Multi-core optical fiber ribbons and methods for making multi-core optical fiber ribbons are described herein. In one embodiment, a multi-core optical fiber ribbon includes at least two core members formed from silica-based glass and oriented in parallel with one another in a single plane. Adjacent core members have a center-to-center spacing ≧15 microns and a cross-talk between adjacent core members is ≦−25 dB. In this embodiment each core member is single-moded with an index of refraction n.sub.c, and a core diameter d.sub.c. In an alternative embodiment, each core member is multi-moded and the center-to-center spacing between adjacent core members is ≧25 microns. A single cladding layer is formed from silica-based glass and surrounds and is in direct contact with the core members. The single cladding layer is substantially rectangular in cross section with a thickness ≦400 microns and an index of refraction n.sub.cl<n.sub.c.

A method of producing a glass preform including: forming a porous glass soot configured by an inner deposition soot deposited on a start material and an outer deposition soot deposited outside the inner deposition soot; and sintering, after the forming, the porous glass soot while doping with fluorine to form a glass body including an inner glass portion and an outer glass layer. An amount of the fluorine, with which the inner deposition soot is doped at the sintering, is equal to or more than 0 g/cm.sup.3 and less than an amount of the fluorine with which the outer deposition soot is doped.

An optical fiber preform includes: a core formed of silica glass which does not contain Ge, wherein the core has at least one of characteristics in spectrometry of (1) an absorption peak is present at a wavelength of 240 nm to 255 nm, and (2) a wavelength at which an ultraviolet transmittance is 50% or lower is longer than 170 nm.

A method is provided that includes: forming a low-index trench region with a first density; forming an inner barrier layer comprising silica around the trench region at a second density greater than the first density; depositing silica-based soot around the first barrier layer to form an overclad region at a third density less than the second density; inserting a core cane into a trench-overclad structure; forming an outer barrier layer comprising silica in an outer portion of the overclad region at a fourth density greater than the third density; flowing a down dopant-containing gas through the trench-overclad structure to dope the trench region with the down dopant, and wherein the barrier layers mitigate diffusion of the down-dopant into the overclad region; and consolidating the trench-overclad and the core cane.

A method for manufacturing a glass fine particle deposit includes: emitting a siloxane gas, a carrier gas, and a combustion gas from a burner; setting volume concentration of a supply volume amount of the siloxane gas per unit time with respect to the sum of the supply volume amount of the siloxane gas per unit time and a supply volume amount of the carrier gas per unit time (C1) to 10.6 volume %<C1<20.0 volume %; and setting volume concentration of the supply volume amount of the siloxane gas per unit time with respect to the sum of the supply volume amount of the siloxane gas per unit time, the supply volume amount of the carrier gas per unit time, and a supply volume amount of the seal gas per unit time (C2) to 5.8 volume %<C2<10.0 volume %.

A method for manufacturing an optical fiber preform includes a deposition step and an introduction step. In the deposition step, glass fine particles are generated from a glass raw material gas in a flame obtained by burning a flammable gas supplied to a burner, and the glass fine particles are deposited to produce a hollow porous glass preform. In the introduction step, a first gas is introduced into an inside of a hollow of the porous glass preform, and a second gas is introduced to an outside of the porous glass preform. In the method, at least one of the first gas and the second gas is a gas containing halogen. In the gas introduction step, the gas containing halogen is introduced so that a first partial pressure of the first gas and a second partial pressure of the second gas are different from each other.

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.

It is an object of the present invention to allow a furnace core tube used for a heat treatment apparatus of a porous glass base material to be used for a long period of time.

A heat treatment apparatus includes: a furnace core tube made of silica glass; a heater provided adjacent to the furnace core tube, the heater heating a heating region; and a moving mechanism supporting a porous glass base material and relatively moving the porous glass base material with respect to the heater in the furnace core tube in a state where the heating region is heated by the heater to make the porous glass base material pass through the heating region. The heat treatment apparatus includes a thin-walled part provided in a region adjacent to a portion located in the heating region in the furnace core tube, the thin-walled part having a thickness of glass less than that of the portion located in the heating region.

A sintering apparatus comprising: a furnace core tube containing a porous glass base material for optical fiber whose longitudinal direction is along the axial direction; and a multi-stage heater in which two or more heaters surround the furnace core tube and are arranged in the axial direction of the furnace core tube to form a heating area in the furnace core tube, is used. The sintering method includes a step in which the base material is heated in the heating area to perform a first dehydration process; and a step in which the base material is moved so that the position in the longitudinal direction of the base material where the dehydration was identified as the most insufficient, is at the position in the axial direction of the furnace core tube where the temperature is highest in the heating area, and then a second dehydration process is performed.

A single mode optical fiber is provided that includes a core region and a cladding region, the cladding region including a depressed-index cladding region, a first outer cladding region, and a second outer cladding region. The first outer cladding region has a lower relative refractive than the second outer cladding region. The single mode optical fiber has a bend loss at 1550 nm for a 15 mm diameter mandrel of less than about 0.75 dB/turn, has a bend loss at 1550 nm for a 20 mm diameter mandrel of less than about 0.2 dB/turn, and a bend loss at 1550 nm for a 30 mm diameter mandrel of less than about 0.005 dB/turn. Additionally, the single mode optical fiber has a mode field diameter of about 9.0 microns or greater at 1310 nm wavelength and a cable cutoff of less than or equal to about 1260 nm.