电光调制器是将高频微波信号加载到光载波上的光电子器件，在高速光通信系统与微波光子链路中发挥着重要作用。日益增长的用户需求与数据传输容量推动着系统整体与核心器件的不断迭代更新。理想的电光调制器应同时具有大调制带宽、低半波电压和紧凑的尺寸。InP基多量子阱 (MQW) 材料同时具有线性电光 (LEO) 效应、二阶电光 (QEO) 效应以及量子限制斯塔克效应 (QCSE)。掺杂包层形成的紧密光学限制与电阻耦合电场加载确保了高效的电光相互作用。此外，InP基电光调制器更容易与半导体激光器、光放大器等光电子器件进行集成，相比于其他材料的光调制器具有显著的优势。然而，当前InP基电光调制器仍存在外延结构、电极结构以及电场加载方式等方面的问题。本论文首次实现了基于新型n-i-n型外延结构的高速马赫-曾德尔(MZ)电光调制器以解决外延结构与电极结构方面的问题。同时，提出了基于高介电常数 (high-k) 材料的薄膜MQW调制器方案以同时满足高电场加载效率与低微波损耗。为降低InP基电光调制器的光学损耗与微波传输损耗，采用本实验室提出的新型n-i-n型外延结构。该结构利用InGaAlAs/InP 材料 II型异质结的高导带不连续性阻挡电子电流，避免了p型掺杂材料的使用，可以在降低光学传输损耗的同时保证低微波损耗。测试得到的波导光学传输损耗仅为1.3 dB/cm，电极与半导体的比接触电阻率低至10?7 Ω·cm2量级，显著优于p-i-n型外延结构。为简化电极制作工艺与调制器驱动方式，并实现高效调制，提出了窄间距GS行波电极 (TWE) 结构。通过对电极参数的优化实现了低微波损耗传输以及良好的阻抗匹配和速度匹配。调制区长度为1 mm的器件6.4-dB电带宽超过50 GHz，并在制备有终端匹配电阻的器件上实现了超过50 GHz的3-dB电光带宽。同时，利用InGaAlAs/InAlAs MQW在高偏置电压下增强的QEO效应与QCSE效应，获得了0.07 V·cm的超低半波电压长度积。进一步，提出了基于high-k材料的薄膜MQW调制器方案以解决电场加载效率与微波信号衰减之间的矛盾。采用high-k材料钛酸钡 (BTO) 代替传统InP基调制器中的掺杂包层，可以在获得高电场加载效率的同时降低微波损耗，从而突破器件的带宽瓶颈。仿真结果表明，薄膜MQW调制器的电光带宽高达240 GHz，并且具有0.5 dB/cm的低波导损耗和0.24 V·cm的低半波电压长度积。
Electro-optic (E-O) modulators are optoelectronic devices for impressing high-frequency microwave signals onto optical carriers, and play an important role in high-speed optical communications and microwave photonic links. The ever-increasing customer demand and data transmission volume call for continuous upgrade of the system and key components. An ideal E-O modulator should simultaneously exhibit large modulation bandwidth, low half-wave voltage and compact size.InP-based multiple quantum well (MQW) materials simultaneously exhibit linear electro-optic (LEO) effect, the quadratic electro-optic (QEO) effect, and the quantum confined Stark effect (QCSE). The tight optical confinement and the resistively coupled electric field loading formed by the doped claddings ensure efficient E-O interaction. In addition, InP-based E-O modulators can be monolithically integrated with other optoelectronic devices such as semiconductor lasers and semiconductor optical amplifiers, showing significant advantages over modulators based on other materials. However, the current InP-based modulators encounter challenges in terms of epitaxial structure, electrode structure and electrical field loading scheme. In this work, high-speed Mach-Zehnder (MZ) E-O modulators based on novel n-i-n epitaxial structure are implemented for the first time to solve the problems of epitaxial structure and electrode structure. In addition, the membrane MQW modulator based on high-k materials is proposed to satisfy both high electric field loading efficiency and low microwave loss.In order to reduce the optical loss and the microwave transmission loss of InP-based modulators, a novel n-i-n epitaxial structure is adopted. The structure is proposed by our laboratory and utilizes the large conduction band discontinuity of type-II InGaAlAs/InP heterojunction to block electron current, thus avoiding the use of p-doped materials. Low microwave loss and reduced optical propagation loss can be guaranteed simultaneously. The measured optical propagation loss of the waveguide is only 1.3 dB/cm, and the specific resistivity of the ohmic contact is as low as 10?7 Ω·cm2, showing significant improvement over the p-i-n epitaxial structure.In order to simplify the electrode fabrication process and modulator driving method, and to achieve high-efficiency modulation, a narrow-gap GS traveling-wave electrode (TWE) structure is proposed and fabricated. Low microwave loss propagation and good impedance matching and velocity matching are realized by optimizing the electrode parameters. A 6.4-dB electrical bandwidth exceeding 50 GHz is obtained on a device with 1-mm-long modulation region, and a 3-dB E-O bandwidth beyond 50 GHz is demonstrated for a modulator with on-chip matching resistor. In addition, with the help of enhanced QEO effect and QCSE of the InGaAlAs/InAlAs MQWs at high bias voltages, an ultralow half-wave voltage-length product (VπL) of 0.07 V·cm is recorded. Finally, a membrane MQW modulator based on high-k material is proposed to attain high electric field loading efficiency and low microwave signal attenuation simultaneously. Employing high-k material barium titanate (BTO) in place of doped claddings in conventional InP-based modulators allows high electric field loading efficiency while reducing microwave loss, thus breaking the bandwidth bottleneck of InP-based modulators. Simulation results reveal that the E-O bandwidth of the membrane MQW modulator is up to 240 GHz, together with a low waveguide loss of 0.5 dB/cm and a low VπL of 0.24 V·cm.
