JPH0333659B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a quartz glass image guide having a two-layer structure.

A quartz glass image guide (multiple fiber), which has a core made of pure silica glass and a cladding made of silica glass with a lower refractive index than the core, is produced by bundling preforms having a core and cladding of the same quality, and then heating and stretching the bundle at once. It is manufactured by fusing optical fibers together. By the way, the low refractive index quartz glass that forms the cladding is manufactured by doping it with fluorine, boron, etc., and the melting temperature of this doped quartz glass decreases as the amount of doping increases. Therefore, if the core pure silica glass is heated to the drawing temperature (approximately 1950 to 2100°C) required for drawing a preform bundle and then drawn, the cladding will flow, resulting in the cores coming into contact with each other or Since it is not possible to obtain a suitable cladding thickness and the cladding is unstable, it is not possible to obtain an image guide that can withstand practical use.

Therefore, we use a three-layer preform with a support made of quartz glass, which has a higher melting temperature than the cladding, formed on the outside of the cladding to avoid problems caused by the cladding's tendency to flow during wire drawing. However, the image guide obtained by this method has a three-layer structure in which there is a quartz glass region generated from the support between the claddings, so the support region serves as a light transmission path in the same way as the core. This causes stray light, which prevents accurate light transmission and causes the transmitted image to appear washed out. Additionally, the need for a support area increases the outer diameter of the entire image guide and impairs its flexibility.

The present inventors have conducted intensive research to produce an image guide in which the cores are uniformly arranged without using a support, and as a result, have completed the present invention.

That is, in the present invention, a skin pipe is densely filled with a large number of preforms having a two-layer structure of a pure silica glass core and a silica glass cladding having a refractive index lower than that of the core, and the inside of the skin pipe is maintained at a reduced pressure. In this state, the core is heated to a temperature at which the core does not substantially flow but the cladding can be fused (hereinafter referred to as the cladding temperature) to fuse the claddings of adjacent preforms together, and then heated to the drawing temperature of the core. The present invention relates to a method for manufacturing a quartz glass image guide having a two-layer structure, which is characterized in that the image guide is drawn using a quartz glass.

In the manufacturing method of the present invention, a bundle of two-layer preforms is not drawn immediately, but is drawn after being heated to the cladding temperature to fuse the claddings together.

The fusion temperature of the cladding varies depending on the amount of dopants such as fluorine doped into the quartz glass forming the cladding, but is usually 1200°C or higher. However, if the temperature at which the core fluidizes is reached, the cladding will overflow and the cores will come into contact with each other, so the temperature must be lower than the fluidizing temperature of the cores. The preferred cladding temperature is approximately
1200~1800℃, especially preferably about 1400~1700℃
It is.

Fusion of clads is carried out by heating a bundle of preforms with a two-layer structure to the cladding melting temperature, but in particular, the bundle of preforms is placed in a quartz glass skin pipe, and the pressure inside the skin pipe is reduced. is preferable. When using this method, the skin pipe is deformed to reduce its diameter by reducing the pressure, and the deformation force further deforms the crud of the preform inside, making the welding work easier and creating a bond between the welded cruds. No air bubbles remain. Therefore, the material of the skin pipe may be any material that deforms at the fusion temperature of the cladding, and may be pure quartz glass, natural quartz, or doped quartz glass. The higher the degree of pressure reduction, the better, but 0.5 atm or less is preferable.
Reducing the pressure can be easily achieved, for example, by using a vacuum pump.

This pre-sealing step results in a preform bundle in which the cores are arranged at approximately equal intervals and the cladding is filled between them. Then, this preform bundle may be drawn at the drawing temperature of pure silica glass, which is the core, in the same manner as in the prior art. The drawing temperature of pure silica glass is usually about 1950-2100°C.

The two-layer preform used in the present invention can be manufactured by a conventionally known external attachment method.
It is also possible to fabricate a preform having a support by an internal attachment method or a rod incubation method, and then remove the support.

When the manufacturing method of the present invention is used, since the fibers are fused together at the cladding temperature, overflow of the cladding does not occur, and the proportion of cores that are approximately equally spaced is increased. When drawing the preform bundle in this state, an image guide with a two-layer structure in which the positional relationship in the preform bundle is maintained is obtained.

The two-layer image guide obtained by the manufacturing method of the present invention has no area based on the support, so there is no risk of stray light, and therefore image clarity can be improved, and the thickness of the support The outer diameter of the image guide can be reduced to that extent, and its flexibility can be greatly improved. Further, when going through the pre-fusion step, the cross-sectional shape of the preform bundle in which the clads are fused together can be made substantially circular, and the cross-sectional shape of the image guide to be folded can also be made circular.

Next, the manufacturing method of the present invention will be explained with reference to Examples, but the present invention is not limited to these Examples.

Example A preform with a two-layer structure consisting of a pure quartz glass core and a fluorine-doped silica glass cladding (core outer diameter 12 mm, clad outer diameter 18 mm)
mm) was prepared. From this preform, the core outer diameter is 240μm, the cladding thickness is 60μm, and the length is
A 300mm two-layer preform was fabricated.

Next, 10,000 of these preforms were made into a length of 320mm.
A bundle assembly was prepared by filling a skin pipe made of pure silica glass with an outer diameter of 39 mm and an inner diameter of 38 mm without any gaps, and then connecting dummy pipes to both ends of the skin pipe. This assembly was set vertically in a high-temperature drawing apparatus, the lower end was sealed with a silicone rubber stopper, and the upper end was connected to a vacuum pump to reduce the pressure inside the assembly to 0.2 atmospheres. Then an electric furnace
The temperature was raised to 1450° C., the assembly was lowered at a rate of 5 mm/〓, and once all the bundle sections had passed through the heating zone, the assembly was raised at the same temperature. When the bundle assembly was observed after repeating this operation three times, it was found that all the spaces between the preforms of the bundle assembly had disappeared, and the bundle assembly had become a perfectly cylindrical preform bundle without bubbles.

This preform bundle was set in a conventional carbon heater high-temperature wire drawing device, and wire was drawn under the conditions of a furnace temperature of approximately 2000°C, a preform bundle feeding temperature of 2 mm/〓, and an Image Gite take-off speed of 1.8 m/〓.
Core outer diameter 8μm, average core spacing (cladding thickness) 1.7μm
A two-layer image guide with a core occupancy of 44% and an outer diameter of 1.25 mm was manufactured. The roundness was 99, and it was almost circular.

Comparative Example 1 A two-layer preform (core outer diameter 12 mm, clad outer diameter 18 mm) manufactured in the same manner as in Example was set at the center of a pure silica glass tube with an inner diameter of 19 mm and an outer diameter of 21 mm, and a normal _H2 A preform with a three-layer structure having a core outer diameter of 12 mm, a clad outer diameter of 18 mm, and a support outer diameter of 20 mm was produced by collapsing with -O ₂ flame.

Support outer diameter made from this preform
10,000 preforms of 360 μm were filled into a skin pipe in the same manner as in the example, and wire was drawn at once under the same conditions as in the example except that no pre-fusion treatment was performed to obtain a core outer diameter of 8 μm, cladding thickness of 1.7 μm, and support thickness of 0.6 μ
A three-layer image guide with an outer diameter of 1.40 mm and a core occupancy of 36% was fabricated. The roundness was 99.

Comparative Example 2 A two-layer preform manufactured in the same manner as in Example was used, and wire was drawn at once under the same conditions as in Example, except that no pre-fusion treatment was performed, resulting in a core outer diameter of 8 μm and a core occupancy of 44. An image guide with a two-layer structure of % was fabricated.

The resulting image guide had uneven core spacing and was highly oval (roundness 70), and had no commercial value.

Test example 1 (Resolution) An eyepiece lens is placed at one end of an image guide with a length of 5 mm, and an objective lens with a viewing angle of 20 degrees and a focal length of 4 mm is placed at the other end, and the EIAJ established by the Electronics Industries Association of Japan is placed 360 mm from the objective lens. Test chart A was placed, a color viewer [DNPmodel-V (used lamp FL-100WX4)] was placed behind the chart, and lines that could be visually identified were examined.

The resolution of the image guide created in the example is
700 lines/360 mm, and the resolution of the image guide obtained in Comparative Examples 1 and 2 is 600 lines/360 mm.
It was a significant improvement over 360mm.

Test Example 2 (Flexibility) The image guides obtained in Example and Comparative Example 1 were bent into a loop shape, the diameter of the loop was gradually reduced, and the diameter of the loop was determined when the image guide broke. I looked into it.

As a result, the image guide obtained in Comparative Example 1 broke when the diameter was 30 mm, but the image guide obtained in Example did not break until the diameter reached 25 mm.

Claims (1)

[Claims] 1. A skin pipe is densely packed with a large number of preforms having a two-layer structure of a pure silica glass core and a silica glass cladding having a refractive index lower than that of the core, and the pressure inside the skin pipe is reduced. The core is heated to a temperature where the core does not substantially flow but the cruds can be fused to fuse the cruds of adjacent preforms while the preform is held at a temperature of A manufacturing method for a two-layered quartz glass image guide. 2 The temperature at which the clads are fused together is 1400 to 1700℃
The manufacturing method according to claim 1.