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Grating-coupled, surface-emitting (GCSE) quantum-cascade lasers (QCLs) are attractive sources for realizing watt-range continuous-wave (CW) output powers in the mid-infrared spectral region. Linear GCSE QCLs generally operate in the antisymmetric longitudinal mode [1] , which produces a double-lobed beam pattern. The antisymmetric mode can be suppressed by using a metal/semiconductor, 2 nd- order distributed-feedback (DFB) grating [2] , allowing symmetric longitudinal-mode operation and a single-lobe beam pattern. However, such designs require rather tight tolerances in grating duty cycle which cause fabrication challenges. Nevertheless, we have previously reported experimental results from GCSE QCLs using this approach – a single-lobe, diffraction-limited beam with a peak pulsed output power of ~ 0.4 W from wide ridges (~ 21 µm) [3] . Alternatively, GCSE devices designed to operate in an antisymmetric mode can exhibit a single-lobe beam by using a central π-phaseshift in the gratings [4] . Here we show that 4.6 µm-emitting GCSE QCLs with a central π-phaseshift can be designed to solely operate in an antisymmetric mode for high CW output power, single-lobed beam operation, while maintaining strong intermodal discrimination over a wide range in grating duty cycle.

Quantum cascade lasers (QCLs) employ intersubband transitions between conduction-band (CB) energy states in multi-quantum-well (MQW) structures, carrier tunneling between adjacent MQW stages, and coherent-light emission from multiple (30–50) stages. Unlike interband-transition semiconductor lasers, QCLs are not affected by Auger recombination in the mid-infrared wavelength range (λ= 3–20 μm); thus, allowing them to readily reach lasing at room temperature (RT). However, using multiple stages leads to high voltages (≥ 10 V) that significantly decrease the wallplug efficiency, η wp . Thus, although pulsed RT operation was obtained in 1996, it took until 2002 to achieve CW RT operation and until 2008 to obtain 1 W CW power.1

Phase-locked array of diode lasers is an attractive way to achieve multi-Watt power single-mode operation. Theoretical analysis of the optical modes of planarized phase-locked quantum cascade laser arrays (QCLA) emitting at 4.8 pm was performed in [1], [2]. These high-index-contract photonic crystal (HC-PC) structures allow global coupling between all array elements. Our numerical modelling predicts that even in the presence of strong thermal lensing under CW operation, 3-element QCLA can achieve more than 1W power.

High-CW-power (i.e., watt-range), mid-infrared (IR) (λ= 3–15 μm) quantum cascade lasers (QCLs) are needed for a wide range of applications, from remote sensing to infrared countermeasures. Many of these applications require single-spatial-mode operation with beam stability to multi-watt-range output powers. Scaling the CW single-mode output power requires optimization of the QCL active region, as well as device architectures allowing for scaling the lateral width. Single-element, edge-emitting QCLs operating in the 4.5–5.0 μm wavelength region generally requires a relatively narrow element width (∼ 5 microns) to maintain stable, single-spatial-mode CW operation up to the 1.5–2.0 watt-range output power levels. Larger width devices operate to high CW output powers (∼ 5W), but suffer from multi-mode operation as well as beam instabilities with drive current [1].

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) quantum-cascade lasers (QCLs) have been studied since 2000 [1] and generally operate in a two-lobe beam pattern [1,2]. Recently, single-lobe surface emission has been reported either from asymmetrically-coated devices [3] or from chirped-grating, ring-cavity devices [4], but at powers ≤ 0.1 W. Here we present the implementation of a novel concept for high-power, single-lobe surface emission, which employs metal-semiconductor, 2nd-order distributed feedback (DFB) gratings [5]. Initial devices demonstrate ∼ 0.4 W single-lobe, single-mode power from 4.75 µm-emitting GCSE QCLs.