Complex sentence: Normally, beams of light pass through each other unperturbed inside an optical material, and if the intensity of the beams is high enough, the beams may affect each other through a variety of non-linear effects.
Split into simpler sentences: Normally, beams of light pass through each other unperturbed. Inside an optical material, and if the intensity of the beams is high enough, the beams may affect each other through a variety of non-linear effects.