Accurate metrology for laser damage measurements in nonlinear crystals

Accurate laser damage measurements are more difficult to perform in nonlinear optical crystals than in glasses due to several effects proper to these materials or greatly enhanced in these materials. Before discussing these effects, we address the topic of error bar determination for probability measurements. Error bars for the measured damage probabilities are especially important when testing small and expensive samples like nonlinear crystals, where only few sites are used for each measurement. The mathematical basics for the numerical calculation of probability error bars corresponding to a chosen confidence level are presented. Effects that possibly modify the maximum light intensity obtained by focusing into a biaxial nonlinear crystal are mainly the focusing aberrations and self-focusing. Depending on focusing conditions, position of the focal point in the crystal, beam propagation direction, and polarization, strong aberrations may change the beam profile and drastically decrease the maximum intensity in the crystal. A correction factor based on former theoretical work is proposed for this effect. The characteristics of self-focusing are quickly reviewed for the sake of completeness, and a note on parasitic second harmonic generation is added at the end. (C) 2008 Society of Photo-Optical Instrumentation Engineers.