Soft-proton exchange on magnesium-oxide-doped substrates: A route toward efficient and power-resistant nonlinear converters

Despite their attractive features, integrated optical devices based on Congruent-melted Lithium Niobate (CLN) suffer from Photo-Refractive Damage (PRD). This light-induced refractive-index change hampers the use of CLN when high-power densities are in play, a typical regime in integrated optics. In bulk devices, the resistance to PRD can be largely improved by doping the lithium-niobate substrates with magnesium oxide. However, the fabrication of waveguides on MgO-doped substrates is not as straightforward as on CLN and either the resistance to PRD is strongly reduced by the waveguide fabrication process (as it happens in Ti-indiffused waveguides) or the nonlinear conversion efficiency is lowered (as it occurs in annealed-proton exchange). Here, we fabricate waveguides starting from MgO-doped substrates using the Soft-Proton Exchange (SPE) technique and we show that this combination represents a promising alternative. We demonstrate that, with a small adaptation of the exchange parameters, SPE allows prod...