PNNL

Abstract

We report divergence, astigmatism and M2 measurements of quantum cascade lasers (QCL) with an emission wavelength of 8.77?m. Emission profiles from the QCL facet showed divergence angles of 62° and 32° FWHM ± 2° for the fast and slow axes, respectively. The observation of far field structure superimposed on the fast axes profiles was attributed to the position of the QCL die with respect to the edge of the laser submount, emphasizing the need for careful placement. Two diffraction-limited Germanium aspheric microlenses were designed and fabricated to efficiently collect, collimate, and focus QCL emission. A confocal system comprised of these lenses was used to measure the beam propagation figure of merit (M2) yielding 1.8 and 1.2 for the fast and slow axes, respectively. Astigmatism at the exit facet was calculated to be about 4 ?m, or less than half a wave. To the best of our knowledge, this is the first experimental measurement of astigmatism and M2 reported for mid-IR QCLs.