Optical characterization of ultra-sensitive TES bolometers for SAFARI

Abstract

We have characterized the optical response of prototype detectors for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI's three bolometer arrays will image a 2'×2' field of view with spectral information over the wavelength range 34-210 µm. SAFARI requires extremely sensitive detectors (goal NEP ~ 0.2 aW/√Hz), with correspondingly low saturation powers (~5 fW), to take advantage of SPICA's cooled optics. We have constructed an ultra-low background optical test facility containing an internal cold black-body illuminator and have recently added an internal hot black-body source and a light-pipe for external illumination. We illustrate the performance of the test facility with results including spectral-response measurements. Based on an improved understanding of the optical throughput of the test facility we find an optical efficiency of 60% for prototype SAFARI detectors.

The SAFARI detector system

“…Figure 2 shows a TES bolometer fabricated at SRON [14] . SAFARI's TES bolometers are two orders of magnitude more sensitive than bolometers used for ground-based observations [19] and testing them requires careful attention to stray-light protection and shielding from electrical, magnetic, and mechanical disturbances [20] . The detector requirements are within reach: we have demonstrated the required NEP≤2×10 -19 W/√Hz [21] and optical efficiency ≥50% for single detectors [22] .…”

Optical performance of prototype horn-coupled TES bolometer arrays for SAFARI

“…These detectors were designed for SAFARI's short-wave band, with the wavelength range λ=34-60 µm. Each detector sat above a hemispherical backshort [6] . In front of the array was an 8×8 array of electroformed rectangular feedhorns, with a pixel pitch of 830 µm, as shown in Figure 4.…”

“…We have successfully measured the optical response of SAFARI prototype detectors all the way to saturation using the cold illuminator [6], [7] . However, because of the low temperature (3.4-34 K) of this black-body emitter, the pass-band of the short-wave detectors is illuminated by the Wien tail of the black-body spectrum.…”

“…We needed the reference detector to have a high saturation power in case the light-pipe attenuation factor was too low, but at the same time be as sensitive as possible in case it was too high. To increase the dynamic range of the reference detector we connected the Ti/Au bilayer in series with the Ta absorber, giving a double IV-curve [6] . We then had a detector that could operate in two regimes.…”

“…In addition to the strict requirements on background and interference, we *audley@physics.org; phone +31 (0)50 363 9361; fax +31 (0)50 363 4033; www.sron.nl require that this facility be flexible and re-configurable so that we can use it for dark and optical tests of single pixels through to the full focal-plane arrays. Through a systematic program of incremental modifications we improved the performance of the SAFARI Detector Test Facility to the point where it is now being used for routine measurements of prototype SAFARI detectors [6] . We have used it to confirm the high optical efficiency of prototype SAFARI single detectors and to understand their optical response in terms of the electromagnetic modes admitted by the feedhorns [7] .…”