Optical fiber coated with an amorphous boron protective layer, a

Details Optical fiber coated with an amorphous boron protective layer, a Optical fiber coated with an amorphous boron protective layer, a

1996-04-24

1998-02-10

Palmer, Phan T.H.

Optical waveguides

Optical fiber waveguide with cladding

Utilizing multiple core or cladding

385123, 65 601, 428366, G02B 602

Patent

active

057178097

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to an optical fiber coated with an amorphous boron protective layer, and a method of depositing such a layer.


BACKGROUND OF THE INVENTION

The conventional structure of an optical fiber is well-known: an optical fiber comprises an optical core, for guiding the majority of the light waves, enclosed in optical cladding, the core and the cladding both being constituted by a silica-based material which is doped to a greater or lesser extent in the core and cladding zones. "Optical cladding" is defined here in a broad sense, i.e. the optical cladding comprises all the silica-based layers which surround the core, regardless of the method by which they have been obtained.
In certain known optical fibers, the optical cladding is directly covered by a resin coating, designed to avoid in part the consequences of the microbends suffered by the fiber, in particular while it is being installed in a cable. During installation and use, the optical fiber is subjected to bending and traction stresses which lead in the long term to mechanical fatigue, i.e. to deterioration of its mechanical properties which consequently results in deterioration of its transmission performance.
In other known optical fibers, designed mainly to be used in moist environments (in particular in underwater applications) where the optical fiber is subjected to the action of water and hydroxyl ions OH--, a protective layer is disposed directly on the optical cladding, under the resin coating. When the optical fiber is subjected to stresses which lead to mechanical fatigue and consequently provoke microcracks in its surface, attack by moisture or by OH.sup.-- ions cause the microcracks in its surface to grow, which weakens the fiber, causing it to break.
In known manner, the protective layer provided on said fibers is generally constituted by carbon that is non-crystalline or turbostratic (i.e. having a structure that causes it to provide much better sealing than does graphite carbon) which carbon is deposited on the optical fiber chemically from the vapor phase, for example.
However, such a carbon protective layer is not entirely satisfactory: although it provides sufficient sealing for the fiber, it presents poor resistance to abrasion because it is not hard enough. It does not therefore protect the fiber in the long term against abrasion due to rubbing against other fibers when a plurality of fibers are installed in a cable, or due to rubbing against the inside wall of the cable in which it is installed. Unfortunately, abrasion causes microcracks to appear in the surface of the fiber, which leads to the risk of the fiber breaking.
To avoid abrasion of the fiber, the resin coating of the fiber must be very thick, in practice, not less than 60 .mu.m thick. Unfortunately, this is very detrimental when, as in the current case, it is desired to increase the capacity of optical fiber cables, so that they can contain up to 50 to 100 optical fibers. Thus a very thick resin coating prevents the cable being compact, with compactness being necessary to enable cables to be used in applications where very little space is available.
U.S. Pat. No. 4,319,803 proposes providing an amorphous boron protective layer, which is supposed to enable the above-mentioned drawbacks to be eliminated.
In addition, a method is known from the article entitled "Chemical Vapor Deposition of Amorphous Boron on Massive Substrates", published in the Journal of Electrochemical Society--Solid-State Science and Technology--February 1976, of depositing amorphous boron chemically from the vapor phase by reducing gaseous boron chloride BCl.sub.3 by means of hydrogen H.sub.2 on substrates that are inert relative to boron chloride, i.e. they do not react with it.
A method of that type, using boron chloride or any other boron halide such as BF.sub.3 (boron fluoride) or BBr.sub.3 (boron bromide), is simple and cheap to implement. In particular, it avoids the use of hydrides (such as SiH.sub.4, recommended in U.S. Pat. No. 4,319,803) as starting reagents, said c

REFERENCES:
patent: 4319803 (1982-03-01), Burmeister et al.
patent: 4735856 (1988-04-01), Schultz et al.
patent: 5000541 (1991-03-01), DiMarcello
patent: 5246746 (1993-09-01), Michalske et al.
patent: 5354348 (1994-10-01), Zushi et al.

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