Modal and modulation characteristics of multimode VCSELs under external feedback for very short reach communications

We report the characteristics of VCSELs while experiencing optical feedback i.e. due to an unisolated coupling to multimode fibers. The experiments and modeling with controlled feedback resulted in a span of phenomena - starting from modified CW lasing, distorted modal distribution as well as flattened small signal modulation response. The connection between the results and the internal laser parameters, especially the nonlinearity, are examined.