PCT/SG2019/050519 WO 2020/081014 material. [0077] The first and third waveguide sections may be formed in a first layer and the second waveguide section may be formed in a second layer. [0078] In use, light may be coupled vertically between an end of the first waveguide section and a proximate end of the second waveguide section and may be further coupled vertically between an opposite end of the second waveguide section and a proximate end of the third waveguide section. [0079] The tunable element may further comprise a layer of cladding material between the first layer and the second layer. [0080] The one or more resistive heating pads may be formed in a third layer. The tunable element may further comprise a layer of cladding material between the second layer and the third layer. [0081] The tunable element may further comprise a layer of cladding between the first layer and the third layer, such that the second layer is further from the third layer than the first layer. [0082] The one or more resistive heating pads may be formed in the second layer. The one or more resistive heating pads may be formed from a resistive region of the second material. [0083] The second waveguide section may comprise a centre portion and two outer portions, wherein the centre portion is thicker than the outer portions. [0084] The first, second and third waveguide sections may be formed in a single layer. [0085] In use, light may be butt-coupled between an end of the first waveguide section and a proximate end of the second waveguide section and may be further butt-coupled between an opposite end of the second waveguide section and a proximate end of the third waveguide section. [0086] The resistive heating pad may be formed in a second layer. [0087] The tunable element may further comprise a layer of cladding material between the first layer and the second layer. [0088] The second waveguide section may comprise a centre portion and two outer portions, wherein the centre portion is thicker than the outer portions and wherein the resistive heating pad may be formed on top of each of two outer portions of the second waveguide section. [0089] The first material may be silicon nitride and the second material may be silicon, amorphous silicon or polycrystalline silicon. [0090] Each resistive heating pad may have a length that is identical to a length of the second waveguide section, such that each resistive heating pad is proximate to the second waveguide section and not to the first or third waveguide sections. [0091] A second further example provides a tunable optical waveguide device comprising