PCT/SG2019/050519 WO 2020/081014 waveguide sections 301, 303 is above the plane of the second waveguide section 402 and in the second example, shown in FIG. 4C, the plane for the first and third waveguide sections 301, 303 is below the plane of the second waveguide section 402. [0062] In the third example cross-section, shown in FIG. 4D, there is not a multi-layer waveguide structure and instead all three waveguide sections 301, 402, 303 are in sub stantially the same plane, with light being butt-coupled between the first and second sections 301, 402 and between the second and third sections 402, 303. As described above, dependent upon the manner in which the vertical coupling (in the examples of FIGs. 4B and 4C) is achieved, butt-coupling may be more lossy and hence the multi layer structure of FIG. 4B or 4C may result in a tunable element that has less optical loss. [0063] FIG. 4E shows a further cross-section through the tunable element 400 shown in FIG. 4A; however, unlike the earlier cross-sections (shown in FIGs. 4B-4D), the cross- section shown in FIG. 4E is taken perpendicular to the direction of propagation of the light through the second section of waveguide 402, as indicated by the dotted line Y- Y ’ in FIG. 4A. This cross-section shows the difference in thickness between the centre portion 402A of the second waveguide section and the outer portions 402B, 402C. As shown in FIG. 4E, the resistive heating pads 404A, 404B are formed in the same plane as the second waveguide section 402 and are positioned either side of the second waveguide section 402 and adjacent to the outer portions 402B, 402C of the second waveguide section 402. As described below, the resistive heating pads 404A, 404B may be initially formed (e.g. deposited) in the same process step and from the same material as the second waveguide section 402 and then a further process step used to dope (i.e. to selectively dope) the portions of the material that are to form the resistive heating pads 404A, 404B. [0064] In a variation on the tunable element 400 shown in FIGs. 4A-4E and described above, is shown in FIGs. 4F and 4G. In this example tunable element 420, instead of the resistive heating pads 424A, 424B being formed from doped silicon (as described above), they are formed from another material that is deposited onto the outer portions 402B, 402B of the second waveguide section 402. FIG. 4F shows a planar view and the cross-section 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. 4F, may be as shown in any of FIGs. 4B-4D and described above. FIG. 4G shows a further cross- section through the tunable element 420 shown in FIG. 4F taken perpendicular to the direction of propagation of the light through the second section of waveguide 402, as indicated by the dotted line Y-Y’in FIG. 4F. This cross-section shows that the two resistive heating pads 424A, 424B deposited onto the distal parts of the outer portions 402B, 402B of the second waveguide section 402. The resistive heating pads 424A,