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

Tobias Hansson,Stefan Wabnitz,Mario Ferraro,Alioune Niang,Alessandro Tonello,Mario Zitelli,Tigran Mansuryan,Fabio Mangini,Vincent Couderc,Rocco Crescenzi

doi:10.1364/oe.398531

Abstract

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 a new exact solution for the nonlinear evolution of first and second order moments of a laser beam of arbitrary transverse shape carried by a graded-index multimode fiber. We have experimentally directly visualized 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 photo-luminescence emission permits us to directly measure the self-imaging period and the beam dynamics. Spatial shift and splitting of the self-imaging process under the action of self-focusing are also revealed.

Highlights

Nonlinear multimode optical fibers (MMFs) are an emerging research field, as they permit new ways for the control of spatial, temporal and spectral properties of ultrashort pulses of light [1]
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
The longitudinal modulation imprinted on the CONCLUSION We studied the dynamics of beam-self imaging in nonlinear GRIN multimode optical fibers

Summary

INTRODUCTION

Nonlinear multimode optical fibers (MMFs) are an emerging research field, as they permit new ways for the control of spatial, temporal and spectral properties of ultrashort pulses of light [1]. Already back in 1992, Karlsson et al have theoretically studied, by means of the variational method, the dynamics of self-imaging in a GRIN MMF under the combined action of diffraction, nonlinearity, and parabolic index profile [23] They derived an approximate analytical solution for a multimode beam, which permit to evaluate the evolution of the beam width and its longitudinal phase delay. [9], and assuming a fixed input beam diameter of 40 μm (FWHM) so that x0 = y0 = 24.0 μm, one obtains that associated power-induced variation of the average width for the radial moment w occurs on a GW scale (and w shrinks down to zero for P0 = 2.5 GW) This means that the nonlinear reduction of the Hamiltonian is not the root cause of spatial beam cleaning. We have generalized the analytcal description to the case of a super-Gaussian initial beam profile: corresponding results are reported in the the Appendix

EXPERIMENTS

10 Normal dispersion

Findings

CONCLUSION