Long-period fiber grating inscription under high-intensity 352 nm femtosecond irradiation: Three-photon absorption and energy deposition in cladding

We inscribed long-period gratings with a strength of 30 dB in a hydrogenated telecom fiber via a three-photon absorption mechanism using high-intensity 352 nm femtosecond radiation and compared their properties with gratings fabricated via a two-photon absorption mechanism by high-intensity 211 and 264 nm laser pulses. Three-photon absorption coefficients for pure fused silica and germanosilicate glass were measured, which allowed us to estimate the distribution of the absorbed light energy in the fiber cladding and the core. It was shown that the deposition of the majority of the absorbed energy in the cladding correlates with the high strength of the recorded gratings.