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 erbiumybtterbium codoped phosphate glass were fabricated using proton-beam writing. Single-mode waveguides were fabricated with fluences ranging from 0.4 to 2.0× 1015 particles 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× 1015 particles 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.