Single-mode optical fiber and method of fabricating the same

Abstract

The present invention relates to a single-mode optical fiber having a configuration which enables lowering of dispersion slope while securing a sufficient MFD. This single-mode optical fiber has a refractive index profile in which an indent with a sufficient width is provided at the center of its core region. In particular, this indent satisfies the following relationship: a ·(Δ n 2 −Δn 1 )/( b·Δn 2 )≧0.04 when the first core portion in the single-mode optical fiber has a mean relative refractive index difference of Δn 1 with respect to the cladding portion and an outer diameter of a while the second core portion has a mean relative refractive index difference of Δn 2 with respect to the cladding portion and an outer diameter of b.

What is claimed is:  
     
         1 . A single-mode optical fiber containing silica glass as a main component, comprising a core region having a predetermined refractive index and a cladding region which is formed around an outer periphery of said core region and whose refractive index is set lower than that of said core region, said core region comprising: 
 a first core portion having an outer diameter of a and a mean relative refractive index difference with respect to said cladding region being a first value Δn 1 ;    a second core portion formed around an outer periphery of said first core portion, said second core portion having an outer diameter of b and a mean relative refractive index difference with respect to said cladding region of a second value Δn 2  greater than the first value Δn 1 , wherein said first core portion and said second core portion satisfy the following relationship:      a ·(Δ n   2   −Δn   1 )/( b·Δn   2 )≧0.04    therebetween; and    a third core portion formed around an outer periphery of said second core portion, said third core portion having an outer diameter of c and a mean relative refractive index difference with respect to said cladding region of a third value Δn 3  smaller than the second value Δn 2 .    
     
     
         2 . A single-mode optical fiber according to  claim 1 , wherein the third value Δn 3  is not less than 0.03%.  
     
     
         3 . A single-mode optical fiber according to  claim 1 , wherein the second value Δn 2  is not less than 0.4%.  
     
     
         4 . A single-mode optical fiber according to  claim 1 , wherein said second core portion and said third core portion satisfy the following relationship:  
       0.1 ≦b/c≦ 0.4  
       therebetween.  
     
     
         5 . A single-mode optical fiber according to  claim 4 , wherein said second core portion and said third core portion satisfy the following relationship:  
       0.1≦ b/c≦ 0.3  
       therebetween.  
     
     
         6 . A single-mode optical fiber according to  claim 1 , wherein said first core portion and said second core portion satisfy the following relationship:  
         a ·(Δ n   2   −Δn   1 )/( b·Δn   2 )≦0.5  
       therebetween.  
     
     
         7 . A single-mode optical fiber according to  claim 1 , wherein said third core portion comprises: 
 an inner core formed around the outer periphery of said second core portion, said inner core portion having an outer diameter of c a  (<c) and a mean relative refractive index difference with respect to said cladding region of a fourth value Δn 3a  smaller than the second value Δn 2 ; and    an outer core spaced from said second core portion by way of said inner core, said outer core having an outer diameter of c and a mean relative refractive index difference with respect to said cladding region being a fifth value Δn 3b  greater than the fourth value Δn 3a  and smaller than the second value Δn 2 , and    wherein said inner core and said outer core satisfy the following relationship:    0.1≦( c   a   −b )·(Δ n   3b   −Δn   3a )/( c·Δn   3b )≦0.8    therebetween.    
     
     
         8 . A single-mode optical fiber according to  claim 7 , wherein the fifth value Δn 3b  is not less than 0.03%.  
     
     
         9 . A single-mode optical fiber according to  claim 7 , wherein the second value Δn 2  is not less than 0.4%.  
     
     
         10 . A single-mode optical fiber according to  claim 7 , wherein said second core portion and said third core portion satisfy the following relationship:  
       0.1≦ b/c≦ 0.4  
       therebetween.  
     
     
         11 . A single-mode optical fiber according to  claim 10 , wherein said second core portion and said third core portion satisfy the following relationship:  
       0.1≦ b/c≦ 0.3  
       therebetween.  
     
     
         12 . A single-mode optical fiber according to  claim 7 , wherein said first core portion and said second core portion satisfy the following relationship:  
         a ·(Δ n   2   −Δn   1 )/( b·Δn   2 )≦0.5  
       therebetween.  
     
     
         13 . 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 Δn 1 ;    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 Δn 2  greater than the first value Δn 1 , said first core portion having a mean relative refractive index difference with respect to said cladding portion of a third value Δn, smaller than the second value Δn 2 ;    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.    
     
     
         14 . A method according to  claim 13 , wherein, in said fifth step, said optical fiber preform obtained at said fourth step is drawn so as to yield a single-mode optical fiber in which said first core portion and said second core portion satisfy the following relationship:  
         a ·(Δ n   2   −Δn   3 )/( b·Δn   2 )≧0.04  
       therebetween.  
     
     
         15 . A method according to  claim 14 , wherein said first step is a step of forming a first soot body to be inner and outer cores of said third core portion after vitrification, said inner core being formed around the outer periphery of said second core portion and having a mean relative refractive index difference with respect to said cladding portion of a fourth value Δn 4  smaller than the second value Δn 1 , said outer core being formed around an outer periphery of said inner core and having a mean relative refractive index difference with respect to said cladding portion of a fifth value Δn 5  greater than the fourth value Δn 4  and smaller than the second value Δn 2 ; 
 wherein said first step includes a process of flowing a halogen gas through the hollow part of said glass tube in which said first soot body is formed, while heating said glass tube, thereby diffusing germanium contained in the 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 glass tube side of said first soot body; and  
 wherein, in said fifth step, said optical fiber preform obtained at said fourth step is drawn so as to yield a single-mode optical fiber in which said inner and outer cores of said third core portion have outer diameters of c a  (<c) and c, respectively, and said inner and outer cores satisfy the following relationship:  
 0.1≦( c   a   −b )·(Δ n   3   −Δn   4 )/( c·Δn   3 )≦0.8  
 therebetween.  
 
     
     
         16 . 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 Δn 1 ;    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 Δn 2  greater than the first value Δn 1 ;    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 Δn 3  smaller than the second value Δn 2 ;    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.    
     
     
         17 . A method according to  claim 16 , wherein, in said sixth step, said optical fiber preform obtained at said fifth step is drawn so as to yield a single-mode optical fiber in which said first core portion and said second core portion satisfy the following relationship:  
         a ·(Δ n   2   −Δn   1 )/( b·Δn   2 )≧0.04  
       therebetween.  
     
     
         18 . A method according to  claim 17 , wherein said third step includes: 
 a first sub-process of forming an inner soot body to be an inner core of said third core portion after vitrification around the outer periphery of said second soot body, said inner core being positioned around the outer periphery of said second core portion and having a mean relative refractive index difference with respect to said cladding portion of a fourth value Δn 4  smaller than the second value Δn 2 ; and    a second sub-process of forming an outer soot body to be an outer core of said third core portion after vitrification around an outer periphery of said inner soot body, said outer core being positioned around the outer periphery of said inner core and having a mean relative refractive index difference with respect to said cladding portion of a fifth value Δn 5  greater than the fourth value Δn 4  and smaller than the second value Δn 5 , and    wherein, in said sixth step, said optical fiber preform obtained at said fifth step is drawn so as to yield a single-mode optical fiber in which said inner and outer cores of said third core portion have outer diameters of c a  (<c) and c, respectively, and said inner and outer cores satisfy the following relationship:    0.1≦( c   a   −b )·(Δ n   5   −Δn   4 )/( c·Δn   5 )≦0.8    therebetween.    
     
     
         19 . 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 Δn 1 , said second core portion having a mean relative refractive index difference with respect to said cladding portion of a second value Δn 2  greater than said first value Δn 1 ;    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 Δn 3  smaller than the second value Δn 2 ;    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.    
     
     
         20 . A method according to  claim 19 , wherein, in said sixth step, said optical fiber preform obtained at said fifth step is drawn so as to yield a single-mode optical fiber in which said first core portion and said second core portion satisfy the following relationship:  
         a ·(Δ n   2   −Δn   1 )/( b·Δn   2 )≧0.04  
       therebetween.  
     
     
         21 . A method according to  claim 20 , wherein said second step includes: 
 a first sub-process of forming an inner soot body to be an inner core of said third core portion after vitrification around the outer periphery of said first soot body, said inner core being positioned around the outer periphery of said second core portion and having a mean relative refractive index difference with respect to said cladding portion of a fourth value Δn 4  smaller than the second value Δn 2 ; and    a later process of forming an outer soot body to be an outer core of said third core portion after vitrification around an outer periphery of said inner soot body, said outer core being positioned around an outer periphery of said inner core and having a mean relative refractive index difference with respect to said cladding portion of a fifth value Δn 5  greater than the fourth value Δn 4  and smaller than the second value Δn 2 , and    wherein, in said sixth step, said optical fiber preform obtained at said fifth step is drawn so as to yield a single-mode optical fiber in which said inner and outer cores of said third core portion have outer diameters of c a  (<c) and c, respectively, and said inner and outer cores satisfy the following relationship:    0.1≦( c   a   −b )·(Δ n   5   −Δn   4 )/( c·Δn   5 )≦0.8    therebetween.    
     
     
         22 . 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 Δn 1 ;    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 Δn 2  smaller than the first value Δn,;    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 Δn 3  smaller than the first value Δn 1 , 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.    
     
     
         23 . A method according to  claim 22 , wherein, in said sixth step, said optical fiber preform obtained at said fifth step is drawn so as to yield a single-mode optical fiber in which said first core portion and said second core portion satisfy the following relationship:  
         a ·(Δ n   1   −Δn   3 )/( b·Δn   1 )≧0.04  
       therebetween.  
     
     
         24 . A method according to  claim 23 , wherein said second step includes: 
 a first sub-process of forming an inner soot body to be an inner core of said third core portion after vitrification around the outer periphery of said first soot body, said inner core being positioned around the outer periphery of said second core portion and having a mean relative refractive index difference with respect to said cladding portion of a fourth value Δn 4  smaller than the first value Δn 1 ; and    a later process of forming an outer soot body to be an outer core of said third core portion after vitrification around an outer periphery of said inner soot body, said outer core being positioned around an outer periphery of said inner core and having a mean relative refractive index difference with respect to said cladding portion of a fifth value Δn 5  greater than the fourth value Δn 4  and smaller than the first value Δn 1 , and    wherein, in said sixth step, said optical fiber preform obtained at said fifth step is drawn so as to yield a single-mode optical fiber in which said inner and outer cores of said third core portion have outer diameters of c a  (<c) and c, respectively, and said inner and outer cores satisfy the following relationship:    0.1≦( c   a   −b )·(Δ n   5   −Δn   4 )/( c·Δn   5 )≦0.8    therebetween.    
     
     
         25 . 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 Δn 1  (<0), said inner cladding portion and said outer cladding portion satisfy the following relationship:      b ·|Δn 1 |/( c−b )≧0.03    therebetween.    
     
     
         26 . A single-mode optical fiber according to  claim 25 , wherein said outer core and said inner cladding satisfy the following relationship:  
         c/b≦ 4  
       therebetween.