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The high-quality surface of an optical element is a prerequisite for a high-power laser system design. Yttrium aluminum garnet (YAG) crystal is one of the most important materials for solid-state laser active medium. Laser-induced damage threshold (LIDT) of the YAG crystals might substantially limit the maximum output power of the whole laser system. In this research, we show the novel possibility of significant LIDT enhancement via plasma etching of YAG crystal surface for picosecond laser pulse durations. The dependence of the LIDT on the etching depth was investigated. With the optimized etching conditions, the LIDT value was increased by more than three times and reached the intrinsic LIDT of the bulk crystal.

Laser damage resistance of a transparent optical component significantly depends on its substrate surface quality. In this work low energy oxygen plasma etching of fused silica (FS) substrates was investigated in terms of plasma ion energy and etching depth. Laser induced damage threshold (LIDT) and surface roughness of experimental samples were measured and compared. The LIDT of plasma treated uncoated FS substrate increased from 5.0 ± 0.3 J/cm2 up to 75 ± 4 J/cm2 for 355 nm laser ns pulses without any surface roughness deterioration. Anti-reflective (AR) optical interference coating for 355 nm wavelength was deposited on plasma etched and non-etched substrates. Measured LIDT of plasma etched and AR coated component was 3.4 times higher comparing to coated non-etched substrate case and reached 14 ± 0.7 J/cm2. These results demonstrate successful application of low energy oxygen plasma etching for improving laser induced damage performance of uncoated and AR coated fused silica substrate without deterioration of surface roughness. This application could be also successfully extended for production of other transparent high power UV laser optics like polarizers, beam splitters, etc.

High surface quality of the optical elements is one of the key factors enabling their effective application in high power laser systems. In our work, commercially polished undoped YAG crystals were etched using low energy oxygen plasma. Surface roughness and optical properties were investigated using two different etching depths. Obtained results demonstrate smoothing of initial crystal surface and 1-4 % decrease of transmittance within UV-VIS spectral range.

Laser induced damage of optical coatings has been one of the most important targets during many decades of intensive research. Different techniques were used and explored with the aim to increase the resistance of multilayer systems to laser pulses. In this work, LIDT results of different “base” structures made by ion beam sputtering of Al2O3, SiO2 and their mixtures are presented, and further enhancement possibilities are discussed by applying additional layer structure using higher bandgap material – fluorides and glancing angle deposited SiO2.

Band-gap and refractive index are known as fundamental properties determining intrinsic optical resistance of multilayer dielectric coatings. By considering this fact we propose novel approach to manufacturing of interference thin films, based on artificial nano-structures of modulated porosity embedded in high band-gap matrix. Next generation all-silica mirrors were prepared by GLancing Angle Deposition (GLAD) using electron beam evaporation. High reflectivity (HR) was achieved by tailoring the porosity of highly resistant silica material during the thin film deposition process. Furthermore, the proposed approach was also demonstrated to work well in case of anti-reflection (AR) coatings. Conventional HR HfO2 and SiO2 as well as AR Al2O3 and SiO2 multilayers produced by Ion Beam Sputtering (IBS) were used as reference coatings. Damage performance of experimental coatings was also analyzed. All-silica based GLAD approach resulted in significant improvement of intrinsic laser damage resistance properties if compared to conventional coatings. Besides laser damage testing, other characteristics of experimental coatings are analyzed and discussed – reflectance, surface roughness and optical scattering. We believe that reported concept can be expanded to virtually any design of thin film coatings thus opening a new way of next generation highly resistant thin films well suited for high power and UV laser applications.

The nonlinear LBO (LiB3O5) crystal is widely used in many Nd : YAG, Yb : KGW and Nd : YLF lasers in order to generate higher optical harmonics. However, the most limiting factor in such applications is the optical resistance of their coated surfaces described by the so-called Laser Induced Damage Threshold (LIDT) parameter. In this work we investigate the “fatigue” (multi-pulse) behaviour of LIDTs in LBO crystals coated with different types of (single AR@355 nm and triple AR@355+532+1064 nm wavelength) anti-reflective coatings while optimising the refractive index designs and selecting appropriate sub-layer materials. All the coatings were produced of different oxide materials (ZrO2, Al2O3, SiO2) and ZrO2-SiO2 mixtures by using the ion beam sputtering (IBS) deposition technique. The optical damage resistance of both fixed and transient refractive index coatings is of special interest. Besides the spectral properties, the resistance to laser irradiation is characterised at the wavelength of 355 nm with laser pulses of nanosecond duration. The conclusions are drawn about the AR coated LBO with the most successful designs by the means of optical resistance.

The laser damage resistance of an optical element in high power laser systems depends significantly on the surface quality of the optical substrate. In this experiment, commercially polished fused silica substrates were etched in argon plasma generated by a RF source and their surface roughness, flatness and optical properties were investigated. This method can be applied in a vacuum chamber prior to deposition of the multilayer coatings without breaking the vacuum. It was shown that by etching the resistance to 355 nm, laser radiation could be improved more than 8 times. However, it strongly related with primary substrate quality. The etching depth from 100 nm suggests the optimum performance of surface quality in terms of surface low roughness, high flatness, and high laser damage threshold. These results are of significant importance for the manufacture of high quality laser optics on fused silica substrates. As an example of an application of our technology, anti-reflective and polarizing optical interference coatings were deposited on etched substrates and the increase of their resistance to laser radiation was measured.

The stability of thin film coatings for applications especially in the UV spectral range is oftentimes a limiting factor in the further development of radiation sources and beam delivery systems. Particularly, functional coatings on laser and conversions crystals as well as resonator mirrors show an insufficient lifetime due to laser-induced degradation. Previous investigations in the power handling capability of UV coatings mostly concentrate on the properties of pure oxide materials and particle mitigation. Recent innovations in ion beam sputtering technology enabled efficient deposition of mixture coatings of different oxide materials. In combination with an advanced thickness monitoring equipment, the described IBS deposition systems are capable of employing designs with sub-layers of a few nm thickness. In the present investigation, the stability of classical designs using pure oxide materials is compared with gradient index design concepts based on mixture materials. Reflecting and transmitting thin film coatings employing classical and gradient index approaches manufactured under comparable conditions are characterized in respect to their power handling capability. The results are analyzed before the background of theoretical expectations regarding contributions from field enhancement and absorptance effects.

Various investigations show that damage threshold of optical coatings by intense ultrashort laser pulses is closely related to the intensity of electric field at layer interfaces. LIDT measurements of high reflectance optical coatings using femtosecond pulses at 800 nm wavelength are presented. ZrO2, HfO2 and Ta2O5 as high refractive index materials for two sets of experiments were chosen. Two different coating designs were investigated: standard quarter-wavelength design with SiO2 overcoat and modified "E-field" non quarter-wavelength design with suppressed electric field. Damage sites were studied using optical and AFM microscopes. Relation between electric field distribution and damage morphology was observed. The results demonstrate, that suppressing electric field at layer interfaces enables to increase LIDT for high reflectance coatings almost twice if compared to standard quarter-wavelength design when using ultrashort laser pulses. However electric field distribution is sensitive to variations in thicknesses of outer layers, so deposition process should be precisely controlled to get improvement in LIDT of coatings.

High power laser systems are one of the most rapidly growing areas in the development of laser technology. This also leads towards higher requirements for environmental stability of optical components and their resistance to laser radiation. There are some reports showing that porous dielectric coatings are more resistant to intense laser radiation, however they have smaller environmental stability than denser coatings, which are more sensitive to laser radiation. The influence of important technological parameters (deposition rate, substrate temperature, energy of ions) on optical and microstructural properties of high reflection dielectric coatings based on Nb2O5/SiO2, and Ta2O5/SiO2 in VIS spectral region is presented. Furthermore the LIDT measurements using repetitive nanosecond laser pulses of Nb2O5/SiO2 and Ta2O5/SiO2 high reflecting optical coatings based on ISO 11254-2 standard are presented.

The performance of optical coatings for UV region (200-300 nm) is closely related to their optical losses. There are a few factors which significantly influence the extinction of deposited coating - deposition vacuum, contamination from filaments of e-beam guns, ion source and finally, the optical properties of selected deposition materials. In this work the contribution of these different factors was investigated and evaluated. HfO2, Al2O3 and SiO2 are the most widely used materials for producing UV optical coatings down to 200 nm. Influence of background oxygen pressure during HfO2 and Al2O3 deposition was evaluated which enabled to reduce extinction of the deposited UV optical coatings.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A quest for higher laser powers is one of the main driving forces in development of laser technology. Unfortunately all laser components have some limit to the intensity of optical radiation that can be applied on them - the so-called laser-induced damage threshold (LIDT). To enable further power scaling of laser devices, novel highly resistant optical components have to be developed. Such components are laser crystals, mirrors, fibers and other components typically coated with periodic dielectric layers made using e-beam, sputtering or sol-gel technologies. The production materials and methods of all the mentioned optics are under constant development, which requires a reliable quality test to provide the feedback to the manufacturing process; one of such tests are the measurements of LIDT. LIDT measurement procedure using repetitive laser pulses, as described in ISO 11254-2 standard, is time- and human resource consuming, if performed without automation. We developed an automated station for the measurements of LIDT that greatly reduces the required human resources and allows fast data collection. In this presentation, we briefly describe the main components of this automated LIDT test station. Furthermore we present the comparison of the latest results obtained on LIDT measurements of ZrO2/SiO2, Nb2O5/SiO2, Ta2O5/SiO2 and TiO2/SiO2 periodic high reflecting dielectric layers performed using repetitive nanosecond laser pulses.

This investigation was aimed at optimization of optical properties, stability and radiation resistance of optical coatings deposited using the standard vacuum coating plant equipped with the ion source for ion assisted deposition. There are some reports showing that porous dielectric coatings are more resistant to intense laser radiation, however they have smaller environmental stability than denser coatings, which are more sensitive to laser radiation. The influence of important technological parameters (deposition rate, substrate temperature, energy of ions) on optical properties and radiation resistance of high reflection dielectric coatings based on Nb 2 O 5 /SiO 2 and Ta 2 O 5 /SiO 2 in VIS spectral region is presented. Keywords: optical damage, optical coatings, ion-assisted deposition, niobium oxide, tantalum oxide, LIDT 1. INTRODUCTION In nearly all of today’s optical systems – from the simplest to the most complex – optical thin films play a critical role. In high-power applications, thin-film dielectric coatings are especially important because they are often limiting factor driving performance. Many of laser systems are based on nonlinear optical technology that requires high peak powers to maximize frequency conversions. The optics used in these systems requires low losses, high efficiency, low absorption, precise spectral properties and high laser damage thresholds. Improved stochiometry, increased environmental stability, smoother surfaces and densities of thin films approaching values of bulk materials are the main benefits of ion-assisted deposition (IAD) technology. However, the information about the influence of IAD on laser induced damage threshold (LIDT) is contradictory.

In this paper, we performed comparison of laser-induced damage threshold (LIDT) for ion-assisted deposition (IAD) and non-IAD coatings in nanosecond and femtosecond pulse ranges. S-on-I tests were performed for 10/sup 4/ pulses at 1-kHz repetition rate. Equivalent BK7 substrates with the same surface roughness were coated for comparison of the two coating. The results were analysed using AFM and XRD. The difference in LIDT values for 130 fs pulses between the IAD optical coating and non-IAD coating deposited on the substrate with the same polishing quality was equal within range of measurement accuracy. In contrast to femtosecond pulses the LIDT values for the nanosecond pulses showed stronger and opposite correlation with the deposition methods.

High density, improved adhesion and environmental stability are the main features of dielectric optical coatings produced using ion-assisted deposition (IAD) technology. However, investigations of resistance of IAD coatings to intensive laser radiation show controversial results. A series of experiments were done to examine the influence of ion gun operation on the transmittance of fused silica substrates. It was shown that operation of ion source introduced extinction in UV spectral range. Optical properties of single hafnia layers and multilayer dielectric mirrors deposited using conventional e-beam evaporation and different modes of IAD were investigated. Microstructural analysis using X-ray diffraction (XRD) measurements and AFM scanning of coated areas was carried out. Single hafnia layers deposited using high energy ion assistance had more amorphous structure with smaller crystallites of monoclinic phase. High reflection UV mirrors deposited using high energy ion assistance had slightly higher mean refractive indices of hafnia, higher extinction than conventional e-beam deposition, but demonstrated slightly higher laser induced damage threshold (LIDT) values measured at 355 nm. Deposition using the lowest energy ions produced the most porous coatings with the best LIDT of 7.7 J/cm 2 .

The ion assisted thin film deposition (IAD) method has been used extensively for more than two decades, but questions about possibility of improving of the laser-induced damage threshold (LIDT) by this method compared with the conventional electron-beam evaporation (non-IAD) method are still not fully answered. A more complete understanding of different factors that can influence laser-induced damage threshold is necessary for continued development of multilayer dielectric coatings optimized for high-power laser applications. To clarify these factors we performed comparison of LIDT for IAD and non-IAD coatings in nanosecond and femtosecond pulse ranges. High reflectance mirrors at 800 nm and 532 nm were tested. Mirror coatings were made of ZrO 2 and SiO 2 . Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. Two lasers were used for the measurements: Nd:YAG (λ = 532 nm, τ = 5 ns) and Ti:Sapphire (λ = 800 nm, τ = 130 fs). Measurements at 800 nm and 532 nm were performed at 1-kHz and 10 Hz pulse repetition rate respectively (S-on-1 test). The damage morphology of coatings was characterized by Nomarski microscopy and relation of LIDT with coating parameters was analyzed.

A comparison of laser induced damage thresholds (LIDT) of ion assisted deposition (IAD) and standard electron beam deposition dielectric coatings on BK7 glass with different surface roughness was performed. Five types of high reflectance mirrors at 800 nm and two types of high reflectance mirrors at 1064 nm were tested. Mirror coatings were made of ZrO 2 and SiO 2 . Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. Two lasers were used for the measurements: Nd:YAG (l = 1064 nm, t = 13 ns) and Ti:Sapphire (l = 800 nm, t = 130 fs ). All measurements were performed at 1-kHz pulse repetition rate (S-on-1 test). A fixed spot size was used for each laser. For 1064 nm it was ~ 70 um and for 800 nm ~ 500 um. The damage morphology and structure of coatings were characterized by an atomic force microscopy (AFM), Nomarski microscopy and X-ray diffraction (XRD).

Commercially polished fused silica substrates were etched and their properties were investigated. Resistance to laser radiation was significantly enhanced and no degradation of surface roughness, flatness or loss in transmission spectra was observed.