Development of a radiometric sensor for the hazard assessment of scattered high-power laser radiation

Abstract

The operation of lasers in free space involves the potential risk of unintentionally exposing the human eye and skin to radiation. In addition to direct exposure, indirect scattered radiation of high-power lasers may pose a threat to operators, working personnel, and third parties. Hazard assessments are usually performed based on laser safety standards. However, these standards would have to be extended for outdoor environments and therefore it is advisable to substantiate models and safety calculations with measurements of the absolute scattered radiant flux under realistic conditions. For the quantification of scattered radiation, a radiometric sensor has been developed. The sensor consists of an optical, electronic, and mechanical unit. Two realizations of the optical detection unit with a side-on photomultiplier (PMT) and a photodiode amplifier (PDA) have been built according to German safety policies. The different detector types facilitate the detection of scattered radiation over a wide power range. The electronic unit includes the data acquisition and processing of the optical detection unit and peripheral devices (i.e. environmental sensors and GPS module). A lock-in amplifier is used to reduce the contribution of background radiation. The optical and electronic units are housed separately in a weather-resistant case on a tripod and a mobile container, respectively. Radiometric calibration is performed for each optical detection unit. The calibration involves a two-step procedure allowing for a direct conversion of the output voltage of the lock-in amplifier into an absolute scattered power considering the detector area and collection solid angle of the optical detection unit. Goniometer-based reflection measurements of solid surface samples are used for the characterization of the performance of the optical detection unit in terms of dynamic range, the influence of background noise, accuracy, and repeatability and contribute to a better understanding of the sensor in future field deployment.