Experimental Studies On Optical Properties Of Titanium-diffused Lithium Niobate Optical Waveguides

Because of excellent electro-optical property,lithium niobate crystal can be used to develop various electro-optic devices,including those ones based on Ti-diffused lithium niobate optical waveguide.A research on electro-optic property of Ti-diffused lithium niobate optical waveguide is of an important significance to improve the performance of the devices based on the Ti-diffused lithium niobate optical waveguide.In this paper,we have studied mainly the waveguiding property of near-stoichiometric Ti-diffused lithium niobate planar optical waveguide,and the electro-optic properties of lithium niobate crystal with Ti diffusion in two surfaces and congruent and near-stoichiometric Ti-diffused lithium niobate planar optical waveguide.The main purpose of present dissertation is to investigate the influence of Ti diffusion doping on the electro-optic property of lithium niobate crystal.The main research works include the following four aspects.(1)Carry out the preparation of near-stoichiometric lithium niobate crystal by Li-rich vapor transport equilibration(Li-rich VTE).The influences of VTE duration and temperature on Li₂O-content at crystal surface as well as the crystal cut effect were studied.The studying results show that the increment of surface Li₂O-content induced by the VTE follows the Arrhenius relationship to the VTE temperature,and has an approximately square-root dependence on the VTE duration,and the relevant empirical expressions were presented.(2)The waveguiding property of near-stoichiometric Ti-diffused lithium niobate planar optical waveguide was investigated.Three methods,including Ti diffusion following VTE,Ti diffusion followed by VTE and Ti diffusion accompanied by VTE,were adopted to fabricate the near-stoichiometric Ti-diffused lithium niobate planar optical waveguides,and characterizations were performed on the composition,waveguiding properties and Ti-concentration profile.The experimental results show that refractive index increment induced by Ti diffusion obeys to an exponential relationship to the Ti-concentration.Different fabrication methods resuilt in different power indices of the exponential relationship due to different ionic environments of Ti dopants in the crystal.(3)The electro-optic property of lithium niobate crystal with Ti diffusion in two surfaces was investigated.A number of lithium niobate crystals with Ti diffusion in two surfaces were prepared by the high-temperature diffusion method.These crystals have different Ti doping concentration.A Mach-Zehnder(M-Z)interferometry system,which is only suitable for measurement of electro-optic coefficient of bulk crystal material,was established,and used to measure the electro-optic coefficients of these crystals with Ti diffusion in two surfaces.For comparision,the measurements were also carried out on a number of bulk Ti-doped lithium niobate crystals with different Ti-doping concentrations of 0-16 mol%.The results show that the electro-optic coefficient decreases slightly with a rise in Ti-doping concentration,regardless of diffusion doping or bulk doping.Comparatively,the decrease in the case of diffusion doping is slightly smaller than that in the case of bulk doping.This is ascribed to different ion environments induced by Ti diffusion doping and bulk doping.In particular,as the Ti doping concentration is that of typical Ti-diffused lithium niobate strip waveguide,?6.4 mol%,the electro-optic coefficient?₃₃of lithium niobate crystal with Ti diffusion in two surfaces decreases by 2%only.And this decrease is within the experimental error,?4%,indicating that at such a doping concentration the electro-optic property of Ti-diffused lithium niobate crystal is well preserved.(4)The electro-optic properties of congruent and near-stoichiometric Ti-diffused lithium niobate planar optical waveguide were investigated.An experimental system,which is valid for measurement of electro-optic coefficient of not only a thin-film or a planar waveguide structure but also a bulk material,was firstly proposed and established on the basis of prism coupling technique,and used to measure the electro-optic coefficients of pure congruent lithium niobate crystal,congruent and near-stoichiometric Ti-diffused lithium niobate planar optical waveguide having a typical surface titanium concentration of 6.4 mol%.The experimental results show that the influence of Ti diffusion doping on the electro-optic property of lithium niobate crystal can be ignored for both cases of congruent and near-stoichiometric optical waveguides,in consisitent with the measurement result of lithium niobate crystal with Ti diffusion in two surfaces.In words,Ti diffusion doping hardly affects the electro-optic property of lithium niobate crystal.The electro-optical properties of Ti diffusion waveguide devices are maintained,which is of great significance for the application and development of related waveguide devices.