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  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 . Recently, the alternative approach to single-frequency lasing based on utilization of the folded-cavity geometry was proposed . 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 .
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