Numerical modeling of femtosecond laser inscribed IR gratings in photonic crystal fibers.

During grating inscription in photonic crystal fibers (PCFs) the intensity of the inscribing laser beam is non-uniformly distributed over the core region due to the interaction with the air holes in the fiber's microstructure. In this paper we model and study the non-uniformity of the index modification and its influence on the grating reflection spectra, taking into account the non-linear nature of the index change. For femtosecond laser inscription pulses at 800 nm, we show that the intensity redistribution in the PCF core region can result in Type II index changes even if the peak intensity of the incident beam is well below the corresponding threshold. Our coupled mode analysis reveals that the non-uniform nature of the index change can seriously affect the reflectivity of the grating due to a limited overlap of the guided mode with the transverse index modulation profile for almost all angular orientations of the PCFs with respect to the inscription beam. We also evaluate the influence of PCF tapering and we found that for the considered PCF a significant increase in the induced index change and reflectivity is observed only for taper diameters below 40 μm.