We present here a detailed study of the mode composition of an Er-doped ring self-sweeping fiber laser. Time-resolved heterodyne measurements prove that the radiation consists of slightly overlapping rectangular mode packets, each of which corresponds to generation of individual longitudinal modes. Optical frequency of each following packet sequentially increases by two free spectral ranges of the laser cavity. The presence of parametric processes during sequential growth of the neighboring mode packets is experimentally demonstrated for the first time in a self-sweeping laser. Instantaneous linewidth of the laser radiation does not exceed 4 kHz during every step of single longitudinal mode generation, which is also accompanied by a small change of the optical frequency (chirp) of similar to 40 kHz associated with an integral change of the laser cavity refractive index. The results are useful for calculations and modeling of laser systems based on dynamic gratings.
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