4. Photonic crystal fibers and optical fibers with new functionality

Toward new areas of optical communication

The transmission wavelength of optical fibers has shifted from 800 nm to longer wavelengths such as 1.3  and 1.5 mm. This is because silica fibers have their minimum loss around 1.5 mm. As a result, efforts have been made to improve fiber structures to construct long distance communication systems in this regime such as the development of dispersion shifted fibers with zero dispersion at 1.5 mm. However the transmission capacity in this wavelength regime is becoming saturated. A shorter wavelength regime is potentially attractive since higher frequency carrier waves can in principle accommodate a larger capacity, however conventional fibers cannot move the zero dispersion wavelength to below 1.3 mm.

Recently, a novel type of optical fiber, called "photonic crystal fiber", that has a structure completely different from that of conventional fibers was proposed in Britain. This may make it possible to realize ultrahigh speed optical communication using visible or 1 mm wavelengths.

Photonic Crystal Fiber

The transverse structure of photonic crystal fiber is characterized by a number of holes in the cladding layer. This type of fiber has various interesting properties that are not observed in conventional optical fiber and include (1) an endlessly single-mode regime and (2) the flexible choice of zero dispersion wavelength and nonlinearity coefficient by the proper design such hole features as their layout area.

Fig. 1 Photonic crystal fiber

Photonic crystal fiber (PCF) can be divided into two types depending on the difference in guiding mechanism. The first is index-cladding PCF (sometimes referred to simply as PCF), whose core layer is made of glass and whose cladding layer is composed of air holes. In conventional fibers, the refractive index of the core layer was reduced by doping some compound in the cladding layer, resulting in the confinement of the lightwave in the core due to total internal reflection. The core index of PCF is also reduced by the surrounding air holes in the cladding, thus providing the same guiding effect as conventional fibers.

The other is photonic bandgap fiber (PBF), whose core is a central air hole and whose cladding is composed of air holes aligned periodically in transversal dimension. Light is confined in the core by means of 2-D Bragg reflection, where the air holes in the core corresponds to a defect in the periodic structure.

Fig. 2 Wavelength dependence of transmission loss in PCF

Figure 2 shows PCF loss versus wavelength. The loss at 1550 and 850 nm is as low as 3.2 and 7.1 dB/km respectively. The loss is expected to be reduced to the same level as that of conventional fibers by improving the fiber structure.

Our future studies will include attempts to gain a deep understanding of various properties of PCF and PBF and transmission experiments in shorter wavelength bands using these fibers.