High Precision Optics Research Articles

Ultrasmooth Ti/Al multilayer with a Ti seed layer for EUV applications

Al-base multilayers have attracted much interest in the extreme ultraviolet (EUV) optics field, but high roughness of this multilayer due to the Al film is still a big concern. Here, a strategy of the seed layer was proposed to reduce the surface roughness and intermixing layer thickness of the Al-base multilayer. Ti film is not only a seed layer, but also an absorption layer in this novel multilayer. An optimized Ti/Al multilayer film structure was designed to work at 21.1 nm, while investigating the use of Ti as a seed layer to reduce the roughness and enhance the peak reflectivity. The experimental results showed that the Ti seed layer effectively reduced the surface roughness and intermixing layer thickness and improved the reflectance. At 21.1 nm, the peak reflectance reached 39.6%, with a bandwidth of only 1.0 nm and an RMS roughness of 0.17 nm. Ti/Al multilayer also exhibits good stability. This multilayer has potential application in high-precision optics, such as corona detection, which requires extreme low light scattering of multilayer mirror.

High-pressure plasma etching up to 9 atm toward uniform processing inside narrow grooves of high-precision X-ray crystal optics

We developed a new etching technique using plasma generated at high pressure up to 9 atm. Operating at 9-atm pressure with a 30-μm-diameter wire electrode, we demonstrated the generation of well-ordered plasma at a narrow gap of ∼10 μm between the electrode and workpiece, and realized a high spatial resolution of <40 μm during processing. This technique should allow for the processing of high-precision X-ray crystal optical devices with compact and complex structures, such as a micro channel-cut crystal monochromator with an extremely narrow (sub-100 μm width) groove for realization of Fourier-transform-limited X-ray lasers with high intensity.

Zero thermal expansion, high specific thermal conductivity, and good machinability of Cu2P2O7/2024Al composite

Negative thermal expansion (NTE) materials can be applied to suppress thermal expansion of other materials, forming composites with zero thermal expansion (ZTE). However, the shortcomings (e.g., high density, narrow working temperature window) inherited from various NTE materials hinder the applications of corresponding ZTE composites. The modified Cu2P2O7 by reducing oxygen deficiencies has a relatively low density and strong NTE over a wide temperature range, which therefore was used to composite with 2024Al. When the volume content of modified Cu2P2O7 is about 50%, the composite Cu2P2O7/2024Al shows ZTE effect, and the linear coefficient of thermal expansion is about −0.014 ppm/K at 273–333 K. The ZTE composite also exhibits a high specific thermal conductivity, significantly superior to other existing ZTE materials. Furthermore, due to the matched moduli between Cu2P2O7 and 2024Al, the ZTE composite displays good machinability. The current ZTE composite may have great potential applications in the fields of high-precision electronics and optics.

Investigation of crystallization in nanolayered TiO2-based superlattices

In the realm of high precision optical applications, such as gravitational wave detectors, the quest for optical coatings with exceptional characteristics, such as high reflectivity, high optical density, and low thermal noise, requires meticulous attention. Achieving the requested optical quality demands a comprehensive approach encompassing design, production, and characterization. A recent study by I.M. Pinto et al. explored the potential of nanolayer-based superlattices as a valuable alternative to the high refractive index component of dielectric Bragg-like mirrors [https://dcc.ligo.org/LIGO-G1301061, https://dcc.ligo.org/LIGO-G1902307]. Following the proposed approach, we have produced and investigated binary nanolayers, employing a combination of TiO2 with other oxides, such as SiO2, Ta2O5, Al2O3, and ZrO2. Within our superlattice structures, each layer's thickness ranges from a few to a few tens of nanometers. It is tailored, together with the number of nanolayers composing the structure, to cater to optical applications involving a light wavelength of 1064 nm (as for the laser of gravitational wave detectors). Our findings indicate that the superlattice structures fulfill the morphological and structural requirements for application in high precision optics when TiO2 is as thin as 2 nm. In this case, the surface attains a level of flatness on the nanometer scale – making spurious light scattering negligible – and its crystallization temperature Tc, a key parameter for reaching high optical quality, exceeds 500 °C. Finally, we demonstrate that further tuning of Tc is possible by varying the interface energy, thus changing the material coupled with TiO2 within the superlattice. This discovery paves the way for even greater control and optimization of these proposed metamaterials.

Study of Interfacial Adhesion and Re-Ir Alloy Coating in Chalcogenide Glass Molding.

Precision glass molding (PGM) has become an efficacious technique to fabricate high-precision optics. Chalcogenide (ChG) glass is increasingly used in thermal imaging, night vision, etc., because of its excellent infrared optical properties. Nevertheless, glass-mold interfacial adhesion has emerged as a pivotal issue within the PGM process. The interfacial adhesion during PGM has the potential to significantly undermine the performance of molded optics and reduce the longevity of molds. It is important to investigate interfacial adhesion behaviors in the PGM. In this study, the interfacial adhesion mechanism between ChG glass and the nickel-phosphorus (Ni-P) mold is analyzed using the cylindrical compression test. The effect of ChG glass internal stress on physical adhesion is investigated by finite element method (FEM) simulation. The spherical preform is proven to be capable of reducing the stress concentration and preventing physical adhesion. More importantly, a rhenium-iridium (Re-Ir) alloy coating is deposited on the Ni-P mold surface by ion sputtering to prevent atomic diffusion and resolve the problem of chemical adhesion. Finally, ChG glass microstructures with high accuracy are fabricated using the spherical ChG glass preform and the Re-Ir-coated Ni-P mold by PGM.

Modeling the hydrodynamic impact on the tool influence function during hemispherical subaperture optical polishing

To fabricate high-precision and accurate optics relative to the optical design surface, a high level of deterministic control of material removal (i.e., the tool influence function, TIF) during subaperture tool polishing is required. In this study, a detailed analysis of the pressure distribution, which is a key component of the TIF, has been performed using finite element analysis to couple together solid mechanics and fluid dynamics. Modeling experimental parameters of recently published work reveals that, when considering tool deformation, which in turn influences the fluid film thickness between the tool and workpiece, the effective pressure profile has a flat-top distribution. This flat-top pressure profile differs from the parabolic pressure distributions predicted by Hertzian mechanics. Moreover, the shear contribution is shown here to be a key contributor to material removal, inducing the removal at the periphery of the contact edge and even outside the generally accepted contact area. Finally, the simulated fluid velocities provide evidence of mixed-mode contact polishing, supporting recent experimental findings that also suggest that onset of hydroplaning contributions lead to material removal drop-off.

Influence of Photon Beam and Motor Vibrations on At-Wavelength X-Ray Speckle Scanning Metrology

Speckle scanning metrology is an effective tool to determine the X-ray wave front error in synchrotron radiation. By measuring wave front radii of curvature, X-ray optics can be high-precision aligned and adaptive optics can be used for compensating these wave front errors to pursue an aberration-free wave front. Photon beam and motor vibrations are the major limitations in characterizing the beam performances. An analytical model is presented in this article to reveal the influences of photon beams and motor vibrations on the measurements of the wave front radii of curvature. It is worth noting that the influence of low-frequency vibration is oscillatory and thus the selection of sampling frequency is strongly related. The experimental results verify the model and reveal the main dependency of the experimental deviations on the amplitude and frequency of vibrations.

Organic abrasive machining system for optical fabrication with 0.1-mm spatial resolution.

High-precision optics for short-wavelength regions, such as x rays and extreme ultraviolet light, generally require nanometer-level figure accuracy on their surfaces. Such optics are finished via a numerically controlled figure correction process in which the dwelling time of the machining tool on the workpiece is controlled. Due to the limitation of the machined spot size, it is difficult to remove mid-spatial-frequency (1 to 10mm-1) errors on an optical surface. To realize a high-spatial-resolution figure correction process for high-precision optics, we have been developing the organic abrasive machining (OAM) technique, which can generate a 100 µm machined spot using a small elastic rotation tool in a slurry that includes acrylic particles. In this study, an OAM apparatus that can measure the machining load was constructed. The effects of the machining and slurry conditions were investigated and high-spatial-resolution machining on a flat glass substrate was demonstrated. The root-mean-square roughness of the surface after OAM processing was below 0.2nm. Patterns with a minimum line and space size of 100 µm were successfully fabricated.

A Mechanically Reliable Transparent Antifogging Coating on Polymeric Lenses

AbstractPolymeric lenses have been increasingly used to replace glass lenses due to advantages of light weight, high refractive index, and ease of making into complicated shapes. However, a severe constraint to their wider application lies with their intrinsic weakness in hardness that can lead to mechanical damages by abrasion. During service, fogging remains another unsolved challenge to optical lenses, which may significantly reduce the users’ visibility or even cause accident. Therefore, it is imperative to develop mechanically reliable and transparent antifogging coating on polymeric lenses. In this work, a two‐step protocol is developed comprising a room‐temperature oxygen plasma treatment of polymer substrate followed by antifogging silica thin film deposition using pulsed laser deposition (PLD). The oxygen plasma treatment modifies the surface chemistry to allow a strong adhesion between the polymer substrate and the silica coating. Due to the porous nature of the PLD deposited nanosilica film, the coating also displays an antireflection effect. This mechanically reliable and highly transparent superhydrophilic silica coating opens great opportunity for the eyewear and high precision optics industries.

Retinal adaptive optics imaging with a pyramid wavefront sensor.

The pyramid wavefront sensor (P-WFS) has replaced the Shack-Hartmann (SH-) WFS as the sensor of choice for high-performance adaptive optics (AO) systems in astronomy. Many advantages of the P-WFS, such as its adjustable pupil sampling and superior sensitivity, are potentially of great benefit for AO-supported imaging in ophthalmology as well. However, so far no high quality ophthalmic AO imaging was achieved using this novel sensor. Usually, a P-WFS requires modulation and high precision optics that lead to high complexity and costs of the sensor. These factors limit the competitiveness of the P-WFS with respect to other WFS devices for AO correction in visual science. Here, we present a cost-effective realization of AO correction with a non-modulated P-WFS based on standard components and apply this technique to human retinal in vivo imaging using optical coherence tomography (OCT). P-WFS based high quality AO imaging was successfully performed in 5 healthy subjects and smallest retinal cells such as central foveal cone photoreceptors are visualized. The robustness and versatility of the sensor is demonstrated in the model eye under various conditions and in vivo by high-resolution imaging of other structures in the retina using standard and extended fields of view. As a quality benchmark, the performance of conventional SH-WFS based AO was used and successfully met. This work may trigger a paradigm shift with respect to the wavefront sensor of choice for AO in ophthalmic imaging.

Trapped light scattering within optical coatings: a multilayer roughness-coupling process.

Despite numerous works devoted to light scattering in multilayer optics, trapped scattering has not been considered until now. This consists in a roughness-coupling process at each interface of the multilayer, giving rise to electromagnetic modes traveling within the stack. Such a modal scattering component is today necessary for completing the energy balance within high-precision optics including mirrors for gyro-lasers and detection of gravitational waves, where every ppm (part per million) must be accounted for. We show how to calculate this trapped light and compare its order of magnitude with the free space scattering component emerging outside the multilayer.

Molecular Dynamic Investigation of the Anisotropic Response of Aluminum Surface by Ions Beam Sputtering

Aluminum optics are widely used in modern optical systems because of their high specific stiffness and high reflectance. With the applied optical frequency band moving to visible, traditional processing technology cannot meet the processing precision. Ion beam sputtering (IBS) provides a highly deterministic technology for high-precision aluminum optics fabrication. However, the surface quality is deteriorated after IBS. The interaction between the bombard atoms and the surface morphology evolution mechanism are not clear, and systematic research is needed. Thus, in this paper, the IBS process for single crystal aluminum with different crystallographic orientations are studied by the molecular dynamics method. The ion beam sputter process is firstly demonstrated. Then, the variation of sputter yield of the three crystal faces is analyzed. The sputter yield difference of different crystal surfaces causes the appearance of the relief structure. Then, the gravel structure generates on the single crystal surfaces and dominates the morphology evolution. The state of the atom diffusion of the specific crystal surfaces will determine the form of the gravel structure. Furthermore, the form and distribution of subsurface damage and stress distribution of three different crystal surfaces are analyzed. Although there are great differences in defect distribution, no stress concentration was found in three workpieces, which verifies that the ion beam sputter is a stress-free machining method. The process of IBS and the mechanism of morphology evolution of aluminum are revealed. The regularity and mechanism will provide a guidance for the application of IBS in aluminum optics manufacture fields.

Black paints covered with multidielectrics: light absorbers.

Black paints are commonly used to provide broadband light absorbers in high-precision optics. We show how multidielectric coatings improve the performances of these absorbers. The coated rough paints still exhibit a quasi-lambertian diffuse reflection, but this scattering pattern can be reduced by several orders of magnitude, which strongly enhances absorption. Predictions are based on an exact electromagnetic theory of light scattering from arbitrary rough multilayers. Results are also compared to useful approximate theories.

Impact of coherence length on the field of view in dark-field holographic microscopy for semiconductor metrology: theoretical and experimental comparisons

Semiconductor manufacturers continue to increase the component densities on computer chips by reducing the device dimensions to less than 10 nm. This trend requires faster, more precise, and more robust optical metrology tools that contain complex and high-precision optics with challenging imaging requirements. Here, we present dark-field digital holographic microscopy as a promising optical metrology technique that uses optics with acceptable complexity. A theoretical analysis and an experimental demonstration of this technique are presented, showing the impact of the coherence length of the light source on the field of view. Finally, we also present the first holographically obtained images of metrology targets.

Automated technology of forming high-precision aspheric optics for multipurpose optical systems

The report presents modern manufacturing technology of high-precision optical parts used in optical-electronic systems for various purposes. The report also presents the complex technology of automated shaping (TESAF) of aspherical, including off-axis surfaces of optical elements for optical-electronic systems. In this paper we consider various methods of forming optical parts with virtually any aspherization value and with different values of the off-axis parameter (off-axis aspheric), with the achievement of surface shape accuracy in the limit λ/60 ÷ λ/80 (λ = 0,6328 μm), by the criterion of the standard deviation. Optical systems manufactured by TESAF technology are already being successfully used at present.

Study on an effective one-dimensional ion-beam figuring method.

Ion-beam figuring (IBF) is a precise surface finishing technique used for the production of ultra-precision optical surfaces. In this study, we propose an effective one-dimensional IBF (1D-IBF) method approaching sub-nanometer root mean square (RMS) convergence for flat and spherical mirrors. Our process contains three key aspects. First, to minimize the misalignment of the coordinate systems between the metrology and the IBF hardware, a mirror holder is used to integrate both the sample mirror and the beam removal function (BRF) mirror. In this way, the coordinate relationship can be calculated using the measured BRF center. Second, we propose a novel constrained linear least-squares (CLLS) dwell time calculation algorithm combined with a coarse-to-fine scheme to ensure that the resultant nonnegative dwell time closely and smoothly duplicates the required removal amount. Third, considering the possible errors induced by the translation stage, we propose a dwell time slicing strategy to divide the dwell time into smaller time slices. Experiments using our approaches are performed on flat and spherical mirrors as demonstrations. Measurement results from the nano-accuracy surface profiler (NSP) show that the residual profile errors are reduced to sub-nanometer RMS for both types of mirrors while the surface roughness is not affected by the figuring process, demonstrating the effectiveness of the proposed 1D-IBF method for 1D high-precision optics fabrication.

Particle identification with DIRCs at PANDA

The DIRC technology (Detection of Internally Reflected Cherenkov light) offers an excellent possibility to minimize the form factor of Cherenkov detectors in hermetic high energy detectors. The PANDA experiment at FAIR in Germany will combine a barrel-shaped DIRC with a disc-shaped DIRC to cover an angular range of 5 to 140 degrees. Particle identification for pions and kaons with a separation power of 3 standard deviations or more will be provided for momenta between 0.5 GeV/c and 3.5 GeV in the barrel region and up to 4 GeV/c in the forward region. Even though the concept is simple, the design and construction of a DIRC is challenging. High precision optics and mechanics are required to maintain the angular information of the Cherenkov photons during multiple internal reflections and to focus the individual photons onto position sensitive photon detectors. These sensors must combine high efficiencies for single photons with low dark count rates and good timing resolution at high rates. The choice of radiation hard fused silica for the optical material and of MCP-PMT photon sensors is essential for DIRC detectors to survive in an environment of radiation and strong magnetic field. The two DIRC detectors differ in the focusing optics, in the treatment of chromatic dispersion and in the electronic readout systems. The technical design of the two DIRC detectors and their validation by testing prototypes in a mixed particle beam at CERN are presented.

Influence of Glassy Carbon Surface Finishing on Its Wear Behavior during Precision Glass Moulding of Fused Silica

Laser technology has a rising demand for high precision Fused Silica components. Precision Glass Moulding (PGM) is a technology that can fulfil the given demands in efficiency and scalability. Due to the elevated process temperatures of almost 1400 °C and the high mechanical load, Glassy Carbon was qualified as an appropriate forming tool material for the moulding of Fused Silica. Former studies revealed that the tools’ surface finishing has an important influence on wear behaviour. This paper deals with investigation and analysis of surface preparation processes of Glassy Carbon moulds. In order to fulfil standards for high precision optics, the finishing results will be characterised by sophisticated surface description parameters used in the optics industry. Later on, the mould performance, in terms of wear resistance, is tested in extended moulding experiments. Correlations between the surface finish of the Glassy Carbon tools and their service lifetime are traced back to fundamental physical circumstances and conclusions for an optimal surface treatment are drawn.

Hygroscopic and thermal stability of high precision replicated epoxy composite mirrors

Replicated lightweight composite mirrors are gaining increasing attention for space applications due to potential weight savings, cost reductions, and faster manufacturing times over traditional glass mirrors. However, dimensional stability remains one of the most critical issues for these resin-based high precision optics. Composite mirrors with a surface figure error (SFE) of 0.03λ were manufactured using a UV-cured replicated layer (RL). The effects of cure state and environmental exposure on the stability of the mirror were evaluated by measuring changes in SFE using interferometric imaging. Higher stability was observed for the replicated epoxy mirror that underwent a secondary cure, after the initial UV cure. SFE for these mirrors increased by only 5-nm (RMS) when exposed to 100% RH for 12 days. The mirrors that were UV cured alone under identical exposure conditions increased in SFE by over 500% of that value. The increased hygroscopic stability is consistent with a reduced amount of unreacted polar epoxide groups that can hydrogen bond with absorbed moisture diffusing into the network. In addition, the mirror with the secondary cure remained stable up to 125°C while the mirror with a lower degree of cure exhibited warpage under identical thermal condition due to additional cure shrinkage.

Effect of motion accuracy on material removal during the CMP process for large-aperture plane optics

Chemical-mechanical polishing (CMP) is a key process in the fabrication of high-precision large-aperture plane optics. To satisfy the growing demand for better polishing efficiency and accuracy in CMP process, there is a new focus on the development of a new CMP machine that applies high-precision motion parts including ultra-precision spindles, guideways, rotary table, and numerical control system. In this article, to show how the run-out error of a rotary table affects the polishing efficiency and accuracy, we propose a material-removing model that creatively analyzes the material-removing process at any point N on a workpiece. To verify the model, we performed a variety of analysis and polishing experiments using the KPJ1700 and KPJ1200 CMP machine. For 6-h-long polishing process on the KPJ1700 CMP machine, the surface accuracy of the 400 × 80 mm UBK7 optic varies from 10.51 to 2.94 λ, and then to 0.32 λ after another 4 h. For 6-h-long polishing process on the KPJ1200 CMP machine, the surface accuracy of the 430 × 430 mm UBK7 optic varies from 5.78 to 3.48 λ, and then to 0.8 λ after another 4 h. The measurement results proved that a smaller run-out error results in higher polishing efficiency and accuracy.