Experimental investigation of the laser conditioning efficiency by nanosecond pulses at 266 and 355 nm in high
reflectivity mirrors used in optical parametric oscillators (OPOs) is present in this report. The high reflection coatings
were deposited on the fused silica substrates. The materials used for e-beam coating deposition were ZrO<sub>2</sub> and SiO<sub>2</sub>.
Laser conditioning was investigated as function of number of pulses, wavelength and conditioning protocol. Ramped-fluence
pre-exposure was used as a method to explore optimal improvement to the damage performance at 266 and
355 nm. No conditioning effect was observed using nanosecond pulses at 266 nm, but the mirror conditioning with
355 nm pulses increased the laser-induced damage threshold (LIDT) by 2.5-3 times. The experimental results support the
concept that the laser conditioning effect observed in high quality optical thin films is associated with intrinsic electronic
defects in the films.