the invention concerns a device for countering and tracking a threat with optical delay device in the form of a homing - head missile , comprising a homing head adapted to receive an incident coherent light beam and in deflecting same to produce a transmitted beam . the invention is characterized in that the homing head comprises a biprism dividing the transmitted beam into two sub - beams , the biprism being associated with an optical delay device introducing an optical path difference between the two sub - beams greater than the coherence length of the incident beam .