Correlation between He-Ne scatter and 2.7-microm 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-microm-diam (D1/e(2)) focal spot of the damage-probe beam. Subsequent damage frequency measurements (1-on-1) were made near normal or at 45 degrees incidence with 100-ns pulses at 2.7-microm 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. Results have implication for nondestructive defect detection and coating quality control.

Pulsed laser-induced damage of metals at 492 nm.

A triaxial flashlamp-pumped dye laser has been used to perform laser damage testing of metal surfaces in the blue-green spectral region. Using LD490 laser dye, the laser produces 0.18-J, 0.5-microsec pulses at 492 nm. The spatial profile of the focused beam is measured in orthogonal directions in the plane of the sample surface. The orthogonal profiles are flat-topped Gaussians with 1/e(2) widths of 270 microm. Multithreshold laser damage test results are presented for polished Mo, diamond-turned high-purity Al alloy, diamond-turned bulk Cu, and diamond-turned electrodeposits of Ag and Au on Cu. Comparisons are made between calculated and experimentally measured slip and melt thresholds.

Measurement of optical coupling coefficients based on thermal expansion.

A fundamentally new method for measuring optical coupling coefficients is described. A sample of a particular shape is subjected to a fixed tensile load and heated with pulsed optical radiation. The resulting peak tensile relaxation is proportional to the total absorbed energy; evolution of the coupling coefficient may thus be observed. A prototype system was constructed, and coupling coefficients were measured as a function of incident energy for CO(2) laser pulses on bare and painted aluminum surfaces. With a sample of known absorptance and thermal properties, the system functions as a sensitive, fast-response, absolute-energy-measurement device.

Pulsed laser damage characteristics of vapor-deposited copper mirrors on silicon carbide substrates.

Pulsed 100-nsec 10.6-microm laser damage characteristics of composite bare copper mirrors were determined. The mirrors were prepared by vapor deposition of a copper film on polished silicon carbide substrates formed by chemical vapor deposition, Comparisons are made with a bulk oxygen-free high-conductivity (OFHC) copper mirror prepared by precision diamond turning. Vapor-deposited Cu on SiC was found to be inherently superior in resisting surface damage related to defects and to thermal stress. However, melt thresholds are comparatively lower. The overheating problem was analyzed by a thermal modeling of thresholds measured on mirrors having different film thicknesses. Results are consistent with reduced thermal conductivity in the SiC substrate as compared with bulk Cu. The effect of the interface was evaluated from samples that had been ion polished prior to deposition and on others where an intermediate chromium layer was deposited. The scattering and figure of the mirrors are also reported.