The SOFIA airborne observatory flies in the lower stratosphere above more than 99.9% of the Earth's water vapor. As
low as this residual water vapor is, it will still affect SOFIA's infrared and sub-millimeter astronomical observations. As
a result, a heterodyne instrument operating at 183 GHz will be used to measure the integrated water vapor overburden in
flight. The accuracy of the measured precipitable water vapor must be 2 microns or better, 3 sigma, and measured at
least once a minute. This presentation will cover the design and the measured laboratory performance of this instrument,
and will discuss other options for determining the water vapor overburden during the SOFIA Early Science shared-risk
Germanium photoconductors offer excellent sensitivity in the 50-140μm spectral range. Coupled with their modest
cooling requirements and their compatibility with the silicon cryo-CMOS readout technology, these detectors are the
most attractive candidates for far IR astronomy in this wavelength range. Over the years we have been pursuing the
advancement of this technology and our initial effort has produced a 2x16 Ge:Sb array with an NEP in the low 10<sup>-18</sup>
W/√Hz range, rivaling the best far IR arrays currently available. Further work has resulted in design and fabrication of
a low noise, 2-side buttable 32x32 (64x64 mosaic) CTIA readout, the first 1k-pixel Ge:Sb fully assembled focal-plane
array, a new hybrid design better suited for far IR photoconductors, and the preliminary design of a 2-side buttable
64x64 (128x128 mosaic) CTIA readout. Our developmental work continues and we believe that sensitivity levels
below 10<sup>-18</sup> W/√Hz are within reach. This paper presents an overview of our progress so far and outlines our roadmap
for further work.
The SB349 is a 32x32 readout multiplexer specifically designed for far IR photodetectors and is capable of operating at
cryogenic temperatures at least as low as 1.8K. This readout is a capacitive-transimpedance amplifier multiplexed to
eight outputs and is buttable on two sides to form a 64x64 mosaic array. It features eight selectable gain settings, auto
zero for better input uniformity, sample-and-hold circuitry, and provisions to block the readout glow. A special, 2-micron cryo-CMOS process was adopted to prevent freeze out and ensure low noise and proper operation at deep
cryogenic temperatures. An overview of the design and the results of the tests performed on this device are reported in
Development of large format, far infrared focal-plane arrays has been identified as a pressing need for future
astronomical instruments. In particular, array sizes as large as 128x128 with sensitivities equal to or better than 10<sup>-18</sup>
W/√Hz are the goals to be achieved within the next fifteen years. As part of our continuing effort to further this
technology, we are developing a 32x32 Ge:Sb photoconductor FPA with a CTIA cryogenic readout multiplexer. A
new, layered-hybrid architecture is employed to block the readout glow, improve heat dissipation and temperature
uniformity across the array, and alleviate the potential problems associated with the large CTE mismatch between the
Ge detector and the Si readout. This is the first 1k-pixel photoconductor FPA of its kind and is meant to be a pathfinder
for future large format FPAs. Based on the test results of a prototype 2x16 Ge:Sb array of similar design, we expect the
sensitivity of this FPA to be as low as 10<sup>-18</sup> W/√Hz. This paper presents the design, characteristics, and the expected
performance of this array.
Germanium detectors are extensively used in astronomical instruments for far infrared observations. To meet the science objectives of future space projects, large-format far IR detectors are needed. As a first step toward this goal, we have fabricated a 2x16 Ge:Sb array with the 1x32, SB-190 CTIA cryogenic readout. The detector design as well as the preliminary results of our parametric tests are presented here. The array exhibits very good noise performance with an NEP as low as 4.0E-18 W/√Hz, and confirms the viability of the design for large format arrays.
We have observed, for the first time, microwave-assisted photoconductivity in high purity GaAs. The enhancement of response appears to be dictated by two distinct mechanisms. First, a broadband enhancement which is believed to be due to detrapping of the free carriers and, therefore, increased photoconductive gain. Secondly, microwave-ionization of the excited states. We expect that both of these mechanisms contribute very little, if any, to the detector noise and, therefore, improve the detector's NEP. In this paper, we report the results of our preliminary tests showing broadband enhancement in response and an indication of enhancement of the excited-state response. Further investigation is currently underway.
SBRC-190 readout multiplexer is a 1×32, multi-gain, capacitive transimpedance amplifier (CTIA) especially suitable for use with infrared detector arrays requiring low-bias levels--such as Ge:Ga far infrared detector arrays. The unit-cell design employs a feedback loop which keeps the bias across the detector constant and, therefore, prevents debiasing--a critical requirement for far infrared detectors. We have tested a number of these multiplexers at various cryogenic temperatures, down to 1.7K. In this presentation we will report the results of our tests and will discuss gain, uniformity, and read noise of the bare mux under correlated-double sampling at 4.2K.
Testing of a 40 to 125 μm Ge:Sb photoconductor array for AIRES (Airborne Infra-Red Echelle Spectrometer) is described. The prototype array is a 2×24 module which can be close-stacked with other modules to provide larger two-dimensional formats. Collecting cones on a 0.08 inch pitch concentrate incident radiation into integrating cavities containing the detectors. The array is read out by two Raytheon SBRC 190 cryogenic multiplexers that also provide a CTIA (capacitive transimpedance amplifier) unit cell for each detector. We have conducted a series of tests to evaluate the array dark current, responsivity and detective quantum efficiency.
We have constructed a far infrared detector array consisting of a 1×16 Ge:Ga and a 1×16 Ge:Sb placed side-by-side to form a linear, 1×32 array. An SBRC-190 readout multiplexer, which is a 1×32, multi-gain, capacitive transimpedance amplifier (CTIA) manufactured by Raytheon Infrared Operation, is wire bonded to the array. The array has been tested in the temperature range of 3.2K to 2.6K under various infrared radiation levels. In this presentation we will discuss the design and will report on the results of the preliminary tests conducted on this array at 3.0K. This developmental effort is intended to test the viability of the SBRC-190 unit-cell design for far infrared detector arrays and help foster improvement and further development of readout electronics based on the CTIA design. In addition, we hope to extend the detector design and develop a two-dimensional, monolithic array.