Claims [Claim 1] A tunable element (300, 320, 400, 420, 501) for an optical waveguide device, the tunable element comprising: a first waveguide section (301) formed from a first material; a second waveguide section (302, 402) formed from a second material and arranged to receive light coupled from the first waveguide section; a third waveguide section (303) formed from the first material and arranged to receive light coupled from the second waveguide section; and one or more resistive heating pads (304, 404A, 404B, 424A, 424B) proximate to the second waveguide section, wherein the second material is more thermo-optically sensitive than the first material. [Claim 2] The tunable element according to claim 1, wherein the first and third waveguide sections (301, 303) are formed in a first layer and the second waveguide section (302, 402) is formed in a second layer. [Claim 3] The tunable element according to claim 2, wherein in use light is coupled vertically between an end of the first waveguide section and a proximate end of the second waveguide section and is further coupled vertically between an opposite end of the second waveguide section and a proximate end of the third waveguide section. [Claim 4] The tunable element according to claim 2 or 3, further comprising a layer of cladding material between the first layer and the second layer. [Claim 5] The tunable element according to any of claims 2-4, wherein the one or more resistive heating pads (304) are formed in a third layer. [Claim 6] The tunable element according to claim 5, further comprising a layer of cladding material between the second layer and the third layer. [Claim 7] The tunable element according to claim 5, further comprising 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. [Claim 8] The tunable element according to any of claims 2-4, wherein the one or more resistive heating pads (304A, 304B, 404A, 404B) are formed in the second layer. [Claim 9] The tunable element according to claim 8, wherein the one or more resistive heating pads (404A, 404B) are formed from a resistive region of the second material. [Claim 10] The tunable element according to claim 8 or 9, wherein the second PCT/SG2019/050519 WO 2020/081014