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

Long-period fibre gratings (LPFGs) have been investigated since the mid-1990s. The first LPFGs were fabricated photochemically by UV laser light at the wavelength coinciding with the maximum of the absorption band of defects in germanosilcate glass; that meant using either KrF excimer laser radiation (248 nm) or the second-harmonic radiation of a CW argon ion laser (244 nm). Such UV laser irradiation led to a refractive index change in the Ge-doped fibre core via a single-quantum photochemical reaction. Later, other methods of LPFG inscription, based on refractive index changes in the fibre core induced by thermal heating, were developed. Newly developed photochemical methods include the excitation of high-energy electronic levels in Ge-doped fused silica due to vacuum UV single-quantum and IR (visible) multiple-quantum techniques, respectively. Two years ago, we suggested the use of high-intensity 264 nm femtosecond pulses from a frequency-quadrupled Nd:glass laser! for LPFG recording. In these experimental conditions, two-quantum excitation (two-photon or two-step, depending on the magnitude of linear absorption at the irradiation wavelength) takes place. Recently, we conducted a detailed investigation of LPFG fabrication by high-intensity femtosecond 264 nm light in both standard telecom and photosensitive fibres [1]. In this study we used the fifth harmonic of a femtosecond Nd:glass laser for 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