Characterizing and manipulation of the modes of a blue laser diode with a double-grating external cavity

Abstract

The lateral modes in linewidth-compressed blue laser diodes are key factors affecting the laser beam quality and are essential to high-efficiency dense spectral beam combing. In this Letter, we use a self-designed high-resolution mode measurement system to study the modes of a blue laser diode coupled to a Littrow-configuration double-grating external cavity which compresses the linewidth to about 30 pm. We found that the linewidth compression leads to only one or two longitudinal mode groups, while the uncompressed spectrum contains more than ten of them. Furthermore, the excited lateral modes depend on both the azimuthal angle φ and the polar angle $\theta$ of the end grating. The order of the prominent lateral modes tends to decrease when the peak wavelength is tuned to the ends of the achievable wavelength range, i.e., from 439.4 nm to 448 nm, by varying $\phi$. On the other hand, increasing $\theta$ increases the order of the dominating lateral modes, from the 1st order to the 7th order for a working current of 400 mA, and introduces asymmetry in the intensity distribution. These results provide a simple way of tuning the mode structure of a linewidth-compressed diode laser and are useful for various high-precision applications.