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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.

A principal possibility to overcome fundamental (intrinsic) limit of pure optical materials laser light resistance is investigated by designing artificial materials with desired optical properties. We explore the suitability of high band-gap ultra-low refractive index material (n less than 1.38 at 550 nm) in the context of highly reflective coatings with enhanced optical resistance. The new generation all-silica (porous/nonporous) SiO2 thin film mirror with 99% reflectivity was prepared by glancing angle deposition (GLAD). Its damage performance was directly compared with state of the art hafnia/silica coating produced by Ion-Beam-Sputtering. Laser-Induced Damage Thresholds (LIDT) of both coatings were measured in nanosecond regime at 355 nm wavelength. Novel approach indicates the potential for coating to withstand laser fluence of at least 65 J/cm2 without reaching intrinsic threshold value. Reported concept can be expanded to virtually any design thus opening a new way of next generation thin film production well suited for high power laser applications.

In this work, the possibilities to engineer the inner nanostructures of individual layer and applications for the formation of coating-based phase retarders were demonstrated. The novel approach of high band-gap birefringent multilayer coatings was proposed and investigated. Zero-order retarders with low optical losses and the potential to withstand high laser fluences at 355 and 266 nm wavelengths were produced and tested in high power laser system.

A simple procedure is demonstrated for fabrication of waveplates which can be released from substrate by laser cutting. Oblique angle deposition, chemical etching and laser inscription steps were used for the final lift-off and release of micro-waveplates in HCl solution., 3 pages

Diffraction gratings that do not reach >99% diffraction efficiency (DE) are a major source of losses in chirped pulse amplification systems in current ultrashort pulse lasers. Here, we suggest an optimization route for diffraction gratings based on a commercially available, 1064 nm laser line high reflectivity (HR) multilayer-dielectric mirror. We modeled optical response of 1700 lines/mm gratings in Littrow configuration imposed on this typical quarter wavelength-based stack structure. We demonstrate that theoretically DE of 99.995% at a single wavelength and >99% in 1003–1034 nm wavelength range can be achieved using the original dielectric mirror layer setup. The high DE spectral range could be broadened up to 992–1060 nm while still maintaining >99% DE, when the dielectric stack with a thinner topmost layer is considered. In both cases the diffraction grating grooves must span the two topmost layers, including a higher refractive index material, i.e. Nb2O5. Alternatively, >99% DE in 1009–1033 nm wavelength range can be also obtained using a thicker topmost layer. In this case, the grating groove spanning just the SiO2 layer is sufficient. However, twice as deep grooves and an accurate control of the filing factor is required. We show through numerical simulations that this structure is more sensitive to angular positioning error. The modelled electric field distributions showed reduced laser damage probability in the latter case. All investigated structures can sustain the bandwidth of sub-100 fs pulse length laser pulses. The proposed optimization route can serve as a guideline to design any diffraction grating based on commercially available dielectric mirrors.

Laser resistance of optical elements is one of the major topics in photonics. Various routes have been taken to improve optical coatings, including, but not limited by, materials engineering and optimisation of electric field distribution in multilayers. During the decades of research, it was found, that high band-gap materials, such as silica, are highly resistant to laser light. Unfortunately, only the production of anti-reflection coatings of all-silica materials are presented to this day. A novel route will be presented in materials engineering, capable to manufacture high reflection optical elements using only SiO2 material and GLancing Angle Deposition (GLAD) method. The technique involves the deposition of columnar structure and tailoring the refractive index of silica material throughout the coating thickness. A numerous analysis indicate the superior properties of GLAD coatings when compared with standard methods for Bragg mirrors production. Several groups of optical components are presented including anti-reflection coatings and Bragg mirrors. Structural and optical characterisation of the method have been performed and compared with standard methods. All researches indicate the possibility of new generation coatings for high power laser systems.

Optical elements for polarization control are one of the main parts in advanced laser systems. The state and intensity of polarized light is typically controlled by optical elements, namely waveplates. Polymers, solid or liquid crystals and other materials with anisotropic refractive index can be used for production of waveplates. Unfortunately, most of aforementioned materials are fragile, unstable when environmental conditions changes, difficult to apply in microsystems and has low resistance to laser radiation. Retarders, fabricated by evaporation process, do not consist any of these drawbacks. In order to manufacture such optical components with high quality, characterisation of deposition parameters are essential. A serial bi-deposition method was employed to coat anisotropic layers for polarisation control. Such waveplate can be deposited on micro optics or other optical elements, essentially improving compact optical systems. The range of available materials is limited by absorption losses for waveplates in UV spectral region. Therefore, the investigation was accomplished with four eligible candidates – TiO2, LaF3, Al2O3 and SiO2. Structural (XPS, XRD) and optical (spectrophotometry, ellipsometry) analysis have shown Al2O3 and SiO2 as the most applicable materials for UV spectral region.

ZrO2 exhibits low optical absorption in the near-UV range and is one of the highest laser-induced damage threshold (LIDT) materials; it is, therefore, very attractive for laser optics applications. This paper reports explorations of reactive sputtering technology for deposition of ZrO2 films with low extinction coefficient k values in the UV spectrum region at low substrate temperature. A high deposition rate (64 % of the pure metal rate) process is obtained by employing active feedback reactive gas control which creates a stable and repeatable deposition processes in the transition region. Substrate heating at 200 °C was found to have no significant effect on the optical ZrO2 film properties. The addition of nitrogen to a closed-loop controlled process was found to have mostly negative effects in terms of deposition rate and optical properties. Open-loop O2 gas-regulated ZrO2 film deposition is slow and requires elevated (200 °C) substrate temperature or post-deposition annealing to reduce absorption losses. Refractive indices of the films were distributed in the range n = 2.05–2.20 at 1,000 nm and extinction coefficients were in the range k = 0.6 × 10−4 and 4.8 × 10−3 at 350 nm. X-ray diffraction analysis showed crystalline ZrO2 films consisted of monoclinic + tetragonal phases when produced in Ar/O2 atmosphere and monoclinic + rhombohedral or a single rhombohedral phase when produced in Ar/O2 + N2. Optical and physical properties of the ZrO2 layers produced in this study are suitable for high-power laser applications in the near-UV range.

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.

In this work we report an experimental investigation of subsurface damage (SSD) in conventionally polished fused silica (FS) substrates which are widely used in laser applications and directly influence performances of optical elements. Two procedures were developed: 1 – acid etching and 2 – superpolishing. Additionally, surface roughness and total integrated scattering (TIS) measurements were performed to find correlation between the main surface properties and laser induced damage threshold (LIDT) as circumstantial evidence of elimination of SSD. Different durations of acid etching have been used to study LIDT of FS substrates. These experiments revealed that the optimal etching time is ~1 min. for a given acid concentration. Laser induced damage threshold of etched and SiO2 layer coated FS samples increased ~3 times, while of the ones that were not coated - 4 times. It has been revealed that for nonetched surface a single nano- to micro-scale absorbing defect ensemble most likely associated with polishing particles within Beilby layer was dominant, while damage morphology in ~1 min etched FS sample had no point defects observed. More than 5 times lower roughness value (RMS) was obtained by superpolishing procedure using colloidal silica abrasive particles. LIDT of such superpolished fussed silica substrates was also strongly increased and compared with conventional CeO2 abrasive polishing.

In the past years the usage of mixed oxides coatings lead to an important improvement of laser damage threshold and quality of optical elements. In this study influence of post treatment procedure - ex-situ annealing - is examined in terms of quality, optical constants and laser induced damage threshold (LIDT) of mixed HfO 2 and SiO 2 coatings. Monolayer thin films containing different fractions of HfO 2 are deposited with ion beam sputtering technology (IBS.) All samples are post annealed at different temperatur es and optimal regimes are defined. Re fractive index and absorption coefficient dispersion is evaluated from transmission spectra measurements. Surface roughness of all samples is characterized before and after deposition and annealing, using atomic for ce microscopy (AFM). Microstr uctural changes are identified from changes in surface topography. Furt her, optical resistance was characterized by 5.7 ns duratio n pulses for 355 nm wavelength laser radiation, performing 1-on-1 sample exposure tests with high resolution micro-focusing approach for monolayer samples and S-on-1 tests for multilayer reflectors. Morphology of damaged sites was analyzed through optical microscopy. Finally, conclusions about annealing effect for mixed HfO

Columnar microstructures were deposited with Ta2O5 material. Optical investigation showed most anisotropic sample to be formed at 60 deg angle. Laser induced damage were also measured for the samples deposited at 0 deg and 60 deg angles.

Despite the growing improvement in optical polishing and deposition technologies optical resistance of the laser components used for high-power UV applications remains insufficient in many cases. In this study influence of different fused silica substrate preparation, post treatment processing and de position techniques are exam ined in terms of surface roughness, optical scattering and laser damage performance. The conventional techniques of polishing, etching, and finally surface cleaning of substrates have been investigated. Further, a part of samples were also coated with SiO 2 monolayer by Ion Beam Sputte ring (IBS) technique. Su rface quality was characterized prior to and after the treatment and deposition processes by the means of total integrated scattering (TIS) and atomic force microscopy (AFM). The experimental results of surface roughness measurements exhibited a good correlation between AFM and TIS methods. Further optical resistance was characteri zed with 10 ns duration pulses for 355 nm wavelength laser radiation performing 1-on-1 sample exposure test with high resolution micro-focusing approach. A dominating damage precursor ensembles produced during manufacturing processes were identified and directly compared. Finally, the conclusions about the quality influencing factors of investigated processes were drawn. Keywords: laser damage threshold, total integrated scattering, polishing, cleaning, etching, coating, ion-beam sputtering.

Recent advances in ion beam sputtering technology enabled efficient deposition of different oxide mixture coatings. In the present investigation, coating materials ZrO2and SiO2were utilized for the synthesis of different mixtures. The goal of the conducted investigation was to explore the potential of ZrO2/SiO2mixtures, especially for applications in the range of the UV spectral. Deposited dielectric mirrors having classical quarter-wave and “rised” design with ZrO2/SiO2mixture for 266nm and 355nm showed good resistance to laser radiation.We also investigated the “fatigue” behavior of LIDT’s in LBO crystals with single, dual and triple wavelength anti-reflective coatings (AR@355, AR@532+1064 and AR@355+532+1064) in order to optimize design and layer materials. The influence of gradient refractive index profiles on damage resistance is of a special interest. We selected a few designs of antireflective coatings which demonstrated the best resistance to laser radiation.An experimental study onrugatefilter coatings showed the need for a more accurate characterization of optical properties of metal oxides and their corresponding mixtures. Santrauka Šiame darbe tiriamas jonapluoščio dulkinimo technologija dengtų ZrO2/SiO2ir jų mišinių optinių savybių pritaikymas kintamo lūžio rodiklio (Rugate tipo) optinėms interferencinėms dangoms formuoti. Mišiniuose naudojant mažą ZrO2frakciją, buvo išplėstas šios medžiagos pritaikymas UV srityje. Remiantis gautais rezultatais, suformuotos didelio atspindžio (HR) ir skaidrinančios (AR) dangos esant λ= 355nm bangos ilgio spinduliuotei, taip pat teoriškai sumodeliuotos ir suformuotos daugiabangio skaidrinimo (AR@ 355+532+ 064) Rugate tipo dangos ant LBO (LiB3O5) netiesinių kristalų. Palygintos šių dangų optinės savybės su tomis dangomis, kuriose panaudoti gryni metalo oksidai (fiksuoto lūžio rodiklio). Atliktas Rugate tipo dangų, pasižyminčių siaura didelio (R > 99,9%) atspindžio zona ties 532nm ir pralaidumu likusioje spektro dalyje, teorinis modeliavimas ir eksperimentinis garinimas. Raktiniai žodžiai:Rugate tipo dangos, metalų oksidai ir jų mišiniai, LBO kristalai, notch filtrai, jonapluoštis dulkinimas

We report an experimental investigation in the laser-induced damage threshold (LIDT) of optical coatings materials. The samples are single layers of Al2O3, Nb2O5, HfO2, SiO2, Ta2O5, ZrO2 deposited through different deposition techniques (evaporation or sputtering with/without ion assistance) and mixtures of Al2O3/SiO2, Nb2O5/SiO2, HfO2/SiO2, Ta2O5/SiO2 and ZrO2/SiO2 on silica substrates. The LIDT is measured at 1030nm, 500fs in single shot mode. The results are expressed and compared in term of LIDT as a function of bandgap and LIDT as a function of refractive index.

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.

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.

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.