Pierluigi Debernardi

We investigate the robustness of single-mode (SM) emission in high-power, large-area rectangular vertical-cavity surface-emitting lasers (VCSELs), emphasizing the impact of self-heating effects. Compared to circular geometries, large-area rectangular VCSELs provide improved heat dissipation thanks to their high geometrical aspect ratio, and higher SM output power by means of their patterned reflectivity obtainable by an array of grating reliefs. Self-heating alters the refractive index of the device. We demonstrate, experimentally and numerically, how the related thermal lensing affects the transverse modes. By misaligning the antinodes of the promoted lasing mode and the surface reliefs, self-heating degrades SM operation if not properly accounted for in the relief position design. Combining thermal and optical models, we propose numerically optimized grating relief geometries ensuring robust SM emission across varying operating temperatures.

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.

In this work we investigate the ultrabroadband dynamics of transverse coupled cavity VCSELs. This study is based on a multimode rate equation model, whose parameters are directly provided by a full-wave vectorial electromagnetic solver. This approach sets a step towards the comprehensive physics-based modeling of transverse-coupled cavity VCSELs, providing a relation between the features of the optical supermodes and the enhancements of the intensity modulation response. The approach emphasizes how the bandwidth enhancement, ascribable to a photon-photon resonance picture, can be triggered by forcing asymmetries in the bias and modulation contacting scheme of the device, and the importance of collecting the fields from each single cavity, providing an interpretation of recent experimental observations and paving the way towards more systematic design strategies.

In this work we investigate patterned large-active area AlGaAs vertical-cavity surface-emitting lasers (VCSELs) targeting high-power single-transverse-mode emission. As a first step, our in-house 3D electromagnetic vectorial mode solver VELMS is validated against preliminary experimental data. Then, VELMS is applied to implement single-mode operation on VCSELs featuring large-area rectangular oxide apertures. A comparative appraisal of different mode selection strategies based on patterning the outcoupling facet of the device is presented, providing a number of alternatives for a broad audience of technologists.

This work reports a multiscale physics-based approach aimed at investigating the benefits of introducing a single tunnel junction (TJ) within conventional AlGaAs Vertical-Cavity Surface-Emitting Lasers (VCSELs). Our comprehensive VCSEL solver VENUS is augmented with a non-equilibrium Green's function (NEGF) approach to extract the band-to-band tunneling rate across the TJ. To showcase the NEGF-VENUS features, we apply it to the commercial pin oxide confined AlGaAs VCSEL previously investigated by VENUS, by inserting a TJ with minimal variations to the optical resonator. Besides finding the optimal position of TJ and oxide aperture, we can also compare the different hole injection schemes in the active region. Our results show the potential of doubling the maximum output power with the same threshold current, with perspectives of further enhancement by stacking more tunnel junctions.

The ultrabroadband operation of transverse-coupled-cavity vertical-cavity-surface-emitting lasers is investigated in this work. Relying on a closed-form expression of the intensity modulation response, our model allows for an extended and fast parametric campaign of the design inputs, as well as an easy interpretation of the bandwidth enhancement.

We analyze the dynamical behavior of multimode VCSELs with elliptical oxide aperture. An advanced electromagnetic solver is employed to determine the electric field modal thresholds and emission frequencies, while mode competition is simulated with a time-domain mode expansion approach that accounts for coherent effects and carrier-photon spatial coupling. Preliminary performance analysis with NRZ PRBS shows an increase in the modulation bandwidth with respect to the single mode case, allowing for bitrates up to 70 Gbit/s.