Determination of the damage growth threshold of multilayer dielectric gratings by picosecond laser pulses based on saturation damage size analysis.

We propose two efficient methods of determining damage growth threshold (DGT) based on the saturation damage size analysis (SDSA) for multilayer dielectric gratings by picosecond pulsed lasers. The damage size at laser fluences above DGT increases with the shot number and finally saturates due to the Gaussian focal spot. The DGT is extracted by mapping the boundary of a saturation damage site obtained at single fluence to the beam profile, which is called the monofluence SDSA method. Meanwhile, the saturation damage size decreases when reducing laser fluence. The fitting and extrapolation of the saturation damage sizes at different fluences are also useful to accurately determine the DGT, which is called the multifluence SDSA method. Although the saturation damage site is asymmetric, the DGTs measured with two SDSA methods are almost identical for the same axis, and both are in very good agreement with those obtained with the growth probability method. The underlying mechanisms and advantages of two SDSA methods are extensively discussed. The consistence of two SDSA methods in determining DGT is attributed to the same morphology of the initial damage and the saturation damage boundary, as well as the local damage dynamics. The relation of the lifetime damage threshold and DGT obtained with the SDSA method is also revealed.