Proceedings Volume 12137

Optics and Photonics for Advanced Dimensional Metrology II

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Proceedings Volume 12137

Optics and Photonics for Advanced Dimensional Metrology II

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Volume Details

Date Published: 31 May 2022
Contents: 10 Sessions, 31 Papers, 24 Presentations
Conference: SPIE Photonics Europe 2022
Volume Number: 12137

Table of Contents

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Table of Contents

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  • Front Matter: Volume 12137
  • Optical Measurements of Length, Form, and Position
  • On-machine and In-process Metrology
  • Light Properties and Optical Metrology
  • Measuring Instruments Using Advanced Photonic Devices
  • Quantitative Imaging for 3D Metrology
  • Optical Micro- and Nanometrology
  • Metrology of Complex Surfaces and Materials
  • Large-Scale Measurements
  • Poster Session
Front Matter: Volume 12137
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Front Matter: Volume 12137
This PDF file contains the front matter associated with SPIE Proceedings Volume 12137, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.
Optical Measurements of Length, Form, and Position
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Holographical image based vibrometry with monochromatic and event based cameras
Simon Hartlieb, Maciej Boguslawski, Tobias Haist, et al.
In this article we show a highly accurate vibration measurement technique based on imaging of multiple light emitters that are attached to the object of interest. Each emitter is holographically replicated to a cluster of spots in the image plane. By averaging the centroids of all replications the position measurement accuracy can be improved. We show, that vibration amplitudes of 100nm can be measured within a measurement field of 148mm×110mm using standard imaging sensors. The standard deviation between our camera setup and a commercial Laser-Doppler-Vibrometer used as reference is σ =0.095 µm in object space, which corresponds to 0.0017 pixels in image space. To overcome the frame rate limitations of standard imaging sensors we also investigate the application of the proposed method to an event based camera. Since the signal no longer consists of grey value images, other approaches have to be developed to reconstruct the object position. One reconstruction approach as well as first experimental results are presented.
Single-shot wavelength meter on a photonic chip for absolute distance measurement using frequency scanning interferometry
C. R. Coggrave, P. D. Ruiz, C. A. Pallikarakis, et al.
A chip-scale solid-state wavelength measuring device based on a silicon photonics platform is presented. It has no moving parts and allows single-shot wavelength measurement with high precision over a nominal bandwidth of 40 nm in the Oband. The wavemeter design is based on multimode interferometer (MMI) couplers and a multi-band Mach–Zehnder interferometer (MZI) structure with exponentially increasing optical path differences and in-phase quadrature detection. The design of the MMI couplers is supported by simulations using the Finite-Difference Time-Domain (FDTD) method. The design, experimental evaluation, and calibration of the device are discussed. Observed performance indicates a spectral support of 38.069 nm (i.e., frequency bandwidth 6.608 THz), with a resolution of 8.3 pm (1σ), corresponding to 1 part in 4,587. This wavelength meter approach has emerged from a need in absolute distance measurements using frequency scanning interferometry, where knowledge of the instantaneous wavelength of a tunable laser is required to relate signal frequency with target range. We also present an adaptive delay line on a chip, demonstrate its use for range measurements, and suggest how the wavelength meter could evolve for real-time applications.
Three-dimensional displacements measurement with two-point source interference
An innovative double corner cube interferometer is proposed to detect the 3-dimensional linear movements as well as straightness of a linear stage. Based on the two-point source interference, two-point light sources superpose and generate an interference pattern on a specific observation plane. The interference pattern will alter accordingly when the relative position of the two light sources in space changes. This measurement system projects an expanded collimated light beam to two corner cube retroreflectors (CCR), one is fixed on the reference frame and the other is attached on the stage for measurement. The two reflected lights from the two CCRs are collected by a focal lens and produce an interference pattern on the focal plane. Then the period and direction of the interference pattern can be extract with Fast Fourier Transform (FFT) for off-axis (X-Y plane) motion measurement, and the four-step phase-shifting method is used for on-axis (Z-axis) motion measurement. The proposed measurement system can measure the three-axis displacement of the two reflectors in real-time. Different from traditional interferometry for 3D measurement which requires multiple sets of interferometers, the proposed system has the advantage to use one simple configuration and a measuring algorithm to achieve three degree-of-freedom measurements. The experiment demonstrates that the resolution for off-axis measurement is about 50 um, and on-axis measurement can reach 2 nm.
Development of a coplanar grating interferometer for displacement and angle measurements
In this study, we propose a grating-based interferometer designed with a coplanar type and double-diffraction configurations to enhance the sensitivity while still simultaneously maintaining nanometric resolution of displacement and angle. According to our proposed method, two heterodyne beams with orthogonally polarized directions are propagated to a transmission grating with a symmetrical angle, and then diffracted. Since the two beams incident to different positions of the grating surface, two detection points are formed for establishing “coplanar type” optical configuration (detection configurations A and B). Each detection configuration has the same optical arrangement. Moreover, through the optical configuration design of “double-diffraction”, fours corresponding diffraction beams will pass through the transmission grating again along the original paths, which could induce double phase variations in each beam. This means the sensitivity and resolution of the proposed interferometer will be enhanced double. When the grating moves along the in-plane direction, changes in the two interference phases in detection configurations A and B are the same, and can be observed from the two detectors. Moreover, the rotation angle of θz can be obtained by comparing the measurement results of in-plane displacement measured by detection configurations A and B. To verify the feasibility and performance of the proposed interferometer, a series of experiments were conducted, with the measurement results obtained from the proposed interferometer compared with the built-in capacitive sensor and linear encoder of two different commercial positioning stages. As can be seen from the results, the displacement and angle resolution of the system was found to be 1.5 nm and 50 nrad while the values of repeatability was found to be 2 nm and 80 nrad together with a long-term stability of about 8 nm and 200 nrad for 10 minutes. Experimental results also demonstrate that the proposed interferometer has the ability to perform precision displacement and angle information simultaneously without changing its optical configuration.
On-machine and In-process Metrology
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Measurement of laser-based powder bed fusion surfaces using light scattering and one-class support vector machines
Measurement of laser-based powder bed fusion (PBF-LB) surfaces provides a promising solution for closed-loop quality control of the final parts. This paper presents a light scattering method to measure PBF-LB surfaces combined with oneclass support vector machines (SVMs) for anomaly (defect) detection during the manufacturing process. With the oneclass SVM method, datasets from solely reference (acceptable) surfaces are used to fit a classification model. Experimental results show that the method is fast and has higher accuracy than our previous work, which is promising for integration into next-generation PBF-LB machines for process quality monitoring.
Mirau-type coherence scanning interferometer with integrated vibration compensation
Coherence scanning interferometry (CSI) is a widespread optical measurement principle for inspection of surface topographies with height resolution down to the nanometer range. However, the corresponding depth scan procedure is sensitive to environmental disturbances. Thus, a highly controlled environment is required and close-to-machine or in-situ applications suffer from higher uncertainty or even failure of the measurement. Several vibration compensation approaches for CSI were introduced in the past decades. These can be mainly divided into active and passive techniques. However, no approach is known so far that compensates such environmental disturbances sufficiently. Thus, precise out of lab measurements are usually not possible. We present a Mirautype CSI system with vibration compensation, where vibrations in the longitudinal direction are detected by an interferometric distance sensor (IDS) integrated into the CSI setup. The interference images of the CSI are then rearranged according the distance values detected by the IDS. This enables a compensation of disturbances and further, precise measurements in close-to-machine environments as we show by measurements obtained in a mechanical shop floor. Furthermore, we demonstrate that a compensation of vibrations with frequencies up to 300 Hz and an amplitude of 2.45 µm is possible with our measurement setup.
Precision measurement of large optics by use of a scanning point multi-wavelength interferometer
M. Wendel
An established measurement system in asphere production, which is a promising approach in high precision freeform manufacturing as-well, is given by a scanning point interferometer based on a multi-wavelength approach. The scanning principle enables for a great flexibility, reduces setup time and costs, and has almost no limitations in spherical departure. Due to the absolute measurement capability, the utilized multi-wavelength approach is beneficial for segmented and interrupted surfaces, which are common apertures of modern application’s optical elements. A new metrology system has recently been developed, based on the before-mentioned multi-wavelength scanning approach, to allow for large surface measurements of up to 850 mm in diameter (part size up to 1 m) with highest accuracy of down to 150 nm peak-to-valley on the max. aperture. The combination of an optimized metrology frame setup with a unique anti-vibration system improves long-term stability, enabling a 3σ RMSi repeatability on a 400 mm surface over 8 hours without recalibration of less than 1 nm.
Light Properties and Optical Metrology
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Exploring the coherence function for optical metrology and beyond
I will report on several strategies to exploit the structure of the coherence function for optical metrology and wave field analysis in general. For the case of optical metrology, I will show that properties like the surface topology or refractive index distributions can be determined by solely investigating the coherence function of light that has previously interacted with the specimen under test. Since the coherence function can be measured by a simple shear interferometer, this suggests that a large variety of metrology tasks can be accomplished by a common-path architecture. A prime advantage of this approach is the combination of interferometric precision with inherent mechanical robustness. Furthermore, because a shear interferometer is essentially an imaging system that provides a twin-image, methods based on sensing the coherence function are very flexible and largely independent from the distance between sensor and object.
A model for speckle noise in phase data from digital holographic measurements
Erwan Meteyer, Pascal Picart, Charles Pézerat
This paper discusses on the analytical modeling of the speckle decorrelation noise in digital Fresnel holographic interferometry. The complex coherence factor between two images from two digitally reconstructed holograms at two different instants is analyzed. The modulus of the coherence factor is established and depends on the local surface deformation. The main influencing parameters of the holographic set-up are discussed. Experiments confirm the validity of the proposed modelling.
Polarimetric femtosecond-laser LiDAR for multispectral material probing
Yu Han, David Salido-Monzú, Jemil Avers Butt, et al.
Polarimetric LiDAR combines polarimetry and non-coherent optical ranging techniques to complement the acquisition of geometrical information with material characteristics. In recent decades, polarimetric LiDAR has been widely explored in material probing, target detection, and object identification. These approaches have so far mainly relied on implementations using a single or very few wavelengths. In this work, we propose, develop and evaluate a polarimetric femtosecond-laser LiDAR that enables extracting multispectral polarization signatures on 7 spectral channels of 40 nm spectral bandwidth and 33 spectral channels of 10 nm spectral bandwidth in the visible and near-infrared range. Multispectral polarization signatures of five material specimens (cardboard, foam, plaster, plastic, and wood board) are obtained and used as input features on a linear support vector machine classification algorithm. The results show that extending polarimetric probing to multiple spectral channels improves the classification capabilities with respect to single-wavelength approaches. The combination of different spectral signature dimensions (polarization, reflectance, and distance) that can be derived from LiDAR measurements is also analyzed, with results indicating their capability to support challenging classification tasks.
Measuring Instruments Using Advanced Photonic Devices
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Surface figure metrology based on geometric phase components
A geometric phase component is very attractive in optical metrology because its meta-surface characteristic enables traditional optical systems to be more compact and multi-functional. In this presentation, we introduce two types of geometric phase components, i.e. a geometric phase lens and a polarization grating in surface figure metrology. Their features of polarized beam splitting and phase retardation play a role of wavefront shearing device, and simple shearing interferometers can be designed. We focus on the instrumentation of a radial shearing interferometer using a geometric phase lens and a lateral shearing interferometer based on a polarization grating. With the aid of a polarization pixelated CMOS camera, each interferometer can provide the phase map corresponding to the sheared wavefront as a snapshot measurement. In the experiment, various wavefronts generated by a deformable mirror and shapes of several mirrors were measured and compared with other commercial devices.
Transfer of an AlGaAs/GaAs crystalline Bragg mirror from a GaAs substrate to a fused silica substrate by direct bonding
V. Hui, A. André, A. Arnoult, et al.
Direct bonding has been a major key technology in many fields nowadays. From microelectronic to optoelectronic technologies, it became a technique used for mass production technology in many different applications. Direct bonding of silicon or silicon dioxide is now a well known process. In this article, we explore this technology through the transfer of a crystalline multilayer made of III-V materials (AlGaAs/GaAs) from its native GaAs substrate upon a fused silica substrate (SiO2). The goal of this work is to explore the conception of crystalline Bragg mirrors with low mechanical loss and high optical quality for precision measurement applications. We present the main results obtained for each step of the transfer process. Various experiment such as AFM characterization have been performed on the wafers to probe surface quality. Chemical wet etching with different experimental conditions have been tested to remove the GaAs substrate. We discuss the main challenges of the process, especially the bonding of two rather different materials from the thermo-mechanic point of view. Focus is also made on the chemistry used in the wet etching part to have a selective etching between GaAs and AlGaAs.
New critical dimension optical metrology for submicron high-aspect-ratio structures using spectral reflectometry with supercontinuum laser illumination
The article presents a novel optical metrology method for accurate critical dimension (CD) measurement of sub-micrometer structures with high spatial resolution and light efficiency. The proposed method takes advantage of the spatially coherent nature of the supercontinuum laser to detect submicron-scale structures with high aspect ratios. By using the method, CD measurement of individual microstructures such as vias and redistribution layers (RDL) becomes achievable when a high magnification optical configuration is incorporated. Proved by a test run on measuring submicron structures with linewidths as small as 0.7 μm and an aspect ratio over 4, the measurement precision of the depth can be kept within a few nanometers.
Quantitative Imaging for 3D Metrology
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Super-resolved 3D optical profiling for surface metrology using structured illumination
Lena Zhukova, Roger Artigas, Guillem Carles
The lateral and axial resolution of optical techniques is bounded by diffraction, making the acquisition of surface topographies of samples with nanometric structures impossible. Super-resolution techniques, such as Structured Illumination Microscopy (SIM), have been developed to overcome this limit, enabling an increase in resolution up to a factor of two and allowing to resolve structures too small for conventional optical microscopes. SIM relies on the projection of structured illumination as periodic fringes with equally spaced phase shifts to recover high-frequency information. A Digital-Micromirror-Device (DMD) can be used to generate structured illumination, providing accurate control and stability of the fringe frequency and phase shifts. Additionally, optical sectioning of the scanned surface is provided, since the projected patterns are only well contrasted within the in focus regions of the sample. To reconstruct a 3D surface, an optical profiler exploits this optical sectioning capability to localise the maximum signal through the axial scan at each point. Whilst SIM based on laser interference has been used to super-resolve the axial dimension, this is not possible with a DMD approach. We explore how DMD-based SIM can be used to enhance the profiler’s ability to super-resolve structures within surface metrology. We modify a DMD-based optical profiler to enable lateral super-resolution of the image stack and explore how the quality of the 3D surface reconstruction can be improved. For this, we combine the super-resolved images with different optical sectioning techniques and assess the lateral resolution of the topographic detail via the characterization of the instrument transfer function (ITF).
Optical Micro- and Nanometrology
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New needs to measure soft matter at micro-/nanoscale: a new possibility thanks to digital holography
The digital holography method has been implemented to several industrial systems with the aim of improving new products by quantitative measurement. Nowadays, digital holography (DH) has been considered an important measurement tool, owing to the abilities of non-contact, label-free, quantitative, high-resolution and real-time. The inherent characteristics of DH makes it a powerful tool for visualization and testing of soft matter, as well as in-situ and real-time characterization of bottom-up fabrication processes. Herein, we report the most useful applications of soft matter, where the capabilities offered by DH, such as three-dimensional (3D) imaging, extended focus, 3D tracking, full-field analysis, high sensitivity, and wide measurement’s range, permit completely non-invasive characterizations on a full-scale. Taking the advantages of DH measurement, the possibility of numerically managing the complex wavefront scattered or transmitted by the sample under investigation allows the extraction of all information through a full-digital modality. Meanwhile, the numerical diffraction propagation process allows object imaging well-in-focus during dynamic process. This also creates the possibility to retrieve phase-contrast maps that enable quantitative measurements of the sample in full-field mode and 3D. Moreover, DH measurement has good ability to manage and remove aberrations in the optical system using simple and flexible methods, thus simplifying the optical apparatus and measurement operations. Owing to these unique features of DH, we have possibility to better study the world of soft-matter.
Measuring angle-resolved dynamic deformation of micromirrors with digital stroboscopic holography
Pooja Thakkar, Clément Fleury, Markus Bainschab, et al.
.We report the use of digital holographic microscopy to study harmonic evolution of dynamic deformation of two orthogonal modes of resonating piezoelectric MEMS mirror with a gimbal frame suspension. For the bending mode, the results show a linear relationship between dynamic deformation and the optical scan angle. For the torsional mode, a hysteretic behavior is observed, showing a significant difference depending on the scan direction. The difference was measured to be 45 nm, representing 18% of the total dynamic deformation of this mode. To investigate this effect, a point-by-point Fourier expansion method of the deformation cycle was employed and mechanical harmonics were extracted. From studying the first harmonic, we conclude that the origin of the hysteresis can be attributed to the phase difference in the oscillation of the two extrema points at the edge of the mirror, defining the peak-to-value deformation. In addition, higher-order harmonic terms (3rd and 5th) were identified and are affecting the hysteresis shape. Next, a modulation transfer function, corresponding to the measured angle-resolved dynamic deformation was estimated. Results show small contrast loss originating from the torsional mode, with an almost negligible effect of the hysteresis. The loss of contrast is dominated by the dynamic deformation of bending mode and was estimated to be 96% already at 0.18 normalized spatial frequency.
Reconstructing phase aberrations for high-precision dimensional microscopy
Philipp-Immanuel Schneider, Phillip Manley, Jan Krüger, et al.
In many industrial sectors, dimensional microscopy enables non-destructive and rapid inspection of manufacturing processes. However, wave-optical effects and imaging errors of the optical system limit the accuracy. With modelbased approaches it is possible to measure the physical position of edges and corners with submicron uncertainty. This requires an accurate model for phase aberrations of the optical system. We present a method to model and quantify those phase aberrations by an efficient inverse measurement.
Metrology of Complex Surfaces and Materials
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High resolution, non-contact surface metrology for freeform optics in digital immersive displays
Alex Sohn, Nelson Cardenas, Neil Naples, et al.
Freeform optical surface shapes have evolved from an intriguing optical design concept to a practical necessity for applications ranging from space and defense to consumer electronics. The demand for freeform solutions is no more strongly felt than in the development of digital immersive displays for augmented and mixed reality, where the combination of exceptionally high performance combines with ergonomic constraints of wearable interactive technologies. Some of these advanced designs call for diffraction-limited performance at large fields of view in off-axis orientations, often through conformal surfaces. Freeform optics are often the only way to correct the resultant aberrations, but their manufacture demands high-precision, high resolution surface figure metrology data such as can be obtained using coherence scanning interferometric microstitching (CSIM).
Full-field optical analysis of 3D printing composite for defect detection
V. Pagliarulo, P. Russo, J. Behal, et al.
Nowadays, due to the competitive economic scenario, industries ever more need to focus on manufacturing speed, increasing efficiency and steady quality. More industrial sectors see additive manufacturing (AM) as a possible way to enhance their processes and increase production efficiency. Thanks to its versatility the Fused Deposition Modelling (FDM), also known as Fused Filament Fabrication (FFF) technique, is one the most attractive processes in Industry 4.0 paradigm. This technique, thanks to its low-cost, is spreading widely in industrial sectors, from biomedical to aerospace to cite some. In this frame a valid solution is to use composite materials Among many, particular attention is paid to thermoplastic systems based on polyether-ether-ketone (PEEK) reinforced with short carbon fibre (CF). The PEEK is a high-performance semi-crystalline thermoplastic polymer that belongs to the polyaryl-ether-ketone family (PAEK). 3D printing, being a novel technology, it must be validated by understanding the behaviours of components and structures. At this purpose, we used different optical techniques for analysing advanced short fibre composites realized by 3D printing. Different CFR-PEEK samples with short carbon fibre at 10% by weight were realized by the FFF technique and characterized in terms of failure mode and mechanical behaviour. Optical tools have been used to retrieve full-field data and expand information about the mechanical behaviour of the investigated material, i.e. 2D Digital Image Correlation (2D-DIC) and Electronic Speckle Pattern Interferometry (ESPI), Optical Microscopy (OM) and Scanning Electron Microscopy (SEM).
Approaching optical metrology with multiple light sources and compressive sensing
André F. Müller, Claas Falldorf, Ralf B. Bergmann
Complex optical surfaces such as aspheres and freeforms are used in optical systems to reduce aberrations or to achieve high optical performance with a compact design and less optical surfaces. Due to limited acceptance angles of conventional interferometric techniques, there is still no satisfactory solution for their form measurement that is at the same time precise, flexible, and fast. Often these surfaces are surveyed by pointwise measurement, or the aperture problem is overcome by elaborately compensating wave front deviations either through compensator lenses or computer-generated holograms. Alternatively, several subapertures are used to capture the whole surface. These, however, have to be recorded in sequence since the superposition of multiple independent wave fields cannot be assigned a time-independent wave front. Instead, we present a compressive sensing approach for Multiple Aperture Shear-Interferometry (MArS) which captures multiple overlapping subapertures simultaneously and allows a flexible measurement of aspheres with multi-spot illumination. MArS uses the mutual intensity as the primary measurand which is still well defined for superposed mutually incoherent wave fields. The mutual intensity is sparse in phase space for there are only a finite number of distinct wave fields at every surface point. Utilizing this sparsity, the presented compressive sensing approach avoids superflously large space-bandwidth products and significantly reduces the number of necessary measurements.
Accuracy-enhanced diffraction image profilometry using foreign aberration for resolving image ambiguity
A new optical surface measuring method based on correlation-based diffractive image profilometry (DIP) is developed for accuracy enhancement by introducing external optical aberration to the microscope. According to the diffraction theory, the diffractive images formed in the microscope mainly depend on the microscopic optical system and the surface features of the tested object. The most critical issue affecting the measurement accuracy of the DIP is that the uniqueness of the diffractive images corresponding to various surface geometric parameters such as different heights and orientations cannot be always guaranteed. This situation can bring undesired uncertainties in surface measurement since undesired ambiguity in image correlation or model estimation may be introduced. To resolve this, a designed foreign aberration is introduced into the microscopic optical system to develop the feature variance of diffractive images for significantly increasing the degree of the image variance, therefore the risk of ambiguity is effectively avoided. Proved by some experimental tests, with this method, the accuracy in measuring height, tilting angle, and tilting direction can be achieved to a level of sub-micrometer and less than 0.01 degrees, respectively.
Large-Scale Measurements
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Simultaneous path following and obstacle avoidance of field-tracked vehicles via model predictive control with deep deterministic policy gradient
Unmanned ground vehicles (UGVs) will be widely adopted in agricultural applications. To accomplish autonomous cruising in farm, path following is an essential skill. However, in the process of field cruising, some obstacles such as wild animals or motorcycles are present. In this study, tracked vehicles are utilized with deep deterministic policy gradient (DDPG) compensating for model uncertainties and achieving collision avoidance simultaneously. Among all, the most important issue is to keep the UGV following the predetermined path in specific agricultural field environment and coping with the uncertainty of the surroundings. Path following and obstacle avoidance of field tracked vehicles are conducted by using model predictive control (MPC) with a controller (agent) trained by DDPG. Therefore, we proposed control algorithm fusion with MPC and model-free DDPG.
Poster Session
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Optical inspection system for simultaneously measuring thicknesses and refractive indexes of multilayered transparent substrates
In this study, an inspection system that can simultaneously measure the thicknesses and refractive index of transparent substrates has been proposed. The proposed inspection system consists of two sub-systems: the transmitted system and the confocal system. When measuring the samples, the transmitted system will compare the spot position before and after placing the samples into the system. To reduce the image processing difficulty and deviation that caused by the transmitted system, we use a slit to transform the shape of spot into rectangle. On the other hand, the confocal system will scan the samples from the top to bottom by moving an objective lens with a precision slider that can change the focus position. We can therefore plot the chart of confocal intensity signal curve to estimate the distance between the peaks that caused by laser focus on the surface of samples. By substituting the measured data of the two sub-systems into the proposed mathematical model, the thicknesses and refractive index of the samples can measured simultaneously. By measuring the thicknesses and refractive index of transparent substrates beyond and below the biological sample, it provides the basis for a possible biological auto-focusing microscope in the future. To prove the system feasibility, we simulated the whole system by optical simulation software Zemax to measure different samples with different thicknesses and materials. The simulation results show that the system deviations of thicknesses and refractive indexes are about 0.005%~3.128% and 1.643%~5.116%, respectively.
Deep convolutional neural network-based surface defect detection for wafer grinding jig
Chao-Ching Ho, Xuan Yan, Sankarsan Mohanty, et al.
Wafer defects are caused by defects on the surface of the Wafer grinding jig. Therefore, this research is aimed at optical inspection of the self-made dataset of the Wafer grinding jig by using the UNet++ network model. Because of the different background distribution of the inner and outer edges in Wafer grinding jig, it is difficult to detect defects solely by Deep learning. Hence, this study uses Image processing to separate the inner and outer edge and then uses UNet++ with transfer learning to detect the location of the defects. In addition, false detection is reduced by tracking the defects as the jig moves regularly. The proposed deep network layer reduction method can decrease the detection time to 48.27% compared to the original network model. The recall rate and IoU of the outer edge are increased by 13.8% and 16.33% through transfer learning. The recall rate and IoU of the inner edge are increased by 2.01% and 1.16% respectively through transfer learning. And the final defect tracking recall rate reached over 75.81%, while the F1 score reached over 77.08% in every frame from 112 to 192 frames.
Development of a linear encoder based on common-optical-path design
In this study, a novel linear encoder (LE) based on common-optical-path design is proposed for displacement measurement. The system configuration of the proposed LE is simple and can be easy to setup, it consists of a laser diode (LD) light source, V-shape prism, beam-splitter, mirror and grating. According to our proposed method, the beams of reference and measurement can be obtained when a light beam outcoming from the LD passes through a specified V-shape prism. By using the design concept of common-optical-path, the reference and measurement beams will move together, which means the two beams will suffer from the same environmental disturbances. Surrounding disturbances can then be compensated in the interference signal making the system much less sensitive to environmental disturbances. Moreover, the proposed LE also takes advantage of a “double-diffraction” optical configuration, which directs diffracted beams to propagate a grating twice, thereby enhancing the phase change induced by grating displacement, effectively improving the sensitivity of the LE. By means of measuring the phase variations of the interfering signals resulting from the moving grating, the displacement information can be acquired. To demonstrate the performances of the proposed LE, two main experiments have been carried out. First, in order to obtain a forward and backward displacement information, a commercial micro-positioning stage and stepper stage were driven with different waveform motions and strokes. This allowed testing of short and long displacements with different motion behaviors. Second, the tests of a step-like function and a long-term stationary were used to infer the system repeatability and stability, respectively. Based on the experimental results, the proposed LE has the ability to measure large displacements with a resolution of 1.7 nm. The repeatability of the system was found to be less than 1.9 nm together with a long-term stability of about 11.1 nm.
Multiwavelength digital holography with meter scale synthetic wavelengths at micrometer precision
Digital holography enables high-precision quality control in machining production and has already been introduced to several multi-axis systems1, 2. To meet the demanding measurement tasks in the quality control of complex components, accuracies in the sub-micrometer range with measurement ranges larger than several centimeters are required. Previous measurements have shown the potential of multiwavelength digital holography to allow unambiguous ranges of few millimeters3 . We present multiwavelength digital holographic measurements using synthetic wavelengths with two meters down to a few micrometers, potentially enabling measurements with meter-scale unambiguity at sub-micrometer accuracy. Measurements on a 10 cm step-height sample have been conducted using the compact digital-holographic sensor HoloTop NX for various multi-axis systems, supplied by an Ondax LMFC single frequency diode laser at 632.852 nm and the tunable laser Hübner C-Wave used in the wavelength range of 480.786 nm – 632.852 nm. The latter offers a frequency stability of 150 MHz on a time scale of several hours. The maximum laser drift during data acquisition was observed to be 0.02 pm. Thus, at the 2 m synthetic wavelength, this results in a maximum synthetic wavelength error of 200 mm. Random noise of 20 mm at the largest used synthetic wavelength of 2 m requires multiple synthetic wavelengths to get down to micrometer precision: Eight nested synthetic wavelengths from 2 µm to 2 m and numerical refocusing of the hologram were used to evaluate a milled sample with multiple step heights, machined on a Hermle C32U machine tool. Ten repetitive measurements confirm a machining uncertainty of 9 µm for this sample at its maximum step height of 10 cm.
Development of a sinusoidal pseudochirp material measure for the characterization of optical surface topography measuring instruments
Optical surface topography measuring instruments are used more and more widely for surface quality control in industry by enabling fast, areal and non-destructive surface topography measurements. However, due to the complexity of the interaction between the surface properties to be measured and the measuring system, their capability to accurately reproduce topographical features of a surface under test is quite often questionable. To understand and investigate the topographic measurement accuracy of different optical surface topography measuring instruments, a physical measurement standard has been developed at PTB which is intended to be used to determine the metrological characteristics of surface topography measuring instruments such as topographic spatial resolution and topography fidelity. The physical standard, fabricated by a diamond turning process, containing nine sinusoidal structures with different amplitudes from 50 nm to 10 μm and varying spatial wavelengths from 2.6 μm to 82.8 μm, is suitable for the characterization of optical instruments with different magnification and numerical apertures. The design of the chirp structures, including the wavelength series, the smallest wavelength for different amplitudes, the slope distribution and the layout are detailed in this paper. The tool path for accurately positioning the cutting tool in fabrication is also described. First measurement results of the instruments response in terms of the features’ aspect ratio, slopes and curvatures, the homogeneity of the field of view of a confocal microscope are presented.
Dynamic short- and large-coherence interferometry to characterize the induced vibrations and topology change of the cryogenic mirror of the Einstein Telescope prototype
The E-TEST project builds a prototype for the Einstein Telescope (ET). ET is a proposed gravitational-wave observatory. E-TEST includes a silicon mirror of 30 cm up to 40 cm diameter, suspended and cooled down at cryogenic temperatures from 20 K to 30 K. During the cooling down, the mirror will be affected by surface topology changes, wavefront deformation, and induced vibrations. We present a metrology device based on short-coherence interferometry to characterize the mirror surface with a sub-nanometer resolution. We design an innovative phase mask to achieve dynamic or single-frame white light interferometry. Moreover, we discuss different interferogram analysis methods. We also discuss the implementation of a long-coherence source to facilitate the measurements with the low-coherence source.
Transient force identification by use of high-speed digital holography
Erwan Meteyer, Pascal Picart, Charles Pézerat
This paper discusses on a full and large field, high spatial resolution, contact-less, and non-intrusive approach for force retrieval at different excitation regimes of mechanical structures. The inverse problem based on the Force Analysis Technique is combined with the full-field measurement to identify the force applied to a structure, in terms of localization and quantification. In the transient regime, the proposed approach is able to retrieve the time dependence of the force and yields good agreement with that from a piezoelectric force sensor.
Adaptive wavefront sensor for accurate characterization and evaluation of circular optical elements
Shack-Hartmann wavefront sensor (SHWFS) is considered as an efficient and complementary tool in the field of optics metrology. It is widely used for evaluating and characterizing rigid glass and soft eye contact lenses. However, the presence of any type of aberrations will cause measurement errors decreasing the precision of the sensor. Therefore, in the current study an experimental configuration based on an active adaptive SHWFS was presented for optical testing of circular optical elements such as thin and soft eye contact lenses. As an adaptive element, deformable mirror (DM), was integrated to the SHWFS setup to dynamically compensate for the wavefront aberrations of the illumination laser beam to provide an ideal plane. The concept was firstly verified by measuring standard thin lenses then applied to measure soft eye contact lenses. For the quantitative evaluation, Zernike polynomials was used to accurately define the dominant modes of wavefront aberrations and thus to calculate the wavefront to be written on the DM. Based on the standard deviation (1σ) between the given and the measured focal lengths of the tested thin glass lenses, the measurements show an improvement of the measurement error from 15.18% difference of the uncompensated wavefront and 3.90% of the referencebased method to only 2.11% after aberration compensation of the illumination beam. While for the contact lenses, the measurement error was 22.93% for uncompensated aberrations, 51% for the reference-based method, and 1.75% after aberrations compensation. The results reveal that the aberrations of the illumination laser beam and the wet cell induced aberrations affect the accuracy of the measurements which can be drastically improved by compensating the existing aberrations utilizing the active adaptive SHWFS setup. In conclusion, adaptive-SHWFS can be considered as an in-production, accurate and complementary tool for testing of optical components.