Optical Characterization of the SPT-3G Focal Plane

TL;DR

This paper details the optical characterization of the SPT-3G focal plane, including frequency response, efficiency, and beam properties, demonstrating high performance suitable for cosmic microwave background measurements.

Contribution

It provides comprehensive measurements of the optical and polarization properties of the SPT-3G detectors, validating their design and performance for CMB observations.

Findings

Detectors' frequency bands match simulations.

High optical efficiencies of 86%, 77%, and 66%.

Polarization efficiency exceeds 90% for most bolometers.

Abstract

The third-generation South Pole Telescope camera is designed to measure the cosmic microwave background across three frequency bands (95, 150 and 220 GHz) with ~16,000 transition-edge sensor (TES) bolometers. Each multichroic pixel on a detector wafer has a broadband sinuous antenna that couples power to six TESs, one for each of the three observing bands and both polarization directions, via lumped element filters. Ten detector wafers populate the focal plane, which is coupled to the sky via a large-aperture optical system. Here we present the frequency band characterization with Fourier transform spectroscopy, measurements of optical time constants, beam properties, and optical and polarization efficiencies of the focal plane. The detectors have frequency bands consistent with our simulations, and have high average optical efficiency which is 86%, 77% and 66% for the 95, 150 and 220 GHz detectors. The time constants of the detectors are mostly between 0.5 ms and 5 ms. The beam is round with the correct size, and the polarization efficiency is more than 90% for most of the bolometers