Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2001-08-03
2002-09-10
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S398000, C065S429000
Reexamination Certificate
active
06446469
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a single-mode optical fiber suitable for a transmission line in digital communications and a method of fabricating the same.
2. Related Background Art
Conventionally, in optical communication systems adopting a single-mode optical fiber (referred to as “SM optical fiber” hereinafter) as their transmission line, light of 1.3-&mgr;m wavelength band or 1.55-&mgr;m wavelength band has often been used. Recently, from the viewpoint of lowering transmission loss, the use of 1.55-&mgr;m wavelength band light has been increasing. Such an SM optical fiber applied to transmission lines for 1.55-&mgr;m wavelength band light (referred to as “1.55-&mgr;m SM optical fiber” hereinafter) has been designed such that its wavelength dispersion (phenomenon in which pulse wave broadens due to the fact that velocity of propagation of light varies according to wavelength) becomes zero (a dispersion-shifted fiber whose zero dispersion wavelength is set 1.55 &mgr;m). Currently, as such a dispersion-shifted fiber, optical fibers having a refractive index profile of a dual-shape type such as that disclosed in Japanese Patent Publication No. Hei 3-18161 or a segmented-core type disclosed in “Relation between Macrobending Losses and Cutoff Wavelength in Dispersion-Shifted Segmented-Core Fiber,”
Electronics Letter,
No. 22, No. 11, p. 574, 1986 have been mainly used.
Also, in recent years, as long-distance transmission has become possible because of the advent of optical amplifier, in order to prevent the four-lightwave mixing, which is one of nonlinear optical effects, there has also been used an optical fiber in which the above-mentioned refractive index profile is modified so as to shift the zero dispersion wavelength from 1.55 &mgr;m to the shorter or longer wavelength side. Here, the nonlinear optical effects refer to phenomenons in which signal light pulse is distorted in proportion to density or the like of optical intensity. They become a factor restricting the transmission rate.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a single-mode optical fiber having a configuration which realizes both reducing of dispersion slope and a sufficient mode-field diameter, and a method of fabricating the same. In order to attain such a single-mode optical fiber, the inventors have studied the conventional single-mode optical fiber as explained in the following.
Since the,conventional 1.55-&mgr;m SM optical fiber has a zero dispersion wavelength set near 1.55 &mgr;m, the wavelength dispersion thereof gradually increases as the signal light wavelength deviates farther from 1.55 &mgr;m. In particular, in the case where the absolute value of its dispersion slope (e.g., differential coefficient, at the zero dispersion wavelength, of a curve indicating dispersion with respect to the signal light wavelength shown in each of
FIGS. 13 and 14
) is large, the ratio of increase in the absolute value of wavelength dispersion unfavorably becomes higher when the zero dispersion wavelength of the SM optical fiber or the wavelength of a light source deviates from 1.55 &mgr;m. Also, in long-distance transmission, since a wavelength dispersion is intentionally generated in order to suppress the above-mentioned nonlinear optical effects, dispersion-shifted fibers in general are designed such that the zero dispersion wavelength thereof is slightly shifted from the signal light wavelength. Accordingly, in the SM optical fiber, which is a transmission line, it is necessary to lower the absolute value of dispersion slope or to effect dispersion compensation (in which signal light is made to pass through an SM optical fiber having a reverse dispersion characteristics) in the transmission line.
In wavelength-divided multiplex transmission (referred to as “WDM” hereinafter) in which a plurality of wavelengths are transmitted as being overlaid on each other in order to increase the transmission rate, since a plurality of wavelengths near the zero dispersion wavelength are used, when the dispersion slope has a large absolute value, the difference among wavelength dispersion values at respective wavelengths may increase so much that dispersion cannot be compensated for. Accordingly, it is important to lower the absolute value of dispersion slope.
In the refractive index profile of the conventional dual shape type or segmented-core type, as the absolute value of dispersion slope is made smaller, the mode-field diameter (referred to as “MFD” hereinafter) decreases or the bending loss increases, thereby making it inevitable for the lateral-pressure characteristic to deteriorate. As the connection characteristic deteriorates when the MFD is made too small, the lower limit thereof has conventionally been set. Also, in long-distance transmission using a light amplifier, nonlinear phenomenons are enhanced as MFD is lowered, thereby demanding increase in MFD. Also, deterioration in the side-pressure characteristic becomes an obstacle when SM optical fibers are formed into a cable. Accordingly, it is important to note that the reducing in absolute value of dispersion slope and the increase in MFD have a trade-off relationship therebetween.
In the refractive index profile of dual shape type or segmented-core type, the refractive index near the center of core region has a constant value or decreases toward the outside. The core of an SM optical fiber having such a refractive index profile comprises an inner core at the center portion thereof and an outer core having a lower refractive index than the inner core. In order to decrease the absolute value of dispersion slope in this refractive index profile, the following three kinds of methods have been known:
(1) Increase the outer core.
(2) Increase the ratio of inner core diameter/outer core diameter.
(3) Decrease relative refractive index difference of the inner core with respect to pure silica glass (where the outer core has a lower refractive index than that of the inner core).
Nevertheless, the method of (1) leads to decrease in MFD, whereas the methods of (2) and (3) cause bending loss (increase in transmission loss in the state where the optical fiber is bent) to increase.
The single-mode optical fiber (SM optical fiber) according to the present invention has a specific configuration in order to overcome the foregoing problems. Specifically, as shown in
FIG. 1
, an SM optical fiber
1
according to the present invention is an optical fiber mainly composed of silica glass, comprising a core region having a predetermined refractive index and a cladding portion
200
which is formed around the outer periphery of the core region and whose refractive index is set lower than that of the core region. The core region comprises a first core portion
110
whose mean relative refractive index difference with respect to the cladding portion
200
is a first value &Dgr;n
1
and whose outer diameter is a; a second core portion
120
which is a glass region whose mean relative refractive index difference with respect to the cladding portion
200
is a second value &Dgr;n
2
greater than the first value &Dgr;n
1
and whose outer diameter is b, formed around the outer periphery of the first core portion
110
; and a third core portion
130
which is a glass region whose mean relative refractive index difference with respect to the cladding portion
200
is a third value &Dgr;n
3
which is smaller than the second value &Dgr;n
2
and whose outer diameter is c, formed around the outer periphery of the second core portion
120
.
In particular, in the SM optical fiber
1
according to the present invention, the first core portion
110
and the second core portion
120
satisfy the following relationship:
a
·(&Dgr;
n
2
−&Dgr;n
1
)/(
b·&Dgr;n
2
)≧0.04 (1)
therebetween.
Here, the refractive index profile
600
of the SM optical fiber
1
does not always have a clear step-like form. In such a case, the outer diameter
a
of the first core portion
110
is defined by a diameter in the
Kato Takatoshi
Takahashi Yuji
Terasawa Yoshiaki
Hoffmann John
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
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