Distributed TES Model for Designing Low Noise Bolometers Approaching SAFARI Instrument Requirements

Transition edge sensors (TES) are the chosen detector technology for the SAFARI imaging spectrometer on the SPICA telescope. The TES are required to have an NEP of $2\mbox{--}3\times 10^{-19}~\mbox{W/}\sqrt{\mathrm{Hz}}$ to take full advantage of the cooled mirror. SRON has developed TiAu TES bolometers for the short wavelength band (30–60?μm). The TES are on SiN membranes, in which long and narrow legs act as thermal links between the TES and the bath. We present a distributed model that accounts for the heat conductance and the heat capacity in the long legs that provides a guideline for designing low noise detectors. We report our latest results that include a measured dark NEP of $4.2\times 10^{-19}~\mbox{W/}\sqrt{\mathrm{Hz}}$ and a saturation power of about 10?fW.     

Effect of Magnetic Fields on TiAu TES Bolometers for the SAFARI Instrument on the SPICA FIR Telescope

At the Netherlands Institute for Space Research (SRON) we are developing Transition Edge Sensors (TES) bolometers for the SAFARI Imaging Spectrometer on the SPICA telescope. With the current design of the SPICA telescope a background magnetic field of about a Gauss with fluctuations of a couple percent is expected. The effect of this external magnetic field on the bolometer needs to be well understood in order to design effective shielding. We present the results of an investigation of magnetic field on the performance of TES bolometers. For this experiment we used a Ti/Au TES bolometer which has a measured NEP of 5.0×10^?19 W/Hz^1/2. The TES is exposed to a parallel and perpendicular magnetic field of varying strength and the baseline and responsivity are determined. We see that the bolometers are sensitive to the perpendicular field but are relative insensitive to the parallel magnetic field. Fluctuations with a period of 30?mG are present as function of perpendicular magnetic field.     

Performance of a Low-Noise Test Facility for the SAFARI TES Bolometer Arrays

We have constructed a test facility for characterizing the focal plane arrays of SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI’s three bolometer arrays are populated with extremely sensitive ( $\mathit{NEP}\sim 2\times 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}$ ) transition edge sensors with a transition temperature close to 100?mK. The extreme sensitivity and low saturation power (～4?fW) of SAFARI’s detectors present challenges to characterizing them. In optimizing the SAFARI Detector System Test Facility we have paid careful attention to stray-light exclusion as well as electrical, magnetic, and mechanical isolation. We present measurements verifying the facility’s performance and analyze them in terms of a two-fluid model of the TES current on the transition to investigate the background power level. We have measured a detector NEP of (5.1±0.4)×10^?19?W?Hz^?1/2, showing that the facility is ready to test the SAFARI prototype arrays and is approaching the performance needed for testing the flight arrays.     

Characterization of NbSi TES Bolometers: Preliminary Results

We present preliminary results on characterisation of voltage biased superconducting bolometers with an NbSi alloy sensor. I(V) curves as well as complex impedance and noise measurements are carried out in a dilution fridge with a pulse tube as a first stage cooler. SQUIDs polarization and addressing are controlled by a cold ASIC in which a SiGe amplifier has been implemented. First characterisations were done on full membrane bolometers and show a noise level of about $10^{-16}\ \mathrm{W}/\sqrt{\mathrm{Hz}}$ . Those detectors are developed for the QUBIC experiment, a B-modes telescope that will be deployed in Antactica in 2013.     

Suppressing Beam Systematics in Antenna-Coupled TES Bolometers for CMB Polarimetry

We have deployed arrays of antenna-coupled TES bolometers for cosmic microwave background (CMB) polarimetry in the BICEP2 and Keck array experiments and will deploy similar detectors in SPIDER and Polar-1. Each pixel receives optical power centered at 146 GHz with a 20% bandwidth (?10 dB edges) through an integrated dual-polarized phased-array antenna. In past deployments, these detectors have shown offsets in the two polarizations’ beam centroids (differential ‘steering’), but we have redesigned the feed networks by strategically spacing lines and adding intentional phase delays to suppress this unwanted effect. We expect that the focal planes in this season’s deployment of Keck and in SPIDER will have detectors with less than 5′ steering, which is 0.5% of the 14.2° nominal antenna’s FWHM (i.e. without lenses). We have also redesigned the antenna-array’s illumination pattern to suppress side-lobe response in anticipation of the Polar-1 experiment, reducing spillover by a factor of 2.5.     

Diffusion Behaviour in Superconducting Ti/Au bilayers for SAFARI TES Detectors

Controlling the critical temperature ( \(T_{C}\) ) of Ti/Au bilayers is vital in the development of practical TES detectors. Previously empirical studies have been done on aging effects in Ti/Au and other superconducting bilayers but no link with theory has been made. Here we attempt to explain the change in \(T_{C}\) with a diffusion mechanism. The change in \(T_{C}\) has been measured for a set of Ti/Au bilayer samples that have been given a variety of bake-out treatments, where we found a trend that can be partly explained by an inter-diffusion mechanism. With an empirical model based on diffusion a safe zone can be defined as a region of bake-out treatments, where the \(T_{C}\) is not affected beyond the requirements. This will shine light on the bake-out and the storage condition boundaries of these detectors.     

Characterization of Low Noise TES Detectors Fabricated by D-RIE Process for SAFARI Short-Wavelength Band

SRON is developing TES detectors based on a superconducting Ti/Au bilayer on a suspended SiN membrane for the short-wavelength band of the SAFARI instrument on SPICA mission. We have recently replaced the wet KOH etching of the Si substrate by deep reactive ion etching. The new process enables us to fabricate the detectors on the substrate and release the membrane at the very last step. Therefore the production of SAFARI large arrays (43 \(\,{\times }\,\) 43) on thin SiN membrane (250 nm) is feasible. It also makes it possible to realize narrow supporting SiN legs of \(\le \) 1 \(\upmu \) m, which are needed to meet SAFARI NEP requirements. Here we report the current–voltage characteristics, noise performance and impedance measurement of these devices. The measured results are then compared with the distributed leg model that takes into account the thermal fluctuation noise due to the SiN legs. We measured a dark NEP of \(\sim \) 0.7 aW/ \(\sqrt{\hbox {Hz}}\) , which is 1.6 times higher than the theoretically expected phonon noise.     

Comparison of the Effects of Magnetic Field on Low Noise MoAu and TiAu TES Bolometers

Recently we have reported on the effects of magnetic field on our low noise (NEP = 4  \(\times 10^{-19}\) W/ \(\surd \) Hz) [1] TiAu TES bolometers that are being developed at SRON for the SAFARI FIR Imaging Spectrometer on SPICA telescope that will be operated in three different wavelength bands: S-band for 30–60 \(\upmu \hbox {m}\) , M-band for 60–110 \(\upmu \hbox {m}\) and L-band for 110–210  \(\upmu \hbox {m}\) . The arrays for the S- and M- band will be based on TiAu TES bolometer arrays, developed by SRON. The L-band array will be based on a MoAu TES bolometer developed by University of Cambridge. We have investigated the effect of the magnetic field on the current, responsivity, speed and critical current for both the TiAu and MoAu TES bolometers in our high accuracy magnetic field set-up. A clear difference in weak link behavior is observed between the two types of TES bolometers in both strength of the effect and period of the oscillations.     

Recent Performance of Scintillating Bolometers Developed for Dark Matter Searches

Scintillating bolometers of sapphire, BGO and LiF have been recently tested both at the Institut d’Astrophysique Spatiale (IAS) and the Laboratorio Subterráneo de Canfranc (LSC) in the frame of the ROSEBUD collaboration. We report on their response to different particles (α, β, γ and nuclear recoils) both in the heat and light channel. Signal output parameters for the different particles have been compared. We have focused on the study of the nuclear recoils discrimination against β/γ background events at low energy. The β/γ and nuclear recoil spectra obtained in the different runs are shown and analyzed. We also discuss the sensitivity of the LiF bolometer for measuring thermal neutron flux at LSC.      

AC Bias Characterization of Low Noise Bolometers for SAFARI Using an Open-Loop Frequency Domain SQUID-based Multiplexer Operating Between 1 and 5 MHz

SRON is developing the Frequency Domain Multiplexing (FDM) read-out and the ultra low NEP TES bolometers array for the infra-red spectrometer SAFARI on board of the Japanese space mission SPICA. The FDM prototype of the instrument requires critical and complex optimizations. For single pixel characterization under AC bias we are developing a simple FDM system working in the frequency range from 1 to 5 MHz, based on the open loop read-out of a linearized two-stage SQUID amplifier and high Q lithographic LC resonators. We describe the details of the experimental set-up required to achieve low power loading (<1?fW) and low noise ( $\mathrm{NEP}\sim 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}$ ) in the TES bolometers. We conclude the paper by comparing the performance of a $4\times 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}$ TES bolometer measured under DC and AC bias.     

Optical Fiber Dual-Reflectometer for Accurate Measurements

In order to measure the reflection coefficients of opt fiber components as accurately as possible, a new "dual-reflectometer" method is proposed. This method is capable of eliminating the major systematic errors in incoherent continuous-wave reflectometers. Experiments have been made using a measurement system with a light-emitting diode (LED) and graded-index multimode fibers. Experimental results show the effectiveness of the method.     

TES Bolometer Array for the APEX-SZ Camera

We will report on the APEX-SZ bolometer camera which houses a 320 element Transition-Edge Sensor (TES) bolometer array designed to survey for galaxy clusters using the 12-meter diameter APEX telescope sited in Chile. Design and fabrication of the TES bolometer array will be discussed, as well as its integration with a frequency-domain SQUID multiplexed readout system. The full configuration of the APEX-SZ camera was deployed in April 2007. A preliminary galaxy cluster map from this deployment will be presented.      

Navy Prototype Optical Interferometer observations of geosynchronous satellites

Using a 15.9 m baseline at the Navy Prototype Optical Interferometer (NPOI), we have successfully detected interferometric fringes in observations of the geosynchronous satellite (geosat) DirecTV-9S while it glinted on two nights in March 2009. The fringe visibilities can be fitted by a model consisting of two components, one resolved (?3.7 m) and one unresolved (～1.1 m). Both the length of the glint and the specular albedos are consistent with the notion that the glinting surfaces are not completely flat and scatter reflected sunlight into an opening angle of roughly 15°. Enhancements to the NPOI that would improve geosat observations include adding an infrared capability, which could extend the glint season, and adding larger, adaptive-optics equipped telescopes. Future work may test the feasibility of observing geosats with aperture-masked large telescopes and of developing an array of six to nine elements.     

Commissioning SCUBA-2 at JCMT and Optimising the Performance of the Superconducting TES Arrays

SCUBA-2 is a state of the art 10,000 pixel submillimeter camera providing wide-field simultaneous imaging at 450 and 850 microns. The instrument is in the final stages of commissioning at the JCMT and is the largest low temperature detector array in operation. Twin focal planes each consist of four 32 by 40 sub-arrays of superconducting Transition Edge Sensor (TES) bolometers, with inline SQUID time-division multiplexed readout. In this paper we discuss the challenges and steps taken to optimise the performance of the SCUBA-2 arrays and maximise the mapping speed of the instrument. We present results of characterising the eight 1280 bolometer arrays and show the performance of the detectors and the instrument.     

Performance of TES X-ray Microcalorimeters with AC Bias Read-Out at MHz Frequencies

At SRON we are developing Frequency Domain Multiplexing for the read-out of superconducting transition edge sensor microcalorimeters for future X-ray astrophysical missions. We will report on the performance of Goddard Space Flight Center pixels under AC bias in the MHz frequency range. Superconducting flux transformers are used to improve the impedance matching between the low ohmic TESs and the SQUID. We connected 5 pixels to the LC filters with resonant frequencies ranging between 1 and 5 MHz. For X-ray photons of 6 keV we measured a best X-ray energy resolution of 3.6 eV at 1.4 MHz, consistent with the integrated Noise Equivalent Power. In addition, we improved the electrical circuit by optimizing the coupling ratio of the impedance matching transformer. In addition, we improved electrical circuit for impedance matching; modified transformer coupling ratio. As a result, we got the integrated noise equivalent power resolution of 2.7 eV at 2.5 MHz. A characterization of the detector response as a function of the AC bias voltage, bias frequency and the applied magnetic field is presented.     

Accurate Measurements of the Zero-Dispersion Wavelength in Optical Fibers

We have developed a frequency-domain phase shift system for measuring the zero-dispersion wavelength and the dispersion slope of single-mode optical fibers. A differential phase shift method and nonlinear four-wave mixing technique were also investigated. The frequency-domain phase shift method is used to produce Standard Reference Materials that have their zero-dispersion wavelengths characterized with an expanded uncertainty (k = 2) of ± 0.060 nm.