In one example, a device includes a bus waveguide to carry a light of a carrier wavelength, a first ring waveguide with a first modulator, a first heater to adjust a resonance wavelength of the first ring waveguide, and a second ring waveguide with a second modulator. The first ring waveguide and the second ring waveguide are coupled to the bus waveguide and are to modulate the light of the carrier wavelength to impart one of at least four optical power levels to the light. In another example, a device includes, a bus waveguide, a first ring waveguide with a first modulator, and a second ring waveguide with a second modulator. The first ring waveguide and the second ring waveguide are coupled to the bus waveguide and are to modulate a light of a carrier wavelength to impart one of at least four optical power levels to the light.