The present invention relates to an optical shape sensor (OS), comprising an optical fiber (F2) having a length defining a longitudinal direction, the optical fiber (F2) having at least two fiber cores (C21, C22) extending along the length of the optical fiber (F2), an optical coupling member (OCM2) arranged at a proximal optical fiber end of the optical fiber (F2), the coupling member (OCM2) having a first distal end face (OF2) optically connected to the proximal optical fiber end, and a proximal second end face (IF2) spaced apart from the first distal end face (OF2) in the longitudinal direction of the optical fiber (F2), the optical coupling member (OCM2) being configured to couple light into each of the fiber cores (C21, C22, C23). An optical interface (OI) at a transition from the optical coupling member (OCM2) to the proximal optical fiber end is partially reflective and substantially transmissive, wherein the optical interface (OI) is arranged in such a distance distally from the proximal second end face (IF2) and is configured such that light is reflected at the optical interface (OI) with a reflection intensity distribution which substantially does not overlap with a reflection intensity distribution of light reflected at the second end face (IF2) of the optical coupling member (OCM2).
