We show that Kerr beam self-cleaning results from parametric mode mixing instabilities, that generate a number of nonlinearly interacting modes with randomised phases -optical wave turbulence, followed by a direct and inverse cascade towards high mode numbers and condensation into the fundamental mode, respectively. This optical self-organization effect is analogue to wave condensation that is well-known in hydrodynamic 2D turbulence.

Multimode optical fibers (MMF) recently attracted a renewed attention, because of their potential for spatial division multiplexing, medical imaging and high-power fiber lasers, thanks to the discovery of new nonlinear optical effects, such as Kerr beam self-cleaning, spatiotemporal mode-locking, and geometric parametric instability, to name a few. The main feature of these effects is that many transverse modes are involved in nonlinear interactions. To advance our understanding, it is necessary to analyse the modal content of beams at the output of MMFs. In this work, based on a computer digital holography method using a phase-only spatial light modulator (SLM) as a correlation filter, we experimentally demonstrate a method of mode decomposition involving a large (≃80) number of fiber modes. To obtain this, we carried out a SLM calibration, and numerically investigated the most critical parameters which affect the fidelity of the decomposition, by comparing experimental and reconstructed beam patterns in both the linear (speckled structures) and in the nonlinear (self-cleaned beams) propagation regime.

The regime of highly chirped dissipative solitons is a powerful technique for generating high-energy femtosecond pulses. In this letter we demonstrate a successful scaling of the pulse energy in the all-normal-dispersion all-fibre configuration by increasing the cavity length and the mode-field diameter of the fibre simultaneously. Using a PM fibre of 10 mkm core diameter, it appears possible to lengthen the cavity up to 40 m, thus generating highly chirped pulses with energy above 50 nJ, whereas further growth is limited by the Raman effect. We have also found that the Raman threshold and compressed pulse duration strongly depend on the bandwidth of the intracavity spectral filter. The threshold length is increased by a reduction of the filter bandwidth at the expense of larger compressed pulse duration (250 fs instead of 150 fs). Further energy upscaling by means of core enlargement is shown to be possible, but for that one should waive the all-fiber design, or use custom-made fibre components.

We experimentally demonstrate the conservation of the average mode number in the process of Kerr beam self-cleaning in a graded-index multimode optical fiber, in analogy with wave condensation in hydrodynamic 2D turbulence.

We experimentally and theoretically investigate complex temporal pulse reshaping that accompanies Kerr beam self-cleaning in multimode optical fibers. We also study the output beam shape dependence on initial conditions.

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Техника генерации сильночирпованных диссипативных солитонов открывает большие возможности для получения мощных лазерных импульсов с короткой длительностью. В данной работе демонстрируется возможность усиления таких импульсов в тейперном волокне с диаметром моды на выходном конце 100 мкм. Сильночирпованные импульсы длительность 10 пс с центральной длиной волны 1040 нм и шириной спектра 13 нм были усилены в полностью волоконной схеме до пиковой мощности 69 кВт и сжаты дифракционными решетками до ≈800 фс. Ключевые слова: синхронизация мод, диссипативный солитон, фемтосекундный лазер, тейпер, волокно, иттербиевый усилитель.