| Abstract [eng] |
The present Ph.D. thesis is the experimental and theoretical analysis of the femtosecond laser pulse induced damage processes in thin film dielectric coatings. Experimental investigations were performed by automated metrological facility designed for S-on-1 laser-induced damage threshold measurements. Femtosecond repetitive pulses (1 kHz) either at fixed 800 nm and 400 nm wavelengths or continuously tunable in 590 nm to 750 nm spectral range were used. The sensitivity of assembled metrological facility was sufficient to determine the influence of various deposition factors (process parameters and coating materials) on LIDT of optical coatings. Our experimental investigations on multi-layer ZrO2/SiO2, HfO2/SiO2, Ta2O5/SiO2, TiO2/SiO2 high reflection coatings and single-layer TiO2 have yielded several important results. To summarize: stepwise change of LIDT values was experimentally observed at the wavelength where two-photon absorption changes to three-photon absorption. This confirms that multiphoton absorption is one of the main damage mechanisms in femtosecond range. The multilayer coatings deposited by IAD and e-beam techniques on substrates having roughness of 0.64 nm or smaller showed similar LIDT values. Moreover, it was also confirmed that suppressing of standing wave electric field intensity at the outer layers of high refractive index improves the optical resistance of high reflectivity coatings also for femtosecond pulses. Furthermore, the model of the pseudo-accumulation effect is introduced for optical surfaces containing absorbing defects. The results of Monte Carlo simulations show that pulse-to-pulse fluctuations of laser beam propagation direction, energy or even mechanical vibrations in the optical systems produce apparently accumulative damage statistics. The exposed area increases shot-by-shot, thus increasing the S-on-1 probability of damage. |
