Abstract: An optical fiber includes: a core portion made of glass; a side core layer made of glass and enclosing the core portion; a cladding portion made of glass and enclosing the side core layer; and a coating layer including a primary layer made of resin and enclosing the cladding portion, and a secondary layer made of resin and enclosing the primary layer. The relationships ?1>?Clad>?2 and 0>?2 are satisfied where ?1 represents relative refractive-index difference of average maximum refractive index of the core portion with respect to average refractive index of the cladding portion, ?2 represents relative refractive-index difference of average refractive index of the side core layer with respect to average refractive index of the cladding portion, and ?Clad represents relative refractive-index difference of average refractive index of the cladding portion with respect to refractive index of pure silica glass.

Abstract: A production method of an optical fiber preform includes forming a porous preform by depositing a silica particle on a circumference of a target rod; and dehydrating and sintering the porous preform by at least three thermal treatment steps. A first and a second thermal treatment steps of the three thermal treatment steps dehydrate the porous preform in an atmosphere including halogen gas or halogen-based compound gas, and a processing temperature at the second thermal treatment step is higher than a processing temperature at the first thermal treatment step.

Abstract: A furnace for dehydrating and sintering a porous glass preform includes a core tube that passes through a center portion of a furnace body to accommodate therein the porous glass preform, a heater that is arranged around the core tube in the furnace body to heat the porous glass preform in the core tube, and a core tube weight dividing and bearing means that is arranged at an outer periphery of the core tube to divide and bear the weight of the core tube in its longitudinal direction. The core tube weight dividing and bearing means includes a plurality of collars that is protruded at a predetermined interval in the longitudinal direction at the outer periphery of the core tube, a first weight receiving means that supports the collars at the outer periphery of the core tube, and a second weight receiving means that bears the weight of the first weight receiving means.

Abstract: A furnace for dehydrating and sintering a porous glass preform includes a core tube that passes through a center portion of a furnace body to accommodate therein the porous glass preform, a heater that is arranged around the core tube in the furnace body to heat the porous glass preform in the core tube, and a core tube weight dividing and bearing means that is arranged at an outer periphery of the core tube to divide and bear the weight of the core tube in its longitudinal direction. The core tube weight dividing and bearing means includes a plurality of collars that is protruded at a predetermined interval in the longitudinal direction at the outer periphery of the core tube, a first weight receiving means that supports the collars at the outer periphery of the core tube, and a second weight receiving means that bears the weight of the first weight receiving means.