Single-mode optical fiber and method of fabricating the same

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 Δn1;
• 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 Δn2 greater than the first value Δn1, said first core portion having a mean relative refractive index difference with respect to said cladding portion of a third value Δn3 smaller than the second value Δn2;
• 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.

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 forming a first soot body to be said first core portion after vitrification, said first core portion having a mean relative refractive index difference with respect to said cladding portion of a first value Δn1;
• a second step of forming a second soot body to be said.second core portion after vitrification around an outer periphery 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 Δn2 greater than the first value Δn1;
• a third step of forming a third soot body to be said third core portion after vitrification around an outer periphery of said second soot body, said third core portion having a mean relative refractive index difference with respect to said cladding portion of a third value Δn3 smaller than the second value Δn2;
• a fourth step of forming a fourth soot body to be said cladding portion having a predetermined refractive index after vitrification around an outer periphery of said third soot body;
• a fifth step of heating and collapsing a composite soot body formed at said fourth step, thereby obtaining a transparent optical fiber preform;
• and
• a sixth step of drawing one end of said optical fiber preform obtained at said fifth 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.

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 forming a first soot body to be said first and second core portions after vitrification around an outer periphery of a cylindrical glass rod, said first core portion having a mean relative refractive index difference with respect to said cladding portion of a first value Δn1, said second core portion having a mean relative refractive index difference with respect to said cladding portion of a second value Δn2 greater than said first value Δn1;
• a second step of forming a second soot body to be said third core portion after vitrification around an outer periphery of said first soot body, said third core portion having a mean relative refractive index difference with respect to said cladding portion of a third value Δn3 smaller than the second value Δn2;
• a third step of forming a third soot body to be said cladding portion having a predetermined refractive index after vitrification around an outer periphery of said second soot body;
• a fourth step of pulling out said glass rod and flowing a halogen gas flow through a hollow part of a tubular soot body composed of said first, second, and third soot bodies, while heating said tubular soot body, thereby diffusing germanium contained in an inner surface side of said first soot body so as to reduce the germanium concentration of the inner surface side of said first soot body as compared with that of the second soot body side of said first soot body;
• a fifth step of heating and collapsing said tubular soot body composed of said first soot body having different germanium concentrations respectively on the inner surface side and second soot body side thereof, second soot body, and third soot body, thereby obtaining a transparent optical fiber preform;
• and
• a sixth step of drawing one end of said optical fiber preform obtained at said fifth 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.

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 forming a first soot body to be said second core portion after vitrification around an outer periphery of a cylindrical glass rod, said second core portion having a mean relative refractive index difference with respect to said cladding portion being a first value Δn1;
• a second step of forming a second soot body to be said third core portion after vitrification around an outer periphery of said first soot body, said third core portion having a mean relative refractive index difference with respect to said cladding portion being a second value Δn2 smaller than the first value Δn1;
• a third step of forming a third soot body to be said cladding portion having a predetermined refractive index after vitrification around an outer periphery of said second soot body;
• a fourth step of pulling out said glass rod, and heating and sintering said first, second, and third soot bodies;
• a fifth step of inserting a cylindrical glass rod to be said first core portion after vitrification into a hollow part of the sintered body obtained at said fourth step, said first core portion having a mean relative refractive index difference with respect to said cladding portion of a third value Δn3 smaller than the first value Δn1, and heating and integrating a composite body including said sintered body and said cylindrical glass rod, thereby obtaining a transparent optical fiber preform;
• and
• a sixth step of drawing one end of the obtained optical fiber preform, 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.

A single-mode optical fiber mainly composed of silica glass, comprising:

• an inner core portion having an outer diameter of a and a predetermined refractive index;
• an outer core portion formed around an outer periphery of said inner core portion, said outer core portion having an outer diameter of b and a lower refractive index than said inner core portion;
• an inner cladding portion formed around an outer periphery of said outer core portion, said inner cladding portion having an outer diameter of c and a lower refractive index than said outer core portion;
• and
• an outer cladding portion formed around an outer periphery of said inner cladding portion, said outer cladding portion having a refractive index higher than that of said inner cladding portion and lower than that of said outer core portion,
• wherein, when said inner cladding portion has a mean relative refractive index difference with respect to said outer cladding portion of Δn1 (<0), said inner cladding portion and said outer cladding portion satisfy the following relationship:
• b·|Δn1|/(c−b)≧0.03
• therebetween.