Photonic crystal fibers (PCFs) are a special class of pure-silica optical fibers. They consist of a silica core, surrounded by a periodic array of air-holes running along the entire length of the fiber. These air-holes permit guidance of light through total internal reflection in the case of index-guiding microstructured fibers (IGMF). And a periodic air-silica cladding exhibits a band gap for photons in the radial direction in photonic band gap fibers (PBGF). Diameter, spacing of the air-holes and geometric air-holes distribution determine the optical properties of the fiber, therefore allowing for tailoring of the fiber according to the intended application.
In this thesis, we numerically investigate the propagation modes and dispersion proper-ties of light in several types of PCFs such as regular index-guiding microstructured fibers, photonic band gap fibers and irregularly structured low- and high-birefringent PCFs. Effects of fiber birefringence, effective index of cladding and the number of guided modes are studied in designing two modes birefringent PCFs. Birefringence of the fiber is realized by adjusting the size and geometry of the air holes around the core regions. We present a design for a birefringent two modes IGMF which can be applied to interferometers and optical sensors.