Efficient Fabrication of Volume Diffraction Gratings in Sapphire with a Femtosecond Laser

Abstract

Volume Bragg gratings were fabricated in a sapphire (Al2O3) crystal using femtosecond laser direct writing. The femtosecond laser filamentation effect was combined with a cylindrical lens focusing method to fabricate gratings by a point-by-point processing method. As the number of pulses increases, spontaneous growth of the filament region with clear change in refractive index was observed along the direction of laser incidence. In addition, the refractive index changed regions are arranged in a periodic pattern to form a volume Bragg grating. By periodically moving the sample stage to connect the filament regions to a surface, a grating with dimensions of 1.2 mm × 1.2 mm × 0.0155 mm can be fabricated within 4 minutes. Under the optimal parameters, the maximum diffraction efficiency of the grating reaches 75.2%. Furthermore, the diffraction efficiency only decreases by 3% after annealing at 1100 °C for 5 h. This study presents a novel strategy for fabricating volume gratings by femtosecond laser direct writing, offering high processing efficiency and robust high-temperature performance.

Keywords: Femtosecond laser, Sapphire, High temperature stability, Volume Bragg grating