Design and characterization of the Cosmology Large Angular Scale Surveyor (CLASS) 93 GHz focal plane

Abstract

The Cosmology Large Angular Scale Surveyor (CLASS) aims to detect and characterize the primordial Bmode signal and make a sample-variance-limited measurement of the optical depth to reionization. CLASS is a ground-based, multi-frequency microwave polarimeter that surveys 70% of the microwave sky every day from the Atacama Desert. The focal plane detector arrays of all CLASS telescopes contain smooth-walled feedhorns that couple to transition-edge sensor (TES) bolometers through symmetric planar orthomode transducer (OMT) antennas. These low noise polarization-sensitive detector arrays are fabricated on mono-crystalline silicon wafers to maintain TES uniformity and optimize optical efficiency throughout the wafer. In this paper, we discuss the design and characterization of the first CLASS 93 GHz detector array. We measure the dark parameters, bandpass, and noise spectra of the detectors and report that the detectors are photon-noise limited. With current array yield of 82%, we estimate the total array noise-equivalent power (NEP) to be 2.1 aW √ s.

The CLASS 150/220 GHz Polarimeter Array: Design, Assembly, and Characterization

“…Similar to the 90 GHz architecture described in [2][3][4], the HF wafer package consists of three separate wafers hybridized together: a photonic choke wafer, a detector wafer, and a backshort assembly. The photonic choke [7] acts as a waveguide interface between the feedhorn array and the detectors.…”

“…As shown in Figure 1, the HF focal plane consists of three identical modules mounted on a mixing chamber plate maintained at a stable bath temperature of ∼ 80 mK by a pulse-tube cooled dilution refrigerator. 2 Each module consists of 85 smooth-walled feedhorns made from a Au-plated CE7 alloy, which is machinable and has lower differential contraction to silicon detector wafers than copper or aluminum [9]. Figure 3 shows the tripod clips, alignment pins, and the side spring used to mount and align the detector wafer onto the feedhorn array.…”

“…In particular, this cabling connects the MUX output to a set of SQUID series array (SSA) amplifiers. The 4K SSA and the warm electronics are identical to the CLASS 90 GHz components as described in [2]. Figure 3 shows the layout of a module during and after assembly.…”

“…We use a polarizing Martin-Puplett Fourier Transform Spectrometer (FTS) to measure the bandpasses of the HF detectors in lab. Refer to [13] for detailed description of the FTS used and [2] for the filters used in the lab-cryostat setup. Figure 4 shows the measured CLASS detector bandpasses compared to simulation and the atmospheric transmission model at the CLASS site in the Atacama Desert with precipitable water vapor (PWV) of 1 mm.…”

“…The Cosmology Large Angular Scale Surveyor (CLASS) maps the microwave sky from the Atacama Desert in Chile at four frequency bands from 40 to 220 GHz, optimized for maximum signal-to-noise based on the atmospheric emission. The addition of a dichroic (150/220 GHz) detector array to the existing 40 [1] and 90 GHz [2] telescopes provides additional sensitivity to CLASS's CMB observations and helps to characterize the dust foreground. The 150/220 GHz high frequency (HF) instrument was delivered to and tested at the CLASS site in June 2019.…”