Coupling modulation of an optical resonator employs a variable modal index to provide modulation of optical signal coupling. A coupling-modulated optical resonator includes an optical resonator having a coupled portion and a bus waveguide having a modulation section adjacent to and coextensive with and separated by a gap from the coupled portion. The modulation section is to modulate coupling of an optical signal between the optical resonator and the bus waveguide according to a variable difference between a modal index of the bus waveguide modulation section and a modal index of the optical resonator coupled portion.