JP4252834B2 - Optical fiber preform manufacturing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing an optical fiber preform used for optical communication.
[0002]
[Prior art]
In general, an optical fiber preform is usually formed by a core material creating step for producing a core corresponding to the center, a part of the core, or a part of the core and the clad, and the outside of the core material produced thereby. It is manufactured through the optical fiber preform material process and the clad manufacturing process. (For example, see Patent Documents 1 to 3)
[0003]
The core material creation process is roughly divided into the following three methods.
The first method is called the MCVD (Modified Chemical Vapor Deposition) method.
As shown in FIG. 4, this is because silicon tetrachloride (SiCl), which is a glass base, is formed in the quartz glass tube 2 from one end 3 to the other end 4 of the quartz glass tube 2 that rotates about the axis 1. ₄ ), source gas such as germanium tetrachloride (GeCl ₄ ) and oxygen (O ₂ ) for controlling the refractive index is supplied, and the heating source 5 is extended along the longitudinal direction of the quartz glass tube 2 from the outside of the quartz glass tube 2. After moving the glass tube 1 relative to the glass tube 1 and oxidizing the raw material gas in the quartz glass tube 1 to directly deposit glass having a predetermined refractive index in the quartz glass tube 1 or after depositing glass particles. In this method, glass particles are sintered to form a deposited glass layer.
The source gas for controlling the refractive index may not be supplied at all, and a fluorine-containing gas may be used.
[0004]
In the above method, since holes remain in the quartz glass tube 1, as shown in FIG. 5, after forming the deposited glasses 6 and 7, the glass tube 2 is heated to a high temperature and the surface tension of the quartz glass tube 1 is increased. A collapse process for closing the central hole is performed. After the completion of the collapsing process, cutting is performed at the position A at the ends of the deposited glasses 6 and 7 as shown in FIG. 5 to produce the core material 8 for the optical fiber preform.
[0005]
Thereafter, as shown in FIG. 6, a handle 9 made of a quartz rod or the like for holding the core material 8 is fused and connected to at least one end of the core material 8 and is sent to the following clad forming process.
In addition, when the thickness of the quartz glass tube 2 is sufficient for spinning into an optical fiber, the core material 8 may become an optical fiber preform without going through the following clad forming process.
[0006]
The second method is called the OVD (Outside Vapor phase Deposition) method.
In this OVD method, as shown in FIG. 7, an oxyhydrogen flame burner 13 that moves relative to the longitudinal direction of the mandrel 12 is disposed outside the mandrel 12 made of alumina or the like that rotates with the shaft 11 as a rotation axis. In addition, a raw material gas such as silicon tetrachloride (SiCl ₄ ) serving as a glass base and germanium tetrachloride (GeCl ₄ ) for controlling the refractive index and a combustion gas such as oxygen and hydrogen are introduced into the burner 13. In this method, the raw material gas is subjected to a flame hydrolysis reaction in the flame of the combustion gas to generate the glass fine particles 14, and the porous glass aggregate 15 by the glass fine particles 14 is deposited on the outer periphery of the mandrel 12.
[0007]
Thereafter, the mandrel 12 is pulled out from the porous glass aggregate 15, and a handle such as a quartz rod (not shown) is attached to the end of the remaining porous glass aggregate 15, and then the porous glass aggregate 15 is heated. Processed into transparent glass. In this case as well, since a hole remains in the center, a collapse process for closing the center hole after transparent vitrification is performed as in the MCVD method.
[0008]
The third method is called the VAD method. In this VAD method, as shown in FIG. 8, glass fine particles generated by a burner 33 on the tip side of a starting material 32 made of a quartz rod or the like that rotates around a central axis 31 made of a quartz material or the like as a rotation axis. In this method, the porous glass aggregate 34 is formed by depositing and growing in the direction. Since this method does not have a hole in the center, it is unnecessary to close the hole. In the manufacturing apparatus shown in FIG. 8, two burners 33 are arranged vertically and the porous glass aggregate 34 is deposited by each burner, but even if the number of burners 33 is more than that, One may be sufficient.
[0009]
The porous glass aggregate 34 manufactured by the VAD method is then dehydrated and sintered (not shown) to form a transparent core material 35 as shown in FIG.
[0010]
FIG. 10 shows an example of the refractive index distribution of the core material formed by dehydrating and sintering the porous glass aggregate 34 manufactured by the VAD method using one burner 33. Shows that the difference between the maximum refractive index of the central portion and the minimum refractive index of the outer peripheral portion is 2%.
[0011]
There are two major methods for creating the clad.
The first method is a method called a rod-in-tube method.
In this method, as shown in FIG. 11, a glass tube 41 having a desired refractive index distribution is prepared, and the core material 42 created in the core material creating step is inserted into the glass tube 41, and then the glass tube In this method, the glass tube 41 is softened by heating in the longitudinal direction, whereby the diameter of the glass tube 41 is reduced.
[0012]
The second method is a method of depositing a porous glass aggregate of glass fine particles on the outer periphery of the core material created in the core material creation step, as in the OVD method. This deposited porous glass aggregate is later made into a transparent glass as in the OVD method.
[0013]
The above clad forming process may be repeated a plurality of times as necessary. In some cases, the first method and the second method of the clad forming step are used in combination.
[0014]
In these methods, it is needless to say that a dopant that changes the refractive index is added to the glass as necessary when depositing glass particles or a glass layer in order to obtain a desired refractive index profile.
[0015]
In all the methods described above, heating is performed and the temperature is raised to a temperature around the softening point of the glass. In particular, glass is deformed by heating in the collapse process of the MCVD method and OVD method, and the rod-in-tube method. In addition, in the VAD method and the OVD method, a viscous flow is caused in the glass fine particles by heating to change to transparent glass. At this time, the deposit of glass fine particles is changed to a transparent glass lump, and the volume is reduced to about 1/10.
[0016]
[Patent Document 1]
JP-A-8-12367 [Patent Document 2]
JP-A-9-159856 [Patent Document 3]
Japanese Patent Laid-Open No. 2002-116338
[Problems to be solved by the invention]
Along with the recent increase in communication demand, more sophisticated optical fibers are being demanded. A dispersion compensating optical fiber used for upgrading an existing line is a typical example.
[0018]
However, the dispersion compensating optical fiber is presumed to have a complicated structure in order to satisfy the characteristics. For example, as seen in Patent Document 3, the core is composed of a center core, a first side core, and a second side core, and the first side core has a refractive index lower than that of the cladding. It is done. Further, the relative refractive index difference Δ1 of the center core is described as 1.0% or more and 1.6% or less. This value is considered to be about 3 to 5 times larger than the relative refractive index difference of the SMF core. Such a structure can be realized by increasing the dopant concentration that increases the refractive index, such as germanium.
[0019]
However, as the dopant concentration increases, other physical property values such as the linear expansion coefficient also change greatly. In particular, when the linear expansion coefficient is greatly different between the core and the clad, the change in volume due to cooling after heating is greatly different between the core and the clad. In any of the above-described methods, it is inevitable that distortion occurs because the shape is changed by heating.
[0020]
Due to the difference between the strain and the linear expansion coefficient of the core and the clad, such an optical fiber preform is likely to crack. In particular, this tendency is remarkable when cooling is performed at a relatively high speed in order to enhance manufacturability. For this reason, such an optical fiber is difficult to manufacture because of its complicated structure, and there is a problem in that the yield is poor.
Accordingly, an object of the present invention is to provide a manufacturing method with good manufacturability and high yield.
[0021]
[Means for Solving the Problems]
In order to achieve the above object, the present invention has the following configuration as means for solving the problems.
[0022]
That is, according to the present invention, after heating the optical fiber preform to 1000 ° C. or more, before the temperature of the material reaches room temperature due to cooling, at least one of the handles is separated to solve the problem.
[0023]
Another aspect of the present invention is a means for solving the problem by stretching the material so that the outer diameter of the material becomes 30 mm or less after the handle of at least one side is separated and before the temperature of the material reaches room temperature by cooling. It is said.
[0024]
Furthermore, another invention is a means for solving the problem by joining a new handle after the handle is detached.
[0025]
Furthermore, another invention of the present invention is a means for solving the problems by including heating operations such as glass fine particle transparency, glass pipe collapse, lot-in-tube processing, and flame polishing.
[0026]
Furthermore, another invention is a means for solving the problem when the heating temperature of the optical fiber preform is equal to or higher than the strain point.
[0027]
Further, another aspect of the present invention is a means for solving the problem by having the temperature of the lowest temperature portion of the material at 300 ° C. or higher when the handle on one side is separated or stretched.
[0028]
Furthermore, another aspect of the present invention is a means for solving the problem when the difference in relative refractive index between the maximum refractive index and the minimum refractive index in the optical fiber preform is 0.5% or more.
[0029]
Furthermore, another invention of the present invention is a means for solving the problem when the cutting method is an oxyhydrogen flame.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
Heating is essential for the production of optical fiber preforms. In the MCVD and OVD methods of collapse and the rod-in-tube method, the glass surface temperature may be 2000 ° C. or higher in order to bring the glass to a temperature above the softening point. The clarification of glass fine particles by the OVD method or the VAD method uses viscous flow on the surface of the glass fine particles, and it is necessary to heat to a temperature at which viscous flow occurs. The strain point of quartz glass (Lica glass) varies depending on the presence of impurities such as dopants contained therein, but is generally 1000 ° C. or higher. Therefore, no matter which method is used, it is essential to heat to a temperature of 1000 ° C. or higher or a strain point or higher.
[0031]
When the thus heated optical fiber preform is cooled, shrinkage occurs with cooling. When having a core with a high refractive index at the center like an optical fiber preform, the core portion may have a different composition due to the presence of the dopant, and the linear expansion coefficient may be significantly different from that of the cladding. Since viscous flow occurs at a temperature higher than the strain point, strain relaxation proceeds, but at temperatures lower than that, relaxation does not proceed and cracks may occur.
[0032]
Such a phenomenon is more conspicuous as the base material has a higher relative refractive index difference with a higher dopant concentration. This is because the linear expansion coefficient is different as the dopant concentration is higher than in silica glass having no dopant. FIG. 12 is a characteristic diagram showing the occurrence frequency of cracks relative to the relative refractive index difference between the core and the clad when the core diameter is about 15 mm and the clad diameter is about 45 mm. If the relative refractive index difference exceeds 0.5%, cracks may occur, and cracks occur at a frequency of 50% at 1.0% and 90% or more at 2.0% or more. For this reason, it is necessary to take measures for preventing cracks in a base material having a relative refractive index difference of 0.5% or more.
[0033]
Further, when the temperature at which the crack was generated was examined, when the relative refractive index difference was 0.5%, the crack was not generated unless the temperature was 300 ° C. or lower. When the relative refractive index difference was 2.0%, cracks did not occur unless the temperature was 600 ° C. or lower. For this reason, if any treatment is performed at least at 300 ° C. or higher, cracks can be prevented.
[0034]
As a result of investigating the form of occurrence of cracks, it was found that the cracks often occur at the joint between the handle attached to both ends and the optical fiber preform material for handling the preform. In particular, when a handle made of glass having a linear expansion coefficient equivalent to that of the outer peripheral portion of the optical fiber preform is bonded, the handle may be broken with cooling.
[0035]
The cause is considered as follows. Such an optical fiber base material has a high germanium concentration in the central core and a larger linear expansion coefficient than the peripheral portion. For this reason, when the temperature of the optical fiber preform is lowered, the core tends to shrink compared to the peripheral portion. However, since the surrounding cladding does not shrink as compared with the core, the core receives tensile strain and the cladding receives compressive strain. It can be considered that a crack is generated when the optical fiber preform cannot withstand these strains.
[0036]
In particular, the optical fiber base material is composed of a core having a predetermined linear expansion coefficient coated with a cladding having a smaller linear expansion coefficient, and one end of this optical fiber base material is equivalent to the cladding. When the handle 4 made of glass having a linear expansion coefficient is joined so as to straddle the core and the clad, the core is surrounded by outer peripheral portions having different linear expansion coefficients. At this time, since the handle is joined by softening the optical fiber base material at a high temperature, the joint between the two becomes weak in terms of strength. For this reason, it is considered that the center portion of the optical fiber base material and the joint portion of the handle 4 cannot withstand distortion, cracks occur, and the handle 4 is broken.
[0037]
Further, it is considered that the crack in the joint portion triggers the entire base material to crack or the base material falls due to the handle being broken.
[0038]
In addition, in an optical fiber preform having a large outer diameter, a temperature difference occurs between the peripheral portion and the central portion of the optical fiber preform. For this reason, the distortion due to the shrinkage of the optical fiber preform is different between the inside and the outside. Due to this difference in strain, cracks may occur from the joints that are weak points. Such a crack is remarkable in a base material having a large relative refractive index difference.
[0039]
However, when examining in more detail, when the handle is joined to both ends of the optical fiber preform, both the handles are less likely to break, and in many cases only one side. This is considered because distortion is released when the handle on one side is broken. Based on this idea, if the handle on one side is cut off before the optical fiber preform is cooled, the strain should be released to some extent and the frequency of cracks should be reduced.
[0040]
In view of this point, the inventors cut the handle before the optical fiber preform was cooled, and the optical fiber preform with a relative refractive index difference between the core and the clad of 2% or less was cracked. The frequency of entering could be reduced to zero.
[0041]
When the relative refractive index difference between the core and the clad exceeded 2%, the above method alone could not prevent cracks. For this reason, when the handles at both ends of the optical fiber preform are cut, cracks can be prevented. This seems to be because the distortion cannot be solved sufficiently by cutting only one side.
[0042]
Further, when the relative refractive index difference between the core and the clad exceeds 2.3%, it is not sufficient to cut the handles at both ends. For this reason, when the optical fiber preform was stretched to an outer diameter of 25 mm, cracks were not generated. This is thought to be because the temperature difference between the outer peripheral portion and the center is reduced by reducing the diameter, and the difference in the radial tension factor due to temperature is reduced.
[0043]
Using an optical fiber preform with a relative refractive index difference of 2.3%, and experimenting with various changes in the stretched diameter after cutting the handles on both ends, the frequency of cracks occurring at an outer diameter of 30 mm or less is 10%. It became the following. For this reason, it is necessary for the optical fiber preform having a large relative refractive index difference to be stretched to an outer diameter of 30 mm or less.
[0044]
These cutting methods are preferably softened and cut using a flame in that they do not give an impact to a cracked optical fiber preform. In particular, an oxyhydrogen flame is optimal in terms of temperature.
[0045]
【Example】
Hereinafter, the present invention will be specifically described with reference to examples.
Example 1
1 to 3, 51 is a core material manufactured according to FIGS. 4 to 9 and 11, and 52 is a handle such as a quartz rod bonded to both ends of the core material 51. Thereafter, both handles 52 are attached to a pair of gripping tools rotating with the same rotation axis, and a porous glass layer 53 to be clad later is deposited on the outer periphery of the core material 51 by the OVD method as shown in FIG. An optical fiber preform 54 having a porous cladding was formed. Thereafter, the optical fiber preform 54 having the porous clad was dehydrated in an electric furnace 55 as shown in FIG.
[0046]
The optical fiber preform 54 thus manufactured is then taken out from the electric furnace, and at a temperature of 800 ° C. or higher, the handle on one side is cut by a flame at the position of line B as shown in FIG. It was done. When not cut, 9 out of 10 cracks were found, but when cut, no cracks were found in 10 pieces. The transparent optical fiber preform thus obtained had a relative refractive index difference of 1.5% between the core and the clad.
When cutting, it is important to cut from the left side of line B in FIG. 1, that is, from the portion of the optical fiber preform. If it is cut from the right side of the line B, the handle and its joint remain in the base material, so it does not make any sense of cutting.
[0047]
(Example 2)
An optical fiber preform 54 having a porous clad having a relative refractive index difference between the core and the clad of 2.4% after being made transparent was prepared in the same manner as in Example 1 and then made transparent in an electric furnace at 1500 ° C. It was conducted. Immediately after removal from the electric furnace, at a temperature of 800 ° C. or higher, the handles at both ends were cut with a flame and stretched to an outer diameter of 5 mm. When 10 were not cut, all 10 cracks were cracked. When cut, 3 cracks were found, but when further stretched, no cracks were generated in 10 cracks.
[0048]
(Example 3)
An optical fiber preform material having a relative refractive index difference of 2% with respect to the cladding and the starting tube was prepared by doping germanium by the MCVD method. After the collapse, the starting tube (glass tube) at both ends of the optical fiber preform was cut at a temperature of 1000 ° C. or higher, and a new handle was joined to one side to obtain the structure shown in FIG. When not cut, 9 out of 10 cracks were found, but when cut, no cracks were found in 10 of them.
[0049]
(Example 4)
In the VAD method, germanium tetrachloride is doped so as to maximize the refractive index of the core axis of the core, soot is deposited from the front end of the starting material in the direction of the rotation axis, and this is dehydrated and sintered. An optical fiber preform having a relative refractive index difference of 2.2% between the refractive index of the axial center having the refractive index profile shown in FIG. In this case, since the linear expansion coefficient was different from that of the quartz starting material, all ten of them were cracked from the joined portion during cooling and spread to the whole. In contrast, when the starting material was melted with a flame before the optical fiber preform was cooled and a new quartz for the handle was attached, none of the 10 cracks occurred during cooling.
[0050]
(Example 5)
When the base material having the shape shown in FIG. 1 and having a relative refractive index difference of 0.5% between the core and the clad is subjected to flame polishing for the purpose of smoothing the surface, the residual strain in the optical fiber base material Cracks may occur due to Therefore, when the handle on one side was cut with a flame immediately after the flame polishing, the frequency of cracks, which had been a ratio of 1 in 50 so far, decreased to 1 in 200.
[0051]
In addition, the said Example of this invention is one Embodiment of this invention, and this invention can be variously deformed in the range which does not deviate from this invention.
[0052]
【effect】
As described above, in the present invention, a handle for holding an optical fiber preform is attached to at least one end of an optical fiber preform made of quartz having different refractive indexes between the axial center and the outer periphery thereof, and the optical fiber In the method of manufacturing an optical fiber preform in which the preform for the optical fiber is manufactured through a thermal history of 1000 ° C. or more, the preform for the optical fiber preform is subjected to a thermal history at 1000 ° C. or higher. Before the temperature of the material for the material reaches room temperature due to cooling, the handle on at least one side is separated from the material for the optical fiber preform. For this reason, this invention has the outstanding effect which can manufacture a highly functional optical fiber with a sufficient yield.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing an embodiment of the present invention.
FIG. 2 is a front view showing an intermediate product in FIG.
3 is an explanatory view showing a production example of another intermediate product in FIG. 1. FIG.
FIG. 4 is a plan view showing an example of a general MCVD method.
FIG. 5 is an explanatory front view showing the intermediate product in FIG. 4;
6 is an explanatory front view showing another intermediate product in FIG. 4; FIG.
FIG. 7 is a front view showing a general OVD method.
FIG. 8 is a schematic explanatory diagram showing a general VAD method.
FIG. 9 is an explanatory view showing an intermediate product manufactured according to FIG. 8;
FIG. 10 is a refractive index profile diagram showing the intermediate product of FIG. 9;
FIG. 11 is a schematic explanatory view showing an example of a general lot-in-tube method.
FIG. 12 is a characteristic diagram showing the occurrence frequency of cracks with respect to a relative refractive index difference by a conventional manufacturing method.
[Explanation of symbols]
51 Core Material 52 Handle 53 Porous Glass Layer 54 Optical Fiber Base Material 55 Electric Furnace

Claims (5)

A handle for holding the optical fiber preform is attached to at least one end of the optical fiber preform made of quartz whose maximum refractive index and minimum refractive index differ by 0.5% or more. In the method of manufacturing an optical fiber preform in which the preform for the optical fiber is manufactured through a thermal history of 1000 ° C. or more, the preform for the optical fiber preform is subjected to a thermal history at 1000 ° C. or higher. When the temperature of the lowest temperature part of the material for the material is 300 ° C. or higher, the handle on at least one side is separated from the material for the optical fiber material , and then the temperature of the lowest part of the material for the optical fiber material is A method for producing an optical fiber preform, wherein the optical fiber preform is stretched so that the outer diameter of the optical fiber preform is 30 mm or less when the temperature is 300 ° C or higher . 2. The optical fiber preform according to claim 1 , wherein after the handle on at least one side is separated from the optical fiber preform, a new handle is joined to the end of the optical fiber preform. Method. The optical fiber preform according to claim 1 or 2 , wherein the heat history is one or more heating operations of glass fine particles, glass pipe collapse, lot-in-tube processing, and flame polishing. Method. The method for producing an optical fiber preform according to any one of claims 1 to 3 , wherein the heat history of the optical fiber preform is a heating temperature equal to or higher than a strain point. Method for manufacturing an optical fiber preform according to any one of claims 1 to 4 separation of at least one side of the handle from the material for the optical fiber preform is characterized by being carried out by an oxyhydrogen flame.

Images