The effect of mode spacing on the speed of quantum-well microcavity lasers

Oxide-confined quantum-well microcavity vertical-cavity surface-emitting lasers (VCSELs) of three-diameters (aperture size dA∼2, 2.5, and 3.5 μm) have been fabricated that operate as nearly single-mode lasers at ultralow thresholds ITH=0.15, 0.16, and 0.20 mA. Relative spectral intensities are measured at a set higher bias current I=0.8 mA for the three VCSEL sizes to determine the dependence on mode spacing between the fundamental and second order modes, which at increasing diameter are Δλ∼2.2, 1.6, and 1.0 nm. By studying the side-mode suppression ratio and the optical microwave frequency response of the microcavity VCSELs throughout a spread-out group of modes, we are able to resolve the dependence of signal amplitude and time response on the difference in mode spacing, Δλ, higher speed response occurring at larger Δλ.