Efficient fabrication of volume diffraction gratings in sapphire with a femtosecond laser

Abstract

Volume Bragg grating was fabricated directly inside sapphire crystals via point-by-point (PbP) processing based on the femtosecond laser filamentation effect. Spontaneous growth of the filament-forming region and significant refractive index changes along the laser incidence direction were observed with increasing the number of scans. In addition, the paper proposes a convenient process for fabricating a 1.2 mm × 1.2 mm × 0.0155 mm grating in 4 min using laser PbP scanning. The grating’s maximum diffraction efficiency reaches 75.2 % under optimal parameters. A VBG preparation scheme ensures stable operation in extreme environments. After annealing at 1100°C for 5 h, the diffraction efficiency only decreased 3 %. This study proposes a strategy to prepare volume grating by femtosecond laser direct writing, offering a solution for fabricating sapphire optical devices.