PCT/SG2019/050519 WO 2020/081014 Description Background [0001] Optical waveguide devices may be formed on a substrate, such as silicon, with the waveguide cladding being formed from a first material (e.g. silicon oxide) and the core material being substantially square or rectangular in cross-section and formed from a second material with a higher refractive index than cladding material (e.g. silicon, Si, or silicon nitride, SiN, etc.). As light is guided substantially within the core material, the structure formed from the core material is typically referred to as the waveguide without any reference to the cladding material which may, in some examples, be air or dielectric with lower refractive index than the core material. The cross-section di mensions of the waveguide (i.e. the core material) may be dependent, at least in part, on the material used to form the waveguide and in particular on the refractive index of the waveguide material. [0002] A tunable waveguide device may, for example, be fabricated using a movable element (e.g. a MEMS device or a larger optical element), the thermo-optic effect or electro-optic effect. In the latter two examples, the tuning is achieved by changing the optical properties (e.g. the refractive index) of the waveguide (i.e. core) material by passing an electrical current, for example, through a heating element positioned close to the waveguide. Inaccuracies in the motion of the movable element or the change in the optical properties (e.g. due to the lack of control of heating effects in a device using the thermo-optic effect) results in tuning errors and this can significantly impair the performance of the tunable waveguide device. [0003] The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known tunable waveguide devices. Summary [0004] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. [0005] A tunable element for an optical waveguide device, such as an Optical Phased Array (OPA), is described. Tunable element comprises three waveguide sections arranged such that light propagates through the first waveguide section, then through the second waveguide section and then through the third waveguide section, with light being either evanescently or directly coupled from one waveguide section to the next. The Title of Invention: OPTICAL WAVEGUIDE TUNING ELEMENT