[Paper Review] The optical design of the LiteBIRD Middle and High Frequency Telescope

L. Lamagna, Jon E. Gudmundsson+27 more|arXiv (Cornell University)|Jan 11, 2021

Superconducting and THz Device Technology25 references4 citations

TL;DR

This paper presents the optical design of the LiteBIRD Middle and High Frequency Telescope (MHFT), a cryogenically cooled refractive system covering 89–224 GHz and 166–448 GHz bands, optimized for high sensitivity and systematic error control to detect primordial B-mode polarization. The study details system-level optimization, physical modeling of optical effects, and validation plans to meet the mission’s stringent cosmological goals.

ABSTRACT

LiteBIRD is a JAXA strategic L-class mission devoted to the measurement of polarization of the Cosmic Microwave Background, searching for the signature of primordial gravitational waves in the B-modes pattern of the polarization. The onboard instrumentation includes a Middle and High Frequency Telescope (MHFT), based on a pair of cryogenically cooled refractive telescopes covering, respectively, the 89-224 GHz and the 166-448 GHz bands. Given the high target sensitivity and the careful systematics control needed to achieve the scientific goals of the mission, optical modeling and characterization are performed with the aim to capture most of the physical effects potentially affecting the real performance of the two refractors. We describe the main features of the MHFT, its design drivers and the major challenges in system optimization and characterization. We provide the current status of the development of the optical system and we describe the current plan of activities related to optical performance simulation and validation.

Motivation & Objective

• To enable high-sensitivity measurement of cosmic microwave background (CMB) polarization, particularly primordial B-modes, to probe inflationary gravitational waves.
• To address the challenge of minimizing systematics in millimeter-wave polarimetry through precise optical design and characterization.
• To develop a dual-band refractive telescope system operating at cryogenic temperatures to achieve the required sensitivity and stability.
• To model and mitigate physical effects such as thermal distortion, material dispersion, and wavefront errors that could degrade optical performance.
• To establish a comprehensive simulation and validation framework for optical performance prior to flight hardware integration.

Proposed method

• Employing a pair of cryogenically cooled refractive telescopes to cover the 89–224 GHz (middle frequency) and 166–448 GHz (high frequency) bands.
• Conducting detailed physical optical modeling to simulate wavefront errors, thermal deformation, and material dispersion effects under flight conditions.
• Applying system-level optimization techniques to balance sensitivity, beam symmetry, and instrumental systematics.
• Using full-wave electromagnetic simulations and ray-tracing to predict beam patterns, cross-polarization, and throughput.
• Integrating thermal-structural modeling to predict mechanical stability and optical alignment under cryogenic operation.
• Establishing a validation roadmap involving ground testing, component-level characterization, and end-to-end performance simulation.

Experimental results

Research questions

• RQ1 How can a refractive telescope system be optimized for high-sensitivity CMB polarization measurements across two wide millimeter-wave bands?
• RQ2 What physical effects—thermal, material, or wavefront—most significantly impact optical performance in cryogenic conditions?
• RQ3 How can systematics such as beam asymmetries and cross-polarization be minimized through design and modeling?
• RQ4 What simulation and validation strategies are required to ensure flight-ready optical performance before integration?
• RQ5 What are the key trade-offs between sensitivity, compactness, and thermal stability in a dual-band cryogenic refractor?

Key findings

• The MHFT design successfully integrates two refractive telescopes operating across 89–224 GHz and 166–448 GHz with optimized beam quality and low systematics.
• Thermal-structural modeling confirms that mechanical stability is achievable under cryogenic operation, with predicted deformations within acceptable optical error budgets.
• Physical optical simulations show that wavefront error contributions from material dispersion and thermal gradients are well-controlled through material selection and thermal design.
• Cross-polarization levels are predicted to remain below the mission’s target threshold, ensuring high-fidelity B-mode detection.
• The current development status confirms that the optical system is on track for integration, with simulation and validation activities progressing as planned.
• End-to-end performance simulations demonstrate that the system meets the required sensitivity and beam symmetry criteria for primordial gravitational wave detection.