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Multilayer optical components for ultrafast laser applications operating in the mid-infrared (MIR) spectral range from 3 to 15 μm exhibit optical and physical properties essentially different from those ones of the coatings operating in the visible-near-infrared spectral range, namely, they should (i) comprise layers from special MIR thin-film materials; (ii) contain much thicker layers; (iii) deal with adhesion problems due to high stresses; (iv) deal with regions of O-H absorption in the MIR range; (v) exhibit positive group delay dispersion; (vi) be ultra-broadband in order to support short pulses. The mid-infrared thin film materials Ge, ZnS, YbF3, and LaF3 were carefully characterized. ZnS/YbF3 and Ge/YbF3 dichroic filters specified in the visible, near- and mid-infrared spectral ranged were designed, produced, and characterized. The filters operate at Brewster angle 67.5°. Almost one-octave broadband dispersive mirrors operating in the mid-infrared spectral range were developed for the first time. The mirrors consist of Ge and YbF3 layers, which have not been used before for manufacturing of multilayer dispersive optics. The mirrors compensate group delay dispersion of ultrashort laser pulses accumulated by propagation through 4 mm ZnSe windows and additional residual phase modulation of an ultrashort laser pulse. Using the mirrors, the 86 fs pulses were compressed up to 55 fs.

The necessity to precisely determine the delay between ultrashort light pulses is ubiquitous in optical metrology. For pulses originating from the same source, optical-interference-based delay tracking using a monochromatic laser affords precisions of a few attoseconds [1] . However, if the pulses originate from separate modelocked oscillators with detuned repetition frequencies – such as widely employed in dual-frequency-comb spectroscopy [2] and electronically controlled optical sampling (ECOPS) [3] – interferometric delay tracking faces two challenges: (i) the optical phase typically fluctuates independently of the repetition frequency, requiring independent control, and (ii) the interference signal is confined to the time window of the overlap, usually < 1ps.

In optical experiments involving variably-delayed ultrashort pulses, the precise calibration of the delay axis is of paramount importance. This is particularly challenging if interferometric delay tracking (with an auxiliary, monochromatic laser [1] ) cannot readily be implemented, such as in the case when the pulses originate from separate modelocked oscillators with detuned repetition frequencies. In the new scheme of electro-optic delay tracking [2] (i) an optical-phase-stable mid-infrared (MIR) waveform is obtained via intrapulse difference frequency generation (IPDFG) driven by an ultrashort near-infrared pulse and (ii) electro-optically sampled (EOS) using a second (independent) near-infrared gate pulse. IPDFG grants (sub-)attosecond lock of the intensity envelope of the driving near-infrared pulse to resulting MIR waveform [3] , while both IPDFG and EOS exhibit (multi-)percent quantum efficiencies [4] being 2 nd -order nonlinear processes. Here, we demonstrate the elongation of a MIR pulse obtained by IPDFG via a narrowband, Fabry-Perot type multilayer optical filter, providing a monochromatic MIR waveform for electro-optic delay tracking which is robust against variations of the initial MIR pulse. The zero crossings of the resulting MIR waveform are reproducible down to a few tens of attoseconds over a time window of 8 ps, which is sufficient for most condensed-phase measurements.

High group delay dispersion (GDD) is often required for ultrafast laser applications. To achieve GDD level higher than -10000 fs2 in a single mirror setting is difficult due to the high sensitivity to unavoidable production inaccuracies. To overcome the problem, total internal reflection (TIR) based dispersive mirrors have been proposed in theory. In this paper, we report our continuous effort to further design, fabricate and measure TIR based dispersive mirrors.

A special dielectric edge filter extremely sensitive to any change in refractive indices, layer thicknesses and angle of incidence has been investigated using holographic pump-probe measurements at different intensity values. Different physical processes overlapping in time were found to occur, namely the Kerr effect, free- electron generation and their subsequent trapping. A numerical model was used to reproduce the experimental results and decouple these processes.

Recent developments in the synthesis, production and characterization of multilayer optical coatings provide precise group-delay dispersion control over broadband spectral ranges. High laser induced damage threshold of dielectric coatings allow exposing them at high intensity laser radiation on the order of 10–1000 GW/cm2. Until now these coatings have been considered in the linear regime only. At the same time, at the intensities close to the laser-induced damage threshold optical coating exhibit nonlinear effects. Namely, intensity dependent addition to the refractive index n 2 I (Kerr effect) cause variation of the reflectance coefficient [1]. The coatings with predictable increase of reflectance at high intensities can be used for the development of innovative elements for ultrafast optics, special edge filters with extremely steep reflectance slope in the vicinity of the central wavelength were designed, produced and carefully investigated. The coatings exhibit predictable increase of reflectance at high intensities.

We report a challenging design, fabrication and post-production characterization problem of a dispersive mirror supporting the spectral range from 2000 nm to 2200 nm and providing a group delay dispersion of -1000 fssup2/sup. The absolute reflectance in the working range is over 99.95%. The reported mirror is a critical element for Tm and Ho based lasers and paves the way for the development of ultrafast 2 µm lasers with sub-100 fs pulse duration.

A specially designed and produced edge filter with pronounced nonlinear effects is carefully characterized. The nonlinear effects are estimated at the intensities close to the laser-induced damage., Comment: 3 pages

A new design approach allowing optimizing absorptance in an individual layer or in a group of layers of an optical coating is proposed. This approach can be used for designing multilayers for solar cells and other absorptive coatings.

In the present work, an innovative optical element developed for the multiterawatt few-cycle light wave synthesizer based on optical parametric amplification is demonstrated. The synthesizer currently produces sub-5-fs, 80-mJ, 18-TW pulses. The required element had to be deposited on 15 mm substrate having 75 mm in diameter; an average value of the GDD of +75 fs 2 and reflectance exceeding 99% in the spectral wavelength region from 570 to 1030 nm. Mechanical stresses of the designed dispersive mirror (DM) caused the substrate bending that distorts wave front quality resulting in the distorting of the beam waveform of laser system. In order to compensate this mechanical stress, the substrate back side was covered by an antireflection coating providing the lowest possible reflection in the working spectral range, having the total physical thickness close to DM thickness and total thicknesses of Nb 2 O 5 /SiO 2 layers close to the corresponding total thicknesses of Nb 2 O 5 /SiO 2 in DM. The produced DM-AR optical element exhibits excellent spectral properties.

We discuss the production of antireflection coating with low reflection zones in the spectral regions around the wavelengths of 546nm, 1060 nm and 11000 nm and additional demands for the reflectance maxima in the visible spectral region. For the coating production we use ZnSe as a substrate material and ZnS/YbF3 as layer materials. Their optical parameters are accurately determined in a wide spectral region using modern characterization and reverse engineering software tools. For the coating production we apply broad band monitoring in the spectral region from 1200 to 2300 nm that does not overlap with the regions where target requirements are specified. In order to select the most practical design that can be reliably manufactured we apply a combination of various modern design approaches and computational manufacturing experiments simulating production runs. Application of the most up-to-date software tools allows us to obtain excellent manufacturing results even from the first production run.

We focus our efforts on development of an advanced monochromatic monitoring strategy to assist the optical coating engineer in finding a single wavelength or a sequence of monitoring wavelengths that meet simultaneously several practical demands, namely, specified input and output swing values, specified amplitude of a monitoring signal variation, and the distance between trigger point and the last signal extremum. Additionally, the most important demand is that the number of different monitoring wavelengths must be as small as possible. Manual construction of such a monitoring strategy is almost impossible because of a large number of conditions to be satisfied. We propose an algorithm that automatically generates a monitoring spreadsheet so that all demands can be satisfied as closely as possible. We consider six typical design problems and obtain a series of solutions for each of them. Then, we provide computational simulations of deposition processes assuming that they are controlled by monochromatic monitoring with the monitoring strategy generated by our algorithm, and we demonstrate how an optical coating engineer can select design solutions that exhibit the highest production yields.

We developed a new method aimed at designing short-pass filters, long-pass filters and filters blocking sidebands of Fabry-Perot bandpasses. The method is an automated version of a non-straightforward empirical approach invented as a result of many years’ experience in design and production of optical coatings. The method allows obtaining near-quarter-wave solutions in a few seconds. In many cases these solutions are more advantageous for deposition systems.

Each complicated coating, in particular, a dispersive mirror consists of dozens of layers. Thin films layers have mechanical stresses. After summing up stresses from all layers, the resulting stress is high enough to bend even a relatively thick substrate. To avoid this effect we suggest depositing an antireflection coating (AR) at the back-side of the substrate which together with suppression of unwanted reflections from the back side will also compensate this stress. We demonstrate unique, extremely thick and sophisticated AR coating consisting of 71 layers with the total physical thickness of 7.5 µm. This AR coating completely compensates stress from the dispersive mirror coated on the front side and minimizes unwanted reflections.

Here we report a highly-dispersive mirror with a group delay dispersion of -10000 fs2 for wavelengths 1025 -1035 nm. This allows power scaling in a 300-fs Yb:YAG oscillator.

Dielectric multilayer coatings exhibiting steep reflectance in an extremely narrow transition zone, highly sensitive to any variations of layer refractive indices and therefore suitable for studying the nonlinear properties are produced and characterized. Increase of reflectance at growing intensity reveals the presence of the optical Kerr effect. A new method calculating intensity dependent spectral characteristics of multilayer optical coatings in the case of nonlinear interaction with high intensity laser pulses is developed. The method is based on the numerical solution of a boundary-value problem derived from the system of Maxwell equations describing the propagation of light through a multilayer system. The method opens a way to synthesis of optical coatings with predictable nonlinear properties. Comparison of our numerical modelling with experimental data enabled us to accurately determine the Kerr coefficients n2 of the widely-used thin-film materials Ta2O5 and Nb2O5.

We propose a reliable reverse engineering approach for a postproduction characterization of complicated optical coatings for ultrafast laser applications. We perform the postproduction characterization on the basis of in situ broadband monitoring data and validate the results using ex situ transmittance data and group delay measurements.

The electronic and optical properties of TiO2 atomic structures representing simulated thin films have been investigated using density functional theory. Suitable model parameters and system sizes have been identified in advance by validation of the results with experimental data. Dependencies of the electronic band gap and the refractive index have been calculated as a function of film density. The results of the performed calculations have been compared to characterized optical properties of titania single layers deposited using different coating techniques. The modeled dependencies are consistent with experimental observations, and absolute magnitudes of simulated values are in agreement with measured optical data.

Periodic multilayer structures of quarterwave and multiple quarterwave stacks with shifted ratios of high and low index layers in the half-wave pairs are considered. Analytical dependencies of the reference wavelength reflectance and the width of high reflectance zone on the number of layers, fraction quarterwave and layer refractive indices are obtained. The structures are used as starting designs for notch filters. Obtained dependencies allow one to estimate in advance parameters required to achieve target spectral characteristics.

We simulate electronic and optical properties of TiO2 structures representing films with different densities. We calculate dependencies of bandgap and refractive index on film density. The observed dependencies are consistent with experimental data.

We study TiO2 films deposited at different substrate temperatures. The films are carefully characterized on the basis of in situ and ex situ measurements. The results will be useful for further laser-induced damage threshold investigations.

We apply a reliable reverse engineering approach for post-production characterization of an optical coupler. We perform reverse engineering on the basis of in situ data analysis and validate results using transmittance and group delay measurements.

We report on design, production and characterization of an extremely broadband multilayer beamsplitter, covering wavelength range from 0.67 - 2.6 µm. The group delay dispersion has small oscillations in the above mentioned working range. We used a new algorithm with floating constants allowing us to obtain a smooth and near constant GDD. The optical element based on the beamsplitter is used for dividing a low-energy super-octave spectrum into several sub-spectral regions which are later amplified and coherently combined.

We explain reasons of oscillations frequently observed in total losses spectra (1 − R − T) calculated on the basis of measurement spectral photometric data of thin film samples. The first reason of oscillations is related to difference in angles of incidence at which spectral transmittance and reflectance are measured. The second reason is an absorption in a thin film. The third reason is a slight thickness non-uniformity of the film. We observe a good agreement between theoretical models and corresponding measurements, which proves above statements on the origins of oscillations in total losses.

Optical properties of metal island films (MIFs) can be combined with interference of dielectric coatings. A set of multilayer designs containing metal clusters reflecting different colours from front and back side of the coating was obtained by numerical optimization. The chosen designs presenting the range of feasible colours were deposited by electron beam evaporation. Spectrophotometric and ellipsometric measurements verified that the produced coatings present an excellent agreement with the optical performance calculated from the designs. Numerical optimization was verified as a useful method in designing of coatings containing MIFs. This approach can ease the implementation of metal clusters into multilayer designs and broaden the applications of MIFs.

We deal with design and production of optimal two-component antireflection (AR) coatings for an ultra broadband spectral range from 450 nm to 1800 nm. We demonstrate the whole design-production chain including design selection, choosing monitoring technique, coating production, and reverse engineering of the deposited coatings. At each step of this chain we provide thorough analysis on the basis of theoretical results and adequate computational manufacturing experiments. In order to produce the designed AR coatings we use magnetron sputtering deposition technique and accurate time monitoring.

Modern efficient optimization techniques, namely needle optimization and gradual evolution, enable one to design optical coatings of any type. Even more, these techniques allow obtaining multiple solutions with close spectral characteristics. It is important, therefore, to develop software tools that can allow one to choose a practically optimal solution from a wide variety of possible theoretical designs. A practically optimal solution provides the highest production yield when optical coating is manufactured. Computational manufacturing is a low-cost tool for choosing a practically optimal solution. The theory of probability predicts that reliable production yield estimations require many hundreds or even thousands of computational manufacturing experiments. As a result reliable estimation of the production yield may require too much computational time. The most time-consuming operation is calculation of the discrepancy function used by a broadband monitoring algorithm. This function is formed by a sum of terms over wavelength grid. These terms can be computed simultaneously in different threads of computations which opens great opportunities for parallelization of computations. Multi-core and multi-processor systems can provide accelerations up to several times. Additional potential for further acceleration of computations is connected with using Graphics Processing Units (GPU). A modern GPU consists of hundreds of massively parallel processors and is capable to perform floating-point operations efficiently.

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.

Modern design approaches enable one to construct a series of theoretical designs with excellent spectral properties for almost all optical coating design problems. Selection of a practical optimal design among a variety of possible theoretical designs becomes a key issue. We demonstrate how preproduction estimations of expected production yields can be used for selection of a practical optimal design. The question of reliability of such estimations is also addressed.

A rigorous definition of the error self-compensation effect is provided. An existence of this effect in the case of coating production with broadband optical monitoring of layer thicknesses is investigated for several widely used types of optical coatings.

Thin metal island films exhibit unique optical properties and possess a high potential in design and fabrication of multilayer coatings with sophisticated spectral performances over wide wavelength and angular ranges. Optical properties of these films are dependent on film thickness. In the present study we consider and solve a problem of designing multilayers which reflect different colors from their front and back sides and have specified average transmittance values. Additionally, in many cases the reflected colors are stable to variations of the incidence angle. In the design process we use optical constants of Ag metal island films, that were carefully determined based on recently proposed characterization approach.

Optical constants of thin metal films are strongly dependent on deposition conditions, growth mode, and thickness. We propose a universal characterization approach that allows reliable determination of thin metal film optical constants as functions of wavelength and thickness. We apply this approach to determination of refractive index dispersion of silver island films embedded between silica layers.

Definition of an error self-compensation mechanism associated with direct broadband optical monitoring is proposed and a research approach for studying this effect is developed. Existence of a strong error self-compensation is demonstrated for cold mirrors.

A method for creating a database of dispersive refractive indices of thin film materials is outlined. Refractive indices for the most widely used metal oxide films produced under different deposition conditions are presented.

Using results of the probability theory, we estimate a number of computational experiments required for choosing the most manufacturable design with a sufficient confidence level. Experiments with broadband optical monitoring are used as examples.

Computational manufacturing experiments are used for estimating expected production yields and for choosing the most manufacturable design and respective optimal monochromatic monitoring strategy.

A new model for the determination of group delay (GD) and GD dispersion of dispersive mirrors is presented. The algorithm based on this model enables one to process interferometric data provided by a white-light interferometer and to obtain GD wavelength dependence over a broad spectral range.

The constrained optimization approach is applied to the design of quasi-rugate optical coatings. These coatings are defined as multilayers with no thin layers where refractive index profiles resemble rugate-type refractive index profiles and where spectral properties are typical for rugate filters. It is shown that all design problems that are usually solved using rugate filters can be solved successfully in the frame of quasi-rugate optical coatings. Comparison between quasi-rugate and two-component multilayer designs is provided.

A semi-empirical formula for estimating numbers of layers of broadband antireflection (AR) coatings is presented. Numbers of layers providing by this formula are in the excellent agreement with results of multiple computational experiments on designing of AR coatings for various combinations of refractive indices and boundary wavelength ratios of AR spectral regions.

A new empirical expression for estimating minimum achievable residual reflectance of antireflection (AR) coatings is presented. The expression gives an accurate approximation of the minimum residual reflectance for normal- and oblique-incidence AR coatings in the visible and infrared spectral ranges.

We present a theoretical approach enabling one to perform a preproduction investigation of the effect of accumulation of thickness errors in the course of optical coating production using broadband optical monitoring. On the basis of this approach we investigate and compare thickness errors that may be associated with such factors as random and systematic errors in measurement data, instabilities of deposition rates, and inaccuracies of on-line algorithms predicting termination instants for layer depositions.

New optical coating design algorithm with the equivalent layers theory is presented. The algorithm is based on the merit-function-constrained optimization in the accessible domain of equivalent phase thicknesses and equivalent refractive indices. It allows for creation of design coatings with sophisticated narrowband spectral characteristics. (

By comparing multilayer and rugate solutions to a typical rugate synthesis problem we demonstrate that rugate synthesis methods are still of a great importance. The new accurate algorithm for the synthesis of rugate coatings with arbitrary refractive index profiles is discussed. It is shown that this algorithm can be successfully applied for solving such classical rugate synthesis problem as reducing sidelobes accompanying stopbands of rugate filters.

We present a new design for an omnidirectional antireflection coating for the visible spectral range. In contrast to classical designs, it combines homogeneous layers and linear gradient index layers into one hybrid design with a full thickness of approximately 500 nm. The coating may be practically produced based on silicon dioxide as low index material and niobium pentoxide as high index material, while intermediate indices may be obtained from corresponding mixtures.

Ultra-short pulse systems are considered as innovative laser sources for a variety of applications in micro material structuring, medicine and diagnostics. Current commercial systems are still lacking in output power limiting the throughput and the economic efficiency within a production line. In the optimization of ultra short pulse sources of the next generation, special effects in optical components during interaction with ultra-short pulses play a major role. Especially, low damage thresholds and non-linear absorptance have already been observed within the activities of the EUREKA-project CHOCLAB II, which are concentrated on the evaluation of multiple-pulse damage and the absorptance of fs-optical components according to the International Standards ISO 11254-2 and ISO 11551. In this paper, a theoretical model on the basis of photo- and avalanche ionization is presented describing the incidence of damage as a consequence of a sufficient high density of conduction band electrons. Furthermore, the influence of the Kerr-effect and conduction band electrons on the optical properties of dielectrics is investigated theoretically. From our calculations, a significant increase in reflectance due to the dominant Kerr-effect can be deduced as well as a noticeable increase in absorptance induced by free electron heating already at energy density values clearly below the damage threshold. Finally, results of an experimental investigation in the influence of the internal field strength in a dielectric layer stack on the damage threshold are described. The experiments clearly support the assumption already stated in other publications, that the field intensity formed by the optical design plays a key role for damage resistance of optical coatings for ultrashort pulses.

A new optical coating design algorithm using the equivalent layers theory is presented. The algorithm is based on the merit function constrained optimization in the accessible domain of equivalent phase thicknesses and equivalent refractive indices.

The work presents a new approach to the design of high quality optical coatings. This approach is based on the optimization of merit function incorporating an additional requirements for stability of optical coating with respect to errors in layer thicknesses.

Depending on the choice of thin film models and measurement data used for the characterization analysis one can obtain essentially different characterization results. It is especially difficult to reliably determine refractive index wavelength dependencies in the case of low accuracy measurement data. We consider possible approaches aimed to improve a stability of refractive index determination. The ways of the verification of characterization results are also discussed. Practical examples used to illustrate the proposed approaches are connected with the most difficult case of the determination of the refractive indices of fluoride films in the VUV spectral region.

The determination of optical parameters of thin films from experimental data is a typical task in the field of optical-coating technology. The optical characterization of a single layer deposited on a substrate with known optical parameters is widely used for this purpose. Results of optical characterization are dependent on not only the choice of the thin-film model but also on the quality of experimental data. The theoretical results presented highlight the effect of systematic errors in measurement data on the determination of thin-film parameters. Application of these theoretical results is illustrated by the analysis of experimental data for magnesium fluoride thin films.