Radially polarized cylindrical vector beam generation in all-fibre narrow linewidth single-longitudinal-mode laser

A new method to generate the radially polarized cylindrical vector beams (CVBs) is proposed and experimentally demonstrated. The method is based on a narrow linewidth single-longitudinal-mode erbium-doped fiber ring laser incorporating an all-fiber fused mode selective coupler (MSC). The MSC, made of a single-mode fiber (SMF) and a home-made few-mode fiber (FMF), is inserted into the cavity of the laser to act as both a mode convertor and a mode splitter. It is able to directly convert the fundamental mode HE11 in the SMF to the radially polarized TM01 mode in the FMF and accordingly output the converted radially polarized CVB. The FMF used in the MSC is specially designed to break the near-degeneracy of the vector modes in the LP11 group, adding to the ease and effectiveness of mode conversion from HE11 mode to TM01 mode in the MSC. The measured linewidth of the laser is approximately 7.8 kHz with a 20 dB linewidth and the mode purity of the radially polarized CVB is about 94%.