High Spectral Density Research Articles

High Coherence at f and 2f of Mid-Infrared Supercontinuum Generation in Silicon Germanium Waveguides

Absorption spectroscopy based on supercontinuum generation in the mid-infrared is a powerful technique to analyze the chemical composition of samples. Furthermore, phase-coherent supercontinuum sources can enable fast data acquisition with coherent, stable pulses that allow single-shot measurements. We report here a numerical study of the coherence of an octave-spanning mid-infrared supercontinuum source that was experimentally obtained in an air-clad SiGe/Si waveguide. We show that engineering two closely spaced zero-dispersion wavelengths that enclose an anomalous dispersion band centered around a fixed pump wavelength can produce supercontinuum pulses with high spectral density and full coherence at the extreme ends of the spectrum. This work is important for absorption spectroscopy, on-chip optical frequency metrology, and f -to-2 f interferometry applications.

Control of supercontinuum generation from filamentation of shaped femtosecond laser pulses

Supercontinuum (SC), as one of the most spectacular phenomena occurring in the nonlinear process of intense femtosecond laser-material interaction, has attracted considerable interest. The broadband SC sources have a variety of applications including the spectroscopy, fluorescence microscopy, remote sensing, and generation of few-cycle pulses. Over the last few decades, the SC has been extensively investigated in various optical media, including liquid, gas, and solid. Especially, ultrabroadband SC sources have achieved remarkable development in the photonic crystal and micro-structured fibers. Even so, the generation of the SC with high brightness, high spatiotemporal coherence and good maneuverability, is still a challenging topic. The SC generation from femtosecond filamentation is a unique white light source with high pulse energy, high brightness and high spatiotemporal coherence, whose spectral range spans from ultraviolet to mid-infrared. In recent years, numerous studies have been conducted to optimize the filamentation and SC. The control of filamentation such as the filament length, number and position, as well as the generation of the ultra-broadband spectrum with high spectral energy density has been realized. To date, the optimal control of SC has been realized by the spatial modulation or time-domain shaping of the femtosecond laser pulse. However, there is no report on the control of SC generation and filamentation by spatiotemporally modulating the femtosecond laser pulses as far as we know. In this work, a spatiotemporal modulation for the femtosecond laser pulse is proposed, which combines the spatial modulation by using microlens array (MLA) and the laser pulse shaping based on liquid crystal spatial light modulator. We investigate the control of the SC generation from the filamentation of the spatiotemporally modulated femtosecond laser pulses in fused silica by using the feedback optimal control based on genetic algorithm. In our experiments, with the increase of the iterative generation, the cut-off wavelength in the blue-side extension of the SC becomes shorter gradually, and the spectral intensity of the SC increases significantly. After the eighth iteration, the increase of the spectral intensity slows. With the number of iterations increasing further, the intensity and broadening of SC spectrum will no longer apparently change. Hence, the feedback optimization control of spectral intensity of SC is realized, and the SC with controllable spectral intensity in a certain range is obtained. The maximum intensity variation of SC is more than three times. By integrating the spectral intensities of SC for different iterative generations, we characterize the increase trend of SC conversion efficiency. During the first few iterations, the conversion efficiency increases rapidly. Then it increases slowly after eighth generations and reaches its maximum after several generations (10th generation). The conversion efficiency has a similar evolution to the spectral intensity of the SC. To explain the physical mechanism, the initial envelope of the shaping pulse with typical iteration generation is calculated. It can be concluded that the spatial modulation of MLA allows for higher incident laser energy and for more filaments’ generation, which increases the energy of SC radiation directly. The peak intensity and envelope distribution of time domain pulse are the main factors affecting the spectral intensity and broadening the SC.

Spectral Efficiency of FBMC Over OFDM System

Significant wireless broadband technology used in various cellular standards is Orthogonal Frequency Division Multiplexing (OFDM) which will make use of Multi Carrier Modulated (MCM) systems. Even though OFDM has numerous advantages, it is hard to employ OFDM for complex networks. It is very hard to establish synchronization in mobile environments as it is difficult to predict the Doppler shifts of different users, which results in inter carrier interference (ICI). Further, filters associated with OFDM carrier have comparatively large sidebands which outcomes in Out of Band (OOB) radiations. Insufficient spectral usage is provided by CP-OFDM by using more guard band. So the problems caused by traditional OFDM/CP-OFDM can be answered by employing a new system termed as Filter Bank Multi Carrier (FBMC) System. It is a form of MCM and it can be considered as an advanced cyclic-prefix (CP-OFDM). In OFDM, whole band gets filtered while in FBMC, each sub carrier band is independently filtered. The primary objective of this work is to relate the performance of 5G modulation technique such as FBMC against OFDM and to suggest an ideal waveform for 5G communication in regard to high spectral efficiency, spectral density, BER and less Peak to Average Power Ratio (PAPR).

Photon number measurement using heterodyne method for a detector's quantum efficiency determination based on spontaneous parametric down-conversion.

Active development of quantum informational components such as quantum computers and quantum key distribution systems requires parameter characterization of single photon detectors. A key property of the single photon detectors is detection efficiency. One of the methods of the detection efficiency measurement, as listed in the international standard ETSI, is the reference-free twin-photon-based Klyshko method. The signal-to-noise ratio (SNR) of this method depends on the combination of the pump wavelength, the nonlinear crystal's axis angle, and the type of detector's sensitive element. When the combination is difficult, one has to deal with the low SNR of the detector counts measurement. To gain the high SNR, one has to average the long record complicated with the "random telegraph signal" noise. This type of noise exhibits high spectral density at a zero frequency, where simple averaging works. The heterodyne based method we have proposed is to perform averaging at the higher frequency of the modulation introduced to the standard Klyshko measurement scheme. The method was numerically simulated and experimentally tested. The 14 times improvement in SNR for the proposed method relative to the simple averaging was demonstrated by the numerical simulation and confirmed experimentally.

Infrared Spectroscopy with a Fiber-Coupled Quantum Cascade Laser for Attenuated Total Reflection Measurements Towards Biomedical Applications.

The development of rapid and accurate biomedical laser spectroscopy systems in the mid-infrared has been enabled by the commercial availability of external-cavity quantum cascade lasers (EC-QCLs). EC-QCLs are a preferable alternative to benchtop instruments such as Fourier transform infrared spectrometers for sensor development as they are small and have high spectral power density. They also allow for the investigation of multiple analytes due to their broad tuneability and through the use of multivariate analysis. This article presents an in vitro investigation with two fiber-coupled measurement setups based on attenuated total reflection spectroscopy and direct transmission spectroscopy for sensing. A pulsed EC-QCL (1200–900 cm) was used for measurements of glucose and albumin in aqueous solutions, with lactate and urea as interferents. This analyte composition was chosen as an example of a complex aqueous solution with relevance for biomedical sensors. Glucose concentrations were determined in both setup types with root-mean-square error of cross-validation (RMSECV) of less than 20 mg/dL using partial least-squares (PLS) regression. These results demonstrate accurate analyte measurements, and are promising for further development of fiber-coupled, miniaturised in vivo sensors based on mid-infrared spectroscopy.

Time resolved characterization of Fabry-Perot quantum cascade lasers for use in a broadband "white light" source.

We report the time resolved characterization of Fabry-Perot quantum cascade lasers (FP-QCLs). We are developing a custom-built broadband laser source in the Mid-LWIR range by combining several high power FP-QCLs for a single snap shot application. This white light source would enable not only stand-off detection applications in a single snapshot but also new data collection modalities such as live, real-time chemical imaging, requiring extremely rapid measurements. In this study, the two FP-QCLs were operated in CW and pulsed modes with varying applied currents and diode temperatures to optimize the best laser operation condition to cover a broad spectral range including spectral features for the analytes of interest. To understand mode behavior of the FP-QCLs in a short period of time, the spectral output for each test condition was temporally resolved. Under most of the conditions, FP mode hopping was observed during the time evolution through the pulse length (3000 ns). Based on the time-resolved spectra, the ideal spectral characteristics for a single snap shot application are discussed, with respect to a broad spectral bandwidth, a flat-top power profile, and high spectral power density.

Enhancing Performance of Orthogonal Frequency Division Multiplexing (OFDM) Based On Fast Fourier Transform (FFT) In Wireless Communication System

Orthogonal Frequency Division Multiplexing (OFDM) is one of the multicarrier transmission techniques used in wireless communication system. It has many benefits such as robust in channel fading and has high spectral density. The main objective of OFDM implementation in wireless communication system is to achieve less or zero Bit Error Rate (BER). However, OFDM design complexity, requirement and selection of the suitable modulation method are among the current issues. Thus, this paper aims to investigate the performance of OFDM in wireless communication by developing two OFDM based system designs. The transmitter, channel and receiver are designed based on OFDM system principles. Forward Error Correction (FEC) method is applied to reduce the BER. Both OFDM designs produce less BER with zero BER for the second OFDM design. The investigation study shows that the performance of OFDM can be enhanced by applying Fast Fourier Transform (FFT) technique. Zero BER can be achieved if the suitable modulation scheme is applied in the system. The developed designs are not complex, suitable to be applied for IEEE 802.11 standard. The BER performance can be influenced by the types of channels, signal to noise ratio (SNR) and various modulation schemes. Thus, this study can be used as a guidance to implement the OFDM in the current or future wireless communication system.

High power GaInNAs superluminescent diodes emitting over 400 mW in the 1.2 μm wavelength range

A high-power superluminescent diode emitting over 400 mW in the 1.2 μm range is reported. The active region is based on a single GaInNAs/GaAs quantum well positioned within a low-confinement vertical waveguide and a lateral ridge waveguide geometry, ensuring single transverse mode operation. The peak wall-plug efficiency and the differential efficiency in the linear region were 22.8% and 0.38 W/A, respectively. The full width at half-maximum spectral width for the maximum output power was 22 nm, corresponding to a spectral power density of 19 mW/nm, a threefold increase compared to continuous wave superluminescent diodes based on a quantum dot active region operating in the same wavelength range. Besides exhibiting excellent optical and electrical properties, the GaInNAs active region enhances operation at elevated temperatures. In this respect, an output power of about 210 mW is demonstrated at operation temperatures as high as 60 °C, while 150 mW is still emitted at 70 °C. The unique combination of parameters demonstrated makes these GaInNAs QW-based superluminescent diodes particularly attractive for hybrid integration with silicon photonic circuitry, enabling the demonstration of compact solutions for sensing, optical coherence tomography, and other emerging concepts exploiting photonic integration technology and requiring single transversal mode operation, good efficiency, broadband high spectral power density, and uncooled operation at elevated temperatures.

Spectral and statistical analysis of wind spectrum for Ali Al-Gharbi area in Iraq

The spectrum known represented as a relationship that’s plotted between the magnitudes or energy for a specific parameter vs. its frequency, the wind spectrum is presented as the sum of wind speed created by events divided either in space, in time, or both. This paper presents a wind speed spectrum demonstration inAli Al-Gharbi location in Iraq. The aim of the present paper is to analysis the wind speed and direction by employing the FFT (Fast Fourier Transform) therefore field measurement of wind speed and direction were collected for one year from Dec 2014 to Dec 2015 in the time interval of 10 minutes at heights of 10, 30 and 50meters. From the performance of the FFT it was found that the values of the peak which contains the highest spectral density was (226236.282 m/sec) at the frequency of (2 Hz) on the 50 m height level throughout the night time but the lowest was (115863.7 m/sec) at the frequency of (2 Hz) at the 10 m height throughout the night time. The dominant wind direction at the area was from west-Northwest and the north-Northwest. The wind speed during morning hours was higher than that at the night time.

High spectral energy density supercontinuum generation in fused silica by interfering two femtosecond laser beams

The filaments formed by interference of femtosecond laser beams are distributed over the cross section of the laser beams, which could allow the incident laser pulse to have a relatively high energy and to generate high power supercontinuum (SC) in fused silica. In this paper, two-beam interference induced filamentation and SC in fused silica are experimentally studied. The spectrum of the SC is shown to be sensitive to cross angle and time delay between the two laser pulses. In our experimental conditions, the spectral energy density of the SC emission can reach the order of μJ nm−1 from 726 to 859 nm and 0.1 μJ nm−1 from 500 to 700 nm. The interference method provides an alternative and flexible approach to obtain the intense SC generation in condensed optical media.

Accelerated nonlinear interactions in graded-index multimode fibers

Multimode optical fibers have recently reemerged as a viable platform for addressing a number of long-standing issues associated with information bandwidth requirements and power-handling capabilities. As shown in recent studies, the complex nature of such heavily multimoded systems can be effectively exploited to observe altogether novel physical effects arising from spatiotemporal and intermodal linear and nonlinear processes. Here, we study for the first time, accelerated nonlinear intermodal interactions in core-diameter decreasing multimode fibers. We demonstrate that in the anomalous dispersion region, this spatiotemporal acceleration can lead to relatively blue-shifted multimode solitons and blue-drifting dispersive wave combs, while in the normal domain, to a notably flat and uniform supercontinuum, extending over 2.5 octaves. Our results pave the way towards a deeper understanding of the physics and complexity of nonlinear, heavily multimoded optical systems, and could lead to highly tunable optical sources with very high spectral densities.

Trace ethane sensor using an economic midinfrared light-emitting diode

Due to the lack of an economic midinfrared laser, especially wavelength around 3 μm, light sources are critical to trace gas sensing for the gases without absorption in the near-infrared zone. We present a trace ethane photoacoustic (PA) sensor using a midinfrared light-emitting diode (MIR LED). MIR LEDs are very attractive sources for PA technology since they are relatively high power spectral density, easy modulation, low power consumption, and low cost. The proposed PA system with a 3.36-μm MIR LED shows good linearity to ethane concentration and reaches a detection limit of 10 parts per million by volume easily. Ideas of further enhancement of the sensitivity are discussed.

Flaws in the use of spectral ripples in cochlear implants

Spectral ripple discrimination is a popular measure of spectral resolution that has been shown to correlate with speech recognition scores in cochlear implant (CI) listeners. In the test, listeners distinguish sounds with varying density of spectral peaks, with some spectral modulation depth. We argue that there are numerous significant flaws with the application of the test specifically in CI listeners. To start, the spectrum is aliased by the CI processor in a way that is similar to frequency aliasing for under-sampled time series. Beyond a critical spectral density, the spectral envelope changes in a chaotic fashion and is no longer under experimenter control. This critical density is exceeded in numerous published studies. Furthermore, the densities linked with “good” performance are not only outliers, but are entirely unrelated to the spectral densities of real speech sounds, and likely exhibit undue leverage over correlation values. Additionally, there are reports of experience and learning effects, inconsistent with the often-stated goals of the test to avoid such factors. We show how artefactual nonlinearities at high spectral densities may unintentionally match the spectral envelope characteristics of speech sounds—an unfortunate result that likely has given spurious results that sustain the use of this test.

Inference for the Lagged Cross‐Covariance Operator Between Functional Time Series

When considering two or more time series of functional data objects, for instance those derived from densely observed intraday stock price data of several companies, the empirical cross‐covariance operator is of fundamental importance due to its role in functional lagged regression and exploratory data analysis. Despite its relevance, statistical procedures for measuring the significance of such estimators are currently undeveloped. We present methodology based on a functional central limit theorem for conducting statistical inference for the cross‐covariance operator estimated between two stationary, weakly dependent, functional time series. Specifically, we consider testing the null hypothesis that the two series possess a specified cross‐covariance structure at a given lag. Since this test assumes that the series are jointly stationary, we also develop a change‐point detection procedure to validate this assumption of independent interest. The most imposing technical hurdle in implementing the proposed tests involves estimating the spectrum of a high dimensional spectral density operator at frequency zero. We propose a simple dimension reduction procedure based on functional principal component analysis to achieve this, which is shown to perform well in a simulation study. We illustrate the proposed methodology with an application to densely observed intraday price data of stocks listed on the New York stock exchange‐20.40

Large Eddy Simulation and Experimental Analysis of Combustion Dynamics in a Gas Turbine Burner

Large eddy simulations (LES) and experiments (planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) and pressure transducer) have been carried out on a gas turbine burner fitted to an atmospheric combustion rig. This burner, from the Siemens SGT-800 gas turbine, is a low NOx, partially premixed burner, where preheat air temperature, flame temperature, and pressure drop across the burner are kept similar to engine full load conditions. The large eddy simulations are based on a flamelet-generated manifold (FGM) approach for representing the chemistry and the Smagorinsky model for subgrid turbulence. The experimental data and simulation data are in good agreement, both in terms of time averaged and time-resolved quantities. From the experiments and LES, three bands of frequencies of pressure fluctuations with high power spectral density are found in the combustion chamber. The first two bands are found to be axial pressure modes, triggered by coherent flow motions from the burner, such as the flame stabilization location and the precessing vortex core (PVC). The third band is found to be a cross flow directional mode interacting with two of the four combustion chamber walls in the square section of the combustion chamber, triggered from general flow motions. This study shows that LES of real gas turbine components is feasible and that the results give important insight into the flow, flame, and acoustic interactions in a specific combustion system.

Ultrafast Maximum-Quantum NMR Spectroscopy for the Analysis of Aromatic Mixtures.

Maximum-quantum (MaxQ) NMR experiments have been introduced to overcome issues related to peak overlap and high spectral density in the NMR spectra of aromatic mixtures. In MaxQ NMR, spin systems are separated on the basis of the highest-quantum coherence that they can form. MaxQ experiments are however time consuming and methods have been introduced to accelerate them. In this article, we demonstrate the ultrafast, single-scan acquisition of MaxQ NMR spectra using spatial encoding of the multiple-quantum dimension. So far, the spatial encoding methodology has been applied only for the encoding of up to double-quantum coherences, and here we show that it can be extended to higher coherence orders, to yield a massive reduction of the acquisition time of multi-quantum spectra of aromatic mixtures, and also to monitor chemical reactions.

Performance of UWB-WBAN System Over Awgn Channel

AbstractThe major constraints in the design of Wireless Body Area Network(WBAN) can be attributed to the battery autonomy, need for high data rate services and low interference from the devices operating within the industrial, scientific and medical (ISM) bands. To meet the demand for high data rate services and low power spectral density to avoid ISM band interference, an ultra-wide band(UWB) system based technology has been proposed. This paper focuses on the design and demonstration of an UWB modem to be used in the WBAN applications and the evaluation of its performance in a near-real world scenarios affected by additive white Gaussian noise(AWGN) interference. The modem is tested with different values of signal to noise ratio(SNR). Results show that the performance of the modem degrades as the value of SNR decreases.

Weld defect identification in friction stir welding using power spectral density

Power spectral density estimates are powerful in extraction of useful information retained in signal. In the current research work classical periodogram and Welch periodogram algorithms are used for the estimation of power spectral density for vertical force signal and transverse force signal acquired during friction stir welding process. The estimated spectral densities reveal notable insight in identification of defects in friction stir welded samples. It was observed that higher spectral density against each process signals is a key indication in identifying the presence of possible internal defects in the welded samples. The developed methodology can offer preliminary information regarding presence of internal defects in friction stir welded samples can be best accepted as first level of safeguard in monitoring the friction stir welding process.

A Thermoneutral Environment Abolishes Age‐Associated Reduction in Heart Rate Variability in Mice

BackgroundCurrent dogma suggests sympathetic tone is the dominant regulator of rodent heart function at standard laboratory ambient temperature (LTA). Recent studies have shown, however, the high surface area to body mass ratio of rodents forces them to use non‐shivering thermogenesis driven by sympathetic stimulation of brown fat to maintain proper thermoregulation at LTA, raising heart rate (HR). When mice are studied at their thermoneutral ambient temperature (NTA) there is a decrease in HR and a formidable vagal component to HR regulation is revealed. HR regulation can be described by heart rate variability (HRV), a group of parameters which quantify the variation of beat‐to‐beat intervals. Additionally, HRV becomes reduced in advanced age due to reduced parasympathetic input.ObjectiveTo unravel the impact of autonomic input on age‐associated changes in HR and HRV over a range of ambient temperatures.MethodsHR was measured in young (3 mo) and aged (30 mo) male C57/BL6 telemetry‐implanted mice via in‐vivo ECG after a 3‐day acclimation period at 20ºC. Another ECG was recorded after a 3‐day acclimation period at 30ºC. HRV was evaluated in both time and frequency domains extrapolated from ECG recordings. Data was analyzed using linear mixed effects models. The double interaction term (temperature and age) in the models was used to evaluate the effect of temperature on HR and HRV by age group while correcting biases stemming from repeated measurements and uneven group sizes.ResultsChanging the ambient temperature altered mean HR of both age groups and several HRV indices in the aged mice. Mean HR of young mice was 595 ± 23 beats per minute (bpm) at 20ºC and decreased to 367 ± 14 bpm at 30ºC (p < 0.0001). Mean HR of aged mice was 622 ± 9.0 bpm at 20ºC and decreased to 323 ± 23 bpm at 30ºC (p < 0.0001) (Figure 1). Coefficient of variance (CV), an indicator of overall time‐domain HRV, was significantly reduced in old mice (2.24 ± 0.3 %) than in young mice (6.96 ± 1.5 %). This difference disappeared at 30ºC because the CV of the aged mice significantly increased to 6.57 ± 0.9 % (p < 0.0001) (Figure 2). Detrended fluctuation analysis (DFA), a measure of correlation within a time series, was significantly lower in the old mice at 30 C (0.81 ± 0.06) than 20ºC (1.1 ± 0.04) (p < 0.008) (Figure 3). The changes in CV and DFA suggest a restoration in complexity in the old mice at 30ºC. Aged mice also showed a significant increase in the high frequency power spectral density compared at 30ºC (175.4 ± 33.2 Hz) compared to 20ºC (20.79 ± 1.72), indicating increased parasympathetic tone at NTA (p < 0.0001) (Figure 4).ConclusionRodents are studied in cold‐stressed conditions at LTA and increased sympathetic stimulation distorts the correct view of autonomic balance. An age‐associated decline in HRV observed at LTA was ameliorated when mice were studied at their NTA. Thus, a true elucidation of autonomic neurotransmitter modulation of cardiovascular function alterations in advanced age require mammals to be at their NTA.Support or Funding InformationFunded by the National Institute of Health/National Institute on Aging Intramural Research Program.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Transformation of non-Gaussian random processes, signals and noise in the nonlinear differential systems by the method of cumulative functions

Reviewed and analyzed the issues linked with the torque and naguszewski cumulant description of random processes. It is shown that if non-Gaussian random processes are given by both instantaneous and cumulative functions, it is assumed that such processes are fully specified. Spectral characteristics of non-Gaussian random processes are considered. It is shown that higher spectral densities exist only for non-Gaussian random processes.