Effective amplifier noise for an optical receiver based on linear mode avalanche photodiodes

The rms noise charge induced by the amplifier for an optical receiver based on the linear-mode avalanche photodiode (APD) was analyzed. It is shown that for an amplifier with a 1-pF capacitor and a noise temperature of 100 K, the rms noise charge due to the amplifier is about 300. Since the noise charge must be small compared to the signal gain, APD gains on the order of 1000 will be required to operate the receiver in the linear mode

Non-universal pairing symmetry and pseudogap phenomena in hole- and electron-doped cuprate superconductors

Experimental studies of the pairing state of cuprate superconductors reveal asymmetric behaviors of the hole-doped (p-type) and electron-doped (n-type) cuprates. The pairing symmetry, pseudogap phenomenon, low-energy spin excitations and the spatial homogeneity of the superconducting order parameter appear to be non-universal among the cuprates, which may be attributed to competing orders. We propose that the non-universal pseudogap and nano-scale variations in the quasiparticle spectra may be the result of a charge nematic (CN) phase stabilized by disorder in highly two-dimensional (2D) p-type cuprates. The CN phase is accompanied by gapped spin excitations and competes with superconductivity (SC). In contrast, gapless spin excitations may be responsible for the absence of pseudogap and the presence of excess sub-gap spectral weight in the momentum-independent quasiparticle spectra of n-type cuprates. The physical implications and further verifications for these conjectures are discussed

Length summation in noise

To investigate the effect of background noise on visual summation, we measured the contrast detection thresholds for targets with or without a white noise mask in luminance contrast. The targets were Gabor patterns placed at 3° eccentricity to either the left or right of the fixation and elongated along an arc of the same radius to ensure equidistance from fixation for every point along the long axis. The task was a spatial two-alternative forced-choice (2AFC) paradigm in which the observer had to indicate whether the target was on the left or the right of the fixation. The threshold was measured at 75% accuracy with a staircase procedure. The detection threshold decreased with target length with slope −1/2 on log-log coordinates for target lengths between 30′ and 300′ half-height full-width (HHFW), defining a range of ideal matched-filter summation extending up to about 200′ (or about 16× the center width of the Gabor targets). The summation curves for different noise contrasts were shifted copies of each other. For the threshold versus mask contrast (TvN) functions, the target threshold was constant for noise levels up to about −22 dB, then increased with noise contrast to a linear asymptote on log-log coordinates. Since the “elbow” of the target threshold versus noise function is an index of the level of the equivalent noise experienced by the visual system during target detection, our results suggest that the signal-to-noise ratio was invariant with target length. We further show that a linear-nonlinear-linear gain-control model can fully account for these results with far fewer parameters than a matched-filter model

Comparison of direct and heterodyne detection optical intersatellite communication links

The performance of direct and heterodyne detection optical intersatellite communication links are evaluated and compared. It is shown that the performance of optical links is very sensitive to the pointing and tracking errors at the transmitter and receiver. In the presence of random pointing and tracking errors, optimal antenna gains exist that will minimize the required transmitter power. In addition to limiting the antenna gains, random pointing and tracking errors also impose a power penalty in the link budget. This power penalty is between 1.6 to 3 dB for a direct detection QPPM link, and 3 to 5 dB for a heterodyne QFSK system. For the heterodyne systems, the carrier phase noise presents another major factor of performance degradation that must be considered. In contrast, the loss due to synchronization error is small. The link budgets for direct and heterodyne detection systems are evaluated. It is shown that, for systems with large pointing and tracking errors, the link budget is dominated by the spatial tracking error, and the direct detection system shows a superior performance because it is less sensitive to the spatial tracking error. On the other hand, for systems with small pointing and tracking jitters, the antenna gains are in general limited by the launch cost, and suboptimal antenna gains are often used in practice. In which case, the heterodyne system has a slightly higher power margin because of higher receiver sensitivity

Collective modes and quasiparticle interference on the local density of states of cuprate superconductors

The energy, momentum, and temperature dependence of the quasiparticle local density of states (LDOS) of a two-dimensional d(x2)-(y2)-wave superconductor with random disorder is investigated using the first-order T-matrix approximation. The results suggest that collective modes such as spin-charge-density waves are relevant low-energy excitations of the cuprates that contribute to the observed LDOS modulations in recent scanning tunneling microscopy studies of Bi2Sr2CaCu2Ox

Pair Interaction Potentials of Colloids by Extrapolation of Confocal Microscopy Measurements of Collective Structure

A method for measuring the pair interaction potential between colloidal particles by extrapolation measurement of collective structure to infinite dilution is presented and explored using simulation and experiment. The method is particularly well suited to systems in which the colloid is fluorescent and refractive index matched with the solvent. The method involves characterizing the potential of mean force between colloidal particles in suspension by measurement of the radial distribution function using 3D direct visualization. The potentials of mean force are extrapolated to infinite dilution to yield an estimate of the pair interaction potential, $U(r)$. We use Monte Carlo (MC) simulation to test and establish our methodology as well as to explore the effects of polydispersity on the accuracy. We use poly-12-hydroxystearic acid-stabilized poly(methyl methacrylate) (PHSA-PMMA) particles dispersed in the solvent dioctyl phthalate (DOP) to test the method and assess its accuracy for three different repulsive systems for which the range has been manipulated by addition of electrolyte.Comment: 35 pages, 14 figure

Modeling the Effect of Oil Price on Global Fertilizer Prices

The main purpose of this paper is to evaluate the effect of crude oil price on global fertilizer prices in both the mean and volatility. The endogenous structural breakpoint unit root test, the autoregressive distributed lag (ARDL) model, and alternative volatility models, including the generalized autoregressive conditional heteroskedasticity (GARCH) model, Exponential GARCH (EGARCH) model, and GJR model, are used to investigate the relationship between crude oil price and six global fertilizer prices. Weekly data for 2003-2008 for the seven price series are analyzed. The empirical results from ARDL show that most fertilizer prices are significantly affected by the crude oil price, which explains why global fertilizer prices reached a peak in 2008. We also find that that the volatility of global fertilizer prices and crude oil price from March to December 2008 are higher than in other periods, and that the peak crude oil price caused greater volatility in the crude oil price and global fertilizer prices. As volatility invokes financial risk, the relationship between oil price and global fertilizer prices and their associated volatility is important for public policy relating to the development of optimal energy use, global agricultural production, and financial integration.volatility;crude oil price;global fertilizer price;non-renewable fertilizers;structural breakpoint unit root test