Process for the production of graded index optical fibres

Claim 1

A process for the production of optical fibres consisting of a core with a graded refractive index and an optical cladding, comprising the preliminary formation of a blank having a unitary structure of a core with a graded refractive index with an optical cladding and larger in diameter than the fibre, comprising: a first stage consisting in preparing a bath of molten glass from starting materials capable of entering into a glass composition showing the phenomenon of separation into two solid interconnected and continuous phases of different composition; said starting materials having an impurity content of less than 10 -4 , calculated taking into account molar proportions of oxides of transition metals; wherein the starting materials comprise, in predetermined proportions, base materials comprising at least oxides of silicon, boron, sodium and doping agents comprising at least one oxide selected from the group of germanium, titanium, phosphorous and aluminum oxides; a second stage comprising a first step of drawing a blank from the glass bath and a second step comprising at least one thermal annealing treatment which results in the appearance of the two solid interconnected and continuous phases of different composition, namely a hard phase rich in silica and a soft phase poor in silica, the soft phase having collected most of the impurities of the initial bath; a third stage comprising the following steps: (a) deep leaching of the blank to eliminate the soft phase and the impurities which it contains, leaving the doping agent emanating from the soft phase as leaching residue to an extent which increases from the periphery to the core of the blank; (b) surface leaching to reduce the concentration of doping agent at the periphery of the blank, thereby forming a unitary optical cladding; a fourth stage comprising a heat treatment in a controlled atmosphere for consolidating the blank.