Measuring the reflections from a high-energy laser focused on melting steel targets for laser safety assessments

In this study, we conducted measurements of reflections from High-Energy Lasers (HEL) focused on melting steel targets to assess laser safety and determine the Ocular Hazard Distance (OHD). Experiments were performed in both a controlled laboratory setting and an outdoor test range. In the laboratory, we utilized a fiber laser with 850 W at 1070 nm focused on steel targets, capturing reflections on an observation screen monitored by a camera sensor at distances of 2.4 m and 4.8 m from the target. For the outdoor tests, a 3 kW-fiber laser was employed 420 m from the targets. Atmospheric turbulence was monitored during these tests, but no significant influence on reflection measurements were observed. The observation screen was positioned first at 2.5 m and second at 9.0 m from the target during these experiments and calibrated for both distances. Analysis of the data revealed that the ratio of radiant intensities at different distances indicated that the measurements at those distances did not fulfill far-field conditions. Notably, we observed that intensity decreased with a 1/r1.25 dependency rather than the expected 1/r2. Using these findings, we established a method for extrapolating data to higher laser powers, ultimately calculating an OHD of 560 m in a worst-case scenario for a 100-kW laser focused on melting steel targets.