Femtosecond laser writing of fibre Bragg gratings in large diameter air-clad optical fibre

摘要

Fibre Bragg gratings were written in large diameter (～300-400 μm) air-clad optical fibre using an amplified Ti:Sapphire femtosecond laser (800 nm, 100 fs, 0.3 μJ/pulse) with the point-by-point method. Utilising multiphoton absorption, we induced modifications in the core of the air-clad fibre. By focusing the femtosecond laser close to the diffraction limit with a 20x oil immersion microscope objective lens, the glass modification region is of the order of 1 μm in the fibre, which has a 5 μm core diameter. A technique to avoid the scattering effect of the large air-silica index contrast, on the femtosecond radiation, was implemented. Grating rejection strengths of 20 dB were achieved with an accompanying insertion loss of 3-4: dB at 1085 nm.