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Net Journal 2

A nano-sized periodic structure to control the refractive index of light

---- What is a photonic crystal?

Masanori Koshiba, Doctor of Engineering

Dr. Koshiba: A number of examples of photonic crystals are found in daily life. One instance is the beautiful colors of butterfly wings (Photo 1) and opals. These are the result of a phenomenon known as Bragg reflection, which involves the strong diffraction or reflection of light of a certain wavelength in a specific direction. Such substances with periodic structures that reflect and transmit only light of specific wavelengths are called photonic crystals. While those with one-dimensional periodic structures have already been put to practical use in applications such as dielectric multilayer filters, we are now studying photonic crystals with two- or three-dimensional periodic structures. Using these will make it possible to create new optical fibers and circuits. Particular advances are being made in the research and development of photonic crystal fibers as a technology close to practical application.

butterfly wings
Photo 1

A conventional optical fiber is a glass filament of approximately 125 micrometers (1 micrometer is 1/1,000 millimeter) in diameter. It has a central core through which light can pass, and a cladding layer with a different refractive index surrounds it. While a photonic crystal fiber is basically the same, multiple holes are positioned systematically in the clad region. Since these holes have periodic structures based on the wavelengths of light, the refractive index of light can be controlled by appropriately adjusting the size and arrangement of the holes. There are two types of photonic crystal fibers - the total reflection type with a glass core and a band gap type with a hollow core (Photo 2), which feature many properties not found in conventional fibers.

photonic crystal fiber
Photo 2

Photonic crystal fibers with complete control over the loss and dispersion of light

---- What specific properties does it have?

Dr. Koshiba: The most remarkable property is that it can minimize the loss of light and freely control the dispersion that hinders high-speed transmission. It can let through light of wavelengths that cannot be transmitted with an ordinary optical fiber, and almost entirely eliminates loss caused by bending. The bandgap type has a hollow core, and the refractive index of light becomes smaller in the air it contains. Although light should not normally be trapped in the core section because of its property of becoming trapped in places with a large refractive index, it stays inside on the principle of Bragg reflection since the surrounding cladding layer serves as a photonic crystal. Also, since light loss in above-ground environments is at its smallest in air, light is transmitted through the core with almost no loss. In other words, photonic bandgap fiber may represent the ultimate low-loss transmission line that mankind can achieve.

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