Filtering dynamics of an interference comb filter in a ytterbium-doped mode-locked fiber laser

Filter plays a key role in a positive dispersion Yb-doped mode-locked fiber laser. It works together with saturable absorbers to maintain the self-consistent evolution of the pulses. Fiber interference filter has a compact structure and is easy to implement. However, its comb-shaped transmission spectrum introduces sidebands at the edges of the pulse spectrum. A numerical model of mode-locked fiber laser using an interference filter is established based on the coupled Ginzburg-Landau equations. The reasons for the appearance of sidebands are analyzed, and the impact of the sidebands on the pulse characteristics is discussed. The simulation results show that the spectral sidebands originate from the comb-shaped transmission spectrum of the interference filter. When the transmittance of the neighboring peaks of the comb filter decreases by 20 dB, the spectral sidebands nearly disappear. The simulation results also indicate that a clear pedestal appears on both sides of the pulse, which occupies a portion of the pulse energy and reduces the pulse peak power by 3%. A method to eliminate the sidebands is proposed, which involves cascading filters with different free spectral ranges, to eliminate the influence of the neighboring transmission peaks of the filter. In the experiment, a filter based on cascaded tapered seven-core fiber is prepared and inserted into a Yb-doped fiber laser mode-locked by nonlinear polarization rotation. Stable mode-locked pulses have been obtained, with a pulse duration and energy of 4.37 ps and 1.2 nJ, respectively. The experimental results agree with the simulation results very well.