基于铌酸锂薄膜的光纤调制器关键理论与技术研究

近年来，铌酸锂薄膜电光调制器因其高速、低半波电压、微型、易集成等优异性能成为了新兴的研究热点。但亚微米尺寸的铌酸锂薄膜光波导在与单模光纤耦合时存在较大的模场失配损耗，阻碍了器件的实用化。而当前采用光栅耦合器与模斑转换器的解决方案要求制作额外的模块以及实现光纤波导间精确而稳固的对准，极大地增加了芯片制作与封装的难度及成本。鉴于此，本项目提出基于铌酸锂薄膜的光纤电光调制器这一独辟蹊径的解决方案，创新性地以键合于D型单模光纤上的铌酸锂薄膜制作高性能马赫-曾德尔型电光调制器，以定向耦合器实现高效的光纤波导间横向耦合取代低效的端面耦合。项目将研究高质量铌酸锂薄膜与D型光纤键合技术，高性能铌酸锂薄膜电光调制器以及光纤波导定向耦合器的关键理论与工艺制作技术，突破端面耦合损耗限制，研制成功低插损的高性能光纤电光调制器，为研制基于铌酸锂薄膜的电光调谐光纤型光子器件，以及实现低插损光子器件开创一条新的路径。

In recent years, lithium niobate on insulator (LNOI) thin-film electro-optic (EO) modulators are becoming an emerging hot research field because of their excellent performances including ultra-high speed, low half-wave voltage, miniaturization, and easy integration. However, their applications are now hampered by the large coupling loss between the submicron-scale LNOI waveguide and the standard single mode fiber (SMF) due to the high difference in the mode field sizes between them. Currently, to address this loss issue, a grating coupler or a mode size converter is usually employed, but this technology requires an additional fabrication of the coupler or converter as well as a precise and stable alignment between the fiber and the LNOI waveguide, resulting in the increase in difficulty and complexity while fabricating and packaging the modulator chip and hence the increase in the cost. In view of this, a novel proposal employing an all-fiber electro-optic modulator based on LNOI to address the loss issue is proposed. The proposed modulator employs Mach-Zehnder interferometer (MZI) configuration structured with the optimized ridge LNOI waveguides to enhance the EO efficiency and driven by travelling-wave electrodes to realize high speed modulation. The light from the fiber core is evanescently coupled into a thin strip LNOI waveguide and then launched into the MZI via a ridge LNOI waveguide with tapered slab height and vice versa. Such all-fiber configuration exempts the need of the low-efficiency butt-coupling with an SMF. We will study the proposed all-fiber LNOI EO modulator from the following aspects, including the technology for the fabrication of high-quality D-shaped SMF, the technology for the bonding of LNOI on the D-shaped SMF, the principle and technology for the design and fabrication of high-performance LNOI EO modulator and directional coupler formed with the LNOI strip waveguide and the D-shaped fiber. With these efforts, it is expected to address the loss issue and realize a low-insertion-loss high-performance all-fiber LNOI EO modulator. Our all-fiber LNOI modulator is feasible in practice. And the proposed integration of fiber and thin film LN enables low-insertion-loss LNOI photonic devices and opens a new door to realize high-speed fiber devices.