Influence of grating parameters on the performance of a high-power blue external-cavity semiconductor laser
The influence of grating parameters, including the diffraction efficiency and ruling density, on the performance of high-power blue external-cavity (EC) semiconductor lasers has been studied. We find that for the Littrow-type EC laser with a zeroth-order diffracted beam as a coupling output, the high zeroth-order diffraction efficiency benefits the high external light extraction efficiency, and the high first-order diffraction efficiency contributes to the low threshold current and the wide wavelength tuning bandwidth. By using a grating with a zeroth-order diffraction efficiency of 76% and a first-order diffraction efficiency of 11.5%, the external light extraction efficiency can reach up to 80% with an output power of more than 1200 mW and a tuning bandwidth of 3.6 nm. When a grating with a first-order diffraction efficiency of 58.5% and a zeroth-order diffraction efficiency of 28.6% is used, the threshold current is reduced by 46% at the central Fabry–Perot resonance wavelength. Moreover, a tuning bandwidth of 8.3 nm ranging from 442.2 nm to 450.5 nm with an output power of more than 500 mW is achieved. Besides, we also find that the high ruling density is helpful in obtaining narrow lasing linewidth, especially for the EC laser system injected with low current. By using a grating with a ruling density of 2400 grooves/mm, the spectral linewidth of the EC laser can be narrowed to be 0.02 nm with an operating current of 300 mA. As the injection current is increased, the lasing linewidth is little affected by the ruling density of the grating due to the broadened feedback spectrum by the flattening of gain spectrum. The results above shed light on the improvement of the performance of EC lasers by selecting appropriate parameters for the grating.