【作者】 刘支华；

【导师】 石邦任；

【作者基本信息】 长春理工大学 ， 光学工程， 2008， 博士

【摘要】 集成声光移频技术主要应用领域包括光纤陀螺、声光频谱分析、相干激光探测等领域。随着技术的发展,光学器件小型化和功能复杂化的趋势的需求,对集成声光移频芯片提出了更高技术指标的要求。文中讨论并研究了基于铌酸锂（LN）衬底集成声光移频芯片技术。研制了在Ti扩散铌酸锂衬底基片上,包含有两个声光波导Bragg调制器组成的声光频移元件和两个无象差非球面波导短程透镜的准直/聚焦集成光学器件。首先,分析了在非对称光波导中电磁场分布的模式,并推导了短程透镜的微分方程及其旋转母线表达式。就短程透镜母线模型建立中所遇到的过渡区和透镜区连接处的弯曲损耗及过渡区到平面波导的弯曲损耗给出了具体的数学解决形式。同时为了减小损耗所应满足的透镜旋转母线一阶导数和二阶导数的连续性,并对卷边函数幂指数的选取进行了讨论,得出了最优化的设计参数,并给出了该参数条件下的短程透镜旋转母线的显式表达。其次,通过对Bragg衍射条件的分析,论证了声波及光波在介质中的最佳传播方向,并给出了波导声光Bragg调制器件的设计参数。考虑压电材料的选取、声光互作用的最佳模式、衍射光和零级光间的可分离条件及如何提高衍射效率等因素,给出了叉指换能器的设计结果。然后,分析了高斯光束在光纤中的传输规律,讨论了对准因素对声光芯片设计的影响。建立了短程透镜基点和主面的表达式方程,给出了短程透镜的具体的物象作图方法。分别利用工程上估算波导和光纤耦合的方法及模场匹配的方法,对短程透镜和光纤的耦合效率进行了推导。最后,论述了集成声光芯片制作工艺和各项指标测试结果。对集成声光芯片的总体技术做了总结并对未来发展趋势做出预测。论文主要创新点为:1)完善和优化了短程透镜母线模型。通过对凹面半径归一化处理,将凹面分成过渡区和透镜区,在不同的区域选取不同的辅助函数,通过优化设计使卷边与平板波导连接处曲率半径达到无限大,显著降低辐射损耗,使透镜能真正消球差和弯曲损耗最小。2)通过声表面波声光衍射布拉格条件与叉指换能器的效率优化,器件在驱动功率为500mW时,调制效率达85%。3)首次将几何光学和矩阵光学的理论引进波导短程透镜的研究中,建立了完整的光波导短程透镜系统几何光学传输理论。运用几何光学的光线追迹和等效光路的方法,在近轴小角度入射的情况下推导了短程透镜的相位传递函数及焦斑模场表示形式;运用计算机仿真技术,绘出了该模型下短程透镜的焦斑图,通过和实际测得的短程透镜焦斑光强分布的比较论证了该理论推导的正确性。4)首次在国内完成了整个LN集成声光芯片的设计和制作,并对芯片的各项性能做了测试和分析。在XYZ方向尺寸分别为46,2.5,30mm的Ti扩散LiNbO₃。光波导基片上制作了光纤耦合结构的声光移频芯片。测试短程透镜传输损耗为1.2dB,衍射光斑半功率点宽度为4.5μm;与单模光纤的耦合插入损耗小于20dB;芯片驱动的中心频率为110MHz;可实现3dB调制带宽为10MHz。更多还原

【Abstract】 Main applications of Integrated Acousto-optic（AO） frequency shifter includes Fiber Gyroscope, AO frequency spectral analysis, coherent laser testing, et al. As optic devices are more and more miniaturized and complicated, higher technology parameters are required to AO frequency shifter. In this paper, AO frequency shifter based on Ti diffused LiNbO₃ is developed. two AO waveguide Bragg modulators to complete AO frequency shifting and two non-aberration and non-spherical waveguide geodesic lens（GL） as collimating and focus integrated optic cell are developed based on Ti diffused LiNbO₃ substrate.The following results have been finished in this work:1. The mode distribution of electromagnetic field in asymmetry waveguide is analyzed. Differential coefficient equation and generatrix expression of geodesic lens is deduced. In profile design of GL, a suitable mathematic solution of lens profile in transitional zone eliminates effectively the curvature singularity of round-edge of lenses. To maintain continuity of first derivative and second derivative of lens profile, selection of power exponent of rim function is discussed. Optimized parameters are gotten. And generatrix expression of geodesic lens is expressed based on these optimized parameters.2. Through analysis of Bragg diffractive condition, the optical transmitting direction of acoustic wave and photonic wave is demonstrated. Design parameters of waveguide AO Bragg modulator are achieved. Cascade surface acoustic wave （SAW） transducer is designed and fabricated considering selection of proper piezoelectric material, optimized reaction model, improvement of diffractive efficiency and clarify separated conditions of diffracted light and undiffracted guided light.3. The rule of Gauss beam transmitting in optical fiber is analyzed. The impact of coupling effect to AO chip design is discussed. The Formulas of waveguide GL cardinal points and principal planes are derived; the geometry graphic imaging formation principles of GL are developed to get the object-image-relation of geodesic lenses. Utilization of geometric optical characters of GL, the equivalent light trail and phase transfer function are given. Simultaneously the distributing model is obtained. The focal spot intensity distribution of theory corresponds with the experiments.4. Integrated AO chip process method, performance indexes, and test results are discussed. A summary of current integrated AO chip technology and trend of AO chip technology are discussed. The innovation points of this research including:1. The profile model of waveguide geodesic lens is optimized. To normalize concave radius, divided to transition area and lens area, radiated loss decreased more, eliminated spherical aberration and curve loss are gained from different auxiliary functions in different region and optimized maximum curvature radius of joint between rim and planar waveguide.2. Optimized SAW AO Bragg diffractive condition and efficiency of SAW transducer, modulated efficiency of 85% is achieved by 500mW input driving power on device.3. Geometric optics and matrix optics is introduced into the research of waveguide geodesic lens, related completed transmitting theory in geodesic lens is obtained from light beam tracing and equivalent photonic circuit method. Under condition of adaxial incident light in small angle, phase transfer function and focus distributing model is obtained. Utilized computer simulation, theoretical focus distribution of GL is described, Which corresponds with the experiments.4. It is the first time in China’s mainland to design and fabrication of LiNbO₃ integrated AO chip, which characterizations are measured and analyzed. Acousto-optic（AO） frequency shifter that consists of a pair of geodesic waveguide lenses to collimate and focus and a pair of cascaded guided-wave AO Bragg cells has been realized in a Ti diffused Y cut LiNbO₃ waveguide substrate 46 cm by 2.5cm by 30 cm in size. Optimized ideal waveguide GL profile and AO parameters are calculated and designed by analytic method. Transmission loss of 1.2dB and spot size （FWHM） of 4.5μm of GL are achieved. All chip has provided a 3dB tunable bandwidth of 10 MHz on driving baseband frequency of 110MHz, coupling insert loss of 20dB with single-mode fiber.更多还原