High-speed low-current-density 850 nm VCSELs

The design of an oxide confined 850 nm VCSEL has been engineered for high speed operation at low current density. Strained InGaAs/AlGaAs QWs, with a careful choice of In and Al concentrations based on rigorous band structure and gain calculations, were used to increase differential gain and reduce threshold carrier density. Various measures, including multiple oxide layers and a binary compound in the lower distributed Bragg reflector, were implemented for reducing capacitance and thermal impedance. Modulation bandwidths > 20 GHz at 25 degrees C and > 15 GHz at 85 degrees C were obtained. At room temperature, the bandwidth was found to be limited primarily by the still relatively large oxide capacitance, while at 85 degrees C the bandwidth was also limited by the thermal saturation of the resonance frequency. Transmission up to 32 Gb/s (on-off keying) over multimode fiber was successfully demonstrated with the VCSEL biased at a current density of only 11 kA/cm(2). In addition, using a more spectrally efficient modulation format (16 QAM subcarrier multiplexing), transmission at 40 Gb/s over 200 m multimode fiber was demonstrated.