CLASS is uniquely built to measure the large-angle polarization patterns in the Cosmic Microwave Background (CMB) from Inflation. A key technology that enables this measurement is called “modulation”.
CLASS modulates the polarization signal of the CMB. This modulation is analogous to AM radio. In fact AM stands for amplitude modulation, in which a radio station’s audio signal is modulated at a specific radio carrier frequency to isolate it from noise and the signals of other stations. Analogously, CLASS uses high frequency amplitude modulation to isolate the CMB polarization signal from the lower frequency atmospheric and instrumental noise.
To illustrate how CLASS uses modulation, we have created the following audio-frequency example. In this example the signal is represented by a voice saying “The Cosmic Microwave Background Polarization”. When unmodulated, the signal is difficult isolate from low-frequency noise. With modulation, the signal is encoded at a distinct (and comically higher) frequency. The ear can then distinguish the high-frequency modulated signal from the low-frequency noise.
CLASS 38 GHz Focal Plane Assembly Timelapse
CLASS maps 70% of the sky at a frequency of 38 GHz, corresponding to a wavelength of 7.9 mm, in the microwave region of the electromagnetic spectrum to provide the most sensitive map of the sky ever made at these frequencies. To achieve this, the CLASS team has developed novel technologies like wide-bandwidth, smooth-walled copper feedhorns (patent pending) and the first transition-edge-sensor (TES) bolometers at such a low frequency. Low-temperature bolometers operating at a temperature just 0.1 degree Celsius above absolute zero, where atoms stop moving altogether, are the most sensitive microwave detectors available.
The video above shows the CLASS 38 GHz focal plane as it was built in stages. The video shows the 36 individual detector chips, microfabricated on silicon wafers at NASA’s Goddard Space Flight Center, as they were assembled into the focal plane, along with associated readout electronics around the edges. Each detector chip and its associate feedhorn form a pixel for the 38 GHz camera and measure the polarization of the cosmic microwave background.