JPS6491111A - High-speed optical modulator - Google Patents

Abstract

PURPOSE:To enable fast optical modulation with a low driving voltage by setting the effective thickness of a modulating electrode constituted on a surface where an optical waveguide is formed equal nearly to or larger than the gap of the modulating electrodes. CONSTITUTION:The thickness (t) of the modulating electrodes is nearly equal to or larger than the gap (g), so the capacity C between the electrodes 11 and 12 is formed of capacities Ca and Cs and the capacity Cg of the gap part between the electrodes. Consequently, the quantity of electric force lines 14 outside a substrate increases relatively to the quantity of electric force lines 13 in the substrate 3 and the specific dielectric constant epsilona of air outside the substrate is smaller than the specific dielectric constant epsilons of the substrate, so the effective dielectric constants epsilon' and eff of the modulating electrodes 11 and 12 become smaller than the effective dielectric constant epsiloneff of the modulating electrodes. Consequently, the effective refractive index to a modulating signal wave is smaller than that of a conventional constituent element, so the difference between the modulating signal wave and light becomes small and the optical modulator which operates at a high speed with the low driving voltage is obtained.