Asymmetrical damage growth of multilayer dielectric gratings induced by picosecond laser pulses.

We experimentally investigated the laser damage growth behavior of multilayer dielectric gratings (MLDGs) by the picosecond pulses at 1053nm. The damage growth threshold of 2.43J/cm is significantly lower than the 20/1 damage threshold of 3.06 J/cm. Once the damage site is initiated, the damage area grows linearly with shot number and saturates after sufficient shots due to the Gaussian spot. The barycenter of the growing damage site deviates from the laser spot center and their distance increases with the shot number, which indicates the asymmetry of the damage growth along the laser propagation axis. The growth rate of the damage site along the laser propagation direction is larger than that in the reverse direction by a factor of ~1.8 for various fluences. The comparison of the experimental and numerical results reveals that the asymmetrical intensity modulation induced by the damage sites causes the asymmetry in growth. The revealed characteristics and mechanisms of the damage growth can be of great significance to predict the lifetime of the MLDGs in high-power laser systems.