The ability to measure both optical efficiency and dynamic response to changes in optical signal is crucial to the development of Transition Edge Sensors (TESs) for far-infrared astronomical instruments. We have devised and implemented a cryogenic test facility for ultra-low-noise far-infrared TESs, designed for the SAFARI grating spectrometer on the cooled-aperture space telescope SPICA. Whilst our experimental arrangement is suitable for the whole of the SAFARI wavelength range, 34-230 μm, we focus here on representative optical measurements at 60-110 μm. Detectors are illuminated with a few-mode beam having modal characteristics identical to those of an ideal imaging telescope. In addition, a fast thermal infrared source allows direct measurement of the TES response to tiny changes in incident optical power. We describe the measured functional forms of TES transient responses both to fast optical pulses and to modulation of the power dissipated in the bilayer, in the presence of background optical loading through to TES saturation.
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.
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