Optical sensor in which a sensing waveguide is thermally isolated from a substrate

Optical sensor 10 comprises a substrate 12; sensing waveguide structure 16 and delivery waveguide 18, each formed integrally with the substrate; and an aperture 30 in the substrate under part of the sensing waveguide structure. The delivery waveguide allows delivery of interrogating probe light 22 into the sensing waveguide structure such that the optical sensor may sense an adjacent gas temperature. The aperture reduces thermal coupling between the substrate and the sensing waveguide structure, e.g. providing faster response to temperature changes. Alternatively a carrier chip (84, Fig.10) supports an optical fibre (25, Fig.10) in a groove (86, Fig.10) in its surface, the surface being in contact with a substrate (12, Fig.10) comprising an integrally formed waveguide such that the optical fibre core is aligned with an end of the waveguide. The sensing waveguide may be a Fabry-Perot or Bragg grating interferometer element.