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Halfwave Plate Polarization Modulators John Ruhl, Case Western Reserve University Shaul Hanany, University of Minnesota 1 Half Wave Plate - Review θ ext ord ord θ t Outpu Detector Input • t=thickness, Δn=index difference, λ=wavelength • half wave plate • Polarization incident on a birefringent HWP is rotated by twice the angle between the incident polarization and one of the crystal axes. • Rotating the HWP rotates the output polarization; a “full cycle” of 180o of polarization rotation is achieved in 90o of HWP rotation. • Rotating HWP at f rotations/second modulates the signal detected 2 by a polarization sensitive detector at a frequency 4f. Half Wave Plates – Implementation Choices 3 Half Wave Plates – Implementation Choices • Material Half-Wave Plate (4 K) • Δnsapphire = 0.31, <n>=3.2 24 cm • 150 GHz:ε=0.7%, Δε=0.07% • size ~30 cm • Location • closest to sky • at aperture stop • temperature Half-Wave Plate (2 K) 30 cm 3 Half Wave Plates – Implementation Choices • Material • Δnsapphire = 0.3, <n>=3.2 • 150 GHz:ε=0.7%, Δε=0.07% • size <~30 cm • Location • closest to sky 200 600 400 • at aperture stop • temperature • Bandwidth • single: • stack: 1 3 5 7 4 9 Half Wave Plates – Implementation Choices • Material • Δnsapphire = 0.3, <n>=3.2 28 cm • 150 GHz:ε=0.7%, Δε=0.07% • size <~30 cm • Location • closest to sky • at aperture stop Stepped, 3 roller bearings (Spider) • temperature • Bandwidth 24 cm • single: • stack: • Mode of Operation • Continuous • Stepped 5 Continuous, magnetic bearing (EBEX) Halfwave Plates – Advantages θ ord ord θ t Outpu Detector Input • • Same beam for orthogonal polarization states Can be placed near the “sky end” of the optical system – limited by diameter – limited by bandwidth • • • I, Q, U from the same detector Q, U at high frequencies, 4x rotation frequency (continuous) Based on simple physics; easy to calculate the “ideal materials” case. 6 Halfwave Plates – Disavantages/Issues • For Cryogenic + Continuous Rotation • power dissipation • wear/tear • (Noise?) • For Continuous Rotation • Removal of IP offsets from time domain data at necessary accuracy • For Stepped • Demonstrate end-to-end recovery of incident Q,U • ‘Bandpass Response’ may complicate foreground removal • Optics • Size (can not be the first element for apertures > ~30 (50) cm) • Diameter/thickness < 150 • Is AR coating good enough (particularly for AHWP)? 7 Half Wave Plates – Tally of Use Rotation Mode Drive Technology T (K) ν bands (#) Plates (#) Maxipol Continuous 300K motor 4K bearing 4 2 1 EBEX Continuous 300K motor, 4K belt + SMB 4 3 5 Stepped 4 K Stepper 4 1 1 ACT/ABS (2010 ?) Continuous 300 K drive 300 1 1 Polarbear (2010 ?) Continuous 300K motor, 50K belt + ball bearing 50 1, then 3 1, then more Clover Continuous 300 K 300 1 3 300K motor + SMB 4 2 5 Stepped 300K Stepper 4 ? ? (future: cont?) (future: SMB?) Experiment (2009) Bicep2/Spider (2010) (2010) BlastPol (2010) Half Wave Plates – Technology Testing Technology Continuous Stepped Experiment EBEX, Polarbear, Clover, ABS, BlastPol Spider, BlastPol Chromatic BICEP2/Spider, Polarbear, ABS Achromatic EBEX, Polarbear, Clover 300K plate Clover, ABS 4K plate First Not first Not aperture EBEX, BICEP2/Spider, Polarbear* BICEP2/Spider, …? EBEX, …? Polarbear, …? Halfwave Plates – TRL / Milestones • Successful experiments →TRL 4 – 5.5 for specific implementations • Near Term Technical Milestones • Cryogenic continuous rotation • Use of a stepped HWP for CMB observations • Use of achromatic HWP 10