Abstract: A three-layer structure optical fiber comprises a first transparent dielectric core layer having a relatively high refractive index, a second transparent dielectric intermediate layer having a relatively low refractive index and a third transparent dielectric cladding layer having an intermediate refractive index wherein the parameter of the optical fiber is determined as follows: ##EQU1## where B is the bandwidth of the signal to be transmitted, z is the transmission distance, C is the velocity of light in vacuum, .eta.o is the refractive index of the core layer, q is the ratio of refractive indices between the core layer and second layer, V = normalized frequency, ##EQU2## p is the ratio of refractive index between the core layer and the third layer, and U.sub.11 =2,405.

Abstract: An optical waveguide for optical communication transmitting light wave energy in a single mode composed of a core of a first transparent dielectric material and successive layers of a second and a third transparent dielectric material coaxially covering the core, the refractive index of the first dielectric material being higher than that of the third dielectric material and the refractive index of the second dielectric material being lower than that of the third dielectric material.