Passive mode-locking and pulse-shortening capabilities of frequency doubling polarization mirror

Abstract

It is shown that a combination of a polarizer, a type II second harmonic generation (SHG) crystal at nearly phase- matched conditions, and a back reflector has pulse shortening and mode-locking capabilities. The latter are due to the intensity dependent change of the polarization state of the fundamental wave by cascade second-order processes in the nonlinear crystal. This change of polarization state causes nonlinear reflection, when the fundamental wave passes back through the polarizer, i.e. the combination of a polarizer a SHG crystal and a dielectric mirror acts for the fundamental wave as a nonlinear mirror, called by us a frequency doubling polarization mirror (FDPM). The dependence of the properties of the FDPM on the nonlinear characteristics and the orientation of the SHG crystal, as well as on the accumulated phase shift between the second harmonic and the fundamental wave within the space between the nonlinear crystal and the back reflector were investigated.