Limitations to beam quality of mid-infrared angled-grating distributed-feedback lasers

The far-field characteristics of mid-infrared angled-grating distributed-feedback (α-DFB) lasers with W active regions are studied as a function of etch depth, stripe width, and optical pumping intensity. Whereas near-diffraction-limited output is obtained for 50 μm stripes at ten times threshold, the beam quality degrades rapidly when either the stripe width or the pump intensity is increased. A key finding is that most of the degradation may be attributed to the onset of Fabry-Perot-like lasing modes that propagate along the direct path normal to the facets. We further show that these parasitic modes may be effectively eliminated by using ion-bombardment to create angled virtual mesas surrounded by loss regions. The bombarded structures show substantial improvement of the beam quality for wide pump stripes and high pump intensities, although in this first demonstration the efficiency decrease was greater than expected