WDM optical communication system

Optical waveguides – With optical coupler – Particular coupling function

Reexamination Certificate

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C385S039000, C359S199200, C359S199200

Reexamination Certificate

active

06687433

ABSTRACT:

The benefit of the filing and priority dates of the International and Japanese Applications is respectfully requested.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a WDM (Wavelength Division Multiplexing) optical communication system suitable for WDM optical communications utilizing a plurality of signals included in a 1.58-&mgr;m wavelength band (1570 nm to 1620 nm).
2. Related Background Art
WDM optical communications are a communication technique enabling large-capacity optical communications by utilizing a plurality of signals having wavelengths different from each other. For the WDM optical communications, light in a 1.55-&mgr;m wavelength band (1530 nm to 1560 nm) is utilized since the transmission loss of silica-based optical fibers which have been widely utilized as transmission lines is small in the 1.55-&mgr;m wavelength band, and since the gain of Er-doped optical fiber amplifier (EDFA: Er-Doped Fiber Amplifier) for amplifying signals is high in the 1.55-&mgr;m wavelength band.
Examples of transmission lines applicable to WDM optical communications in the 1.55-&mgr;m wavelength band include a single-mode optical fiber having a zero-dispersion wavelength in a 1.3-&mgr;m wavelength band (1260 nm to 1350 nm), a dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1.55-&mgr;m wavelength band, and a hybrid transmission line in which they are mixed. Since the single-mode optical fiber has a large positive dispersion in the 1.55-&mgr;m wavelength band, the single-mode optical fiber and a dispersion-compensating optical fiber (dispersion compensator) having a large negative dispersion in the 1.55-&mgr;m wavelength band are often combined together, so as to compensate for dispersion in the 1.55-&mgr;m wavelength band.
On the other hand, it has been known that the dispersion-shifted optical fiber, in general, is likely to deteriorate the waveform of each signal due to nonlinear optical phenomena, such as four-wave mixing in particular, in optical communications in the 1.55-&mgr;m wavelength band since it has a very small absolute value of dispersion (nearly zero) in the 1.55-&mgr;m wavelength band and a small effective area. Since such a waveform deterioration caused by nonlinear phenomena cannot be restored, the occurrence of nonlinear optical phenomena must be suppressed to a minimum. For suppressing the occurrence of nonlinear optical phenomena, the power of each signal may be lowered. In the case of long-distance optical communications, however, if the signal power is lowered, then the intervals between repeaters must be shortened, whereby the cost rises along with the increase in optical amplifiers and the like which are disposed. Hence, as another effective method of suppressing nonlinear optical phenomena, optical communications may be carried out in a wavelength band, other than the 1.55-&mgr;m wavelength band, in which the absolute value of dispersion is sufficiently high to maintain effects of non-linear dispersion within tolerable limit.
On the other hand, larger capacities are demanded in optical communications. From this viewpoint, the research and development aimed at enlarging the amplification bandwidth by use of optical fiber amplifiers has been under way. Also, the research and development of optical fiber amplifiers capable of amplification in wavelength bands other than the 1.55-&mgr;m wavelength band has been under way, and an optical fiber amplifier which can amplify signals in the 1.58-&mgr;m wavelength band, for example, has been realized.
From the foregoing technical background, WDM optical communications utilizing a plurality of signals included in the 1.58-&mgr;m wavelength band in place of or in addition to the 1.55-&mgr;m wavelength band have been taken into consideration. The transmission loss of silica-based optical fibers is relatively small even in the 1.58-&mgr;m wavelength band, so that there are no inconveniences in terms of transmission loss.
As configurations of WDM optical communication system which transmit signals in the 1.58-&mgr;m wavelength band, those described in a literature—A. K. Srivastava et al., ECOC'98, postdeadline paper, pp. 73-75 (1998)—, a literature—Y. 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, have been known. Each of the transmission lines of WDM optical communication systems described in these literatures is constituted by a dispersion-shifted optical fiber alone.
SUMMARY OF THE INVENTION
The inventors have studied conventional WDM optical communication systems and, as a result, have found a problem as follows. In the dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1 .55-&mgr;m wavelength band, the absolute value of dispersion in the 1.58-&mgr;m wavelength band is about 2 to 3 ps
m/km, whereby four-wave mixing is relatively hard to occur. Employing such a dispersion-shifted optical fiber in a transmission line can increase the power of each signal, thereby making it possible to elongate repeater intervals. If each of the signals have a higher power while the number of signals (number of channels) subjected to wavelength multiplexing increases, however, then cross-phase modulation (XPM), which is another nonlinear optical phenomenon, becomes remarkable. In addition, for-wave mixing has also been in a serious problem, if a channel spacing of signals becomes lower or signal input power becomes much higher.
In order to overcome problems such as the one mentioned above, it is an object of the present invention to provide a WDM optical communication system which: effectively suppresses the waveform deterioration resulting from nonlinear optical phenomena, such as four-wave mixing and cross-phase modulation in particular, of each signal in the 1.58-&mgr;m wavelength band in a transmission line including a dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1.55-&mgr;m wavelength band.
The WDM optical communication system according to the present invention is a WDM (Wavelength Division Multiplexing) optical communication system for transmitting a plurality of signals included in the 1.58-&mgr;m wavelength band (1570 nm to 1620 nm). This WDM optical communication system comprises at least one hybrid transmission unit for transmitting the plurality of signals. This hybrid transmission unit comprises at least a single-mode optical fiber and a dispersion-shifted optical fiber, whereas these optical fibers are arranged such that signals emitted from an optical transmitter successively pass though the single-mode optical fiber and the dispersion-shifted optical fiber. For enabling bidirectional communications of signal, the hybrid transmission unit may comprise a dispersion-shifted optical fiber and two single-mode optical fibers disposed so as to sandwich the dispersion-shifted optical fiber therebetween. Namely, the hybrid transmission unit in the WDM optical communication system according to the present invention is configured such that signals pass through a single-mode optical fiber before entering the dispersion-shifted optical fiber, regardless of the traveling direction of signal.
The single-mode optical fiber has a zero-dispersion wavelength in the 1.3-&mgr;m wavelength band. (1260 nm to 1350 nm) and an effective area A
SMF
at a wavelength of 1.58 &mgr;m. The dispersion-shifted optical fiber has a zero-dispersion wavelength in the 1.55-&mgr;m wavelength band (1530 nm to 1565 nm). If the zero-dispersion wavelength of the dispersion-shifted optical fiber is set to the 1.55-&mgr;m wavelength band, then the accumulated part of dispersion in this wavelength band can be made substantially zero. Preferably, the dispersion-shifted optical fiber has a dispersion with an absolute value of 0.5 ps
m/km or more at a wavelength of 1.58 &mgr;m. It is because of the fact that dispersion is intentionally generated to a certai

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