Localized moderate temperature Er3+ doping into optical crystals

Possibility of localized doping by Er3+ diffusion at moderate (less then 500 degrees C) temperature is demonstrated for lithium niobate and sapphire. The doping is achieved by immersing the substrate wafers into reaction melt containing smalt amounts of erbium salt. A crucial point of the presented technology is a crystallographic orientation of the used wafers. Though in the Z and Y-cuts of lithium niobate the content of incorporated erbium did not exceed the concentration achieved using standard high temperature (or high energy) approaches, Lithium niobate X-cuts contained up to 10 weight % of erbium. Similar results were obtained also for the corresponding cuts of sapphire. This strong anisotropy of the doping is explained on the basis of crystal structure of the particular cuts. The in-diffused layers in all the cuts are rather shallow, however, erbium can be diffused deeper into the substrates by post-diffusion annealing in air. The moderate-temperature approach enabled us to fabricate APE waveguides in erbium doped lithium niobate without deteriorating the samples surfaces. The samples were characterized by RES, SEM, NDP and mode spectroscopy.