On the influence of hexagonal lattice photonic crystal fiber parameters on femtosecond grating inscription

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.