Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-05-08
2002-11-26
Ngo, Hung N. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
Reexamination Certificate
active
06487353
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a single-mode optical fiber usable as a transmission line in optical communications and the like; and, in particular, to a dispersion-shifted optical fiber suitable for wavelength division multiplexing (WDM) transmission.
2. Related Background Art
In general, a WDM transmission system using optical fiber networks is a system enabling long-distance, large-capacity optical data communications, and is constituted by a transmitter/receiver for transmitting and receiving WDM signals of a plurality of wavelengths (light signals), an optical amplifier such as an optical fiber amplifier for amplifying the WDM signals, an optical fiber which is a transmission medium, and the like. In such a WDM transmission system, the wavelength band that can optically be amplified in the optical amplifier is from 1530 nm to 1560 nm, whereas the low-loss wavelength band in the optical fiber is from 1400 nm to 1700 nm. As a consequence, the wavelength band utilizable as the WDM signals in the conventional WDM transmission system has substantially been limited to a width of about 30 nm from 1530 nm to 1560 nm.
The amplification of WDM signals by the optical amplifier increases the optical power of each light signal in the optical fiber acting as the transmission medium, thereby causing nonlinear phenomena such as four-wave mixing, self phase modulation, modulation instability, and the like. In particular, the four-wave mixing causes power variations among the individual signal components, whereas the self phase modulation distorts the pulse waveform of each light signal upon an interaction with the chromatic aberration of the optical fiber (hereinafter referred to as dispersion), whereby the occurrence of such nonlinear phenomena limits the normal transmission of light signal.
SUMMARY OF THE INVENTION
The inventors have studied the case where a conventional dispersion-shifted optical fiber is employed in a WDM transmission system and, as a result, have found problems as follows.
Namely, for effectively s Oppressing the four-wave mixing, it is preferred that the wavelength of each light signal be different from the zero-dispersion wavelength of the optical fiber. For effectively suppressing the self phase modulation, on the other hand, it is preferred that the absolute value of dispersion value of the optical fiber with respect to each light signal be not so large.
When the four-wave mixing and the self phase modulation are compared with each other, the distortion in pulse waveform of each light signal caused by the self phase modulation can be alleviated to a certain extent by a dispersion-compensating technique in which a dispersion-compensating optical fiber (having a dispersion characteristic with a polarity opposite to that of the dispersion value of the optical fiber acting as the transmission medium) is inserted in the optical transmission line through which each light signal propagates, so that the dispersion value of the optical transmission line as a whole becomes nearly zero. By contrast, no technique has been known for compensating for the crosstalk between individual light signals caused by the four-wave mixing. Therefore, as compared with the self phase modulation, it is more important to suppress the four-wave mixing.
In view of the increase in noise components caused by the modulation instability, on the other hand, it is preferred that the zero-dispersion wavelength be set on the longer wavelength side from the wavelength band of each light signal. Further, letting N
2
be the nonlinear refractive index of the optical fiber, A
eff
be the effective area thereof, P be the power of light the propagating therethrough, and L
eff
be the effective length of the optical fiber, the amount of occurrence of nonlinear phenomena in the optical fiber is given by the following expression (1):
N
2
·
P·L
eff
|A
eff
· (1)
Among these parameters, the nonlinear refractive index N
2
is determined by the material of the optical fiber, whereby it is necessary for the effective area A
eff
of the optical fiber to increase in order to reduce the amount of occurrence of nonlinear phenomena.
Here, as shown in Japanese Patent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), the above-mentioned effective area A
eff
is given by the following expression (2):
A
eff
=
2
π
(
∫
0
∞
E
2
r
ⅆ
r
)
2
/
(
∫
0
∞
E
4
r
ⅆ
r
)
(
2
)
where E is the electric field accompanying the propagating light, and r is the radial distance from the center of the core region.
In the conventional WDM transmission system, as a result of the foregoing studies, it is preferred that the zero-dispersion wavelength of the optical fiber be restricted to the range of 1560 nm to 1600 nm, and that its effective area A
eff
be 50 &mgr;m
2
or more. Further, for suppressing the increase in loss upon cabling the optical fiber, it is preferred that its bending loss be smaller, whereby its cutoff wavelength must be set to an appropriate value.
In the conventional WDM transmission system, the channel spacing between the individual light signals included in the WDM signals is about 1 nm, whereby the actual multiplicity has been limited to about 30 waves. For enhancing the transmission capacity, however, it is desirable that the wavelength multiplicity be increased. In this case, while a method of narrowing the channel spacing and a method of enlarging the wavelength bandwidth can be considered, the latter is preferred in view of the above-mentioned suppression of four-wave mixing.
On the other hand, the amplification wavelength band of the optical fiber amplifier has been expanding along with the advance in technology, thereby making it possible to amplify the WDM signals in a wider wavelength band of 1530 nm to 1610 nm (see, for example, A. Mori, et al., “1.5 &mgr;m Broadband Amplification by Tellurite-Band EDFAs,” OFC '97, PD1). In contrast, the zero-dispersion wavelength of the conventional dispersion-shifted optical fiber lies within the range of 1560nm to 1600 nm as mentioned above. Therefore, in a WDM transmission system employing the optical fiber amplifier having thus expanded amplification wavelength band and the conventional dispersion-shifted optical fiber, there is a possibility that the zero-dispersion wavelength of the conventional dispersion-shifted optical fiber may lie within the wavelength band that can be amplified by the optical fiber amplifier (the signal wavelength band of WDM signals), so that the four-wave mixing may occur strongly, whereby the WDM signals may not be transmitted normally.
In order to overcome the problems such as those mentioned above, it is an object of the present invention to provide a dispersion-shifted optical fiber suitable for an optical transmission line in which an optical fiber amplifier having an expanded amplification band is installed.
The dispersion-shifted optical fiber according to the present invention is a single-mode optical fiber comprising a core region extending along a predetermined axis, and a cladding region provided on the outer periphery of the core region; wherein a zero-dispersion wavelength is set within a range of 1610 nm or more but 1750 nm or less, preferably 1610 nm or more but 1670 nm or less, so that the optical fiber is employable in a WDM transmission system including an optical amplifier having an expanded amplification wavelength band. Particularly, in order to suppress the occurrence of nonlinear phenomena across a signal wavelength band by slightly generating a dispersion, the zero-dispersion wavelength is preferably set within a range of 1640 nm or more but 1750 nm or less, further preferably 1640 nm or more but 1670 nm or less. Also, the dispersion-shifted optical fiber according to the present invention has a cutoff wavelength of 1.1 &mgr;m or more at a length of 2 m and has, with respect to light having a wavelength of 1550 nm, which is light in a signal wavelength b
Kato Takatoshi
Nishimura Masayuki
Sasaoka Eisuke
McDermott & Will & Emery
Ngo Hung N.
Sumitomo Electric Industries Ltd.
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