Cooling an optical fiber to 4.5 K by indirect thermal contact with a liquid-helium flow and spectroscopic temperature measurements.

We have made it possible to cool a 10-m-long optical fiber to 4.5 K in a compact helium-flow cryostat. The cryostat is vacuum loaded and the cooling principle is based on thermal contact with a copper holder cooled by the liquid-helium flow. To measure the temperature directly, we employed two different spectroscopic methods depending on the temperature regions. One was photon-counting Raman spectroscopy and the other was Brillouin spectroscopy. We used certain devices to improve the cooling efficiency and confirmed that the fiber was uniformly cooled to 4.5 K.