Hybrid Silicon Photonic-Lithium Niobate Electro-Optic Mach-Zehnder Modulator Beyond 100 GHz Bandwidth (1803.10365v2)

Abstract: Electro-optic modulation, the imprinting of a radio-frequency (RF) waveform on an optical carrier, is one of the most important photonics functions, being crucial for high-bandwidth signal generation, optical switching, waveform shaping, data communications, ultrafast measurements, sampling, timing and ranging, and RF photonics. Although silicon (Si) photonic electro-optic modulators (EOMs) can be fabricated using wafer-scale technology compatible with the semiconductor industry, such devices do not exceed an electrical 3-dB bandwidth of about 50 GHz, whereas many applications require higher RF frequencies. Bulk Lithium Niobate (LN) and etched LN modulators can scale to higher bandwidths, but are not integrated with the Si photonics fabrication process adopted widely over the last decade. As an alternative, an ultra-high-bandwidth Mach-Zehnder EOM based on Si photonics is shown, made using conventional lithography and wafer-scale fabrication, bonded to an unpatterned LN thin film. This hybrid LN-Si MZM achieves beyond 100 GHz 3-dB electrical bandwidth. Our design integrates silicon photonics light input/output and optical components, including directional couplers, low-radius bends, and path-length difference segments, realized in a foundry Si photonics process. The use of a simple low-temperature (200C) back-end integration process to bond a postage-stamp-sized piece of LN where desired, and achieving light routing into and out of LN to harness its electro-optic property without any etching or patterning of the LN film, may be broadly-useful strategies for advanced integrated opto-electronic microchips.

High-Frequency Electro-Optic Modulators: A Hybrid Approach Using Lithium Niobate and Silicon Photonics

The paper presents a significant advancement in the field of electro-optic modulation (EOM) by integrating lithium niobate (LN) thin films onto a silicon photonics platform. This integration achieves an electrical 3-dB bandwidth exceeding 100 GHz, a notable enhancement over traditional silicon photonic EOMs, which typically cap at around 50 GHz. This hybrid Mach-Zehnder Modulator (MZM) design leverages wafer-scale fabrication, conventional lithography, and low-temperature bonding techniques to combine the strengths of LN, a well-established material for high-speed modulation, with the scalable silicon photonics technology.

The authors address the limitations of standalone LN modulators, characterized by expensive and labor-intensive traditional fabrication processes incompatible with modern silicon photonics. By employing an unpatterned LN thin film in conjunction with silicon photonic components such as directional couplers and waveguides, the researchers have circumvented the intricate etching and patterning requirements normally associated with LN-based devices. This methodological innovation potentially streamlines the integration of high-bandwidth modulators with other silicon photonic components, essential for advanced optoelectronic microchip applications.

Technically, the presented MZM utilizes a silicon photonic platform with 220 nm silicon-on-insulator wafers. Post-processing and precise lithography yield highly accurate optical pathways determined by silicon waveguide features. Importantly, this approach does not necessitate LN film patterning or etching, simplifying the bonding process and ensuring consistent optical propagation paths. Notably, the article details the usage of coplanar waveguide electrodes, a strategic choice to mitigate piezoelectric resonance issues inherent in conventional LN substrates.

The hybrid modulators demonstrated advantageous electro-optic features. The device showcased an estimated optical propagation loss of -0.6 dB/cm in the hybrid region and maintained a low inter-layer transition loss, contributing to its effectiveness in high-frequency applications. RF measurements evidenced a greater-than-100 GHz bandwidth, corroborated by both theoretical simulations and experimental data. The modulator's performance was characterized using a modulation index derived from the optical spectrum, affirming the stability and reliability of the hybrid design over a wide frequency range.

From a fabrication standpoint, the methodological choice to omit etching or sawing LN and instead rely on silicon photonics features underscores a practical, cost-effective production protocol. This approach does not compromise the considerable bandwidth, enabling potential applications in various high-frequency domains, such as digital communications and RF photonic systems.

In terms of implications, the marriage of lithium niobate's electro-optic modulating capabilities with the scalability and integration potential of silicon photonics presents a noteworthy advancement for integrated photonic systems. The device could pioneer new applications in analog-to-digital conversion and millimeter-wave instrumentation, though further exploration into optimizing the voltage-length product (V L) might improve efficiency and reduce operational power requirements.

Moving forward, this research sets a foundation for the exploration of hybrid materials in EOM technology. It underscores the potential for expanded adoption of LN-based modulators within silicon photonics platforms, promising enhancements in bandwidth without the cumbersome requirements of traditional approaches. Future developments could look toward incorporating more complex optoelectronic components in this hybrid system, exploring miniaturization while sustaining performance, ultimately advancing the integration of photonics and electronics in compact, multifunctional microchips.