Robotization of medical acts often requires the evaluation of contacts between a robotic system and a patient, for safety or efficiency reasons. When contact occurs with a stiff environment, instabilities and vibrations can appear and a passive compliance is therefore needed. In this paper, we propose to embed compliance in a force sensor and to develop a novel force sensor with large compliance, i.e. a zero stiffness at contact transition to ease robot control. To get at the same time a satisfying measurement range and low off-axis sensitivity, an optical measurement process based on an optical line generated thanks to additive manufacturing is exploited. A compliant sensor body allowing the desired stiffness profile is presented and the specific optical measurement technique is developed. Finally, a prototype of the proposed force sensor is evaluated experimentally.