TUTORIAL - HIGH INDEX GUIDING FIBERS
M-TIR is analogous to total internal reflection known from standard optical fibers. It relies on a high index core region, typically pure silica, surrounded by a lower effective index provided by the microstructured region. The effective index of such a fiber can, in the simple case, be approximated by a standard step index fiber, with a high index core and a low index cladding. However, the refractive index of the microstructured cladding in PCFs exhibits a wavelength dependency very different from pure silica - an effect which allows PCFs to be designed with a complete new set of properties not possible with standard technology. As an example, the strong wavelength dependence of the refractive index allows design of endlessly single-moded fibers, where only a single mode is supported regardless of optical wavelength. Furthermore, it is possible to alter the dispersion properties of the fibers, thereby making it possible to design fibers with an anomalous dispersion at visible wavelengths.
More complex index structures can also be constructed by utilizing arrangements of holes of different size in various periodic or nonperiodic structures. In addition, highly asymmetric core fibers can be fabricated thereby creating fibers with very high level of birefringence.
The technology also makes it possible to create highly nonlinear fibers which can be used for e.g. super continuum generation.
