The self-induced laser frequency sweeping (“self-sweeping” for short) effect in fiber lasers was observed in 2011 . In this case the lasers generate self-sustained pulses and its frequency drifts slowly from pulse to pulse in one direction and jumps back after reaching a border of sweeping region. The self-sweeping operation is based on formation of long-lived dynamical phase and gain gratings. The gratings are connected to modulation of population inversion inside the active medium induced by the field of a standing wave formed in a linear cavity . Dynamical gratings inscribed in the cavity by previously generated longitudinal modes are kept in the active medium for a relatively long time as compared with lifetime of a single mode and affects generation of the next spectral components. Due to broad sweeping range (more than 10 nm) and simplicity, self-sweeping fiber lasers are attractive sources for applications demanding tunable radiation. As a result, the self-sweeping laser is the simplest sort of tunable laser without any tuning elements and electrical drivers for frequency tuning. Currently the self-sweeping operation was observed in almost all common fibers: ytterbium [1-2], bismuth , erbium , holmium  and thulium . It should be noted that the self-sweeping operation has been demonstrated up to now only in quasi-three-level laser systems. In the present paper, we demonstrate for the first time self-sweeping operation in a four-level laser system based on neodymium.
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