Out-of-band laser damage to an anti-reflection coated Germanium window

Thermal infrared imaging systems are increasingly vulnerable to the threat of disruption or damage from laser weapons. For laser weapons operating within the waveband of the imager, in-band, the effect is primarily dazzling of the focal plane array. In contrast, for those not in the same waveband, out-of-band, the critical interaction is thermal heating of the outer optical components leading to probable substrate damage. In this article, we analyze the impact of out-of-band high power laser radiation on an infrared telescope operating in the longwave waveband region (LWIR). In particular, we illustrate the stages from coating ablation to subsequent substrate damage. A 1.07 micron continuous-wave fiber laser illuminated the telescope for durations up to a few seconds at a maximum power of approximately one kilowatt. Both visible and LWIR imagers recorded the impact on the outer lens. Our results indicate a predictable chronological sequence of events leading to material damage and heat transfer effects across the substrate as we increased the laser power and duration time.