This PDF file contains the front matter associated with SPIE Proceedings Volume 10749, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Speckle interferometry is generally known as a method that measures the deformation on an object with rough surfaces. In this paper, a three-dimensional (3-D) shape-measurement method is proposed for superfine structures beyond the diffraction limit using the basic feature of speckle interferometry. Because the differential coefficient distribution of the shape of such an object can be detected by giving a known lateral shift to the measured object in speckle interferometry, the shape can be reconstructed by integrating the differential coefficient distribution. Based on experimental results obtained using diffraction gratings as measured objects, it is confirmed that the proposed method can measure 3-D shapes that are beyond the diffraction limit of the lens.

A trustworthy speckle contrast calculation is fundamental in many applications, such as “laser speckle contrast Imaging” (LSCI), which is a non-invasive technique commonly employed to estimate relative blood speed. In LSCI, the local contrast of a speckle image is calculated using spatial, temporal analysis or a combination of both. In this work, we focus on the spatial analysis. To calculate the local spatial contrast, typically, a sliding window of 5x5 pixels is used to calculate the standard deviation (σ_s) and the mean intensity (<I>_s) of those 5x5 pixels and the calculated contrast K_S=σ_s/(<I>_s) is assigned to the central pixel of the sliding window. In this work, we show that this experimental procedure to calculate the local speckle contrast does not match the corresponding spatial theoretical model and we propose an alternative method that considers correlations of the central pixel with the other ones. We have found a better agreement of the contrast measurement with our numerical calculation.

Speckle contrast analysis had been used for multiples purposes, for instance, laser speckle contrast imaging (LSCI) has been used to estimate the relative blood flow speed in a non-invasive way. The speckle contrast can be calculated using a spatial or temporal algorithm or a combination of both. Our work focuses into the contrast temporal algorithm. A contrast image calculated with the temporal contrast algorithm requires a sequence of L equal-sized frames. The contrast images are performed pixel by pixel, however, the experimental contrast calculation does not match with the current temporal theoretical model especially when the exposure time T is smaller than the correlation time τ_c. In this work, we propose to correlate neighboring pixels along the temporal axis to improve the contrast calculation. The contrast measurements using our proposal provide a better agreement than the current models.

There are a lot of surface shape measurement methods. Appropriate method is selected from them in consideration of measurement accuracy, range, speed, etc. We have recently developed a system using a method among them called “linear scanning confocal surface shape measurement system” [1] for measuring micro objects. In this paper, we describe the method to measure the horizontal dimensions of micro objects concurrently with the measurement of the vertical dimension using this system. Generally, it is difficult to get the focused image for the horizontal dimensional measurement of micro objects because of insufficient depth of field of the optics. However, as the system has plural images at multiple Z positions for vertical dimension measurement, we thought that the focus problem could be solved by calculating the focused image from these images. As a concrete example of micro objects, microelectrodes of semiconductor products [2] are realistic. Microelectrodes are called bumps and electrically connects the integrated circuit chip and the package substrate. For guaranteeing the reliability of electrical connection, dimensional measurements of the bumps are required. Currently, as the density of integrated circuits increases, the arrangement density of bumps also increases. Therefore precise dimensional measurement of bumps is becoming important more and more. By using the bumps as the object under test and measuring its diameter, position and height, we confirmed the performance of developed dimensional measurement method on the system. The results showed that the system could measure a lot of bumps within a second with an accuracy of 1μm or less.

One of the problems of interferometric methods is the difficulty of measuring surface shape with sharp boundaries due to the wavelength-limited dynamic range of the measurement. To circumvent this limitation multiwavelength methods or techniques based on hologram capturing at the different tilt of the illumination beam are applied. In this work we examine the performance of the digital holographic interferometry with multi-inclination illumi- nation in the numerical and real experiments. Lensless implementation of the technique implies the wavefront propagation by numerical algorithms. In this regard the speckle scattering in the Fresnel diffraction area caused by surface roughness and the impact of distance from the object to the registration plane are analyzed. Since shape measurement is based on the calculation of phase difference for the wavefronts recorded with tilt of the object illuminating beam, the requirements to preciseness of measurements of the angle of incidence of this beam are considered. The algorithm of the inclination angle determination are developed. The performance of noise suppression techniques, namely sine-cosine and BM3D methods are considered for high noisy conditions, when the phase distributions are formed by reflecting object with a great roughness and height differences.

We report exceptionally good repeatability in non-contact Frequency Space Moire (FSM) Fiber Optic Wafer Thickness Metrology. The FSM method, which is a frequency domain interferometry technique, takes advantage of the low frequency beat-like pattern observed by grating spectrograph in the recorded spectra of the light reflected from the thick samples, and subsequently filtered through the well characterized etalon. In this paper we focus on semiconductor application of the FSM method. The FSM experimentally achieved static repeatability of 0.35 nm for the measurement of thickness of the nominally 508 um thick blanket silicon wafer, which is about 1 part per million, for the acquisition time of 10 ms.

Optical 3D surface measurement based on structured light illumination has been widely used in machine vision and industrial detection. The structured light 3D measurement is based on the assumption that the surface of the object reflects diffusely. However, most engineering materials do not satisfy this condition, and the influence of the optical properties of the measured surface on the measurement accuracy is often neglected. In this paper, I analyzed and discussed the influence of the optical properties of the surface material on the measurement. The method for calculating and comparing the phase and position of the center of mass of the impulse response is proposed to explain the error that occurs from the use of surfaces with diffuse transmissions and diffuse scattering. I designed a 3D measuring system to obtain the distribution of phase and the position of the impulse response. The results of the comparative experiments show that the optical properties of the measured surface affect the 3D measurement result. It is thus necessary to consider the optical properties of the measured surface in precise 3D structured light measurement.

A single shot RGB Multi-wavelength Polarizing Interferometer (MPI) is proposed for measuring moving precision surfaces with micro/nano-scale structured pattern or defects. The interferometer is combined with four CMOS cameras, each with an integrated Bayer filter, to capture four color phase shifted interferograms at a single exposure time. The phase shifting mechanism is achieved by using thin film linear polarizers and birefringent quarter-wave plates. The 2π phase ambiguity range is extended by using a synthetic wavelength produced from the RGB. Measurement of step height standard samples are also presented and compared to measurement obtained by Coherence Scanning Interferometer (CSI).

The generation of a heterodyne carrier frequency via offset-lock in an optical phase-locked loop (OPLL) is a widespread technique in communication, spectroscopy and other fields. Commercial state-of-the-art laser-Doppler vibrometers (LDV) generate heterodyne frequency carrier by acoustooptic devices (Bragg cells) efficiently with the slow shear mode up to 409 MHz. Therefore, these LDVs are limited in measurement bandwidth and it is impossible to adjust the heterodyne carrier frequency to the optimal value in respect to the requested demodulation bandwidth. For RF-MEMS (radio-frequency microelectromechanical systems) testing, carrier frequencies in heterodyne LDVs have to exceed 1 GHz to enable the unambiguous reconstruction of the measured vibration, which is restricted by the conventional heterodyning techniques. Recently, we demonstrated a LDV microscope with the generation of a variable heterodyne carrier frequency up to 200 MHz by offset-lock in an OPLL with visible DBR semiconductor lasers. In this paper, we demonstrate the increase of heterodyne-frequency-carrier generation by conventional RF electronics up to 1.4 GHz and discuss the decisive OPLL parameters for the application of this technique to ultra-high-frequency laser-Doppler vibrometry. Our LDV microscope shows an (out-of-plane) vibration amplitude sensitivity of less than 1 pm/ √ Hz for vibration frequencies higher than 50 MHz, which enables the vibration measurement of most RF-MEMS. First measurements of resonances of a piezoelectric transducer are presented.

Time averaging technique applied to different interferometric methods is one of the most commonly used measurement technique in vibration testing. The information on amplitude of vibration is encoded in so called fringe envelope function described by the J₀ Bessel function in case of harmonic motion. In the presented paper the author proposes novel solution for amplitude distribution evaluation. It is based on the analysis of the Bessel pattern modulation distribution. For modulation evaluation Temporal Phase Shifting method was used. Conducted numerical error analysis prove usefulness of the proposed approach.

Every object possesses a unique acoustical spectrum/signature that is determined by its dimensions, materials, defects, and history. The signature can be read and analyzed by exciting the part and scanning the surface using Laser Doppler Velocimetry (LDV). Defects, tampering, inferior materials, and spec failures change the signature and can be detected by reading the signature. Finite element analysis (FEA) employed with the computer design and knowledge of material makeup of an object can predict its signature. Additional security is possible by programming a hidden authentication key printed during manufacturing. Such a readable signature offers many opportunities, especially in additive manufacturing. This paper describes the development and potential applications of this photonic tool.

This paper describes work I have done in the area of speckle metrology. This began with the redesign in 1969 of the speckle interferometer invented at the National Physical Laboratory in Teddington, UK, and culminated in the development of a real-time digital holography system in 1989. Along the way, I developed a system for heterodyne readout of halo fringes in speckle photography that made it practical for two-dimensional strain measurement at high temperatures. My most recent contributions have been the illustration of digital holography with a speckled reference beam and what may be called transverse digital holography, a technique that derives from one of the first speckle interferometry configurations.

3D displacement and strain measurement for the fast flapping wings is of crucial importance to micro air vehicle design. This research introduces a high-resolution photomechanics testing method for robotic flapping wings. First, the dynamic 3D shape of a fast flapping flight process is reconstructed by the high-speed structured light technique with defocused binary stripe illumination. Then, the 3D displacement map is extracted through geodesic analysis. Finally, the Green- Lagrange strain tensor is computed by Kirchhoff-Love shell theory. Experiments will demonstrate the success of the method by testing a robotic bird with a wings’ flapping frequency over 20 cycles/second.

Fringe Projection Profilometry and Phase Measuring Deflectometry Systems are well-established technologies for noncontact 3D surface measurements. A common challenge in those systems is to obtain the absolute surface information using few measurement frames. In practice both techniques often generate a series of sinusoidal fringe patterns with different frequencies and use a camera vision system to capture the (by the object) deformed patterns. The images containing the deformed patterns are then processed to obtain the 3D surface information. Sinusoidal multi-frequency techniques have been for years and are an on-going area of research, where various algorithms have been developed to measure the unwrapped phase map (at the shortest fringe period). Commonly, temporal phase shifting techniques are used to extract the phase at different frequencies. However, obtaining the phase map for every fringe period requires multiple measurement frames. In this work, the advantages of efficient grey level (GL) coding techniques are presented, where a focus is given on the reduction of frames for noncontact 3D surface measurements. A further focus is given on additional features of GL coding as e.g. the error-detecting and the error-correcting properties of those codes that make GL coding an interesting candidate for harsher measurement environments.

Zonal integration techniques are commonly used for surface shape reconstruction from slope measurements for phase measuring deflectometry (PMD) systems. The stability of zonal surface reconstruction algorithms relies on the symmetry of numerical differential equations for uniformly sampled grids. Modified zonal integration methods for non-uniformly sampled slope data can yield accurate results for the measurement of a single camera yet fail to produce accurate results for stereo deflectometry. In this publication, we identify and quantify a large systematic bias in the deflectometry measurement that is introduced during resampling or interpolation of phase data onto a uniform sample grid. This interpolation is commonly done in all forms of deflectometry and cannot be avoided in stereo phase measuring deflectometry. This surface reconstruction bias is largely independent of the integration method and the accuracy of the integration algorithm under different grid assumptions. This bias is a significant contributor to the well-known micronlevel low spatial-frequency errors in deflectometry that are commonly attributed to calibration errors. We show the dependence of the bias on different system parameters including surface shape, curvature, and the geometry of the system components, providing guidance on setup geometry for stereo PMD.

Interference microscopy for biospecimen characterization based on the moiré phenomena is described. Two scenarios of incoherent multiplicative superimposition are employed: (1) object information carrying interferogram is superimposed with reference one or (2) two object interferograms are superimposed (with equal or opposite phase signs). Second strategy provides additional doubling of underlying phase function of interest. Superimposition can be performed using two experimental interferograms or single real interferogram and numerically designed reference structure in so-called digital moiré regime. Multiplicative superimposition of two periodic intensity distribution yields moiré pattern containing low spatial frequency difference beat term (moiré fringes – macrostructure) and high spatial frequency sum beat term (microstructure). The macrostructure was studied in great majority of previously reported moiré techniques. In this contribution we point attention onto the sum beat spatial frequency component and report efficient means for its recovery using numerically advanced digital filtering (variational/empirical decomposition approaches). Its single-frame (single-pattern) phase demodulation by 2D Hilbert spiral transform with enhanced accuracy follows - this feature comes from the fact that sum beat moiré term has high spatial frequency which is generally beneficial in single-frame fringe analysis and its phase function is be doubled. In particular spatial phase change is better sampled by fringes for denser interferogram and more importantly numerical filtering of fringe term of interest from background intensity is easier. We propose and preliminarily evaluate experimental proof-of-concept strategy where two interferograms with slight difference in spatial frequency are simultaneously recorded in two halves of CCD camera and superimposed multiplicatively. Proposed moiré technique opens up new possibilities in interference microscopy based bio-phase imaging mainly due to its data-driven enhanced phase sensitivity (fringe doubling effect) and real-time operation. Evaluation employing numerical simulations and validation using experimental recordings of phase bio-samples, i.e., prostate cancer cells are enclosed.

Digital holography is now often used for biomedical imaging because of its ability of post-computational focusing and the quantitative phase imaging of a transparent biological cell. The spectral information is also important to observe the functional property and the structure of a biomedical cell. In the multi-spectral digital holography, a target is illuminated by light with each wavelength, the corresponding interference fringes are recorded, and then the image reconstruction iterates for each wavelength. The most common method is to use the wavelength switching, but when an object is changing temporally, the wavelength switching should be faster than the changing speed of the object. In this paper, we demonstrate a capturing method of the digital holography with fast wavelength switching based on a white-light continuum laser with acousto-optic modulator and a high-speed camera.

Multi-beam Spatially Multiplexed Interferometric Microscopy (MB-SMIM) is presented as the implementation of a robust, fast and accurate single-shot phase-estimation algorithm with a simple, low-cost and stable way to convert a bright-field microscope into a holographic one using partially-coherent illumination. Ronchi diffraction grating is used to generate two-beam and multi-beam interference. To ensure effectiveness of the process object field should be divided into analyzed specimen area (1/3) and clear reference area (other 2/3). Using partially coherent light source one is able to produce pure three-beam interference of -1 diffraction order of specimen area, and 0/+1 diffraction orders of both clear reference areas. Opposite sign orders create doubled spatial frequency term overlapping with the regular SMIM interferogram. Phase demodulation based on the single-frame two-dimensional Hilbert-Huang transform or regular multi-frame phase-shifting follows. Single-frame processing is more beneficial for increased spatial frequency term when doubled number of fringes “samples” the same phase variation as in regular SMIM interferogram. Adding (or subtracting) two three-beam interferograms phase shifted by π allows one to accurately isolate doubled (or single) spatial frequency fringe pattern. Altogether, MB-SMIM adds quantitative phase imaging capability to a commercially available non-holographic microscope with improved phase reconstruction: noise reduction, spatial frequency doubling and phase change disambiguation capabilities. Proposed method enables common-path optical generation of doubled spatial frequency fringes only by slight object field manipulation – no mechanical doubling of incident angle is needed. The technique has been validated using a 20X/0.46NA objective in Olympus BX-60 upright microscope for prostate cancer cells phase imaging.

In this work, we introduce an opto-geometric parameter for the morphologic characterization of cell populations during their early adhesion process. Using measurements for a cell population of the maximal optical phase for each cell and its substrate-contact-surface, we show, experimentally, that a relationship exists between these variables. This connection is primarily associated with morphological cell characteristics. It is also shown that using the relationship obtained above, we can derive a morphological parameter, which, for the cell populations studied, results in a monodisperse Gaussiansize distribution, which would allow for the use of regular statistical variables. This result is in total contrast with the polydisperse distribution obtained if only the contact surface area between cell and substrate is used. In addition, optical phase measurements where accomplished by phase shifted interferometry using a Mirau-type interference microscope. The cellular system studied consisted of Osteoblast-like cells, plated on 316VM medical-grade stainless steel polished surfaces. These cell populations were studied within the same culture conditions of cell type, plating time and substrate roughness conditions. The existence of a relationship between maximal optical phase and substrate contact area agrees entirely with the accepted spreading model for cell adhesion; in particular, considering the close link between the optical phase time change and cell thickness reduction.

Living cells as phase objects require not only non-invasive measurement but also quantitative phase information during dynamic biopsy. Digital Holographic Microscopy (DHM), measuring three-dimensional morphology without changing the active condition of cells and in situ inspection, is becoming excellent tools for biology research. We have described a DHM method for quantitative, unlabeled observation of living cell subjected to fluid shear stress (FSS) in flowing fluid. The holographic recording system combined with the fluid shear system is improved. The numerical reconstruction technique firstly employed deep learning Convolutional Neural Network model filter, which achieved automatically processing large scale the spectrum of holograms immediately. Osteocytes as the experimental samples were observed and their morphological changes under the stimulation of FSS was successfully measured.

A novel dual beam Optical Coherence Tomography (OCT) system is presented for microfluidics applications. The dual sensing-light-sheet approach presented here achieves high velocity flow measurement because the rapid re-imaging by the second sensing-light-sheet allows for particle tracking between each image-frame of the pair. We have demonstrated measurements of microchannel flow using 10 μm particles in a 700 μm deep channel with peak velocities of approximately 0.28 metres-per-second (m/s), approximately two orders of magnitude higher than previously reported OCT systems operating at a few millimeters-per-second.

A novel, single shot, low cost, multidirectional lateral shear interferometer for extended range wave front sensing has been developed. It exploits the Fresnel diffraction field formed by five lowest diffraction orders of a simple amplitude checker grating. The Fresnel pattern encodes information on four directional partial derivatives of the wavefront under test. It has been theoretically and experimentally shown that for larger gradient phase objects or shear amounts only diagonal derivative information is easily accessible. The x and y direction gradient maps are strongly amplitude modulated. Therefore their demodulation becomes a formidable task. The same feature has been found in widely used quadriwave interferometer developed at ONERA, France. The results of analytical studies and experimental works including fringe pattern processing and phase demodulation are presented.

Two multi-path interferometers were developed using cleaved silica microspheres. A microsphere on top of a singlemode fiber tip was cleaved with a focused ion beam. The asymmetry introduced in the structure generates a new set of optical paths due to random reflections inside the microsphere. The obtained reflection spectrum presents a random-like interferometric behavior with strong spectral modulation of around 3 dB amplitude. Two distinct regions can be observed when a fast Fourier transform is applied. The first involves two cavities at a lower frequency and the second region involves a band of frequencies that is originated by the random interferometric reflections. These two spectral characteristics can be separated using low-pass and high-pass filters, respectively. A correlation method was used to obtain a temperature response from the two-cavity component. A similar structure was also created in a microsphere of multimode fiber. The microsphere was cleaved by polishing the structure with a certain angle. The interference between the different optical paths can be seen as the superposition of several two-wave interferometers, which can be discriminated through signal processing. Temperature sensing was also explored with this structure. The sensitivity to temperature is more than 3-fold for smaller cavities. Moreover, a sensitivity enhancement is also verified if a correlation method is used.

The author had a very enjoyable career working with holography, interferometry, and optical testing and during this talk several projects the author worked on during his career will be discussed including 1) Use of computer generated holograms for testing aspheric optics, 2) temporal phase-shifting interferometry, 3) lateral shearing interferometry and adaptive optics, 4) computerized interference microscope, and 5) dynamic (single-shot) phase-shifting interferometry. It will be shown that as technology improves many things that were very difficult to do 30 or 40 years ago are now very simple to do and many ideas that were useless many years ago are now useful.

The pursuit of low noise in interference microscopy for topography measurement is relevant to many surface types, ranging from super-polished optical surfaces to weakly-reflecting or scattering textures that require enhanced signal sensitivity. Noise is a random error source that may be quantified using a repeatability test. Here we propose a noise density normalized to the square root of the number of data points per unit time, to evaluate performance independent of measurement speed and areal filtering. Consistent with standardized vocabulary, we also distinguish between measurement noise, which is specific to a part and environmental conditions, and instrument noise, which is an apparatus specification corresponding to measurement noise under the best possible conditions. To illustrate these ideas, we present results from a commercial phase-shifting interference microscope showing an RMS measurement noise of 0.03 nm for a 1-second data acquisition of 1 million surface topography image points, after application of a 3x3-pixel median filter. The results follow the expected inverse square root dependence on the data acquisition time.

Recently, a technique for calibration of the Modulation Transfer Function (MTF) of a broad variety of metrology instrumentation has been established. The technique is based on test samples structured according to binary pseudorandom (BPR) one-dimensional sequences and two-dimensional arrays. The inherent power spectral density of BPR gratings and arrays, has a deterministic white-noise-like character that allows a direct determination of the MTF with a uniform sensitivity over the entire spatial frequency range and field-of-view of an instrument. As such, the BPR samples satisfy the characteristics of a test standard: functionality, ease of specification and fabrication, reproducibility, and low sensitivity to manufacturing error. Here we discuss our recent developments working with support of the U.S. Department of Energy on industrialization of the technique. The goal is to develop affordable BPR test samples, application procedures, and data processing software, suitable for thorough characterization of optical interferometers and microscopes, as well as x-ray, electron (scanning and transmission), and atomic force microscopes. We report on the development of BPR array test samples optimized for advanced characterization (including the instrumental MTF and aberrations) and operation optimization of large aperture optical interferometers. We describe the sample fabrication process and tests to verify the compliance to desired surface topography. The data acquisition and analysis procedures for application of the technique for precise focusing of Fizeau interferometer are discussed in detail.

Understanding measurement uncertainty is critical for qualifying an instrument for a specific task and for improving the instrument’s performance. Specifying that uncertainty requires a precise definition of the particular quantity being measured and the conditions under which it is measured. These should both be relevant to the end user’s application. In this paper I re-examine and summarize some of the definitions and test methods that have been used to characterize Fizeau interferometers and attempt to encourage a discussion about how to best specify the performance of Fizeau interferometers for today’s demanding applications.

Standard interferometers are not suitable for measuring highly aspherical lenses due to the obligatory null configuration. In a tilted wave interferometer (TWI), however, the surface under test is illuminated from multiple directions by a point light source array ^1,2. Thus, there is at least one source for each location on the test object, which illuminates the specimen perpendicular so an evaluable measuring signal is generated. In combination with the stitching approach, even large convex apertures can be evaluated. However, the system must be calibrated before measuring with a TWI. During this calibration, a known sphere is measured in several positions. The obtained information describes the entire imaging system with a finite number of Zernike polynomials. High-frequency structures in the system components are not covered and can lead to calibration errors. For this reason, they must be reduced to a minimum. In order to control high-frequency aberrations, the wavefront of each individual lens and the entire system must be determined. In classical methods with a return mirror, many error effects are encountered, such as air turbulences at large distances. The proposed model for the mathematical determination of wavefront aberration uses single measurements of the front and back surface. This reduces measurement uncertainties and simplifies the measuring effort. In addition, the costs for otherwise necessary return mirrors are also omitted. However, the lenses used must be verified additional for impurities or streaks³. Different measurement configurations are compared with the proposed method.

Spectrally controlled interferometry (SCI) is a method which presents a host of advantages over traditional coherent and white light interferometry. As its name suggests, the source spectrum is precisely controlled to produce localized fringes whose location and phase are tunable. The approach has been demonstrated to produce accurate interferometric measurements of planar and spherical optics in the presence of detrimental back reflections over a large range of cavity sizes. Phase shifted measurements of single surfaces can be done without any means of mechanical phase shifting. Additionally, existing systems can be converted to be SCI compatible as the method is implemented entirely at the source level of the instrument. Previous demonstrations of this method have applied temporal phase shifting, but use of the SCI method does not preclude the use of alternative measurement techniques. While traditionally, SCI measurements are acquired by shifting the phase of the spectrum modulation function, here we present an alternative method for phase shifting via mean wavelength shift. It is a convenient extension of SCI because typically source parameters are already controlled electronically and shifting mean wavelength of the source adds no additional complication or modification to the existing hardware. By utilizing wavelength shifting novel architectures for instantaneous measurements become possible. In this paper we present two methods of instantaneous surface measurements: using carrier fringe approach and simultaneous PSI by mean wavelength shift. Various phase measurements of multiple surface cavities via both methods are presented to demonstrate the capability. Comparisons are made to traditional SCI and standard coherent phase shifting measurements. Limitations and sources of noise are addressed as well.

Quantitative phase imaging (QPI) has made a tremendous contribution to microbiology and medicine. Recently, QPI has been used for in-vitro fertilization optimization, and cell culture monitoring, among others. These accomplishments have been made using phase measurements. QPI is based on well-known optical metrology techniques. The challenge in QPI is developing phase measuring systems that can be used in the medical or microbiological environment. For such applications, two-beam interferometers are not suitable. In this context, we propose a common path interferometric system that provides quantitative phase measurements. The method combines the diffraction phase microscopy with the arbitrary phase shifting technique. The phase-shifted images are obtained by means of a shifted diffraction grating and an amplitude filter that allows interference between the undiffracted light and the first spatial frequency at the Fourier plane. The results show a reduction in the noise of the resultant phase when using the arbitrary phase shifting technique. This system can potentially be used to quantify nanoscale motions in living cells.

For accurate radius of curvature measurements of spherical specimens on an interferometric radius bench, the axial displacement between the cat’s eye position and the confocal position needs to be measured at the height of the optical axis of the form measuring interferometer. We propose an alignment technique which allows for the alignment of the Abbe-offset well below 100 μm within a few seconds based on fringe nulling. The alignment technique bases on a special designed optical element and a pre-adjustment procedure. The implementation of the alignment technique in a radius measurement setup with a repeatability of 30 nm will be shown.

We implemented a Michelson interferometer capable to follow temporal phase variations. The system uses polarization phase shifting techniques and is coupled to the pixelated polarization sensor. By considering the space-time stability obtained, the systems allow a 3D FFT phase demodulation algorithm that combines the spatial carrier due of the pixelated polarization mask and a low pass filtering in time. In the system implementation, we reduced the usage of polarization components by observing the back propagated interferogram and, in the speckle reduction part, a laser combined with a spatial filter and speckle reducer commercially available commonly employed for projection. Experimental results presenting the enhancement in space and time are presented.

We present a method for shape measurements, which is based on sampling the temporal coherence function of the light using a shear interferometer. In contrast to standard White Light Interferometry our method has low demands regarding the spatial coherence of the light and is robust with respect to exterior disturbances. We sample the coherency function of the light with the help of of a test object with steps of a couple of micrometers. Due to its common path principle the system is intrinsically vibration tolerant.

We discuss the application of new synchronized, and virtually synchronized multiprobe real space and frequency domain low coherence interferometers for measurement of wafer thickness and topography. The recently developed low coherence frequency domain interferometers have been proved to be fast and effective tool for measurement of wafer topography, thickness and individual layers within multilayer structures. In our paper we present simple models describing performance of multiprobe metrology and compare it with other solutions.

One of the most time-consuming aspects of measuring the radius of curvature of a test article is repeatedly moving between the cateye and confocal positions for stage readings and measurements. Here we introduce a unique short coherence dynamic Fizeau interferometer with internal path matching that facilitates long term stability of the cateye reference position relative to the confocal position, eliminating the need to take repeated cateye measurements. The path matching assembly is equipped with a high-quality encoder and has an athermalized design for long-term stability of the path match locations. In addition, the interferometer uses a polarization based pixelated camera for single frame acquisition of the measurements. Combined with the short coherence source, the single frame measurements facilitate rapid scans of the fringe modulation as a function of stage position for accurately locating the relative position of the test article along the optical axis. Radius of curvature measurement repeatability of less than a micron has been demonstrated. This paper will discuss the theory of operation and present the results of measurement studies.

Micro-electro-mechanical systems (MEMS) are used today in a multitude of applications. The production of MEMS is a fully automated wafer process consisting of deposition, patterning and etching. Especially for safetyrelevant MEMS devices such as airbag or ESP sensors, a 100% inspection is required. After an optical inspection of the MEMS structures, they are sealed and protected by bonding a silicon cap wafer on top. This bonding can lead to an additional stress in the wafer stack that interferes with the MEMS function, which is tested by an electrical test. This stress can cause individual elements to stick together or to be bent. In the case of a failed test, the problem cannot be located on the MEMS structures because they are opaque to visible light. The defects are to be detected by a topography measurement. This publication discusses the challenge of topography measurement using an low-coherence interference microscope, which is necessary to achieve a high lateral and axial resolution. It turns out that dispersion does not have such a large influence on the measurement signal as the uneven thickness of the cap. The influence of thickness variation could be reduced by measuring both cap surfaces and subtracting them from the MEMS topography. A comparative measurement of the MEMS without a cap shows that the residual deformation is of a similar magnitude. Removing the cap, however, will cause any stress that may be present to dissolve.

Since the moment when it was possible to achieve the necessary power density to start the process of deep penetration welding, accompanied by a keyhole, there is hope - and need - to measure the depth of this vapor channel. In the decades in which the technology of deep penetration welding has been used, various approaches have been developed that allow a message about the depth of the keyhole. All these approaches have one thing in common, the basics of determining the depth are based on secondary information, such as the dimension of the melt pool, or the strength of the emissions from the plasma or the metal vapor. Except by means of X-ray or destructive testing no method has been developed so far to determine the real keyhole depth. With the IDM system (In-Process Depth Meter) it is now possible to bring a system to market, which can measure the depth of the keyhole in industrial laser welding applications. It is important to bring up, that not only laser welding as a highly dynamic process, requiring a sensor with high temporal and spatial resolution can profit from this sensor technology. The use of this method in laser applications has risen in the last years. Since its first appearance in 2008 [1], application examples were shown for laser cutting [2], selective laser melting [3], laser micro machining [4], laser drilling [5] and laser welding [6]. For the latter, a huge potential is foreseen [8].

Multi-wavelength phase unwrapping techniques have traditionally been used to unwrap the phase at the shortest measurement wavelength, where numerous techniques have been developed with distinct advantages for a given application. Nevertheless, multi-wavelength techniques are more than phase unwrapping approaches: super-sensitive multiwavelength interferometers have a lower uncertainty than conventional interferometers, multi-wavelength techniques can break the Nyquist limit and thereby relax the requirements on the measurement system, and multi-wavelength techniques have also unconventional applications as e.g. optical encryption. This work discusses different multi-wavelength techniques, derives new noise criteria with no approximations, and outlines important, but still little researched areas of multi-wavelength phase unwrapping techniques.

Phase-shifting algorithms (PSAs) are usually derived for static or quasi-static conditions, where the temporal phase step is the only significant variation expected between successive frames. When these assumptions are valid, choosing the right algorithm often translates into faster acquisition times or robustness against systematic errors (such as detuning, random noise, and distorting harmonics). In practice, however, one may need to cope with dynamic conditions that require more complex phase-demodulation approaches. In this work, we present a PSA designed for robust quadrature filtering assuming temporal variations of the background and contrast functions. The frequency transfer function (FTF) formalism allows us to design its spectral response and to assess its robustness against systematic errors. This procedure is conceptually and computationally easy to generalize for many-step algorithms. Finally, a work-in-progress application for high-dynamic range (HDR) in fringe-projection profilometry is presented as proof of concept.

We propose a synthesis of phase-shifting algorithms for Fizeau interferometry of high numerical-aperture spherical surfaces. Several approaches have been reported for spherical surface measurements such as phase shifting, carrier fringe, and wavelength tuning. The commonly used method of phase shifting with mechanical phase modulation suffers from spatial nonuniformity of the phase step within the observing aperture. We operated seven algorithms designed for different phase steps on the same set of interference fringes recorded in a one phase-shift sequence, and determined the object phase from each of the algorithms depending on the aperture position. The resultant phase distribution showed minimum systematic errors.

Digital interferometer is widely used for evaluating optical surfaces due to its outstanding sub-nanometer accuracy and precision. In this paper, we will summarize its advantages and then describe its applications in industry, especially in both absolute flat and cylindrical surface and measurements. Inner surfaces measurement of cylindrical ring can be achieved without map stitching, by a Fizeau interferometer with a 90° conical mirror. The alignment of this arrangement, however, is very crucial to the accomplishment. Any small misplacement of 90° cone or hollow cylinder from their ideal settings may result in large measurement errors. These errors are not intuitive and hard to be removed if their origins are not well understood. In other words, it is very important to know how these measurement errors are generated from the optical misalignment in order to eliminate them. Transmission flat has normally 1/20 wavelength PV. However, when a flat surface under test is better or much better than the transmission flat, we need the absolute flat measurement. We developed a new method to be easily able to achieve the accuracy of 1/100 wavelength PV. We have dedicated our efforts to do so. The theoretical analysis, computer simulations, and experimental validation are presented in the paper.

The low-coherence interferometry which is usually combined with the wide-field optical microscopy is well known technique for surface profile measurement, micro step measurement and so on. One of the problems is that its axial measurement range is typically limited by its depths of field of imaging, which is determined by the numerical aperture of their objective lens and the central wavelengths of their light source. If a low-coherence interference fringe is far outside the depth of field, the measurement accuracy inevitably decreases, regardless of how well adjusted the reference mirror is. To solve this problem and improve the axial measurement range of the low-coherence interferometry in this study, an object scanning measurement scheme involving a Linnik interferometer was developed. To calibrate the system in the proposed technique, image post-processing is performed for a well-conditioned state to ensure that a lowcoherence interference fringe is generated within the depth-of-field, so that three-dimensional objects with high-aspectratio structures can be scanned along the axial direction. During object scanning, this state is always monitored and corrected by adjusting the reference mirror. By using this scheme, the axial measurement range can be significantly improved up to the working distance of the objective lens without compromising the measurement accuracy. The working distance is typically longer than 10 mm, while the depth-of-field of the microscope is generally around 0.01 mm, although it varies depending on the imaging system. In this report, the experimental setup of an object scanning low-coherence interferometry is presented, a series of experimental verifications is described, and the results are discussed.

In this paper, a novel approach was implemented to image the marine plankton using the in-line digital holographic technology. the digital holography can image all plankton in a certain volume and more information can be recorded including the intensity and phase information. Moreover, the lensless system cases no aberrations and reduces the complexity of structure. In the process of hologram reconstruction, numerical algorithms were developed based on the angle spectrum theory. In the experiments of marine plankton, some technical issues, such as reconstruction algorithm, numerical refocusing, zero-order term suppression, were discussed. We can obtain the reconstructing image layer by layer at different distances by changing the distance step, which demonstrates that digital holographic imaging is capable of digital refocusing. Digital holographic imaging has clear advantages over other optical methods for the analysis of marine plankton, which contributes to further microorganism identification in the oceanographic observation by using the digital image processing and microscopy techniques.

When an optical surface has local deformations, it is not practical to measure the entire surface at one time, because an average of the entire surface is obtained, ignoring the local deformations. In a previous work we used the principle of the Chelmer’s test, for analyzing just local defects of an optical surface, the technique uses a reflective spatial light modulator (RSLM), which allows the screen to remain at a fixed position, and only two apertures are used at a time. Our proposed method measures only obtain the local or zonal defects in the wavefront. The use of an RSLM provides a very flexible way of selecting a particular zone to be tested. The advantage of the proposed dynamic Chelmer’s test is that using an RSLM it is possible to select a pair of holes in real time, for any zone, and with different sizes and orientation of the holes. In this work, we compare the results obtained previously, with three different techniques: the first one is the traditional analysis of fringes used in Chelmer’s experiment. The deviations of the interference fringes from a straight line were calculate, according to the equation of Guenther. The second is an analysis by computer, where we developed a computer program to analyze an interferogram produced by a commercial interferometer (ZYGO), with the computer program is possible to know the position of the dark fringes. The ideal separation the dark fringes are obtained by analyzing an area of the interferogram that has no deformations. Finally we analyze the profile of the interference fringes corresponding to the local areas using the commercial program “Durango Interferometry Software”[1].

We present a polarimetry method based on measurement of the geometric phase. We implement a dual-path Mach-Zehnder interferometer to measure the geometric phase introduced by polarization optics composed of retarders. The method can be used either to determine a polarization state of light or to measure the Jones matrix of retarders and polarization elements.

In this research an interferometric system was developed that generates four simultaneous interferograms with independent phase shifts using modulated polarization. The proposed system consists of three coupled interferometers: the first system is a polarized Mach-Zehnder interferometer, which generates the pattern, the second and the third interferometer system, function as replicators of the first pattern, so the four patterns are generated. To show the novelty of the developed system, the calculation of optical path difference (OPD) for phase samples are shown.

Digital holographic micro-tomography (DHMT) is widely used for three-dimensional (3-D) detections in biomedical research, from which the information of refractive indexes (RI) distributions of cells obtained can reflect some pathological states. When measuring cells, a capillary is usually used to contain and rotate them. The accuracy of diffraction tomography methods, which is commonly used, is likely to decrease when the object induces complex scattering effects, especially when measuring cell clusters. The nonlinear algorithm based on multi-slice wave propagation method (WPM) we proposed earlier is preferred in these situations. In this paper, we perform 3-D measurements of cell clusters using the setup-rotating system and the WPM-based nonlinear algorithm. Quantitative 3-D distributions of RI can be obtained accurately to display the structures of the cell clusters. The experimental results demonstrate that the WPM-based nonlinear algorithm can provide high accuracy for cell clusters.

This paper proposes an improvement of the measurement method of focusing high gradient microobjects of small and large radius of curvature. The measurement process is carried out in a Fourier digital holographic microscope with spherical illumination, which ensures maximum usage of numerical aperture of the optical system. The interference pattern is the result of interference between the deformed, quasi-spherical object and spherical reference waves. The noise influence on the phase measurement is analyzed and minimized with block-matching 3-D (BM3D) filtering method. The accurate shape reconstruction of the object and localization of the imaging reference plane are provided by (1) the aberration compensation procedure, and (2) the calibration procedure. To obtain the final shape, the Local Ray Approximation approach is used. In this work an effective BM3D method was applied in the process of the reconstruction of the object beam, improving accuracy of the measurement method. The validation of our approach is presented for spherical objects of large radius of curvature and high gradient of shape.