Femtosecond laser ablation processing of x-cut LiNbO₃ substrates for optical communication devices

We have studied femtosecond laser ablation characteristics of LiNbO₃ for the first time. LiNbO₃ is ferroelectric material with large optical nonlinearity and Pockels effect. The femtosecond laser ablation is very useful to fabricate various optical devices including the optical modulator and the tunable optical filter for optical communication systems because the thermal damage around the irradiated area is small due to the short pulse width, and the sub-wavelength structures may be formed by the multi-photon excitation. In our experiments, the femtosecond Ti:Sapphire laser system (Energy 0.14 mJ/pulse, Wavelength 800 nm, Pulse duration 60 fs, Repetition rate 1 kHz) based on the chirped-pulse amplification (CPA) technique was used. The aperture with a diameter of 5 mm was imaged onto the LiNbO₃ surface by the objective lens in the air. We observed ablation holes by the scanning electron microscope and the profilometer. We have found no damage around the holes and the clear boundary between ablated area and non-ablated area was observed. Those features are very useful for precise material processing. The bottom face of the holes was relatively flat. The etching rate was 0.93 micrometer/pulse and proportional to the number of the laser pulse. The results showed that the femtosecond laser ablation is an innovative tool for manufacturing LiNbO₃-based optical devices.