An optical fiber coating apparatus that provides increased gyre stability and reduced gyre strength, thereby providing a more reliable coating application process during fiber drawing includes a cone-only coating die having a conical entrance portion with a tapered wall angled at a half angle , wherein 225, and a cone height L.sub.1 less than 2.2 mm, and a cylindrical portion having an inner diameter of d.sub.2, wherein 0.1 mmd.sub.20.5 mm and a cylindrical height of L.sub.2, wherein 0.05 mmL.sub.21.25 mm; a guide die having an optical fiber exit, the guide die disposed adjacent the cone-only coating die such that a wetted length (L.sub.5) between the optical fiber exit of the guide die and the entrance of the cone-only coating die is from 1 mm to 5 mm; and a holder for holding the cone-only coating die and the guide die in a fixed relationship defining a coating chamber between the guide die and the cone-only coating die, the coating chamber having an inner radius L.sub.6 from the optical fiber axis to an inner wall of the holder that is from 3 mm to 10 mm.

An optical fiber coating apparatus that provides increased gyre stability and reduced gyre strength, thereby providing a more reliable coating application process during fiber drawing includes a cone-only coating die having a conical entrance portion with a tapered wall angled at a half angle , wherein 225, and a cone height L.sub.1 less than 2.2 mm, and a cylindrical portion having an inner diameter of d.sub.2, wherein 0.1 mmd.sub.20.5 mm and a cylindrical height of L.sub.2, wherein 0.05 mmL.sub.21.25 mm; a guide die having an optical fiber exit, the guide die disposed adjacent the cone-only coating die such that a wetted length (L.sub.5) between the optical fiber exit of the guide die and the entrance of the cone-only coating die is from 1 mm to 5 mm; and a holder for holding the cone-only coating die and the guide die in a fixed relationship defining a coating chamber between the guide die and the cone-only coating die, the coating chamber having an inner radius L.sub.6 from the optical fiber axis to an inner wall of the holder that is from 3 mm to 10 mm.

A method of forming a crystalline core/crystalline clad (C4) optical fiber. The method comprises coextruding a cladding mixture of a plasticizer and a binder with a yttrium aluminum garnet (YAG) core. The coextrusion dynamically clads a polycrystalline cladding onto the YAG core to yield a green C4 optical fiber. The C4 optical fiber is then densified, preferably in two steps sintering and hot isostatic pressing. The resulting optical C4 fiber has greater power capacity than a glass fiber labor host.

A tri-layer optical fiber. The fiber has a core which is preferably a single crystal and outer cladding, to reduce signal loss. The cladding process begins with high porosity particles, and is known to introduce porosity in the cladding and at the core/cladding interface. The porosity increases refraction and signal loss. The invention interposes a film layer intermediate the core and cladding. The film layer is sputter coated and preferably of the of same material as the cladding. The film prevents diffusion of the porous cladding into the core, minimizing porosity and improving signal transmission.

A tri-layer optical fiber. The fiber has a core which is preferably a single crystal and outer cladding, to reduce signal loss. The cladding process begins with high porosity particles, and is known to introduce porosity in the cladding and at the core/cladding interface. The porosity increases refraction and signal loss. The invention interposes a film layer intermediate the core and cladding. The film layer is sputter coated and preferably of the of same material as the cladding. The film prevents diffusion of the porous cladding into the core, minimizing porosity and improving signal transmission.

Coated optical fibers and uses of such fibers as sensors in high temperature and/or high pressure environments. The coated optical fiber has improved sensing properties at elevated pressure and/or temperature, such as enhanced acoustic sensitivity and/or a reduced loss in acoustic sensitivity. The use of the coated optical fibers in various sensing applications that require operation under elevated pressure and/or temperature, such as, acoustic sensors for various geological, security, military, aerospace, marine, and oil and gas applications are also provided.

A coating device includes: a fiber passage through which a glass fiber passes downward in a vertical direction; a first flow path which is a flow path allowing a primary resin to flow toward the fiber passage and includes a first branch path horizontally dividing the primary resin moving in a horizontal direction; a first temperature controller which is disposed along the first flow path and controls a temperature of the first flow path; a second flow path which is a flow path allowing a secondary resin to flow toward the fiber passage, includes a second branch path horizontally dividing the secondary resin moving in the horizontal direction, and is located below the first flow path; and a second temperature controller which is disposed along the second flow path and controls a temperature of the second flow path.

A coating device includes: a fiber passage through which a glass fiber passes downward in a vertical direction; a first flow path which is a flow path allowing a primary resin to flow toward the fiber passage and includes a first branch path horizontally dividing the primary resin moving in a horizontal direction; a first temperature controller which is disposed along the first flow path and controls a temperature of the first flow path; a second flow path which is a flow path allowing a secondary resin to flow toward the fiber passage, includes a second branch path horizontally dividing the secondary resin moving in the horizontal direction, and is located below the first flow path; and a second temperature controller which is disposed along the second flow path and controls a temperature of the second flow path.