The invention relates to a system for detecting a deformation on a fan (1) for an aircraft engine comprising a rotor (2) equipped with a plurality of blades (3) made of a composite material including woven fibres. This system is noteworthy in that at least one of the fibres of each of the blades (3) is an optical fibre (4) comprising at least one portion defining a Bragg grating, the system furthermore comprising an emitter/receiver (6) linked to the optical fibre (4) and able to send an optical signal down this optical fibre (4) and to receive in response an optical signal from the optical fibre (4) and a detection module (7, 17) linked to the emitter/receiver for detecting a deformation on the fan (1, 11) when the optical signal received exhibits a correlation with a predetermined signature of a damped shock on a blade at a determined rotation speed. This deformation may arise from an impact of a foreign object on a blade of the fan or be consequent upon the development of an internal defect.