Composite filtering effect in a SESAM mode-locked fiber laser with a 3.2-GHz fundamental repetition rate: switchable states from single soliton to pulse bunch.

Abstract

States that are switchable from single soliton to pulse bunch in a compact semiconductor saturable absorber mirror (SESAM) mode-locked fiber laser with a fundamental repetition rate of 3.2 GHz are experimentally investigated and further studied via simulations. A composite filtering effect comprising an intracavity low-finesse Fabry-Perot (FP) filter, an artificial optical low-pass filter, and a gain filter implements the state switching to pulse bunch. A numerical model is proposed to clarify the mechanism underlying the switching. It reveals that, for pulse interval ∆T > τA (relaxation time of the SESAM) in a pulse bunch, the laser operates in pulse-bound build up. In an inverse mechanism the state returns to single soliton, in which the ∆T is obtained from the free spectral range Ωc of the intracavity FP filter by mechanically controlling the distance between the SESAM and gain fiber. This pulse bunch regime of operation ought to be amenable to a quasi-steady-state treatment. It represents an alternative emergence trait in the temporal domain between a main soliton with strong sidelobes in both sides and a bound soliton pair with weak sub-sidelobes. Another profile of the pulse bunch state is that the side peak amplitude in the autocorrelation trace is more than 50%, which is distinct and larger than that in the conventional bound state regime in fiber lasers. The optical spectra, radio frequency spectra, and frequency chirp are further analyzed. These numerical results agree well with the experimental ones within the variation range of the crucial values of Ωc and enable the explicit understanding of such behavior in SESAM mode-locked high-repetition-rate fiber lasers.