High 90 per cent efficiency Bragg gratings formed in fused silica by femtosecond Gauss-Bessel laser beams

Direct laser write of volume Bragg gratings with diffraction efficiency (absolute) ∼90% is demonstrated using Gauss-Bessel laser beams in fused silica glass. Axial multiplexing of ∼ 90 µm long segments of modified optical material was demonstrated and thick Bragg gratings of aspect ratio depth/period depth/period ≈234 were achieved with period d = 1.5 µm. Typical fabrication scanning speeds were up to 50 mm/s for gratings with cross sections up to five millimeters made within 1 h time. Potential applications of high efficiency Bragg gratings in a low nonlinearity medium such as silica are discussed.