The Doping Concentration Optimization of Er-Doped LiNbO₃ Crystals for LNOI Lasers and Amplifiers

Photonics on a lithium-niobate-on-insulator (LNOI) has emerged as one of the most attractive fields within integrated optics. Recently, the lasers and amplifiers on LNOI have made a series of breakthroughs and important progress; however, their output efficiencies are still at a low level. In this paper, a series of erbium-doped lithium niobate (Er:LN) crystals were grown from the congruent melts with different concentrations ranging from 0.1 to 4.0 mol % by the Czochralski method. The absorption and emission intensities became stronger with the increase of erbium ion concentration and approached saturation when it is up to 2.0 mol %. Judd–Ofelt and McCumber theories were carried out to discuss the optical characteristics of erbium ions in Er:LN crystals, providing a basis for evaluating the performance of Er:LN devices. Besides, the UV absorption edges and the OH– absorption bands showed that there is a threshold around 2.0–2.65 mol % of erbium ions in Er:LN crystals, indicating a change of defect structures. It was concluded that the optimal doping concentration of erbium ions in Er:LN crystals is about 2.0 mol %, which provides significant guidance for the development of highly efficient LNOI lasers and amplifiers.