The invention concerns a device for countering and tracking a threat in the form of a homing-head missile, comprising a homing head adapted to receive an incident coherent light beam and to deflect same to produce a transmitted beam. The invention is characterized in that the homing head comprises a biprism including two prisms made of different materials and adapted to divide the transmitted beam into two sub-beams, the refractive index difference between the prisms being adapted to introduce an optical path difference between the two sub-beams which is greater than the coherence length of the incident beam.