the probe is aligned with one of the windows 21 of the splice tray t 39 the probe is then activated to force the optical head 23 against the associated fibre and the mandrel 24 in this position the operating position the optical fibre is urged into the recess 26 by the mandrel 24 when the optical fibre is positioned in the recess 26 the rounded v shaped portion 26b subjects the fibre to a tight bend of short arcuate length a kink this causes light carried by the fibre to leak out of the fibre over a very small region thereof in the vicinity of the rounded v shaped portion 26b light is therefore tapped out of the fibre from practically a point source and then travels through the block 25 in a narrow but slightly diverging beam this beam is directed to the photodetector by total internal reflection from an angled side surface 25a of the block 25 the output of the photodetector is coupled to an optical receiver circuit not shown and fed via an umbilical cable to the electronics of a power meter not shown associated with the probe it will be apparent that modifications could be made to the arrangement described above with reference to fig6 and 7 for example each window 1 could be provided with a mandrel rather than having the mandrel forming part of the probe also the.