Small holes, big effects
Optical fibres are made from thin, flexible, strands of glass about the width of a human hair and are used to transmit light from place to place in the same way that copper wires carry electrical signals and power.
Conventional optical fibres are typically made from two different layers of silica glass: an inner core that is 'doped' by adding very small amounts of material (usually germanium), and an outer cladding of pure silica.
Dopants such as germanium raise the refractive index of the core relative to the cladding region. This difference in refractive index confines light to the core via a process known as total internal reflection. In this way, light is channelled down the core, only emerging when it reaches the end of the fibre. Over the past 25 years, optical fibres have revolutionised communications, transmitting more information over greater distances than could ever be achieved in copper wires, and are also vital in many technologies such as imaging endoscopes and high power laser transport for cutting and drilling applications.
In recent years, two new types of optical fibre have revolutionised this dynamic 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 do not rely on dopants for guidance. Instead, the cladding region is peppered with many small air holes, that run the entire fibre length. These fibres are typically separated into two classes, defined by the way in which they guide light:
· Holey fibres, in which the core is solid and light is guided by a modified form of total internal reflection as the air holes lower the effective refractive index of the cladding relative to that of the solid core.
· Photonic band-gap fibres, in which guidance in a hollow core can be achieved via photonic band-gap effects. The many varieties of microstructured fibres are discussed in more detail in the following sections.
Microstructured fibre types investigated and fabricated at the ORC
Endlessly single-mode holey fibres
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 endlessly single-mode guidance, whereby only the fundamental mode is guided, regardless of the wavelength.
This, together with the flexibility of holey fibre fabrication techniques, which can be used to create extremely large core sizes simply by creating large scale structures, enables the creation of large-mode-area fibres with excellent beam quality. In this type of fibre the air holes are typically arranged on a hexagonal lattice with a single missing air hole – however, even larger cores can be created by omitting more than one air holes from the centre of the fibre.These fibres have applications in high power beam delivery where good beam quality, high damage threshold and low nonlinear effects are essential.
Small core non-linear holey fibres
Using a two step process to further reduce the structure scale, very small core dimensions can be achieved. Even though silica glass does not possess an intrinsically high nonlinearity, the combination of small core sizes and high NA in these fibres leads to tight mode confinement and high optical sensitivities even for modest powers. This, together with the ability to tailor the dispersive properties of the fibre, enables highly efficient nonlinear processes such as supercontinuum generation, all optical switching and pulse compression to name but a few.
Highly multimode, jacketed air clad
By taking advantage of the large index contrast between air and glass, holey fibre technology offers a simple way of creating all glass, multi-mode fibres with large cores and high values of NA simply by using a single ring of large air holes to form a high NA cladding. These fibres have applications in high power beam delivery where high damage threshold and low nonlinear effects are essential.
Active fibres with high NA cladding
Active holey fibre devices can be created by incorporating elements made from doped glass in the fibre preform. Double-clad holey fibre lasers can be created by adding a ring of air holes to form a secondary, high NA cladding. This removes the requirement for low index polymer coatings, which have poor reliability at high optical powers.
Photonic band-gap fibres
In contrast to all the fibres described above, in which the mechanisms responsible for guidance share similarities with conventional fibres, photonic band-gap fibres represent a fundamentally different 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. 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. The ability to guide light in a hollow core opens up potential applications that were previously inconceivable. In addition to applications such as gas sensing such waveguides offer access to lower values of nonlinearity than is possible in conventional solid core waveguides, which has advantages for high power applications.
Microstructured fibres made from other glasses
Microstructured fibres can also be made from non-silica glasses, such as tellurite and chalcogenide glasses, which offer optical properties not available in silica, such as mid-IR transmission and high values of refractive index and non-linearity. These so called soft-glasses possess lower melting points than silica glass and can, as a result, be extruded to form a preform in one step. This not only simplifies fabrication, but also enables the fabrication of novel geometries.
Small core, extreme nonlinearities
By extruding the preform, fibres with small, near air-suspended cores and high air filling fractions that are ideally suited for exploiting the high non-linearity in non silica glasses. Such fibres offer access to nonlinear effects at remarkably low powers and short device lengths.
Solid microstructured fibres
The novel optical properties of holey fibres are not restricted to air/glass structures. In theory, two uniform solid materials can be used to create a solid microstructure with similarly unusual optical properties if the refractive index contrast between the two materials is sufficiently high. Using compatible glasses, two types of solid microstructured fibre have been fabricated at the ORC; solid holey fibres with hexagonally arranged low index regions; and bragg fibres, in which concentric, alternating rings of glass are used to confine light to the core. This technique has the advantage that large reductions in scale can be achieved with negligible structural distortion, in addition to the mechanical advantages offered by a solid fibre, for polishing/splicing etc.
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