Efficiency-optimized monolithic frequency stabilization of high-power diode lasers

High-power GaAs-based diode lasers produce optical energy with extremely high efficiencies, but their spectrum is too broad for many applications (4-5 nm with 95% power content). Narrow spectra (<1 nm) can be achieved using monolithically integrated gratings and recent advances in semiconductor technology have largely eliminated the losses associated with this step. However, it remains challenging to develop designs that simultaneously achieve high power, high efficiencies and narrow spectra over a wide operation range. We review here the design choices necessary for optimized performance, using 975 nm broad-area lasers with uniform overgrown distributed feedback (DFB) gratings as a worked example, focusing on the role of the grating coupling strength. The steps necessary to minimize losses in the grating region are also reviewed. In addition, we show that special measures are needed to sustain narrow spectra in broad-area DFB lasers as they can support multiple longitudinal, lateral and vertical modes.