Design and analysis of phase gratings for laser beams coherent combination

The aperture-coded or multilevel phase grating can obtain higher diffraction efficiency for laser beam shaping than general binary phase grating (BPG). In this paper, a multiple phase grating (MPG) for coherent addition of 5x5 laser beams is designed. The grating can be fabricated by the gray-tone photolithography technology, which employs the computer controlled spatial-light-modulator (SLM) as a mask. This fabrication method is simple, cheap and some aberrations in photolithography process can be reduced in one-step exposure. We have simulated the exposure dose of the gray-tone photolithography for fabricating the multilevel phase grating, and the effects of fabrication error were analysed. The simulation results showed that the coupling efficiency of MPG for laser beams was 78.74%.