Strong self-focusing of axial symmetric laser beam due to quadratic nonlinearity

Strong self-focusing and defocusing of optical radiation with Gaussian spatial and temporal distribution due to cascading second harmonic generation without a significant change in the beam profile and the pulse shape for the fundamental wave under the condition of group velocity matching in the axial-symmetric case for a bulk medium is considered, and the maximum intensity of an optical beam increases in 70 times (or more) in comparison with the intensity of incident optical radiation. This result is obtained for pulse duration, which belongs to a time interval from microseconds to picoseconds, for which the dispersion of group velocity is negligible. Self-focusing of laser radiation allows the realization of the mode that is similar to Kerr-lens mode locking, but for laser systems generating the optical radiation with a duration from microseconds to picoseconds. (C) 2012 Optical Society of America