Single HfO2 layers were deposited on fused silica substrates using Hf as a starting material by PIAD technology. Inductively coupled RF plasma source was used, generating pure oxygen, as well as argon-oxygen mixture plasma. Mean plasma ion energies were varied from ~120 to ~270 eV. Optical properties, stress values, direct absorption for 1064 nm, surface roughness, film structure were determined for the samples and were also compared to pure e-beam and IBS formed HfO2 single layers. It is shown, that by selecting proper plasma parameters it’s possible to obtain HfO2 layer with 185 MPa compressive stress, suitable for making dense multilayer coatings with moderate compressive stresses. However, simultaneously obtained high optical absorption and extinction implies necessity for further investigations.
Remote sensing, range finding, optical communications have strong demand for compact, eye-safe lasers. Co2+:MgAl2O4 crystals can be used as a passive Q-switchers to obtain pulses of compact Er:glass lasers and might be one of the limiting factors, determining their maximum output power. This study presents oxygen plasma etching of commercially-polished Co2+:MgAl2O4 crystals, including investigation on their spectrophotometric, surface and LIDT (R(1)-on-1) properties using two different lasers and beam diameters - 172 µm and 55 µm at 1540 nm. Measurements higher fluence laser and smaller 55 µm laser beam diameter allowed determination of all etched crystals and revealed dramatic increase of their surface LIDT comparing to untreated sample.
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 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.
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.
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 Nb2O5/SiO2 and Ta2O5/SiO2 in VIS spectral region is
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.
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.
An influence of substrate temperature and working gas in coating plant during evaporation process on the laser-induced damage threshold (LIDT) of high reflection dielectric coatings was experimentally investigated. Also a LIDT comparison of ion assisted deposition (IAD) and conventional electron-beam evaporation (non-IAD) coatings fabricated under the same substrate temperature (300 °C) was performed. A set of different type high reflection mirrors were tested for LIDT at 532 nm for 3.4 ns pulses: one type of non-IAD and six types of IAD evaporated at different substrate temperatures and different working gases. All coatings were made on BK7 glass substrates from ZrO2 and SiO2. The computer controlled test station for LIDT measurements according to the requirements of current ISO 11254-2 standard was used. All measurements were performed at 10 Hz pulse repetition rate (S-on-1 test). The tests were performed at fixed spot size. Strong LIDT dependence on substrate temperature of was observed.
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/cm2.
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 ZrO2 and SiO2. 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 ZrO2 and SiO2. 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).
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