Light intensity profiles of surface-emitting InGaAsP LED's: Impact on coupling to optical fibers

Light intensity profiles of 1.3-µm surface-emitting InGa AsP/InP LED's were measured using an infrared vidicon imaging technique. Typical light spot profiles were shown to have uniform intensity only over a small spot, with significant Gaussian tails around the periphery. Profiles were measured versus current. Narrowing of the light spot via current crowding was observed. A previous model for current crowding in stripe-geometry lasers was modified to calculate intensity profiles of circular-contact LED's. The model qualitatively predicts the observed changes in the profile shape with varying current and epitaxial-layer parameters. Measured efficiencies η of coupling LED power to optical fibers are usually lower than calculated for an ideal Lambertian source of uniform intensity. The observed profile is shown here to be responsible for the reduced η. For example, η of a uniform 25-µm spot is calculated to be ～30-percent higher than that of a spot with typical Gaussian spreading and a width of 25 µm at half-maximum intensity, both butt-coupled to a 50-µm core, graded-index fiber. Lensing schemes to improve η were designed assuming uniform light spots, A ray-tracing calculation shows here that spherical lenses improve η equally well in the presence of Gaussian tails around the light spot.