Numerical simulation of an adaptive beam-shaping technique using a phase grating overlapped via a spatial light modulator for precision square–flat-top beam

Abstract

Beam shape is a key parameter in process optimisation for all laser applications. Among these shapes, a square–flat-top beam is fundamental and is in high demand. To obtain a beam with an aimed structure, various beam-shaping methods have been developed. Among them, an adaptive beam-shaping technique that uses phase grating encoded on a spatial light modulator with spatial frequency filtering in the Fourier plane in a 4f system has been developed. In this paper, using precise and simple beam shaping to produce a square and flat-top beam, we examine in detail the phase grating structure via simulations. The directions of the grating vectors inside and outside of the aimed area, i.e. kg-inside and kg-outside, and the normal vectors of the square and flat-top area, i.e. k1 and k2, critically affect the separation of the extracted and residual components on the Fourier plane. To extract the high spatial frequency component for precise shaping, a non-parallel configuration of the grating vectors to the normal vectors of the square beam is found to be effective. This method ensures precision beam shaping as well as keeping of pulse width and wavefront over the shaped area.