Optical Metrology System Research Articles

Determination of the Stress Distribution in Timber Elements Reinforced with Self-Tapping Screws Using an Optical Metrology System

The use of self-tapping screws with continuous threads to reinforce and join timbers in addition to glulam elements represents an effective, simple and economic method. The high withdrawal resistance of the screws as well as its continuous bond with the wood enables an effect similar to steel reinforcement in concrete structures. Within the framework of a research project, funded by the German Research Foundation (DFG) at the Chair of Structures and Structural Design in cooperation with the Institute for Building Material Research of the RWTH Aachen University, several tests have realized to investigate the bond behaviour, the force transfer and the anchorage length of the screws. Herewith the stress distribution will be determined with an optical 3D field measuring system, based on the digital image correlation (DIC) method, which measures the strain by observing the surface of the test specimens. To ensure the accuracy of the measurements for a wide measuring field of wood surface, a comparison study was conducted involving traditional electrical sensors (i.e. strain gauges and LVDTs). The results of this study, which confirm the accuracy regarding the determination of stress distribution for small deformations on the wood surface with optical 3D field measuring system, will be presented in this article.

Common mode noise rejection properties of amplitude and phase noise in a heterodyne interferometer

High precision metrology systems based on heterodyne interferometry can measure the position and attitude of objects to accuracies of picometer and nanorad, respectively. A frequently found feature of the general system design is the subtraction of a reference phase from the phase of the position interferometer, which suppresses low frequency common mode amplitude and phase fluctuations occurring in volatile optical path sections shared by both the position and reference interferometer. Spectral components of the noise at frequencies around or higher than the heterodyne frequency, however, are generally transmitted into the measurement band and may limit the measurement accuracy. Detailed analytical calculations complemented with Monte Carlo simulations show that high frequency noise components may also be entirely suppressed, depending on the relative difference of measurement and reference phase, which may be exploited by corresponding design provisions. While these results are applicable to any heterodyne interferometer with certain design characteristics, specific calculations and related discussions are given for the example of the optical metrology system of the LISA Pathfinder mission to space.

Simulation and System Design of a 3D Metrology Optical System Based on a Bidirectional OLED Microdisplay

Innovative display technologies enable a wide range of different system applications. specifically, in metrology, medical, and automotive applications microdisplays were increasingly used. In the last decades OLED microdisplays were in the focus of display development. A new class of OLED microdisplays with an integrated photodiode array is the latest development. The so-called bi-directional OLED microdisplays combine light-emitting devices (AM-OLED microdisplay) and photo sensitive detectors (photodiode matrix) on one single chip based on OLED-on-CMOS-technology. Currently this kind of display is still a prototype. Based on such a novel bidirectional OLED microdisplay, we present for the first time a system simulation and design of a 3D optical surface metrology system. The first step is the full characterization of the microdisplay. Depending on the characterization results the future system parameters are determined. Based on the characterization results and the application parameters the system design parameters are defined. The functionality of the system is simulated, and a theoretical proof of concept is presented. An example for our application on 3D optical surface metrology system is evaluated.

Remotely measuring a thin dielectric coating on a metallic cylinder

A laser optical metrology system is described that remotely measures at high rate the presence and thickness of a thin-film lubricant on metallic cylinders with diameters on the order of 0.5 mm. Applications include remote measurement of hypodermic needle dielectric coating thickness in a clean room environment. High accuracy computer simulation of the electric field scattered from a coated cylinder by an incident laser beam is demonstrated using the condition numbers of the matrices defined by the boundary value matching equations derived from the eigenfunction expansion of the exact solution to Maxwell's equations. Dielectric coatings from 1 μm to 50 μm are seen to be readily observed and accurately measured using a remotely placed CMOS array. Distinctive signatures are shown for film thicknesses in the range from 0 to 10 μm, and an appropriate location for CMOS detector placement is determined from the scattering patterns.

Verification of an optical metrology system (ARAMIS) by comparing experimental data with FE calculations and continuum approaches

AbstractOptical 3D field measuring systems, based on DIC (Digital Image Correlation) deliver kinematic quantities. They are appropriate devices helping to understand the material behavior especially at high strains. One problem of these systems is the missing information about material points located behind the observed surface of the specimen, which is needed to assign the full deformation gradient. Different ways of dealing with raw point coordinate data that is confidently given by the metrology system lead to different kinematic results. To ensure the correctness of strain‐like results determined by the system, they are compared to analytical and finite element calculations. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Development of a low cost high precision fabrication process for glass hybrid asphericaldiffractive lenses

The hybrid aspherical diffractive singlet achromat design can be used to reduce chromaticaberration in compact optical systems. In this paper, the development of a compressionmolded, low cost and high precision hybrid diffractive glass lens is described. Specifically,an aspherical diffractive lens designed to compensate for chromatic aberration wasfabricated by precision glass molding. The diffractive features are integrated on theaspherical surface to avoid mold alignment during fabrication. As part of the effort to lowermanufacturing cost, the diffractive profiles were directly fabricated by single-pointdiamond turning without polishing. A thin layer of platinum–iridium coatingwas applied to the mold surfaces to protect the mold inserts from degradationduring the molding process. In order to reduce thermal shrinkage error, the hybridlens was fabricated using a two-step precision molding process on a commercialglass molding machine. The geometry of the molded hybrid aspherical diffractivelens was measured using an optical profilometer and the results demonstrated amatch to the design mold profile with a replication error of 0.16% in the radialdirection and 6.3% in the axial direction. In addition, an optical metrology system toevaluate the diffraction efficiency and chromatic focal shift was constructed andthe measured results showed that the hybrid lenses indeed function as designed.

Digital Laser Frequency Control and Phase-Stabilization Loops in a High Precision Space-Borne Metrology System

We report on the design, implementation, and characterization of fully digital control loops for laser frequency stabilization, differential phase-locking, and performance optimization of the optical metrology system on-board the LISA Pathfinder space mission. The optical metrology system consists of a laser with modulator, four Mach-Zehnder interferometers, a phase-meter and a digital processing unit for data analysis. The digital loop design has the advantage of easy and flexible controller implementation and loop calibration, automated and flexible locking and resetting, and improved performance over analog circuitry. Using the practical ability of our system to modulate the laser frequency allows us to accurately determine the open-loop transfer function and other system properties. Various noise sources and their impact on system performance are investigated in detail.

The LISA Pathfinder interferometry—hardware and system testing

Preparations for the LISA Pathfinder mission have reached an exciting stage. Tests of the engineering model (EM) of the optical metrology system have recently been completed at the Albert Einstein Institute, Hannover, and flight model tests are now underway. Significantly, they represent the first complete integration and testing of the space-qualified hardware and are the first tests on an optical system level. The results and test procedures of these campaigns will be utilized directly in the ground-based flight hardware tests, and subsequently during in-flight operations. In addition, they allow valuable testing of the data analysis methods using the MATLAB-based LTP data analysis toolbox. This paper presents an overview of the results from the EM test campaign that was successfully completed in December 2009.

Coupling characterization and noise studies of the optical metrology system onboard the LISA Pathfinder mission

We describe the first investigations of the complete engineering model of the optical metrology system (OMS), a key subsystem of the LISA Pathfinder science mission to space. The latter itself is a technological precursor mission to LISA, a spaceborne gravitational wave detector. At its core, the OMS consists of four heterodyne Mach-Zehnder interferometers, a highly stable laser with an external modulator, and a phase meter. It is designed to monitor and track the longitudinal motion and attitude of two floating test masses in the optical reference frame with (relative) precision in the picometer and nanorad range, respectively. We analyze sensor signal correlations and determine a physical sensor noise limit. The coupling parameters between motional degrees of freedom and interferometer signals are analytically derived and compared to measurements. We also measure adverse cross-coupling effects originating from system imperfections and limitations and describe algorithmic mitigation techniques to overcome some of them. Their impact on system performance is analyzed within the context of the Pathfinder mission.

Ultrastable assembly and integration technology for ground- and space-based optical systems

Optical metrology systems crucially rely on the dimensional stability of the optical path between their individual optical components. We present in this paper a novel adhesive bonding technology for setup of quasi-monolithic systems and compare selected characteristics to the well-established state-of-the-art technique of hydroxide-catalysis bonding. It is demonstrated that within the measurement resolution of our ultraprecise custom heterodyne interferometer, both techniques achieve an equivalent passive path length and tilt stability for time scales between 0.1 mHz and 1 Hz. Furthermore, the robustness of the adhesive bonds against mechanical and thermal inputs has been tested, making this new bonding technique in particular a potential option for interferometric applications in future space missions. The integration process itself is eased by long time scales for alignment, as well as short curing times.

Fiber-optical measurement of form deviations of rotation-symmetric parts

The quality inspection of products poses a challenge towards existing production metrology techniques in terms of accuracy and speed, especially when it comes to inline or in-process measurements. In this terms, optical metrology proves its capabilities, in particular for sensible and fragile work pieces. For the inspection inside small spaces and cavities or for complex geometries, many optical metrology system are to inflexible or cannot be miniaturized enough. Within this work a set-up for a fiber-optic low-coherence interferometer system is described. Due to the high degree of miniaturization of the sensing probe, the system can be used for demanding measuring tasks like roundness of small drilling holes or the run-out of shafts. Different evaluation measurement are presented, which underline the potential of the described system for highly accurate, non-contact inspection of rotational parts.

Challenges in the Measurement and Data-Processing Chain of the LISA Mission

The LISA Mission (Laser Interferometer Space Antenna) is currently under mission formulation with a launch date planned in 2020. The purpose of the mission is the observation of gravitational waves at frequencies between 0.1 mHz and 1 Hz by measuring distance fluctuations between inertial reference points, represented by cubic proof masses. In order to provide a sufficient sensitivity of the instrument, distance fluctuations between two inertial reference points must be measured with a strain accuracy of around 10−20 Hz−1/2. This is achieved by setting up a laser interferometer with a base-length of 5⋅106 km and a path-length measurement noise in the order of 10 pm Hz−1/2. For a correct evaluation of the data on the ground, it is essential that the science data telemetry preserves all required frequency domain information. That is, any on-board data-processing and down-sampling must be done with great care in order not to introduce aliasing or other artifacts into the data stream. As an additional complication, most of the optical metrology data is dominated by laser phase noise which is about eight orders of magnitude larger than the required instrument sensitivity. However, by applying a method called “time-delayed interferometry” during the ground data processing, this laser phase noise can be eliminated from the data. This method has already been demonstrated in a detailed simulation environment, but it requires a very careful filtering, synchronization, and interpolation of the individual data streams. Last but not least, a calibration of system parameters is necessary in many areas of the LISA measurement system. The system design must therefore ensure that all data required for these calibrations is available on-ground in a quality that allows a successful computation of the calibration coefficients within a reasonable time-frame. The data streams do not only include data from the optical metrology system, but also from the drag-free and attitude control system which are used to derive other information, such as the charge state of the proof mass. This yields a strong coupling between the different disciplines since data that is only used for housekeeping purposes in other missions becomes an essential part of the science data stream for the LISA mission. This paper gives an overview of the LISA measurement and data-processing chain. It highlights the most challenging areas that have been identified so far and describes the intended solution methods.

Flat-topped spectrum obtained from partially coherent hollow Gaussian Schell-model beams passing through a slit

The anomalous spectral behaviors of partially coherent polychromatic hollow Gaussian Schell-model beams (HGSMBs) diffracted by a slit are investigated. Besides spectral switches, asymmetrical dual-peak spectral split and multi-peak spectral split are observed. In the vicinity of the positions where spectral switches occur, flat-topped spectrum can be obtained, the bandwidth of which is adjustable by changing the transverse coherence width and the observation position. The flat-topped spectrum has potential application for creating probe beams in optical metrology systems and generating broadband optical sources in fiber optic sensor interface systems.

Analysis and Optomechanical Characterisation of a MEMS Micromirror Device

Abstract: Advances in our analysis and characterisation of a micro‐electromechanical system (MEMS) micromirror device are presented. The micromirror device, to be used as an integral component of an interferometer, has a characteristic dimension of 510 μm and operates in a dynamic mode with maximum out‐of‐plane displacements in the order of 10 μm at oscillation frequencies of up to 1.3 kHz. Developments are carried out by analytical, computational and experimental methods. Analytical and computational nonlinear geometrical models are developed in order to determine the optimal loading‐displacement operational characteristics of the micromirror. Because of the operational mode of the micromirror, the experimental characterisation of its loading‐displacement transfer function requires utilisation of advanced optical metrology methods. Optoelectronic holographic microscopy (OEHM) methodologies, based on multiple wavelengths being developed to perform such characterisation, are described. The micromirror device is suitable for optical phase measurements and applicable to development of miniaturised optical metrology systems for characterisation of shapes and deformations.

The end-to-end testbed of the optical metrology system on-board LISA Pathfinder

LISA Pathfinder is a technology demonstration mission for the Laser Interferometer Space Antenna (LISA). The main experiment on-board LISA Pathfinder is the so-called LISA Technology Package (LTP) which has the aim to measure the differential acceleration between two free-falling test masses with an accuracy of 3 × 10−14 ms−2 Hz−1/2 between 1 mHz and 30 mHz. This measurement is performed interferometrically by the optical metrology system (OMS) on-board LISA Pathfinder. In this paper, we present the development of an experimental end-to-end testbed of the entire OMS. It includes the interferometer and its sub-units, the interferometer backend which is a phasemeter and the processing of the phasemeter output data. Furthermore, three-axes piezo-actuated mirrors are used instead of the free-falling test masses for the characterization of the dynamic behaviour of the system and some parts of the drag-free and attitude control system (DFACS) which controls the test masses and the satellite. The end-to-end testbed includes all parts of the LTP that can reasonably be tested on earth without free-falling test masses. At its present status it consists mainly of breadboard components. Some of those have already been replaced by engineering models of the LTP experiment. In the next steps, further engineering and flight models will also be inserted in this testbed and tested against well-characterized breadboard components. The presented testbed is an important reference for the unit tests and can also be used for validation of the on-board experiment during the mission.

Optical metrology alignment and impact on the measurement performance of the LISA Technology Package

Aside from LISA Pathfinder's top-level acceleration requirement, there is a stringent independent requirement for the accuracy of the optical metrology system. In case of a perfectly aligned metrology system (optical bench and test masses) it should rather be independent of residual displacement jitter due to control. However, this ideal case will not be achieved as mechanical tolerances and uncertainties lead to a direct impact of test mass and spacecraft displacement jitter on the optical measurement accuracy.In this paper, we present a strategy how to cover these effects for a systematic requirement breakdown. We use a simplified nonlinear geometrical model for the differential distance measurement of the test masses which is linearized and linked to the equations of motion for both the spacecraft and the two test masses. This leads from test mass relative displacement to a formulation in terms of applied force/torque and thus allows to distinguish the absolute motion of each of the three bodies. It further shows how motions in each degree of freedom couple linearly into the optical measurement via DC misalignments of the laser beam and the test masses. This finally allows for deriving requirements on the alignment accuracy of components and on permittable closed-loop acceleration noise.In the last part a budget for the expected measurement performance is compiled from simulations as no measurement data is available yet.

Comparison of Measurement Techniques for Linewidth Metrology on Advanced Photomasks

This paper compares electrical, optical, and atomic force microscope (AFM) measurements of critical dimension (CD) made on a chrome on quartz photomask. Test structures suitable for direct, on-mask electrical probing have been measured using the above three techniques. These include cross-bridge linewidth structures and pairs of Kelvin bridge resistors designed to investigate dimensional mismatch. Overall, the results show very good agreement between the electrical measurements and those made with a calibrated CD-AFM system, while the optical metrology system overestimates the measured width. The uncertainty in each of the measurements has been considered, and for the first time an attempt has been made to describe the levels and sources of uncertainty in the electrical measurement of CD on advanced binary photomasks.

A high-precision measurement technique for evaluating alcohol concentrations using an optical metrology system based on a position sensing detector

This study develops a high-precision, non-destructive optical metrology system based on a position sensing detector for measuring the refractive index of a liquid solution such that its alcohol concentration can be derived. A series of experiments are performed to measure the refractive indices of samples of known alcohol concentrations in the range 0–99.5%. The experimental results indicate that the refractive index and the alcohol concentration are related by a quadratic function. According to this mathematical function, the refractive index of a sample with an alcohol concentration of 95% is n 95%=1.36442. The measured value of the refractive index of the same sample is found to be n alcohol-95%=1.36587. The deviation of just 0.00145 between the two sets of results confirms the accuracy of the analytical prediction method. For a sample with a known alcohol concentration of 37%, the error between the analytically predicted concentration value and the actual value is found to be less than 0.7%. Overall, the results confirm that the proposed system provides a low-cost, straightforward and highly precise approach for measuring the alcohol concentration of liquid samples.

3D Stacking of Multiple Silicon Plates using Direct Wafer Bonding

In this paper we present a method to stack several tens of structured and curved silicon plates by direct wafer bonding into large geometries. We have developed a number of new processes and methods for the manufacturing and assembly of structured silicon plates. In this paper we will report on all of the process steps, like structuring of the wafers (grooving and dicing), surface protection, artificial wedging of the plates, metallic coating and detection and removal of sub-micrometer particles on ribbed surfaces. In addition we will describe methods to pre-bend plates, to align and to stack them. This also includes new optical and interferometric metrology systems that are used to perform inspection of the bonded surfaces. Furthermore, we report to have stacked 35 structured and curved silicon plates on top of each other. Such a stack is stiff enough to maintain its shape.

Real-time measurement of glucose concentration and average refractive index using a laser interferometer

This study presents a non-destructive, highly precise optical metrology system for measuring the average refractive index of a liquid solution such that its glucose concentration can be derived. The metrology system is employed to measure the average refractive indices of samples with known glucose concentrations ranging from 0 to 200 g/l. By applying a regressional analysis technique to the experimental results, an analytical expression is derived to describe the relationship between the refractive index and the glucose concentration. An excellent agreement is observed between the experimentally determined values of the glucose concentration and the analytically derived results. For an assumed laser interferometer resolution of 1 nm, the measurement resolution of the proposed metrology system is found to be at least F=0.05 g/l, which is significantly better than that of F=2 g/l obtained using the polarimetric glucose sensor presented by Lo and Yu [A polarimetric glucose sensor using a liquid-crystal polarization modulator driven by a sinusoidal signal. Opt Commun, 2006; 259: 40–8].