Influence of spherical aberration of the active medium on the performance of Nd:YAG lasers

In general, the temperature distribution in Nd:YAG laser rods deviates from a parabolic functional form due to the temperature dependence of thermal conductivity and the inhomogeneity of pumping light distributions. An analytical expression is derived which relates the resulting radially dependent refractive power to the shape of the pumping light distribution. To first order, the refractive power has a parabolic radial dependence and this spherical aberration considerably affects the laser´s output characteristics. The performance of both stable and unstable resonators is investigated theoretically by numerically solving the Fresnel-integrals, including the amplification, the refractive power and the aberration of the rod. High output power and good beam quality cannot be achieved simultaneously when aberration is present. Experiments for both resonator types were performed with a pulsed Nd:YAG laser capable of a maximum output of 320 watts of average power