Aperture For Use Research Articles

Investigation of the Noise Characteristics of Vertical-Cavity Surface-Emitting Laser with a Rhomboidal Oxide Current Aperture for Use in a Cs-Based Compact Atomic Magnetometer

Investigation of the Noise Characteristics of Vertical-Cavity Surface-Emitting Laser with a Rhomboidal Oxide Current Aperture for Use in a Cs-Based Compact Atomic Magnetometer

Region-of-interest intra-arc MV imaging to facilitate sub-mm positional accuracy with minimal imaging dose during treatment deliveries of small cranial lesions.

To automate the generation of region-of-interest (ROI) apertures for use with megavoltage imaging for online positional corrections during cranial stereotactic radiosurgery. Digitally reconstructed radiographs (DRRs) were created for a 3D-printed skull phantom at 5degree gantry angle increments for a three-arc beam arrangement. At each angle, 3000 random rectangular apertures were generated, and 100 shifts on a grid were applied to the anatomy within the frame. For all shifts, the mutual information (MI) between the shifted and unshifted DRR was calculated to derive an average MI gradient. The top 10% of apertures that minimized registration errors were overlaid and discretely thresholded to generate imaging plans. Imaging was acquired with the skull while implementing simulated patient motion on a linac. Control point-specific couch motions were derived to align the skull to its planned positioning. Apertures with a range of repositioning errors less than 0.1mm possessed a 42% larger average MI gradient when compared with apertures with a range greater than 1mm. Dose calculations with Monte Carlo exhibited an 84% reduction in the dose received by 50% of the skull with the 50% thresholded plan when compared to a constant 22×22cm2 imaging plan. For all different imaging plans (with and without motion), the calculated median 3D-errors with respect to the tracking of a metal-BB fiducial positioned at isocenter in the skull were sub-mm except for the 80% thresholded plan. Sub-mm positional errors are achievable with couch motions derived from control point-specific ROI imaging. Smaller apertures that conform to an anatomical ROI can be utilized to minimize the imaging dose incurred at the expense of larger errors.

Optimisation of trench filling in GaN towards vertical HEMT structures

In this work we present studies how to fill epitaxially trenches in a GaN layer in order to form a current aperture for use in 3D high power field effect transistors. By optimising various growth parameters in our metal organic vapour phase epitaxy, we achieved an almost complete and smooth filling of 5 μm wide and 500 nm deep trenches etched into a GaN:Mg layer by reactive-ion etching. By using GaN:Si marker layers, the filling process could be excellently monitored. Eventually, a high electron mobility transistor structure was deposited on top of the optimized structures enabling an electrical assessment of this process.

Investigation of the noise characteristics of vertical-cavity surface-emitting laser with a rhomboidal oxide current aperture for use in a Cs-based compact atomic magnetometer

The possibility of using vertical-emitting lasers with intracavity contacts (IC-VCSEL) and a rhomboidal oxide current aperture for creating a non-zero magnetic field optically pumped atomic magnetometers (OPM) with a 133Cs vapor cell for magnetoencephalographic (MEG) systems were demonstrated. Relative intensity noise (RIN) and polarization resolved RIN of the IC-VCSEL in the 895 nm range with different mirror losses (linewidth) in the frequency range from 1 Hz to 100 kHz were experimentally investigated. Lasers with low mirror loss (narrow linewidth) have polarization resolved RIN comparable to amplitude noise. For IC-VCSEL with an output optical power of 0.8 mW and a linewidth of 55 MHz, the noise level measured is 148 dB/Hz in 1 Hz bandwidth at 40 kHz frequency. The ultimate sensitivity of OPM based on two-beam MZ-scheme with studied VCSELs was estimated as ~ 11 fT/sqrt(Hz)sqrt. Keywords: vertical-cavity surface-emitting laser (VCSEL), magnetoencephalographic (MEG) systems, optically pumped atomic magnetometers (OPAM), relative intensity noise (RIN), polarization-resolved RIN.

The quadrant method measuring four points is as a reliable and accurate as the quadrant method in the evaluation after anatomical double-bundle ACL reconstruction.

The quadrant method was described by Bernard et al. and it has been widely used for postoperative evaluation of anterior cruciate ligament (ACL) reconstruction. The purpose of this research is to further develop the quadrant method measuring four points, which we named four-point quadrant method, and to compare with the quadrant method. Three-dimensional computed tomography (3D-CT) analyses were performed in 25 patients who underwent double-bundle ACL reconstruction using the outside-in technique. The four points in this study's quadrant method were defined as point1-highest, point2-deepest, point3-lowest, and point4-shallowest, in femoral tunnel position. Value of depth and height in each point was measured. Antero-medial (AM) tunnel is (depth1, height2) and postero-lateral (PL) tunnel is (depth3, height4) in this four-point quadrant method. The 3D-CT images were evaluated independently by 2 orthopaedic surgeons. A second measurement was performed by both observers after a 4-week interval. Intra- and inter-observer reliability was calculated by means of intra-class correlation coefficient (ICC). Also, the accuracy of the method was evaluated against the quadrant method. Intra-observer reliability was almost perfect for both AM and PL tunnel (ICC>0.81). Inter-observer reliability of AM tunnel was substantial (ICC>0.61) and that of PL tunnel was almost perfect (ICC>0.81). The AM tunnel position was 0.13% deep, 0.58% high and PL tunnel position was 0.01% shallow, 0.13% low compared to quadrant method. The four-point quadrant method was found to have high intra- and inter-observer reliability and accuracy. This method can evaluate the tunnel position regardless of the shape and morphology of the bone tunnel aperture for use of comparison and can provide measurement that can be compared with various reconstruction methods. The four-point quadrant method of this study is considered to have clinical relevance in that it is a detailed and accurate tool for evaluating femoral tunnel position after ACL reconstruction. Case series, Level IV.

Design of the polar neutron-imaging aperture for use at the National Ignition Facility.

The installation of a neutron imaging diagnostic with a polar view at the National Ignition Facility (NIF) required design of a new aperture, an extended pinhole array (PHA). This PHA is different from the pinhole array for the existing equatorial system due to significant changes in the alignment and recording systems. The complex set of component requirements, as well as significant space constraints in its intended location, makes the design of this aperture challenging. In addition, lessons learned from development of prior apertures mandate careful aperture metrology prior to first use. This paper discusses the PHA requirements, constraints, and the final design. The PHA design is complex due to size constraints, machining precision, assembly tolerances, and design requirements. When fully assembled, the aperture is a 15 mm × 15 mm × 200 mm tungsten and gold assembly. The PHA body is made from 2 layers of tungsten and 11 layers of gold. The gold layers include 4 layers containing penumbral openings, 4 layers containing pinholes and 3 spacer layers. In total, there are 64 individual, triangular pinholes with a field of view (FOV) of 200 μm and 6 penumbral apertures. Each pinhole is pointed to a slightly different location in the target plane, making the effective FOV of this PHA a 700 μm square in the target plane. The large FOV of the PHA reduces the alignment requirements both for the PHA and the target, allowing for alignment with a laser tracking system at NIF.

Effect of pinhole shape on projection resolution

We are designing a dual-resolution pre-clinical SPECT system based on square-pinhole apertures for use in applications with a small field-of-view (FOV), such as cardiac imaging of mice. Square pinholes allow for increased sensitivity due to more efficient projection tiling on the detector compared to circular pinholes. Aperture fabrication techniques cannot produce a perfect square, giving the square pinholes some amount of roundedness at the corners. This work investigates how this roundedness affects the physical properties of projection images in terms of spatial resolution. Different pinhole full-acceptance angles and roundedness values were simulated. To facilitate a fair comparison, properties of the non-square pinholes were manipulated to yield pinholes with approximately the same sensitivity (to within 0.1%) and FOV (to within 0.5%) as those of the square pinholes, subsequently referred to as matched apertures. The aperture size (flat-to-flat edge length) of each non-square aperture was increased until its sensitivity was approximately equal to that of the square pinhole. Next, the full acceptance angle was increased until the FOV of each non-square aperture was approximately equivalent to that of the square pinhole. Sensitivity was calculated to include both the geometric and penetrative sensitivity of a point source, as well as the packing faction of the multi-pinhole collimator. Using the sensitivity-matched and FOV-matched apertures, spatial resolution was estimated. For the 0.3 mm, 0.5 mm, and 1 mm edge-length square apertures studied, the full-width at half-maximum widened as pinhole shape changed from square to circle, while full-width tenth-maximum showed little change. These results indicate that a perfect square pinhole shape is more desirable than a rounded-square pinhole with regard to spatial resolution when sensitivity and FOV-matched pinholes are compared.

Dynamic metamaterial aperture for microwave imaging

We present a dynamic metamaterial aperture for use in computational imaging schemes at microwave frequencies. The aperture consists of an array of complementary, resonant metamaterial elements patterned into the upper conductor of a microstrip line. Each metamaterial element contains two diodes connected to an external control circuit such that the resonance of the metamaterial element can be damped by application of a bias voltage. Through applying different voltages to the control circuit, select subsets of the elements can be switched on to create unique radiation patterns that illuminate the scene. Spatial information of an imaging domain can thus be encoded onto this set of radiation patterns, or measurements, which can be processed to reconstruct the targets in the scene using compressive sensing algorithms. We discuss the design and operation of a metamaterial imaging system and demonstrate reconstructed images with a 10:1 compression ratio. Dynamic metamaterial apertures can potentially be of benefit in microwave or millimeter wave systems such as those used in security screening and through-wall imaging. In addition, feature-specific or adaptive imaging can be facilitated through the use of the dynamic aperture.

Fabrication of Self-Supporting Annular Apertures for Use in the Transmission Electron Microscope

Fabrication of Self-Supporting Annular Apertures for Use in the Transmission Electron Microscope

Multiple-input multiple-output radar aperture optimisation

The authors describe a process for designing low-cost, light-weight antenna apertures for use in multiple-input multiple-output (MIMO) radars. In a MIMO radar system, two or more transmitters emit independent waveforms, with the resulting reflections received by an array of receivers. Recently, MIMO radar has become a subject of great interest. In part, this interest is due to the potential for MIMO techniques to reduce radar weight and cost, while maintaining performance (as compared with conventional radar approaches). However, the size of these reductions has not yet been quantified. Here, the authors describe a process for designing optimal radar apertures. This process treats the design problem as one of minimising an objective function under performance constraints. The objective function is based upon a first-order model for the relationship between cost (or weight) and performance, and is derived for systems employing active, element-digitised arrays. A systematic process for optimising the aperture's design with respect to this objective function is presented, and equations describing the optimal aperture are derived. These equations provide insight into the optimal relationship between various aperture characteristics, such as the number of transmitters, number of receivers, module power level and virtual array length.

Projection of diffraction patterns for use in cold-neutral-atom trapping

Scalar diffraction theory is combined with beam-propagation techniques to investigate the projection of near-field diffraction patterns to spatial locations away from the aperture for use in optically trapping cold neutral alkali-metal atoms. Calculations show that intensity distributions with localized bright and dark spots usually found within a millimeter of the diffracting aperture can be projected to a region free from optical components such as a cloud of cold atoms within a vacuum chamber. Calculations also predict that the critical properties of the optical dipole atom traps are not only maintained for the projected intensity patterns but also can be manipulated and improved by adjustment of the optical components outside the vacuum chamber.

An easily built diffuse illumination system effective at both very low and moderate magnifications, for observing in situ stained slides

Effective study of in situ stained sections often requires illumination that is difficult to achieve with commonly used research microscopes. One must be able to switch quickly and conveniently from the very lowest to moderate magnifications. At all magnifications contrast due to light scatter must be minimized, so that the weak staining that signifies low gene expression can be observed reliably. For the lowest power objectives (e.g., 1.25x or 2x) many microscopes require that the condenser be removed to illuminate the full field of view. This is not only very inconvenient when switching magnifications, but without a condenser the low numerical aperture of the illuminating light beam results in unwanted contrast due to light scatter. We have devised a simple system that diffusely illuminates the full field of view of the lowest power objective (1.25x) and has high enough numerical aperture for use with the 25x and 40x objectives. A key feature is the use of a large diameter ring light (internal diameter 5.8 cm), placed on the microscope base, to illuminate a large diameter diffuser placed just below the slide.

A millimeter-wave cassegrain antenna using shaped beam waveguide feed

As a way to increase the efficiency of the focused beam-fed Cassegrain antenna with a large aperture for use at millimeter-wave frequencies, a method is proposed for shaping the surface of two focusing reflectors in the focused beam waveguide feed. The reflector surface shaping method is presented. Since the reflector is used at millimeter-wave frequencies, it is necessary to take into account the effect of the aberration caused by the deformation of the reflector surface on the radiation characteristics. Both the sub- and main reflectors have quadratic surfaces. The aberration in the main reflector due to self weight deformation can be corrected with the deformation of the subreflector in the homologic design. Hence, in order to validate the present beam shaping method, it is necessary to evaluate the effects of the aberration caused by the alignment error of each focusing reflector on the radiation characteristics. In this paper, a method of shaping the two focusing reflectors is applied to the new 10-m antennas for the millimeter-wave interferometer of Nobeyama National Observatory (model No. 6). The alignment errors in the parallel shift in three axis directions and the rotating motion about the axes are assumed, and the aberration and the gain degradation are quantitatively investigated when each focusing reflector is deformed. It is shown that the grain degradation is small in the range of realizable alignment errors and that the method of shaping the surfaces of two focusing reflectors is effective as a reflector shaping method for a millimeter-wave large-aperture antenna. © 2000 Scripta Technica, Electron Comm Jpn Pt 1, 84(4): 39–47, 2001

High-accuracy aperture-area measurement facilities at the National Institute of Standards and Technology

The uncertainty in the measurement of aperture area can limit high-accuracy radiometric and photometric measurements. Relative total uncertainties in some measurements have now been determined at or below the 0.1% level, making substantially smaller aperture-area measurement uncertainties necessary. The National Institute of Standards and Technology (NIST) has recently implemented an absolute aperture-area measurement facility and a relative aperture-area measurement facility; the two facilities together are designed to determine aperture areas with low uncertainty. The absolute instrument measures the aperture-area using optical edge detection, along with high-precision positioning of the optical edge relative to the sensor, resulting in an expected relative combined standard uncertainty of less than 10-4. The relative instrument uses optical flux transfer to compare aperture areas and also has an expected relative combined standard uncertainty of less than 10-4. The absolute instrument will be used to measure the area of standard apertures for use with the relative instrument.

Focused ion-beam fabrication of fiber probes with well-defined apertures for use in near-field scanning optical microscopy

We present a focused ion-beam (FIB) fabrication method for very clean and well-defined subwavelength fiber probes with metallic apertures of a desired diameter for use in near-field scanning optical microscopy. Such probes exhibit improved features compared to probes coated with metal by the conventional angled evaporation technique. Examples of FIB fabricated fiber probes are shown and images of a test sample are presented using one of the probes in a near-field microscope.

Making refractory blocks with conical apertures

In the production of refractory blocks with conical apertures for use in melting tanks in the production of sheet glass with the formation of the ribbon on molten metal (hot polishing), it is usual to employ the following technical operations: tamping of the block blank in convered metal molds, drilling the aperture around the conductor with a diameter of 50 mm from one or the other side, drilling of the aperture diameter 80 mm to a cone depth of &, reaming the cone with a diameter of the upper base equal to 80 mm and the lower 120 mm and a depth &. After each operation it is necessary to rotate the block and change the drilling tool. Upon completion of the operations of making the aperture the block is subjected to drying and firing in accordance with the factory standards.

A laser drilled aperture for use in an ultrahigh vacuum gas doser

A laser drilled aperture for use in an ultrahigh vacuum gas doser

A method for selecting detector apertures for use in SRIXE

A method is presented that will help in selecting the most appropriate detector aperture size for use in SRIXE (synchrotron radiation induced X-ray emission) measurements. The criterion for selection involves maximizing the fluorescence yield while minimizing background from scattered radiation. The method uses previously published expansions of the scattering cross sections, integrated over solid angle (into 90°), and solving for the value of the aperture aspect ratio where the crossover occurs between terms of the cross section that are proportional to solid angle and terms that change faster than the solid angle.

Utility of low-index high-dispersion glasses for Cooke triplet design

Four Cooke triplet systems have been designed for an 85-mm focal length and f/2.8 relative aperture for use as a 35-mm slide projector objective. Two designs use the existing glasses. In the other two designs the EDF glass of the negative lens was replaced by hypothetical low-index high-dispersion glasses. The performance of the triplet systems is found to improve considerably by using low-index high-dispersion glasses for the middle negative lens.

A simple electronic aperture for rastered-beam depth profiles

A simple, inexpensive electronic aperture for use in depth profiling with ion microprobes and ion-scattering spectrometers is described. The system derives a gate signal from a photocell which views the center of an oscilloscope display driven by the ion beam raster voltages.