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

2. Polarisation modulator baseline: Reflecting Half-Wave Plate (RHWP)
A bit challenging !

3. Polarisation modulator: Some of the present requirements1
Very large dimensions   1.2 m !! 2
Broadband performance  Bandwidth ~180% !! 3
Robust and light device: mechanical rotation needed 4
Modulation efficiency:  80%? 5
Low absorption losses (also differential losses): thermal emissivity 6
Polarisation systematic effects: deep understanding / control needed 7
...

4. RHWP: Bands and efficiency
see G. Siringo et al., Laboca Experiment
D - Phase shift between s & p pol
- Modulation efficiency
Freq [GHz]
BW [%] 60 33
100 20
140 14
180 11
220 9
340
5.9
540
3.7
820
2.4
Bandwidth such that the averaged e~0.8
 Dn=20GHz (independent on frequency)
d=5.3mm, f=45  nn=20(2n+1)GHz
Example
 Cross-Pol issues to be solved

5. (50 cm diameter wire-grid example)
RHWP: Feasibility
D. Chuss
(2008)
(50 cm diameter wire-grid example)
500mm diameter wire grids has been built (see VPM - D.Chuss later)
 Is it possible to go up to ~1.2m? 1 2
 RHWP bandwidth needs to be improved 3
 It would be very fragile, will the wires bend ?

6. 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:
Freq [GHz] 60
100
140
180
220
340
540
820
BW [%] 33 20 14 11 9
5.9
3.7
2.4
BW+ [%] 33 20 14 11 12 17 15
 Increase from 3.7% to 15% in BW

7. 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.

8. 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 ?

9. (Example of 3-plate sapphire recipe, no ARC)
Birefringent HWPs: Pancharatnam designs
- Recipes based on birefringent plates:
~10cm
(Example of 3-plate sapphire recipe, no ARC)
Limits on maximum diameters available :
 Quartz Ø ~110mm, Sapphire Ø ~280 mm 1 2
Bandwidth: 5-plate recipe ~100%

10. (Example of inductive stack)
Mesh Half-Wave Plate: Air-gap design
G. Pisano et al., Applied Optics v47, n33 (2008)
- Recipes based on metal grids geometry/spacing:
~4cm
(Example of inductive stack)
Dimension in principle achievable but very thin substrates required  Present limits in diameter ~200mm 1 2
 Present max bandwidth ~70% 3
 Too fragile, it can vibrate

11. (Example of embedded mesh-HWP)
Mesh HWP: Dielectrically embedded design
Pol 1
Pol 2
20cm
(Example of embedded mesh-HWP)
 Present hot-pressing working up to 300mm (near future 500mm)
Alternative ‘cold bonding’ for bigger diameters under study 1 2
Bandwidth similar to air-gap
Very robust & light although it might bend with diameters >1m
 Flatness problem 3

12. Other types and other possible solutions of RHWPs
Dielectrically embedded RHWP
Twist reflectors
- Dielectrically embedded Mesh RHWP
Hard & Soft surfaces
Artificial surfaces

13. Modified RHWPs: Dielectrically embedded RHWP
 Photolithographic Wire-grid
Anti-Reflection Coating 
Dielectric substrate 
 Mirror 1
 Dimensions: should be feasible using photolithography
(2 evaporated/etched substrates + cold bonding) * 2
 Bandwidth: same as the free-standing one ?
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

14. 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)
R.Kastner
IEEE TAP (1982)
K.C Hwang
IEEE MWCL (2010)
Corrugated
metal surface
Meander-grooved
metal surface
Meander-strips on dielectric/ metal surfaces 1
Dimensions:  Ok: depends on CNC machines, photolithography 2
Bandwidth:  a) ~10% , b) 15% , c) 24%  All too narrow

15. Other RHWPs: Dielectrically embedded Mesh-RHWP
- Can we improve the bandwidth using multi-layered embedded grids ?
Anti-Reflection Coating 
Dielectric substrates 
 C/L grids
 Mirror
 Present hot-pressing working up to 300mm
Alternative ‘cold bonding’ for bigger diameters not ready yet 1 2
 Bandwidth: same as the free-standing one ?
What about the off-axis behaviour ?
Very light & robust (held by a mirror) 3

16. 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 ?

17. 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?

18. 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?

19. In the view of the imminent proposal writing:
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?
- ...