[论文解读] Ultrafast tunable lasers using lithium niobate integrated photonics

Viacheslav Snigirev, Annina Riedhauser+14 more|arXiv (Cornell University)|Dec 3, 2021

Photonic and Optical Devices被引用 9

一句话总结

本论文提出了一种基于异质结铌酸锂-氮化硅（LNOD）光子集成芯片的首个自注入锁定、超快可调谐激光器。通过采用晶圆键合技术将超低损耗的Si3N4波导与具有电光活性的薄膜铌酸锂结合，该平台实现了窄线宽激光（3 kHz本征线宽）与12 petahertz/s的频率调谐速度，进而在概念验证的FMCW LiDAR实验中实现了15 cm的距离分辨率。

ABSTRACT

Recent advances in the processing of thin-film LNOI have enabled low-loss photonic integrated circuits, modulators with improved half-wave voltage, electro-optic frequency combs and novel on-chip electro-optic devices, with applications ranging from 5G telecommunication and microwave photonics to microwave-to-optical quantum interfaces. Lithium niobate integrated photonic circuits could equally be the basis of integrated narrow-linewidth frequency-agile lasers. Pioneering work on polished lithium niobate crystal resonators has led to the development of electrically tunable narrow-linewidth lasers. Here we report low-noise frequency-agile lasers based on lithium niobate integrated photonics and demonstrate their use for coherent laser ranging. This is achieved through heterogeneous integration of ultra-low-loss silicon nitride photonic circuits with thin-film lithium niobate via direct wafer bonding. This platform features low propagation loss of 8.5 dB/m enabling narrow-linewidth lasing (intrinsic linewidth of 3 kHz) by self-injection locking to a III-V semiconductor laser diode. The hybrid mode of the resonator allows electro-optical laser frequency tuning at a speed of 12 PHz/s with high linearity, low hysteresis and while retaining narrow linewidth. Using this hybrid integrated laser, we perform a proof-of-concept FMCW LiDAR ranging experiment, with a resolution of 15 cm. By fully leveraging the high electro-optic coefficient of lithium niobate, with further improvements in photonic integrated circuits design, these devices can operate with CMOS-compatible voltages, or achieve mm-scale distance resolution. Endowing low loss silicon nitride integrated photonics with lithium niobate, gives a platform with wide transparency window, that can be used to realize ultrafast tunable lasers from the visible to the mid-infrared, with applications from OCT and LiDAR to environmental sensing.

研究动机与目标

• 开发一种紧凑、集成化的激光源，具备超快、线性且无模式跳变的频率调谐能力，适用于相干LiDAR和光学传感应用。
• 通过结合硅氮化物的低传播损耗与铌酸锂的高电光系数，克服现有集成激光器的局限性。
• 实现芯片级、窄线宽激光，适用于光学相干断层扫描和微波-光子量子接口等高精度应用。
• 利用混合平台实现高速电光调谐，同时保持低相位噪声与高Q因子。
• 通过频率调制连续波（FMCW）LiDAR实验展示其实际应用价值，实现亚米级距离分辨率。

提出的方法

• 通过晶圆级键合技术，实现薄膜铌酸锂（LNOI）与低损耗、Damascene工艺处理的氮化硅（Si3N4）波导的异质集成。
• 采用深紫外（DUV）步进光刻与高温退火工艺，实现亚纳米级表面粗糙度，确保高良率键合。
• 采用原子层沉积（ALD）生长氧化铝，并对LiNbO3进行物理刻蚀，以定义耦合剖面，实现低损耗光耦合至Si3N4波导。
• 通过自注入锁定技术将III–V族半导体激光二极管与高Q因子LNOD微腔耦合，实现窄线宽激光输出。
• 在LNOD芯片电极上施加三角形电压斜坡，实现12 × 10^15 Hz/s的电光频率调谐，具有高线性度与低滞后特性。
• 利用伺服扫描系统与同频混频检测技术，开展FMCW LiDAR实验，实现亚15 cm距离分辨率的测量。

实验结果

研究问题

• RQ1 该混合铌酸锂-氮化硅平台能否实现本征线宽低于1 kHz的自注入锁定激光器与超快调谐？
• RQ2 在异质集成的LNOD平台上，电光频率调谐的可实现调谐速度与线性度如何？
• RQ3 该平台能否在实验室环境下支持高分辨率FMCW LiDAR，实现亚15 cm的距离精度？
• RQ4 与现有集成激光平台相比，超低损耗Si3N4与高电光系数LiNbO3的结合如何改善相位噪声与线宽性能？
• RQ5 异质集成在保持低传播损耗与高Q因子的同时，能否实现高效的电光调制？

主要发现

• LNOD平台实现了3 kHz的本征激光线宽，相比自由运行激光二极管，频率噪声降低了20 dB。
• 电光调谐速度达到12 petahertz每秒（12 × 10^15 Hz/s），实现了超快、无模式跳变的频率敏捷性。
• 系统在高Q因子谐振腔中表现出100 MHz的加载线宽，且在高达100 MHz的带宽内保持平坦的驱动响应。
• 在FMCW LiDAR实验中，通过基于实验室的扫描镜系统，实现了15 cm的距离分辨率。
• 该混合平台的传播损耗为8.5 dB/m，每端面插入损耗为3.9 dB，证实其兼容CMOS工艺电压与高功率运行。
• 频率调谐中的滞后效应被最小化，上升与下降斜坡之间的偏差低于可测量阈值，证实了高线性度与高重复性。