JP2766995B2 - Manufacturing method of optical fiber preform - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing an optical fiber preform, and more particularly to an improvement in a method for doping a core with a high concentration of a rare earth element.

<Conventional technology> A conventional optical fiber can be used as a functional device such as an optical amplifier device or a laser device by doping a rare earth element in a core of quartz glass and imparting an amplification effect by this. Things are provided.

In order to obtain an optical fiber serving as such a functional element, it is necessary to dope the core with a rare earth element at a high concentration. In this case, the doping amount of the rare earth element cannot be sufficiently increased by the usual method of adding a dopant at the same time as forming a core soot by VAD (Vaperphase Axial Deposition). Therefore, in the prior art, a so-called molecular stuffing method of preparing an alcohol solution containing a rare earth element in advance and immersing core soot in the alcohol solution to dope the rare earth element may be applied.

FIG. 5 shows the steps in the case where an optical fiber preform is manufactured by applying the conventional VAD method and the molecular stuffing method. In this manufacturing method, first, in the first step, VA
A core soot is prepared by the method D, and then the core soot is completely immersed in an alcohol solution containing a rare earth element (for example, Nd or Er) prepared in advance in the second step. Subsequently, in the third step, the core soot impregnated with the alcohol solution is left at room temperature to sufficiently evaporate the alcohol. Thereby, the rare earth element is deposited on the core soot. In the fourth step, after removing water in the core soot doped with the rare earth element, the core soot is sintered and vitrified to obtain a core rod. In the fifth step, the core rod is slightly stretched in order to adjust the outer diameter, and subsequently, in the sixth step, a clad soot is deposited on the core rod. Next, in a seventh step, after removing moisture in the clad soot, the clad soot is sintered and vitrified to produce an optical fiber preform having a clad formed on a core.

By sintering in an atmosphere of He gas and chlorine gas, dehydration and transparency can be performed simultaneously.

<Problems to be Solved by the Invention> In the above steps of producing the core soot and the clad soot, a raw material such as SiCl ₄ or GeCl ₄ is heated in an oxyhydrogen flame to cause a hydrolysis reaction, thereby depositing glass particles. Therefore, in the process itself, moisture is naturally mixed into the soot. Then, when such a soot containing water is sintered as it is, OH groups remain in the core, and as a result, absorption loss increases and the characteristics as an optical fiber deteriorate.

Therefore, as described above, even in the prior art, in the step of sintering the core soot or the clad soot, Cl, B
2. Description of the Related Art A dehydration treatment is performed in which an OH group is replaced with a halogen element by performing a heat treatment in an atmosphere containing a halogen element such as r or F.

However, conventionally, a step of once vitrifying the core soot and depositing the clad soot on the core rod is employed. When the clad soot is produced (at the time of the sixth step in FIG. 5), a hydrolysis reaction is performed, so that moisture enters the core rod. In this case, since the core rod has already been sintered, it is not porous but solid. Therefore, even if the dehydration treatment is performed at the stage of sintering the clad soot (during the seventh step in FIG. 5), it is difficult to smoothly replace the OH groups impregnated in the core rod with halogen elements. Therefore, in the conventional manufacturing method, the dehydration treatment of the optical fiber preform is insufficient, and there is a limit in improving the characteristics of absorption loss.

<Means for Solving the Problems> The present invention has been made in view of such circumstances, and is a rare earth element having improved properties such as absorption loss by sufficiently removing OH groups in a core. The purpose is to obtain an element-doped optical fiber preform.

Therefore, in the method for manufacturing an optical fiber preform of the present invention,
The porous core soot produced by the VAD method is impregnated with an alcohol solution containing a rare earth element, and is dried to evaporate the alcohol.Consequently, the clad soot is directly deposited on the core soot. The core soot and the clad soot are both vitrified by sintering after dehydration.

<Effect> According to the above method, in a state where the clad soot is deposited on the core soot, both the core soot and the clad soot are in a porous state, so if this is dehydrated using a halogen element such as Cl, Since these halogen elements enter the soot and replace the OH groups, the water in any soot is sufficiently discharged.

<Example> FIG. 1 is a process chart showing a procedure of a method for manufacturing an optical fiber preform of the present invention.

In the method for manufacturing an optical fiber preform of the present invention, first, the first
In the step (3), as shown in FIG. 2, a porous core soot a, which is a semi-sintered body of glass fine particles, is produced by a well-known VAD method. At this time, Ge, P,
A dopant such as Al can also be added at the same time.

In the next second step, the core soot a is immersed in an alcohol solution b containing a rare earth element prepared in advance. For example, to dope Nd as a rare earth element, core soot is immersed in a solution of NdCl ₃ in methanol. In this case, as shown in FIG. 3, only the lower end portion of the core soot a is locally immersed in the alcohol solution b, and the portion of the core soot a above the solution surface is made of the alcohol solution b by using capillary action. Suck up. As the alcohol solution b is gradually impregnated into the core soot a, the air remaining in the core soot a is also gradually discharged, so that the rare earth element can be made uniform as compared with the case where the core soot a is immersed in the alcohol solution at once. Can be doped.

In the third step, the core soot impregnated with the alcohol solution is left at room temperature to sufficiently evaporate the alcohol. Thereby, the rare earth element is deposited on the core soot.

In the fourth step, as shown in FIG. 4, the cladding soot c is directly deposited on the core soot a doped with the rare earth element.

In the next fifth step, dehydration treatment is performed on the core soot with the clad soot deposited. In this case, since both the core soot and the clad soot are in a porous state, if this is dehydrated using a halogen element such as Cl, these halogen elements smoothly enter the soot and are replaced with OH groups. Therefore, the water in each soot is sufficiently discharged. After this dehydration treatment, the core soot and the clad soot are both 1500 ° C.
An optical fiber preform is obtained by sintering in an atmosphere of about He gas and chlorine gas to perform dehydration and transparent vitrification.

In this embodiment, a method for manufacturing a single-mode optical fiber preform in which a clad is formed on a core is described. However, a so-called side core having a smaller refractive index is formed on a central core. The present invention can also be applied when producing a convex type optical fiber preform. That is, by depositing the side core soot directly on the core soot and performing the dehydration treatment in this state, the moisture remaining in the central core can be sufficiently removed.

<Effects of the Invention> According to the present invention, the clad soot is directly deposited on the core soot, and both the soots are dehydrated in a porous state, so that sufficient dehydration can be performed. Therefore,
Since the number of OH groups remaining in the core is greatly reduced, excellent effects such as the ability to manufacture a rare earth element-doped optical fiber preform with improved absorption loss characteristics are exhibited.

[Brief description of the drawings]

1 to 4 show an embodiment of the present invention. FIG. 1 is a process chart showing a procedure of a method of manufacturing an optical fiber preform of the present invention. FIG. 2 is an explanatory view of a VAD method. FIG. 3 is an explanatory view of doping of a rare earth element by a molecular stuffing method, and FIG. 4 is an explanatory view of a case where a clad soot is directly deposited on a core soot. FIG. 5 is a process chart showing a procedure of a conventional method for manufacturing an optical fiber preform. a ... clad soot, b ... alcohol solution, c ...
Clad soot.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Minoru Yoshida 4-3-1 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Wire & Cable Co., Ltd. Itami Works (72) Masaaki Morisawa 4--3, Ikejiri, Itami-shi, Hyogo Mitsubishi Electric (72) Inventor Masataka Nakazawa 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Yasuo Kimura 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Japan (56) References JP-A-1-96021 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C03B 1/00-5/44 C03B 8/00- 8/04 C03B 19 / 12,19 / 14,20 / 00 C03B 37/00-37/16

Claims (2)

(57) [Claims] 1. A porous core soot manufactured by a VAD method is impregnated with an alcohol solution containing a rare earth element, and is dried to evaporate the alcohol.
Deposit clad soot directly on this core soot,
Next, a method for producing an optical fiber preform characterized by dehydrating and then sintering to vitrify both the core soot and the clad soot. 2. The method of manufacturing an optical fiber preform according to claim 1, wherein a center core soot is applied instead of the core soot and a side core soot is applied instead of the clad soot. Production method.