IR femtosecond pulsed laser-based fiber Bragg grating inscription in a photonic crystal fiber using a phase mask and a short focal length lens.

Fiber Bragg grating inscription with infrared femtosecond pulsed lasers in photonic crystal fiber is far from being trivial due to the presence of air holes in the cladding region and the non-linear nature of the absorption process inducing the required refractive index changes. We have studied this problem numerically and experimentally for a phase mask-based writing setup equipped with short focal length cylindrical lenses, which are often used for through-coating and high temperature stable grating writing. We have shown that for a cylindrical lens with a focal length f of 10 mm, the hexagonal lattice PCF needs to be translated away from the beam waist position by around 15 µm to efficiently deliver the energy to the core region. We have also investigated the importance of the PCF's angular orientation and we have shown that for some optimal positions the same behavior is observed for cylindrical lenses with different focal lengths. Finally, we have succeeded in writing a 4 dB strong grating in a photonic crystal fiber with a 1030 nm femtosecond pulsed laser in around 4 seconds, using an acylindrical lens with f = 10 mm.