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Photonic crystal fibers (PCFs) offer great design flexibility as their internal microstructure can be tailored to achieve a
wide range of optical guiding properties adapted to many different applications. Fiber Bragg grating fabrication in such
fibers is now extensively investigated to enable new fiber sensor and all-fiber laser applications. Grating writing in PCF
is not necessarily straightforward. This is due, to a large extent, to the air hole microstructure in the fiber cladding that
impedes the inscribing beam intensity to reach the fiber core in sufficient amounts. This issue is more pronounced for
multi-photon absorption based grating inscription techniques, for which the intensity of the light reaching the core is
crucial to induce the desired refractive index change.
We performed a numerical study of transverse light propagation through the cladding to the core for various hexagonal
lattice PCFs. A numerical tool based on commercial FDTD software was developed for that purpose. To assess the
influence of the PCF microstructured cladding, we defined a figure of merit to quantify the amount of laser light reaching
the core: the "transverse coupling efficiency" (TCE). We studied the influence of the hexagonal lattice parameters, in
particular the air hole radius and pitch, on the energy reaching the core for various angular orientations of the fiber with
respect to the impinging laser beam. We conducted this study for ultraviolet and infrared femtosecond laser sources. As a
result we have identified favorable PCF lattice parameters and a fiber orientation that would allow efficient femtosecond
grating inscription. We show that the microstructure of a PCF can not only have a limiting, but also a constructive
influence on the laser energy reaching the core of the fiber and thus on the efficiency with which gratings can be
inscribed.
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