Direct imaging of the end-of-range and surface profiles of proton-beam written erbium-doped waveguide amplifiers by atomic force microscopy

Buried channel waveguide amplifiers in erbium∕ybtterbium codoped phosphate glass were fabricated using proton-beam writing. Single-mode waveguides were fabricated with fluences ranging from 0.4to2.0×1015particles∕cm2. The end-of-range and surface profiles of the waveguides were investigated using atomic force microscopy. The waveguiding effect was investigated using the end-fire coupling technique. From the near-field mode profiles, the refractive index profiles of these waveguides were recovered using the propagation mode near-field method. From these results, it can be deduced that for phosphate glass waveguides fabricated with fluences <1.0×1015particles∕cm2, the ionization from the electronic stopping, rather than the nuclear damage processes, is the major contributing factor leading to an increase in the refractive index near the end of range.