The mechanism of superluminal propagation in a long-cavity Brillouin fiber laser system was theoretically analyzed and experimentally evaluated. Considering the impact of the pump power spectral density and the modulation frequency, the formulation of the fractional advancement was deduced. The limitation factors for superluminal propagation, such as the cavity length, gain coefficient, and effective mode field area, were also studied in detail. Consequently, the simulations and experiments indicate that the multiple-longitudinal-mode operation is the main limitation to the distance of superluminal propagation based on Brillouin lasing oscillation. Besides, the fractional advancement of light pulses with different modulation frequencies or depths was thoroughly investigated to optimize advancement efficiency. This research could serve as the theoretical underpinning for long-distance superluminal propagation. (C) 2016 Optical Society of America
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