Spectral Modulation Research Articles

Fast hyperspectral single-pixel imaging via frequency-division multiplexed illumination.

Hyperspectral imaging that detects 3D spectra-spatial information has been used in a wide range of applications. Among reported techniques, multiplexed spectral imaging with a single-pixel detector provides as a photon-efficient and low-cost implementation; however, the previous spectral modulation schemes are mostly complicated and sacrifice the imaging speed. Here, we propose a fast and compact hyperspectral single-pixel imaging technique based on programmable chromatic illumination. A multi-wavelength LED array modulated by independent carriers achieves stable and accurate spectral modulation up to MHz in a frequency-division multiplexed manner, hence allowing the full use of the spatial light modulation speed. Additionally, we propose a multi-channel deep convolutional autoencoder network to reconstruct hyperspectral data from highly-compressed 1D measurement. Experimental reconstructions of 12 spectral channels and 64 × 64 pixels are demonstrated for dynamic imaging at 12 fps image rate. The proposed imaging scheme is highly extensible to a wide spectrum range, and holds potential for portable spectral imagers in low-light or scattering applications.

Quantitative measurement of corrosion at the nanoscale by in situ spectral modulation interferometry

This research study presents a novel application of a quantitative phase microscopy technique, spectral modulation interferometry (SMI), for in situ nanoscale characterization of corrosion of an aluminum alloy in real time. SMI offers high sensitivity, rapid image acquisition, and speckle-free images; thus, real-time quantification of surface topography evolution during corrosion can be obtained accurately to evaluate the temporally- and spatially-dependent corrosion rates. With an innovative additive-manufactured fluid cell, experiments were performed in situ under flowing solution conditions. Electrochemical tests via stepwise polarization and solution chemistry through collected aliquots of outflow solution were also performed alongside the nanoscale SMI experiment to simultaneously provide a corroborating corrosion rate measurement. Based on the quantitative 3D height profiles across the corroded surface, pit formation resulting from rapid local corrosion was predominant, appearing at different times and are heterogeneously distributed across the surface. The computed time-dependent dissolution rates of aluminum also varied as the experiment proceeded, with the combination of linear and nonlinear surface normal distributions. An initial mean linear dissolution rate of [0.40 ± 0.007] μmol m−2 s−1 transitioned to a more rapid mean rate of [1.95 ± 0.035] μmol m−2 s−1, driven by the anodic polarization. Dissolution rates from the three performed methods follow similar trends and there is the visibility of linking the nanoscale in situ SMI data to the electrochemical corrosion measurements and ex situ chemical solution analysis. At the end of the corrosion period, rates of 118 μmol m−2 s−1, 71 μmol m−2 s−1, and 2.45 μmol m−2 s−1 were obtained from electrochemical measurements, ex situ solution analyses, and in situ SMI corrosion measurement, respectively. Finally, these experimental results validate the applicability of SMI for in situ nanoscale characterization of a corroding alloy surface.

Quantitative study on the interference of OH radicalmeasurement in the air pollution complex

<p indent="0mm">OH radical is the most important gas-phase oxidant in the troposphere, which oxidizes the primary pollutants to secondary pollutants and causes the regional secondary pollution. Hence, it is crucial to acquire comprehensive knowledge of the radical chemistry to mitigate the secondary air pollution. However, the well-established photochemistry is insufficient to describe the chemical processes in the complex air mixture. For instance, the model severely underpredicts OH radical concentration in forest areas. <italic>In situ</italic> measurement of OH radical is an efficient way to quantify the atmospheric oxidation capacity and improve our understanding of atmospheric chemical mechanisms. While due to its high reactivity and low concentration, measurement of OH radical is one of the most challenging tasks in the atmospheric research. Laser-induced fluorescence (LIF) is the most widely used technique to measure OH due to its high sensitivity and selectivity. However, studies have shown that under high volatile organic compounds (VOCs) conditions, LIF technique based on traditional spectral modulation methods may suffer from interference, whose origin has not yet been known. In this study, based on the Peking University Laser-Induced Fluorescence system (PKU-LIF), a chemical modulation method was developed, to separate interference by quantifying the chemical background signals, through chemical removing of the ambient OH prior to be sampled into the detection cell, thereby enabling accurate measurement of OH radical. During the winter campaign in Beijing in 2016, simultaneous OH radical measurement based on both the chemical modulation method and the spectral modulation method was achieved for the first time, and two sets of OH radical concentrations were obtained. It was found that, the two sets of OH radical concentrations agreed within the combined uncertainties of 31% (correlation slope: 0.88±0.01), indicating that there was no significant interference in the measurement of OH based on the traditional spectral modulation method during this winter campaign. It is worth nothing that whether the LIF measurement of OH radicals will suffer from interference and the degree of interference highly depend on the specific instrument configurations and the environmental conditions. Therefore, when applying LIF technique to measure the concentration of OH radical under complex environments in the future, the chemical modulation method is required to achieve accurate OH measurement.

Optical Lock-in Spectrometry Reveals Useful Spectral Features of Temporal Light Modulation in Several Light Source Technologies

ABSTRACT This paper presents a study of the spectral characteristics of temporal light modulation in several technologies of lighting products. An optical lock-in spectrometer was designed for this purpose and integrated in a spectral radiant flux measurement facility. It was applied to incandescent and fluorescent lamps, as well as lamps based on white phosphor-converted LEDs and tunable RGB LEDs. The results are well correlated with the light emission processes of each technology. For incandescent lamps, the spectral modulation follows a 1/λ relationship in agreement with the blackbody radiation laws. Measurements performed on halophosphate and tri-phosphor tubes agree well with published data. The modulation and phase spectra of fluorescent lamps reveal a variable modulation rate across the visible range, directly related to the fluorescence lifetimes of the different luminophores, which were estimated from our data using a model of single exponential decay.The spectral modulation of white phosphor-converted LED lamps is nearly constant across the visible spectrum, demonstrating that their color parameters can be assessed from the lock-in modulation amplitude spectrum. In the case of tunable RGB LED lamps using PWM, the spectral modulation widely differs from the steady-state spectral distribution and changes with the user settings, confirming the possible occurrence of temporal color artifacts. Optical lock-in spectrometry can be used to improve spectral and color measurements of solid-state lighting, opening new opportunities for laboratory and remote sensing applications. Other foreseeable applications of optical lock-in spectrometry are also presented.

Spectrally modulated polarimetry with wavelength domain analysis.

Spectrally modulated Stokes polarimeters use a pair of high-order crystal retarders to generate a spectrally dependent modulation of the polarization of light. In these systems, the detected intensity versus wavenumber spectrum is usually referred to as a channeled spectrum, and the Fourier inversion of this spectrum allows the determination of Stokes parameters of light without needing any other mechanical or active device for polarization control. This work proposes a spectrally modulated polarimeter beyond the concept of a channeled wavenumber spectrum, so effectively detaching the spectral modulation from the Fourier analysis technique. The wavelength domain analysis we use is best suited for dispersive spectrometers offering intensity versus wavelength measurements. The technique is illustrated with the measurement of very small optical rotations produced by sucrose solutions. The proposed technique is easily extendable to spectrally modulated Mueller polarimeters.

Creating alignment echoes using a phase-shaped femtosecond laser pulse

In recent years, molecular alignment echoes induced by a pair of time-delayed femtosecond laser pulses have been proposed and have aroused wide research interest. However, we demonstrate that an alignment echo can be alternatively produced by a shaped femtosecond laser pulse with a V-style spectral phase modulation. The full, fractional, and imaginary alignment echoes are formed by the excitation of the tailored two time-delayed sub-pulses. Both the delay time and the ratio of intensity between the two sub-pulses can be easily manipulated by designing the modulation parameters to induce various types of echoes. We further show that the optimal ratio of intensity between the two sub-pulses, which results in the maximal alignment degrees of the full echo, closely correlates with the energy of the sub-pulse. When the pulse energy is relatively low, the maximal alignment degree of the full echo is obtained when the two sub-pulses have equal intensity. The optimal ratio of intensity increases with the excitation energy of the first pump pulse.

A secondary modulation mechanism for aerofoil tonal self-noise generation

Acoustic emission of a NACA 0012 aerofoil is investigated over a range of free-stream velocities. Acoustic spectra show a dominant tone and two sets of weaker side tones characterised by different frequency intervals. The frequency of the dominant tones in the acoustic spectra varies with velocity in a ladder-type structure. With increasing Reynolds number, the spectrum becomes progressively more broadband in nature. Through synchronised particle image velocimetry and acoustic measurements, the aeroacoustic noise generation mechanisms, resulting in different spectral characteristics and modulation types, are further investigated. A separation bubble and related significant velocity fluctuations are observed on the pressure side. Pressure side velocity spectra show characteristics similar to the acoustic ones, whereas velocity spectra on the suction side feature broadband characteristics. These findings confirm that noise emission is dominated by pressure side events for the Reynolds number range of this study, i.e. $2 \times 10^{5}$ – $7 \times 10^{5}$ . As the acoustic emission is defined by coherent flow structures, the proper orthogonal decomposition method is adopted to facilitate the understanding of the relation between the complex flow field and acoustic emission. Side tones in the acoustic spectra are attributed to two different modulation mechanisms in the aeroacoustic source region near the trailing edge. By aligning the sound pressure time history and the time coefficients of the dominant modes, the primary modulation of the dominant tone is found to be related to the amplitude modulation of the high-frequency velocity fluctuations associated with the acoustic feedback loop. A secondary modulation is attributed to periodic variation of the separation bubble and, therefore, variation in the roll-up of the shear layer, which results in a modulation of the amplitude of the velocity fluctuations associated with the convecting vortices at the trailing edge.

VO2:Ge based thermochromic solar absorber coatings

Flat plate solar collectors face the problem of overheating and the ensuing high thermal stresses and general collector damage lead to high maintenance costs. To address this challenge, absorber coatings with a passive optical switch at critically high operating temperatures are proposed. The remarkable near- and mid-infrared spectral modulation of thermochromic VO 2 makes it a promising material for smart selective absorbers with temperature dependent optical selectivity. Single VO 2 films readily achieve thermal emittance modulations of 40% points (p.p.) at a phase transition temperature of 66 °C. With 5 at.% Ge added to the VO 2 thin film, the transition temperature is increased to 82 °C. When VO 2 and VO 2 :Ge films are combined with a selective CuCoMnO x layer and an SiO 2 top layer, a thermal emittance modulation of 31 p.p. and 33 p.p. is retained, respectively. However, a simultaneous increase of the solar absorptance α sol by 7 p.p. and 5 p.p. limits the drop in collector efficiency at elevated temperatures. Simulations show that adding an absorber layer between the Al substrate and thermochromic VO 2 , decreasing solar absorptance over the phase transition is achieved. In line with these findings, a TiAlSiN//VO 2 :Ge//SiO 2 multilayer is prepared. Below the phase transition temperature, the efficiency of the thermochromic collector is similar to that of standard collectors. At the transition temperature, α sol decreases from 0.93 to 0.91 and ϵ th increases from 0.08 to 0.24. Such a decrease in α sol accompanied by an increase in ε th for increasing temperature has been achieved for the first time. The subsequent drop in selectivity, limits the maximum stagnation temperature of the thermochromic collector to 160 °C, 17 °C lower than for a standard collector in identical conditions. The absorber durability is confirmed through accelerated aging tests in dry and humid conditions alike. • Thermochromic VO 2 and VO 2 :Ge based selective absorber coatings are deposited. • ε th increases at 66 °C for VO 2 and 82 °C for VO 2 :Ge with 5 at.% Ge. • First absorber designs with both increasing ε th and decreasing α sol are reported. • High collector efficiencies and reduced stagnation temperatures are achieved. • Accelerated aging test in dry and humid conditions are successfully completed.

Passive OFE multi‐Gbps VLC transmission using POF as a feeder line

AbstractThis paper presents a novel in‐home communication system employing plastic optical fibers (POFs) acting both as transmission medium and access point for wireless links to user devices, hence no separate luminaires or optical–electrical–optical conversions is required. All active devices, such as lasers/detectors, can be remotely placed from the room ceilings, resulting in fully passive optical front‐ends, which is beneficial for maintenance and upgrading. We approach two concepts, the first aims to realize small but high‐capacity hotspots in a room, the second creates larger coverage areas to increase the number of users and allow user's mobility. The wavelength division multiplexing concept is used to maximize the wireless throughput and to serve different wireless access points on the ceilings from a single POF feeder. A transmission link comprising eye‐safe transceivers, 1‐mm core diameter step‐index POF, and free space link, achieving multi‐gigabit throughputs using spectral efficient modulation formats, is demonstrated.

Design of luminescence in (Sr,Gd)Li(Al,Mg)3N4:Eu2+ deep red phosphors via crystal field engineering for full-spectrum WLEDs

Red phosphor, with longer wavelength, is highly desirable for full-spectrum WLEDs. Targeted deep red phosphors (Sr,Gd)Li(Al,Mg)3N4:Eu2+ were designed from the initial model of SrLiAl3N4:Eu2+ by structural modification. The correlations among structural evolution, crystal-field environment, and luminescence properties were elucidated. Replacing Sr2+ with Gd3+ in (Sr,Gd)LiAl3N4:Eu2+ leads to the enhanced crystal field splitting, larger Stokes shift, and increased structural polyhedron distortion differences, consequently resulting in spectral red-shift and broadening. For further spectral tuning, Mg, with lower electronegativity, was also introduced to modify the local crystal structure, consequently resulting in a further red-shift towards 675 nm and enhanced photoluminescence intensity in (Sr,Gd)Li(Al,Mg)3N4:Eu2+. What's more, w-LEDs were fabricated by using blue LED chip, blue, green, red and deep red ((Sr,Gd)Li(Al,Mg)3N4:Eu2+) phosphors whose color rendering index were Ra 96.0 and R9 97.7. All above results demonstrate that the partial replacements of Sr2+ by Gd3+ and Al3+ by Mg2+ are effective methods for spectral modulation and (Sr,Gd)Li(Al,Mg)3N4:Eu2+ phosphors are suitable for high-quality full-spectrum WLEDs.

Fault feature extraction method of gear based on spectral amplitude modulation and autocorrelation analysis

Due to the strong noise interference and amplitude modulation effect, it is difficult to extract the impact characteristics of the fault gear from the spectrum of the fault signal. To tackle these issues, a gear fault feature extraction method based on spectral amplitude modulation (SAM) and autocorrelation analysis is proposed. First, the SAM method is used to decompose the signal into different components according to energy, and the optimal component with the most fault information is determined by combining kurtosis index and magnitude order selection range. Then, the optimal component is denoised using the autocorrelation function. Finally, the fault feature frequency is extracted by calculating the squared envelope spectrum of the denoised signal. The superiority of the method is verified by simulating the signal. Furthermore, the effectiveness and superiority of the method in gear fault diagnosis are verified by comparison with the fast kurtogram, cepstrum pre-whitening, and SAM.

Robust and swiftly multicolor Zn2+-electrochromic devices based on polyaniline cathode

Multifunctional electrochromic (EC) devices are very important for the current research on smart electronic devices. Herein, we report a robust and swiftly multicolor EC energy storage device design based on polyaniline (PANI) cathode. To achieve this goal, we systematically studied the EC properties of PANI in aqueous and organic Zn 2+ electrolytes for the first time. Compared with Zn 2+ aqueous electrolyte, the PANI film measured in organic Zn 2+ electrolyte delivered a fast-switching time (τ c /τ b = 2.0/2.4 s), high optical contrast (72.2%), high coloration efficiency (205 cm C −1 ), and good cycle stability (optical contrast remains 92.7% after 10,000 cycles). The excellent EC performance could be attributed to the good wettability of the interface between organic Zn 2+ electrolyte and PANI film, which supports rapid charge transfer kinetics. The PANI cathode in the organic Zn 2+ electrolyte also provided a multicolor electrochromism (light yellow, green and dark green) along with a high specific capacitance of 26.4 mAh m −2 and 72.2% spectral modulation range at a current density of 0.1 A m −2 , excellent rate capability of 86.4% capacity retention and 81.3% spectral modulation even at an 8-fold current-density increase. The laboratory prototype device also verified the EC energy storage performance of organic Zn 2+ intercalated/deintercalated PANI, confirming that they are suitable for highly robust multicolor EC energy storage applications. • The EC properties of PANI film in aqueous and organic Zn 2+ electrolytes for the first time were systematically revealed. • The PANI film delivered a fast-switching time, high optical contrast, high coloration efficiency, and excellent cycle stability (optical contrast remains 92.7% after 10,000 cycles). • The laboratory prototype device based on PANI showed a good EC performance as well as energy storage.

Dynamic spectroscopic characterization for fast spectral variations based on dual asynchronous undersampling with triple optical frequency combs

A dynamic spectroscopic characterization scheme for fast spectral variations based on dual asynchronous undersampling with triple optical frequency combs is proposed and demonstrated here. The tri-comb scheme leverages two of the combs to simultaneously sample the fast spectral variations applied to the third comb. The repetition rate differences for both pairs of combs are relatively small, in order to maintain sufficiently large spectral measurement ranges. The corresponding sampling rates could be much slower than a fast time-varying spectral modulation to be measured. Nevertheless, despite of the frequency ambiguity in the undersampled dual-comb results, it is shown that it is possible to identify the original frequency of spectral variations based on the difference in the undersampled frequency domain information. Experimental results show that our scheme can increase the frequency demodulation range by at least two orders of magnitude while retaining the accuracy and spectral measurement range of similar DCS schemes. Demonstration based on a tri-comb mode-locked fiber laser is carried out to measure the dynamic spectral response from a fiber Bragg grating (FBG), to which up to 110 kHz of vibration is applied. This scheme could further extend the capability of dual-comb spectroscopic techniques, and it also illustrates the potential of the low-complexity multi-comb laser source for novel yet practical applications.

Pulse shaping by spectral-domain polarization gratings.

We consider the spectral-domain counterparts of spatial-domain polarization gratings and study their effect on the temporal evolution of femtosecond-scale light pulses. These devices divide an incident light pulse to several orders via spectral polarization modulation, permitting pulse splitting and shaping with controlled time-domain polarization dynamics.

Spectral processing deficits appear to underlie developmental language disorders

More than 50 years of research has suggested something is awry in the auditory functioning of children with language disorders, yet there remains no consensus on exactly what the problem is. To address this gap in knowledge, we have proposed a model termed the Auditory Bases of Phonological Acquisition. This model is based on the language feature known as dual patterning, which describes two levels of structure: one consisting of meaningful words and how they can be combined and the other consisting of word-internal, meaningless units and how they can be combined. We hypothesized that acquisition of this second level—phonological structure—is most affected by deficits in auditory functions, especially those involving the spectral domain. To test this hypothesis, we assessed children with and without language disorders between 7 and 10 years old on their abilities to detect spectral and temporal modulation and to perform relevant language tasks. The largest group differences were found at younger ages for spectral modulation detection (SMD). SMD thresholds were also found to correlate most strongly with language measures, especially those based on phonological structure. Outcomes suggest that keen sensitivity to spectral structure in the speech signal is necessary for acquiring precise phonological representations.

Portable automated rapid testing in Spanish: Validation and norms of central hearing processes on a Mexican population

Portable automated rapid testing of central auditory processing measures implemented in the app PART (https://braingamecenter.ucr.edu/games/p-a-r-t/) were adapted for Spanish-speaking populations. This adaptation included the generation of a corpus equivalent to the CRM to test a speech-on-speech masking task, in addition to frequency modulation tasks, temporal gap, temporal, spectral and spectro-temporal modulations, and pure tone detection in noise. This battery of tests was selected for its potential clinical utility and its previous validation across several settings including remote testing with participant-owned headphones, tablets or smartphones. Here, we tested 96 young adults in Mexico City (ages 18–45) with normal hearing. As expected, distributions of scores were similar to previously reported norms for the English-language version of the battery in English-speaking populations. Scores for the newly developed Spanish-language speech-on-speech masking tests were also similar to data previously reported in English-speaking populations. Furthermore, there was no significant effects of age or subjective hearing handicap (HHIA) in any of the tests. This study can be now used as a normative dataset, and the adapted battery can be used as an accsessible tool (available freely online) for valid and reliable testing of central auditory processing measures in Spanish-language populations.

Topographical distribution of spectrotemporal receptive field properties in the bat primary auditory cortex

Spectrotemporal modulations are a prominent feature of natural sounds including animal vocalizations and human speech. Echolocating bats must process spectrotemporal cues such as echo delays and spectrum properties to navigate their environment; however, the neuronal networks in the primary auditory cortex (A1) that process features of natural sounds remain incompletely understood. Here, we investigated the topographical distribution of tuning properties throughout A1 of Mexican free-tailed bats. While the majority of the bats’ A1 neurons are tuned to downward FM sweeps, the entire cortex is required to capture and categorize patterns of spectral details embedded within each biosonar echo. We captured the neural responses of the A1 to complex acoustic stimuli using linear, 16-channel translaminar microelectrode arrays. We analyzed 145 spectrotemporal receptive fields (STRFs) across A1, as well as the spectral and temporal modulation transfer functions (sMTF and tMTF, respectively) to determine neural tuning preferences in the spectral and temporal dimensions. We found evidence of neuronal sub-categories that were classified as having simple or complex (multi-peaked) STRFs. For simple STRFs, sMTFs and tMTFs were evaluated as a function of best frequency, and spatial location and revealed tonotopic trends similar to those reported for non-specialized animals.

Assessing bottom-up and top-down effects on speech-in-noise recognition by adolescents with normal hearing or cochlear implants

The terms “bottom-up” and “top-down” effects are commonly used when measuring speech recognition in competing sounds to refer to listeners’ abilities to exploit details of the sensory input and to use linguistic constraints, respectively. The magnitude of each effect has been assessed for listeners with cochlear implants, but typically in separate studies. We varied the availability of bottom-up and top-down information in a single experiment by generating sentences with three kinds of clause structure, from less to more restrictive, in three different babbled backgrounds, with sex and number of talkers varied. Next we examined the factors that might explain abilities to use bottom-up and top-down information in speech-in-noise recognition. Predictor variables included spectral modulation detection abilities, lexical-syntactic knowledge, verbal working memory, and phonological sensitivity. Participants were 14-year-olds with normal hearing (NH; 46) or with cochlear implants (CIs; 46). Adolescents with cochlear implants showed deficits in their abilities both to segregate the target speech from background babble and to benefit from restrictive syntactic constructions. For adolescents with NH, only verbal working memory explained variability in abilities to use bottom-up or top-down information in speech-in-noise recognition. For adolescents with CIs, only spectral modulation detection abilities explained their abilities to apply these effects.

Engineering nonlinear epileptic biomarkers using deep learning and Benford\u2019s law

In this study, we designed two deep neural networks to encode 16 features for early seizure detection in intracranial EEG and compared them and their frequency responses to 16 widely used engineered metrics to interpret their properties: epileptogenicity index (EI), phase locked high gamma (PLHG), time and frequency domain Cho Gaines distance (TDCG, FDCG), relative band powers, and log absolute band powers (from alpha, beta, theta, delta, low gamma, and high gamma bands). The deep learning models were pretrained for seizure identification on the time and frequency domains of 1 s, single-channel clips of 127 seizures (from 25 different subjects) using “leave-one-out” (LOO) cross validation. Each neural network extracted unique feature spaces that were interpreted using spectral power modulations before being used to train a Random Forest Classifier (RFC) for seizure identification. The Gini Importance of each feature was calculated from the pretrained RFC, enabling the most significant features (MSFs) for each task to be identified. The MSFs were extracted to train another RFC for UPenn and Mayo Clinic’s Seizure Detection Kaggle Challenge. They obtained an AUC score of 0.93, demonstrating a transferable method to identify and interpret biomarkers for seizure detection.

Generation of pulse trains with nonconventional temporal correlation properties

We apply time dependent spectral phase modulation to generate pulse trains that are spectrally and temporally partially coherent in an ensemble averaged sense. We consider, in particular, quadratic spectral phase modulation of Gaussian pulses, and demonstrate two particular types of nonuniformly correlated pulse trains. The controlled partial temporal coherence of the nonstationary fields is generated using a pulse compressor and experimentally verified with frequency resolved optical gating (FROG). We show that the correlation characteristics of such pulse trains can be retrieved directly from the FROG spectrograms provided one has certain a priori knowledge of the pulse train. Our results open a pathway for experimental confirmation of several correlation induced effects in the temporal domain.