薄膜铌酸锂波导器件的声光调制

声光效应描述的是声波和光波的相互作用，它被广泛应用于制作移频器、可调滤波器和声光偏转器件，在激光调Q/锁模、信号处理和光束调节方面也有很多应用。在所有声光材料中，铌酸锂是最吸引人的材料之一，这是因为其高压电系数使得通过叉指式换能器（IDT）有效地激发表面声波成为可能。和目前在其它压电材料诸如砷化镓上制备的先进的声光调制器相比，铌酸锂具备更高的机电耦合系数，这会大大简化表面声波器件的设计，并且用更小的器件尺寸就可以匹配射频（RF）驱动源。另外，铌酸锂突出的声光效应可以增强光子-声子相互作用。这些性质使得铌酸锂在实现惯性传感器、全息存储/成像和光束偏转方面成为很有潜力的材料之一。

然而，由于钛扩散和质子交换方法制备的波导对光的限制很弱，在传统块状铌酸锂上制作的声光器件的声光相互作用受到了很大限制。例如波导，其导模尺寸在几十平方微米、波导半径在厘米量级，这导致了器件尺寸相对较大从而制约了高集成度。

在过去二十年，单晶铌酸锂薄膜（LNOI，键合在低折射率衬底的薄膜铌酸锂）由于其高折射率差的层状结构成为了很有前途的集成光学平台。由于近年来在制备工艺上的不懈努力，已有关于低损耗波导、高品质因子谐振腔和高性能小型化的光子器件的报道。研究铌酸锂薄膜的声光性质和制作高效率声光调制器对于发展下一代LNOI声光器件/应用是十分必要的。

为了证明这种在薄膜铌酸锂上尝试的可行性，来自于卡耐基梅隆大学电子与计算机工程学院的蔡璐彤博士和Gianluca Piazza教授开展了在LNOI上制备声光调制器件的概念验证工作。研究结果发表在Photonics Research, 2019年第7卷第9期上 (L. Cai, et al., Acousto-optical modulation of thin film lithium niobate waveguide devices)。

LNOI上的声光调制器由IDT（用来产生表面声波）、形成声子谐振腔的两个反射器和光子波导器件组成。所有的器件都在厚度为500 nm的铌酸锂薄膜上单片集成。铌酸锂下方的氧化物厚度为1.5 μm，这是因为虽然更厚的氧化物会降低波导损耗，但是也会对声波器件性能产生负面影响，而1.5 μm的厚度可以很好地平衡波导损耗和声波的机电耦合性能。声光相互作用由于表面声波在声子谐振腔中形成驻波而得到提高。将光学波导放在最大应变/应力的位置，通过材料的光弹性质得到强声光调制，其中光学波导的尺寸仅是声波波长的很小一部分。

表面声波和波导模式传播方向的选择是使折射率在波导位置产生最大改变，同时在沿波导方向保持相同的折射率变化。两个光栅被用来将光耦合进/出芯片，这样做也便于未来在芯片上集成激光器和探测器。通过测量马赫-曾德尔干涉仪中不同阶的调制信号，提取出了薄膜铌酸锂的光弹系数，测量到的系数表明LNOI保持了块状铌酸锂优良的声光性质。探究材料性质对于今后设计声光和光机械系统具有重要的指导意义。利用优秀的声光性质可以制造出很多其他声光器件，这些器件有望胜过基于块状铌酸锂的声光器件。研究人员还展示了可以在高品质因子（Q > 300,000）低损耗（损耗小于 0.7 dB/cm）的光学谐振腔中实现的高效率声光相互作用，它的调制强度和马赫-曾德尔干涉仪声光调制器相比提高了10 dB增益。

与应用于宽带通信网络的LNOI宽带电光调制器不同，此兆赫兹声光调制器有许多其他极具影响力的应用，包括惯性传感器（倾向于低频率）、全息存储/成像和光束偏转（应用于激光雷达）。另外，在高品质因子谐振腔中的光信号操控对于芯片上受激布里渊散射和光频梳具有巨大潜在应用价值。

Gianluca Piazza教授相信，该技术对于发展新型小型化集成器件很有吸引力，它将会解决现在已经出现在无线和光学通信、经典计算、人工智能和量子信息科学的诸多问题和挑战。

未来工作将致力于将垂直腔面发射激光器和光探测器集成到声光调制器上，并将器件工作频率范围延伸到高频率宽带调制来实现光束偏转。

声光调制器的示意图。叉指式换能器、声反射器和光子波导器件单片集成在薄膜铌酸锂上。

Acousto-optical modulation of thin film lithium niobate waveguide devices

Acousto-optic (AO) effect, which describes the interaction between light and sound waves, is extensively used to build up frequency shifter, tunable filter and light beam deflectors, leading to applications including laser Q-switching/mode-locking, signal processing and manipulation of light beams. Among all the acousto-optic active materials, lithium niobate (LN) is one of the most attractive materials because of its high piezoelectric coefficients enabling efficient generation of surface acoustic waves (SAW) through interdigital transducer (IDT) technology. Compared with the state-of-art AO modulator fabricated on other piezoelectric materials such as GaAs, LN exhibits a higher electromechanical coupling coefficient, which dramatically simplifies the design of SAW devices and facilitates matching to radio frequency (RF) driving sources in a much smaller form factor. In addition, the strong photo-elastic effect enables efficient photon-phonon interactions in such material. These properties have made LN one of the most promising materials for realizing integrated AO devices for inertial sensing, holographic storage/imaging and beam steering applications. However, conventional AO devices built on bulk LN wafers suffer from the limited AO interaction due to the weak light confinement in waveguide structures created by titanium-indiffusion and proton exchange methods. Typical waveguide mode size is in the dozens of square micrometers (μm2), and the bend radius on the order of centimeters, resulting in relatively large device footprint and hindering high density integration.

In the past two decades, lithium niobate on insulator (LNOI, thin film lithium niobate bonded on a low refractive index substrate or buffer layer) has emerged as a promising platform for integrated optics due to its high index contrast stack layer structure. Thanks to the persisting effort made in fabrication methods and processes, low-loss waveguides and high-Q resonators have been demonstrated and a variety of photonic devices with high performance and small footprint have flourished in recent years. Investigating the thin film LN AO properties and forming highly efficient AO modulation devices are essential to develop next generation of AO devices/applications in LNOI.

To confirm the feasibility of the new thin film LN approach, Dr. Lutong Cai and Prof. Gianluca Piazza from the Department of Electrical and Computer Engineering at Carnegie Mellon University carried out the first proof-of-concept work of implementing AO modulation devices in LNOI. This work is published in Photonics Research, Volume 7, No. 9, 2019 (L. Cai, et al., Acousto-optical modulation of thin film lithium niobate waveguide devices).

The LNOI AO modulator consists of IDTs (generating SAW), two acoustic reflectors forming a photonic cavity, and photonic waveguide devices. All the components are fabricated and monolithically integrated on a 500 nm thick film of lithium niobate. The thickness of the insulating oxide below the LN is chosen to be 1.5 μm, so as to achieve a good trade-off between the waveguide loss and SAW electromechanical coupling. In fact, as a thicker oxide layer lowers the waveguide loss, it deleteriously impacts the SAW devices. AO interactions are enhanced by forming standing SAWs inside a photonic cavity. The optical waveguide, which is now a very small fraction of the acoustic wave, is placed at the maximum strain/stress locations to attain strong AO modulation through photo-elastic effect. The SAW and waveguide mode propagation directions are optimized in such a way that the refractive index change is maximized while kept uniform along the waveguides. Two grating couplers are used to couple light to/from the chip as well as facilitate future integration with on-chip lasers and detectors. The photo-elastic coefficient of thin film lithium niobate is extracted by measuring modulation signals of different orders in a Mach-Zehnder interferometer, indicating that LNOI preserves the excellent acousto-optical properties of bulk lithium niobate (LN). Such material property study is of paramount importance in guiding the design of any AO and optomechanical systems. By harnessing the good AO properties, a great wealth of other AO devices can be built to outperform existing counterparts made in bulk LN. The researchers also showcase that the efficient AO interaction can be implemented in a high-Q racetrack resonator (> 300,000), exhibiting loss as low as 0.7 dB/cm, to further enhance the modulation strength with a gain up to 10 dB (compared with Mach-Zehnder type).

Different from the high-bandwidth electro-optical modulators demonstrated in LNOI that are used in broadband communication networks, the MHz AO modulators demonstrated in this work have a wealth of different and impactful applications including inertial sensing (which favors low frequency), holographic storage/imaging, and light beam steering (applications like Lidar). In addition, the strong regulation of optical signal in a high-Q resonator showcased in this work has enormous potential for on-chip stimulated Brillouin scattering (SBS) applications and broadband frequency comb generation.

Prof. Gianluca Piazza believes that this technology will be extremely attractive for the development of new classes of miniaturized and integrated devices that address emerging challenges in wireless and optical communication, classical computing, artificial intelligence, and quantum information science.

Future work will focus on the 3D integration of vertical-cavity surface-emitting laser (VCSEL) and photodetectors onto the AO modulators and extending its operation to high frequency and bandwidth modulation for light beam steering.

Schematic diagram of the acousto-optic modulator. The split interdigital transducers, acoustic reflectors, and photonic waveguide devices are monolithically integrated on the thin film lithium niobate.