Contents I. Introduction
Index–Guiding microstructured optical fibers (IG MOFs), also known as holey or photonic crystal fibers, consist of a solid core surrounded by a cladding with multiple periodic air channels (holes) oriented along the fiber axis. IG MOFs guide light by means of total internal reflection [1], [2] and have unique properties that are very important for applications in laser physics, nonlinear optics, optical communication systems and optical technology. The unique properties of IG MOFs allow the possibility of controlling their chromatic dispersion in a wide wavelength range by varying a hole diameter d and a hole-to-hole spacing, pitch Λ. To date, IG MOFs with zero dispersion wavelengths in the visible and near-infrared wavelengths [3] –[6], with two zero dispersions [7], [8], with varying dispersion [8]–[10], with flattened and near zero dispersion [11]–[13], and even with ultra-flattened and near zero chromatic dispersion for wavelengths of about 1.0 to 1.8 μm [14]–[19] have been reported. IG MOFs with ultra-flattened dispersion can be used in wide-band supercontinuum generation, dispersion compensation, ultra-short soliton pulse transmission, optical parametric amplification, wavelength-division multiplexing or pulse reshaping [15]– [20]. However, the fabrication of a such type of IG MOFs is a very complicated process. For IG MOFs with the same air-hole diameter d [14], [15], [18] it is necessary to fabricate at least 11 air-hole rings in the cladding with a relative hole diameter d/Λ of about 0.26. At this very small relative hole diameter, the confinement losses [21] in the IG MOFs, even for a large number of air-hole rings in the cladding, are very high. For example, the overall losses of the IG MOFs reported in [15] were of about 2 dB/m at 1500 nm. Other designs of IG MOFs consist of fibers with four or five rings of air-holes with different air-hole diameters in the cladding [16], [19] . Theoretical calculations show that such IG MOFs have low losses, and ultra-flattened and near zero chromatic dispersion for wavelengths of about 1.0 to 1.8 μm. However, the fabrication of such IG MOFs has not been reported in the literature until now. As it was mentioned before, there are some possibilities to obtain IG MOF with exotic dispersion properties, for example, by changing the hole diameter d, the pitch Λ , or the arrangement of air-holes. Thus, IG MOFs with regular or irregular cladding structures can exhibit new or exotic properties.
