Power scaling of quantum cascade lasers via multiemitter beam combining

Different approaches to power scaling of 4.5- to 5-µm emitting quantum cascade (QC) lasers by multiemitter beam combining are investigated. Spectral beam combining of linear arrays of QC lasers consisting of several individual emitters located side by side is demonstrated as a first variant, using an external cavity equipped with a diffraction grating and a partially transmitting output mirror providing wavelength-selective feedback to each emitter. In this way, spectral beam combining of up to eight individual QC lasers is achieved with an optical coupling efficiency of 60% for an array of six emitters. The resulting beam quality (M2 < 2 for both fast and slow axes) is close to that observed for single emitters. As a second approach, a linear array of QC lasers is coupled to a custom-made array of silicon microlenses positioned in front of the output facets of the QC lasers. This technique produces a set of closely spaced parallel output beams, strongly overlapping in the far field, without introducing any coupling losses. The resulting beam divergence is given by the aperture size of the microlenses, which is limited by the center-to-center spacing of the QC lasers (500 µm in our case).