High-Performance Hybrid Silicon and Lithium Niobate Mach-Zehnder Modulators for 100 Gbit/s and Beyond (1807.10362v2)

Abstract: Optical modulators are at the heart of optical communication links. Ideally, they should feature low insertion loss, low drive voltage, large modulation bandwidth, high linearity, compact footprint and low manufacturing cost. Unfortunately, these criteria have only been achieved on separate occasions.Based on a Silicon and Lithium Niobate hybrid integration platform, we demonstrate Mach-Zehnder modulators that simultaneously fulfill these criteria. The presented device exhibits an insertion loss of 2.5 dB, voltage-length product of 2.2 Vcm, high linearity, electro-optic bandwidth of at least 70 GHz and modulation rates up to 112 Gbit/s. The high-performance modulator is realized by seamless integration of high-contrast waveguide based on Lithium Niobate - the most mature modulator material - with compact, low-loss silicon circuits. The hybrid platform demonstrated here allows for the combination of 'best-in-breed' active and passive components, opening up new avenues for enabling future high-speed, energy efficient and cost-effective optical communication networks.

• The paper introduces a novel hybrid integration of silicon and lithium niobate using BCB bonding and dry etching, achieving a reduced insertion loss of approximately 2.5 dB and a 2.2 V∙cm voltage-length product.
• The paper demonstrates outstanding modulation performance with an electro-optic bandwidth over 70 GHz and successful data transmission at 100 Gbit/s (OOK) and 112 Gbit/s (PAM-4) with low bit-error rates.
• The paper underscores the scalability and cost-effectiveness of CMOS-compatible fabrication, paving the way for advanced integrated microwave photonics in next-generation optical networks.

High-Performance Hybrid Silicon and Lithium Niobate Mach–Zehnder Modulators

The research delineates the development and characterization of high-performance hybrid silicon and lithium niobate (LN) Mach–Zehnder modulators (MZMs) that exhibit promising capabilities for future optical communication networks. Silicon photonics on the silicon-on-insulator (SOI) platform and lithium niobate have independently served the photonics industry due to their distinct advantages. This paper explores their synergistic integration to tackle persistent challenges in optical modulators, such as insertion loss, drive voltage, linearity, and electro-optic (EO) bandwidth, while remaining cost-effective.

Key Advancements in Device Design

Utilizing benzocyclobuten (BCB) adhesive bonding and LN dry etching, the proposed MZM structure incorporates vertical adiabatic couplers (VACs) that facilitate efficient optical power transfer between silicon waveguides and LN membranes. This layout achieves optimal overlap between optical modes and active materials, resulting in reduced on-chip insertion losses of approximately 2.5 dB and exceptional modulation performance. The coupling efficiency of the VACs exceeds 97%, demonstrating minimal loss during optical mode conversion.

Performance Metrics

The hybrid modulators achieve remarkable voltage-length products of 2.2 V∙cm, demonstrating improved modulation efficiency compared to conventional LN devices. The EO bandwidth surpasses 70 GHz, limited primarily by the capabilities of measuring equipment, with simulations indicating potential extensions beyond 100 GHz. In data transmission tests, the MZMs successfully demonstrated on-off keying (OOK) at rates of up to 100 Gbit/s and four-level pulse amplitude modulation (PAM-4) at 112 Gbit/s, with bit-error rates below industry-specified thresholds.

The devices' spurious free dynamic range (SFDR) tests showcase competitive linearity, with values reaching 99.6 dB Hz$^2/3$ at 1 GHz. These results stand in comparison with, and in some cases surpass, commercial LN MZMs, indicating the hybrid devices' commercial viability and robustness in high-frequency applications.

Implications and Future Directions

This research signifies a substantial step toward scalable, energy-efficient, high-performance modulation for optical networks. The compatibility of the hybrid Si/LN platform with existing CMOS fabrication processes enables potential cost reductions and seamless integration into existing silicon photonic systems. The modulator's architecture may accommodate advances in integrated microwave photonics (MWP), offering options for sophisticated modulation techniques necessary in diverse applications ranging from telecommunications to quantum computing.

Furthermore, the adaptation of this hybrid design could yield more efficient electro-optic modulators, with future iterations potentially exploring reduced energy consumption scenarios or improved bandwidth capabilities. The hybrid integration method presented offers a foundation for future investigations into customizable optical circuits at terabit-per-second scales, alongside possible inclusion of other nonlinear materials to enhance photonic performance.

In conclusion, the work contributes a pivotal framework to future developments in photonics, positioning the silicon/lithium niobate integration as a strategically advantageous option for next-generation optical modulators focused on maximizing performance metrics while ensuring cost-effectiveness and adaptability to mass production processes.