A coated optical fiber is generally manufactured by coating the surface of a glass optical fiber pulled from a preform (also referred to as the optical fiber base material) with resin. The coating is formed by first applying ultraviolet curable resin to the periphery of the glass optical fiber and then curing the ultraviolet curable resin by ultraviolet (UV light) irradiation. The ultraviolet curable resin is also used in various industrial fields other than the optical fiber coating. The ultraviolet curable resin is also used, for example, for curing paint or printing ink and for protective coating of various products including electronic circuits. Generally, the ultraviolet curable resin is efficiently cured in a short time by irradiation of ultraviolet light having a wavelength of about 200 nm to 400 nm.
A high-pressure mercury lamp or a metal halide lamp (hereinafter described as the high-pressure mercury lamp or the like) has heretofore been used as a light source of UV light in an optical fiber coating and curing apparatus (coated optical fiber manufacturing apparatus). The high-pressure mercury lamp has high-purity mercury (Hg) and a small amount of noble gas sealed in a luminous tube made of quartz glass. The high-pressure mercury lamp has a main wavelength of 365 nm, and efficiently emits ultraviolet light of 254 nm, 303 nm, and 313 nm. Meanwhile, the metal halide lamp similarly used has metal sealed in the form of halide in addition to mercury in a luminous tube, and emits an ultraviolet spectrum over a wide range from 200 nm to 450 nm.
As a method for coating a glass optical fiber with ultraviolet curable resin, a Wet-on-Wet method and a Wet-on-Dry method are known. The Wet-on-Wet method is a method in which multiple ultraviolet curable resin layers are cured all at once by single ultraviolet irradiation after the ultraviolet curable resin layers are applied onto a glass optical fiber. On the other hand, the Wet-on-Dry method is a method in which a single ultraviolet resin layer is applied onto a glass optical fiber and then cured by ultraviolet irradiation, and ultraviolet resin layer application and ultraviolet irradiation are further performed thereon.
In the Wet-on-Wet method used in a manufacturing process of an optical fiber, in which multiple resins are applied and then cured, multiple coating layers are efficiently cured with the ultraviolet light having a wavelength of about 200 nm to 400 nm described above. Therefore, for example, a photopolymerization initiator that absorbs ultraviolet light of about 200 nm to 350 nm is added to the upper secondary layer, and a photopolymerization initiator that absorbs ultraviolet light of about 200 nm to 350 nm and 350 nm or more is added to the lower primary layer. Thus, the respective coating layers sufficiently absorb ultraviolet light and are cured by ultraviolet irradiation using the high-pressure mercury lamp or the like described above.
To reduce power consumption in curing the ultraviolet curable resin, there has recently been an attempt to use an ultraviolet semiconductor light emitting element such as an ultraviolet laser diode (UV-LD) or an ultraviolet light emitting diode (UV-LED) as a UV light source, instead of the conventionally used high-pressure mercury lamp or the like.
For example, Patent Document 1 discloses an ultraviolet irradiation device including an ultraviolet irradiation unit including a light guide path capable of emitting ultraviolet light from the side and a semiconductor light emitting element that is an ultraviolet light source. With this device, the ultraviolet light introduced into the light guide path can be linearly emitted to a coating resin from the side of the light guide path. Thus, a sufficient cumulative amount of light can be obtained even when a small number of ultraviolet light sources are used. As a result, curability of the coating resin and the life of the device can be improved.
Moreover, Patent Document 2 discloses a method for improving curability of a coating resin in a case where a semiconductor light emitting element with a single wavelength is used as a resin curing light source, as compared with a high-pressure mercury lamp having a wide wavelength range, and the like, the method including using a coating resin obtained by adding a photopolymerization initiator having an absorption region at an emitted light wavelength of the semiconductor light emitting element to an ultraviolet curable resin composition.