Long-period gratings in a standard telecom fibre fabricated by high-intensity femtosecond UV and near-UV laser pulses

Abstract

We inscribed long-period gratings in a hydrogenated SMF-28 Corning fibre using a point-by-point technique and as a light source high-intensity (100–1000 GW cm−2) femtosecond (220–250 fs) pulses at the fourth, fifth and third harmonics (264, 211 and 352 nm, respectively) of Nd:glass laser radiation. The strength of the transmission loss peak reaches 30 dB in gratings recorded with a period of 300 µm and a length of 2 cm, which is significantly higher than the strength of long-period gratings inscribed with the same parameters by any other photochemical method. The analysis of the absorbed energy distribution revealed that such long-period gratings are induced via a two-photon absorption mechanism (or three-photon in the case of 352 nm excitation) and characterized by preferential energy deposition in the cladding, which leads to a high asymmetry of refractive index change (relative to the fibre core) and results in high birefringence properties of gratings formed by high-intensity UV and near-UV radiation.