For a number of years much research work has been carried out in the field of producing optical wave guides, since these are very advantageous in optical telecommunication systems because of their high transmitting capacity.
These wave guides or optical fibers comprise a glass sheath and a glass core having a reflective index higher than that of the sheath. When the fibers are monomode fibers the core has a constant composition. In contrast, in the case of multimode optical fibers the composition of the core varies radially either in stages or continuously.
The known processes for producing monomode and multimode fibers, with a jump in index or an index gradient, use either glass-making methods or high temperature deposition methods.
Amongst the methods using glass-making techniques, French Pat. No. 2.368.444 illustrates a process which consists in forming a blank of the future fiber, whereafter the blank is subjected to a purification treatment comprising a number of stages, including a separation into two solid phases and a washing of the phase which collected the majority of the impurities; after the purification treatment, the fiber is obtained by drawing the blank. However, that process has the disadvantage of resulting in optical fibers generally having fairly high losses, an ill-defined core-sheath interface and an index profile which is difficult to control, and therefore average or mediocre transmission bands. The process also requires considerable purification of the starting products, and this raises certain problems, since such products are in the solid state and are therefore difficult to purify and handle without pollution.
The methods using high temperature deposition techniques for doped silica layers, for example, the known processes for deposition by flame hydrolysis, such as those illustrated in French Pat. Nos. 2.088.486 and 2.178.175, or processes of deposition in the vapor phase have the disadvantages of being expensive, requiring high temperatures, generally above 1400.degree. C., and producing only fibers of limited length.
Moreover, those methods have certain difficulties of performance as regards mastering the index profile of the layers deposited. Further, they always have a yield lower than that of 1, and require the use of a glass or silica support.
The invention relates more particularly to a process for manufacturing a glass tube comprising at least one doped silica layer which enables preforms to be obtained which can be used for the manufacture of monomode or multimode optical fibers, with a jump in index or an index gradient, while obviating the disadvantages of the known processes.