A system for detecting deformation of a fan for an aeroengine, the fan including a rotor including plural blades made of composite material including woven fibers. At least one of the fibers in each of the blades is an optical fiber including at least one portion defining a Bragg grating. The system further includes a transceiver connected to the optical fiber and configured to send an optical signal into the optical fiber and to receive an optical signal in response from the optical fiber, and a detector module connected to the transceiver to detect deformation of the fan when the received optical signal presents correlation with a predetermined signature of a damped impact on a blade at a determined speed of rotation. This deformation may be the result of a foreign object impacting against a blade of the fan or may follow from variation in an internal defect.