This PDF file contains the front matter associated with SPIE Proceedings Volume 9238, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.

In high power laser systems, the laser-induced damage threshold (LIDT) in optical coating is very important parameters for obtaining high performances. Recently, LIDT was found to have strong temperature dependences in the bulk, surface of substrates, and in coatings. These temperature dependences of LIDT were carefully measured, and the damage formation model was constructed regarding to this temperature dependence. To explain this temperature dependence of LIDT, the temperature dependences of the initial electron generation and electron multiplication in the avalanche process were taken into account. On the other hand, LIDT in optical coating is very sensitive to organic contaminations accumulated in coating layers during storage and using condition. This paper also introduces the oil-contamination problem in LFEX laser system for First Ignition scheme in the laser fusion. We have analyzed contaminants and evaluated the effects of the contamination. We also developed new cleaning methods to remove contamination from the coating, and we have succeeded to prevent the degradation in LIDT for the duration of evacuation with Silica-gel in the chamber. The quantitative analysis of contamination on LIDT was made. We have investigated the characteristics of LIDT in dielectric coatings under the controlled contamination with several materials.

The laser induced damage densities measured on fused silica surface are found to be higher when produced with multiple longitudinal mode pulses than those produced by single longitudinal mode pulses at 1064 nm. The enhancement of the three-photon absorption due to the intensity spikes related to longitudinal mode beating might favor the damaging process at this wavelength. At 355 nm the picture is different. The absorption is supposed to be linear and an opposite behavior occurs. Evidences of a process leading to the possible annealing of a part of absorbent defects are discussed in this paper.

Electric field intensity formed in a multilayer mirror has been investigated numerically based on the analytical calculation, for the case where two laser beams with different wavelengths enter the mirror simultaneously. In this case, the intensity changes with time due to the interference between the two waves. By taking into account it, a design method of a multilayer mirror for high power lasers is proposed.

The laser induced ultrafast dynamics in fused silica under few-cycle UV laser excitation is investigated in the present study. Using sub-10 fs UV laser pulses, we characterize the free carrier dynamics in the fused silica before laser damage. After laser excitation, free carrier in the conductive band is found to turn into self-trapped excitons within about 300fs. It is possible that the trapped exciton will result in the incubation effect under the condition of ultrafast high-frequency pulsed UV laser exposure.

Laser damage of TYPE-I KDP plate was investigated. High-purity large-aperture KDP crystals used for second harmonic frequency generation in high power laser systems were prepared by rapid growth. The different parts of the KDP boule, spanning the growth history including early, middle and late growth stages, were examined for their bulk defect properties and laser damage behaviors. Ultra-microscopy was employed to analyze the preexisting laser scattering defects, and the correlations between scattering and laser damage initiations/ growth were identified. The laser damage fluence was dominated by the defect scale or the scattering intensity. Simulation of thermal response of the defects under laser radiation indicated the micro-explosion occurrence. Thermal annealing and laser conditioning were applied to reduce defect density and improve laser damage resistance. Based on the above techniques, laser induced damage threshold (LIDT) of 400 mm aperture TYPE-I crystal plate exceeded 22J/cm² (1064nm, 3ns), which met the requirements of the high power laser systems.

In this paper, laser induced reactions of bulk defects in DKDP crystals were real-time detected by ultra-microscopy while high power laser irradiation with different photon energy, and the defect elimination processes were observed. It’s found that there were two kinds of bulk defects that can be eliminated: submicron-scale defect and nanoscale defect clusters. The decrement of submicron-scale defects was related to the laser parameters, such as laser fluence and photon energy. The defect decrement could achieve its maximum value at appropriate laser fluence, while the photon energy was fixed. It indicated that up to ~47% of defects could be eliminated by laser irradiation at 3.50eV (355nm). While the laser fluence was fixed, the amount of defects reduced by laser irradiation at 3.50eV was larger than that at 1.17eV (1064nm). The nanoscale defect clusters were hard to be eliminated by laser irradiation at 1.17eV, while most of them could be reduced by laser irradiation at 3.50eV.

Within ESA’s ADM-Aeolus and EarthCARE missions Doppler-wind Lidar systems will be operated in the Earth’s orbit to measure global wind profiles. The active instrument will be based on a Nd:YAG laser, frequency tripled by nonlinear optical crystals. Different crystals are therefore to compare and qualify in regard of their space acceptability. A dedicated set-up to measure the maximum conversion efficiencies and its stability during longterm operation for KTP crystals (SHG) and BiBO crystals (SHG and THG) is presented in this work. In order to detect gray-tracking and its influence on thermal lensing in situ, measurements with a Shack-Hartmann sensor and a co-aligned HeNe laser were performed. Conversion efficiencies were 76±3 % at SHG for KTP and BiBO crystals and 48±2 % at THG with a combination of two BiBO crystals. During longterm experiments of 60 million laser pulses, conversion efficiencies were demonstrated to be stable over time (±1 % at SHG with KTP and ±2 % at THG with BiBO). The occurrence of gray-tracking was detected in the KTP crystal and the resulting thermal lensing with an exponential saturation over time was observed in situ.

We present the results of the investigation of a Nd:glass rod laser amplifier 320 mm long and 150 mm in diameter. The gain and depolarization distributions over the amplifier aperture as well as the stored energy have been measured for different values of the pump energy. The small signal gain averaged over the rod aperture was 2.3 when the pump energy was 48 kJ. The corresponding stored energy was 500 J. We have determined the maximal pulse repetition rate that is quiet safe from the point of the thermal shock damage. It is about 1 shot per 5 minutes. The calculated focal length of the thermal lens exceeds 1.5 km in such a regime. We have shown that the use of a couple of such amplifiers gives the possibility to increase the energy of laser pulses up to 650 J.

In new laser facilities, broadband ultra-short pulses need to be transported in large and energetic beams to the experiment chambers through high performance optical components. We studied laser resistance of electronbeam- deposited materials in order to design meter scale high reflective optics according to petawatt-system requirements. We report intrinsic laser resistance of several dielectric materials, spectral performances of high reflective coatings and their laser induced damage threshold at different pulse durations from 150ps to 11fs.

Laser-induced damages on optical components will cause wavefront modulation effect and further influence the downstream propagation of transmitted beam. In this paper, a method named polygon-binaryzation modeling (PBM) is applied to describe this effect taking place on dielectric coating. Both theoretical and experimental studies are carried out. In theory, the principle of PBM is introduced. Downstream beam propagation behind damage site could be simulated based on the wavefront model. In experiment, a pump-and-probe testing bench is designed. Wavefront properties of coating damages with opaque and transparent morphologies are detected. Furthermore, based on the experimental parameters, PBM is simulated. The good match between simulation and experiment demonstrates the validity of our method.

Laser damage growth on the exit surface of fused silica optics is considered as exponential, the growth coefficient depending essentially on fluence. In this presentation, experiments with large beams have been carried out at 351 nm under nanosecond pulses. A statistical analysis has then been conducted leading to a refined representation of the growth. The effect of several parameters has also been taken into account to describe precisely the growth phenomenon. Finally, the two main parameters reporting the growth are the mean fluence and the size of the damage sites. Contributions of other parameters have been estimated too: the number of neighbors around the damage site, the shot number.... From experimental results, a model based on a statistical approach has been developed that permits the description of a complete sequence of growth. At the end, the knowledge of damage initiation and damage growth permit the determination of the lifetime of optical components illuminated with successive shots.

The Z-Backlighter lasers at Sandia National Laboratories are kilojoule class, pulsed systems operating with ns pulse lengths at 527 nm and ns and sub-ps pulse lengths at 1054 nm (www.z-beamlet.sandia.gov), and are linked to the most powerful and energetic x-ray source in the world, the Z-Accelerator (http://www.sandia.gov/z-machine/). An important Z-Backlighter optic is a flat, fused silica optic measuring 32.5 cm × 32.5 cm × 1 cm with an antireflection (AR) coating on both sides. It is used as a debris shield to protect other Z-Backlighter laser optics from high-velocity particles released by the experiments conducted in the Z-Accelerator. Each experiment conducted in the Z-Accelerator releases enough debris to cloud the surface of a debris shield, which means that a debris shield cannot be used for more than one experiment. Every year, the large optics coating facility [1] at Sandia provides AR coatings for approximately 50 debris shields, in addition to AR coatings for numerous other meter-class Z-Backlighter lenses and windows. As with all Z-Backlighter optical coatings, these AR coatings must have a high laser-induced damage threshold (LIDT) in order to withstand the powerful Z-Backlighter laser fluences. Achieving a good LIDT depends not only on the coating deposition processes but also on the polishing and cleaning processes used to prepare the coated and uncoated surfaces [2]. We spend a lot of time, both before and after the coatings have been deposited, manually cleaning the optics, including the debris shields, even though they are an expendable type of optic. Therefore, in this study we have tested new cleaning methods in addition to our current method to determine their impact on the LIDT of AR coatings, and conclude whether a shorter-duration or less labor-intensive cleaning process would suffice.

The damage morphology change condition of 1064nm reflector under multi-shot was investigated. Two typical kind of damage, scald and delaminate, were careful characterized in the damage growth process by SEM. The scald damage tends to become delaminate damage under some certain condition. Huge experiments supports that this morphology change condition has a close connection with scald initial fluence, subsequent fluence and shot number. The relationship among these factors is for the first time achieved to offer the “safety lines” for components.

Experiments showed that high density point and linear bulges emerged on the surface of fused silica after thermal annealing in an oven. To analyze the source of annealing bulges , the surface morphology was studied by atomic force microscopy, and the depth profile of impurities were analyzed by secondary ion mass spectroscopy. The results showed that impurities in redeposition layer migrated from subsurface to surface during annealing. The potential of using thermal annealing to reveal surface structure was developed.

The figures of the polishing pad are of great significance for the figures of optical workpieces in the continuous polishing process. Three main factors which affect the figures of the pad, including the polishing pad creep deformation, the calibration plate grinding and the ambient change in temperature, are analyzed in this work. Processing parameters including the eccentricity between the polishing pad and the calibration plate, the calibration plate thickness, angular velocities of the pad are introduced in this analytic process. With this method, the figures of the polishing pad in the continuous polishing can be obtained. This work provides theoretical guidance for the deterministic processing of the continuous polishing process.

Damages on or near polished substates can be easily observed by TIRM(Total Internal Reflection Microscopy). In our experiments we found that there was a strong dependence of scratch visibility on the angle(φ) between scratches and normal direction of the incident plane. In this paper, the scattered field distribution of a scratch and the imaging properties of a microscope are analyzed. We believe that it is the anisotropy of illumination in TIR-illumination mode that causes the visibility changes of a scratch. After taking this directionality into consideration, we propose an experimental method for TIRM to take picutures with all scratches in all directions in one image.

With the purpose of understanding nanosecond laser induced damage mechanisms when working with multiple longitudinal mode pulses, an accurate measurement of the temporal profiles is required. In this study, the use of a streak camera with a wide bandwidth is justified through the knowledge of the Nd:YAG spectral characteristics. A statistical and phenomenological analysis of multiple longitudinal modes intensity profiles is then performed through experiments and modeling. The resolution limitation of our photodiodes is also discussed.

Novolak resists which are implanted with B, P, and As ions, respectively, were irradiated with a pulsed 532nm laser. Regardless of the implanted ion species and density, more than 74 % of the laser power was found to absorb into the Si wafer surface. For the laser irradiation of 1 pulse, the ion-implanted resist with a density of 5.0x10¹³ atoms/cm² was completely stripped in the same way as that of a non-implanted resist. The optical absorption of the resist surface increased as the density of the ion-implantation increased. In case of the ion-implanted resist with a density of 5.0x1015 atoms/cm², the resist was stripped by 20 pulses irradiation without occurring laser-induced surface damage. A scanning removal of the highly ion-implanted resist was also successfully stripped by using an optimized irradiation condition. A highly ion-implanted resist was continuously stripped by the scanning laser irradiation with 20 pulses.

High-efficiency cavity-dumped ytterbium-doped yttrium aluminum garnet (Yb:YAG) laser was developed. Although the high quantum efficiency of ytterbium-doped laser materials is appropriate for high-efficiency laser oscillation, the efficiency is decreased by their quasi-three/four laser natures. High gain operation by high intensity pumping is suitable for high efficiency oscillation on the quasi-three/four lasers without extremely low temperature cooling. In our group, highest efficiency oscillations for continuous wave, nanosecond to picosecond pulse lasers were achieved at room temperature by the high gain operation in which pump intensities were beyond 100 kW/cm².

High-average-power, high-repetition-rates picosecond-pulsed regenerative ytterbium-doped yttrium aluminum garnet (Yb:YAG) laser amplifiers were developed. The architecture used in the amplifiers, which are named as thin-rod, has a unique cooling scheme like slab lasers and also has a unique pumping scheme like photonic crystal fiber lasers, is suitable for high-average power Ytterbium lasers. This architecture also has high gain characteristics which is appropriate for the regenerative spectral and pulse shaping on high-repetition-rate, ultrashort-pulse amplifications.