WO 2020/081014 PCT/SG2019/050519 [0045] 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 302 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. [0046] In an example, the first material is silicon nitride, SiN, and S 1=2.4x10 5 and the second material is silicon, Si, and S2=l.8xl0 $^{4}$. In this example, the thermo-optic sen sitivity of the second material is an order of magnitude larger than the thermo-optic sensitivity of the first material. Consequently, any change in refractive index in the first and/or third sections of waveguide which are caused by heating from the resistive heating pad, will result in a phase change (and hence phase error) that is very small (e.g. negligible) compared to the phase change caused by heating of the second waveguide section. [0047] Silicon and silicon nitride provide just one example combination of materials which may be used to form the three waveguide sections 301-303. In other examples, poly crystalline or amorphous silicon may be used for the second waveguide section 302. In another example, the first material is SiON and the second material is silicon, poly crystalline or amorphous silicon. In a further example, the first material is SiON and the second material is SiN. In further examples, any combination of materials may be used where the second material has a higher thermo-optic sensitivity than the first material. In other examples, the first and third sections may not be fabricated from a single material but instead may be formed from a first material combination (e.g. a multi-layer structure that forms the waveguide core). Similarly, the second section may not be fabricated from a single material but instead may be formed from a second material combination (e.g. a second multi-layer structure, different to that used for the first and third sections, that forms the waveguide core). [0048] 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 302 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. 3B and 3C. The fab rication process is described below with reference to FIGs. 6A-6D. [0049] FIGs. 3B, 3C and 3D show three different alternative cross-sections through the tunable element 300 of FIG. 3A and in all these diagrams, the cross-section is taken through the centre of the waveguide sections 301-303 and along the direction of propagation of the light through the waveguide sections 301-303, as indicated by the dotted line X-X’ in FIG. 3A. In all examples, the waveguide sections 301-303 are surrounded by a cladding material 203 and in all examples shown, the length of the second waveguide section 302 (as measured along the direction of propagation of the light) is the same as the length of the resistive heating pad 304 and the ends of the