Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-11-27
2003-09-09
Nguyen, Khiem (Department: 2839)
Optical waveguides
Optical fiber waveguide with cladding
Reexamination Certificate
active
06618532
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmission line through which a plurality of wavelengths of optical signals propagate in a transmission system utilizing a wavelength division multiplexing (WDM) technique.
2. Related Background Art
WDM transmission systems enable high-speed, large-capacity optical communications by transmitting a plurality of wavelengths of optical signals (WDM signals). Silica type optical fibers utilized as optical transmission lines in such WDM transmission systems have been known to lower their transmission loss near a wavelength of 1.55 &mgr;m (1550 nm). Also, since optical amplifiers amplifying optical signals in a wavelength band of 1.55 &mgr;m have been in actual use, optical signals in the 1.55-&mgr;m wavelength band are used in general.
In optical transmission lines, if chromatic dispersion occurs in a wavelength band in use (e.g., 1.55-&mgr;m wavelength band), then signal waveforms are deformed, whereby transmission characteristics may deteriorate. Therefore, from the view point of preventing signal waveforms from being deformed, it is desirable that the absolute value of chromatic dispersion in the wavelength band in use be smaller. If the absolute value of chromatic dispersion in the wavelength band in use is not greater than a predetermined value, on the other hand, then four-wave mixing, which is a kind of nonlinear optical phenomena, is likely to occur. The occurrence of four-wave mixing generates cross talk and noise, thereby eventually deteriorating transmission characteristics. For suppressing the occurrence of four-wave mixing, the repeater intervals for arranging optical amplifiers may be shortened, and the optical power of optical signals emitted from the optical amplifiers may be lowered. However, for realizing this, it is necessary to install a large number of optical amplifiers, which lowers the cost performance of the whole system.
For dealing with problems such as those mentioned above, U.S. Pat. No. 5,894,537 discloses an optical transmission line in which respective sections having positive and negative chromatic dispersions at a predetermined wavelength in the wavelength band in use are alternately disposed along its longitudinal direction. When such an optical transmission line is used, transmission characteristics can be restrained from deteriorating due to chromatic dispersion if the average chromatic dispersion observed as the whole system is set such that its absolute value does not exceed a predetermined value. It has also been presumed that the deterioration of transmission characteristics caused by nonlinear optical phenomena such as four-wave mixing can be suppressed if the absolute value of chromatic dispersion is set to a predetermined value or higher in most of the sections of the optical transmission line.
Also, a reference—Y. Kubo, et al., “Dispersion Flattened Single-Mode Fiber for 10,000 km Transmission System,”ECOC′90 (1990)—which will herein after be referred to as Kubo reference, describes an optical transmission line including respective sections in which both chromatic dispersion and dispersion slope at a predetermined wavelength in the wavelength band in use are positive and negative. When such an optical transmission line is employed, the average chromatic dispersion observed as the whole system can be set such that its absolute value does not exceed a predetermined value in a wide wavelength band. As a result, the deterioration in transmission characteristics caused by chromatic dispersion is suppressed over this wide wavelength band.
SUMMARY OF THE INVENTION
The inventors have studied the above-mentioned conventional techniques and, as a result, have found problems as follows. The optical transmission line disclosed in the above-mentioned U.S. Pat. No. 5,894,537 can set the average chromatic dispersion observed as the whole system, such that its absolute value does not exceed a predetermined value at a predetermined wavelength in a wavelength band in use. At other wavelengths in the wavelength band in use, which are different from the above-mentioned predetermined wavelength, however, the absolute value of average chromatic dispersion observed as the whole system becomes greater, thereby yielding a possibility that the deterioration of transmission characteristics caused by chromatic dispersion cannot effectively be suppressed over the whole wavelength band in use. In particular, as the difference between the above-mentioned predetermined wavelength and other wavelengths included in the wavelength band in use is greater, the absolute value of average chromatic dispersion observed as the whole system becomes greater. Therefore, even when the optical transmission line disclosed in the above-mentioned U.S. Pat. No. 5,894,537 is utilized, there is a possibility that a WDM transmission system having a wide wavelength band in use is hard to realize.
The optical transmission line disclosed in the above-mentioned Kubo reference can set the average chromatic dispersion observed as the whole system, such that its absolute value does not exceed a predetermined value in a wide wavelength band. When the optical transmission line is seen section by section, however, there may exist a point where the absolute value of chromatic dispersion falls short of the predetermined value, thus leaving a possibility that transmission characteristics may locally deteriorate due to nonlinear optical phenomena such as four-wave mixing. Also, depending on the sequence of connection of respective optical fibers whose chromatic dispersion and dispersion slope are both positive and negative (the positional relationship of thus arranged two kinds of optical fibers observed in the traveling direction of propagating optical signals), accumulative chromatic dispersion may increase locally, thus yielding a possibility of transmission characteristics deteriorating due to an interaction between the accumulative chromatic dispersion and nonlinear optical phenomena.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide an optical transmission line comprising a structure which can yield favorable transmission characteristics over a wider wavelength band in use.
The optical transmission line according to the present invention comprises one or more first waveguides and one or more second waveguides having optical characteristics opposite to each other at a predetermined wavelength in a wavelength band in use, and also comprises a structure in which the first and second waveguides are alternately arranged along a traveling direction of optical signals in the wavelength band in use. Namely, the optical transmission line includes a section in which the first and second waveguides are arranged adjacent each other along the traveling direction of the optical signals in the wavelength band in use. In particular, each first waveguide has a chromatic dispersion with a sign opposite to that of the chromatic dispersion of each second waveguide and a dispersion slope with a sign opposite to that of the dispersion slope of each second waveguide.
The optical transmission line according to the present invention is disposed in at least one of places between an optical transmitter for emitting a plurality of wavelengths of optical signals and a receiver for receiving the optical signals, between the optical transmitter and a repeater station including an optical amplifier or the like, between repeater stations, and between the repeater station and the receiver. Also, the optical transmission line can be constituted by a unitary optical fiber having no junction, as well as a configuration in which a plurality of optical fibers functioning as each of the first waveguides and a plurality of optical fibers functioning as each of the second waveguides are fusion-spliced in a predetermined sequence. Along the traveling direction of optical signals, the unitary optical fiber as the optical transmission line is alternately formed with one or mo
Ishikawa Shinji
Kato Takatoshi
Nishimura Masayuki
Sasaoka Eisuke
Tanaka Shigeru
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