Development status of the polarization modulator using 330 mm diameter sapphire-based achromatic half-wave plate for LiteBIRD

We report on the breadboard model of a polarization modulator unit (PMU) using a sapphire-based achromatic half-wave plate (HWP) for the low-frequency telescope (LFT) of LiteBIRD, the JAXA-led space mission to probe cosmic inflation by observing the polarization of the cosmic microwave background. The PMU is a key component to reduce 1/f noise and the systematic effects between the two orthogonal polarized detectors. For the HWP, we glued together the surfaces of five 330 mm diameter sapphire plates using hydro catalysis bonding, working as HWP in LFT bands. We also fabricated anti-reflective sub-wavelength structures using ultra-short pulsed laser ablation. For the rotation mechanism, we use a superconducting magnetic bearing (SMB) and contactless synchronous motor to levitate, and rotate the HWP without any contact. Optical measurements show that fabricated HWP archives broadband transmission and polarization efficiency to obtain a sensitivity close to an ideal HWP. We investigate and characterize each component of the rotation mechanism, SMB, encoder to measure the rotation angle of HWP, and the holder mechanism. We improved the design of the rotation mechanism and reduced the total mass from 34.7 kg to 21.7 kg, which is significant reduction for the mass limited satellite mission. The knowledge of characterization for each component can be scaled to the size of the flight model of 480 mm diameter.