Characterization of an Ultra-Sensitive Transition-Edge Sensor for Space-Borne Far-IR/Sub-mm Spectroscopy

We measured the noise equivalent power (NEP) of an ultra-sensitive membrane-isolated transition-edge sensor (TES) designed for the SpicA FAR Infrared instrument (SAFARI) along with several Johnson noise Thermometry Devices (JTDs) simultaneously in a “dark” experimental setup. Our goal was to characterize the dark power ${\rm P}_{\rm D}$ present in our experimental setup and its effect on the measured NEP. By employing resistive attenuators, filters, lossy coaxial connections, and a light-tight box, we reduced ${\rm P}_{\rm D}$ to 0.25 fW in our setup. For the TES, G was measured to be (325$\pm$25) fW/K at ${\rm T}_{\rm C}=78\ {\rm mK}$ giving an upper-limit expected NEP of $(3.5\pm 0.25)\times 10^{-19}\ {\rm W/Hz}^{1/2}$ . We measured an actual NEP on the TES equal to $(6.5\pm 1.5)\times 10^{-19}\ {\rm W/Hz}^{1/2}$ , which is approximately double the expected value and likely due to residual electrical dark power in our setup.