Surfaces Of Optical Elements Research Articles

Ellipsometric Diagnostics of a Transient Surface Layer in Optical Glass

Optical properties of a transient layer with a broken structure that arises at the surface of optical glass at its treatment have been considered. Rather often, the surface of optical elements is considered to be perfect, although the actual inhomogeneous surface structure can have a significant effect for precision physical experiments or novel technological problems. Furthermore, the simulation of the surface layer structure and the corresponding optical characteristics, as well as the study of a possibility to determine those parameters from the results of optical researches, is also of theoretical interest, which is demonstrated in this work. Ellipsometric measurements of optical glass specimens with a broken surface layer are carried out. When modeling the angular dependences of the ellipsometric parameters tan ф and cos б, the near-surface specimen region is considered as a stack of 500 thin layers, and the matrix method of light reflection in this structure with regard for the interference phenomenon is used in calculations. Five models are tested for the optical profile of a nonuniform layer, whose parameters are fitted to achieve the minimum of the target function describing the discrepancy between the calculated and measured data. It is found that the theoretical models describe the optical properties of the specimens more accurately, if they make allowance for the inhomogeneous surface layer. Nevertheless, the solution of the inverse ellipsometric problem turns out ambiguous, so that additional measurements are required for the final choice of a model that would be adequate to the actual morphological structure of the broken layer to be made. However, the key advantage of the applied method consists in that it allows a direct registration of the optical response of the system.

Measurement and Analysis of the Stepwise Curved Surface of Diffractive Optical Elements by a Constant Speed Confocal Probe

Diffractive optical elements (DOEs) play an important role in improving imaging quality and reducing system volume. The measurement of DOE surface topography is of great significance to the evaluation of DOE quality and the optimization of its manufacturing process. However, there are still some difficulties in measuring the large curvature and the large sag of curve-based DOEs. In this paper, considering the geometrical feature information and measurement problems of curve-based DOEs, a measurement method based on a confocal measurement principle for DOE measurement is proposed. The proposed measurement data processing method is verified by a high-precision motion axis system and the stability of an ultra-precision lathe. The results show that the proposed measurement method can reconstruct and accurately evaluate the three-dimensional surface topography of DOEs.

Stokes parameters polarization scattering properties of optical elements surface of different material

The micro-morphology and refractive index of the optical element surface not only affect the quality of the transmitted beam in the optical system, but also cause the polarization modulation of the beam. To characterize the effect of surface properties of optical elements on polarization modulation characteristics, an improved polarization bidirectional reflectance distribution function (pBRDF) is obtained based on the theory of Micro-facet scattering, then a Stokes parametric polarization scattering mathematical model between the object surface and the modulated beam is established. The effect regulations of incident angle, surface roughness, scattering angle and scattering azimuth on the scattering distribution of Stokes parameters are analyzed by numerical simulation. The modified J.A. Woollam ellipsometry is used to measure and analyze Stokes parameters of samples with different materials and roughness. the distribution relationships between scattering angle and degree of linear polarization and polarization angle are obtained. Comparing with results of simulation and experiment, and it coincide well. It provides technical and theoretical support for the subsequent surface quality assessment and defect detection testing based on scattering.

Numerical and experimental investigation of glow discharge cleaning on SSRF beamline

Synchrotron radiation induced carbon contaminations on optical elements is widely considered to be a major concern for a beamline. The purpose of this paper is to study on glow discharge cleaning (GDC) of the mirror box, striving to prevent carbon contamination formation fundamentally by obtaining a cleaner vacuum chamber. In order to theoretically understand GDC, a 2D fluid model of Ar/O2 discharge was established, and the discharge physical parameters were obtained under different conditions (Ar/O2 mixing ratio, electrode arrangement). GDC system was designed and built on the basis of simulation to verify the GDC results. Moreover, we analyzed the influence of GDC on the surface of the optical components. It is found that the average density of metastable particles increases with the increase of oxygen-to-argon ratio. The arrangement of a single electrode is more reasonable for GDC compared with dual electrodes. From the results of quadrupole mass spectrum analysis, we identify that the amount of oil molecules could be reduced by 84% after being cleaned by glow discharge, indicating that GDC has a remarkable effect on removing oil molecules on the surface of the mirror box. However, we found that GDC has some damage to the optical element surface.

Transparent block copolymer thin films for protection of optical elements via chemical vapor deposition

In this study, glycidyl methacrylate and 1H, 1H, 2H, 2H-perfluorodecyl acrylate copolymer p(GMA-co-PFDA) thin-films fabricated via Initiated Chemical Vapor Deposition (iCVD) were investigated as protective coatings on optical BK7 glass substrates and commercial optical filters. Durability tests based on military standards MIL-F-48616 and MIL-C-48497A were performed to evaluate performance of coatings for the protection of surfaces of optical elements. Cross-linked p(GMA-co-PFDA) copolymer coatings successfully passed all durability tests showing excellent mechanical properties and protection against humidity, salt water, swelling in water, and resistance to organic solvents while providing excellent adhesion to substrate. iCVD process enabled fine tuning of film morphology, mechanical properties and hydrophobicity by controlling the process parameters. Fabricated films were hydrophobic and highly transparent (>98%) in the wavelength range from 300 nm to 1000 nm. Optical transmittance measurements before and after coating process proved that while providing chemical and physical protection, p(GMA-co-PFDA) copolymer thin-films do not cause any detectable change in optical performance of commercial narrow band and wide band filters.

Segmentation of shallow scratches image using an improved multi-scale line detection approach

Along with developing modern technology, the demands for optical element surface develop towards the characteristics of large scale and high precision. However, it is challenging to evaluate the surface defects since some shallow scratches in optical element surface images are usually characterized by low contrast and blurry outlines. This property makes the machine vision inspection extremely difficult. So, this paper proposes a novel multi-scale line detection method that can efficiently extract shallow scratches. Firstly, to decrease the influence of the surrounding region, a new multi-scale line detector combines all the responses at different scales by setting different weights for each scale. Then, based on the scratches features, we utilize morphological operations to get the full continuum of the scratches area. Experimental results show that our model can ideally extract the contours of shallow scratches that are very close to the optical microscope results observed by specialists.

基于高斯脉冲激光空间分辨测量光学元件表面激光损伤阈值研究

基于高斯脉冲激光空间分辨测量光学元件表面激光损伤阈值研究

Design of an axisymmetric optical element generating a prescribed illuminance distribution and wavefront

Design of an axisymmetric refracting optical element to transform an incident beam into an output beam with the prescribed illuminance distribution and prescribed wavefront is considered. The wavefront of the output beam is represented by the eikonal function defined in some plane behind the optical element. The computation of the optical element surface is reduced to the solution of two explicit ordinary differential equations of the first order. As examples, we consider the design of two optical elements to transform a spherical beam from a point Lambert light source into a beam with the uniform illuminance and plane wavefront and a beam with the complex wavefront to focus into an on-axis line.

Development of a Free Space, LED Illuminated Spectral-domain Optical Coherence Tomography Setup

Free-space spectral domain optical coherence tomography has been demonstrated using an 8 mW ultra-bright 850 nm light-emitting diode with a 40 nm spectral width. The system detects longitudinal reflectivity of surface and sub-surface layers of optical elements to depths of a millimetre with high fidelity. Development stages included mathematical analysis of light interference by superposition of electric field phasors of reference and sample arms of a Michelson interferometer. A method by which depth-resolved reflectivity is acquired is described. A locally assembled Czerny Turner monochromator was aligned such that the interferometer output beam is dispersed into its spectral components before image re-construction. Calibration of the 2048-pixel detecting charge-coupled device line camera was performed using a Mercury vapour lamp with 8 spectral lines spanning from the ultra-violet to yellow region of the electromagnetic spectrum. Processing of interference fringe signals from spectral domain data is described and an analysis of variations in frequency of the interference fringe signal and threshold illumination with depth into the sample presented. A test of sensitivity of the depth imaging algorithm to low-amplitude signals is also reported.

Manifestation of Crystal Lattice Distortions in the IR Reflection Spectra of Abrasion-Treated ZnSe Ceramics

The Fourier IR reflection spectra of ZnSe ceramics prepared by hot pressing (HP), physical vapor deposition (PVD), and PVD combined with hot isostatic pressing (HIP) are presented. The optical constants of polished and dry-ground specimens were used for comparison. The grinding treatment simulated the erosion of the outer surface of optical elements made of zinc selenide under the influence of solid dust particles and deposits. In the polished specimens residual stresses showed up in the IR reflection spectra of the ZnSePVD and ZnSeHIP ceramics, which had well-defined orientation of grains, but were not present in the spectra of the ZnSeHIP ceramics as a result of mutual compensation of the stresses in the randomly oriented grains of the material. The stresses, which appeared as a shift of the absorption bands calculated by the Kramers–Kronig method, increased significantly after abrasive treatment of the specimens. For all the treated ceramics the intensity of the absorption bands resulting from the anharmonicity of the vibrations in the distorted crystal lattice increased by several times. The last effect also depends on the production prehistory of the ceramics.

Aspherical Lens Design Using Computational Lithography

Optical lithography is often applied to fabricate three-dimensional (3D) patterns because of its capability on the scale of optical wavelengths. An aligner, a laser writer, and a stepper with an optical projection system are used as the exposure tool. The focusing capability must enable exposure of step heights of ≥50μm, which is normally achieved using an optical projection system with a large focal depth. Recently, a projection lens was developed that can produce a variable step height using an adjustable numerical aperture, which has led to the fabrication of micro-lens arrays (MLAs) with step heights as high as 100μm. These MLAs are used in various fields but mostly only as condensing lenses. However, wider applications await the realization of optical aberration reduction. The introduction of aspherical lenses would be a major step in this direction; however, aspherical lenses are difficult to produce because of the limitations in processing and polishing the aspherical surfaces of optical elements made of glass. Alternatively, the fabrication of 3D patterns using optical lithography involves a photomask whose optical density distribution corresponds to the 3D profile after exposure and development. A density distribution corresponding to the desired 3D shape can be achieved using a computer to control the dot density of a microdot mask pattern. In this study, we present a design method for low-aberration aspherical MLAs that satisfies the Rayleigh standard using computational lithography.

Structural variation in a synchrotron-induced contamination layer (a-C:H) deposited on a toroidal Au mirror surface.

A carbon layer deposited on an optical component is the result of complex interactions between the optical surface, adsorbed hydrocarbons, photons and secondary electrons (photoelectrons generated on the surface of optical elements). In the present study a synchrotron-induced contamination layer on a 340 mm × 60 mm Au-coated toroidal mirror has been characterized. The contamination layer showed a strong variation in structural properties from the centre of the mirror to the edge region (along the long dimension of the mirror) due to the Gaussian distribution of the incident photon beam intensity/power on the mirror surface. Raman scattering measurements were carried out at 12 equidistant (25 mm) locations along the length of the mirror. The surface contamination layer that formed on the Au surface was observed to be hydrogenated amorphous carbon film in nature. The effects of the synchrotron beam intensity/power distribution on the structural properties of the contamination layer are discussed. The I(D)/I(G) ratio, cluster size and disordering were found to increase whereas the sp2:sp3 ratio, G peak position and H content decreased with photon dose. The structural parameters of the contamination layer in the central region were estimated (thickness ≃ 400 Å, roughness ≃ 60 Å, density ≃ 72% of bulk graphitic carbon density) by soft X-ray reflectivity measurements. The amorphous nature of the layer in the central region was observed by grazing-incidence X-ray diffraction.

High-performance facility and techniques for high-precision machining of optical components by ion beams

The paper describes a high-performance facility for ion beam processing of the surface of optical elements. The facility is equipped with three technological ion sources, which allow working with inert or reactive gases, and a five-axis goniometer. Two sources (KLAN-103M and KLAN-163M) have a wide-aperture quasi-parallel ion beam and a third ion source with the focused ion beam (width of the output beam is Ø1–15mm by changing the diaphragm). For a focusing ion source an optimal diaphragm material (sapphire) and the Ar energy range (less than 400eV), which minimizes deposition on the surface of the workpiece the material sputtered from the edge of the diaphragm, are found. The concept of a movable workpiece has allowed realizing within one vacuum chamber three methods of ion beam surface treatment: aspherization, local shape errors correction and ion polishing. A detailed description of all the surface treatment methods is given. An algorithm for selecting the ion beam diameter and scanning steps for the local shape errors correction depending on the lateral dimension of the surface errors is proposed. The facility allows to produce an optical surface of any complex (convex/concave) shape, including higher-order aspheres with an asymmetrical profile and a diameter of up to 300mm, with a subnanometer precision of the surface shape and the effective roughness in the range of spatial frequencies q∈[2.5·10−2–6.0·101μm−1] down to σeff=0.14nm.

Comparison of Surface Modification of CR-39 Polymer Film Using RF and DC Glow Discharges Plasma

Surfaces of optical elements are deposited by antireflection coatings (ARCs) to decrease the reflection of light. Surface needs treatment before depositing the ARC one of treatment processes by plasma for adhesion improvement and surface hardening. A comparison of RF and DC glow discharges treated CR-39 polymer films gives insight into the mechanism of these surface processes. The surface properties of the plasma-treated samples are examined by microscopy techniques include contact angle measurements, scanning electron microscopy (SEM), atomic force microscopy (AFM), infrared (IR) spectroscopy and refractive index measurements. Results show that the plasma treatment modifies the polymer surface in both composition and morphology. It is found that the surface wettability is enhanced after plasma treatment. It is found that, RF plasma is more effective than DC plasma in CR-39 surface modification, as it implants more oxygen atoms into the surface and makes the contact angle declining to a lower level.

Full field of view polarization effect measurement and error analysis of non-polarized channels of spaceborne directional polarimetric camera

The optical system of spaceborne directional polarimetric camera that employs an ultra wide-angle lens for its multiangle, a filter wheel for its multispectral and also its multipolarization capability, a CCD itself for its imaging has a certain polarization effect, which can affect the radiometric accuracy of the non-polarized channels of the instrument. The transmittance of the oblique light rays that are incident on the optical element surfaces is sensitive to the orientation of the electric field, contributing to the linear polarization effect of optical system. The precise polarization measurement and calibration of the passive imaging polarimetry are in urgent need to eliminate the instrumental polarization effect and to improve its radiometric accuracy for observation scenes. The non-polarized channel radiometric model considering the linear polarization effect is deduced in detail by analyzing the instrumental principle and optical structures. Moreover, the reasonably simplified model is established based on the actual lens characteristics. A calibration method in which completely linearly polarized light with different kinds of polarization angles irradiates sparsely the instrument within full field of view and subsequently fits the response in the least square sense, is proposed and simulated. In addition, the measured relative errors of the intensity of incident light with different kinds of polarization states caused by the calibration deviations of instrumental principal physical parameters are analyzed and calculated, such as the azimuthal angle of single pixels, explicit optic polarization rate and low frequency spatial relative transmittance. The actual instrumental parameter values and their calibration deviation amounts are acquired by carrying out the laboratory calibration experiment for instrument and combining the least square fitting. Furthermore, the maximum radiometric calibration relative error caused by the deviation of the physical parameter called explicit optic polarization rate is calculated to be 0.4%, fulfilling completely the requirement of radiometric relative accuracy 5% and retaining abundant tolerance. The study provides a theoretical basis and an experimental guidance in high accurately measuring radiation, calibrating and processing data for the instrumental non-polarized channels with full field of view.

An orthogonal phase-shifting interferometry and its application to the measurement of optical plate

An optical interferometry named orthogonal phase-shifting interferometry (OPSI) was proposed. Compared with other interferometers, OPSI works well at air turbulence and complex environment. Phase recovery method was calculated by Jones matrix for measurement of optical elements surface shape and roughness. In order to control piezo-electric transducer (PZT) shifter, the property between voltage and phase of PZT shifter was calculated. Based on this interferometry, the roughness of an unknown transparent optical plate was measured. The results show that the error of peak to valley (PV) value is 0.0087λ (λ=1064nm) and the error of root mean square (RMS) value is 0.0006λ compared with WYKO(a commercial interferometer of Veeco),and this demonstrates the high stability and precision of the orthogonal phase-shifting interferometry.

Estimation of parameters for evaluating subsurface microcracks in glass with in-line digital holographic microscopy.

In this work, a new method for imaging subsurface damage (SSD) is proposed, which is, to the best of our knowledge, the first application of the in-line digital holographic microscopy (IDHM) to the reconstruction of the subsurface damage in glass. By combination of the in-line arrangement and an objective lens to image the hologram on the CCD surface, the method is characterized by its high resolution in both the lateral and depth directions. Then the three-dimensional reconstruction of the microcracks within the glass was realized by numerically focusing en-face images at different depths, and the sizes of SSD along the transversal and depth directions were estimated. Based on the experimental results, the cracks can be divided into two categories: one is that the cracks begin from the surface of optical elements, the other is totally within the components. To indicate the propagation or development trajectory of the cracks and predict the magnitude of the laser-induced damage threshold, the relative intensity distributions of the light scattered by the cracks compared with the ones without cracks were also reconstructed. In this case all the required parameters for evaluating SSD are obtained with our IDHM system, so that the SSD produced in the manufacturing process can be reduced or removed more easily to optimize the performance of the optical component and extend its lifetime. These results provide the guidance for the optical system design of precision measurements.

Precision aspherization of the surface of optical elements by ion-beam etching

Ion-beam methods are developed for forming supersmooth aspherical optical surfaces of polished fused quartz with a precision on the level of 3 nm relative to the standard deviation and 0.2–0.3 nm for the mean-square surface roughness in the range of spatial frequencies of 0.01–100 μm−1.

Parallel Optical Profilometry Assisted Conception of an Opto-Electronic Illuminator Using a VCSEL and Diffractive Optics

Micro-opto electromechanical systems are of growing importance in the development of new miniaturised projection, imaging and detection technologies. Because of the increasing complexity of construction involving layers of microelectronics, optoelectronics and micro-optics, di erent types of defects can appear that need to be understood and controlled in order for the miniaturised system to operate correctly within the given speci cations. We have recently developed a miniaturised structured light (micro-opto electromechanical systems) projector consisting of a multilayer sandwich made of a rst layer of 3 bars of 12 VCSEL laser sources, a second layer of Fresnel collimating lenses (di ractive optical elements) and a third layer of Fourier di ractive optical elements to produce the structured light pattern. The main defects that had to be controlled were alignment errors, the rst ones being between the emitting surface and the collimating lenses and the second one concerning the lateral alignment of the photomasks used to produce the 4 levels of the Fourier elements. We demonstrate how 3D geometrical shape characterization of the emitting structures and di ractive optical elements surface structures using coherence scanning interferometry played a major role in the conception and fabrication of a prototype micro-optoelectromechanical projection system to understand the source of the alignment errors and to minimise them.

基于有限元法的面形反演模型

In order to inversed compute the optical element surface profile using the finite element method and the surface shape interference test results, the composition of interference test results was decomposed, and the testing principles of the rotational averaging method were analysised. The feasibility of using simplified geometrical model to caculate the deformation of an optical element with arbitrary given surface shape was discussed. Surface profile inversion model was proposed based on finite element method. A finite element analysis model was then presented based on surface-surface contact modeling. A plane mirror supported by three rigid spheres as the simulation and experimental model, the rotationally asymmetrical surface deviation was quantitative analysis using the numerical method presented in this paper and the N-step rotational averaging method. The obtained results show that the root mean square of rotationally asymmetric surface is 2.933 nm obtained through numerical method and the root mean square of rotationally asymmetric surface is 2.75 nm obtained through N-step rotational averaging method. Subtracted result of the two rotationally saymmetric surface show that the root mean square of the subtraction are 6.31% of the numerical method result and 6.73% of the N-step rotational averaging method result. Finally, comparing the optical element surface profile computed by proposed inversion model with the surface profile tested by N-step rotational averaging method. Results show that the root mean square of optical element surface profile are 3.535 nm and 3.351 nm respectively. Subtracted result of the two surface profile show that the root mean square of the subtraction are 11.67% of the inversiong result and 11.06% of the result tested by N-step rotational averaging method. The proposed inversion model is accurate and reliable.