Apodization Factor Research Articles

A new diamond-shaped periodic permanent magnet electromagnetic acoustic transducer for side-lobe suppression of shear horizontal ultrasonic waves

Periodic permanent magnet electromagnetic acoustic transducers (PPM EMATs) effectively generate shear horizontal (SH) ultrasonic guided waves in metallic media. Depending on their construction parameters, PPM EMATs can generate ultrasonic waves towards unwanted directions, due to the presence of side lobes in their radiation pattern. In this paper, a new PPM-like EMAT is proposed with the objective of reducing the side-lobe level in the radiation pattern of PPM EMATs. This was achieved by altering the magnets’ dimension in the PPM array following a diamond-like shape, reducing their width as one gets further from the centre of the transducer. The radiation behaviour of the EMAT was evaluated by means of an apodization factor, defined as the aperture reduction ratio. The side-lobe level showed a maximum reduction for an apodization factor of approximately 2. Four different configurations of the new PPM-like EMAT design and two reference conventional PPM EMATs were fabricated and experimentally evaluated on an aluminium plate. Experimental radiation patterns were compared to analytical model results showing good agreement. Relative to the conventional PPM EMAT, the new apodized diamond-shaped PPM EMAT presented a side lobe suppression of up to approximately 8 dB.

Two-mirror system for tunable apodization.

Here we present an optical system composed of two mirrors such that at the input/output, the light is a plane wave but with a user-defined apodization factor. The model presented is an analytic closed form with no numerical approximations or iterations. We test the model with illustrative scenarios, and the results are as expected; the system is stigmatic with the desired apodization factor. Thus, this system has several potential applications in high contrast imaging.

3D Internal Visualization of Concrete Structure Using Multifaceted Data for Ultrasonic Array Pulse-Echo Tomography.

This research proposes a 3D internal visualization using ultrasonic pulse-echo tomography technique to evaluate accurately the state of concrete structures for their efficient maintenance within a limited budget. Synthetic aperture focusing technique (SAFT) is used as a post-processing algorithm to manipulate the data measured by the ultrasonic pulse-echo technique. Multifaceted measurements improve the weakness of the existing ultrasonic pulse-echo tomography technique that cannot identify the area beyond a reflector as well as the area located far away from measuring surfaces. The application of apodization factor, pulse peak delay calibration and elimination of trivial response not only complements the weaknesses of the SAFT algorithm but also improves the accuracy of the SAFT algorithm. The results show that the proposed method reduces the unnecessary surface noise and improves the expressiveness of the reflector’s boundaries on the resulting images. It is expected that the proposed 3D internal visualization technique will provide a useful non-destructive evaluation tool in combination with another structure evaluation method.

Improving non-line-of-sight image reconstruction with weighting factors.

Non-line-of-sight (NLOS) imaging is a light-starving application that suffers from highly noisy measurement data. In order to recover the hidden scene with good contrast, it is crucial for the reconstruction algorithm to be robust against noises and artifacts. We propose here two weighting factors for the filtered backprojection (FBP) reconstruction algorithm in NLOS imaging. The apodization factor modifies the aperture (wall) function to reduce streaking artifacts, and the coherence factor evaluates the spatial coherence of measured signals for noise suppression. Both factors are simple to evaluate, and their synergistic effects lead to state-of-the-art reconstruction quality for FBP with noisy data. We demonstrate the effectiveness of the proposed weighting factors on publicly accessible experimental datasets.

Tunable Magnetization Chains Induced With Annular Parabolic Mirrors

Based on the inverse Faraday effect, tunable light-induced magnetization chains are investigated by focusing narrow annular azimuthally polarized vortex beams with a 4 $\pi$ focusing system composed of double annular parabolic mirrors. Using Richards–Wolf vector diffraction theory, approximate analytical expressions of all parameters of the magnetization chains are theoretically calculated, realizing that all parameters are tunable. The calculated results show that the parameters are independent of the amplitude distribution function of the incident beam and the apodization factor of the focusing system. Hence, different kinds of long (about $100\lambda$ ) chains with sub-wavelength lateral size can be achieved by adjusting the central angular position, the angular width, and the phase difference of two counter-propagating incident beams. Compared with lenses, parabolic mirrors can generate multi-needle fields that lenses cannot.

Gaussian apodization factor analysis for resolution of the embedded peaks in real complicated chromatographic datasets

Gaussian apodization factor analysis (GAFA) has been developed as an enhanced algorithm to assess the peak purity of the two-dimensional data, by weighting the fixed-size moving window via Gaussian formula. In GAFA method, submatrices are extracted by Gaussian apodization moving window. Therefore, each submatrix mainly characterizes the spectrum and by performing factor analysis on this Gaussian weighted submatrix, the number of principal components for each evaluated spectrum, is determined. This precise and quick determination of a rank map is successfully used for extract pure components from hyphenated chromatographic data. An algorithm based on GAFA was applied to resolve different types of overlapped simulated and real complex data of GC–MS. This algorithm finds spectra of pure component with GAFA one by one and eliminates obtained components from a data matrix and search for next pure component spectra until all the components are determined.

Multicapillary Gas Chromatography-Temperature Modulated Metal Oxide Semiconductor Sensors Array Detector for Monitoring of Volatile Organic Compounds in Closed Atmosphere Using Gaussian Apodization Factor Analysis.

A unique metal oxide semiconductor sensor (MOS) array detector with eight sensors was designed and fabricated in a PTFE chamber as an interface for coupling with multicapillary gas chromatography. This design consists of eight transfer lines with equal length between the multicapillary columns (MCC) and sensors. The deactivated capillary columns were passed through each transfer line and homemade flow splitter to distribute the same gas flow on each sensor. Using the eight ports flow splitter design helps us to equal the length of carrier gas path and flow for each sensor, minimizing the dead volume of the sensor's chamber and increasing chromatographic resolution. In addition to coupling of MCC to MOS array detector and other considerations in hardware design, modulation of MOS temperature was used to increase sensitivity and selectivity, and data analysis was enhanced with adapted Gaussian apodization factor analysis (GAFA) as a multivariate curve resolution algorithm. Continues air sampling and injecting system (CASI) design provides a fast and easily applied method for continues injection of air sample with no additional sample preparation. The analysis cycle time required for each run is less than 300 s. The high sample load and sharp injection with the fast separation by MCC decrease the peak widths and improve detection limits. This homemade customized instrument is an alternative to other time-consuming and expensive technologies for continuous monitoring of outgassing in air samples.

Comparison of different theories for focusing through a plane interface: comment.

In light focusing through a dielectric interface, Wiersma et al. [J. Opt. Soc. Am. A14, 1482 (1997)JOAOD60740-323210.1364/JOSAA.14.001482] claim that the Debye-Wolf diffraction theory and the m-theory predict axial focal fields with "little difference." We found a possible mistake of using an inaccurate apodization factor in the m-theory integral. Here we correct the apodization factor, which then leads to better agreement on axial intensity distributions between the two theories than reported.

Research status and application on the focusing properties of cylindrical vector beams

Cylindrical vector beams are spatially inhomogeneously polarized, whose intensity in the center is zero, and can produce special field components in the vicinity of focus when they are focused by an objective lens. In the case of optical system with high apodization factor, radial polarization can achieve tight focus by adapting pupil filtering and image restoration technology compared with linear and circular polarization. The properties of cylindrical vector beams are introduced. Based on electric dipole radiation model and vector diffraction theory, focal field properties for cylindrical vector beams focused by a high-NA objective lens are discussed. The method to achieve tight focus by cylindrical vector beams is presented. Furthermore, we consider it feasible for the super resolution laser polarized differential confocal microscopy by adapting the differential confocal microscopy, and put forward the prospective development.

Rigorous theory on elliptical mirror focusing for point scanning microscopy

A rigorous elliptical mirror focusing formula based on spherical wave transformation is derived as a kind of imaging technique with high NA for potential applications in molecule imaging, spectroscopy and industrial artifact microscopy. An apodization factor is given and used to compare the energy conversation rules in lens transmission and parabolic and elliptical mirror reflections. Simulation results indicate that the axial HFWHM of elliptical and parabolic mirrors is about 80% of the corresponding HFWHM of lens in case of NA = 1 and φs = 0, and the side lobe noise is also slightly lower than that of lens, but the transverse HFWHM of mirrors is comparatively wider despite the width of main lobe is still smaller. In comparison with parabolic mirror based system, an elliptical mirror based system is potentially promising in aberration control of incident beam when the aperture of mirror is enlarged to adapt a large stage or specimen container at a small beam shading ratio.

Spring constant modulation in a zone plate tweezer using linear polarization

At large NAs a micro-Fresnel zone plate produces a focal spot that is more elliptical than that produced by an objective lens with the same NA. Using this anisotropy we demonstrate a method for modulating the spring constant of an optical trap by rotating the linear input polarization. The focal spot ellipticity is enhanced by the apodization factor of the zone plate and its extremely high NA. By measuring the positions of trapped particles we obtain two-dimensional histograms of particle position. These indicate that the trap spring constant is 2.75 times larger perpendicular to the incident polarization than along it. The elliptical focal spot distribution can be rotated by rotating the incident polarization, allowing the spring constant along a given direction to be modulated.

Ray-tracing and physical-optics analysis of the aperture efficiency in a radio telescope

The performance of telescope systems working at microwave or visible-IR wavelengths is typically described in terms of different parameters according to the wavelength range. Most commercial ray-tracing packages have been specifically designed for use with visible-IR systems and thus, though very flexible and sophisticated, do not provide the appropriate parameters to fully describe microwave antennas and to compare with specifications. We demonstrate that the Strehl ratio is equal to the phase efficiency when the apodization factor is taken into account. The phase efficiency is the most critical contribution to the aperture efficiency of an antenna and the most difficult parameter to optimize during the telescope design. The equivalence between the Strehl ratio and the phase efficiency gives the designer/user of the telescope the opportunity to use the faster commercial ray-tracing software to optimize the design. We also discuss the results of several tests performed to check the validity of this relationship that we carried out using a ray-tracing software, ZEMAX, and a full Physical Optics software, GRASP9.3, applied to three different telescope designs that span a factor of approximately 10 in terms of D/lambda. The maximum measured discrepancy between phase efficiency and Strehl ratio varies between approximately 0.4% and 1.9% up to an offset angle of >40 beams, depending on the optical configuration, but it is always less than 0.5% where the Strehl ratio is >0.95.

High-numerical-aperture focusing of radially polarized doughnut beams with a parabolic mirror and a flat diffractive lens.

Recently, Dorn et al. [Phys. Rev. Lett. 91, 233901 (2003)] demonstrated the significance of radially polarized doughnut beams in obtaining very small focal spots (with an area of approximately 0.26 lambda2) with high-numerical-aperture (NA) aplanatic microscope objectives. We propose two simple alternative ways to focus such radially polarized beams: a parabolic mirror and a flat diffractive lens. Because of their large apodization factor for a high NA, a significant further reduction in spot area (up to a factor of 1.76 at a NA of 1) compared with the aplanatic system can be achieved.