Artificial spectral filtering in dissipative soliton fiber lasers with invisible bandpass filters

We numerically study the artificial spectral-filtering effect in dissipative soliton fiber lasers without intracavity spectral filter. It is found that in the dissipative soliton lasers with real saturable absorbers (SAs), the dynamic spectral filtering of the real SAs serves as an artificial spectral filter and contributes to the pulse shaping. While in the dissipative soliton lasers with artificial SAs, such as nonlinear polarization rotation, the spectral filtering introduced by the intracavity polarization-dependent components acts as an artificial spectral filter and shapes the pulses to get mode-locking. The investigation of the artificial spectral-filtering effect reveals the operating mechanisms of the dissipative soliton fiber lasers without visible bandpass filter.