Quantum Cascade Lasers (QCLs), operating in continuous wave (cw) at room temperature (rt) in 3-3.5μm spectral range, which overlaps the spectral fingerprint region of many hydrocarbons, is essential in spectroscopic trace gas detection, environment monitoring, and pollution control. A 3μm QCL, operating in cw at rt is demonstrated. This initial result makes it possible, for the most popular material system (AlInAs/GaInAs on InP) used in QCLs in mid-infrared and long-infrared, to cover the entire spectral range of mid-infrared atmospheric window (3-5μm). In 0.79 Ga 0.21 As/In 0.11 Al 0.89 As strain balanced superlattice, which has a large conduction band offset, was grown. The strain was balanced with composite barriers (In0.11Al0.89As /In0.4Al0.6As) in the injector region, to eliminate the need of extremely high compressively strained GaInAs, whose pseudomorphic growth is very difficult.