Manipulation of optical modes in quantum dot laser diodes by selective oxidation of high aluminum content AlGaAs layers

We describe the effects created by selective oxidation of high aluminium content AlGaAs layers at the facets of 5-stack quantum dot edge-emitting 50μm stripe lasers. The steam oxidation affects only the facet areas of the devices, where unpumped sections are created. These unpumped regions alone enable reduction of the width of the lasing near-field spatial profile of up to 65% and the reduction of threshold in long devices by up to 30%. These effects are attributed to saturable absorber-type behaviour, where the absorber saturates first at the location of highest optical intensity, so allowing lasing over a smaller spatial area. Secondly, a combination of self-heating at the facets and the saturable absorption generates novel saw-toothed wavelength-time profiles. A model for the behaviour behind all of these results is proposed and backed up with experimental data.