Optical device for detecting the acceleration of a support which is movable relative to a frame of reference, this frame of reference comprising at least one preferential direction of displacement of the support, the device comprising: - a frame intended to be fixed on the support, - a flexible element, elongated and having two opposite ends (51, 52), which are a first end, which is connected to the frame 4, and a second end which, upon flexion of the flexible element, is free to displace itself relative to a reference axis extending longitudinally to the flexible element, the flexible element being composed of two parts (54, 55) substantially identical to one another, referred to as first part and second part, which: - are each of substantially elongated shape and are flexible, - have their longitudinal axes (54A, 55A) which extend substantially in a same plane, referred to as first plane, - are each made up of at least one flexible optical fibre (54, 55), the first part of the flexible element being used to transmit the light from the first end of this flexible element toward the second end of said flexible element, in such a manner that this light exits by an end face, referred to as exit face, of this first part, this device being characterized in that: - the said first part and second part of the flexible element are maintained spaced apart by a predetermined distance, referred to as first distance, in a way so as to determine a preferential plane of deformation, referred to as second plane, approximately perpendicular to the first plane and parallel to the reference axis, and - the second part is adapted to collect, by an end face referred to as entrance face, the light coming from the exit face illuminating the second end of the flexible element and transmit it in the direction of the first end of this flexible element, - means of detection are adapted to determine the displacement of the second end starting from the quantity of light received by the entrance face. - a frame intended to be fixed on the support, - a flexible element, elongated and having two opposite ends (51, 52), which are a first end, which is connected to the frame 4, and a second end which, upon flexion of the flexible element, is free to displace itself relative to a reference axis extending longitudinally to the flexible element, - are each of substantially elongated shape and are flexible, - have their longitudinal axes (54A, 55A) which extend substantially in a same plane, referred to as first plane, - are each made up of at least one flexible optical fibre (54, 55), - the said first part and second part of the flexible element are maintained spaced apart by a predetermined distance, referred to as first distance, in a way so as to determine a preferential plane of deformation, referred to as second plane, approximately perpendicular to the first plane and parallel to the reference axis, and - the second part is adapted to collect, by an end face referred to as entrance face, the light coming from the exit face illuminating the second end of the flexible element and transmit it in the direction of the first end of this flexible element, - means of detection are adapted to determine the displacement of the second end starting from the quantity of light received by the entrance face.