Frequency-domain Spectroscopy Research Articles

Hemodynamics and Tissue Optical Properties in Bimodal Infarctions Induced by Middle Cerebral Artery Occlusion.

Various infarct sizes induced by middle cerebral artery occlusion (MCAO) generate inconsistent outcomes for stroke preclinical study. Monitoring cerebral hemodynamics may help to verify the outcome of MCAO. The aim of this study was to investigate the changes in brain tissue optical properties by frequency-domain near-infrared spectroscopy (FD-NIRS), and establish the relationship between cerebral hemodynamics and infarct variation in MCAO model. The rats were undergone transient MCAO using intraluminal filament. The optical properties and hemodynamics were measured by placing the FD-NIRS probes on the scalp of the head before, during, and at various time-courses after MCAO. Bimodal infarction severities were observed after the same 90-min MCAO condition. Significant decreases in concentrations of oxygenated hemoglobin ([HbO]) and total hemoglobin ([HbT]), tissue oxygenation saturation (StO2), absorption coefficient (μa) at 830 nm, and reduced scattering coefficient (μs’) at both 690 and 830 nm were detected during the occlusion in the severe infarction but not the mild one. Of note, the significant increases in [HbO], [HbT], StO2, and μa at both 690 and 830 nm were found on day 3; and increases in μs’ at both 690 and 830 nm were found on day 2 and day 3 after MCAO, respectively. The interhemispheric correlation coefficient (IHCC) was computed from low-frequency hemodynamic oscillation of both hemispheres. Lower IHCCs standing for interhemispheric desynchronizations were found in both mild and severe infarction during occlusion, and only in severe infarction after reperfusion. Our finding supports that sequential FD-NIRS parameters may associated with the severity of the infarction in MCAO model, and the consequent pathologies such as vascular dysfunction and brain edema. Further study is required to validate the potential use of FD-NIRS as a monitor for MCAO verification.

High-Voltage FDS of Thermally Aged XLPE Cable and Its Correlation with Physicochemical Properties.

This paper aims to investigate the influence of thermal aging on a crosslinked polyethylene (XLPE) cable, and the relationships between the macroscopical high-voltage dielectric and the microscopical physicochemical properties are also elucidated. To better simulate thermal aging under working condition, the medium-voltage-level cable is subjected to accelerated inner thermal aging for different aging times. Then, high-voltage frequency domain spectroscopy (FDS) (cable sample) and analyses of microscopic physical and chemical properties (sampling from the cable), including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and elongation at the break (EAB), are conducted at different cable aging stages. The dielectric test results show that after a certain aging time, the high-voltage FDS curves of the cable have layered characteristics, and this phenomenon is more obvious as the aging degree increases. Moreover, the slope and the integral of the high-voltage FDS curves rise with aging time. The mechanism is deduced by the physicochemical results that thermo-oxidative aging results in increasing polar groups and dislocation defects in the crystal region, which leads to the above phenomenon. On the one hand, the appearance of polar groups increases the density of the dipole. On the other hand, the destruction of the crystal region increases the probability and amplitude of dipole reversal. In addition, the breaking of molecular bonds and the increase in the amorphous phase also reduce the rigidity of the XLPE molecular main chain. The above factors lead to obvious delamination and larger dielectric parameters of the thermally aged cable. Finally, according to the experimental results, an on-site diagnosis method of cable insulation thermal aging based on high-voltage FDS is discussed.

Spin-wave interconversion via thermoelectric point-contact control

The magnonic functionalities of magnonic devices, such as logic gates, majority gates, and multiplexers, critically depend on anisotropic spin-wave dispersions. Further, each magnonic function utilizes the advantages of distinct spin-wave modes. However, thus far, no method of directly combining magnonic functions to construct an integrated magnonic circuit has been proposed. Therefore, this paper presents a method for the interconversion of surface spin-waves and backward volume spin-waves using a thermoelectrically controlled magnetization gradient. The results of time- and frequency-domain spectroscopies, heat transfer analyses, and micromagnetic simulations demonstrate the effectiveness of the proposed method, through which spin-wave interconversion via thermoelectric control may be employed to develop integrated magnonic circuits.

Non-invasive monitoring of blood oxygenation in human placentas via concurrent diffuse optical spectroscopy and ultrasound imaging.

Direct assessment of blood oxygenation in the human placenta can provide information about placental function. However, the monitoring of placental oxygenation involves invasive sampling or imaging techniques that are poorly suited for bedside use. Here we show that placental oxygen haemodynamics can be non-invasively probed in real time and up to 4.2 cm below the body surface via concurrent frequency-domain diffuse optical spectroscopy and ultrasound imaging. We developed a multimodal instrument to facilitate the assessment of the properties of the anterior placenta by leveraging image-reconstruction algorithms that integrate ultrasound information about the morphology of tissue layers with optical information on haemodynamics. In a pilot investigation involving placentas with normal function (15 women) or abnormal function (9 women) from pregnancies in the third trimester, we found no significant differences in baseline haemoglobin properties, but statistically significant differences in the haemodynamic responses to maternal hyperoxia. Our findings suggest that the non-invasive monitoring of placental oxygenation may aid the early detection of placenta-related adverse pregnancy outcomes and maternal vascular malperfusion.

Widely Wavelength-Swept Single-Longitudinal-Mode Fiber Laser With Ultra-Narrow Linewidth in C+L-Band

This study proposes and demonstrates a widely wavelength-swept single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) enabled by a dual-coupler-ring based compound-cavity (DCR-CC) filter, a fiber Fabry-Pérot tunable filter (FFP-TF) and a C+L-band erbium-doped fiber amplifier (EDFA). Moreover, it develops a novel methodology utilizing the signal-flow graph combined with the matrix algebra to analyze the DCR-CC filter and obtain its essential design parameters for SLM selection of the EDFL. The C+L-band EDFA is constructed to provide a gain range >90 nm. On the other hand, it demonstrates that, under a sweep-rate ≤100 Hz, a lasing wavelength-swept range >87 nm is obtained, the SLM operation is permanently guaranteed for every instantaneous lasing wavelength. Furthermore, the instantaneous lasing parameters of an optical signal-to-noise ratio ≥66 dB, a relative intensity noise ≤−153.33 dB/Hz@≥3 MHz, a linewidth ≤625 Hz and a power fluctuation ≤0.704 dB are achieved. The distinguished performance of the proposed fiber laser enables it to have a significant potential for application in optical frequency domain reflectometry, spectroscopy and Lidar system.

Parameters Extraction and Mechanism Analysis of the Moisture in Oil–Paper Based on the Spectral Decomposition of Dielectric Response

Frequency-domain dielectric spectroscopy (FDS) is widely used in the insulation assessment of oil-immersed power equipment, and the moisture in insulating paper could be qualitatively predicted by mathematical approximation. However, due to the still lots unexplored underlying mechanisms, until now, there is still no convincing and physical method for the diagnosis of moisture content. With the aid of spectral decomposition methods, this article aims to explore the effects and mechanism of moisture on the independent dielectric response spectra. First, the measured dielectric response curve is unfolded and reconstructed by derivation of the real part curve. Second, the independent polarization and conductance spectra of damp samples are extracted, and then, the relationships between moisture content in insulating paper and spectral parameters are established quantitatively. The results show that moisture introduces a new polarization in the midfrequency region at room temperature, and both polarization and conductance increase with the moisture content. The multiple adsorption model and the proton exchange model of moisture are employed to illustrate the changes of the polarization and the conductance spectra, respectively. This research provides a methodological reference for the moisture assessment of oil–paper insulation.

DC-biased dielectric measurements using an existing frequency-domain spectroscopy (FDS) instrument and series battery

We present a simple method for adding DC bias to frequency-domain spectroscopy (FDS) measurements by inserting a battery in series in the ‘high’ lead to the device under test. An instrument designed for FDS measurements with pure AC can be used in this way without modification, even for DC voltages above the instrument’s AC range. Issues and limitations of the method are discussed, along with some alternative methods. Experimental results from FDS measurements on well defined linear objects are compared with/without the DC bias, to check that the measurement is not disturbed by the DC source. The only detected difference was the expected effect of including the battery impedance in series with the specimen in the measurement. This effect was negligible for typical lab-specimen capacitances, around and below power frequency. The nature of battery impedance is further described, as multiple small batteries in series can strongly affect the results for large specimens and at high frequencies.

Checking of the condition of transformers and the efficiency of oil regeneration with RVM (return voltage measurement)

Up to now the insulation in HV power transformers has been made from oil/paper therefore the diagnosis of this kind of insulation will be also important in the next forty-fifty years. The lifetime of this equipment strongly depends on the condition of the insulation system. The ageing process of oil/paper insulating systems is a very complex and complicated phenomenon. In order to get a well-supported decision on the further operation of aged transformers, relevant information would be necessary on the condition of the oil-paper insulation. We also know that the oil-paper insulation has almost always inhomogeneous condition considering the temperature, moistening and ageing processes. The classical methods (insulation resistance, loss tangent, etc.) characterise the insulation by single measured value. This single value is not sufficient for relevant characterisation and diagnosis of such a complex system with lots of tons of insulation and with almost always inhomogeneous distribution of temperature, moisture and ageing product. If we measure the polarization spectra in three states (new insulation in equilibrium and uniform polarization spectrum, new insulation with not uniform distribution of polarization spectrum, later reaching again the uniform distributions) the shape of three polarization spectra will be different. Comparing the polarization and classical methods of this three case we can realize that sometimes the “classical single values” are almost the same but the polarization spectra are different. The three response methods provide very practical information about the insulation system (e.g. moisture, ageing). The moistening and accumulation of ageing by-products change the distribution of interfacial polarisation in the range of long time-constants. These promising response methods measure the polarisation distribution in the range of long time-constant (with other words, in the low frequency range). The three test methods were: Return Voltage measurement (RVM), the C and tgδ measurement in range of some tens of mHz to 50 Hz (FDS=Frequency Domain Spectroscopy), and the measurement of DC charging and discharging currents (PDC=polarisation and depolarization currents) up to some thousands of sec. These equivalent methods (RVM, FDS and PDC) are able to follow the changing of condition of insulation contrary to classical methods. Therefore a little bit surprising that until now the convenient standards is missing considering the polarization methods. In an earlier Hungarian research work (Budapest University of Technology) almost all the necessary measurements have been realized, consequently we are in possession of fundamental data considering the polarization methods. This paper would like to show a short review about the RVM technique, the correct interpretation of RVM data and a case study for the checking of the efficiency of the oil reclamation with RVM technique.

Novel Characteristic Quantities for Determining the Moisture State of Oil-Impregnated Cellulose Insulation Using the Extended Debye Model

Despite the technical popularity of the dielectric response (DR) methods used for the nonintrusive diagnosis of the cellulose insulation condition, it is still considered a challenging and tough task so far to trustworthily interpret the details of the measurement results for further understanding and tracing the insulation changes behind. From a novel perspective of parametric study on the circuital DR equivalence of cellulosic insulation—extended Debye model (EDM), the new dielectric characteristic quantities and the quantification approach to determine the cellulosic moisture condition are investigated in the present study. First, a kind of tailor-made scale-down bushing model is prepared, with its cellulosic condenser body absorbing differently controlled quantities of water to simulate the dampness of real-life insulation, of which the frequency-domain spectroscopy (FDS) is measured under controlled temperatures. Then, the EDM is employed for a parameterized study on the FDS details with the moisture changes, by identifying the model parameters against the FDS curves, whereby the two time-domain dielectric spectra—polarization/depolarization current and return voltage—are transferred from the FDS so that a set of hybrid time–frequency characteristic quantities is proposed, which is revealed to be indicative of the dampness variations of cellulosic insulation. Finally, the principal component analysis and the feature recognition algorithm based on minimum distance are applied to determine the moisture content of the condenser body, which verifies the holistic effectiveness of the proposed method.

Estimation of Moisture Content in XLPE Cable Insulation Using Electric Modulus

In this article, a novel approach is proposed for accurate moisture content (MC) estimation of cross-linked polyethylene (XLPE) insulation employing electric modulus. Additionally, the variation of real and imaginary part of electric modulus in the argand plane is fitted using Cole-Cole (C-C) model for understanding of distributed relaxation behavior of XLPE insulation. Based on the frequency variation of dielectric parameters, five moisture sensitive parameters are identified and using these parameters, five empirical relationships are proposed for quantitative assessment of MC in XLPE insulation. It is observed that parameters obtained from the complex modulus spectrum yields better accuracy in estimation of MC compared to the parameters obtained from traditional frequency domain spectroscopy (FDS).

Fast noniterative data analysis method for frequency-domain near-infrared spectroscopy with the microscopic Beer–Lambert law

By using the microscopic Beer–Lambert law (MBL), we propose a fast and effective analysis method for retrieving the absorption coefficient changes of a sample from broadband frequency-domain diffuse optical spectroscopy (FD-DOS) or frequency-domain near-infrared spectroscopy (FD-NIRS) data. To verify the advantages of our proposed method, we built a broadband frequency-domain single-distance diffuse optical imaging (DOI) system for measuring the amplitude and the phase of transmitted light. Fifteen liquid phantoms with different absorption and scattering coefficients are prepared, and the changes in the absorption coefficient Δμa are retrieved by using the conventional nonlinear least-squares fitting (NLSF) and the proposed MBL methods. Results show that the computation time and the precision of the MBL method are much better than those of the NLSF method, respectively.

Study on Hydration and Dehydration of Ezetimibe by Terahertz Spectroscopy with Humidity-Controlled Measurements and Theoretical Analysis.

Understanding the solid-state transitions of active pharmaceutical ingredients (APIs) is essential for quality control since differences in their forms affect the bioavailability of APIs. Terahertz (THz) frequency-domain spectroscopy is suitable for such an application since it can sensitively probe the lattice phonon modes originating in the crystal structures. THz absorption spectra were obtained for ezetimibe (EZT) and ezetimibe monohydrate (EZT-MH), which have similar crystalline structures and belong to the same space group. The observed absorption spectrum of EZT matched well with the solid-state density functional theory (ss-DFT)-simulated spectrum for the structures at 0 K and room temperature (modeled using constrained unit cell volumes). However for EZT-MH, the ss-DFT spectrum of the room-temperature structure showed better correlation with the experimental THz spectrum than that of the simulated spectrum of the 0 K structures, suggesting that the EZT-MH crystal has greater anharmonic character. Gibbs free-energy curves were calculated, and EZT-MH was found to be more stable than pure EZT and water in a broad temperature range. The hydrate stability may be influenced by the existence of more hydrogen bonds in EZT-MH. The hydration and dehydration of EZT in a pure API tablet and formulation tablets were monitored using a THz spectrometer with a humidity-controlled sample chamber. The effect of the excipient in the formulation tablet on hydration was successfully confirmed by showing that the solid-state transition of the API with excipients is significantly slower than that without it. Under a relative humidity of 60%, hydration of EZT in a pure EZT tablet occurred in 200 min, while the hydration of EZT in a formulation tablet was 50 times slower.

Correction to: Parameter Recovery in Frequency-Domain Time-Resolved Fluorescence Spectroscopy; Resolution of the Prototropic Forms of 5-Carboxyfluorescein in the Physiological pH Range

Correction to: Parameter Recovery in Frequency-Domain Time-Resolved Fluorescence Spectroscopy; Resolution of the Prototropic Forms of 5-Carboxyfluorescein in the Physiological pH Range

Dielectric Response Model for Transformer Insulation Using Frequency Domain Spectroscopy and Vector Fitting

This paper proposes a rational approximation-based approach to find positive real parameters for the extended Debye model (EDM), aimed at condition assessment of insulation systems of power transformers. The EDM can model the slow and fast polarization phenomenon, including relaxation mechanisms with different relaxation times within a composite dielectric material. In the proposed approach, the complex permittivity of the transformer’s composite insulation is approximated via rational functions, as given by the vector fitting (VF) software tool, and the EDM parameters are identified from the obtained poles/residues. To guarantee positive real parameters, i.e., a physically realizable circuit, VF is internally modified to calculate the final residues of the rational approximation via a constrained linear least-squares problem without resorting to further post-processing algorithms, as in existing methods, hence without affecting fitting accuracy. The effectiveness of the parametrized EDM is demonstrated in two ways: (a) by reconstructing frequency domain spectroscopy (FDS) curves provided via measurements in new oil-immersed power transformers and (b) by the comparison of the calculated polarization current given by EDM versus real measurements in time domain. The achieved fitting accuracy in most of the cases is above 99 percent for the reconstructed FDS curves, while the polarization current waveform is reproduced with good agreement.

A BPNN Model-Based AdaBoost Algorithm for Estimating Inside Moisture of Oil–Paper Insulation of Power Transformer

The traditional method for transformer moisture diagnosis is to establish empirical equations between feature parameters extracted from frequency domain spectroscopy (FDS) and the transformer’s moisture content. However, the established empirical equation may not be applicable to a novel testing environment, resulting in an unreliable evaluation result. In this regard, it is acknowledged that FDS combined with machine learning is more suitable for estimating moisture content in a variety of test environments. Nonetheless, the accuracy of the estimation results obtained using the existing method is limited by the algorithm’s inability to generalize. To address this issue, we propose an AdaBoost algorithm-enhanced back-propagation neural network (BP_AdaBoost). This study creates a database by extracting feature parameters from the FDS that characterize the insulation states of the prepared samples. Then, using the BP_AdaBoost algorithm and the newly constructed database, the moisture estimation models are trained. Finally, the results of the estimation are discussed in terms of laboratory and field transformers. By comparing the proposed BP_AdaBoost algorithm to other intelligence algorithms, it is demonstrated that it not only performs better in generalization, but also maintains a high level of accuracy.

Influence of temperature on frequency domain spectroscopy detection of transformer bushings

Frequency domain spectroscopy (FDS) technology has been widely used to evaluate the insulation state of electrical equipment. However, there are still many in-adaptability when using FDS technology to evaluate the insulation state of transformer bushings. Especially, the result will be affected by temperature in the quantitative diagnosis of moisture degree. The influence of different temperatures on FDS detection data of transformer bushings has been studied to solve this problem in this paper. To obtain the FDS test data curves of transformer bushings at different temperatures, the transformer bushings experiment platform was built in the laboratory. And the bushing samples at different temperatures was measured by FDS detection method, which started at 30 °C and rising at 10 °C intervals until the temperature reached 80 °C. The result shows that the temperature will have a serious impact on the FDS detection data, especially the R0 and C5 in the extended Debye model. To solve the influence of temperature, FDS data correction curves with different temperature has been built in this paper. And its feasibility has been verified by FDS data of transformer bushings with 0% and 1.89% water content. The obtained results can promote the application of FDS method in the evaluation of transformer bushings insulation state.

Research on broadband equivalent modeling for the dielectric response of transformer bushings based on the improved artificial bee colony algorithm

As a new nondestructive diagnosis method of insulation state, frequency domain spectroscopy (FDS) has been widely used in the field detection of power equipment. However, the quantitative correlation between FDS measurement results and insulation state is not clear for transformer bushing, which seriously affects its detection effect. In order to solve this problem, the FDS modeling and parameter identification of transformer bushing has been studied in this paper. Firstly, the transformer bushing model was built in the laboratory, and the FDS measurement data of transformer bushings with different water content were collected. Then, a fusion algorithm of improved artificial bee colony algorithm and sequential quadratic programming algorithm was used to identify the parameters of transformer bushing model. After modeling and analyzing the FDS measured data, it is found that there is a good consistency between the measured spectrum of complex capacitance and the reconstructed spectrum of the model, and the goodness of fit of the model to the measured FDS exceeds 0.95. In addition, the identified values of resistance and capacitance in the equivalent model are within the reasonable range of 109Ω and 10−9F respectively. These results show that the extended Debye model and parameter identification algorithm can accurately model the frequency domain dielectric response of transformer bushing in a wide frequency range, and can provide an effective tool for more accurate analysis of transformer bushings insulation state.

Rapid Identification of Soybean Varieties by Terahertz Frequency-Domain Spectroscopy and Grey Wolf Optimizer-Support Vector Machine

Different soybean varieties vary greatly in their nutritional value and composition. Screening for superior varieties is also essential for the development of the soybean seed industry. The objective of the paper was to analyze the feasibility of terahertz (THz) frequency-domain spectroscopy and chemometrics for soybean variety identification. Meanwhile, a grey wolf optimizer-support vector machine (GWO-SVM) soybean variety identification model was proposed. Firstly, the THz frequency-domain spectra of experimental samples (6 varieties, 270 in total) were collected. Principal component analysis (PCA) was used to analyze the THz spectra. After that, 203 samples from the calibration set were used to establish a soybean variety identification model. Finally, 67 samples from the test set were used for prediction validation. The experimental results demonstrated that THz frequency-domain spectroscopy combined with GWO-SVM could quickly and accurately identify soybean varieties. Compared with discriminant partial least squares (DPLS) and particles swarm optimization support vector machine, GWO-SVM combined with the second derivative could establish a better soybean variety identification model. The overall correct identification rate of its prediction set was 97.01%.

Partial damp diagnostic method of oil-paper insulating bushing based on frequency domain spectroscopy

Partial damp diagnostic method of oil-paper insulating bushing based on frequency domain spectroscopy

A Method for Discriminating the Moisture Status of OIP Bushing Based on Dissado-Hill and GWO-HMM Model

An accurate discrimination on moisture status of oil-impregnated paper (OIP) bushings is crucial for the maintenance and replacement schedule of bushings. Based on frequency-domain spectroscopy (FDS) measurement and Dissado–Hill (DH) relaxation model, this article proposes a hybrid approach of hidden Markov model and gray wolf optimization (GWO-HMM) for MS estimation of bushings subjected to the ununiform moisture distribution and dynamic time-series modeling. First, simulation models of moisture diffusion and FDS of the OIP bushing were constructed using finite element modeling approach. Then, the GWO algorithm was employed to explore dielectric parameters influenced by moisture in the DH model. Then, the GWO-HMMs was adopted as a classification tool to discriminate the MS of the simulation and experimental data of the bushings. Classification results confirm that the average identification accuracies of the proposed method are 97.17% and 96.81% over these two datasets, which demonstrates the effectiveness of the proposed moisture estimate method for OIP bushings.