An optical shape sensing method comprising: providing first and second optical fibre portions arranged to receive light from at least one common laser such that a path length of the light from the laser to the first optical fibre portion differs from a path length of the light from the laser to the second optical fibre portion by a first distance; wherein the first and second optical fibre portions are spaced apart from each other in a first direction; and wherein the first and second optical fibre portions comprise a plurality of reflectors spaced along the length of the optical fibre portions , wherein the reflectors are arranged in sets of reflectors , each set including one reflector from each of the optical fibre portions , 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 are in neutral positions; supplying light from the laser to the optical fibre portions ; detecting an interferometric signal from the optical fibre portions 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 in the first direction based on the optical path length differences, between the reflectors in the first and second optical fibre portions , for the sets of reflectors . providing first and second optical fibre portions arranged to receive light from at least one common laser such that a path length of the light from the laser to the first optical fibre portion differs from a path length of the light from the laser to the second optical fibre portion by a first distance; wherein the first and second optical fibre portions are spaced apart from each other in a first direction; and wherein the first and second optical fibre portions comprise a plurality of reflectors spaced along the length of the optical fibre portions , wherein the reflectors are arranged in sets of reflectors , each set including one reflector from each of the optical fibre portions , 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 are in neutral positions; wherein the first and second optical fibre portions are spaced apart from each other in a first direction; and wherein the first and second optical fibre portions comprise a plurality of reflectors spaced along the length of the optical fibre portions , wherein the reflectors are arranged in sets of reflectors , each set including one reflector from each of the optical fibre portions , 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 are in neutral positions; supplying light from the laser to the optical fibre portions ; detecting an interferometric signal from the optical fibre portions 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 in the first direction based on the optical path length differences, between the reflectors in the first and second optical fibre portions , for the sets of reflectors . <EOS>