Performance Characterisation of an Optical Fibre for ALPHA

Optical fibres present a simple, time-saving replacement to the use of a periscope for sending light over a large distance. This is a requirement for laser cooling of beryllium ions which sympathetically cool positrons in ALPHA, an important step in the production of trappable antihydrogen. The transmission losses of an LMA-10-PM fibre and its ability to correct the beam profile are presented here. A maximum transmission of (32.1±0.3)% was found, however it is believed that this value could be improved with stretching of the fibre. It was also found that increasing the diameter of bends in the excess length of fibre improved the transmission up to a diameter of 38cm, above which the transmission appeared to decrease again. It was shown that defects in the beam profile before entering the fibre were corrected when the fibre output was measured. Temperature effects were also investigated and it was found that an increase of 4.4°C approximately halved the transmission from the fibre. In future, if the environment temperature continues to be unstable, a non-polarisation-maintaining fibre would be a more suitable choice.