Transverse modes under external feedback and fiber coupling efficiencies of VCSEL's

We report a detailed study on the transverse mode behavior of vertical-cavity surface-emitting lasers (VCSEL's) under strong external feedback. Backreflections from a glass facet result, periodically depending on the feedback phase, in variations of the transverse mode pattern of strongly index guided multitransverse-mode-emitting lasers. As a result, butt-coupling efficiencies of these lasers strongly depend on laser-fiber distance. For typical active VCSEL diameters around 15-μm fiber-coupled powers into standard 50 μm-core diameter graded-index (GI) silica fibers vary by nearly 10 dB. Even for high-numerical aperture 100-μm-core diameter fibers, power variations of up to 2 dB are observed.