Beam quality investigation in Nd:YAG crystal fiber amplifier pumped at >110w

We present results of beam quality investigation in Nd:YAG crystal fiber amplifier seeded by ns, sub-ns and ps laser pulses counter-propagating to continuous pump of < 110 W power at 808 nm wavelength. The maximum amplified power of 44 W and energy of 3.2 mJ has been achieved with ns seed. We observed gain rise to < 75 with ps seed of 1.6 mW average power when tuned emission spectra to ~ 1064.3 nm. Beam propagation coefficient M 2 approach 1.2 at the maximum pump power with high quality seed pulse emitted by 6 ps fiber laser. Amplification of sub-ns seed pulses from microchip laser led to an M2 increase from ~1.2 to ~< 1.5. New analytical solution for temperature distribution in end-pumped thin long single crystal fiber with temperature dependent thermal conductivity coefficient is found for polynomial transverse pump distribution. The analytical relation between thermal coefficients of refractive index at zero stresses and zero strains is found for YAG type cubic crystals. Using plane strain approximation the analytical expression for thermal radial and tangential changes of refractive index is found, and the relation between different expressions for so-called photoelastic constants C r , θ is established. The methods of numerical calculation of rays and Gaussian beam propagation in a graded-index medium of active element are developed. The error in widely used formula for M 2 of Gaussian beam with quartic phase aberration is corrected. It is shown that beam quality degradation can be explained by active thermal lens in power amplifier when changes of transverse beam shape or beam width during amplification are taken into account.