PCT/SG2019/050519 WO 2020/081014 device 500 shown in FIG. 5 is an OPA (optical phased array) and comprises a single input waveguide (shown on the left of FIG. 5), a plurality of tunable elements 501 (eight in the example shown in FIG. 5) and a plurality of output waveguides (shown on the right of FIG. 5). The tunable elements 501 shown in FIG. 5 may be as described above with reference to any of FIGs. 3A-3G and FIGs. 4A-4G or any combination of the tunable elements (or parts thereof) described above. As shown in FIG. 5, in the tunable waveguide device 500, an arrangement of splitters 502 is formed which is configured to split incoming light into a plurality of separate optical paths (eight in the example shown in FIG. 5) and each of these separate optical paths comprises a tunable element 501 (e.g. a tunable element 300, 320, 400, 420). In the example shown in FIG. 5, the tunable element 501 in each path has a resistive heating structure 504 (and hence also a second waveguide section 502) of a different length, although the widths of the resistive heating structure 504 (e.g. the one or more resistive heating pads, as described above) may be substantially the same. Thermal isolation trenches 506 are formed between each of the tunable elements 500 to reduce any heating from one resistive heating pad 504 spreading and causing changes in refractive index in an adjacent tunable element or an adjacent optical path. The second waveguide section 502 in the tunable element 500 may be a second waveguide section 302 as described above with reference to FIGs. 3A-3G or a second waveguide section 402 as described above with reference to FIGs. 4A-4G. Similarly, the resistive heating structure 504 in the tunable element 500 may be a resistive heating pad 304 as described above with reference to FIGs. 3A-3E or two resistive heating pads 304A, 304B, 404A, 404B as described above with reference to any of FIGs. 3F-3G and 4A-4E. [0067] It will be appreciated that the diagrams in FIGs. 3A-3G, 4A-G and 5 do not nec essarily show the entire structure or material stack and there may be additional layers or elements not shown in the diagrams (e.g. additional metal layers to provide tracks to electrically connect to the resistive heating pad(s)). [0068] FIGs. 6A-6D show four different example methods of fabrication of an improved tunable element 300 as described herein. FIGs. 7A-7D show four different example methods of fabrication of an improved tunable element 400 as described herein. FIGs. 8A-8D show four different example methods of fabrication of an improved tunable element 320, 420 as described herein. Whilst these methods are described in isolation of the rest of the tunable waveguide device, it will be appreciated that in many examples the improved tunable element 300, 320, 400, 420 is fabricated at the same time as the rest of the tunable waveguide device. [0069] In the example methods shown in FIGs. 6A-6D, 7A-7D and 8A-8D there are many stages in common and in all methods, the stages performed are substantially the same (with the omission, in some examples, of some cladding layers); however they may be