JPH0557562B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a silica glass-based multiple fiber suitable as an optical transmission path for an industrial image scope.

Conventional technology Multiple fibers manufactured using core rods made of pure silica glass have excellent radiation resistance, but the drawing temperature of the core rods is low.
Due to the high temperatures reaching over 2000°C, advanced technology is required to manufacture multiple fibers.

On the other hand, core rods made of dopant-containing silica glass can increase the refractive index difference with the cladding layer by changing the type of dopant, the amount of dopant, etc. It has the advantage of making it possible to manufacture multiple fibers with excellent flexibility.

Problems that need to be solved By the way, quartz glass rods containing Dohant can generally be manufactured using the CVD method, VAD method, etc., but regardless of which method they are manufactured, the conventional rods are It is a graded index type rod with a maximum refractive index at It is of the type that rapidly decreases as the trend progresses.

Therefore, when multiple fibers are manufactured using such core rods, the resulting multiple fibers are bright only at the center of each fiber as a pixel, and rapidly darken as the distance from the center increases. There was a problem in that the blurring phenomenon became so large that the transmitted image became extremely difficult to see.

Means for Solving the Problem The present invention provides a multiple fiber having a graded index type core, which overcomes the above problems and is therefore capable of transmitting clear and bright images even with a small diameter. This is what I am trying to do.

That is, the present invention has a structure in which a large number of silica glass optical fibers each having a cladding layer made of pure silica glass with a drawing temperature of at least 1800°C are fused to each other on a core having a radius r. The refractive index distribution of the maximum refractive index value at the center of the core is
When n ₀ and the minimum refractive index value at the outermost part of the core are n ₂ , the refractive index n ₁ at a position at a radius of 0.65r from the central axis of the core satisfies the following formula (1), and at a position at a radius of 0.7r. The refractive index at is also at least n ₂ + 0.65 (n ₀ − n ₂ )
be doped with germanium as a dopant in a graded index form, and each of the above optical fibers shall have a core space factor of at least 20% in the cross section of the optical fiber. It is a quartz glass-based multiple fiber characterized by:

n ₁ ≧ n ₂ +0.65 (n ₀ − n ₂ )...(1) Actions and Effects Each optical fiber included in the multiple fiber of the present invention has a core space factor of at least 20%. Therefore, the amount of light transmitted by the entire core is sufficient, and the degree of decrease in refractive index of the core is smaller than that of conventional ones in the area from the center to a radius of 0.7r, and the refraction exceeds a certain value. Therefore, each pixel in the multiple fiber has sufficient brightness for practical use, not only at least in an area up to a radius of 0.7r from its center, but even in an area slightly outside of that area. The fact that each core of the individual optical fibers contained in a multiple fiber has a graded index profile of refractive index means that it is made of doped silica glass. .
If the cladding layer is also made of doped silica glass, it has the advantage of increasing the refractive index difference between the core and the cladding layer. Each optical fiber as a whole tends to be abnormally deformed due to drawing during fiber manufacturing, and the core of the optical fiber may also become abnormally deformed without maintaining its circular shape, resulting in generally low production yields. Additionally, the core used in the present invention contains a larger amount of germanium dopant than conventional graded index cores, even near the outermost layer of the core, making it easier to draw optical fiber bundles at high temperatures. Another problem is that germanium in the core volatilizes and bubbles are likely to form in the resulting multiple fiber. In contrast, in the present invention, the cladding layer is made of high-purity quartz glass whose viscosity when softened is extremely higher than that of doped quartz glass, that is, whose drawing temperature is at least 1800°C, and which has a low viscosity. Since the core material is surrounded by a cladding layer, it does not deform more than the deformation of the cladding layer during wire drawing. Furthermore, since the cladding layer itself has high viscosity, the rate of abnormal deformation even during wire drawing is extremely low. Moreover, the high viscosity of the cladding layer prevents the germanium in the core from volatilizing. As a result,
The multiple fiber of the present invention has the effect that it can be manufactured with high yield without containing bubbles or abnormal deformation.

EXAMPLES The present invention will be explained below based on the drawings. 1st
2 is a partially enlarged sectional view of FIG. 1, and FIG. 3 is a refractive index distribution curve in the core of the optical fiber constituting the conventional multiple fiber and the present invention. shows.

In FIGS. 1 and 2, 1 is a multiple fiber, and 2 is an optical fiber constituting the multiple fiber 1. In FIG. A large number of optical fibers 2 each consist of a core 21 and a cladding layer 22 provided thereon, and are fused to each other by fusion of adjacent cladding layers. 3 is a skin layer provided on the outermost side of the multiple fibers.

The multiple fibers of the present invention include a large number of optical fiber preforms, for example 1,000 to 500,000, having a cross-sectional shape similar to the optical fibers described above, in a skin pipe made of natural quartz glass or synthetic quartz glass, preferably synthetic quartz glass. The manufacturing method can be carried out by filling the skin pipes in an aligned state, and then drawing the skin pipes together.

In FIG. 3, curve 1 is the refractive index distribution curve in the core of the embodiment of the present invention, and curve 2 is that of the conventional example. In curve 1, the difference (Δn) between the refractive index n ₀ (maximum refractive index) at the center of the core and the refractive index n ₂ (minimum refractive index) at the outermost part of the core, that is, (n ₀ −n ₂ ), is:
It is 0.015-0.035, preferably 0.02-0.028.

In the refractive index distribution shown in curve 1, the refractive index decreases slowly in the section from the center r ₀ of the core to the radius r ₁ , that is, 0.65r, and from the radius r ₁ of the core to the radius r, that is, the outermost part of the core. In the section, the refractive index decreases rapidly. In other words, the change in refractive index is small in the interval r ₀ to r ₁ . Moreover, the refractive index n ₁ at the position of _the core radius r ₁ is n _{2 +}
It has a value of 0.65 (Δn) (= n ₂ + 0.65 × 0.025 = n ₂ + 0.016) or more, that is, n ₂ + 0.016, and furthermore, n ₂ + 0 at a position of radius 0.7r of the core. Satisfies the value of .016. In addition, in the present invention, the core 2
If the cross section of 1 is not circular as shown in FIG. 2, it may be approximated to a circle having a corresponding area.

In the present invention, each of the above optical fibers 2 is
It is essential that the core space factor in the optical fiber cross section is at least 20%. Core space factor is 20
If it is less than %, the amount of light transmitted through the core is insufficient, making it difficult to transmit bright images. Incidentally, if the core space factor is too large, the cladding layer becomes too thin unless flexibility is sacrificed, and a blurring phenomenon occurs in the transmitted image, making it difficult to obtain a clear image. Therefore, the core space factor is preferably 50% or less, particularly 25 to 45%.

The refractive index distribution of the core described above is achieved by using germanium as a dopant that effectively increases the refractive index of silica glass, and by using normal methods such as VAD or CVD using GeCl ₄ as a raw material. This can be achieved by adjusting the above refractive index distribution.

In the present invention, the cladding layer 22 is made of high-purity quartz glass as described above, and synthetic quartz glass with a purity of 99.99% by weight or more is particularly preferred as a constituent material.

Regarding the thickness d ₁ (average thickness of the portion shown in Figure 2) of the cladding layer 22 of each optical fiber 2, if it becomes too thin, a blurring phenomenon will occur in the transmitted image, making it difficult to obtain a clear image. On the other hand, if it gets too hot, the outer diameter of the multiple fiber will become too large, so it is generally 0.5 to 5 μm, especially 1.0 to 4 μm, and even more.
It is preferable to set it as 1.5-2.5 micrometers. Further, the average outer diameter d ₂ (average diameter of the portion shown in FIG. 2) of each optical fiber 2 is 4 to 15 μm, particularly 5 to 13 μm, and more preferably 8 to 13 μm.
The thickness is preferably 11 μm.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of an embodiment of the present invention, and Fig. 2 is a cross-sectional view of an embodiment of the present invention.
3 is a partially enlarged sectional view of the figure, and FIG. 3 shows refractive index distribution curves in the cores of optical fibers constituting the conventional multiple fiber and the present invention multiple fiber. 1 and 2, 1... multiple fibers, 2... optical fibers constituting multiple fiber 1, 21... cores 21 and 22 of optical fiber 2, 22... cladding layer of optical fiber 2, 3... In FIG. 3, the skin layer provided on the outermost side of the multiple fiber 1, Curve 1...Refractive index distribution curve in the core of the embodiment of the present invention Curve 2...Refractive index distribution curve in the core of the conventional example.

Claims (1)

[Claims] 1. On the core of radius r, the drawing temperature is at least
It has a structure in which a large number of silica glass optical fibers each having a cladding layer made of pure silica glass at 1800°C are fused together, and the refractive index distribution of the core has a maximum refractive index value at the center of the core of n ₀ , When the minimum refractive index value at the outermost part of the core is _n2 , the refractive index _n1 at a position at a radius of 0.65r from the central axis of the core satisfies the following formula, and the refractive index at a position at a radius of 0.7r is also at least n ₂ +0.65 (n ₀ - n ₂ ), doped with germanium as a dopant in a graded index form, and each of the above optical fibers has a cross section of the optical fiber. A silica glass-based multiple fiber characterized in that the core space factor in n ₁ ≧ n ₂ + 0.65 (n ₀ − n ₂ ) is at least 20%. 2. The silica glass-based multiple fiber according to claim 1, wherein the cladding layer has a thickness of 0.5 to 5 μm.