Near-field measurement of Four-Wave Mixing sidebands and noise performance degradation in elliptical multimode VCSELs for datacom

We study transverse mode coupling in 850 nm elliptical aperture multimode VCSELs. We perform near-field measurements of lasing mode sidebands observed in the optical spectrum and we are able to unequivocally identify Four-Wave Mixing (FWM) as the cause of coupling. Our dynamical model is able to reproduce the spectral features observed in the optical and Relative Intensity Noise (RIN) spectra. Our results allow to identify the main causes of noise degradation in multimode VCSELs and highlight the importance of a suitable model, including mode coupling, for VCSEL design for datacom.