The rapid progress in photonic integrated circuits (PICs), which integrate various optical devices and functions onto a single chip, has transformed optical communication and computing technologies.
For many years, silicon-based PICs have been the dominant technology due to their cost-effectiveness and compatibility with existing semiconductor manufacturing processes, despite their limitations in electro-optical modulation bandwidth. However, silicon-on-insulator optical transceiver chips have been successfully commercialized, facilitating data transmission through millions of optical fibers in modern data centers.
Recently, the lithium niobate-on-insulator wafer platform has emerged as a superior alternative for photonic integrated electro-optical modulators because of its high Pockels coefficient, crucial for fast optical modulation. Despite this, the high cost and complex production of lithium niobate have restricted its broader adoption.
Lithium tantalate (LiTaO3), a close relative of lithium niobate, offers similar excellent electro-optic properties but is more scalable and cost-effective. It is already widely used in 5G radiofrequency filters, making it a promising candidate for broader application.
Recently, researchers 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 new platform utilizes the material’s advantages, making high-quality PICs more affordable and accessible. Their findings are published in Nature.
The team created a wafer-bonding method for lithium tantalate that is compatible with silicon-on-insulator production lines. They then used diamond-like carbon to mask the thin-film lithium tantalate wafer and etched optical waveguides, modulators, and ultra-high quality factor micro resonators using a combination of deep ultraviolet (DUV) photolithography and dry-etching techniques. These techniques, initially developed for lithium niobate, were adapted to work with the harder and more inert lithium tantalate by optimizing etch parameters to reduce optical losses.
This approach allowed the fabrication of high-efficiency lithium tantalate PICs with an optical loss rate of just 5.6 dB/m at telecom wavelengths. Notably, the electro-optic Mach-Zehnder modulator (MZM), a device used in high-speed optical fiber communication, demonstrated a half-wave voltage-length product of 1.9 V cm and an electro-optical bandwidth of 40 GHz.
According to Chengli Wang, the study’s first author, “Along with maintaining high electro-optical performance, we also generated soliton microcombs on this platform. These microcombs, which consist of multiple coherent frequencies, combined with electro-optic modulation capabilities, are particularly suitable for parallel coherent LiDAR and photonic computing applications.”
The lithium tantalate PIC’s lower birefringence allows for compact circuit designs and broad operational capabilities across all telecommunications bands, paving the way for scalable, cost-effective manufacturing of advanced electro-optical PICs.