Arrays For Imaging Applications Research Articles

Scattered direction of guided waves in composites with steering effects

We studied the scattered direction of the guided wave in composites and found a negative reflection phenomenon, whereby the reflection and incident directions lie on the same side of the interface’s normal direction. Scattering sources, such as delamination and stiffeners, can generate the scattered wave, and the scattered direction is dependent on the incident angle and interface direction due to the velocity anisotropy and steering effect. When the interface direction is not aligned with the principal material directions, the slowness curve becomes asymmetric about the interface. At a certain incident angle, the reflection angle is negative and negative reflection occurs, which is related to the tangent line of the slowness curve parallel to the interface direction. We experimentally proved the existence of negative reflection, which provides a basis for improving damage imaging and phased array applications. We also quantitatively verified the relationship between the damage direction and the reflection angle, which is significant for imaging compensation.

A Broadband and High Gain Tapered Slot Antenna for W-Band Imaging Array Applications

A broadband and high gain tapered slot antenna (TSA) by utilizing a broadband microstrip- (MS-) to-coplanar stripline (CPS) balun has been developed for millimeter-wave imaging systems and sensors. This antenna exhibits ultrawideband performance for frequency ranges from 70 to over 110 GHz with the high antenna gain, low sidelobe levels, and narrow beamwidth. The validity of this antenna as imaging arrays is also demonstrated by analyzing mutual couplings and 4-element linear array. This antenna can be applied to mm-wave phased array, imaging array for plasma diagnostics applications.

Interferometric Technique for Measuring Terahertz Antenna Phase Patterns

A quasi-optical interferometric technique capable of measuring antenna phase patterns without the need for a heterodyne receiver is presented. It is particularly suited to the characterization of terahertz antennas feeding power detectors or mixers employing quasi-optical local oscillator injection. Examples of recorded antenna phase patterns at frequencies of 1.4 and 2.5 THz using homodyne detectors are presented. To our knowledge, these are the highest frequency antenna phase patterns ever recovered. Knowledge of both the amplitude and phase patterns in the far field enable a Gauss-Hermite or Gauss-Laguerre beam-mode analysis to be carried out for the antenna, of importance in performance optimization calculations, such as antenna gain and beam efficiency parameters at the design and prototype stage of antenna development. A full description of the beam would also be required if the antenna is to be used to feed a quasi-optical system in the near-field to far-field transition region. This situation could often arise when the device is fitted directly at the back of telescopes in flying observatories. A further benefit of the proposed technique is simplicity for characterizing systems in situ, an advantage of considerable importance as in many situations, the components may not be removable for further characterization once assembled. The proposed methodology is generic and should be useful across the wider sensing community, e.g., in single detector acoustic imaging or in adaptive imaging array applications. Furthermore, it is applicable across other frequencies of the EM spectrum, provided adequate spatial and temporal phase stability of the source can be maintained throughout the measurement process. Phase information retrieval is also of importance to emergent research areas, such as band-gap structure characterization, meta-materials research, electromagnetic cloaking, slow light, super-lens design as well as near-field and virtual imaging applications.

Submicron thin-film amorphous silicon photoconductive light sensors

Advanced photodetector and imaging array applications require photosensors with ever-decreasing dimensions to achieve high spatial resolution. Thin film, intrinsic hydrogenated amorphous silicon submicron wire, gap, and square photoconductors, in which the minimum device dimension has been scaled down to a minimum of 60 nm, maintain the 10 5 photo-to-dark conductivity ratio of their large-area (mm-size and above) counterparts at a carrier generation rate of 10 21 cm −3 s −1. The devices are fabricated using electron beam lithography. Ohmic behavior is observed in all devices independently of their device dimension. The dark conductivity is independent of the critical dimension of the photoconductor, while the photoconductivity increases sublinearly with decreasing device dimensions.

Experimental verification of the reduction of coupling between dipole antennas by using a woodpile substrate

In this paper, the mutual coupling between dipole antennas placed on top of a woodpile structure is experimentally investigated. The coupling between dipoles backed by a woodpile substrate is compared with that obtained when the dipole antennas are placed on a dielectric substrate and when they radiate into free space. On average, between 2 and 7 dB reduction of the mutual coupling is achieved with respect to the dielectric substrate case when the woodpile substrate is used. The free space coupling is also reduced in the H-plane configuration case. However, the E-plane coupling slightly increases due to modification of the radiation pattern. Imaging array applications could potentially benefit from the use of EBG substrates due to the improved isolation between pixels.

MULTI-COLOR, BROADBAND QUANTUM WELL INFRARED PHOTODETECTORS FOR MID-, LONG-, AND VERY LONG-WAVELENGTH INFRARED APPLICATIONS

Quantum well infrared photodetectors (QWIPs) have been widely investigated for the 3–5 μm mid-wavelength infrared (MWIR) and 8–12 μm long-wavelength infrared (LWIR) atmospheric spectral windows as well as very long wavelength infrared (VLWIR: λc > 14 μm) imaging array applications in the past decade. The mature III-V compound semiconductor growth technology and the design flexibility of device structures have led to the rapid development of various QWIP structures for infrared focal plane arrays (FPAs) applications. In addition to the single-color QWIP with narrow bandwidth, multi-color or broadband QWIPs required for advanced IR sensing and imaging applications have also emerged in recent years. Using band gap engineering approach, the multi-color (2, 3, and 4-color) QWIPs with multi-stack quantum wells and voltage-tunable asymmetrical coupled quantum well structures for detection in the MWIR, LWIR, and VLWIR bands have been demonstrated recently. The triple-coupled (TC-) QWIP employs the quantum confined Stark effect to tune the peak detection wavelength by the applied bias voltage, A typical single-color QWIP exhibits a rather narrow spectral bandwidth of 1 to 2 μm. For certain applications, such as spectroscopy, sensing of a broader range of infrared radiation is highly desirable. Using the stacked quantum wells with different well width and depth, the digital-graded superlattice barrier (DGSLB) or the linear-graded barrier (LGB) structures, broadband (BB-) QWIPs covering the 8–14 μm atmospheric spectral window have been reported recently. In this chapter, the basic operation principles of a QWIP, and the design, fabrication, and characterization of multi-color and broadband QWIPs based on the GaAs/AlGaAs and InGaAs/AlGaAs material systems for the MW/LW/VLWIR applications are depicted.

Millimeter-wave tapered-slot antennas on synthesized low permittivity substrates

This paper presents 30-GHz linear-tapered slot antennas (LTSA) and 94-GHz constant-width slot antennas (CSWA) on synthesized low dielectric constant substrates (/spl epsiv//sub r/=2.2). The performance of tapered-slot antennas (TSA) is sensitive to the effective thickness of the substrate. We have reduced the effective thickness by selectively machining holes in the dielectric substrate. The machined substrate antenna radiation patterns were significantly improved independent of the machined hole size or lattice as long as the quasi-static effective thickness remained the same, even if the hole/lattice geometry is comparable to a wavelength. The method was applied at 94 GHz on a CSWA with excellent radiation pattern improvement, making it suitable for f/1.6 imaging array applications.

Double-slot antennas on extended hemispherical and elliptical silicon dielectric lenses

Far-field patterns and Gaussian-beam coupling efficiencies are investigated for a double-slot antenna placed on hemispherical lenses with varying extension lengths. The radiation patterns of a double-slot antenna on a silicon dielectric lens are computed using ray-tracing inside the dielectric lens and electric and magnetic field integration on the spherical dielectric surface. The measured radiation patterns at 246 GHz and Gaussian-beam coupling efficiencies show good agreement with theory. The theoretical results are presented in terms of extension-length/radius and radius/ lambda , and therefore result in universal design curves for silicon lenses of different diameters and at different frequencies. The theoretical and experimental results indicate that for single units, there exists a wide range of extension lengths which result in high Gaussian-coupling efficiencies (50-60%) to moderately high f 's. These Gaussian-coupling efficiencies can be increased to 80-90 degrees % with the use of a lambda /sub m//4 matching-cap layer. For imaging array applications with high packing densities, an extension-length/radius of 0.38 to 0.39 (depending on frequency) will result in peak directivity and a corresponding Gaussian-coupling efficiency 15-20% lower than for single units.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparison of Computer Simulations with Measurements From a-Si:H Imaging Arrays.

ABSTRACTThe design and characterization of flat panel a-Si:H arrays for imaging applications in radiotherapy and diagnostic radiology has already begun. Future improvements in design and performance will be heavily reliant on the results of computer simulations capable of accurately predicting the behavior of the arrays in a variety of operating conditions. These results will allow the design of new arrays to be optimized for the particular application before they are committed to fabrication. This paper outlines the results of an investigation into the effects of varying a number of array parameters and operating conditions using a simulation code which models the operation of a-Si:H devices. The results are compared with actual measurements from arrays fabricated at Xerox PARC.

Integrated horn antennas for millimeter-wave applications

This paper reviews the development of integrated horn antennas since their introduction in 1987. The integrated horn is fabricated by suspending a dipole antenna on a thin dielectric membrane in a pyramidal cavity etched in silicon. Recent progress resulted in optimized low and high-gain designs with single and double-polarizations for remote-sensing and communication applications. A fullwave analysis technique have resulted in an integrated antenna with performance comparable to that of waveguide-fed corrugated horn antennas. The integrated horn design can be easily extended to large arrays for imaging and phased array applications while still leaving plenty of room for the rf and w processing circuitry. Theoretical and experimental results at microwave frequencies and at 90, 240 and 802 GHz will be presented.

Radiation properties of slot and dipole elements on layered substrates

This paper presents calculations of the radiation properties of slot and dipole antennas on electrically thick, grounded, dielectric substrates. These structures offer the possibility of simplifying the fabrication of imaging array antenna structures which operate at millimeter wave and far infrared freqeuncies. They also offer the possiblility of good beam patterns which can be tailored to suit a specific need. We present an analysis of practical layered structures which have beam patterns that are suitable for millimeter wave and far-infrared imaging array applications. We discuss considerations of the choice of dielectric layers with regard to beam patterns, surface wave losses, and the type of element used. The effects of dielectric and ground plane losses in high-gain structures are also considered. Efficiencies and beam patterns for three and five layer structures are presented, although the analysis techniques are extendable to an arbitrary number of layers. It is found that in combination with the use of a twin element configuration, both slot and dipole antennas can overcome the problems of losses to surface waves in the substrate. Consequently, they can be made to efficiently radiate to air on these layered dielectric structures.

Use of broadside twin element antennas to increase efficiency on electrically thick dielectric substrates

This paper describes a technique for using twin element antennas (dipoles and slots) to increase the efficiency of antennas fabricated on electrically thick dielectric substrates. We present calculations which show that the efficiency of both the slot and dipole antennas can be increased by the proper spacing of elements placed broadside to each other. We consider the use of substrates that are odd integral multiples of a quarter of a dielectric wavelength thick and give results for an e=4 substrate with thicknesses of one, three, and five quarter wavelengths. These thicknesses can be used when working at millimeter wave frequencies and yield substrate dimensions which can be handled and processed easily, while still yielding radiation-to-air efficiencies of about 70%. We also show calculated beam patterns for the elements which appear to be suitable for imaging array applications.