Optical waveguides – Optical fiber waveguide with cladding
Reissue Patent
2000-11-17
2003-04-22
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000, C385S127000
Reissue Patent
active
RE038086
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dispersion compensating fiber and an optical transmission system including the same, which are applied to an optical fiber transmission network capable of long-distance and high-bit-rate optical communication utilizing the 1.55 &mgr;m-band wavelength-multiplexing signal light.
2. Related Background Art
From social needs based on the coming of advanced information society, research and development has been conducted vividly heretofore as to high-bit-rate high-speed communication such as video communication and long-distance communication such as international communication utilizing the optical fiber transmission network.
In the case of the optical fiber transmission network to realize such long-distance and high-bit-rate optical communication, first, its transmission lines need to be optical fibers that permit only single-mode propagation. It is because mode dispersion (represented by dispersion due to a difference between group velocities of respective propagation modes) inevitably takes place in the case of multimode communication.
Thus, the first countermeasure was the single-mode optical fiber permitting only single-mode propagation. This single-mode optical fiber is free of occurrence of mode dispersion, but chromatic dispersion represented by the sum of material dispersion (dispersion due to wavelength dependence of refractive index specific to a material of optical fiber) and structural dispersion (dispersion due to wavelength dependence of group velocity of propagation mode) confines transmission capacity. Specifically, even if the wavelength of light emitted from a light source is said to be single, though rigorously speaking, it will have a certain spectral width. When a light pulse having this spectral width propagates in the single-mode optical fiber having predetermined chromatic dispersion characteristics, the width of the light pulse if broadened, so as to deform the pulse shape. This chromatic dispersion is expressed as a transmission delay time difference per unit spectral width (nm) and unit optical fiber length (km) in units of ps/km
m).
It is, however, known that silica normally used as a material for optical fiber shows zero material dispersion near the wavelengths of 1.26 to 1.29 &mgr;m. Since the structural dispersion varies depending upon parameters of optical fiber, the optimum design of the parameters of optical fiber permits the material dispersion and the structural dispersion to cancel each other near the wavelengths of 1.3 to 1.32 &mgr;m, thereby realizing zero chromatic dispersion. Therefore, use of single-mode optical fiber allows longer-distance and larger-bit-rate optical communication near the wavelength 1.3 &mgr;m than use of multimode optical fiber does. In practice, the single-mode optical fibers are used in optical communication of the communication distance of several hundred km and the communication capacity of several hundred Mbit/sec.
However, transmission loss of optical fiber is minimum in the 1.55 &mgr;m wavelength band, from which there have been desires for optical communication utilizing the 1.55 &mgr;m-band light. This resulted in developing a dispersion shifted fiber in which the wavelength where the chromatic dispersion was zero (zero-dispersion wavelength) was shifted into this wavelength band. In the dispersion shifted fiber, because the material dispersion cannot be changed so much, the index profile thereof is designed optimally to change the value of structural dispersion, thereby setting the zero-dispersion wavelength in the vicinity of 1.55 &mgr;m. This dispersion shifted fiber, together with an erbium (Er)-doped optical fiber amplifier, is employed in the long distance optical fiber transmission system with the transmission capacity being several Gbit/sec, utilizing the 1.55 &mgr;m-band wavelength division multiplexing (WDM) signal light.
On the other hand, there are many single-mode optical fibers already installed heretofore. Therefore, needs exist for optical communication in the 1.55 &mgr;m wavelength band utilizing the existing single-mode optical fiber transmission network. Thus, an attempt has been made to cascade-connect connect a dispersion compensating fiber having negative chromatic dispersion and negative dispersion slope to a single-mode optical fiber having positive chromatic dispersion in the 1.55 &mgr;m wavelength band, thereby canceling out the chromatic dispersion and dispersion slope as the whole of optical transmission line (for example, as in the bulletin of Japanese Laid-open Patent Application No. 6-11620).
In a graph to show the chromatic dispersion, the dispersion slope is given as a slope of the graph.
SUMMARY OF THE INVENTION
The inventors investigated the above-stated prior art and found the following problems. Specifically, with the above-stated dispersion shifted fiber, the chromatic dispersion thereof becomes zero at a predetermined wavelength near the wavelength 1.55 &mgr;m. However, the chromatic dispersion is not zero in the regions before and after the wavelength (the zero-dispersion wavelength) and the chromatic dispersion increases with increasing wavelength in general when a sign of chromatic dispersion is positive. In other words, the dispersion slope (which is the wavelength dependence of chromatic dispersion and is expressed in units of (ps/km
m
2
)) has a positive sign in this condition. This would be a problem in the case of communication by the wavelength division multiplexing (WDM) system for multiplexing signal light components of mutually different wavelengths in order to further raise the transmission speed to higher rates. Namely, there is such a tendency that among the 1.55 &mgr;m-band wavelength-multiplexing signal light (having a plurality of wavelengths) the chromatic dispersion becomes larger (positive) for signal light components of longer wavelengths while the chromatic dispersion becomes smaller (negative) for signal light components of shorter wavelengths (i.e., there is such a trend as to have positive dispersion slope), which results in the limit of increase in transmission speed in the WDM method.
On the other hand, studies on dispersion-flattened optical fibers the both chromatic dispersion and dispersion slope of which become nearly zero in the 1.55 &mgr;m wavelength band are reported, for example, in Kubo et al., “Characteristics of double cladding type low-dispersion SM fiber,” C-374, Abstracts (The spring meeting, 1990); Institute of Electronics, Information and Communication Engineers of Japan, and P. K. Bachmann et al., “Dispersion-Flattened Single-Mode Fibers Prepared with PCVD: Performance, Limitations, Design Optimization, ” J. of Lightwave Technol., Vol. LT-4, No. 2, pp. 858-863 (1986). However, the dispersion=flattened fibers need to be fabricated with extremely precise control of the size, such as the core diameter, and the refractive index profile and are hard to fabricate, thus not coming to the stage of practical application yet.
A dispersion compensating optical fiber according to the present invention is optically connected to the conventional optical fiber (an optical transmission line being a compensated object), so as to compose an optical transmission system. It is, therefore, an object of the present invention to provide a dispersion compensating fiber enabling the long-distance and high-bit-rate optical communication by optically connecting the dispersion compensating fiber according to the present invention to the conventional optical fiber transmission line in respectively appropriate lengths, thereby improving the overall chromatic dispersion and dispersion slope of the optical transmission line in the 1.55 &mgr;m wavelength band (i.e., making absolute values of chromatic dispersion and dispersion slope closer to zero), and to provide an optical transmission system comprising it.
The dispersion compensating fiber according to the present invention is used for compensated objects mainly including dispersion shifted fibers the zero-dispersion
Kanamori Hiroo
Kashiwada Tomonori
Koyano Yasushi
Nishimura Masayuki
Onishi Masashi
McDermott & Will & Emery
Palmer Phan T. H.
Sumitomo Electric Industries Ltd.
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