Numerical investigation of thermal effects of high energy laser irradiation on a germanium lens

High energy lasers (HEL) are increasingly seen as a versatile tool to counter observation systems by directly damaging or dazzling the electro-optical(EO) or infrared (IR) sensors used for detection, recognition, tracking, and targeting. The main mechanism through which high energy lasers affect their target is by heat. In the case of thermal sensors which use germanium optics, this heat is applied on the outer optical component as germanium isn't transmissive at the typical wavelengths at which current HEL lasers operate. Germanium is a brittle material and therefore is prone to shattering due to mechanical stresses caused by local thermal expansion of the hotspot. Furthermore, germanium becomes opaque to thermal IR radiation at elevated temperatures and starts emitting radiation itself. This mechanism allows an out-of-band high-energy laser to indirectly dazzle thermal infrared sensors which is referred to as pseudo-in-band dazzling. Because this effect depends on the temperature of the germanium, the sensor can remain dazzled some time after the laser irradiation has stopped while the germanium lens is cooling down. To be able to assess the effectiveness of a laser beam to dazzle or destroy a germanium lens, one must know the evolution of the heat distribution throughout the lens. In this work a thermal simulation model is presented that takes in account several aspects that influence the propagation of heat in a realistic lens. The simulation results are compared to experimentally obtained results from an earlier measurement campaign. The potential impact of the incident radiation distribution on the heating and cooling times are discussed.