Abstract

A high-purity and super-stable vortex beam generator with a large mode field diameter based on chiral fiber gratings is proposed and fabricated. The double-clad fiber (DCF) is twisted with arc-discharge heating, and its core mode is completely converted to the inner cladding OAM_1,1 mode which only contains a single orbital angular momentum (OAM). To optimize the design of DCF, the equivalent mode field diameter (EMD) and optimal coupling length (OCL) of DCF with different refractive index profiles are solved by using the fiber mode analysis method. A homemade arc discharge heating fiber twisting device was used to fabricate twisted DCF. Experimental results demonstrate an EMD of 44 μm and a conversion efficiency of up to 99.5% for the generated OAM_1,1 vortex beam, which is independent of the ambient refractive index and input polarization state. The twisted DCF has the characteristics of high conversion efficiency, no adjustment, and compact structure, and can be used for STED microscopic imaging and optical manipulation in complex liquid environments.