Femtosecond nonlinear losses in multimode optical fibers

TL;DR

This paper investigates femtosecond nonlinear losses in multimode optical fibers, revealing how sub-threshold intensities cause significant nonlinear absorption influenced by spatial self-imaging, with an analytical model matching experimental results.

Contribution

It provides the first detailed experimental analysis of sub-threshold nonlinear absorption in multimode fibers and introduces an effective N-photon absorption model for this regime.

Findings

Nonlinear absorption significantly affects beam dynamics below breakdown threshold.

Spatial self-imaging enhances nonlinear optical losses in graded-index fibers.

The N-photon absorption model accurately describes experimental data.

Abstract

Research on multimode optical fibers is arousing a growing interest, for their capability to transport high-power laser beams, coupled with novel nonlinear optics-based applications. However, when beam intensities exceed a certain critical value, optical fiber breakdown associated with irreversible modifications of their refractive index occurs, triggered by multiphoton absorption. These processes have been largely exploited for fiber material microstructuration. Here we show that, for intensities slightly below the breakdown threshold, nonlinear absorption strongly affects the dynamics of a propagating beam as well. We experimentally analyze this sub-threshold regime, and highlight the key role played by spatial self-imaging in graded-index fibers for enhancing nonlinear optical losses. We characterize the nonlinear power transmission properties of multimode fibers for femtosecond pulses propagating in the near-infrared spectral range. We show that an effective N-photon absorption analytical model is able to describe well the experimental data.