B-Pol Satellite experiment: HWP technologies comparison

1. B-Pol Satellite experiment: HWP technologies comparison The University of Manchester Giampaolo Pisano Radioastronomy Technology Group Jodrell Bank Centre for Astrophysics, University of Manchester, UK B-Pol Meeting - Paris, 29-30 July 2010

2. Half-Wave Plates: Requirements • In sensitive multi-pixel array applications there are many demanding • requirements for the HWP characteristics, let’s mention the most general ones: Large dimensions: (up to 30 cm in diameter) to achieve meaningful sensitivities. 1 Robust and light device: mechanical rotation needed; should not vibrate. 2 Anti-reflection coatings (ARCs): to achieve broadband performance. 3 Low absorption losses: to minimise the thermal emission seen by the detectors ( also very low differential losses between fast/slow axes). 4 Polarisation systematic effects introduced by the device: deep understanding and good control both needed. 5

3. Sapphire Achromatic HWPs: Pancharatnam designs G. Pisano et al., Applied Optics v45, n26 (2006) G. Savini et al., Applied Optics v45, n35 (2006) - Recipes based on birefringent plates: • Limited maximum diameters • Very expensive plates 1 (Quartz Ø ~110mm; Sapphire Ø ~280 mm)  Very robust but heavy 2 (Ex: 3-plates sapphire, no ARC) • New ARCs to be synthesised • - For sapphire (n~3.4) •  new materials needed (n~1.8) 3 ~10cm •  Low absorption (2-4%) • Can be achieved by cooling at • cryogenic temperatures but : • differential loss axes ~0.1% • (not reducible) 4

4. G. Pisano et al., Applied Optics v45, n26 (2006) G. Savini et al., Applied Optics v45, n35 (2006) Sapphire Achromatic HWPs: Modelling & FTS Tests Results Fast axis Transmission Absorption calculation Results Extrapolated Minimum Cross Polarisation Measured Cross-Polarisation X-pol ~-25/-30dB  We have excellent understanding of the polarisation systematics 5

5. Sapphire Achromatic HWPs: Quasi-Optical Tests Co-Pol & Cross-Pol Beams - VNA measurements: Horn-OMT pixel + HWP Averaged Cross-Pol: -29dB (Across beam and frequency band)  We can characterise/control the polarisation systematics with high accuracy 5

6. Mesh Half-Wave Plate: Air-gap design G. Pisano et al., Applied Optics v47, n33 (2008) Capacitive Stack Dj = p • Limits in diameter ~100mm 1 • ARCs not required ! 3 • It is very fragile and can vibrate ! 2 Inductive Stack • Absorption can be very low at room T (1.5%) • Differential losses (0.6%) could be equalised 4

7. Mesh HWP : Air-gap design results G. Pisano et al., Applied Optics v47, n33 (2008) Fast Axis Transmission Slow Axis Transmission Differential Phase-Shift Cross-Polarisation X-pol ~ -25dB  We have very good understanding of the polarisation systematics 5

8. Mesh HWP: Dielectrically embedded design(v.1) G. Pisano et al., to be submitted to Appl. Opt. Capacitive Stack Dj = p • Present diameter ~200mm (soon 300mm !) 1 • ARC commercially available 3 Pol 1 • Very robust & light 2 Inductive Stack Pol 2 • Absorption should be low at room T • Differential loss could be equalised 4

9. Mesh HWP: Dielectrically embedded design(v.2) G. Pisano et al., to be submitted to Appl. Opt. 20cm Recipe with Inductive and Capacitive lines on the same grid • One C/L stack only •  Grids # reduced by a factor 2 • Grids easier to align  should have lower losses

10. Conclusions • HWP studies: • Birefringent vs Mesh options • We know how to model and build both • Mesh HWP the most promising •  We should be able to meet all the requirements