PURPOSE: To prepare a heat-resistant optical fiber which will not cause the fracture of a covering material, the increase in transmitting loss, etc. even at about 160°C, by covering a light transmitting glass fiber with a heat curable organopolysiloxane composition.
CONSTITUTION: Immediately after the melt spinning of a parent material, the outer periphery of the light transmitting glass fiber 10 is coated with the first covering layer 15 with a thickness about 20W50μ consisting of a organopolysiloxane composition curable at normal temp. (e.g., phenylpolysiloxane composition) having a larger refractive index than that of the glass forming the outermost layer of the glass fiber. The second covering layer 16 with a thickness about 50W300μ consisting of a organopolysiloxane composition curable at normal temp. (preferably dimethylpolyciloxane composition) is applied to the first covering layer. This layer 16 is then covered with a heat curable organopolysiloxane) about 10,000 degree of polymerization) composition by extrusion so that the third covering layer 17 with a thickness about 100W300μ is formed, to give a heat-resistant fiber.