Air Force Research Laboratory Kirtland AFB United States
Broad area quantum cascade lasers may preferentially operate in only a few high order modes rather than in a strongly multimode regime. We demonstrate that in such cases the high order mode can be converted into a single-lobed near diffraction-limited beam using an appropriate optic placed in the near field. Alternatively, high order modes can be strongly attenuated via slight sidewall etching. The result is to encourage fundamental mode lasing also leading to a very good far field beam. A third manner to influence the transverse modes is with feedback from an external mirror. The latter case can be used to create a plethora of modes albeit ones substantially less than the diffraction limit. The investigations demonstrate that broad area quantum cascade lasers BA-QCLs have the potential to simultaneously produce high power simultaneously with high brightness.