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

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