Optical waveguides – Optical fiber waveguide with cladding – With graded index core or cladding
Reexamination Certificate
2000-12-13
2003-12-02
Kim, Robert H. (Department: 2871)
Optical waveguides
Optical fiber waveguide with cladding
With graded index core or cladding
C123S184550, C123S184550
Reexamination Certificate
active
06658190
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber applicable to an optical transmission line for optical communication systems.
2. Background of the Invention
Single-mode optical fibers have conventionally been used as optical transmission lines in optical communications. Such single-mode optical fibers have a zero-dispersion wavelength in the vicinity of a wavelength of 1.3 &mgr;m, a positive dispersion slope in the 1.55-&mgr;m wavelength band, and a dispersion of about 18 ps
m/km at a wavelength of 1.55 &mgr;m.
Single-mode optical fibers having optical characteristics such as those mentioned above are defined in G652 and G654 standards of ITU-T, and have a simple refractive index profile composed of a core and a cladding. The 1.55-&mgr;m wavelength band (1500 nm to 1600 nm) is applied to a signal wavelength band since silica glass, which is the main ingredient of optical fibers, has a low attenuation in this wavelength band. On the other hand, as mentioned above, a single-mode optical fiber has a positive dispersion in the 1.55-&mgr;m wavelength band. Hence, in order to compensate for this positive dispersion, an example constructing an optical communication system by combining a dispersion-compensating optical fiber having a negative dispersion with a large absolute value in the 1.55-&mgr;m wavelength band and the single-mode optical fiber is reported in M. Murakami, et al., EOCC'98, pp. 313-314 (1998), for instance.
SUMMARY OF THE INVENTION
The inventors have studied conventional optical fibers and, as a result, have found a problem as follows. Namely, the single-mode optical fibers defined in the above-mentioned G652 and G654 standards have an effective area which is greater than that of dispersion-compensating optical fibers and the like, and is about 80 &mgr;m
2
at 1550 nm. Therefore, the single-mode optical fibers are relatively effective in reducing nonlinear optical phenomena.
Meanwhile, for elongating repeater intervals in an optical communication system, optical signals incident thereon are required to increase their power. Here, optical fibers utilized in optical transmission lines between repeaters must further increase their effective area, so as to fully restrain nonlinear optical phenomena from occurring even when optical signals having a high power propagate through the optical fibers.
However, the optical fibers defined in G652 and G654 standards cannot fully suppress the occurrence of nonlinear optical phenomena. Therefore, it has been difficult to carry out optical communications over a longer distance by utilizing the conventional optical fibers.
For overcoming the problem such as that mentioned above, it is an object of the present invention to provide an optical fiber comprising a structure suitable for long-distance optical communications, and an optical communication system including the same.
The optical fiber in accordance with the present invention is an optical waveguide which is mainly composed of silica glass and is disposed in at least one of areas between an optical transmitter for outputting an optical signal and an optical receiver for receiving the optical signal, between the optical transmitter and a repeater including an optical amplifier or the like, between repeaters, and between a repeater and the optical receiver. Applicable to this optical fiber is any of an optical fiber having a matched type refractive index profile obtained when the cladding region surrounding the outer periphery of the core region is constituted by a single layer, and an optical fiber having a depressed cladding type refractive index profile obtained when the cladding region is constituted by at least an inner cladding in contact with the core region and an outer cladding having a refractive index higher than that of the inner cladding.
This optical fiber has, as characteristics at a wavelength of 1.55 &mgr;m (1550 nm), an effective area of at least 110 &mgr;m
2
, a dispersion of 18 to 23 ps
m/km, and a dispersion slope of 0.058 to 0.066 ps
m
2
/km, whether it has the above-mentioned matched type refractive index profile or depressed cladding type refractive index profile.
In particular, it is preferred in the optical fiber having a matched type refractive index profile that the relative refractive index difference of the core region with respect to the cladding region be +0.15% to+0.30%. Obtained in this case is an optical fiber having a cutoff wavelength of 1.3 &mgr;m to 1.75 &mgr;m, and an effective area of at least 110 &mgr;m
2
at a wavelength of 1.55 &mgr;m.
On the other hand, the optical fiber having a depressed cladding type refractive index profile comprises a core region, an inner cladding region disposed at the outer periphery of the core region, and an outer cladding disposed so as to surround the outer periphery of the inner cladding, and has an effective area of at least 110 &mgr;m
2
at a wavelength of 1.55 &mgr;m. Here, the inner cladding and outer cladding constitute a cladding region surrounding the outer periphery of the core region, the inner cladding has a refractive index lower than that of the core region, and the outer cladding has a refractive index higher than that of the inner cladding.
In any of the optical fibers having the above-mentioned refractive index profiles, the effective area is preferably at least 120 &mgr;m
2
, more preferably 150 &mgr;m
2
, at a wavelength of 1.55 &mgr;m. Enlarging the effective area as such effectively restrains nonlinear optical phenomena from occurring even when the power of incident optical signal (1.55-&mgr;m wavelength band) is enhanced, thereby enabling optical communications over a longer distance.
Preferably, this optical fiber has a transmission loss of 0.30 dB/km or less at a wavelength of 1.38 &mgr;m (1380 nm). Further preferably, the cutoff wavelength (the cutoff wavelength of LP
11
mode measured in a state where an optical fiber having a length of 2 m is loosely wound about a mandrel having a radius of 140 mm by one turn) is 1.3 &mgr;m to 1.75 &mgr;m. In this case, a single mode is assured, in a cable having over 1 km length, in the 1.55-&mgr;m wavelength band, and also the bending loss is restrained from increasing (which is advantageous for cabling). For realizing long-distance optical communications, it is preferred that the transmission loss at a wavelength of 1.55 &mgr;m be 0.180 dB/km or less at most.
For satisfying the condition concerning cutoff wavelength mentioned above, the core region preferably has an outside diameter of 11.5 &mgr;m to 23.0 &mgr;m. If the outside diameter (fiber diameter) of the cladding region is set to 130 &mgr;m to 200 &mgr;m, then microbend loss can be reduced, and the probability of breakage can be lowered.
In the optical fiber having a depressed cladding type refractive index profile, the ratio
2
b/
2
a
of the outside diameter
2
b
of the inner cladding to the outside diameter
2
a
of the core region is preferably 1.1 to 7. This is because of the fact that the cutoff wavelength can be shortened without increasing the bending loss and that the effective area can be enlarged while in a state where the single mode is assured in the 1.55-&mgr;m wavelength band even if the outside diameter of the core region is enlarged. Preferably, the refractive index differences of the core region and inner cladding with respect to the outer cladding are +0.15% to +0.50% and −0.15% to −0.01%, respectively. Under such a condition, an optical fiber having a cutoff wavelength of 1.3 &mgr;m to 1.75 &mgr;m and an effective area of at least 110 &mgr;m
2
at a wavelength of 1.55 &mgr;m is obtained.
Preferably, in the optical fiber in accordance with the present invention, the core region is made of silica glass which is not intentionally doped with impurities (hereinafter referred to as pure silica glass), whereas the cladding region (composed of the inner and outer claddings in the case of the optical fiber having a depressed cladding type refractive index profile) is made of silica
Hirano Masaaki
Kato Takatoshi
Kim Robert H.
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
Wang George Y.
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