High-power vortex beam generation enabled by a phased beam array fed at the nonfocal-plane.

High-power vortex beams have extensive applications in optical communication, nonlinear frequency conversion, and laser processing. To overcome a single beam's power limitation, generating vortex beams, based on a phased beam array, is an intuitive idea that requires locking each beamlet's phase to a specific different value. Conventionally, the intensity profiles of the focal plane (far field) are used for extracting the cost functions in active phase control systems. However, as for generating vortex beams, the cost function extraction method at the focal plane suffers because the same intensity profile of the beam array could correspond to different phase distributions in near field. Thus, the accurate phase control signals are difficult to obtain. In this paper, a new concept of extracting cost functions at the non-focal-plane is firstly presented and analyzed in detail by numerical simulation. This cost function extraction method is an efficient way of generating vortex beams with different topological charges, including second-order Bessel-Gaussian beams. The new concept could provide a valuable reference and contribute to the practical implementation of generating vortex beams by coherent beam combining technology.