Optical communication system

Details Optical communication system Optical communication system

2001-04-13

2002-09-17

Kim, Robert H. (Department: 2882)

Optical waveguides

Optical fiber waveguide with cladding

C385S001000, C385S003000

Reexamination Certificate

active

06453103

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical communication system suitable for wavelength division multiplexing (WDM) communications utilizing a plurality of signal light components in a 1.58-&mgr;m wavelength band (1565 nm to 1620 nm).
2. Related Background Art
WDM communications area type of optical communications enabling large-capacity communications by utilizing a plurality of signal light components having wavelengths different from each other. In WDM communications, a 1.55-&mgr;m wavelength band (1530 nm to 1565 nm) has conventionally been utilized as a signal light wavelength band in the WDM communications since silica type optical fibers, which are typically used as a transmission line, have a low transmission loss in the 1.55-&mgr;m wavelength band, and since Er-doped optical fiber amplifiers (EDFA: Er-Doped Fiber Amplifier) have a large gain in the 1.55-&mgr;m wavelength band.
The optical transmission line employed in WDM communications in the 1.55-&mgr;m wavelength band can utilize not only the above-mentioned silica type optical fibers, but also single-mode optical fibers having a zero-dispersion wavelength in a 1.3-&mgr;m wavelength band (1260 nm to 1350 nm), dispersion-shifted optical fibers having a zero-dispersion wavelength in the 1.55-&mgr;m wavelength band, or hybrid transmission lines in which these optical fibers are mixed. Since the above-mentioned single-mode optical fibers have a large positive dispersion in the 1.55- &mgr;m wavelength band, dispersion-compensating optical fibers having a large negative dispersion in the 1.55-&mgr;m wavelength band are utilized as a dispersion compensator for compensating for the dispersion of the single-mode optical fibers in the 1.55-&mgr;m wavelength band.
SUMMARY OF THE INVENTION
The inventors have studied conventional optical communication systems in detail and, as a result, have found problems as follows.
Namely, since dispersion-shifted optical fibers exhibit a very small absolute value of dispersion (substantially zero) in the 1.55-&mgr;m wavelength band and have a small effective area in general, signal light waveforms are likely to deteriorate due to nonlinear optical phenomena, such as four-wave mixing in particular, in the case of WDM communications in the 1.55-&mgr;m wavelength band. Since such deterioration of signal light waveforms caused by nonlinear optical phenomena cannot be restored, it is necessary that the occurrence of such nonlinear optical phenomena be suppressed as much as possible. For suppressing the occurrence of nonlinear optical phenomena, the power of signal light maybe lowered. In this case, however, repeater intervals have to be shortened in long-distance optical communications, whereby the cost rises. Therefore, as another potent method, optical communications may be carried out in a wavelength band, different from the 1.55-&mgr;m wavelength band, where dispersion occurs to a certain extent.
On the other hand, further larger capacity is demanded in the field of optical communications. From this viewpoint of achieving a larger capacity, the research and development aimed at widening the bandwidth that can be amplified by optical fiber amplifiers or utilizing wavelength bands other than the 1.55-&mgr;m wavelength band has been under way. As optical fiber amplifiers which can amplify signal light in wavelength bands other than the 1.55-&mgr;m wavelength band, those capable of amplifying signal light in the 1.58-&mgr;m wavelength band have been realized, for example.
From the background as in the foregoing, WDM communications in the 1.58-&mgr;m wavelength band in place of or in addition to the 1.55-&mgr;m wavelength band have been considered for practical use. In this case, the transmission loss of silica type optical fibers is relatively low even in the 1.58-&mgr;m wavelength band, whereby there are no inconveniences in particular.
Configurations of optical communication systems which transmit signal light in the 1.58-&mgr;m wavelength band are described, for example, in a literature—A. K. Srivastava, et al., ECOC'98, postdeadline paper, pp. 73-75 (1998)—, a literature—Yano, et al., ECOC'98, pp. 261-262 (1998)—, a literature—T. Sakamoto, et al., OAA'98, TuB3, pp. 88-91 (1998)—, and a literature—M. Jinno, et al., IEEE Photon. Technol. Lett., Vol. 10, No. 3, pp. 454-456 (1998)—, for example. In each of the optical communication systems described in these literatures, the optical transmission line is constituted by a dispersion-shifted optical fiber alone. Since the dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1.55-&mgr;m wavelength band has a dispersion with an absolute value of about 2 to 3 ps
m/km in the 1.58 -&mgr;m wavelength band, four-wave mixing is relatively hard to occur therein.
On the other hand, dispersion-shifted optical fibers have a transmission loss slightly higher than that of optical fibers yielding a low transmission loss, such as single-mode optical fibers having a zero-dispersion wavelength near a wavelength of 1.3 &mgr;m. Therefore, if such a low-loss optical fiber is employed in a part behind a dispersion-shifted optical fiber, then total loss can be lowered. However, the low-loss optical fibers have a dispersion with a large absolute value in the 1.58-&mgr;m wavelength band in general. In this case, if a system is constructed carelessly, then it may not function as an optical communication system.
For solving the problems mentioned above, it is an object of the present invention to provide an optical communication system comprising a structure for effectively restraining signal light waveforms from deteriorating due to the occurrence of nonlinear optical phenomena, such as cross-phase modulation in particular, in WDM communications in the 1.58-&mgr;m wavelength band even when it is an optical communication system including a dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1.55-&mgr;m wavelength band.
The optical communication system according to the present invention comprises at least one hybrid transmission unit. As a first configuration, this hybrid transmission unit has a dispersion-shifted optical fiber and a first high-dispersion optical fiber, whereas the dispersion-shifted optical fiber is disposed upstream the first high-dispersion optical fiber such that a WDM signal successively propagates through the dispersion-shifted optical fiber and the first high-dispersion optical fiber. The dispersion-shifted optical fiber is an optical fiber with a length L
DSF
having a zero-dispersion wavelength in the 1.55-&mgr;m wavelength band (1530 nm to 1565 nm) and exhibiting, at a wavelength of 1.58 &mgr;m, a dispersion D
DSF
with an absolute value of 0.5 ps
m/km or more. The first high-dispersion optical fiber is an optical fiber with a length L
1
exhibiting, at a wavelength of 1.58 &mgr;m, a dispersion D
1
having an absolute value greater than that of the dispersion D
DSF
of the dispersion-shifted optical fiber. Here, the dispersion-shifted optical fiber and the first high-dispersion optical fiber may be disposed such that a repeater including a coupler and an optical amplifier, for example, is interposed therebetween.
In particular, with respect to at least signal light having the shortest wavelength in signal light having a bit rate B included in a signal light wavelength band in which wavelength ranges from 1.565 &mgr;m to 1.610 &mgr;m, the hybrid transmission unit of the first configuration comprising the dispersion-shifted optical fiber and first high-dispersion optical fiber satisfies the following condition:
&Dgr;&phgr;
XPM
·D
T
≦18000 (unit:(
ps
m
)·(
Gb/s
)
2
)
D
T
=(
D
DSF
·L
DSF
+D
1
·L
1
)·
B
2
where &Dgr;&phgr;
XPM
is the total phase shift amount of cross-phase modulation in the signal light having the shortest wavelength under the influence of signal light having the other wavelengths, and DT is the total dispersion in the hybrid transmission unit.
On the other hand, the optical communication system accordi

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