Laser damage thresholds of optical coatings

Since the very beginning of laser technology, Laser Induced Damage Thresholds (LIDT) of optical components were always an obstacle for the application of laser systems operating at high power levels. Also, further progresses in the development of new high power laser concepts are often directly limited by the availability of advanced optical components with high quality and LIDT-values. Nowadays, in the course of the development of optical materials with excellent quality and power handling capability, the problem of laser induced damage has shifted from the bulk to the surface of the optical component. The optical surface is objected to various production steps and environmental influences, which modify its structure and composition. Especially, the thin film coating, which is deposited on the optical surface to adapt its reflectance and transmittance to the application, contributes predominantly to the reduction of the LIDT-values. As a consequence, the measurement and optimization of the power handling capability of thin films is considered as one of the primary research areas in modern optics technology and is supported by an extensive scientific community.In the present paper, a brief review will be given on selected fundamental damage mechanisms in thin films considering different operation conditions of modern laser systems. Also, the current standards for the measurement of LIDT will be described, and examples illustrating some practical aspects of high power optical coatings will be presented. Finally, recent trends in laser technology will be discussed in respect to research in laser induced damage.     

Comparison of algorithms used for optical characterization of multilayer optical coatings

Two algorithms used for the on-line and off-line characterization of multilayer optical coatings are experimentally compared using test samples produced by two different deposition processes and different monitoring approaches. One of these algorithms, called the triangular algorithm, demonstrates its superiority in all considered situations. We performed experiments with multilayer samples formed by high-density thin films, which allowed us to neglect possible errors in the film refractive indices and concentrate only on errors in the thicknesses of the layers of the produced coatings.     

Light scattering losses of high reflection dielectric multilayer optical devices

Light scattering losses from dielectric multilayer are becoming increasingly important for designing high precision performance optical devices. In this paper, we applied the bi-directional reflectance distribution function of optical multilayer and analyzed the total reflectance scattering losses based on both the completely correlated and non-correlated interface models to compare with a high reflection 17-layer optical multilayer deposited on roughness of 2.8 nm substrates. The experimental result supports the completely correlated interface model as firstly the wavelength dependence is in good agreement with the phase change of the calculated result and secondly the calculated scattering intensity of the completely correlated interface scattering model is approximately the same as that of the measured scattering spectrum, while the intensity using the non-correlated interface scattering model is significantly higher than the measured result in the high transmission ranges and lower in the main band of the high reflection range.     

Patterning of multilayer dielectric optical coatings for multispectral CCDs

The development of optical sensors for spacecraft applications requires that all components be as lightweight as possible. One method to reduce the weight of a multispectral optical system is to eliminate beamsplitting optics and multiple detectors by patterning a filter array directly onto a CCD. However, techniques commonly used in the production of these filter arrays result in decreased image resolutions. This can greatly impact the performance of sensors used for applications such as planetary probes. To address this issue, we have studied the patterning of multilayer dielectric optical coatings in a small scale, two dimensional array, which will allow development of a four color sensor with a resolution one-half that of monochromatic sensors (compared to one-fourth or less for a four color striped array). We have developed ion milling techniques for the preparation of optical filter arrays which are patterned on a scale as small as 7.5 μm, enabling each pixel of a CCD to have its own associated filter. This paper presents details of the fabrication of these multispectral arrays, and discusses problems associated with pixel-sized filters.     

Measurement of mechanical properties of multilayer coatings by nanoindentation

Abstract

Multilayer protective coatings of alternate aluminium and titanium diboride TiB₂ layers have been tested by nanoindentation to measure both hardness and Young's modulus values. The initial results show that the values obtained depend upon the depth of indentation. An alternative view is presented to show that by considering the percentage of each coating in contact with the indenter a single relationship between either hardness or Young's modulus and the amount of aluminium layer penetrated can be produced. This technique allows the influence of the percentage ceramic on the results obtained to be identified. Comparison of the nanoindentation results with three point bending tests show how the coating structure influences the results obtained.     

Defect induced laser damage in oxide multilayer coatings for 248 nm

Photothermal displacement microscopy is used for the detection of μm-sized defects in Al₂O₃/SiO₂ multilayer coatings highly reflective for 248 nm. It is shown that for high quality coatings the global (averaged over several cm²) laser-induced damage threshold for coatings of different quality is determined by the density and absorption strength of the light absorbing defects. This is confirmed by a measurement of local (averaged over 0.01 mm²) damage thresholds with the pulsed photoacoustic mirage technique allowing a direct correlation of local damage with photothermally detected thin film defects. An analysis of laser-damaged spots reveals damage-craters of 13 μm diameter at the onset of detrimental irradiation effects. The formation of these craters is explained by a thermoelastic model describing the tensile stress in the film system resulting from evaporation of a defect located at the interface between thin film and substrate.     

Femtosecond dispersion compensation with multilayer coatings: toward the optical octave

Dispersive multilayer coatings have found widespread use, particularly in the compensation of material dispersion in femtosecond oscillators and amplifiers. Other than prism or grating sequences, only chirped mirrors allow for the compensation of a much more general spectral dependence of the dispersion. The current state of the art in ultrabroadband mirror design for dispersion compensation is reviewed. Approaches to expand the utility of chirped-mirror coatings toward the coverage of an even-wider bandwidth beyond the optical octave are discussed.     

Hemostatic multilayer coatings

Spray layer-by-layer assembly is used to create hemostatic films containing thrombin and tannic acid. The spray assembly technique enables coating of porous and absorbent commercial gelatin sponges with these films. Coated sponges are able to promote instantaneous hemostasis in a porcine spleen bleeding model.     

Markov-chain Monte Carlo approach to the design of multilayer thin-film optical coatings

We reconsider the problem of locating the globally optimal solution of a multilayer-optical-coating design problem, within some predetermined space of parameters, with the aim of obtaining a robust technique that requires a minimum of user intervention. The approach we adopt centers on exploring the space of the parameters of interest by using a Markov-chain Monte Carlo sampling algorithm. This technique enables one to locate the global optimum automatically with high confidence and without the need for a good starting design. It also allows the trivial inclusion of prior constraints on the variables and provides a natural means for investigating the robustness of the optimal solution.     

Effects of light scattering in optical coatings on energy losses in LED devices

The effect of light scattering in phosphor-containing optical coatings on energy losses in chip-on-board LED devices is considered. The energy losses have been evaluated by numerical simulation with the refractive index of the coating material (n = 1.4–1.8) and the average radius of dispersed phosphor particles (R = 5–50 μm) being the variable parameters. It is established that the energy losses most significantly depend on the level of total internal reflection at the coating/air interface. In the LED device configuration studied, the optimum refractive index of the coating material is n = 1.4. It is also found that, in order to reduce the energy losses related to light scattering on phosphor particles, it is expedient to use phosphors with coarse particles (R = 50 μm).     

Multiscale analysis of the laser-induced damage threshold in optical coatings

We have investigated the influence of laser beam size on laser-induced damage threshold (LIDT) in the case of single- and multiple-shot irradiation. The study was performed on hafnia thin films deposited with various technologies (evaporation, sputtering, with or without ion assistance). LIDT measurements were carried out at 1064 nm and 12 ns with a spot size ranging from a few tens to a few hundreds of micrometers, in 1-on-1 and R-on-1 modes. These measurements were compared with simulations obtained with the statistical theory of laser-induced damage caused by initiating inclusions. We show how to obtain information on the initiating defect properties and the related physical damage mechanisms with a multiscale study. Under certain conditions, it is possible with this method to discriminate different defects, estimate their densities, and follow the evolution of the defects under multiple irradiation. The different metrology implications of our approach, particularly for obtaining a functional LIDT of optical components are discussed.     

Characterization of multilayer anti-fog coatings

Fog formation on transparent substrates constitutes a major challenge in several optical applications requiring excellent light transmission characteristics. Anti-fog coatings are hydrophilic, enabling water to spread uniformly on the surface rather than form dispersed droplets. Despite the development of several anti-fog coating strategies, the long-term stability, adherence to the underlying substrate, and resistance to cleaning procedures are not yet optimal. We report on a polymer-based anti-fog coating covalently grafted onto glass surfaces by means of a multistep process. Glass substrates were first activated by plasma functionalization to provide amino groups on the surface, resulting in the subsequent covalent bonding of the polymeric layers. The anti-fog coating was then created by the successive spin coating of (poly(ethylene-maleic anhydride) (PEMA) and poly(vinyl alcohol) (PVA) layers. PEMA acted as an interface by covalently reacting with both the glass surface amino functionalities and the PVA hydroxyl groups, while PVA added the necessary surface hydrophilicity to provide anti-fog properties. Each step of the procedure was monitored by XPS, which confirmed the successful grafting of the coating. Coating thickness was evaluated by profilometry, nanoindentation, and UV visible light transmission. The hydrophilic nature of the anti-fog coating was assessed by water contact angle (CA), and its anti-fog efficiency was determined visually and tested quantitatively for the first time using an ASTM standard protocol. Results show that the PEMA/PVA coating not only delayed the initial period required for fog formation but also decreased the rate of light transmission decay. Finally, following a 24 hour immersion in water, these PEMA/PVA coatings remained stable and preserved their anti-fog properties.     

AC losses in multilayer superconducting tapes

ac losses have been investigated experimentally as well as theoretically in tapes having on the surface several normal conducting and/or superconducting layers. The superconducting layers under investigation have been Nb₃Sn on a niobium substrate and Nb₃Ge on a stainless steel substrate. It has been proved that the layered structure of the tapes is well reflected by the stepwise character of the ac losses dependence on the amplitude of the surface magnetic field. The magnetic flux passing through a surperconducting layer or surface barrier into the inside of the tape enhances the losses in the passed barrier or layer.     

Localized defects in multilayer coatings

We present a non-linear continuum model of the growth of localized defects in multilayer coatings nucleated by particles on the substrate. The model is valid when the deposition and etch fluxes are near normal incidence so that shadowing effects are negligible. Three-dimensional simulations of defects in Mo/Si multilayer films nucleated by arrays of lithographically patterned particles are shown to be in good agreement with experimental measurements. Our results confirm that incorporating ion beam etching in the multilayer deposition process significantly suppresses the defect growth. This has a potentially important application in the fabrication of defect-free masks for extreme ultraviolet lithography.     

Measurement of the optical damage threshold in fused quartz

The damage thresholds of five different types of quartz glass used for the production of spectroscopic cuvettes for liquids were determined with single temporal and spatial mode nanosecond pulses at 532 nm. One of the glasses had a damage threshold of ?420 J/cm(2), which was more than twice that of the other glasses.     

Spectroscopic ellipsometry of multilayer dielectric coatings

Multilayer dielectric coatings deposited by e-beam evaporation have been characterised by the phase modulated spectroscopic ellipsometer (PMSE). Measurements have been done on various multilayer thin films devices e.g., high reflectivity mirror, narrow band filter, beam combiner, beam splitter, etc. consisting of several bilayers of TiO₂/SiO₂. Results have been shown here for the first two samples. The measured Ellipsometry spectra are fitted with theoretical spectra generated assuming appropriate models regarding the sample structures. Optical constants of the substrates and the SiO₂ films have been supplied and trial dispersion relations have been used for the optical constants of the TiO₂ layers. The fittings have been done by minimising the squared difference (χ²) between the measured and calculated values of the ellipsometric parameters (ψ and Δ) and accurate information have been derived regarding the thickness and refractive indices of the different layers.