Dual gratings based on surface plasmons for optical beam shaping

To shape the beam with two-lobe far-field property from a dual side Bragg reflection waveguide semiconductor laser, a dual grating structure based on surface plasmons was prepared on the optical outlet facet of a Bragg reflection waveguide. It could combine the two lobes into a single lobe and to increase the optical intensity and quality of the beam. An Au-SiO2 grating was used to couple photons into surface plasmons and to combine the two lobe beams into a single beam. The surface plasmons also were taken to increase the extraordinary optical transmission. On the other hand, the Au-Si3N4 grating was used to help the outlet surface plasmons couple back to photons, meanwhile collimating the outlet beam to increase the far-field property. Numerical simulation results indicate when the parameters for Au-SiO2 show a depth of 50 nm, a filling factor of 0.5 and a duration of 350 nm, and those for Au-Si3N4 show 70, 0.5 and 660 nm, respectively, the outlet far-field beam will has a 6.1° divergence, which means the divergence angle shrinks by 3.6 times as that without the dual grating structure. The far-field optical transmission power reaches 62% of the model source, that is 1.59 times of the power of a single lobe far-field to the structure without the dual grating. Moreover, the cavity facet reflectivity has reduced to 12.4%, 0.53 times as the structure without the dual grating. It concludes that the dual grating structure has optimized the far field properties of dual side Bragg reflection waveguide semiconductor lasers.