Patent ID: 6446469
Filing Date: 2002-09-10
Classification: C03B,G02B

Abstract:
A method of fabricating a single-mode fiber mainly composed of silica glass comprising, at least, a first core portion, a second core portion formed around an outer periphery of said first core portion, a third core portion formed around an outer periphery of said second core portion, and a cladding portion formed around an outer periphery of said third core portion, said method comprising:a first step of preparing a glass tube to be said cladding portion having a predetermined refractive index, and making a material gas containing at least Si and Ge flow through a hollow part of said glass tube, while heating said glass tube, thereby forming a first soot body to be said third core portion after vitrification on an inner surface of said glass tube, said third core portion having a mean relative refractive index difference with respect to said cladding portion of a first value &Dgr;n1; a second step of making a material gas containing at least Si and Ge flow through the hollow part of said glass tube in which said first soot body is formed, while heating said glass tube, thereby forming a second soot body to be said first and second core portions after vitrification on an inner surface of said first soot body, said second core portion having a mean relative refractive index difference with respect to said cladding portion of a second value &Dgr;n2 greater than the first value &Dgr;n1, said first core portion having a mean relative refractive index difference with respect to said cladding portion of a third value &Dgr;n3 smaller than the second value &Dgr;n2; a third step of flowing a halogen gas through the hollow part of said glass tube in which said first soot body and said second soot body are formed, while heating said glass tube, thereby diffusing germanium contained in an inner surface side of said second soot body so as to reduce the germanium concentration of the inner surface side of said second soot body as compared with that of the first soot body side of said second soot body; a fourth step of heating and collapsing said glass tube in which said first soot body and said second soot body having different germanium concentrations respectively on the inner surface side and first soot body side thereof are formed, thereby obtaining a transparent optical fiber preform; and a fifth step of drawing one end of said optical fiber preform obtained at fourth step, while heating said one end, so as to yield a single-mode optical fiber having at least said first core portion of an outer diameter a, said second core portion of an outer diameter b and said third core portion of an outer diameter c.