On the possibility of observing bound soliton pairs in a "wave-breaking-free" mode-locked fiber laser

Abstract

To generate ultrashort pulses with higher energy in passively mode-locked fiber lasers, several groups have recently focused on the development of laser cavities operating in normal path-averaged dispersion, with a view to achieve wave-breaking-free operation. The mode-locking mechanism in fiber laser play a key role in providing scalability limitation: for instance, nonlinear polarization evolution can be overdriven. Furthermore, the differential gain of the doped fiber decreases along with the traveled distance. From all of these limitations, it follows that power scalability is always bounded for virtually any mode-locked laser cavity design. In general, the consequence is the emergence of multiple pulsing operation for high pumping powers. Interactions between these pulses could lead to the formation of stable bound pulses, although the pulses are highly chirped and energetic. The present study follows the experimental report of "parabolic bound pulses" in a high-power ytterbium-doped fiber laser operating in the normal dispersion regime. We explained through detailed numerical simulations how such bound pulses can be observed in a regime where wave-breaking-free operation was initially expected. The vectorial laser model takes explicitly into account gain saturation, finite amplification bandwidth, and nonlinear polarization evolution, in addition to self-phase modulation, group velocity dispersion and linear birefringence. We show that, for a given pumping power, the propagation regime can indeed appear like a "wave-breaking-free" regime, with only one minimum of the pulse duration per roundtrip. However, increasing the pumping power, pulse splitting finally occurs and leads to the stable binding of two highly-chirped, energetic pulses. The influence of gain filtering and total averaged cavity dispersion is analyzed in detail.