A laser device for intra-cavity frequency conversion of a radiation with a laser fundamental frequency (ω) by a second harmonic generation to a converted radiation with the second harmonic frequency (2ω) of the fundamental frequency (ω), the device comprising: a laser cavity, formed by a first and a second laser cavity end reflectors; wherein said first and second laser cavity end reflectors are highly reflective of a radiation equal with the laser fundamental frequency (ω), and wherein said first and second laser cavity end reflectors provide for a predetermined level of said fundamental frequency (ω) power circulating inside the cavity to effect an efficiency of non-linear frequency conversion corresponding to said fundamental frequency (ω) power circulating inside the cavity; a cavity intermediate reflector (31; 32; 33) which is spectrally selective between the fundamental frequency (ω) and the second harmonic frequency (2ω) and which divides the laser cavity into a first section of the laser cavity formed between the cavity intermediate reflector (31; 32; 33) and the first laser cavity end reflector and a second section of the laser cavity formed between the cavity intermediate reflector (31; 32; 33) and the second laser cavity end reflector; an active medium provided within the first section of the laser cavity; at least one non-linear crystal, adapted to generate the converted radiation with the second harmonic frequency (2ω) of the fundamental frequency (ω), provided within the second section of the laser cavity; wherein the cavity intermediate reflector (31; 32; 33) and the second laser cavity end reflector are arranged such that the converted radiation with the second harmonic frequency (2ω) is maintainable for a round trip through the at least one non-linear crystal; and are further arranged such that the converted radiation with the second harmonic frequency (2ω) is out-coupled from the laser device uni-directionally from the second section of the laser cavity; tuning means adapted to control the position of at least one of the elements of the second laser cavity end reflector; wherein the second laser cavity end reflector comprises an interferometric layout providing spectrally selective reflection for the radiation equal with the laser fundamental frequency (ω), having a value of reflectivity (RmC(λ), Rm(λ)) nearly to 100% within a spectral range close to the free spectrum range of the laser cavity, and having a lower reflectivity (RmC(λ), Rm(λ)) outside the spectral range such that the laser cavity restricts the spectrum of the laser radiation to a single longitudinal mode; and wherein the interferometric layout forming the second laser cavity end reflector is configured as a Michelson interferometer. a laser cavity, formed by a first and a second laser cavity end reflectors; wherein said first and second laser cavity end reflectors are highly reflective of a radiation equal with the laser fundamental frequency (ω), and wherein said first and second laser cavity end reflectors provide for a predetermined level of said fundamental frequency (ω) power circulating inside the cavity to effect an efficiency of non-linear frequency conversion corresponding to said fundamental frequency (ω) power circulating inside the cavity; a cavity intermediate reflector (31; 32; 33) which is spectrally selective between the fundamental frequency (ω) and the second harmonic frequency (2ω) and which divides the laser cavity into a first section of the laser cavity formed between the cavity intermediate reflector (31; 32; 33) and the first laser cavity end reflector and a second section of the laser cavity formed between the cavity intermediate reflector (31; 32; 33) and the second laser cavity end reflector; an active medium provided within the first section of the laser cavity; at least one non-linear crystal, adapted to generate the converted radiation with the second harmonic frequency (2ω) of the fundamental frequency (ω), provided within the second section of the laser cavity; wherein the cavity intermediate reflector (31; 32; 33) and the second laser cavity end reflector are arranged such that the converted radiation with the second harmonic frequency (2ω) is maintainable for a round trip through the at least one non-linear crystal; and are further arranged such that the converted radiation with the second harmonic frequency (2ω) is out-coupled from the laser device uni-directionally from the second section of the laser cavity; tuning means adapted to control the position of at least one of the elements of the second laser cavity end reflector; wherein the second laser cavity end reflector comprises an interferometric layout providing spectrally selective reflection for the radiation equal with the laser fundamental frequency (ω), having a value of reflectivity (RmC(λ), Rm(λ)) nearly to 100% within a spectral range close to the free spectrum range of the laser cavity, and having a lower reflectivity (RmC(λ), Rm(λ)) outside the spectral range such that the laser cavity restricts the spectrum of the laser radiation to a single longitudinal mode; and wherein the interferometric layout forming the second laser cavity end reflector is configured as a Michelson interferometer.