Nonlinear beam self-imaging and self-focusing dynamics in a GRIN multimode optical fiber: theory and experiments

Beam self-imaging in nonlinear graded-index multimode optical fibers is of interest for many applications, such as implementing a fast saturable absorber mechanism in fiber lasers via multimode interference. We obtain an exact solution for the nonlinear evolution of first and second order moments of a laser beam carried by a graded-index multimode fiber, predicting that the spatial self-imaging period does not vary with power. Whereas the amplitude of the oscillation of the beam width is power-dependent. We have experimentally studied the longitudinal evolution of beam self-imaging by means of femtosecond laser pulse propagation in both the anomalous and the normal dispersion regime of a standard telecom graded-index multimode optical fiber. Light scattering out of the fiber core via visible fluorescence emission and harmonic wave generation permits us to directly confirm that the self-imaging period is invariant with power. Spatial shift and splitting of the self-imaging process under the action of self-focusing are also emphasized

High-power passively mode-locked dissipative soliton fiber laser featuring cladding-pumped non-CVD thulium-doped fiber

International audienceWe are reporting on the characterization of a thulium-doped fiber laser applying new powder technology in the mode-locking regime. A high average output power of 185 mW at a repetition rate of 9 MHz was achieved directly from the oscillator, which resulted in 21 nJ of pulse energy. The single-pulse operation regime was confirmed by careful numerical modeling of the laser cavity

Spatial beam reshaping and spectral broadening in quadratic crystals

Nonlinear optics in crystals with quadratic susceptibility has been largely explored along the last decades, with a particular emphasis on spatial solitons. When in the initial part of the propagation, the nonlinear length is much shorter than the diffraction length, rather than solitons, in these crystals it is possible to observe strong beam reshaping and spectral broadening. This mechanism of nonlinear beam evolution can be induced by combining high laser energies and large input diameters, so to reduce the contribution of diffraction in the initial steps of the propagation

Multimode optical fiber beam-by-beam cleanup

We introduce and experimentally demonstrate the concept of all-optical beam switching in graded-index multimode optical fibers. Nonlinear coupling between orthogonally polarized seed and signal beams permits to control the spatial beam quality at the fiber output. Remarkably, we show that even a weak few-mode control beam may substantially enhance the quality of an intense, highly multimode signal beam. We propose a simple geometrical representation of the beam switching operation, whose validity is quantitatively confirmed by the experimental mode decomposition of the output beam. All-optical switching of multimode beams may find important applications in high-power beam delivery and fiber lasers

Spatio-temporal reshaping in multimode fibers

Multimode fibers received recently a renewed interest because of the ability to control the multimode propagation and to select, at the output, a quasi-single mode supporting the main part of the energy. In this paper we present results on spatial Kerr-beam self-cleaning in multimode optical fiber. We show how a speckled beam obtained because of multimode propagation can be transformed into a quasi-single-mode emission under the effect of the peak power increas

Acheminement d'impulsions femtosecondes par fibre optique

National audienceCe travail concerne le déport à l'extrémité d'une fibre optique monomode standard de longueur métrique d'impulsions ultra-brèves de durée égale à la limite de Fourier. Dans le dispositif étudié, deux tronçons de fibre encadrent une ligne hybride à dispersion anormale à GRISMs. Cette architecture permet de compenser simultanément la dispersion et l'automodulation de phase de type Kerr qui accompagnent la propagation dans les fibres. En sortie de deuxième fibre émerge un train d'impulsions de durée 45 fs de puissance moyenne 30 mW à partir d'un simple oscillateur femtoseconde de durée initiale 150 fs @ 80 MHz, à 830 nm. L'apport de compensation de la dispersion jusqu'à l'ordre trois inclus est mis en évidence expérimentalement et par simulations numériques

Transport par fibre optique de longueur métrique d'impulsions lumineuses de durée inférieure à 20 fs et d'énergie supérieure à 1 nJ

National audienc

Pulse compression and fiber delivery of 45 fs Fourier transform limited pulses at 830 nm

International audienceA specific scheme is used for fiber delivery of ultrashort pulses using conventional elements. Starting from a standard femtosecond Ti:Al2O3 oscillator (150 fs @ 830 nm), perfectly compressed ultrashort pulses with a duration of 45 fs are produced at the output of a standard two meter long single-mode fiber. The setup allows compensating independently and simultaneously second and third orders of chromatic dispersion as well as management of self-phase modulation in the fiber. It includes an optimized dispersion compensation line made of the assembly of diffraction gratings and prisms. The unsurpassed performances of the device are experimentally and numerically highlighted. Fiber delivery of sub-30 fs multinanojoule pulses is discussed

Mesure d'impulsions femtosecondes ultra-courtes par interférométrie spectrale résolue temporellement : " Wideband-SPIRIT "

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