Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-01-21
2002-01-01
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S127000, C385S124000
Reexamination Certificate
active
06335995
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a single-mode optical fiber (hereinafter referred to as S-mode optical fiber) used for transmitting light in long-haul optical communications or the like and, in particular, to a dispersion-shifted fiber suitable for wavelength-multiplexing transmission.
2. Related Background Art
Conventionally, optical communication systems employing a S-mode optical fiber as their transmission line have often utilized light in the wavelength band of 1.3 &mgr;m or 1.55 &mgr;m as their signal light for communications. Recently, in order to reduce transmission loss in the transmission line, the light in the wavelength band of 1.55 &mgr;m has been in use more and more. The S-mode optical fiber employed in such a transmission line for light in the wavelength band of 1.55 &mgr;m (hereinafter referred to as 1.55-&mgr;m S-mode optical fiber) has been designed such that its wavelength dispersion (phenomenon in which pulse wave spreads due to the fact that velocity of propagation of light changes depending on its wavelength) is nullified (namely, to yield a dispersion-shifted fiber whose zero-dispersion wavelength is 1.55 &mgr;m). For example, as such a dispersion-shifted fiber, Japanese Patent Publication No. 3-18161 discloses a dispersion-shifted fiber having a dual-shape core type refractive index profile in which a core is constituted by an inner core layer and an outer core layer having a refractive index lower than that of the inner core layer. Further, Japanese Patent Application Laid-Open No. 63-43107 and No. 2-141704 propose a dispersion-shifted fiber having a depressed cladding/dual-shape core type refractive index profile in which, in addition to the double core structure mentioned above, a cladding is constituted by an inner cladding layer and an outer cladding layer having a refractive index higher than that of the inner cladding layer.
On the other hand, long-haul light transmission has recently become possible with the advent of wavelength division multiplex (WDM) transmission and optical amplifiers. Under such circumstances, however, influences of nonlinear optical effects cannot be neglected. Accordingly, in order to eliminate the nonlinear optical effects, it has been proposed to deform the refractive index profiles mentioned above, thereby shifting their zero-dispersion wavelength toward the shorter or longer wavelength side of their signal wavelength band (Japanese Patent Application Laid-Open No. 7-168046 and U.S. Pat. No. 5,483,612). Here, a nonlinear optical effect is a phenomenon in which a signal light pulse is distorted in proportion to density of light intensity or the like. This phenomenon becomes a factor restricting transmission speed, as well as a relay distance in a relaying transmission system.
SUMMARY OF THE INVENTION
As a result of studies concerning the above-mentioned prior art, the inventors have discovered the following problems. Namely, in the above-mentioned dispersion-shifted fibers proposed for wavelength division multiplex transmission, the zero-dispersion wavelength is set to a level different from the wavelength level of signal wavelength band so as to restrain nonlinear optical effects from occurring, while their effective core cross-sectional area A
eff
is set on the order of 55 &mgr;m
2
. Though the conventional dispersion-shifted fibers for wavelength division multiplex transmission are sufficient for the conventional applications, it may be difficult for the prior art to keep a suitable transmission quality in the conventional transmission distance in view of further advance in wavelength multiplexing which will occur as communications become more sophisticated.
Here, as disclosed in Japanese Patent Application Laid-Open No. 8-248251, effective core cross-sectional area A
eff
is given by the following expression:
A
eff
=
2
π
(
∫
0
∞
E
2
r
ⅆ
r
)
2
/
(
∫
0
∞
E
4
r
ⅆ
r
)
wherein E is an electric field accompanying propagated light, and r is a radial distance from a core center.
It is an object of the present invention to provide a dispersion-shifted fiber which can effectively restrain the nonlinear optical effects from occurring, and is suitable for long-haul light transmission.
The dispersion-shifted fiber according to the present invention is a S-mode optical fiber mainly composed of silica glass, whose zero-dispersion wavelength is shifted toward the shorter or longer wavelength side of a signal light wavelength band. The object to be transmitted through the dispersion-shifted fiber according to the present invention is at least one light component whose center wavelength is within the range of 1,500 to 1,600 nm (signal light wavelength band). In this specification, light in a 1.55-&mgr;m wavelength band equals to light in the signal light wavelength band. The dispersion-shifted fiber has a zero-dispersion wavelength out of a wavelength band of 1.53 &mgr;m (1,530 nm) to 1.56 &mgr;m (1,560 nm) and has, as various characteristics at 1,550 nm, a dispersion level of 1.0 to 4.5 ps
m/km in terms of absolute value, a dispersion slope not greater than 0.13 ps
m
2
/km in terms of absolute value, an effective core cross-sectional area A
eff
of 70 &mgr;m
2
or more, and a transmission loss not greater than 0.25 dB/km with respect to light in a wavelength band of 1.55 &mgr;m.
Here, when the dispersion level in terms of absolute value is smaller than 1.0 ps
m/km, waveform distortion caused by four-wave mixing, unstable modulation, and the like cannot practically be neglected in long-haul light transmission over 20 km or more. When the dispersion level in terms of absolute value is greater than 4.5 ps
m/km, by contrast, waveform distortion caused by wavelength dispersion and by self phase modulation cannot practically be neglected in long-haul light transmission over 20 km or more.
In the dispersion-shifted fiber according to the present invention, the absolute value of dispersion slope is not greater than 0.13 ps
m
2
/km. Accordingly, it is possible to transmit-signal lights in which the variation in the amount of waveform distortion due to the dispersion wavelength in signal lights is effectively decreased.
The amount of nonlinear optical effects generated is in proportion to nonlinear optical effect constant (N
2
/A
eff
). Accordingly, at the same propagating light condition, nonlinear optical effects are effectively restrained from occurring when the nonlinear optical effect constant (N
2
/A
eff
) is made smaller. On the other hand, since nonlinear refractive index N
2
is substantially defined by a main material of the optical fiber, it is difficult for the optical fiber made of the same main material to change the nonlinear refractive index N
2
from its conventional level so as to restrain the nonlinear optical effects from occurring.
Therefore, in the dispersion-shifted fiber according to the present invention, the effective core cross-sectional area A
eff
is increased to 70 &mgr;m
2
or greater, thereby the amount of nonlinear optical effects generated becomes smaller than that of the conventional dispersion-shifted fiber by at least 20%.
FIG. 1
is a graph showing a relationship between effective core cross-sectional area A
eff
and nonlinear optical constant (N
2
/A
eff
) in a dispersion-shifted fiber having a typical composition. From
FIG. 1
, it can be seen that nonlinear optical constant (N
2
/A
eff
), which is 5.8×10
−10
(1/W) when effective core cross-sectional area A
eff
is 55 &mgr;m
2
, becomes 4.6×10
−10
(1/W) when effective core cross-sectional area A
eff
is 70 &mgr;m
2
, thus being reduced by about 20%. Accordingly, as compared with the conventional dispersion-shifted fiber, the dispersion-shifted fiber according to the present invention can increase the degree of wavelength multiplexing in signal light.
In general, refractive index N of a medium under strong light changes depending on light intensity. Acc
Ishikawa Shinji
Kato Takatoshi
Sasaoka Eisuke
Palmer Phan T. H.
Pillsbury & Winthrop LLP
Sumitomo Electric Industries Ltd.
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