High brightness from unstable resonator mid-IR semiconductor lasers

Abstract

We compare the performance of several optically pumped unstable resonator semiconductor lasers with cavity lengths of 4, 5, and 6 mm and operating in the mid-IR at 4.7 μm. The unstable resonator lasers (URLs) were fabricated by polishing a diverging cylindrical mirror on one of the facets. In general, the URL beam quality (BQ) was 1–2 times diffraction limited when operated at pump powers below 30 W (URL power ≤6 W) As the pump power is increased, the BQ is observed to degrade; at 60 W the URL’s were ∼3.5 times diffraction limited (URL power ≈11 W). The highest brightness URL, a 4-mm long device, was compared with an equivalent 4-mm long Fabry–Perot (FP) laser operated at different cavity widths. The performance of the broad area URL (500 μm width), as assessed by power-in-the-bucket measurements, was superior to both wide stripe (500 μm) and narrow pump stripe (100 μm) FP lasers.