Scaling Characteristics of Multijunction VCSELs

Increasing the number of active regions is an effective approach for scaling optical output power in Vertical-Cavity Surface-Emitting Lasers (VCSELs). LiDAR applications utilizing Time-of-Flight (ToF) mapping methods require power-efficient VCSELs with high throughput and fast rise times to achieve high spatial resolution and extended detectable ranges. This paper presents a comprehensive study on the performance characteristics of multijunction VCSELs. Single-, triple-, and five-junction 940nm VCSELs are designed and fabricated into both single aperture dies and densely packed arrays. Electro-optical characterization results demonstrate the scaling behavior of these devices, where the differential quantum efficiency increases from 65% for the single-junction VCSEL to 360% for the five junction VCSEL, corresponding to the slope efficiencies of 0.9W/A and 5.0W/A. On the other hand, a broadening in spectral response and far-field beam divergence angle is observed by increasing the number of active junctions, which is primarily caused by the stimulation of additional higher order transverse modes. This study aims to analyze the effects of increasing the number of active junctions on VCSEL performance and to address the associated challenges in the development of multijunction VCSELs.