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---- What are the advantages of photonic crystal fiber over conventional optical fiber?

Dr. Koshiba: First, it is resistant to bending because of the holes in the clad layer. The bending loss of conventional optical fiber increases dramatically when the bending radius becomes 20 millimeters or smaller. This radius is therefore limited to approximately 30 millimeters at the time of wiring. Conversely, photonic crystal fiber has minimal bending loss and can be bent almost at a right angle (Fig. 1 is a graph showing the extremely low bending loss), thus making high-density wiring or indoor extension / wiring easy.

Fig. 1
Fig. 1

Second, the wavelength range that can be transmitted on a single mode is much broader than that of the conventional type. While conventional fibers use invisible light in a near-infrared region with wavelengths of 1.3 to 1.7 micrometers, photonic crystal fibers can use light of up to 0.4 micrometers, which is in the range of visible light.  The availability of a broader band is exactly what leads to the efficient use of frequency resources. These fibers also have other advantages (such as the ability to flexibly control nonlinearity and realization of a high polarization-maintaining property), and steady and precise processing is also possible as they are made of high-quality glass.

Such properties also make photonic crystal optical circuits greatly advantageous.  In the optical fiber systems currently in use, light of different wavelengths is multiplexed and transmitted to one optical fiber by wavelength division multiplexing as mentioned at the beginning. When the number of multiplexed wavelengths becomes too large, however, the size of the optical multiplexer/demultiplexer that multiplexes/demultiplexes light at the entrance and exit of transmission may become too big.  Photonic crystal optical circuits are thought to solve this problem. Since light can be transmitted without loss using photonic crystals even if its path is bent at a right angle, it is possible to form an optical circuit similar in size to a conventional integrated circuit.  It is considered possible to use this as a platform for future ultra-high integrated optical circuits.

In the tenth year since the creation of photonic crystal fibers, now is the time to expand its application to technologies that benefit society.

---- Photonic crystal technology is expected to play a leading role in the next generation of optical networks. What kind of development is expected in terms of commercialization and practical application for the technology?

Masanori Koshiba, Doctor of Engineering

Dr. Koshiba: It has been almost 15 years since the emergence of the concept of photonic crystals, and 10 years since the creation of the first photonic crystal fiber. The development of optical communications technology is a national policy, and basic research in the field of nanophotonics has been led by industry, educational institutions and the administration. It is now time to produce products that can be used in the real world. NTT is already putting holey fibers based on total internal reflection into practical application and is preparing to commence an access-system service. Our laboratory is also shifting to a research phase for practical application of photonic crystal fibers. Besides the fibers, we are also engaged in research and development of microminiature photonic crystal optical circuits that can withstand wavelength division multiplexing. The possibility of application for photonic crystal fibers is also expanding outside the field of communications. We hope to establish it as a technology to contribute to society while keeping the expansion of its fields of application in mind.

(2006/03/31)

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