Fundamental mechanisms and statistical nature of laser-induced damage in dielectrics are investigated and light scattering micro-tomography using He-Ne laser beam is developed for the purpose of damage observation.
Nonlinear coupled equations of electron density and optical intensity for the generation of highly excited conduction electrons by strong laser pulse are obtained on the basis of avalanche ionization and multi-photon ionization processes. Expressions for nonlinear transmittance of intensity and fluence are obtained from the approximate solution of the coupled equations and they are compared with experimentally measured transmittance. From this comparison and the statistical nature in damage experiment, it is concluded that the main mechanism for damage in BK-7 glass induced by Nd-glass laser pulse is avalanche ionization. The measured threshold fluence for damage is 1.0± .2 MJ/㎠.
Light scattering tomography developed in this study is found to be efficient for the observation of small defects distributed in transparent media, however, multiple scattering process of the scanning laser beam requires refined optical and (or) electronic signal processing to improve tomographic image quality.
