Distributed feedback（DFB） quantum cascade lasers（QCLs） in continuous-wave（CW） mode emitting atλ≈7.6μm are presented.Holographic lithography was used to fabricate the first-order distributed feedback grating. For a high-reflectivity-coated QCL with 14.5-μm-wide and 3-mm-long cavity,CW output powers of 300 mW at 85 K and still 10 mW at 270 K are obtained.Single-mode emission with a side-mode suppression ratio（SMSR） of about 30 dB and a wide tuning range of ~300 nm in the temperature range from 85 to 280 K is observed.
Surface-emitting distributed feedback quantum-cascade lasers operating at λ≈7.8 μm are demonstrated. The metal-covered second-order grating is shallow-etched into the surface of a thin InGaAs contact and cladding layer. This forms a hybrid waveguide and used to achieve relatively low waveguide losses and high coupling strengths. The devices exhibit stable single-mode operation from 90 to 130 K with a side mode suppression ratio above 20 dB. A slope efficiency of 194 mW/A is obtained at 90 K, which is twice higher than that of the Fabry-Perot counterpart.
A 7.8-μm surface emitting second-order distributed feedback quantum cascade laser （DFB QCL） structure with metallized surface grating is studied. The modal property of this structure is described by utilizing coupled-mode theory where the coupling coefficients are derived from exact Floquet-Bloch solutions of infinite periodic structure. Based on this theory, the influence of waveguide structure and grating topography as well as device length on the laser performance is numerically investigated. The optimized surface emitting second-order DFB QCL structure design exhibits a high surface outcoupling efficiency of 22% and a low threshold gain of 10 cm-1. Using a π phase-shift in the centre of the grating, a high-quality single-lobe far-field radiation pattern is obtained.
An analysis of a surface emitting distributed-feedback quantum cascade laser（DFB QCL） based on a surface-plasmon waveguide is presented.The second-order grating realized by the sole patterning of the top metal provides strong feedback.The analysis is based on a coupled-mode theory derived from exact Floquet-Bloch solutions of the infinite periodic structure.The surface outcoupling efficiency and threshold gain for the optimized device design are 43%and 12 cm-1,respectively,which represent great improvements on the conventional dielectric waveguide based DFB QCL with typical values of 17.5%and 20 cm-1.
The tapered quantum cascade lasers operating at about 9.0/zm are reported. In contrast to the common ridge waveguide laser, tapered devices give rather small horizontal beam divergence. Performances of devices with identical 11 μm ridge waveguide sections and different tapered gain sections are comparatively studied. The optimal taper angle of 3° leads to a relative high output power and a very small horizontal beam divergence of 7.1 °.
