180 research outputs found
Confocal Ellipsoidal Reflector System for a Mechanically Scanned Active Terahertz Imager
We present the design of a reflector system that can rapidly scan and refocus a terahertz beam for high-resolution standoff imaging applications. The proposed optical system utilizes a confocal Gregorian geometry with a small mechanical rotating mirror and an axial displacement of the feed. For operation at submillimeter wavelengths and standoff ranges of many meters, the imaging targets are electrically very close to the antenna aperture. Therefore the main reflector surface must be an ellipse, instead of a parabola, in order to achieve the best imaging performance. Here we demonstrate how a simple design equivalence can be used to generalize the design of a Gregorian reflector system based on a paraboloidal main reflector to one with an ellipsoidal main reflector. The system parameters are determined by minimizing the optical path length error, and the results are validated with numerical simulations from the commercial antenna software package GRASP. The system is able to scan the beam over 0.5 m in cross-range at a 25 m standoff range with less than 1% increase of the half-power beam-width
Time-Delay Multiplexing of Two Beams in a Terahertz Imaging Radar
We demonstrate a time-delay multiplexing technique
that doubles the frame rate of a 660â690-GHz imaging radar
with minimal additional instrument complexity. This is done by
simultaneously projecting two offset, orthogonally polarized radar
beams generated and detected by a common source and receiver.
Beam splitting and polarization rotation is accomplished with a
custom designed waveguide hybrid coupler and twist. A relative
time lag of approximately 2 ns between the beamsâ waveforms is
introduced using a quasi-optical delay line, followed by spatial
recombination using a selectively reflective wire grid. This delay is
much longer than the approximately 20-ps time-of-flight resolution
of the 30-GHz bandwidth radar, permitting the two beamsâ
reflected signals from a compact target to be easily distinguished
in digital post-processing of the single receiver channel
Optimization of a metallo-dielectric EBG waveguide
n this contribution we have presented a new metallo-dielectric EBG waveguide. The initial concept is based on Bragg fiber, which cross section is usually very large in terms of the wavelength. To reduce the waveguide dimensions and make it feasible at THz frequencies, we use a combination of a metallic and a dielectric waveguide. Standard resonant geometries will have high bend coupling between the TE01 and EH11 modes, thus reducing the losses introduced by the bend. Here we present two ways of optimizing the waveguide structure to break the degeneracy between these modes. One considers the periodicity along Ï and the other along Ï. The results can be extended to structures with two or three periods to reduce the metallic losses. These structures will have an overall propagation and bend loss lower than an overmoded circular waveguide working on the TE11 mode
Design of a Low Loss Metallo-Dielectric EBG Waveguide at Submillimeter Wavelengths
In this letter we show the viability of using concentric cylindrically-periodic (CP) dielectrics to realize low-loss propagation at submillimeter wavelengths. Most of the power in the CP Waveguide (WG) is confined to an air core propagating a TE_(01) mode. The TM_(11) mode degeneracy is removed over a very wide bandwidth by a series of periodically spaced dielectric rings that are optimized in thickness and spacing. The new waveguide differs from classical Bragg fibers in that we use only a small number of dielectric layers (2-4), and terminate the outer layer with an external metal coating. We include several designs and describe the optimization procedure that was used to realize structures with low propagation and bend loss
Submillimetre wave 3D imaging radar for security applications
There is ongoing worldwide interest in finding solutions to enhance the security of civilians at airports, borders and high risk public areas in ways which are safe, ethical and streamlined. One promising approach is to use submillimetre wave 3D imaging radar to detect concealed threats as it offers the advantages of high volumetric resolution (~1 cm3) with practically sized antennas (<0.5 m) such that even quite small objects can be resolved through clothing. The Millimetre Wave Group at the University of St Andrews has been developing submillimetre wave 3D imaging radars for security applications since 2007. A significant goal is to achieve near real-time frame rates of at least 10 Hz, to cope with dynamic scenes, over wide fields of view at short range with high pixel counts. We review the radar systems we have developed at 340 and 220 GHz and the underpinning technologies which we have employed to realise these goals.PostprintNon peer reviewe
Penetrating 3-D Imaging at 4- and 25-m Range Using a Submillimeter-Wave Radar
We show experimentally that a high-resolution imaging radar operating at 576â605 GHz is capable of detecting weapons concealed by clothing at standoff ranges of 4â25 m. We also demonstrate the critical advantage of 3-D image reconstruction for visualizing hidden objects using active-illumination coherent terahertz imaging. The present system can image a torso with <1 cm resolution at 4 m standoff in about five minutes. Greater standoff distances and much higher frame rates should be achievable by capitalizing on the bandwidth, output power, and compactness of solid state Schottky-diode based terahertz mixers and multiplied sources
Dielectric Covered Planar Antennas at Submillimeter Wavelengths for Terahertz Imaging
Most optical systems require antennas with directive patterns. This means that the physical area of the antenna will be large in terms of the wavelength. When non-cooled systems are used, the losses of microstrip or coplanar waveguide lines impede the use of standard patch or slot antennas for a large number of elements in a phased array format. Traditionally, this problem has been solved by using silicon lenses. However, if an array of such highly directive antennas is to be used for imaging applications, the fabrication of many closely spaced lenses becomes a problem. Moreover, planar antennas are usually fed by microstrip or coplanar waveguides while the mixer or the detector elements (usually Schottky diodes) are coupled in a waveguide environment. The coupling between the antenna and the detector/ mixer can be a fabrication challenge in an imaging array at submillimeter wavelengths. Antennas excited by a waveguide (TE10) mode makes use of dielectric superlayers to increase the directivity. These antennas create a kind of Fabry- Perot cavity between the ground plane and the first layer of dielectric. In reality, the antenna operates as a leaky wave mode where a leaky wave pole propagates along the cavity while it radiates. Thanks to this pole, the directivity of a small antenna is considerably enhanced. The antenna consists of a waveguide feed, which can be coupled to a mixer or detector such as a Schottky diode via a standard probe design. The waveguide is loaded with a double-slot iris to perform an impedance match and to suppress undesired modes that can propagate on the cavity. On top of the slot there is an air cavity and on top, a small portion of a hemispherical lens. The fractional bandwidth of such antennas is around 10 percent, which is good enough for heterodyne imaging applications.The new geometry makes use of a silicon lens instead of dielectric quarter wavelength substrates. This design presents several advantages when used in the submillimeter-wave and terahertz bands: a) Antenna fabrication compatible with lithographic techniques. b) Much simpler fabrication of the lens. c) A simple quarter-wavelength matching layer of the lens will be more efficient if a smaller portion of the lens is used. d) The directivity is given by the lens diameter instead of the leaky pole (the bandwidth will not depend anymore on the directivity but just on the initial cavity). The feed is a standard waveguide, which is compatible with proven Schottky diode mixer/detector technologies. The development of such technology will benefit applications where submillimeter- wave heterodyne array designs are required. The main fields are national security, planetary exploration, and biomedicine. For national security, wideband submillimeter radars could be an effective tool for the standoff detection of hidden weapons or bombs concealed by clothing or packaging. In the field of planetary exploration, wideband submillimeter radars can be used as a spectrometer to detect trace concentrations of chemicals in atmospheres that are too cold to rely on thermal imaging techniques. In biomedicine, an imaging heterodyne system could be helpful in detecting skin diseases
Surface wave control for large arrays of microwave kinetic inductance detectors
Large ultra-sensitive detector arrays are needed for present and future
observatories for far infra-red, submillimeter wave (THz), and millimeter wave
astronomy. With increasing array size, it is increasingly important to control
stray radiation inside the detector chips themselves, the surface wave. We
demonstrate this effect with focal plane arrays of 880 lens-antenna coupled
Microwave Kinetic Inductance Detectors (MKIDs). Presented here are near field
measurements of the MKID optical response versus the position on the array of a
reimaged optical source. We demonstrate that the optical response of a detector
in these arrays saturates off-pixel at the dB level compared to the
peak pixel response. The result is that the power detected from a point source
at the pixel position is almost identical to the stray response integrated over
the chip area. With such a contribution, it would be impossible to measure
extended sources, while the point source sensitivity is degraded due to an
increase of the stray loading. However, we show that by incorporating an
on-chip stray light absorber, the surface wave contribution is reduced by a
factor 10. With the on-chip stray light absorber the point source response
is close to simulations down to the dB level, the simulation based on
an ideal Gaussian illumination of the optics. In addition, as a crosscheck we
show that the extended source response of a single pixel in the array with the
absorbing grid is in agreement with the integral of the point source
measurements.Comment: accepted for publication in IEEE Transactions on Terahertz Science
and Technolog
Silicon Micromachined Microlens Array for THz Antennas
5 5 silicon microlens array was developed using a silicon micromachining technique for a silicon-based THz antenna array. The feature of the silicon micromachining technique enables one to microfabricate an unlimited number of microlens arrays at one time with good uniformity on a silicon wafer. This technique will resolve one of the key issues in building a THz camera, which is to integrate antennas in a detector array. The conventional approach of building single-pixel receivers and stacking them to form a multi-pixel receiver is not suited at THz because a single-pixel receiver already has difficulty fitting into mass, volume, and power budgets, especially in space applications. In this proposed technique, one has controllability on both diameter and curvature of a silicon microlens. First of all, the diameter of microlens depends on how thick photoresist one could coat and pattern. So far, the diameter of a 6- mm photoresist microlens with 400 m in height has been successfully microfabricated. Based on current researchers experiences, a diameter larger than 1-cm photoresist microlens array would be feasible. In order to control the curvature of the microlens, the following process variables could be used: 1. Amount of photoresist: It determines the curvature of the photoresist microlens. Since the photoresist lens is transferred onto the silicon substrate, it will directly control the curvature of the silicon microlens. 2. Etching selectivity between photoresist and silicon: The photoresist microlens is formed by thermal reflow. In order to transfer the exact photoresist curvature onto silicon, there needs to be etching selectivity of 1:1 between silicon and photoresist. However, by varying the etching selectivity, one could control the curvature of the silicon microlens. The figure shows the microfabricated silicon microlens 5 x5 array. The diameter of the microlens located in the center is about 2.5 mm. The measured 3-D profile of the microlens surface has a smooth curvature. The measured height of the silicon microlens is about 280 microns. In this case, the original height of the photoresist was 210 microns. The change was due to the etching selectivity of 1.33 between photoresist and silicon. The measured surface roughness of the silicon microlens shows the peak-to-peak surface roughness of less than 0.5 microns, which is adequate in THz frequency. For example, the surface roughness should be less than 7 microns at 600 GHz range. The SEM (scanning electron microscope) image of the microlens confirms the smooth surface. The beam pattern at 550 GHz shows good directivity
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