Didelės energijos hibridiniai femtosekundiniai lazeriai Yb legiruotų šviesolaidžių bei YAG kristalų pagrindu.

In this dissertation, sub-picosecond hybrid laser systems are investigated, which are based on an all-in-fiber seed source and end-pumped double-pass Yb:YAG amplifier cascade. These laser systems could be widely used in the science, industry, and military sectors. A numerical model for the Yb:YAG amplifier was presented and experimentally verified – optical amplification, beam degradation due to thermal effects, and gain narrowing effects were verified. Three different laser systems were experimentally demonstrated based on the modeling data. These systems featured such laser parameters: (1) pulse energy 104 uJ, pulse repetition rate 200 kHz, pulse duration 764 fs; (2) pulse energy 1 mJ, pulse repetition rate 14 kHz, pulse duration 815 fs; (3) pulse energy 35 uJ, pulse repetition rate 1 MHz, pulse duration 318 fs. It was experimentally demonstrated that spatial filtering and amplification of the circular polarization radiation minimizes the beam degradation due to the thermal effects in such amplifiers. The last chapter of the dissertation focused on the minimization of power losses due to the depolarization effect in the additional Yb:YAG amplifier cascade. An innovative depolarization compensation method using a spatially variable waveplate was presented. Power losses were reduced from 18% to 3%. In such a system, 116 uJ pulse energy, 1 MHz pulse repetition rate, and 441 fs duration pulses were achieved

Peculiarities of second harmonic generation with chirped femtosecond pulses at high conversion efficiency

Frequency doubling of an infrared laser radiation in non-linear optical crystals is a widely used technique to obtain light in the visible range. The second harmonic generation process is influenced by several well-known parameters. In this article we study the effect of group delay dispersion on the second harmonic generation process for femtosecond pulses. We show, both through simulation and experiments, that for certain parameters even a small amount of chirp can have a detrimental effect on the conversion efficiency as well as the second harmonic beam quality. We also check the effect of higher order dispersion. By properly accounting for those effects the crystal length and focusing conditions can be optimized to reach high conversion efficiency, while maintaining low sensitivity to chirp variations and good beam quality

Investigation of materials for supercontinuum generation for subsequent nonlinear parametrical and Raman amplification at 1 MHz repetition rate

In the present work we performed research of supercontinuum generation in several commonly used and new supercontinuum generation crystals for subsequent nonlinear amplification, using 1-3 mu J energy pulses of 300 fs duration at 1 MHz repetition rate. Obtained supercontinuum spectra spanning over 480-1950 nm wavelength range at pump pulse energies as low as 200nJ in KGW and YVO4 crystals. We present simple experimental setups of stimulated Raman amplification and optical parametric amplification using supercontinuum seeds obtained from several selected crystals. We achieved total energy conversion efficiencies up to 9% both for optical parametric amplification setup and for stimulated Raman amplification setups. The optical parametric amplifier was tunable in the 680-980 nm spectral range and produced ultrashort pulses of 23-44 fs duration. Raman amplifier produced more than 130 mW average power at 1194 nm wavelength and featured broadened spectrum corresponding to Fourier transform limited similar to 100 fs pulse duration. We demonstrated that low power and low energy femtosecond lasers could be efficiently employed for the nonlinear wavelength conversion