Embodiments of the current disclosure include small diameter single-mode optical fibers having gratings and methods of forming thereof. In some embodiments, methods of forming a small diameter single-mode optical fibers having gratings include providing an optical fiber having a core and cladding with a combined outer diameter of 100 μm to 125 μm and a coating having a thickness of less than or equal to 20 μm, wherein the coating comprises one of: (i) a high-modulus coating layer surrounding the cladding region; or (ii) a low-modulus coating layer surrounding the cladding region and a high-modulus coating layer surrounding the low-modulus coating layer; and exposing the core, through the coating, to a pattern of ultraviolet radiation to form an optical grating within the core.

Provided is a colored optical fiber of which a primary layer is easily formed. The colored optical fiber includes a bare optical fiber, a primary layer formed of an ultraviolet curing resin covering the bare optical fiber, and a secondary layer formed of an ultraviolet curing resin covering the primary layer. Young's modulus of the primary layer is smaller than 70% with respect to a saturated Young's modulus of the primary layer. The saturated Young's modulus of the primary layer is larger than or equal to 0.84 MPa.

Provided is a colored optical fiber of which a primary layer is easily formed. The colored optical fiber includes a bare optical fiber, a primary layer formed of an ultraviolet curing resin covering the bare optical fiber, and a secondary layer formed of an ultraviolet curing resin covering the primary layer. Young's modulus of the primary layer is smaller than 70% with respect to a saturated Young's modulus of the primary layer. The saturated Young's modulus of the primary layer is larger than or equal to 0.84 MPa.

A resin composition comprises: a base resin containing an oligomer, a monomer, and a photopolymerization initiator; and inorganic oxide particles, wherein the inorganic oxide particles are lump-shaped aggregated particles, and the volume average particle size of the inorganic oxide particles measured by an X-ray small angle scattering method is 5 nm or more and 800 nm or less.

A resin composition comprises: a base resin containing an oligomer, a monomer, and a photopolymerization initiator; and inorganic oxide particles, wherein the inorganic oxide particles are lump-shaped aggregated particles, and the volume average particle size of the inorganic oxide particles measured by an X-ray small angle scattering method is 5 nm or more and 800 nm or less.

An apparatus for curing a coating composition disposed on a glass fiber includes a diffuse reflector surrounding a coating composition disposed on a glass fiber. The diffuse reflector defines a cavity having a sidewall extending from a first end to a second end. The first end has a first opening and the second end has a second opening. The glass fiber passes through the cavity from the first opening to the second opening. The sidewall has an interior surface facing the coating composition disposed on the glass fiber. The interior surface includes a scattering material. A light source integrated with the diffuse reflector. The light source directs light to the scattering material. The scattering material diffusely reflects at least 90% of the light. The diffusely reflected light has sufficient intensity to cure the coating composition.

An apparatus for curing a coating composition disposed on a glass fiber includes a diffuse reflector surrounding a coating composition disposed on a glass fiber. The diffuse reflector defines a cavity having a sidewall extending from a first end to a second end. The first end has a first opening and the second end has a second opening. The glass fiber passes through the cavity from the first opening to the second opening. The sidewall has an interior surface facing the coating composition disposed on the glass fiber. The interior surface includes a scattering material. A light source integrated with the diffuse reflector. The light source directs light to the scattering material. The scattering material diffusely reflects at least 90% of the light. The diffusely reflected light has sufficient intensity to cure the coating composition.

A resin composition for primary coating of an optical fiber includes a photopolymerizable compound and a photopolymerization initiator. The photopolymerizable compound includes a urethane oligomer and an N-vinyl compound represented by the formula (I) below:

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(where R represents an alkyl group having 1 to 5 carbon atoms).

A resin composition for secondary coating of an optical fiber includes a photopolymerizable compound including a urethane (meth)acrylamide, and a photopolymerization initiator. The urethane (meth)acrylamide has a (meth)acrylamide group at at least one end of a urethane bond.

A system for precoating a preform for drawing optical fiber including a diameter sensor to determine a diameter of pulled optical fiber, a cooling system to cool the optical fiber once it is pulled from a furnace, a coating system to apply a coating to the optical fiber once it has cooled and an ultra-violet lamp to cure the coating.