Optical waveguides – With splice – Fusion splicing
Reexamination Certificate
2002-03-22
2003-11-11
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With splice
Fusion splicing
C385S028000, C065S501000
Reexamination Certificate
active
06644870
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical fiber transmission line comprising at least two optical fibers fusion-spliced to each other.
2. Related Background Art
A WDM (Wavelength Division Multiplexing) optical communication system enables long-haul transmission of large amounts of data through propagation of signals for multiple channels of multiplexed wavelengths. In order to enable transmission of large amounts of data over long haul, it is preferred that the optical fiber transmission line, which comprises a transmission medium, have a small absolute value for the accumulated chromatic dispersion in a signal wavelength band. However, in an optical fiber transmission line comprising only a single type of optical fiber, it is difficult to obtain a small absolute value for the accumulated chromatic dispersion in the signal wavelength band. Accordingly, an optical fiber transmission line formed by interconnecting two types of optical fiber having different chromatic dispersion characteristics is generally used.
Specifically, such optical fiber transmission line is formed by connecting a standard single-mode optical fiber having a positive chromatic dispersion and a positive dispersion slope in the signal wavelength band (such as a wavelength band of 1.55 &mgr;m) to a dispersion compensator that compensates for the chromatic dispersion in this single-mode optical fiber. This construction results in a small absolute value for the accumulated chromatic dispersion in the entire optical fiber transmission line that includes the single-mode optical fiber and the dispersion compensator. For the dispersion compensator, dispersion compensating optical fiber that has a negative chromatic dispersion and a negative dispersion slope in the signal wavelength band may be used. For example, the dispersion compensator, disclosed in Japanese Patent Application Laid-Open No. 2000-91991, comprises an optical fiber transmission line formed by interconnecting two or more types of dispersion compensating optical fibers in accordance with a predetermined length ratio. Using this dispersion compensator, the absolute value of the accumulated chromatic dispersion over a wide signal wavelength band may be reduced in the entire optical fiber transmission line by compensating for not only chromatic dispersion, but for the dispersion slope as well.
When interconnecting optical fibers having different mode field diameters, such as when connecting single-mode optical fiber with dispersion compensating optical fiber or when connecting two different types of dispersion compensating optical fiber, fusion splicing technology, in which the end surfaces are fusion-spliced after being heated, is generally employed. For example, in the fusion-splicing technology disclosed in Japanese Patent Application Laid-Open No. H3-130705, a first optical fiber having a large core diameter and a small relative refractive index difference of the core with respect to the cladding is fusion-spliced (this process shall hereinafter be referred to as the ‘fusion-splicing’ process) with a second optical fiber having a small core diameter and a large relative refractive index difference between the core and the clad, the area near the fusion-spliced position is heated, and the dopants in each fiber are diffused (hereinafter referred to as the ‘dopant diffusion process’). In this way, the difference between the mode field diameter of the first optical fiber and that of the second optical fiber at the fusion-spliced position is kept small, and the connection loss between the first and second optical fibers is reduced.
Therefore, where a single-mode optical fiber (corresponding to the first optical fiber) and a dispersion compensating optical fiber (corresponding to the second optical fiber) are fusion-spliced, the connection loss can be reduced by performing the dopant diffusion process after fusion-splicing. Where the difference between the respective mode field diameters before fusion-splicing (hereinafter the ‘minimum mode field diameter’) of the first and second optical fibers is relatively large, this process is performed with the intention of reducing the difference between the respective mode field diameters of the first and second optical fibers at the fusion-spliced position. On the other hand, where the difference between the respective minimum mode field diameters of the first and second optical fibers is relatively small, the dopant diffusion process is not performed.
SUMMARY OF THE INVENTION
As a result of studying the conventional optical fiber transmission line, the inventors have discovered the matters described below.
During the manufacturing process for the conventional optical fiber transmission line, if the difference between the respective minimum mode field diameters of optical fibers are relatively small, as when dispersion compensating optical fibers are fusion-spliced, the dopant diffusion process is not carried out after fusion-splicing. However, the inventors have discovered that, even where the difference between the respective minimum mode field diameters of the optical fibers to be fusion-spliced to each other is relatively small, if the Ge-concentration in the core region of each fiber is large, and the minimum mode field diameter of each optical fiber is small, the connection loss is large when the first and second optical fibers have been fusion-spliced, and this connection loss has a wavelength-dependency. An optical fiber transmission line having these connection loss characteristics is not desirable for long-haul WDM transmission.
In order to overcome these problems, it is an object of the invention to provide an optical fiber transmission line comprising optical fibers fusion-spliced to each other wherein the difference between the respective minimum mode field diameters of the fibers is small and each fiber has a small mode field diameter and a core region doped with a high concentration of Ge, and wherein the optical fiber transmission line has superior connection loss characteristics.
In this specification, ‘optical fiber transmission line’ refers to a transmission line comprising first and second optical fibers fusion-spliced to each other and have a mode field diameter difference of 1 &mgr;m or less, and includes not only the case in which both the first and second optical fibers are laid in a relay space, but also the case in which the fibers are located in a relay station or the like while one or both of them are wound in a coil configuration and modularized.
The optical fiber according to the present invention comprises a first optical fiber and a second optical fiber fusion-spliced to each other. The first optical fiber has a core region doped with a high concentration (10 mol % or more) of Ge, and a small mode field diameter in which the minimum value at a wavelength of 1550 nm is 7 &mgr;m or less. The second optical fiber as well has a core region doped with a high concentration (10 mol % or more) of Ge, and a small mode field diameter in which the minimum value at the wavelength of 1550 nm is 7 &mgr;m or less. Accordingly, the difference between the respective minimum mode field diameters of the first and second optical fibers is 1 &mgr;m or less.
In particular, in this optical fiber transmission line, the mode field diameter of the first optical fiber, at a position separated by at least 2 mm from the fusion-spliced position between the first and second optical fibers, is enlarged so as to reach a value of 110% of the minimum mode field diameter of the first optical fiber. Similarly, the mode field diameter of the second optical fiber, at a position separated by at least 2 mm from the fusion-spliced position between the first and second optical fibers, is enlarged so as to reach a value of 110% of the minimum mode field diameter of the second optical fiber.
As described above, a conventional optical fiber transmission line, which is formed by fusion-splicing first and second optical fibers having a small difference between their respective mo
Fukuda Keiichiro
Iwata Noriko
Kato Takatoshi
Palmer Phan T. H.
Sumitomo Electric Industries Ltd.
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