Optical and electrical transient response of ultra-low-noise far-infrared transition edge sensors for the SAFARI instrument on SPICA

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.