Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
1998-02-11
2001-01-30
Lee, John D. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S124000, C385S127000
Reexamination Certificate
active
06181858
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a single-mode optical fiber applied to a transmission line for optical communications or the like and, in particular, to a dispersion-shifted fiber suitable for wavelength division multiplexing (WDM) transmission.
2. Related Background Art
Conventionally, in optical communication systems employing single-mode optical fibers as their transmission lines, light in the wavelength band of 1.3 &mgr;m or 1.55 &mgr;m has often been utilized as signal light for communications. Recently, from the viewpoint of reducing transmission loss in transmission lines, the light in the 1.55-&mgr;m wavelength band has been in use more and more. The single-modeoptical fiber applied to such a transmission line for light in the wavelength band of 1.55 &mgr;m (hereinafter referred to as 1.55-&mgr;m single-mode optical fiber) is designed so as to nullify its wavelength dispersion (phenomenon in which pulse wave broadens because the propagating speed of light varies depending on its wavelength) for light in the wavelength band of 1.55 &mgr;m (thus yielding a dispersion-shifted fiber having a zero-dispersion wavelength of 1.55 &mgr;m). As such a dispersion-shifted fiber, for example,
Japanese Patent Publication No. 3-18161 discloses a dispersion-shifted fiber having a refractive index profile of a dual-shape-core structure, whose core region is constituted by an inner core and an outer core having a refractive index lower than that of the inner core. Also, Japanese Patent Application Laid-Open No. 63-43107 and Japanese Patent Application Laid-Open No. 2-141704 disclose a dispersion-shifted fiber having a refractive index profile of a depressed cladding/dual-shape-core structure, whose cladding region is constituted by an inner cladding and an outer cladding having a refractive index greater than that of the inner cladding. Further, V. A. Bhagavatula et al., OFC' 95 Technical Digest, Paper ThH1, 1995, and P. Nouchi et al., ECOC' 96, Paper MoB.3.2, 1996 disclose a dispersion-shifted fiber having a refractive index profile of a ring-shaped core structure.
Recently, on the other hand, since long-haul transmission has become possible with the advent of wavelength division multiplexing (WDM) transmission and optical amplifiers, there have been proposed, in order to avoid nonlinear optical effects, dispersion-shifted fibers employing a refractive index profile of the above-mentioned dual-shape-core structure or depressed cladding/dual-shape-core structure, whose zero-dispersion wavelength is shifted to the shorter wavelength side or longer wavelength side than the center wavelength of signal light (Japanese Patent Application Laid-Open No. 7-168046 and U.S. Pat. No. 5,483,612). Here, the nonlinear optical effects refer to phenomena in which signal light pulses are distorted in proportion to density of light intensity or the like due to nonlinear phenomena such as four-wavemixing (FWM), self-phase modulation (SPM), cross-phase modulation (XPM), or the like. Transmission speed and relaying intervals in repeating transmission systems are restricted by the nonlinear optical effects.
Japanese Patent Application Laid-Open No. 8-248251 proposes an optical fiber having a configuration which suppresses the occurrence of the above-mentioned nonlinear optical phenomena, which may be generated when light having a high power is incident on the optical fiber, thereby reducing the distortion in optical signals caused by these nonlinear optical phenomena. Such an optical fiber has a refractive index profile whose effective core cross-sectional area A
eff
is designed to be greater than about 70 &mgr;m
2
.
Here, as disclosed in Japanese Patent Application Laid-Open No. 8-248251, the effective core cross-sectional area A
eff
is given by the following expression (1):
A
eff
=
2
π
(
∫
0
∞
E
2
r
ⅆ
r
)
2
/
(
∫
0
∞
E
4
r
ⅆ
r
)
(
1
)
wherein E is an electric field accompanying propagated light, and r is a radial distance from the core center.
On the other hand, dispersion slope is defined by the gradient of a graph indicating a dispersion characteristic in a predetermined wavelength band.
SUMMARY OF THE INVENTION
Having studied the foregoing prior art, the inventors have found the following problems.
In general, while the dispersion slope increases as the effective core cross-sectional area A
eff
is greater, no consideration has been made in the conventionally proposed dispersion-shifted fibers so as to optimize their dispersion slope value, which relates to the occurrences of distortion in signal light waveform due to dispersion and nonlinear optical effects, from the viewpoint of reducing distortion in the whole waveform.
Accordingly, in view of future advances in wavelength division multiplexing accompanying more sophisticated communications, expected is a situation where it is difficult to keep a transmission quality by simply employing a conventional dispersion-shifted fiber.
In order to overcome the problems such as those mentioned above, it is an object of the present invention to provide a dispersion-shifted fiber for WDM transmission, suitable for long-haul submarine cables or the like, which has a structure for effectively restraining the nonlinear optical phenomena from occurring.
The dispersion-shifted fiber according to the present invention is a single-mode optical fiber for propagating signal light in a 1.55 &mgr;m wavelength band (namely, a wavelength in the range of 1,500 nm to 1,600 nm) comprising a core region extending along a predetermined reference axis and a cladding region disposed around the outer periphery of the core region. This dispersion-shifted fiber has a zero-dispersion wavelength shifted to a shorter wavelength side or longer wavelength side from the center wavelength (1,550 nm) of the 1.55-&mgr;m wavelength band.
In particular, as characteristics at the center wavelength (1,550 nm) of the 1.55-&mgr;m wavelength band, the dispersion-shifted fiber according to the present invention has, at least, a dispersion whose absolute value is 1.0 to 4.5 ps
m/km, a dispersion slope of 0.05 to 0.09 ps
m
2
/km, an effective core cross-sectional area of at least 70 &mgr;m
2
, and a cutoff wavelength of at least 1,300 nm at a fiber length of 2 m.
In general, at a time of wavelength division multiplexing transmission, if the dispersion slope is small, a four-wave mixing which greatly distorts the waveform of a signal light is apt to occur. When the dispersion slope is large, on the other hand, the waveform of signal light is greatly distorted due to the synergistic effect of accumulated dispersion and self-phase modulation.
As a result of studies, the inventors have found that, in the case where, at a wavelength of 1,550 nm, the absolute value of dispersion is1.0 to4.5 ps
m/km and the effective core cross-sectional area is 70 &mgr;m
2
or greater, the total amount of distortion in signal light wave form can be reduced in a long-haul transmission if the dispersion slope is 0.05 to 0.09 ps
m
2
/km. Here, the total amount of distortion refers to the sum of the distortion in signal light waveform caused by the four-wave mixings and the distortions in signal light waveform caused by the synergistic effect of accumulated dispersion and self-phase modulation. Thus, the dispersion-shifted fiber according to the present invention can restrain the distortion from occurring due to the nonlinear optical effects, thereby allowing high-quality signal transmission to be realized.
Further, in the dispersion-shifted fiber according to the present invention, the core region is constituted by an inner core having a first refractive index, and an outer core disposed around the outer periphery of the inner core and having a second refractive index higher than the first refractive index; whereas a cladding region having a refractive index lower than the second refractive index is disposed around the outer periphery of the outer core. It
Kato Takatoshi
Okuno Toshiaki
Sasaoka Eisuke
Kang Juliana K.
Lee John D.
Pillsbury Madison & Sutro LLP
Sumitomo Electric Industries Ltd.
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