New, low-cost, and high-efficiency photonic integrated circuits

The rapid development of photonic integrated circuits (PICs) has combined multiple optical devices and functions on a single chip, completely changing optical communication and computing systems.

For decades, silicon-based PICs have dominated the field due to their cost-effectiveness and integration with existing semiconductor manufacturing technologies, despite their limitations in electro-optic modulation bandwidth. Nevertheless, silicon optical transceiver chips on insulators have been successfully commercialized, driving information flow through millions of glass fibers in modern data centers.

Recently, the lithium niobate wafer platform on insulators has become a high-quality material for photonic integrated electro-optic modulators due to its strong Pockels coefficient, which is crucial for high-speed optical modulation. However, high costs and complex production requirements have hindered the wider adoption of lithium niobate, limiting its commercial integration.

Lithium tantalate (LiTaO 3) is a close relative of lithium niobate and has the potential to overcome these obstacles. It has similar excellent electro-optical quality, but has advantages in scalability and cost compared to lithium niobate, as it has been widely used in 5G RF filters in the telecommunications industry.

Now, scientists led by Professor Tobias J. Kippenberg from the Federal Institute of Technology in Lausanne and Professor Ou Xin from the Shanghai Institute of Microsystems and Information Technology (SIMIT) have created a new type of PIC platform based on lithium tantalate. PIC utilizes the inherent advantages of materials to make high-quality PIC more economically feasible, thereby changing the field. This breakthrough was published in the journal Nature.

Researchers have developed a lithium tantalate wafer bonding method that is compatible with silicon on insulator production lines. Then, they covered the thin film lithium tantalate chip with diamond-like carbon and continued to etch the optical waveguide, modulator, and ultra-high quality factor microresonator.

Etching is achieved by combining deep ultraviolet (DUV) lithography with dry etching technology, which was originally developed for lithium niobate and then carefully adjusted to etch harder and more inert lithium tantalate. This adjustment involves optimizing etching parameters to minimize optical losses, which is a key factor in achieving high-performance photonic circuits.

Through this method, the team was able to manufacture efficient lithium tantalate PIC with an optical loss rate of only 5.6 dB/m at telecommunication wavelengths. Another highlight is the electro-optic Mach Zehnder modulator (MZM), which is a widely used device in high-speed fiber optic communication today. The half wave voltage length product of lithium tantalate MZM is 1.9 V cm, and the electro-optic bandwidth reaches 40 GHz.
"While maintaining efficient electro-optical performance, we have also generated soliton micro combs on this platform," said Chengli Wang, the first author of the study. "These soliton micro combs have a large number of coherent frequencies, making them particularly suitable for applications such as parallel coherent lidar and photon computing when combined with electro-optical modulation functions."

The birefringence (dependence of refractive index on optical polarization and propagation direction) of lithium tantalate PIC is reduced, enabling dense circuit configurations and ensuring broad operational capabilities in all telecommunications frequency bands. This work paves the way for the scalable, cost-effective manufacturing of advanced optoelectronic PICs.

Source: Laser Net