A resonator for thermo optic devices is formed in the same process steps as a waveguide and is formed in a depression of a lower cladding while the waveguide is formed on a surface of the lower cladding. Since upper surfaces of the resonator and waveguide are substantially coplanar, the aspect ratio, as between the waveguide and resonator in an area where the waveguide and resonator front one another, decreases thereby increasing the bandwidth of the resonator. The depression is formed by photomasking and etching the lower cladding before forming the resonator and waveguide. Pluralities of resonators are also taught that are formed in a plurality of depressions of the lower cladding. To decrease resonator bandwidth, waveguide(s) are formed in the depression(s) of the lower cladding while the resonator is formed on the surface. Thermo optic devices formed with these resonators are also taught.