Ultrahigh-sensitive temperature sensor based on modal interference in a metal-under-clad ridge waveguide with a polymer upper cladding.

We propose a highly sensitive temperature sensor based on modal interference in a metal-under-clad ridge waveguide (MUCRW) with polydimethylsiloxane as the upper cladding. The proposed sensor exploits the interference between the fundamental and the first higher order TE modes of the MUCRW. The increased fractional modal power in the ambient medium due to the metal under-cladding along with the high thermo-optic coefficient of the upper cladding results in a very significant change in the modal characteristics of the two interfering modes with temperature variation. Moreover, the effect of temperature change is more pronounced for the higher order mode compared with the fundamental mode, resulting in an ultrahigh sensitivity of the modal interference to the ambient temperature. The sensitivity of the proposed sensor structure is found to be as high as 8.35  nm/°C, which, to the best of our knowledge, is the highest reported sensitivity in any integrated optic waveguide-based temperature sensor.