An optical coupler, comprising: an adiabatic waveguide structure having a proximal end and a distal end, the adiabatic waveguide structure comprising: a first waveguide comprising: an input section (112, 322) at the proximal end; a first coupling section (114, 324) contiguous with the input section and extending toward the distal end; and a first laterally displaced section (116, 326) contiguous with the first coupling section and terminating adjacent to the distal end; wherein the first waveguide narrows along the first coupling section, from the input section to the first laterally displaced section; and a second waveguide separate from the first waveguide, the second waveguide comprising: a second laterally displaced section (122, 342) adjacent to the proximal end; a second coupling section (124, 344) contiguous with the second laterally displaced section and extending toward the distal end; and an output section (126, 346) contiguous with the second coupling section and terminating at the distal end; characterised in that the second waveguide widens along the second coupling section, from the second laterally displaced section to the output section, wherein the first and second waveguides are thereby shaped so that before the waveguide widths vary, the second waveguide is brought gradually into proximity with the first waveguide, and after the two waveguide widths vary, the first waveguide is gradually moved away from the second waveguide before being terminated. a first waveguide comprising: an input section (112, 322) at the proximal end; a first coupling section (114, 324) contiguous with the input section and extending toward the distal end; and a first laterally displaced section (116, 326) contiguous with the first coupling section and terminating adjacent to the distal end; an input section (112, 322) at the proximal end; a first coupling section (114, 324) contiguous with the input section and extending toward the distal end; and a first laterally displaced section (116, 326) contiguous with the first coupling section and terminating adjacent to the distal end; wherein the first waveguide narrows along the first coupling section, from the input section to the first laterally displaced section; and a second waveguide separate from the first waveguide, the second waveguide comprising: a second laterally displaced section (122, 342) adjacent to the proximal end; a second coupling section (124, 344) contiguous with the second laterally displaced section and extending toward the distal end; and an output section (126, 346) contiguous with the second coupling section and terminating at the distal end; a second laterally displaced section (122, 342) adjacent to the proximal end; a second coupling section (124, 344) contiguous with the second laterally displaced section and extending toward the distal end; and an output section (126, 346) contiguous with the second coupling section and terminating at the distal end; characterised in that the second waveguide widens along the second coupling section, from the second laterally displaced section to the output section, wherein the first and second waveguides are thereby shaped so that before the waveguide widths vary, the second waveguide is brought gradually into proximity with the first waveguide, and after the two waveguide widths vary, the first waveguide is gradually moved away from the second waveguide before being terminated.