JP2000143273A - Production of optical fiber preform - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an optical fiber preform containing desired elements only in the central part of a core with a simple process. SOLUTION: Glass particulates generated from a burner 11 for the core is adhered to the bottom end of a supporting rod 20, by which a core central part 31 is deposited. Only the surface of this core central part 31 is thereafter heated and sintered by burner 12 for sintering and the glass particulates from a burner 13 for the core are adhered to the circumference of this sintered part 32. Further, the glass particulates from a burner 14 for the clad are adhered to the circumference of the core outer peripheral part 33, by which a clad part 34 is deposited. As a result, a porous glass body 30 which is columnar as a whole and is embedded with such cylindrical sintered part 32 as to cover the circumference of the core central part 31 is obtained. Only the core central part 31 is immersed into a solution after end of such VAD process and this soln. is permeated into this part by capillarity so that the solution is penetrated only into the range blocked by the cylindrical sintered part 32, i.e., the inside of the core central part 31.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an optical fiber preform which is used as a preform for making an optical fiber by drawing, and more particularly to a method of manufacturing a preform for making an optical amplifier type optical fiber. And a method of manufacturing a base material containing a predetermined element only in a predetermined region at the center of the base material.

[0002]

2. Description of the Related Art In an optical fiber preform, a predetermined element may be contained only in a core portion or a central region in the core portion. For example, in a preform for an optical amplifier type optical fiber, an element such as erbium is contained in a core portion or a central region of the core portion. When such a base material is drawn to form an optical fiber, an optical amplifier type optical fiber having an optical amplification effect is obtained. That is, when the quartz glass core is doped with a rare earth element such as Er or Nd, these dopants function to absorb the excitation light and amplify the signal light. Normal,
As a 1.5 μm band optical amplifier type optical fiber, one in which at least Er is added to a core portion is known, and as a 1.3 μm band optical amplifier type optical fiber, N is added to a core portion.
Those to which d, Pd, and the like are added are known. 1.5μ
The m-band optical amplifier type optical fiber is called an erbium-doped optical fiber because of the addition of Er.

In these optical amplifier type optical fibers such as erbium-doped optical fibers, the signal light is amplified by absorbing the pump light. However, the minimum required pump light intensity is required to amplify the signal light. Exists and its value is called the threshold. If the excitation light intensity is smaller than this threshold value, the signal light is not amplified and conversely absorbed because the excitation of erbium ions is insufficient. However, the excitation light intensity in the single-mode optical fiber has a shape that is higher at the center in the radial direction and becomes lower toward the periphery. Therefore, as shown in the refractive index distribution of FIG. 3, it is necessary to add erbium, aluminum, or the like only to the central region 53 in the core region 51 having a higher relative refractive index difference than the peripheral cladding region 52.

[0004] Therefore, conventionally, a porous soot serving as a central portion of a core is first deposited by a VAD method, and then a rare earth element such as Er is impregnated into the porous soot by a liquid immersion method. -Transparent vitrification, an outer peripheral portion of the core and a clad portion are deposited on the glass base material by an external method, and this is dehydrated and sintered to produce an optical fiber base material for drawing.

Further, after depositing a core portion by using the MCVD method (internal mounting method), a rare earth element such as Er is impregnated into the inner core portion by a liquid immersion method, and thereafter, sintering is performed. A method of realizing the optical fiber preform by realizing it is also considered.

[0006]

However, in the conventional manufacturing method combining the VAD method and the liquid immersion method, it is necessary to deposit the core portion in two stages, that is, the central portion and the outer peripheral portion. There are problems that the process is complicated, that air bubbles are easily generated at the interface between the core central portion and the core outer peripheral portion, and that stable production cannot be achieved. In other words, after the core central portion is once deposited, it is immersed in liquid, transparent vitrified, and then the core outer peripheral portion is deposited therearound. As a result, the glass base material after sintering tends to generate bubbles, cracks, and the like at its interface.

The manufacturing method combining the MCVD method and the liquid immersion method has a problem that the polarization characteristics when formed into an optical fiber are not good. That is, in this manufacturing method, although the solidification is performed in the final step, it is difficult to solidify the solid shape into a perfect circle, so that the core portion becomes elliptical.
As a result, when the optical fiber is used, there is a problem that the polarization characteristic is poor due to the non-circularity of the core.

In view of the above, the present invention can manufacture an optical fiber preform containing a predetermined element only in a predetermined region at the center of a core while simplifying the process, and furthermore, the characteristics of an optical fiber made therefrom are improved. It is an object of the present invention to provide a method of manufacturing an optical fiber preform that can be excellent.

[0009]

In order to achieve the above object, in a method of manufacturing an optical fiber preform according to the present invention, at least three burners are vertically arranged, and the first burner is arranged at a lowermost position. The resulting glass fine particles are deposited as a core center at the lowermost end of the rotating support rod, and the surface of the deposited core center is sintered by a second burner located above the first burner, and further sintered. Glass fine particles generated from a third burner located above the second burner,
A VAD step of growing a columnar porous glass preform at the lower end of the support rod by depositing the surface on the outer periphery around the center of the sintered core, and the columnar porous glass A liquid immersion step of immersing the base material in a desired solution only in the central portion of the core protruding from the lower end thereof and soaking the solution only in the central portion of the core.

Although the VAD method and the liquid immersion method are combined, at least the entire core portion is deposited at a time by the VAD method (that is, only the core portion, or in addition to the entire core portion, the outer periphery of the core portion). Is also deposited).
Then, the core portion deposits glass fine particles from the two burners as a core central portion and a core outer peripheral portion, respectively.
The surface in the center of the core is sintered by a burner placed between the two burners. As a result, as soon as the core central portion is deposited, the surface thereof is sintered, and the core outer peripheral portion is deposited on the surface of the sintered core central portion. Thus, the columnar porous glass base material grows at the lower end of the support rod.

In the columnar porous glass base material thus grown, the center of the core protrudes from the lower end thereof.
Only the center of the protruding core is soaked in the desired solution. Although the center of the core is a porous glass body, the surface of the core is sintered to form a portion having a high bulk density, so that the solution permeating the center of the core exceeds the portion having a high bulk density. It does not penetrate,
It will penetrate only into the core. Therefore, it is possible to easily dope only the central portion of the core portion with Er or the like.

As a result, the core can be formed in a single VAD process as compared with the conventional manufacturing method combining the VAD method and the liquid immersion method, so that the process is simple and the core portion can be easily formed. Dust is not mixed with the outer periphery of the core and bubbles are not generated, and an optical fiber preform having excellent characteristics can be obtained. Compared with the conventional manufacturing method combining the MCVD method and the liquid immersion method, an optical fiber preform having excellent characteristics because it does not perform a solidification process that is likely to produce a non-circular core portion. Can be easily manufactured.

[0013]

Next, embodiments of the present invention will be described in detail with reference to the drawings. First, FIG.
As shown in FIG. 5, a columnar porous glass body 30 including a core part and a part of a clad part is grown at the lower end of the support rod 20 by the VAD method. Therefore, near the lower end of the support rod 20,
Four burners 11 to 14 are vertically arranged. The lowermost burner 11 and the third burner 13 from the bottom are burners for forming a core portion, and the uppermost burner 14 is a burner for forming a clad portion. That is, hydrogen gas / oxygen gas for forming an oxyhydrogen flame, silicon tetrachloride as a glass material, and germanium tetrachloride as a dopant are introduced into the burners 11 and 13, and fine particles (glass fine particles) of silicon dioxide and germanium dioxide are introduced. Is generated. Hydrogen gas / oxygen gas for forming an oxyhydrogen flame and silicon tetrachloride as a glass material are introduced into the burner 14 to generate silicon dioxide fine particles (glass fine particles). The burner 12 disposed second from the bottom is a sintering burner that is a mere sintering burner, in which hydrogen gas / oxygen gas is introduced to form a heating flame.

The fine glass particles from the burner 11 are
The core central part 31 adheres to the lower end of the zero. Since the support rod 20 is rotated and pulled up in accordance with the growth of the core central part 31, the core central part 31 grows in a columnar shape. When the core central portion 31 grows in this manner, the portion excluding the lowermost growth end (the upper portion thereof) is exposed to the flame of the sintering burner 12. Therefore, the surface portion of the columnar core central portion 31 is sintered to form a sintered portion 32 having a high bulk density.

Since the core burner 13 is disposed above the sintering burner 12, glass particles from the core burner 13 adhere to the sintering portion 32 on the surface of the core center portion 31 and the outer peripheral portion of the core. 33 are formed. Further, a clad burner 14 is disposed above the core burner 13, and glass particles from the clad burner 14 adhere to the surface of the core outer peripheral portion 33 to form a clad portion 34.

When the porous glass body 30 having a columnar shape as a whole grows in the axial direction, a cylindrical sintered portion 32 is formed around a core center portion 31 located at the center of the porous glass body 30.
Will be embedded. Further, as shown in the figure, the tip (lower end) has a shape in which the core central portion 31, the core outer peripheral portion 33, and the clad portion 34 are stepped and project downward.

Next, the porous glass body 30 thus produced is immersed in a solution 40 as shown in FIG. That is, an aqueous solution 40 of a rare earth such as erbium or aluminum is used.
Then, the core central portion 31 at the lower end of the porous glass body 30 is immersed, and the aqueous solution 40 penetrates upward from the core central portion 31 by capillary action. Since the core central portion 31 projects most downward, only the core central portion 31 can be immersed in the solution 40. Since the periphery of the core central portion 31 is covered with the sintered portion 32 having a high bulk density, the core central portion 3
The solution 40 that has permeated into 1 does not permeate beyond this sintered portion 32 and stays in the core central portion 31. As a result, a porous glass body 30 in which only the core central portion 31 is doped with erbium or aluminum can be obtained.

After drying the porous glass body 30,
After dehydration (removal of OH groups), the whole is sintered to form a transparent glass. Thereafter, a thick clad portion is formed around the base material by an external method. An optical fiber can be obtained by dehydrating, sintering, and vitrifying the base material, and then drawing the fiber to form a fiber. In this optical fiber, erbium or the like is doped in the central region of the core, and erbium or the like is not doped in the periphery of the core where the excitation light intensity is smaller than the threshold value, so that the signal light is absorbed in the periphery of the core. The optical fiber can be used as an optical amplifier-type optical fiber in which signal light is favorably amplified without being performed.

In the above description, the four burners 11 to 14
Is used to deposit three layers, but it is also possible to deposit more layers using a larger number of burners and grow the entire layer up to the clad portion. Conversely, the burner 14 is removed, and the burner 11 and the burner 11 are removed. 13, only the central portion 31 and the outer peripheral portion 33 of the core can be deposited. In the latter case, it is necessary to attach a clad part. Therefore, after immersion in the same manner as described above, drying, dehydration and sintering are performed, and a clad part is deposited around the periphery by an external method. The solution to be immersed is not limited to one containing erbium or aluminum, and the optical fiber to be made is not limited to erbium-doped optical fiber. In addition, various changes can be made without departing from the spirit of the present invention. Further, the present invention provides a porous glass body having a core outer peripheral portion omitted, that is, in the case where the porous glass body 30 in the above example has a structure including a core portion 31 and a clad portion 34, The present invention can also be applied to a method of manufacturing an optical fiber preform in which a predetermined element is permeated only into the core portion 31.

[0020]

EXAMPLES Further, specific examples will be described in detail. As shown in FIG. 1, V is controlled by using four burners 11 to 14.
A porous glass body 30 was made by the AD method. The core burner 11 deposited a core portion 31 of silicon dioxide to which 10 mol of germanium dioxide was added. The core central part 31 was heated by the burner 12, and only its surface was sintered and vitrified. Similarly, the core burner 13 generates fine particles of silicon dioxide to which 10 mol of germanium dioxide is added, and the fine particles are formed in the core central portion 3.
No. 1 was deposited on the outer periphery of the vitrified sintered portion 32, and a part of the clad portion 34 was further deposited on the outer periphery by the burner 14. The flow conditions in each of these burners 11 to 14 are as shown in Table 1. The baking method is used for supplying silicon tetrachloride and germanium tetrachloride. With such a VAD process, a diameter of 200 m
m and a porous glass body 30 having a length of 4000 mm.

[Table 1] The unit SLM (Standard Litere Minutes) is a flow rate in a standard state: liter / minute, and the unit SCCM (Stan
dard CC Minutes) is the standard flow rate: cc / min.

The porous glass body 30 thus obtained is shown in FIG.
As shown in FIG. 5, only the core central portion 31 was immersed in the solution 40 so as to be immersed. The solution 40 is an aqueous solution containing erbium trichloride and aluminum trichloride.
Immersion time. Thereafter, a drying process was performed for 72 hours by a method of gradually increasing the temperature from room temperature to 120 ° C. The porous glass body 30 after this drying treatment
Was dehydrated in a chlorine gas atmosphere, heated and sintered in a helium gas atmosphere to form a transparent glass. The base material of the transparent glass did not show any bubbles or cracks, and had a good appearance. The transparent glass base material was drawn into a fiber. The optical fiber thus obtained has a conversion efficiency of 92%.
This resulted in a highly efficient erbium-doped optical fiber.
Polarization dispersion (PMD) of this erbium-doped optical fiber
Was as good as 0.2 ps / km (picoseconds / route kilometer).

[0022]

COMPARATIVE EXAMPLE For comparison, a porous glass body of germanium dioxide-added silicon dioxide serving as a core center was deposited by a VAD method, and this porous glass body was immersed in an alcohol solution containing erbium and aluminum. Thereafter, the substrate was dehydrated and sintered in a fluorine-added atmosphere to obtain a transparent glass base material to which 0.01% by weight of erbium and 0.5% by weight of aluminum were added. Since this base material constitutes only the central portion of the core, a porous glass body of germanium dioxide-added silicon dioxide was deposited around the core as the outer peripheral portion at a predetermined magnification. This was sintered and turned into a transparent glass, but bubbles were generated at the interface between the core central part obtained in the first VAD step and the core outer peripheral part provided in the subsequent external step, and the transparent glass for the core was formed. No material usable as a base material was obtained.

[0023]

As described above, according to the method for manufacturing an optical fiber preform of the present invention, an optical fiber preform containing a desired element only in the central portion of a core can be produced in a simple process with a good yield. Can be made. Moreover, the optical fiber made from the base material has excellent characteristics and high reliability.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a VAD process according to an embodiment of the present invention.

FIG. 2 is a schematic view showing a liquid immersion process according to the embodiment.

FIG. 3 is a graph showing a refractive index distribution.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Burner for core central part deposition 12 Burner for sintering 13 Burner for core peripheral part deposition 14 Burner for cladding part deposition 20 Support rod 30 Porous glass body 31 Core central part 32 Sintering part 33 Core peripheral part 34 Cladding part 40 Solution 51 core region 52 cladding region 53 central region

Claims (1)

[Claims] At least three burners are vertically arranged, and glass fine particles generated from a lowermost first burner are deposited at a lowermost end of a rotating support rod as a core center,
The deposited core central surface is sintered by a second burner located above the first burner, and the glass fine particles generated from the third burner located above the second burner are separated by the above-mentioned method. A VAD process of growing a cylindrical porous glass preform at the lower end of the support rod by depositing the outer periphery of the core around the center of the sintered core; and a columnar porous glass preform. A method of immersing the material in a desired solution only in the central portion of the core protruding from the lower end thereof, and impregnating the solution only in the central portion of the core.