Basic Detection Techniques Front-end Detectors for the Submm

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					Basic Detection Techniques

Front-end Detectors for the Submm

Wolfgang Wild

Lecture on 21 Sep 2006
Contents overview
•   Submm / THz regime
     • Definition and significance
     • Science examples
•   Submm detection: direct + heterodyne
•   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

                                  Basic Detection Techniques – Submm receivers   2
Submillimeter Wavelength Regime I

•   λ ~ 0.1 … 1 mm

•   Between infrared/optical and radio waves

•   Submm technology is relatively new (~ 20 years)
    (Compare to optical technology: ~ 300 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


                             Basic Detection Techniques – Submm receivers   3
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




                               Basic Detection Techniques – Submm receivers   4
Why submillimeter ?
Sub-/Millimeter vs. optical astronomy

Item           Sub-/Millimeter                  Optical / IR

Wavelength     0.1 mm to 3 mm                   0.4 to 30 μm
Frequency      100 GHz to 3 THz                 10 to 600 THz

Targets          Cold medium                    Hot medium
                  (10-100K)                    (a few 1000K)
               Molecular clouds                     Stars
              Extended structures               Point sources


  Sub-/Millimeter astronomy studies the Cold Universe.
  And most of the sky is dark and cold …


                                  Basic Detection Techniques – Submm receivers   5
Radiation at (sub)mm wavelengths



  Continuum:
   cold dust at 10-100 K
   (black body of 30K peaks at
   0.1 mm)
  Lines: pure rotational
   transitions of molecules

     Sub-/mm radiation
     probes cold molecular
     clouds of gas and dust

                                       Energy levels of CO and CS
                                 Basic Detection Techniques – Submm receivers   6
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




                                   Basic Detection Techniques – Submm receivers   7
Atmospheric transmission at 5000m altitude




         pwv = precipitable water vapour, i.e. the column height
         of condensed water vapour
                               Basic Detection Techniques – Submm receivers   8
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

                               JCMT Spectral Survey IRAS16293- 2422




                                     Basic Detection Techniques – Submm receivers   9
Cazaux et al. 2003
                     Basic Detection Techniques – Submm receivers   10
Basic Detection Techniques – Submm receivers   11
Optical vs. Submm/Far-Infrared


  Orion Trapezium Region at Optical Wavelengths    Highlighted Region at IR




                                                  Basic Detection Techniques – Submm receivers   12
Basic Detection Techniques – Submm receivers   13
Molecular gas in M31

                                       CO line emission
                                       traces molecular
                                       gas.

                                       This is where new
                                       stars form.




                                       Nieten et al. 2005
                       Basic Detection Techniques – Submm receivers   14
Dust and CO at z=6.4 !




  Sloan survey:
  optical image
                                          Z=6.4
  Contours: dust

  => Heavy elements
  formed shortly after
  Big Bang

  IRAM 30m MAMBO
  Bertoldi et al. 2003

                         Basic Detection Techniques – Submm receivers   15
Bertoldi et al. 2003




                       Basic Detection Techniques – Submm receivers   16
          Two Main Detection Schemes for
               Sub-/mm Radiation

• Incoherent detection  direct detectors (bolometer)

      • total power detection
      • no phase information  used on single antenna
      • low spectral resolution

• Coherent detection  heterodyne receiver

      • frequency down conversion
      • high spectral resolution
      • phase information  single antenna and interferometer

    Heterodyne technique and receivers will be treated here.


                             Design of a Scientific Instrument   06 June 2006   17
Heterodyne Signal Chain


                electrical
                             Intermediate
                             Frequency (IF)
           Heterodyne                               Spectrometer/
                                                    Correlator                    Data acquisition
           Instrument

optical    “Front End”                               “Backend”




• Convert incoming radiation into electronic signal (IF) for further processing

• Spectral information is preserved (spectral resolution Δf/f determined
  by backend)

• Heterodyne detection achieves spectral resolution > 106

                                              Design of a Scientific Instrument   06 June 2006       18
                          Principle of Heterodyne Mixing

Heterodyne principle = mixing of two frequencies (signal + local oscillator)
                       to produce (sum and) difference signal
                       (intermediate frequency = IF)

Mixing needs non-linear element (e.g. diode, SIS junction) = mixer
                                                  f IF = | f LO - f RF |

                                                  Double sideband mixer:
                                                  both sidebands converted to same IF

                      LO                          Single sideband mixer:
    IF           RF         RF
                                                  Only one sideband converted to IF
          //                          freq        Sideband separating mixer:
0   fIF          Lower       Upper                two sidebands converted to different
               sideband    sideband               IF outputs
                 (LSB)       (USB)



                                             Design of a Scientific Instrument   06 June 2006   19
                              Heterodyne Mixing
Combine strong LO signal             VLO= cos(LOt) (e.g. 996 GHz)
+
A weak RF signal                     VS= cos(St+) (e.g. 1002 GHz)
Gives total power absorbed           P ~ VS VLO cos((S - LO)t + )+….
Amplitude and phase information conserved in IF signal
                                                                                  IF signal
Detect radiation at frequencies where no amplifiers are available
        Local               Signal Spectrum                         IF Spectrum
       Oscillator
                    Power




                                                           Power
        996           1000         1004                            4            8
                       Frequency (GHz)                             Frequency (GHz)
              Mixing needs strong non-linear detector charcteristic
                                         Design of a Scientific Instrument   06 June 2006   20
         Block Diagram of a Heterodyne Receiver



                                         LO signal                           IF signal out
                                         (e.g. 646 GHz)                      (e.g. 4 GHz)
                         LO ref in
                                             Local
                                             oscillator


                                                                                 to correlator
Cal                 Optics
                                                                                 or spectrometer
source
                                     Mixer       IF amp(s)
Astronomical RF signal   4K
(e.g. 650 GHz)

  Components:         Optics                               • IF amplifier(s)
                     • Mixer                                • Dewar and cryogenics
                     • Local Oscillator (LO)                • Bias electronics
                     • Calibration source                   • Spectrometer(s)


                                             Design of a Scientific Instrument   06 June 2006      21
A Heterodyne Receiver




         Design of a Scientific Instrument   06 June 2006   22
A heterodyne receiver for space

                                                                        Telescope
                                                                          Beam
HIFI = Heterodyne Instrument
       for the Far-Infrared

Will fly on the Herschel
Space Observatory in 2008




                      7 LO Beams
                                                                   ~ 50 cm


                               Design of a Scientific Instrument   06 June 2006   23
HIFI Signal Path


           Local
          Oscillator                                   Telescope
            Unit


                       LOU         optics
                                                          Focal
                                   mixer                  Plane
                                                           Unit

                                     IF

                                                              IF
                    LSU      HRS            WBS         spectrometers


          Local Oscillator          ICU              Instrument
           Source Unit                               Control Unit
                             To Astronomer

                                   Design of a Scientific Instrument   06 June 2006   24
Main components of a heterodyne front-end

•   Optics  own lecture on “quasi-”optics

•   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




                                   Basic Detection Techniques – Submm receivers   25
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)


                                   Basic Detection Techniques – Submm receivers   26
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.

                                   Basic Detection Techniques – Submm receivers   27
Sensitivity of a receiving system

Question: What is the smallest detectable signal ?

The answer is the
Radiometer formula (Sensitivity):    Tmin = c1 Tsys / (t )1/2

                                                                   system bandwidth
                     system temperature                          integration time

Received noise power from an antenna / receiver system:
Noise power     Wsys = WA + Wrx = k Tsys  = k (TA + Trx ) 
                Tsys = TA + Trx

                                  receiver noise temperature

                                  antenna temperature (signal, atmosphere,
                                  antenna losses)
                                         Basic Detection Techniques – Submm receivers   28
Noise Contributions from Receiver Components
Question:            What is the noise contribution from different
                     receiver components ?

Receiver as a series of linear two-ports:
                                                                    T: noise temp
                                                                    G: Gain
          Optics          Mixer        1st IF amplifier

                                                                                           To
          T 1 , G1        T2, G2          T3, G3                       Tn, Gn              detector




Trx   =    T1        +   T2 / G1   +   T3 / (G1 G2 ) + … + Tn / ( G1 G2 …. Gn )


 Receiver noise temperature determined by first few elements
 Cooled optics for high frequencies

                                            Basic Detection Techniques – Submm receivers      29
HIFI signal chain
                                                                    HIFI Dual IF System - one polarisation
                                                                                    N. D. Whyborn, 021016

                                          4K
                             2 K level                                     15 K level                                                                     290 K (SVM)
                                         level



                              mixer &              IF-1                level                IF-2                                                                    warm
                                                                                                            cryoharness               IF up-converter                         spectrometers
                              isolator           amplifier          trimming              assembly                                                                 harness

                          max. level:    -118 dBm/MHz                  -93 dBm/MHz                               -69 dBm/MHz                                            -90 dBm/MHz
                          min. level:    -128 dBm/MHz                 -103 dBm/MHz                               -79 dBm/MHz                                        -100 dBm/MHz
   2.4 - 4.8 GHz IF




                          IF gain:       -1 dB    29 dB            -3 dB                  +30 dB               -6 dB                  -16 dB               -3 dB    -2 dB

                                                                                                                                               10.4 GHz

                      6H                                                                                                                              8 - 5.6
                                                                                                                                                       GHz
                      6L
                                                                                                                          2.4 - 4.8
                                                                                                                            GHz

                                                                                                                                                                                 HRS-V
                      5

                      4

                      3
   4 - 8 GHz IF




                                                                                                                                                                                 WBS-V
                      2

                      1
                                                                               (+31 dB)     (-10 dB)
                          IF gain:       -1 dB    25 dB            -5 dB                  +21 dB               -8 dB                                       -3 dB    -2 dB
                          max. level:    -118 dBm/MHz                  -98 dBm/MHz                               -85 dBm/MHz                                            -90 dBm/MHz
                          min. level:    -128 dBm/MHz                 -108 dBm/MHz                               -95 dBm/MHz                                        -100 dBm/MHz


                                                             N.B. There is an identical arrangement for the other polarisation.




                                                                                                       Basic Detection Techniques – Submm receivers                                           30
Sub-/millimeter Optics


  Main function: coupling of the antenna signal into mixer

  Used components:

  • Lenses (e.g. PTFE, quartz)
  • Mirrors (plane and focusing)
  • Feed horn
  • Grids (polarization separation)
  • quarter / half-wave plates
  • Martin-Puplett Interferometers


  Gaussian optics used in sub-/mm regime (separate lecture)



                                      Basic Detection Techniques – Submm receivers   31
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

                                  Basic Detection Techniques – Submm receivers   32
The SIS mixer

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

Superconductor-Insulator-Superconductor (SIS) Tunnel Junctions

                                                            S     I   S




    Cross section of a typical
    Niobium SIS tunnel junction



  • insulator thickness <= 1nm : tunneling


                                             SEM view of junction top electrode
                                             (1x1 µm²)
                                       Basic Detection Techniques – Submm receivers   33
Bandgap structure of an SIS mixer


 Energy gap  in density of states:                                   „Semiconductor“ model for SIS
  no current below Vbias = 2/e                                      „Quasiparticle Excitations“ ~ Electrons
    low shot noise

 root singularity in density of states:
  large current flow at VGap                                             Superconductor 1      Ins.    Superconductor 2
     extremely sharp nonlinearity                                        at V ~ VGap                   grounded



                 200



                 150
  Current [A]




                                                   RN = dI / dV
                 100
                               Rsg= 2mV/Isg

                  50

                 Isg
                       0
                           0           2   VGap   4               6

                                     Bias Voltage [mV]

                                                                      (Cooper pair tunneling effects not shown !)
                                                                          Basic Detection Techniques – Submm receivers     34
SIS mixer principle = photon assisted tunneling

                       



      F + eU
                            F
                   h




           Photon assisted tunneling (Dayem&Martin)
           series of steps at V = UGap – nh/e

           Frequency limit for mixing at h = 4 (1400 GHz for Nb)

           LO power: PLO ~ (h/e)²/RN (800 GHz, 20 Ohms: 0.5µW)


                                    Basic Detection Techniques – Submm receivers   35
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)




                                    Basic Detection Techniques – Submm receivers   36
Example of a waveguide SIS mixer (540-700 GHz)


                                            10 mm




                                                                  Junction
 Lens          Feed horn                      Magnet
                                                                  holder



                           Basic Detection Techniques – Submm receivers   37
Precision machining


0.1 mm



Human
 hair
         Backshort cavity   Mixer backpiece            Terahertz mixer




           With SIS chip
           and tunnel junction
                                    Basic Detection Techniques – Submm receivers   38
HIFI mixers 800-960 GHz and 960-1120 GHz




                 These mixers will fly on the
                 Herschel Space Observatory




                        Basic Detection Techniques – Submm receivers   39
HIFI mixer design


                       magnet
                                     Pressure unit

                                              IF-board

                                                      Re-alignment
                                                      spring

 Magnet pole shoes

     Device mount with
     backshort, substrate
     channel and
     alignment spring

    ESD protection, bias and LF filtering

                                                 Corrugated horn
     Cover for bias/ESD PCB


                                             Basic Detection Techniques – Submm receivers   40
Example of a quasioptical mixer structure



                                                                                  10 mm




  Antenna
              SIS junction   Stripline      Mixer chip                     Lens
  structure


                                         Basic Detection Techniques – Submm receivers   41
Quasi-optical mixer implementation



                                  Quasi-optical mixer for the
                                  Space instrument HIFI

                                  Chalmers Technical University
                                  Gothenburg, Sweden

                                  1.5 THz


Silicon lens        IF board

 Main challenges:      - chip alignment on lens
                       - optical properties, beam direction



                                  Basic Detection Techniques – Submm receivers   42
Hot electron bolometer (HEB) principle

 Thin superconducting film


 Square law power detector


 thermal time constant t = C/G
 C: thermal capacitance
 G: thermal conductivity


 Mixer operation: can detect beat
 frequency between LO and signal


 has to be very fast (ps) for few GHz IF
 (needed for spectroscopy)



                                     Basic Detection Techniques – Submm receivers   43
Hot electron bolometer (HEB) mixer

 Principle of operation
 • Radiation heats electrons  R
 • Cooling either by phonons or out-diffusion
 • Direct or heterodyne detection
                                                           1 m x 0.15 m (W x L)
 Limitations
 • IF bandwidth, sensitivity
                         radiation


                     e            S
                 e       e   ph

            e
                                  ph
                         L
            Substrate

           Hot Electron Bolometer

                                       Basic Detection Techniques – Submm receivers   44
Submm mixer noise temperatures

                                                          HIFI space
                                                          instrument

                                                          Jan 2006




  • Mixer noise increases with frequency (increased losses)



                               Basic Detection Techniques – Submm receivers   45