The demand for optical networks has surged in recent years owing to the significant growth in global internet traffic. High-performance electro-optic (EO) modulators is one of the most crucial components of optical signals processing. They aid the conversion of electrical signals into optical signals and are more effective at low driving voltages and high speeds. While modulators have generally operated at longer communication wavelengths exceeding 1000-nm near-infrared range, there is a growing interest in photonic platforms operating at relatively shorter communication wavelengths.
Recently, chip-scale optical devices operating at shorter communication wavelengths below 1000 nm, such as light detection and ranging (LiDAR), have been developed for high-speed applications like bio-sensing, 3D cameras and autonomous driving. Most LiDARs use an optical phased array with an operating wavelength of 905 nm because fog-, snow- and rain-induced signal deterioration at wavelengths below 1000 nm is less than near-infrared wavelengths. Although Si modulators are compatible with Si photonic platforms, it has a limited transparency window at communication wavelengths below 1000 nm. Therefore, it is necessary to explore other materials for potential application at wavelengths below 1000 nm.
Lithium niobate (LN) has emerged as potential candidates for modulators in the visible-wavelength region. Modulators made from these materials can operate across a wide wavelength range based on EO effects. However, they exhibit undesirable properties, such as small changes in the refractive index, difficulties in fine processing and inadequate data rates. Additionally, most modulators exhibit high half-wave voltage-length product (VπL), low VπL has not been achieved at shorter wavelengths below 1000 nm.
Herein, Dr. Shun Kamada, Ms. Rieko Ueda, Ms. Chiyumi Yamada, Mr. Kouichi Tanaka, Dr. Toshiki Yamada and Dr. Akira Otomo from the National Institute of Information and Communications Technology in Japan developed a superior EO-polymer modulator operating at wavelengths below 1000 nm for high-speed optical communication. Briefly, they commenced their research work by identifying EO polymers that can be utilized in the visible region. Thus, a novel side-chain polymer consisting of an aminobenzene donor and tricyanofuran (TCF) acceptor connected via a single bond was synthesized. Their work is currently published in the journal, Optics Express.
The research team demonstrated the working and capability of the newly developed EO-polymer modulator. The resulting EO chromophore exhibited remarkable properties in absorption peak wavelength, hyperpolarizability, larger static dipole moment, and narrow absorption band characterized by a significant decrease in the longer wavelength region. As a result, the proposed modulator featured a higher modulation efficiency with low VπL of 0.52 V·cm, excellent thermal stability, high glass transition temperature of 164 °C, and low optical absorption loss of 2.6 dB/cm at a wavelength of 640 nm.
Despite being highly efficient, this modulator can also enable efficient light manipulation and ultra-high-speed data communication for optical devices operating in wavelength ranges of visible to below 1000 nm near-infrared. For example, LiDAR used for autonomous driving and bio-sensing, operate at wavelengths of 905 nm and 785 nm, respectively. Furthermore, it is worth noting that photonic platforms in the visible wavelength are required for neural probes utilized in some applications.
In a nutshell, the authors are the first to experimentally report the lowest VπL modulator-based EO polymer operating at wavelength below 1000 nm. Unlike inorganic materials, the molecular structure of EO polymers can be modified to customize the transparency window across the visible-near-infrared region. In a statement to Advances in Engineering, the authors explained that the newly presented EO-polymer modulator will pave the way for developing high-performance photonic devices in the visible wavelength region.
Kamada, S., Ueda, R., Yamada, C., Tanaka, K., Yamada, T., & Otomo, A. (2022). Superiorly low half-wave voltage electro-optic polymer modulator for visible photonics. Optics Express, 30(11), 19771-19780.