A radiation source includes an initial source, a non-linear optical frequency converter, a conversion detector and a controller. The controller is arranged such that in a first mode of operation, the controller generates the conversion control signal in dependence on the conversion measurement signal such that the resonance frequency is controlled to follow the initial centre frequency. The controller is selectably switchable between the first mode of operation and a second mode of operation. In the second operating mode the power of the converted radiation is controlled such that either the power of the converted radiation has a smaller relative variation over time than the power of the initial radiation in the beam direction in front of the coupling mirror or the power of the initial radiation in the beam direction behind the coupling mirror has a smaller relative variation over time than the power of the initial radiation in the beam direction in front of the coupling mirror.

The present disclosure provides a method of optimising an optical system of a mode-locked laser oscillator or a regenerative, multi-pass or single pass amplifier. The method may include the steps of identifying crystallographic axes of an active laser gain medium crystal, cutting the crystal, and orienting the crystal in the optical system in a predetermined orientation relative to a propagation vector of a laser pulse depending on the required output of the optical system.

An ultraviolet (UV) light source includes a light emitting element that is configured to generate light of a first frequency, a nonlinear optical element that is configured to receive the light of the first frequency from the light emitting element and generate UVC or far-UVC light of a second frequency from the light of the first frequency, and an output coupling element that is configured to selectively outcouple the UVC or far-UVC light from the nonlinear optical element as output light. Related light sources and fabrication methods are also discussed.