We propose a new hole-assisted polymer optical fiber design to eliminate the influence of dopant diffusion and to increase the UV writing efficiency in fiber Bragg grating inscription. The optical waveguide is formed inside a solid core surrounded by a ring of 3 large air holes in enhanced UV photosensitive PMMA with double-cladding. We determined a map of the single-mode and multi-mode phase transitions using a finite-element- based vectorial optical mode solver. We obtained a wide range of geometrical configuration for the single-transverse-mode (HE11) propagation in the visible. The design is optimized to operate at the minimum optical loss wavelengths of 580 nm and 770 nm.