The recently observed laser wavelength self-sweeping is known for its large sweeping range up to tens of nanometers, single- or few-longitudinal mode operation and self-pulsing output [1-5]. The self-sweeping can be explained by spatial hole burning along the active fiber. The spatial hole burning cause also weak refractive index modulation [6-7]. Self-swept fiber lasers recently attained attention thanks to its practical applications in laser spectroscopy and fiber sensor systems as well as revealing physical insight view into important laser physics phenomena, e.g., the origin of self-Q-switching regime and longitudinal mode-instabilities in fiber lasers.
We present evaluation of the reflectivity of superimposed transient fiber Bragg gratings (FBG) using a numerical model of fiber laser with refractive index gratings build-up. In contrast to single refractive index grating demonstrated earlier [6-7] we present more realistic case of several superimposed gratings with damped modulation depth. We show that the 100 % reflectivity and thus sudden enhancement of the cavity Q-factor can be easily achieved despite small modulation depth of the gratings. The differences of reflectivities of the single FBG and of the multiple superimposed damped FBGs are discussed. This work was supported by the Czech Science Foundation, project No. 16-13306S.
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