Signal Processing Protocols Research Articles

Measuring linear birefringence via rotating-sample transmission Stokes spectropolarimetry

Linear birefringence is a fundamental property of optically anisotropic media, defined by the difference in refractive index experienced by light polarized along orthogonal directions. It is usually manifested in microscopically aligned molecular systems, where a preferential direction of light–matter interaction is created. For instance, the anisotropic structure of calcite crystal causes the famous double-refraction phenomenon. Another common example is commercial adhesive tapes, which are polymeric materials possessing birefringent properties due to their manufacturing processes. The intrinsic relation between birefringence and molecular alignment forges a new analytical route to study materials such as polymeric thin films. Therefore, the capacity of measuring linear birefringence and its fast axis is of paramount importance for the science of anisotropic molecular systems. In this contribution, a comprehensive approach to acquire linear birefringence using rotating-sample transmission Stokes spectropolarimetry is presented and applied to transparent adhesive tapes as a case study. The experimental setup comprises a thermal light source and a spectropolarimeter capable of determining wavelength distributions of Stokes parameters. The samples are carefully aligned in a rotating mount and subjected to a fixed broadband vertically polarized light beam. Then, the transmitted light is analyzed using a rotating retarder type of spectropolarimeter. Through systematic variation of the sample’s angular position, the Stokes parameters of transmitted light are measured for each transmitted wavelength as a function of the sample’s angular position. The linear retardance and fast axis direction relative to the tape’s long axis are then determined from the modulation of Stokes parameters over sample rotation. The model derivation, experimental procedure, and signal processing protocol are described in detail, and the approach is verified with a simple correlation between linear retardance and the number of stacked layers of tape.

An Immersion Ultrasonic Testing‐Based Research Platform for Studying Electroplating Kinetics

AbstractElectroplating is widely exploited in numerous application areas. Understanding the kinetics of electroplating, which is essentially a dynamic phenomenon, is crucial to making use of it appropriately in the real world. Currently, there exist a variety of research tools for studying electroplating kinetics, each with its own pros and cons. In this paper, we introduce a novel, immersion ultrasonic testing‐based technique that carries out in situ, direct monitoring of the thickness increase of an electroplated layer. Through careful design and optimization of the measurement setup and the signal processing protocol, the measurement resolution of the technique was able to reach a sub‐micron level. Via a number of demonstrative zinc plating experiments that were performed under different conditions, the measurement accuracy of the technique was thoroughly validated by a number of independent methods. All in all, the technique can become a promising alternative tool for studying the kinetics of different electroplating processes, supplementing the existing toolbox with the capability of providing a new class of information.

Real-Time Digital Biometric Monitoring during Elite Athletic Competition: System Feasibility with a Wearable Medical-Grade Sensor

Introduction: Real-time digital heart rate (HR) monitoring in sports can provide unique physiological insights into athletic performance. However, most HR monitoring of elite athletes is limited to non-real-time, non-competition settings while utilizing sensors that are cumbersome. The present study was undertaken to test the feasibility of using small, wearable medical-grade sensors, paired with a novel technology system, to capture and process real-time HR data from elite athletes during professional competition. Methods: We examined the performance of the BioStamp nPoint® sensor compared to the Polar chest strap HR sensor in 15 Professional Squash Association (PSA) tournament matches in 2019–2020. Fourteen male professional squash players volunteered for the study (age = 23.8 ± 4.9 years; height = 177.9 ± 7.1 cm; weight = 71 ± 7.0 kg), which was approved by the PSA in accordance with their Code of General Conduct and Ethics. Algorithms developed by Sports Data Labs (SDL; Detroit, MI, USA) used proprietary data collection, transmission, and signal processing protocols to produce HR values in real-time during matches. We calculated the mean and maximum HR from both sensors and used widely accepted measures of agreement to compare their performance. Results: The system captured 99.8% of HR data across all matches (range 98.3–100%). The BioStamp’s mean HR was 170.4 ± 20.3 bpm, while the Polar’s mean HR was 169.4 ± 21.7 bpm. Maximum HR ranged from 182 to 202 bpm (Polar) and 185 to 203 bpm (BioStamp). Spearman’s correlation coefficient (r_s) was 0.986 (p < 0.001), indicating a strong correlation between the 2 devices. The mean difference (d) in HR was 1.0 bpm, the mean absolute error was 2.2 bpm, and the percent difference was 0.72%, demonstrating high agreement between device measurements. Conclusions: It is feasible to accurately measure and monitor real-time HR in elite athletes during competition using BioStamp’s and SDL’s proprietary system. This system facilitates development and understanding of physiological digital biomarkers of athletic performance and physical and psychosocial demands in elite athletic competition.

Silicon photonics-based laser Doppler vibrometer array for carotid-femoral pulse wave velocity (PWV) measurement.

Pulse wave velocity (PWV) is a reference measure for aortic stiffness, itself an important biomarker of cardiovascular risk. To enable low-cost and easy-to-use PWV measurement devices that can be used in routine clinical practice, we have designed several handheld PWV sensors using miniaturized laser Doppler vibrometer (LDV) arrays in a silicon photonics platform. The LDV-based PWV sensor design and the signal processing protocol to obtain pulse transit time (PTT) and carotid-femoral PWV in a feasibility study in humans, are described in this paper. Compared with a commercial reference PWV measurement system, measuring arterial pressure waveforms by applanation tonometry, LDV-based displacement signals resulted in more complex signals. However, we have shown that it is possible to identify reliable fiducial points for PTT calculation using the maximum of the 2nd derivative algorithm in LDV-based signals, comparable to those obtained by the reference technique, applanation tonometry.

Structured Light in Turbulence

Optical communication is an integral part of the modern economy, having all but replaced electronic communication systems. Future growth in bandwidth appears to be on the horizon using structured light, encoding information into the spatial modes of light, and transmitting them down fibre and free-space, the latter crucial for addressing last mile and digitally disconnected communities. Unfortunately, patterns of light are easily distorted, and in the case of free-space optical communication, turbulence is a significant barrier. Here we review recent progress in structured light in turbulence, first with a tutorial style summary of the core concepts, before highlighting the present state-of-the-art in the field. We support our review with new experimental studies that reveal which types of structured light are best in turbulence, the behaviour of vector versus scalar light in turbulence, the trade-off of diversity and multiplexing, and how turbulence models can be exploited for enhanced optical signal processing protocols. This comprehensive treatise will be invaluable to the large communities interested in free-space optical communication with spatial modes of light.

On coherent structures in gas–solid fluidization

In this work, experimental data from the novel high speed particle image velocimetry (HSPIV) measurements and pressure fluctuations complemented by numerical predictions from TFM simulations are processed using advanced signal processing protocols to identify and characterize coherent structures in a gas–solid fluidized bed with Geldart D particles. The time-frequency properties of the bed dynamics were studied using the wavelet coherent analysis (WCA) and the Hilbert Huang transform. The WCA of numerical pressure drop signals at different measurement positions showed the existence of spatial-temporal coherent structures. At close measurement positions phase locked coherent phenomenon due to bubble generation dominate most of the frequency and time scales due the proximity to the distributor. At a wider measurement spacing the fast traveling waves are attenuated due to gas bubble/void coalescence and acceleration and only pockets of highly coherent oscillations are visible mostly in the frequency range of 0.5–3Hz at certain times. Multi-resolution of the particle fluctuations realized with the Hilbert–Huang transform. The structures were resolved into the micro, meso and macro scales of fluidization based on the energy and frequency distributions. The meso scale structures form the main contribution to the normal axial Reynolds stresses and bubble granular temperature and ultimately the mixing.

Rate equation reformulation including coherent excitation: application to periodic protocols based on spectral hole-burning

A large number of signal-processing protocols are based on recording a spectral pattern via spectral hole-burning in an inhomogeneously broadened absorption profile. We present a simulation method specifically designed for periodic excitation sequences leading to the creation of a spectral pattern. This method is applicable to any multilevel atomic structure. The atomic variables’ coherent dynamics are solved for a single temporal excitation step. The result is expressed as an equivalent population transfer rate. This way, the whole sequence is described as a matrix product and the steady state of the system under periodic excitation is easily derived. The propagation through the atomic medium is fully decoupled from the temporal evolution. We apply this method to the engraving of a spectral grating in a large-absorption Tm:YAG sample for wideband spectral analysis.

Optical coherence microscopy as a novel, non-invasive method for the 4D live imaging of early mammalian embryos

Imaging of living cells based on traditional fluorescence and confocal laser scanning microscopy has delivered an enormous amount of information critical for understanding biological processes in single cells. However, the requirement for a high numerical aperture and fluorescent markers still limits researchers’ ability to visualize the cellular architecture without causing short- and long-term photodamage. Optical coherence microscopy (OCM) is a promising alternative that circumvents the technical limitations of fluorescence imaging techniques and provides unique access to fundamental aspects of early embryonic development, without the requirement for sample pre-processing or labeling. In the present paper, we utilized the internal motion of cytoplasm, as well as custom scanning and signal processing protocols, to effectively reduce the speckle noise typical for standard OCM and enable high-resolution intracellular time-lapse imaging. To test our imaging system we used mouse and pig oocytes and embryos and visualized them through fertilization and the first embryonic division, as well as at selected stages of oogenesis and preimplantation development. Because all morphological and morphokinetic properties recorded by OCM are believed to be biomarkers of oocyte/embryo quality, OCM may represent a new chapter in imaging-based preimplantation embryo diagnostics.

Computations on Cipher Speech for Secure Biometrics

Signal Processing in Encrypted Domain (SPED) is a blending of signal processing and cryptography. SPED is a magnificent tool for processing encrypted data. It provides privacy and security for highly confidential and sensitive data. In this paper we propose a system model for speech authentication, where authentication is done on cipher speech rather than raw speech in the data set. Here we have tested the proposed system with Advanced Encryption Standard (AES) ciphers and Rivest, Shamir and Adleman (RSA) ciphers with secure signal processing protocols like Secure Inner Product (SIP) and Secure Log Sum (SLS). The test results with AES ciphers and RSA ciphers with secure authentication protocols SIP and SIS are detailed in this paper. The experimental result shows that the SLS protocol works well for speech authentication irrespective of the encryption schemes and the feature selection. The performance of the system is evaluated using False Acceptance Rate (FAR) and False Rejection Rate (FRR) measures.

Fast assessment of planar chromatographic layers quality using pulse thermovision method

The main goal of this paper is to demonstrate capability of pulse thermovision (thermal-wave) methodology for sensitive detection of photothermal non-uniformities within light scattering and semi-transparent planar stationary phases. Successful visualization of stationary phases defects required signal processing protocols based on wavelet filtration, correlation analysis and k-means 3D segmentation. Such post-processing data handling approach allows extremely sensitive detection of thickness and structural changes within commercially available planar chromatographic layers. Particularly, a number of TLC and HPTLC stationary phases including silica, cellulose, aluminum oxide, polyamide and octadecylsilane coated with adsorbent layer ranging from 100 to 250μm were investigated. Presented detection protocol can be used as an efficient tool for fast screening the overall heterogeneity of any layered materials. Moreover, described procedure is very fast (few seconds including acquisition and data processing) and may be applied for fabrication processes online controlling. In spite of planar chromatographic plates this protocol can be used for assessment of different planar separation tools like paper based analytical devices or micro total analysis systems, consisted of organic and non-organic layers.

Optimal signal processing for continuous qubit readout

The measurement of a quantum two-level system, or a qubit in modern terminology, often involves an electromagnetic field that interacts with the qubit, before the field is measured continuously and the qubit state is inferred from the noisy field measurement. During the measurement, the qubit may undergo spontaneous transitions, further obscuring the initial qubit state from the observer. Taking advantage of some well known techniques in stochastic detection theory, here we propose a novel signal processing protocol that can infer the initial qubit state optimally from the measurement in the presence of noise and qubit dynamics. Assuming continuous quantum-nondemolition measurements with Gaussian or Poissonian noise and a classical Markov model for the qubit, we derive analytic solutions to the protocol in some special cases of interest using It\={o} calculus. Our method is applicable to multi-hypothesis testing for robust qubit readout and relevant to experiments on qubits in superconducting microwave circuits, trapped ions, nitrogen-vacancy centers in diamond, semiconductor quantum dots, or phosphorus donors in silicon.

One-dimensional time-domain finite-element modelling of nonlinear wave propagation for non-destructive evaluation

This one-dimensional time-domain finite-element model achieves accurate quantitative modelling of ultrasonic wave propagation in multi-layered structures. First, a sinusoidal wave toneburst is sent into a single layer of material exhibiting inherent material nonlinearity characterised by the nonlinear parameter β and thick enough for the toneburst received in through transmission to be resolved. The signal processing protocol that yields the theoretically correct quantitative value of β involves measuring the received toneburst for several propagation distances as well as the use of scaling factors taking into account the fast Fourier transform implementation, input signal windowing and material damping. Using that model configuration, model parameters (element size, time step, frequency step, input pressure, etc.) are then optimised and chosen quantitatively to generate accurate results. Finally, these model parameters are used for cases of interest where the configuration is not such that the exact β value can be obtained – e.g. thinner sample, pulse-echo etc. but where confidence in the results remains. This quantitative model that can be used for multi-layered structures provides a tangible resource useful to NDE engineers: a new prediction tool expected to enable them to choose the experimental set-up, driving frequency and post-processing method that would optimise kissing bond detection capability.

Improvement of the determination of element concentrations in quartz-hosted fluid inclusions by LA-ICP-MS and Pitzer thermodynamic modeling of ice melting temperature

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has become an essential analytical tool for the study of paleofluid chemistry through the analysis of individual fluid inclusions. The calculation of major and trace element concentrations in fluid inclusions is usually based on empirical equations whose significance and accuracy are questionable. In addition, methods for estimation of analytical uncertainties element concentration in individual fluid inclusions are lacking. This study describes a method based upon Pitzer’s thermodynamic model for the calculation of major element (Na, K, Mg and Ca) concentrations in low-to moderate-salinity fluid inclusions. A signal processing protocol, used in combination with the new method is also developed to calculate the concentration, for each inclusion, and uncertainty for each major and trace element. In order to validate the proposed method, synthetic and natural fluid inclusions (from Alpine quartz veins) were ablated with a 193nm ArF excimer laser and analyzed with a quadrupole ICP-MS, equipped with an octopole collision–reaction cell. The difference between the calculated and actual element concentration (i.e. accuracy) does not exceed 20% and the calculated relative standard deviation (i.e. precision) for all element concentrations is ∼10% in standards (glasses, solutions in capillary tubes and synthetic fluid inclusions). The element concentrations obtained with this new method for the Alpine fluid inclusions are in good agreement with those previously measured using Laser Induced Breakdown Spectroscopy (LIBS) or crush-leach methods. Finally, the calculated concentrations and associated uncertainties determined for each element in individual fluid inclusions show that the sensitivity of LA-ICP-MS analysis is high enough to reflect small variations of major and trace element concentrations in the Alpine paleofluid, initially considered to have a constant chemistry. The new approach presented in this paper highlights small but significant variations in the paleofluid chemistry that were not previously detected with conventional LA-ICP-MS data processing.

Monitoring protein fouling on polymeric membranes using ultrasonic frequency-domain reflectometry.

Novel signal-processing protocols were used to extend the in situ sensitivity of ultrasonic frequency-domain reflectometry (UFDR) for real-time monitoring of microfiltration (MF) membrane fouling during protein purification. Different commercial membrane materials, with a nominal pore size of 0.2 μm, were challenged using bovine serum albumin (BSA) and amylase as model proteins. Fouling induced by these proteins was observed in flat-sheet membrane filtration cells operating in a laminar cross-flow regime. The detection of membrane-associated proteins using UFDR was determined by applying rigorous statistical methodology to reflection spectra of ultrasonic signals obtained during membrane fouling. Data suggest that the total power reflected from membrane surfaces changes in response to protein fouling at concentrations as low as 14 μg/cm2, and results indicate that ultrasonic spectra can be leveraged to detect and monitor protein fouling on commercial MF membranes.

Orthogonal decomposition of chemo-sensory cues

Most practices of gas and mixture identification follow the conventional signal processing protocol of mapping the sensor signal to a stand-alone feature upon each presentation of a gas sample. This single-feature representation scheme, however, may not convey enough qualitative information to successfully represent distinct chemical analytes. Following this argument, it is suggested that having additional qualitative information can significantly benefit these results. In this paper, we explore a multi-feature processing scheme for chemo-sensory response cues. The method consists of decomposing the sensor response signal to a set of orthogonal Bessel functions. Bessel functions give us the possibility of decomposing the sensor signal response by a finite number of series of orthogonal functions that are localized in the magnitude of the oscillations’ changes with the time. We believe, therefore, that expressing the sensor response signals by these coefficients is advantageous. We demonstrate the utility of this novel scheme in identifying and quantifying distinct analytes using metal-oxide gas sensors.

Acoustic density measurements of consolidating cohesive sediment beds by means of a non-intrusive “Micro-Chirp” acoustic system

A non-intrusive “Micro-Chirp” acoustic system and a signal-processing protocol have been developed to estimate the bulk density of consolidating cohesive sediment beds. Using high-frequency (300–700 kHz) Chirp acoustic waves, laboratory measurements were conducted with clay–water mixtures. Because acoustic echo strength is proportional to variations in acoustic impedance, and the speed of sound in the clay bed hardly changed during consolidation, the bulk density could be successfully estimated without disturbing the sediment bed. Based on acoustic signal analysis, this study demonstrates that the reflection coefficient and bulk density at the water–sediment interface increase with consolidation time, and that a single speed of sound value can be used for practical bulk density estimation in muddy environments.