Thermal profiles of a single-element quantum cascade laser (QCL) and a five-element QCL array at different bias currents, under quasi-continuous-wave (QCW) conditions, are obtained using the charge-coupled device (CCD)-based thermoreflectance imaging technique. Peak temperature changes of 55 K and 105 K are measured on the single-element QCL (operating at 1.2 A) and the central element of the QCL array (operating at 4.2 A), respectively. The average facet temperature of the single QCL device shows a linear relationship with the dissipated power, indicating an effective thermal resistance of Rth = 3.0 ± 0.2 K/W (7%) for the device. The thermal transient behavior of the single QCL device, in response to a 35 μs-wide heating pulse, is also measured. From the transient curve, an effective thermal time constant of τth = 9.5 ± 0.4 μs (4%) is obtained. Experimental results are compared to the results obtained from heat-transfer models for both the single-element and array devices. Thermal profiles show a thermal lensing effect at the facet of the single-element QCL. In the array device, a more pronounced heating is observed at the center of the device while the temperature gradually decreases away from the central element.

Mid-infrared quantum cascade lasers (QCLs) are a growing industry and are being introduced into the marketplace primarily for low-output-power operation. High-power (> 1 W) continuous wave (CW) QCLs are expected to become commercially viable as they become more efficient and the necessary thermal dissipation requirements are achieved. However, there is a relative lack of knowledge regarding the degradation and failure mechanisms of QCLs under high power CW operation [1,2]. QCLs are expected to have different degradation and failure modes than diode lasers, because nonradiative recombination at the facets is not an issue. To push towards wider commercial adoption, lifetesting and failure analyses of high-power QCLs are performed. Previously reported QCL lifetests were carried out at relatively low output powers (~200 mW) and revealed activation energies as high as 1.2 eV, with the primary failure mechanism being reported to be oxidation of the front facet [3]. Here, we report on initial constant-power lifetest studies of QCLs emitting at λ ~ 5.0 μm and operating at 5 times the output power previously reported (i.e. at 1W CW). To mitigate the failure mechanism previously observed and improve device output, both facets have coatings: a high-reflectivity (HR) back-facet coating and a 14% low-reflectivity (LR) front-facet coating. The devices are mounted epi-side-down on copper with indium and tested under constant-power operation in a controlled environment.

Grating-coupled surface-emitting (GCSE) lasers generally operate with a double-lobed far-field beam pattern along the cavity-length direction, which is a result of lasing being favored in the antisymmetric grating mode. We experimentally demonstrate a GCSE quantum-cascade laser design allowing high-power, nearly single-lobed surface emission parallel to the longitudinal cavity. A 2nd-order Au-semiconductor distributed-feedback (DFB)/distributed-Bragg-reflector (DBR) grating is used for feedback and out-coupling. The DFB and DBR grating regions are 2.55 mm- and 1.28 mm-long, respectively, for a total grating length of 5.1 mm. The lasers are designed to operate in a symmetric (longitudinal) grating mode by causing resonant coupling of the guided optical mode to the antisymmetric surface-plasmon modes of the 2nd-order metal/semiconductor grating. Then, the antisymmetric modes are strongly absorbed by the metal in the grating, causing the symmetric mode to be favored to lase, which, in turn, produces a single-lobed beam over a range of grating duty-cycle values of 36%–41%. Simulations indicate that the symmetric mode is always favored to lase, independent of the random phase of reflections from the device's cleaved ends. Peak pulsed output powers of ∼0.4 W were measured with nearly single-lobe beam-pattern (in the longitudinal direction), single-spatial-mode operation near 4.75 μm wavelength. Far-field measurements confirm a diffraction-limited beam pattern, in agreement with simulations, for a source-to-detector separation of 2 m.