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FPP Instrument: Review of quasi-optical Polarisation Modulators. The University of Manchester. Giampaolo Pisano Radioastronomy Technology Group Jodrell Bank Centre for Astrophysics, University of Manchester, UK FPP Workshop - Henri Poincaré Institute, Paris , 8 th -9 th October 2010.

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fpp instrument review of quasi optical polarisation modulators
FPP Instrument: Review of quasi-optical Polarisation Modulators

The University

of Manchester

Giampaolo Pisano

Radioastronomy Technology Group

Jodrell Bank Centre for Astrophysics, University of Manchester, UK

FPP Workshop - Henri PoincaréInstitute, Paris, 8th-9th October 2010

slide3

Polarisation modulator: Some of the present requirements

Very large dimensions 1.2 m !!

1

Modulation efficiency:  80%?

Robust and light device: mechanical rotation needed

Broadband performance  Bandwidth ~180% !!

3

4

2

Low absorption losses (also differential losses): thermal emissivity

5

Polarisation systematic effects: deep understanding / control needed

6

...

7

slide4

RHWP: Bands and efficiency

see G. Siringoet al., Laboca Experiment

D - Phase shift between s & p pol

- Modulation efficiency

Example

d=5.3mm, f=45  nn=20(2n+1)GHz

  • Bandwidth such that the averaged e~0.8
    •  Dn=20GHz (independent on frequency)

 Cross-Pol issues to be solved

slide5

RHWP: Feasibility

D. Chuss

(2008)

  • 500mm diameter wire grids has been built (see VPM - D.Chuss later)
    •  Is it possible to go up to ~1.2m?

1

 It would be very fragile, will the wires bend ?

 RHWP bandwidth needs to be improved

3

2

(50 cm diameter wire-grid example)

slide6

RHWP: Bandwidth increase

  • If we could use filters within the 30% bandwidth to select sub-bands
  • where the average modulation efficiency is >80%:
  •  Increase in effective bandwidth
  • Example 540GHz channel:

 Increase from 3.7% to 15% in BW

slide7

Other known polarisation modulators

  • Variable Phase Delay modulators (VPM)
  • Birefringent HWPs
  • Mesh HWPs (Air-gap or dielectrically embedded)
  • Note: we are not considering the following devices because they
  • are relatively ‘narrow’ band (30-40%):
      • Waveguide polarisation modulators/rotators:
        • Faraday rotators, rotating waveguides
      • Microstrip devices:
        • MEMS switches, SC switches. Etc.
slide8

Similar polarisation modulator: Variable Phase Delay Modulator

D. Chuss

(2008)

  • This type of modulator does not modulate Q and U at the same time
  •  Can this apply in our case ?
slide9

Birefringent HWPs:Pancharatnam designs

- Recipes based on birefringent plates:

  • Limits on maximum diameters available :
        •  Quartz Ø ~110mm, Sapphire Ø ~280 mm

1

Bandwidth: 5-plate recipe ~100%

2

(Example of 3-plate sapphire recipe, no ARC)

~10cm

slide10

Mesh Half-Wave Plate: Air-gap design

G. Pisano et al., Applied Optics v47, n33 (2008)

- Recipes based on metal grids geometry/spacing:

~4cm

  • Dimension in principle achievablebut very thin substrates required  Present limits in diameter ~200mm

1

 Too fragile, it can vibrate

 Present max bandwidth ~70%

3

2

(Example of inductive stack)

slide11

Mesh HWP: Dielectrically embedded design

20cm

  • Present hot-pressing working up to 300mm (near future 500mm)
    • Alternative ‘cold bonding’ for bigger diameters under study

1

Bandwidth similar to air-gap

2

Pol 1

  • Very robust & light although it might bend with diameters >1m
    •  Flatness problem

(Example of embedded mesh-HWP)

3

Pol 2

slide12

Other types and other possible solutions of RHWPs

  • Dielectrically embedded RHWP
  • Twist reflectors
  • - Dielectrically embedded Mesh RHWP
  • Hard & Soft surfaces
  • Artificial surfaces
slide13

Modified RHWPs: Dielectrically embedded RHWP

 Dimensions: should be feasible using photolithography

(2 evaporated/etched substrates + cold bonding) *

1

 Photolithographic Wire-grid

Anti-Reflection Coating 

Dielectric substrate 

 Bandwidth: same as the free-standing one ?

2

 Mirror

  • Very light & robust (held by a mirror)

3

  • (*) - 2 m diameter evaporator chambers available
    • Possible to print masks on 2m width acetate
    • Printer resolution will allow to build grids with 50um period and 25um strip:
      •  Wire-grid efficiency still >90% at 1THz frequency
slide14

Other RHWPs: Twist reflectors

- They are meant to provide 180º phase-shift and work off-axis

a)

b)

c)

K.C Hwang

El.Lett. (2008)

K.C Hwang

IEEE MWCL (2010)

R.Kastner

IEEE TAP (1982)

Dimensions:  Ok: depends on CNC machines, photolithography

1

Bandwidth:  a) ~10% , b) 15% , c) 24%  All too narrow

2

Meander-grooved

metal surface

Corrugated

metal surface

Meander-strips on dielectric/ metal surfaces

slide15

Other RHWPs: Dielectrically embedded Mesh-RHWP

- Can we improve the bandwidth using multi-layered embedded grids ?

  • Present hot-pressing working up to 300mm
    • Alternative ‘cold bonding’ for bigger diameters not ready yet

1

Anti-Reflection Coating 

 Bandwidth: same as the free-standing one ?

What about the off-axis behaviour ?

2

 C/L grids

Dielectric substrates 

 Mirror

  • Very light & robust (held by a mirror)

3

slide16

Other RHWPs: Hard & Soft surfaces

- Corrugated surfaces are part of the family of Hard & Soft surfaces

P.S. Kildal

- Could we design a very broadband RHWPs using this kind of surfaces ?

slide17

Other RHWPs: Artificial surfaces (Metasurfaces)

- Many more complex surfaces are used to control the propagation of waves at

grazing incidence:

P.S. Kildal (2009)

- The surface impedance can be customised:

Q. Wu (2010)

Can we tailor the phase characteristics in order to design very broadband RHWPs?

slide18

RHWP: Improving efficiency

D - Phase shift between s & p pol

  • - D does not depend only on the path
  • difference between s & p polarisations
  • We are implicitly assuming the metallic
  • reflection to give a phase-shift of p

Artificial surface

 Could we improve the RHWP performance (bandwidth and cross-pol) using a frequency dependent ‘artificial’ surface’ instead of a flat mirror?

slide19

Discussion..

In the view of the imminent proposal writing:

- Can we keep the wire-grid RHWP as baseline with the present performance?

- Can we improve the RHWP bandwidth ?

- Shall we investigate the dielectrically embedded RHWP ?

- How can we reduce the cross-pol effects ? Flatter efficiencies across bands.

- Other ideas?

- ...