Researchers at EPFL, led by Professor Tobias J. Kippenberg and Professor Xin Ou at the Shanghai Institute of Microsystem and Information Technology, have developed a new, cost-effective photonic integrated circuit (PIC) based on lithium tantalate. This material has similar electro-optic qualities to lithium niobate, but is more scalable and cost-effective. The new PIC can make high-quality PICs more economically viable, revolutionizing optical communications and computing systems. The team’s method is compatible with existing silicon-on-insulator production lines, and the resulting PICs have an optical loss rate of just 5.6 dB/m at telecom wavelength. The research is published in Nature.
Advancements in Photonic Integrated Circuits
Photonic Integrated Circuits (PICs), which amalgamate multiple optical devices and functionalities on a single chip, have been instrumental in transforming optical communications and computing systems. For many years, silicon-based PICs have been the standard due to their cost-effectiveness and compatibility with existing semiconductor manufacturing technologies. However, these silicon-based PICs have limitations, particularly in their electro-optical modulation bandwidth.
Despite these limitations, silicon-on-insulator optical transceiver chips have been successfully commercialized, facilitating information traffic through millions of glass fibers in modern datacenters. Recently, the lithium niobate-on-insulator wafer platform has emerged as a superior material for photonic integrated electro-optical modulators due to its strong Pockels coefficient, which is essential for high-speed optical modulation. However, high costs and complex production requirements have hindered the widespread adoption of lithium niobate, limiting its commercial integration.
Lithium Tantalate: A Promising Alternative
Lithium tantalate (LiTaO3), a close relative of lithium niobate, offers a promising alternative. It possesses similar excellent electro-optic qualities but has an advantage over lithium niobate in terms of scalability and cost. Lithium tantalate is already being widely used in 5G radiofrequency filters by telecom industries, indicating its potential for broader commercial applications.
A team of scientists led by Professor Tobias J. Kippenberg at EPFL and Professor Xin Ou at the Shanghai Institute of Microsystem and Information Technology (SIMIT) have developed a new PIC platform based on lithium tantalate. This PIC leverages the material’s inherent advantages and has the potential to make high-quality PICs more economically viable.
Innovative Fabrication Method
The researchers developed a wafer-bonding method for lithium tantalate that is compatible with silicon-on-insulator production lines. They then masked the thin-film lithium tantalate wafer with diamond-like carbon and proceeded to etch optical waveguides, modulators, and ultra-high quality factor microresonators.
The etching was achieved by combining deep ultraviolet (DUV) photolithography and dry-etching techniques, initially developed for lithium niobate and then carefully adapted to etch the harder and more inert lithium tantalate. This adaptation involved optimizing the etch parameters to minimize optical losses, a crucial factor in achieving high performance in photonic circuits.
High-Efficiency Lithium Tantalate PICs
With this approach, the team was able to fabricate high-efficiency lithium tantalate PICs with an optical loss rate of just 5.6 dB/m at telecom wavelength. Another highlight is the electro-optic Mach-Zehnder modulator (MZM), a device widely used in today’s high-speed optical fiber communication. The lithium tantalate MZM offers a half-wave voltage-length product of 1.9 V cm and an electro-optical bandwidth reaching 40 GHz.
The lithium tantalate PIC’s reduced birefringence (the dependence of refractive index on light polarization and propagation direction) allows dense circuit configurations and ensures broad operational capabilities across all telecommunication bands. This work paves the way for scalable, cost-effective manufacturing of advanced electro-optical PICs.
Potential Applications and Future Directions
The team also generated soliton microcomb on this platform. These soliton microcombs feature a large number of coherent frequencies and, when combined with electro-optic modulation capabilities, are particularly suitable for applications such as parallel coherent LiDAR and photonic computing.
The development of lithium tantalate PICs marks a significant advancement in optical technologies with potential for widespread commercial applications. The work, funded by the Swiss National Science Foundation (SNSF) and the European Research Council (ERC), is published in the journal Nature.
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