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

Summary form only given. 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]. In this paper, we demonstrate room -temperature lasing of the quantum -cascade lasers with U-shaped cavity (UQCLs) and present the approach for realization of the hop -free frequency -swept light source in the mid infrared range. The experimental lasers were fabricated with heterostructures described elsewhere [4] designed for lasing near 8 i.tm. The U-shaped cavities were fabricated with rounding radii of either 75 or 100 i.tm and the linear part of -1 to -3 mm. This corresponded to the full length of cavity of -2 mm to 6 mm. The UQCLs demonstrated lasing at room temperature with threshold current density ranging from 12 to 25 kA/cm 2 in pulsed mode (70 ns pulse width and 48 kHz repetition rate). The maximum output peak power in single -frequency regime was 6 mW. The sidemode suppression ratio (SMSR) was well above 20 dB (see fig.1 a). In order to study the lasing spectra in more detail, we performed spectrally -resolved optical pulse waveform measurements with the wavelength step of 1.6 A. The typical resulting time evolution of the UQCL lasing spectrum is shown in fig.lb. The heat -map in the fi gure clearly demonstrates linear chirp of 0.82 A/ns. The chirp value was increasing with the pumping amplitude and was attributed to the UQCL self -heating during the pulse. The highest chirp values demonstrated in our experiments were well exceeding 1 A/ns for all samples that opens up new avenues for development of the hop-free frequency-swept light sources in the mid -infrared range