Fundamental mode emission in tapered quantum cascade lasers for high brightness

Currently, the highest power quantum cascade lasers (QCLs) profit from (i) the efficient active region design, minimizing the energy fraction transferred into heat, (ii) relatively high number (40) of cascades, increasing the mode confinement factor,, and (iii) narrow buried-heterostructure (BH) waveguides, which cool efficiently the active region due to the high lateral heat flow. This combination allows Watt-level continuous wave (cw) emission powers at room temperature. Moreover, narrow waveguides often support the fundamental TM00 lateral mode, resulting into a nearly Gaussian laser beam. Power scaling is possible in this configuration only by laser length, which is limited by the needs for robust construction and transparent anti-reflection coatings. Broad-area (BA) QCLs with a small number of cascades can also deliver a Watt-level cw powers at room temperature, profiting from the enhanced vertical heat flow. Power can also be scaled for these lasers via the laser width, and is nearly unlimited. The usual drawback of the BA lasers is poor beam quality. In this paper we report on the first demonstration of the room temperature cw operation of the BA QCL at the fundamental TM00 mode. The stable and reproducible transverse mode selection is achieved by the double-taper waveguide geometry. The laser emits at 4.6 mu m and has demonstrated room-temperature cw power of 100 mW.