Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-03-21
2004-02-17
Snow, Walter E. (Department: 2862)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000
Reexamination Certificate
active
06694079
ABSTRACT:
TECHNICAL FIELD
This invention relates to dispersion-shifted optical fiber having large effective core area and low dispersion slope.
The present invention is based on patent applications filed in Japan (Japanese Patent Application No. H11-212949/1999, Japanese Patent Application No. H11-230137/1999, Japanese Patent Application No. 2000-64008, Japanese Patent Application No. 2000-224491, and Japanese Patent Application No. 2000-224492), and the particulars described in those Japanese patent applications are incorporated as part of this specification.
BACKGROUND ART
In a long-haul system such as an optical amplifier repeater transmission system employing optical fiber amplifiers, it is important that the nonlinear optical effects be reduced. A parameter called the nonlinear optical coefficient is a parameter that serves as an index to the degree of nonlinear optical effect. The nonlinear optical coefficient is expressed by n
2
/Aeff, where n
2
is the nonlinear refractive index and Aeff is the effective core area. The value of n
2
becomes roughly constant depending on the material, and expanding the Aeff is an effective technique for reducing nonlinear optical effects.
In wavelength division multiplexed transmission systems which can handle high-volume transmission, on the other hand, there is a need to suppress the chromatic dispersion value and reduce dispersion slope. It is well known that, in a wavelength division multiplexed transmission system, when a zero-dispersion wavelength exists in the transmission bandwidth, transmission quality declines due to a nonlinear optical effect called four-wave mixing. However, because large chromatic dispersion values are accompanied by signal waveform deterioration, it is necessary to suppress that value to a certain size. In order to satisfy these conflicting demands, optical fiber called non-zero dispersion-shifted optical fiber in which the chromatic dispersion value in the wavelength band used is controlled to within a narrow range has been developed.
In a wavelength division multiplexed transmission system, furthermore, reducing the dispersion slope is also important. By dispersion slope, which indicates the wavelength dependency of the chromatic dispersion value, is meant the slope of the curve obtained by plotting wavelength (nm) on the horizontal axis and chromatic dispersion value (ps/km·nm) on the vertical axis. In a wavelength division multiplexed transmission system, if the dispersion slope of the transmission line (optical fiber) is large, the difference in chromatic dispersion value between wavelengths will be great. For that reason, by taking a very large dispersion value, depending on the wavelength, difficulties are encountered such as the transmission quality being greatly different between different channels. Accordingly, there is a need to make the dispersion slope smaller.
The specific values for the characteristics demanded in the Aeff and dispersion discussed above will be different according to the system employed. In a system in which transmissions are made over very long haul, such as submarine systems, a reduction in nonlinear optical effect resulting from Aeff expansion is sought. In a system extending for from several tens to several hundreds of km, on the other hand, there is sometimes a need to suppress the dispersion value in a wide wavelength band by dispersion slope reduction. In terms of the minimum conditions demanded for the transmission line in a light communication system, furthermore, the optical fiber should be substantially single-mode, and the bending loss should be held down to 100 dB/m or lower.
That being so, proposals have recently been made on ways to effect some degree of Aeff expansion and dispersion slope decrease using various refractive index distribution shapes (refractive index profiles), as in Japanese Patent Application Laid-Open No. H10-62640/1998, Japanese Patent Application Laid-Open No. H10-293225/1998, Japanese Patent Application Laid-Open No. H8-220362/1996, and Japanese Patent Application Laid-Open No. H10-246830/1998, for example.
In
FIGS. 10A
to
10
C are diagrammed examples of shapes of refractive index distribution of such dispersion-shifted optical fiber.
In
FIG. 10A
is represented one example of a dual shape core (step) type of refractive index distribution, in a core
14
is formed, with the symbol
11
designating the center core portion and a step core portion
12
provided about the outer circumference thereof having a lower refractive index than the center core portion
11
. Furthermore, about the outer circumference of that core
14
, clad
17
is provided having a lower refractive index than the step core portion
12
.
In Japanese Patent Application Laid-Open No. H8-220362/1996, the present applicant disclosed the use of the smaller diameter solution, for the purpose of expansion of Aeff, in dispersion-shifted optical fiber having a dual shape core type refractive index distribution.
It has been known for some time that, when the core diameter of a dispersion-shifted optical fiber is expanded while maintaining the similarity of refractive index distribution shape, two or more solutions exist wherewith the chromatic dispersion value becomes the desired value. At such time, of the solutions within a range wherein the bending loss and cutoff wavelength characteristics are comparatively practical, the solution wherewith the core diameter is relatively thin is called the smaller diameter solution, and the solution wherewith the core diameter is relatively large is called the larger diameter solution.
In
FIG. 10B
is represented an example of a segmented core type of refractive index distribution shape, wherein a core
24
is configured with an intermediate portion
22
of low refractive index provided about the outer circumference of the center core portion
21
of high refractive index, and a ring core portion
23
having a higher refractive index than the intermediate portion
22
but a lower refractive index than the center core portion
21
provided about the outer circumference of that intermediate part
22
. Also, about the outer circumference of that ring core portion
23
is provided a first clad
25
having a lower refractive index than the intermediate portion
22
, and about the outer circumference of that first clad
25
is provided a second clad
26
having a higher refractive index than the first clad
25
but a lower refractive index than the intermediate portion
22
, thus configuring clad
27
.
In Japanese Patent Application Laid-Open No. H11-119045/1999 (published), furthermore, the present applicant disclosed a dispersion-shifted optical fiber well suited to optical communication systems wherein the reduction of the dispersion slope is more rigorously demanded than the expansion of the Aeff, by using the larger diameter solution in a segmented core type of refractive index distribution shape.
In
FIG. 10C
is represented an example of an O ring type refractive index distribution shape, wherein a core
34
is configured with a two-layer structure, with a peripheral core portion
32
of high refractive index provided about the outer circumference of a center core portion
31
of low refractive index at the center. About the outer circumference of that core
34
is provided clad
37
of lower refractive index than the peripheral core portion
32
, thereby configuring a three-layer structure convex type refractive index distribution shape inclusive of the clad
37
.
In the dispersion-shifted optical fibers conventionally proposed, however, under such conditions as that they are substantially single-mode and that the bending loss is held down to 100 dB/m or lower, it is very difficult to sufficiently realize both Aeff expansion and dispersion slope reduction simultaneously.
Looking at the dual shape core type of optical fiber wherein the smaller diameter solution is used disclosed in Japanese Patent Application Laid-Open No. H8-220362/1996, for example, the dispersion slope is in the neighborhood of 0.10 ps/km
m
2
at minimum, wherefore this op
Kutami Hiroshi
Matsuo Shoichiro
Tanigawa Shoji
Fujikura Ltd.
Snow Walter E.
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