on waveplate polarimeters for high precision cmb and mm astronomy measurements
DESCRIPTION
On waveplate polarimeters for high precision CMB and mm astronomy measurements. Maria Salatino Physics Department “Sapienza Università di Roma” Rencontres de Moriond, La Thuile, 13 th -20 th March 2010. Summary of the presentation. cosmological + astrophysical signals. - PowerPoint PPT PresentationTRANSCRIPT
On waveplate polarimeters for
high precision CMB and
mm astronomy measurementsMaria Salatino
Physics Department “Sapienza Università di Roma”
Rencontres de Moriond, La Thuile, 13th-20th March 2010
cosmological +cosmological + astrophysical signalsastrophysical signals
Rotating Half Wave PlateRotating Half Wave Plate(HWP) + fixed polarizer(HWP) + fixed polarizer
components temperatures;components temperatures;non ideal parameters;non ideal parameters;internal reflections;internal reflections;reflections between componentsreflections between components
POLARIZEDPOLARIZEDDETECTEDDETECTEDSIGNALSIGNAL
new description of the HWP
SummarySummary of the of thepresentationpresentation
optimizationoptimizationexperimental setupexperimental setup(temperatures of(temperatures ofdistinct components, distinct components, bkg ...)bkg ...)
B-POL,B-POL,SPIDER,SPIDER,EBEX...EBEX...
Mueller formalism
Assumptions
PILOT
Principle of operation: rotating HWP followed
by a fixed polarizer
The HWP polarimeter:The HWP polarimeter: ideal behaviorideal behavior
HWP
]4sin4cos[2
1)(
)]([)(
210det
idealTRAS,
det
SSSS
SMMDW inHWPhorpol
polarizer
detector
t with
Stokes + Mueller matrix formalism
Ongoing and future polarization experiments
BPOL
PILOT
SPIDER
BRAINEBEX LSPE
QUBIC
HWP
polarizer
detector
The HWP polarimeter: real behaviorThe HWP polarimeter: real behavior
TPOL
1. Temperatures of the components + background
emission of polarized radiations
If they were constant, given typical temperatures ofCMBP experiments optical devices,…..
TBKG
they would add simply an offset contribution but…
HWP
polarizer
detector
The HWP polarimeter: real behaviorThe HWP polarimeter: real behavior
TPOL
2. Non ideal parameters (absorbing coefficients, HWP refraction indeces, ...)
Spectral dependence of theabsorbing coefficients(Savini et al., 2006)
ax , ay ≠ 0,1ne(), no()
TBKG
THE MUELLER FORMALISMTHE MUELLER FORMALISM
rotating HWP or QWP
sapphire HWP
3
2
1
0
00
00
0010
0001
S
S
S
S
) () Cos Sin(
) () -Sin Cos(
Advantages: simple description for the radiation detected by a polarimeter
Drawbacks: independence by the incidence angle; doesn’t depend on the frequencyassumption: 100% transmitted radiation.
)n(nc
π ν d)k(k oeoe
2 frequency incidence wave;d thickness crystalc speed of lightneextraordinary refraction indexnoordinary refraction index
x
y
y
x
oo
o
ooo
ee
e
ee
H
E
H
E
d)(k) Cosη
dk jSin(
d)Sin(kjηd) Cos(k
d) ) Cos(kη
dkj Sin(
d) Sin(kd) jCos(k
00
00
00
00 characteristic impedence of the medium
input and output waveIn vacuum
1 eo ηη
xx EH yy EH
THE ADACHI FORMALISM THE ADACHI FORMALISM
Advantages:Spectral dependence of
ν..n
ν.ν.ν..n
e
o
5
3122104
10313873)(
1011102210740533)(
(Savini G. et al., Applied Optics, 2006)
(Adachi S. et al., 1960)
OPTICALLY ACTIVEOPTICALLY ACTIVEMULTIPLE REFLECTIONSMULTIPLE REFLECTIONS (OAMR)(OAMR)
Ingredients: dielectric reflection; optical action HWP; spectral dependence Adachi; Fresnel equations for an
anisotropic medium.
2 output waves from the HWP:the reflected and transmittedcomponent.
air
air
HWP
A new Mueller matrix for the transmittedfield by a HWP and for the reflected one;
Complicate expression, function of neno
(Salatino M. et al., in preparation)
0.737- 0.028 0. 0.
0.028- 0.737- 0. 0.
0. 0. 0.737 0.008
0. 0. 0.008 0.737
1.- 0. 0. 0.
0. . 1- 0. 0.
0. 0. 1. 0.
0. 0. 0. 1.
ideal HWP real HWP
HWP
polarizer
detector
The HWP polarimeter: real behaviorThe HWP polarimeter: real behavior
TPOL
3. Internal reflections
ax , ay ≠ 0,1
(Salatino M. et al., in preparation)
birefringency +multiple reflections
OAMR
1 plate -> complete solution + new Mueller matrix2 or more plates -> approximate solutions
TBKG
HWP
polarizer
detector
The HWP polarimeter: real behaviorThe HWP polarimeter: real behavior
TPOL
4. Reflections between components
emission of polarized radiations
ax , ay ≠ 0,1
Normal incidenceInput: monochromatic wave
Non monochromatic waves ->small circular polarization (few % S1)
TBKG
Detected signal due to CMB onlyLambda CDM model + T/S=0.1expected signal: 10-7 K
Real HWP (absorption + OAMR)Similar signal amplitude butdifferent height of the peaks(2theta component)
Ideal HWP Signal when staring at a given pixel, while HWP rotates
Real HW
P
POL ->Cos(2 theta)
BKG->Cos(2 theta)
The radiation reflected by the HWP produces a cos(2*theta) which contaminatesthe CMBP Stokes parameters.Necessity of cooling down the polarizer and to reduce the background.
e=1%150 GHz
Total
The result of the addition of the modulated emission of BKG and Pol depends on the relative angle between the CMBP polarization angle and the orientation of the wire grid.
Spurious10 mK @ 2
CMB0.6 K @ 2+4
Spurious0.3 K @ 2
Total signal dominated by HWP emission@ 2
Can we separate it ?
NON LINEARITIESThe disturbance signals at 2theta (produced by unpolarized background, waveplate emission and reflected polarizer emission) are easily separated from the sky signal at 4theta.
However if they are too large they challenge the linearity of thedetectors and the dynamic range of the data acquisition system.
Bolometers become non linear and start to saturate so that a pure 2 theta signal acquires a 4theta component.
out
in
2
2
NON LINEARITIES
Greater effect on S1 with respect to S2,limits on the bkg
HWP WITH ANTI REFLECTION COATING
HWP
The efficiency of the polarimeter increasesby a factor > 2.
The spurious signal reflected by the HWP isreduced by at least one order ofmagnitude.
However here there could be a component modulated at 4 theta; which would be extremely dangerous!
Naive model
WORK IN
PROGRESS
PROGRESS
Possible solution…
HWP
detector arrays
HWP
detector + pol arrays
Reduction of the detected emission reflected from the HWP +polarized emissionwith differentphases
He tank
secondarymirror
polarizer
124
cm
70 cm
vapo
r co
oled
shi
elds
fiber
glas
s cy
linde
rs
tertiarymirror
Polarized Instrument for Long Wavelenght Observation of the Tenuous interstellar medium
Targets: cirrus clouds at high Galactic latitudes, Galactic plane
1st flight Kiruna (Sweden)
?
Off-axis telescope (70 cm)
The PILOT Cryogenic Waveplate Rotator
innovative MECHANICAL SYSTEM driven by a DC motor running at room temperature
the control of its position is assured by a 3-bit optical encoder fed by optical fibers
(Salatino et al., 2008)
HWP at 4K
CONCLUSIONS AND FUTURE DEVELOPMENTS (1/2) A study of the sistematics introduced in a CMBP polarimeter has been done
taking into account the emission of the polarizer and the unpolarized radiative background;
both the thermal emission of the polarizer and the unpolarized bkg are modulated at 2;
this has been done by means of a new description of the HWP which takes into account multiple reflections, the transmitted and reflected fields inside an anisotropic medium, ….
the non linear behavior of the bolometers introduces a component -> constraints on the bkg;
possible dangerous signals at 4 can be reduced with 1 polarizer per bolometer;
in the PILOT experiment a DC motor running at room temperature will rotate the HWP at 4K.
CONCLUSIONS AND FUTURE DEVELOPMENTS (2/2)
The HWP description has been done with some assumptions:
normal incidence ; input monochromatic wave; no ARC.
Future work: slant incidence -> effect of the mixing of extraordinary and ordinary ray on the
incoming ray; input non monochromatic wave; full model ARC ; achromatic HWP (multi-plates) with multi-layer ARCs.
--------------------------------------------------------
Full simulation of a balloon/ satellite experiment -> multi-pixel analysis of these sistematics.