Fabrication And Characterization Of Near-stoichiometric LinbO₃Waveguide

NS LiNbO₃（LN） crystal mainly displays two advantages over the congruent LNcrystal, One is that NS crystal only needs a little amount of MgO （>0.8mol%） toprevent the optical damage while the congruent material requires an threshold MgOconcentration as high as4.9mol%to resist the photorefractive effect. On the otherhand, NS material requires a much lower coercive field strength for ferroelectricmicrodomain inversion in a periodically poled LiNbO₃waveguide.Li-rich vapor transport equilibration（VTE） treatment can be used to make LNcrystal reach the NS ratio. Here, NS Ti:LN planar waveguides with different Ti filmthickness are fabricated by co-working of Ti⁴⁺-diffusion and Li-rich VTE treatment ona Z-cut congruent LN crystal, and the refractive index profile of the NS Ti:LNwaveguide is then characterized by prism-coupler method. The result shows that therefractive index profile is correlated with the Ti⁴⁺concentration profile（mol%） andthere exists a linear relationship,△n_e≈2.38×10³C_Ti.However, the refractive index profile in strip waveguide can be hardly measuredby the prism-coupler method because of the narrow guiding area in the Ti-strip.Therefore, in this paper, a near-field method is employed to construct the refractiveindex profile of strip waveguide. Based upon the measured results, waveguidingcharacteristics including loss figure, mode field distribution, Ti⁴⁺depth profiles andrefractive index profile of passitive and native strip waveguides have all beenconcerned. Like the congruent Ti:LN waveguide, the Ti⁴⁺-induced extraordinaryrefractive index increase in these NS strip waveguides follow a sum of two errorfunctions in the width direction and Gaussian function in the depth direction, and therefractive index models are then constructed. On the basis of the profile models, modefield profiles of the NS waveguides are further calculated using variational methodand compared with the measured data. The results show that the calculated mode sizesare in good agreement with the measured, implying that the refractive index profilemodels proposed are close to the practical scenario. In addition, the relationshipbetween Ti⁴⁺-concentration profile and the refractive index increase profile are alsocalculated and analyzed, and there also exist linear relationships,△n_e≈2.0×10³C_Ti.