Optical fiber and optical transmission path

Abstract

Under condition that a non-circularity ratio is 5% or lower and a thermal expansion coefficient of a glass which forms the core is α1 and a thermal expansion of a glass which forms the cladding is α2, the difference of coefficients is controlled such that a formula −2.5×10 −7 /° C.≦α1−α2≦1.0×10 −7 /° C. is satisfied so as to maintain a polarization mode dispersion to be 0.03 ps/km 0.5 or lower. The difference of coefficients is further controlled such that a formula −1.5×10 −7 /° C.≦α1−α2≦0/° C. is satisfied so as to maintain a polarization mode dispersion to be 0.015 ps/km 0.5 or lower. By doing this, birefringence is reduced by adjusting the thermal expansion coefficient in a core and a cladding; thus providing an optical fiber, and an optical transmission path using the optical fiber, having preferable PMD for high speed transmission.

What is claimed is:  
     
         1 . An optical fiber which is made mainly from a silica glass having a layer of core and a layer of cladding, wherein: 
 a formula −2.5×10 −7 /° C.≦α1−α2≦1.0×10 −7 /° C. is satisfied; and    a polarization mode dispersion is 0.03 ps/km 0.5  or lower;    under condition α1 is a thermal expansion coefficient of glass which forms the layer of core and α2 is a thermal expansion coefficient of glass which forms the layer of cladding.    
     
     
         2 . An optical fiber according to  claim 1 , wherein: 
 a formula −1.5×10 −7 /° C.≦α1−α2≦0/° C. is satisfied; and    the polarization mode dispersion is 0.015 ps/km 0.5  or lower.    
     
     
         3 . An optical fiber according to  claim 1 , wherein: 
 the layer of cladding is a silica glass to which at least a fluorine is doped;    the layer of core is an approximately pure silica glass, or the layer of core is a silica glass doped with a germanium and/or a fluorine; and    a transmission loss at a wavelength of 1550 nm is 0.20 dB/km or lower.    
     
     
         4 . An optical fiber according to  claim 3 , wherein 
 a maximum concentration of germanium which is doped into the layer of core is 1.5 wt %;    a maximum concentration of fluorine which is doped into the layer of core is also 1.5 wt %.    
     
     
         5 . An optical fiber according to  claim 3 , wherein a germanium together with a fluorine is doped into the layer of cladding so as to improve hydrogen characteristics.  
     
     
         6 . An optical fiber according to  claim 5 , wherein 
 a formula C2−C1≧−0.5 wt % is satisfied,    under condition that C2 is the concentration of germanium in of the layer of cladding and C1 is the concentration of germanium in the layer of core.    
     
     
         7 . An optical fiber according to  claim 5 , wherein 
 a maximum concentration of germanium which is doped into the layer of core is 1.5 wt %;    a maximum concentration of fluorine which is doped into the layer of core is also 1.5 wt %.    
     
     
         8 . An optical fiber according to  claim 1 , wherein: 
 an optical fiber preform is twisted and drawn; and    a polarization mode dispersion is 0.01 ps/km 0.5  or lower.    
     
     
         9 . An optical transmission path formed by combining the optical fiber according to  claim 1  as a first optical fiber, and a second optical fiber which compensates for a chromatic dispersion and/or a dispersion slope of the first optical fiber.  
     
     
         10 . An optical fiber which is made mainly from a silica glass having a layer of core and two layers of cladding with a refractive index in an outer layer of cladding higher than a refractive index in an internal layer of cladding, wherein; 
 a formula −2.5×10 −7 /° C.≦α1−α2≦1.0×10 −7 /° C. is satisfied; and    a polarization mode dispersion is 0.03 ps/km 0.5  or lower;    under condition α1 is a thermal expansion coefficient of glass which forms the layer of core and α2 is a thermal expansion coefficient of glass which forms the layer of cladding having the lowest thermal expansion coefficient of said two layers of cladding.    
     
     
         11 . An optical fiber according to  claim 10 , wherein; 
 a formula −1.5×10 −7 /° C.≦α1−α2≦0/° C. is satisfied; and    a polarization mode dispersion is 0.015 ps/km 0.5  or lower;    under condition that α1 is the thermal expansion coefficient of glass which forms the layer of core and α2 is the thermal expansion coefficient of the cladding layer having the lowest thermal expansion coefficient of said two layers of cladding.    
     
     
         12 . An optical fiber according to  claim 10 , wherein: 
 at least one of the layers of cladding is a silica glass to which at least a fluorine is doped;    the layer core is an approximately pure silica glass, or the layer of core is a silica glass doped with a germanium and/or a fluorine; and    a transmission loss at a wavelength of 1550 nm is 0.22 dB/km or lower.    
     
     
         13 . An optical fiber according to  claim 12 , wherein 
 a maximum concentration of germanium which is doped into the layer of core is 1.5 wt %;    a maximum concentration of fluorine which is doped into the layer of core is also 1.5 wt %.    
     
     
         14 . An optical fiber according to  claim 12 , wherein a germanium together with a fluorine is doped into any one of the layer of cladding or all of the layers of cladding so as to improve hydrogen characteristics.  
     
     
         15 . An optical fiber according to  claim 14 , wherein 
 a formula C2−C1≧−0.5 wt % is satisfied,    under condition that C2 is the concentration of germanium in of the layer of cladding having the least amount of germanium of said two layers of cladding and C1 is the concentration of germanium in the layer of the core.    
     
     
         16 . An optical fiber according to  claim 14 , wherein 
 a maximum concentration of germanium which is doped into the layer of core is 1.5 wt %;    a maximum concentration of fluorine which is doped into the layer of core is also 1.5 wt %.    
     
     
         17 . An optical fiber according to  claim 10 , wherein: 
 an optical fiber preform is twisted and drawn; and    a polarization mode dispersion is 0.01 ps/km 0.5  or lower.    
     
     
         18 . An optical transmission path formed by combining the optical fiber according to  claim 10  as a first optical fiber, and a second optical fiber which compensates for a chromatic dispersion and/or a dispersion slope of the first optical fiber.  
     
     
         19 . An optical fiber which is made mainly from a silica glass member having three layers of core and two layers of cladding, with a refractive index in an outer layer of cladding equal to or greater than a refractive index in an internal layer of cladding, and a refractive index in a central layer of core higher than a refractive index in an intermediate layer of core, and a refractive index in the intermediate layer of core lower than a refractive index in an outer layer of core, and a refractive index in the outer layer of core higher than a refractive index of at least the internal layer of cladding of said two layers of cladding, wherein: 
 a formula −2.5×10 −7 /° C. ≦α1−α2≦1.0×10 −7 /° C. is satisfied; and    a polarization mode dispersion is 0.03 ps/km 0.5  or lower;    under condition that a coefficient of thermal expansion of a glass member which forms the layer of core having the highest coefficient of thermal expansion of said three layers of core is α1 and a coefficient of thermal expansion of a glass member which forms the layer of core having the lowest coefficient of thermal expansion of said three layers of core is α2.    
     
     
         20 . An optical fiber according to  claim 19 , wherein; 
 a formula −1.5×10 −7 /° C.≦α1−α2≦0/° C. is satisfied; and    a polarization mode dispersion is 0.015 ps/km 0.5  or lower.    
     
     
         21 . An optical fiber according to  claim 19 , wherein: 
 at least one of the layers of cladding is a silica glass to which at least a fluorine is doped;    each of said three layers of core consist of an approximately pure silica glass, or a silica glass doped with a germanium and/or a fluorine; and    a transmission loss at a wavelength of 1550 nm is 0.25 dB/km or lower.    
     
     
         22 . An optical fiber according to  claim 21 , wherein 
 the maximum concentration of germanium which is doped into any of the three layers of core is 1.5 wt %;    the maximum concentration of fluorine which is doped into any of the three layers of core is also 1.5 wt %.    
     
     
         23 . An optical fiber according to  claim 21 , wherein a germanium together with a fluorine is doped into any one of the layers of cladding or all of the layers of cladding so as to improve hydrogen characteristics.  
     
     
         24 . An optical fiber according to  claim 23 , wherein 
 a formula C2−C1≧−0.5 wt % is satisfied,    under condition that C2 is the concentration of germanium in of the layer of cladding having the least amount of germanium of said two layers of cladding and C1 is the concentration of germanium in the layer of the core having the least amount of germanium of said three layers of core.    
     
     
         25 . An optical fiber according to  claim 23 , wherein 
 the maximum concentration of germanium which is doped into any of the three layers of core is 1.5 wt %;    the maximum concentration of fluorine which is doped into any of the three layers of core is also 1.5 wt %.    
     
     
         26 . An optical fiber according to  claim 17 , wherein: 
 an optical fiber preform is twisted and drawn; and    a polarization mode dispersion is 0.01 ps/km 0.5  or lower.    
     
     
         27 . An optical transmission path formed by combining the optical fiber according to claim  17  as a first optical fiber, and a second optical fiber which compensates for a chromatic dispersion and/or a dispersion slope of the first optical fiber.  
     
     
         28 . An optical fiber which is made mainly from a silica glass member having four layers of core and two layers of cladding, with a refractive index in an outer layer of cladding equal to or greater than a refractive index in an internal layer of cladding, and a refractive index in a central layer of core lower than a refractive index in an inner-intermediate layer of core, and a refractive index in the inner-intermediate layer of higher than a refractive index in an outer-intermediate layer of core, and a refractive index in the outer-intermediate layer of core lower than a refractive index in an outer layer of core, and a refractive index in the outer layer of core higher than a refractive index in at least the internal layer of cladding of said two layers of cladding, wherein; 
 a formula −2.5×10 −7 /° C.≦α1−α2≦1.0×10 −7 /° C. is satisfied; and    a polarization mode dispersion is 0.03 ps/km 0.5  or lower;    under condition that a coefficient of thermal expansion of the glass member which forms the layer of core having the highest coefficient of thermal expansion of said four layers of core is α1 and a coefficient of thermal expansion of a glass member which forms the layer of cladding having the lowest coefficient of thermal expansion of said two layers of cladding is α2.    
     
     
         29 . An optical fiber according to  claim 28 , wherein; 
 a formula −1.5×10 −7 /° C.≦α1−α2≦0/° C. is satisfied; and    a polarization mode dispersion is 0.015 ps/km 0.5  or lower.    
     
     
         30 . An optical fiber according to  claim 28 , wherein: 
 at least one of the layers of cladding is a silica glass to which at least a fluorine is doped;    each of said four layers of core consists of an approximately pure silica glass, or a silica glass doped with a germanium and/or a fluorine; and    a transmission loss at a wavelength of 1550 nm is 0.30 dB/km or lower.    
     
     
         31 . An optical fiber according to  claim 30 , wherein 
 the maximum concentration of germanium which is doped into any of the four layers of core is 1.5 wt %;    the maximum concentration of fluorine which is doped into any of the four layers of core is also 1.5 wt %.    
     
     
         32 . An optical fiber according to  claim 30 , wherein a germanium together with a fluorine is doped into any one of the layers of cladding or all of the layers of cladding so as to improve hydrogen characteristics.  
     
     
         33 . An optical fiber according to  claim 32 , wherein 
 a formula C2−C1≧−0.5 wt % is satisfied,    under condition that C2 is the concentration of germanium in of the layer of cladding having the least amount of germanium of said two layers of cladding and C1 is the concentrations of germanium in the layer of the core having the least amount of germanium of said four layers of core.    
     
     
         34 . An optical fiber according to  claim 32 , wherein the maximum concentration of germanium which is doped into any of the four layers of core is 1.5 wt %; 
 the maximum concentration of fluorine which is doped into any of the four layers of core is also 1.5 wt %.    
     
     
         35 . An optical fiber according to  claim 28 , wherein: 
 an optical fiber preform is twisted and drawn; and    a polarization mode dispersion is 0.01 ps/km 0.5  or lower.    
     
     
         36 . An optical transmission path formed by combining the optical fiber according to  claim 28  as a first optical fiber, and a second optical fiber which compensates for a chromatic dispersion and/or a dispersion slope of the first optical fiber.