JPS61264303A - Single mode optical fiber for 1.5mu band - Google Patents

Abstract

PURPOSE:To obtain an optical fiber which evades stress concentration and has a superior transmission band by adding an additive which compensates the difference in coefficient of linear expansion between a core and a clad to the clad together with fluorine. CONSTITUTION:A material to be added to the clad itself is selected to compensate the difference in coefficient of linear expansion between the core and clad, and thus stress concentration on the interface is reduced; and the coefficient of linear expansion of quartz glass to which fluorine is added is smaller than that of pure quartz glass. For the purpose, another additive is added to the clad together with the fluorine to approximate the coefficient of linear expansion to that of the pure quartz glass, reducing the stress concentration. Consequently, the additive which increases the coefficient of linear expansion of the clad is necessary. When the additive has such property that the refractive index is increased, the increase in refractive index is preferably small. In this case, P2O5 approximates the coefficient of linear expansion to that of the pure quartz glass without causing any deterioration in light transmission loss, so P2O5 and fluorine are added as additive to the clad.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a single mode optical fiber having a zero dispersion wavelength in the 1.5 micron band.

[Background technology and problems] Silica-based glass optical fiber has a light length of 1.5 to L, S
Transmission loss is minimum in the micron range. Therefore, in order to maximize the repeater spacing in a transmission system using optical fibers, it is necessary to use light with a wavelength of 1.5 microns.

In this case, if you want to send information at a high transmission rate, the transmission is much wider than multimode fiber.

A single mode optical fiber with a bandwidth is used. In order to transmit information at very high transmission speeds, it is necessary to minimize the dispersion effect of optical fibers at the optical wavelengths used.

The currently commonly used wavelength is 1. :A single-mode optical fiber for N-meters is designed so that material dispersion and structural dispersion cancel each other out and the dispersed phase becomes zero at a wavelength of around 1.3 microns.

On the other hand, there are two methods for using a single mode optical fiber in the 1.5 micron band and reducing dispersion at this wavelength. One is to design the optical fiber structure to have zero dispersion in the 1.5 micron band, and the other is to design the optical fiber structure to have zero dispersion in the 1.5 micron band.
The purpose of this method is to use a light source with a wavelength of 5 μ band and a very narrow spectral width.

Literature (1) Malyon+P, J+ and Mcdan
na, Ap, I02kmunrepeated mon
ollode fiber system expert
imentat 140 Mbit/s wlt
h an Injectron-locked
1.52μm 1aser translitter,
Electron, Lett, 1982. +8pp 4
'45-447. (1102k non-relay, +40Mb
lt/s single mode fiber experimental system).

As will be described in detail later, the present invention aims to obtain a single mode optical fiber having zero dispersion in the t, S micron band.

In a single mode optical fiber, the zero dispersion wavelength is set to 1.
To move from 3 microns to 5 microns, it is necessary to make the core diameter of the optical fiber thinner and increase the refractive index difference Δn between the core and the cladding, as described in the following document .

Literature (ZICOMnlL, G, LlnC,, and F
rench, W.G.

“Tailoring zero chroa+atl
c dlsperslon Int.

the 1.5-1.8μs+ Iowloss 5p
ectral regon ofslngle+mod
e f(bres), EIectron, Lett
, l979. l5pp 334-335. (Moving the atmospheric dispersion of single mode fiber to the low loss region of the 1.5 to 1.68 m wavelength band).

Increasing Δn causes a problem of transmission loss for two reasons. One is that increasing Δn increases light scattering loss by adding more germanium to the core, and the other is that increasing Δn increases the linear expansion coefficient of the core and cladding. This increases the difference, leading to increased stress at the core-cladding interface within the optical fiber, which causes increased transmission loss. This is stated in the following document (3).

Literature (3) Anslle, B. J., et al.
” Monomode fiber with ultimate
ra-1ow 1oss and minimum d
ispersionat 1.55”EIectron
, Lett, 198218 pp 842-844゜(
Single-mode fiber with ultra-low loss and minimal dispersion in the 1.55 μ-wavelength band).

One way to solve the former problem is to add germanium to the core to lower the refractive index of the cladding instead of increasing the refractive index. The most extreme case is an optical fiber whose core is made of pure quartz and whose cladding has a refractive index lower than that of quartz. This structure inherently has the potential for lower loss than germanium-filled core optical fibers, and is already well known for zero-dispersion optical fibers in the 1.3 micron band from the following document (4). is added.

Reference (4) R,C5encslts at al, “F
abrlcatlon offlow-1oss sin
gle-mode flber"In Technlc
alDlgest, Topical Nsetlog
on (lptlcal FiberCommunlc
atlon 1984. TU 13. (Structure of low loss single mode fiber).

As a method of resolving the latter stress at the interface between the core and the cladding, it has been proposed to reduce the stress concentration at the interface by making the refractive index distribution of the core triangular rather than step-like. It has zero dispersion at 55 microns and achieves a certain degree of low loss. On this point, see the reference cited above (31).

The present inventors combined and applied the above-mentioned technical methods to prototype a single mode optical fiber having a zero dispersion wavelength in the 1.5 micron band and extremely low transmission loss, as shown in FIG.

The figure shows the refractive index distribution, but the central part 1 of the core is made of pure silica glass, the cladding 2 is made of fluorine-containing silica glass, and the interface between the central part 1 of the core and the cladding 2 gradually refracts. Jacket J! consists of layer 3 with varying ratio and further has cladding 2! No. 14 with a high refractive index. Jacket J! 14 is a protective layer that is unrelated to light propagation;
Since a quartz tube is usually used, it often has the same refractive index as the core as shown in the figure, but it may also be made of glass that has the same refractive index as the cladding 2.

In an optical fiber with such a structure, the center part 1 of the core where the light energy is most concentrated is made of pure silica glass, so the scattering loss of light can be extremely small. Since stress concentration at the cladding interface can be avoided, an increase in loss due to stress at the interface can be reduced.

Note that a method of adding fluorine to lower the refractive index than that of pure silica glass is already known.

[Objective of the Invention] As explained above, in a 1.55 micron band zero dispersion optical fiber with a pure silica glass core, it is possible to avoid stress concentration at the interface between the core and the Clarad, and to create a single mode optical fiber with an excellent transmission band. Although it is possible to achieve this, the present invention attempts to obtain a single-mode optical fiber that can avoid the problem of stress concentration and has an excellent transmission band using a different method.

[Means for solving the problem] The above method gradually changes the amount of fluorine added to, for example, pure silica glass at the interface between the core and the cladding, thereby gradually lowering the linear expansion coefficient and increasing the fluorine-doped cladding. The purpose of this invention is to match the coefficient of linear expansion, suppress sudden changes in the coefficient of linear expansion, and avoid concentrated stress occurring at the interface between the core and cladding. This is intended to compensate for the difference in linear expansion coefficient of the cladding and reduce the stress concentration that occurs at the interface.

As already mentioned, in single-mode optical fibers with pure silica glass cores, fluorine-doped silica glass is usually used as the cladding. The linear expansion coefficient of fluorine-doped silica glass is smaller than that of pure silica glass. Therefore, other additives are added to the cladding at the same time as fluorine to bring the coefficient of linear expansion closer to that of pure silica glass, thereby reducing stress concentration.

For this reason, an additive that increases the coefficient of linear expansion of the cladding is required. It goes without saying that when the additive has the property of increasing the refractive index, it is preferable that the increase in the refractive index be small.

P2O5 and fluorine were added to the cladding as additives so that the addition of P2O5 would not cause deterioration of optical transmission loss and the coefficient of linear expansion would approach that of pure silica glass.

Examples will be described below.

[Example As shown in Figure 1, core diameter (a') 4.6 microns Cladding diameter (C) 125 microns Relative refractive index difference (Δn) 0.65% Fluorine concentration in cladding 2.4% by weight Cladding P of
2O5 concentration: 0.8% by weight Difference in linear expansion coefficient between core and cladding: 0.4 Average 0.32dB/km, minimum 0.29dB/km, zero dispersion wavelength 1.55±0.0
It was 5 microns.

Next, a comparative example to be compared with this example will be explained.

In this comparative example, the core is made of pure silica glass and the cladding is made of fluorine-doped silica glass.

Core diameter (^') 4.6 microns Clad diameter (C) +25. Micron relative refractive index difference (Δn) 0.85% Fluorine concentration of cladding 2.0% by weight Linear expansion coefficient of cladding 4.5XIO-' Linear expansion coefficient difference between core and cladding 1.0XIO-' Optical fiber of this comparative example 1
The average transmission loss at a wavelength of 1.55 microns was 0, 35 dB/, and the minimum was 0.
32dB/++, zero dispersion wavelength is 1.55±0.05
It was a micron.

From the differences in linear expansion coefficients and transmission losses between the core and the cladding in the Examples and Comparative Examples as described above, the effect of making the linear expansion coefficient of the cladding closer to that of the core in the present invention is fully recognized.

Ge20a is used as an additive in addition to fluorine in the cladding.
+Al2203 etc. are also possible.

Note that the following document (■) describes a 1.5 micron band zero dispersion optical fiber in which phosphorus and fluorine are added to the cladding, but this is about a germanium-doped silica glass core, and the cladding is doped with phosphorus and fluorine. The silica glass of the cladding is added with the purpose of lowering the melting temperature and making it easier to manufacture the preform.

Literature ■Pearson A, D, et al, “0p
tical trans+n1sslon Indi
spersion-shifted slngle m
IEEE
J, Light wave Tech.

1984 Mol LT-2pp 34G-349, (
Distributed mobile single mode fiber and cable optical transmission).

[Effect 1.55 micron band zero dispersion single mode light] We have described the method of reducing the transmission loss of the fiber, but this configuration can also be applied to a 1.3 micron band f dispersion single mode optical fiber with a pure silica glass core. It is. However, 1
．． In the 3-micron zero-dispersion optical fiber, Δn may be smaller than in the 1.5-micron zero-dispersion optical fiber, so the increase in transmission loss due to stress concentration at the interface between the core and the cladding is small, so the effect of the present invention is 1. It is particularly effective for .5 micron zero dispersion single mode optical fiber.

Furthermore, the present invention is particularly useful for manufacturing optical fibers by the VAD method. Depending on the VAD method, it is difficult to change the refractive index distribution of an optical fiber cut linearly or in an arbitrary shape in the radial direction, but the structure of the present invention can be easily implemented using the VAD method. .

The present invention compensates for the difference in linear expansion coefficient so that the linear expansion coefficient of the cladding is as close as possible to the linear expansion coefficient of the core, and a single mode optical fiber having excellent characteristics can be obtained.

[Brief explanation of the drawing]

FIG. 1 shows the refractive index distribution and each radical method of the single mode optical fiber of the present invention. FIG. 2 shows the previously proposed refractive index distribution and each radical method that causes less stress concentration. ^... Core center diameter, 4'... Core diameter, 6...
Interface, C... Clad diameter, 1... Core center, 1'
...Core, 2...Clad. Continued from page 41 Inventor Futoshi Mizutani 1, Taya-cho, Totsuka-ku, Yokohama

Claims (1)

[Claims] In a 1.5-micron band zero-dispersion single-mode optical fiber whose core is made of pure silica glass and whose cladding is made of fluorine-doped silica glass, the cladding is doped with fluorine and an additive that compensates for the difference in linear expansion coefficient between the core and the cladding. A single mode optical fiber for the 1.5 micron band.