  We study the dynamical behavior of medium-size multimode VCSELs with an
elliptical oxide aperture, selected for the best trade-off between high output
power and modulation speed for datacom applications, with a focus on their
relative intensity noise (RIN) performance. Our experimental results, collected
for various VCSELs, outline the presence of several peaks in the RIN spectra
within the bandwidth of the transmission system, which can limit the eye
opening under direct current modulation. Here, we present a rigorous model to
explain for the first time the origin of these peaks. In particular, the
frequencies of the spectral RIN peaks are analytically described as the result
of the non-trivial interaction among transverse modes by addressing the laser
dynamics and the related noise features through a time-domain mode expansion
approach, accounting for coherent effects in multimode competition, spatial
hole burning, and carrier diffusion. The laser modulation performance is
addressed through dynamical simulations with PAM2 and PAM4 modulations, which
clearly demonstrate the potential for high-bitrate optical interconnects.
Finally, we address the effect of the oxide aperture aspect ratio via
electromagnetic simulations, demonstrating how the ellipticity affects the
modal frequency detuning and the RIN, thus providing design guidelines for
VCSELs with low RIN performance and outlining a clear roadmap for a
substantially improved bandwidth-power trade-off in these devices.
