JPS5815041A - Production of base material for optical fiber - Google Patents

Abstract

PURPOSE:To make the production of an intended base material for optical fibers possible in the stage of production of a base raw material for optical fibers that generates double refraction in the core part by forming a thin film of glass on the inside wall of a quartz glass tube then reducing the pressure in the tube down to the pressure below the atmospheric pressure and melt sticking the tube by heating thereby making the tube solid. CONSTITUTION:A quartz glass tube 1 of 5-50mm. outside diameter and 0.3-5mm. thickness is prepared as a base material 1 for optical fibers, and a thin film 2 of glass for formation of the clad of optical fibers and a thin film 3 of glass for formation of the core are formed by a chemical deposition method on the inside wall of said tube. Silica glass contg. B2O3 of a softening point lower than that of the softening point of quartz glass is used for the film 2 and glass of a softening point higher than that of the film 2 is used for the film 3. Such tube is heated with a burner 4 to melt stick the end part, after which the inside is evacuated and the burner 4 is moved to make the tube solid with the films 2, 3, whereby the tube is formed to the base material having an elliptical shape in section wherein the outside circumference attains C2/a>=200/(100-r)-1 where the rate of ellipse is r, the minor axis diameter of the elliptical shape C2 and the diameter of the core formed a.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an optical fiber preform O, and more specifically, to an optical fiber used in the process of manufacturing an optical fiber that produces birefringence in the core, which is the essential part of optical transmission of the optical fiber. The present invention relates to a method of manufacturing a base material, that is, a preform rod.

One possible use of optical fibers is to transmit polarized waves without disturbance, and to connect them to optical integrated circuits, to use them in measurement devices, optical switches, and the like. As described above, for a circular optical fiber that can propagate light without disturbing the plane of polarization (preserving the plane of polarization), it is necessary that the difference in the propagation phase constants in the orthogonal principal axis directions of the core forming the optical fiber is large. In order to obtain such a difference in phase conduction propagation constants, a method of creating a difference in thermal stress strain applied to the core has been considered.

However, the conventionally known method involves forming a glass layer to serve as the core and cladding on the inner wall of the glass tube by chemical vapor deposition (CVD), and then solidifying the inside of the glass tube, at least without applying pressure. A rod is made, and a portion of the rod is polished to form a non-circular optical fiber preform, which is then heated and drawn to form a thin optical fiber. This is a method of producing birefringence in the core due to the non-circularity of the preform lot and the difference in coefficient of thermal expansion between the materials of the glass tube and the glass layer formed on the inner wall.

However, this method requires a polishing process to make a part of the solid rod non-circular, and it is not possible to generate sufficiently large birefringence, especially in order to generate birefringence in the core. Although it is desirable that the cladding or the outer jacket be elliptical, it is difficult to control the shape arbitrarily.

Therefore, an object of the present invention is to form a glass thin film different from at least the material of the glass tube on the inner wall of a circular quartz glass tube, and heat and weld this to form a solid optical fiber base material.
In other words, the method of making preformed lots is economical;
It is an object of the present invention to realize a manufacturing method in which at least a portion of the glass thin film portion has an elliptical shape using a simple method.

Another object of the present invention is to provide a circular center layer, an intermediate layer formed on the outer periphery of the center layer and having an elliptical outer periphery, and an outer periphery of the intermediate layer, without requiring a polishing process or a process of deforming the initial glass tube. It is an object of the present invention to realize a method for manufacturing an optical fiber base material in which the ellipticity of the outer layer of the intermediate layer can be arbitrarily set from the same outermost layer formed in the center layer.

In order to achieve the above-mentioned purpose, a method of forming a glass thin film on the inner wall of a glass tube serving as a base material and heat-welding it to produce a solid preform for Hikari 7 Eyepa, One end of the tube on which the glass thin film is formed is heated and crushed, and the heated part is removed from the crushed one end while rotating with the pressure inside the crushed glass tube lower than the external pressure. Particularly, in the present invention, the preform lot obtained by the present invention has a cross-sectional structure in which the center layer is circular and the middle layer has an elliptical outer circumference. In the above method, a part of the intermediate layer is made of a material having a softening point lower than that of the glass tube serving as the base material so that the outermost layer is formed of a circular or nearly circular layer. The material forming the core portion is made of a material having a higher softening point than the material having a lower softening point.

Note that the center layer and the intermediate layer are not necessarily limited to a single layer as described in the embodiments, and may be formed of a plurality of layers. In addition, in the present invention, an ellipse is defined as 01 for the long axis and C for the short axis.
When it is set to 1, the ellipticity less than that is considered circular.

According to the method of the present invention, an optical fiber base material with arbitrary reproducibility of the ellipticity of the center layer or intermediate layer can be realized by specifying the radius, thickness, degree of vacuum of the glass tube, and material and amount of the glass thin film. can do.

In particular, when the central layer is circular and the intermediate layer is elliptical, it is easy to make the refractive index of the central layer different in the orthogonal axes, resulting in an optical fiber that easily preserves the plane of polarization. can do.

Incidentally, the final optical fiber can be easily produced by simply drawing the preform lot obtained by the above method while heating it, and having a cross-sectional structure similar to the above-described cross-sectional structure.

The present invention will be explained in detail below using the drawings.

FIG. 41 is a diagram showing the steps of the method for manufacturing an optical fiber preform according to the present invention.

First, (l) a quartz glass tube 1, which is a base material for an optical fiber, is prepared. If the diameter of this glass tube is large and the thickness is thin, - means the amount of subsequent pressure reduction is 1! It is necessary because it is difficult to obtain the specified shape in (3)! ! In some cases, a step of reducing the diameter is included. Desirably, the outer diameter is 5 to 50 cm and the thickness is α3 to 5 m.

A glass thin film 2, which will serve as an intermediate layer of the optical fiber, and a glass thin film 3, which will serve as a center layer, are formed on the inner wall of the glass tube 1 by a chemical deposition (CVD) method. The intermediate layer may have optical fiber cladding or jacket and crut functionality. Also, only the core of the central layer optical fiber, or the core and the crut (i.e., forming the optical transmission part)
may form.

These materials and thicknesses will be explained in detail later. Both ends of the glass tube with the glass thin film obtained in the above process are attached to a glass lathe stand and rotated at a constant rotational speed (3). In the figure, the inner part of the tube attached to the mount is heated with a heating burner 4 to crush it. Then, an exhaust tank 5 is provided at the other opening of the glass tube, and while adjusting the exhaust pipe 6 and the exhaust valve 7, the pressure inside the tube is reduced and maintained at a constant pressure. The amount of pressure reduction is determined by a U-shaped tube 9 whose one end is inserted into the quartz tube 8.
It is measured by the difference in liquid level of 0. In this state, the heating source (burner) 4 is gradually moved to form a solid preform rod. The degree of this pressure reduction is K I III as shown in Figure 3.
IHt O~20■H, set to about 0.

The preformed rod is melted by a heating burner 4 and drawn from one side to produce an optical fiber in which a portion of the inner layer is elliptical.

FIGS. 2(a), φ)e(C), and (d) are traced photographs of cross sections of preforms made by the above method, and the manufacturing conditions for each are as follows.

The initial quartz tubes have the same outer diameter of 20 mm and thickness of 1.5 mm.

After forming a glass thin film as shown below, the oxyhydrogen burner 2 was gradually heated to 0.17 m while rotating at a speed of 50 revolutions per minute.
It moved at a speed of l sec.

In (a), the core is made of silica glass doped with Daruma, and the decompression amount is 9- (hereinafter expressed as ■H, 0) at the height of water, and a center layer with an ellipticity of about 50% is obtained. It will be done.

In the figure (b), the amount of pressure reduction in the tube is increased to 27-H,0, and the thickness of the Daruma layer doped in the core of the hollow preform is about 15 μm. (C) and (d) have silica glass in the core (center layer) and cluttering (
The intermediate layer (interlayer) is doped with boron (BgOm) to form nine-silica glass, and the core is circular and the cladding is oval (C), and conversely the core is oval and the cladding is circular (factor (d)). This is what is shown. These manufacturing methods are achieved by shrinking the starting quartz tube once to prevent rotation of the elliptical axis as described above, and by optimally selecting the amount of shrinkage of the kernel and the thickness of the core layer and cladding layer. .

Figure 3 shows the measurement results of the ellipticity when changing the thickness of the quartz tube and the degree of vacuum.In each curve, the core was made of silica glass doped with Daruma, and Figure 2 (a)
, It has a two-layer structure as shown in (b). The roughness of Daruma is about 15 m0/%, the outer diameter of the starting quartz tube is 201J, and the inner diameter of 17 m is common for both a% curves, and the thickness of the gelm layer of the hollow preform is about 10 μm. 13.5■, IZ8■, 9.7 before heat welding (calarapus)
Curve 11 shows the ellipticity of the core of a solid preform obtained by shrinking it to the outer diameter of ■ and then reducing the pressure.
゜12.13 is 13.5■, 128■, 9,7 respectively
This is an experimental example of ■. Curve 12 is a fiber using silica glass doped with silica for the core and boron doped for the cladding, and has a three-layer structure as shown in Fig. 2 (C). The hollow preform had a thickness of 18 μm.

The silica layer serving as the core has a thickness of approximately 8 μm, and the starting quartz tube has the same outer diameter of 201111% and inner diameter of 17.
It is from ■. After forming a cladding layer and a core layer on this tube, it was shrunk to about 13.1-. Thereafter, curve 12 shows the ellipticity of the 9-clap ink obtained by changing the amount of vacuum as a function of the amount of vacuum.

From the above example, the elliptical shape of the inner elliptical layer formed in a part of the preformed rod is determined by controlling the degree of vacuum, the material of the dopant, the diameter and thickness of the starting quartz tube, and the radius of the hollow part. I understand. That is, the higher the degree of vacuum, the greater the ellipticity, and the thicker the hollow tube, the lower the ellipticity. As can be seen from the above example, the thickness of the quartz glass tube is 0.3 m to 5 m, the outer diameter is 5 mm to 50111 mb, and the difference between the outside pressure and the pressure inside the tube is 1 H, 0 to 30 H, 0. Now, the relationship between the zero-order ellipticity r, the degree of pressure reduction P (txxHt O), and the thickness of each layer, which is realized by the method of the present invention at a welding temperature of 1700 t to 2000 t, will be quantitatively explained.

Figure 4 shows the ellipticity of the inner diameter of the outermost layer (same as the outer circumference of the center or middle layer) of the preformed rod obtained by the method of the present invention, and the ellipticity of each layer before solidification and each layer after solidification. It shows the relationship between the thickness and the vertical axis shows the ellipticity of the inner elliptic layer of the preform, and the horizontal axis shows (b'/X(d'/C'), and these are shown in Figure 5. As shown in Fig. 6, the outer diameter V inner diameter 1' of the quartz tube before solidification, the outer diameter d/ of the preform, and the average radius c' (tz r J of the elliptical inner layer), and the manufacturing conditions are , welding temperature 18001: degree of vacuum 8@H, 0 moving speed of heating burner 0.8 wl/sec, glass thin film is silica glass containing Gem, 8.0. as a dopant. In the figure, ., O9Δ represent the initial quartz tube diameters of 14.18 and 20 mm, respectively.

From Figure 4, the ellipticity r is It turns out that there is a relationship between

The above equation U) is the welding temperature (1700-2000'c),
Movement speed of u heat source 4 (0.02~0.2mg/Sec
), it also holds true even if the composition of the deposited glass is varied within the practical range of optical fiber manufacturing. In addition, above (1)
The equation is a constant determined by the degree of decompression, and Figure 7 shows the relationship between Hashimoto and the degree of pressure P obtained experimentally.
According to the figure, in order to make a preformed rod having an inner elliptic layer with a predetermined ellipticity r, a quartz tube is used as the base material, the welding temperature is 1700-2000°C, and the moving speed of the heating source is 0.02°C.
~0.2■H,0, the diameter and pressure of the degree of pressure reduction P11 are X - (b'/a') X (d'/C') ・--
−−・−・−・−・The settings may be made based on (4). Note that even if the deposited glass layer is not one type but multiple layers, the above equation always holds true if the deposited glass layer is sufficiently thin compared to the thickness of the quartz glass tube when it is hollow.

One of the major advantages of the method of the present invention is that, as shown in FIG. 2(C), FIG. 8, and FIG. An optical fiber base material in which the center layer has a nearly circular shape can be easily realized.

In Figure 8, the center layer 3 is the core, the intermediate layer 2 is the cladding, the outermost layer 1 is the jacket, the core 3 and the cladding 2 form an optical transmission part, and the jacket 3 and the cladding 2 produce birefringence in the core. It's tight. In the one in Figure 9, the center layer is composed of a core 3 and a crat 2-2, and the middle layer 2-1 is a jacket.
The outermost layer 3 forms a support part, and the core 3 has a circular outer periphery.
and cladding 2-2 form an optical transmission section, and jacket 2-2
1 and the outermost layer have the function of producing core 3 birefringence.

In this way, in order to make the ellipticity of the intermediate layer higher than that of the center layer, the intermediate layer should be made of a material that has a lower softening point than the core material.This means that the intermediate layer material ICB, O, S10. This is achieved by adding The softening point decreases with increasing amount of B, 0.0, but for rounding with a large difference in thermal expansion coefficient, the amount of B*Oa is preferably 3 mol to V30 mol. To make it circular, K is the ellipticity of the intermediate layer of the preform base material r1
The thickness of the glass thin film formed on the inner wall of the glass tube may be set in advance so that the short axis length of the intermediate layer ellipse is C-8, and the circular diameter of the center layer is a. .

These requirements are due to the following reasons. When a glass tube with a thin glass film formed on it is heated and welded under reduced pressure, the temperature distribution is initially high on the outside, and because the tube is thick, the outside is less affected by coercion and tries to maintain its circular shape due to surface tension. do. The inside is mainly controlled by the degree of decompression and tends to become flat. If the heating continues, the temperature inside will also increase, making it easier to deform. Then, due to the pressure, the tube shrinks while becoming flat, and the hollow part becomes smaller. During this time, the viscosity of the intermediate layer having a low softening temperature is gradually reduced. Therefore, when the hollow part disappears, the center layer floats in the intermediate layer with reduced viscosity. At this time, the center layer is not under pressure, so the shape of the center layer tends to become circular mainly due to surface tension. This is because, during the cooling process, an intermediate layer is filled between the ellipse formed inside the initial quartz tube and the circular core at the center, and is deformed and solidified.

Therefore, the factors that determine these shapes include the softening point of the intermediate layer during heat welding, the viscosity, and the relative thickness of the intermediate layer and the central layer. and the relationship between the ellipticity of the innermost station in the outermost layer (in other words, the outer periphery of the middle layer).

First, the conditions for making the outer circumference of the intermediate layer an ellipse are determined by the conditions of the above-mentioned equation U).

Next, in order for the ellipticity r of the center layer to become smaller than the ellipticity of the outer periphery of the intermediate layer, that is, to approach a circle, the softened intermediate layer must be Among the layers, it is necessary for the center layer to easily change freely into a stable circular shape due to surface tension, etc., and for this purpose, the softening point temperature of the center layer, middle layer, and outermost layer must be adjusted to al.
, α.

and α, then αi〉αsnow, α$〉α1 °0−0″□′.(6)
Normally, to satisfy this condition, the outermost layer must be made of quartz glass, and the center must have a high refractive index, so it should be made of Sin or 810. Gem.

Or P, 0. The intermediate layer is made of glass containing B2O as a dopant, and the intermediate layer is 3 m thick with B2O as a dopant.
o/- to 30m0/- including 8 i 0. is desirable.

In order to improve the roundness of the center layer, in addition to the softening point mentioned above, the ellipticity of the middle layer and the ratio of the materials of the middle layer and the center layer are affected, and there is a certain relationship between them. The experimental Kl bow angle 3° is shown in the quartz tube (inner diameter [7
■, outer diameter 12■) inner wall by KCVD method)'
(middle layer) 17 molti B, 0. and 83 morchi 8i
0. After forming a glass thin film of 150 μm thick, a 100 mol d8i0. The ellipticity of the intermediate layer and the ellipticity of the center layer (core) are shown when the glass thin films of 2 are formed with different thicknesses x (*m) and welded and solidified at a degree of vacuum of 10 μH, 0. In this case, the ellipticity of the intermediate layer (cladding) is 4
It is 59G. Note that the core diameter in the figure indicates the radius when the core is preformed. That is, it can be seen that when the ellipticity is constant, the ellipticity of the center layer (core) is determined by the relative ratio of the thickness of the intermediate layer and the thickness of the center layer.

Figure 11 shows the minor axis diameter of the intermediate layer (cladding) and the central layer (coca The ratio of the diameters is experimentally determined.In the figure, the horizontal axis shows the ellipticity r of the outer periphery of the cladding, and the vertical axis shows the short axis diameter C3 of the cladding; From this measurement result, at the boundary where the core is circular, it tends to become circular depending on the way you walk, so when setting the ellipticity of the intermediate layer as r and setting the thickness, diameter, and degree of vacuum of the quartz tube, In order to make the center layer circular, the thickness of the glass thin film formed by the CVD method may be set appropriately.

Although the above description has been made regarding the case of the cross-sectional structure shown in FIG. 8, the same relationship holds true when manufacturing an optical fiber base material having the cross-sectional structure shown in FIG. 1.

Next, 11 specific examples of the manufacturing method according to the present invention will be illustrated.

Example 1 81 on the inner wall surface of a quartz tube (outer diameter 18 φ, middle diameter 15 w φ)
0 Goose-B, 0. -Gem, was deposited to a thickness of 50 .mu.m (this amount of deposition corresponded to an outer diameter (2d') of 7 m.phi. after heating bath deposition and an average diameter (2C') of the deposited glass layer of 3.1 .phi.

(Note that the outer diameter of the preform is medium to small because fine quartz powder is scattered from the quartz outer wall surface due to heating during welding.)
do. In order to set the ellipticity r:tsos of the deposited glass layer at this ζ, using equation (1), the degree of vacuum is set to 8■H,0,X, volume 5. Got an O & 1 Nine. Therefore, by writing b//a'g5,0x-7-cap11, the ellipticity rw501 is obtained. For this reason 21
According to the welding results, a preform with a middle layer ellipticity of 51 inches was obtained by setting the inner diameter of the pipe before welding to &1 mφ and the outer diameter of the pipe before welding to I L2wmφ.

Example 2 Inner wall surface K of quartz tube (inner fi 18wφ, inner diameter 15+wφ)
IRK, 1! $4 ru IB, 0. +85-v-le%s
iO.

The glass is 180 μm 100 molar S10. The glass was deposited using the CVD method and heated to a diameter of 5 m.
A preform rod was formed by welding the inside of the tube to a quartz tube with an outer diameter of 11 mm while reducing the pressure to 8 mm at a water level compared to 1 atmospheric pressure. The outer diameter of the obtained preformed rod is 9.9 mm, the core is circular with 10.3 mm, the cladding is oval with an ellipticity of 40 g6, and the short axis diameter is 1.5 mm.
Met.

Example 3 15% of the inner wall of the same quartz tube as described in Example 2 was applied in order.
B, O, 1,000 85 mole g B io, 18 cass.
0 μm% lOO Morch 8 i 0. glass &2μm1
44 Le $Ge o, + 9 @%As% 810. f
A quartz tube having an inner diameter of 5 .phi. and an outer diameter of 11 w.phi. is formed by depositing the material by a cVD method of 0.3 .mu.m and then heating it. Next, the inside of the tube was reduced in pressure by 8 sa*H10 compared to atmospheric pressure, and then heated and welded to obtain a solid preform rod.

The obtained preformed rod had an outer diameter of 9.9 mφ and a center part of 8
i0. The layers Sip, 10GeO, and Sip were concentric circles, each having a radius of 0.321111% α095, and the outer circumference of the 5iO10B, 01 layer had an ellipticity of 279 g.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the steps of the manufacturing method of the present invention, FIG. 2 is a diagram showing the relationship with a nine-light ellipsoid obtained by the manufacturing method of the present invention, and FIG. 4 is a diagram showing the optical fiber base material. Diagrams showing the relationship between layer thickness and ellipticity of the inner elliptic layer, Figures 5 and 6r
IA is a cross-sectional view of the glass tube and a cross-sectional view of the preform for explanation of FIG. 4, FIG. 7 is a diagram showing the relationship between the degree of vacuum in the glass tube and the ellipticity of the inner elliptic layer, and FIGS. 8 and 9. is a cross-sectional view of the preform obtained by the present invention, Figure 1llo is a diagram showing the relationship between the core diameter and core ellipticity of the preform produced by the manufacturing method by Kinoto v4, and Figure 11 is the clad minor axis of the preform according to the present invention. FIG. 3 is a diagram showing the relationship between the ratio of the diameter to the core diameter and the cladding ellipticity. 1-... Quartz glass tube (outermost layer), 2... Clad (
middle layer), 3-... core (center layer), 4... heating source,
5...Exhaust tank, 6...Exhaust port, 7...Exhaust control valve, 8...Quartz tube inside, 9...0-shaped tube, 1G...
·water. Kaya I Figure (1) (Z) (4) ￥J z Ou (0-2<b) (f) 3rd day I 3A /L (Next,・Line, Clear,) PC outpost (H2O) 'fJ 'g Figure'yfJqFigure 1 10 days Fu8shi7t-1-Riko Ryogyui L(,/wL, N￥117
1 Figure 7 Futsure round circle base ('/,) Procedural amendment (method) Patent application No. 112137 of 1981 Name of the invention Manufacturing method of optical fiber base material Person who makes the amendment); Pl, f510) Co., Ltd. Jt I'm
Address - 1) Shigeyo Katsu, Osamu Contents of amendment to the "Brief explanation of drawings" column of the specification subject to personal amendment

Claims (1)

[Claims] 1. A method for manufacturing an optical fiber base material in which a glass thin film layer is formed on the inner wall of a quartz glass tube serving as a base material, and then the quartz glass tube is solidified by heat welding, A method for manufacturing an optical fiber base material, characterized in that the pressure inside the quartz glass tube is made lower than the external pressure during the heat welding. 2 In the manufacturing method described in item 1, the thickness of the quartz tube at the start of heat welding is 0.3 to 5 m, and the outer diameter of the tube is 5 fi.
~50■ The difference between the outside pressure and the pressure inside the pipe is 1m1In, o~
A method for producing an optical fiber base material, characterized in that the temperature is 30■H,0. λ In the method described in item 1, when forming a glass thin film layer, first a first glass thin film layer lower than the softening point of the quartz glass is formed on the inner wall of the quartz glass tube, and then the first glass thin film layer is formed on the inner wall of the quartz glass tube. A method for producing an optical fiber base material, comprising forming a second glass thin film layer having a softening point higher than that of the first glass thin film layer. -' A method for manufacturing an optical fiber base material, characterized in that the first glass thin film is formed of silica glass containing B and O in the method described in Table 3. & In the method described in item 3 or 4, the thickness of the first and second glass thin film layers is such that the thickness of the optical fiber I (the outer layer of the layer formed of the first glass thin film layer of the base material is A method for manufacturing an optical fiber preform, characterized in that the optical fiber preform is formed as follows, where r is the ellipticity, C1 is the short axis diameter of the ellipse, and a is the diameter of the core formed by the second thin glass layer.