Passive mode-locking of cw lasers due to frequency sliding

Rapid progress in the field of subpico- and femtosecond pulse generation has resulted in the shortest pulse duration of 7.5 fs [1]. The main mode-locking technique used here is the Kerr-lens mode-locking [2]. However, one of the distinctive feature of modern investigations is the renovation of the interest toward passive mode-locking due to saturable absorbers, in particular, multilayer quantum-well semiconductor absorbers [3, 4]. It was shown, that such absorbers are capable to provide sub-10 fs ultra-short pulse generation, in spire the absorber’s recovery time exceeds 100 fs. This result has an explanation in terms of soliton mode-locking with slow saturable absorber as a stabilizing mechanism [5]. Last time an ultra-short pulse stabilization mechanism due to spectral filter with a transmission band shifted in respect to the generation field carrier frequency (a so called sliding frequency method [6] which is used in fiber optics lines) has received a great lot of attention.