A thermal microring optical sensor is configured such that a portion of the optical resonator and its associated waveguide are encased within a cladding structure to minimize scattering losses along the waveguide and also provide improved evanescent coupling efficiency between the waveguide and the resonator. Functioning as a thermal sensor, incoming radiation modifies the temperature of the resonator, which changes its resonant frequency and, as a result, the percentage of light that it evanescently couples from the waveguide. The cladding structure also functions as a mechanical support for the resonator disk, eliminating the need for a pedestal to suspend the disk above the support substrate. Thermally-induced buckling of the optical waveguide is also reduced by encasing the susceptible portion of the waveguiding within the cladding structure.