Aperture Separation Research Articles

Development and verification of the signal to noise ratio for a layer of turbulence in a multi-layer atmosphere.

Wide-field image correction in systems that look through the atmosphere generally requires a tomographic reconstruction of the turbulence volume to compensate for anisoplanatism. The reconstruction is conditioned by estimating the turbulence volume as a profile of thin homogeneous layers. We present the signal to noise ratio (SNR) of a layer, which quantifies how difficult a single layer of homogeneous turbulence is to detect with wavefront slope measurements. The signal is the sum of wavefront tip and tilt variances at the signal layer, and the noise is the sum of wavefront tip and tilt auto-correlations given the aperture shape and projected aperture separations at all non-signal layers. An analytic expression for layer SNR is found for Kolmogorov and von Kármán turbulence models, then verified with a Monte Carlo simulation. We show that the Kolmogorov layer SNR is a function of only layer Fried length, the spatio-angular sampling of the system, and normalized aperture separation at the layer. In addition to these parameters, the von Kármán layer SNR also depends on aperture size, and layer inner and outer scales. Due to the infinite outer scale, layers of Kolmogorov turbulence tend to have lower SNR than von Kármán layers. We conclude that the layer SNR is a statistically valid performance metric to be used when designing, simulating, operating, and quantifying the performance of any system that measures properties of layers of turbulence in the atmosphere from slope data.

A nanophotonic interferometer

The transmission of light through sub-wavelength apertures (zero-mode waveguides, ZMW) in metal films is well-explored. It introduces both an amplitude modulation as well as a phase shift to the oscillating electromagnetic field. We propose a nanophotonic interferometer by bringing two ZMW (∼100 nm diameter) in proximity and monitoring the distribution of transmitted light in the back-focal plane of collecting microscope objective (1.3 N.A.). We demonstrate that both an asymmetry induced by the binding of a quantum dot in one of the two ZMW, as well as an asymmetry in ZMW diameter yield qualitatively similar transmission patterns. We find that the complex pattern can be quantified through a scalar measure of asymmetry along the symmetry axis of the aperture pair. In a combined experimental and computational exploration of detectors with differing ZMW diameters, we find that the scalar asymmetry is a monotonous function of the diameter difference of the two apertures, and that the scalar asymmetry measure is higher if the sample is slightly displaced from the focal plane of the collecting microscope objective. An optimization of the detector geometry determined that the maximum response is achieved at an aperture separation that is comparable to the wavelength on the exit side of the sensor. For small separations of apertures, on the order of a quarter of the wavelength and less, the signal is strongly polarization dependent, while for larger separations, on the order of the wavelength or larger, the signal becomes essentially polarization-independent.

Validity of Orifice equation and impact of building parameters on wind-induced natural ventilation rates with minute mean wind pressure difference

Natural ventilation is gaining more popularity in recent decades as a sustainable strategy to reduce energy consumption and improve indoor air quality. Due to variable turbulent characteristics, there is a lack of a practical guide that helps engineers accurately predict and maximize wind-induced ventilation rates. The current work intends to investigate the factors that influence the ventilation rates by focusing on cases that have a minor wind pressure coefficient difference. This paper presents an experimental study of single- and double-sided wind-induced ventilation mainly through two openings of a reduced scale building with three different building aspect ratios in a boundary-layer wind tunnel. A continuous dose method of tracer gas technique was used to evaluate the ventilation rate. Important factors that affect ventilation rates such as velocity in the vicinity of the wall where the opening is assumed to exist (expressed as “nearby velocity”) and surface wind pressure were measured. Flow visualization was also performed to help to understand the ventilation mechanism caused by turbulence.The experimental results showed that building aspect ratios of 1:2 and 1:3 generally led to higher ventilation rates than that of 1:1. Linear correlation between aperture separation and ventilation rates was reconfirmed regardless of openings symmetry. We noticed that the Orifice equation works well when the wind pressure coefficient difference is higher than 0.1. There is a positive correlation between the fluctuation of wind pressure coefficient and ventilation rates. The nearby velocity of the sealed model was uncorrelated with ventilation rates in this experiment.

Приемо-передающий облучатель зеркальных антенн систем спутниковой связи

The paper presents an approach to the synthesis of a transceiving feed for axisymmetric double reflector antennas based on the aperture separation of C- and Ka-band signals of coaxial and circular waveguides with a maximum separation of the extreme bands of 10 : 1. The development and selection of a coaxial feed design for constructing a feed system of the transceiving reflector antenna was carried out. Optimal parameters of the coaxial feed were determined and experimentally proved to provide a feed angle level of the subreflector of 30◦ and a VSWR of 1.25 for the C- and Ka-band outputs in the receiving channels and 1.15 and 1.3 for the C- and Ka-band transmission channels, respectively, with the bandwidth of Сreception — 21 %, Сtransmission — 18.2 %, Каreception — 8.6 %, Каtransmission — 7.8 %. The results of practical implementation are presented.

Method for Measuring the Second-Order Moment of Atmospheric Turbulence

The turbulence moment of order m (μm) is defined as the refractive index structure constant Cn2 integrated over the whole path z with path-weighting function zm. Optical effects of atmospheric turbulence are directly related to turbulence moments. To evaluate the optical effects of atmospheric turbulence, it is necessary to measure the turbulence moment. It is well known that zero-order moments of turbulence (μ0) and five-thirds-order moments of turbulence (μ5/3), which correspond to the seeing and the isoplanatic angles, respectively, have been monitored as routine parameters in astronomical site testing. However, the direct measurement of second-order moments of turbulence (μ2) of the whole layer atmosphere has not been reported. Using a star as the light source, it has been found that μ2 can be measured through the covariance of the irradiance in two receiver apertures with suitable aperture size and aperture separation. Numerical results show that the theoretical error of this novel method is negligible in all the typical turbulence models. This method enabled us to monitor μ2 as a routine parameter in astronomical site testing, which is helpful to understand the characteristics of atmospheric turbulence better combined with μ0 and μ5/3.

Wind driven “pumping” fluid flow and turbulent mean oscillation across high-rise building enclosures with multiple naturally ventilated apertures

This paper presents a study on the impact of incoming wind velocity magnitudes and horizontal aperture separations on natural ventilation flows in a single-sided wind-driven naturally ventilated building with two apertures (SS2) on the rear wall or the front wall. Both the velocity fields and the contaminant concentration fields were simulated and investigated. The present study is based on CFD simulations with unsteady Reynolds-averaged Navier-Stokes (URANS) SST k-ω model. A vortex shedding flow mechanism has been identified when the two apertures are mounted either on the rear wall or on the front wall, through which the air flow oscillates at a certain rate. CFD results further demonstrate that the oscillating frequency increases with the incoming wind magnitude and is nearly independent of the horizontal aperture separations. For the same building configuration, the root mean square (r.m.s) of the non-dimensional ventilation flow rate is independent of the incoming wind speed, whereas it decreases when the horizontal aperture separation is reduced. The time-averaged contaminant concentration fields also validated the different ventilation performance of different aperture configurations. This novel ventilation mechanism could be applied to alleviate the poor ventilation performance of buildings with single-sided apertures.

Two-dimensional numerical simulation of wind driven ventilation across a building enclosure with two free apertures on the rear side: Vortex shedding and “pumping flow mechanism”

This paper investigates the single-sided wind driven natural ventilation with two apertures on the rear wall, regarding different horizontal aperture separations, magnitudes of incoming wind speed and various side ratios (L/D, length/width). In the present work, CFD simulations are performed with Reynolds-Averaged Navier-Stokes (RANS) approach, and pressure correction linked equation is adopted to provide closure. The simulation results indicate that wind flow behind the rear wall is essentially driven by the pumping mechanism due to vortex shedding, in which wind flow direction alternates at a mean rate. The frequency of the “pumping” flow increases dramatically with the wind flow velocity at a wide range of Reynolds numbers. The decrease of free aperture separation reduces the enclosure flow rate whilst the frequency of the wind flow almost maintains at a constant value. The increase of sidewall length has similar impacts on the ventilation frequency and the dimensionless ventilation flow rate. Numerical investigations further demonstrate that non-dimensional ventilation rate can be promoted by increasing incoming wind velocities when the wind speed is less than 5 m/s. This promotion will be restrained when the wind speed is further increased. The simulation results of built enclosure flow rate, frequency of enclosure flow can be applied to improve the ventilation performance of buildings merely with rear wall openings.

Hand anthropometry and the limits of aperture separation determine the utility of Weber's law in grasping and manual estimation.

Recent work proposed that biomechanical constraints in aperture separation limit the utility of Weber's law in determining whether dissociable visual codes support grasping and manual estimation. We tested this assertion by having participants precision grasp, manually estimate and complete a method of adjustment task to targets scaled within and beyond the range of their maximal aperture separation (i.e., from 20 to 140% of participant-specific maximal aperture separation: MAS). For grasping and manual estimation tasks, just-noticeable-difference (JND) scores were computed via the within-participant standard deviations in peak grip aperture, whereas method of adjustment JNDs werecomputed via the within-participant standard deviations in response output. Method of adjustment JNDs increased linearly across the range of targets; that is, responses adhered to Weber's law. Manual estimation JNDs linearly increased for targets 20-100% of MAS and then decreased for targets 120-140% of MAS. In turn, grasping JNDs for targets 20% through 80% of MAS did not differ and were larger than targets 100-140% of MAS. That manual estimation and grasping showed a decrease in JNDs for the largest targets indicates that participants were at their biomechanical limits in aperture shaping, and the fact that the target showing the JND decrease differed between tasks (i.e., manual estimation = 100% of MAS; grasping = 80% of MAS) is attributed to the fact that grasping-but not manual estimation-requires a safety-margin task-set. Accordingly, manual estimations and grasping across a range of functionally 'graspable' targets, respectively, adhered to and violated Weber's law-a result interpreted to reflect the use of dissociable visual codes.

Vision for action and perception elicit dissociable adherence to Weber's law across a range of 'graspable' target objects.

A number of studies have reported that grasps and manual estimations of differently sized target objects (e.g., 20 through 70mm) violate and adhere to Weber's law, respectively (e.g., Ganel et al. 2008a, Curr Biol 18:R599-R601)-a result interpreted as evidence that separate visual codes support actions (i.e., absolute) and perceptions (i.e., relative). More recent work employing a broader range of target objects (i.e., 5 through 120mm) has laid question to this claim and proposed that grasps for 'larger' target objects (i.e., >20mm) elicit an inverse relationship to Weber's law and that manual estimations for target objects greater than 40mm violate the law (Bruno et al. 2016, Neuropsychologia 91:327-334). In accounting for this finding, it was proposed that biomechanical limits in aperture shaping preclude the application of Weber's law for larger target objects. It is, however, important to note that the work supporting a biomechanical account may have employed target objects that approached -or were beyond-some participants' maximal aperture separation. The present investigation examined whether grasps and manual estimations differentially adhere to Weber's law across a continuous range of functionally 'graspable' target objects (i.e., 10,…,80% of participant-specific maximal aperture separation). In addition, we employed a method of adjustment task to examine whether manual estimation provides a valid proxy for a traditional measure of perceptual judgment. Manual estimation and method of adjustment tasks demonstrated adherence to Weber's law across the continuous range of target objects used here, whereas grasps violated the law. Thus, results evince that grasps and manual estimations of graspable target objects are, respectively, mediated via absolute and relative visual information.

Interferometric synthetic aperture ladar using code division multiple access apertures.

This paper describes a multi-static interferometric synthetic aperture ladar (IFSAL) for high-resolution, high-precision 3D imaging. Code division multiple access apertures with periodic, pseudorandom noise waveforms are used to create aperture diversity and overcome the ambiguity associated with the aperture separation requirements for interferometric synthetic aperture ladar. The basic theory for mapping relative aperture phase to a high-precision elevation profile is derived for a multi-static IFSAL system and subsequent processing steps are presented. An analytic model and computer simulation are developed to demonstrate the 3D imaging capability of an IFSAL system.

Manual estimations of functionally graspable target objects adhere to Weber's law.

The manual estimation task requires that participants separate the distance between their thumb and forefinger until they perceive it to match the size of a target object. Ganel and colleagues (Curr Biol 18:R599-R601, 2008a) demonstrated that manual estimations yield just-noticeable-difference (JND) scores that linearly increased with increasing target object size; that is, responses adhered to Weber's law and thus evince response mediation via relative and perception-based visual information. In turn, more recent work has reported that the size of a target object influences whether JNDs provide a reliable metric for evaluating the nature of the visual information supporting manual estimations. In particular, Bruno et al. (Neuropsychologia 91:327-334, 2016) reported that JNDs for 'large' target objects (i.e., 80 and 120mm) violate Weber's law due to biomechanical limits in aperture opening. It is, however, important to recognize that the absolute size of the 'large' target objects employed by Bruno et al. may have exceeded some participants' functional aperture separation and resulted in a biomechanical strategy serving as the only viable response mode. Hence, the present investigation employed a manual estimation task wherein target object sizes were proportionately matched to decile increments (i.e., 10, 20, …, 70 and 80%) of individual participants' maximal aperture separation. Results showed that JNDs increased linearly with increasing target object size. Accordingly, we propose that manual estimations of target objects within a functionally 'graspable' range adhere to Weber's law and are mediated via relative and perception-based visual information.

Impact of aperture separation on wind-driven single-sided natural ventilation

This paper presents a study of the impact of horizontal aperture separation in single-sided ventilation flows with two apertures (SS2). The study is based on wind tunnel measurements and dimensional analysis. The results show that the SS2 ventilation flow rate, scaled with incoming wind velocity and aperture area, depends on the incoming wind angle relative to the aperture façade, θ, and on the aperture separation scaled by building width, s′. For most wind angles, the ventilation flow increases as the square-root of s′. This study also identified a novel flow driving mechanism – vortex shedding: when the ventilation openings are on the leeward side of the building and the wind is nearly head-on, the flow is driven by a pumping mechanism due to vortex shedding.

Enantioselective Optical Trapping of Chiral Nanoparticles with Plasmonic Tweezers

Enantiomer separation is a critical step in many chemical syntheses, particularly for pharmaceuticals, but prevailing chemical methods remain inefficient. Here, we introduce an optical technique to sort chiral specimens using coaxial plasmonic apertures. These apertures are composed of a deeply subwavelength silica channel embedded in silver and can stably trap sub-20 nm dielectric nanoparticles. Using both full-field simulations and analytic calculations, we show that selective trapping of enantiomers can be achieved with circularly polarized illumination. Opposite enantiomers experience distinct trapping forces in both sign and magnitude: one is trapped in a deep potential well, while the other is repelled with a potential barrier. These potentials maintain opposite signs across a range of chiral polarizabilities and enantiomer–aperture separations. Our theory indicates that the interaction of chiral light and chiral specimens can be mediated by achiral plasmonic apertures, providing a possible route to...

Inter-aperture correlation in MIMO free space optical systems

We present a unified framework for determining the inter-aperture separations in multiple-input–multiple-output (MIMO) free space optical (FSO) systems such that the transmitter–receiver paths are resolvable. The analysis framework is also useful in determining the amount of spatial correlation for a given set of system configuration parameters and aperture separations. It is applicable to both point apertures and also apertures with larger diameters and can be used at both transmit and receive arrays. We show that the results obtained via theoretical derivations are in good agreement with those in the literature obtained via measurements or simulations. The theoretical calculations reveal that even under strong turbulence conditions and very long link distances, aperture separations at the order of a few tens of centimeters are sufficient to have resolvable paths with independent fading gains. Furthermore, the channel correlations increase relatively slowly with decreasing inter-aperture separations which are below these values. We also provide design guidelines to obtain resolvable paths for several commonly used system configurations.

Isoplanatic angle in finite distance: theory analysis on measurement feasibility

Previous studies have shown that the isoplanatic angle (θ₀) can be measured by using plane-wave scintillation. The problem of measuring the isoplanatic angle in a finite distance using spherical-wave scintillation is considered in this Letter. Based on theoretical analysis and numerical evaluation, we found that by selecting suitable aperture size and aperture separations, the isoplanatic angle can be estimated through spherical-wave scintillation and covariance of irradiance in three received apertures using a point source. The error of θ₀ measured by this method is less than 6% in typical turbulence models.

Design of a Large Single-Aperture Dipole Magnet for HL-LHC Upgrade

An upgrade of the low-beta insertion system for the ATLAS and Compact Muon Solenoid experiments is proposed in the high luminosity Large Hadron Collider upgrade project. It includes final beam focusing quadrupoles, beam separation and recombination dipoles, and larger aperture matching section quadrupoles. KEK is in charge of the conceptual design of the large aperture separation dipole D1. The latest design parameters are a main field of ~ 5 T at 1.9 K with Nb-Ti superconducting technology, a coil aperture of 160 mm, and a cos-theta one-layer coil with Large Hadron Collider dipole cable. Because the new D1 is expected to be operated in a very high radiation environment, radiation resistance and a cooling scheme are being carefully considered. The collaring-yoke structure is adopted to provide the mechanical support for the single-layer Nb-Ti coil. We summarize the design study of this magnet, including i) the very large iron saturation effect on field quality due to the large aperture and limited size of the iron yoke, ii) the stray field at the outer surface of the iron cryostat, and iii) the stress management from room temperature assembly to final operation.

Experimental observation of spatial correlation of scintillations from an extended incoherent source

The optical irradiance scintillation for an extended source and a dual-aperture system with variable aperture separation is analyzed with respect to the effect of turbulence. The scintillation index and the correlation between the dual-aperture signals emanating from an extended source are used to study the turbulence spectral model and the deviation from the common Kolmogorov atmospheric weak-turbulence model. Signal decorrelation as a function of aperture separation is presented. The relative errors introduced by using dual-aperture systems are also discussed.

Variation of fluorescence in tissue with temperature

Previous studies demonstrated a decrease in fluorescence intensity as tissue temperature increased. In vitro samples were increased from room temperature and in vivo canine liver from body temperature. This study investigated variations in fluorescence intensity with temperatures starting at 14°C and compared in vivo and in vitro results for consistency. A fiber optic-based noninvasive system was used to characterize the temperature effect on tissue fluorescence in hamster dorsal skin in vivo, and in sclera and cornea of enucleated pig eyes in vitro. As tissue was allowed to progress through the temperature range of 14-42°C, the spectra of auto-fluorescence with respect to temperature was sampled every 1-2 minutes. A pulsed nitrogen laser was used to excite fluorescence through a fiber optic probe with a source-detector aperture separation of 370 µm. Fluorescence intensity decreased as temperature increased from 14 to 42°C in a phantom containing Rhodamine B dye. Results from both in vivo and in vitro tissue followed the same trend of decreasing intensity as tissue temperature increased from 14°C. Spectral intensity lineshape changed around 450 nm due to absorption from tissue. Cooling a tissue increased fluorescence intensity of skin in vivo, in all experiments. In vitro results were consistent with in vivo measurements.

A twin aperture resistive quadrupole for the LHC

The European Organization for Nuclear Research (CERN) is constructing the Large Hadron Collider (LHC). The LHC's cleaning insertions require 48 twin aperture resistive quadrupoles. These 3.1 m long magnets have a gradient of 35 T/m for an inscribed circle of 46 mm diameter and an aperture separation distance of 224 mm. This magnet project is part of the Canadian contribution to the LHC. A prototype magnet was delivered in May 1998 and measured at CERN. Design changes were made based on the results. Due to the small apertures and the complicated geometry, the mechanical precision of the laminations and stacks is the main issue in the production of these quadrupoles. Series production will start in October 1999. The design and the measurement results are described in this paper.

Simultaneous implementation of Leendertz and Duffy's methods for in-plane displacement measurement

Leendertz two-beam illumination and Duffy's two-aperture arrangements are two well established optical configurations in speckle interferometry for in-plane displacement measurement. However, the measurement sensitivities of both these techniques are quite different. In Leendertz method the interbeam angle between the two illumination beams determines the sensitivity, while in Duffy's arrangement, it is governed by the subtense formed by aperture separation at the object. In the present paper, we implement both these configurations simultaneously for in-plane displacement measurement. The proposed optical arrangement extends the range of in-plane displacement measurement considerably. Detailed theoretical and experimental results are presented.