Photonic Crystal Fibre for Dispersion Controll

Abstract

The idea to prepare photonic crystal fibres (PCF) as their name show, goes back to the birth of photonic crystals. The ability to tailor structures on the micro and nano scale range, in the late 1980s, provided the opportunity to investigate the relation between the structure of matter and light. Three dimensional photonic crystal structures are periodic, dielectric structures in which light may behave similar way as electron waves in a crystal lattice: under suitable circumstances, the periodic potential presented by the crystal may open up a frequency band in which the propagation of electron waves with certain energies are prohibited. This frequency band in the case of photonic crystals is generally called photonic bandgap (PBG) as a nomenclature borrowed from solid state physics. Within this framework, the photonic crystal emerged and became an extensively studied scientific area since 1987 (Yablonovitch, 1987; John, 1987). To introduce two dimensional photonic crystal waveguides into the practice which implies a waveguide being uniform along the propagation direction holding a two dimensional periodic photonic crystal lattice around the core as shown in Fig. 1, was justified by the novel optical properties of these waveguides compared to traditional optical fibres. One may distinguish two type of photonic crystal PCFs. One type is index guiding PCF (Knight et al., 1996) which means that the core refractive index is larger than the average index of the cladding therefore the propagation occurs due to the total internal reflection on the corecladding interface and only the fibre structure recalls the structure of photonic crystals. The other type of PCFs is bandgap guiding fibres (Knight, Broeng, Birks & Russell, 1998; Cregan et al., 1999) where the fibre core has lower refractive index than the average index of the surrounding cladding region and the propagation occurs due to the bandgap guidance and not due to the total internal reflection which would not be possible in such a structural arrangement. Both type of PCFs hold very interesting new properties for manipulating and controlling the propagating light or light pulses. Index guiding photonic crystal fibres with small-core sizes can have a large waveguide contribution to the material dispersion, therefore anomalous dispersion can be achieved well below the 1.27 μm region where material dispersion alone is negative (Mogilevtsev et al., 1998). This allows the dispersion management of the fibres even in the visible wavelength range with a suitable cladding design by changing the hole size and hole-to-