Basic Detection Techniques Front-end Detectors for the Submm Andrey Baryshev/Wolfgang Wild

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Basic Detection Techniques Front-end Detectors for the Submm

  • Andrey Baryshev/Wolfgang Wild

  • Lecture on 21 Sep 2006

Contents overview

  • Submm / THz regime

    • Definition and significance
    • Science examples
  • Submm detection: direct + heterodyne

  • Heterodyne receiver systems

    • Signal chain, block diagram
    • Heterodyne principle
    • Noise temperature and sensitivity
    • Heterodyne frontend
      • Mixers
      • Local oscillators
      • IF amplifiers
    • Spectrometers: Filterbank, AOS, Autocorrelator, FFT
    • Overview submm astronomy facilities
    • Examples of heterodyne receiver systems
      • ALMA 650 GHz
      • HIFI space instrument
  • Direct detection systems

    • Signal chain, block diagram
    • Types of direct detectors and operating principles
    • Noise equivalent power (NEP)
    • Examples of a direct detection system
  • Quasi optics

Submillimeter/THz Wavelength Regime I

  • λ ~ 0.1 … 1 mm

  • Photon energy corresponds 2-20 K in temperature scale (hF =kT)

  • Between infrared/optical and radio waves

  • Submm technology is relatively new (~ 20 years)

  • (Compare to optical technology: ~ 400 years)

  • Submm astronomy is crucial for understanding star and planet formation

  • Range of 0.1… 0.3 mm is one of the last unexplored regimes in astronomy

Submillimeter Wavelength Regime II

  • Technically challenging and interesting

    • Challenging: small λ means high precision fabrication
    • Interesting: Combination of optical and electronic techniques
  • Submm astronomy and technology are very dynamic fields

Why submillimeter ? Sub-/Millimeter vs. optical astronomy

Radiation at (sub)mm wavelengths

The Earth atmosphere at submm wavelenghts

  • The Earth atmosphere is only partially transparent for submillimeter wave radiation

  • Several atmospheric “windows” exist

  • Water vapor and oxygen cause strong absorption

  •  dry, high observatory sites

  •  airplane, balloon and space platforms

Atmospheric transmission at 5000m altitude

Submillimeter astronomy – star formation

  • New stars form in molecular clouds

  • These clouds are best observed in the infrared and submm regime since they are cold and have high optical extinction

  • Star and planet formation is associated with a rich interstellar chemistry  many lines observable in IR/submm/mm

Optical vs. Submm/Far-Infrared

Molecular gas in M31

Dust and CO at z=6.4 !

Heterodyne Signal Chain

A heterodyne receiver for space

HIFI Signal Path

Main components of a heterodyne front-end

  • Optics  last part of this college

  • Submillimeter wave mixer

    • SIS = Superconductor-Insulator-Superconductor
    • HEB = Hot-Electron-Bolometer
    • (Schottky = Semiconductor-metal contact diode)
  • Local Oscillator

    • Multiplier chain
    • Quantum-Cascade-Laser (QCL)
  • Intermediate frequency (IF) amplifiers

Sensitivity and Noise Temperature

  • In radio and submm astronomy, the signal unit “Temperature” is used.

  • This is really a signal power W = k T Δν (k Boltzman constant)

  • Usually the signal power is much smaller than the noise power (“noise temperature”) of the receiving system.

  • The noise temperature of a system is defined as the physical temperature of a resistor producing the same noise power.

  • Difference measurements are used to detect the signal, e.g.

  • (sky + signal source) minus (sky)

The “ideal” submillimeter wave receiver

  • Converts all incoming radiation into an electric signal

  •  no photons “lost”

  •  has no own noise contribution

  • However: Heisenberg’s uncertainty principle (ΔE x Δt ≥ h/2π) makes such a noiseless mixer impossible.

  • Why ? – A heterodyne mixer measures signal amplitude and phase. This corresponds to number of photons and time in the photon picture which – according to the uncertainty principle – cannot be measured simultaneously with infinite precision. This uncertainty results in a minimum noise of a heterodyne mixer, the “quantum limit”.

  • Current best mixers are ~few times worse than the quantum limit.

Sensitivity of a receiving system

Noise Contributions from Receiver Components

HIFI signal chain

Sub-/millimeter Optics

Cryogenic submillimeter mixers

  • SIS = Superconductor-Insulator-Superconductor

  • - used in mm and submm from ~70 GHz to ~1200 GHz

  • - very good performance

  • - theory well understood

  • - submm detector of choice at ground-based and space

  • telescopes

  • HEB = Hot-Electron-Bolometer

  • - used above ~1200 GHz into THz regime

  • - performance better than SIS above 1200 GHz

  • - theory not well understood

  • - active research on-going

The SIS mixer

  • The SIS mixer (Superconductor-Insulator-Superconductor) element is a sandwich structure with a very thin insulator.

Bandgap structure of an SIS mixer

SIS mixer principle = photon assisted tunneling

Some formulas

300, 400, 800 GHz photon steps

Different RF power

Typical SIS mixer responce

SIS mixer implementation

  • Task: Couple the astronomical signal to the (very small, ~1 μm2) tunnel junction. Two ways are used:

  • Feedhorn and waveguide (waveguide mixer)

  • or

  • A lens and antenna structure (quasi-optical mixer)

Example of a waveguide SIS mixer (540-700 GHz)

Precision machining

HIFI mixers 800-960 GHz and 960-1120 GHz

HIFI mixer design

Example of a quasioptical mixer structure

Quasi-optical mixer implementation

Hot electron bolometer (HEB) principle

Hot electron bolometer (HEB) mixer

Typical I-V cirves

Submm mixer noise temperatures

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