Design considerations in electrically-pumped, single-epitaxial VCSELs at 1.55 μm with Sb-based mirrors

The development of vertical-cavity surface-emitting lasers (VCSELs) at the telecommunications important wavelength of 1.55 μm has been hindered by the absence of a substrate that is suitable for both technologically-developed distributed Bragg reflectors (DBRs) and quantum well active regions. AlGaAsSb-based DBRs, however, which can be lattice-matched to InP and have a refractive index-contrast that is similar to AlGaAs-based DBRs at 1.3 μm-1.55 μm, offer the opportunity to combine the mature InP-based active region technology with high reflectivity mirrors in a single growth. We have recently reported, in fact, electrically-pumped, Sb-based vertical-cavity lasers operating at 1.55 μm and produced in a single epitaxial growth. These lasers, which employ two n-type, lattice-matched AlGaAsSb mirrors and an AlInGaAs-based active region, had room temperature threshold current densities of 1.4 kA/cm² and an external quantum efficiency of ~18%. The VCSELs, unfortunately, suffered from a very high operating voltage. We present design changes that have significantly reduced this voltage and discuss future improvements that could lead to further reductions