Guided modes in a photonic crystal fibre

Photonic crystal fibre (PCF) is a unique form of low-loss optical waveguide, which works by trapping light in a defect within a two-dimensional silica/air photonic crystal material (see figure 1)[1]. The photonic crystal material is formed by a 2 dimensional array of small closely-spaced (1-5µm) air holes which run down the length of the silica fibre. The defect is a region of pure silica embedded in the holey material. Although it works by total internal reflection, as do conventional fibres, we have found that PCF has properties which are fundamentally different to those displayed by other fibres. These properties are determined by the design of the unit cell of the crystal cladding material - the shape, the pitch and the air-filling fraction - as well as by the size of the defect core. The photonic crystal is a new type of optical material, so there is considerable scope to fabricate fibre with novel properties. For example, we have demonstrated a fibre with a simple hexagonal array of air holes in the cladding which is single-moded over an extraordinary bandwidth range, extending from 337nm to beyond 1.55µm, a result which is supported by theoretical considerations. Figure 1 Optical micrograph of a photonic crystal fibre, of diameter 80µm. Figure 2 Higher-order mode in the core of a photonic crystal fibre. By increasing the size of the air holes in the structure one increases the effective index difference between the silica core and the silica/air cladding, increasing the number of guided modes. Likewise, the use of more complex unit cell structures as illustrated here can result in more guided modes (figure 2). This paper describes more fully the dependence of the number of guided modes on the design of the photonic crystal cladding