Radiation safety considerations for ultrafast lasers beyond laser machining

Abstract

The interaction of ultrafast lasers with plasmas has been studied for many years, primarily with respect to fundamental emission mechanisms. Only in recent years has ionizing radiation emerged as a safety concern in ultrafast laser-based material processing, where high pulse energies, repetition rates, and average powers, combined with continuous material supply, can lead to sustained X-ray emission. These processing-specific findings have informed German radiation protection legislation, which mandates notification or approval for laser systems exceeding irradiances of $1 \times 10^{13}~W/cm^2$. However, this threshold does not distinguish between material processing and other ultrafast laser applications. In this work, we show that the conditions required for X-ray generation are highly specific and are typically only met during material processing. We assess the applicability of existing radiation studies to non-processing environments and present experimental results demonstrating negligible or no dose production under representative laboratory conditions, such as ultrafast laser interactions with underdense gas or stationary solid targets. We conclude that current legislation generalizes a processing-specific hazard to all ultrafast laser applications and does not adequately reflect the relevant physical conditions.

Figures (4)

• Figure 1: Experimental setup and representative diagnostic images. a, Schematic overview of the experimental setup for dose estimation. The laser beam is focused by a lens to generate either an air plasma or to drill holes in stationary overcritical targets at normal incidence. The targets are mounted on translation stages for alignment. X-rays are measured using an OD-02 detector and a Silix Science detector positioned at distances of 20cm and 10cm, respectively. For overcritical targets (tungsten and steel), a second OD-02 detector was placed in the forward direction (in laser propagation direction). b, Pulse duration measurement obtained with an autocorrelator. c, Image of plasma emission projected onto a screen placed behind the air plasma. d, Optical image of ten holes drilled into a 1mm thick tungsten target. e, Microscope image of a hole drilled into a steel target.
• Figure 2: X-ray measurements in air at nominal wavelength-normalized intensities $I\lambda^2$ of up to e16Wcm µm. a, Photon spectrum and absolute dose measured with the Silix Science detector during laser operation. b, Dose rate measured with the OD-02 detector with laser operation (signal) and without laser operation (background).
• Figure 3: Spectrum and absolute doses measured during drilling of 1mm thick, stationary tungsten targets at nominal wavelength-normalized laser irradiance $I\lambda^2$ of up to e16Wcm µm. a, Spectrum and absolute dose integrated for all ten holes measured with the Silix Science detector. b, Evolution of the average dose per hole with standard deviation measured with the OD-02 detector. c, Evolution of the total integrated dose for all ten holes measured with the OD-02 detector. d, Temporal evolution of the average dose rate with standard deviation, obtained by numerical differentiation of the data shown in panel b. Mild smoothing (three-point moving average) was applied to mitigate noise amplification inherent to numerical differentiation. The dotted line indicates the zero line.
• Figure 4: Spectrum and absolute doses measured during drilling of 3mm thick, stationary steel targets at nominal laser pulse intensities of up to e16Wcm µm. a, Spectrum and integrated absolute dose for all five holes measured with the Silix Science detector. b, Evolution of the average dose per hole with standard deviation measured with the OD-02 detector. c, Evolution of the total integrated dose for all five holes measured with the OD-02 detector. d, Evolution of the absolute dose for each of the five individual holes measured with the OD-02 detector.