Tapered QC lasers reduce beam divergence

Jan. 1, 2008
Although mid- to far-infrared quantum-cascade (QC) lasers are ideal for gas sensing, free-space optical communications, and imaging, their high beam divergence (typically about 48º for ridge-waveguide devices) can be a limiting factor for resolution and efficiency.

Although mid- to far-infrared quantum-cascade (QC) lasers are ideal for gas sensing, free-space optical communications, and imaging, their high beam divergence (typically about 48º for ridge-waveguide devices) can be a limiting factor for resolution and efficiency. To significantly reduce this divergence, researchers at the Universität Würzburg (Würzburg, Germany) created lasers with an index-guided tapered gain section using a unique processing method.

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In the growth process, 35 active periods on an indium phosphide (InP) substrate are embedded in tapered structures in an indium gallium arsenide waveguide core facing the substrate and adjacent to a 2-µm-thick InP top cladding. This approach differs not in the QC growth stage, but in the processing of the grown structure: instead of just evaporating a tapered contact on the grown structure (as in the typical gain-guided approach for semiconductor diode lasers), the researchers completely etch through the grown layers leaving a tapered-shaped device. Because they etch through the whole active region and achieve a semiconductor/air interface, current spreading does not take place and divergence is minimized. The 200 mW tapered QC lasers have a horizontal beam divergence of only 6.6º in the single-lobe far field at a wavelength of approximately 8.9 µm. Contact Julia Semmel at [email protected].

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