SAFARI is one of the focal-plane instruments for the European/ Japanese far-IR SPICA mission proposed for the ESA M5 selection. It is based on three arrays with in total 3550 TES-based bolometers with noise-equivalent powers (NEP) of 2∙10-19 W/Hz. The arrays are operated in three wavelength bands: S-band for 30-60 µm, M-band for 60-110 µm and L-band for 110-210 µm, and have high optical efficiency. SRON is developing Frequency Domain Multiplexing (FDM) for readout of large AC biased TES arrays for both the SAFARI instrument, and the XIFU instrument on the X-ray Athena mission. In FDM for SAFARI, the TES bolometers are AC biased and read out using 24 channels. Each channel contains 160 pixels of which the resonance frequencies are defined by in-house developed cryogenic lithographic LC filters. FDM is based on the amplitude modulation of a carrier signal, which also provides the AC voltage bias, with the signal detected by the TES. To overcome the dynamic range limitations of the SQUID pre-amplifier, baseband feedback (BBFB) is applied. BBFB attempts to cancel the error signal in the sum-point, at the input coil of the SQUID, by feeding back a remodulated signal to the sum-point, and therefore improving the dynamic range of the SQUID pre-amplifier.
Previously we have reported on a detailed study of the effects of electrical crosstalk using our first iteration of a prototype of the full 160 pixel FDM experiment and the successful readout of 132 pixels using our 176 pixel FDM system. After the demonstration it is important to perform more detailed measurements to consolidate the system. For instance, one of the important next steps is to expose the FDM system to an optical infrared source. The cold part of the FDM system consists of a detector chip with 176 pixels with a designed NEP of 7∙10-19 W/Hz and two matching LC filter chips, each of which contains 88 carefully placed high-Q resonators, with a total of 176 different resonance frequencies, and a single-stage SQUID. The warm electronics consist of a low-noise amplifier (LNA) and a digital board on which the generation of the bias carriers, the demodulation of the signal and remodulation of the feedback signal are performed. The optical experiment will be conducted in a Leiden Cryogenics dilution refrigerator with a cooling power of about 200µW at 100 mK. This system contains multiple optical sources. These include a conical black body radiator which can be operated in the range of 3-34K and a light pipe through which the experiment can be illuminated from outside the cryostat. Dark measurements are conducted in a Janis ADR system with a base temperature of 50mK.
In this paper we describe the experimental tests and results of the more detailed testing of our 176 pixel TES bolometer system.