We demonstrate m GaAs/Al Ga As quantum cascade lasers (QCLs) operating up to 320 K. Metal-organic vapor phase epitaxy has been used throughout for the growth of the devices. Detailed comparison has been carried out for the QCLs with various waveguides and grown on (100) GaAs substrates with different miscut angles towards (111)A. Introduction of InGaP cladding layers into the optical waveguide significantly improves the QCL performance due to a better optical confinement and lower waveguide losses compared with the GaAs-based waveguide. A 20-m-wide 4-mm-long device with high reflectivity coating on the laser back facet demonstrates room-temperature pulsed output power of 220 mW and a lowered threshold current density of 6.5 kA/cm . Index Terms-Midinfrared quantum cascade lasers (QCLs), optical waveguide.

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We report measurements which give direct insight into the origins of the transparency current for λ ~5 µm In0.6Ga0.4As/In0.42Al0.58As quantum cascade lasers in the temperature range of 80-280 K. The transparency current values have been found from broadband transmission measurements through the laser waveguides under sub-threshold operating conditions. Two active region designs were compared. The active region of the first laser is based on double-LO-phonon relaxation approach, while the second device has only one lower level, without specially designed resonant LO-phonon assisted depopulation. It is shown that transparency current contributes more than 70% to the magnitude of threshold current at high temperatures for both designs.

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