Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-09-05
2003-05-20
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S024000
Reexamination Certificate
active
06567595
ABSTRACT:
TITLE OF THE INVENTION
Optical Fiber Line, Optical Transmission Line, Method of Making Optical Cable, and Method of Laying Optical Transmission Line
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber line for transmitting a plurality of wavelengths of optical signals in a wavelength division multiplexing (WDM) transmission system, an optical transmission line, a method of making an optical cable, and a method of laying an optical transmission line.
2. Related Background Art
Along with demands for higher speed and larger capacity in optical communications, wavelength division multiplexing transmission techniques for transmitting a plurality of wavelengths of optical signals as being multiplexed have come into use.
For enhancing the transmission quality of such wavelength division multiplexing transmission, optical fiber lines to become a transmission medium are required to have the following characteristics. As the absolute value of chromatic dispersion in the optical fiber line in a signal wavelength band (e.g., 1.55-&mgr;m wavelength band) is greater, the pulse waveform of optical signals is more likely to deform, thereby deteriorating the transmission quality. Therefore, from such a viewpoint, it is desirable that the absolute value of chromatic dispersion in the optical fiber line be smaller. If the absolute value of chromatic dispersion in the signal wavelength band is smaller, on the other hand, then four-wave mixing, which is a kind of nonlinear optical phenomena, is more likely to occur, which causes cross talk and noise, thereby deteriorating the transmission quality. Therefore, from such a viewpoint, it is desirable that the absolute value of chromatic dispersion in the optical fiber line be greater.
For satisfying the two contradictory demands mentioned above, reference 1—K. Nakajima, et al., “Design of Dispersion Managed Fiber and its FWM suppression Performance,” OFC'99 Technical Digest, ThG3 (1999)—, for example, discloses an optical fiber line whose chromatic dispersion is periodically changed so as to become positive and negative in the longitudinal direction, so that the absolute value of chromatic dispersion is sufficiently small in the line as a whole but locally greater. Described as a method of periodically changing the chromatic dispersion of optical fiber line so as to become negative and positive in reference 1 are methods in which core and cladding diameters are periodically changed, methods in which dopant concentrations are periodically changed, and the like.
SUMMARY OF THE INVENTION
The inventors have studied the conventional techniques mentioned above and, as a result, have found a problem as follows. Namely, the optical fiber lines in accordance with the above-mentioned conventional techniques necessitate a complicated manufacturing step in which the core or cladding diameter is periodically changed or a dopant concentration is periodically changed, which is also very hard to control. Along with the complexity in manufacturing steps and the difficulty in their control, the manufacturing cost rises as well. Therefore, in the case where an optical transmission line including a plurality of optical fiber lines is to be constructed in order to realize optical communications with a larger capacity, if the optical fiber lines in accordance with the above-mentioned conventional techniques are used therefor, then there will occur a problem that the manufacturing cost rises greatly.
Hence, it is an object of the present invention to overcome the above-mentioned problem, and provide an optical fiber line which has a higher transmission quality and can be constructed inexpensively, an optical transmission line, a method of making an optical cable, and a method of laying an optical transmission line.
The optical fiber line in accordance with the present invention comprises a plurality of positive dispersion optical fibers, having a positive chromatic dispersion in a signal wavelength band, selected from a positive dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of D
A
(>0) and a standard deviation of &sgr;
A
; and a plurality of negative dispersion optical fibers, having a negative chromatic dispersion in the signal wavelength band, selected from a negative dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of D
B
(<0) and a standard deviation of &sgr;
B
; the positive and negative dispersion optical fibers being alternately arranged and coupled in a longitudinal direction.
Without any complicated manufacturing steps and difficult control, positive and negative dispersion optical fibers are alternately coupled in this optical fiber line, whereby the absolute value of cumulative chromatic dispersion can be made sufficiently small in the optical fiber line as a whole, whereas chromatic dispersion can be kept from locally becoming zero. Also, since the positive and negative dispersion optical fibers are selected from the positive and negative dispersion optical fiber groups, respectively, the cumulative dispersion in the optical fiber line as a whole can fall within a predetermined range if the average values D
A
and D
B
and the standard deviations &sgr;
A
and &sgr;
B
are appropriately adjusted.
Preferably, each of the positive dispersion optical fibers has a chromatic dispersion of 2 ps
m/km or greater, whereas each of the negative dispersion optical fibers has a chromatic dispersion of −2 ps
m/km or smaller. In this manner, the absolute value of chromatic dispersion can be made locally greater.
Preferably, the average value of dispersion slope in the plurality of positive dispersion optical fibers and the average value of dispersion slope in the plurality of negative dispersion optical fibers have polarities different from each other. In this manner, a wavelength range in which the absolute value of chromatic dispersion becomes sufficiently small can be widened in the optical fiber line as a whole.
Preferably, each of the positive dispersion optical fibers and negative dispersion optical fibers has an effective area exceeding 50 &mgr;m
2
. In this manner, nonlinear optical phenomena can be restrained from occurring.
Preferably, the absolute value of dispersion slope in each of the positive dispersion optical fibers and the absolute value of dispersion slope in each of the negative dispersion optical fibers are each smaller than 0.03 ps
m
2
/km. In this manner, a wavelength range in which the absolute value of chromatic dispersion becomes sufficiently small can be widened in the optical fiber line as a whole.
Preferably, the ratio of the mode field diameter of any of the negative dispersion optical fibers to the mode field diameter of any of the positive dispersion optical fibers is at least 0.8 but not exceeding 1.2. In this manner, loss can be lowered at junctions between the positive and negative dispersion optical fibers.
Preferably, each of the positive dispersion optical fibers and negative dispersion optical fibers has a length of 5 km or shorter. Since the interval between two repeaters connected to each other by an optical fiber line is usually about several tens of kilometers, an optical fiber line in which a plurality of positive dispersion optical fibers and a plurality of negative dispersion optical fibers are alternately coupled will be laid between the two repeaters if each of the positive and negative dispersion optical fibers has a length of 5 km or shorter. As a consequence, even when chromatic dispersion fluctuates more or less among the individual optical fibers, the absolute value of chromatic dispersion in the optical fiber line as a whole can be made sufficiently small in terms of statistics.
An optical transmission line will be constructed if a plurality of optical fiber lines, each mentioned above, are contained therein.
In this optical transmission line, a plurality of optical cables are arranged adjacent each other in a longitudinal direction
Knauss Scott A.
McDermott & Will & Emery
Sanghavi Hemang
Sumitomo Electric Industries Ltd.
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