The invention relates to a plasma chemical vapor deposition (PCVD) apparatus for deposition of one or more layers of silica onto an interior wall of an elongated hollow glass substrate tube. The apparatus comprises a microwave generator, a plasma generator receiving microwaves from said generator in use, a cylindrical cavity extending through said generator, and a cylindrical liner positioned in the cavity. The substrate tube passes through the liner in use. The cylindrical liner has at least one section having a reduced inner diameter over a part of the length of the liner, the at least one section providing a contact zone for the substrate tube. The microwave generator is configured to generate microwaves having a wavelength Lw in the range of 40 to 400 millimeters, wherein a length of said at least one section having the reduced inner diameter is at most 0.1×Lw.

A device for manufacturing an optical preform by means of an internal vapour deposition process including an energy source, a hollow substrate tube having a supply side and a discharge side and the energy source being moveable along a length of the hollow substrate tube, and an elongation tube connected to the hollow substrate tube at the discharge side thereof, wherein the hollow substrate tube extends into an interior of the elongation tube and an internal diameter of the elongation tube is at least 0.5 millimeters larger than an external diameter of the hollow substrate tube.

An optical fiber having a core comprising silica and greater than 1.5 wt % chlorine and less than 0.5 wt % F, said core having a refractive index Δ.sub.1MAX, and an inner cladding region having refractive index Δ.sub.2MIN surrounding the core, where Δ.sub.1MAX>Δ.sub.2MIN.

Disclosed are methods for manufacturing a precursor for a primary preform for optical fibers via an internal plasma deposition process. An exemplary method includes creating a first plasma reaction zone having first reaction conditions in the interior of a hollow substrate tube to deposit non-vitrified silica layers on the inner surface of the hollow substrate tube, and subsequently creating a second plasma reaction zone having second reaction conditions in the interior of the hollow substrate tube to deposit vitrified silica layers on the deposited, non-vitrified silica layers. Thereafter, the hollow substrate tube is removed from the deposited, vitrified silica layers to yield a deposited tube.

The present invention relates to a method for manufacturing a preform for optical fibers, which method comprises the sequential steps of: i) deposition of non-vitrified silica layers on the inner surface of a hollow substrate tube; ii) deposition of vitrified silica layers inside the hollow substrate tube on the inner surface of the non-vitrified silica layers deposited in step i); iii) removal of the hollow substrate tube from the vitrified silica layers deposited in step ii) and the non-vitrified silica layers deposited in step i) to obtain a deposited tube; iv) optional collapsing said deposited tube obtained in step iii) to obtain a deposited rod comprising from the periphery to the center at least one inner optical cladding and an optical core; v) preparation of an intermediate layer by the steps of: * deposition of non-vitrified silica layers on the outside surface of the deposited tube obtained in step iii) or deposited rod obtained in step iv) with a flame hydrolysis process in an outer reaction zone using glass-forming precursors, and subsequently; * drying and consolidating said non-vitrified silica layers into a vitrified fluorine-doped silica intermediate cladding layer; and * in case preceding step iv) was omitted collapsing; to provide a solid rod comprising from the periphery to the center the intermediate layer, at least one inner optical cladding and an optical core; wherein a fluorine-comprising gas is used during the deposition and/or drying and/or consolidating and wherein the intermediate layer has a ratio between the outer diameter of the intermediate cladding layer (C) to the outer diameter of the optical core (A) that is at least 3.5; vi) deposition of natural silica on the outside surface of the intermediate cladding layer of the solid rod obtained in step v) by melting natural silica particles in an outer deposition zone to produce an outer cladding whereby a preform is obtained.

The present invention relates to a method for manufacturing a primary preform for optical fibers by means of an internal plasma chemical vapor deposition (PCVD) process in a hollow silica substrate tube, the hollow substrate tube having a supply side and a discharge side. An exemplary method comprises depositing doped or non-doped silica layers on the inner surface of the hollow substrate tube by supplying glass-forming gasses to the interior of the hollow substrate tube via the supply side thereof, and by creating a plasma reaction zone in the interior of the hollow substrate tube by means of microwave radiation having a microwave power, wherein the microwave power is decreased during the depositing. The present invention further relates to a method for forming an optical fiber and to the primary preform and optical fiber directly obtainable by such methods.

A method of forming an optical fiber preform includes: flowing a silicon halide and an oxidizer inside of a substrate tube, wherein a molar ratio of the silicon halide to the oxidizer is from about 1.5 to about 5.0; applying a plasma to the substrate tube to heat the substrate tube to a temperature of from about 1000° C. to about 1700° C.; and depositing silica glass comprising a halogen inside the substrate tube.

A method for activating an inner surface of a hollow glass substrate tube for manufacturing an optical fiber preform including depositing a plurality of activation glass layers on the inner surface of the hollow substrate tube by a PCVD process, wherein a total thickness of the deposited activation glass layers is between 10 microns and 250 microns, and etching the deposited activation glass layers to remove at least 30% of the deposited activation glass layers.

The invention relates to a method and an apparatus for performing a plasma chemical vapour deposition process. The apparatus comprises a mainly cylindrical resonator being provided with an outer cylindrical wall and an inner coaxial cylindrical wall defining therebetween a resonant cavity that is operable at an operating frequency. The resonant cavity extends in a circumferential direction around a cylindrical axis of the inner and outer cylindrical wall. Further, the outer cylindrical wall includes an input port connectable to an input waveguide. In addition, the inner cylindrical wall includes slit sections extending in a circumferential direction around the cylindrical axis. A greatest dimension defining the aperture of the slit sections is smaller than half the wavelength of the operating frequency.

A method and apparatus for performing a plasma chemical vapour deposition process including a mainly cylindrical resonator having an outer cylindrical wall and an inner coaxial cylindrical wall defining therebetween a resonant cavity operable at an operating frequency. The resonant cavity extends in a circumferential direction around a cylindrical axis of the inner and outer cylindrical wall. The outer cylindrical wall includes an input port connectable to an input waveguide. The inner cylindrical wall includes slit sections extending in a circumferential direction around the cylindrical axis. A greatest dimension defining the aperture of the slit sections is smaller than half the wavelength of the operating frequency.