica glass or synthetic quartz glass, for example, 500 to 500,000 fibers, which have a circular cross section and have the same structure and cross-sectional space factor ratio as the optical fiber 2. They can be manufactured by filling them in an aligned state into a skin pipe made of glass, preferably synthetic quartz glass (the material for forming the skin layer 3 in FIGS. 1 and 3), and then drawing the skin pipe together. It is essential that the first cladding layer 22 has a cross-sectional area factor of 5 to 25% in the above-mentioned optical transmission body base material (therefore, in the optical fiber 2). If the cross-sectional area factor is less than 5%, the above-mentioned effect of the same layer will be poor, and if it is more than 25%, the refractive index of the same layer 22 will be sufficiently small due to the above-mentioned cracking and overflow problems. This becomes difficult. Therefore, the cross-sectional area factor of the first cladding layer 22 is 5 to 15%.
It is preferable that The refractive index difference Δn between the minimum refractive index of the first cladding layer 22 and the refractive index of the core 21 is at least 0.014. The minimum refractive index difference Δn between the first cladding layer 22 and the second cladding layer 23 is at least
Preferably it is 0.002, especially at least 0.004. Also, the minimum refractive index of the second cladding layer 23 and the third
The minimum refractive index difference Δn with the cladding layer 24 is preferably at least 0.008, in particular at least 0.010. First cladding layer 22 and second cladding layer 23
is preferably made of quartz glass doped with fluorine and/or boron, or a dopant containing at least one thereof as a main component. Particularly preferred is pure silica glass doped with BCl ₃ , BF ₃ or a mixture thereof as a dopant precursor. The third cladding layer 24 is preferably made of pure silica glass, particularly one with a high purity of 99.99% by weight or more. The doped silica glass constituting the cladding layers 22 and 23 generally exhibits low viscosity at high temperatures where pure silica glass softens, so these layers may be abnormally deformed at high temperatures during wire drawing in the manufacture of multiple optical transmitters. be. In this case, if a third cladding layer 24 made of pure silica glass is provided,
Since the first cladding layer and the second cladding layer are surrounded by the highly viscous third cladding layer, they do not deform more than the deformation of the third cladding layer. In the present invention, the thickness of the first cladding layer 22
T ₁ , the thickness T ₂ of the second cladding layer 23, and the thickness T ₃ of the third cladding layer 24 are each, for example, from 0.05 to
1.5 μm, 0.2-2.5 μm, and 0.01-1.0 μm, especially
The thickness is preferably about 0.1 to 1.0 μm, 0.5 to 2.0 μm, and 0.08 to 0.5 μm. The multiple optical transmission body of the present invention has, for example, a structure of a pipe for the third cladding layer and a core 21 in which doped quartz glass for the second cladding layer 23 and doped quartz glass for the first cladding layer 22 are attached thereon. It can be manufactured by obtaining an optical transmitter base material by the rod-in-tube method using a rod, and then filling a large number of the base materials in a lined manner into a skin pipe, and drawing the skin pipe together. The table below shows the structures and performances of the multiple optical transmission bodies of Examples 1 to 9 and Comparative Examples 1 to 3. Note that the number of pixels (the number of fused optical fibers) of the multiple optical transmission bodies of each example and comparative example is 5000. The transmission image quality was evaluated using the method described below. Samples with a length of 5 m were taken from the multiple optical transmission bodies of each example and comparative example, and lenses were attached to each end to create an image scope (objective lens viewing angle: 60 degrees, eyepiece lens magnification: 40x). , 30W located 5m away from the objective lens
I looked directly into the fluorescent light. In general, the poorer the optical confinement effect of each cladding layer in a multiple optical transmission body, the more
Strong coloring appears to exist around the light emitting part of the fluorescent lamp. Therefore, the fluorescent lamps were graded as follows according to the degree of coloring around the light emitting part. Excellent: No coloration observed. Good: Very faint red or green coloration is observed. Fair: Very strong coloring is observed.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a sectional view of a reference example, FIG. 3 is a sectional view of an embodiment of the present invention, FIG. 2a is a partially enlarged sectional view of FIG. 1, and FIG. 2b is a cross-sectional view of FIG. FIG. 4a is a partially enlarged sectional view of FIG. 3, and FIG. 4b is a diagram showing the refractive index distribution of each portion in FIG. 4a. 1: Multiple optical transmission body, 2: Optical fiber constituting multiple optical transmission body 1, 21: Core, 2
2: first cladding layer, 23: second cladding layer, 2
4: Third clad layer, 3: Skin layer.

Claims (1)

[Scope of Claims] 1. A core made of pure silica glass, and a first cladding layer made of doped silica glass, a second cladding layer made of doped silica glass, and a third cladding layer made of pure silica glass, in this order. It has a structure in which a large number of silica glass-based optical transmitter base materials are fused together, and the first cladding layer has a lower refractive index than the second cladding layer, and the second cladding layer has a lower refractive index than the third cladding layer. the first cladding layer has a refractive index that is at least 0.014 less than the refractive index of the core;
Moreover, the quartz glass-based multiple optical transmission body is characterized in that the space factor in the quartz glass-based optical transmission body matrix is 5 to 25%. 2. The multiple optical transmission body according to claim 1, wherein the core has a cross-sectional area factor of at least 20%. 3. Claims 1 to 2, wherein the first cladding layer and the second cladding layer are made of silica glass doped with a dopant containing fluorine and/or boron, or at least one thereof as a main component. The multiple optical transmission body according to any one of paragraphs.