Photonic crystal fibers technology provides us with new way to obtain fibers with much higher non-linearity than conventional techniques. Upper limits of non-linear coefficients obtainable in silica-based photonic crystal fibers have been already investigated. Unique dispersion characteristic and enhanced non-linearity make this kind of fibers an ideal candidate for non-linear optical devices in telecommunication applications, for measurement and sensing and for supercontinuum generation. However, there are limitations given by material properties, which obstruct us from achieving theoretical limits of these fibers. Extremely small core and high air-filling fraction are here needed for reach higher non-linearity, so when material properties of conventional silica restrict us, there is a requirement on a novel matter. This could be poly-methyl metacrylate (PMMA), a common material for plastic optical fibers manufacturing. These microstructured polymer optical fibers are a recent technology, which gives us with new possibilities in core size, fiber geometry and related air-filling fraction. By this kind of fiber, we could be closer to ideal non-linear fiber, which is core strain surrounded by air, than even before. But new kind of fiber brings new issues, like which effect in fiber will be dominant or how will be coupled light affected by outer influences - and what difference will be between predicted and real values in general. This is a large task and hopefully, there will be answer at least for a small part in this paper.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.