Autocorrelation Technique Research Articles

Femtosecond nonlinear optical refraction of single crystal alumina doped with copper ions

The single crystal alumina was fabricated by the Czochralski method. The copper ions were doped into the alumina sample to form the Cu:Al2O3 composite material. The doping energy is 35keV and the ion concentration is 1.2×1017 ions/cm2. The suppercontinuum spectrum was investigated to show the larger nonlinear refraction of this composite material. The nonlinear optical refractive property of the Cu:Al2O3 composite material was investigated by using the Kerr-lens autocorrelation technique for the first time. The nonlinear optical refractive indices, n2=2.8×10−15cm2/W at 800nm, is larger than the value of the single crystal alumina. The mechanism of the nonlinear optical refraction has been discussed and it is due to the optical Kerr effect.

Validity of EMH; A Case Study of KSE-100 Index

The emerging markets offer major investments opportunities for a range of investors over the last decades especially after the global financial crises,which attracted the attention of investors and financial researchers towards the market efficiency.This research paper is designed to verify other researchers work, because some of them have provided contradictory results to test the market efficiency of Pakistani stock index (KSE-100). Average daily observations are considered for the period of twenty two years (November 02, 1991 to December 31, 2012). Unit Root tests (ADF, PP and KPSS), Runs test, Serial Autocorrelation (L-Jung-Box Q statistic) techniques are used to analyze the market’s informational weak form efficiency. Return time series is not normally distributed because it is negatively skewed and leptokurtic. All of the tests applied provide sufficient statistical evidence to reject the Random Walk Hypothesis thus KSE-100 shares index is informational weak form inefficient.

Blind sequence‐length estimation of low‐SNR cyclostationary sequences

Several existing direct-sequence spread spectrum (DSSS) detection and estimation algorithms assume prior knowledge of the symbol period or sequence length, although very few sequence-length estimation techniques are available in the literature. This study presents two techniques to estimate the sequence length of a baseband DSSS signal affected by additive white Gaussian noise. The first technique is based on a known autocorrelation technique which is used as reference, and the second technique is based on principal component analysis. Theoretical analysis and computer simulation show that the second technique can correctly estimate the sequence length at a lower signal-to-noise ratio than the first technique. The techniques presented in this study can estimate the sequence length blindly which can then be fed to semi-blind detection and estimation algorithms.

Single-station cross-correlation analysis of ambient seismic noise: application to stations in the surroundings of the 2008 Iwate-Miyagi Nairiku earthquake

We introduce the single-station cross-correlation (SC) technique of processing ambient seismic noise and compare its results with the established cross-correlation (CC) and autocorrelation (AC) techniques. While CC is the correlation of the signals of two seismic stations with each other and AC is the correlation of a signal with itself, SC is the correlation of two different components of a single three-component seismic sensor. The comparison of the three different correlation techniques shows that CCs give the best results at frequencies below 0.5 Hz and that SCs give the best results at higher frequencies. In all three processing techniques, ambient seismic noise is correlated in order to reconstruct the Green's function describing the wave propagation between the first and the second sensor. By relating the coda parts of the daily Green's functions with the long-term reference Green's functions, shear wave velocity changes are determined. Here, we apply this technique to the data of 20 seismic stations in the surroundings of the fault zone of the Iwate-Miyagi Nairiku earthquake (M W =6.9), which occurred on 2008 June 13, UTC (2008 June 14, Japan Standard Time) in the northern part of the Japanese island Honshu. The data range from 2008 January to 2011 June and therefore include the Tohoku earthquake (M W =9.0), which occurred on 2011 March 11, off the coast of northern Honshu. The data are analysed in five different frequency ranges between 0.125 and 4.0 Hz. The data show coseismic velocity changes for both earthquakes followed by a post-seismic velocity recovery. In general, the coseismic velocity changes increase with frequency. For the Iwate-Miyagi Nairiku earthquake, the strongest velocity changes occur close to the fault zone. Quickly recovering coseismic velocity changes can be separated from changes not recovering during the study period. For the Tohoku earthquake, the complete area is affected by coseismic velocity changes. A modelling of the depth of the coseismic velocity changes indicates that the Iwate-Miyagi Nairiku earthquake can be explained either by large shallow velocity changes or by small, but deep changes. For one station, the observations can only be explained by assuming deeper changes. For the Tohoku earthquake, the modelling shows that different parts of the study area are affected in different ways, some showing shallow changes, others deeper changes. Furthermore, seasonal velocity variations occur, which are compatible for the different stations above 0.5 Hz, with velocity maxima in autumn.

Nonlinear optical refraction of Al2O3 single crystal doping with nickel nanoparticles measured by the Kerr-lens autocorrelation technique

The nonlinear refraction of a nickel doped α-Al2O3 single crystal was measured with a 800 nm pulse using the Kerr-lens autocorrelation technique. The sample was fabricated by ion implantation using a metal vapor vacuum arc ion source. The value of the nonlinear refractive index, n2, of the sample was determined to be 7.9 × 10−16 cm2 W−1. The mechanisms of nonlinear refraction of the bulk material and the nanoparticles have been discussed through the UV–vis spectrum and supercontinuum spectra.

Very long baseline interferometry and observations of gravitational lenses using intensity fluctuations: an analysis based on intensity autocorrelation

A novel interferometric technique that uses the spectrum of the current fluctuations of a quadratic detector, a type of detector commonly used in Astronomy, has recently been introduced. It has major advantages with respect to classical interferometry. It can be used to observe gravitational lenses that cannot be detected with standard techniques. It can be used to carry out very long baseline interferometry. Although the original theoretical analysis, that uses wave interaction effects, is rigorous, it is not easy to understand. The present article therefore carries out a simpler analysis, using the autocorrelation of intensity fluctuations, which is easier to understand. It is based on published experiments that were carried out to validate the original theory. The autocorrelation analysis also validates simple numerical techniques, based on the autocorrelation, to model the angular intensity distribution of a source. The autocorrelation technique also allows a much simpler detection of the signal. In practice, the gravitational lens applications are the ones that can readily be done with presently available telescopes. We describe a practical example that shows that presently available VLBI radio-astronomical data can be used to observe microlensisng and millilensing in macrolensed Quasars. They may give information on the dark matter substructures in the lensing galaxies.

The Study and Implementation of the Anti-Doppler Technology of AUV Communication System

Due to the complexity of the underwater acoustic channel, a reliable mobile underwater acoustic communication system must have high resistance to Doppler and multi-path extension. In this paper, an AUV underwater acoustic control and communication system based on frequency-hopping frequency-shift keying (FH-FSK) is proposed. The system uses pulse pair method to estimate Doppler frequency shift, and analyzes the performance of the algorithm via simulation. The system has been developed based on chip OMAP-L138, with lake test conducted. Result of the tests shows that the system can estimate and compensate the Doppler frequency shift generated by the platform's movement precisely, and has strong resistance to interference to transmit AUV control messages effectively.

Nocturnal river water temperatures: Spatial and temporal variations

Nocturnal water temperature (Tw) affects the behaviour of aquatic biota and metabolism of whole rivers. However, night-time water temperature (nTw) is poorly understood because spot samples are typically taken during daylight hours, or Tw series are aggregated in ways that mask sub-daily properties. This paper examines 15-minute measurements of Tw and air temperature (Ta) collected at 36 sites in the Rivers Dove and Manifold, English Peak District. Data were stratified by day and night then analysed using hysteresis, auto-correlation and logistic regression techniques. Daily hysteresis loops show lagged responses between nTw and previous daylight air temperatures (dTa), plus the influence of groundwater and discharge variations. Logistic regression models were modified using a seasonal factor and explained between 80 and 94% of the variance in daily maximum nTw; minimum nTw were predicted with less skill, particularly for headwater sites in summer. Downstream variations in model parameters also reflect the influence of groundwater and/or riparian shade, and prevailing weather conditions. A case is presented where an intense summer storm resulted in the propagation of a thermal wave that produced maximum Tw at some sites during hours of darkness. Hence, our findings show that Tw management by riparian shade has to be seen in a catchment wide context, with anticipated benefits normalised for weather variability, extreme rainfall events, local influence of groundwater, and channel structures.

Nonlinear refraction of lithium niobate crystal doped with different metal nanoparticles

The nonlinear optical refractive property of metal-implanted lithium niobate crystals has been investigated by using the Kerr-lens auto-correlation technique. The lithium niobate crystals fabricated by the Czochralski method were implanted with metal ions of Cu, Ag, Fe and Ni by using metal vapor vacuum arc ion source implanter. The magnitudes of the nonlinear refraction index of lithium niobate crystals have been improved by the metal ions implantation. The potentiation was analyzed from two aspects: the resonance interaction and the displacements in lithium niobate.

Measurement of nonlinear optical refraction of composite material based on sapphire with silver by Kerr-lens autocorrelation method

Silver nanoparticles synthesized in a synthetic sapphire matrix were fabricated by ion implantation using the metal vapor vacuum arc ion source. The optical absorption spectrum of the Ag: Al2O3 composite material has been measured. The analysis of the supercontinuum spectrum displayed the nonlinear refractive property of this kind of sample. Nonlinear optical refraction index was identified at 800 nm excitation using the Kerr-lens autocorrelation (KLAC) technique. The spectrum showed that the material possessed self-defocusing property (n(2) = -1.1 × 10(-15) cm(2)W). The mechanism of nonlinear refraction has been discussed.

Measurement of Coherence Decay in GaMnAs Using Femtosecond Four-wave Mixing

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, spin-flip scattering) is described, using experiments on GaMnAs as a prototype III-Mn-V system. Spectrally-resolved and time-resolved experimental configurations are described, including the use of zero-background autocorrelation techniques for pulse optimization. The etching process used to prepare GaMnAs samples for four-wave mixing experiments is also highlighted. The high temporal resolution of this technique, afforded by the use of short (20 fsec) optical pulses, permits the rapid spin-flip scattering process in this system to be studied directly in the time domain, providing new insight into the strong exchange coupling responsible for carrier-mediated ferromagnetism. We also show that spectral resolution of the four-wave mixing signal allows one to extract clear signatures of (s,p)-d hybridization in this system, unlike linear spectroscopy techniques. This increased sensitivity is due to the nonlinearity of the technique, which suppresses defect-related contributions to the optical response. This method may be used to measure the time scale for coherence decay (tied to the fastest scattering processes) in a wide variety of semiconductor systems of interest for next generation electronics and optoelectronics.

Measurement of Coherence Decay in GaMnAs Using Femtosecond Four-wave Mixing

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, spin-flip scattering) is described, using experiments on GaMnAs as a prototype III-Mn-V system. Spectrally-resolved and time-resolved experimental configurations are described, including the use of zero-background autocorrelation techniques for pulse optimization. The etching process used to prepare GaMnAs samples for four-wave mixing experiments is also highlighted. The high temporal resolution of this technique, afforded by the use of short (20 fsec) optical pulses, permits the rapid spin-flip scattering process in this system to be studied directly in the time domain, providing new insight into the strong exchange coupling responsible for carrier-mediated ferromagnetism. We also show that spectral resolution of the four-wave mixing signal allows one to extract clear signatures of (s,p)-d hybridization in this system, unlike linear spectroscopy techniques. This increased sensitivity is due to the nonlinearity of the technique, which suppresses defect-related contributions to the optical response. This method may be used to measure the time scale for coherence decay (tied to the fastest scattering processes) in a wide variety of semiconductor systems of interest for next generation electronics and optoelectronics.

Determination of the Site Characterization Properties in Eastern Segment of the North Anatolian Fault Zone in Turkey based on the MMSPAC Method

Erzincan (a small city in Eastern Turkey) is located in the conjunction of three active faults: North Anatolian, North East Anatolian and East Anatolian Fault Zones. Erzincan city center is in a pull-apart basin underlain by soft sediments which significantly amplify the ground motions. Combination of the tectonic and geological behaviour of the region have led to destructive earthquakes such as the 27 December 1939 (Ms=8.0) and the 13 March 1992 (Mw=6.6) events resulting in extensive losses. In this study, in order to perform site characterization in the region, we employ the passive microtremor survey technique at nine sites on three sides of the Erzincan city center in order to gain quantitative estimates of 1D shear-wave velocity profiles for key parts of the basin. The sites are aligned as two profiles in the North-South and East-West directions allowing construction of approximate 2D profiles. At each site we performed surface wave dispersion curve analysis using the Multi-Mode Spatial Autocorrelation (MMSPAC) technique on either single or nested four-station arrays together with horizontal to vertical spectral ratio measurements at array centers. The combination allows resolution of Vs for typically the upper 4 m from high-frrequency data, and as deep as 480 m using low-frequency SPAC and HVSR data in combination. The survey showed that a rate of production of two sites per day is achievable. We present our results in the form of one and two dimensional velocity structures and fundamental frequencies obtained from the HVSR spectra, a combination suitable for input to earthquake ground-motion prediction studies.

RETRACTED ARTICLE: STFT Analysis of the Particle Transport on the IR-T1 Tokamak Plasma Sheath

The time-resolved frequency component analysis has been performed using short time Fourier transform. Fourier-based techniques and auto-correlation have been employed to analyze the frequency of the MHD fluctuations. The time evolution of potential fluctuation, and electric field and turbulent transport have been measured by using two arrays of the Langmuir probes in both the radial and poloidal directions. The experiments have been done in different regimes as Limiter biasing and RHF and both of them. The analyses have been done by the fast Fourier transport (FFT) method and spectral features of them are obtained with the help of the standard auto-correlation technique. The results show that radial turbulent transport decreases about 60 % after positive biasing application while it increases about 40 % after negative biasing. The effect of positive biasing on poloidal turbulent transport displays an increase of about 55 % while the negative bias voltage decreases the poloidal turbulent transport about 30 %. Consequently, confinement is improved and plasma density rises significantly due to the applied positive biasing in IR-T1. But the results are reversed when negative biasing is applied. Also, in this work, the results of applied RHF (L = 3) are compared with biasing results and analysed.

Femtosecond photoelectron point projection microscope

By utilizing a nanometer ultrafast electron source in a point projection microscope we demonstrate that images of nanoparticles with spatial resolutions of the order of 100 nanometers can be obtained. The duration of the emission process of the photoemitted electrons used to make images is shown to be of the order of 100 fs using an autocorrelation technique. The compact geometry of this photoelectron point projection microscope does not preclude its use as a simple ultrafast electron microscope, and we use simple analytic models to estimate temporal resolutions that can be expected when using it as a pump-probe ultrafast electron microscope. These models show a significant increase in temporal resolution when comparing to ultrafast electron microscopes based on conventional designs. We also model the microscopes spectroscopic abilities to capture ultrafast phenomena such as the photon induced near field effect.

Methods for Space-Time Analysis and Modeling

With increasing availability of spatio-temporal data and the democratization of Geographical Information Systems (GIS), there has been a demand for novel statistical and visualization techniques which can explicitly integrate space and time. The paper discusses the nature of spatio-temporal data, the integration of time within GIS and the flourishing availability of spatial and temporal-explicit data over the Internet. The paper attempts to answer the fundamental question on how these large datasets can be analyzed in space and time to reveal critical patterns. The authors further elaborate on how spatial autocorrelation techniques are extended to deal with time, for point, linear, and areal features, and the impact of parameter selection, such as critical distance and time threshold to build adjacency matrices. The authors also discuss issues of space-time modeling for optimization problems.

Spatial autocorrelation in the European air navigation system

The fragmentation of the European air navigation system has been identified as a cause of inefficiencies and additional costs. One of the problems is that since the different national systems are not independent of their neighbours, there might be issues of spatial autocorrelation, where the efficiency of a system is dependent on the efficiency of others, and thus an inefficient system might negatively impact the economic efficiency of those around it. This article will explore that possibility by applying spatial autocorrelation techniques to data envelopment analysis efficiency results for 2010.

ADCP에서의 스펙트럼 모멘트 추정에 관한 연구

ADCP(Acoustic Doppler Current Profiler) 에서의 거리 별 유속이나 유체의 교란 정도 등에 관한 정보는 수신 도플러 스펙트럼의 첫 번째 및 두 번째 스펙트럼 모멘트를 추정함으로서 얻을 수 있다. 그러나 펄스페어(Pulse-pair) 방법으로 불리는 상관함수 추정에 의한 모멘트 추정 기법은 일반적으로 가장 많이 사용되고 있으나 수신 신호의 특성이 대칭형 도플러 스펙트럼이며 하나의 첨두치를 갖는 것을 전제 조건으로 한다. 유체의 교란정도가 심하게 나타나는 상황에서는 이러한 조건이 충족되지 않을 수 있기 때문에 심각한 문제가 발생할 수 있다. 따라서 본 논문에서는 이러한 문제점을 분석하기 위하여 여러 가지 형태를 가지는 다양한 도플러 스펙트럼 수신 신호들을 모의 구현하였으며 이러한 신호들을 이용하여 펄스페어 추정 방법에 의하여 얻어진 결과 값들을 FFT(Fast Fourier Transform) 추정 방법에 의한 결과 값들과 비교하고 그 차이점을 분석하였다. 수신 신호가 도플러 주파수 영역에서 심하게 편이 되거나 다중 첨두치를 가지는 경우에는 펄스 페어 방법에 의한 추정치들은 무의미한 값들로 나타날 수 있음을 알 수 있다. The current velocity and turbulence information in each range cell can be obtained from the first and second Doppler spectrum moment estimates. However, the very widely used correlation method often called as the pulse-pair method has the inherent restrictions under the highly turbulent conditions since it does not satisfy the assumptions that the return Doppler spectrum should be symmetric and have a single peak value. Therefore, in this paper, the quality of pulse-pair estimates were compared with that of FFT estimates for problem analysis using various shapes of simulated Doppler spectra. It can be known that the pulse-pair method often yields meaningless results if the received signals are severely biased or multi-peak Doppler spectra in the Doppler frequency domain.

The use of spatial autocorrelation analysis to identify PAHs pollution hotspots at an industrially contaminated site

The identification of contamination "hotspots" are an important indicator of the degree of contamination in localized areas, which can contribute towards the re-sampling and remedial strategies used in the seriously contaminated areas. Accordingly, 114 surface samples, collected from an industrially contaminated site in northern China, were assessed for 16 polycyclic aromatic hydrocarbons (PAHs) and were analyzed using multivariate statistical and spatial autocorrelation techniques. The results showed that the PCA leads to a reduction in the initial dimension of the dataset to two components, dominated by Chr, Bbf&Bkf, Inp, Daa, Bgp, and Nap were good representations of the 16 original PAHs; Global Moran's I statistics indicated that the significant autocorrelations were detected and the autocorrelation distances of six indicator PAHs were 750, 850, 1,200, 850, 750, and 1,200 m, respectively; there were visible high-high values (hotspots) clustered in the mid-bottom part of the site through the Local Moran's I index analysis. Hotspot identification and spatial distribution results can play a key role in contaminated site investigation and management.

197 nm femtosecond laser-pulse duration: comparison of autocorrelation measurements

An easy and reliable way is presented to measure the duration of UV femtosecond laser pulses of λ < 200 nm. The used autocorrelation techniques are based on two-photon absorption (TPA) in different TPA media, especially calcium fluoride (CaF2). For 197 nm, the laser-pulse energy transmission and the laser-induced fluorescence of self-trapped excitons at 278 nm are applied. Both methods yield nearly the same second-order autocorrelation functions allowing to analyze the investigated laser pulse and obtain its duration of \((350\pm10)\) fs.