All-Fiber Fused Coupler for Stable Generation of Radially and Azimuthally Polarized Beams

The stable generation of TE
₀₁
and TM
₀₁
beams is demonstrated through a novel all-fiber fused coupler fabricated from a standard single mode fiber (SMF) and a custom air-core fiber. The fundamental mode in the SMF is directly coupled to the TM
₀₁
or TE
₀₁
mode by appropriately phase matching the modes in the fibers resulting in efficiency of ~67% and ~85%, and polarization purity of 70% and 82%, respectively. The phase matching is achieved through pre-tapering the SMF with precise control of the tapering ratio. The air-core fiber ensures selective excitation of the desired modes, thereby improving propagation stability and polarization quality of the TE
₀₁
and TM
₀₁
beams.