second waveguide section 402 and substantially in the same plane (or layer) as the second waveguide section 402. FIG. 3F shows a planar view and the cross-section along the direction of propagation of the light through the waveguide sections 301-303, as indicated by the dotted line X-X’ in FIG. 3F, may be as shown in any of FIGs. 3B-3D with the omission of the resistive heating pad 304. FIG. 3G shows a further cross-section through the tunable element 320 shown in FIG. 3F taken perpendicular to the direction of propagation of the light through the second section of waveguide 302, as indicated by the dotted line Y-Y’in FIG. 3F. This cross-section shows that the two resistive heating pads 304A, 304B formed in the same plane as the second waveguide section 302 and are positioned either side of the second waveguide section 202. The resistive heating pads 304A, 304B may be formed from any suitable material, for example, TiN, and may be the same thickness as, or thicker or thinner than the centre waveguide section 302. [0057] FIG. 4A shows a schematic diagram (in planar view) of another example improved tunable element 400. This tunable element 400 is a variant on that described above with reference to FIGs. 3A-3E and as is described in more detail below. In this variant, shown in FIG. 4A, the first and third waveguide sections 301, 303 are as described above with reference to FIGs. 3A-3E; however the second waveguide section 402 differs in cross-section from the second waveguide section 302 described above. The second waveguide section 402 in this variant is a rib waveguide and hence has a portion 402A in the centre of the waveguide that is thicker than the two outer portions 402B, 402C and this is also shown in FIG. 4E (described below). The thickness of the centre portion 402A may be approximately the same thickness as the second waveguide section 302 in the earlier examples. Furthermore, the resistive heating pad arrangement in tunable element 400 differs from the resistive heating pad 304 in tunable element 300. In particular, instead of a single resistive heating pad 304 in a layer above or below the second waveguide section 302, there are two resistive heating pads 404A, 404B, one either side of the second waveguide section 402 and sub stantially in the same plane (or layer) as the second waveguide section 402 and these may, for example, be formed from doped silicon, instead of being formed from one or more metals (e.g. TiN). Any suitable dopants may be used (e.g. p-type or n-type) and the thermal efficiency will depend upon the resistance of the heating pads and hence on the doping concentration. The direction of propagation of the light is shown by the arrows in FIG. 4A although as described above (e.g. with reference to FIG. 1B), in various examples the light may also propagate in the opposite direction through the tunable element 400. [0058] The two resistive heating pads 404A, 404B may be referred to as resistive regions of the second material. These are formed by implanting impurities into the region (i.e. PCT/SG2019/050519 WO 2020/081014