This PDF file contains the front matter associated with SPIE Proceedings Volume 11817, including the Title Page, Copyright information and table of contents

The ability to measure small thermal expansion of precision components can be critical to the performance of precision instruments such as optical telescopes. The methods available to measure nanometer and sub-nanometer dimensional changes on optical elements is very limited. This white paper explores the ability of the capacitance gauge to measure expansion of optical elements in the sub-nanometer range.

Precision-machined parts in aviation, automotive, and manufacturing industries have tightly controlled tolerances for the dozens of small geometries spread throughout a single part. These parts can also have stringent specifications for any defects along the surface. The sheer number of measurements needed on each part paired with the volume of parts demands the ability to take not only one measurement quickly, but dozens in a rapid process. Coupling a polarized structured light technique with robotic automation allows for accurate measurements at volume. This paper will discuss automated optical measurements of stationary parts, parts moving on a production line, and rotationally symmetric parts on a rotary stage. The paper will also look at ongoing projects combining the automated polarized structured light method with bright field techniques to accomplish automatic defect identification and inspection.

We describe a technique for measuring the instrument transfer function (ITF) of an interferometric microscope, allowing both characterization and data processing to increase the fidelity and effective resolution of the tool. The technique, based on test samples structured as two-dimensional (2D) binary pseudo-random arrays (BPRAs), employs the unique properties of the BPRA patterns in the spatial frequency domain. The inherent 2D power spectral density of the pattern has a deterministic white-noise-like character that allows direct determination of the ITF with uniform sensitivity over the entire spatial frequency range and field-of-view of an instrument. As such, the BPRA samples satisfy the characteristics of a test standard: functionality, ease of specification and fabrication, reproducibility, and low sensitivity to manufacturing error. We discuss the results of the development and application of highly randomized (HR) BPRA test samples with elementary feature sizes in the range from 80 nm and up to 2.5 μm, optimized for the ITF characterization of interferometric microscopes broadly used for 2D optical surface profiling. The data acquisition and analysis procedures for different applications of the ITF calibration technique developed are also discussed.

Nowadays, the pandemic situation caused by the COVID-19 virus inspired many companies, research institutes and lighting designers to adopt UV radiation as a new tool in their projects and research. The promising germicidal effect of the UV-C radiation, also on the Coronavirus, raises the question of the reliable measurement of the UV radiation. However, this complex task needs an expertise and appropriate equipment. The system suitable for UV measurements consists of a high-precision spectroradiometer with stray light correction and irradiance probes or PTFE-coated integrating spheres for total radiant flux measurements. However, the reliable and traceable calibration of the system is the challenging factor. So far, no national metrological institute has been able to offer a reference standard for the total radiant flux in the UV-B and UV-C spectral region. Therefore, we have realized traceable UV LED calibration standards, which complete measurement system for UV radiation presented here. The UV LED calibration standards have been developed for the typical peak wavelengths of 280 nm (UV-C), 305 nm (UV-B) and 365 nm (UV-A). The traceability of the radiant flux is ensured by the precise calibration of the spectroradiometer with the irradiance probe and a subsequent integrative measurement using a goniospectroradiometer. Such UV LED calibration standards can be used for monitoring and for absolute calibration of UV measurement equipment consisting of the stray light corrected spectroradiometer and the integrating sphere. The largest contribution to the measurement uncertainties of the systems containing integrating spheres is the fluorescence of the coating material. Special manufacturing procedure with optically pure Polytetrafluorethylen (PTFE) enabled us to produce new integrating spheres with permanently low fluorescence.

Over the past few decades, tremendous efforts have been devoted to developing various techniques for fringe analysis, and they can be broadly classified into two categories: (1) phase-shifting (PS) methods which require multiple fringe patterns to extract phase information and (2) spatial phase demodulation methods which allow phase retrieval from a single fringe pattern, such as the Fourier transform (FT), windowed Fourier transform (WFT), and wavelet transform (WT) methods. Compared with spatial phase demodulation methods, the multiple-shot phase-shifting techniques are generally more robust and can achieve pixel-wise phase measurement with higher resolution and accuracy. Furthermore, the phase-shifting measurements are quite insensitive to non-uniform background intensity and fringe modulation. Nevertheless, due to their multi-shot nature, these methods are difficult to apply to dynamic measurements and are more susceptible to external disturbance and vibrations. Thus, for many applications, phase extraction from a single fringe pattern is desired, which falls under the purview of spatial fringe analysis. Recently, we demonstrated that the use of convolutional neural networks can substantially enhance the accuracy of phase demodulation from a single fringe pattern. Moreover, we find the powerful learning ability of deep neural network (DNN) enables the phase unwrapping, super-fast 3D shape measurement of transient events, multi-view fringe projection and so on. From comparative results, the DNN shows better performance over traditional state-of-the-art methods in terms of the phase accuracy and efficiency. We believe the deep learning technique is a powerful technique to handle fringe images and will find wide applications in 3D measurements with structured-light illumination.

An innovative self-interferometric pupil ellipsometry(SIPE) has been demonstrated to overcome the spectral sensitivity and throughput limitations for optical critical dimensions (OCD) metrology in the advanced semiconductor devices. The two orthogonally polarized lights from the target structure on wafer were combined through suitably devised polarization state analyzer to generate an interferometric fringe pattern on the pupil surface of the SIPE optical system. The measured fringe pattern was processed with our novel holographic reconstruction algorithm to extract the ellipsometric information (Ψ and Δ) with the entire incident angles 0 to 70º and azimuthal angle 0 to 360º separately. In contrast to conventional ellipsometry tools, no mechanical movements were required to obtain the multi-angular information. To verify the usefulness of SIPE system and the algorithms, both experimental and theoretical validation have been performed for patterned wafers as well as for SiO2 mono-layered wafers. We first measured the non-patterned wafers of various different thicknesses, and found that the obtained values from SIPE, commercial ellipsometry tool, and theoretical simulation present a good agreement for wide spectral and angular ranges. Furthermore, we show that the large amount of angle resolved information from SIPE technique can greatly enhance the ability to overcome the OCD ellipsometry’s recent challenges such as spectral sensitivity issues, parameter correlation and structural asymmetry problems, etc. In short, the proposed system and algorithms, which are completely new approaches, show a capability to overcome current metrology challenges and we strongly believe that the SIPE is a promising metrology solution that can be eventually replacing the traditional OCD metrology tools..

We present a new displacement probe for a gantry-type profilometer to measure the ground surface form. The system is based on heterodyne interferometry with an acousto-optic modulator (AOM) and I/Q demodulation scheme. When the light from the single-frequency laser diode passes through the AOM following the Bragg condition angle, it is divided into the non-diffracted zero-order and a diffracted first-order beam with a frequency shift as the amount of the AOM driving frequency. One beam is used for the test and the other for the reference beam. Orthogonally reflected beams pass through the AOM one more time, and these beams make a heterodyne signal with a double modulation frequency. In this case, the only backscattered beam from the ground surface satisfies the Bragg condition, producing the modulated interference. Therefore, the AOM tends to reduce the stray light noises from the other incidence angles, acting as a transmission filter for backscattered light. The interference light arrives at the high-speed photodiode and it is then demodulated using the I/Q demodulation to extract the phase value. Although the backscattered ray from the rough glass surface has a very low intensity, the I/Q interferometer can detect the signal because the phase can be acquired regardless of the intensity change. Preliminary experiments confirmed that the system can measure from backscattered light of ground glass surface form.

High sensitivity displacement interferometer has wide applications in gravitational wave detection area, performing as crucial part in test mass dynamics measurement and seismic motion monitoring for low-noise observatory operation. With advances in heterodyne laser interferometry, sensitivities at levels of sub-nm/√Hz over sub-Hz frequencies can be achieved. However, the breakthrough towards picometer level still needs various techniques in noise characterization and suppression. In this article, a compact heterodyne laser interferometer design as well as benchtop prototype system is presented. Common noise sources and their effects are investigated, including laser frequency noise, non-linear OPD noise, thermo-elastic noise, as well as readout noise from phasemeters and photoreceivers to determine the sensitivity limits in our system. Furthermore, each individual noise source is characterized with dedicated instruments and the coupling coefficients are determined respectively. By subtracting the individual noise contributions, the interferometer sensitivity reaches a sensitivity at the picometer level above 100 mHz frequency. We will present our progress and current results.

This work presents a versatile digital holography software (HOLO4D) that provides an all-in-one solution for holographic reconstruction, simulation, de-twining and lens distortion removal in a user-friendly graphical interface. The software accepts holograms for both, in-line and off-axes schemes with the option to locate and specify the cross-term in the latter case. Both amplitude and phase shifts are recovered from holograms generated by plane waves using multiple functionalities for propagation. These include the angular spectrum method, sinc-interpolation, upsampling-lowpass filter and zero-padding processes that can be applied in a variety of combinations to optimize holographic reconstructions. The virtual image that obscures the real image in these reconstructions is suppressed using a subtraction division plus mean (SDPM) normalization procedure or by iteratively updating the complex-valued wavefront in the detector plane. The use of lenses to manipulate the effective position of the hologram introduces optical distortions. These are removed by calibrating a dot-pattern hologram reconstruction in a step-by-step auto-detection procedure and mapping the distortion characteristics to the corresponding object hologram reconstruction. Hologram simulations enable determining the maximum field-of-view, fringes captured, and the lateral resolution possible based on the imaging sensor, illumination and setup specifications. The numerical package is used to process holograms of 80 μm thick hair strands, scan the depth-of-field and locate the precise three-dimensional (3D) location of reconstructed objects. The results demonstrate the versatility of the software to reconstruct, de-twin, undistort and simulate holograms in a user friendly manner and its applicability as a robust tool for 3D visualization, education, and holographic research.

We present analysis and experimental evaluation of a novel system based on fiber optic frequency space low coherence interferometer (FSLCI), employing selectable filter frequency space Moire technology d in [1]. Our system is using novel polarization-maintaining-fiber-based filters allowing user to select the working range from 0 cm. We discuss in detail tradeoffs of this technology due to losses on optical switches. The system is more robust than the earlier disclosed design based on free space Fabry Perot filter [1] . The use of the new design eliminated etalon described in Ref 1 and eliminated artifacts resulted from non-sinusoidal transmission dependence on light frequency of the etalon. Reference: 1. Walecki, Wojtek J., et al. In Applied Optical Metrology II, vol. 10373, p. 103730N. International Society for Optics and Photonics, 2017.

A simple and inexpensive method to measure vibrations in mechanical structures is presented by means of Fabry-Perot interferometry and Doppler effect. The sensor consists of a transparent hydrogel sphere attached to an single-mode optical fiber from a 50:50 fiber coupler, laser light with a wavelength of 658 nm, and a photodetector which is connected to an oscilloscope. The vibrometric sensor works in the time domain by detecting the number of interference fringes. The tip of the prototype is composed of mechanical couplings that join the fiber with the hydrogel sphere. The sensor allows knowing the vibration frequency at which a mechanical part is oscillating. The analysis presented in this work shows several advantages over conventional methods, such as low cost, real-time measurement, and simplification in experimental setup. The sensor system is capable of capturing vibrations of up to 5000 Hz.

We demonstrate the use of physics-informed machine learning algorithms for the adaptive, real-time characterization of aero-optical systems. From deep learning algorithms to nonlinear control methods, the optical sciences are an ideal platform for integrating data-driven control and machine learning for robust characterization and system identification. For the specific case of aero-optics, the ability to extract dominant coherent structures, transients and turbulent behaviors is critical for a diverse number of applications, including the complex and dynamic aero-optic effects on airborne-based laser platforms. Specifically, aero-optical beam control relies on the development of low-latency predictors that can quickly predict aberrated wavefronts to feed into an adaptive optic control loop. We propose develop a number of data-driven methods, including the dynamic mode decomposition (DMD), for real-time forecasting and control.

The 50th anniversary of Dennis Gabor’s Nobel prize (1971) and the upcoming 60th anniversary of the work of both Yuri N. Denisyuk (1962) and Emmett N. Leith, Juris Upatnieks (1962) are excellent reasons to look back at the inventions and the great pioneers of holography. In the talk at first I present my personal look at the timeline of optical and digital holography with focus on 2D and 3D holographic microscopy. Next I will address several metrological problems connected with 2D and 3D quantitative phase imaging based on data gathered by means of digital holography microscopy, holographic tomography and combined holographic tomography and optical coherence tomography .

The design of next generation gravitational wave observatories considers operation at cryogenic temperatures to enhance their sensitivity by reducing thermal noise fluctuations. Inertial sensors are used on the observatory platforms to measure local seismic noise and counteract its effects by active control or subtraction in post-processing. Measuring the displacement of a test mass in a resonator system allows for creation of a compact accelerometer system. Currently, there are no commercial inertial sensors available that are capable of operating at cryogenic temperatures and providing the required sensitivities for gravitational wave observatories. Materials such as fused silica exhibit very low losses at room temperature. However, this changes significantly at lower temperatures. Unlike fused silica, the Q factor of crystalline silicon structures is expected to remain high at low temperatures, making it a likely candidate for use in these types of inertial sensors. We are working to fabricate compact mechanical resonators from Si wafers to test their mechanical response. Micro-fabrication consists of optimizing the photolithography and Bosch etching processes for through-wafer Si etching on a 280 μm, 500 μm, and 1 mm wafer. Successful etching on 280 μm wafers has been achieved. We report on the design, model, and fabrication progress of these resonators.

While material parameters are fairly well known for conventional solids, more variability occurs with materials produced by additive manufacturing. A method is described to determine these parameters from the unique vibrational resonanceresponse spectra, which can be measured experimentally and predicted by simulations. The experimental spectra, obtained by excitation of the component with a piezo-electric transducer and measurement with a laser Doppler vibrometer, are defined by the actual values of material parameters although only some of the resonance modes are captured in any particular measurement. On the other hand, the simulated spectra, predicted by finite element analysis based on the CAD file of the part, contain all possible vibration peaks, but their specific frequencies, unlike the experimental values, depend on the assumed values of the material parameters. Thus, the two sets contain different number of peaks and are measured on two differently-scaled frequency scales. The main re-scaling factor is the ratio of elasticity to density, and a nonlinear least squares regression that maximizes the correlation between the two sets of peaks yields the optimal pairwise assignment. A linear regression over pairwise-assigned peaks yields the Young’s modulus that gives the best match between the two spectra. Unlike the elasticity, the Poisson’s ratio affects different modes differently, and inaccuracies in the Poisson ratio lead to increased deviations from linearity in the experiment vs. simulations regression, and minimization of the correlation coefficient yields the best-fitting value of the Poisson ratio as well. The accuracy, sources of errors, and potential limitations are discussed.

We describe a new photonics procedure developed for non-destructive testing of additive manufactured (AM) parts and present a case study for inspection of AM brackets to detect and reject parts containing printing defects. This is an update of a previously presented paper of the work in progress to develop an Authentication Sensing System Using Resonance Evaluation Spectroscopy (ASSURES). The principle of operation is based on the concept that a part’s vibration spectrum (set of resonant frequencies) is uniquely determined by its dimensions, material parameters, and interior flaws. The vibration spectrum of a part can be measured remotely with a laser vibrometer, and the presence of defects or a change in material parameters can be inferred from a change in the part’s vibration spectrum (shifts in the resonant frequencies). The spectrum of a part can be measured and compared in a few seconds to a known “good” part, a “good” reference spectrum, or to other parts in the batch.

A microplasma is defined as a plasma (i.e., a mixture of ions and electrons) with one critical dimension (e.g., depth, or length, or radius) in the mm or the sub-mm (m) range. Over the years, we designed and fabricated a variety of microplasmas using technologies ranging from those borrowed from semiconductor fabrication on crystalline-Silicon substrates to 3D printing on polymeric substrates. In this presentation, the determination of excitation temperatures (Texc) is described. Determination of Texc is important because it is affecting detection limits and is described here.

We propose an instrument model for coherence scanning interferometry using familiar Fourier optics methods, the spectrum of plane waves, and the assumption that the light source spectral bandwidth is the dominant factor in determining fringe contrast as a function of optical path length. The model is straightforward to implement, is computationally efficient, and reveals many of the common error sources related to the optical filtering properties of the imaging system. We quantify the limits of applicability of the model related to the geometrical approximations for conventional Fourier optics, particularly for high numerical apertures, and when using the fringe contrast for determining surface heights. These limitations can be overcome by using a three-dimensional imaging model.

A powerful technique for interference fringe analysis uses tunable light sources. Originally developed to solve the problem of phase shifting in large aperture systems, the technique has evolved to the simultaneous measurement of multiple surfaces and optical thickness of optical assemblies and components. Here we review the principles and current state of the art for swept-wavelength interferometry for optical testing, including recent advances in digital holographic refocusing and environmental robustness using model-based data analysis. Applications for swept-wavelength interferometry span the full optical metrology space; we provide examples of the measurement of glass substrates for rigid data storage drives to planar waveguides for augmented and mixed reality.

Presented for the first time are studies of the effect of optical medium density on the possibility of suppression of light field shift of a coherent population trapping (CPT) resonance excited with multi-component radiation generated by modulation of the injection current of a pumping single-frequency diode laser. It is shown experimentally and theoretically that the possibility of suppressing CPT resonance light shift depends on the optical density of the medium.

A coherent beam of incident light that impinges on a turbid medium or a rough surface, generates a characteristic interference pattern called "speckle". In this research work, was modeled the speckle pattern due to volumetric scattering within a turbid medium by using Monte Carlo simulations in OpticStudio^® when the optical parameters (OP) of the medium were kept constant. A variable number of analysis rays from the light source was considered in order to evaluate the adequacy of the statistical distribution of intensities and its agreement to fully developed speckle (FDS) as predicted by the theory. In the non-sequential mode of OpticStudio^®, it was implemented an optical setting of diffuse reflection geometry composed of: a coherent light source (Source Ellipse), a scattering volume (Rectangular Volume), and a detector (Rectangle Detector) with dimensions typical of a realistic sensor. The source was configured with a coherence length of 50x103 mm, a linear polarization along the x-axis (Jx =1), and a diameter beam of 1 mm. The OP of the scattering volume were defined using the Henyey-Greenstein scattering model with the following parameters: mean path MP = 0.1 mm, transmission T = 0.9, and anisotropy factor g= 0.95. Detector settings were established as: dimension= 4.8x3.8 mm², resolution= 1328x1048 pixels, and Polarization Flag= 1. The study was performed for 2, 5, 10, 15, 20, 25, 30, 50, 75, 100 and 500 million analysis rays launched from the light source. The goodness of fit between simulated normalized histograms of intensity and the negative exponential probability density function of speckle patterns predicted by the theory was determined by using the software Minitab®. It was demonstrated that a good agreement between these previous mentioned quantities is achieved for the higher number of analysis rays. This study provides a guideline about a threshold number of analysis rays that should be used in OpticStudio^® when simulations of coherent scattering in turbid media are performed. This study could also impact in different fields of speckle metrology by predicting results using OpticStudio^® during the modeling specific optical configurations.

Direct wavefront sensing is commonly performed by using a popular Shack-Hartmann wavefront sensor. On the other hand, indirect wavefront sensing is performed based on an image quality metric by acquiring a sequence of images in which pre-determined amount of aberrations modes are incorporated. Both the sensing approaches have their advantages and disadvantages depending on specific applications. In the present work, we propose simultaneous realization of both the sensing approaches with broader applications by using a multiplexed programmable binary diffraction grating pattern. We present proof-of-concept simulation results that demonstrate the working of the proposed multiplexed grating array based wavefront sensor (MGAWS) and its flexibility in easy switching between both the sensing approaches to estimate the wavefront accurately.

An improved programmable grating array based wavefront sensor (GAWS) is proposed which is capable of estimating the incident wavefront more accurately, by generating an array of uniform intensity +1 order spots with negligible contribution from unwanted higher order spots. The duty cycle of each grating element of the proposed sensor is effectively varied in order to independently control the intensity of each +1 order spot. Furthermore, random binarisation technique is implemented on the diffraction grating array to reduce the contribution from undesirable higher order spots by disintegrating them into noise. Proof-of-principle simulation results are presented to demonstrate the working of the proposed GAWS in comparison to the conventional GAWS, for non-uniform intensity of the +1 order spots.

The roundness profile extraction strategy of the cylindricity measuring instrument (CMI) only uses 5-10 cross-section to evaluate the cylindricity, which can not exactly express the catachrestic of the cylindrical surface error. On the contrary, the measurement result of the stitching interferometry has the advantage of high resolution and high precision, which is able to meet the requirement of bird-cage extraction. So, in this paper, we build a two-dimensional (2D) Gaussian filter weighting function to extract the cylindrical profile, according to its special shape characteristic. Besides, an experiment has been carried out by taking a ceramic plug gauge as the workpiece. The extracted results of 1-50 undulation per revolution (UPR) in circumference direction and 1-10 UPR in generatrix direction demonstrate a good performance of our method.