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Astronomical Observational Techniques and Instrumentation. Professor Don Figer Quantum-Limited Detectors. Aims for this lecture. Motivate the need for future detectors Describe physical principles of future detectors Review some promising technologies for future detectors.
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Astronomical Observational Techniques and Instrumentation Professor Don Figer Quantum-Limited Detectors
Aims for this lecture • Motivate the need for future detectors • Describe physical principles of future detectors • Review some promising technologies for future detectors
Improving Detectors • Detector properties limit sensitivity in most applications. • For instance, dark current and read noise are important in low flux applications. • Detectivity is a measure of system effectiveness.
Detectivity in Narrowband Applications Figure 5. Detectivity as a function of quantum efficiency and read noise for narrowband astrophysics applications.
Detectivity in Narrowband Applications with Low Dark Current
Detectivity in Spectroscopic Applications with Low Dark Current
Very Low Light Level - ExoPlanet Imaging • The exposure time required to achieve SNR=1 is dramatically reduced for a zero read noise detector, as compared to detectors with state of the art read noise.
Key Capabilities for Future Improvement • photon-counting (zero read noise) • wavelength-resolving • polarization-measuring • low power • large area • in-pixel processing • high dynamic range • high speed • time resolution
Key to Single-Photon Counting • A photon-counting system requires that the ratio of signal from a single photon to the noise of the system be big enough to detect. • This can be achieved by: • increasing numerator (e.g., charge gain) • decreasing denominator (e.g., cooling, better circuits) • decreasing what is “big enough” (e.g., better processing) • combination of all
Superconductors • Most metals have descreased resistance with lower temperature, but they still have finite resistance at T=0 K. • Superconductors lose all resistance to electrical current at some temperature, Tc. Examples include: Pb, Al, Sn, and Nb. • Electrons in superconductors bond as “Cooper pairs” that do not interact with the ion lattice below Tc because the required interaction energy exceeds the thermal energy in the crystal. • In general, Tc<4.2 K. • Recent developments have produced “high” temperature superconductors, for which Tc>77 K (temperature of liquid nitrogen).
Geiger-Mode Imager:Photon-to-Digital Conversion Digitallyencodedphotonflight time Pixel circuit Digital timing circuit photon APD/CMOS array APD Lenslet array Focal-plane concept Quantum-limited sensitivity Noiseless readout Photon counting or timing
Gain of an APD M Ordinary photodiode Linear-mode APD Geiger-mode APD 100 10 1 0 Breakdown Response to a photon ∞ I(t) M 1
on avalanche Operation of Avalanche Diode on Linear Geiger mode mode quench Geiger Linear mode mode Current Current Current Current arm Vdc + DV off off V V br br Voltage Voltage
Single photon input time APD output Discriminator level time time Digital comparator output time time Dark count – from dark current Photon absorbed but insufficient gain – missed count Successful single photon detection Performance Parameters • Photon detection efficiency (PDE) • The probability that a single incident photon initiates a current pulse that registers in a digital counter • Dark count Rate (DCR)/Probability (DCP) • The probability that a count is triggered by dark current instead of incident photons
APD Charge Gain • Show animation with thumping euro-techno disco music http://techresearch.intel.com/spaw2/uploads/files/SiliconPhotonics.html
32x32 Timing Circuit Array 0.35-mm CMOS process fabricated through MOSIS 1.2 GHz on-chip clock Two vernier bits 0.2-ns timing quantization 100-mm spacing to match the 32x32 APD array Pixels Time bin Vernier bits Counter Timing image/histogram measuring propagation of electronic trigger signal
Shortcomings of Conventional Imaging • When the 3D world is projected into a flat intensity image, there is a huge information loss. • Image processing algorithms attempt to use intensity edges to infer properties of 3D objects. • Consequences of lost information for automated image segmentation and target detection/recognition: • Depth ambiguity • Sensitivity to lighting, reflectivity patterns, and point of observation • Obscuration and camouflage
Microchip laser Geiger-mode APD array Ladar Imaging System • Imaging system photon starved • Each detector must precisely time a weak optical pulse • Sub-ns timing, single photons Color-codedrange image
Laser Radar Brassboard System (Gen I) • 4 4 APD array • External rack-mounted timing circuits • Doubled Nd:YAG passively Q-switched microchip laser (produces 30 µJ, 250 ps pulses at = 532 nm) • Transmit/receive field of view scanned to generate 128 128 images Taken at noontime on a sunny day
Conventional vs Ladar Image Conventional image 3D image
Foliage Penetration Experiment View from 100 m tower Laser radar on tower elevator Objects under trees
Transition Edge Sensors (TES) • A TES is similar to a bolometer, in that photon energy is detected when it is absorbed in a material that changes resistance with temperature. • The difference is that a TES is held at a temperature just below the transition temperature at which the material becomes supconducting. • The effective change in resistance when photons are absorbed is very large (and easy to detect). • One of the disadvantages of using TES’s is that the transition temperature is usually very low, requiring exotic cooling techniques.
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Superconducting Tunneling Junctions (STJs) • An STJ uses the current response of a Josephson junction (aka STJ) when struck by a photon to detect light. • The junction is similar to semiconducting junction and is composed of superconductor-insulator-superconductor. • The gap energy is generally much less than for silicon, so optical photons induce charge gain that depends on photon energy.
