Spectral Characterization of a 90 GHz CLASS Pixel

TL;DR

This paper presents a method to accurately characterize the spectral response of a 90 GHz detector pixel for the CLASS experiment, separating filter effects from other components using Fourier transform spectroscopy.

Contribution

It introduces a novel approach to distinguish the bandpass filter's impact from other microwave components in detector pixels.

Findings

Band edges at 80 GHz and 108 GHz identified

Fourier transform spectroscopy effectively separates component effects

Method improves detector array optimization

Abstract

The Cosmology Large Angular Scale Surveyor (CLASS) is an experiment designed to measure the polarization of the cosmic microwave background on large angular scales to probe cosmic reionization and search for the inflationary $B$-mode signal. CLASS is a multi-frequency ensemble of telescopes with bands centered at 40, 90, 150, and 220 GHz. Each telescope has arrays of feedhorn-coupled transition edge sensor bolometers at the focal plane. The frequency response is primarily defined by the on-chip bandpass filter with additional contributions coming from the feedhorn, orthomode transducer, and 180-degree hybrid. In this study, we compare simulations and measurements of the frequency response of single pixel witness devices in the 90 GHz band with and without the bandpass filter. For the first time, we can separate the effects of the bandpass filter from the other microwave components using Fourier transform spectroscopy and design splits of the pixel. The results show that the -3 dB band edges are at 80 GHz and 108 GHz. The measurements demonstrate a robust method for characterizing the spectral response of individual components, which is crucial for optimizing the performance of future detector arrays.