Radiation source and a method for generating electromagnetic radiation at a plurality of frequencies

A radiation source for generating electromagnetic radiation at a plurality of frequencies, wherein the radiation source comprises:

• an initial source, wherein the initial source is arranged to generate electromagnetic initial radiation comprising an initial centre frequency during operation of the radiation source;
• a non-linear optical frequency converter, wherein the frequency converter comprises a converter resonator which is resonant for the initial radiation and comprises an input coupling mirror which is partially transparent for the initial radiation and a non-linear optical crystal which is arranged in the converter resonator, wherein the frequency converter is located in a beam path of the initial radiation so that during operation of the radiation source the initial radiation is coupled into the converter resonator through the coupling mirror, wherein the frequency converter is arranged such that during operation of the radiation source the frequency converter generates converted radiation at a converted frequency from the initial radiation, the converted frequency being different from a frequency of the initial radiation, and wherein the converter resonator comprises an actuator, and wherein the actuator is located and arranged such that a resonance frequency of the converter resonator is tunable by the actuator in dependence on a conversion control signal;
• a conversion detector, wherein the conversion detector is located in a beam path of the converted radiation or in the beam path of the initial radiation in the beam direction behind the coupling mirror so that the conversion detector detects a measure of the power of the converted radiation during operation of the radiation source and outputs a conversion measurement signal representing the measure of the power of the converted radiation;
• and
• a controller, wherein the controller is connected to the conversion detector such that the controller receives the conversion measurement signal during operation of the radiation source, wherein the controller is connected to the actuator such that the actuator receives the conversion control signal from the controller during operation of the radiation source, and wherein the controller is arranged, during operation of the radiation source in a first mode of operation, to generate the conversion control signal in dependence on the conversion measurement signal such that the resonance frequency is controlled to follow the initial centre frequency,
• wherein the controller is set up in such a way that the controller is selectably switchable between the first mode of operation and a second mode of operation during operation of the radiation source,
• wherein the controller is arranged to generate the conversion control signal in the second mode of operation such that the power of the converted radiation is controlled, that either the power of the converted radiation has a smaller relative temporal fluctuation 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 after the coupling mirror has a smaller relative temporal fluctuation than the power of the initial radiation in the beam direction in front of the coupling mirror.