Characterising the optical response of ultra-low-noise far-infrared 60-110 $\mu$m transition edge sensors

TL;DR

This paper presents a cryogenic test facility for precise optical measurements of ultra-low-noise far-infrared TESs, demonstrating characterization techniques and analyzing their optical response in the 60-110 μm range for space telescope applications.

Contribution

We developed a specialized cryogenic setup for detailed optical testing of ultra-low-noise TESs and provided comprehensive characterization methods for their optical response in the far-infrared.

Findings

Successful measurement of optical efficiency and saturation.

Observation of thermalisation through transient response comparison.

Identification of stray light challenges in the setup.

Abstract

Far-infrared Transition Edge Sensors (TESs) are being developed for the SAFARI grating spectrometer on the cooled-aperture space telescope SPICA. In support of this work, we have devised a cryogenic (90 mK) test facility for carrying out precision optical measurements on ultra-low-noise TESs. Although our facility is suitable for the whole of the SAFARI wavelength range, 34-230 $\mu$m, we focus on a representative set of measurements at 60-110 $\mu$m using a device having a Noise Equivalent Power (NEP) of 0.32 $\mathrm{aW/\sqrt{Hz}}$. The system is able to perform a range of measurements: (i) Dark electrical characterisation. (ii) Optical efficiency with respect to a partially coherent beam having a modal composition identical to that of an ideal imaging telescope. (iii) Optical saturation and dynamic range. (iv) Fast optical transient response to a modulated thermal source. (v) Optical transient response in the presence of high-level background loading. We describe dark measurements to determine the operating characteristics of a TES, and then compare predicted optical behaviour with measured optical behaviour. By comparing electrical and optical transient response, we were able to observe thermalisation in the device. We comment on the challenge of eliminating stray light.