Precision of degree of polarization estimation in the presence of additive Gaussian detector noise

We address the problem of degree of polarization (DOP) estimation in images limited by additive Gaussian detector noise. We derive and analyze the probability density function (PDF) of the pixelwise DOP estimate, which is shown to have significantly different statistical properties than when noise is Gamma distributed (speckle). We then determine the Cramer-Rao Lower Bound and the maximum likelihood estimator of the DOP. We deduce from this study practical solutions for characterizing and reducing the noise in these images.     

Dual-frequency single-axis laser using a lead lanthanum zirconate tantalate (PLZT) birefringent etalon for millimeter wave generation: beyond the standard limit of tunability

We demonstrate the generation of optically carried, broadly tunable, millimeter-wave signals with a dual-frequency single-axis Nd:YAG laser. A frequency difference as high as 127 GHz is reached thanks to an intracavity electro-optically tunable etalon made of lead zirconate tantalate (PLZT) ceramic. We show that the available frequency range is actually limited by the bandwidth of the amplification medium, namely, far beyond the usually accepted free spectral range value in the case of a single-axis laser. Both coarse discrete and fine continuous tunabilities are obtained with the same voltage-controlled device, opening the way to widely tunable low-phase-noise optically carried submillimeter or even terahertz sources.     

Modeling the effects of interband and intraband transitions on phase and gain stabilities of quantum dot semiconductor optical amplifiers

In this paper, the effects of interband and intraband transitions on the gain and phase stabilities in quantum dot semiconductor optical amplifier (QD-SOA) are investigated both temporally and spectrally employing electrical and optical pumping schemes. For this purpose, the carrier rate equations in different energy states coupled to the traveling wave optical field equation have been numerically solved to derive the dynamical behavior of QD-SOA. Our results show that the gain and phase response can be stabled under optical pumping (OP) scheme because the role of the interband and intraband transitions on the dynamics of QD-SOA is reduced. This behavior leads to high-speed pattern effect-free cross-phase modulation (XPM) in QD-SOA. It is found that optically pumped QD-SOA can have high performance in phase based applications. Moreover, it is shown that under OP scheme although the QD-SOA has lower gain value and slower gain recovery time, the ultrafast cross-gain modulation (XGM) without pattern effect is possible and the phase is recovered within a shorter time compared to EP scheme. The behavior arises from the different capacity of the carrier reservoir for pumping schemes.     

The triel bond: a potential force for tuning anion–π interactions

Using ab-initio calculations, the mutual influence between anion–π and B···N or B···C triel bond interactions is investigated in some model complexes. The properties of these complexes are studied by molecular electrostatic potential, noncovalent interaction index, quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. According to the results, the formation of B···N or B···C triel bond interactions in the multi-component systems makes a significant shortening of anion–π distance. Such remarkable variation in the anion–π distances has not been reported previously. The strengthening of the anion–π bonding in the multi-component systems depend significantly on the nature of the anion, and it becomes larger in the order Br^? > Cl^? > F^?. The parameters derived from the QTAIM and NBO methodologies are used to study the mechanism of the cooperativity between the anion–π and triel bond interactions in the multi-component complexes.     

A perturbed hard-sphere equation of state extended to imidazolium-based ionic liquids

A perturbed hard-sphere equation of state (EOS) has been previously employed to predict pressure–volume–temperature properties of some ionic liquids (ILs) with phosphonium-, pyridinium-, and pyrrolidinium cations. In this work, we have extended the considered EOS to another class of ILs in compressed states. This class consists of 14 imidazolium-based ILs. The predicted densities were compared with those obtained from the experiment, over a broad pressure range from 0.1 to 200 MPa. From 1,122 data points examined for the aforementioned ILs, the total average absolute deviation was found to be 1.05%.     

LES study of diesel flame/wall interaction and mixing mechanisms at different wall distances

In this paper, the flame-wall interaction of reacting diesel spray under engine like conditions is investigated using large eddy simulations. The aim of this study is to understand the influence of the distance between the wall and the spray nozzle on the air entrainment rate, which is a key variable in formation/oxidation process of soot. Three experimental cases are investigated, a free jet case and two wall impingement cases with a distance from nozzle to wall of 30 mm and 50 mm, which are considered as characteristic wall impingement distances for light- and heavy-duty bores in diesel engines, respectively. The optical soot measurements imply a positive influence of wall on the rate of soot oxidation. Numerical simulations are employed to elucidate importance of different mechanisms for the air entrainment, i.e., air entrainment prior to flame lift-off position, enhanced mixing due to the wall impingement and enhanced mixing by the entrainment wave. The results show that oxidation process after the end of injection is driven by a different mixing mechanism depending on the distance to the wall. The 30 mm case resulted in a “mixing boost”, where the dominant mixing mechanism is the wall impingement vortex mixing, which gives rise to the fastest soot decay among the cases. The mixing in the 50 mm case is governed by a late wall impingement vortex mixing, giving rise to a low, but a constant air entrainment rate, i.e., a “mixing plateau”. The free jet case resulted in mixing governed by the entrainment wave mechanism. Both wall impingement cases have faster soot oxidation rate compared with the free jet case, but due to a different underlying mixing process. LES is shown to be able to replicate the line-of-sight measurements of natural OH* chemiluminescence and distribution of soot region from the optical soot diagnostics.     

Interactive effects of local smoothing window size and fundamental frequency on shimmer calculation

Slow amplitude modulation of human voice was approximated by a sinusoidal wave. The theoretical effects of smoothing window size, F₀, and modulation frequency on window amplitude average as well as calculated shimmer were mathematically derived. Subsequently, the theoretical predictions were tested using idealized and real voice signals from normal speakers. The theoretical and experimental results suggest that shimmer (when calculated using a smoothing window) is a function of window duration and modulation frequency. Window duration when defined as a constant number of pitch periods varies from speaker to speaker depending on their F₀. It may not be desirable to use local smoothing windows with a constant number of cycles for shimmer computation, especially if voices with known low-frequency amplitude modulations but notably different fundamental frequencies are compared.     

Light reflection from finite conductors with a Gaussian random roughness using Kirchhoff and geometric optics approximations

A comparison between exact solutions and two approximate models, Kirchhoff approximation (KA) and geometric optics approximation (GOA), for reflection from random Gaussian rough conductive metallic surfaces for three regimes of correlation length in both cases of polarization TM and TE has been reported. The phenomenon of excitation of surface plasmons (SPs) has been shown only at TM polarization for KA. The domains of validity of both approximate models have been quantified. It is shown that accuracy and efficiency of any approximate method depended of various parameters: surface roughness, polarization, SPs effects, wavelength, and surface materials. KA is efficient for weakly rough surfaces with correlation length greater than wavelength, but when rms height increases GOA is more suitable than KA. Excitation of SPs is observed only at TM-polarized light, only for weakly rough surfaces with correlation length less than wavelength using KA and not GOA.