Aperture Length Research Articles

Diffractive focusing of hard x rays generated by femtosecond laser-driven plasma

Current focusing systems for hard x rays produced by laser-based sources provide focal spots of the order of 30 µm or larger. We propose a diffractive system that can improve this resolution. This system consists of a hollow cylinder perforated in a metal sheet, with the aperture size and length dependent on the desired focusing properties. Various configurations of this diffractive lens are analyzed and compared using numerical algorithms for beam propagation. Nanometric transverse resolutions (FWHM=550λ, which corresponds to 39 nm at λ=0.071 nm) and intensity gains larger than 14 are predicted for a configuration with a working distance of 1 mm on each side of the lens.

Design of Dual-Band Multimode Profiled Smooth-Walled Horn Antenna for Satellite Communication

This letter is concerned with design and development of a new dual-band multimode profiled smooth-walled horn (DBPH) at Ku-band (uplink and downlink frequency bands). The horn is analyzed and optimized using mode-matching-technique-based software (Mician ? Wave wizard). Predicted radiation patterns and properties of the DBPH are compared to corresponding measured results showing good agreement. Relatively higher edge of coverage (EOC) gain and low cross-polar level at EOC is achieved with optimized DBPH by synthesizing higher order modes TE1m and TM1m in proper ratio of amplitude and phase. An innovative modified hyperbolic profile is used to optimize the various performance objectives. The DBPH also provides higher bore-sight gain, higher aperture efficiency, and slightly wider beam (i.e. lower roll-off) in comparison to a conical horn having similar aperture dimension and length. The horn would be used as an element of a direct radiating horn cluster for multibeam satellite antenna application.

Reproduction of the vermetid gastropod Dendropoma maximum (Sowerby, 1825) in Moorea, French Polynesia

Vermetidae are sessile molluscs with tubular shells cemented to the substrate. Although they are common inhabitants of coral reefs and rocky shores, little is known about the basic biology of most species. Dendropoma maximum is the largest vermetid species and is common across its Indo-Pacific range. Here we report on a study of reproductive and morphological traits in D. maximum from the lagoon around Moorea, French Polynesia. Both body length and diameter of shell aperture (generally the only visible part of the animal in the field) were good predictors of body weight and this relationship did not differ between males and females. Shell apertural diameter is therefore a good index of body size in D. maximum. The sex ratio became increasingly dominated by females with increasing body size, which is suggestive of protandric hermaphroditism. Probability of brooding, the number and size of brooded egg capsules, and the number of embryos per capsule were all positively related to female size. Females released veliger larvae that lived without additional food for up to 10 days, but were observed to feed when offered cultured phytoplankton on the final day. This is the first direct observation of feeding by larvae in this genus, and a planktotrophic larval stage has implications for potential dispersal in D. maximum.

Tunable figure of merit for a negative-index metamaterial with a sandwich configuration

Metamaterials with a free-standing Ag-SU-8-Ag sandwich configuration, perforated with a periodic array of cross-dipole apertures, are fabricated. The transmittance spectra of both experiment and simulation have indicated the existence of two transmission bands in mid-IR frequencies. The high-frequency transmission peak is the guided mode associated with the surface plasmon polariton (SPP), and its location strongly depends on the arm length of the cross-dipole. The lower-frequency transmission peak is proposed to be the antisymmetric plasmon polariton mode, and its location hardly shifts as the size of the cross-dipole varies. Numerical simulations indicate that, on increasing the arm length of the cross-dipole aperture, the coaction of the electric dipole-like mode and the SPP-associated guided mode results in a better figure of merit.

The relay propagation of partially coherent cosh–Gaussian–Schell beams in turbulent atmosphere

We have studied the relay propagation of a partially coherent cosh–Gaussian–Schell beam in turbulent atmosphere. Analytical expressions for both the cross-spectral density at the relay system and average intensity at the target are derived. By using the analytical expressions some special cases are studied and some numerical simulation comparisons are made, especially the effects of the coherence of the beam, turbulence strength, aperture and its size on the relay propagation. Our study shows that the effects of diffraction and coherence of initial beam on the intensity profiles at the relay system are so small that they can be neglected when the effects due to turbulence are large enough. Even though the correction to the receiving beam at relay system is important, it is not necessary to improve the receiving beam when the effect of turbulence over the travel path is strong. A high peak intensity at target can be obtained by optimizing these factors, such as propagation distance, aperture and spatial correlation length.

Theory and simulation of ionospheric effects on synthetic aperture radar

Space-based synthetic aperture radar (SAR) necessarily involves imaging through the ionosphere. At low frequencies (VHF, UHF and L-band) the ionosphere will degrade the SAR image. Previous work has shown that the amount of image degradation strongly depends on the integrated strength of ionospheric turbulence, CkL. The focusing, sidelobes and integrated sidelobe ratio all depend on CkL in a manner that can be directly predicted by a simple analytic theory, which is reviewed and extended to cover any synthetic aperture length. Simulations of the ionosphere, using a thin phase screen parabolic equation approach, are performed under different ionospheric conditions for a number of possible SAR systems and the results compared with the analytic theory. It is concluded that, provided that the scattering is weak, the theory represents a good predictor of SAR performance, even at UHF. The known statistics of CkL can therefore be used to predict the performance of any trans-ionospheric SAR without performing a simulation.

A simple method for evaluating the wavefront compensation error of diffractive liquid-crystal wavefront correctors

A simple method for evaluating the wavefront compensation error of diffractive liquid-crystal wavefront correctors (DLCWFCs) for atmospheric turbulence correction is reported. A simple formula which describes the relationship between pixel number, DLCWFC aperture, quantization level, and atmospheric coherence length was derived based on the calculated atmospheric turbulence wavefronts using Kolmogorov atmospheric turbulence theory. It was found that the pixel number across the DLCWFC aperture is a linear function of the telescope aperture and the quantization level, and it is an exponential function of the atmosphere coherence length. These results are useful for people using DLCWFCs in atmospheric turbulence correction for large-aperture telescopes.

Aberration effects on femtosecond pulses generated by nonideal achromatic doublets

There are three main effects that affect the femtosecond pulse focusing process near the focal plane of a refractive lens: the group velocity dispersion (GVD), the propagation time difference (PTD), and the aberrations of the lens. In this paper we study in detail these effects generated by nonideal achromatic doublets based on a Fourier-optical analysis and Seidel aberration theory considering lens material, wavelength range, lens surface design, and temporally and spatially uniform and Gaussian intensity distributions. We show that the residual chromatic aberration in achromatic lenses, which has been neglected so far, has a considerable effect on the focusing of pulses shorter than 20 fs in the spectral range between the UV and IR, 300 to 1100 nm, and is particularly important in the blue and UV spectral range. We present a general fitted function for an estimation of the pulse stretching parameter, which depends only on the numerical aperture and focal length of the doublet as well as the wavelength of the carrier of the pulse.

The Neogene flora from Badaogou of Changbai, NE China — Most similar living relatives of selected taxa and relations to the European record

The diatomites exposed in the surroundings of Badaogou town, in the counties of Linjiang and Changbai, Jilin Province of China, bear a rich Neogene leaf assemblage. Previously, the most similar living relatives (MSLRs) of Sassafras paratsumu Chen (Lauraceae), Acer rotundatum Huzioka (Sapindaceae, subfam. Aceroideae, sect. Platanoidea), and A. trifoliatum Geng (Sapindaceae, subfam. Aceroideae, sect. Trifoliata) have been determined based solely on gross morphology. Cuticle preservation in the plant material from Badaogou allows the comparison of epidermal features. Thus, Sassafras tzumu (Hemsley) Hemsley is confirmed as the unambiguous MSLR of S. paratsumu. The cuticular features of Acer rotundatum best match A. mono Maximowicz and A. cappadocicum Gleditsch. Only the stoma and aperture length are slightly shorter in A. mono than in A. cappadocicum, therefore more closely resembling A. rotundatum. Based on gross morphology, earlier workers already regarded A. triflorum Komarov as the MSLR of Acer trifoliatum. The cuticle, however, bears some differences. In A. trifoliatum the stomata are slightly bigger, the adaxial surface is only faintly, if at all, striate, and the anticlinal walls are straight; in A. triflorum, however, the anticlinal walls are undulate and the surface shows distinct striation. Compared to other East Asian members of sect. Trifoliata – A. griseum (Franchet) Pax, A. mandshuricum Maximowicz, and A. maximowiczianum Miquel – the combination of features match best with A. triflorum. This includes leaflet shape and size, trichome bases of simple, filiform trichomes girdled by roundish cells, and the presence of uniserial, short-celled, glandular trichomes in intercostal areas abaxially. The modern counterparts of the fossil taxa are elements of the modern flora of China.

A new species of Plagiodontes from Argentina, and new data on the anatomy of four other species in the genus (Gastropoda: Orthalicidae, Odontostominae)

Plagiodontes weyrauchi sp. nov. is among the smallest species in the genus Plagiodontes (Doering, 1877). Statistical multivariate analyses confirmed its distinctiveness from other species, mostly because of its high number of whorls (8–9.75), relatively small width, and low proportions of last whorl and aperture length to shell length; multivariate discriminant analysis applied to this species and its most similar relative, P. multiplicatus parvus, correctly identified 100% of specimens. Pallial and genital anatomy supports P. weyrauchi as a different species, mainly because of the position of the ureteric pore, the small general size of the genital system, the presence of an accessory lobe alongside the verge, and the vaginal internal structure. Internal anatomy of four other small-sized taxa in the genus was studied. Two of them have a verge with no accessory lobe [P. dentatus (Wood, 1828) and P. patagonicus (d'Orbigny, 1835)], while the remaining two have an accessory lobe with different characteristics [P. rocae Doering, 1881 and P. multiplicatus parvus (Hylton-Scott, 1952)]. Internal structure of the vagina also shows specific differences among Plagiodontes species; their phylogenetic significance will be further investigated.

Influence of the ionospheric plasma density fluctuations on the subsurface sounding of the Martian soil by a synthetic aperture radar

We consider the problem of subsurface sounding of the planetary interior with an ultrawideband synthetic aperture radar through the ionosphere containing random density fluctuations. Numerical simulation is performed for a variety of typical parameters of the medium and the instrument. The impact of the ionospheric fluctuations on the sounding is estimated. The question of optimal choice of the synthetic aperture length is addressed.

개구면을 통한 마이크로스트립-수직 슬롯 라인 결합 구조의 회로망 해석과 모델링

A general analysis of a microstrip-to-vertically mounted slotline(VMS) coupler is presented with a view to developing an equivalent circuit, and the efficient evaluation of the related circuit element values. Based on this theory, the effects of frequency and structure parameters such as aperture length and VMS width on the characteristics of the coupler are studied. In order to check the validity of the proposed analysis and design theory, a C-band linearly tapered slot antenna fed by an aperture-coupled back-to-back microstripline-to- VMS coupling structure is optimally designed using a hybrid genetic algorithm. Moreover, the computed characteristics from the network analysis is compared to the measurement and simulation results. The obtained results fully validate the efficiency and accuracy of the proposed network model.

Measurement Guideline of Fresnel-Field Antenna Measurement Method

In this letter, a parametric analysis of the Fresnel-field antenna measurement method is performed for a square aperture. As a result, the optimum number of Fresnel fields for one far-field point is guided as Mopt=Nopt=D 2 /λR+5, where D is the antenna diameter, λ is the wavelength, and R is the distance between the source antenna and the antenna under test. For the aperture size 5 ≤ Lx/λ ≤ 20, the tolerable distances for gain errors of 0.5 dB and 0.2 dB can be guided as R0.5 dB ≈ 1.2Lx/λ and R0.2 dB ≈ 2.0Lx/λ, where Lx is the lateral length of the square aperture. The tolerable distances for 20 ≤ Lx/λ ≤ 200 are also proposed. This measurement guideline can be fully utilized when performing the Fresnel-field antenna measurement method.

Estimation of the Minimum Number of Tracks for SAR Tomography

Synthetic aperture radar tomography (SARTom) is the natural extension of SAR interferometry to solve for multiple phase centers within a resolution cell and obtain the 3-D representation of a scene. This paper deals with the determination of the minimum number of tracks required to perform SARTom. Through the prolate spheroidal wave functions, the number of equivalent targets of a volumetric source is derived, and from it, the minimum number of observations required to apply subspace superresolution methods is computed. The minimum tomographic aperture length is also investigated. The results are validated on real data acquired in L-band by the experimental SAR system of the German Aerospace Center.

Effective lateral modulations with applications to shear modulus reconstruction using displacement vector measurement

High accuracy in measuring target motions can be realized by combined use of our previously developed lateral Gaussian envelope cosine modulation method (LGECMM) and displacement vector measurement methods that enable simultaneous axial and lateral displacement measurements, such as the multidimensional autocorrelation method (MAM). In this paper, LGECMM is improved by using parabolic functions and Hanning windows instead of Gaussian functions in the apodization function, i.e., parabolic apodization and Hanning apodization. The new modulations enable decreases in effective aperture length (i.e., channels) and yield more accurate displacement vector measurements than LGECMM due to increased echo signal-to-noise ratio and lateral spatial resolution. That is, on the basis of a priori knowledge about ultrasound propagation using the focusing scheme and shape of the apodization function, we stopped using Fraunhofer approximation. As practical applications of the modulations, for an agar phantom that is deformed in a lateral direction, stable and accurate 2-D shear modulus reconstructions are performed using our previously developed direct inversion approach together with 2-D strain tensor measurements using MAM.

Acoustic characterization of high intensity focused ultrasound fields: A combined measurement and modeling approach

Acoustic characterization of high intensity focused ultrasound (HIFU) fields is important both for the accurate prediction of ultrasound induced bioeffects in tissues and for the development of regulatory standards for clinical HIFU devices. In this paper, a method to determine HIFU field parameters at and around the focus is proposed. Nonlinear pressure waveforms were measured and modeled in water and in a tissue-mimicking gel phantom for a 2 MHz transducer with an aperture and focal length of 4.4 cm. Measurements were performed with a fiber optic probe hydrophone at intensity levels up to 24,000 W/cm(2). The inputs to a Khokhlov-Zabolotskaya-Kuznetsov-type numerical model were determined based on experimental low amplitude beam plots. Strongly asymmetric waveforms with peak positive pressures up to 80 MPa and peak negative pressures up to 15 MPa were obtained both numerically and experimentally. Numerical simulations and experimental measurements agreed well; however, when steep shocks were present in the waveform at focal intensity levels higher than 6000 W/cm(2), lower values of the peak positive pressure were observed in the measured waveforms. This underrepresentation was attributed mainly to the limited hydrophone bandwidth of 100 MHz. It is shown that a combination of measurements and modeling is necessary to enable accurate characterization of HIFU fields.

Linear array beamforming in harbor environments.

The use of linear arrays of sources to focus low‐frequency sound on a target zone of limited spatial extent in a very shallow water environment is a topic of current interest for applications such as harbor defense. While linear array beamforming provides a starting point, the analytical expressions for beam response of a line array found in standard acoustics texts are developed for arrays of compact simple sources in a free‐field environment. In contrast, the typical harbor environment is characterized by very shallow water depths, varying bottom conditions, irregular vertical boundaries, and the presence of multiple scattering bodies. In addition to environmental conditions, the size and cost of low‐ frequency sources limit the validity of the simple source assumption and impose practical limits on the number of sources and maximum aperture length. Standard beamforming calculations are compared to measured data from a June 2008 harbor acoustic propagation experiment to quantify limitations of the linear array theory in estimating the beam response and three‐dimensional sound field generated by an array of a small number of noncompact low‐frequency sound sources in a real harbor environment. The relative impacts of environmental factors and source characteristics are also discussed.

Road crossing structures for amphibians and reptiles: Informing design through behavioral analysis

Road traffic causes significant amphibian and reptile mortality, which could be mitigated through the installation of road crossing structures that facilitate safe passage, but only if reptiles and amphibians are willing to use them. Through a series of behavioral choice experiments with frogs and turtles, we examined how aperture diameter, substrate type, length, and light permeability influenced individuals’ preferences for specific attributes of crossing structures, and how individuals responded to various heights of barrier fences. Snapping turtles ( Chelydra serpentina), green frogs ( Rana clamitans), and leopard frogs ( Rana pipiens) preferred larger diameter tunnels (>0.5 m) whereas painted turtles ( Chrysemys picta) preferred tunnels of intermediate (0.5–0.6 m) diameter. Green frogs preferred soil- and gravel-lined tunnels to concrete- and PVC-lined tunnels. Painted turtles showed non-random choice of different lengths of tunnel, possibly indicating some avoidance of the longest tunnel (9.1 m); although no species preferred to exit via the longest tunnels (9.1 m), members of all four species used such tunnels. Green frogs preferred tunnels with the greatest light permeability. Fences 0.6 m in height were effective barriers to green frogs, leopard frogs, and snapping turtles, whereas 0.3 m fences excluded painted turtles. We conclude that tunnels > 0.5 m in diameter lined with soil or gravel and accompanied by 0.6–0.9 m high guide fencing would best facilitate road crossing for these and likely other frog and turtle species.

Randomised trial comparing ocular lubricants and polyacrylamide hydrogel dressings in the prevention of exposure keratopathy in the critically ill

To compare the cost and effectiveness of the two most popular forms of eye care in intensive care, ocular lubricant (Lacrilube) and polyacrylamide hydrogel dressings (Geliperm); for the prevention of exposure keratopathy in the critically ill. A prospective randomised contralateral eye study was conducted at the Intensive Care Unit (ICU), Royal London Hospital, London, UK. Eighty eyes of 40 patients were recruited. Each patient received both Lacrilube and Geliperm allocated at random to different sides. A daily ophthalmology ward round was conducted. The outcome measures included the greatest palpebral aperture length, conjunctival oedema, and any exposure keratopathy. There was no statistically significant difference in the maximum corneal exposure score between the eyes treated with Lacrilube and Geliperm (P = 0.38). No significant difference in degree of chemosis or palpebral aperture was identified. Our data suggest that Geliperm is as effective as Lacrilube in the prevention of exposure keratopathy in the critically ill. We also note that nursing staff must be fully trained in its application for eye care.

Weighted near‐field focusing in an array‐based GPR

This paper presents a 3‐D imaging technique for an ultra‐wideband (UWB) ground penetrating radar (GPR) with a single transmit antenna and a linear receive array. The video impulse GPR working in the frequency band of 0.3–3 GHz has been designed in IRCTR for landmine detection, i.e., for a near‐field application. Installed on a vehicle it can image in one mechanical scan a strip of 84 cm width due to the length of array aperture. The imaging is done by software means only. The developed imaging technique combines a real aperture focusing in the array plane with a synthetic aperture focusing in the mechanical scan direction. To compensate for parasitic time delays in the array channels, a calibration procedure is also described. Owing to directional properties of transmit antenna, the distribution of signal strength over the array is nonuniform that requires an amplitude correction when focusing the real aperture. The authors analyzed how this affects the footprint of the focused array, its cross‐range resolution capability and the image quality of antipersonnel plastic landmines which were buried under different array channels. The analysis bases on experimental data sets acquired in the facilities of IRCTR and TNO‐DSS. As a result, the authors propose a weighted array focusing that improves the cross‐range resolution and provides proper imaging of typical buried landmines.