Technical Insight
Cascade VCSEL with 130% Slope Efficiency (Research Review)
The relatively low roundtrip gain in VCSELs can be increased by monolithic stacking of active regions, electrically coupled by integrated Esaki tunnel junctions for carrier recycling (see inset in Figure). Thus, the threshold current density can be scaled down while simultaneously obtaining high differential quantum efficiencyeven greater than unity. Lasers of this type are interesting solutions for optical links, low-noise devices and analog systems, to high-power applications and long-wavelength VCSELs. The top-surface emitting InGaAs/AlGaAs quantum-well VCSEL contains three active pn-junctions in series, coupled by two highly doped GaAs tunnel junctions. The top and bottom mirror reflectivity is around 97 % and 99.95 %, respectively. The figure shows the optical output characteristic of a 9 m active diameter device emitting at a wavelength centered on 980 nm. For the first time, a differential quantum efficiency above 100% is demonstrated for CW operation at room temperature. Optimizing the mirror doping levels and tunnel junctions will further improve the device performance. T. Knodl et al. University of Ulm, Germany Elec. Lett., 2001, 37, p. 31
