Undercut Ridge Structures: a Novel Approach to 1.3/1.55 Μm Vertical-Cavity Lasers Designed for Continuous-Wave Operation

Until now, long-wavelength (1.3/1.55 μm) vertical-cavity lasers with two dielectric mirrors have suffered from too low an injection efficiency (which is the result of lateral current injection) and a high thermal resistance (induced by the low thermal conductivity of the dielectric materials). A novel structure for such devices is proposed, which has improved both thermal and electrical characteristics: an undercut InGaAsP layer enables one to define a diaphragm which funnels current paths towards the centre of the cavity and confines the optical mode, thus increasing injection efficiency. Thermal behaviour is improved by realisation of a mesa-like top structure embedded in electrolytic gold. For these lasers, the temperature-dependent threshold condition is first determined theoretically, and some simple design rules are established using efficient simulation softwares. Such devices including two (SiO2/Si) dielectric mirrors were fabricated and operate at 1.3 μm in the continuous-wave regime up to -15°C with threshold currents of 8 mA for 6 μm wide diaphragms