Professor, Department of Applied Physics and Physico-informatics, Faculty of Science and Technology, Keio University
Photonics polymers
—Fundamental principles led to breakthrough—
“White is not a color. It appears white because of the light scattering. The smaller the size of constituent particles of a substance is, the less light scatters. So, milk could be transparent,” says Prof. Koike. Light reflects and refracts with millimeter-sized substances, and it scatters with micrometer-sized substances. However, it does not scatter with nanometer-sized substances; therefore, nanometer-sized substances are transparent. Prof. Koike has overcome the difficulties in dealing with light by reviewing the fundamental principles of light.
In the early 80’s, plastics were thought to be unsuitable for optical fibers due to the high scattering loss. Prof. Koike proposed using plastic optical fibers for high-speed data transmission with rates in excess of 1 Gbps. In order to realize the high-speed data transmission, waveforms of light have to be constant through the whole length of the fiber, so, he proposed the concept of graded-index (GI) fiber as a fiber core structure, where a refractive index decreases with increasing radial distance from the center of the fiber. The idea was based on the fact that the light in the center of the fiber travels slower than light in the outer part; therefore, both lights reach the end of the fiber at the same time. However, the addition of impurities into the polymers is needed to change the refractive index. At the time, most of the researchers were focused on reducing the impurities in order to decrease scattering loss and did not accept his idea. “Scattering loss is caused not by the amount of impurities but the particle size of impurities. If the particle size of the impurities is reduced to about a nanometer, the particles do not scatter light, and thus, the fiber remains transparent,” says Prof. Koike. He tried to prove his theory using polymethyl methacrylate (PMMA), which has the highest transparency. However, the experimental value of excess light scattering was 10 times higher than the theoretical value, and therefore, his research did not go well.
Prof. Koike, who had given up once on plastic optical fiber research, decided to start all over with defining “what is scattering”. He studied Einstein’s fluctuation theory of light scattering and the theory involving the structures of polymers and light scattering, which Debye published in 1947. After fabricating PMMA without coarse impurity particles, Prof. Koike researched and determined the cause of excess light scattering in 1992. He verified that “stereoregularity of PMMA”, “molecular weight”, “remaining low-molecular-weight impurities such as additives and monomers” and “bridging by gel effects” were not the true causes of excess light scattering. He theoretically and experimentally determined that voids formed during polymerization below the secondary phase transition temperature caused excess light scattering. The voids can be eliminated by heat treatment at high temperatures above the secondary phase transition temperature. He says, “It was good that my research did not go well. If it went well, I would not have gone over the basic question, i.e. what is light scattering. The essentials of a true breakthrough cannot be found in superficial research but in fundamental principles.” It took him 10 years to finally develop Graded Index plastic optical fibers by determining the nature of excess light scattering and fabricating PMMA with high transparency. In 1996, he developed perfluorinated fibers that reduce absorption loss by replacing hydrogen with fluorine in the polymers. In 2000, high-speed data transmission plastic optical fibers, which surpassed glass fibers, were commercialized.
Prof. Koike explored the field of “photonics polymer”, clarified interaction between polymers and lights, and developed various materials, such as a zero-birefringence optical polymer, which drastically reduces the manufacturing cost of liquid crystal display (LCD), and a highly scattering optical transmission polymer, which makes LCD backlights twice brighter than the conventional ones. His aim is to develop a true broadband environment, not Fiber-to-the-Home, but Fiber-to-the-Display, with photonics polymers. In the Fiber-to-the-Display architecture, remote medical care can be realized because clear pictures can be sent to the display in real time. He says, “If a TV is connected to a hospital via optical fibers, it would be possible to access the hospital remotely just by pressing a button on the side of the TV whenever medical care is needed. If an elderly person gets sick in the middle of the night, do you think they can turn a computer on and type on the keyboard? Technology must be a useful tool to make our lives easier; it becomes useless if it is complicated to use even though it is advanced. The essence of technology is to make our lives secure and comfortable.”
