Research On Electro-Optic Characteristics Of BaTiO₃ Crystal Thin Film Waveguide

In an optical communication system,optical modulator is a device for generating the high-speed digital pulse light signal,playing the role of the heart of the communication system.The functional mechanism of an optical modulator is to modulate the physical characteristics of the light signal through the high-speed microwave signal,so that the cw laser beam can become the optical pulse signal alternating with it synchronously.At present,there are two kinds of optical modulators:one is electro-optic modulator,the other is electro-absorption modulator,which uses an external driving electrical signal to change the optical refractive index and the optical absorption characteristics of the optical signal channel,respectively.The lithium niobate crystal waveguide electro-optic modulators have commonly been employed in optical communications systems and the maximum modulation bandwidth has reached 40GHz,and its electrooptic property limits the continuous improvement of modulation bandwidths,so the new materials for realizing high modulation bandwidth electro-optic modulators are demanded.The ferroelectric crystal-barium titanate film has become one of the new research hotspots at present due to its ultrahigh electro-optic coefficient and excellent stability,and the compatibility with semiconductor materials.A corporation of a barium titanate（BTO）crystal film waveguide and a drive voltage can implement the modulation of a laser signal,in which the electrooptic coefficient r₅₁ of up to 600-800 pm/V plays the critical role.However,in previous researches,the inappropriate electrooptic modulation theoretical models have led to the inaccurate values of the electrooptic coefficient and further led to an impairment to the researches on the electrooptic modulation devices.This paper is to investigate the electrooptic characteristics of barium titanate crystal film waveguide to attract an efficient model relating the correlative electrooptic coefficient r₅₁ and birefringence b_eo to the driving voltage,so that the much more accurate values of r₅₁ and b_eo can be measured.The above results can provide the theoretical and practical evidences to the ever increases of electrooptic modulation efficiency and further build a foundation for the researche and development of the high-performance electrooptic modulators.The central researches and outcomes are summarized below.First,for two types of barium titanate crystal thin films with different growth directions,a-axis and c-axis,established a synchronous test and analysis model,in which the key parameters,electro-optic coefficientr₅₁and birefringenceb_eo,are synchronized.The reason for involving birefringence in this step of work is that the birefringence of BTO crystal film plays a critical role in the electrooptic modulation theory,and barium titanate crystal is an anisotropic crystal with birefringence effect depending on the axis direction and the experimental condition.However,such an important property of an electrooptic crystal material is always ignored in researches in the past decades so that the experimental results are inaccurate that is not helpful for investigating the electro-optic properties of BTO crystal film waveguide,bringing deterioration to the later research on low voltage and high bandwidth electro-optic modulators.Second,the full-BTO crystal thin film rib waveguides and further an embedded waveguide/electrode structure model that can realizes a two-dimensional electrooptic interaction effect are proposed and studied,and then the parameters of the device are discussed to optimize the device structure model with the highest modulation efficiency and the minimum optical transmission loss.Third,with the established theoretical models,to simulate and compare the performance of BTO crystal film waveguides,both the Si₃N₄/BTO rib waveguide and the full BTO rib waveguide are respectively analyzed through systematic simulations and compared with the practical tests.It found that the full BTO rib waveguide structure much lower bending optical loss than the Si₃N₄/BTO rib waveguide,which is sustainable to designs of the devices including the bending waveguides such as the Mach-Zehnder（MZ）type intensity electrooptic modulators.After systematic analyses for two waveguide/electrode structures,the coplanar waveguide（CPW）structure and the newly designed embedded waveguide/electrode structure,the embedded waveguide/electrode device model with the highest modulation efficiency is obtained by optimizing the structure.Illustratively,if r₅₁=400 pm/V andb_eo=-0.009are given,for the embedded full-BTO rib waveguide/electrode device structure in the c-axis growth grown film,the minimum figure-of-merit of V_π²L≈7.385V²?c m is obtained with the nonlinear modulation model,and V_π=5 V,it equals to V_πL=1.477V?c m.In contrast,for the a axis grown BTO crystal film,when the electric angle is set to be 45°,the minimum modulation figure-of-merit of V_πL=0.311V?c m can be obtained with a quasi-linear electrooptic modulation model.Fourth,in terms of the aforementioned theoretical study and simulation results,design and fabricate a device sample of full-BTO straight rib waveguide electrooptic modulator with 4.0μm rib width and 0.1μm height,8.0μm and 0.1μm embedding depth,then obtain the optic-electrical field interaction efficiency of 0.78 and the optical propagation loss of0.5dB/cm for the waveguide.With both the linear and ellipse polarization modulations,carry out experimental measurements for the optical phase modulations under the driving voltages to have multiple relations between the electrooptic coefficient and the birefringence,and further obtain the accurate measurement values of these parameters with the overlap of their modulation relations.For the linear polarization electrooptic modulation that the higher measurement accuracy than the ellipse polarization electrooptic modulation,the measurement values and accuracies are r₅₁=425±5pm/V andb_eo=-（0.0221±0.0006）.Therefore,an efficient metrology of the electrooptic properties is realized.Fifth,the accuracies of synchronous measurement theory and method for the electro-optic coefficient r₅₁ and birefringence b_eo of c axis grown BaTiO₃ crystal thin film are analyzed,then the simulation data and test results are combined to obtain the range of measurement errors.Consequently,based on the accuracies of the instruments,the measurement accuracy of both r₅₁ and b_eo are confirmed to be in the range of±5.0%,which are in accord with the above real measurement results.