Butt-coupling efficiency of VCSELs into multimode fibers

We report a detailed study on butt coupling efficiencies of vertical-cavity surface-emitting lasers (VCSELs) to standard graded index multimode silica fibers. Coupling efficiency strongly depends on active laser diameter as well as index guiding and transverse mode spectrum of the laser. For typical active laser diameters of 16-20-/spl mu/m coupling efficiencies of about 90% are obtained with weakly index guided proton-implanted vertical-cavity lasers (VCLs) whereas stronger index guided oxidized lasers show considerably lower coupling efficiencies between 75% and 55%, depending on driving currents.