Method and installation for making an optical fiber

batchwise process for the manufacture of an optical fiber made of polymers, this fiber comprising a core and a sheath, the core being formed from a first polymer based on methyl methacrylate and optionally on a (meth)acrylic ester other than methyl methacrylate and the sheath being formed from a second polymer having a lower refractive index than that of the core, this process being characterized in that the process is carried out in an in-line plant ranging from a device for the purification of the starting materials to a spinning device, involving the intermediacy of the various devices of the in-line plant and the various transfer means connecting the various devices of the in-line plant, this plant being leaktight to the external air and to dust and sheltered from light, in particular ultraviolet radiation, and in that the process comprises the following stages:

• (1) beads of the first polymer are prepared by suspension polymerization of Purified methyl methacrylate and optionally of at least one purified (meth)acrylic ester other than methyl methacrylate in demineralized, filtered and deoxygenated water, the polymerization being carried out in the presence of at least one radical polymerization initiating agent, of at least one chain-transfer agent and of at least one suspending agent and in the virtually complete absence of polymerization inhibitor and of impurities, such as:
• (a) biacetyl, in an amount reduced to at most 1 ppm with respect to the total amount of monomers,
• (b) transition metal ions capable of giving strong light absorption in the visible region,
• (c) dust and particles, the various abovementioned starting materials used in the suspension polymerization having been filtered before polymerization with a filtration threshold of 0.1 micron;
• the polymerization also being carried out with stirring, under an atmosphere of an inert gas;
• (2) on conclusion of stage (1), the beads are separated and washed using demineralized and dedusted water and are dried under an atmosphere of a dedusted and preferably inert gas, and the dried beads are stored under this atmosphere in at least one intermediate tank;
• (3) at least a portion of the beads obtained on conclusion of stage (2) is transferred, still under an atmosphere of an inert and dedusted gas, from the intermediate tank or tanks to a coextrusion device and the core of the fiber, starting from said beads, and the sheath of the fiber, starting from a polymer having a lower refractive index than that of the core, are coextruded;
• (4) the fiber obtained at the outlet of the coextrusion device is gradually cooled, so as to avoid quenching the first polymer intended to constitute the core of the fiber, and the fiber is drawn, in order to obtain a fiber with a mean total diameter which can vary from 250 to 2 000 microns.