A method for manufacturing an optical fiber preform includes: producing a core preform including a core portion made of transparent glass and a first cladding layer obtained by adding fluorine to the core portion; and forming, on an outer periphery of the first cladding layer, a second cladding layer made of glass having a refractive index higher than that of the first cladding layer. Further, a refractive index profile is formed in the first cladding layer due to a fluorine concentration profile, the refractive index profile being provided at least near a boundary surface with the second cladding layer and having a profile such that a refractive index difference between a refractive index of the first cladding layer and a refractive index of the second cladding layer decreases in accordance with a reduction in a distance from the boundary surface with the second cladding layer.

A method for manufacturing an optical fiber preform includes: producing a core preform including a core portion made of transparent glass and a first cladding layer obtained by adding fluorine to the core portion; and forming, on an outer periphery of the first cladding layer, a second cladding layer made of glass having a refractive index higher than that of the first cladding layer. Further, a refractive index profile is formed in the first cladding layer due to a fluorine concentration profile, the refractive index profile being provided at least near a boundary surface with the second cladding layer and having a profile such that a refractive index difference between a refractive index of the first cladding layer and a refractive index of the second cladding layer decreases in accordance with a reduction in a distance from the boundary surface with the second cladding layer.

[Problem] To provide a radiation curable resin composition which is suitable as a primary material for optical fibers, while having a high curing rate by means of irradiation of radiation [Solution] A radiation polymerizable composition for forming an optical fiber primary coating layer, said composition containing: (A) a urethane oligomer containing a structure represented by formula (I) (in formula (I), R represents a vinyl group; and * represents a bonding hand); (B) one or more compounds selected from among (i) maleic acid anhydride, (ii) a compound represented by formula (II) (in formula (II), R.sup.I represents a single bond or an alkanediyl group having from 1 to 6 carbon atoms; and R.sup.2 represents a hydrogen atom, a hydroxy group or a specific group represented by formula (II-1) or formula (II-2)), and (iii) a compound represented by formula (III) (in formula (III), R.sup.5 represents an alkanediyl group having from 1 to 6 carbon atoms); and (C) a radiation polymerization initiator.

[Problem] To provide a radiation curable resin composition which is suitable as a primary material for optical fibers, while having a high curing rate by means of irradiation of radiation [Solution] A radiation polymerizable composition for forming an optical fiber primary coating layer, said composition containing: (A) a urethane oligomer containing a structure represented by formula (I) (in formula (I), R represents a vinyl group; and * represents a bonding hand); (B) one or more compounds selected from among (i) maleic acid anhydride, (ii) a compound represented by formula (II) (in formula (II), R.sup.I represents a single bond or an alkanediyl group having from 1 to 6 carbon atoms; and R.sup.2 represents a hydrogen atom, a hydroxy group or a specific group represented by formula (II-1) or formula (II-2)), and (iii) a compound represented by formula (III) (in formula (III), R.sup.5 represents an alkanediyl group having from 1 to 6 carbon atoms); and (C) a radiation polymerization initiator.

Described herein are systems, methods, and articles of manufacture for a spatially nonuniform scattering profile along its length, whose backscattering signal can be used for sensing even after fiber attenuation increases due to the conditions in the sensing environment. In one embodiment, the fiber has been pre-exposed to the conditions that produce attenuation, and the spatially nonuniform profile compensates for this. Subsequent exposure then results in very little or at least acceptable levels of additional attenuation. An exemplary fiber comprises a fiber length and an optical back scatter along the fiber length greater than a Rayleigh back scattering over the fiber length, wherein the optical back scatter does not decrease along the fiber length by more than 3 dB after exposure to a hydrogen-rich first environment having a given pressure and temperature. An exemplary method comprises drawing a fiber, applying a UV coating, post-processing the fiber using an interferogram, measuring optical back scatter enhancement dependence based on a UV dosage, incrementally increasing the reflectivity, exposing the fiber to a hydrogen-rich first environment.

Described herein are systems, methods, and articles of manufacture for a spatially nonuniform scattering profile along its length, whose backscattering signal can be used for sensing even after fiber attenuation increases due to the conditions in the sensing environment. In one embodiment, the fiber has been pre-exposed to the conditions that produce attenuation, and the spatially nonuniform profile compensates for this. Subsequent exposure then results in very little or at least acceptable levels of additional attenuation. An exemplary fiber comprises a fiber length and an optical back scatter along the fiber length greater than a Rayleigh back scattering over the fiber length, wherein the optical back scatter does not decrease along the fiber length by more than 3 dB after exposure to a hydrogen-rich first environment having a given pressure and temperature. An exemplary method comprises drawing a fiber, applying a UV coating, post-processing the fiber using an interferogram, measuring optical back scatter enhancement dependence based on a UV dosage, incrementally increasing the reflectivity, exposing the fiber to a hydrogen-rich first environment.

[Problem] To provide a radiation curable resin composition that is suitable as a primary material of an optical fiber and has a fast curing rate by irradiation with radiation.

[Solution] A radiation polymerizable composition for forming a primary coating layer of an optical fiber, the radiation polymerizable composition comprising: (A) a urethane oligomer comprising a structure represented by formula (I) below:

##STR00001## wherein R is a vinyl group and * is a bond; (B) one or more compounds of: (i) maleic anhydride, (ii) a compound represented by formula (II):

##STR00002## wherein R.sup.1 is a single bond or an alkanediyl group comprising 1 to 6 carbon atoms, and R.sup.2 is a hydrogen atom, a hydroxy group, or a group represented by predetermined formula (II-1) or formula (II-2), or (iii) a compound represented by formula (III):

##STR00003## wherein R.sup.5 is an alkanediyl group comprising 1 to 6 carbon atoms; and (C) a radiation polymerization initiator.

[Problem] To provide a radiation curable resin composition that is suitable as a primary material of an optical fiber and has a fast curing rate by irradiation with radiation.

[Solution] A radiation polymerizable composition for forming a primary coating layer of an optical fiber, the radiation polymerizable composition comprising: (A) a urethane oligomer comprising a structure represented by formula (I) below:

##STR00001## wherein R is a vinyl group and * is a bond; (B) one or more compounds of: (i) maleic anhydride, (ii) a compound represented by formula (II):

##STR00002## wherein R.sup.1 is a single bond or an alkanediyl group comprising 1 to 6 carbon atoms, and R.sup.2 is a hydrogen atom, a hydroxy group, or a group represented by predetermined formula (II-1) or formula (II-2), or (iii) a compound represented by formula (III):

##STR00003## wherein R.sup.5 is an alkanediyl group comprising 1 to 6 carbon atoms; and (C) a radiation polymerization initiator.

A method of fabricating an optical fiber, the method including providing a core portion including a doped portion having greater than or equal to 1.6 wt. % of a halide dopant and eliminating seed precursor sites at an exterior surface of the core portion, the seed precursor sites forming seeds in the optical fiber, wherein the eliminating the seed precursor sites includes one or more of: (i) fabricating the core portion by densifying an exterior portion of a silica soot body prior to exposing the silica soot body to the halide dopant, and (ii) exposing the exterior surface of the core portion to a reactive etchant. The method further including forming an optical fiber preform by applying cladding material to the exterior surface of the core portion and drawing the fiber preform into the optical fiber.

A curing apparatus for curing a coating composition disposed on an optical fiber, the curing apparatus including a first light source and a second light source such that the second light source is spaced from the first light with a gap. The curing apparatus further including a first reflector and a second reflector such that the second reflector is spaced from the first reflector with the gap. Furthermore, a spacer is disposed within the gap, the spacer being formed of a material configured to reflect at least about 90% of light emitted from the first light source and from the second light source, and incident on the spacer, to an optical fiber such that the reflected light has sufficient intensity to cure a coating on the optical fiber.