Near-field measurement and four-wave mixing in single-polarization elliptical multimode VCSELs

We investigate the impact of transverse mode coupling caused by four-wave mixing (FWM) in elliptical multimode vertical cavity surface emitting lasers (VCSELs) with polarization control. Our study includes relative intensity noise (RIN) and near-field measurements, with a particular focus on the near-field pattern of sidebands of the lasing modes observed in the optical spectrum. Our model, which includes coherent mode coupling via FWM and spatial hole burning, shows a very good agreement between numerical and experimental results, providing a detailed explanation of the fine spectral features observed in the optical and RIN spectra. These results identify the primary causes of RIN degradation in multimode VCSELs and highlight the importance of considering FWM when approaching the design of VCSELs for high-speed and datacom applications.