80 research outputs found
Interferometric Polarization Control
A signal conditioning module provides a polarimeter capability in a photometric system. The module may include multiple variable delay polarization modulators. Each modulator may include an input port, and a first arm formed to include a first reflector and first rooftop mirror arranged in opposed relationship. The first reflector may direct an input radiation signal to the first rooftop mirror. Each modulator also may include an output port and a second arm formed to include a second reflector and second rooftop mirror arranged in opposed relationship. The second reflector can guide a signal from the second rooftop mirror towards the output port to provide an output radiation signal. A beamsplitting grid may be placed between the first reflector and the first rooftop mirror, and also between the second reflector and the second rooftop mirror. A translation apparatus can provide adjustment relative to optical path length vis-a-vis the first arm, the second arm and the grid
A Quasioptical Vector Interferometer for Polarization Control
We present a mathematical description of a Quasioptical Vector Interferometer (QVI), a device that maps an input polarization state to an output polarization state by introducing a phase delay between two linear orthogonal components of the input polarization. The advantages of such a device over a spinning wave-plate modulator for measuring astronomical polarization in the far-infrared through millimeter are: 1. The use of small, linear motions eliminates the need for cryogenic rotational bearings, 2. The phase flexibility allows measurement of Stokes V as well as Q and U, and 3. The QVI allows for both multi-wavelength and broadband modulation. We suggest two implementations of this device as an astronomical polarization modulator. The first involves two such modulators placed in series. By adjusting the two phase delays, it is possible to use such a modulator to measure Stokes Q, U, and V for passbands that are not too large. Conversely, a single QVI may be used to measure Q and V independent of frequency. In this implementation, Stokes U must be measured by rotating the instrument. We conclude this paper by presenting initial laboratory results
Precision control of thermal transport in cryogenic single-crystal silicon devices
We report on the diffusive-ballistic thermal conductance of multi-moded
single-crystal silicon beams measured below 1 K. It is shown that the phonon
mean-free-path is a strong function of the surface roughness
characteristics of the beams. This effect is enhanced in diffuse beams with
lengths much larger than , even when the surface is fairly smooth, 5-10
nm rms, and the peak thermal wavelength is 0.6 m. Resonant phonon
scattering has been observed in beams with a pitted surface morphology and
characteristic pit depth of 30 nm. Hence, if the surface roughness is not
adequately controlled, the thermal conductance can vary significantly for
diffuse beams fabricated across a wafer. In contrast, when the beam length is
of order , the conductance is dominated by ballistic transport and is
effectively set by the beam area. We have demonstrated a uniformity of 8%
in fractional deviation for ballistic beams, and this deviation is largely set
by the thermal conductance of diffuse beams that support the
micro-electro-mechanical device and electrical leads. In addition, we have
found no evidence for excess specific heat in single-crystal silicon membranes.
This allows for the precise control of the device heat capacity with normal
metal films. We discuss the results in the context of the design and
fabrication of large-format arrays of far-infrared and millimeter wavelength
cryogenic detectors
Diffraction Considerations for Planar Detectors in the Few-Mode Limit
Filled arrays of bolometers are currently being employed for use in astronomy
from the far-infrared through millimeter parts of the electromagnetic spectrum.
Because of the large range of wavelengths for which such detectors are
applicable, the number of modes supported by a pixel will vary according to the
specific application of a given available technology. We study the dependence
of image fidelity and induced polarization on the size of the pixel by
employing a formalism in which diffraction due to the pixel boundary is treated
by propagating the second-order statistical correlations of the radiation field
through a model optical system. We construct polarized beam pattern images of
square pixels for various ratios of p/\lambda where p is the pixel size and
\lambda is the wavelength of the radiation under consideration. For the limit
in which few modes are supported by the pixel (p/\lambda<1), we find that the
diffraction due to the pixel edges is non-negligible and hence must be
considered along with the telescope diffraction pattern in modeling the
ultimate spatial resolution of an imaging system. For the case in which the
pixel is over-moded (p/\lambda>1), the geometric limit is approached as
expected. This technique gives a quantitative approach to optimize the imaging
properties of arrays of planar detectors in the few-mode limit.Comment: 20 pages, 8 figure
Compact Radiative Control Structures for Millimeter Astronomy
We have designed, fabricated, and tested compact radiative control structures, including antireflection coatings and resonant absorbers, for millimeter through submillimeter wave astronomy. The antireflection coatings consist of micromachined single crystal silicon dielectric sub-wavelength honeycombs. The effective dielectric constant of the structures is set by the honeycomb cell geometry. The resonant absorbers consist of pieces of solid single crystal silicon substrate and thin phosphorus implanted regions whose sheet resistance is tailored to maximize absorption by the structure. We present an implantation model that can be used to predict the ion energy and dose required for obtaining a target implant layer sheet resistance. A neutral density filter, a hybrid of a silicon dielectric honeycomb with an implanted region, has also been fabricated with this basic approach. These radiative control structures are scalable and compatible for use large focal plane detector arrays
Detector Modeling and CMB Polarimetry Technology Development at GSFC
Pixel size limits the resolution in the focal plane. This should be accounted for in optical design. Alternatively, this reduces the effective number of independent detectors. Polarization and scattering are intrinsically related, and both are more severe at low pnambda. Future work: Quantification of the pixel cross-coupling- calculate a theoretical covariance matrix to predict performance of future detector arrays
Scalable background-limited polarization-sensitive detectors for mm-wave applications
We report on the status and development of polarization-sensitive detectors
for millimeter-wave applications. The detectors are fabricated on
single-crystal silicon, which functions as a low-loss dielectric substrate for
the microwave circuitry as well as the supporting membrane for the
Transition-Edge Sensor (TES) bolometers. The orthomode transducer (OMT) is
realized as a symmetric structure and on-chip filters are employed to define
the detection bandwidth. A hybridized integrated enclosure reduces the
high-frequency THz mode set that can couple to the TES bolometers. An
implementation of the detector architecture at Q-band achieves 90% efficiency
in each polarization. The design is scalable in both frequency coverage, 30-300
GHz, and in number of detectors with uniform characteristics. Hence, the
detectors are desirable for ground-based or space-borne instruments that
require large arrays of efficient background-limited cryogenic detectors.Comment: 7 pages, 3 figures, Presented at SPIE Astronomical Telescopes and
Instrumentation 2014: Millimeter, Submillimeter, and Far-Infrared Detectors
and Instrumentation for Astronomy VII. To be published in Proceedings of SPIE
Volume 915
Silicon-Based Antenna-Coupled Polarization-Sensitive Millimeter-Wave Bolometer Arrays for Cosmic Microwave Background Instruments
We describe feedhorn-coupled polarization-sensitive detector arrays that
utilize monocrystalline silicon as the dielectric substrate material.
Monocrystalline silicon has a low-loss tangent and repeatable dielectric
constant, characteristics that are critical for realizing efficient and uniform
superconducting microwave circuits. An additional advantage of this material is
its low specific heat. In a detector pixel, two Transition-Edge Sensor (TES)
bolometers are antenna-coupled to in-band radiation via a symmetric planar
orthomode transducer (OMT). Each orthogonal linear polarization is coupled to a
separate superconducting microstrip transmission line circuit. On-chip
filtering is employed to both reject out-of-band radiation from the upper band
edge to the gap frequency of the niobium superconductor, and to flexibly define
the bandwidth for each TES to meet the requirements of the application. The
microwave circuit is compatible with multi-chroic operation. Metalized silicon
platelets are used to define the backshort for the waveguide probes. This
micro-machined structure is also used to mitigate the coupling of out-of-band
radiation to the microwave circuit. At 40 GHz, the detectors have a measured
efficiency of 90%. In this paper, we describe the development of the 90 GHz
detector arrays that will be demonstrated using the Cosmology Large Angular
Scale Surveyor (CLASS) ground-based telescope
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