Design of a Fourier-transform spectrometer for detector characterization on CMB polarization telescope

Abstract

We present the design of a Fourier-transform spectrometer (FTS) tailored for operation within the frequency range of 75 GHz to 300 GHz, offering a spectral resolution surpassing 2 GHz. This FTS can be used for characterizing detectors on a Cosmic Microwave Background (CMB) Polarization Telescope for the observation of CMB B-mode polarization. The FTS instrument is divided into the primary part, housing a Martin-Puplett interferometer (MPI) module, and the coupled part, which incorporates a beam expanding module and a beam steering module. An in-depth performance analysis focusing on the MPI module, involving considerations such as the orientation of the beam splitter, dihedral angle of the roof-top mirrors, and positioning uncertainty of the scanning mirror, has been conducted by using a simulation tool. Results indicate that the spectral characteristic remains unaffected by variations in the Reflection/Transmission ratio of the beam splitter, with any deviation from the 1:1 ratio uniformly decreasing intensity. The impact of dihedral angle errors of roof-top mirrors (below 0.02 degrees) and positioning uncertainty in the scanning mirror (lower than 3 × 10-3 mm) on the system performance is acceptable for the design of the FTS. The beam expanding module is capable of enlarging the output beam of the MPI module to roughly a 150 mm-radius. The beam steering module allows for directional adjustments within the range of +17 degrees to -17 degrees relative to the normal of the target plane. Preliminary testing on the MPI prototype reveals a spectral resolution of 1.5 GHz for the measured frequency.