The role of highly non-linear index change mechanism during femtosecond grating writing in microstructured optical fibers

New methods for fiber Bragg grating inscription in optical fibers use femtosecond laser sources, which can induce refractive index changes even in non-photosensitive fibers and which allow achieving gratings that remain stable at high temperatures. The index change takes place as a result of a highly non-linear multi-photon absorption process. Although such gratings were successfully inscribed in conventional fibers, there are still challenges involved when attempting to fabricate femtosecond gratings in microstructured optical fibers (MOFs). The air holes are usually impeding the delivery of optical power to the core region, which results in a lower grating writing efficiency. In this paper we report on our numerical computations that aim to estimate the influence of the MOF's holey cladding on the induced index change during interferometric grating inscription with an infrared (IR) femtosecond laser source. For high power femtosecond laser pulses at 800 nm the refractive index change in silica stems from a highly non-linear five photon absorption process. Using empirical data on refractive index changes from literature and intensity distribution data from our transverse coupling simulations we propose an approach to reconstruct the non-linear refractive index modification in the MOF core region. We then study the influence of the MOF angular orientation on the induced index change and we model the impact of MOF tapering as a possible way to increase the grating writing efficiency.