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.
We give an overview of the baseline detector system for SAFARI, the prime focal-plane instrument on board the proposed space infrared observatory, SPICA. SAFARI's detectors are based on superconducting Transition Edge Sensors (TES) to provide the extreme sensitivity (dark NEP≤2×10<sup>-19 </sup>W/√Hz) needed to take advantage of SPICA's cold (<8 K) telescope. In order to read out the total of ~3500 detectors we use frequency domain multiplexing (FDM) with baseband feedback. In each multiplexing channel, a two-stage SQUID preamplifier reads out 160 detectors. We describe the detector system and discuss some of the considerations that informed its design.
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
The SAFARI Detector Test Facility is an ultra-low background optical testbed for characterizing ultra-sensitive
prototype horn-coupled TES bolmeters for SAFARI, the grating spectrometer on board the proposed SPICA satellite.
The testbed contains internal cold and hot black-body illuminators and a light-pipe for illumination with an external
source. We have added reimaging optics to facilitate array optical measurements. The system is now being used for
optical testing of prototype detector arrays read out with frequency-domain multiplexing. We present our latest optical
measurements of prototype arrays and discuss these in terms of the instrument performance.
SRON is developing ultra-low noise Transition Edge Sensors (TESs) based on a superconducting Ti/Au bilayer on a
suspended SiN island with SiN legs for the SAFARI instrument aboard the SPICA mission. We successfully fabricated
TESs with very narrow (0.5-0.7 μm) and thin (0.25 μm) SiN legs on different sizes of SiN islands using deep reactiveion
etching process. The pixel size is 840x840 μm<sup>2</sup> and there are variety of designs with and without optical absorbers.
For TESs without absorbers, we measured electrical NEPs as low as <1x10<sup>-19</sup> W/√Hz with response time of 0.3 ms and
reached the phonon noise limit. Using TESs with absorbers, we quantified the darkness of our setup and confirmed a
photon noise level of 2x10<sup>-19</sup> W/√Hz.
For the read out of the Transition Edge Sensors (TES) bolometer arrays of the SAFARI instrument on the Japanese background-limited far-IR SPICA mission SRON is developing a Frequency Domain Multiplexing (FDM) read-out system. The next step after the successful demonstration of the read out of 38 TES bolometers using FDM was to demonstrate the FDM readout of the required 160 TES bolometers. Of the 160 LC filter and TES bolometer chains 151 have been connected and after cooldown 148 of the resonances could be identified. Although initial operation and locking of the pixels went smoothly the experiment revealed several complications. In this paper we describe the 160 pixel FDM set-up, show the results and discuss the issues faced during operation of the 160 pixel FDM experiment.
The Far-Infrared Fourier transform spectrometer instrument SAFARI-SPICA which will operate with cooled optics in a low-background space environment requires ultra-sensitive detector arrays with high optical coupling efficiencies over extremely wide bandwidths. In earlier papers we described the design, fabrication and performance of ultra-low-noise Transition Edge Sensors (TESs) operated close to 100mk having dark Noise Equivalent Powers (<i>NEPs</i>) of order 4 × 10<sup>−19</sup>W/√Hz close to the phonon noise limit and an improvement of two orders of magnitude over TESs for ground-based applications. Here we describe the design, fabrication and testing of 388-element arrays of MoAu TESs integrated with far-infrared absorbers and optical coupling structures in a geometry appropriate for the SAFARI L-band (110 − 210 μm). The measured performance shows intrinsic response time τ ~ 11ms and saturation powers of order 10 fW, and a dark noise equivalent powers of order 7 × 10<sup>−19</sup>W/√Hz. The 100 × 100μm<sup>2</sup> MoAu TESs have transition temperatures of order 110mK and are coupled to 320×320μm<sup>2</sup> thin-film β-phase Ta absorbers to provide impedance matching to the incoming fields. We describe results of dark tests (i.e without optical power) to determine intrinsic pixel characteristics and their uniformity, and measurements of the optical performance of representative pixels operated with flat back-shorts coupled to pyramidal horn arrays. The measured and modeled optical efficiency is dominated by the 95Ω sheet resistance of the Ta absorbers, indicating a clear route to achieve the required performance in these ultra-sensitive detectors.
At SRON we are developing the Frequency Domain Multiplexing (FDM) for the read-out of the TES-based
detector array for the future infrared and X-ray space mission. We describe the performances of a multiplexer
designed to increase the experimental throughput in the characterisation of ultra-low noise equivalent power
(NEP) TES bolometers and high energy resolving power X-ray microcalorimeters arrays under ac and dc bias.
We discuss the results obtained using the TiAu TES bolometers array fabricated at SRON with measured dark
NEP below 5 · 10<sup>−19</sup>W/
Hz and saturation power of several fW.
SPICA is an infra-red (IR) telescope with a cryogenically cooled mirror (~5K) with three instruments on board, one of
which is SAFARI that is an imaging Fourier Transform Spectrometer (FTS) with three bands covering the wavelength of
34-210 μm. We develop transition edge sensors (TES) array for short wavelength band (34-60 μm) of SAFARI. These
are based on superconducting Ti/Au bilayer as TES bolometers with a T<sub><i>c</i></sub> of about 105 mK and thin Ta film as IR
absorbers on suspended silicon nitride (SiN) membranes. These membranes are supported by long and narrow SiN legs
that act as weak thermal links between the TES and the bath. Previously an electrical noise equivalent power (NEP) of
4×10<sup>-19</sup> W/√Hz was achieved for a single pixel of such detectors. As an intermediate step toward a full-size SAFARI
array (43×43), we fabricated several 8×9 detector arrays. Here we describe the design and the outcome of the dark and
optical tests of several of these devices. We achieved high yield (<93%) and high uniformity in terms of critical
temperature (<5%) and normal resistance (7%) across the arrays. The measured dark NEPs are as low as 5×10<sup>-19</sup> W/√Hz
with a response time of about 1.4 ms at preferred operating bias point. The optical coupling is implemented using
pyramidal horns array on the top and hemispherical cavity behind the chip that gives a measured total optical coupling
efficiency of 30±7%.
SRON is developing an electronic system for the multiplexed read-out of an array of transition edge sensors (TES) by
combining the techniques of frequency domain multiplexing (FDM) with base-band feedback (BBFB). The astronomical
applications are the read-out of soft X-ray microcalorimeters and the far-infrared bolometers for the SAFARI instrument
on the Japanese mission SPICA. In this paper we derive the requirements for the read-out system regarding noise and
dynamic range in the context of the SAFARI instrument, and demonstrate that the current experimental prototype is
capable of simultaneously locking 57 channels and complies with these requirements.
The next generation of space missions targeting far-infrared wavelengths will require large-format arrays of extremely
sensitive detectors. The development of Transition Edge Sensor (TES) array technology is being developed for future
Far-Infrared (FIR) space applications such as the SAFARI instrument for SPICA where low-noise and high sensitivity is
required to achieve ambitious science goals.
In this paper we describe a modal analysis of multi-moded horn antennas feeding integrating cavities housing TES
detectors with superconducting film absorbers. In high sensitivity TES detector technology the ability to control the
electromagnetic and thermo-mechanical environment of the detector is critical. Simulating and understanding optical
behaviour of such detectors at far IR wavelengths is difficult and requires development of existing analysis tools.
The proposed modal approach offers a computationally efficient technique to describe the partial coherent response of
the full pixel in terms of optical efficiency and power leakage between pixels. Initial wok carried out as part of an ESA
technical research project on optical analysis is described and a prototype SAFARI pixel design is analyzed where the
optical coupling between the incoming field and the pixel containing horn, cavity with an air gap, and thin absorber layer
are all included in the model to allow a comprehensive optical characterization. The modal approach described is based
on the mode matching technique where the horn and cavity are described in the traditional way while a technique to
include the absorber was developed. Radiation leakage between pixels is also included making this a powerful analysis
Transition edge sensor (TES) is the selected detector for the SAFARI FIR imaging spectrometer (focal plane arrays covering a wavelength range from 30 to 210 μm) on the Japanese SPICA telescope. Since the telescope is cooled to <7 K, the instrument sensitivity is limited by the detector noise. Therefore among all the requirements, a crucial one is the sensitivity, which should reach an NEP (Noise Equivalent Power) as low as 3E<sup>-19</sup> W/Hz^0.5 for a base temperature of >50 mK. Also the time constant should be below 8 ms.
We fabricated and characterized low thermal conductance transition edge sensors (TES) for SAFARI instrument on SPICA. The device is based on a superconducting Ti/Au bilayer deposited on suspended SiN membrane. The critical temperature of the device is 155 mK. The low thermal conductance is realized by using narrow SiN ring-like supporting structures. All measurements were performed having the device in a light-tight box, which to a great extent eliminates the loading of the background radiation. We measured the current-voltage (IV) characteristics of the device in different bath temperatures and determine the thermal conductance (G) to be equal to 1.66 pW/K. This value corresponds to a noise equivalent power (NEP) of 1E<sup>-18</sup> W/√Hz. The current noise and complex impedance is also measured at different bias points at 25 mK bath temperature. The measured electrical (dark) NEP is 2E<sup>-18</sup> W/√Hz, which is about a factor of 2 higher than what we expect from the thermal conductance that comes out of the IV curves. Despite using a light-tight box, the photon noise might still be the source of this excess noise. We also measured the complex impedance of the same device at several bias points. Fitting a simple first order thermal-electrical model to the measured data, we find an effective time constant of about 65 μs and a thermal capacity of 3-4 fJ/K in the middle of the transition
Superconducting Tunnel Junctions (STJs) have been extensively investigated as photon detectors covering the range from near-infrared to X-ray energies. A 10×12 array of Tantalum/Aluminium junctions has been integrated into the S-Cam3 camera for ground based astronomy. With this camera, the European Space Agency has performed multiple astronomical observations of optical sources using the William Herschel 4.2m telescope at La Palma and the Agency's 1-m Optical Ground Station telescope at Tenerife. Compared to its predecessor, this new instrument features a 10"×12" field-of-view, an optimized IR rejection reducing baseline noise and increasing optical light throughput and ultra-stable operations. In this paper, we review the instrument's architecture and describe the system's performance and in particular the energy resolution and count-rate capabilities of the detector arrays. Finally, we shall present first astronomical images taken during the Optical Ground Station's 2005 and 2006 campaigns which demonstrate the system's timing, photometric and spectroscopic capabilities.
The requirement on energy resolution for detectors in future X-ray satelite missions such as XEUS (X-ray Evolving
Universe Spectroscopy mission) is <2eV in the soft x-ray range of 50-2000 eV, with a detection efficiency >80%. In
addition, the requirements for field of view and angular resolution demand a detector array of typically 150x150 micron
sized pixels in a 30x30 pixel format. DROIDs (Distributed Read Out Imaging Devices), consisting of a superconducting
absorber strip with superconducting tunnel junctions (STJs) as read-out devices on either end, can fulfill these
requirements. The amplitudes of the two signals from the STJs provide information on the absorption position and the
energy of the incoming photon in the absorber. In this paper we present the development status of Ta/Al 1-D DROIDs, as
well as the the short term development program that should result in a full size XEUS array.
In this paper we present the preliminary results from experiments with Distributed Read Out Imaging Devices (DROIDs) in the optical and in the X-ray regime. For the optical results DROIDs of different lengths ranging from 200 to 700 μm have been used with an STJ lay-up of Ta/Al/AlOx/Al/Ta with thicknesses of 100/30/1/30/100 nm. With this data the behavior with different absorber length has been investigated to determine an optimal absorber size for a DROID array to be used in the optical wavelength regime. The optimum absorber size has been found to be 30x400 um. The X-ray data has been obtained with a similar device structure but with 60 nm aluminium trapping layers to increase the trapping of quasiparticles in the STJs. In this paper we only present the data obtained with the standard DROID size of 400μm. With this device an extensive set of measurements have been performed which involves; a scan in photon energy ranging from 50eV to 1900 eV, a scan in temperature and a scan in bias voltage. We report here only results from the preliminary analysis of the data obtained with readout electronics comprising the normal preamplifier and subsequent shaping stage. For the final analyzes the pulses resulting from the STJs have been digitized and are ready to be analyzed. The pulses have been used to estimate the decay time of the STJs which appear to be very short. This is probably caused by the poor trapping of quasiparticles. Detailed results on this process will be presented however at a later date.