A co-doped optical fiber is provided having an attenuation of less than about 0.17 dB/km at a wavelength of 1550 nm. The fiber includes a core region in the fiber having a graded refractive index profile with an alpha of greater than 5. The fiber also includes a first cladding region in the fiber that surrounds the core region. Further, the core region has a relative refractive index of about −0.10% to about +0.05% compared to pure silica. In addition, the core region includes silica that is co-doped with chlorine at about 1.2% or greater by weight and fluorine between about 0.1% and about 1% by weight.

A glass sheet includes a first main surface and a second main surface opposite to the first main surface in a thickness direction. X represented by the following formula (1) is −0.29<X<0.29: A×Δ.sup.1H/.sup.30Si+B×ΔNa.sub.2O+C×ΔSn+D×ΔF=X (1). F.sub.0-3 determined according to the following formula (II) is 0.02 or more: F.sub.0-3=[average fluorine concentration (wt %) by SIMS at depth of 0 to 3 μm in first main surface]×3 (II).

An optical fiber manufacturing method includes: a drawing step of heating one end portion of an optical fiber preform to melt and deform the one end portion and drawing an optical fiber, wherein in the drawing step, drawing is performed while applying pressure to a melted-deformed portion that is melted and deformed.

An optical fiber can include a core comprising silica co-doped with nitrogen and chlorine and an outer cladding surrounding the core. In some aspects, the core can be characterized by an annealing temperature of less than or equal to about 1150° C. and/or the core can include a relative refractive index Δ.sub.core in a range of from about 0.15% to about 0.45%.

An optical fiber according to an embodiment has a structure capable of reducing an increase in transmission loss. The optical fiber includes a glass part extending in a direction of a central axis, and the glass part is comprised of silica-based glass, includes a core and a cladding, and has residual stress approximately uniform throughout a cross section of the glass part orthogonal to the central axis, the core having the central axis and being doped with chlorine with a mass fraction of 1% or more, the cladding surrounding the core and having a refractive index lower than a maximum refractive index of the core.

A radiation-resistant laser optical fiber preform core rod at least includes one type of activated ion (Yb.sup.3+, Er.sup.3+) and one or more types of co-doped ion (Al.sup.3+, P.sup.5+, Ge.sup.4+, Ce.sup.3+, F.sup.−), and —OD group of 16-118 ppm. Irradiation resistance of core rod glass can be effectively improved by sequentially performing pre-treatments, i.e. deuterium loading, pre-irradiation and thermal annealing on a preform core rod. Electron paramagnetic resonance test shows that, under the same radiation condition, the radiation induced color center concentration in a preform core rod treated by the method above is lower than in an untreated core rod by one or more orders of magnitude. The obtained core rod can be used for preparing a radiation-resistant rare earth-doped silica fiber, and has the advantages of high laser slope efficiency, low background loss, being able to be used stably in a vacuum environment for a long time, for example.

An optical fiber according to an embodiment includes a core, a cladding, and a coating layer. At the boundary between the core and the cladding, the local sound velocity decreases in the direction from the core side toward the cladding side. At least in the cladding, the local sound velocity changes continuously in a radial direction. Further, the line width of the Brillouin gain of the light beam guided by the fundamental mode is 60 MHz or more.

An optical fiber containing alkali metal elements or the like in which Rayleigh scattering loss can be reduced is provided. An optical fiber includes a core composed of silica glass and a cladding which surrounds the core, has a refractive index lower than a refractive index of the core, and is composed of silica glass containing fluorine. The core contains a first group of dopants and a second group of dopants having a diffusion coefficient lower than a diffusion coefficient of the first group of dopants. The difference between the maximum value and the minimum value of residual stress in the optical fiber is 150 MPa or less.

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 glass powder composite includes a first glass powder, and a second glass powder having a different solubility from that of the first glass powder depending on pH, wherein both the first glass powder and the second glass powder have ion sustained-release properties.