PICs (photonic integrated circuits) have revolutionized optical communication and computing systems by integrating multiple optical components and functions on a single chip.
For decades, silicon-based PICs have dominated the field, thanks in large part to their cost-effectiveness and ability to integrate seamlessly with existing semiconductor fabrication technologies. Despite some limitations in electro-optical modulation bandwidth, optical transceiver chips on silicon optical insulators have been successfully commercialized, and they transmit massive amounts of information traffic over millions of optical fibers in modern data centers.
In recent years, photonic integrated circuit (PIC) electro-optical modulators based on lithium niobate on insulator wafer platforms have risen to prominence due to their remarkable Pukes effect coefficients, a property that is extremely critical for enabling high-speed optical signal modulation. Nevertheless, the high cost and complex production process of lithium niobate materials have limited its application in a wider range of fields and hindered its further integration at the commercial level.
Tantalum niobate (LiTaO3), a close relative of lithium niobate, shows potential to break through these barriers. Not only does it possess excellent electro-optical properties comparable to lithium niobate, but it also has advantages in terms of cost and large-scale production, largely thanks to the fact that the telecom industry has already made extensive use of it in 5G RF filters.
Currently, a research team led by Prof. Xin Ouyang, a professor at the Swiss Federal Institute of Technology in Lausanne (ETH) and the Shanghai Institute of Microsystems and Information Technology (SIIT) of the Chinese Academy of Sciences (CAS), has developed a new type of PIC platform based on tantalum niobate. This platform not only fully utilizes the intrinsic advantages of tantalum niobate material, but also has the potential to lead an industry-wide change by improving the economics of high-quality PICs.
The research team developed a wafer bonding technology for tantalum niobate that is compatible with existing silicon-on-insulator production lines. They then masked the thin-film tantalum niobate wafers using a diamond-like carbon material and proceeded to etch out optical waveguides, modulators, and microresonators with very highly quality factors.
This etching process was realized through a combination of deep-ultraviolet (DUV) lithography and dry etching techniques that were originally developed for lithium niobate materials and then carefully adapted to accommodate the harder, more chemically stable tantalum niobate material. During the adaptation process, the researchers optimized the etching parameters to minimize optical losses, which are critical for achieving high performance in photonic circuits.
With this approach, the team succeeded in fabricating high-efficiency tantalum niobate photonic integrated circuits (PICs) with an optical loss of only 5.6 dB/m in the telecom band. Another noteworthy achievement is the electro-optical Mach-Zönder modulator (MZM), a device widely used in today's high-speed fiber-optic communications. Made from tantalum niobate, the MZM has a half-wave voltage length product of 1.9 volt-centimeters and an electro-optical bandwidth of up to 40 gigahertz.
"While maintaining the highly efficient electro-optical performance, we also succeeded in generating soliton microcombs on this platform," the researcher said. "These soliton microcombs have numerous coherent frequencies and are particularly suitable for applications such as parallel coherent laser radar (LiDAR) and photonic computing when they are combined with electro-optical modulation techniques."
Tantalum niobate photonic integrated circuits (PICs) reduce birefringence, the property of refractive index to vary with the direction of polarization and propagation of light, which allows for more compact circuit designs and ensures a wide range of operational capabilities across all telecom bands. This research opens up new avenues for realizing large-scale and cost-effective production of advanced electro-optical PICs.
