Beam shaping diffractive optical elements for high power laser applications

Diffractive Optical Elements (DOEs) are lightweight, thin optical components with many applications in laser beam-shaping. In this paper we consider the application of DOEs for coupling of high power Nd:YAG laser light to fibre For the laser system in question intra-cavity DOEs are considered for the generation of a super-Gaussian cavity mode, while an extra-cavity element is considered for shaping the beam to produce a profile suitable for fibre coupling. The arrangement to be considered in our application involves coupling a 100mJ, 20ns pulse laser beam of 5mm diameter into 3 fibres, each with a core diameter of 400µm, positioned in an equilateral triangle formation with a centre to centre spacing of 2mm. The threshold power density for the fibres is 4.5GW/cm². 512x512 pixel DOEs with 16 phase levels have been optimized using the iterative Fourier transform algorithm (IFTA). The optimized element produces spots with a radius of 14 diffraction orders. The modeled efficiency of the element is 91.4% with a peak power of 1.26GW/cm². Experimental measurements using a low power 633nm source equate to a peak power of 2.65GW/cm² for the high power laser, well within the damage threshold.