Optical broadband monitoring of conventional and ion processes

Reverse engineering of e-beam deposited optical filters based on multi-sample photometric and ellipsometric data

A post-production characterization approach based on spectral photometric and ellipsometric data related to a specially prepared set of samples is proposed. Single-layer (SL) and multilayer (ML) sets of samples presenting building blocks of the final sample were measured ex-situ, and reliable thicknesses and refractive indices of the final ML were determined. Different characterization strategies based on ex-situ measurements of the final ML sample were tried, reliability of their results was compared, and the best characterization approach for practical use, when preparation of the mentioned set of samples would be a luxury, is proposed.

Improving optical thickness monitoring by including systematic and process-influenced transmittance deviations

During optical monitoring using broadband transmittance measurement, the accuracy depends on how both the substrate and the optical path are aligned. We present a correction procedure to improve the accuracy of the monitoring, even if the substrate has features such as absorption or if there is misalignment of the optical path. The substrate in this case can either be a test glass or a product. The algorithm is proven by experimental coatings which were produced with and without the correction. Additionally, the optical monitoring system was also used to perform an in situ quality check. The system allows a detailed spectral analysis of all substrates with a high position resolution. Both plasma and the temperature effects on the central wavelength of a filter are identified. This knowledge enables the optimization of the following runs.

Noble Infrared Optical Thickness Monitoring System Based on the Algorithm of Phase-Locked Output Current–Reflectivity Coefficient

With the rapid development of modern science and technology, the application of infrared (IR) optical thin films is increasingly widespread, including in aerospace, autonomous driving, military development, and the fight against Coronavirus Disease 2019 (COVID-19), in which thin film devices play an important role. Similarly, with the improvement of user requirements, the precision requirements for the preparation of optical films are also developing and improving. In this paper, the IR optical thickness monitoring system is studied with the noble algorithm introducing the phase-locked amplifier current and single film reflectivity coefficient. An optical control system is developed to monitor the IR thin film accurately, which aids the IR narrow-band filter film research.

Large area ion beam sputtered dielectric ultrafast mirrors for petawatt laser beamlines

The latest advances in petawatt laser technology within the ELI Beamlines project have stimulated the development of large surface area dielectrically coated mirrors meeting all demanding requirements for guiding the compressed 30 J, 25 fs HAPLS laser beam at 10 Hz repetition rate and a center wavelength of 810 nm entirely in vacuum. We describe the production and evaluation of Ta₂O₅/HfO₂/SiO₂ ion beam sputtered coated (440 × 290 × 75) mm³ beam transport mirrors. No crazing was observed after thirty vacuum-air cycles. A laser induced damage threshold of 0.76 J/cm² (fluence on mirror surface) was achieved and maintained at high shot rates.

Enhancement of the damage resistance of ultra-fast optics by novel design approaches

Dielectric components are essential for laser applications. Chirped mirrors are applied to compress the temporal pulse broadening crucial in the femtosecond regime. However, the design sensitivity and the electric field distribution of chirped mirrors is complex often resulting in low laser induced damage resistances. An approach is presented to increase the damage resistance of pulse compressing mirrors up to 190% in the NIR spectral range. Layers with critical high field intensity of a binary mirror design are substituted by ternary composites and quantized nanolaminates, respectively. The deposition process is improved by an in situ technique monitoring the phase of reflectance.

Tunable optical properties of amorphous Tantala layers in a quantizing structure

Plasma deposition techniques like ion-beam-sputtering (IBS) are state of the art to manufacture high quality optical components for laser applications. Besides the well optimized process and monitoring systems, the coating material selection is integral to achieve optimum optical performances. Applying the IBS technology, an approach is presented to create novel materials by the direct application of binary oxides in a quantizing structure. By reducing the physical thickness of the high refractive index material to a few nm, within a classical high-low index stack, the electron confinement can be changed. Optical characterizations of the manufactured samples with decreasing quantum well thicknesses result in an increasing blue shift of the absorption gap and offer a method to approximate the effective mass of the high refractive index material in conjunction with theoretical models. Laser-induced damage threshold tests of coating samples prepared with different well thicknesses indicate an increase of the measured threshold values with optical gap energy.

Synthesis, fabrication and characterization of a highly-dispersive mirrors for the 2 µm spectral range

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 fs². 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.

Photometric calibration of an in situ broadband optical thickness monitoring of thin films in a large vacuum chamber

To improve the in situ monitoring of thin films at the Laboratoire des Matériaux Avancés, a broadband optical monitoring of the coated thin films was developed and installed in the biggest ion-beam sputtering machine in the world. Due to the configuration of the coating machine and the chamber strain under vacuum, a standard calibration procedure is impossible and a double-beam optical system is not suitable. A novel theoretical and practical solution to calibrate the measurements was found and is described in this paper. Some relevant results achieved thanks to this technique are discussed as well.

Kerr effect in multilayer dielectric coatings

We report the utilization of the optical Kerr effect in multilayer dielectric coatings, previously discussed only theoretically. We present the design and realization of multilayer dielectric optical structures with layer-specific Kerr nonlinearities, which permit tailoring of the intensity-dependent effects. The modulation depth in reflectance reaches up to 6% for the demonstrated examples of dielectric nonlinear multilayer coatings. We show that the nonlinearity is based on the optical Kerr effect, with the recovery time faster than the laser pulse envelope of 1 ps. Due to high flexibility in design, the reported dielectric nonlinear multilayer coatings have the potential to open hitherto unprecedented possibilities in nonlinear optics and ultrafast laser applications.

Broadband beamsplitter for high intensity laser applications in the infra-red spectral range

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.

Frequency tripling mirror

A frequency tripling mirror (FTM) is designed, fabricated and demonstrated. The mirror consists of an aperiodic sequence of metal oxide layers on a fused silica substrate tailored to produce the third harmonic in reflection. An optimized 25-layer structure is predicted to increase the reflected TH by more than five orders of magnitude compared to a single hafnia layer, which is a result of global compensation of the phase mismatch of TH and fundamental, field enhancement and design favoring reflection. Single pulse conversion efficiencies approaching one percent have been observed with the 25-layer stack for fundamental wavelengths in the near infrared and 55 fs pulse duration. The FTM is scalable to higher conversion, larger bandwidths and other wavelength regions making it an attractive novel nonlinear optical component based on optical interference coatings.

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.

Highly-dispersive mirrors reach new levels of dispersion

A highly-dispersive mirror with the unprecedented group delay dispersion of -10000 fs² in the wavelength range of 1025-1035 nm is reported. Reproducible production of a coating with such a high dispersion was possible due to the recently developed robust synthesis technique. Successful employment of the new highly-dispersive mirror in an oscillator is demonstrated.

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.

Stress compensation with antireflection coatings for ultrafast laser applications: from theory to practice

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.

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.

Quality control of oblique incidence optical coatings based on normal incidence measurement data

We demonstrate selection of reliable approaches for post-production characterization of oblique incidence multilayer optical coatings. The approaches include choice of input information, selection of adequate coating model, corresponding numerical characterization algorithm, and verification of the results. Applications of the approaches are illustrated with post-production characterization of oblique incidence edge filter, oblique incidence beam splitter and oblique incidence 43-layer quarter-wave mirror.

Laser damage resistance of ion-beam sputtered Sc2O3/SiO2 mixture optical coatings

We report on the correlation between the laser damage resistance, the optical and the physical properties of Sc(2)O(3)/SiO(2) mixture coatings. Several sets of samples with ten different mixture ratios have been prepared by ion-beam sputtering. The atomic compositions of the mixture thin films are quantified employing x-ray photoelectron spectroscopy depth profiles. Laser-induced damage thresholds are determined with single subpicosecond pulses (500 fs) at 1030 nm. Furthermore, Son1 multishot measurements are realized in the ultraviolet wavelength range (355 nm) at pulse durations of 5 ns. In addition, the influence of two different substrate polishing qualities on the radiation resistance of the composite thin films is discussed.

Computational manufacturing as a tool for the selection of the most manufacturable design

Applications of computational manufacturing experiments (CMEs) for selection of the most manufacturable designs among a variety of different design solutions are demonstrated. We compare design solutions with respect to estimations of their production yields. Computational experiments are performed using two simulation software tools. In the course of CMEs, we take into account all major factors causing errors in our deposition process. Real deposition experiments are in agreement with CMEs; the most manufacturable design exhibits better target performances compared to other designs.

Computational manufacturing as a key element in the design-production chain for modern multilayer coatings

We propose a general approach that allows one to reveal factors causing production errors in the course of the deposition process controlled by broadband optical monitoring. We consider computational experiments simulating the real deposition process as a crucial point of this approach. We demonstrate application of the approach using multiple experimental deposition runs of the selected multilayer coatings.

On the reliability of reverse engineering results

Determination of actual parameters of manufactured optical coatings (reverse engineering of optical coatings) provides feedback to the design-production chain and thus plays an important role in raising the quality of optical coatings production. In this paper, the reliability of reverse engineering results obtained using different types of experimental data is investigated. Considered experimental data include offline normal incidence transmittance data, offline ellipsometric data, and online transmittance monitoring data recorded during depositions of all coating layers. Experimental data are obtained for special test quarter-wave mirrors with intentional errors in some layers. These mirrors were produced by a well-calibrated magnetron-sputtering process. The intentional errors are several times higher than estimated errors of layer thickness monitoring, and the reliability of their detection is used as a measure of reliability of reverse engineering results. It is demonstrated that the most reliable results are provided by online transmittance data.

Design, production, and reverse engineering of two-octave antireflection coatings

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.

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.

Characterization of zirconia- and niobia-silica mixture coatings produced by ion-beam sputtering

ZrO2-SiO2 and Nb2O5-SiO2 mixture coatings as well as those of pure zirconia (ZrO2), niobia (Nb2O5), and silica (SiO2) deposited by ion-beam sputtering were investigated. Refractive-index dispersions, bandgaps, and volumetric fractions of materials in mixed coatings were analyzed from spectrophotometric data. Optical scattering, surface roughness, nanostructure, and optical resistance were also studied. Zirconia-silica mixtures experience the transition from crystalline to amorphous phase by increasing the content of SiO2. This also results in reduced surface roughness. All niobia and silica coatings and their mixtures were amorphous. The obtained laser-induced damage thresholds in the subpicosecond range also correlates with respect to the silica content in both zirconia- and niobia-silica mixtures.

Indirect broadband optical monitoring with multiple witness substrates

We present an indirect broadband optical monitoring approach based on using several witness substrates that are brought to a measurement position in a special sequence. Different witness substrates are used to monitor not groups of successive design layers but specially chosen design layers. An attractive feature of the presented monitoring approach is the ability to reliably control thin dielectric and metal layers. Considered examples demonstrate a good accuracy of the proposed approach.

Dual wavelengths monitoring for optical coatings

A new monitoring method based on the use of dual wavelengths monitoring is proposed. Firstly, the sensitivity of each layer in an optical coating for the monitoring wavelength is calculated by admittance equations. Then two appropriate monitoring wavelengths are chosen to make sure that every layer has a sensitive terminal point. The thickness error of the layer can be compensated. For quarter-wave multilayer and nonquarter-wave multilayer optical coatings, the advantage of this new monitoring method has been demonstrated by both the theoretical analyses and experimental results.

In situ optical characterization and reengineering of interference coatings

A new optical monitoring system has been developed that allows recording of transmission spectra in the wavelength range between 400 and 920 nm of a growing optical coating during deposition. Several kinds of thin film sample have been prepared by use of a hybrid monitoring strategy that is essentially based on a combination of quartz monitoring and in situ transmission spectra measurements. We demonstrate and discuss the applicability of our system for reengineering procedures of high-low stacks and measurements of small vacuum or thermal shifts of optical coatings.

Interest of broadband optical monitoring for thin-film filter manufacturing

Broadband optical monitoring for thin-film filter manufacturing is more and more developed thanks to better performances of spectrometers with array detectors. We compare this optical monitoring with turning point monitoring and quartz monitoring of different designs. The sensitivity to thickness errors and to refractive index errors is evaluated. We show that real time determination of deposited thickness is a valuable criterion. We also present our experimental setup of transmittance and reflectance broadband optical monitoring. The use of a 400-1000 nm range combined with a signal-to-noise ratio of ~2500 in transmittance and 1000 in reflectance permits us to expect the manufacturing of high-performance non-quarter-wave designs. A first manufacturing of an 18-layer non-quarter-wave high-pass filter is provided.