This PDF file contains the front matter associated with SPIE Proceedings Volume 10713, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Laser damage measurements with multiple pulses at constant fluence (S-on-1 measurements) are of high practical importance for design and validation of high power photonic instruments. Mimicking the usual operation conditions, they allow observing possible modifications of the laser damage behavior during operation. In fact, nanosecond S-on-1 tests often reveal the “fatigue effect”, i.e. a decrease of the laser damage threshold with increasing pulse number. When irradiating with ultraviolet wavelengths, the fatigue effect is caused by cumulative material modifications. Systematic improvement of the concerned optical materials can only be achieved if the material modifications operated by the laser irradiation are identified. In this presentation we will show our latest results on the material modifications observed by photoluminescence in the bulk of fused silica. Causing the modifications and pumping the photoluminescence at 266 nm, modifications in the color center concentrations can be observed before the occurrence of damage. These observations can thus help to predict imminent fatigue laser damage under certain irradiation conditions. The lifetime and the nature of the observed modifications differ for low OH and high-OH silica types. Although bulk fatigue damage is only limiting at 266 nm, we also made first investigations using 355 nm as modification wavelength. However, the lifetime of the modifications causing the reduced laser damage threshold is much longer than the lifetime of the modified color centers, indicating that the observed modifications only accompany the initial stage of the problematic and still unknown modifications that weaken the damage threshold.

In this paper, the concept of a mesoscopic method with high-speed and high-sensitivity is proposed for characterization of surface defects for large optics. The technology is a comprehensive integration of laser scattering method and highly sensitive photothermal method. The principle, experimental setup and preliminary measurement results are presented in detail in the paper. A statistical model for evaluation of mapping results of defects is also proposed to show the effectiveness of the comprehensive metrology method. The proposed method can detect non-destructively surface defects with high-speed and high-sensitivity at the mesoscopical level. It is a promising novel tool for mapping defects in meter size optics and hence it can provide clues to eliminate defects during the manufacturing processes and march toward “defect-free” optics.

Laser-induced damage in optical components has always been a key challenge in the development of high-power laser systems. In picosecond regime, the laser-matter interactions are quite complex and the damage mechanism is not yet understood. Therefore, it is necessary to investigate the laser induced damage of optical components in picosecond regime. Our previous study on the laser induced damage in HfO2/SiO2 high-reflective (HR) coatings in 30-ps laser pulses reported the damage morphologies to be high-density micrometer-scale pits, which are similar to the morphologies of HR coatings irradiated by 355-nm pulses in nanosecond regime. Thus, it makes sense to analyze the damage mechanism of HR coatings in picosecond regime by comparing the damage results with those tested with 355-nm pulses. In this study, laser induced damage of HfO₂/SiO₂ HR coatings are performed by 355-nm, 7-ns pulses and 1064-nm, 30-ps pulses, respectively. Different angles of incidence (AOIs) are operated in the tests, in order to modulate the electric field (E-intensity) distributions in the coating stacks. Damage morphologies and cross-sectional profiles are characterized using scanning electron microscope (SEM) and focused ion beam (FIB), respectively. The laser-induced damage thresholds (LIDTs) and morphologies tested with two different laser pulses are compared. The damage locations are compared with corresponding E-field distributions and the damage reasons are discussed.

Chemical etching is usually utilized to improve the laser damage performance of optical glass by mitigating microcracks, while it inevitably produces some reaction products (RPs). In this paper, two K9 glasses with good quality and two K9 glasses with micro-cracks are etched, statically or dynamically (high-frequency ultrasonic agitation). The morphologies of cracks and RPs are characterized, and the laser-induced damage thresholds (LIDTs) are measured. The results show that with the increase of etching time, the LIDT increases slightly at first and then decreases gradually for the glass with RPs, and the LIDT increases at first and then stabilizes for the glass without RPs. Using finite-difference time-domain method, the light intensities around crack, RP and their combination are simulated, respectively. The results indicate that the light intensity enhancement factor (LIEF) increases at first and then decreases with the decrease of crack aspect ratio, and the LIEF increases with RP radius. The LIEF for the combination is generally larger than that for one crack or one RP, which greatly depends on the relative distance between the crack and the RP. Experimental and simulated results complement each other, revealing the influence mechanism of crack and RP on the LIDT. This work would contribute to improving the LIDT of optical glass by chemical etching.

We report on progress for increasing the laser-induced damage threshold of dichroic beam combiner coatings for high transmission at 527 nm and high reflection at 1054 nm (22.5° angle of incidence, S-polarization). The initial coating consisted of HfO₂ and SiO² layers deposited with electron beam evaporation, and the laser-induced damage threshold was 7 J/cm² at 532 nm with 3.5 ns pulses. This study introduces different coating strategies that were utilized to increase the laser damage threshold of this coating to 12.5 J/cm².

The properties of coatings deposited by electronic beam (e-beam) technique can be easily influenced by environmental humidity, causing spectrum shift, residual stress evolution, and wave front errors. In this work, HfO₂/SiO₂ multilayer coatings with different overcoat layer deposition process were prepared. The optical spectrum shift caused by atmosphere-vacuum effect of the prepared samples was investigated by spectrometer. The laser-induced damage resistance was studied and the damage morphologies were characterized by Scanning Electron Microscope (SEM). The surface morphology and global coating stress of the films were analyzed by Atomic Force Microscope (AFM) and Zygo interferometer, respectively. The experimental results demonstrate that by a capping SiO₂ layer employed by plasma ion assisted deposition (PIAD), considerable stability concerning the environmental stability of e-beam coatings can be improved due to delayed water vapor transport rate. A relatively smaller grain size can be obtained as well. Moreover, the laser- induced damage threshold (LIDT) shows no significant differences.

It is hard to judge quality of CaF₂ window from its nominal specification, because suppliers are used to testing products performance under relatively weak spectral lines from lamp, which can not tell the real operation behavior of CaF₂ window under intense laser irradiation. We report a method of testing the transmissivity of three different grade commercially available CaF₂ windows under high pulse repetition rate laser irradiation at 193nm, and the lab prototype of test module distinguishes them well with a repeatability better than 2%.

The recent advances in the development of Holmium monoclinic double tungstate thin-disk lasers are reviewed. The thin-disk is based on a 250-μm-thick 3 at. % Ho:KY(WO₄)₂ active layer grown on a (010)-oriented KY(WO₄)₂ substrate. When pumped by a Tm-fiber laser at 1960 nm with a single-bounce pump geometry, the continuous-wave Ho:KY(WO₄)₂ thin-disk laser generates an output power of 1.01 W at 2057 nm corresponding to a slope efficiency η of 60% and a laser threshold of only 0.15 W. The thin-disk laser is passively Q-switched with a GaSb-based quantum-well semiconductor saturable absorber mirror. In this regime, it generates an average output power of 0.551 W at ~2056 nm with η = 44%. The best pulse characteristics are 4.1 μJ / 201 ns at a repetition rate of 135 kHz. The laser performance, beam quality and thermo-optic aberrations of such lasers are strongly affected by the Ho³⁺ doping concentration. For the 3 at.% Ho³⁺-doped thin-disk, the thermal lens is negative (the sensitivity factors for the two principal meridional planes are -1.7 and -0.6 m^-1/W) and astigmatic. The Ho:KY(WO₄)₂ epitaxial structures are promising as active elements in mode-locked thin-disk lasers at ~2060 nm.

Laser damage resistance is a key factor for the improvement of high power laser system. The PETAL laser, developed by the CEA-CESTA (France), uses meter scale reflective optics to compress, transport and focalize sub-picosecond laser pulses at 1053nm with high-energy [1]. In the case of defect-free material, laser-induced damage in the sub-picosecond regime is known to be deterministic since the threshold depends only on the electronic structure of the irradiated materials, the pulse duration and the enhancement of the electric fields in thin film coatings. Based on this consideration, a mono-shot technique has been investigated to assess the intrinsic damage resistance of optical component with only one laser shot. On the other hand, while considering real optical components, manufacturing processes included nanoscale defects in the functional coating. These defects can be ejected when irradiated and strongly reduce the laser damage resistance of optics: rasterscan procedure has then been developed to determine defect-induced damage densities. These densities are found to be high even for fluences well below the intrinsic Laser-Induced Damage Threshold and they increase with the fluence. These experiments bring new information on the operating characteristics of optics in short pulse regime. Once damage is triggered, its evolution under subsequent irradiations has also been studied. Growth experiments have been compared to numerical simulations. The investigations on growth behavior allow a better estimation of the functional lifetime of an optic in its operating conditions. The whole of results, damage initiation and damage growth, is discussed to the light of the laser damage observed on PETAL optics.

A mid-infrared optical parametric chirped pulse amplification (OPCPA) system generating few-cycle pulses with multi-GW peak power at a 1 kHz repetition rate is presented. The system is pumped by a high-energy 2-μm picosecond source to exploit the high nonlinearity of ZnGeP2 (ZGP) crystals for parametric amplification. Employing a dispersion management scheme based on bulk materials and a spatial light modulator pulses as short as 75 fs are obtained in the idler at a center wavelength of 5 μm. The maximum generated pulse energy of 1.2 mJ translates into a peak power of 14 GW. Moreover, damage considerations of ZGP crystals at high 2 μm pump pulse intensities in the few-ten picosecond range are explored.

In high power laser system, the upstream flaw could induce light intensification in the downstream, thus damaging the optical component. In most of the research, the shape of the defect model is ideal, for example, Gaussian shape. However, the defect in the real system is non-ideal with different shapes. In this paper, the light intensification effect caused by defects with different shapes are compared by numerical simulation. Results show the shape dependence of downstream light intensification caused by flaws. When only the linear effect is considered, the change of defect shape could change the maximum light intensification factor and the downstream location for the maximum intensity. When the nonlinear effect is also considered, the light intensification effect will be more sensitive to the shape of defects. This research can provide some reference for the beam quality control and defect management in the high power laser systems.

Subsurface defects and contaminations will be generated during the grinding and polishing processes of optical components. Combined modulation is one of the important factors for the laser-induced damages of fused silica. In this paper, by using 2D finite-difference time-domain method, the light intensity distribution modulated by both radial crack and contaminant is studied on front/rear surface, respectively. The results show that the light intensity distribution is significantly affected by the aspect ratio of radial crack and the relative position between radial crack and contaminant. The simulations of the combined modulation on rear surface show that larger LIEFs are generated at certain relative positions compared with those in the single modulation of radial crack or contaminant. Meanwhile, with the increase of distance, the LIEFs are wave-like up and down fluctuations, and gradually tend to stable values. When there is no total internal reflection, the LIEF in contaminant on the crack wall rises significantly with increase of distance, the maximum LIEF occurs when the contaminant is near the intersecting line between radial crack and rear surface. The simulation of the combined modulation on front surface show that the variation of LIEFs in global domain are not very prominent.

Refractive index (RI) distribution plays a crucial role in the propagation of light trough any transparent medium except a vacuum. Gradient-index (GRIN) optical components made from heterogeneous materials with the continuously varying RI are finding an increasing role in optical communications, imaging, biomedical and industrial sensing. A diverse majority of existing methods for RI measurements are intended to perform point-wise RI measurements of homogeneous media. Withal only a few methods are devoted to measure RI gradient profiles (RIGP) of a GRIN medium. In this work we present a design of a noninvasive method aimed at the measurement of the RIGP of GRIN materials with the onedimensional RI variations. The physical principle of the proposed technique is based on the relaying a line structured laser beam displacements, which occur due to the difference of the refraction across medium, to the corresponding RIGP. Two of the main advantages of the proposed method are that the RIGP is rendered from a single measurement and without knowing of any RI value of a sample under the test, respectively. We show that the precision of up to 10^-4 in RIGP measurement can be achieved due to the ability of localizing laser stripe displacements with the subpixel accuracy, as well as using of a machine vision based triangulation system and corresponding image processing algorithms. The simplicity, robustness and measurement speed (indeed, real time) of the proposed method confirm that it can be utilized for real-time quality control of GRIN materials during the manufacturing process.

Quasi-CW laser damage behaviors of indium tin oxide (ITO) single-layer and polyimide (PI) on ITO bi-layer were investigated. The ITO single-layer with thickness of 25nm was deposited on fused silica substrate by magnetron sputtering, and the PI/ITO bi-layer was prepared by spin coating 80nm PI film on the 25nm ITO single-layer. Single-shot, with radiation time of 120 seconds, laser induced damage threshold (LIDT) of the samples were determined according to ISO 21254. The damage morphologies were mapped by optical profiler. It showed interesting phenomena that the PI top layer increased LIDT of the sample. The typical damage morphologies were blisters, and the height of the blisters increased as the laser power density increases. The formation and evolution of the blisters were analyzed.