Ultralow Beam Divergence and Increased Lateral Brightness in Optically Pumped Midinfrared Laser (Postprint)

An optically pumped edge-emitting semiconductor laser emitting near 4.1 m was designed with weak transverse mode confinement resulting in an exceptionally large transverse optical mode size. Consequently, a laser device is reported that exhibits a fast-axis divergence angle of 4.2 full-width at half-maximum FWHM. More notably, a large reduction of the lateral axis divergence is also observed as a result of the increased lateral coherency. The reduced confinement factor and differential gain serves to suppress lateral filamentation. Despite the higher threshold and lower power, a lateral divergence angle of 3.2 FWHM and increased lateral brightness is achieved.