Optical fiber preform, method of manufacturing optical fiber preform, and method of manufacturing optical fiber

Abstract

A porous layer is formed by depositing a silica glass particle around a core rod. The porous layer is dehydrated. The dehydrated porous layer is sintered under a decreased pressure until the dehydrated porous layer becomes a translucent glass layer containing a closed pore. The translucent glass layer is vitrified under an ambient atmosphere including an inert gas other than a helium gas.

1 . (canceled) 
     
     
         2 . An optical fiber preform including a core layer and a cladding layer surrounding the core layer, wherein
 the cladding layer is in a state of a translucent glass containing a closed pore inside, and   a leading edge of the cladding layer on a side of starting a drawing is in a state of a transparent glass free of the closed pore.   
     
     
         3 . The optical fiber preform according to  claim 2 , wherein
 an average density of a first portion of the state of the translucent glass is equal to or more than 1.8 grams per cubic centimeter and less than 2.2 grams per cubic centimeter.   
     
     
         4 . The optical fiber preform according to  claim 2 , wherein
 a second portion of the state of the transparent glass of the leading edge of the cladding layer on the side of starting the drawing includes at least a leading-edge tapered portion.   
     
     
         5 . The optical fiber preform according to  claim 2 , wherein
 when placing the optical fiber preform in a drawing furnace before starting the drawing process, the second portion on the side of starting the drawing process is entirely placed in the drawing furnace.