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. In these experiments, the laser cavity comprises a significant length of normally dispersive fiber, either passive or with high gain. Theoretically, optical wave breaking is suppressed if a pulse develops a monotonic frequency chirp as it propagates. Such a pulse propagates self-similarly and accumulates a linear chirp. It is thus a way to overcome the known limitations of the pulse energy in mode-locked lasers that operate in "soliton" and "dispersion-managed (DM) soliton" regimes. However, pulse shaping in a laser cavity is much more complex than in a transmission fiber link, and detailed numerical simulations are required to distinguish among various possible regimes of operation. It is well known that either gain bandwidth limitation or saturation are necessary conditions for stable pulsed solutions. On the other hand, the limited bandwidth of the gain medium provides a practical limit to the scalability of single pulse energy. The mode-locking mechanism can also 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 [1]. We here explain through detailed numerical simulations how such bound pulses can be observed in a regime where wave-breaking-free operation was initially expected. Our 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 [2]. 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.

© 2007 IEEE