Demonstration of Type A volume Bragg gratings inscribed with a femtosecond Gaussian-Bessel laser beam

Abstract

To our knowledge, we report on the first demonstration of Type A VBGs inscribed in silver-containing phosphate glasses by femtosecond laser writing. The gratings are inscribed plane-by-plane by scanning the voxel of a 1030 nm Gaussian-Bessel inscription beam. This results in a refractive-index modification zone, induced by the appearance of silver clusters, extending over a much larger depth than those obtained with standard Gaussian beams. As a result, a high diffraction efficiency of 95% at 632.8 nm is demonstrated for a 2-µm period transmission grating with a 150-µm effective thickness indicating a strong refractive-index modulation of 1.78 × 10−3. Meanwhile, a refractive-index modulation of 1.37 × 10−3 was observed at a wavelength of 1.55 µm. Thus, this work opens the avenue for highly effective femtosecond-written VBGs suitable for industrial applications.