氮化硅薄膜材料的光学透明波长范围宽、光学吸收系数低、损伤阈值高、制备与CMOS（Complementary Metal Oxide Semiconductor）工艺兼容，这些优秀特性使其成为一种理想的新型光波导材料。基于氮化硅薄膜波导的光学相控阵芯片不仅光学损耗低，还可以工作于近红外波段，与低成本的硅探测器兼容。在光学相控阵芯片中，输出天线阵列的间距越小，无栅瓣扫描角度范围越大。但是，当天线阵列长度达到mm量级、天线间串扰低于-20dB时，氮化硅波导天线的间距在1550nm下通常大于2μm（1.3倍波长），而在905nm波长下通常大于4μm（4.4倍波长），因而光学相控阵的无栅瓣扫描角度范围低于45°，无法实现高性能的光束扫描。如何构造出串扰低、密度高的波导阵列及高性能的平板波导衍射光栅天线是目前近红外波段氮化硅波导集成光学相控阵芯片领域需要解决的关键科学技术问题。本论文采用理论和实验结合的方法，研究了近红外波段多层氮化硅波导集成光学相控阵芯片的关键结构，分析了波导阵列结构对串扰和扫描角度的影响，并探讨了高向上衍射效率的平板波导衍射光栅的结构设计与制备。
提出将非对称双层氮化硅材料结构体系用于周期性波导阵列，探讨了波导阵列的周期单元结构对串扰的影响规律。理论分析表明，在1.0mm波导长度和905nm工作波长下，当U2、U3和U4周期结构的双层氮化硅波导阵列串扰优于-23.0dB时，波导间距分别为1.320、1.160和1.053μm。U3和U4周期结构的波导阵列间距均小于1.3倍波长，可以实现超低串扰和高集成度的多层氮化硅波导阵列。以U4周期波导阵列结构参数为基础，制备了多层氮化硅波导阵列，其最近邻与次近邻串扰均优于-22.0dB，接近于设计值-23.0dB；但由于加工偏差，波导的实际宽度相比设计值偏窄了19%~25%，致使周期间串扰劣化至-11.2dB。理论计算证实了加工偏差是导致波导阵列周期间串扰劣化的主要原因，若加工工艺参数与设计参数一致，则能够实现超低串扰和高密度的波导阵列制备。
相比于包层中无空气隙的非对称双层氮化硅波导，空气隙波导的趋肤深度更浅、单模波导有效折射率范围更大，为此在U4周期波导阵列结构的基础上提出了包层中带有空气隙的非对称双层氮化硅周期性波导阵列结构。理论分析表明，在1.0mm波导长度和905nm工作波长下， 当U4周期空气隙多层氮化硅周期性波导阵列结构的串扰优于-23.0dB时，波导间距低达0.889μm（0.98倍波长），波导阵列的密度显著提升。这些研究结果为超低串扰、超高密度多层氮化硅波导阵列的研发提供了科学依据。
提出以非对称双层氮化硅周期性波导阵列为天线的一维光学相控阵架构，从理论和实验上研究了光学相控阵芯片的波导阵列排布结构和串扰对输出光束远场衍射参数的影响。理论分析表明，在905nm工作波长下，以U4周期波导阵列为天线的一维光学相控阵的主瓣发散角随天线总宽度增加而减小，当通道数为64路时主瓣发散角为1.07°；无栅瓣扫描角度范围随天线间距减小而增加，当输出波导阵列天线的间距为1.100μm时，无栅瓣且主瓣功率占比高于50%的扫描角度范围理论值可达51.0°，无栅瓣且主瓣功率占比高于80%的扫描范围为20°。测试结果表明，以U4周期波导阵列结构为天线的一维光学相控阵芯片的扫描角度范围不小于-10.75°~10.75°，主瓣发散角平均值为1.07°，旁瓣抑制比优于-9.0dB，与理论结果基本一致。光学相控阵芯片制备的工艺偏差、芯片内部与边缘散射、波导输出光束的相位分布偏离理想分布等原因会导致芯片旁瓣抑制劣于理论值。采用多组一维多层氮化硅周期性光学相控阵芯片可拼接构成线扫描泛光激光雷达，其全固态二维扫描视场角可达90°。
作为光学相控阵芯片的输出光束整形器，平板波导衍射光栅通常基于错位双层衍射结构以实现高于0.90的向上衍射效率，加工需至少三次光刻。但实际上多步光刻间的套刻偏差常导致衍射光栅的向上衍射效率降低、远场发散角增加，影响其光束整形效果。本文以多层氮化硅材料为基础，提出了一种基于三层氮化硅的平板波导衍射光栅结构，从理论和实验上研究了衍射光栅的结构与关键性能参数之间的关系。理论研究表明，三层氮化硅平板衍射光栅在905nm工作波长下可实现高达0.92的向上衍射效率和低达0.103°的远场发散角；当光栅的出射角度在φ方向（相控阵角度扫描方向）0°~22°范围内变化时，向上衍射效率在0.72~0.92范围内变化，近场有效长度在408~467μm范围内变化。实验研究表明，三层氮化硅平板衍射光栅芯片的向上衍射效率和远场发散角分别为0.91和0.154°，与理论计算数据基本相近。该衍射光栅加工仅需两步光刻，套刻偏差容忍度高，并且当刻蚀深度超过衍射层而未及第二层芯层时，光栅的衍射强度不随刻蚀深度的变化而改变。研究表明，基于简单的两次光刻刻蚀工艺能够在三层氮化硅薄膜材料制备出实现具有高向上衍射效率和低远场发散角等优良光学衍射性能的平板波导衍射光栅。
本论文的研究为近红外波段集成光学相控阵芯片性能的提高、制备难度的降低提供了依据，为基于多层氮化硅薄膜材料的光学相控阵光束扫描元件的研发提供了理论和实验支撑，为推动高性能的多层氮化硅光学相控阵芯片在自动驾驶激光雷达中的应用提供了科学依据。

Silicon nitride has become a novel optical waveguide material due to its excellent properties such as a wide transparency wavelength range, low loss, high damage threshold, and compatibility with CMOS processes. The optical phased array (OPA) chip based on silicon nitride thin film waveguide has low optical loss, and covers a wide waveband including the near-infrared band, which is compatible with the low-cost silicon detectors. In an OPA chip, the smaller the spacing of the output antenna array, the larger the grating-lobe-free steering angle range. However, when the length of the antenna array reaches the millimeter level and the crosstalk between antennas is less than -20 dB, the spacing of silicon nitride waveguide antennas is usually greater than 2 μm (1.3 times the wavelength) at 1550 nm and greater than 4 μm (4.4 times the wavelength) at 905 nm, resulting in a grating-lobe-free steering angle range below 45°, unable to achieve high-performance beam scanning. Consequently, how to construct the low-crosstalk and high-density waveguide arrays and the high-performance slab waveguide diffraction grating antennas are the key scientific and technological issues that need to be addressed in the field of near-infrared integrated OPA chips based on silicon nitride waveguide. This thesis investigates the key components of the multi-layered silicon nitride waveguide-based near-infrared integrated OPA chips using a combination of theoretical and experimental approaches. The impact of the waveguide array structure on the crosstalk and steering angle is analyzed, and the design and manufacture of the upward efficiency slab waveguide diffraction gratings are discussed.
A periodical waveguide array based on asymmetric double-layer silicon nitride is proposed, and the influence of the unit cell structure on the crosstalk investigated. The theoretical analysis shows that when the crosstalk of the U2, U3, and U4 periodical waveguide arrays based on double-layer silicon nitride are below -23.0 dB, the waveguide spacing is respectively 1.320, 1.160, and 1.053 μm, under a waveguide length of 1.0 mm and a wavelength of 905 nm. Here, the waveguide spacings of the U3 and U4 periodical waveguide arrays are all less than 1.3 times the wavelength, enabling the ultralow-crosstalk and high-density multi-layer silicon nitride waveguide arrays. Then a double-layer silicon nitride waveguide array was fabricated based on the structural parameters of the U4 periodical waveguide array. The nearest neighbor and second nearest neighbor crosstalks are below -22.0 dB, which are consistent with the design value of -23.0 dB. However, due to the processing deviation, the actual waveguide width was 19%~25% narrower than the design value, leading to an inter-period crosstalk degradation to -11.2 dB. Theoretical calculations confirm that process deviation is the primary reason for the inter-period crosstalk degradation, indicating that if the actual structure parameters match the design parameters, the ultralow-crosstalk and high-density waveguide arrays can be achieved.
Compared with the asymmetric double-layered silicon nitride waveguides without air gaps in the cladding, waveguides with air-gaps have shallower skin depths and wider effective refractive index range for single-mode operation. On this basis, a double-layer silicon nitride U4 periodical waveguide array with air gaps in the cladding is proposed. The theoretical analysis indicates that when the crosstalk of the air-gap U4 periodical waveguide array structure is below -23.0 dB, the waveguide spacing is as low as 0.889 μm (0.98 times the wavelength), significantly improving the density of the waveguide array. These results provide a scientific basis for developing ultralow-crosstalk, ultra-high-density multi-layer silicon nitride waveguide arrays.
A one-dimensional (1D) OPA architecture with the antennae of asymmetric double-layer silicon nitride periodical waveguide arrays is proposed. And the influence of the waveguide array arrangements and the crosstalk on the OPA far-field diffraction properties is investigated theoretically and experimentally. Theoretical analysis shows that, as the total width of the U4 periodical waveguide array increases, the main lobe divergence angle of the 1D OPA decreases. When the channel number is 64, the divergence angle is 1.07°. As the waveguide array spacing reduces, the grating-lobe-free steering angle range increases. At a spacing of 1.100 μm, the grating-lobe-free steering angle range exceeds 51.0° with a main lobe power ratio above 50% , and the grating-lobe-free steering angle range remains 20° when the main lobe power ratio is above 80%. Experiments reveals that, the 1D OPA chip with the antennae of U4 periodical waveguide array antennas exhibits a steering angle range of -10.75° to 10.75°, with an average main lobe divergence angle of 1.07° and sidelobe suppression ratio below -9.0 dB, consistent with the theoretical predictions. However, due to the process variations, the scattering inside and on the edge of the chip, and the deviations from the ideal phase distribution, the sidelobe level was higher than predicted. By integrating multiple sets of these 1D OPA chips, a line-scanning flash LiDAR (light detection and ranging) may be realized with a solid-state two-dimensional steering field-of-view up to 90°.
As a beam shaping component of the OPA chips, slab waveguide diffraction gratings typically employ an offset dual-layer diffractive structure to achieve upward efficiency higher than 0.90, requiring at least three photolithography steps in the fabrication. In practice, overlay errors between the multiple lithography processes often lead to a decrease in the upward efficiency and an increase in the far-field divergence angle, thus degrading the beam-shaping performance. 
A triple-layer silicon nitride slab waveguide diffraction grating is proposed, and the relationship between the structure parameters and the key performance is investigated. The theoretical study demonstrates that the slab grating can achieve an upward efficiency up to 0.92 and a far-field divergence angle as low as 0.103° at the 905nm wavelength. The upward efficiency varies between 0.72 and 0.92, while the near-field effective length varies between 408 and 467 μm when the φ direction (the direction of the OPA steering) exit angle varies within 0° to 22°. The experimental results show that the upward efficiency and the far-field divergence angle of the slab grating chip was 0.91 and 0.154°, respectively, close to the design values. This fabrication process of the grating is straightforward. The tolerance for overlay errors between the two photolithography steps is high, and the diffraction intensity remains unchanged when the etch depth exceeds the diffractive layer but does not reach the core layer. The results suggest that triple-layer silicon nitride slab diffraction gratings with high upward efficiency and low far-field divergence angles can be produced by simple two-step photolithography and etching processes.
This work contributes to the near-infrared integrated OPA chips by improving the performance and reducing the preparation difficulty, and also paves the way for the application of OPA-based beam steering technology in the autonomous driving light detection and ranging (LiDAR) systems.