The mechanism and intrinsic conditions of high-energy wave-breaking-free pulse generation in fiber lasers mode-locked by a nonlinear polarization rotation technique are investigated numerically and experimentally. Both numerical and experimental results show that the pulses along the two orthogonal polarization axes of the fiber have a large difference in pulse energy. The numerical simulations show that the ratio of the energy of two components is limited and ranges from about 8 to about 65. The slope of the instantaneous frequency at the central position of the pulse decreases rapidly with the increase of the pulse duration and energy, whereas the slope at the pulse edge changes slightly. The accumulation of instantaneous frequency throughout the pulse width approaches a constant in a higher pulse energy regime. Understanding the mechanism and intrinsic conditions of the wave-breaking-free pulse generation could be useful in generating high-energy pulses delivered from fiber lasers.
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