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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 Poincaré Institute, Paris , 8 th -9 th October 2010.

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FPP Instrument: Review of quasi-optical Polarisation Modulators

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  1. 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

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

  3. 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

  4. 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

  5. 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)

  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:  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. 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

  10. 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)

  11. 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

  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  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

  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) 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

  15. 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

  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. 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? - ...

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