Ultra-low-noise transition edge sensors for far infrared wavelengths: optical design, measurement and stray light control

Ultra-low-noise Transition Edge Sensors (TESs) have been selected for the far-infrared Fourier transform spectrometer SAFARI on the space telescope SPICA, now under study as an M5 mission, operating in three wavelength bands: S-band from 34-60 μm, M-band from 60-110 μm and L-band from 110-210 μm. We report the fabrication and optical characterisation of a linear TES array for the SAFARI M-band, integrated with micromachined reflective backshorts and profiled pyramidal optical feedhorns. The design and construction of the cryogenic optical test facility used to illuminate the devices under test are described, featuring a variable temperature blackbody load, band-defining filters and an optical aperture. We observe effective numbers of optical modes, Nef f = 0.41 ± 0.03, and near-unity optical efficiencies in TES-backshort assemblies, with some loss of efficiency in the presence of horns. Stray light control measures are discussed in the context of a significant reduction achieved in long wavelength stray light detected by these devices.