Single-mode optical fiber with plural core portions

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:

• 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 .DELTA.n.sub.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 .DELTA.n.sub.2 greater than the first value .DELTA.n.sub.1, wherein said first core portion and said second core portion satisfy the following relationship:
• a.multidot.(.DELTA.n.sub.2 -.DELTA.n.sub.1)/(b.multidot..DELTA.n.sub.2).gtoreq.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 .DELTA.n.sub.3 smaller than the second value .DELTA.n.sub.2.