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Improved Absolute Polarization Calibrator for BICEP CMB Polarimeters

A. R. Polish, P. A. R. Ade, Z. Ahmed, M. Amiri, D. Barkats, R. Basu Thakur, C. A. Bischoff, D. Beck, J. J. Bock, H. Boenish, V. Buza, B. Cantrall, J. R. Cheshire, J. Connors, J. Cornelison, M. Crumrine, A. J. Cukierman, E. Denison, L. Duband, M. Echter, M. Eiben, B. D. Elwood, S. Fatigoni, J. P. Filippini, A. Fortes, M. Gao, C. Giannakopoulos, N. Goeckner-Wald, D. C. Goldfinger, S. Gratton, J. A. Grayson, A. Greathouse, P. K. Grimes, G. Hall, G. Halal, M. Halpern, E. Hand, S. A. Harrison, S. Henderson, T. D. Hoang, J. Hubmayr, H. Hui, K. D. Irwin, J. H. Kang, K. S. Karkare, S. Kefeli, J. M. Kovac, C. Kuo, K. Lasko, K. K. Lau, M. Lautzenhiser, A. Lennox, T. Liu, S. Mackey, N. Maher, K. G. Megerian, L. Minutolo, L. Moncelsi, Y. Nakato, H. T. Nguyen, R. O'Brient, S. N. Paine, A. Patel, M. A. Petroff, T. Prouve, C. Pryke, C. D. Reintsema, T. Romand, M. Salatino, A. Schillaci, B. Schmitt, B. Singari, A. Soliman, T. St. Germaine, A. Steiger, B. Steinbach, R. Sudiwala, K. L. Thompson, C. Tsai, C. Tucker, A. D. Turner, C. Verges, A. G. Vieregg, A. Wandui, A. C. Weber, J. Willmert, W. L. K. Wu, H. Yang, C. Yu, L. Zheng, C. Zhang, S. Zhang

TL;DR

This work develops and validates an improved absolute polarization calibration pipeline for BICEP3 by combining a Rotating Polarized Source (RPS) with an In-Situ Absolute Angle Calibrator (ISAAC). Key advances include a benchtop calibration procedure that eliminates reconfiguration, a ±1 arcsecond absolute optical encoder that reduces mechanical backlash to about $0.0005^\circ$, and in-situ end-to-end verification with ISAAC. The analysis identifies diffraction off the wire-grid aperture as the dominant systematic that couples pointing errors into polarization errors, and proposes hardware upgrades (low-sidelobe corrugated horn, internal stops) to suppress this effect. With remaining systematics reduced below $0.02^\circ$, the approach enables measuring uniform cosmic birefringence at the target precision of order $0.05^\circ$, significantly strengthening BICEP3’s constraints on parity-violating electromagnetism in the early universe.

Abstract

Cosmic birefringence is a hypothesized parity violation in electromagnetism that predicts a frequency-independent polarization rotation as light propagates. This would rotate the light from the Cosmic Microwave Background, producing an unexpected EB correlation. However, cosmic birefringence angle is degenerate with instrument polarization angle, and breaking this degeneracy requires an absolute polarization calibration. We calibrate the BICEP3 telescope (a 95GHz CMB polarimeter) by observing a rotating polarized source (RPS) with both the telescope and a small test receiver called the In-Situ Absolute Angle Calibrator (ISAAC).

Improved Absolute Polarization Calibrator for BICEP CMB Polarimeters

TL;DR

This work develops and validates an improved absolute polarization calibration pipeline for BICEP3 by combining a Rotating Polarized Source (RPS) with an In-Situ Absolute Angle Calibrator (ISAAC). Key advances include a benchtop calibration procedure that eliminates reconfiguration, a ±1 arcsecond absolute optical encoder that reduces mechanical backlash to about , and in-situ end-to-end verification with ISAAC. The analysis identifies diffraction off the wire-grid aperture as the dominant systematic that couples pointing errors into polarization errors, and proposes hardware upgrades (low-sidelobe corrugated horn, internal stops) to suppress this effect. With remaining systematics reduced below , the approach enables measuring uniform cosmic birefringence at the target precision of order , significantly strengthening BICEP3’s constraints on parity-violating electromagnetism in the early universe.

Abstract

Cosmic birefringence is a hypothesized parity violation in electromagnetism that predicts a frequency-independent polarization rotation as light propagates. This would rotate the light from the Cosmic Microwave Background, producing an unexpected EB correlation. However, cosmic birefringence angle is degenerate with instrument polarization angle, and breaking this degeneracy requires an absolute polarization calibration. We calibrate the BICEP3 telescope (a 95GHz CMB polarimeter) by observing a rotating polarized source (RPS) with both the telescope and a small test receiver called the In-Situ Absolute Angle Calibrator (ISAAC).
Paper Structure (12 sections, 1 equation, 9 figures)

This paper contains 12 sections, 1 equation, 9 figures.

Table of Contents

  1. Introduction
  2. Absolute polarization calibration
  3. The Rotating Polarized Source
  4. Calibrating the Rotating Polarized Source
  5. Benchtop calibration procedure
  6. Absolute optical encoder
  7. The In-Situ Absolute Angle Calibrator
  8. Calibrating BICEP3
  9. Successful measurements of BICEP3
  10. 2024–25 RPS tests at Pole
  11. Mitigation of RF systematics
  12. Conclusions

Figures (9)

  • Figure 1: A labeled photo of the RPS. A) Amplified, modulated thermal noise source B) Wire grid polarizer (cross-pol of $-$83dB) C) Rotation stage D) Absolute rotary encoder E) Precision tilt-meter
  • Figure 2: Left, An RPS calibration in progress. Right, the view through the USB microscope, where the 15$\mu$m grid wires are easily visible.
  • Figure 3: The absolute optical encoder reveals mechanical imperfections in the RPS rotation stage. This plot compares the angle commanded to the stage against the angle measured by the encoder, as it rotates through 360$^\circ$ in both directions. The separation between the two traces shows backlash, and the overall shape is due to elastic deformation as the uneven weight of the RPS shifts around.
  • Figure 4: A: ISAAC mounted on the BICEP3 Far Field Flat mirror. B: RPS on MAPO mast. C: Detail view of RPS with alignment tabs
  • Figure 5: Top: Beam maps of an orthogonal pair of detectors. Bottom: Beam amplitudes (points) and modcurve model fits (lines). Figure taken from spie_proc.
  • ...and 4 more figures