ALBERT

Description: We present a case study of observed and modelled C-band microwave backscatter signatures for a complexly-layered snow cover on smooth, land-fast, first-year sea ice. We investigate how complexly-layered snow affects the backscatter, by comparing signatures with those for a simple snow cover, and through model sensitivity analysis. Backscatter signatures are obtained using a surface-based scatterometer, on sea ice in Hudson Bay, Canada. Coincident in-situ snow and ice geophysical measurements, and on-ice meteorological observations, describe the snow cover formation and structure. A multi-layer snow and ice backscatter model is used to iteratively add and subtract components of the complex snow cover to assess their impacts on the overall backscatter. For incidence angles between 20° and 70° the backscatter from a complex snow cover on smooth first-year sea ice is significantly higher than backscatter from a simple snow cover on similar sea ice. Sensitivity analysis suggests that rough ice layers formed within the complex snow cover and those superimposed at the sea ice interface, are the physical mechanisms that affect an increase in surface and volume backscattering. This has implications for sea ice mapping, geophysical inversion, and snow thickness studies. This article is protected by copyright. All rights reserved.

Description: We report on the design and performance of a broad-band, high-power 540-640-GHz fix-tuned balanced frequency tripler chip that utilizes four planar Schottky anodes. The suspended strip-line circuit is fabricated with a 12-micron-thick support frame and is mounted in a split waveguide block. The chip is supported by thick beam leads that are also used to provide precise RF grounding. At room temperature, the tripler delivers 0.9-1.8 mW across the band with an estimated efficiency of 4.5%-9%. When cooled to 120 K, the tripler provides 2.0-4.2 mW across the band with an estimated efficiency of 8%-12%.

Description: This paper discusses the construction of solid-state frequency multiplier chains utilized far teraherz receiver applications such as the Herschel Space Observatory . Emphasis will he placed on the specific requirements to be met and challenges that were encountered. The availability of high power amplifiers at 100 GHz makes it possible to cascade frequency doublers and triplers with sufficient RF power to pump heterodyne receivers at THz frequencies. The environmental and mechanical constraints will be addressed as well as reliability issues.

Description: In this paper, we report a MEMS preconcentrator (PC) - gas chromatograph (GC) that is a crucial part of the Spacecraft Atmosphere Monitor (S.A.M.). The S.A.M. is a highly miniature gas chromatograph - mass spectrometer (GC-MS) for monitoring the atmosphere of crewed spacecraft for both trace organic compounds and the major constituents of the cabin air. The S.A.M. instrument is the next generation of GC-MS, based on JPLs Vehicle Cabin Air Monitor (VCAM), which was launched to the International Space Station (ISS) in April 2010 and successfully operated for two years [1, 2]. The S.A.M. employs a unique MEMS PC-GC technology that replaces the macro PC-GC unit in the VCAM. We report herein the current progress of the MEMS PC-GC for the S.A.M. instrument.

Description: 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.

Description: Planetary Atmospheric Chemistry Exploration Sounder (SPACES), a high-sensitivity laboratory breadboard for a spectrometer targeted at orbital planetary atmospheric analysis. The frequency range is 520 to 590 GHz, with a target noise temperature sensitivity of 2,500 K for detecting water, sulfur compounds, carbon compounds, and other atmospheric constituents. SPACES is a prototype for a powerful tool for the exploration of the chemistry and dynamics of any planetary atmosphere. It is fundamentally a single-pixel receiver for spectral signals emitted by the relevant constituents, intended to be fed by a fixed or movable telescope/antenna. Its front-end sensor translates the received signal down to the 100-MHz range where it can be digitized and the data transferred to a spectrum analyzer for processing, spectrum generation, and accumulation. The individual microwave and submillimeter wave components (mixers, LO high-powered amplifiers, and multipliers) of SPACES were developed in cooperation with other programs, although with this type of instrument in mind. Compared to previous planetary and Earth science instruments, its broad bandwidth (approx. =.13%) and rapid tunability (approx. =.10 ms) are new developments only made possible recently by the advancement in submillimeter circuit design and processing at JPL.

Description: Sources in the THz range are required in order for NASA to implement heterodyne instruments in this frequency range. The source that has been demonstrated here will be used for an instrument on the SOFIA platform as well as for upcoming astrophysics missions. There are currently no electronic sources in the 2 3- THz frequency range. An electronically tunable compact source in this frequency range is needed for lab spectroscopy as well as for compact space-deployable heterodyne receivers. This solution for obtaining useful power levels in the 2 3- THz range is based on utilizing power-combined multiplier stages. Utilizing power combining, the input power can be distributed between different multiplier chips and then recombined after the frequency multiplication. A continuous wave (CW) coherent source covering 2.48 2.75 THz, with greater than 10 percent instantaneous and tuning bandwidth, and having l 14 W of output power at room temperature, has been demonstrated. This source is based on a 91.8 101.8-GHz synthesizer followed by a power amplifier and three cascaded frequency triplers. It demonstrates that purely electronic solid-state sources can generate a useful amount of power in a region of the electromagnetic spectrum where lasers (solid-state or gas) were previously the only available coherent sources. The bandwidth, agility, and operability of this THz source has enabled wideband, high-resolution spectroscopic measurements of water, methanol, and carbon monoxide with a resolution and signal-to-noise ratio unmatched by other existing systems, providing new insight in the physics of these molecules. Further - more, the power and optical beam quality are high enough to observe the Lamb-dip effect in water. The source frequency has an absolute accuracy better than 1 part in 1012, and the spectrometer achieves sub-Doppler frequency resolution better than 1 part in 108. The harmonic purity is better than 25 dB. This source can serve as a local oscillator for a variety of heterodyne systems, and can be used as a method for precision control of more powerful but much less frequency-agile quantum mechanical terahertz sources.

Description: 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.