Characterizing the optical response of ultra-low-noise far-infrared 60-110 μm transition edge sensors

Far-infrared Transition Edge Sensors (TESs) are being developed for the SAFARI grating spectrometer on the cooled-aperture space telescope SPICA. In support of this work, we have devised a cryogenic (90 mK) test facility for carrying out precision optical measurements on ultra-low-noise TESs. Although our facility is suitable for the whole of the SAFARI wavelength range, 34 µm-230 µm, we focus on a representative set of measurements at 60 µm-110 µm using a device having a noise equivalent power of 0.32 aW Hz^-1/2. The system is able to perform a range of measurements: (i) dark electrical characterization, (ii) optical efficiency with respect to a partially coherent beam having a modal composition identical to that of an ideal imaging telescope, (iii) optical saturation and dynamic range, (iv) fast optical transient response to a modulated thermal source, and (v) optical transient response in the presence of high-level background loading. We describe dark measurements to determine the operating characteristics of a TES and then compare the predicted optical behavior with the measured optical behavior. By comparing electrical and optical transient responses, we were able to observe thermalization in the device. We comment on the challenge of eliminating stray light.