Reducing thermal lensing in diode-pumped laser rods

We present a diode-end-pumped Nd:YAG laser with multiple composite rods. Composite rods (rods with undoped endcaps) were used in order to remove the part of thermal lens formed by the bending of the pump face of the rod. We give an analytical expression for determining the fraction of the thermal lens contributed by the end effect in a homogeneously doped rod, such that the improvement in a composite-doped rod can be determined. For the Nd:YAG rods used in this work this part of the thermal lens constituted approximately 47% of the total thermal lens in the homogeneously doped rod leading to the maximum pump power being increased by almost 90%, on introduction of composite rods. The laser was demonstrated, giving a maximum output power of over 16 W and an optical efficiency of over 42% in fundamental mode operation.