Self-focusing limitation of brightness in amplification of ultrashort pulses in neodymium glass and yttrium aluminum garnet

An experimental investigation was made of ultrashort pulses generated in yttrium aluminum garnet (YAG) and amplified in YAG and neodymium glass. When the power density was less than 3–5 GW/cm², the main mechanism which limited the amplified power was large-scale self-focusing of the amplified radiation in the active media, which altered greatly the laser beam divergence. Conditions were found under which this self-focusing was avoided completely. The investigated self-focusing produced, around the main beam, a scattered-radiation halo of divergence much greater than the beam divergence. The proportion of the laser beam energy lost to the halo increased with the amplification, so that beginning from a certain stage further amplification simply resulted in a reduction in the brightness. In this situation the enrgy density in the amplified beam could still be below the damage threshold of the active medium.