25 GHz Small-Signal Modulation Bandwidth with a 7-Element Electrically-Parallel Vertical-Cavity Surface-Emitting Laser Array

Abstract

Small arrays of VCSELs have emerging applications in free-space optical communications for a plethora of gadgets from smartphones to appliances to sensing systems within the realm of the Internet-of-Things. For data communication we require eye-safe line-of-sight beams or cones of light radiation that can reliably and inexpensively send data (as across optical fiber) from device-to-device or serve as an illumination source or both. Via our simplified VCSEL epitaxial designs [1] we investigate various configurations of top-emitting single 980 nm VCSELs and VCSEL arrays with varying VCSEL-to-VCSEL spacing, mesa shapes, and emitting diameters. We report record combinations of small-signal modulation bandwidth (f3dB) and optical output power (L) for an electrically-parallel compact VCSEL array with 7 VCSELs in a hexagonal configuration. The VCSELs in the array are identical with oxide aperture diameters estimated to be ϕ ∼3.5 μm ± 0.5 μm. We summarize key room temperature (RT) performance figures-of-merit in Fig. 1. We achieve an f3dB of 20 GHz at a bias of about I ∼35 mA with an operating L exceeding 25 mW. The f3dB is as high as 24.8 GHz with L ∼50 mW when I ∼65 mA. The maximum wall plug efficiency (WPE) is 0.32 at I ∼16 mA. We anticipate increased WPE and optical output power for a given I with heat sinking, advanced packaging, and substrate-emitting array designs. Previous best array results include an f3dB of 7.5 GHz at L = 120 mW for a 28-element 850 nm array [2] and an f3dB of 23 GHz at L = 25 mW for a single 980 nm VCSEL. In our presentation we include additional experimental results on our large ϕ single VCSELs and on our triple, septuple, and novemdecuple VCSEL arrays from RT to 85 °C.