Influence of a relaxation time of optical semiconductor saturable absorber on spectral properties of ultrashort pulses

In the present work, the formation of ultrashort pulses in a fiber laser resonator with the effect of slow saturable absorption and spectral filtering was studied. It has been shown that in a resonator with normal chromatic dispersion, the finite relaxation time of a saturable absorber leads to a spectral shift of the generated pulses with respect to the central length of the spectral filter. The numerical results are verified using two experimental laser sources: a ring fiber laser with a semiconductor saturable absrober mirror and a fiber laser with a nonlinear amplifying loop mirror. The results obtained are relevant for designing sources of ultrashort pulses in applications for which the spectral properties of radiation are crucial parameters.