AbstractUsage of air holes in optical fibers has become a hot subject in fiber optics because of the possibilities for novel transmission properties. Although photonic crystal fibers based on photonic bandgap guidance are the most drastic innovation in this subject, optical fibers containing air holes but not having photonic crystal structures are also being intensively studied. Such air-silica microstructured fibers are more practical than the photonic bandgap fibers because the lack of photonic crystal structure makes the fabrication far easier. Even without the photonic bandgap, the microstructured fibers can exhibit valuable properties in terms of group velocity dispersion and nonlinearity, because the index contrast between air and silica is 10 or more times as large as that of the conventional optical fibers based on doped silica glasses. However, one of the major challenges for practical applications of the air-silica microstructured fibers has been their high transmission losses, which have been several tens to hundreds times higher than those of the conventional fibers. As a solution to this problem, we have proposed a more practical structure called hole-assisted lightguide fiber (HALF). In addition to the air holes for realizing novel optical properties, this structure has a material index profile for waveguiding, and hence is closer to the conventional fibers than the other microstructured fibers are. As a result, novel optical properties can be realized without severe degradation in transmission loss. In experiments, an anomalous group velocity dispersion as large as +35 ps/nm/km at 1550 nm wavelength, which would be unattainable in the conventional fibers, has been realized with a loss of 0.41 dB/km, which is comparable to those of the conventional fibers. Analyses of the losses of the fabricated HALFs suggest that the loss should be lowered by mitigating the effect of the drawing tension and minimizing the power fraction in the holes. It is also shown that the full-vector finite element method realizes accurate modeling of the properties such as dispersion and macrobend loss.
Photonic crystal and microstructured fibers: Making fibers better by leaving bits out