We report our most recent progress and demonstration of a frequency domain multiplexing (FDM) readout technology for transition-edge sensor (TES) arrays, both of which we have been developing in the framework of the X-IFU instrument on board the future Athena X-ray telescope. Using Ti/Au TES micro-calorimeters, high-Q LC filters and analog/digital electronics developed at SRON and low-noise two-stage SQUID amplifiers from VTT Finland, we demonstrated the feasibility of our FDM readout technology, with the simultaneous readout of 37 pixels with an energy resolution of 2.23 eV at an energy of 6 keV. We finally outline our plans for further scaling up and improving our technology in the future.

We have characterized and mapped the electrical cross talk (ECT) of a frequency division multiplexing (FDM) system with a transition edge sensor (TES) bolometer array, which is intended for space applications. By adding a small modulation at 120 Hz to the AC bias voltage of one bolometer and measuring the cross talk response in the current noise spectra of the others simultaneously, we have for the first time mapped the ECT level of 61 pixels with a nominal frequency spacing of 32 kHz in a 61 × 61 matrix and a carrier frequency ranging from 1 MHz to 4 MHz. We find that about 94% of the pixels show an ECT level of less than 0.4%. Only the adjacent pixels reach this level, and the ECT for the rest of the pixels is less than 0.1%. We also observe higher ECT levels, up to 10%, between some of the pixels, which have bundled long, parallel coplanar wires connecting TES bolometers to inductor-capacitor filters. In this case, the high mutual inductances dominate. To mitigate this source of ECT, the coplanar wires should be replaced by microstrip wires in the array. Our study suggests that an FDM system can have a relatively low ECT level, e.g., around 0.4% if the frequency spacing is 30 kHz. Our results successfully demonstrate a low electrical cross talk for a space FDM technology.

We demonstrate multiplexing readout of 60 transition edge sensor (TES) bolometers operating at 90 mK using a frequency division multiplexing readout chain with bias frequencies ranging from 1 to 3.5 MHz and with a typical frequency spacing of 32 kHz. The readout chain starts with a two-stage SQUID amplifier and has a noise level of 9.5 pA/ Hz. We compare current-voltage curves and noise spectra of TESs measured in a single-pixel mode and in a multiplexing mode. We also map the noise equivalent power (NEP) and the saturation power of the bolometers in both modes, where there are 43 pixels that do not show more than 10% difference in NEP and 5% in saturation power when measured in single pixel and multiplex modes. We have read out a TES with an NEP of 0.45 aW/ Hz in the multiplexing-mode, which demonstrates the capability of reading out ultra-low noise TES bolometer arrays for space applications.

In this paper we present the results of our 176-pixel prototype of the FDM readout system for SAFARI, a TES-based focal-plane instrument for the far-IR SPICA mission. We have implemented the knowledge obtained from the detailed study on electrical crosstalk reported previously. The effect of carrier leakage is reduced by a factor two, mutual impedance is reduced to below 1 nH and mutual inductance is removed. The pixels are connected in stages, one quarter of the array half of the array and the full array, to resolve intermediate technical issues. A semi-automated procedure was incorporated to find all optimal settings for all pixels. And as a final step the complete array has been connected and 132 pixels have been read out simultaneously within the frequency range of 1-3.8MHz with an average frequency separation of 16kHz. The noise was found to be detector limited and was not affected by reading out all pixels in a FDM mode. With this result the concept of using FDM for multiplexed bolometer read out for the SAFARI instrument has been demonstrated.