[en] Roles of absorbing defects and structural defects in multilayer under laser radiation were investigated. The HfO₂/SiO₂ dielectric mirrors for 355 nm were prepared by conventional electron beam deposition. Two kinds of HfO₂ with different purity were chosen as the high index material, and impurity content of the materials was accessed by glow discharge mass spectrometer (GDMS) and X-ray photoelectron spectroscopy (XPS). Laser damage testing was performed in both the ''1-on-1'' and ''s-on-1'' regime, using 355 nm pulsed laser at a pulse length of 8 ns. It was found that the single-shot laser-induced damage threshold (LIDT) is much higher than that of the multi-shot, and the absorbing defects hindered the improvement of laser damage resistivity greatly in the single-shot process, but in multi-shot mode the main factor influencing LIDT is accumulation of irreversible changes of structural defects. Optical microscopy and surface profiler were employed in mapping laser-induced damage morphology features after irradiation

[en] Laser conditioning effects of the dielectric mirror coatings with different designs were investigated. Simple quarter-wave ZrO₂:Y₂O₃/SiO₂ mirrors and half-wave SiO₂ over-coated ZrO₂:Y₂O₃/SiO₂ mirror coatings were fabricated by E-beam evaporation (EBE). The absorbance of the samples before and after laser conditioning was measured by surface thermal lensing (STL) technology and the defects density was detected under Nomarski microscope. The enhancement of the laser damage resistance was found after laser conditioning. The dependence of the laser conditioning on the coating design was also observed and the over-coated sample obtained greatest enhancement, whereas the absorbance of the samples did not change obviously. During the sub-threshold fluence raster scanning, the minor damage about defects size was found and the assumption of pre-damage mechanism, based on the functional damage concept, was put forward. The improvement of the laser induced damage threshold (LIDT) was attributed to the benign damage of the defects and the dependence on the coating design owed to the damage growth behavior of different coating designs

[en] The characterization of repetitive frequency (S-on-1) laser resistance of optical coatings was investigated experimentally based on damage pattern divided into slight damage and severe damage. It was discovered that, when only the slight damage pattern was counted, a peak would be seen in the damage probability curve, and the laser fluence at this peak was stable as pulse sequence S changed. The stable laser fluence at this peak, therefore, could be employed to characterize the repetitive frequency laser resistance of optical coatings. This method is different from the method proposed by the International Organization for Standardization [ISO 11254-2(2001)] and has the advantage of simplicity in practical applications. According to our analysis, it is a method that can obtain the functional damage threshold of optics.

[en] The effects of annealing on structure and laser-induced damage threshold (LIDT) of Ta₂O₅/SiO₂ dielectric mirrors were investigated. Ta₂O₅/SiO₂ multilayer was prepared by ion beam sputtering (IBS), then annealed in air under the temperature from 100 to 400 deg. C. Microstructure of the samples was characterized by X-ray diffraction (XRD). Absorption of the multilayer was measured by surface thermal lensing (STL) technique. The laser-induced damage threshold was assessed using 1064 nm free pulsed laser at a pulse length of 220 μs. It was found that the center wavelength shifted to long wavelength gradually as the annealing temperature increased, and kept its non-crystalline structure even after annealing. The absorbance of the reflectors decreased after annealing. A remarkable increase of the laser-induced damage threshold was found when the annealing temperature was above 250 deg. C

[en] TiO₂ films are deposited by electron beam evaporation as a function of oxygen partial pressure. The packing density, refractive index, and extinction coefficient all decrease with the increase of pressure, which also induces the change of the film's microstructure, such as the increase of voids and H₂O concentration in the film. The laser-induced damage threshold (LIDT) of the film increases monotonically with the rise of pressure in this experiment. The porous structure and low nonstoichiometric defects absorption contribute to the film's high LIDT. The films prepared at the lowest and the highest pressure show nonstoichiometric and surface-defects-induced damage features, respectively

[en] ZrO₂ films were deposited by electron-beam evaporation with the oxygen partial pressure varying from 3x10^-3 Pa to 11x10^-3 Pa. The phase structure of the samples was characterized by x-ray diffraction (XRD). The thermal absorption of the films was measured by the surface thermal lensing technique. A spectrophotometer was employed to measure the refractive indices of the samples. The laser-induced damage threshold (LIDT) was assessed using a 1064 nm Nd: yttritium-aluminum-garnet pulsed laser at pulse width of 12 ns. The influence of oxygen partial pressure on the microstructure and LIDT of ZrO₂ films was investigated. XRD data revealed that the films changed from polycrystalline to amorphous as the oxygen partial pressure increased. The variation of refractive index at 550 nm wavelength indicated that the packing density of the films decreased gradually with increasing oxygen partial pressure. The absorptance of the samples decreased monotonically from 125.2 to 84.5 ppm with increasing oxygen partial pressure. The damage threshold values increased from 18.5 to 26.7 J/cm² for oxygen partial pressures varying from 3x10^-3 Pa to 9x10^-3 Pa, but decreased to 17.3 J/cm² in the case of 11x10^-3 Pa

[en] The damage behaviors of HfO₂/SiO₂ high reflective optical thin film fabricated by electron beam evaporation (EBE) illuminated using 1064 nm laser and 532 nm laser showed some difference. It was found that the damage was usually caused by absorptive defects and absorption near the interfaces under the irradiation of 1 064 nm laser, and the damage was caused by electronic defects and absorptive ones under irradiation of 532 nm laser and the former had lower damage threshold. Therefore, the key point to enhance the laser resistance of optical thin films at 1064 nm was to prevent the presence of the absorptive defects, and to those at 532 nm was to eliminate the electronic defects. (authors)

[en] The influence of organic contamination in vacuum on the laser-induced damage threshold (LIDT) of coatings is studied. TiO₂/SiO₂ dielectric mirrors with high reflection at 1064 nm are deposited by the electron beam evaporation method. The LIDTs of mirrors are measured in vacuum and atmosphere, respectively. It is found that the contamination in vacuum is easily attracted to optical surfaces because of the low pressure and becomes the source of damage. LIDTs of mirrors have a little change in vacuum compared with in atmosphere when the organic contamination is wiped off. The results indicate that organic contamination is a significant reason to decrease the LIDT. N₂ molecules in vacuum can reduce the influence of the organic contaminations and prtectect high reflectance coatings

[en] The development of high-power laser systems requires optical components that function at peak performance. Here, a high-precision, double beam, 1064 nm laser photometer setup was developed to measure the following different forms of optical loss from Nd-glass samples: total loss, volume loss, and the residual reflection and surface loss. The double beam design and a lock-in technique were utilized to decrease the impact of light-source instabilities and signal noise, respectively. The stability of the signal was further improved by decreasing the amount of optical absorption along the light path and by increasing the detection responsivity. Paired samples were symmetrically placed to eliminate beam displacement, and a laser scattering imaging technique was used to determine the influence of surface defects on the optical performance. Using the above techniques, multiple measurements of the transmittance and reflection values of the sample were taken, which showed our transmittance measurement to be highly precise, exhibiting a relative standard deviation of less than 0.06%. Different types of optical loss were distinguished and obtained from the transmittance and reflection measurements of samples with different thicknesses. A comparison of the optical performance from test points with and without surface defects allowed us to determine the influence of surface defects on the optical performance. (paper)

[en] A high laser-induced damage threshold (LIDT) TiO₂/SiO₂ high reflector (HR) at 1064 nm is deposited by e-beam evaporation. The HR is characterized by optical properties, surface, and cross section structure. LIDT is tested at 1064 nm with a 12 ns laser pulse in the one-on-one mode. Raman technique and scanning electron Microscope are used to analyze the laser-induced modification of HR. The possible damage mechanism is discussed. It is found that the LIDT of HR is influenced by the nanometer precursor in the surface, the intrinsic absorption of film material, the compactness of the cross section and surface structure, and the homogeneity of TiO₂ layer. Three typical damage morphologies such as flat-bottom pit, delamination, and plasma scald determine well the nanometer defect initiation mechanism. The laser-induced crystallization consists well with the thermal damage nature of HR