Angled-grating distributed-feedback-laser output is nearly diffraction-limited
High-power continuous-wave semiconductor lasers traditionally have not produced diffraction-limited beams.
High-power continuous-wave semiconductor lasers traditionally have not produced diffraction-limited beams. One way to approach single-mode operation involves an angled-grating distributed-feedback (α-DFB) laser, in which a diffraction grating is etched into the laser cavity at an angle to the facets. Along this line, researchers at the Naval Research Laboratory (Washington, DC) have produced nearly diffraction-limited output from a α-DFB laser emitting near 3.4 µm. For pulsed optical pumping of a 50-µm-wide stripe at 78 K, the far-field beam divergence angle was only 1.4°. The slope efficiency was 64% of that for a conventional Fabry-Perot laser on the same bar. Beam quality was also substantially better out to stripe widths of at least 800 µm. In contrast to shorter-wavelength a-DFB lasers, however, the spectral linewidth decreased by only a factor of two under the same pumping conditions. The researchers believe this is related to the presence of much-higher background internal losses relative to the diffraction losses at longer wavelengths. Contact Robert Bartolo at email@example.com.