JPS6132266B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing an optical transmission fiber (hereinafter simply referred to as optical fiber).

The loss of optical fibers has been rapidly reduced, and communication systems using fibers as transmission paths are about to be put into practical use. Important transmission characteristics required of optical fiber include low loss and wide bandwidth. The transmission band of a fiber is usually evaluated by the frequency characteristics of the baseband, and a large band means a large transmission capacity. Various methods have been proposed to increase the transmission band of the fiber, but the most promising method is to have the refractive index distribution shape f(r) in the core part 1 (light concentration region) shown in Figure 1. , the band velocity is expanded by matching the group velocities of each propagable mode.

The refractive index distribution shape that realizes this is given by the following formula: The one where f(r)≈(r/a) ² is well known.

There are difficult problems when manufacturing fibers having such a distribution shape by a vapor phase deposition method (CVD method).
Also, when trying to obtain the required transmission band, f
It is necessary to change the functional form of (r).

In general, Ge, Al, P,
Methods of doping the core portion of optical fibers by changing raw materials that increase the refractive index, such as Ti and Ga, have been used, but there are problems in manufacturing, such as the inability to stably obtain the required transmission band. be.

The present invention aims to solve these problems and easily obtain the required transmission band. The characteristic point is that the refractive index distribution in the cross section of the core of the optical fiber is almost constant with no change in a certain region near the center of the core, and is formed so that it has a circularly symmetrical function in the surrounding core region. In the method of manufacturing optical transmission fibers by doping the core portion of the optical fiber and having a certain circularly symmetric function by vapor phase growth, Ge, Al,
This is a method for manufacturing an optical transmission fiber that includes at least one type of doping material such as P, Ti, or Ga, and after forming the fiber by increasing the amount by changing it stepwise according to a certain circularly symmetrical function, the doping material is formed from a certain region without changing. . A fiber with a distribution shape as shown in formula (1) is manufactured by doping the core with elements that increase the refractive index, such as Ge, Al, P, Ti, Ga, etc., using a vapor phase growth method. In this case, the common method is to increase the doping element content in stages at the center of the core, as shown in Figure 2. . By the way, as shown in Figure 3
In the manufacturing method called CVD internal sooting, a mixture of raw materials such as Si compounds, doping compounds, and carrier gas O ₂ is fed into the quartz pipe, the glass tube is heated with a burner, and the quartz pipe is heated by a gas phase oxidation decomposition reaction. A layer of glass is laminated inside. Thereafter, as shown in FIG. 4, the glass pipe is crushed and solidified by external heating. The glass rod solidified by this collapse, which is generally called a collapse, is heated at a high temperature and spun to a predetermined fiber diameter to obtain an optical fiber having a distribution shape. According to this general method, the transmission band of the optical fiber varies widely from 200 MHZ.Km to 1000 MHZ.Km, and it is difficult to stably obtain an optical fiber having a constant transmission band. On the other hand, when trying to control the fiber transmission band to a certain predetermined value, the conventional method uses the function f in equation (1).
(r) must be controlled to a certain function, but
For this purpose, it is necessary to precisely control the refractive index distribution in detail, which is difficult to manufacture.

The present invention solves these problems and relates to a method of manufacturing an optical fiber that can easily and stably obtain the required transmission band. The example is the fifth example.
As shown in FIG. The feature of the present invention is that, as shown in FIG.
o is set and formed without any change from a certain region, and then the doping element content is changed stepwise according to a certain circularly symmetric function. Figure 6 shows
Two types of doping elements, P and B, are contained in a certain amount in a part of the core of the optical fiber, and the Ge content is changed stepwise according to a certain circularly symmetric function, so that This is an example in which only a certain area is set and formed without change.

Table 1 shows various characteristics of the optical fibers produced according to the examples, FIGS. 5, 6, and 7, and the optical fibers produced by the conventional method shown in FIG. In the table,
The frequency characteristics (MHZ.Km) of the fiber characteristics were evaluated by injecting light whose intensity was modulated at a given frequency and measuring the amount of attenuation of the frequency component. Specifically, we measured the frequency when it was attenuated by 6 dB and calculated it by converting it into length. It can be said that a fiber with a larger frequency characteristic value has a wider band. In addition, the pulse characteristics in the table are when a single pulse is input into the fiber.
The shape of the emitted pulse was measured and it was observed whether pulse cracking occurred. Communication systems using optical fibers are often based on pulse code modulation (PCM). In this case, for example, when the above-mentioned pulse cracking phenomenon occurs, a code error occurs and the quality of the signal deteriorates. Therefore, it has been an extremely important problem to stably produce fibers that are free from pulse cracks.

As shown in Examples 1, 2, 3, 4, 5, 6, and 7 in Table 1, the method according to the present invention involves changing the doping element stepwise according to a certain circular symmetry function, and then changing the doping material from a certain region. Set without
The resulting fiber has a frequency characteristic of 600
It was found that a wide band of ~1000MHZ.Km was obtained, and a fiber with no pulse cracking was obtained in terms of pulse characteristics. As shown in Comparative Examples 1, 2, 3, and 4, the fibers made using the conventional setting method were 200 to 300 MHZ.Km.
This fiber has a narrow band and pulse cracking, making it unsuitable for optical communication systems.

As described above, according to the present invention, there is an advantage that an optical fiber having a low loss and a wide band can be obtained as shown in the embodiments.

[Brief explanation of the drawing]

FIG. 1 shows a cross-sectional view of an optical fiber and the distribution of refractive index, and shows the core of one optical fiber and the cladding of two optical fibers. Figure 2 is a characteristic diagram showing the relationship between doping element content change and optical fiber core diameter according to the conventional method, and Figures 3 and 4 show the glass tube 3 as an example of the general CVD and internal soot deposition method. 5, 6, and 7 are characteristic diagrams showing the relationship between changes in doping element content and optical fiber core diameter in examples of the present invention.

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Claims (1)

[Claims] 1. Introducing a gas mixture of raw materials and doping into a rotating quartz pipe together with _O2 , causing the gas mixture to react with a heat source moving along the quartz pipe, and sequentially depositing a glass layer on the inner wall surface of the quartz pipe; The light is formed by increasing the amount of the doping material in the core part that increases the refractive index stepwise in a CVD internal sooting method in which the material is solidified and then spun, and the doping material is constantly supplied from a certain region. Method of manufacturing transmission fiber.