For single-mode fibers, there is a change in refractive indices between core and cladding.
In large mode area PCF fibers the single-mode transport is introduced by microstructuring the fiber cladding. In our case the PCF fibers have a hexagonal structure of periodic holes that lead to a single-mode transport within the fiber. The fibers are categorized by their core diameter (e.g. 5 µm or 10 µm). Detailed calculations reveal that for this type of fibers the mode field diameter MFD is almost wavelength-independent. It is larger than the MFD of comparable standard fibers. In contrast, the numerical aperture has a significant wavelength-dependency. This is important to consider when collimating a beam for a large wavelength range.