The National Institute of Information and Communications Technology developed a highly efficient optical modator that uses organic electro-optic polymer (hereinafter referred as EO polymer) to transmit visible light. Near-infrared light could be used by conventional EO polymer optical modators (wavelength 1,550 nm), etc. They can’t be used in visible light (wavelengths 380 nm-780 nm) due to their high absorption loss. NICT developed an EO-polymer that has a low absorption loss in visible sunlight and the high electro-optic coefficient needed for the optical modulator. This EO polymer was used to fabricate an optical modulator. The optical modulator was operated in visible light (red wavelength 640 nm). The optical modulator shown for visible light is smaller than the conventional EO-polymer optical modulator.

The EO polymer optical modulator of visible light will be used in next-generation display devices, such as stereoscopic displays or smart glasses. Optics Express published the research.

High-performance optical networks are in high demand due to increased internet traffic. NICT is currently developing a low-voltage, high-speed optical modulator with EO polymer. Key devices in telecommunications are optical modulators, which convert electrical signals into optical signals. EO polymer optical modators are not compatible with conventional lithium niobate modulators. They can only be used with near infrared light to optical communications.

This research resulted in the development of an EO polymer that has high electro-optic coefficient and low absorption. This was possible thanks to NICT’s precise measurement technology and molecular designs based on a vast molecular structure database that has been accumulated over many decades. This EO polymer, which has less than 1/20,000 loss per unit of visible light than conventional EO polymers, was designed to be both short and rigid in order to reduce absorption.

NICT designed an interferometer structure for Mach-Zehnder using a microfabrication process. Waveguide sizes for operation at visible light are smaller than those for near-infrared. A ridge-type waveguide has been adopted, which guarantees one mode, even though the waveguide’s width is large. This allows for high accuracy processing, but also allows for a greater fabrication tolerance.

The developed optical modulator was given an electric signal, and the operation of the modulation was tested. The figure of merit for optical modulators was 0.52 Vcm at wavelength 640nm (red). This figure is less than a third of the traditional figure, which indicates extreme efficiency (small size, low driving voltage). The operating wavelength is also significantly shorter than it was before.

Future prospects

This groundbreaking research result can lead to innovations in light beam control technology and display technology. An optical phased array can be made from the optical modulator that can control the propagation light’s phase. An optical phased array can be used for stereoscopic displays. Next-generation smart glasses will feature compact and efficient display devices. A low-cost, silicon-based photodetector with visible light can also be used, which reduces the overall system’s cost.

Kamada Shun stated, “I will continue to encourage research and development for the optical phased array (next-generation display)”. We also plan to develop EO polymers in green and blue and expand their application to full-color stereoscopic displays.