Radiation safety considerations for ultrafast lasers beyond laser machining

TL;DR

This paper examines radiation safety in ultrafast laser applications, showing that X-ray emission risks are mainly associated with material processing and are negligible in other laboratory conditions, challenging current legislation.

Contribution

It demonstrates that X-ray generation thresholds are specific to processing conditions and argues that existing safety regulations overgeneralize this hazard to all ultrafast laser uses.

Findings

X-ray emission occurs primarily during material processing.

Laboratory experiments show negligible X-ray doses in non-processing conditions.

Current legislation may overstate the radiation hazard for non-processing ultrafast laser applications.

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.