Abstract: Natural or synthetic silica is deposited on a preform set into rotation in front of a plasma torch which moves back and forth substantially parallel to a longitudinal direction of the preform, a first feed duct feeds the plasma with grains of natural or synthetic silica while a second feed duct feeds the plasma with a fluorine or chlorine compound, preferably a fluorine compound, mixed with a carrier gas Any sodium or lithium contained in the grains of natural or synthetic silica react with the fluorine or chlorine of the fluorine or chlorine compound, thereby making it possible to improve the optical quality of fibers drawn from a preform built up with natural or synthetic silica, and to do so at reduced cost.

Abstract: An optical fiber preform comprising a core of radius R, and a sheath comprising cladding and outer sheath, wherein the outer sheath comprises an inner zone, and a peripheral zone made of silica doped with alumina and of thickness lying in the range 0.08 R to 2.2 R, and the proportion by weight of alumina in the peripheral zone is such that the equivalent concentration of alumina in the outer sheath as a whole lies in the range 100 ppm to 1000 ppm by weight of aluminum relative to silica. A method of manufacturing said preform, and an associated optical fiber.

Abstract: In the method, a suspension of silica particles in water is provided, the suspension is gelled, the resulting gel is dried under microwaves, the dried gel is fractioned into silica granules having a diameter lying in the range 10 .mu.m to 1000 .mu.m, having an apparent density of about 0.5 g/cm.sup.3 to 0.6 g/cm.sup.3, and a porosity of less than 20%, and the granules are screened in the range 100 .mu.m to 500 .mu.m. The suspension in water of silica particles is made by mixing silica soots having a size of less than 0.1 .mu.m and silica grains having a size greater than 1 .mu.m. Its maximum concentration in total weight is greater than that of a suspension of synthetic soots on their own, thereby making it possible to increase the production capacity of the method. Advantageously, the out-of-range granules rejected by the screening are recycled as silica grains for the aqueous suspension, thereby making it possible to increase the accumulated production efficiency of the method.

Abstract: A method of manufacturing a silica powder using a sol-gel technique, wherein a suspension consisting of synthetic silica soot, having a specific surface area of less than 80 m.sup.2 /g, is made in water to form a concentration of 50% to 75% by weight of silica; the suspension is gelled; the resulting gel is dried without forming a crust and with microwaves and broken up into silica grains having an apparent density of 0.5 g/cm.sup.3 to 0.6 g/cm.sup.3 ; and screening of the dry gel is performed to provide grains having a desired size.

Abstract: A device for doping silica powder with a dopant includes an arrangement for receiving silica soot particles or previously fabricated silica granules. The particles or granules contained in the receiving arrangement are heated to a temperature sufficient to allow agglomeration of the soot particles into granules followed by densification of the granules. The receiving arrangement is fed with a precursor gas of the dopant so that the granules are heated in an atmosphere containing the precursor gas, which causes the dopant to be incorporated into the granules during their densification to produce densified and doped silica grains. The receiving arrangement is mobile so that, during their movement, each of the granules that it contains is subject to movement such that substantially all of its outside surface is exposed to the atmosphere containing the precursor gas.

Abstract: The method of manufacturing a preform having a supporting core by effecting successive passes with a plasma torch while material is being supplied, and by effecting at least one glazing step with the plasma torch and without material being supplied includes:a) a first glazing step effected at slow speed from a first end of the preform to the second end thereof;b) pre-separation of the preform adjacent to the first end, thereby reducing its diameter to substantially that of the supporting core;c) a second glazing step effected from the first end to the second end of the preform; andd) complete separation of the preform at the place of pre-separation.

Abstract: A device for producing spherical silica grains includes a cylinder having its axis inclined to the horizontal and an inside wall made from a material that is significantly softer than silica. The cylinder has an entry chute for non-spherical silica grains and an exit chute for spherical cylinder grains. A rotary member is rotated at high speed by a drive shaft having its axis coincident with the axis of the cylinder. The rotary member includes at least one paddle vertically aligned with the entry chute so that silica grains entering via the entry chute are thrown by the paddle against the inside wall of the cylinder and follow a path most of which is against the wall before leaving via the exit chute.

Abstract: A method of applying a silica having fluorine therein by spraying silica simultaneously with a fluorine-containing gas by a POD method plasma torch.

Abstract: A method for producing spherical silica grains using a cylinder having its axis inclined to the horizontal and having an inside wall made from a material that is significantly softer than silica. The cylinder has an entry chute for non-spherical silica grains and an exit chute for spherical cylinder grains. According to the method, a rotary member is rotated at high speed by a drive shaft having its axis coincident with the axis of the cylinder. The rotary member includes at least one paddle vertically aligned with the entry chute so that silica grains entering via the entry chute are thrown by the paddle against the inside wall of the cylinder and follow a path most of which is against the wall before leaving via the exit chute. As a result, the silica graining have a spherical shape.