Download Halfwave Plate Polarization Modulators

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