PCT/SG2019/050519 WO 2020/081014 doping) and this modifies the local resistivity of this part of the structure formed from the second material. As described above, the first and third waveguide sections 301, 303 are formed from a first material having a first value of dn/dT, Sl, and the second (or centre) waveguide section 402 is formed from a second material having a second value of dn/dT, S2. The second material is more thermo-optically sensitive than the first material, i.e. S2>Sl. [0059] The first and third waveguide sections 301, 303 may be formed in the same layer and so may be co-planar. The second waveguide section 402 may be formed in a separate process step (as it is formed from a different material) and may be formed in a separate layer to the first and third waveguide sections, as shown in FIGs. 4B and 4C. The fab rication process is described below with reference to FIGs. 7A-&D. [0060] FIGs. 4B, 4C and 4D show three different alternative cross-sections through the tunable element 400 of FIG. 4A and in all these diagrams, the cross-section is taken through the centre of the waveguide sections 301, 402, 303 and along the direction of propagation of the light through the waveguide sections 301, 402, 303 as indicated by the dotted line X-X’ in FIG. 4A. In all examples, the waveguide sections 301, 402, 303 are surrounded by a cladding material 203 and in all examples shown, the length of the second waveguide section 402 (as measured along the direction of propagation of the light) is the same as the length of the resistive heating pads 404A, 404B and the ends of the second waveguide section 402 and the ends of the resistive heating pads 404A, 404B are aligned (in a direction parallel to the direction of propagation of the light). In other examples (not shown in the figures), however, the length of the second waveguide section 402 (as measured along the direction of propagation of the light) may be longer (e.g. only slightly longer) than the length of the resistive heating pad pads 404A, 404B, such that the resistive heating pads 404A, 404B terminate before the end(s) of the second waveguide section 402. [0061] In the first two examples, as shown in FIGs. 4B and 4C, a multi-layer waveguide structure is formed, with the second waveguide section 402 being in a plane (or layer) which is parallel to, but spaced from, the plane (or layer) of the first and third waveguide sections 301, 303. Light is coupled vertically from the end of the first waveguide section 301 and into the second waveguide section 402 (i.e. into a proximate end of the second waveguide section 402) and then coupled vertically from the other end of the second waveguide section 402 and into the third waveguide section 303 (i.e. into a proximate end of the third waveguide section 303). To improve the ef ficiency of the vertical coupling, the waveguide sections may overlap by a small amount (i.e. the first and second waveguide sections overlap by a small amount and the second and third waveguide sections overlap by a small amount), as shown in FIGs. 4B and 4C. In the first example, shown in FIG. 4B, the plane for the first and third