Over the past 25 years, conventional optical fibres have revolutionised communications and have become vital components in many technologies, from sensing and medical imaging to high power applications, including laser welding and machining and active devices such as fibre lasers and amplifiers. In recent years, two new types of optical fibre have enlivened this well-established field, bringing with them a wide range of novel optical properties. These new fibres, known collectively as microstructured fibres, can be made entirely from one type of glass as they need not rely on dopants for guidance. Instead, the cladding region is peppered with many small holes that run the entire fibre length, creating a longitudinally uniform air/glass microstructure. Although the first microstructured fibres were fabricated in the early 1970’s by Kaiser et al. at Bell labs [1, 2], the current burst in activity stems from the first demonstration of guidance within such structures in 1996 [3]. This explosive growth is fuelled by the many unusual and useful optical properties exhibited by these fibres, many of which are impossible to achieve using conventional methods.
Microstructured fibres are separated into two distinct categories, defined by the way in which they guide light: (1)Holey fibres, which guide light due to average index effects, and (2) Photonic crystal or Band-gap fibres, which guide light due to photonic band-gap effects. Typical examples of these two fibre types are shown respectively in Figs 1.1 (a) and (b). In both fibre types, the cladding is formed by an array of air holes that run the full fibre length. These air holes are typically arranged on a hexagonal lattice and the defining
(a) (b)
Figure 1.1: (a) SEM image of a typical index guidingHoley fibreand (b) SEM image of a typicalPhotonic band-gap fibre. Both fibres were made at the ORC.
parameters are the hole-to-hole spacing Λ, and the hole diameter d.
In holey fibres, the air holes that define the cladding act to lower the average, or effective refractive index of this region relative to that of the solid core. As a result, light is guided within the core via a modified form of total internal reflection. Whilst this mechanism is similar to the way in which conventional step-index fibres guide light, the wavelength scale features in a holey fibre lead to a strongly wavelength dependent cladding index. This property is responsible for the host of unusual optical properties unique to holey fibres, including single-mode guidance at all wavelengths [4], which has great practical significance for broadband and short wavelength applications, in addition to a host of novel dispersive properties [5, 6, 7, 8]. Furthermore, simply by scaling the cladding geometry, single-mode holey fibres with effective mode areas (AFMeff) ranging from approximately 1.5 to 600 µm2 at 1550 nm can be created [9, 10]. Such small values of AFMeff have enormous potential for highly nonlinear applications, whilst large-mode-area fibres are essential for applications involving high optical powers.
In contrast to holey fibres, in which the mechanisms responsible for guidance share sim- ilarities with conventional fibres, photonic band-gap fibres represent a fundamentally differ- ent class of waveguide. In a photonic band-gap fibre, the cladding air holes are arranged in a perfectly periodic fashion. For certain geometries the cladding can form a two-dimensional photonic crystal with band-gaps at well defined optical frequencies [11]. Wavelengths within
the band-gap cannot propagate in the cladding region and are thus confined to the core of the fibre. The most attractive property of this fibre type arises from the fact that the core need not be defined by a high index region, as is necessary in an index-guiding fibre. Instead, the fibre core can be created by a low-index defect, and via careful design of the cladding can result in a fibre in which light is guided within a hollow air-core [12]. Such waveguides offer potentially lower values of loss and nonlinearity than is possible in con- ventional solid core waveguides, which has obvious advantages for long-haul transmission and high power applications.
Note that holey fibres are often referred to asphotonic crystalfibres within the literature. However, the average index effects responsible for guidance in holey fibres do not rely on any periodicity within the air hole lattice [13]. Consequently, here the termphotonic crystal
is reserved for band-gap fibres, in which periodicity is essential to the guidance mechanism. In the following sections of this chapter, these two classes of microstructured fibre are discussed in more detail. Holey fibres are considered within Section 1.2, in which the unique modal properties and flexible fabrication techniques that give rise to a wide range of novel fibre designs and optical properties are discussed. A summary of the numerical techniques used to model the modal properties of holey fibres is presented in Section 1.4. The second class of microstructured fibre, the photonic band-gap fibre, is discussed in Section 1.3. However, please note that band-gap fibres are only discussed briefly and are not considered in any detail in the work presented in this thesis.