An optical shape sensing method comprising: providing first and second optical fibre portions (202, 204) arranged to receive light from at least one common laser (112a, 112b, 122b) such that a path length of the light from the laser (112a, 112b, 122b) to the first optical fibre portion differs from a path length of the light from the laser (112a, 112b, 122b) to the second optical fibre portion by a first distance; wherein the first and second optical fibre portions (202, 204) are spaced apart from each other in a first direction; and wherein the first and second optical fibre portions (202, 204) comprise a plurality of reflectors spaced along the length of the optical fibre portions (202, 204), wherein the reflectors are arranged in sets of reflectors, each set including one reflector from each of the optical fibre portions (202, 204), wherein the reflectors in each set are aligned with one another in a second direction that is perpendicular to the first direction when the optical fibre portions (202, 204) are in neutral positions; supplying light from the laser (112a, 112b, 122b) to the optical fibre portions (202, 204); detecting an interferometric signal from the optical fibre portions (202, 204) at a common detector; determining, for each set of reflectors, optical path length differences between the reflectors in the set of reflectors based on the detected interferometric signal; determining a curvature of the optical fibre portions (202, 204) in the first direction based on the optical path length differences, between the reflectors in the first and second optical fibre portions (202, 204), for the sets of reflectors. providing first and second optical fibre portions (202, 204) arranged to receive light from at least one common laser (112a, 112b, 122b) such that a path length of the light from the laser (112a, 112b, 122b) to the first optical fibre portion differs from a path length of the light from the laser (112a, 112b, 122b) to the second optical fibre portion by a first distance; wherein the first and second optical fibre portions (202, 204) are spaced apart from each other in a first direction; and wherein the first and second optical fibre portions (202, 204) comprise a plurality of reflectors spaced along the length of the optical fibre portions (202, 204), wherein the reflectors are arranged in sets of reflectors, each set including one reflector from each of the optical fibre portions (202, 204), wherein the reflectors in each set are aligned with one another in a second direction that is perpendicular to the first direction when the optical fibre portions (202, 204) are in neutral positions; wherein the first and second optical fibre portions (202, 204) are spaced apart from each other in a first direction; and wherein the first and second optical fibre portions (202, 204) comprise a plurality of reflectors spaced along the length of the optical fibre portions (202, 204), wherein the reflectors are arranged in sets of reflectors, each set including one reflector from each of the optical fibre portions (202, 204), wherein the reflectors in each set are aligned with one another in a second direction that is perpendicular to the first direction when the optical fibre portions (202, 204) are in neutral positions; supplying light from the laser (112a, 112b, 122b) to the optical fibre portions (202, 204); detecting an interferometric signal from the optical fibre portions (202, 204) at a common detector; determining, for each set of reflectors, optical path length differences between the reflectors in the set of reflectors based on the detected interferometric signal; determining a curvature of the optical fibre portions (202, 204) in the first direction based on the optical path length differences, between the reflectors in the first and second optical fibre portions (202, 204), for the sets of reflectors.