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 homemade 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%. (letter)