Quantum-Cascade Lasers with U-Shaped Resonator: Single Frequency Generation at Room Temperature

Single-frequency generation of the quantum-cascade lasers (QCLs) is the key for many applications including gas analysis and free space communications in the mid-infrared range. At the moment, the main approach to single-frequency lasing of QCLs is based on distributed feedback with modulation of either real part of refractive index [1] or it's imaginary counterpart. The last is typically achieved with ‘plasmon’ coupling through the grating manufactured on the metal contact of the QCL ridge [2]. Recently, the alternative approach to single-frequency lasing based on utilization of the folded-cavity geometry was proposed [3]. This approach relies on monolithic coupling of Fabry-Perrot (FP) and whispering gallery mode (WGM) resonators in one laser cavity. The resulting lasing frequency corresponds both FP and WGM cavities simultaneously that ensures single-frequency lasing. However, with this approach single frequency lasing was observed only at cryogenic temperatures up to 240 K [3].