An optical fiber comprises a glass fiber comprising a core and a cladding, a primary resin layer being in contact with the glass fiber and covering the glass fiber, and a secondary resin layer covering the primary resin layer, wherein the Young's modulus of the primary resin layer is 0.04 MPa or more and 1.0 MPa or less at 23 C.2 C., the secondary resin layer consists of a cured product of a resin composition comprising a base resin containing a urethane (meth)acrylate oligomer, a monomer, and a photopolymerization initiator and hydrophobic inorganic oxide particles, and the content of the inorganic oxide particles is 1% by mass or more and 60% by mass or less based on the total amount of the resin composition.

An optical fiber comprises a glass fiber comprising a core and a cladding, a primary resin layer being in contact with the glass fiber and covering the glass fiber, and a secondary resin layer covering the primary resin layer, wherein the Young's modulus of the primary resin layer is 0.04 MPa or more and 1.0 MPa or less at 23 C.2 C., the secondary resin layer consists of a cured product of a resin composition comprising a base resin containing a urethane (meth)acrylate oligomer, a monomer, and a photopolymerization initiator and hydrophobic inorganic oxide particles, and the content of the inorganic oxide particles is 1% by mass or more and 60% by mass or less based on the total amount of the resin composition.

The systems and methods of forming optical fiber coatings with reduced defects include moving a bare optical fiber through first and second coating sub-systems. The first coating sub-system forms a first coating on the bare optical fiber by depositing a first coating material and then curing the deposited first coating material with actinic light. This process also results in the formation of stray actinic light. The process also includes moving the coated optical fiber through a second coating sub-system to form a second coating on the first coating. A light-blocking device resides between the first and second coating sub-systems to block the stray actinic light. Without the light-blocking device, the stray actinic light can enter the second coating sub-system and reach the second coating material therein and form a gel therefrom, which in turn leads to defects in the coated optical fiber exiting the second coating sub-system.

A resin composition comprises a base resin containing an urethane (meth)acrylate oligomer, a monomer having a phenoxy group, and a photopolymerization initiator, and hydrophobic inorganic oxide particles, wherein the viscosity is 300 mPa.Math.s or more and 4200 mPa.Math.s or less at 45 C. and the content of the monomer having a phenoxy group is 1% by mass or more and 30% by mass or less based on the total amount of the base resin.

A resin composition comprises a base resin containing an urethane (meth)acrylate oligomer, a monomer having a phenoxy group, and a photopolymerization initiator, and hydrophobic inorganic oxide particles, wherein the viscosity is 300 mPa.Math.s or more and 4200 mPa.Math.s or less at 45 C. and the content of the monomer having a phenoxy group is 1% by mass or more and 30% by mass or less based on the total amount of the base resin.

The optical fiber comprises a glass fiber comprising a core and a cladding, a primary resin layer in contact with the glass fiber and covering the glass fiber, and a secondary resin layer covering the primary resin layer, wherein the primary resin layer contains a cured product of a resin composition containing a urethane (meth)acrylate oligomer, a monomer, a photopolymerization initiator and an aromatic acid compound, a content of the aromatic acid compound is 20 ppm or more and 12000 ppm or less based on a total amount of the resin composition, and Young's modulus of the primary resin layer is 0.6 MPa or less at 23 C.2 C.

The optical fiber comprises a glass fiber comprising a core and a cladding, a primary resin layer in contact with the glass fiber and covering the glass fiber, and a secondary resin layer covering the primary resin layer, wherein the primary resin layer contains a cured product of a resin composition containing a urethane (meth)acrylate oligomer, a monomer, a photopolymerization initiator and an aromatic acid compound, a content of the aromatic acid compound is 20 ppm or more and 12000 ppm or less based on a total amount of the resin composition, and Young's modulus of the primary resin layer is 0.6 MPa or less at 23 C.2 C.

An optical fiber comprises a glass fiber comprising a core and a cladding, a primary resin layer being in contact with the glass fiber and covering the glass fiber, and a secondary resin layer covering the primary resin layer, wherein the Young's modulus of the primary resin layer is 0.04 MPa or more and 1.0 MPa or less at 23 C.2 C., the secondary resin layer consists of a cured product of a resin composition comprising a base resin containing a urethane (meth)acrylate oligomer, a monomer, and a photopolymerization initiator and hydrophobic inorganic oxide particles, and the content of the inorganic oxide particles is 1% by mass or more and 60% by mass or less based on the total amount of the resin composition.

An optical fiber comprises a glass fiber comprising a core and a cladding, a primary resin layer being in contact with the glass fiber and covering the glass fiber, and a secondary resin layer covering the primary resin layer, wherein the Young's modulus of the primary resin layer is 0.04 MPa or more and 1.0 MPa or less at 23 C.2 C., the secondary resin layer consists of a cured product of a resin composition comprising a base resin containing a urethane (meth)acrylate oligomer, a monomer, and a photopolymerization initiator and hydrophobic inorganic oxide particles, and the content of the inorganic oxide particles is 1% by mass or more and 60% by mass or less based on the total amount of the resin composition.

Provided are a surface-treated glass cloth capable of enhancing insulation reliability when used to prepare a prepreg, a prepreg and a printed wiring board using the surface-treated glass cloth. The surface-treated glass cloth includes a surface treatment layer on a surface, a glass constituting the glass cloth has a composition containing 52.0 to 60.0 mass % of SiO.sub.2, 15.0 to 26.0 mass % of B.sub.2O.sub.3, 9.0 to 18.0 mass % of Al.sub.2O.sub.3, 1.0 to 8.0 mass % of MgO, 1.0 to 10.0 mass % of CaO, 0 to 6.0 mass % of SrO, 0 to 6.0 mass % of TiO.sub.2, and 0.1 to 3.0 mass % in total of F.sub.2 and Cl.sub.2, based on the total amount of the glass, the glass cloth has a surface coverage of 75.0 to 100.0% and a thickness of 8 to 95 ?m, and the surface treatment layer contains a silane coupling agent having a methacrylic group and contains no surfactant.