Compensation of depolarization in high power yb:yag amplifier using a nanostructured optical elements

High intensity and high power femtosecond laser systems suffer from undesirable thermal phenomena in the active element: depolarization and bi-focusing. After the propagation through polarizing elements the depolarized light acts as optical losses. Subsequently, beam profile is deteriorated, severely limiting the output power and thus applicability of such high-power lasers. The main goal of this work was to compensate depolarization in Yb:YAG crystal double-pass amplifier pumped by 270 W laser diodes radiations. For this purpose the nanostructured optical element – nanogratings formed in a fused silica glass by direct laser writing using femtosecond laser was used as a depolarization compensator. Phase delay profile created in specially designed sample repeats that of the Yb:YAG amplifier but with opposite sign. In this work, the degree of depolarization and bi-focusing in Yb:YAG crystal, used in double-pass amplifier was measured. The dependences of these parameters on the seed signal power, as well as beam profiles of depolarized and non-depolarized beams on seed power were also measured. At the maximum pump power (270 W) and amplified signal power (131 W), the highest degree of depolarization observed was 19,3%, which was estimated as a power loss of 25,3 W. Moreover, due to the induced bi-focusing, divergence ratio in two perpendicular beam cross sections X and Y increased to 1,46 while beam profile became elliptical. Depolarization losses and bi-focusing were reduced with depolarization compensator. The best compensation results were achieved for 67 W amplified signal power and resulted in depolarization reduction from 14,3% to 1,3%. In this case, bi-focusing was eliminated such as the ratio of divergences in perpendicular directions was reduced from 1,31 to initial 0,98. The symmetry of the beam profile was also restored to the original. To summarize, depolarization compensators based on direct laser writing of nanogratings in fused silica is suitable for efficient compensation of depolarization, bi-focusing and beam profile distortions occurring in thermally loaded active elements and can be implemented in high power ultrafast laser systems

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