Ionizing radiation hazards from the tight focusing of ultrashort laser pulses in ambient air Available to Purchase

Manipulating unconfined ultrashort pulsed laser beams is increasingly common in academic research laboratories. Normally, health risks associated with beam propagation and alignment can be mitigated using protective eyewear and following standard laser safety guidelines. A recent investigation [1] has uncovered an additional, unexpected health risk. While working with High Numerical Aperture (HNA) reflective optics in ambient air, it was demonstrated that the laser pulse intensity at the focus was sufficient to accelerate electrons to relativistic energies and generate high dose-rate ionizing radiation. Non-linear effects in these conditions usually distort the propagating wavefront and render difficult the proper focusing of the beam. We have shown this does not hold true when focusing the output of the ALLS 100Hz Mid-IR laser beamline (2.8 mJ, 12 fs pulse centered at 1800 nm) with a reflective optic having a Numerical Aperture (NA) close to 1. Using this configuration, a radiation dose of 100's of mGy per minute was measured at 50 cm from the focus in ambient air, which exceed public dose limits in a very short time. Non-negligible dose measurements have also been observed with longer pulse durations, shorter wavelengths and standard off-the-shelf tight focusing optics from traditional optical suppliers. In this specific case, when using HNA reflective optics with ultrashort pulsed laser beams, the Laser Safety Officer (LSO) should carefully assess the risk related to hazardous ionizing radiation emitted by the focused beam.