PCT/SG2019/050519 WO 2020/081014 phase changes that are introduced by the one or more tuning elements 112, cause the light to recombine differently (e.g. as a consequence of interference between the light from the different optical paths). An example tunable waveguide device 100 is shown in FIG. 1A. [0031] The tuning element 112 is capable of changing the phase of the light that is output and this phase change is achieved by changing the optical properties (i.e. the refractive index) of the waveguide (i.e. core) material within the tuning element by heating the waveguide. The heating may be achieved passing an electrical current through a resistive heating pad (which may, for example, be formed from a metal, a doped silicon strip, or other material) that is positioned close to (e.g. above or beside) the waveguide. [0032] Whilst FIG. 1A shows an example tunable waveguide device in which the light is split and then recombined using two different splitter structures 108, 110, in other examples, such as the second example tunable waveguide device 120 shown in FIG. 1B, the waveguides forming each of the multiple optical paths may be terminated with a reflective element 114 such that the same splitter structure 108 is used both to split and to recombine the light. Whilst the two example devices 100, 120 shown in FIGs. 1A and 1B show light being split into two optical paths, in other examples, the light may be split into more than two optical paths (e.g. into eight optical paths) using any suitable splitter arrangement. [0033] In the example devices 100, 120 shown in FIGs. 1A and 1B, there is a single tuning element 112 in one of the multiple optical paths 104, 106; however in other examples there may be tuning elements in more than one of the multiple optical paths and in various examples there may be tuning elements in each of the multiple optical paths. In examples where a tunable element is included within more than one of the multiple optical paths, the tunable elements within different optical paths may introduce different phase changes (e.g. through different dimensions of tunable element and/or passing different currents through the resistive heating pads within the tunable element). Various techniques, such as trenches formed in the cladding material, may be used to provide thermal isolation between adjacent tunable elements. [0034] FIG. 2A shows a schematic diagram (in planar view) of a tunable element in a waveguide device (e.g. tunable element 112). The tunable element comprises a waveguide (i.e. waveguide core) 202 and a resistive heating pad 204. The direction of propagation of the light is shown by the arrows in FIG. 2A 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. [0035] FIGs. 2B and 2C show two example cross-sections through the tunable element of FIG. 2A. The first cross-section, shown in FIG. 2B, is taken perpendicular to the