JPH0561606B2 - - 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 core rods are drawn at high temperatures.
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, dopant-containing silica glass rods can generally be manufactured by CVD, VAD, etc., but regardless of which method they are manufactured, 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, in the present invention, a large number of silica glass optical fibers each having a cladding layer made of doped silica glass on a core of radius r and a support layer made of silica glass having a drawing temperature of at least 1800° C. It has a fused structure, and the refractive index distribution of the core is a radius from the central axis of the core, where n ₀ is the maximum refractive index value at the center of the core, and n ₂ is the minimum refractive index value at the outermost part of the core. The refractive index (n ₁ ) at the position of 0.65r satisfies the following formula (1), and the refractive index at the position of radius 0.7r also satisfies at least n ₂
+0.65 (n ₀ - n ₂ ) and is doped with germanium as a dopant in a graded index form, and each of the above optical fibers has a core in the cross section of the optical fiber. It is a quartz glass-based multiple fiber characterized by a space factor of at least 20%.

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. . Therefore, when the cladding layer is also made of doped silica glass as in the present invention, there is an advantage that the difference in refractive index between the core and the cladding layer can be increased. Both the core and cladding layers are made of doped silica glass.
Generally, doped silica glass exhibits low viscosity when softened by heating, so the drawing process during multiple fiber manufacturing may cause abnormal deformation of the entire individual optical fiber.
There is also a problem in that the core in the optical fiber is also abnormally deformed. Furthermore, the core used in the present invention is doped with a larger amount of germanium than conventional graded index cores, and contains germanium near the outermost layer of the core, making it easier to draw optical fiber bundles at high temperatures. Finally, there is also the problem that this germanium tends to volatilize and form bubbles in the obtained multiple fiber. In contrast, in the present invention, a support layer made of quartz glass having a drawing temperature of at least 1800° C. is provided on the cladding layer. The viscosity of this quartz glass when softened is extremely high compared to doped silica glass. Therefore, since the low-viscosity core and cladding layers are surrounded by the above-mentioned high-viscosity quartz glass, it is difficult to form a support layer during drawing. No abnormal deformation. Since the support layer itself has a high viscosity, the rate of abnormal deformation even when wire is drawn is generally extremely low. Moreover, the main feature is high viscosity when drawing the support layer,
In addition to this, the presence of the cladding layer also prevents the volatilization of germanium in the core. Furthermore, since a cladding layer made of doped silica glass with a low refractive index is present on the core with a high refractive index, a large aperture ratio can be obtained. As a result, the present invention can provide multiple fibers that do not contain bubbles, are free from abnormal deformation in each pixel, and are further made thinner and more flexible.

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, a cladding layer 22 provided thereon, and a support layer 23, and are fused to each other by fusion of adjacent support 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 (△n) 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 gradually in the area from the center of the core (r ₀ ) to the radius r ₁ , that is, 0.65r, and
The refractive index rapidly decreases in the section from r ₁ to the radius r, that is, the outermost part of the core. In other words, the change in refractive index is small in the interval r ₀ to r ₁ . Moreover, the refractive index at the position of core radius r ₁ is
For example, n ₁ is n ₂ +0.65 ( _{△ n} ) (=n ₂ +0.65×0.025=n ₂ +
0.016) or more, that is, n ₂ + 0.016, and furthermore, n ₂ + at the core radius of 0.7r.
It satisfies the value of 0.016. In the present invention, if the core 21 does not have a circular cross section as shown in FIG. 2, it may be approximated to a circular shape 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 and the support layer become too thin unless flexibility is sacrificed, causing a bleeding phenomenon in the transmitted image and making it difficult to obtain a clear image. Therefore, the core space factor is 50% or less,
In particular, it is preferably 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 doped silica glass as described above, but preferably quartz glass doped with a dopant containing fluorine and/or boron or at least one thereof as a main component. ,for example
These include those manufactured using raw material gases such as BCl ₃ , BF ₃ , and SiF ₄ .

The thickness d ₂ of the cladding layer 22 of each optical fiber 2 (second
Regarding the average thickness of the part shown in the figure, 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 becomes too thick, the outer diameter of the multiple fibers will be too large. Therefore, it is generally preferable to set the thickness to 0.5 to 2 μm, particularly 0.9 to 1.3 μm.

The support layer 23 has a drawing temperature of at least
Made of 1800℃ quartz glass. In the present invention, the above-mentioned drawing temperature means that a pipe with an inner diameter of 23 mm and an outer diameter of 26 mm manufactured using the quartz glass to be tested is heated and softened to form a reduced pipe of an inner diameter of 2.3 mm and an outer diameter of 2.6 mm per minute.
It is defined as the lowest temperature at which the drawing tension is 500g or less when drawing at 0.5m. Examples of the quartz glass having the above-mentioned high drawing temperature include natural quartz glass and synthetic quartz glass, but preferably synthetic quartz glass, particularly high-purity quartz glass having a purity of 99.99% by weight or more.

If the thickness of the support layer 23 is too small, it will not be possible to prevent the above-mentioned abnormal deformation of the core and cladding layer during the production of the multiple fiber of the present invention, while if the thickness is too large, the multiple fiber The outer diameter of is too large. Therefore, the thickness of the support layer 23 is 0.05 to 1.5 μm, particularly 0.1 to 0.7 μm.
It is preferable to set it to m.

In the present invention, the average outer diameter of each optical fiber 2
d ₃ (average diameter of the part shown in Figure 2) is 4 to 15μ
m, particularly preferably from 5 to 13 μm, more preferably from 6 to 9 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: core of optical fiber 2, 22: cladding layer of optical fiber 2, 2
3: Support layer of optical fiber 2, 3: Skin layer provided on the outermost side of multiple fiber 1 In FIG. 3, curve 1: refractive index distribution curve in the core of the embodiment of the present invention, curve 2: core of the conventional example refractive index distribution curve.

Claims (1)

[Claims] 1. A large number of silica glass optical fibers each having a cladding layer made of doped silica glass on a core having a radius r, and a support layer made of silica glass having a drawing temperature of at least 1800° C. They have a mutually fused structure, and the refractive index distribution of the core is from the central axis of the core, where n ₀ is the maximum refractive index value at the center of the core, and n ₂ is the minimum refractive index value at the outermost part of the core. The refractive index (n ₁ ) at a position with a radius of 0.65r satisfies the following formula, and the refractive index at a position with a radius of 0.7r also has a value of at least n ₂ + 0.65 (n ₀ − n ₂ ). Each optical fiber is doped with germanium as a dopant in a graded index form, and each of the above optical fibers has a core space factor of at least
A quartz glass-based multiple fiber characterized by 20%. n ₁ ≧n ₂ +0.65 (n ₀ −n ₂ ) 2 The silica glass-based multiple fiber according to claim 1, wherein the cladding layer has a thickness of 0.5 to 2 μm. 3. The silica glass-based multiple fiber according to any one of claims 1 to 2, wherein the support layer has a thickness of 0.05 to 1.5 μm. 4. The multiple fiber according to any one of claims 1 to 3, wherein each optical fiber has an outer diameter of 4 to 15 μm. 5. The cladding layer is made of quartz glass doped with a dopant containing fluorine and/or boron or at least one thereof as a main component. Quartz glass multiple fiber.