Optical performance of an ultra-sensitive horn-coupled transition-edge-sensor bolometer with hemispherical backshort in the far infrared

The next generation of far infrared space observatories will require extremely sensitive detectors that can be realized only by combining extremely low intrinsic noise with high optical efficiency. We have measured the broad-band optical response of ultra-sensitive TES bolometers (NEP$\approx2\rm\ aW/\sqrt Hz$) in the 30--60-$\mu\rm m$ band where radiation is coupled to the detectors with a few-moded conical feedhorn and a hemispherical backshort. We show that these detectors have an optical efficiency of 60% (the ratio of the power detected by the TES bolometer to the total power propagating through the feedhorn). We find that the measured optical efficiency can be understood in terms of the modes propagating through the feedhorn with the aid of a spatial mode-filtering technique.