Abstract
We report divergence, astigmatism, and beam propagation factor (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) measurements of quantum cascade lasers (QCLs) with emission wavelengths of 8.77 mum. Emission profiles from the facet showed full-width at half-maximum divergence angles of 62deg and 32degplusmn2deg for the fast and slow axes, respectively. Diffraction-limited Ge 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 M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> yielding 1.8 and 1.2 for the fast and slow axes, respectively. Astigmatism at the exit facet was calculated to be about 3.4 mum, or less than half a wave. To the best of our knowledge, this is the first experimental measurement of astigmatism and M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> reported for midinfrared QCLs.
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