GaN-Based Lasers on SiC: Influence of Mirror Reflectivity on L-I Characteristics

Threshold current density and differential slope efficiency as function of end-mirror reflectance have been measured to estimate the internal losses of GaN-based laser diodes. An Al-coated fiber tip is used as an external micro-mirror to vary the reflectance of the end facets allowing for a continuous adjustment of mirror losses of a particular laser. The effective reflectance of the external resonator is modeled as function of facet reflectivities, emission wavelength, micro-mirror distance and laser mode shape. In contrast to other methods, this eliminates all ambiguities usually arising from the comparison of devices of varying length or mirror coatings. In GaN-lasers with high threshold current transparency loss for the gain medium is not negligible and, therefore, ith dependence on mirror reflectivity alone is not sufficient to determine internal losses. Our measurements of one-facet slope efficiency yields internal losses 〈αint〉 between 27 cm and 46 cm whereas for derivation from current threshold a combination of transparency and internal losses in the range of 58 cm to 67 cm has been obtained.