The ends of the fibre were held inside a pair of gas cells for the evacuation and delivery of gas to the system, inside which a pair of electrodes were positioned near the fibre ends to deliver a DC high voltage of up to 40 kV with a variable current. The electrical characteristics were then measured using a multimeter. The anode gas cell featured a large calcium fluoride window perpendicular to the fibre, and the cathode had a sapphire window for the output signal. The fibre used had a hollow core inner diameter of approximately 120 μm, and a low loss guidance band from 3–4 μm (< 0:18 dB/m at 3:5 μm). The spectrometer (Bentham DTMc300) had a grating with 300 grooves/mm and a cryogenically cooled InSb detector for the range at 3–4 μm.