A new family of optical fibers, largely in the research stage, relies on internal microstructures to control the propagation of light in ways impossible with conventional fibers. These are often called photonic fibers, but also have been called “microstructured” or “holey” fibers because their internal structures typically have holes running along their length.
Figure below shows how microstructured fibers are made. Hollow glass tubes and solid rods are stacked together with the desired proportions and enclosed in an outer tube. In the design shown, a single solid rod is at the center of the array. Then the glass is fused together and drawn into a fiber. Careful processing produces finished fiber with fine holes running along its length.
Microstructured fibers developed from the phenomenon of the photonic bandgap, which arises in materials with internal structures that make it impossible for light to propagate at certain wavelengths. In that sense, the photonic bandgap is analogous to the electronic bandgap, a range of energy levels that electrons can’t occupy in semiconductors.
Semiconductor bandgaps are used to confine electrons; photonic bandgaps are used to confine the path of light.
The two basic components of a microstructured fiber are a glass matrix and holes containing air (or some other gas or liquid). The relative packing and size of the holes and glass can range from nearly solid glass with a few tiny holes that act as flaws to a thin lattice of glass spread through a volume that is largely air. Specialists divide microstructured fibers into two broad classes, depending on whether the light is confined in a central solid area or within a central hole.
Photonic crystal fibers, like the one shown in Figure above, have a solid core surrounded by a layer containing holes running the length of the fiber. The central solid region is a defect in a sense, because it lacks the holes present in the surrounding microstructure. The microstructured zone is a photonic bandgap material with anaverage refractive index lower than that of the solid core. That makes photonic crystal fibers act somewhat like a conventional solid fiber, with a high-index core surrounded by a lower index cladding. However, the light guiding of the structure depends on the size and spacing of the holes, which determine the effective refractive index of the holey cladding layer.
9 Photonic bandgap fibers have a hollow core surrounded by a photonic bandgap cladding, which reflects all light at certain wavelengths. In this way, it guides light through the air-filled core, which has a lower refractive index than the surrounding material. Such light guiding is impossible in conventional fibers because the refractiveindex of air is lower than that of any conventional solid, so these fibers can only be described in terms of photonic bandgaps.
These types of fibers can be designed to have properties impossible in standard fibers.
Photonic crystal fibers with a large fraction of the cladding filled with air and small features can confine light in effective mode areas as small as one square micrometer, which is useful for producing nonlinear effects. Large-holed microstructures also can produce high waveguide dispersion for use in dispersion compensation or shifting.
Other photonic crystal structures can confine light in a larger core than otherwise possible, reducing nonlinear effects.
Photonic bandgap fibers have received less attention because they are a more recent development, but they offer other possibilities. Guiding light in air should allow very low attenuation and reduce nonlinear effects. It also could allow light transmission at wavelengths where no usable transparent solids are available. Because the photonic bandgap effect is wavelength-dependent, such fibers would guide some wavelengths but not others.