A method for manufacturing an optical preform including the steps of providing a substrate tube having deposited layers of glass on an inside surface thereof, increasing an outer diameter of the substrate tube by means of applying a traversing heat source to heat the substrate tube to above a softening temperature thereof and by providing an internal pressure in the substrate tube higher than an ambient pressure, and collapsing the substrate tube of increased outer diameter by means of applying the traversing heat source to heat the substrate tube to above the softening temperature thereof such that an optical preform is manufactured.

The core region of an optical fiber is doped with chlorine in a concentration that allows for the viscosity of the core region to be lowered, approaching the viscosity of the surrounding cladding. An annular interface region is disposed between the core and cladding and contains a concentration of fluorine dopant sufficient to match the viscosity of the core. By including this annular stress accommodation region, the cladding layer can be formed to include the relatively high concentration of fluorine required to provide the desired degree of optical signal confinement (i.e., forming a low loss optical fiber).

A method for manufacturing a precursor for a primary preform for optical fibers by an internal plasma deposition process including the steps of providing a hollow substrate tube, creating a first plasma reaction zone having first reaction conditions and depositing non-vitrified silica layers along at least a portion of the inner surface of the substrate tube, subsequently creating a second plasma reaction zone having second reaction conditions different from the first reaction conditions and depositing vitrified silica layers along at least a portion of the substrate tube, and cooling the substrate tube to produce the precursor for a primary preform.

The present invention relates in a first aspect to a method for etching a primary preform or core rod. The present invention moreover relates in a second aspect to the etched primary preform thus obtained and moreover to a final preform and optical fibers obtained therefrom and to a method of preparing optical fibers therefrom.

An optical fiber containing an alkali metal and capable of reducing Rayleigh scattering loss is provided. An optical fiber has a core and a cladding made of silica glass and enclosing the core. The cladding contains fluorine and has a refractive index lower than the refractive index of the core. The core contains first group dopants selected from the group of Na element, K element, or a compound thereof at an average concentration of 0.2 ppm or more and 10 ppm or less. The core also contains second group dopants for reducing the viscosity of silica glass and having a diffusion coefficient of 110.sup.12 cm.sup.2/s or more and smaller than the diffusion coefficient of the first group dopants, by an average concentration of 0.2 ppm or more at a temperature of 2000 C. to 2300 C.

An exemplary method for preparing a primary preform by etching and collapsing a deposited tube includes mounting a deposited tube on a lathe and introducing the deposited tube into a central aperture of a furnace mounted on the lathe, wherein the furnace and the deposited tube are movable in axial direction with respect to each other, and creating within the furnace a hot zone that moves in translation back and forth over the length of the deposited tube during one or more cycles, wherein (i) during at least one cycle at least part of the outside of the deposited tube is etched by supplying a fluorine-containing etching gas to an annular region between the outer surface of the deposited tube and the central aperture of the furnace, and (ii) during at least one cycle the deposited tube is collapsed.

A method activates the inner surface of a substrate tube via plasma etching with a fluorine-containing etching gas. An exemplary method includes the steps of (i) supplying a supply flow of gas to the interior of a substrate tube, wherein the supply flow includes a main gas flow and a fluorine-containing etching gas flow, (ii) inducing a plasma via electromagnetic radiation to create a plasma zone within the substrate tube's interior, and (iii) longitudinally reciprocating the plasma zone over the length of the substrate tube between a reversal point near the supply side and a reversal point near the discharge side of the substrate tube. The flow of the fluorine-containing etching gas is typically provided when the plasma zone is near the supply side reversal point.

A method for manufacturing an optical fiber preform includes: adding an alkali metal element or an alkaline earth metal element to an inner surface of a glass pipe made of silica-based glass; reducing a diameter of the glass pipe after the adding; etching an inner surface of a continuous section of the glass pipe in a longitudinal direction after the reducing; and collapsing the glass pipe after the etching. At least one of the adding, the reducing, the etching, and the collapsing includes performing a local etching on an inner surface of a section of the glass pipe that is shorter than the continuous section.

A method for manufacturing an optical fiber preform made of silica-based glass, the method including: forming a core portion; and forming a cladding portion surrounding the core portion, the cladding portion having a refractive index lower than a refractive index of the core portion, wherein the forming the core portion includes: adding an alkali element group consisting of an alkali metal element and an alkaline earth metal element to an inner surface of a glass pipe made of silica-based glass; and integrating the glass pipe and a glass rod disposed inside the glass pipe to form an integrated rod after the adding.

A method of manufacturing a hollow core optical fiber including (a) a consolidated tube presenting step including presenting a consolidated cladding tube including a consolidated cladding first end, a consolidated cladding second end, a consolidated cladding longitudinal axis, and a consolidated cladding inner surface, the consolidated cladding inner surface defining a consolidated cladding interior and including consolidated cladding recesses (i) positioned around the consolidated cladding longitudinal axis and (ii) extending from the consolidated first end to the consolidated second end; and (b) a capillary tube coupling step comprising coupling preform capillary tubes to the consolidated cladding inner surface within the consolidated cladding recesses thus creating an optical fiber preform, each of the preform capillary tubes disposed within a different one of the consolidated cladding recesses.