Investigation of laser damage resistance of a sapphire-substrate-based resonant waveguide grating exposed to sub-picosecond pulses

We investigate the laser induced damage threshold (LIDT) of a sapphire-substrate-based resonant waveguide grating (RWG) at a pulse duration of 500 fs and a wavelength of 1 µm. The RWG structure consists of a single-layer waveguide of Tantalum Pentoxide (Ta 2 O 5 )deposited on a micro-structured c-cut sapphire (α − Al 2 O 3 ) substrate. The study examines the influence of the spectrum and the polarization of the laser pulses on the resistance of the RWG on laser-induced damage. The measurements were conducted at an angle of incidence of 10.8°, where TE-polarized radiation experiences a guided-mode resonance, while for TM polarized radiation it corresponds to an off-resonance condition. At the resonance, the LIDT for single-shot test was measured to be (101.5 ± 9) mJ⁄cm 2 , whereas off-resonance, the LIDT was measured to be (580 ± 52) mJ⁄cm 2 . Moreover, a comprehensive and simple model, supported by numerical simulations of the electric-field distribution within the RWG, is proposed to interpret the measured LIDT values. The close agreement observed between experimental and simulation results confirms the reliability of the proposed model and demonstrates its capability to predict the LIDT of RWG with a relatively good accuracy. To the best our knowledge, this is the first study highlighting the effect of the energy spectral density of sub-picosecond laser pulses on the analysis and interpretation of LIDT measurements, particularly related to resonant optical components.