Scalable, High-Speed Photonics with TFLN Modulators

Integrated photonics is poised to revolutionize a wide range of applications, from high-speed optical interconnects and data centers to advanced signal processing and computing. A crucial component in many photonic integrated circuits (PICs) is the electro-optic modulator (EOM), particularly thin-film lithium niobate (TFLN) modulators, which convert electrical signals into optical signals.

Lithium niobate (LN) has long been the material of choice for EOMs due to its excellent electro-optic properties, broad optical transparency window, and high-speed operation capabilities. However, integrating LN with other photonic materials, such as silicon (Si) or silicon nitride (SiN), in a scalable and cost-effective manner has been a persistent challenge. Traditional LN EOM devices require specialized fabrication methods and materials that differ from those used in the global silicon microelectronics industry.

In a recent publication, researchers from the University of California, San Diego, and LIGENTEC SA have made a significant breakthrough in this area. Their paper, titled “High-Performance Hybrid Lithium Niobate Electro-Optic Modulators Integrated with Low-Loss Silicon Nitride Waveguides on a Wafer-Scale Silicon Photonics Platform,” details the successful fabrication of high-performance hybrid LN EOMs integrated with low-loss SiN waveguides using a wafer-scale process.

Heterogeneous Integration

The key innovation of this research lies in the adoption of a “hybrid-mode” design. In this approach, the optical mode—how light is guided—is distributed between a thin LN film and a SiN waveguide. The SiN waveguides serve as a passive photonic layer, while the LN layer provides electro-optic modulation.

The fabrication process, developed by the authors, leverages LIGENTEC’s longstanding expertise in SiN photonics. It begins with a silicon wafer onto which two SiN layers are deposited. A thin film of LN is then bonded to the wafer. A crucial step in the process is the formation of air trenches in the silicon substrate beneath the active region of the modulator. This reduces radio frequency (RF) losses and enhances the device’s high-frequency performance.

This heterogeneous integration strategy offers several advantages:

• Scalable and Cost-Effective: The process is compatible with industrialised silicon wafer fabrication techniques, enabling wafer-scale production and reducing costs.
  
• High Performance: This platform offers standardised, reproducible and robust PDK components which are validated over hundreds of fabrication runs. Combining this with LN’s excellent electro-optic properties provides a high performance platform.
  
• Design Flexibility: SiN waveguides enable the integration of additional passive photonic components on the same chip.

Performance Analysis

The fabricated devices exhibited impressive performance metrics, summarized in the table below:

The researchers tested three different modulator designs, varying the electrode configuration and the presence of air trenches in the substrate. The design, incorporating air trenches and slow-wave electrodes, demonstrated the best overall performance and achieved high bandwidth.

Comparison with Previous Work

The results presented in this study build upon the previous work on heterogeneous LN modulators and represent a significant advancement. This research demonstrates an industrialised, scalable and robust fabrication process while also achieving high bandwidth, low loss and efficient modulation. 

Implications and Future Directions

This research paves the way for the development of advanced photonic integrated circuits for a wide range of applications. The ability to integrate high-performance LN modulators with a robust and industrialized SiN platform unlocks new possibilities for:

• High-speed optical communication systems
  
• Next-generation data centers
  
• Microwave photonics and signal processing
  
• Quantum information processing
  

The authors also suggest future research directions, including further optimization of the device design and fabrication process to enhance performance and integration complexity.

Author Spotlight

This groundbreaking work was made possible by the collaborative efforts of researchers at the University of California, San Diego, and LIGENTEC SA. The LIGENTEC team—including Forrest Valdez, Viphretuo Mere, Camiel Op de Beeck, and Pieter Wuytens—played a crucial role in designing and fabricating the SiN waveguide components, leveraging LIGENTEC’s state-of-the-art SiN photonics platform. Their expertise in low-loss waveguide technology was instrumental in achieving the high performance demonstrated in this publication.

Learn More

For a deeper dive into the research and findings, read the full paper on arXiv.