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