giampaolo pisano radioastronomy technology group jodrell bank centre for astrophysics, university of...
TRANSCRIPT
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?
- ...