An optical fiber comprising a core region 100 doped with Cl which raises
the refractive index; and a cladding region 200, provided at the outer periphery
of the core region 100, having a cladding layer 201 doped with F
which lowers the refractive index is formed. The cladding region 201 to
become the outermost cladding layer is configured such that the doping amount of
F successively decreases within an outer peripheral part 205 including the
outer periphery thereof to a predetermined doping amount which is the minimum doping
amount of F within the cladding layer 201. Therefore, the stress within
the optical fiber is dispersed into the outer peripheral part 205 having
an enhanced viscosity, whereby the stress concentration into the core is suppressed.
Since the favorable tension range at the time of drawing the optical fiber becomes
wider at this time, tension control is facilitated.
