Optical fiber

Abstract

An optical fiber in which the increase of attenuation can be reduced is offered. An optical fiber is made of silica glass and includes a core and a cladding enclosing the core. The refractive index of the cladding is smaller than that of the core. The core includes chlorine and any of the alkali metal group, The chlorine concentration is 1 ppm or more in the whole region of the core. In the whole region of the core, the absolute value of rate of radial change of the chlorine concentration is smaller than 2000 ppm/μm.

1 . An optical fiber made of silica glass and containing a core and a cladding,
 the cladding enclosing the core and having a refractive index smaller than the refractive index of the core,   the core containing chlorine as well as an alkali metal or an alkaline-earth-metal element,   the core having a chlorine concentration of 1 ppm or more in the whole region thereof the chlorine concentration being measured with an EPMA using an electron beam having a diameter of 1 μm, and   the absolute value of rate of radial change in the chlorine concentration being smaller than 2000 ppm/μm.   
     
     
         2 . An optical fiber as set forth in  claim 1 , wherein
 the absolute value of the rate of radial change in the chlorine concentration of the whole region of the core is less than 1000 ppm/μm.   
     
     
         3 . An optical fiber as set forth in  claim 1 , wherein
 the absolute value of the rate of radial change in the chlorine concentration of the whole region of the core is less than 500 ppm/μm.   
     
     
         4 . An optical fiber as set forth in  claim 1 , wherein
 the absolute value of the rate of radial change in the chlorine concentration of the whole region of the core is less than 300 ppm/μm.   
     
     
         5 . An optical fiber as set forth in  claim 1 , wherein
 the absolute value of the rate of radial change in the chlorine concentration of the whole region of the core is more than 100 ppm/μm.   
     
     
         6 . An optical fiber as set forth in  claim 1 , wherein
 the absolute value of the rate of radial change in the chlorine concentration of the whole region of the core is more than 200 ppm/μm.   
     
     
         7 . An optical fiber as set forth in  claim 1 , wherein
 the core contains fluorine.   
     
     
         8 . An optical fiber as set forth in  claim 7 , wherein
 the average fluorine concentration in the core is 500 to 4000 ppm.   
     
     
         9 . An optical fiber as set forth in  claim 1 , wherein
 the average value of the concentration of alkali metal or alkaline-earth-metal element in the core is 0.2 to 200 ppm.   
     
     
         10 . An optical fiber as set forth in  claim 1 , wherein
 the core contains, for alkali metal or alkaline-earth-metal element, one or more elements selected from a group consisting of sodium, potassium, rubidium, cesium, and calcium.