Advanced algorithms for monochromatic monitoring of complex optical coatings.

The paper proposes to use combinations of two algorithms for the on-line prediction of the termination times of layer depositions in the case of complex optical coatings with large numbers of layers with significantly different layer thicknesses. Notch filters with very thin and thick coating layers are considered as examples of coatings where such combinations of algorithms can be useful. Computational manufacturing experiments are used to estimate expected levels of thickness errors and the strength of the error self-compensation effect for various notch filter designs. It is shown that the estimates obtained can be used to select the optimal combination of the considered algorithms.

Pre-production estimates for the choice of strategy and algorithm for monochromatic monitoring of the production of optical coatings.

The paper proposes an approach for the pre-production selection of the most promising combination of a monochromatic monitoring strategy and a monitoring algorithm used for predicting termination of layer deposition. The approach is based on the previously introduced estimate of the strength of the expected error self-compensation effect and the new estimate, to the best of our knowledge, of the strength of the cumulative effect of thickness error growth presented in this paper. The application of the proposed approach is demonstrated using computational manufacturing experiments with various types of optical coating designs.

Turning point monitoring of narrow bandpass filters: the enormous strength of the error self-compensation effect.

It is generally accepted that turning point optical monitoring provides a very strong error self-compensation effect when used to control the deposition of narrow bandpass filters with quarter-wave or multiple-quarter-wave layer optical thicknesses. However, how strong this effect is has never been assessed before, to our knowledge. A recently developed general approach to estimating the strength of thickness error correlation and the strength of the associated error self-compensation effect allows, possibly for the first time, such an assessment. In this work, it is proved that, indeed, in the case of monitoring narrowband filters, the latter effect has enormous strength, many times greater than the strength of the error self-compensation effects when manufacturing all other types of optical coatings using various monochromatic and broadband monitoring techniques.

Atomistic Simulation of the Ion-Assisted Deposition of Silicon Dioxide Thin Films.

A systematic study of the most significant parameters of the ion-assisted deposited silicon dioxide films is carried out using the classical molecular dynamics method. The energy of the deposited silicon and oxygen atoms corresponds to the thermal evaporation of the target; the energy of the assisting oxygen ions is 100 eV. It is found that an increase in the flow of assisting ions to approximately 10% of the flow of deposited atoms leads to an increase in density and refractive index by 0.5 g/cm3 and 0.1, respectively. A further increase in the flux of assisting ions slightly affects the film density and density profile. The concentration of point defects, which affect the optical properties of the films, and stressed structural rings with two or three silicon atoms noticeably decrease with an increase in the flux of assisting ions. The film growth rate somewhat decreases with an increase in the assisting ions flux. The dependence of the surface roughness on the assisting ions flux is investigated. The anisotropy of the deposited films, due to the difference in the directions of motion of the deposited atoms and assisting ions, is estimated using the effective medium approach.

Investigation of the error self-compensation effect associated with direct broad band monitoring of coating production.

Computational manufacturing experiments are used to detect the types of optical coatings that are showing the presence of a strong error self-compensation effect in the coating production with direct broad band monitoring. It is shown that predictions made on the basis of these experiments coincide with the predictions of the previously developed rigorous mathematical approach to the investigation of the error self-compensation effect.

Error self-compensation mechanism in the optical coating production with direct broad band monitoring.

The main theoretical results related to the investigation of the error self-compensation mechanism associated with direct broad band monitoring of optical coating production are presented. The presented results are illustrated using the production of Brewster angle polarizer where this effect is especially strong. Specific properties of the design merit function required for the presence of the error self-compensation effect are discussed and the mechanism of thickness errors correlation by the direct broad band monitoring is described. It is also discussed how one can check whether a strong error self-compensation effect may be expected for a given coating design and specific parameters of the monitoring procedure that will be used for coating production.

Production of Brewster angle thin film polarizers using a ZrO2/SiO2 pair of materials.

The production of polarizers for high-intensity applications based on a ZrO2/SiO2 pair of thin film materials is discussed. A special approach to accurate determination of a ZrO2 refractive index and the application of direct broadband optical monitoring enable obtaining good manufacturing results.

Computational experiments on atomistic modeling of thin-film deposition.

The molecular dynamic algorithm for simulation of thin-film growth is reported. The achieved performance of this algorithm is sufficient for detailed investigations of structural effects in thin films with practically meaningful dimensions.

Modified sequential algorithm for the on-line characterization of optical coatings.

We present a new algorithm for the on-line determination of thicknesses of deposited layers that can be used in the course of coating production with broadband optical monitoring. The proposed algorithm can be considered as a modification of the well-known sequential algorithm. The main idea of the new algorithm is to re-calculate thicknesses of some of the previously deposited layers along with the determination of the thickness of the last deposited layer. The algorithm implies analytical estimations that enable recalculating only those layer thicknesses that can be found with better accuracy than before. Simulation and computational manufacturing experiments confirm high accuracy of the proposed algorithm.

Automated construction of monochromatic monitoring strategies.

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.

Sensitivity-directed refinement for designing broadband blocking filters.

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.

Reverse engineering of multilayer coatings for ultrafast laser applications.

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.

Reliable optical characterization of e-beam evaporated TiO2 films deposited at different substrate temperatures.

We studied e-beam evaporated TiO2 films deposited at two different substrate temperatures between 120°C and 300°C. We reliably characterized the film samples on the basis of in situ and ex situ measurements. We carried out annealing on the samples and studied the induced changes in the properties of the films. The results can be useful for further laser-induced damage threshold investigations.

Design and production of antireflection coating for the 8-10 µm spectral region.

A special design procedure allowing to trap layer thicknesses inside specified limits is applied for designing of antireflection coating (AR) for the infrared spectral band of 8-10 µm. The obtained AR design has no too thick layers that may cause delaminating of the deposited AR coating. A special monitoring procedure taking into account wavelength positions of monitoring signal extrema is applied for coating deposition. The manufactured coating features excellent AR properties in the requested spectral region and possesses high mechanical stability.

High-dispersive mirrors for high power applications.

We report on the development and manufacturing of two different types of high-dispersive mirrors (HDM). One of them provides a record value for the group delay dispersion (GDD) of -4000 fs2 and covers the wavelength range of 1027-1033 nm, whereas the other one provides -3000 fs2 over the wavelength range of 1020-1040 nm. Both of the fabricated mirrors exhibit a reflectance of >99.9% and are well suited for intracavity applications. Mirrors of the second type have been successfully employed in a Kerr-lens mode-locked Yb:YAG thin-disk oscillator for the generation of 200-fs pulses with multi-10-W average power.

Robust synthesis of dispersive mirrors.

A synthesis technique allowing to obtain a set of robust designs is reported. The robust synthesis is based on simultaneous optimization of spectral characteristics of multiple designs located in a small neighborhood of a so-called pivotal design. Efficiency of this technique is demonstrated by the synthesis and successful experimental realization of a high dispersive mirror. The fabricated dispersive mirror covers 690-890 nm wavelength range and provides the dispersion of -300 fs2 at 800 nm.

Phase optimization of dispersive mirrors based on floating constants.

A novel floating constants phase-optimization technique is developed and applied to the design of dispersive mirrors. This technique reduces the dispersive mirror's sensitivity to layer thickness errors. To demonstrate the significant improvement in design stability, we theoretically and experimentally compare our new phase-optimization approach to the conventional one. The fabricated dispersive mirror has a reflectivity of >99.99% and provides an accurate dispersion control over a bandwidth of around 60 nm.

Double-angle multilayer mirrors with smooth dispersion characteristics.

We report the feasibility of precision broadband dispersion control with multilayer mirrors produced in a single coating run. Inherent fluctuations of the group-delay dispersion (GDD) are suppressed by using the mirrors at two different angles of incidence. With a specialized version of the needle optimization algorithm, we have designed the multilayer structure to yield a complementary pair with a resultant GDD substantially free from spectral oscillations characteristic of broadband chirped multilayers. Since the mirrors employed at two different incidence angles are produced in a single deposition run, their overall dispersion is more robust to errors in layer thicknesses than that of previous complementary mirror pairs manufactured in two different steps. This offers the potential for improving production yield and quality of femtosecond dispersion control. We have successfully used the first "double-angle" mirrors for compressing pulses to a duration of 4.3 fs.

Comparison of dispersive mirrors based on the time-domain and conventional approaches, for sub-5-fs pulses.

Dispersive mirrors based on time-domain approach are compared with mirrors resulting from conventional phase target designs. Phase targets have been applied to complementary-pair dispersive mirrors, used for sub-5-fs pulse compression. While the phase approach has hither to afforded the best performance for the shortest pulses, our new approach, based on time-domain targets and tailored for a specific input spectrum, appears to provide comparable performance for pulse compression for a pulse duration 4.6 fs. Experimental studies using dispersive mirrors made to both designs are described.

Time-domain approach for designing dispersive mirrors based on the needle optimization technique. Theory.

We combine powerful and well-proven needle-optimization technique with time-domain optimization approach in order to obtain a new efficient method of designing dispersive mirrors. We also propose a new optimization criterion targeted at reaching shortest possible pulses with maximum possible energy at the exit of a compressor containing such mirrors. Proposed optimization criterion includes two parameters allowing one to adjust the relative weights of the mentioned targets with a high flexibility. The obtained results are compared with solutions of the "classical" optimization approach based on the optimization of a merit function comparing theoretical reflectance and group delay dispersion with target ones. The new approach allows obtaining simpler solutions providing better characteristics of the output pulse.