Giampaolo Pisano

Radioastronomy Technology Group

Jodrell Bank Centre for Astrophysics, University of Manchester, UK

FPP Workshop - Henri Poincaré Institute, Paris, 8th-9th October 2010

FPP Instrument:

Review of quasi-optical Polarisation Modulators

The University of Manchester

Polarisation modulator baseline: Reflecting Half-Wave Plate (RHWP)

A bit challenging !

Polarisation modulator: Some of the present requirements

3 Robust and light device: mechanical rotation needed

1 Very large dimensions 1.2 m !!

5 Low absorption losses (also differential losses): thermal emissivity

6 Polarisation systematic effects: deep understanding / control needed

4 Modulation efficiency: 80%?

2 Broadband performance Bandwidth ~180% !!

7 ...

RHWP: Bands and efficiency see G. Siringo et al., Laboca Experiment

Freq [GHz] BW [%]

60 33

100 20

140 14

180 11

220 9

340 5.9

540 3.7

820 2.4

- Modulation efficiency

D - Phase shift between s & p pol

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

d=5.3mm, f=45 nn=20(2n+1)GHzExample

Cross-Pol issues to be solved

RHWP: Feasibility

2 RHWP bandwidth needs to be improved

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

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

(50 cm diameter wire-grid example)

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

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

15

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.

Similar polarisation modulator: Variable Phase Delay Modulator

- This type of modulator does not modulate Q and U at the same time

Can this apply in our case ?

D. Chuss(2008)

Birefringent HWPs: Pancharatnam designs

- Recipes based on birefringent plates:

Limits on maximum diameters available :

Quartz Ø ~110mm, Sapphire Ø ~280 mm

1

~10cm

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

2 Bandwidth: 5-plate recipe ~100%

Mesh Half-Wave Plate: Air-gap design G. Pisano et al., Applied Optics v47, n33 (2008)

Dimension in principle achievable but very thin substrates required

Present limits in diameter ~200mm

1

2 Present max bandwidth ~70%

~4cm

(Example of inductive stack)

- Recipes based on metal grids geometry/spacing:

3 Too fragile, it can vibrate

Mesh HWP: Dielectrically embedded design

Very robust & light although it might bend with diameters >1m

Flatness problem

3

(Example of embedded mesh-HWP)

Pol 1

Pol 2

20cm

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

Other types and other possible solutions of RHWPs

- Dielectrically embedded RHWP

- Twist reflectors

- Dielectrically embedded Mesh RHWP

- Hard & Soft surfaces

- Artificial surfaces

Modified RHWPs: Dielectrically embedded RHWP

Dielectric substrate

Anti-Reflection Coating Photolithographic Wire-grid

Mirror

2 Bandwidth: same as the free-standing one ?

1 Dimensions: should be feasible using photolithography

(2 evaporated/etched substrates + cold bonding) *

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

Other RHWPs: Twist reflectors

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

R.Kastner IEEE TAP (1982)

Corrugated metal surface

Meander-grooved metal surface

K.C HwangIEEE MWCL (2010)

K.C HwangEl.Lett. (2008)

Meander-strips on dielectric/ metal surfaces

a) b) c)

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

1 Dimensions: Ok: depends on CNC machines, photolithography

Other RHWPs: Dielectrically embedded Mesh-RHWP

Dielectric substrates

Anti-Reflection Coating

C/L grids

Mirror

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

2 Bandwidth: same as the free-standing one ?

What about the off-axis behaviour ?

Very light & robust (held by a mirror)3

Present hot-pressing working up to 300mm

Alternative ‘cold bonding’ for bigger diameters not ready yet

1

Other RHWPs: Hard & Soft surfaces

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

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

P.S. Kildal

Other RHWPs: Artificial surfaces (Metasurfaces)

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

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

P.S. Kildal (2009)

- The surface impedance can be customised:Q. Wu (2010)

RHWP: Improving efficiency

- 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

D - Phase shift between s & p pol

Could we improve the RHWP performance (bandwidth and cross-pol)

using a frequency dependent ‘artificial’ surface’ instead of a flat mirror?

Artificial surface

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?

- ...