Frequency red shift induced by the enhancement of asymmetric absorption of saturable absorber in a Yb-doped mode-locked fiber laser

The mechanism of frequency red shift from gain spectral center is revealed in a SESAM mode-locked Yb-doped fiber laser for the first time as far as we know, which is due to the asymmetric absorption of SESAM. The phenomenon is demonstrated by numerical simulations and confirmed by experimental results, which is attributed to a greater loss of the frequency components in the temporal rising edge, as a consequence pulses become asymmetric. With the balance between the asymmetric absorption of SESAM and the pulling effect of gain spectral center, pulses in the laser are able to be stabilized and the spectra are ultimately red shifted. The increase of temporal asymmetry, as well as the enhancement of frequency red shift as results of the increase of saturation energy are also demonstrated.