Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-02-26
2003-07-08
Font, Frank G. (Department: 2877)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000, C385S127000
Reexamination Certificate
active
06591048
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a dispersion compensating optical fiber that is connected to, for example, a single-mode optical fiber having zero dispersion in the vicinity of a wavelength of 1.3 &mgr;m (hereinafter, referred to simply as single-mode optical fiber) and is used in making wavelength division multiplexing optical transmission by the use of light in a 1.5 &mgr;m wavelength band or other predetermined wavelength bands, and the invention relates to an optical transmission line that uses the dispersion compensating optical fiber.
BACKGROUND ART
Single-mode optical fibers have been constructed as a transmission network of optical communications all over the world. Nowadays, the amount of communications information is showing a tendency to increase sharply with the development of the information society. With the increase in information, wavelength division multiplexing transmission (WDM transmission) has been widely accepted in the field of telecommunications such that the era of the WDM transmission has now been entered. The WDM transmission is an optical transmission method suitable for large capacity high-speed communication because the WDM transmission can transmit light having a plurality of wavelengths by means of one optical fiber.
However, when the WDM transmission is made by the use of existing single-mode optical fibers and by the use of a signal light beam in a 1.3 &mgr;m-wavelength band, it does not coincide, in a wave range, with a 1.55 &mgr;m-wavelength band that is the gain band of a general optical amplifier using an erbium doped optical fiber. (Note that the 1.55 &mgr;m-wavelength band is a wavelength band substantially centering around a wavelength of 1550 nm, including wavelengths of, for example, 1530 nm to 1570nm; the term “1.55 &mgr;m-wavelength band” is hereinafter used with this meaning.) Thus, a problem resides in that the optical amplifier cannot be used, and an obstacle is caused to long distance optical communication (long distance optical transmission). Therefore, recently, WDM transmission has been made in the 1.55 &mgr;m-wavelength band using the existing single-mode optical fibers.
However, when optical communication is carried out in the 1.55 &mgr;m-wavelength band by using the existing single-mode optical fiber, the single-mode optical fiber usually has a positive dispersion of about 17 ps
m/km in a wavelength of 1.55 &mgr;m that is substantially a center wavelength in the 1.55 &mgr;m-wavelength band, and, in addition, has a positive dispersion slope in the 1.55 &mgr;m-wavelength band. For this reason, a disadvantageous case occurred in which the waveform distortion of an optical signal becomes large as the optical signal propagates through the single-mode optical fiber, and, especially when the WDM optical transmission is made, it becomes difficult to separate/distinguish the signal on the signal receiving side, thus lowering the quality of the optical communication and destroying the reliability thereof.
Then, in order to resolve the problem, recently, a dispersion compensating optical fiber that has large negative dispersion in the 1.55 &mgr;m-wavelength band has been modularized and has been connected to the single-mode optical fiber transmission line, in order to compensate for the dispersion and the dispersion slope in the 1.55 &mgr;m-wavelength band by a short dispersion compensating optical fiber. This aims to enable the WDM transmission in the 1.55 &mgr;m-wavelength band by compensating the positive dispersion and the positive dispersion slope that the single-mode optical fiber has in the 1.55 &mgr;m-wavelength band so as to restrain the deterioration of a transmission signal caused by wavelength dispersion.
However, as described above, the conventional dispersion compensating optical fiber is designed to compensate for the dispersion and dispersion slope of the single-mode optical fiber by its short length, and therefore, generally, the mode field diameter in the 1.55 &mgr;m-wavelength band becomes small in design so as to easily cause a nonlinear phenomenon. Additionally, a problem resides in that a transmission loss in the 1.55 &mgr;m-wavelength band is enlarged to be 0.4 dB/km or more, and a polarization mode dispersion value (PMD value) in the 1.55 &mgr;m-wavelength band also reaches a relatively large value of 0.2 ps/km
½
or more. The single-mode optical fiber is relatively excellent in low nonlinearity.
Today, there is a demand to increase the amount of communications information even more. However, if the WDM transmission uses only the 1.55 &mgr;m-wavelength band, there is a limit to the number of transmissible wavelengths, and a saturation state will eventually be reached. Therefore, there are demands for new optical transmission lines in which a 1.5 &mgr;m-wavelength band (i.e., a wavelength band of, for example, 1520 through 1620 nm, including the conventional 1.55 &mgr;m-wavelength band; the term “1.5 &mgr;m-wavelength band” is hereinafter used with this meaning) is made usable by extending a usable wavelength band used for the wavelength division multiplexing transmission to both sides of the conventional 1.55 &mgr;m-wavelength band (1530 through 1570 nm, for example), or a wavelength band (wavelength range) other than the 1.5 &mgr;m-wavelength band is also made usable.
However, if the conventional dispersion compensating optical fiber is connected to the single-mode optical fiber, the dispersion of, for example, wavelengths 1530 through 1570 nm in the 1.55 &mgr;m-wavelength band is compensated to be almost zero, but, instead, the dispersion in other wavelength ranges including a 1.3 &mgr;m-wavelength band that has shown almost zero dispersion before the connection to the conventional dispersion compensating optical fiber is displaced from the neighborhood of zero. The optical transmission line formed by connecting the conventional dispersion compensating optical fiber to the single-mode optical fiber in this way cannot serve as an optical transmission line capable of making the WDM transmission using the other wavelength ranges that include the 1.3 &mgr;m-wavelength band, for example.
Then, the present inventor has reoriented the conventional concept that the dispersion compensation of the single-mode optical fiber is merely carried out with the short dispersion compensating optical fiber, and has studied ways to realize an optical fiber provided with a function suitable for long distance optical transmission of the dispersion compensating optical fiber in such a way as to improve the characteristics of the mode field diameter, bend loss, and polarization mode dispersion of the dispersion compensating optical fiber itself in the 1.5 &mgr;m-wavelength band.
If such an optical fiber is realized, a problem caused by the nonlinearity can be controlled, and excellent signal transmission in the 1.5 &mgr;m-wavelength band can be made in the optical transmission line in which the single-mode optical fiber and the dispersion compensating optical fiber are connected to each other.
Additionally, the present inventor thought that an optical transmission line having small dispersion in a wavelength band, for example, near 1.3 &mgr;m that is a zero dispersion wavelength of the single-mode optical fiber will be formable if the dispersion characteristics of the dispersion compensating optical fiber to be connected to the single-mode optical fiber are made appropriate.
The present invention was made in consideration of the foregoing. A first object of the present invention is to provided a dispersion compensating optical fiber in which the characteristics of a mode field diameter, bend loss, polarization mode dispersion, etc., in the 1.5 &mgr;m-wavelength band are excellent, and, by connecting to a single-mode optical fiber or connecting to an optical fiber having almost the same dispersion characteristic in the 1.5 &mgr;m-wavelength band as the single-mode optical fiber, an optical signal can be transmitted for a long distance while compensating the dispersion in the 1.5 &mgr;m-wavelength band of lig
Font Frank G.
Mooney Michael P.
The Furukawa Electric Co. Ltd.
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