In the examples provided herein, a system has a first racetrack resonant waveguide structure, positioned to enable an input light signal to couple from a first waveguide; and a second racetrack resonant waveguide structure, positioned to enable the input light signal to couple between the first racetrack resonant waveguide structure and the second racetrack resonant waveguide structure, and further positioned to enable an output light signal to couple from the second racetrack resonant waveguide structure to a second waveguide. The system also has a primary heating unit, positioned to heat a primary region including a first portion of the first racetrack resonant waveguide structure and a first portion of the second racetrack resonant waveguide structure, to change a central frequency and a passband width for the system.