Adaptive generation of a near-diffraction-limited square flattop beam with dual phase only liquid crystal spatial light modulators

We demonstrate the adaptive conversion of a quasi-Gaussian beam into a near-diffraction-limited square flattop beam in the near field based on the combination of dual phase only liquid crystal spatial light modulators (LC-SLMs) and a stochastic parallel gradient descent (SPGD) algorithm. The SPGD algorithm iteratively adjusts the phase distributions of the dual phase only LC-SLMs to reduce the variance between the target and the actual beam shape. One phase only LC-SLM adaptively redistributes the intensity and the other adaptively compensates the wavefront of the output beam. The shaping effect and influencing factors have been studied in detail. The theoretical and experimental results show that the technique is capable of adaptive creation, on demand, of a near-diffraction-limited flattop beam that closely matches the desired target parameters.