Introduction
Integrated electro-optic modulators (EOMs) play a crucial role in modern photonic systems, enabling high-speed data transmission, waveform generation, and quantum light manipulation. However, achieving high performance across multiple metrics such as bandwidth, drive voltage, optical power handling, and low loss has proven challenging in traditional integrated photonics platforms. This tutorial explores the hybrid integration of thin-film lithium niobate (TFLN) with silicon and silicon nitride photonics, enabling high-performance EOMs to be seamlessly incorporated into existing photonic circuits.
The Need for High-Performance EOMs
Silicon photonics has emerged as a scalable and low-cost integrated photonics technology, but its native modulators suffer from limitations. Silicon modulators cannot achieve high-contrast modulation with bandwidths exceeding 50-60 GHz, and they exhibit amplitude-phase coupling (chirp) effects. Low-voltage silicon microresonator modulators cannot handle optical powers above a few milliwatts without nonlinear impairments. Furthermore, silicon modulators do not operate at wavelengths below 1 μm.
On the other hand, silicon nitride and other short-wavelength photonic platforms have not demonstrated high-bandwidth and low-voltage performance. Overcoming these limitations requires the integration of materials with strong electro-optic properties, such as lithium niobate (LN).
Hybrid TFLN Integration
The hybrid integration approach developed by Mookherjea's group at UC San Diego involves bonding a thin film of lithium niobate (TFLN) onto silicon or silicon nitride photonic circuits. Crucially, the TFLN layer remains unetched, preserving its excellent electro-optic properties. The light is guided in hybrid modes, distributed between the TFLN region and the underlying silicon or nitride waveguides. The waveguide width is varied to controllably push and pull light into the TFLN region, enabling efficient electro-optic modulation.
Outside the bonded TFLN regions, compact bends, couplers, and other devices can be implemented using standard silicon or nitride components from process design kits (PDKs). This approach leverages the scalability and maturity of silicon photonics while integrating the high-performance electro-optic capabilities of TFLN.
The fabrication process involves constructing the silicon or silicon nitride photonic features at a foundry, followed by a low-temperature direct bonding process to incorporate the TFLN layer over the desired regions. Traveling-wave electrodes are then formed to enable high-speed modulation.
Figure 1a shows the cross-section of the different layers, while Figure 1b illustrates a typical Mach-Zehnder modulator structure with a hybrid TFLN region and silicon nitride waveguides. The patterned electrode structure ensures efficient RF-optical mode index matching over a large modulation bandwidth. Figure 1c provides a photograph of a microchip with hybrid TFLN/silicon nitride modulators.
Classical Photonics Applications
The hybrid TFLN integration approach has enabled the demonstration of high-performance EOMs across multiple wavelength bands, including 1.3 μm, 1.55 μm (traditional fiber-communication wavelengths), and 780 nm. These modulators exhibit 3-dB modulation bandwidths exceeding 100 GHz and half-wave voltages (Vπ) of approximately 2-3 V at 1.3 μm and 1.55 μm, and 1 V at 780 nm.
At 1550 nm, optical powers above 100 mW can be modulated at these high radio frequencies (RF). Both aluminum (Al) and gold (Au) electrodes have been designed, with the former capable of being buried under the bonded layer for improved performance.
These high-performance integrated EOMs surpass traditional silicon photonic modulators in terms of bandwidth and drive voltage, enabling higher data rates, increased efficiency, and improved analog link performance. At shorter wavelengths where silicon modulators cannot operate, these devices can generate high-bandwidth waveforms useful for sensing, spectroscopy, and quantum optics applications.
Quantum Photonics Applications
In addition to classical photonics applications, the hybrid TFLN integration approach has enabled advancements in quantum photonics. In a recent work, a monolithic silicon photonic microchip was developed to improve the purity of room-temperature photon-pair generation at 1550 nm.
The microchip features seamless waveguide interconnections between hybrid TFLN/silicon photonic EOMs for pump-pulse preparation and high-Q silicon microresonators for efficient parametric nonlinearity used for biphoton generation. By precisely controlling the joint spectrum of the biphoton generation process, near-optimal purity was achieved on an integrated microchip for the first time, without the need for externally generated short pump pulses from mode-locked lasers.
Conclusion
The hybrid integration of thin-film lithium niobate and silicon photonics has emerged as a powerful approach for incorporating high-performance electro-optic modulators into scalable photonic circuits. By leveraging the strengths of both platforms, this technology enables advancements in classical and quantum photonics, paving the way for more efficient and capable integrated photonic systems across a wide range of applications.
Reference
[2] S. Mookherjea, "Hybrid Integration of Thin-film Lithium Niobate and Silicon Photonics," Department of Electrical & Computer Engineering, University of California, San Diego, La Jolla, CA 92093-0407, USA, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.