Cascade pumping schemes that utilize single-QW gain stages enhanced both the power conversion efficiency and the output power level of GaSb-based diode lasers that emit near and above 3 μm at room temperature. The cascade lasers discussed in this work had densely stacked type-I QWs gain stages characterized by high differential gain. The 3 μm emitting devices demonstrated CW threshold current densities near 100 A/cm2, a twofold improvement over the previous world record, that resulted in peak power conversion efficiencies increasing to 16% at 17°C. Comparable narrow ridge two-stage devices generated more than 100 mW of CW power with ~10% power conversion efficiencies. Three-stage multimode cascade lasers emitted 960 mW of CW output power near 3 μm and 120 mW CW near 3.3 μm.
Cascade GaSb-based type-I quantum well diode lasers were designed and fabricated. Cascade pumping was achieved utilizing efficient interband tunneling through "leaky" window in band alignment at GaSb/InAs heterointerface. The 100% efficient carrier recycling between stages was confirmed by twofold increase lightcurrent characteristics slope of two-stage 2.4 – 3.3 μm cascade lasers as compared to reference single-stage devices. Moderate internal optical loss increase was observed in cascade lasers with interband injector located near the optical mode peak. Cascade pumping scheme increased the continuous wave output power of room temperature operated 2.4 - 3 μm semiconductor lasers and led to improved power conversion efficiency.
Recent progress and state of GaSb based type-I lasers emitting in spectral range from 2 to 3.5 μm is reviewed. For lasers
emitting near 2 μm an optimization of waveguide core width and asymmetry allowed reduction of far field divergence
angle down to 40-50 degrees which is important for improving coupling efficiency to optical fiber. As emission
wavelength increases laser characteristics degrade due to insufficient hole confinement, increased Auger recombination
and deteriorated transport through the waveguide layer. While Auger recombination is thought to be an ultimate limiting
factor to the performance of these narrow bandgap interband lasers we demonstrate that continuous improvements in
laser characteristics are still possible by increasing hole confinement and optimizing transport properties of the
waveguide layer. We achieved 190, 170 and 50 mW of maximum CW power at 3.1, 3.2 and 3.32 μm wavelengths
respectively. These are the highest CW powers reported to date in this spectral range and constitute 2.5-fold
improvement compared to previously reported devices.