Spectral beam combining of quantum cascade lasers in an external cavity

In this contribution, we demonstrate that spectral beam combining in an external cavity (EC), a technique which has been applied previously to shorter wavelength diode laser bars [1], is also applicable to mid-infrared QC lasers. Within this concept, the output of multiple emitters from a 4.6 ?m emitting QC laser chip is combined in a single, collinear beam. The average power of an EC-QC laser module realized that way surpasses the output of a corresponding single emitter by more than a factor of 4. Furthermore, the EC-concept allows a certain degree of wavelength tuning during operation. The EC, consisting of a collimating lens, a grating and a partially reflecting outcoupling mirror, forces each laser to emit at a unique wavelength defined by its offset relative to the main optical axis. The EC approach further ensures the collinear directional and spatial overlap of the individual QC laser output beams forming a single combined output beam.