Etching an optical fiber fusion splice

Details Etching an optical fiber fusion splice Etching an optical fiber fusion splice

2000-06-06

2003-04-29

Hoffmann, John (Department: 1731)

Glass manufacturing

Processes of manufacturing fibers, filaments, or preforms

Process of manufacturing optical fibers, waveguides, or...

C065S429000

Reexamination Certificate

active

06553791

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical-fiber fusion-splicing method for fusion-splicing optical fibers with mode field distributions being different from each other at low splice loss and with less reduction of its mechanical strength.
2. Description of the Related Art
The optical communication using a quartz single mode optical fiber has an advantage of large transmission capacity. A wave-length division multiplexing communication (WDM communication) using wave length region of 1.55 &mgr;m or therearound, which is a minimum loss wave length of the single mode optical fiber, is known as a technique which makes full use of the advantage of the large transmission capacity, and is put into practical use. In the WDM communication, it is desirable that the transmission rates of the waves of those wavelengths are equal to one another. To this end, there is an approach in which the dispersion characteristic of the single mode optical fiber is controlled by appropriately selecting the structure and physical properties of the single mode optical fiber. To be more specific, the dispersion characteristic of the single mode optical fiber is determined by its refractive index distribution and materials constituting the single mode optical fiber. Therefore, a desired dispersion characteristic of the single mode optical fiber is obtained by properly designing the distribution and selecting the materials. A single mode optical fiber used for a 1.55 &mgr;m band is generally called a 1.55 &mgr;m-band dispersion shifted fiber.
If a more precise control is required for the dispersion characteristic of the transmission path in order to increase the transmission capacity of the WDM communication, a dispersion shifted fiber sometimes is spliced, at its terminal or mid-point, to another optical fiber having a refractive index distribution, which is different from that of the dispersion shifted fiber, such as a dispersion compensation fiber. When optical fibers having different refractive index distributions, or different mode field distributions, are spliced, a spliced loss is caused, at the splice point, due to the mismatching of the mode field distributions. The splice loss cancels the advantage of the optical fiber, i.e., low loss. Therefore, the matching of the mode field distributions of the optical fibers at the splice point is very important in realizing a long distance communication, e.g. a communication using an under-water optical-fiber cable. A high mechanical strength is also required for the under-water optical-fiber cable in order to prevent the optical-fiber cable from being broken by a large tension applied when it is laid. For this reason, where the splice points of the optical fibers are contained in the under-water optical-fiber cable, it is required that the splice points of the optical fibers are passed on a tensile test in which the spliced parts are tensioned by 2% to 3% in its elongation during the cable manufacturing process. This figure is two or three times as large as a tensile strength required for the ordinary splice points of the optical fibers.
A technique to match the mode field distributions of two single mode optical fibers to be spliced, viz., to make a mode field distribution of one single mode optical fiber coincident in configuration with that of the other single mode optical fiber by expanding the mode field distribution of the former is described by Kihara et al., in their paper in “The Transactions of the Institute of Electronics, Information and Communication Engineers, B-I, Vol. J-75-B-1, No. 7, pp467-470, July, 1991)”. The technique discussed in the paper is constructed such that the dopants contained in the core of the optical fiber are diffused by heating the optical fiber to make the refractive index distribution of the optical fiber flat in configuration and consequently to expand the mode field distribution of the optical fiber.
FIG. 8
shows a relationship between an expansion of the mode field diameter of the optical fiber and heating time when the optical fiber is heated at 1400° C., which is described in the paper. The paper teaches that for a 1.55 &mgr;m-band dispersion shifted fiber and a 1.3 &mgr;m-band single mode optical fiber, the fusion characteristics by the heating and the dopant diffusion characteristics vary depending on the refractive index distributions and the materials constituting the optical fibers, and that variations of the mode field diameters are different even when the optical fibers are heated for the equal heating time. In the paper, there is a description “an exact measurement of heating temperature and its control are difficult at present”. The paper does not refer to a reliability of the splice point of the single mode optical fibers fusion spliced, and hence it is clear that the difficulty of the exact measurement of heating temperature and its control has not yet been removed.
SUMMARY OF THE INVENTION
Accordingly, the present invention has an object to provide an optical-fiber fusion-splicing method for fusion-splicing optical fibers with different mode field distributions at low splice loss and with less reduction of its mechanical strength. That is, an object of the invention is to provide an optical-fiber fusion-splicing method for fusion-splicing optical fibers with different refractive index distributions at low splice loss and high mechanical strength.
According to the present invention, there is provided an optical-fiber fusion-splicing method comprising the steps of: removing the coating of one end part of each of two optical fibers with different mode field distributions to be spliced; heating the exposed end part of at least one of the optical fibers, to thereby vary a mode field distribution thereof; cleaving the exposed end part of at least one of the optical fibers so that the mode field distributions at the splice end faces of the two optical fibers are substantially coincident with each other in configuration; fusion-splicing the two optical fibers; and immersing the heated portion of the exposed end part of the fusion-spliced optical fiber in a hydrofluoric acid solution, to thereby effect a surface treatment thereof. Thus, a low loss splice is realized by making the mode field distributions at the cleaved end faces of the two optical fibers are made to be substantially coincident with each other in configuration. A high strength splice is realized by the surface treatment using a hydrofluoric acid solution.
In the optical-fiber fusion-splicing method, the heating of the exposed end part of the optical fiber is carried out such that the coated end part and the exposed end part of the optical fiber are gripped, and a portion of the optical fiber located between the gripping positions is heated.
In the optical-fiber fusion-splicing method, a temperature at which the exposed end part of the optical fiber is heated is selected to be a temperature lower than a melting point of the optical fiber but higher than a temperature at which the dopants of the optical fiber are substantially diffused. A proper diffusion of the dopants is secured by using hydrocarbon gas for the heat source, to thereby expand the mode field distribution.
In the optical-fiber fusion-splicing method, the surface treatment of the exposed end part of the optical fiber is carried out in a manner that a container of which the width is shorter than an exposed end part of a fusion-spliced optical fiber is filled with a hydrofluoric acid solution, and the exposed end part of the fusion-spliced optical fiber is immersed, while being held in a straightened state, in a portion of the hydrofluoric acid solution, which stands up the rim of the container by its surface tension.
In the optical-fiber fusion-splicing method, the surface treatment is carried out in a manner that the exposed end part of the fusion-spliced optical fiber is immersed in a hydrofluoric acid solution containing hydrofluoric acid of about 10% by volume ratio for a time of five minutes or longer but ten minutes or sho

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