# Soft-proton exchange on Magnesium-oxide-doped substrates a route toward efficient and power-resistant nonlinear converters

## TL;DR

This paper introduces a novel waveguide fabrication method using Soft-Proton Exchange on MgO-doped lithium niobate, enhancing resistance to photo-refractive damage and maintaining high nonlinear efficiency for high-power integrated optics.

## Contribution

It demonstrates for the first time that SPE on MgO-doped substrates produces effective, nonlinear, and PRD-resistant waveguides comparable to CLN, offering a new fabrication route.

## Key findings

- SPE on MgO-doped substrates yields near-identical refractive-index profiles to CLN.
- SPE maintains high nonlinearity and PRD resistance in MgO-doped waveguides.
- The fabrication process is compatible with existing CLN techniques.

## Abstract

Despite its attractive features, Congruent-melted Lithium Niobate (CLN) suffers 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. 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 effective as for CLN: 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, for the first time, waveguides starting from MgO-doped substrates using the Soft-Proton Exchange (SPE) technique and we show that this third way represents a promising alternative. We demonstrate that SPE allows to produce refractive-index profiles almost identical to those produced on CLN without reducing the nonlinearity in the substrate. We also prove that the SPE does not affect substantially the resistance to PRD. Since the fabrication recipe is identical between CLN and MgO-doped substrates, we believe that SPE might outperform standard techniques to fabricate robust and efficient waveguides for high-intensity-beam confinement.