Patent ID: 7382541

Claim:
A gradient-index rod lens having a core/clad structure that is obtained by providing a mother glass rod with a refractive index distribution through ion exchange, the mother glass rod comprising a mother composition of a core glass and a mother composition of a clad glass that covers the mother composition of the core glass, wherein the clad glass and the core glass each contain an alkali metal oxide but are substantially free from lead, the clad glass and the core glass each contain an alkali metal oxide and have a basic composition including components other than the alkali metal oxide, and the clad glass and the core glass are different from each other in the basic composition, the mother composition of the clad glass has a higher thermal expansion coefficient and a higher glass-transition temperature than those of the mother composition of the core glass, and a difference Δα between a thermal expansion coefficient α1 of the mother composition of the clad glass and a thermal expansion coefficient α2 of the mother composition of the core glass, specifically Δα=α1−α2, is larger than 2×10 −7 /° C. but smaller than 20×10 −7 /° C.