Apparatus and method for generating electromagnetic useful radiation

A device for generating an electromagnetic useful radiation comprising a useful frequency, wherein the device comprises:

• a first radiation source configured such that the first radiation source generates and emits electromagnetic pump radiation at a pump frequency in an operation of the device;
• an external optical resonator;
• a controller;
• a pumping means;
• and
• an amplifying medium,
• wherein the resonator is arranged and configured:
• so that the resonator has a tunable resonance frequency,
• so that, in the operation of the device, the pump radiation emitted by the first radiation source is coupled into the resonator, and
• so that in the operation of the device the useful radiation is coupled out of the resonator,
• wherein the controller is operatively coupled to at least one of the first electromagnetic radiation source and the resonator and is arranged so that, in the operation of the device, the controller sets at least the pump frequency or the resonance frequency such that the resonance frequency is, at least initially, essentially equal to the pump frequency, such that, in the resonator, an electromagnetic resonator radiation oscillates at least initially at the resonance frequency, and then changes the resonance frequency of the resonator and thus the useful frequency of the useful radiation over a tuning bandwidth,
• wherein the device is configured such that the pump frequency does not follow a change in the resonance frequency,
• wherein the amplifying medium is arranged in the resonator so that in the operation of the device the amplifying medium is pumped by the pumping means and the resonator radiation is amplified in the amplifying medium,
• wherein the amplification is smaller than a threshold amplification required by the resonator and the amplifying medium for a laser action of the resonator.