Optical cleaning of lithium niobate crystals

For the optical cleaning, a Gaussian laser beam at 532 nm is focused onto the crystal. The 1/e-radius at the beam waist is 40 mum along the z- axis of the crystal. Afterwards, the impact of the cleaning treatment on the optical damage behavior of the crystals is inspected: A focused beam (1/e-radius of 20 mum) at 514 nm is directed onto the sample. After a 150- sec exposure at I2 =(10³-10⁴) W/cm², the output beam profile is measured in two ways: First, a pinhole behind the sample blocks the stray light, and a photodiode records the power P_in. Second, a micro-disk blocks the beam center, so that the power P_out is recorded. The results after cleaning of a crystal with an intensity I₂=15 W/cm² for 340 h; the illuminating beam was moved for 1 mm in +z-direction during the cleaning treatment. The threshold of the optical damage is increased by more than three orders of magnitude due to the optical cleaning.