Optical field distribution in close-confined laser structures

An experimental and theoretical study has been made of the radiation patterns from GaAs lasers with a "close-confined" structure. The lasers were grown by liquid-phase epitaxy and consisted of a p+ - p-n structure with Al in the p+ region. Photometric measurements were made of the intensity distribution at the emitting facet (microscope observations) and the radiation pattern in a plane perpendicular to the junction. The electromagnetic field distribution in the vicinity of the p-n junction and the radiation pattern were calculated by solving the wave equation for a three-layer structure with complex dielectric constants in each layer. The theory predicts enhanced confinement of the radiation by the increased dielectric discontinuity due to the heterojunction, in agreement with the low lasing threshold and high efficiencies of the close-confined diodes. Another consequence of the theory is that as the thickness d of the p region of the cavity is increased, higher order modes can propagate with efficiences much greater than in ordinary lasers without the heterojunction. The third mode, corresponding to three intensity maxima along a line perpendicular to the junction, was observed for d = 5 \mu . The good agreement found between the observed and calculated radiation patterns indicates that confinement of the radiation by dielectric discontinuities is an important factor in explaining the low-threshold currents found in close-confined lasers.