Mode-Locked Fibre Lasers with High-Energy Pulses

The recent explosive development of the physics and technology of fibre lasers as well as a vast expansion of their application areas greatly stimulate the quest for and the study of more advanced operational modes of such lasers, including generation of ultra-short and high-energy pulses. Since the invention of the laser researchers have continuously strived to generate shorter laser pulses. Simultaneously achieving a short duration and a high pulse energy is certainly more challenging than improving one of these parameters independently. However, it is this combination that becomes increasingly important in a wide range of scientific, technological, medical, and other applications. High energies and ultra-short pulse durations are both associated with high field intensity that often makes physical system non-linear. In fibre lasers, there are specific properties relevant to both shortening of the pulse duration and increasing their energy. The main obstacles on the road to shorter pulses are a relatively high dispersion and non-linearity of fibre resonators. In addition, the path to high-energy ultra-short pulses is, typically, further complicated by relatively low energy damage thresholds of standard fibre components, such as splitters, isolators, and so forth. While the effects of dispersion can be compensated by different rather advanced means, the nonlinear effects in fibres are much more difficult to manage. Thus, nonlinearity plays a critical role in the design of advanced fibre laser systems, but paradoxically, it is somewhat undesirable to many engineers because of its very limited controllability. Substantial efforts have been made to reduce the resonator nonlinearity, e.g. by using large-mode-area fibres, and this direction presents an important modern trend in laser technology. On the other hand, understanding and mastering nonlinear physical fibre systems offer the potential to enable a new generation of laser concepts. Therefore, it is of great importance to study physics and engineering design of laser systems based on nonlinear photonic technologies. In particular, new nonlinear approaches and solutions pave a way for development of advanced mode-locked fibre lasers with ultra-short high-energy pulses. Presently, passive mode locking is one of the key methods of ultra-short pulse (USP) generation. As recently as a few years ago, femtoand pico-second pulses extracted directly from the master oscillator operating in a passive mode-locking regime had relatively low energies, typically, not exceeding at few dozens of nJ and, in some special cases, hundreds