Investigations Of Second Harmonic Generation In Ion Implanted LiNbO₃ Ridge Waveguides

With the advancement of theory and experimental techniques in the field of integrated optics,the application of integrated photonic devices based on lithium niobate crystals and waveguides has attracted more and more attention in scientific research and engineering.Lithium niobate is a ferroelectric oxide crystal.It has excellent electro-optic,acousto-optic and nonlinear optical properties in the visible to mid-infrared region.It is a commonly used integrated optical waveguide material.Lithium niobate can form a periodic polarization flip structure in the crystal through the electric field polarization,avoiding the deviation effect of different polarized light in phase matching,and improving the conversion efficiency of the nonlinear process.The combination of periodic polarization reversal structure and lithium niobate two-dimensional channel optical waveguide can produce high conversion efficiency nonlinear devices,which have application value in many fields such as quantum information processing,laser wavelength conversion,etc.This paper mainly studies the nonlinear generation and characterization of periodically poled lithium niobate ridge waveguides implanted with carbon ions.Ion implantation can produce planar optical waveguides on lithium niobate crystals.Injecting ions can modulate the refractive index of the implanted region of the lithium niobate crystal,thereby forming a variety of optical waveguide structures.In the published results on the second-order nonlinearity of ion-implanted LiNbO3 waveguides,the oxygen(O)ion-implanted LiNbO3 channel waveguide achieves the second-harmonic generation near the 980nm wavelength with a very high transmission loss.The normalization coefficient of waveguide was 34.5%/(W.cm2).The lattice damage of the sample caused by ion implantation can be reduced by subsequent annealing treatment,so as to further improve the nonlinear conversion efficiency of the sample.The ridge waveguide structure has high conversion efficiency due to its good light-limiting ability and small mode volume.Wet etching,dry etching,and the like mainly achieve the ridge waveguide structure prepared in lithium niobate.Wet etching utilizes the anisotropic etching characteristics of lithium niobate in a hydrofluoric acid solution to achieve high-quality microstructure preparation by masking.Dry etching includes ion beam milling,reactive ion beam etching,and focused ion beam etching.Recent studies over the past decade have shown that Precise blade dicing can also produce low-loss ridge waveguide structures on the surface of lithium niobate crystals.This is also the method used in this article.In this dissertation,high energy carbon ion implantation and precise blade dicing are used to form ridged optical waveguides on periodically poled lithium niobate crystals,and the second harmonic generation efficiency and transmission loss of lithium niobate waveguides are studied by fiber waveguide coupling.This study has provided new research ideas for the development of lithium niobate integrated optical devices.The main findings are as follows:1.Diced ridge waveguides based on 15MeV carbon ion-irradiated periodically oled LiNbO3.Carbon(C)ions were used as the implanted ions,and the injected energy was 15 MeV.Periodically polarized LiNbO3 waveguides were prepared on Mg-doped LiNbO3 crystals by precise blade dicing.An optical fiber-waveguide coupling platform was set up to study the second harmonic fundamental wavelength,conversion efficiency,and the loss of the waveguide after different annealing conditions systematically.The research results show that the ridge waveguide generates the second harmonic near 1600nm(fundamental wavelength),and the highest conversion efficiency of the ridge waveguide obtained is 77.9%/W/cm2.This efficiency can already be compared with commercial proton exchange lithium niobate devices.2.Diced ridge waveguides based on 4/7 MeV carbon ion-irradiated periodically oled LiNbO3.Select 4MeV and 7.5MeV C ions injected into the lithium niobate sample simultaneously,and then prepare the ridge waveguide based on periodic polarization LiNbO3 by precise blade dicing.The distribution of the second-order nonlinear coefficient(d33)of the waveguide in the depth direction was measured by the reflection method.The results show that the quasi-phase-matched second harmonic in the waveguide region produces the highest conversion efficiency of 30.4%/W/cm2,which corresponding to a waveguide width of 6.9 ?m.