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.
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<sup>2</sup> and reflectance exceeding 99% in the spectral wavelength region from 570 to 1030 nm.<p> </p> 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<sub>2</sub>O<sub>5</sub>/SiO<sub>2</sub> layers close to the corresponding total thicknesses of Nb<sub>2</sub>O<sub>5</sub>/SiO<sub>2</sub> 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/YbF<sub>3</sub> 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.
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.
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.
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
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.
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 key role of the design total optical thickness in the synthesis process is demonstrated by practical examples and confirmed by rigorous theoretical results. A special development of the needle optimization procedure called <i>the synthesis by gradual evolution</i> is proposed. This synthesis process does not require any starting design. It automatically constructs a set of designs with various combinations of three major design parameters: merit function value, number of design layers, design total optical thickness. This provides the optical coating engineer with additional opportunities in choosing the most practical design.
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.