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On waveplate polarimeters for high precision CMB and mm astronomy measurements

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.

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On waveplate polarimeters for high precision CMB and mm astronomy measurements

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  1. On waveplate polarimeters for high precision CMB and mm astronomy measurements Maria Salatino Physics Department “Sapienza Università di Roma” Rencontres de Moriond, La Thuile, 13th-20th March 2010

  2. Summary of the presentation cosmological + astrophysicalsignals • new description of the • HWP RotatingHalf Wave Plate (HWP) + fixed polarizer Assumptions Mueller formalism optimization experimental setup (temperatures of distinct components, bkg ...) componentstemperatures; non idealparameters; internalreflections; reflectionsbetweencomponents B-POL, SPIDER, EBEX... PILOT POLARIZED DETECTED SIGNAL

  3. The HWP polarimeter: idealbehavior HWP Principle of operation: rotating HWP followed by a fixed polarizer polarizer with Stokes + Mueller matrix formalism detector

  4. Ongoing and future polarization experiments SPIDER QUBIC BPOL EBEX LSPE BRAIN PILOT

  5. The HWP polarimeter: realbehavior TBKG • Temperatures of • the components + • background HWP emission of polarized radiations polarizer TPOL If they were constant, given typical temperatures of CMBP experiments optical devices,….. detector

  6. they would add simply an offset contribution but…

  7. The HWP polarimeter: realbehavior TBKG • Non ideal parameters • (absorbing coefficients, • HWP refraction indeces, ...) HWP ax , ay ≠ 0,1 ne(n), no(n) Spectral dependence of the absorbing coefficients (Savini et al., 2006) polarizer TPOL detector

  8. THE MUELLER FORMALISM rotating HWP or QWP sapphire HWP • Advantages: • simple description for the radiation detected by a polarimeter • Drawbacks: • independence by the incidence angle; • f doesn’t depend on the frequency; • assumption: 100% transmitted radiation.

  9. THE ADACHI FORMALISM (Adachi S. et al., 1960) • frequency incidence wave; • d thickness crystal; • c speed of light; • neextraordinary refraction index; • noordinary refraction index. • characteristic impedence • of the medium input and output wave In vacuum Advantages: Spectral dependence of f (Savini G. et al., Applied Optics, 2006)

  10. OPTICALLY ACTIVE MULTIPLE REFLECTIONS (OAMR) air HWP • Ingredients: • dielectric reflection; • optical action HWP; • spectral dependence Adachi; • Fresnel equations for an • anisotropic medium. air A new Mueller matrix for the transmitted field by a HWP and for the reflected one; Complicate expression, function of ne , no, f 2 output waves from the HWP: the reflected and transmitted component. (Salatino M. et al., in preparation)

  11. ideal HWP real HWP

  12. The HWP polarimeter: realbehavior TBKG 3. Internal reflections HWP ax , ay ≠ 0,1 polarizer TPOL birefringency + multiple reflections OAMR 1 plate -> complete solution + new Mueller matrix 2 or more plates -> approximate solutions detector (Salatino M. et al., in preparation)

  13. The HWP polarimeter: realbehavior TBKG 4. Reflections between components HWP ax , ay ≠ 0,1 emission of polarized radiations TPOL Normal incidence Input: monochromatic wave polarizer Non monochromatic waves -> small circular polarization (few % S1) detector

  14. Detected signal due to CMB only Lambda CDM model + T/S=0.1 expected signal: 10-7 K Ideal HWP Signal when staring at a given pixel, while HWP rotates Real HWP Real HWP (absorption + OAMR) Similar signal amplitude but different height of the peaks (2theta component)

  15. e=1% 150 GHz BKG-> Cos(2 theta) POL -> Cos(2 theta) Total The radiation reflected by the HWP produces a cos(2*theta) which contaminates the CMBP Stokes parameters. Necessity of cooling down the polarizer and to reduce the background.

  16. CMB 0.6 mK @ 2q+4q Spurious 10 mK @ 2q Spurious 0.3 mK @ 2q Total signal dominated by HWP emission @ 2q Can we separate it ? 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.

  17. NON LINEARITIES The 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 the detectors 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. 2q out in 2q+4q+..

  18. NON LINEARITIES Greater effect on S1 with respect to S2, limits on the bkg

  19. HWP WITH ANTI REFLECTION COATING Naive model The efficiency of the polarimeter increases by a factor > 2. The spurious signal reflected by the HWP is reduced by at least one order of magnitude. However here there could be a component modulated at 4 theta; which would be extremely dangerous! HWP WORK IN PROGRESS

  20. Possible solution… HWP HWP Reduction of the detected emission reflected from the HWP + polarized emission with different phases detector arrays detector + pol arrays

  21. Polarized Instrument for Long Wavelenght Observation of the Tenuous interstellar medium vapor cooled shields He tank polarizer 124 cm ? Targets: cirrus clouds at high Galactic latitudes, Galactic plane fiberglass cylinders 1st flight Kiruna (Sweden) secondary mirror tertiary mirror 70 cm Off-axis telescope (70 cm)

  22. 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 HWP at 4K (Salatino et al.,2008)

  23. 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 2q; • 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 4q component • -> constraints on the bkg; • possible dangerous signals at 4q 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.

  24. 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.

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