Single-frequency and Kerr-lens mode-locked Nd:YLF/ppKTP ring laser

In this work, we demonstrate indeed that complex cascaded second-order nonlinear processes occurring inside the ppKTP crystal are responsible of the KLM operation of a sigma-polarized (lambda=1321nm) Nd:YLF/ppKTP ring laser, over the full range of the ppKTP temperature tuning. The preliminary analysis of the laser emission versus the ppKTP temperature or versus the intra-cavity power demonstrates unambiguously the tendency to cascaded Kerr-lens mode locking (KLM) effects, which threshold occurs at about 5 W of diode pump power. Mid-IR spectra recorded with an optical spectrum analyzer (OSA) reveal emission bandwidths greater than BW=3 nm under the pulsing temporal regime, in correlation with a broadband background transmission from the scanning Fabry-Perot interferometer. While passive KLM mode-locking due to pure cascaded second-order nonlinear effects was first reported in a standing-wave cavity operating close to its stability region, our unidirectional ring laser operates well within the cavity stability range and is rather designed for cw operation.