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Inscription of long-period fibre gratings by high-intensity femtosecond radiation at 211 nm

In this study we used the fifth harmonic of a femtosecond Nd:glass laser for long-period fibre gratings (LPFG) fabrication in a hydrogenated SMF-28 fibre. In this case, the total excitation energy of two-quantum excitation reaches the value of about 11.7 eV, which is significantly higher than the bandgap energy value of 7.1 eV for Ge-doped fused silica