Terahertz focal plane arrays for astrophysics and remote ...€¦ · Terahertz focal plane arrays...
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IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
Terahertz focal plane arrays for astrophysics and remote sensing
Christopher Groppi Arizona State University
School of Earth and Space Exploration
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
Emission at 115 GHz from the CO molecule was first detected in 1970. Astronomers have been imaging THz light ever since.
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
This is what we can do today: a 120 square degree image of 13CO(1-0) at 110 GHz. Beamsize is ~1 arcminute in this image.
Every pixel is an integrated high resolution spectrum.
Full Moon
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
We can do the same trick looking at the thermal emission from small, solid particles (“dust”). Dust emits a blackbody spectrum,
peaking at THz frequencies at typical molecular cloud temperatures.
350 GHz continuum
SWIR image 1.2, 1.6, 2.1 microns
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
Blackbody emission from dust
Dust is 1% by mass of a molecular cloud, but is easy to detect and provides information about the cloud temperature and structure.
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
How do molecules emit light?
Blue / Higher Frequency Red / Lower Frequency
• Molecular clouds are 99% molecular gas, mostly H2.
• We can’t see H2 (no dipole moment) so we look at other, less abundant molecules instead (like carbon monoxide).
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
How do we make images? • Until recently, virtually all telescopes were
equipped with single pixel THz detectors.
• Images are made using the “On The Fly” Mapping technique. • Antenna is raster scanned across the source at a fixed
angular rate. • Receiver is read out rapidly (several Hz). • Lots of short integrations at closely spaced intervals are
convolved with a Gaussian kernel the size of the beam on a Nyquist sampled grid.
• Much faster than point by point mapping, since multiple spatial pixels share the same reference.
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
Tradeoff between spatial resolution and mapping speed. • Large antennas have smaller beams, resulting in better spatial
resolution in the final image.
• This also results in more pixels in an image of a given angular size. Bigger telescopes map a given area slower.
• If you want your cake and eat it too (wide areas AND high spatial resolution) you need multiple spatial pixels.
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
This is what we can do today: a 120 square degree image of 13CO(1-0) at 110 GHz. Beamsize is ~1 arcminute in this image.
Every pixel is an integrated high resolution spectrum.
Full Moon
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
Coherent array receivers • SEQUOIA was built for the 15m FCRAO antenna in
Massachusetts. • Operates from 85-115 GHz. • 32 cryogenic HEMT amplifier based pixels (16 in each linear
polarization).
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
PoleStar array for AST/RO
•810 GHz
•4 Pixel SIS superconducting mixers
•Solid state local oscillator source (~0.3 mW)
•L-band IF 1-2 GHz
•Trec~550-650 K
•4 channel array AOS backend spectrometer
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
2 8 channel downconverter modules
Omnisys Spectrometer 64x250 MHz complete system
Prototype 8 channel bias system (1 6U card with power supplies)
Spectrometer and bias control computer
LO System with 8 way power divider LO Optics LO Beamsplitter & dewar window CTI 350 cooler Sumitomo 4K cooler
Supercam System
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
Magnet DC connector
Bias DC connector Gilbert GPPO blind mate IF connectors
1x8 Mixer Module
Electromagnets
Horn Extension Block
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
Ground beamlead
SOI SIS Chip
Beamlead alignment tabs
IF Beamlead
Magnet probes Input matching network WBA13 MMIC chip
Bias circuitry
Output coax
A Closer Look
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
Low Noise Amplifiers
N. Wadefalk, J. Kooi, H. Mani & S. Weinreb,
Caltech
32 dB Gain, 5 K Noise at 8mW power dissipation
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
• 1x8 Downconverter Module (Caltech: G. Jones and J. Bardin)
• Total power metering
• 250 MHz and 500 MHz bandwidth modes (1 GHz with filter change)
• Digital attenuators
• Low cost surface mount components
Supercam IF Processing
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
• Real-Time FFT system • Virtex 4 SX55 FPGA • 4x 500 MHz or 2x 1 GHz per board • 1024 channels • power consumption 25W per board • Ethernet interface • SuperCam spectrometer initially uses 8
identical boards for 64 x 250 MHz operation
• Allan time (with IF processor) ~250 s
Supercam Spectrometer (Omnisys)
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
LO Multiplexing 64-Way Waveguide Power Divider
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
CNC Metal Micromachining
350 GHz TWT
650 GHz Sideband Separating Mixer
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
Technological Challenges and Proposed Solutions for Even Larger Coherent Arrays
1. Mechanical and Electrical Complexity Solution: Use 2D integration to simplify design
2. Detector yield and focal plane assembly Solution: Use simple, robust SIS device design with self aligning SOI chips
3. Heat load from LNAs becomes dominant with large pixel count Solution: Develop integrated ultra-low power dissipation LNAs
4. Economical and fast WG and feed fabrication Solution: use drillable feeds (e.g. Leech et al, this conference), CNC micromachining for WG.
5. RF and DC interconnects and wire count Solution: Develop multi-conductor cryo-ribbon interconnects
6. Magnetic field for SIS devices Solution: Use engineered permanent magnets to replace individually adjustable electromagnets
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
KAPPA FPU Concept
SiGe MMIC Bonding Pad
Waveguide
Via to surface mount G3PO
connector
6 mm
IF Microstrip
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
KAPPA SiGe LNA 500 um
800 um
SiGe MMIC Chip
• 0.5-4.5 GHz • 16 dB Gain • 7K noise temperature (predicted)
9K (measured) • 2 mW power dissipation
• |S11|<-11 dB • On-chip bias tee • Small chip size • 2 stage version also fabricated
(32 dB gain, 4-5 mW power dissipation)
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
LNA Integration
IEEE mm-wave and THz Workshop April 27, 2012 Tempe AZ
IF Flex Circuit • 16 channel stripline
flex circuit with microstrip terminations.
• 1m circuit has ~4 dB (simulated) loss at 4.5 GHz.
• Heat load with 9 micron thick copper conductors: ~5 mW (heat sunk at 15K, 150mm length.
• Heat load could be further reduced with cryogenically compatible metal replacing copper (i.e. phosphor-bronze).
10mm wide
Rogers Ultralam 3000 flexible LCP dielectric: 200um total thickness
18mm wide Stripline to microstrip transition