JPH0577618B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an optical fiber base material, and more specifically, to an optical fiber used in the process of manufacturing an optical fiber that produces birefringence in its core, which is an essential part of optical fiber transmission. 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, to connect them to optical integrated circuits, to use them in measurement devices, optical switches, and the like. In order for a circular optical fiber to be able to 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 uses chemical vapor deposition (CVD) to deposit the core on the inner wall of the glass tube.
After forming a glass layer to serve as a cladding, the inside of the glass tube was solidified to form a rod without reducing the pressure at least, and a portion of this was ground by polishing to form a non-circular optical fiber preform (preformed rod). After that, it is heated and drawn to make a thin optical fiber.
This makes the pre-home lot non-circular and
This method produces birefringence in the core due to the difference in thermal expansion coefficient between the materials of the glass tube and the glass layer formed on the inner wall.

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

Therefore, an object of the present invention is to form at least a glass thin film different from 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, that is, a preformed rod. An object of the present invention is to realize an economical and simple manufacturing method in which at least a portion of the glass thin film portion has an elliptical shape.

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 step or a step 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, which is composed of an outermost peripheral layer formed in the intermediate layer and in which the ellipticity of the outer periphery of the intermediate layer can be arbitrarily set.

In order to achieve the above object, the present invention provides a method for manufacturing a solid optical fiber preform by forming a glass thin film on the inner wall of a glass tube serving as a base material and heat welding the thin film. One end of the tube on which the glass thin film has been formed is heated and crushed, and the heating section is gradually moved from the crushed one end while rotating with the pressure inside the glass tube with the crushed one end lowered than the outside pressure. It is characterized by solidification.

In particular, in the present invention, the cross-sectional structure of the preformed rod obtained by the present invention is such that the central layer is circular, the intermediate layer has an elliptical outer periphery, and the outermost layer is a layer having a circular or nearly circular shape. In the above method, a part of the material of the intermediate layer is formed of a material having a softening point lower than that of the glass tube serving as the base material, and a material forming the core part is formed of a material having a lower softening point than the softening point of the glass tube serving as the base material. It is made of a material that has a higher softening point than that of the other materials.

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 the ellipticity γ, where the major axis of the ellipse is C ₁ and the minor axis is C ₂ .
(C ₁ −C ₂ /C ₁ +C ₂ ×100) is 3% or more. Anything below that is circular.

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

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

Incidentally, the final optical fiber can be easily produced by simply drawing the preformed 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. 1 is a diagram showing the steps of a method for manufacturing an optical fiber preform according to the present invention.

First, (1) a quartz glass tube 1, which will be the base material of the optical fiber, is prepared. If this glass tube has a large diameter and a small thickness, it will be difficult to obtain a predetermined shape in the subsequent pressure reduction step (3), so a step of reducing the diameter may be included if necessary. Desirably, the outer diameter is 5 mm to 50 mm and the thickness is 0.3 mm to 5 mm.

Chemical deposition (CVD) on the inner wall of the glass tube 1 above
By this method, a thin glass film 2 serving as the intermediate layer of the optical fiber and a thin glass film 3 serving as the central layer are formed. The intermediate layer may have a fiber optic cladding or jacket and cladding function. Further, the central layer may form only the core of the optical fiber, or the core and the crack (that is, form the optical transmission part).

These materials and thicknesses will be explained in detail later. The glass tube with the glass thin film obtained by the above process is attached at both ends to a glass lathe table and rotated at a constant rotational speed (3). In the figure, the end 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 control valve 7 through the exhaust pipe 6, the pressure inside the tube is reduced and maintained at a constant pressure. The amount of pressure reduction is measured by the difference in the liquid level of the liquid 10 using a U-shaped tube 9 whose one end is inserted into the interior 8 of the quartz tube. In this state, the heating source (burner) 4 is gradually moved to form a solid preformed rod. The degree of this pressure reduction is set to about 1 mmH ₂ O to 20 mmH ₂ O, as shown in FIG. The preformed rod is melted by a heating burner 4 and drawn from one side to produce an optical fiber in which part of the inner layer is elliptical.

Figures 2a, b, 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, gradually turn on the oxyhydrogen burner 2 while rotating at a speed of 50 revolutions per minute.
It moved at a speed of 0.17mm/sec.

A is made of silica glass doped with germa as a core, and the amount of pressure reduction is 9 mm at the height of water (hereinafter mmH ₂
O), and a central layer with an ellipticity of about 50% is obtained. In Figure b, the amount of pressure reduction in the tube is increased to 27 mmH ₂ O, and the thickness of the gela layer doped in the core of the hollow preform is approximately 15 μm. For c and d, silica glass is used as the core (center layer) and boron (B ₂ O ₃ ) is used as the cladding (middle layer).
It is made of silica glass doped with , and the core is circular and the clasp is elliptical.
And, conversely, Figure d shows the core as an ellipse and the cladding as a circle. In these manufacturing methods, as described above, the starting quartz tube is shrunk to a certain extent so that the ellipse axis does not rotate, and this can be accomplished by optimally selecting the amount of shrinkage and the thicknesses of the core layer and cladding layer.

FIG. 3 shows the measurement results of the ellipticity when the thickness of the quartz tube and the degree of vacuum were changed, and the operating conditions for each curve are as follows. Curves 11, 12, 13
The core is made of silica glass doped with germa, and has a two-layer structure as shown in FIGS. 2a and 2b. The concentration of germa is approximately 15 mol%. For each curve, the outer diameter of the starting quartz tube is 20 mm and the inner diameter is 17 mm, and the thickness of the gel layer of the hollow preform is about 10 μm. Curve 11 shows the ellipticity of the core of the solid preform obtained by shrinking it to an outer diameter of 13.5 mm, 12.8 mm, or 9.7 mm before heat welding (calapus) and then reducing the pressure. 11,1
3 are experimental examples of 13.5 mm, 12.8 mm, and 9.7 mm, respectively. Curve 12 has a three-layer structure of fibers using silica for the core and boron-doped silica glass for the cladding as shown in FIGS. 2c and d. Boron was doped at approximately 12 mol% and had a thickness of 18 μm in the hollow preform.
The silica layer that forms the core has a thickness of approximately 8 μm, and the starting quartz tube has the same outer diameter of 20 mm and inner diameter of 17 mm.
mm. After forming a cladding layer and a core layer on this tube, it was shrunk to about 13.1 mm. Thereafter, the collapse was performed while changing the amount of pressure reduction, and curve 12 shows the obtained ellipticity of the cladding as a function of the amount of pressure reduction.

From the above example, it can be seen that the elliptical shape of the 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. In other words, the higher the degree of decompression, the greater the ellipticity;
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
mm to 5 mm, outer diameter is 5 mm to 50 mm, difference between external pressure and pressure inside the pipe is 1 mm H ₂ O to 30 mm H ₂ O, welding temperature
The method of the invention is realized at 1700°C to 2000°C. Next, the relationship between the ellipticity γ, the degree of reduced pressure P (mmH ₂ O), and the thickness of each layer 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 after solidification. This shows the relationship between the thickness and the vertical axis shows the ellipticity of the elliptical layer of the preform, and the horizontal axis shows (b'/a') x (d'/c'), which are shown in Figures 5 and 5. As shown in Fig. 6, the outer diameter b′ of the quartz tube before solidification is
Inner diameter a′, outer diameter d′ of the preform, average radius of the elliptical layer
c' (=√ ₁ , ₂ ), and the manufacturing conditions were: welding temperature 1800℃, degree of vacuum 8mmH ₂ O heating burner moving speed 0.8mm/sec, glass thin film doped with GeO ₂ and B ₂ O _3. It is made of silica glass. In the figure, ●, ○, △ indicate the initial quartz tube diameter of 14, 18 and
20mm is shown respectively.

From Figure 4, the ellipticity γ has the following relationship: γ=100e ^-A(x-1)2 [%] }...(1) x=(b'/a') x (d'/c') I understand.

Equation (1) above can be applied even if the welding temperature (1700 to 2000°C), the moving speed of the heating source 4 (0.02 to 0.2 mm/sec), and the composition of the deposited glass are changed within the range that is realistically implemented in the production of optical fibers. To establish. In addition, in the above formula (1), A is a constant determined by the degree of reduced pressure, and Figure 7 shows the relationship between the above ellipticity and the degree of reduced pressure P, which was experimentally determined. From the figure, A = 0.344. /P...(2) is found.

Therefore, in order to make a preformed rod having an elliptical layer with a predetermined ellipticity γ, a quartz tube should be used as the base material, the welding temperature should be 1700 to 2000°C, and the moving speed of the heating source should be 0.02 to 0.2 mmH ₂ O. For example, the degree of pressure reduction P, the diameter and thickness of each layer are γ=100e ^- 0.344/P(x-1) ² ...(3) x=(b'/a')×(d'/c')... It should be set 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 great advantages of the method of the invention is that FIG.
And, as shown in FIGS. 8 and 9, it is possible to easily realize an optical fiber base material in which the layer structure of the preform cross section has a nearly circular outermost layer, an elliptical outer circumference of the middle layer, and a nearly circular shape of the center layer. It is.

In Figure 8, the central layer 3 is the core, the intermediate layer 2 is the cladding, and 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 cause birefringence in the core. . In the one shown in Fig. 9, the center layer is composed of a core 3 and a cladding 2-2, the middle layer 2-1 is a jacket, the outermost layer 3 forms a support part, and the outer periphery is a circular core 3 and a cladding 2-2. to form the optical transmission part, and the jacket 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 hollow layer higher than that of the central layer, the intermediate layer should be made of a material with a lower softening point than the core material.This is done by adding B ₂ O ₃ to the intermediate layer material. This is achieved by
As the amount of B ₂ O ₃ increases, the softening point decreases, but
Dopants are used to increase the difference in thermal expansion coefficients.
The amount _{of B2O3} is preferably 3 mol% to 30 mol%. Furthermore, in order to make the central layer circular, the ellipticity of the intermediate layer of the preform base material is γ, the length of the minor axis of the intermediate layer ellipse is c ₂ , and the circular diameter of the central layer is a. The thickness of the glass thin film formed on the inner wall of the glass tube may be set in advance so that C ₂ /a≧200/100−γ−1 (5).

These requirements are due to the following reasons. When a glass tube on which a thin glass film has been formed is heated and welded while reducing pressure, the temperature gradient is initially higher on the outside, and because the tube is thicker, the outer side is less affected by the reduced pressure and maintains its circular shape mainly due to surface tension. shall be. The inside is mainly controlled by the degree of vacuum and tends to become flat. If the heating continues, the temperature inside will also increase, making it easier to deform. Therefore, by reducing the pressure, the tube becomes flat and shrinks, reducing the hollow space. During this time, the viscosity of the intermediate layer having a low softening temperature is gradually reduced. Therefore, when the hollow part disappears, the central layer floats in the intermediate layer with reduced viscosity. At this time, since the central portion is not depressurized, a force acts that tends to shape the central layer into a circular shape mainly due to surface tension. This is because, during the cooling process, the intermediate layer is filled between the ellipse formed inside the initial quartz tube, the center, and the circular core, and is solidified.

Therefore, the factors that determine these shapes include the softening point of the intermediate layer during heat welding, the viscosity, the relationship between the relative thicknesses of the intermediate layer and the central layer, and the maximum The relationship between the ellipticity of the inner circumference of the outer layer (and therefore the outer circumference of the middle layer) can be considered.

First, the conditions for making the outer circumference of the intermediate layer into an ellipse are determined by the conditions of the above-mentioned equation (1).

Next, in order for the ellipticity γ of the center layer to become smaller than the ellipticity of the outer periphery of the middle layer, that is, to approach a circle, the process of solidifying and solidifying during welding must be softened for the reasons mentioned above. Among the intermediate layers, it is necessary for the central layer to easily change freely into a stable circular shape due to surface tension, etc., and for this purpose, the softening point temperatures of the central layer, intermediate layer, and outermost layer must be set to α 1 and α ₁ , respectively.
If α ₂ and α ₃ are α ₁ >α ₂ , α ₃ >α ₂ ……(6). Usually, to satisfy this condition, the outermost layer is made of quartz glass, and the center part needs to have a high refractive index, so it is made of SiO ₂ or SiO ₂ with GeO ₂ or P ₂ O ₅ as a dopant. configure,
As the intermediate layer, 3 mol% B ₂ O ₃ is used as a dopant.
SiO ₂ containing 30 mol% 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. It is experimentally determined that

Figure 10 shows a glass thin film of 17 mol% B ₂ O ₃ and 83 mol% SiO ₂ which will become the cladding (intermediate layer) formed by CVD on the inner wall of a quartz tube (inner diameter 6.7 mm, outer diameter 12 mm).
After forming 150 μm, 100 μm becomes the core (center layer).
The ellipticity of the intermediate layer and the ellipticity of the center layer (core) are shown when glass thin films of mol % SiO ₂ are formed with varying thickness x [μm] and welded into a solid at a reduced pressure of 100 mmH ₂ O. In this case, the ellipticity of the middle layer (clad) is 45
%. In addition, the core diameter in the figure shows the radius when it becomes a preformed lot. 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 diameter of the central layer (core) when the ellipticity of the central layer becomes 5% or less when the ellipticity of the intermediate layer changes as shown in Figure 10. This is an experimentally determined ratio. In the figure, the horizontal axis shows the ellipticity γ of the outer periphery of the cladding, and the vertical axis shows the ratio C ₂ /a between the minor axis diameter c ₂ of the cladding and the diameter a of the core. From this measurement result, it can be seen that there is a relationship of c ₂ /a=200/(100−γ)−1 at the boundary where the core is circular. Therefore, when c ₂ /a is larger than 200/(100-γ)-1, the core tends to become circular due to surface tension, so the ellipticity of the intermediate layer is set as γ, and the thickness of the quartz tube is When setting the thickness, diameter, and degree of vacuum, in order to make the center layer circular, the thickness of the glass thin film layer by CVD method should be set so that c ₂ /a200/(100-γ)-1 in the preformed state. All you have to do is set .

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. 9.

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

Example 1 On the inner wall of a quartz tube (outer diameter 18 mmφ, middle diameter 15 mmφ)
A glass thin film of SiO ₂ −B ₂ O ₃ −GeO ₂ is deposited to a thickness of 50 μm (this amount of deposition corresponds to an outer diameter of 2d′7 mmφ after heat welding and an average diameter of the deposited glass layer of 2c′3.1 mmφ. (The outer diameter of the preform is slightly smaller because fine quartz powder is scattered from the outer quartz wall surface.) Here, the ellipticity γ of the deposited glass layer is 50
%, use equation (1) to set the degree of pressure reduction to 8 mmH ₂
O, x=5.0 was obtained. Therefore, if b'/a'=50×3.1/7=2.21, ellipticity γ=50% can be obtained. For this reason, 2a' (inner diameter of the tube before welding) was set to 5.1 mmφ, and 2b' (outer diameter of the tube before welding) was set to 11.2 mmφ, and according to the welding results, a preform with an intermediate layer ellipticity of 51% was obtained.

Example 2 15 mol% B ₂ O ₃ + 85 mol% SiO ₂ glass was applied to the inner wall surface of a quartz tube (inner diameter 18 mmφ, inner diameter 15 mmφ) in this order.
A 180 μm, 100 mol% SiO ₂ glass is deposited by a 3.5 μm CVD method and heated to form a quartz tube with an inner diameter of 5 mm and an outer diameter of 11 mm. Next, the inside of the tube was welded while reducing the pressure to 8 mmH ₂ O at the level of water compared to atmospheric pressure to form a preformed rod. The outer diameter of the obtained preformed rod was 9.9 mmφ, the core was circular with a diameter of 10.3 mm, and the cladding was elliptical with an ellipticity of 40%, and the minor axis diameter was 1.5 mm.
It was φ.

Example 3 On the inner wall surface of the same quartz tube as described in Example 2, 15 mol% B ₂ O ₃ + 85 mol% SiO ₂ glass was applied to 180 μm, 100 mol% SiO ₂ glass was applied to 3.2 μm, and 4 mol% GeO ₂ + 96 mol% SiO ₂ glass is deposited to a thickness of 0.3 μm by CVD and then heated to form a quartz tube with an inner diameter of 5 mmφ and an outer diameter of 11 mmφ. Next, the pressure inside the tube is 8 mmH ₂ O compared to atmospheric pressure.
A solid preform rod was obtained by heating and welding under reduced pressure. The resulting preformed rod has an outer diameter of
9.9mmφ, the central SiO ₂ layer and SiO ₂ +GeO ₂ layer are concentric circles with radii of 0.32mm and 0.095mm, respectively.
The outer circumference of the _three SiO ₂ +B ₂ O layers had an ellipticity of 27%.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the steps of the manufacturing method of the present invention, Figure 2 is a cross-sectional diagram of the optical fiber base material obtained by the manufacturing method of the present invention, and the degree of vacuum inside the glass tube in the process of the present invention. A diagram showing the relationship between the ellipticity of the elliptic layer of the optical fiber base material, Figure 3 is a diagram showing the relationship between the ellipticity, the thickness of the quartz tube, and the degree of vacuum, and Figure 4 is the layer constituting the optical fiber base material. Figures 5 and 6 are a cross-sectional view of a glass tube and a preform for explaining Figure 4, and Figure 7 is a diagram showing the relationship between the thickness of the glass tube and the ellipticity of the elliptical layer. Figures 8 and 9 are cross-sectional views of preforms obtained by the present invention;
FIG. 10 is a diagram showing the relationship between the core diameter and core ellipticity of the preform produced by the manufacturing method according to the present invention, and FIG.
The figure shows the relationship between the ratio of the short axis diameter of the clad to the core diameter and the ellipticity of the clad of the preform according to the present invention. 1... Quartz glass tube (outermost layer), 2... Clad (middle layer), 3... Core (center layer), 4... Heat source, 5... Exhaust tank, 6... Exhaust port, 7... Exhaust control valve, 8... Inside of quartz tube, 9... U-shaped tube, 1
0...Water.

Claims (1)

[Claims] 1. A method for manufacturing a solid optical fiber base material by forming a glass thin film layer on the inner wall of a quartz glass tube serving as a base material, and then heat welding the quartz glass tube, wherein the heat welding The thickness of the quartz glass tube at the start is 0.3 mm to 5 mm, the outer diameter of the quartz glass tube is 5 mm to 50 mm, and the pressure inside the quartz glass tube is 1 mm below the external pressure during the heat welding.
A method for producing an optical fiber base material, characterized in that _H2O is lowered by 30 mm _H2O .