Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
1998-01-15
2003-03-18
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S126000, C065S413000, C065S414000, C065S415000
Reexamination Certificate
active
06535679
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber which can be applied to a long-haul large-capacity optical communication system and, more particularly, to a dispersion-shifted fiber which is suitable for wavelength division multiplexing (WDM) communication and whose zero-dispersion wavelength is set within a desired range.
2. Related Background Art
A conventional optical communication system to which a single-mode optical fiber is applied as a transmission line often uses light in a 1.3- or 1.55-&mgr;m wavelength band as communication signal light. Recently, however, use of light in a 1.55-&mgr;m wavelength band increases from the viewpoint of a reduction in transmission loss in the transmission line. In particular, for a single-mode optical fiber (to be referred to as a 1.55-&mgr;m single-mode optical fiber hereinafter) applied to a transmission line for light in the 1.55-&mgr;m wavelength band, since the transmission loss in a silica-based single-mode optical fiber is minimized for light in the 1.55-&mgr;m wavelength band, the wavelength dispersion (phenomenon that a pulse wave spreads due to the light propagation speed difference depending on wavelengths) is also designed to be zero for light in the 1.55-&mgr;m wavelength band. Such a 1.55-&mgr;m single-mode optical fiber whose zero-dispersion wavelength is shifted near the 1.55-&mgr;m wavelength band is generally called a dispersion-shifted fiber.
As a conventional dispersion-shifted fiber, the sectional structure and composition of a dispersion-shifted fiber whose zero-dispersion wavelength is shifted near 1.55 &mgr;m, and a method of manufacturing the same are disclosed in, e.g., Japanese Patent No. 2533083 (first prior art). The dispersion-shifted fiber of the first prior art has an inner core made of GeO
2
—SiO
2
(SiO
2
containing germanium dioxide), an outer core made of SiO
2
, and a cladding made of F—SiO
2
(SiO
2
containing fluorine). The refractive index profile of the dispersion-shifted fiber of the first prior art is a so-called matched type profile which has no depressed part in a portion corresponding to the cladding. In this specification, an optical fiber having this matched type profile will be referred to as a matched fiber. On the other hand, a refractive index profile having a depressed part in a portion corresponding to the cladding is called a depressed cladding type profile. In this specification, an optical fiber having this depressed cladding type profile will be particularly referred to as a depressed fiber. In the structure of the dispersion-shifted fiber of the first prior art, only setting of the zero-dispersion wavelength near 1.55 &mgr;m can be realized.
Japanese Patent Laid-Open No. 63-208005 (second prior art) discloses a dispersion-shifted fiber having a depressed cladding type profile, which has, around a core, a first cladding having a refractive index lower than that of the core, and, around the first cladding, a second cladding having a refractive index higher than that of the first cladding. The object of the dispersion-shifted fiber of the second prior art is to suppress wavelength dispersion over a wide wavelength band of 1.3 to 1.5 &mgr;m.
SUMMARY OF THE INVENTION
In recent years, extensive studies of construction of high-speed large-capacity transmission systems have been made, and particularly, transmission systems employing wavelength division multiplexing (WDM) have received a great deal of attention. In this scheme, a plurality of signal light components having different wavelengths are simultaneously transmitted through one transmission line, so the quantity of data which can be transmitted largely increases.
To realize such a transmission system, various new specifications are required of an optical fiber to be used as a transmission line. The above-mentioned conventional dispersion-shifted fiber cannot cope with the requirements anymore.
The present inventors have examined the conventional dispersion-shifted fiber and found the following problems. The mode field diameter (MFD) of the conventional dispersion-shifted fiber is about 8 &mgr;m. When the power of signal light increases, the influence of nonlinear optical effects tends to be generated. In addition, a variation in wavelength dispersion among various dispersion-shifted fibers applied to a transmission system is large. For this reason, when the signal light wavelength matches the zero-dispersion wavelength of the optical fiber, noise tends to be generated due to four-wave mixing as a nonlinear optical effect.
The nonlinear optical effect is a phenomenon that a signal light pulse is distorted as, e.g., the density of light intensity increases, and this is a major factor for restricting the transmission rate.
When, e.g., fluorine is added to adjust the refractive index of silica glass as a major component of the optical fiber, bubbles may be formed in the preform, or the preform itself may deform in sintering (making the preform transparent) the porous preform of the optical fiber. Flaws formed on the transparent glass surface (preform surface) due to the added impurity may break the optical fiber at the time of drawing.
The present invention has been made to solve the above problems, and has as its object to provide an optical fiber having a large MFD and a structure for effectively suppressing the influence of nonlinear optical effects, and a method of manufacturing the same which effectively prevents bubble occurrence in a transparent preform, deformation of the preform, and flaws on the preform surface during the manufacture of the optical fiber (drawing process).
An optical fiber according to the present invention is a dispersion-shifted fiber whose MFD is 8.6 &mgr;m or more, and preferably, 9 &mgr;m or more, and whose zero-dispersion wavelength is shifted to the long or short wavelength side of 1.55 &mgr;m, i.e., a typical signal light wavelength. The optical fiber is a single-mode optical fiber mainly containing silica glass. In this dispersion-shifted fiber, the zero-dispersion wavelength is shifted from the signal light wavelength by a predetermined amount to intentionally generate wavelength dispersion and suppress the influence of nonlinear optical effects. Therefore, a transmission system which allows variations in zero-dispersion wavelength among dispersion-shifted fibers can be constructed.
According to the first embodiment according to the present invention, there is provided an optical fiber comprising a first core
10
(inner core) having a first refractive index n
1
, a second core
20
(outer core) provided around an outer periphery of the inner core
10
and having a second refractive index n
2
lower than the first refractive index n
1
, a first cladding
30
(inner cladding) provided around an outer periphery of the outer core
20
and having a third refractive index n
3
lower than the second refractive index n
2
, and a second cladding
40
(outer cladding) provided around an outer periphery of the inner cladding
30
and having a fourth refractive index n
4
higher than the third refractive index n
3
and lower than the second refractive index n
2
, as shown in FIG.
1
.
In particular, an optical fiber
1
according to the first embodiment has a depressed cladding type profile, as is apparent from the above-described structure. The outer core
20
has an outer diameter of 25 to 40 &mgr;m.
This depressed cladding type profile can be realized when the following basic composition is employed: the inner core
10
is made of silica glass containing at least germanium dioxide as an index increaser (GeO
2
—SiO
2
); the outer core
20
, silica glass essentially containing no germanium dioxide (SiO
2
) or silica glass containing germanium dioxide (GeO
2
—SiO
2
); the inner cladding
30
, silica glass containing at least fluorine (index reducer) (F—SiO
2
); and the outer cladding
40
, silica glass containing at least fluorine (F—SiO
2
). When the sectional area (plane perpendicular to the signal light propagation direction) of the outer core
2
Danzuka Toshio
Urano Akira
Yokoyama Yoshio
Bovernick Rodney
Kang Juliana K.
Sumitomo Electric Industries Ltd.
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