Spectral Modulation Research Articles

Experimental demonstration of a temporal pulse shaping method based on nonlinear chirp

We present a general temporal shaping method based on spectral phase-only modulation for ultrafast laser sources. We explain the working principle of this technique and use it experimentally to generate a ramp-shaped pulse at the output of a laser source delivering 30 μJ 200 fs pulses at 500 kHz. This pulse is then launched inside a multipass cell to demonstrate non-linear wavelength shifting. A spectral tunability of 11 nm around the center wavelength of 1030 nm is achieved.

Optimization of femtosecond fiber laser pulses with selected machine-learning algorithms

We investigate the possibility of optimization of ultrashort laser pulses by spectral phase modulation using machine learning algorithms. We compare the efficiency of two algorithms: Grey Wolf Optimizer and Genetic Algorithm.

Optical Spectral Modulation of CdSe/ZnS Quantum Dot-Based UAV Identification

Optical Spectral Modulation of CdSe/ZnS Quantum Dot-Based UAV Identification

Aluminum-based plasmonic metasurface for computational spectrometry with full coverage of visible light.

Reconstructive spectrometers/spectral cameras have immense potential for portable applications in various fields, including environmental monitoring, biomedical research and diagnostics, and agriculture and food safety. However, the performance of these spectrometers/spectral cameras is severely limited by the operational bandwidth, spectral diversity, and angle sensitivity of the spectral modulation devices. In this work, we propose a compact spectrometer based on plasmonic metasurfaces that operate across the entire visible wavelength range, covering wavelengths from 400 to 750 nm. We experimentally demonstrate the effective spectral reconstruction achieved by the designed metasurface spectrometer, exhibiting angle tolerance to the incident light within the range of ± 12°. Our results highlight the potential for constructing broadband, large field-of-view hyperspectral cameras.

Dispersive wave manipulation by the spectral Heaviside step phase modulation

We investigate the dispersive waves (DWs) emitted from shaped pulses with spectral Heaviside step phases (HSPs). The spectrally HSP-modulated pulse exhibits a unique double-peak structure, where the intensity and separation of the twin peaks are determined by the modulation depth and frequency detuning. By tailoring the parameters of the HSP suitably, we can control the DW emission with regard to resonant frequency and conversion efficiency. As the intensity ratio or relative separation of neighboring peaks is elaborately chosen, the DW emission can be effectively boosted, or a solitonic cage can be constructed for realizing temporal reflections and refractions associated with spectral broadening and multi-peak spectra of the output DWs. These findings offer a straightforward and efficient approach for controlling the DW emission, which is highly relevant to the advancement of supercontinuum generation and wavelength conversion technology.

Asynchronies in Auditory and Language Development Obscure Connections to Phonological Deficits in Children

Purpose: For half a century, psycholinguists have been exploring the idea that developmental language disorders may have their roots in suprathreshold auditory dysfunctions, but results are inconclusive. Typical studies focus on relationships between temporal processing abilities and measures of various language skills at the time of testing, a proximal account. This study expanded that focus by testing three novel hypotheses: (a) Spectral processing impairments may be more responsible for language-learning deficits than temporal processing impairments. (b) Phonological sensitivity is likely the specific language skill most strongly affected by auditory (dys)functions. (c) Poor auditory functioning observed at young ages may wholly or partly recover, reducing the magnitude of relationship between those recovered functions and persistent language skills at older ages. Method: Sixty-six children (31 boys, 35 girls) 7–10 years of age participated: 36 with typical language and 30 with reading or speech disorder; from this sample two subsamples were designated: younger (7–8 years) and older (9–10 years) children. Four auditory measures were obtained of spectral modulation detection (0.5 and 2.0 cycles per octave) and temporal modulation detection (16 and 64 Hz). Four language measures were obtained, two lexicosyntactic and two phonological. Results: Younger children showed deficits in all auditory skills, but most strongly for spectral modulation detection at 0.5 cycles per octave; that measure was the only one for which older children showed deficits. Spectral modulation detection was the auditory function most strongly correlated with a language skill, and that language skill was phonological sensitivity. Conclusions: Early impairments in suprathreshold auditory functions, especially spectral processing, interfere with language acquisition at early stages, especially phonological sensitivity. Although auditory functions can recover to some extent, impairments in language skills persist, indicating that a distal account may more appropriately explain the relationship. Supplemental Material: https://doi.org/10.23641/asha.24730128

Dynamic Spectral Modulation Enabled by Conductive Polymer-Integrated Plasmonic Nanodisk-Hole Arrays.

The electrically driven optical performance modulation of the plasmonic nanostructure by conductive polymers provides a prospective technology for miniaturized and integrated active optoelectronic devices. These features of wafer-scale and flexible preparation, a wide spectrum adjustment range, and excellent electric cycling stability are critical to the practical applications of dynamic plasmonic components. Herein, we have demonstrated a large-scale and flexible active plasmonic nanostructure constructed by electrochemically synthesizing nanometric-thickness conductive polymer onto spatially mismatched Au nanodisk-hole (AuND-H) array on the poly(ethylene terephthalate) (PET) substrate, offering low-power electrically driven switching of reflective light in a wide wavelength range of 550-850 nm. The composite structure of the polymer/AuND-H array supports multiple plasmonic resonance modes with strong near-field enhancement and confinement, which provides an excellent dynamic spectral modulation platform. As a result, the PPy/AuND-H array achieves 18.4% reversible switching of spectral intensity at 780 nm and speedy response time, as well as maintains a stable dynamic modulation range at two-potential cycling between -0.6 and 0.1 V after 200 modulation cycles. Compared to the case of the PPy/AuND-H array, the PANI/AuND-H array obtains a more extensive intensity modulation of 25.1% at 750 nm, which is attributed to the significant differences in the extinction coefficient between the oxidized and reduced states of PANI, but its modulation range degrades apparently after 20 cycles driven at applied voltages between -0.1 and 0.8 V. Additionally, the cycling stability could be further improved by reducing the modulation voltage range. Our proposed electromodulated composite structure provides a promising technological proposal for dynamically plasmonic reconfigurable devices.

Index modulation-based efficient technique for underwater wireless optical communications

This paper presents and experimentally demonstrates energy and spectral efficient index modulation (IM) scheme for underwater wireless optical communications (UWOC). The proposed scheme utilizes IM with direct-current optical orthogonal frequency division multiplexing which is based on discrete Hartley transform (DHT-OFDM) instead of conventional discrete Fourier transform-based OFDM (DFT-OFDM). Although DHT-OFDM has been previously studied in various applications such as optical and wireless communications, this is the first work that practically investigates it with the UWOC system. In this paper, we first demonstrate that DHT-OFDM can inherit all advantages of conventional DFT-OFDM with less computational complexity and higher robustness to the inter-carrier interference (ICI). Therefore, the DFT process is employed at receiving end instead of DHT to guarantee low-complexity frequency-domain estimation and equalization processing and ensure that shorter required pilot overheads are required to track the UWOC channel effects. Furthermore, IM-DHT-OFDM has been presented to provide better flexibility of spectral and energy efficiency (SE/EE) to UWOC and offers higher robustness to the ICI effect as the transmitted data is modulated and physically transmitted via some active subcarriers while others must be kept idle. For instance, IM-DHT-OFDM provides up to 50% spectral efficiency improvement compared to DFT-OFDM and DHT-OFDM with better performance over the UWOC system. Simulation and experimental tests, performed over 2-meter UWOC, are provided to demonstrate the outperformance of the proposed schemes against the existing benchmarks in terms of bit error rate (BER), achieved SE, peak-to-average ratio (PAPR), and computational complexity.

Periodical Ultra-Modulation of Broadened Laser Spectra in Dielectrics at Variable Ultrashort Laser Pulsewidths: Ultrafast Plasma, Plasmonic and Nanoscale Structural Effects

Self-phase modulation (SPM) broadening of prompt laser spectra was studied in a transmission mode in natural and synthetic diamonds at variable laser wavelengths (515 and 1030 nm), pulse energies and widths (0.3–12 ps, positively chirped pulses), providing their filamentary propagation. Besides the monotonous SPM broadening of the laser spectra versus pulse energy, which was more pronounced for the (sub)picosecond pulsewidths and more nitrogen-doped natural diamond with its intra-gap impurity states, periodical low-frequency modulation was observed in the spectra at the shorter laser pulsewidths, indicating dynamic Bragg filtering of the supercontinuum due to ultrafast plasma and nanoplasmonic effects. Damping of broadening and ultra-modulation for the longer picosecond pulsewidths was related to the thermalized electron-hole plasma regime established for the laser pulsewidths longer, than 2 ps. Unexpectedly, at higher pulse energies and corresponding longer, well-developed microfilaments, the number of low-intensity, low-frequency sideband spectral modulation features counterintuitively increases, thus indicating dynamic variation of the periods in the longitudinal plasma Bragg gratings along the filaments due to prompt secondary laser–plasmon interactions. The underlying sub- and/or near-wavelength longitudinal nanoscale Bragg gratings produced by femtosecond laser pulses in this regime could be visualized in less hard lithium niobate by atomic force microscopy cross-sectional analysis in the correlation with the corresponding sideband spectral components, supporting the anticipated Bragg filtering mechanism and envisioning the corresponding grating periods.

Lenslet-array-based snapshot hyperspectral imaging polarimeter using opposite spectral modulation

We present a lenslet-array-based snapshot hyperspectral imaging polarimeter (LSHIP) that integrates spectral polarization modulation (SPM) into integral field imaging spectrometry. The LSHIP can simultaneously acquire three-dimensional spatial and spectral data-cubes for linear Stokes parameters in a single snapshot without any moving parts. The dual-scan technique is achieved through a custom-fabricated Wollaston prism that can simultaneously generate pairs of micro-spectra with opposite spectral modulations, maintaining the system's intrinsic spectral resolution during intensity spectrum reconstruction. We describe the principle and experimental setup of the LSHIP, as well as the system calibration and validation using measurements of polarized color blocks and a sphere made of polytetrafluoroethylene material. The prototype can cover the spectral range of 450–650 nm with 157 sample points, capturing approximately 3700 SPM micro-spectra spatially distributed on the detector in a single integration time. These results demonstrate the potential of the LSHIP for various applications, including remote sensing and biomedical diagnosis.

Soliton and bound-state soliton mode-locked fiber laser based on polarization-dependent grating.

We demonstrate the generation of solitons and bound-state solitons in a passively mode-locked fiber laser based on the nonlinear polarization rotation effect by polarization-dependent helical grating. The CO2-laser-inscribed grating has a high polarization-dependent loss of 24.4 dB at 1558.4 nm, which has facilitated the achievement of stable mode locking. The soliton laser could generate 548.9 fs pulses at 1560.59 nm with a spectrum bandwidth of 5.45 nm and a signal-to-noise ratio of 75.2 dB. Through adjustment of the polarization controller and pump power, a bound-state soliton mode-locked pulse with a spectral modulation period of 3.11 nm was achieved and the temporal interval between the two solitons was 2.19 ps. Furthermore, its repetition rate can be easily manipulated by varying the pump power. The results indicated that the polarization-dependent helical grating is an excellent polarizer that could be applied in an ultrafast fiber laser.

Exploring the Ability of Hubble Space Telescope WFC3 G141 to Uncover Trends in Populations of Exoplanet Atmospheres through a Homogeneous Transmission Survey of 70 Gaseous Planets

We present analysis of the atmospheres of 70 gaseous extrasolar planets via transit spectroscopy with Hubble’s Wide Field Camera 3 (WFC3). For over half of these, we statistically detect spectral modulation that our retrievals attribute to molecular species. Among these, we use Bayesian hierarchical modeling to search for chemical trends with bulk parameters. We use the extracted water abundance to infer the atmospheric metallicity and compare it to the planet’s mass. We also run chemical equilibrium retrievals, fitting for the atmospheric metallicity directly. However, although previous studies have found evidence of a mass–metallicity trend, we find no such relation within our data. For the hotter planets within our sample, we find evidence for thermal dissociation of dihydrogen and water via the H− opacity. We suggest that the general lack of trends seen across this population study could be due to (i) the insufficient spectral coverage offered by the Hubble Space Telescope’s WFC3 G141 band, (ii) the lack of a simple trend across the whole population, (iii) the essentially random nature of the target selection for this study, or (iv) a combination of all the above. We set out how we can learn from this vast data set going forward in an attempt to ensure comparative planetology can be undertaken in the future with facilities such as the JWST, Twinkle, and Ariel. We conclude that a wider simultaneous spectral coverage is required as well as a more structured approach to target selection.

Dual-Channel Switchable Metasurface Filters for Compact Spectral Imaging with Deep Compressive Reconstruction.

Spectral imaging technology, which aims to capture images across multiple spectral channels and create a spectral data cube, has been widely utilized in various fields. However, conventional spectral imaging systems face challenges, such as slow acquisition speed and large size. The rapid development of optical metasurfaces, capable of manipulating light fields versatilely and miniaturizing optical components into ultrathin planar devices, offers a promising solution for compact hyperspectral imaging (HSI). This study proposes a compact snapshot compressive spectral imaging (SCSI) system by leveraging the spectral modulations of metasurfaces with dual-channel switchable metasurface filters and employing a deep-learning-based reconstruction algorithm. To achieve compactness, the proposed system integrates dual-channel switchable metasurface filters using twisted nematic liquid crystals (TNLCs) and anisotropic titanium dioxide (TiO2) nanostructures. These thin metasurface filters are closely attached to the image sensor, resulting in a compact system. The TNLCs possess a broadband linear polarization conversion ability, enabling the rapid switching of the incidence polarization state between x-polarization and y-polarization by applying different voltages. This polarization conversion facilitates the generation of two groups of transmittance spectra for wavelength-encoding, providing richer information for spectral data cube reconstruction compared to that of other snapshot compressive spectral imaging techniques. In addition, instead of employing classic iterative compressive sensing (CS) algorithms, an end-to-end residual neural network (ResNet) is utilized to reconstruct the spectral data cube. This neural network leverages the 2-frame snapshot measurements of orthogonal polarization channels. The proposed hyperspectral imaging technology demonstrates superior reconstruction quality and speed compared to those of the traditional compressive hyperspectral image recovery methods. As a result, it is expected that this technology will have substantial implications in various domains, including but not limited to object detection, face recognition, food safety, biomedical imaging, agriculture surveillance, and so on.

Nonlinear characterization of enhanced and generalized Hjorth’s feature space for bearing condition monitoring

The degradation assessment of rolling bearings provides a reasonable maintenance plan for the safe operation of mechanical equipment. The general strategy for bearing condition monitoring is to construct a health indicator (HI) to characterize different degradation stages. A preferable HI that can sensitively detect initial faults and track machine degradation is crucial to developing timely maintenance strategies for mechanical equipment to avoid catastrophic accidents. However, many developed and reported HIs are still insensitive to early faults, resulting in delayed maintenance schedules. To identify the incipient defects as early as possible, a novel HI constructed by nonlinear characterization of enhanced and generalized Hjorth’s feature space based on extended probability entropy is proposed in this paper. Firstly, the time-frequency spectral amplitude modulation helps to enhance the characteristics of the original signal with the amplitude editing in the time-frequency domain. Then, three new features of generalized Hjorth’s parameter combinations are designed and combined with other similar feature combinations to construct a high-dimensional enhanced and generalized Hjorth’s feature space. On this basis, a set of low-dimensional sensitive features is obtained by nonlinearly characterizing high-dimensional features through extended probability entropy after these features are standardized. Finally, a novel HI is developed by calculating the distance between the minimum volume ellipse (MVE) center of the low-dimensional feature subspace based on nonlinear characterization and the low-dimensional feature vector of the real-time monitoring signal. The performance of the proposed approach is verified in three cases, whose experimental results indicate that the proposed HI is more sensitive and significant in detecting early faults compared to some current HIs.

Behavioral characterization of the cochlear amplifier lesion due to loss of function of stereocilin (STRC) in human subjects

Loss of function of stereocilin (STRC) is the second most common cause of inherited hearing loss. The loss of the stereocilin protein, encoded by the STRC gene, induces the loss of connection between outer hair cells and tectorial membrane. This only affects the outer hair cells (OHCs) function, involving deficits of active cochlear frequency selectivity and amplifier functions despite preservation of normal inner hair cells. Better understanding of cochlear features associated with mutation of STRC will improve our knowledge of normal cochlear function, the pathophysiology of hearing impairment, and potentially enhance hearing aid and cochlear implant signal processing. Nine subjects with homozygous or compound heterozygous loss of function mutations in STRC were included, age 7–24 years. Temporal and spectral modulation perception were measured, characterized by spectral and temporal modulation transfer functions. Speech-in-noise perception was studied with spondee identification in adaptive steady-state noise and AzBio sentences with 0 and -5 dB SNR multitalker babble. Results were compared with normal hearing (NH) and cochlear implant (CI) listeners to place STRC−/− listeners’ hearing capacity in context. Spectral ripple discrimination thresholds in the STRC−/− subjects were poorer than in NH listeners (p < 0.0001) but remained better than for CI listeners (p < 0.0001). Frequency resolution appeared impaired in the STRC−/− group compared to NH listeners but did not reach statistical significance (p = 0.06). Compared to NH listeners, amplitude modulation detection thresholds in the STRC−/− group did not reach significance (p= 0.06) but were better than in CI subjects (p < 0.0001). Temporal resolution in STRC−/− subjects was similar to NH (p = 0.98) but better than in CI listeners (p = 0.04). The spondee reception threshold in the STRC−/− group was worse than NH listeners (p = 0.0008) but better than CI listeners (p = 0.0001). For AzBio sentences, performance at 0 dB SNR was similar between the STRC−/− group and the NH group, 88 % and 97 % respectively. For -5 dB SNR, the STRC−/− performance was significantly poorer than NH, 40 % and 85 % respectively, yet much better than with CI who performed at 54 % at +5 dB SNR in children and 53 % at + 10 dB SNR in adults. To our knowledge, this is the first study of the psychoacoustic performance of human subjects lacking cochlear amplification but with normal inner hair cell function. Our data demonstrate preservation of temporal resolution and a trend to impaired frequency resolution in this group without reaching statistical significance. Speech-in-noise perception compared to NH listeners was impaired as well. All measures were better than those in CI listeners. It remains to be seen if hearing aid modifications, customized for the spectral deficits in STRC−/− listeners can improve speech understanding in noise. Since cochlear implants are also limited by deficient spectral selectivity, STRC−/− hearing may provide an upper bound on what could be obtained with better temporal coding in electrical stimulation.

Controlling beam dynamics with spectral quadratic phase modulation in the fractional Schrödinger equation.

The propagation dynamics of Gaussian beams and finite energy Airy beams with spectral quadratic phase modulation (QPM) modeled by the fractional Schrödinger equation (FSE) are numerically investigated. Compared with beam propagation in the standard Schrödinger equation, the focusing property of beams under FSE is influenced by the QPM coefficient and the Lévy index. For symmetric Gaussian beams, the focusing position increases and the focusing intensity decreases for the larger QPM coefficient or smaller Lévy index. For asymmetric Airy beams, multiple focusing positions occur, and the tendency of focusing intensity is opposite to that of Gaussian beams. Our results show the promising application of the FSE system for optical manipulation and optical splitting by controlling the QPM.

Parietal cortical alpha/beta suppression during prospective memory retrieval.

Prospective memory (PM) impairment is among the most frequent memory complaints, yet little is known about the underlying neural mechanisms. PM for a planned intention may be achieved through strategic monitoring of the environment for cues, involving ongoing attentional processes, or through spontaneous retrieval. We hypothesized that parietal spectral power modulation accompanies prospectively encoded intention retrieval, irrespective of PM retrieval approach. A cognitively engaging arithmetic-based ongoing task (OGT) was employed to encourage spontaneous retrieval, with a focal, internally generated PM cue to eliminate OGT/PM trial differentiation based on perceptual or conceptual PM cue features. Two PM repetition frequencies were used to vary the extent of strategic monitoring. We observed a transient parietal alpha/beta spectral power reduction directly preceding the response, which was distinguishable on a single trial basis, as revealed by an OGT/PM trial classification rate exceeding 70% using linear discriminant analysis. The alpha/beta idling rhythm reflects cortical inhibition. A disengagement of task-relevant neural assemblies from this rhythm, reflected in alpha/beta power reduction, is deemed to increase information content, facilitate information integration, and enable engagement of neural assemblies in task-related cortical networks. The observed power reduction is consistent with the Dual Pathways model, where PM strategies converge at the PM retrieval stage.

Perceptual and neural measures of spectral and temporal cue strength in speech perception

Many listeners experience difficulty understanding speech in noise that may result from limited ability to use important acoustic cues, such as spectral and temporal features. One way of assessing deficits related to perceptual or neural processing is to enhance or degrade specific cues and measure the impact on behavior or neural coding. To do so, we combined a target-word identification task with simultaneous EEG measurement in 20 young and 20 older normal-hearing listeners. Coordinate Response Measure sentences were manipulated in the spectral modulation (SM) and temporal modulation (TM) domains (neutral, degraded, enhanced) and presented in four-talker babble in both passive and attention conditions. Multivariate temporal response function (mTRF) analyses were used to evaluate cortical tracking of the speech envelope. Speech perception in younger listeners improved for both SM and TM enhancement, but older listeners only benefited from TM enhancement. Both groups showed stronger cortical tracking for attention versus passive listening, but only older listeners showed tracking changes across SM and TM conditions. Cortical tracking and perceptual results in older listeners are consistent with cognitive aging theories suggesting that cortical processing changes with age involve both compensation and dedifferentiation to help maintain auditory perceptual performance.

Implications of Narrow Spectra of Fast Radio Bursts

Fast radio bursts (FRBs) are millisecond-duration radio transients with extremely high brightness temperatures at cosmological distances, and the physical origin and the radiation mechanism of FRBs are still unknown. The observed spectral bandwidth of some FRBs appeared narrow compared with their peak frequencies, which could be used to constrain the radiation mechanism and the astrophysical environment of FRBs. In this work, we investigate some of the possible physical origins of the narrow spectra from the perspectives of intrinsic radiation mechanisms, coherent processes, radiative transfers, and interference processes. We find that: (1) If the observed narrow spectra of FRBs are attributed to the intrinsic radiation mechanism by a single charged particle, the particle’s deflection angle should be much smaller than the radiation beaming angle. (2) Coherent process can cause narrow spectra. For the bunching mechanism, the narrow spectra might arise from the radiating bunches with a quasiperiodic distribution. For the maser mechanism, the negative absorption process can naturally cause a narrow spectrum. (3) Most absorption and scattering processes seem not to significantly change the observed spectra based on the current observation of some FRB repeaters. (4) Scintillation and plasma lensing in the FRB source environment can modulate the spectra, leading to the narrow spectra and the burst-to-burst variation of spectra. A planet-like object can generate the spectral modulation via gravitational lensing at the gigahertz band, but the observed burst-to-burst variation of the spectra does not support this scenario.

Influence of optical surface distortion in a cylindrical Offner stretcher on the far-field signal-to-noise ratio.

In a stretcher, the surface distortion of the optical elements can introduce spectral phase modulations into the laser, which can affect the laser's signal-to-noise ratio. In this paper, by combining ray tracing methods and angular spectrum diffraction methods, the impact of the mid-frequency surface distortion of the optical elements in an cylindrical Offner stretcher on the far-field signal-to-noise ratio of the laser is simulated. The results show that reducing the spatial chirp on the convex cylindrical mirror can effectively improve the far-field signal-to-noise ratio of the laser, and two methods to improve the far-field signal-to-noise ratio are presented.