JP3587032B2 - Manufacturing method of optical fiber preform - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing an optical fiber having a high relative refractive index difference and a complicated refractive index distribution by an external method (OVD method), and particularly to a high-performance optical fiber used for a long-distance large-capacity communication system. It will not be provided.
[0002]
[Prior art]
As one of the conventional methods of manufacturing an optical fiber preform, a transparent glass rod is used as a starting member, and this starting member is installed horizontally or vertically in a container, and this is rotated around the axis of the starting member, A so-called external particle deposition method in which a gaseous glass material is supplied to a burner for synthesizing glass fine particles to synthesize glass fine particles, and is deposited on a starting member that reciprocates relatively to the burner for synthesizing glass fine particles. There is a method of obtaining an (optical fiber preform), heating and melting it to form a vitreous glass (Japanese Patent Application Laid-Open No. 2-172838).
FIG. 1 shows an example of an apparatus for manufacturing an optical fiber preform by this method. In the apparatus shown in FIG. 1, a starting member 2 is vertically held by a rod 7 held in a container 1, and the starting member 2 rotates around an axis, and reciprocates vertically by an elevating device 5. I have. A glass synthesis burner 3 is mounted on the side surface of the container 1 at right angles to the rotation axis of the starting member 2, and synthesizes glass fine particles from the raw material supplied from the raw material supply device 8 and deposits the glass particles on the starting member 2. The glass composite burner 3 can be moved back and forth by a burner moving device 4. In addition, an exhaust port 6 is provided on the side surface of the container 1 opposite to the glass synthesis burner 3, and exhaust gas containing surplus glass fine particles during synthesis is exhausted.
[0003]
When an optical fiber preform is manufactured by such an apparatus, as shown in FIG. 2, a constant outer diameter portion 10 having a constant outer diameter is provided at an intermediate portion of a glass particle deposit body 9 in which glass particles are deposited on a starting member 2. Although formed, outer diameter unsteady portions 11 having different outer diameters are formed at the upper and lower ends. At this end, the burner is folded back, and the outer diameter is small and the heat capacity is small. If the heating power of the burner is constant, the temperature of the glass fine particle deposit locally increases. . For example, when the moving speed of the target is constant, even if the temperature of the constant outer diameter portion is adjusted to about 900 ° C., the temperature may rise to about 1100 ° C. at the end portion, and the glass fine particle deposition may occur. There has been a problem that the bulk density of the body is distributed, the hardness is irregular, and soot cracks occur due to the difference in the heat shrinkage between the transparent glass rod and the glass fine particle deposition layer.
[0004]
In addition, the method of the present invention solves the drawbacks of the above method and suppresses a local rise in temperature at the end of the glass particle deposit when depositing glass particles on a starting member made of a transparent glass rod. Provided is a method of manufacturing an optical fiber preform from which a material can be obtained, and a method of manufacturing an optical fiber preform with a good yield when drawing an optical fiber preform while suppressing an increase in the outer diameter unsteady portion at the end. Therefore, by controlling the heating power of the glass composite burner at the outer diameter unsteady portion at the end of the glass fine particle deposit, control is performed so as to suppress the local rise in temperature at the outer diameter unsteady portion at the end. Although it has been proposed to prevent the occurrence of cracks in the base material (Japanese Patent Application No. 9-131997), the case where a dopant is added has not been recognized.
[0005]
[Problems to be solved by the invention]
The present invention has been developed to solve the problems in the prior art as described above. In general, an MCVD method or an OVD method is suitable for a method for manufacturing an optical fiber having a high relative refractive index difference and a complicated refractive index distribution. In particular, since the OVD method can increase the size of a base material, Although it is advantageous for increasing the productivity, when an optical fiber having a high relative refractive index difference is produced by the OVD method, there is a problem that cracks easily occur in the glass porous preform to be synthesized. That is, in order to produce an optical fiber having a high relative refractive index difference, it is necessary to greatly change the amount of dopant added in the radial direction. When synthesizing a glass porous base material, when the amount of dopant changes, the shrinkage amount between the glass fine particles changes, so that strain is generated in the glass porous base material during synthesis and cracks are easily generated. Particularly when depositing glass particles containing a high concentration of dopant on a glass particle deposition layer containing no dopant, the difference in thermal shrinkage between the two becomes large, so that the frequency of soot cracking becomes even higher. was there. Further, in the production of the porous glass preform by the OVD method, there is a problem that cracks are formed at both ends and the cracks are generated from the cracks in the entire length.
[0006]
The present invention provides, for example, a method of manufacturing a base material for an optical fiber having a high relative refractive index difference and a complicated refractive index distribution by an OVD method, when supplying a dopant-containing raw material gas at an end portion. An object is to suppress the occurrence of cracks by lowering the dopant concentration.
[0007]
[Means for Solving the Problems]
The above objects can be effectively achieved by the following inventions:
(1) By moving the burner for glass synthesis relatively to the axial direction of the starting rod and reciprocating within a certain range in the axial direction, a glass porous body is synthesized on the starting rod, and the glass porous material for the optical fiber is formed. A method for manufacturing an optical fiber preform, wherein the flow rate of a dopant source gas is reduced to 0 to 70%, preferably 0 to 40% of a steady portion at a reciprocating end.
(2) Start reducing the flow rate of the source gas of the dopant or return the flow rate to the steady-state flow rate by setting the distance from the end to 30 to 250%, preferably 50 to 150% of the outer diameter of the porous base material being synthesized. The method for producing an optical fiber preform according to the above (1), wherein
[0008]
(3) The production of the optical fiber preform according to the above (1) or (2), wherein the change in the flow rate of the dopant source gas is changed stepwise or as a function of the distance from the terminal. Method.
(4) The optical fiber preform according to any one of (1) to (3), wherein the H ₂ / O ₂ flow rate of the burner flame is adjusted simultaneously with the change in the flow rate of the dopant source gas. Method.
[0009]
According to the invention of the above (1), by reducing the amount of the source gas of the dopant at the end portion and lowering the dopant concentration, the proportion of the high dopant concentration portion in the total surface area of the soot is reduced, and the distortion of the entire soot surface is reduced. The occurrence of cracks can be suppressed by reducing the number of cracks. In addition, especially at the end, the outer diameter changes in the longitudinal direction as compared with the steady part, so that the temperature distribution on the deposition surface is likely to change in the longitudinal direction, and thus distortion is likely to occur. Furthermore, since the yield of the dopant changes according to the temperature distribution, a complicated strain distribution is formed at the end. Therefore, reducing the dopant concentration at the end has the greatest effect on suppressing cracks. Here, if the flow rate of the source gas of the dopant is larger than 70% of the steady portion, there is a problem that cracks are easily generated.
[0010]
In the invention of the above (2), since the length of the outer diameter taper at the end changes depending on the outer diameter of the porous base material, the distance from the end where the change of the dopant raw material gas flow starts and ends at the base material is determined. Specified based on the outer diameter of If it is less than 30%, the effect of suppressing cracks is insufficient, and if it exceeds 250%, there arises a problem that the yield of the base material is reduced.
[0011]
In the above invention (3), the rate of change in the flow rate of the dopant source gas is specified. The effect of suppressing cracks is greater when the flow rate of the dopant source gas is as small as possible at the end, but when the flow rate changes rapidly, distortion tends to occur because the temperature of the deposition surface changes greatly. Become. It is effective to select the flow rate change speed according to the magnitude of the flow rate change. Normally, the flow rate is changed linearly from the steady portion to the end or from the end to the steady portion.
[0012]
The invention (4) specifies that the change in soot bulk density due to the change in the amount of dopant at the end is suppressed by adjusting the H ₂ / O ₂ flame. In other words, it is effective for suppressing cracks to reduce a change in the temperature of the deposition surface caused by a change in the flow rate of the dopant source gas by adjusting the intensity of the H ₂ / O ₂ flame.
[0013]
The present invention relates to a method for producing an optical fiber preform in which a transparent glass rod is used as a starting member and glass fine particles are deposited on the surface of the starting member. The fine particle deposit is referred to as an optical fiber preform or an optical fiber porous preform.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic sectional view showing an example of an apparatus for performing the method of the present invention. In the apparatus shown in FIG. 1, a starting member 2 is vertically held by a rod 7 held in a container 1, and the starting member 2 rotates around an axis, and reciprocates vertically by an elevating device 5. I have. The starting member 2 made of a transparent glass rod is, for example, a glass rod-shaped member is installed horizontally or vertically in a container, and is rotated around an axis as a starting member. The glass particles are supplied to the burner to synthesize glass particles, and a glass particle stack having a core and a clad layer (corresponding to a target refractive index pattern) having different refractive indices is produced. It can be manufactured by forming A glass synthesis burner 3 is mounted on the side surface of the container 1 at right angles to the rotation axis of the starting member 2, and synthesizes glass fine particles from the raw material supplied from the raw material supply device 8 and deposits the glass particles on the starting member 2.
[0015]
The glass composite burner 3 can be moved back and forth by a burner moving device 4. The glass synthesis burner 3 is usually a multi-tube burner in which a plurality of tubes are combined, and silicon tetrachloride, oxygen, hydrogen, and a dopant carrier gas are supplied from each of the tubes to synthesize glass fine particles, and the starting member is used. It is configured to be deposited on the surface. Then, as the outer diameter of the glass fine particle deposit increases, the burner is retracted to adjust the distance between the burner and the deposition surface, and furthermore, the gas flow rate, particularly the heating efficiency, is maintained so that the surface temperature is maintained within an appropriate range. The hydrogen flow, which greatly contributes, is gradually increased to increase the thermal power of the burner. In addition, an exhaust port 6 is provided on the side surface of the container 1 opposite to the glass synthesis burner 3, and exhaust gas containing surplus glass fine particles during synthesis is exhausted.
[0016]
When the optical fiber preform is manufactured by the apparatus of the type shown in FIG. 1, the temperature of the deposition surface at the constant outer diameter portion of the glass fine particle deposit is usually 500 to 700 ° C., preferably 550 to 650 ° C. Glass fine particles are deposited. When this temperature is lower than 500 ° C., the bulk density of the glass fine particle deposit becomes small, and the glass fine particle deposit is easily broken. Further, when the temperature is higher than 700 ° C., the bulk density of the glass fine particle deposit becomes too large, and OH groups and impurities cannot be sufficiently removed in dehydration and sintering, thereby deteriorating the transmission loss of the fiber.
[0017]
The optical fiber having a high relative refractive index difference and a complicated refractive index difference of the present invention is manufactured by the OVD method as described above. For example, using the device shown in FIG. 1, having a relative refractive index difference of 0.6 to 3.0%, a ring type Δn profile as shown in FIG. The optical fiber preform having a bulk density of 0.2 to 0.8 g / cm ^{3 and} a thickness of 200 mm or more is synthesized using Ge and F as dopants.
While the quartz starting member is traversed up and down, a raw material gas such as SiCl ₄ and GeCl _{4 is} caused to flow along with the H ₂ / O ₂ flame from the glass synthesis burner to deposit fine glass particles on the starting member. At the ends where the traverse is turned in the opposite direction (both ends), the flow rate of the dopant source gas is reduced to 0 to 70%, preferably 0 to 40% of the steady state. When the content is 0% at the end, the maximum effect is obtained in suppressing cracks. However, during sintering, the dopant is diffused to the end, so that the dopant concentration is reduced at the end of the stationary portion and the yield is reduced. Cracks can be prevented in a range of more than 0% to 70%, particularly in a range of 0 to 40%. On the other hand, when it is more than 70%, cracks are easily generated.
[0018]
That is, for example, when manufacturing an optical fiber preform having a ring type Δn profile as shown in FIG. 3A, a raw material gas not containing GeCl ₄ as a dopant is first supplied from a burner to produce a pure gas. An SiO ₂ layer is formed, and then a ring portion is synthesized by supplying a source gas containing GeCl ₄ using Ar as a carrier gas to synthesize a ring portion (SiO ₂ —GeO ₂ ), and further outside the ring portion. A pure SiO ₂ layer is synthesized using only SiCl ₄ as a source gas.
4 and 5 show examples of the flow rate pattern of the supply gas amount. FIG. 4 shows a case where the flow rate of the raw material gas for the dopant is gradually increased in accordance with the increase in the outer diameter at the constant outer diameter portion, and the gas supply amount is set to 0% at the end portion. In this case, since the supply amounts of O ₂ and H ₂ are constant, the temperature of the deposition surface of the glass fine particles becomes slightly unstable. In FIG. 5, the supply temperature of the source gas of the dopant at the end is gradually reduced to 0%, and at the same time, the supply amounts of O ₂ and H ₂ are also increased to adjust the surface temperature of the deposition surface. In FIG. 9, the supply amount of the source gas of the dopant is gradually reduced, but is not set to 0%, but is suppressed to, for example, 20%. On the other hand, the supply amounts of O ₂ and H ₂ are also reduced to adjust the temperature. The state is shown.
[0019]
In the method of supplying the dopant source gas as described above, the distance from the end where the change in the flow rate of the dopant source gas starts, the distance from the end that returns from the end and returns to the steady-state flow rate, and the source gas of the dopant. It is possible to change the flow rate change rate, and it is also possible to adjust the bulk density of the burner by changing the flow rate of the H ₂ / O ₂ gas in accordance with the change in the flow rate of the dopant source gas. Actually, when the flow rate of the raw material gas of the dopant at the end portion was set to 0% of the steady portion and the distance from the end where the flow rate was changed was set to 50% of the outer diameter of the glass porous base material to be synthesized, the synthesis was performed. No cracks occurred in the material. Cracks also occurred at the ends where synthesis was performed under the same gas flow conditions as for the steady part.
[0020]
【Example】
Hereinafter, the present invention will be described specifically with reference to examples.
(Example 1)
A glass base material having a ring type Δn profile as shown in FIG. 3- (a) on a starting member (quartz rod) of φ17 mm × 480 mm using an octuple burner with an apparatus having the configuration shown in FIG. Was prepared. The main manufacturing conditions at this time are: SiCl ₄ flow rate 2.80 SLM, H ₂ flow rate 50 SLM, O ₂ flow rate 70 SLM, GeCl ₄ flow rate 900 SCCM in the ring part (Ar carrier flow rate, condenser temperature 66 ° C.), reciprocating traverse speed of starting member Was 300 mm / min, and the spindle rotation speed was 40 rpm. The procedure for synthesizing a porous glass body (soot body) is as follows. From the beginning until the 80th traverse, the raw material gas flowing from the burner is made only SiCl ₄ to form a layer made of pure SiO ₂ , and then the raw material to which GeCl ₄ is added Gas was passed from the burner to synthesize a ring portion (SiO ₂ —GeO ₂ ) up to 150 traverses. Further, a layer made of pure SiO ₂ was synthesized up to 230 traverses again using only SiCl _{4 as a} source gas outside. In the conventional method shown in FIG. 10, when synthesizing the ring portion, the GeCl ₄ flow rate during one traverse is always supplied constantly to perform the synthesis. Under such manufacturing conditions, cracks occurred during the synthesis of the ring portion, and a good base material could not be obtained. On the other hand, as shown in FIG. 6, when synthesizing the ring portion, the supply of GeCl _{4 to} the burner was stopped at both ends 50 mm in one traverse, and the glass particles containing no GeO ₂ (50 mm at both ends). When soot was deposited, a good base material (soot body) without cracks could be obtained.
In this embodiment, the dopant reduction region is 50 mm, which corresponds to 135% of the base material outer diameter of 37 mm.
[0021]
In traversing the three stages described above, the supply amount of H ₂ and O ₂ was constant, especially when the ring portion synthesized in the portion of the both end portions 50mm and the supply amount is 0 GeCl ₄ source gas, H _2, and O ₂ was supplied as it was. At this time, the temperature of the constant outer diameter portion was 600 ° C., and the temperature of the unsteady portion (both ends 50 mm) was 650 ° C.
[0022]
(Example 2)
In the synthesis of the ring portion, the supply amounts of H ₂ and O ₂ in the non-stationary outer diameter portion at the end of 50 mm were reduced to 80% and 90%, respectively, as shown in FIG. Except that the surface temperature was kept constant at 600 ° C. and the gas flow rate of the dopant was set to 0 at the end, thereby preventing the deposition surface temperature from changing and causing strain and cracking easily. An optical fiber preform was produced under the same conditions as in Example 1.
[0023]
(Example 3)
At the time of synthesis of the ring portion, the supply amount of GeCl ₄ in the unsteady portion having an outer diameter of 50 mm at the end portion is reduced with a gradient as shown in FIG. 8, and at the same time, the supply amounts of H ₂ and O ₂ are also reduced with a gradient. (Reduced to 80% and 90%, respectively, with respect to the constant outer diameter portion), and an optical fiber preform was manufactured under the same conditions as in Example 1 except that the deposition surface temperature was kept constant at 600 ° C. In this example, if the gas flow rate was suddenly changed, the glass composition and the deposition surface temperature would change suddenly, causing distortion and cracking easily. Therefore, the gas flow rate was gradually changed.
[0024]
(Example 4)
In the synthesis of the ring portion, the supply amounts of H ₂ and O ₂ in the non-stationary outer diameter portion at the end of 50 mm are reduced to 85% and 95%, respectively, as shown in FIG. By keeping the surface temperature constant at 600 ° C. and setting the gas flow rate of the dopant to 20% of the steady portion at the end, the deposition surface temperature is prevented from changing, thereby preventing distortion and cracks from easily occurring. Prepared an optical fiber preform under the same conditions as in Example 1.
[0025]
【The invention's effect】
According to the method of manufacturing an optical fiber preform of the present invention, glass particles are deposited on the starting member by adjusting the supply flow rate of the raw material gas of the dopant to the glass synthesis burner at the end of the porous glass preform. At this time, the occurrence of local distortion at the end of the porous glass preform can be suppressed, and an optical fiber preform of good quality can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view showing one example of an apparatus for manufacturing an optical fiber preform for carrying out a method of the present invention.
FIG. 2 is an explanatory diagram showing a state of deposition of glass fine particles on a starting member.
FIGS. 3A to 3E are schematic views showing examples of a ring-shaped Δn profile formed by the method of the present invention (the vertical axis represents a relative refractive index difference).
FIG. 4 is an explanatory diagram (constant H ₂ and O ₂ supply amounts) showing an example of a gas supply amount control pattern according to the method of the present invention.
FIG. 5 is an explanatory diagram (increase of H ₂ and O ₂ supply amounts) showing an example of a gas supply amount control pattern according to the method of the present invention.
FIG. 6 is an explanatory diagram illustrating a control pattern of a gas supply amount adopted in the first embodiment.
FIG. 7 is an explanatory diagram illustrating a control pattern of a gas supply amount adopted in the second embodiment.
FIG. 8 is an explanatory diagram illustrating a control pattern of a gas supply amount adopted in a third embodiment.
FIG. 9 is an explanatory diagram illustrating a control pattern of a gas supply amount adopted in a fourth embodiment.
FIG. 10 is an explanatory diagram showing a control pattern of a gas supply amount adopted by a conventional method.
[Explanation of symbols]
1. Container, 2. 2. Starting member; 3. glass synthetic burner; Burner moving device, 5. 5. lifting device; Exhaust port, 7. Rod, 8. 8. raw material supply device; 10. glass porous base material; Outer diameter constant portion, 11. Outer diameter unsteady part

Claims (4)

By moving the burner for glass synthesis relatively to the axial direction of the starting rod and reciprocating within a certain range in the axial direction, a glass porous body is synthesized on the starting rod, and a glass porous preform for an optical fiber is formed. A method of manufacturing an optical fiber preform, wherein the flow rate of a dopant source gas at a reciprocating end is reduced to 0 to 70% of a steady state. 2. The method according to claim 1, wherein the distance from the terminal to start decreasing the flow rate of the dopant raw material gas or to return the flow rate to the steady state flow rate is 30 to 250% of the outer diameter of the porous base material being synthesized. A method for producing the optical fiber preform according to the above. 3. The method of manufacturing an optical fiber preform according to claim 1, wherein the change in the flow rate of the dopant source gas is changed stepwise or as a function of the distance from the terminal. Method for manufacturing an optical fiber preform according to claim 1, characterized in that to adjust the H _{2 /} O ₂ flow rate at the same time the burner flame and changes in the flow rate of the source gas of the dopant.

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