Keywords: 
• 光学涡旋 /
• 拓扑荷数 /
• 分数阶涡旋光束

Abstract: The generation of fractional vortex beams and their propagation have been interesting research topics in recent years.In this paper we introduce a new type of fractional double-vortex beam,which is generated by the coaxial superposition of the vortex beams with two different fractional topological charges,and its total intensity distribution is of double-ring.We study the generation of this kind of beam theoretically and experimentally.It is shown that the rings of the fractional double-vortex beams carry different orbital angular momenta,from each other and propagate independently.The fractional double-vortex beams possess diverse manipulations as compared with the vortex beams with integer or single fractional charges.Therefore,the fractional double-vortex beam will be of great significance in optical rotation and manipulation of microscopic particles.