Correlation between He-Ne scatter and 2.7-µm pulsed laser damage at coating defects

A reported correlation between defect-initiated pulsed laser damage and local predamage scatter in multilayer infrared mirror coatings has been analyzed in detail. Examination of a much larger data base confirms the previous result on dielectric-enhanced reflectors with polished substrates over a wide range of energy densities above the damage onset. Scatter signals from individual undamaged defects were detected using a He-Ne scatter probe with a focal spot that nearly coincides with the 150-micron-diam (D1/e-squared) focal spot of the damage-probe beam. Subsequent damage frequency measurements (1-on-1) were made near normal or at 45-deg incidence with 100-ns pulses at 2.7-micron wavelength. The correlation is characterized by an increase in damage frequency with increasing predamage scatter signal and by equivalence of the defect densities indicated by the two probes. Characteristics of the correlation are compared with a simple model based on focal spot intensity profiles. Conditions that limit correlation are discussed, including variable scatter from defects and background scatter from diamond-turned substrates. The results have implications for nondestructive defect detection and coating quality control.