Spectral Envelope Research Articles

Gain-Switched Optical Frequency Comb Source Using a Hybrid Integrated Self-Injection Locking DFB Laser

A hybrid integrated gain-switched (GS) optical frequency comb (OFC) source with eight pure continuous comb lines within 3-dB of the spectral envelope peak and a narrow linewidth of 615 kHz is proposed and demonstrated by coupling a silicon nitride microring reflector (MRR) to a commercially available distributed feedback (DFB) laser. Due to the self-injection locking effect induced by the MRR, the carrier-to-noise ratio (CNR) and phase correlation between comb lines are greatly enhanced. In addition, benefiting from the unique merit of gain switching, the resultant comb can also achieve a large frequency spacing adjustment range from 5.55 GHz to 8.7 GHz. This work explores a cost-efficient and robust OFC source for numerous applications, such as radio over fiber (RoF) and coherent optical communication.

Forward masking of spectrotemporal modulation detection.

Recent work has suggested that there may be specialized mechanisms in the auditory system for coding spectrotemporal modulations (STMs), tuned to different combinations of spectral modulation frequency, temporal modulation frequency, and STM sweep direction. The current study sought evidence of such mechanisms using a psychophysical forward masking paradigm. The detectability of a target comprising upward sweeping STMs was measured following the presentation of modulated maskers applied to the same carrier. Four maskers were tested, which had either (1) the same spectral modulation frequency as the target but a flat temporal envelope, (2) the same temporal modulation frequency as the target but a flat spectral envelope, (3) the same spectral and temporal modulation frequencies as the target but the opposite sweep direction (downward sweeping STMs), or (4) the same spectral and temporal modulation frequencies as the target and the same sweep direction (upward sweeping STMs). Forward masking was greatest for the masker fully matched to the target (4), intermediate for the masker with the opposite sweep direction (3), and negligible for the other two (1, 2). These findings are consistent with the suggestion that the detectability of the target was mediated by an STM-specific coding mechanism with sweep-direction selectivity.

Photon coupling-induced spectrum envelope modulation in the coupled resonators from Vernier effect to harmonic Vernier effect

Abstract The Vernier effect and harmonic Vernier effect have attracted ever-increasing interest due to their freely tailored spectrum envelope in tunable laser, modulator, and precision sensing. Most explorations have mainly focused on configuring two isolated optical resonators, namely the reference and tunable resonator. However, this configuration requires a stable reference resonator to guarantee robust readout, posing a significant challenge in applications. Here, we discover the coupled-resonators configuration enabling a reference-free envelope modulation to address this problem. Specifically, all parameters of one resonator theoretically span a hypersurface. When the resonator couples to another one, photon coupling merit an escaped solution from the hypersurface, resulting in an envelope modulation independent of reference. We have first experimentally verified this mechanism in a coupled air resonator and polydimethylsiloxane resonator by inserting a semi-transparent 2-mercaptobenzimidazole-modified silver nanowire network. In addition, this novel mechanism provides a new degree of freedom in the reciprocal space, suggesting alternative multiplexing to combine more envelope modulations simultaneously. This study facilitates the fundamental research in envelope multiplexing. More importantly, the combination of silver nanowire network and flexible microcavity experimentally progress the spectral envelope modulation in optoelectronic integration inside resonators.

2.1 μm, 1.52 μJ square-wave noise-like pulse from an all-fiber figure-of-9 Ho-doped oscillator and single-stage amplifier.

A 2.1 μm, high energy square-wave noise-like pulse (NLP) in an all-fiber Ho-doped fiber laser is proposed, which consists of an oscillator and a single-stage amplifier. In the figure-of-9 oscillator, mode-locking is achieved based on the nonlinear amplifying loop mirror, employing a long gain fiber to provide sufficient gain in 2.1 μm band and optimizing the cavity length to obtain maximum pulse energy output. With appropriate pump power and polarization state, the oscillator emits a 175.1 nJ square-wave NLP with center wavelength of 2102.2 nm and spike width of 540 fs. The 3-dB spectral width and pulse envelope width are 11.2 nm and 6.95 ns, respectively. The single-stage amplifier employs a bi-directional pump scheme. After amplification, 5.8 W NLP with a slope efficiency of 56.8% is obtained. The pulse energy of NLP is scaled to 1.52 μJ, which is the highest pulse energy of NLP at 2.1 μm to the best of our knowledge. The obtained high-energy square-wave NLP-fiber laser has great potential in mid-infrared laser generation.

Relative humidity sensor based on cascaded Fabry-Perot interferometers and Vernier effect

We study and experimentally demonstrate a highly sensitive humidity sensor, whose sensitivity has an amplification effect, through the use of the Vernier effect produced by a cascaded Fabry-Perot interferometer (FPI). A section of capillary was spliced with single-mode optical fiber, and then the capillary was immersed in polyimide (PI) solution to form PI film to construct the structure of the sensor. The proposed sensor comprises an air cavity, a PI cavity, and an air-PI mixing cavity. Owing to the approximate free spectral range of the air cavity and air-PI mixing cavity, the sensor produces a Vernier effect. Theoretical analysis shows that the relative humidity sensitivity of the sensor is only related to PI film. The experimental results show that in the range of 40~85% relative humidity, the sensitivity of the spectral envelope of the sensor can reach ~344.4 pm/% RH, which is 3.77 times higher than that measured by the sensor interference fringe. The experimental results are basically consistent with the theoretical analysis.

Audio Watermarking for Security and Non-Security Applications

The digitization of audiovisual data is significantly increasing. Thus, to guarantee the protection of the intellectual properties of this digital content, watermarking has appeared as a solution. Watermarking can be used in reality in several types of applications that target two different contexts: the first for security applications and the second for non-security ones. In this paper, we carry a big interest in studying these two types of applications. Moreover, we propose a first digital watermarking scheme for security copyright protection applications, where we have involved neural network architecture in the insertion and detection processes, and integrated some masking phenomena of the human psychoacoustic model with linear predictive coding spectral envelope estimation of the audio file. Experiments proved the efficiency of exploiting perceptual masking with spectral envelope consideration in terms of imperceptibility and robustness results. In addition, we suggest a second audio watermarking technique for non-security content characterization applications based on a deep learning classification architecture. In this scheme, the extracted watermark advises about the audio class: music or speech, speaker gender, and emotion. The reported results indicated that the suggested scheme achieved a higher performance at the classification level, as well as at the watermarking properties.

Stability and non-linear dynamic analysis of Kerr optical frequencycombs in dual-coupled microcavities with high-order dispersion

We investigate the stability and nonlinear dynamics of the Kerr optical frequency comb inside a dual coupled microcavity with high-order dispersion effect based on the theoretical model of coupled nonlinear Schrödinger equation. The effects of different high-order dispersion parameters on the evolution and spectral characteristics of the optical field in the cavity are also explored. Theoretical results indicate that the addition of the third-order dispersion enlarges the stability domain of the parametric space and transforms the periodically varying soliton breathers and chaos into stable bright soliton. In order to obtain an accurate Kerr optical frequency comb spectral envelope, higher-order dispersion should be considered. Moreover, high-order dispersion terms have a significant effect on the spectral characteristics of the optical frequency comb, such as the spectral envelope frequency shift and the dispersive wave spectral position. Specifically, the third-order dispersion and positive fourth-order dispersion can broaden the spectrum and enhance the dispersive waves; while the negative fourth-order dispersion can suppress the dispersive wave generation and obtain a symmetric soliton frequency comb; the fifth-order dispersion can regulate the drift direction and speed of the optical solitons. The theoretical results are of great value for dispersion regulation and design and also for stability studies in double-coupled microcavity experiments.

Resolution Analysis for Power System Measurement and Transient Identification

Averaging techniques are frequently applied when sampling quasi-periodic signals to enhance signal fidelity. This article investigates the use of spectral envelope preprocessing for evaluating power consumption and harmonic content drawn by ac loads. A statistical characterization of a preprocessor system’s resolution given analog input noise is derived. Resolution is defined here as the ability to detect transient step changes in power consumption. With these results, the concept of effective number of bit (ENOB) is extended to this averaging system. Design trade-offs in the number of quantization bits, sampling frequency, noise level, and minimum resolvable transient are explored. The conclusions reached here generalize to any system in which spectral envelopes are extracted.

Propagation properties of circularly symmetric Airy beam modulated by spectral asymmetric envelope

An asymmetric envelope function for modulating the spectrum of circular Airy beam is proposed in this work. The propagation properties of the modified circular Airy beam are investigated in both theory and experiment. The three parameters of the asymmetric hyperbolic secant function can be used to adjust the ratio of the high frequency components to the low frequency components in Fourier space, and thus tuning the propagation properties of this modified circular Airy beam. The results demonstrate that the focal position is affected mainly by the high frequency components. The maximum focal intensity will not be enhanced continuously by increasing the proportion of the high frequency components. It depends on the ratio of the high frequency components to the low frequency components when the center frequency is determined. Therefore, using an asymmetric envelope in Fourier space is much more reasonable than using the high pass filtering or symmetric Gaussian envelope. The FWHM decreases significantly with the increase of center frequency. When the parameters are chosen appropriately, the size of focal spot will be reduced significantly, the maximum focal intensity, especially the abruptly autofocusing property will be enhanced greatly and the focal position can remain almost the same as the focal position of the common circular Airy beam. The maximum focal intensity of the proposed beam is 3.4 times that of the common circular Airy beam and the abruptly autofocusing property of the proposed beam is much better than that of the beam using the symmetric Gaussian envelope. The phase-only encoding method in Fourier space is used to generate the proposed beam in experiment. The experimental results are in reasonable agreement with the simulation results. It indicates that the modified beam can be generated conveniently by using the same method as that used to generate the common circular Airy beam.

Thermal-optic tuning cascaded double ring optical sensor based on wavelength interrogation

Based on the Vernier effect of the cascaded double ring resonator (CDRR) sensor, a sensor consisting of a microfluidic control system, a sensing ring, and a reference ring with a micro-heater for thermal tuning is proposed in this paper. In wavelength interrogation, a broadband spectrometer or a large tunable range laser is not required. The shift of the output spectrum caused by the refractive index change of the sample is converted into the change of the electric power by the thermal tuning heater. Due to the Vernier effect, the sensitivity of the sensor is 20 times higher than that of the single ring. The spectral envelope of the thermal-optic tuning of the sensor was fitted by a Gaussian function in the wavelength range of 8 nm. The experimental results showed that the sensitivity was 33.703 W/RIU, and the limit of detection was 1.34×10−5 RIU.

The Fabrication of an Eccentric Three-Core Fiber and Its Application as a Twist Sensor

The fabrication and application for twist sensing of an eccentric three-core fiber (3CF) were demonstrated. The fiber was made by a stack-and-draw technique, in which silica rods and core canes were put in a tube and drawn on a fiber drawing tower. Three cores formed a Mach–Zehnder interferometer (MZI), where the lights transmitted in the three cores interfered with each other, resulting in the formation of envelopes on spectrum. Because two of the cores were off-axis, phase differences among the cores varied with twist due to different stretches on each core, which caused shift of the spectral envelopes of the interference signal. Wide-range twist measurement can be realized with relatively high sensitivity by tracking lower dips of the envelopes. Experimental results revealed that the dips shift quadratically with twist angle, which means that the sensitivity increases with twist. The compensation of temperature influence was also implemented by inscribing a Bragg grating on one of the cores with femtosecond laser. Because the fiber can be mass-produced, it is suitable for twist sensing in practical application for its low cost.

Differentiable Measures for Speech Spectral Modeling

Autoregressive models for the envelope of speech power spectral densities (PSDs) are refined by the self-supervised spectral learning machine (S3LM) provided with differentiable spectral objective functions, including the Itakura-Saito divergence (ISD), the Kullback-Leibler divergence (KLD), the reverse KLD (RKLD) and the log spectral distortion (LSD), which display more significant results. However, in order to assess the models more perceptually, a method is proposed based upon perturbations around perfect reconstruction analysis-synthesis configurations. In the cross-excitation analysis-synthesis assessment (CEASA) method, the residual signals generated by analysis filters of the spectral models are injected as excitation into the synthesis filters derived from the same and other models in order to be evaluated by the perceptual evaluation of speech quality (PESQ) and Itakura divergence (ID), which are averaged over a set of models obtained using the objective functions mentioned above. The results lead to a superior performance when the RKLD is used as the loss function for the estimation of the spectral models with the ISD ranking close behind. The focus of these divergences on the spectral peaks is argued and pointed as the most important factor for this behavior. Specifically, using the PESQ scores obtained with CEASA, the RKLD loss is found to improve the performance by 1.0%, 4.0% and 19.3% with respect to the open-loop analysis, the KLD and the LSD models, respectively, while the corresponding improvements for the ISD loss are 0.1%, 3.0% and 18.2%, and the RKLD models excel the ISD models by 1.0% on average. Even though the spectral measures alone are not able to unequivocally distinguish the better of the two, CEASA is shown to have enough sensitivity to distinguish their performances. In summary, the learning machine S3LM fits models for the short-term spectral envelope of speech and, for the evaluation of its performance under several differentiable loss functions, the CEASA assessment tool has been developed. In addition, CEASA may be used for other assessments connected with speech analysis and synthesis.

Financial Cycles in Euro Area Economies: A Cross‐Country Perspective Using Wavelet Analysis*

AbstractWe study the cross‐country dimension of financial cycles for six euro area countries using wavelet analysis. Estimated wavelet cohesions show that cycles in equity prices and interest rates display stronger synchronization across countries than real output cycles, whereas credit variables and house prices show lower cross‐country synchronization. We propose a wavelet‐based extension to the spectral envelope that is similar to a frequency‐based time‐varying principal component analysis. The country loadings show that, contrary to all other variables, cycles in loans to households and house prices in Germany and the Netherlands are negatively or less strongly correlated with the common cycles.

Umbilical Vein Pulse Wave Spectral Analysis: A Possible Method for Placental Assessment Through Evaluation of Maternal and Fetal Flow Components.

Placental blood flow analysis is complicated by having both maternal and fetal flow components. Using the Fast Fourier Transform (FFT) of the umbilical venous pulse wave spectra (PW) envelope, we could simultaneously assess maternal/fetal blood flow in the placenta and investigate if normal and intrauterine growth restriction (IUGR)/pre-eclamptic pregnancies could be distinguished. This retrospective study included normal gestations (N=11) and gestations with IUGR, pre-eclampsia, or both (N=13). Umbilical vein PW were acquired and spectral envelopes were identified as a function of time and analyzed by FFT. Base-10 logarithms of the ratios of the maternal/fetal spectral peaks (LRSP) were compared in normal and IUGR/pre-eclamptic populations (two-tailed t-test). Body mass index (BMI), gestational age at scan time, placental position, and weight-normalized umbilical vein blood volume flow (two-tailed t-test, analysis of variance [ANOVA] analysis) were tested. P < .05 was considered significant. The LRSP for normal and IUGR/pre-eclamptic pregnancies were 0.141 ± 0.180 and -0.072 ± 0.262 (mean ± standard deviation), respectively (P=.033). We detected differences between normal gestations and combinations of LRSP and weight-normalized umbilical venous blood flows. Placental effects based on LRSPs and blood flow may act synergistically in cases with both pre-eclampsia and IUGR (P=.014). No other significant associations were seen. In this preliminary study, we showed that umbilical venous flow contains markers related to placental maternal/fetal blood flow, which can be used to assess IUGR and pre-eclampsia. When coupled with umbilical cord blood flow, this new marker may potentially identify the primary causes of the two conditions.

Research on Multimodal Music Emotion Recognition Method Based on Image Sequence

The work of music performance system is to control the light change by identifying the emotional elements of music. Therefore, once the identification error occurs, it will not be able to create a good stage effect. Therefore, a multimodal music emotion recognition method based on image sequence is studied. The emotional characteristics of music are analyzed, including acoustic characteristics, melody characteristics, and audio characteristics, and the feature vector is constructed. The recognition and classification model based on neural network is trained, the weight and threshold of each layer are adjusted, and then the feature vector is input into the trained model to realize the intelligent recognition and classification of multimodal music emotion. The threshold of the starting point range of a specific humming note is given by the center clipping method, which is used to eliminate the low amplitude part of the humming note signal, extract the short-time spectral structure features and envelope features of the pitch, and complete the multimodal music emotion recognition. The results show that the calculated kappa coefficient k is greater than 0.75, which shows that the recognition and classification results are in good agreement with the actual results, and the classification and recognition accuracy is high.

Correlation between optical fibre diameter and characteristics of tilted fibre Bragg grating-assisted sensors

We report the results of a study on the influence of the fibre cladding diameter on the accuracy and resolution of tilted fibre Bragg grating-assisted refractometers. Tilted fibre Bragg gratings are an essential element used to develop high-precision fibre sensors for environmental monitoring. Comparative research was performed using one of comprehensive processing algorithms employing spectral envelope analysis. It was shown that the sensor accuracy decreases with decreasign fibre cladding diameter. At the same time, an increase in the diameter deteriorates the spectral pattern contrast, thus impeding the development of high-efficiency sensor elements.

Seismic hazard assessment of the urban area of Ambato, Ecuador, in deterministic form

Seismic microzonation of the urban area of Ambato, Ecuador, was done in 2018 in a probabilistic and a deterministic manner. This type of calculation is presented in the first part of the article. For this purpose, three geologic faults and three strong-motion equations were considered. For each geologic fault, recurrence periods are determined using two methods. It is seen that a magnitude 6.3 earthquake associated with the blind faults traversing Ambato may occur in 80 to 100 years, and one of magnitude 6.5 in the next 300 years. Geophysical and geotechnical studies of the urban area of the city of Ambato are presented. These permitted the acquisition of curves with the same period of soil vibration and equal speed of the shear wave in the first 30 m, plus the classification of soils of the city. Later, six models of strong soil movements were considered and horizontal acceleration spectra of the soil were obtained in a mesh of points separated every 500 m, for each soil profile. Average spectra were found for soil profiles C, D and E when making comparisons with the spectra found in the 2018 study. Based on the results of the present study and those from 2018, new spectral forms are proposed for the urban area of the city of Ambato (called spectral envelopes) and compared to spectra reported by seismic regulations in force in Ecuador (NEC-15).

The Spectral Envelope: An Application to the Decoupling Problem in Economics

In this paper we introduce the technique of spectral envelope into the analysis of the decoupling problem among economies. Decoupling refers to the phenomenon that the business cycles in emerging market economies are de-synchronized from cyclical movements in the advanced economies. The analysis shows that all the countries considered in the sample have a common cycle of approximately 42 months. The results point to a strong possibility of a common global cycle of a periodicity between 3 and 4 years.

Ordinal scaling of temporal audio descriptors and perceptual significance of attack temporal centroid in timbre spaces.

Temporal audio features play an important role in timbre perception and sound identification. An experiment was conducted to test whether listeners are able to rank order synthesized stimuli over a wide range of feature values restricted within the range of instrument sounds. The following audio descriptors were tested: attack and decay time, temporal centroid with fixed attack and decay time, and inharmonicity. The results indicate that these descriptors are susceptible to ordinal scaling. The spectral envelope played an important role when ordering stimuli with various inharmonicity levels, whereas the shape of the amplitude envelope was an important parameter when ordering stimuli with different attack and decay times. Linear amplitude envelopes made the ordering of attack times easier and caused the least amount of confusion among listeners, whereas exponential envelopes were more effective when ordering decay times. Although there were many confusions in ordering short attack and decay times, listeners performed well in ordering temporal centroids even at very short attack and decay times. A meta-analysis of six timbre spaces was therefore conducted to test the explanatory power of attack time versus the attack temporal centroid along a perceptual dimension. The results indicate that attack temporal centroid has greater overall explanatory power than attack time itself.

Scattering Attenuation of the Martian Interior through Coda Wave Analysis.

We investigate the scattering attenuation characteristics of the Martian crust and uppermost mantle to understand the structure of the Martian interior. We examine the energy decay of the spectral envelopes for 21 high-quality Martian seismic events from Sol 128 to Sol 500 of InSight operations. We use the model of Dainty et al. (1974b) to approximate the behavior of energy envelopes resulting from scattered wave propagation through a single diffusive layer over an elastic half-space. Using a grid search, we mapped the layer parameters that fit the observed InSight data envelopes. The single diffusive layer model provided better fits to the observed energy envelopes for High Frequency (HF) and Very High Frequency (VF) than for the Low Frequency (LF) and Broadband (BB) events. This result is consistent with the suggested source depths (Giardini et al., 2020) for these families of events and their expected interaction with a shallow scattering layer. The shapes of the observed data envelopes do not show a consistent pattern with event distance, suggesting that the diffusivity and scattering layer thickness is non-uniform in the vicinity of InSight at Mars. Given the consistency in the envelope shapes between HF and VF events across epicentral distances and the tradeoffs between the parameters that control scattering, the dimensions of the scattering layer remain unconstrained but require that scattering strength decreases with depth and that the rate of decay in scattering strength is fastest near the surface. This is generally consistent with the processes that would form scattering structures in planetary lithospheres.