An optical sensor for sensing information relating to an analyte liquid or gas, has a a planar substrate having a refractive index nc. The planar substrate supports a ridge waveguide having an unclad top portion having a refractive index nr. The substrate serves as cladding layer for the ridge waveguide at a location where the ridge waveguide contacts the substrate. A Bragg grating inscribed in the ridge waveguide has two modes for providing information relating to both temperature and refractive index of the surrounding analyte liquid or gas. A cladding mode has a different response to the analyte when compared to a Bragg resonance response. Both modes have a same reaction to temperature, wherein said Bragg grating is formed within the unclad region of ridge waveguide, wherein nc