Multimode fiber laser dynamics

High-power sources of femtosecond pulses are interesting for material processing, remote sensing, spectroscopy, laser particle acceleration, and different high-field physics branches. For these aims, fiber systems have the advantages of large single pass gain, high mode quality, and low environmental sensitivity. The harvesting of femtosecond pulse energy can be provided by spatial scaling of the fiber mode thus reducing the peak intensity and, thereby, nonlinear effects. Such a method of mode size scaling faces the issue of mode structure and quality degradation. We demonstrated that a mode quality control could be provided by mechanisms of nonlinear mode self-cleaning in mode-locked solid-state oscillators and passive multimode fibers (MMF). The main idea here is not suppressing the higher-order modes but their synthesizing in a coherent and stable large-area mode. Such an approach can be named a spatial mode condensation in a MMF, and it is realized as a distributed Kerr-lens mode-locked fiber laser (see Figure below).

The mastering of these multidimensional nonlinear wave patterns has an interdisciplinary character and bridges different areas, ranging from nonlinear wave collapse and turbulence phenomena in plasmas, the formation of highly coherent structures, such as solitons in nonlinear optics (the so-called ”light bullets”), to liquid crystals and Bose-Einstein condensates (BEC). These soliton-like structures could provide unprecedented energy condensation, and lead to a breakthrough in photonic networks’ information capacity and multimode microresonators for optical comb generation. We demonstrated that multidimensional wave pattern stabilization can be substantially enhanced by introducing graded dissipation in an MMF. The Figure below demonstrates the mode-cleaning of a femtosecond pulse in a short MMF without (a) and with (b, c) graded dissipation. (a): In a nondissipative fiber, a multitude of spatial modes is excited, so that mode cleaning can only be provided by propagating high-power pulses in a sufficiently long MMF. (b): In an actively doped MMF one may reach mode cleaning in an extremely short fiber (z), without strong dependence on input pulse power and transverse beam size (r). (c): The control of a graded-index MMF characteristics results in a stable comb of multiple femtosecond pulses (t).

We analyzed the analogy between the stabilization of spatio-temporal patterns (solitons) in an MMF with a weakly-dissipative BEC confined by a cigar-shaped potential. We found that dissipative

factors, such as graded dissipation, saturable gain, and kinetic cooling (spectral dissipation), play a crucial role in stabilizing spatio-temporal solitons. In photonics, one can treat such a stabilization as a manifestation of mode-cleaning in an MMF, which is enhanced by dissipation, and can be realized on extremely short propagation distances.

The main practical significance of the obtained results lies in the possibility of energy/mass scaling of coherent multidimensional photon/matter wave structures, and the demonstration of stable spatiotemporal mode-locking in MMF lasers.

Selected publications:

V. L. Kalashnikov, S. Wabnitz, Distributed Kerr-lens mode locking based on spatiotemporal dissipative solitons in multimode fiber lasers, Phys. Rev. A, vol. 102, 023508 (DOI: 10.1103/PhysRevA.102.023508, also arXiv:2004.00990 [physics.optics])

V. L. Kalashnikov, S. Wabnitz, A “metaphorical” nonlinear multimode fiber laser approach to weakly-dissipative Bose-Einstein condensates, arXiv:2010.11165 [physics.optics]

F. Mangini, M. Ferraro, M. Zitelli, V. Kalashnikov, A. Niang, T. Mansuryan, F. Frezza, A. Tonello, V. Couderc, A.B. Aceves, S. Wabnitz, Giving light a new twist with standard optical fibres: rainbow spiral emission, arXiv:2010.00487 [physics.optics]

Related projects:

MASTEDIS – Mastering spatiotemporal dissipative solitons

STEMS – Spatiotemporal multimode complex optical systems