A novel optical fibre sensor has been developed which is capable of monitoring ultra violet light output from a high power (>100 W) light source. This sensor comprises of a large core multimode fibre which has had its cladding removed and been coated with a Phosphor doped polymer. Experiments are described which show the sensors response to varying ultra violet light powers and the phosphor's absorption spectrum. A significant application for this sensor is the detection of microwave radiation in a cavity or waveguide of a microwave source using a miniaturised version of the tube and fibre. A proposal for this design is included.