25-W Monolithic Spectrally Stabilized 975-nm Minibars for Dense Spectral Beam Combining

Low-fill-factor laser bars composed of five narrow-stripe broad-area lasers (30 μm × 6000 μm) with monolithic spectral stabilization are presented. Each laser on the bar emits at a unique wavelength from 970 to 980 nm with a channel spacing of 2.5 nm. Narrow spectral line width c1 nm per emitter is obtained by implementing 40th Bragg order distributed feedback surface gratings with varied grating periods. The design and fabrication of these laser bars is reviewed, with optimal grating etch depth determined using simulations that combine 2D-simulation of the reflectivity and optical losses of a 40th-order surface grating with coupled mode theory. The selected etch depth delivers a grating coupling coefficient of κL ~ 0.2, which is shown to be sufficient to suppress lasing in Fabry-Perot modes for narrow line operation across all wavelength channels. However, additional optical scattering losses of the gratings are found to limit output power and efficiency. The final realized minibars deliver 25-W continuous wave output power with a conversion efficiency >40% and operate with a beam parameter product of ~1.6 mm mrad per single emitter, twofold improved in comparison with typical 90-μm wide broad area laser. Such laser minibars are an attractive source for brilliant dense spectral beam combining.