Doughnut-shaped ASE from polymer clad fiber doped with a laser dye

A doughnut-shaped beam was generated with polymer clad fibers (PCFs), and their cladding layers were doped with a laser dye. Doping was performed using dye vapor in a closed vacuum. The doughnut-shaped beam was the amplified spontaneous emission (ASE) from the doped cladding layer, and was observed only by coaxial pumping and not by side pumping.