evices or, polarization-maintaining fibers. Alternatively, a "photonic bandgap" fiber can be created, in which the light is confined by a photonic bandgap cre- ated by the microstructured cladding-such a bandgap, properly designed, can confine light in a lower-index core and even a hollow (air) core [35]. In general, regular structured fibers such as PCFs, have a cross section (normally uni- form along the fiber length) microstructured from one, two or more materials. Most com- monly, the microstructure is periodically arranged over much of the cross section, usually as a "cladding" surrounding a core where light is confined. For example, the fibers dem- onstrated in [36] consisted of a hexagonal lattice of air holes in a silica fiber, with a solid or hollow core at the center where light is guided. Other arrangements include concentric rings of two or more materials. Recently, it has been recognized that the periodic structure is actually not the best solu- tion for many applications. This idea has been applied to reduce bend losses, and for achieving structured optical fibers with propagation losses below that of the SSMF. PCFs with different designs of the hole pattern can have very remarkable properties, strongly depending on the design details: Single-mode guidance over very wide wavelength regions (endlessly single-mode fiber) is obtained for small ratios of hole size and hole spacing [37]. Extremely small or extremely large mode areas are possible. These lead to very strong or very weak optical nonlinearities. PCFs can be made with a low sensitiv- ity to bend losses even for large mode areas [38]. Certain hole arrangements result in a photonic bandgap, where guidance is pos- sible even in a hollow core, as a higher refractive index in the inner part is no longer required. Asymmetric hole patterns can lead to extremely strong birefringence for polariza- tion-maintaining fibers [39]. This property can also be combined with large mode areas. Strongly polarization-dependent attenuation (polarizing fibers) [40]