Assessing noise performance in multimode VCSELs with elliptical oxide apertures through spatially resolved near-field measurements

We experimentally and theoretically characterize multimode elliptical oxide aperture VCSELs. Optical spectra display sideband peaks and Relative Intensity Noise spectra reveal spurious peaks that limit the laser performance for high-speed optical communications. For the first time, we analytically explain these characteristics as nontrivial four-wave-mixing processes, through a model based on mode expansion of the electric field, accounting for coherent mode coupling. We validate these findings with spatially resolved near-field measurements performed with a lensed fiber and a high-resolution optical spectrum analyzer. We extract the spatial profile of sidebands observed in the optical spectrum and we demonstrate that they originate from four-wave mixing between optical modes.