**1. Introduction**

Optical fibers work as one of the most important parts for data transmission medium and light-signal processing component in optical communications. Microstructure holey optical fibers (MHOFs) from very new advanced technology have attracted considerable attention in research communities during recent years for the purpose of enhancing the performance of existing optical communication system [1-5]. Basically, the MHOFs can be made of a single material, in contrast to all other types of conventional optical fibers, which are normally manufactured with two or more materials. Only by properly designing the geometrical structures of the MHOF, numerous novel properties, such as single mode operation over the entire wavelength range of interest, zero or anomalous group-velocity dispersion, and large effective area, can be obtained. Moreover, various combinations of material composition in the MHOF may offer more opportunities for new and reliable fiber-optic devices.

Since the invention of the MHOFs, lots of applications have been explored and realized recently. Some of the novel applications include second-harmonic generation with broader bandwidth compared to some conventional fibers, polarization-preserving capability with elliptical air holes, fiber lasers, nonlinear-optic devices, and even highly sensitive fiber sensors. This special optical fiber, which is different from the conventional step-index fiber or graded-index fiber, can be fabricated by changing multiple parameters such as the diameter of each small air hole, its shape and location coordinates, the spacing between the adjacent holes, and the number of air-hole cladding layers. Because of the structural uniqueness, the MHOF can be designed to provide small or nearly constant dispersion and highly nonlinearity with single-mode operation characteristic over a wide wavelength range in optical communications.

In this chapter, following a brief review about the principle of light-guiding of the conventional optical fiber, general features of the MHOF will be introduced in section 3.

© 2012 Kim, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Kim, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Showing the arrangement method of small-hole cladding on a transverse plane, modelling of the MHOF without and with an air core is addressed. For the electromagnetic analysis of the proposed microstructures with complicated geometries, the utilizations of a finite difference method (FDM) and a finite-difference time-domain (FDTD) technique are considered in the investigation of optical guidance properties, such as normalized propagation constant, mode field distribution, effective area, and chromatic dispersion. Employing the two numerical methods provides cross-verification and additive confidence in the accuracy of the results. Based on the FDTD and the FDM techniques, how the cladding structure of the HMOF affects optical guidance properties are analyzed in section 4. Then the investigation is extended to find out the effects of the size and shape of the core in the holey fiber on optical propagation properties in section 5.
