Ricerc@Sapienza

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.