JPH0243690B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an optical fiber, and more specifically, to an optical fiber base material used in the process of manufacturing an optical fiber that produces birefringence in the core that forms the essential part of optical transmission of the optical fiber. , namely, a method of manufacturing a preformed 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 (maintaining 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 method produces birefringence in the core due to the non-circularity of the preformed rod 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 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 create a solid optical fiber base material, that is, a preform rod. 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 an economical and 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 a circular center layer and an elliptical 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 middle 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 turned on from the crushed one end while rotating with the pressure inside the glass tube with the crushed glass tube lower than the external pressure. It is characterized by moving and becoming solid.

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 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 section).

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 the water (hereinafter mm).
H ₂ O), and a central layer with an ellipticity of about 50% is obtained. Figure b shows the amount of pressure reduction in the pipe at 27mm.
The thickness of the germanium layer doped in the core of the hollow preform is approximately ₁₅ μm. In c and d, the core (center layer) is made of silica glass and the cladding (intermediate layer) is made of silica glass doped with boron (B ₂ O ₃ ).The core is circular and the cladding is oval. Figure d shows the core in an elliptical shape and the cladding in a circular shape. 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 approximately
It is 10 μm. Before heat welding (calaps)
Curves 11, 12, and 13 show the ellipticity of the core of the solid preform obtained by shrinking the core to an outer diameter of 13.5 mm, 12.8 mm, and 9.7 mm and then decompressing it. Curves 11, 12, and 13 are 13.5 mm and 12.8 mm, respectively. This is an experimental example of 9.7 mm.
Curve 12 has a three-layered fiber structure as shown in FIGS. 2c and d, using silica for the core and boron-doped silica glass for the cladding. The 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 as above.
belongs to. After forming a cladding layer and a core layer on this tube, it was shrunk to about 13.1 mm. After that, the collapse was performed by changing the amount of decompression, and the ellipticity of the obtained cradle was shown as a function of the amount of decompression.
2 curve.

From the above example, 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. That is, the higher the degree of pressure reduction, 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
The method of the present invention is realized at a welding temperature of 0.3 mm to 5 mm, an outer diameter of 5 mm to 50 mm, a difference between the external pressure and the pressure inside the tube of 1 mmH ₂ O to 30 mm H ₂ O, and a welding temperature of 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 preform rod obtained by the method of the present invention, for each layer before solidification and for each layer 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 Figure 6, the outer diameter b′ and inner diameter a′ of the quartz tube before solidification, the outer diameter d′ of the preform, and the 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 silica glass containing as. In the figure, ●, 〇, △ indicate the initial quartz tube diameter of 14, 18 and
Each represents a value of 20mm.

From Figure 4, we can see that the ellipticity γ is γ=100e ^-A(X-1)2 [%] x=(b'/a')×(d'/c')...(1) Ru.

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 pressure reduction, and FIG. 7 shows the relationship between the ellipticity and the degree of pressure reduction P obtained experimentally.
From the same figure, A=0.344/P...(2) can be found.

Therefore, in order to make a preform rod having an elliptical layer with a predetermined ellipticity γ, use a quartz tube as the base material, set the welding temperature to 1700 to 2000°C, and set the moving speed of the heating source to 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') x (d'/c') ...(4 ). Note that even if the deposited glass layer is not one type but multiple layers, the above formula 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.
Also, 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 shape in which the outermost layer is close to a circle, the outermost middle layer is elliptical, and the center layer is close to a circle. 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 clad 2-2, the middle layer 2-1 is a jacket, the outermost layer 3 forms a support part, and the outer periphery is circular. 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 middle layer higher than that of the center layer, the middle layer should be made of a material with a lower softening point than the core material, which is done by adding B ₂ O ₃ to the middle 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 B ₂ O ₃ 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 and the circular core at the center and 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, in order for it to approach a circle, it must be softened in the process of solidifying and solidifying during welding 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 , respectively. ₂ and α ₃ , it is sufficient if 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 thicknesses x [μm] and welded and solidified at a reduced pressure of 10 mm H 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 short axis diameter c ₂ of the cladding and the core diameter a. 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 diameter and depressurization degree, 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 / a 200 / 100 - γ - 1 in the preformed state. good.

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 was deposited to a thickness of 50 μm (this amount of deposition corresponded to an outer diameter (2d′) of 7 mmφ after heat welding and an average diameter (2c′) of the deposited glass layer of 3.1 mmφ.
Note that the outer diameter of the preform is slightly smaller because fine quartz powder is scattered from the outer quartz wall surface due to heating during welding. Here, in order to set the ellipticity γ of the deposited glass layer to 50%, using equation (1), the degree of vacuum is set to 8.
_mmH2O , x=5.0 was obtained. Therefore, b′/a′=
If 5.0×3.1/7=2.21, ellipticity γ=50% can be obtained. Therefore, 2a′ (pipe inner diameter before welding) is 5.1mmφ,
2b' (tube outside diameter 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 sequentially applied to the inner wall surface of a quartz tube (outer diameter 18 mmφ, inner diameter 15 mmφ).
A 180 μm, 100% SiO ₂ glass is deposited by 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 height of the 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 an ellipse with an ellipticity of 40%, and the minor axis diameter was 1.5 mmφ.
It was hot.

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 8mmH 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%.

By performing well-known heating drawing using the preform rods obtained in each of the above Examples, an optical fiber having a cross-sectional structure similar to the above-mentioned cross-sectional structure is obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the steps of the manufacturing method of the present invention, FIG. 2 is a cross-sectional diagram of the optical fiber base material obtained by the manufacturing method of the present invention, and FIG. 3 is a diagram showing the inside of a glass tube in the process of the present invention. Figure 4 is a diagram showing the relationship between the degree of pressure reduction and the ellipticity of the elliptic layer of the optical fiber base material, Figure 4 is a diagram showing the relationship between the thickness of the layers constituting the optical fiber base material and the ellipticity of the elliptical layer, and Figure 5 and Figure 6 is the fourth
A cross-sectional view of the glass tube and a cross-sectional view of the preform for explanation of the figures, FIG. 7 is a diagram showing the relationship between the degree of vacuum in the glass tube and the ellipticity of the ellipsoidal layer, and FIGS. 8 and 9 are according to the present invention. 10 is a diagram showing the relationship between the core diameter and core ellipticity of the preform obtained by the manufacturing method according to the present invention, and FIG. 11 is a diagram showing the relationship between the clad minor axis diameter and the core of the preform according to the present invention. FIG. 3 is a diagram showing the relationship between diameter ratio and cladding ellipticity. 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)

[Scope of Claims] 1. The pressure inside the base material tube having the first thin film layer formed on the inner wall and the second thin film layer further formed on the inner wall thereof is lowered to lower than the external pressure, and the pressure is reduced by heat welding. the first thin film layer is made of a material having a lower softening point temperature than the materials constituting the second thin film layer and the base material tube;
And when the ellipticity of the outer circumference of the first thin film layer is r, the major axis diameter and minor axis diameter are c ₁ and c ₂ respectively, and the diameter of the second thin film layer is a, c ₂ /a≧200 /(100-r)-1 However, the above-mentioned base material is formed and drawn so as to satisfy the relationship r≡( _c1 - _c2 )/( _c1 + _c2 )×100. Method of manufacturing fiber. 2. In the method for manufacturing an optical fiber according to claim 1, the formation of the first thin film layer comprises:
_A method for producing optical fibers by depositing silica glass containing _B2O3 . 3. The method for manufacturing an optical fiber according to claim 1, wherein a quartz glass tube is selected as the base tube. 4. In the method for manufacturing an optical fiber according to claim 1, the formation of the second thin film layer comprises:
A method of manufacturing optical fibers by depositing silica glass containing _GeO2 . 5. The method of manufacturing an optical fiber according to claim 1, wherein the second thin film layer is formed by depositing a plurality of thin film layers having different compositions.