Download Halfwave Plate Polarization Modulators

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts

Rotation matrix wikipedia, lookup

Transcript
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