Method for conditioning pulsed laser radiation for subsequent amplification, comprising: (a) coupling of an input laser radiation into a multipass active spectral filter, said multipass active spectral filter comprising a laser beam splitter and a ring delay line, (b) splitting of the input laser radiation into two parts by the laser beam splitter and delivering one part of the input laser radiation to an output of the multipass active spectral filter and the other part of the input laser radiation into the ring delay line, (c) executing a predetermined spectral conditioning of radiation, power conditioning of radiation, and preventing from oscillation inside said ring delay line, (d) splitting of radiation propagating in the ring delay line into a first part and a second part by the laser beam splitter; delivering said first part to the output of the multipass active spectral filter and delivering said second part back to the ring delay line, further, steps (c) and (d) are repeated in each roundtrip of radiation in the ring delay line, wherein said predetermined spectral conditioning of radiation in said ring delay line is performed by spectral filtering destined to narrow the spectrum with a spectral filtering element of a bandwidth not broader than 0.1nm and with a Gaussian, Lorentzian, or other bell-shaped transmission or reflection function, said power conditioning in said ring delay line is performed by amplification of radiation with an optical amplifier, said preventing from oscillation inside the ring delay line is executed by modulating losses in the ring delay line with an optical switch, (e) forming a conditioned laser radiation as a series of pulses with variable durations at the output of said multipass active spectral filter formed from said one part of the input laser radiation, which is directed to the output of the multipass active spectral filter, and said first parts of the laser radiation propagating in the ring delay line after each roundtrip, wherein the repetition of the spectral conditioning is needed for obtaining the variable pulse duration, and $ (f) selecting a pulse (61, 102 or 118) with a predetermined duration in the sub-nanosecond or a-few-nanosecond range from the series of pulses with variable durations. (a) coupling of an input laser radiation into a multipass active spectral filter, said multipass active spectral filter comprising a laser beam splitter and a ring delay line, (b) splitting of the input laser radiation into two parts by the laser beam splitter and delivering one part of the input laser radiation to an output of the multipass active spectral filter and the other part of the input laser radiation into the ring delay line, (c) executing a predetermined spectral conditioning of radiation, power conditioning of radiation, and preventing from oscillation inside said ring delay line, (d) splitting of radiation propagating in the ring delay line into a first part and a second part by the laser beam splitter; delivering said first part to the output of the multipass active spectral filter and delivering said second part back to the ring delay line, further, steps (c) and (d) are repeated in each roundtrip of radiation in the ring delay line, wherein said predetermined spectral conditioning of radiation in said ring delay line is performed by spectral filtering destined to narrow the spectrum with a spectral filtering element of a bandwidth not broader than 0.1nm and with a Gaussian, Lorentzian, or other bell-shaped transmission or reflection function, said power conditioning in said ring delay line is performed by amplification of radiation with an optical amplifier, said preventing from oscillation inside the ring delay line is executed by modulating losses in the ring delay line with an optical switch, said predetermined spectral conditioning of radiation in said ring delay line is performed by spectral filtering destined to narrow the spectrum with a spectral filtering element of a bandwidth not broader than 0.1nm and with a Gaussian, Lorentzian, or other bell-shaped transmission or reflection function, said power conditioning in said ring delay line is performed by amplification of radiation with an optical amplifier, said preventing from oscillation inside the ring delay line is executed by modulating losses in the ring delay line with an optical switch, (e) forming a conditioned laser radiation as a series of pulses with variable durations at the output of said multipass active spectral filter formed from said one part of the input laser radiation, which is directed to the output of the multipass active spectral filter, and said first parts of the laser radiation propagating in the ring delay line after each roundtrip, wherein the repetition of the spectral conditioning is needed for obtaining the variable pulse duration, and $ (f) selecting a pulse (61, 102 or 118) with a predetermined duration in the sub-nanosecond or a-few-nanosecond range from the series of pulses with variable durations.