Superconducting bolometer

Model QNbTES/X bolometer

     See below for notes relating to the above specification.

Our new superconducting bolometers replace our semiconductor (composite germanium) bolometers. Offering the same high sensitivity, in addition they provide much greater linear dynamic range and speed of response along with lower susceptibility to environmental noise. They are provided with a new read-out unit which controls and optimises the detector operating parameters automatically, and has both analogue and digital output options. The bolometer incorporates a niobium superconducting thermistor attached to an absorber deposited onto a thin silicon nitride (SiN) support substrate. The SiN has very low thermal conductivity at cryogenic temperatures and serves to isolate the thermistor and absorber from the cryogenic environment. Radiation incident on the absorber causes its temperature to rise as does that of the thermistor. The thermistor is operated at a temperature where it is making the transition from the normal state to the superconducting (zero resistance) state. The resistance thus changes sharply over a very small temperature range, making it an extremely sensitive thermometer. This resistance is monitored by the read-out system. The bolometer is most commonly mounted in an optical integrating cavity behind Winston Cone coupling optics along with low-pass filters which ensure that unwanted higher frequencies are efficiently rejected. It is important that the field of view of the coupling optics be matched to the incoming beam geometry. This ensures not only that all available signal power is available for detection but also that the bolometer is not unduly de-sensitised by exposure to an unnecessarily high background power.

The bolometer Bolometer and Winston cone in cryostat

Detectors can be purchased as a stand-alone unit or as part of a fully assembled, tested and calibrated detector system ready for use. As with all our cryogenic detector systems, cooling can be provided either by a mechanical (pulse tube) cooler or by liquid helium. Mechanically cooled systems are complete and - apart from a vacuum pump for initial evacuation of the system - require only an electricity supply to operate.

Our liquid helium bath cryostats are designed and built to our specifications by our sister company, Thomas Keating Ltd. These cryostats offer the convenience of very long run-times. Cryostat operation is straightforward, and detector systems come with a comprehensive range of safety devices as standard. Superior cryogenic performance allows significant long-term saving on the cost of liquid helium. Larger cryostats are suitable for multi-channel detector systems or when very long liquid helium hold times are required.

Each detector system is evaluated and designed on an individual basis to provide optimum performance in all applications. Direction of view, throughput, field-of-view, filter selection, cryogenic performance and many more aspects of the detector system can be custom designed often at no extra cost.

Notes on specification

• Detector optical responsivity is specified at 275 GHz

• Detector optical NEP is specified at 275 GHz (80 Hz modulation)

• The high frequency limit of the range of a bolometer system is set by low-pass filters mounted at both 77 K and 4.2 K. We offer a range of standard filters with transmission edges at 0.3, 0.6, 1, 2, 3, 6, 10, 13.5 and 18 THz (10, 20, 33, 66, 100, 200, 300, 450 and 600 cm^-1 respectively). Further information can be found here.