An optical coupler comprising: a substrate; a first waveguide element provided in a first layer with respect to the substrate, wherein the first waveguide element comprises a first end and a second end, and wherein the first end of the first waveguide element is coupled to input/output light to/from a first end of the optical coupler; wherein the first waveguide element tapers in width from having a first width at its first end to having a second width at its second end (45 b); a second waveguide element provided in a second layer, the second layer arranged adjacent to the first layer, wherein the second waveguide element comprises a first end and a second end, and wherein the first end of the second waveguide element is coupled to input/output light to/from a second end of the optical coupler; wherein the first waveguide element is configured to extend from the first end of the optical coupler towards the second end of the optical coupler, and wherein the second waveguide element is configured to extend from the second end of the optical coupler towards the first end of the optical coupler, such that the first waveguide element overlaps with the second waveguide element to adapt light passing between the first end of the first waveguide element and first end of the second waveguide element, wherein the second waveguide element comprises: an expansion region (43 1); and a compression region (43 3); wherein the expansion region (43 1) comprises an increasing width from a third width at the first end of the second waveguide element to a larger, fourth width, at an interface with the compression region (43 3), and wherein the compression region (43 3) reduces in width from the fourth width to having a smaller, fifth width, at the second end of the second waveguide element; characterised in that the first waveguide element is configured to overlap with the second waveguide element such that the second end of the first waveguide element overlaps at least partially with the expansion region (43 1) of the second waveguide element. a substrate; a first waveguide element provided in a first layer with respect to the substrate, wherein the first waveguide element comprises a first end and a second end, and wherein the first end of the first waveguide element is coupled to input/output light to/from a first end of the optical coupler; wherein the first waveguide element tapers in width from having a first width at its first end to having a second width at its second end (45 b); a second waveguide element provided in a second layer, the second layer arranged adjacent to the first layer, wherein the second waveguide element comprises a first end and a second end, and wherein the first end of the second waveguide element is coupled to input/output light to/from a second end of the optical coupler; wherein the first waveguide element is configured to extend from the first end of the optical coupler towards the second end of the optical coupler, and wherein the second waveguide element is configured to extend from the second end of the optical coupler towards the first end of the optical coupler, such that the first waveguide element overlaps with the second waveguide element to adapt light passing between the first end of the first waveguide element and first end of the second waveguide element, wherein the second waveguide element comprises: an expansion region (43 1); and a compression region (43 3); wherein the expansion region (43 1) comprises an increasing width from a third width at the first end of the second waveguide element to a larger, fourth width, at an interface with the compression region (43 3), and wherein the compression region (43 3) reduces in width from the fourth width to having a smaller, fifth width, at the second end of the second waveguide element; characterised in that the first waveguide element is configured to overlap with the second waveguide element such that the second end of the first waveguide element overlaps at least partially with the expansion region (43 1) of the second waveguide element. an expansion region (43 1); and a compression region (43 3); wherein the expansion region (43 1) comprises an increasing width from a third width at the first end of the second waveguide element to a larger, fourth width, at an interface with the compression region (43 3), and wherein the compression region (43 3) reduces in width from the fourth width to having a smaller, fifth width, at the second end of the second waveguide element; characterised in that the first waveguide element is configured to overlap with the second waveguide element such that the second end of the first waveguide element overlaps at least partially with the expansion region (43 1) of the second waveguide element. wherein the expansion region (43 1) comprises an increasing width from a third width at the first end of the second waveguide element to a larger, fourth width, at an interface with the compression region (43 3), and wherein the compression region (43 3) reduces in width from the fourth width to having a smaller, fifth width, at the second end of the second waveguide element; characterised in that the first waveguide element is configured to overlap with the second waveguide element such that the second end of the first waveguide element overlaps at least partially with the expansion region (43 1) of the second waveguide element.