Disclosed herein is a coated optical fiber comprising: a siliceous optical fiber including a core inside a cladding; and a cured coating comprising: a thermally cured polyorganosilsesquioxane; and an oxide powder dispersed in said cured polyorganosilsesquioxane, wherein said oxide powder has a refractive index from about 1.2 to about 2.7 and includes a plurality of particles having a particle size less than about 100 nanometers, said cured coating having adhesion to said siliceous optical fiber and further having transparency to ultraviolet radiation. Also disclosed herein is a fiber optic device comprising the optical fiber.