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.
LiteBIRD, the next-generation cosmic microwave background (CMB) experiment, aims for a launch in Japan’s fiscal year 2032, marking a major advancement in the exploration of primordial cosmology and fundamental physics. Orbiting the Sun-Earth Lagrangian point L2, this JAXA-led strategic L-class mission will conduct a comprehensive mapping of the CMB polarization across the entire sky. During its 3-year mission, LiteBIRD will employ three telescopes within 15 unique frequency bands (ranging from 34 through 448 GHz), targeting a sensitivity of 2.2 μK-arcmin and a resolution of 0.5° at 100 GHz. Its primary goal is to measure the tensor-toscalar ratio r with an uncertainty δr = 0.001, including systematic errors and margin. If r ≥ 0.01, LiteBIRD expects to achieve a > 5σ detection in the ℓ = 2–10 and ℓ = 11–200 ranges separately, providing crucial insight into the early Universe. We describe LiteBIRD’s scientific objectives, the application of systems engineering to mission requirements, the anticipated scientific impact, and the operations and scanning strategies vital to minimizing systematic effects. We will also highlight LiteBIRD’s synergies with concurrent CMB projects.
We present the on-sky commissioning and science verification of DESHIMA 2.0: the first science-grade integrated superconducting spectrometer (ISS) for ultra-wideband mm-submm spectroscopy. With an instantaneous band coverage of 205-392 GHz at a spectral resolution of F/dF = 500, DESHIMA 2.0 will be applied to emission line surveys and redshift measurement of dusty star-forming galaxies, spectroscopic Sunyaev–Zeldovich effect observations of galaxy-clusters, and other new science cases that utilize its ultra-wide bandwidth. Compared to its predecessor (DESHIMA 1.0), DESHIMA 2.0’s superconducting filterbank chip with a x4 higher optical efficiency, x4 wider instantaneous bandwidth, x20 faster position switching on the sky, and a remotely-controlled optics alignment system. DESHIMA 2.0 is currently installed on the ASTE 10-m telescope at 4860 m altitude with excellent sky transmission, and is being commissioned for science operation. In the conference we will report the on-sky performance and latest results in the science-verification campaign at ASTE.
LiteBIRD is an JAXA-led strategic L-class satellite mission designed to measure the primordial B modes of the cosmic microwave background radiation (CMB) to test cosmic inflation. The LiteBIRD Low-Frequency telescope employs a polarization modulator unit (PMU). The PMU is placed at the telescope aperture to modulate the incoming CMB polarization signal by using a continuous rotating half-wave plate to reduce the impact of 1/f noise and differential systematic effects. The current PMU design employs three cryogenic holder mechanisms that hold the rotor until the superconducting magnet bearing cools below its critical temperature after launch. They also serve a conductive path to the rotor when they are held. Minimizing the heat dissipation of this holder is one of the key development goals of the PMU due to the limited cooling power on the satellite system. In this paper, We report on the detailed design of the holder and the developed cryogenic stepping motor that actuates the holder. Also, we conducted the preliminary thermal characterization at around 7 K. The preliminary estimated total heat dissipation of the holder is 2.39 ± 0.09 mW when we activated it for 532 s.
We present designs and fabrications of sub-wavelength anti-reflection structures on alumina for infrared filters at three observational frequency bands near 30, 125, and 250 GHz, which are widely used for instruments measuring cosmic microwave background (CMB) radiation from the ground. The three observational windows contain the two observational bands in each receiver, and the corresponding fractional bandwidth is about 60%. We used laser ablation to directly machine on alumina substrate. This technology is robust against the use of an optical element at the cryogenic temperature with which all the CMB telescope receivers have to comply. Based on the fabricated 9 (3 × 3) pyramidal structures, we computed the expected averaged transmittance of above 0.97 for each of the three filter designs including anticipated absorptive loss, the loss tangent of 4 × 10−4, and the incident angle up to 20 degrees. The band averaged instrumental polarization, the fractional difference between the p and s-state incident polarization states, is computed and they are less than ±4 × 10−3 for the bands and incident angles.
We used laser ablation to fabricate sub-wavelength structure anti-reflection coating (SWS-ARC) on a 5 cm diameter alumina lens. With an aspect ratio of 2.5, the SWS-ARC are designed to give a broad-band low reflectance response between 110 and 290 GHz. SWS shape measurements give 303 μm pitch and total height between 750 and 790 μm height, matching or exceeding the aspect ratio design values. Millimeter-wave transmittance measurements in a band between 140 and 260 GHz show the increase in transmittance expected with the ARC when compared to finite element analysis electromagnetic simulations. To our knowledge, this is the first demonstration of SWS-ARC on an alumina lens, opening the path for implementing the technique for larger diameter lenses.
We present transmission and loss measurements of 3D printed alumina and reflectance measurement of a sample with 3D printed sub-wavelength structures anti-reflection coatings (SWS-ARC). For a band between 160 and 700 GHz we find an index of refraction n = 3.11 ± 0.01 and loss tan δ = 0.002 ± 0.003. Transmission measurements between 160 and 250 GHz of a sample with SWS-ARC 3D printed on one side give a reduction of reflectance from a maximum of 64% to a maximum of 31% over the band, closely matching predictions. These first measurements of the index and loss over this frequency band suggest that the material could be useful for astrophysical applications.
We present the development of a breadboard model achromatic half-wave plate (AHWP) for the LiteBIRD Low-Frequency Telescope (LFT). LiteBIRD is a JAXA-led strategic L-class satellite mission to probe the cosmic microwave background polarization. The breadboard model (BBM) polarization modulator unit (PMU) of the LFT uses an AHWP, which achieves an observational frequency coverage 34-161 GHz using a five-layer sapphire stack with a diameter of 330 mm based on the Pancharatnam recipe. The sub-wavelength structures on both end surfaces mitigate the reflection over broadband frequencies. The designed single sapphire plate thickness is 4.95 mm, with a corresponding half-wave shift center frequency of 97.5 GHz. We use hydro-catalysis bonding to glue the sapphire surfaces and assemble the five-layer AHWP. The AHWP is successfully assembled and measured. The transmittance and polarization properties are consistent with the theoretical prediction that neglects the effect of the bonding interfaces. In this work, we present the AHWP design, the assembly process, and the polarimetric characterization. We also discuss the path-forward for this BBM AHWP including the cryogenic and vibrational tests, and the development plan for the flight-size engineering model.
LiteBIRD is the Cosmic Microwave Background (CMB) radiation polarization satellite mission led by ISAS/JAXA. The main scientific goal is to search for primordial gravitational wave signals generated from the inflation epoch of the Universe. LiteBIRD telescopes employ polarization modulation units (PMU) using continuously rotating half-wave plates (HWP). The PMU is a crucial component to reach unprecedented sensitivity by mitigating systematic effects, including 1/f noise. We have developed a 1/10 scale prototype PMU of the LiteBIRD LFT, which has a 5-layer achromatic HWP and a diameter of 50 mm, spanning the observational frequency range of 34-161 GHz. The HWP is mounted on a superconducting magnetic bearing (SMB) as a rotor and levitated by a high-temperature superconductor as a stator. In this study, the entire PMU system is cooled down to 10 K in the cryostat chamber by a 4-K Gifford-McMahon (GM) cooler. We propagate an incident coherent millimeter-wave polarized signal throughout the rotating HWP and detect the modulated signal. We study the modulated optical signal and any rotational synchronous signals from the rotation mechanism. We describe the testbed system and the preliminary data acquired from this setup. This testbed is built to integrate the broadband HWP PMU and evaluate the potential systematic effects in the optical data. This way, we can plan with a full-scale model, which takes a long time for preparation and testing.
We develop a continuously rotating achromatic half-wave plate (HWP) for LiteBIRD. An achromatic HWP is made of five-layer sapphire plates following a Pancharatnam design. The two surfaces employ broadband anti-reflection (AR) sub-wavelength structures (SWS) fabricated with ultra-short pulsed laser ablation. For designing AHWP with SWS, we fabricated three representative structures using laser ablation. One has a symmetric SWS shape and the other two have different asymmetric shapes in ordinary and extraordinary directions. We modeled five-layer AHWP with SWS based on fabricated shapes and numerically evaluated their transmittance, modulation efficiency, and phase of the modulated signal using the rigorous coupled-wave analysis (RCWA) method. We also added instrumental polarization (IP) as the figure-of-merit, which is a conversion of unpolarized to polarized light. IP creates an undesired modulated signal, which may cause a non-linear response in a bolometric detector. The typical cause of IP is the imperfection of AR SWS. From calculations, we did not find a significant difference in IP among the three cases. However, we found the impact on the modulation efficiency because the retardance depends on the SWS shapes. Furthermore, the retardance depends on frequency. We numerically analyzed the impact of the extra retardance from SWS on the overall AHWP performance. We show one of the three cases has the broadest modulation efficiency by compensating for the frequency dependence of the retardance from the SWS and the AHWP sapphire stacks.
This conference presentation was prepared for the Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI conference at SPIE Astronomical Telescopes + Instrumentation, 2022.
LiteBIRD is a future space mission designed to observe the polarization of the cosmic microwave background (CMB) radiation. LiteBIRD employs polarization modulator units (PMUs) at telescope apertures to mitigate 1/f noise and systematic uncertainties. The PMU employed in the Low-Frequency Telescope (LFT) consists of a broadband achromatic half-wave plate (HWP) and a cryogenic rotation mechanism. A superconducting magnetic bearing, which is a rotor levitation type bearing, is used to eliminate physical friction. A contactless AC synchronous motor consisting of SmCo permanent magnets and copper coils is employed as the drive mechanism. One of the technical challenges for the PMU development is to reduce the heat dissipation generated by the rotation mechanism during cryogenic operation. We evaluated the heat dissipation owing to the eddy currents generated from the rotor in the rotation mechanism at room temperature. We performed a rotor spindown measurement using a breadboard model of the PMU. We established that eddy currents generated from the motor coil were dominant in the rotor at room temperature, and its estimated value was 3.91 ± 0.91 mW.
LiteBIRD, the Lite (Light) satellite for the study of B-mode polarization and Inflation from cosmic background Radiation Detection, is a space mission for primordial cosmology and fundamental physics. JAXA selected LiteBIRD in May 2019 as a strategic large-class (L-class) mission, with its expected launch in the late 2020s using JAXA's H3 rocket. LiteBIRD plans to map the cosmic microwave background (CMB) polarization over the full sky with unprecedented precision. Its main scientific objective is to carry out a definitive search for the signal from cosmic inflation, either making a discovery or ruling out well-motivated inflationary models. The measurements of LiteBIRD will also provide us with an insight into the quantum nature of gravity and other new physics beyond the standard models of particle physics and cosmology. To this end, LiteBIRD will perform full-sky surveys for three years at the Sun-Earth Lagrangian point L2 for 15 frequency bands between 34 and 448 GHz with three telescopes, to achieve a total sensitivity of 2.16 μK-arcmin with a typical angular resolution of 0.5° at 100 GHz. We provide an overview of the LiteBIRD project, including scientific objectives, mission requirements, top-level system requirements, operation concept, and expected scientific outcomes.
We have developed a prototype half-wave plate (HWP) based polarization modulator (PMU) for Cosmic Microwave Background polarization measurement experiments. We built a 1/10 scaled PMU that consists of a 50 mm diameter five-layer achromatic HWP with a moth-eye broadband anti-reflection sub-wavelength structure mounted on a superconducting magnetic bearing. The entire system has cooled below 20 K in a cryostat chamber that has two millimeter-wave transparent windows. A coherent source and the diode detector are placed outside of the cryostat and the millimeter-wave goes through the PMU in the cryostat. We have measured the modulated signal by the PMU, analyzed the spectral signatures, and extracted the modulation efficiency over the frequency coverage of 34-161 GHz. We identified the peaks in the optical data, which are synchronous to the rotational frequency. We also identified the peaks that are originated from the resonance frequency of the levitating system. We also recovered the modulation efficiency as a function of the incident electromagnetic frequency and the data agrees to the predicted curves within uncertainties of the input parameters, i.e. the indices of refraction, thickness, and angle alignment. Finally, we discuss the implication of the results when this is applied to the LiteBIRD low-frequency telescope.
Pancharatnam base achromatic half-wave plate (AHWP) achieves high polarization efficiency over broadband. It generally comes with a feature of which the optic axis of AHWP has dependence of the electromagnetic frequency of the incident radiation. When the AHWP is used to measure the incident polarized radiation with a finite detection bandwidth, this frequency dependence causes an uncertainty in the determination of the polarization angle due to the limited knowledge of a detection band shape and a source spectral shape. To mitigate this problem, we propose new designs of the AHWP that eliminate the frequency dependent optic axis over the bandwidth of which the polarization efficiency also achieves the same broadband width. We carried out the optimization by tuning the relative angles among the individual half-wave plates (HWP) of the five- and nine-layer AHWP. The optimized set of the relative angles achieves the frequency independent optic axis and covers the fractional bandwidth of 1.3 and 1.5 for five- and nine-layer AHWPs, respectively. We also study the susceptibility of the alignment accuracy, which can be chosen based on the requirement in each application.
LiteBIRD has been selected as JAXA’s strategic large mission in the 2020s, to observe the cosmic microwave background (CMB) B-mode polarization over the full sky at large angular scales. The challenges of LiteBIRD are the wide field-of-view (FoV) and broadband capabilities of millimeter-wave polarization measurements, which are derived from the system requirements. The possible paths of stray light increase with a wider FoV and the far sidelobe knowledge of -56 dB is a challenging optical requirement. A crossed-Dragone configuration was chosen for the low frequency telescope (LFT : 34–161 GHz), one of LiteBIRD’s onboard telescopes. It has a wide field-of-view (18° x 9°) with an aperture of 400 mm in diameter, corresponding to an angular resolution of about 30 arcminutes around 100 GHz. The focal ratio f/3.0 and the crossing angle of the optical axes of 90◦ are chosen after an extensive study of the stray light. The primary and secondary reflectors have rectangular shapes with serrations to reduce the diffraction pattern from the edges of the mirrors. The reflectors and structure are made of aluminum to proportionally contract from warm down to the operating temperature at 5 K. A 1/4 scaled model of the LFT has been developed to validate the wide field-of-view design and to demonstrate the reduced far sidelobes. A polarization modulation unit (PMU), realized with a half-wave plate (HWP) is placed in front of the aperture stop, the entrance pupil of this system. A large focal plane with approximately 1000 AlMn TES detectors and frequency multiplexing SQUID amplifiers is cooled to 100 mK. The lens and sinuous antennas have broadband capability. Performance specifications of the LFT and an outline of the proposed verification plan are presented.
Sapphire, alumina, and silicon present the following characteristics that make them suitable as optical elements for millimeter and sub-millimeter applications: low-loss, high thermal conductivity at cryogenic temperatures, and high refractive index ~3. However, the high index also leads to high reflection. We developed a technique to machine sub-wavelength structures (SWS) as a broadband anti-reflection coating on these materials through laser ablation. We describe here the status of our development: transmission measurements of fabricated samples in a diameter of 34.5 mm agree with predictions, and we are now focusing on increasing the fabrication area with high processing rate. This is motivated by the need of ~500 mm diameter optical elements for the next-generation cosmic microwave background polarization experiments. We show our large area machining method on the alumina and sapphire over an area of < 5200 mm2 with the processing rate of < 4:0 mm3=min:, and the transmission measurements are consistent with the predictions.
We report the development of an optical encoder and its readout system for a cryogenically-cooled continuously rotating half-wave plate (HWP) polarization modulator unit (PMU) in the LiteBIRD low-frequency telescope. LiteBIRD is a cosmic microwave background polarization satellite mission to probe B-mode polarization, which originates from primordial gravitational waves, observing from the second Lagrange point (L2). LiteBIRD employs a continuously-rotating HWP to mitigate systematic effects. The knowledge of the position angle of the HWP is in a one-to-one relationship to the incident polarization angle. The required reconstruction accuracy is about 1 arcmin and the targeted rotational frequency stability is 1 mHz. A unique development constraint comes from a telemetry bandwidth limitation between the Earth and L2, and thus we implement a digital process to reduce the data volume assuming a future implementation of on-board processing of the encoder data before the downlink. The demonstrations were done experimentally using a breadboard model of the PMU: a readout system using FPGA (Spartan-6) and a rotational mechanism using a superconducting magnetic bearing and AC motor. We acquired the encoder data from the rotational mechanism operating under two conditions: liquid nitrogen at room pressure and below 10 K in a cryostat. We demonstrated the reconstruction of the position angle accuracy < 0.5 arcmin and the corresponding data volume of 0.12 GB/day, which is at least an order of magnitude smaller than the total data volume per day. We further discuss the sources of the position angle uncertainty and its implications to the observations.
LiteBIRD is a JAXA-led Strategic Large-Class mission designed to search for the existence of the primordial gravitational waves produced during the inflationary phase of the Universe, through the measurements of their imprint onto the polarization of the cosmic microwave background (CMB). These measurements, requiring unprecedented sensitivity, will be performed over the full sky, at large angular scales, and over 15 frequency bands from 34 GHz to 448 GHz. The LiteBIRD instruments consist of three telescopes, namely the Low-, Medium-and High-Frequency Telescope (respectively LFT, MFT and HFT). We present in this paper an overview of the design of the Medium-Frequency Telescope (89{224 GHz) and the High-Frequency Telescope (166{448 GHz), the so-called MHFT, under European responsibility, which are two cryogenic refractive telescopes cooled down to 5 K. They include a continuous rotating half-wave plate as the first optical element, two high-density polyethylene (HDPE) lenses and more than three thousand transition-edge sensor (TES) detectors cooled to 100 mK. We provide an overview of the concept design and the remaining specific challenges that we have to face in order to achieve the scientific goals of LiteBIRD.
We present a breadboard model development status of the polarization modulator unit (PMU) for a low-frequency telescope (LFT) of the LiteBIRD space mission. LiteBIRD is a next-generation cosmic microwave background polarization satellite to measure the primordial B-mode with the science goal of σr < 0.001. The baseline design of LiteBIRD consists of reflective low-frequency and refractive medium-and-high-frequency telescopes. Each telescope employs the PMU based on a continuous rotating half-wave plate (HWP) at the aperture. The PMU is a critical instrument for the LiteBIRD to achieve the science goal because it significantly suppresses 1/f noise and mitigates systematic uncertainties. The LiteBIRD LFT PMU consists of a broadband achromatic HWP and a cryogenic rotation mechanism. In this presentation, we discuss requirements, design and systematic studies of the PMU, and we report the development status of the broadband HWP and the space-compatible cryogenic rotation mechanism.
LiteBIRD is a candidate for JAXA’s strategic large mission to observe the cosmic microwave background (CMB) polarization over the full sky at large angular scales. It is planned to be launched in the 2020s with an H3 launch vehicle for three years of observations at a Sun-Earth Lagrangian point (L2). The concept design has been studied by researchers from Japan, U.S., Canada and Europe during the ISAS Phase-A1. Large scale measurements of the CMB B-mode polarization are known as the best probe to detect primordial gravitational waves. The goal of LiteBIRD is to measure the tensor-to-scalar ratio (r) with precision of r < 0:001. A 3-year full sky survey will be carried out with a low frequency (34 - 161 GHz) telescope (LFT) and a high frequency (89 - 448 GHz) telescope (HFT), which achieve a sensitivity of 2.5 μK-arcmin with an angular resolution 30 arcminutes around 100 GHz. The concept design of LiteBIRD system, payload module (PLM), cryo-structure, LFT and verification plan is described in this paper.
We present our design and development of a polarization modulator unit (PMU) for LiteBIRD space mission. LiteBIRD is a next generation cosmic microwave background (CMB) polarization satellite to measure the primordial B-mode. The science goal of LiteBIRD is to measure the tensor-to-scalar ratio with the sensitivity of δr < 10-3. The baseline design of LiteBIRD is to employ the PMU based on a continuous rotating half-wave plate (HWP) at a telescope aperture with a diameter of 400 mm. It is an essential for LiteBIRD to achieve the science goal because it significantly reduces detector noise and systematic uncertainties. The LiteBIRD PMU consists of a multi-layered sapphire as a broadband achromatic HWP and a mechanism to continuously rotate it at 88 rpm. The whole system is maintained at below 10K to minimize the thermal emission from the HWP. In this paper, we discuss the current development status of the broadband achromatic HWP and the cryogenic rotation mechanism.
LiteBIRD is a satellite project to measure the polarization of the CMB with an unprecedented accuracy. LiteBIRD observes all sky for three years at the sun-earth second Lagrange point. The goal of LiteBIRD is to observe the B-mode polarization at large angular scales and to measure the tensor-to-scaler ratio r with an accuracy less than 0.001, exploring the energy scale of the inflation. In order to mitigate the system 1/f noise and systematics, we plan to use continuous rotating half-wave plates (HWPs) as a polarization modulator at each aperture of two telescopes. One of the telescopes, called a low frequency telescope (LFT), covers the frequency range from 34 to 270 GHz, requiring the HWP to have a high modulation efficiency in the wide bandwidth. We employ a Pancharatnam-type achromatic HWP (AHWP) to achieve the broadband coverage. The AHWP consists of nine layer stacked HWPs with the optic axes mutually rotated by the angles optimized for the LFT bandwidth. In this paper, we report our development status of the nine layer AHWP and measurement results on the modulation efficiency and the phase as a function of frequency.
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